Article

Feeding by grey seals on endangered stocks of Atlantic cod and white hake

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Abstract

High natural mortality is preventing the recovery of collapsed stocks of Atlantic cod and white hake in the southern Gulf of St Lawrence, Canada. Predation by grey seals has been proposed as an important cause of this high mortality. We determined the contribution of cod and hake to the diet of grey seals collected along the west coast of Cape Breton Island and in the Cabot Strait, an area where overwintering cod aggregate. Along the coast of Cape Breton Island, the contribution of hake and cod to the diet was 30 and 17%, respectively, by weight using stomach contents and 13 and 9%, respectively, based on intestine contents. In the Cabot Strait, when overwintering aggregations of cod were present, cod accounted for 68% (range 57–80%) of the male diet from stomachs, and 46% (range: 31–64%) of the diet determined from intestines. Among females, cod represented 14% (range: 0–34%) and 9% (range: 3–54%) of the diet from stomachs and intestines, respectively. In Cabot Strait, white hake accounted for up to 17% of the diet by weight from stomachs, and up to 6% of the diet determined from intestines. The mean length of cod consumed by seals was 28 cm (SD = 8.6) along the coast of Cape Breton Island, and 39 cm (SD = 5.7) in Cabot Strait. The mean length of hake consumed by seals was 29 cm (SD = 7.0) along the coast of Cape Breton Island, and 35 cm (SD = 5.6) in Cabot Strait. Cod and hake are more important to the diet of males than that of females. The contribution of cod to the diet of grey seals foraging in the cod overwintering area is much greater than has been reported elsewhere.

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... The abundance of grey seals foraging in this ecosystem has increased dramatically over the past 50 yr (Fig. 8), coincident with the increases in natural mortality of cod and other large demersal fishes in the sGSL (Chouinard et al. 2005, Swain et al. 2009). With the exception of skates, these fishes are all known to be important prey of grey seals foraging in the sGSL (Hammill et al. 2007(Hammill et al. , 2014. Skates lack the bony parts normally used to identify prey in seal diet studies in the sGSL but have also been identified as prey of grey seals in this region (Benoît & Bowen 1990, Beck et al. 2007. ...
... In late fall and winter, foraging by grey seals, in particular males, appears to be associated with overwintering aggregations of cod in the Cabot Strait (Harvey et al. 2012). The contribution of cod to the diet of seals foraging in the vicinity of these aggregations is much greater than has been reported elsewhere: 52 to 79% of prey weight based on prey recovered from stomachs and 25 to 63% based on prey recovered from intestines (Hammill et al. 2014). Flatfish, likely mainly American plaice, comprised 20 to 60% of the diet of these seals based on intestines. ...
... The mean length of cod in the nearshore diet samples was 26 to 28 cm (Hammill et al. 2007). The cod consumed in winter offshore samples were considerably larger, ranging between 10 and 76 cm and averaging 34 to 45 cm (Hammill et al. 2014). The majority of cod in the offshore samples were estimated to be greater than 35 cm in length (51, 67 and 75%, depending on year). ...
Conference Paper
The population of Atlantic cod in the southern Gulf of St. Lawrence collapsed in the early 1990s due to overfishing. Fishing mortality has been at negligible levels since then, but the population has continued to decline. This lack of recovery results from dramatic increases in the natural mortality of adult cod. Among the potential causes for this high mortality, only the hypothesis that it reflects increased predation by grey seals received strong support. Grey seal abundance has increased roughly 40-fold since 1960. Grey seals feed intensively on cod aggregations in the southern Gulf and appear to prey preferentially on the large cod with high natural mortality. Grey seals and cod co-existed in the Northwest Atlantic in earlier centuries when both occurred at high abundance. The current high natural mortality of southern Gulf cod appears to represent an emergent Allee effect, resulting from the recovery of grey seal abundance at a time when cod abundance is at exceedingly low levels. Even in the absence of fishing, recovery of the southern Gulf cod population is not possible under current ecosystem conditions.
... The proportion of White Hake occurring in inshore areas has declined over time, with White Hake virtually absent from these areas in recent years (Swain et al. 2016). This shift in the distribution of adult White Hake is strongly related to risk of predation by Grey Seal, an important predator of White Hake (Hammill et al. 2014;. As seal abundance increased, White Hake distribution shifted into deep waters where risk of predation by Grey Seal remained low. ...
... Given the extremely high M currently experienced by adult White Hake, this population remains viable only because of the coincident increase in recruitment rates to extremely high levels. The high adult M has been attributed to increased predation by Grey Seal (Hammill et al. 2014. The increase in recruitment success may be due to a relaxation of density-dependent constraints on productivity. ...
... The failed recovery of this population even with fishing mortality reduced to negligible levels is due to extremely high natural mortality, which has reached levels near 90% annually. This high natural mortality has been attributed to predation by Grey Seal (Hammill et al. 2014. ...
Technical Report
Full-text available
The southern Gulf of St. Lawrence (sGSL) White Hake Designatable Unit (DU) has been assessed as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). This DU consists primarily of White Hake occurring in the Northwest Atlantic Fishery Organization (NAFO) Division 4T. The Recovery Potential Assessment for this stock found that extremely high natural mortality was preventing the recovery of this stock, while fishing mortality with a bycatch limit of 30 t has a negligible effect on the population trajectory. However, if fishing effort is increased as proposed with the expansion of the Redfish fishery, the impacts of bycatch fisheries on this population may no longer be considered negligible. This report aimed to examine existing information from surveys and fisheries to evaluate whether increased catch levels of Redfish would result in increased bycatch of White Hake. Significant overlap in the spatial distribution of White Hake and Redfish was noted in along the deep waters of the Laurentian Channel. This is partly the result of diet interdependencies among these species and the shift of White Hake to deeper waters to avoid predation. Bycatch of White Hake associated with Redfish was lower at depths greater than 380 m, whereas bycatch was much greater in the months of June, July, and December. Bycatch did not differ significantly based on gear type, however bycatch was overall greater in the southern region of the Laurentian Channel. Overall, the mean value of bycatch was 10.5% in the Redfish fishery. Even in the absence of fishery removals, the sGSL White Hake stock is expected to decline due to extremely high natural mortality. The sGSL White Hake population was projected forward 25 years assuming that productivity would remain at recent levels. SSB was estimated to decline by 38.7% with no catch and 39.3% with annual bycatch of 20 t, the recent level. With annual bycatch of 150 t to 350 t, SSB was estimated to decline by 43% to 48%. With bycatch of 500 t to 1,500 t, SSB declined by 53% to 70%. At present, the White Hake stock is sustained by unusually high recruitment rates which depend largely on a single cohort each year (age 4). The extinction risk for this stock (below 2,000 t) is 22 to 26% with no bycatch up to 150 t, and increases to 30% and 49% at bycatch levels of 350 t to 1,500 t respectively. If recruitment rates were to decline even slightly to the levels seen in the 2000s, the extinction risk for this stock would increase. At the present 30 t bycatch limit for White Hake, White Hake will become a choke species for the future Redfish fishery.
... Grey seals in eastern Canadian 52 waters compose a single genetic population estimated to number 424,300 individuals in 2016 and 53 distributed across 3 breeding areas: Sable Island, coastal areas of Nova Scotia, and the GSL 54 (Hammill et al., 2017). Since the end of an intensive hunt in the 1960s, the population has been 55 slowly increasing and predation by grey seals may be impacting the recovery of groundfish 56 stocks which collapsed due to overfishing (Swain et al., 2019;Hammill et al., 2014). The total 57 allowable catch for grey seals in Canada is set at 60,000, however only 1,612 seals were 58 harvested commercially in 2016 which is less than 1% of the Northwest Atlantic population 59 (Government of Canada, 2016a). ...
... The lack of significant differences in most trace elements 405 concentrations among spring YY (5-6 mos), juveniles (1 -4 yrs) and adults (11 -29 yrs) suggests 406 a similar diet after weaning for a broad range of ages of grey seals in the GSL. Indeed, a previous 407 study from the same region showed no significant difference in grey seals' diet as a function of 408 age with seals from 1 -33 yrs old (Hammill et al., 2014). 409 ...
... shown to eat a greater proportion of higher trophic-level fish, such as cod and hake, which likely 440 have higher mercury concentrations than lower trophic-level species, like sandlance and herring, 441 which are consumed in greater proportions by female seals (Hammill et al., 2014). 442 ...
Article
We measured concentrations of 19 trace elements and mercury speciation in grey seals (Halichoerus grypus) from the Gulf of St. Lawrence (GSL), Canada. With interest growing in commercializing grey seal products for human consumption in this region, our goal was to measure essential and non-essential trace elements in grey seals to evaluate health concerns and nutritional benefits. From 2015 to 2019, 120 grey seals were sampled by hunters and researchers at 4 sites in the GSL. Muscle, liver, heart and kidney samples were analyzed for 10 non-essential elements (Sb, As, Be, B, Cd, Pb, Hg, Ni, Tl, Sn) and 9 essential elements (Co, Cr, Cu, Fe, Mg, Mn, Mo, Se, Zn). Both total mercury (THg) and methylmercury (MeHg) were analyzed for a subset of samples. Results showed a two-step bioaccumulation pattern with lower element concentrations in the muscle (Fe, Mg, Se) and livers (Cd, Cr, Hg, Mn, Mo, Se) of young-of-the-year harvested in the winter (<6 wks old) compared to older animals feeding at sea. We did not observe progressive age-dependent bioaccumulation for older seals (∼5 mos–29 yrs). Sex-specific differences were not very pronounced, but a few elements were 30–70% higher in the muscle (THg, MeHg) and liver (Mn, Zn) of male seals. Comparison to Canadian dietary reference intakes shows that a weekly portion of liver from young-of-the-year (<6 wks old) is a good source of essential elements (Cu, Fe) and that muscle and liver from this age category do not exceed reference values for toxic elements (As, Cd, Pb, MeHg). Discussions with regional public health professionals are on-going to develop dietary recommendations for the consumption of older grey seals.
... The abundance of grey seals foraging in this ecosystem has increased dramatically over the past 50 yr (Fig. 8), coincident with the increases in natural mortality of cod and other large demersal fishes in the sGSL (Chouinard et al. 2005, Swain et al. 2009). With the exception of skates, these fishes are all known to be important prey of grey seals foraging in the sGSL (Hammill et al. 2007(Hammill et al. , 2014. Skates lack the bony parts normally used to identify prey in seal diet studies in the sGSL but have also been identified as prey of grey seals in this region (Benoît & Bowen 1990, Beck et al. 2007. ...
... In late fall and winter, foraging by grey seals, in particular males, appears to be associated with overwintering aggregations of cod in the Cabot Strait (Harvey et al. 2012). The contribution of cod to the diet of seals foraging in the vicinity of these aggregations is much greater than has been reported elsewhere: 52 to 79% of prey weight based on prey recovered from stomachs and 25 to 63% based on prey recovered from intestines (Hammill et al. 2014). Flatfish, likely mainly American plaice, comprised 20 to 60% of the diet of these seals based on intestines. ...
... The mean length of cod in the nearshore diet samples was 26 to 28 cm (Hammill et al. 2007). The cod consumed in winter offshore samples were considerably larger, ranging between 10 and 76 cm and averaging 34 to 45 cm (Hammill et al. 2014). The majority of cod in the offshore samples were estimated to be greater than 35 cm in length (51, 67 and 75%, depending on year). ...
Article
Improved understanding of the dynamics of populations at low abundance is needed in the face of global biodiversity loss. We examined the dynamics of depleted demersal fish populations in the southern Gulf of St. Lawrence, Canada. Twenty years ago, a number of these populations collapsed due to overexploitation. Since then, others have declined to low abundance. Despite negligible levels of fishing mortality and strong rates of production of small juvenile fish, these populations have shown no sign of recovery and some continue to ecline. Lack of recovery is due to dramatic increases in the natural mortality of larger individuals in these populations. In some of these fishes, natural mortality has risen to levels typical of high-turnover forage fishes rather than long-lived demersal fishes. We hypothesize that these high levels of mortality reflect a ‘predator pit’ or predation-driven Allee effect, resulting from the severely depleted abundance of these fishes and the high and rising abundance of their marine mammal predators, in particular grey seals. Recovery of collapsed demersal fish populations does not appear to be possible under current conditions in this ecosystem, even in the absence of fishing. Our results indicate a need for more precautionary management regimes in order to avoid population collapses that are not reversible by reducing exploitation.
... This did not happen despite greatly reduced fishing mortality (including long periods under moratorium) because the sizeselective pressure due to fishing has been replaced by sizeselective predation. Available information indicates the largest cod remaining in the population are still subject to very high predation by grey seals (Halichoerus grypus), especially on cod's overwintering concentrations (Hammill et al. 2014). Because grey seal have a generalist fish diet, essentially any fish >15-20 cm long (Hammill et al. 2007(Hammill et al. , 2014 can be eaten. ...
... Available information indicates the largest cod remaining in the population are still subject to very high predation by grey seals (Halichoerus grypus), especially on cod's overwintering concentrations (Hammill et al. 2014). Because grey seal have a generalist fish diet, essentially any fish >15-20 cm long (Hammill et al. 2007(Hammill et al. , 2014 can be eaten. The grey seal population is still increasing (Hammill et al. 2014;, and they can be expected to continue preying on cod as a function of availability as well as consuming fish prey formerly eaten by cod. ...
... Because grey seal have a generalist fish diet, essentially any fish >15-20 cm long (Hammill et al. 2007(Hammill et al. , 2014 can be eaten. The grey seal population is still increasing (Hammill et al. 2014;, and they can be expected to continue preying on cod as a function of availability as well as consuming fish prey formerly eaten by cod. Thus, the high level of predation on the largest remaining individuals appears to be continuing the selection against large size-at-age cod (Swain 2011;. ...
Article
The American lobster (Homarus americanus) population in southern Gulf of St. Lawrence has long been subjected to high exploitation, and yet its population is currently at a high and increasing abundance level. The lobster fishery management is based on effort-control, with a short season, mandatory release of egg-bearing females, and strict enforcement of regulations. Another important factor is the high survival of lobster returned to the water. The combination of a minimum legal size limit and either an upper size limit for females or an effective size limit due to entrance-ring size on the traps has resulted in a slot fishery after which the larger, most fecund animals have low vulnerability to the fishery. These efforts to protect large individuals have had a positive effect on lobster larval production, which may lead to even higher adult population numbers. Comparisons with the management of snow crab (Chionoecetes opilio) and Atlantic cod (Gadus morhua) quota-based fisheries were made to try to explain the different trajectories that these three species’ populations have taken since the 1960s.
... Some improvements have been also done to account for complete digestion using feeding experiments on captive individuals. The factors applied are called number correction factors (NCFs) and began to be commonly used when dealing with seal diet (Tollit et al., 1997;Bowen, 2000;Grellier and Hammond, 2006;Harris, 2007;Lundström et al., 2007;Gosch et al., 2014;Hammill et al., 2014). Even if it is believed that seal diet estimates corrected for partial and complete digestion are reliable, these processes can vary with individual seals but also with prey and NCFs may not be available for some fish species in certain geographic areas (Tollit et al., 1997;Grellier and Hammond, 2006;Bowen and Iverson, 2013). ...
... The second possible method to study grey seal diet is the analysis of hard prey remains in stomach and/or intestine contents. This technique has been used to determine seal diet mainly in Canada and the Baltic Sea (Bjørge et al., 1981;Andersen et al., 2007;Lundström et al., 2007;Lundström et al., 2010;Benoît et al., 2011a;Kauhala et al., 2011;Suuronen and Lehtonen, 2012;Hammill et al., 2014) but also cetacean (Bjørge et al., 1981), bird and fish diets (Bundy, 2005;Bundy and Fanning, 2005). ...
... However in the West of Scotland, despite the decrease in cod stock from 1985 to 2002, the consumption of cod by grey seals has not changed Harris, 2007) and this contrasts with the idea of switching. Some researchers have demonstrated that grey seals movements follow fish distribution or aggregations (Harvey et al., 2012;Hammill et al., 2014); this could be an evidence of a certain diet preference. Grey seal diet varies spatially also at a small spatial scale (Lundström et al., 2007;Lundström et al., 2010;Brown et al., 2012;Lundström, 2012). ...
Thesis
Full-text available
It has been several decades that groundfish stocks have decreased around the UK. Meanwhile, grey seal population has increased. This has created a controversy between fishers and conservationists as regards to the role grey seals have played in the stock depletion. Currently, opinions are still divided, and further studies need to be done to mitigate these conflicts. A bioeconomic model able to quantify the economic impact of grey seal predation on West of Scotland demersal fisheries for cod, haddock and whiting was developed. The biological part of the model accounts for seal predation and fishing catches and is linked to an economic model accounting for fleet revenues and costs. Three scenarios are tested. The “status quo F” model assesses seal predation impacts on fleet revenues at the biological equilibrium. Two dynamic models are also studied to determine seal impacts when fleet behaviour is considered: the maximum economic yield scenario (MEY) where the fishery net profit is maximised and the bioeconomic equilibrium (BE) model where the profits are dissipated in the long-run. Cod is the fish the most impacted by grey seal predation so is the key stock in evaluating fishery effects. While the biological impacts can be important, seal predation is not economically important at the fishery level but some fleets are more sensitive than others. The large whitefish trawlers are likely to be the only fleet that could benefit from a reduction in grey seal predation. The following increase in its revenues would be certainly improved by fishery regulations.
... on gadoid recovery as well as bottom-up processes that link physical dynamics, primary production, and prey abundance to body growth, recruitment, survival, and recovery of gadoid stocks (e.g. Hammill et al., 2014;Neumann et al., 2014). Rindorf (oral presentation) summarized the key processes at work as (i) low abundance of prey fish impairs growth and increases cannibalism, (ii) at low abundance, gadoids may end up in predatory pits from which they can only escape if there is a large year-class, and (iii) not knowing natural mortality makes predicting recovery highly uncertain. ...
... The question of top-down control of gadoid stock dynamics by seals was taken up. A study on grey seal (Halichoerus grypus) diet composition on cod overwintering grounds in the southern Gulf of St Lawrence revealed the highest ever reported contributions of cod to grey seal diet (Hammill et al., 2014). Seal feeding aggregations co-occur in time and space with large demersal fish aggregations and comparison of diet compositions and prey abundance indicates that seals prefer large cod and white hake (Urophycis tenuis; Benoit et al., oral presentation), actively targeting water layers of high cod abundance (Harvey et al., poster presentation). ...
... All areas are characterized by dramatic increases in a seal species as top predators. However, there is a contrast between areas because the Grey seal Halichoerus grypus, a large piscivore able to eat large groundfish (Hammill et al., 2014), mainly occurs in the southern GSL, but also in small numbers around Anticosti and the west coast of Newfoundland in the northern GSL, while the Harp seal Phoca groenlandica, which is known to eat smaller fish as well as small invertebrates, feeds mainly in the northern regions of the GSL (Hammill and Stenson, 2000). ...
... In contrast, seal abundance has increased considerably, and many species showed strong negative regression coefficients with the seal predation index. The notable exceptions are Greenland and Atlantic halibut, two species that are very uncommon in the diets of these seals (Hammill and Stenson, 2000;Hammill et al., 2007Hammill et al., , 2014Stenson, 2012). The predation effect also seems stronger in the SGSL, where most fish taxa showed high negative coefficients with the Grey seal index. ...
Article
Full-text available
The analysis of biomass time-series from standardized surveys, particularly comparing patterns of variation and their supposed relationship with drivers, can provide insight into how fish species and communities as a whole respond to environmental, trophic, and fishing-related drivers of change in ecosystems. Here we describe and compare the common patterns of temporal variations in demersal fish communities among three distinct ecoregions of the Gulf of St Lawrence (GSL; Atlantic Canada) for 1990–2013, a period during which many commercial groundfish stocks collapsed due to fishing and subsequently failed to recover. Dynamic factor analysis was used to estimate common trends in biomass densities, to assess synchrony among species, and to investigate the influence of key drivers on those trends. The analyses revealed similar temporal patterns of variations for all regions: the trends fluctuated between periods of negative and positive slopes with transitions around the late 1990s or early 2000s and again in the late 2000s. Over the last two decades, the effect of predation by seals was more important than an effect of fishing, especially in the southern GSL. Warming trends in the upper layer of the water column and in the deep water masses may have affected the dynamics of species in those communities, especially in the northern GSL. Fluctuations in the biomasses of two important commercial species (Greenland halibut Reinhardtius hippoglossoides and Atlantic halibut Hippoglossus hippoglossus) influenced trends in all regions. The effect of weakly exploited and non-commercial species appeared more important late in the series, and the species associated with trends varied according to the region.However, the rarity ofmanysmall-demersal species from the bottom-trawl surveys in the northern GSLmaylimit detection of changes in this community.
... The exact causes for increased mortality in some depleted cod populations remain to some extent unclear and debatable, and the amount of unintentional fishing mortality through bycatches is not always well known. Nonetheless, the weight of evidence suggests that natural mortality has increased and that it can be partially attributed to predation of cod by seals [4,[17][18][19]. In eastern Canada, concurrently with a 97% reduction in cod biomass [3], the abundance of grey seals (Halichoerus grypus) increased 50-fold [20] while that of harp seals (Pagophilus groenlandicus) has increased fourfold [21]. ...
... In eastern Canada, concurrently with a 97% reduction in cod biomass [3], the abundance of grey seals (Halichoerus grypus) increased 50-fold [20] while that of harp seals (Pagophilus groenlandicus) has increased fourfold [21]. Despite large reductions in cod abundance, the proportion of cod in seal diets in some areas has remained substantial [17,19,22]. This general pattern suggests that relative cod mortality by seals can increase as cod abundance decreases; that is, their predator-prey dynamics exhibit elements characteristic of a type-II or type-III functional response [2]. ...
Article
Full-text available
Negative density-dependent regulation of population dynamics promotes population growth at low abundance and is therefore vital for recovery following depletion. Inversely, any process that reduces the compensatory density-dependence of population growth can negatively affect recovery. Here, we show that increased adult mortality at low abundance can reverse compensatory population dynamics into its opposite-a demographic Allee effect. Northwest Atlantic cod (Gadus morhua) stocks collapsed dramatically in the early 1990s and have since shown little sign of recovery. Many experienced dramatic increases in natural mortality, ostensibly attributable in some populations to increased predation by seals. Our findings show that increased natural mortality of a magnitude observed for overfished cod stocks has been more than sufficient to fundamentally alter the dynamics of density-dependent population regulation. The demographic Allee effect generated by these changes can slow down or even impede the recovery of depleted populations even in the absence of fishing.
... Grey seals in 68 eastern Canadian waters compose a single genetic population estimated to number 424,300 69 individuals in 2016 and distributed across 3 breeding areas: Sable Island off Nova Scotia, coastal 70 areas of Nova Scotia, and the GSL (Hammill et al., 2017). Since the end of an intensive hunt in 71 the 1960s, the population has been slowly increasing and increased predation by grey seals may 72 be impacting the recovery of cod and other groundfish stocks which collapsed due to overfishing 73 (Hammill et al., 2014;Swain et al., 2019)). The total allowable catch for grey seals in Canada is 74 set at 60,000, however only 1,612 seals were harvested commercially in 2016 which is less than 75 1% of the Northwest Atlantic population (Government of Canada, 2016a). ...
... shown to eat a greater proportion of higher trophic-level fish, such as cod and hake, which likely 466 have higher mercury concentrations than lower trophic-level species, like sandlance and herring, 467 which are consumed in greater proportions by female seals (Hammill et al., 2014). 468 ...
Preprint
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We measured baseline levels of 19 trace element and mercury speciation for grey seals (Halichoerus grypus) from the Gulf of St. Lawrence (GSL), Quebec, Canada. With interest growing in commercializing grey seal products for human consumption in this region, the goal of this study was to measure essential and non-essential trace elements in grey seals to evaluate health concerns and nutritional benefits. From 2015 to 2019, 120 grey seals were sampled by hunters and researchers at 4 sites in the GSL. Muscle, liver, heart and kidney samples were analyzed for 10 non-essential elements (Sb, As, Be, B, Cd, Pb, Hg, Ni, Tl, Sn) and 9 essential elements (Co, Cr, Cu, Fe, Mg, Mn, Mo, Se, Zn). Both total mercury (THg) and methylmercury (MeHg) were analysed for a subset of samples. Many elements were undetected in liver (Sb, As, Be, B, Cr, Co, Pb, Ni, Tl, Sn) and muscle tissues (same, plus Cd, Mn, Mo). Results showed lower element concentrations in the muscle (Fe, Mg, Se) and livers (Cd, Cr, Hg, Mn, Mo, Se) of young-of-the-year harvested in the winter (< 6 weeks old) compared to older animals feeding at sea. For older seals (~ 5 months to 29 years), we did not observe progressive age-dependent bioaccumulation. Sex-specific differences were not very pronounced, but a few elements were 30 - 70% higher in the muscle (THg, MeHg) and liver (Mn, Zn) of male seals. Comparison to Canadian dietary reference intakes shows that a weekly portion of liver from young-of-the-year (< 6 weeks old) is a good source of essential elements (Cu, Fe) and that muscle and liver from this age category does not exceed reference values for toxic elements (As, Cd, Pb, MeHg). Ongoing discussions with regional public health professionals will help to develop dietary recommendations for the consumption of older grey seals.
... The proportion of White Hake occurring in inshore areas has declined over time, with White Hake virtually absent from these areas in recent years (Swain et al. 2016). This shift in the distribution of adult White Hake is related to risk of predation by Grey Seal, an important predator of White Hake (Hammill et al. 2014;. As seal abundance increased, White Hake distribution shifted into deep waters where risk of predation by Grey Seal remained low. ...
... The natural mortality of large adult individuals has increased to very high levels in many fish species in the sGSL . This elevated natural mortality has been attributed to predation by Grey Seal in these species (e.g., , Neuenhoff et al. 2019 including White Hake (Hammill et al. 2014, Swain et al. 2016). ...
... The asterisk (*) denotes articles that apply to more than one category of community change or restoration status. (Chouinard et al. 2005, Benoit and Swain 2008, Swain et al. 2011, Hammill et al. 2014, Neuenhoff et al. 2019. Grey seals were hunted to near extirpation in the early 1900s and their abundance remained low into the 1980s when fishing pressure was high (Bowen et al. 2003, Savenkoff et al. 2007). ...
Article
In recent decades, anthropogenic and natural disturbances have increased in rate and intensity around the world, leaving few ecosystems unaffected. As a result of the interactions among these multiple disturbances, many biological communities now occur in a degraded state as collections of fragmented ecological pieces. Restoration strategies are traditionally driven by assumptions that a community or ecosystem can be restored back to a pre-disturbance state through ecological remediation. Yet despite our best efforts, attempts to restore fragmented communities are often unsuccessful. One explanation, the humpty-dumpty effect, suggests that once a community is disassembled , it is difficult to reassemble it even in the presence of all the original pieces. This hypothesis, while potentially useful, often fails to incorporate the multitude of other critical mechanisms that affect our abilities to put fragmented communities back together. Here, we extend the original humpty-dumpty analogy to incorporate eco-evolutionary changes that can hinder successful restoration. A systematic literature review uncovered few studies that have explicitly considered how the original humpty-dumpty effect has affected restoration success in the 30 years since its inception. Using case studies, we demonstrate how the application of our extended eco-evolutionary humpty-dumpty framework may determine the success of restoration actions via ecological and evolutionary changes in fragments of communities. Lastly, given continued anthropogenic disturbances and projected climatic changes, we make five recommendations to facilitate more successful restoration efforts given our revised eco-evolutionary humpty-dumpty effects framework. These guidelines, combined with clearly defined management goals are aimed at both keeping ecological communities as intact as possible while ensuring that future ecosystem restorations might more successfully put the ecological community pieces back together.
... Rosen and Tollit, 2012). Recent sampling of grey seals foraging on cod aggregations indicate that cod can be the dominant component of the diet (up to 64-80% by weight for males and 35-54% for females) with a high proportion being large cod (Hammill et al., 2014). In conclusion, whilst there is general agreement on the abundance levels of the three grey seal herds ( Fig. 3c) and their annual fish consumption in ESS (Fig. 4a), there is considerable uncertainty with respect to the diet composition. ...
Article
Two hypotheses have been proposed to account for trophic dynamic control of the eastern Scotian Shelf ecosystem off Atlantic Canada: (1) top-down: fishery induced trophic cascade and (2) bottom-up: climate variability. We evaluate the evidence in support of these hypotheses: including observations on top-down drivers (fishing effort and predation by grey seals), bottom-up drivers (nutrient supply and water column stratification), and the several trophic levels (groundfish, macro-invertebrates, small pelagic fish, and plankton). There is limited support for the fishery-induced trophic cascade hypothesis. The predictions of the climate variability hypothesis are generally met for the lower and middle trophic levels, but the ongoing high levels of natural mortality of groundfish are not accounted for. We propose an alternative hypothesis encompassing concurrent top-down and bottom-up processes, and conclude that many species of groundfish (including cod) and small pelagic fish stocks (including herring) will not recover with the ongoing high levels of natural mortality generated by grey seal predation. Predictions on future trends in abundance of the commercially important macro-invertebrate species (lobster, snow crab, and shrimp) are not possible based on the available evidence.
... Increases in the abundance of Grey Seal occurring in the sGSL have been linked with important increases in the mortality of several demersal fish stocks that are declining in abundance or failing to recover from fishery-induced collapse (Benoît et al. 2011a;Swain and Benoît 2015). Though Grey Seals are predators of pelagic fishes such as Herring (Hammill et al., 2007(Hammill et al., , 2014a, a link has yet to be made between changes in pelagic fish productivity and Grey Seals in the NW Atlantic. ...
Technical Report
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Herring (Clupea harengus) is a key prey species in the diets of numerous fishes, marine mammals, seabirds, and large pelagic predators in many North Atlantic ecosystems, including the southern Gulf of St. Lawrence (sGSL; NW Atlantic, Canada). Like many small pelagic forage fish species, predation can be a significant source of mortality in Herring, rivaling or exceeding fishery removals. Changes in the abundance of Herring and their predators can therefore lead to important non-stationarity in Herring productivity, which, if unaccounted for, can bias the perception of stock status and its response to given levels of fishing. In this report we review the available information on interannual and seasonal trends in the abundance, distribution and diet of the major predators of Herring in the sGSL. This information was assembled in support of an assessment framework review for sGSL fall-spawning Herring that took place on April 13-15, 2015. Sufficient information was available to estimate the annual consumption of sGSL Herring by Northern Gannets (Morus bassanus), Cormorants (Phalacrocorax sp.), Grey Seals (Halichoerus grypus), Bluefin Tuna (Thunnus thynnus), White Hake (Urophycis tenuis) and Atlantic Cod (Gadus morhua). The reliability of those estimates varied between predators and was best for the groundfish predators. Consumption by cetacean predators could only be estimated for the mid-1990s, where it represented 37% of total estimated consumption by predators, indicating that consumption estimates for other years could be significantly biased low. For 2013, consumption by all predators other than cetaceans was almost equal to landings in the fishery, confirming the relative importance of natural mortality for this forage fish. Consumption of Herring by Cod, a historically important predator, peaked in the mid-1980s and has been declining since then. The declines since 1990 in consumption by Cod and White Hake have been offset by estimated increases in consumption by Grey Seals and Northern Gannets. The amount of Herring consumed annually by Bluefin Tuna is somewhat uncertain, but is likely to have increased to a relatively important level since 2000. Additional abundance surveys for certain key predators (e.g., cetaceans, Bluefin Tuna) and ongoing monitoring of predator diets will improve consumption estimation, ideally eventually to a point where this information can be more directly incorporated in the assessment of sGSL Herring. For now, the available information is most useful for identifying key predators, changes over time in their relative importance and an indication of the overall scale of predation mortality for sGSL Herring.
... The findings of this study augment previous research on grey seal and fisheries overlap in the North East Atlantic. While, much of the research on resource competition has focused on estimating biomass removal by seals and modelling the impacts of seal populations on species of high commercial value [1,4,22,23], Cronin et al. [14] developed a novel approach that employed tracking technologies to assess spatial overlap of grey seals with fisheries. They found a significantly low rate of spatial overlap, which implied that direct competition for the resource was far less than expected. ...
Article
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Seals and humans often target the same food resource, leading to competition. This is of mounting concern with fish stocks in global decline. Grey seals were tracked from southeast Ireland, an area of mixed demersal and pelagic fisheries, and overlap with fisheries on the Celtic Shelf and Irish Sea was assessed. Overall, there was low overlap between the tagged seals and fisheries. However, when we separate active (e.g. trawls) and passive gear (e.g. nets, lines) fisheries, a different picture emerged. Overlap with active fisheries was no different from that expected under a random distribution, but overlap with passive fisheries was significantly higher. This suggests that grey seals may be targeting the same areas as passive fisheries and/or specifically targeting passive gear. There was variation in foraging areas between individual seals suggesting habitat partitioning to reduce intra-specific competition or potential individual specialisation in foraging behaviour. Our findings support other recent assertions that seal/fisheries interactions in Irish waters are an issue in inshore passive fisheries, most likely at the operational and individual level. This suggests that seal population management measures would be unjustifiable, and mitigation is best focused on minimizing interactions at nets.
... Given that grey seals individually consume 1,300-2,200 kg of food annually (Kastelein et al. 1990;Hammill and Stenson 2000;Trzcinski et al. 2006), prey consumption by grey seals now greatly exceeds any recorded commercial fisheries landings, and the seals now occupy the dominant predatory role in much of Atlantic Canada (Bundy et al. 2009;Morissette et al. 2009;Ouellet et al. 2016). Moreover, because all medium to large demersal fishes and all forage fish species are prey of grey seals (Hammill and Stenson 2000;Hammill et al. 2007Hammill et al. , 2014, the seals not only function as predators of piscivorous fishes but also as competitors for prey. These predatory and potential competitive interactions are, of course, one-way. ...
Article
Diet composition, diet similarity, and major predator–prey interactions between the 15 most abundant demersal and pelagic fishes (N = 12,163 stomachs) during the period 1999–2003 were described for Northumberland Strait, a semi-enclosed, marine coastal ecosystem. Of the five pelagic species, Rainbow Smelt Osmerus mordax ate benthic prey (shrimps and polychaetes); Atlantic Herring Clupea harengus and American Shad Alosa sapidissima consumed small copepods and crab zoeae; and Atlantic Mackerel Scomber scombrus and Alewives Alosa pseudoharengus ate small copepods and moderate amounts of benthic prey and small fishes. Three demersal species were strongly piscivorous but with minimal diet overlap: Sea Ravens Hemitripterus americanus consumed small benthic fishes (especially flounders); Winter Skate Leucoraja ocellata consumed sand lances Ammodytes sp. and Rainbow Smelt; and White Hake Urophycis tenuis consumed Atlantic Herring and Atlantic Mackerel. Cluster analysis revealed seven feeding guilds, of which four contained a single species; consequently, the potential for compensation of predatory function was nonexistent for these guilds. The three multispecies guilds comprised consumers of (1) small copepods and crab zoeae; (2) small benthic invertebrates and polychaetes; and (3) shrimps and crabs. Based on diets, the principal route of energy transfer seemed to be the benthic–detrital pathway; however, confirmation requires representative abundance estimates for the various fishes. Moreover, abundances of many fish species have changed (i.e., most have decreased) since this study was completed, with unknown consequences for food web structure and functioning.
... For pregnant females, these results indicate a diverse prey assemblage geared towards opportunistic foraging. They also point to a potential bias of previous scat analysis: that they may underrepresent soft-bodied prey such as cephalopods and cartilaginous fishes such as winter and thorny skates [9,10,12,66]. This discrepancy may be accounted for in two ways. ...
Article
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Gray seals (Halichoerus grypus) have been rapidly recolonizing the Northeast US coast, eliciting concern from the fishing industry. However, the ecological effect of this recovery is still unknown and as such, research is needed to better understand how the diet composition of gray seals in US waters will contribute to the ecological impact. While previous research on seal diets has focused on the analysis of hard prey remains, stable isotope analysis presents an alternative method that can be used to describe marine mammal diets when direct observation is impossible. To address this issue, we used stable isotope analysis of gray seal pup vibrissae and lanugo from Monomoy Island, Cape Cod, MA during the 2015/2016 winter breeding season to estimate adult female diet composition during pregnancy. Stable isotope mixing models (SIMM) suggested adult female gray seals were consuming greater amounts of cephalopod prey and less sand lance than previously indicated from analysis of hard prey remains. However, using SIMMs to estimate the diet composition of gray seals remains difficult due to the large number of isotopically similar prey species and uncertainty in tissue-specific, stable isotope trophic enrichment factors. Even so, by combining prey sources into ecologically informative groups and integrating prior information into SIMMs it is possible to obtain additional insights into the diet of this generalist predator.
... Length and age composition of the fishery catch was based on length-frequency 139 distributions and length-stratified subsamples of ageing material (otoliths) obtained by port 140 samplers and at-sea observers. By-catch of commercial-sized Atlantic cod (≥ 43 cm TL) in 141 fisheries for invertebrates (lobster, snow crab, scallop and shrimp) appears to be negligible 142 (Swain et al. 2011a), as is the catch in recreational fisheries (Swain et al. 2015b). of Atlantic cod otoliths found in grey seal digestive tracts, the length composition of consumed cod, to the diet was much greater in these samples than in those collected in the nearshore areas 168 in summer (Hammill et al. 2014b). The average contribution of 5+ Atlantic cod to the diet based 169 on these winter samples is 41% by weight (Supplementary material 2). ...
Article
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Most stocks of Atlantic cod (Gadus morhua) in the Northwest Atlantic collapsed in the early 1990s, with little sign of recovery since then. In the southern Gulf of St. Lawrence (sGSL), the failed recovery is due to severe increases in the natural mortality of adult Atlantic cod. We examined the role of predation by grey seals (Halichoerus grypus) in this failed recovery by directly incorporating grey seal predation in the population model for Atlantic cod via a functional response. Estimated predation mortality of adult Atlantic cod increased sharply during the cod collapse and has continued to increase, comprising the majority of mortality since the late 1990s. While predation by grey seals appeared to play a minor role in the collapse of Atlantic cod, we found it to be the main factor preventing recovery. Our results are consistent with the hypothesis that failed recovery is due to predation-driven Allee effects, a demographic effect due to the decline in cod abundance and an emergent effect resulting from increasing grey seal abundance. Under current conditions, extirpation of sGSL Atlantic cod appears likely unless there is a large decline in the abundance of grey seals.
... When a collapse occurs, attempts are made to determine the drivers that preceded the crash and/or are limiting recovery of the stock from a depleted state. These can range from climate change (Brander, 2013;Pershing et al., 2015), predation (Bundy et al., 2009;Hammill et al., 2014), poor management (Hennessey, 2000;Costello et al., 2008), overfishing (Hutchings, 2000;Worm et al., 2006;many others), habitat degradation (Seitz, 2014), regime shifts (Anderson and Piatt, 1999;Steneck et al., 2004;Frank et al., 2005), and lack of a forage base (Smith et al., 2011;Essington et al., 2015). Even when a cause of decline can be pinpointed, management responses such as moratoria and rebuilding imperatives are costly, politically challenging to enact, and difficult to continue when stocks fail to recover in a timely fashion (Layzer, 2006;Khan and Neis, 2010;Boettiger et al., 2016). ...
Article
Winter Flounder populations have declined throughout the southern New England/Mid-Atlantic (SNE/MA) region since the 1980s, and evidence suggests near extirpation of some local populations. Previous research has focused on the correlation between temperature and declining stock-productivity, supporting the hypothesis that a warming climate is a primary driver of the species decline. Our objective was to critically investigate several potential drivers of Winter Flounder's regional decline and collapse in the SNE/MA region by evaluating them in relation to management actions, spawning stock biomass (SSB), recruitment, productivity (PROD) and fishing mortality (F). Results indicate that the SNE/MA stock has remained below the 40% unfished biomass threshold (B40%) since 1984, and that F was above the maximum sustainable fishing rate (FMSY) for most of the period between 1995 and 2010. We found a negative relationship between F and SSB between 1981 and 2000. Correlative analysis between young-of-the-year PROD and a number of biological and physical parameters resulted in 8 (out of 21) significant relationships. However, after correcting for multiple comparisons only two, the abundance of Striped Bass and Summer Flounder, remained significant. The PROD and recruitment analyses did not indicate strong environmental drivers and suggested recruitment compensation during the early 2000s in some surveys. In summary, we did not find evidence of a global environmental driver, such as temperature, explaining the decline of Winter Flounder. Rather, our analysis indicates that long-term overexploitation and failure of management to control harvest rates preceded and was likely a primary driver in the species decline and lack of recovery. Finally, following decades of overfishing, attempts to rebuild the Winter Flounder fishery likely will require a longer-term commitment than management has shown to date.
... This indicates that this top predator has an impact on the ecosystem and vice versa (Hårding et al., 2018). Some studies from the northern Atlantic indicate that seals have a crucial role in preventing recovery of depleted cod stocks (Benoit m.fl., 2011;Hammill et al., 2014;Cook et al., 2015;Trijoulet et al., 2018;Aarts et al., 2019;Neuenhoff et al., 2019), whereas other studies suggest limited impact of seals on cod stocks (Trzcinski et al., 2006;Alexander et al., 2015;Houle et al., 2016;Baudron et al., 2019). ...
Article
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Stocks of Atlantic cod (Gadus morhua) in the Baltic Sea, Kattegat and Skagerrak (N. Europe) have been strongly exploited for decades bringing them into an enduringly depleted status. Scientific cod stock related advice for targeted and mixed fisheries is provided on an annual basis by the International Council for Exploration of the Sea. This advice forms a basis for ministerial decisions on, e.g., the total allowable catch and management plans. Despite measures to reduce fishing-induced mortality of cod, such as catch and effort restrictions, increased gear selectivity, closed areas and seasons, clear signs of recovery are yet to be seen. Thus, traditional advice for the management of these stocks may have to be complemented by advice on supporting measures focusing on other pressures hampering the recovery of cod. The present study elaborates on potential supportive measures for cod stock recovery in the Baltic Sea, Kattegat, and Skagerrak (including local populations where applicable), based on current knowledge. The list of measures presented here is the outcome of in-depth discussions on the state-of-the-art knowledge, among cod experts and further with stakeholders with the aim to follow principles of ecosystem-based fisheries management. Following the identification of different pressures on and prerequisites for the separate stocks, the listed measures differ between stocks and include cod bycatch mortality reduction, alterations in fisheries affecting food sources for cod, restocking, protection of juvenile habitats, and reduced predation. The literature review and the list of measures are intended to provide decision-support for managers and policymakers aiming to provide conditions for the cod stocks to recover.
... There is evidence that this is currently happening in the cod pop ulation from the southern Gulf of St Lawrence. Concurrently with a one-order-ofmagnitude increase in the abundance of the grey seal (Halichoerus grypus), a known cod predator (Hammill et al. 2014), cod have moved to deeper water and their condi tion (energy reserves) has diminished (Swain et al. 2015), possibly as a result of lower prey availability and/or of the hypoxic conditions at these depths. ...
Chapter
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This chapter shows that cod physiology is profoundly influenced by temperature, and a cod living at 0 °C is in many ways a different animal than a cod living at 12 °C. Cod populations also occur along inshore‐offshore gradients that generate an even more complicated array of environmental factors that this species must deal with. The chapter reviews published information that allows the revisiting of Atlantic cod physiological ecology from the perspective of Fry's concept of aerobic metabolic capacity and classification of the environment. It provides a basic knowledge enabling the reader unfamiliar with physiology to comprehend the mechanisms by which that factor affects cod functioning and performance. The chapter emphasizes the physiology of juvenile and adult cod: the physiology of cod larvae is only partially covered. It also reviews the impact of food availability and of the timing of food availability on survival and growth of cod larvae, juveniles and adults.
... In the SGSL, several groundfish stocks are currently controlled at low abundance levels by a surge in predation pressure (e.g. Hammill et al. 2014, Neuenhoff et al. 2019. Even though sources of variability in juvenile mackerel predation mortality within the system remain to be identified, our data suggest that predation during the first growth season could interact with larval growth conditions to determine Atlantic mackerel recruitment. ...
Article
The growth-survival paradigm predicts that year-class strength is determined by growth-dependent mortality during the larval stage. In Atlantic mackerel Scomber scombrus, the possibility that strong growth-dependent mortality extends into the early juvenile stage has not previously been tested because of the difficulty in sampling young-of-the-year (YOY) juveniles. The present study determined the timing of the ‘endpoint’ during the early ontogeny, when growth-selective mortality decreases and recruitment is set. We relied on regurgitations from one of the main predators of mackerel, the northern gannet Morus bassanus, as a source of YOY juveniles. Early growth trajectories of YOY mackerel were reconstructed from the otolith microstructure and were compared to those of 1-yr-old (OYO) juveniles from the same cohort for the year classes of 2015 and 2017. In both cohorts, the early growth trajectory of OYO fish was faster than that of YOY juveniles, indicating that growth-selective mortality extended beyond the larval stage. For the 2017 cohort, the comparison of larval growth trajectories between 2-mo-old YOY, 3mo-old YOY and OYO juveniles indicated that strong selection for fast growth persisted until the pre-wintering period, but that winter mortality likely did not play an important role in shaping year-class strength. These findings suggest that in Atlantic mackerel, the endpoint when the relative strength of cohorts is fixed occurs at the age of 3 mo. These results highlight the importance of considering growth-dependent mortality processes occurring beyond the larval stage to obtain a better understanding of causes of recruitment variability.
... The breeding colony on SI has grown rapidly since the 1960s when just a few thousand pups were born on the island (Bowen et al., 2007;den Heyer et al., 2020). The population associated with the SI breeding colony is now over 300 000 individuals (Rossi et al., 2021), and the impact of this population on prey species has been focus of much research (e.g., Trzcinski et al., 2006;O'Boyle and Sinclair, 2012;Hammill et al., 2014;Neuenhoff et al., 2019). Recently the grey seal breeding colony has also been shown to contribute to the ecology of the island by fertilising vegetation that supports a population of feral horses (McLoughlin et al., 2016). ...
Article
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Elevated surface chlorophyll-a (chl-a) concentration ([chl-a]), an index of phytoplankton biomass, has been previously observed and documented by remote sensing in the waters to the southwest of Sable Island (SI) on the Scotian Shelf in eastern Canada. Here, we present an analysis of this phenomenon using a 21-year time series of satellite-derived [chl-a], paired with information on the particle backscattering coefficient at 443 nm (b bp (443), a proxy for particle suspension) and the detritus/gelbstoff absorption coefficient at 443 nm (a dg (443), a proxy to differentiate water masses and presence of dissolved organic matter) in an attempt to explain some possible mechanisms that lead to the increase in surface biomass in the surroundings of SI. We compared the seasonal cycle, 8 d climatology and seasonal trends of surface waters near SI to two control regions located both upstream and downstream of the island, away from terrigenous inputs. Application of the self-organising map (SOM) approach to the time series of satellite-derived [chl-a] over the Scotian Shelf revealed the annual spatio-temporal patterns around SI and, in particular, persistently high phytoplankton biomass during winter and spring in the leeward side of SI, a phenomenon that was not observed in the control boxes. In the vicinity of SI, a significant increase in [chl-a] and a dg (443) during the winter months occurred at a rate twice that of the ones observed in the control boxes, while no significant trends were found for the other seasons. In addition to the increase in [chl-a] and a dg (443) within the plume southwest of SI, the surface area of the plume itself expanded by a factor of 5 over the last 21 years. While the island mass effect (IME) explained the enhanced biomass around SI, we hypothesised that the large increase in [chl-a] over the last 21 years was partly due to an injection of nutrients by the island's grey seal colony, which has increased by 200 % during the same period. This contribution of nutrients from seals may sustain high phytoplankton biomass at a time of year when it is usually low following the fall bloom. A conceptual model was developed to estimate the standing stock of chl-a that can be sustained by the release of nitrogen (N) by seals. Comparison between satellite observations and model simulations showed a good temporal agreement between the increased abundance of seal on SI during the breeding season and the phytoplankton biomass increase during the winter. We found that about 20 % of chl-a standing stock increase over the last 21 years could be due to seal N fertilisation, the remaining being explained by climate forcing and oceanographic processes. Although without in situ measurements for ground truthing, the satellite data analysis provided evidence of the impact of marine mammals on lower trophic levels through a fertilisation mechanism that is coupled with the IME with potential implications for conservation and fisheries.
... The Atlantic Herring Clupea harengus is a vital prey species for many predators in the southern Gulf of St. Lawrence (sGSL), including grey seals Halichoerus grypus (Hammill and Stenson 2000;Hammill et al. 2007Hammill et al. , 2014, seabirds (Cairns et al. 1991), cetaceans (Fontaine et al. 1994;Benoît and Rail 2016), Atlantic Cod Gadus morhua (Hanson and Chouinard 2002), White Hake Urophycis tenuis (Benoît and Rail 2016), and Bluefin Tuna Thunnus thynnus (Pleizier et al. 2012). Atlantic Herring are socioeconomically important and are targeted by the largest finfish fishery in the sGSL, which takes place largely on the spawning grounds. ...
Article
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Predation can be a significant source of natural mortality for small pelagic fish species, rivaling or exceeding fishery removals. Failure to account for changes in natural mortality can introduce uncertainty in the assessment and management of these stocks. In this study, a 10-year span of hydroacoustic data was used to detect Bluefin Tuna Thunnus thynnus on two major fall spawning grounds of Atlantic Herring Clupea harengus, an economically and ecologically valuable forage fish species in the southern Gulf of St. Lawrence (sGSL). Average Bluefin Tuna detections increased 22-fold from 2002 to 2012 on both spawning grounds independently of Atlantic Herring density or aggrega-tion size. This increase is directionally consistent but larger than changes in other Bluefin Tuna population indices. Preliminary estimates of annual Atlantic Herring consumption doubled across the time series, reaching values of 4,300-20,000 metric tons in recent years. This would suggest that Bluefin Tuna are among the most important consumers of Atlantic Herring in the sGSL. These findings are key for an ecosystem-based approach to the assessment and management of both Atlantic Herring and Bluefin Tuna in the sGSL.
... Herring is a vital pelagic prey species for numerous predators in the sGSL including grey seals (Halichoerus grypus, Hammill and Stenson 2000;Hammill et al. 2007Hammill et al. , 2014b, seabirds (Cairns et al. 1991), cetaceans (Fontaine et al. 1994;Benoît and Rail 2016), Atlantic cod (Gadus morua, Hanson and Chouinard 2002), white hake (Urophycis tenuis, Benoît and Rail 2016) and Atlantic Bluefin tuna (Thunnus thynnus, Pleizier et al. 2012). Of these major predators, abundances of cod, grey seals and Bluefin tuna have changed drastically in the sGSL in the last decades. ...
... The differences in diet between areas describe a shift from herring dominance in the central parts to percid and cyprinid (especially bream) dominance in the Gulf of Finland, and importance of perch in the western Baltic Proper. The prey of the Baltic grey seals is distinguished from the diets of grey seals of the Atlantic, where herring is a minor dietary component but gadoids, flatfish and sandeels are frequently consumed [64,65]. ...
Article
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The growing grey seal (Halichoerus grypus) population in the Baltic Sea has created conflicts with local fisheries, comparable to similar emerging problems worldwide. Adequate information on the foraging habits is a requirement for responsible management of the seal population. We investigated the applicability of available dietary assessment methods by comparing morphological analysis and DNA metabarcoding of gut contents (short-term diet; n = 129/125 seals, respectively), and tissue chemical markers i.e. fatty acid (FA) profiles of blubber and stable isotopes (SIs) of liver and muscle (mid- or long-term diet; n = 108 seals for the FA and SI markers). The methods provided complementary information. Short-term methods indicated prey species and revealed dietary differences between age groups and areas but for limited time period. In the central Baltic, herring was the main prey, while in the Gulf of Finland percid and cyprinid species together comprised the largest part of the diet. Perch was also an important prey in the western Baltic Proper. The DNA analysis provided firm identification of many prey species, which were neglected or identified only at species group level by morphological analysis. Liver SIs distinguished spatial foraging patterns and identified potentially migrated individuals, whereas blubber FAs distinguished individuals frequently utilizing certain types of prey. Tissue chemical markers of adult males suggested specialized feeding to certain areas and prey, which suggest that these individuals are especially prone to cause economic losses for fisheries. We recommend combined analyses of gut contents and tissue chemical markers as dietary monitoring methodology of aquatic top predators to support an optimal ecosystem-based management.
... Our results also showed that predation mortality of Baltic fish by grey seals was higher in the Env0 scenario, in which cod, sprat and herring abundances had their lowest values. Thus, maintaining cod populations at relatively high abundance levels could reduce cod predation mortality by seals (e.g., Hammill et al. (2014)). Interestingly, the differences in seal diet composition between the datasets used in our model showed a decrease in the consumption of larger prey like cod and an increase in smaller prey (e.g. ...
Article
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Seal populations are recovering in many regions around the world and, consequently, they are increasingly interacting with fisheries. We used an Ecopath with Ecosim model for the offshore Central Baltic Sea to investigate the interactions between the changes in fish stocks and grey seal (Halichoerus grypus) population under different fishing and environmental scenarios for the twenty-first century. The assumed climate, eutrophication and cod (Gadus morhua) fisheries scenarios modified seal predation impacts on fish. Fish biomass and catches are more affected by fishing mortality and the environment than by seal predation. Our results highlight that the impacts of the increasing seal population on lower trophic levels are complex; thus, we emphasize the need to consider a range of possible ecosystem contexts when evaluating potential impacts of top predators. Finally, we suggest that an increasing seal population is not likely to hinder the preservation of the main Baltic fish stocks. Electronic supplementary material The online version of this article (10.1007/s13280-018-1131-y) contains supplementary material, which is available to authorized users.
... Gray seals are social pinnipeds that aggregate at shared terrestrial haul outs in between single-and multi-day movements offshore (Breed et al. 2009). Gray seals feed primarily on demersal fishes (Beck et al. 2007, Breed et al. 2009), some of which are commercially valuable (Trzcinski et al. 2006, O'Boyle & Sinclair 2012, Hammill & Stenson 2014. In the past, perceptions of competition with fisheries motivated hunting and statefunded bounty programs that greatly reduced gray seal populations in the Northwest Atlantic (Lelli et al. 2009), including extirpation from the US coastline (Wood et al. 2020). ...
Article
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White sharks Carcharodon carcharias and gray seals Halichoerus grypus are re-establishing their ecological roles within the Northwestern Atlantic Ocean, presenting an opportunity to understand gray seal movement and at-sea behavior under predation risk. As with other shark-seal hotspots, movements to and from terrestrial haul outs can be risky for gray seals, thereby eliciting antipredator strategies. We investigated the movement and coastal behavior of gray seals on Cape Cod (USA) in relation to seasonal and diel changes in white shark activity. Analyzing 412 trips to sea by 8 seals and more than 25000 acoustic detections from 23 individual white sharks, we observed seasonally homogeneous movements in seal behavior during months with greater shark presence. During riskier months, seal behavior manifested in near-exclusive nocturnal foraging, reduced offshore ranging, and limited at-sea activity. On these nocturnal trips to sea, seals returning to haul outs tended to avoid daybreak and traversed during diel minima in shark activity. However, seals tended to depart haul outs at dusk when shark presence was maximal. As conservation efforts succeed in rebuilding depleted populations of coastal predators, studying re-emerging predator-prey interactions can enhance our understanding about the drivers of movement and behavior.
... Following federal protections, gray seals have recolonized the New England coastline and populations are growing rapidly (Hayes et al. 2017;Moxley et al. 2017). This growing population has renewed concerns about various environmental problems that could be attributed to seals and potential effects on fisheries (e.g., Hammill et al. 2014). Most diet studies in the western north Atlantic are from Canadian waters, where seals are considered benthic generalists consuming prey such as sand lance (Ammodytes spp. ...
Article
While it is often assumed that individuals in generalist populations are equivalent, recent research indicates that individual dietary specialization can be common in marine predators. Gray seals ( Halichoerus grypus , [Fabricius, 1791]) were considered locally extinct in United States waters by 1958 but have since recolonized the region. While considered generalists, less is known about gray seal foraging ecology in the United States. To address this, we used carbon and nitrogen stable isotope analyses to investigate the foraging niches of adult gray seals in Massachusetts, USA. We examined skin, fur and blood components to investigate seasonal variability and individual consistency in foraging niches, and serially-sampled vibrissae to quantify the degree of individual foraging specialization in this population. Our results suggest that seals shift from coastal foraging habitats pre-molt to offshore habitats post-molt, with a coincident shift from higher to lower trophic level prey. Adult gray seals also exhibited individual consistency in foraging niches independent of population level shifts and reflect a generalist population composed of individual foraging specialists. These findings serve as a baseline for subsequent research on gray seals in United States waters that could help to determine the mechanisms that promote individual specialization in this population.
... Improving estimates of predation and natural mortality is of the utmost importance to fisheries assessment and management, particularly for vulnerable populations 6,7 . A prime example of a species for which detailed information about predation and mortality is needed is the anadromous Atlantic salmon (Salmo salar). ...
Article
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Predation and mortality are often difficult to estimate in the ocean, which hampers the management and conservation of marine fishes. We used data from pop-up satellite archival tags to investigate the ocean predation and mortality of adult Atlantic salmon (Salmo salar) released from 12 rivers flowing into the North Atlantic Ocean. Data from 156 tagged fish revealed 22 definite predation events (14%) and 38 undetermined mortalities (24%). Endothermic fish were the most common predators (n = 13), with most of these predation events occurring in the Gulf of St. Lawrence and from the Bay of Biscay to the Irish Shelf. Predation by marine mammals, most likely large deep-diving toothed whales (n = 5), and large ectothermic fish (n = 4) were less frequent. Both the estimated predation rates (ZP) and total mortality rates (ZM) where higher for Atlantic salmon from Canada, Ireland, and Spain (ZP = 0.60–1.32 y−1, ZM = 1.73–3.08 y−1) than from Denmark and Norway (ZP = 0–0.13 y−1, ZM = 0.19–1.03 y−1). This geographical variation in ocean mortality correlates with ongoing population declines, which are more profound for southern populations, indicating that low ocean survival of adults may act as an additional stressor to already vulnerable populations.
... The predator driving adult M in sGSL cod appears to be grey seals (Halichoerus grypus), which have rapidly increased in abundance over recent decades (Swain and Benoît, 2015). While the diet of grey seals is difficult to establish due to spatial, seasonal and individual heterogeneities, diet samples of seals taken in the vicinity of overwintering sGSL cod aggregations show high proportions of cod (Hammill et al., 2014). Moreover, simulations based on the energetic requirements of seals and the spatiotemporal overlap of seals with sGSL cod have demonstrated that grey seals could account for a high proportion of sGSL cod M, even if the contribution of cod to the seal diet was modest . ...
Article
A suite of statistical catch-at-age models with time-varying natural mortality (M), selectivity and catchability were fitted to data for eastern Georges Bank Atlantic Cod (Gadus morhua) and Georges Bank Yellowtail Flounder (Limanda ferruginea) to address hypotheses regarding nonstationarities in these systems. Model averaging and ensemble methods were used to combine the results of converged models for each species. Model averaging was based on both predictive (i.e., cross-validation, AIC) and retrospective performance metrics. Models allowing large shifts in M provided the best fit to observed data for both species and were also regarded as the most plausible. For Atlantic Cod, variance in biomass estimates among the individual models was very high and multimodel estimates were sensitive to the relative weight given to competing models. In contrast, converged Yellowtail Flounder models produced broadly similar output and multimodel estimates were more robust to model weights. Our results suggest that the plausibility of competing models should be scrutinized, particularly when structural uncertainty is high.
Article
For the period 1999–2003, diet composition, diet similarity, and major predator–prey interactions between the 15 most abundant demersal and pelagic fishes (N = 12,163 stomachs) were described for Northumberland Strait - a semienclosed, marine, coastal ecosystem. Of the five pelagic species, Rainbow Smelt Osmerus mordax ate benthic prey (shrimps and polychaetes), Atlantic Herring Clupea harengus and American Shad Alosa sapidissima consumed small copepods and crab zoea, and Atlantic Mackerel Scomber scombrus and Alewife A. pseudoharengus ate small copepods and moderate amounts of benthic prey and small fishes. Three demersal species were strongly piscivorous but with minimal diet overlap: Sea Raven Hemitripterus americanus consumed small benthic fishes (especially flounders); Winter Skate Leucoraja ocellatus consumed sand lance (Ammodytes sp.) and Rainbow Smelt; and White Hake Urophycis tenuis consumed Atlantic Herring and Atlantic Mackerel. Cluster analysis revealed seven feeding guilds of which four contained a single species; consequently, the potential for compensation of predatory function was nonexistent for these guilds. The three multispecies guilds were comprised of consumers of: small copepods and crab zoea; small benthic invertebrates and polychaetes; and shrimps and crabs. Based on diets, the principal route of energy transfer seemed to be the benthic-detrital pathway; however, confirmation requires representative abundance estimates for the various fishes. Moreover, abundances of many fish species have changed (most have decreased) since this study was completed, with unknown consequences for food web structure and functioning. Received 10 Apr 2017 accepted 06 Sep 2017 revised 10 Aug 2017
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Niche-based approaches to community analysis often involve estimating a matrix of pairwise interactions among species (the "community matrix"), but this task becomes infeasible using observational data as the number of modeled species increases. As an alternative, neutral theories achieve parsimony by assuming that species within a trophic level are exchangeable, but generally cannot incorporate stabilizing interactions even when they are evident in field data. Finally, both regulated (niche) and unregulated (neutral) approaches have rarely been fitted directly to survey data using spatio-temporal statistical methods. We therefore propose a spatio-temporal and model-based approach to estimate community dynamics that are partially regulated. Specifically, we start with a neutral spatio-temporal model where all species follow ecological drift, which precludes estimating pairwise interactions. We then add regulatory relations until model selection favors stopping, where the "rank" of the interaction matrix may range from zero to the number of species. A simulation experiment shows that model selection can accurately identify the rank of the interaction matrix, and that the identified spatio-temporal model can estimate the magnitude of species interactions. A forty-year case study for the Gulf of St. Lawrence marine community shows that recovering grey seals have an unregulated and negative relation with demersal fishes. We therefore conclude that partial regulation is a plausible approximation to community dynamics using field data, and hypothesize that estimating partial regulation will be expedient in future analyses of spatio-temporal community dynamics given limited field data. We conclude by recommending ongoing research to add explicit models for movement, so that meta-community theory can be confronted with data in a spatio-temporal statistical framework. This article is protected by copyright. All rights reserved.
Technical Report
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The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assessed White Hake in Canadian waters, and based on a combination of genetic, behavioural (spawning locations and seasons), and meristic information suggested the existence of two populations (designatable units (DUs)). The definition of geographic boundaries between these two populations is complicated because of spatial overlap in their distribution. One population named the Southern Gulf of St. Lawrence (DU1) encompasses all of NAFO Div. 4T and the northern portion of Subdivision 4Vn. The second population identified as Atlantic and northern Gulf of St. Lawrence (DU2) includes the Scotian Shelf (NAFO Div. 4VWX), waters off southern and eastern Newfoundland, northern Gulf of St. Lawrence (NAFO Div. 4RS) as well as waters greater than 200 m in NAFO Div. 4T. The areas of overlap are: along the southern edge of the Laurentian Channel in NAFO Div. 4T at depths greater than 200 m and within NAFO Subdiv. 4Vn. The Atlantic and northern Gulf of St. Lawrence population was designated as threatened in November of 2013. The reason provided by COSEWIC for this designation is: • Adults in this population are estimated to have declined by approximately 70% over the past three generations. Most of this decline occurred before the mid-1990s. The population has remained fairly stable since then, and there has been little overall trend in area of occupancy. Restrictions on fisheries since the mid to late 1990s over most of their range may be responsible for stabilizing their numbers. Following on the COSEWIC assessment, Fisheries and Oceans Canada (DFO) Science Branch was asked to undertake a Recovery Potential Assessment (RPA) for the White Hake populations assessed as threatened or endangered. This Research Document supports the RPA for the Atlantic and northern Gulf of St. Lawrence DU (ANGSL). It describes the current state of knowledge of White Hake in the northern Gulf of St. Lawrence (nGSL) portion of the ANGSL DU in terms of its biology, ecology, abundance, distribution and trends, habitat requirements and threats. Main sources of data were DFO and industry surveys, commercial landings, at-sea observer data, and stomach content databases. White Hake in the nGSL is not considered as a separate stock. There is no established directed fishery and it is not assessed under a regional peer review process. Limited information is available on White Hake in the nGSL. The species is broadly distributed in water depths between 175 and 350 meters, with concentrations along the slope of the channels of the nGSL. An index of abundance, based on data from the DFO RV summer survey, was developed for this RPA. This index, estimated for the 1985-2014 period, indicates that the abundance of White Hake in the nGSL declined rapidly from the mid 80s to early 90s, and has since remained at low levels. The information contained in this document may be used to support the development of recovery plans and decision- making with regard to the issue of permits, agreements and related conditions under the SARA.
Article
A recent data-recovery exercise uncovered a large amount of stomach-content data (n = 8209) from mainly demersal fishes collected seasonally in the Miramichi River Estuary, southern Gulf of St. Lawrence (sGSL), 1991–1993. Of the demersal species captured, all but Pleuronectes putnami (Smooth Flounder) represent transient species. Within the estuary, stomach samples were collected from all species (except Osmerus mordax [Rainbow Smelt]) captured by trawling and in commercial trap nets. Rainbow Smelt stomachs were collected during trawl surveys (May through October) in nearby coastal waters. The summer diet analyses were limited to Smooth Flounder (small bivalve specialist), Pseudopleuronectes americanus (Winter Flounder), and Rainbow Smelt. Many juvenile Urophycis tenuis (White Hake) entered the estuary during late September to feed and then departed during November. Numerous Gadus macrocephalus (Greenland Cod), Myoxocephalus scorpius (Shorthorn Sculpin), Winter Flounder, juvenile Clupea harengus (Atlantic Herring), Microgadus tomcod (Atlantic Tomcod), Rainbow Smelt, Morone saxatilis (Striped Bass), and Zoarces americanus (Ocean Pout) entered the estuary during autumn to overwinter, spawn, and feed. Winter Flounder, Smooth Flounder, Striped Bass, and Atlantic Herring fasted during winter. Crangon septemspinosa (Seven-spined Bay Shrimp) was the most important invertebrate prey (up to 95% of total prey mass) of all transient species. Large specimens of Greenland Cod, Shorthorn Sculpin, Ocean Pout, and White Hake also ate substantial numbers of small Rainbow Smelt, Atlantic Herring, Striped Bass, and Atlantic Tomcod. When combined with published data for the adjacent coastal zone, C. septemspinosa represents a nexus in 2 food webs and fits the description of a keystone species for the sGSL coastal zone and adjacent estuaries.
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Elevated surface chlorophyll-a concentration, an index of phytoplankton biomass, has been previously observed and documented by remote sensing in the waters to the southwest of Sable Island (SI) on the Scotian Shelf in eastern Canada. Here, we present a detailed analysis of this phenomenon using a 20-year time series of satellite-derived chlorophyll-a concentration (chl-a), paired with information on the particle backscattering coefficient at 443 nm (bbp(443)) and the detritus/gelbstoff absorption coefficient at 443 nm (adg(443) ) in an attempt to explain the possible mechanisms that lead to the increase in surface biomass in the surroundings of SI. We compared the seasonal cycle, climatology and trends of surface waters near SI to two control regions located both upstream and downstream of the island, away from terrigenous inputs. Application of the self-organizing maps approach (SOMs) to the time series of satellite-derived chl-a over the Scotian Shelf revealed the annual spatio-temporal patterns around SI and, in particular, persistently high phytoplankton biomass during winter and spring in the leeward side of SI, a phenomenon that is not observed in the control boxes. Time series analysis of the satellite archive evidenced a long-term increase in chl-a and adg(443), and a long-term decrease in bbp(443) in all regions. In the close vicinity of SI, the increase of chl-a and adg(443) during the winter months occurred at a rate twice that of the ones observed in the control boxes. In addition to the increase of the chl-a and adg(443) within the plume southward of SI, the surface area of the plume itself has also expanded by a factor of five over the last 20 years. While the island mass effect (IME) is certainly contributing to the enhanced biomass around SI, we hypothesize that the large increase in chl-a over the last 20 years is due to an injection of nutrients by the island’s grey seal colony, which has increased by about 300 % over the last twenty years. The contribution of nutrients from seals may sustain high phytoplankton biomass at a time of year when it is usually low. A conceptual model was developed to describe the annual variation of seal abundance on SI and estimate the standing stock of chl-a concentration that can be sustained by the release of nitrogen. Comparison between satellite observations and model simulations showed a very good agreement between the seal population increase on SI during the breeding season and the phytoplankton biomass increase during the winter. In addition, the 20-year satellite-derived trend in chlorophyll-a concentration showed a good agreement with the increasing trend in seal population on SI during the same time period. The satellite data analysis supports the concept of top-down control of marine mammals over lower trophic levels through a fertilisation mechanism, although these results could not be confirmed without in situ measurements for ground truthing. Our findings challenge the idea that the IME is restricted to islands with strong bathymetric slope located in oligotrophic waters of mid-latitudes and tropics, and demonstrate that enhanced marine production can occur in other oceanic regions, with potentially substantial implications for conservation and fisheries.
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1. We constructed a size- and trait-based dynamic marine community model of the Celtic Sea / Biologically Sensitive Area, including grey seals Halichoerus grypus (Fabricius 1791) and harbour seals Phoca vitulina vitulina (Linnaeus 1758) to examine potential resource conflict between seals and commercial trawl fisheries. The model incorporates seal diet preference, population size and commercial fishery catch, with survey data to quantify ecological interactions between seals and fisheries. 2. Total annual consumption by seals was an order of magnitude less than the catch of the modelled trawl fishery. Increasing fishing pressure reduced fish spawning stock biomass (SSB) much more than a proportionally equivalent increase in seal predation. For most fish species, quadrupling seal predation showed little effect on the predicted fishery catch. 3. These results arise from relatively low seal abundance and partial niche partitioning. The fishery harvested a wider range of fish length and species than seals consumed. The fish community was dominated by small fish lengths and seals predated on these more than suggested by their calculated diet preference. 4. Seal predation disproportionately affected several fish species not targeted by the fishery, but seal predation did not significantly affect the SSB of any of the species that constitute 90% of the total landings of the fishery. 5. Synthesis and applications. Predation of fish by grey and harbour seals is unlikely to harm commercial trawl fisheries in south-west Irish waters. This conclusion differs from those of some model-based studies of other North Atlantic systems, demonstrating the need for ecosystem-specific evidence in considering such conflicts. In systems with low niche overlap between seals and fisheries, the two are largely decoupled in effect, leaving fishing pressure as the overwhelming determinant of targeted fish stock status.
Article
Numerous studies have shown that, at spatial scales of metres to several kilometres, animals balance the trade-off between foraging success and predation mortality by increasing their use of safer but less profitable habitats as predation risk increases. However, it is less clear whether prey respond similarly at the larger spatiotemporal scales of many ecosystems. We determine whether this behaviour is evident in a large marine ecosystem, the southern Gulf of St. Lawrence (sGSL, 75 000 km(2) ) over a 42-year period. This ecosystem is characterized by a recent increase in the abundance of a large marine predator, the grey seal (Halichoerus grypus Fabricius), by more than an order of magnitude. We compared changes in spatial distribution over the 1971-2012 period between important prey of grey seals (Atlantic cod, Gadus morhua L.; white hake, Urophycis tenuis Mitchill; and thorny skate, Amblyraja radiata Donovan) and non-prey fishes. Distribution was modelled using generalized additive models incorporating spatially variable effects of predation risk, density dependence and water temperature. Distributions of cod, hake and skate were strongly related to risk of predation by seals, with distribution shifting into lower risk areas as predation risk increased. Non-prey species did not show similar changes in habitat use. Spatial variation in fish condition suggests that these low-risk areas are also less profitable for cod and skate in terms of food availability. The effects of density dependence and water temperature were also important in models, but did not account for the changes in habitat use as the risk of predation increased. These results indicate that these fish are able to assess and respond to spatial variation in predation risk at very large spatial scales. They also suggest that non-consumptive 'risk' effects may be an important component of the declines in productivity of seal prey in this ecosystem, and of the indirect effects at lower trophic levels.
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Diet composition of grey seals in the Gulf of St. Lawrence (Gulf) and around the coast of Newfoundland, Canada, was examined using identification of otoliths recovered from digestive tracts. Prey were recovered from 632 animals. Twenty-nine different prey taxa were identified. Grey seals sampled in the northern Gulf of St. Lawrence fed mainly on capelin, mackerel, wolffish and lumpfish during the spring, but consumed more cod, sandlance and winter flounder during late summer. Overall, the southern Gulf diet was more diverse, with sandlance, Atlantic cod, cunner, white hake and Atlantic herring dominating the diet. Capelin and winter flounder were the dominant prey in grey seals sampled from the east coast of Newfoundland, while Atlantic cod, flatfish and capelin were the most important prey from the south coast. Animals consumed prey with an average length of 20.4 cm (Range 4.2-99.2 cm). Capelin were the shortest prey (Mean = 13.9 cm, SE = 0.08, N = 1126), while wolffish were the longest with the largest fish having an estimated length of 99.2 cm (Mean = 59.4, SE = 2.8, N = 63). In the early 1990s most cod fisheries in Atlantic Canada were closed because of the collapse of the stocks. Since then they have shown limited sign of recovery. Diet samples from the west coast of Newfoundland indicate a decline in the contribution of cod to the diet from the pre-collapse to the postcollapse period, while samples from the southern Gulf indicate little change in the contribution of cod.
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Despite 2 decades of very low fishing levels, numerous NW Atlantic demersal fish stocks have failed to recover from collapsed states or are presently collapsing. In the southern Gulf of St. Lawrence, adult natural mortality (M) appears to be the demographic rate that most limits population productivity in at least 3 species: Atlantic cod Gadus morhua, white hake Urophycis tenuis and winter skate Leucoraja ocellata. The causes of elevated M are not well understood, though there is indirect evidence consistent with an effect of predation by grey seals Halichoerus grypus. However, direct evidence is lacking due to uncertainty in the seal diet. Consequently, Monte Carlo simulations were undertaken using data on the spatial overlap between the seals and the fishes and a seal food-consumption model, to estimate the plausibility that different seal-diet compositions could explain observed M levels. Under the simulation assumptions, we find that predation could explain up to 20 to 50% of M in adult white hake and cod even if these species comprise a small percentage of grey seal diets (< 25%). If seals consume some of these fish only partially, by selectively feeding on soft tissues, a predation effect becomes more plausible. Predation can also plausibly explain the observed elevated M in adult winter skate, even if they comprise a negligible (< 0.1%) percentage of the grey seal diet. Though the simulations deal with the factors that shape the potential for predation, a greater understanding of prey selection is required to conclude whether grey seals are actually adversely impacting these fish populations.
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Northwest Atlantic grey seals form a single stock, but for management purposes are often considered as 2 groups. The largest group whelps on Sable Island, 290 km east of Halifax, Nova Scotia. The second group referred to as ‘non-Sable Island’ or ‘Gulf’ animals whelps primarily on the pack ice in the southern Gulf of St. Lawrence, with other smaller groups pupping on small islands in the southern Gulf and along the eastern shore of Nova Scotia. Estimates of pup production in this latter group have been determined using mark-recapture and aerial survey techniques. The most recent visual aerial surveys flown during January-February 1996, 1997 and 2000 in the southern Gulf of St Lawrence and along the Eastern Shore resulted in pup production estimates of 11,100 (SE = 1,300), 7,300 (SE = 800) and 6,100 (SE = 900) in 1996, 1997 and 2000 respectively after correcting for births and including counts of pups on small islands. Incorporating information on pup production, reproduction rates and removals into a population model indicates that the Gulf component increased from 15,500 (95% CI = 14,600-16,300) animals in 1970 to 62,700 (95% CI = 49,800-67,800) animals by 1996 and then declined to 22,300 (95% CI = 17,200-28,300) animals in 2000. On Sable Island the population has increased from 4,800 (95% CI = 4,700-4,900) animals in 1970 to 212,500 (95% CI = 159,600-276,200) in 2000. The total Northwest Atlantic grey seal population is estimated to number around 234,800 animals in 2000.
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Diet estimation in marine mammals relies on indirect methods including recovery of prey hard parts from stomachs and feces, quantitative fatty acid signature analysis (QFASA), stable isotope mixing models, and identification of prey DNA in stomach contents and feces. Experimental evidence (9 species/13 studies) shows that digestion strongly influences the proportion and size of otoliths that can be recovered in feces. Number correction factors (NCF) and digestion coefficients have been experimentally determined to reduce the biases in fecal analysis. Correction factors and coefficients have not been determined for diet estimated from stomach contents. QFASA estimates which prey species and amounts must have been eaten to account for the fatty acid composition of the predator. Experimental studies on mammals and seabirds (9 species/10 studies) indicate that accurate estimates of diet can be determined using QFASA. Stable isotope mixing models provide rather coarse taxonomic resolution of diet composition. Prey DNA analysis shows promise as a method to estimate the species composition of diet, but further development and testing is needed to validate its use. Obtaining a representative sample from marine mammal populations is a significant challenge. Therefore, the use of complementary methods is recommended to obtain the most informative results.
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We examined the digestive tract contents from 145 grey seals (Halichoerus grypus) collected be- tween 2001 and 2004 in the Baltic Sea. We compensated for biases introduced by erosion of otoliths, both by using additional hard-part structures other than otoliths, and species-specific size and nu- merical correction factors. In the absence of numerical correction factors based on feeding experi- ments for some species, we used correction factors based on a relationship between otolith recovery rate and otolith width. A total of 24 prey taxa were identified but only a few species contributed substantially to the diet. The estimated diet composition was, independently of the prey number estimation method and diet composition estimation model used, dominated by herring (Clupea harengus), both by numbers and biomass. In addition to herring, common whitefish (Coregonus lavaretus) and sprat (Sprattus sprattus) were important prey, but cyprinids (Cyprinidae), eelpout (Zoarces viviparus), flounder (Platichtys flesus) and salmon (Salmo salar) also contributed sig- nificantly. Our results indicated dietary differences between grey seals of different age as well as between seals from the northern (Gulf of Bothnia) and the southern (Baltic Proper) Baltic Sea. Lundström, K., Hjerne, O., Alexandersson, A. and Karlsson O. 2007. Estimation of grey seal (Halichoerus grypus) diet composition in the Baltic Sea. NAMMCO Sci. Publ. 6:177-196.
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Many pinnipeds forage considerable distances from mainland haul-out sites for much of the year, yet little is known about the composition of their offshore diets. This may result in an incomplete and potentially biased view of the diet of wide-ranging species such as the grey seal Halichoerus grypus. On the Scotian Shelf, offshore fish assemblages differ from those nearer the coastal mainland sites and thus we expected that grey seal diets would also differ. We studied the composition of grey seal diets at Sable Island, an offshore site 160 km east of Nova Scotia, Canada. Otoliths, squid beaks and other hard parts, representing more than 24 types oi prey, were recovered from 365 of 393 grey seal faeces collected between July 1991 and January 1993. Despite the large number of taxa found, 3 prey-sand lance Ammodytes dubius (69.2%), Atlantic cod Gadus morhua (15.5%) and flatfishes (Pleuronectiformes, 10.7%)-accounted for 95.4% of the estimated wet weight food consumed by grey seals. Despite within-year variation in the species composition of grey seal diets, sand lance was the most important food in all months sampled. Comparisons of prey species abundance in research trawl surveys to that in grey seal diets indicated that more abundant and more widely distributed species accounted for most of the prey eaten.
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Excessive and unsustainable fishing mortality was the predominant factor in the depletion of Northwest Atlantic cod (Gadus morhua) stocks. However, despite imposition of severe catch restrictions for over a decade, stocks have mostly failed to recover at predicted rates. A number of explanations have been considered. Our analysis of demographic characteristics of 12 of these stocks indicates that recent productivity over the northern portion of the range is much lower than 20 years previous when several stocks recovered from less severe declines. Main contributing factors are, in rank order, increased natural mortality, decreased body growth, and in a few cases, reduced recruitment rates. Continued fishing in directed and bycatch fisheries is also an important factor. Under current conditions, we estimate negative or very low (< 2% per year) average growth rates in eight stocks. If fishing ceases, growth rates of > 5% would be expected in six stocks, with > 10% in four of these. Although productivity is low, we conclude that fishing mortality is further delaying recovery.
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Nine prey species (n = 7431) were fed to four captive female Steller sea lions (Eumetopias jubatus (Schreber, 1776)) in 11 feeding trials over 75 days to investigate the effectiveness of different methods used to determine diet from prey hard remains. Trials aimed to replicate short (1-2 days) and long feeding bouts, and consisted of single species and mixed daily diets. Overall, 25.2% ± 22.2% (mean ± SD, range 0%-83%) otoliths were recovered, but recovery rates varied by species (ANOVA, P = 0.01) and were linearly related to otolith robustness (R2 = 0.88). Squid beaks were recovered at higher frequencies (mean 96%) than the otoliths of all species. Enumerating both non-otolith skeletal structures and otoliths (together termed bones) increased species recovery rates by twofold, on average (P < 0.001), with increases up to 2.5 times for Pacific herring (Clupea pallasii Valenciennes in Cuvier and Valenciennes, 1847) and 3-4 times for salmonids. Using bones reduced interspecific differences (P = 0.08), but recovery varied among sea lions. Bones were distributed over more scats per meal (mean 2.9 scats, range 0-5) than otoliths (mean 1.9 scats, range 0-4). In three different 15-day mixed diet trials, biomass reconstruction (BR) indices performed better than frequency of occurrence indices in predicting diet fed. Applying our experimentally derived numerical correction factors (to account for species differences in complete prey digestion) further improved BR estimates, resulting in all 12 unweighted comparisons within 5% (for otoliths) and 12% (for bones) of the actual diet fed.
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In 1993, six Canadian populations of Atlantic cod (Gadus morhua) had collapsed to the point where a moratorium was declared on fishing. It has been argued that the collapses were caused by poor recruitment of cod to the fishery. Yet we are unable to detect a difference between the recruitment of year classes that should have contributed most to the spawning stock at the time of the collapse and recruitment levels in earlier years. A power analysis shows that we would have almost certainly detected an overall reduction of recruitment of 20%. There are considerable differences in the abundance trends as determined by research surveys and reconstructed from the commercial catch at age data (called ''virtual population analysis'' [VPA]) for each stock. VPA-based abundances consistently depict lower recruitment levels than do survey-based estimates in recent years. More important is the observation that from the early 1980s the VPA-based trend shows a decline where none is apparent in the survey-based trend. One explanation of these differences would be an increase in discarding of young fish as fishing mortality increased. We test the hypothesis that the mortality for young cod is unrelated to the fully recruited fishing mortality. This hypothesis is rejected; in each of the six stocks, high juvenile mortality was associated with high adult mortality. This is consistent with the discarding hypothesis. We suggest that age-specific abundance trends estimated from research surveys and VPA should be compared for all stocks where such data exist, and that high priority should, be given to the measurement of discarding levels and the extent to which catch misreporting is related to changes in fishing mortality.
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The grey seal (Halichoerus grypus) population on the Scotian Shelf has grown significantly over the past 20 yr, thus increasing the potential for competitive interactions between grey seals and fisheries. The relative contribution (percent wet weight) and size of prey eaten were estimated from otoliths and squid beaks recovered from 143 stomachs that contained food of the 528 collected from 1988 to 1990. Although 22 taxa were found, only four species (Atlantic herring, silver hake, Atlantic cod, and squid) accounted for 80% of the estimated weight of food eaten. The mean length of prey eaten ranged from 19 to 35 cm for six species. Only 17% of the cod and none of the pollock and squid eaten were of the length taken in commercial fisheries. However, about 80% of the silver hake and herring eaten were of commercial size. Offshore at Sable Island, northern sand lance, silver hake, and squid (in order of importance) accounted for 86.1% of the wet weight ingested by seals during summer; sand lance and cod accounted for 96.1% of prey eaten in winter. At inshore locations, herring, cod, and pollock made up 90% of the diet in summer; Atlantic mackerel, cod, squid, and herring made up 83% of the diet in winter.
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More than 10 years after the collapse of Atlantic cod (Gadus morhua) fisheries in Canada, the role of increased seal populations in the decline and lack of recovery of the stocks continues to be discussed. Using removals and abundance indices from synthetic populations, we found that sequential population analysis can uncover trends in natural mortality. We used this approach to examine variation in natural mortality (M) of southern Gulf of St. Lawrence cod. M increased from the late 1970s to the mid-1990s but declined slightly recently. Results were consistent with previous work indicating that M increased in the 1980s. Changes in estimated M for this cod stock matched fluctuations in grey seal (Halichoerus grypus) abundance. The increase in grey seal abundance from the late 1970s to the mid-1990s corresponded with the increase in estimated M of cod over this time period. The correspondence between seal abundance and M of cod supports the hypothesis that seal predation may be a cause of increased M. However, the diet information available suggests that seals consume mainly juvenile cod, whereas our evidence for an increase in M is for larger cod (ages 3 years and older).
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The recovery of sagittal fish otoliths and cephalopod beaks from fecal samples is an important source of information about the diets of marine mammals. Nevertheless, diet reconstructions are biased to some extent because of the partial and complete digestion of these prey structures. Although some authors have used correction factors to account for partial digestion of otoliths, none to date have corrected for the number of otoliths and cephalopod beaks that are completely digested, termed number correction factors (NCFs). Data from nine studies of captive pinnipeds show that corrections for the complete digestion of otoliths and cephalopod beaks range from 1.0 to 25.0 in the 28 prey species. Correction factors ranged from 1.0 to 10.0 in cases where seals could exercise by swimming during the experiment. In several species, NCFs vary inversely with prey length. The effect of applying NCFs will depend on the relative proportion of prey species in the diet and the NCFs of these species. Nevertheless, estimates of the species composition of marine mammal diets will benefit from the use of NCFs. Finally, standardization of experimental protocols and attention to the estimation of variability are needed to provide more reliable NCFs.
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We examined the digestion of hard remains of between one and four different size ranges of nine key North Sea prey taxa fed to seven captive harbour seals (Phoca vitulina). Percentage length reduction (mean 27.5%) and recovery rates (mean 42%) of experimental otoliths varied between species and were positively correlated to fish size and otolith robustness (mass/length). Mean length reduction of egested otoliths increased systematically with increasing size of ingested whiting and sandeel otoliths (p < 0.001), indicating that the size of larger fish may be underestimated. Intraspecific variation in otolith digestion was high (CV = 0.48-1.30), and to control for the artificial conditions of a captive study, external morphological features of otoliths were used to grade the degree of digestion and provide grade-specific correction factors. Bootstrap simulations were used to estimate 95% confidence intervals around correction factors and when partitioned indicated that calculation errors were in general less important than resampling errors. The application of species-, size-, and grade-specific correction factors progressively improved reconstructed estimates of prey biomass fed. As a consequence, estimates of prey size and diet composition require otoliths from faeces to be graded and more complex correction factors applied.
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Atlantic cod (Gadus morhua) was the dominant demersal fish and most important predator in the southern Gulf of St. Lawrence ecosystem as recently as the 1980s. However, productivity of southern Gulf cod has declined, and the population is no longer viable even in the absence of fishing. We conducted population projections taking into account uncertainty in current abundance-at-age and uncertainty or variability in each of the components of population productivity (i.e., rates of recruitment, individual growth, and adult natural mortality). We defined extirpation as a spawning stock biomass less than 1000 t (< 0.3% of historical levels). Based on these projections, at its current level of productivity, this population is certain to be extirpated within 40 years in the absence of fishing and in 20 years with fishery removals at the level of the total allowable catch in 2007 and 2008 (2000 t). Elevated natural mortality of adult cod (M) is the main factor contributing to the low productivity of this stock. Because M appears to be increasing, our projections are likely overly optimistic.
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Quantitative fatty acid signature analysis (QFASA) has been proposed as a technique for determining the long-term diet of animals. The method compares the fatty acid (FA) profiles of predators and potential prey items to estimate relative prey intake. We tested the assumptions of a key step in QFASA, the correction of predator FA signatures for metabolic processes through sets of calibration coefficients (CCs). We conducted long-term controlled feeding studies with captive Steller sea lions consuming herring and eulachon and northern fur seals consuming herring. We compared the results with data from harbour seals eating herring to evaluate the effects of phylogeny and prey type on individual CCs. Even within the limited extended dietary FA subset recommended for use by other researchers, we found that at least 41% of the CCs differed by family (otariid vs. phocid seals) and 58% differed by predator species (sea lion vs. fur seal), suggesting that CCs may be highly species-specific. We also found that 64% of the CCs differed by prey type (sea lions consuming herring vs. eulachon), which raises some fundamental implementation issues. We also found significant differences in diet predictions when the herring- and eulachon-derived sets of CCs were applied to an actual multi-species diet. CCs are presently used as a simple mathematical attempt to describe potentially complex biochemistry. The results of this study raise questions regarding the validity of using CCs derived from an alternative predator species, and highlight some fundamental issues regarding QFASA methodology that need to be addressed through further controlled studies.
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G. 2001. Delineating stocks of Atlantic cod (Gadus morhua) in the Gulf of St Lawrence and Cabot Strait areas using vertebral number. – ICES Journal of Marine Science, 58: 253–269. Cod (Gadus morhua) from several populations overwinter in the Cabot Strait area at the entrance to the Gulf of St Lawrence. We examined the utility of vertebral number for estimating the stock composition of cod aggregations in this overwintering area. Stock-specific mean vertebral numbers were estimated using samples obtained in the 1994–1997 spring (spawning) and summer (feeding) seasons when the populations were geographically separated. Mean vertebral number was significantly lower in the southern Gulf population than in each other population (northern Gulf, southern Newfoundland, Sydney Bight, eastern Scotian Shelf), but did not differ significantly among the other populations. Stock-specific vertebral counts were stable over the study period and, for three of the five stocks (including the large migratory Gulf of St Lawrence stocks), over several year classes. Stock composition of the overwintering aggregations was estimated using samples obtained from synoptic bottom-trawl surveys of the Cabot Strait area in January of 1996 and 1997. In both years, mean vertebral number was significantly lower along the south flank of the Laurentian Channel in NAFO (Northwest Atlantic Fisheries Organization) Divisions 4T and 4Vn than in areas on the north side of the Channel, suggesting that the south side of the Channel was dominated by southern Gulf cod. Maximum likelihood estimates of stock composition based on vertebral number indicated that cod in overwintering aggregations in the 4T–4Vn area were almost entirely (98%) from the southern Gulf stock while southern Gulf cod were almost entirely absent (<0.1%) from the aggregations on the north side of the Laurentian Channel. Bootstrapping indicated that the precision of these estimates was high. Simulations indicated that their bias was negligible. These results suggest that there is virtually no stock mixing across the Laurentian Channel during the overwintering period despite the aggregation of cod from several populations in this small area each year. We conclude that vertebral number provides a useful complement to other types of characters (microsatellite DNA markers, chemical composition of otoliths) that have been used to delineate these cod stocks.
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Identifying foraging areas of individuals and correlating them with potential food resources allows for a better understanding of predator-prey relationships. Herein, we examine whether grey seal movements were associated with overwintering concentrations of several commercial fish species in the Cabot Strait, Atlantic Canada, using data from satellite transmitters deployed on grey seals (between 1993 and 2005) and winter bottom-trawl survey data (1994 to 1997). The distribution of searching effort by male grey seals varied throughout the winter. In early winter, males concentrated their movements around St. Paul's Island. In late winter, they were found to the southeast of this area, where females also occurred. The fish community differed between apparent foraging and non-foraging areas. Densities of small plaice, hake and redfish, large herring and cod of all sizes were relatively high in the male grey seal foraging zones; female foraging zones were characterized by higher densities of small plaice and redfish and large cod. Areas where grey seal foraging was not concentrated were characterized by high densities of medium and large redfish as well as large turbot and witch flounder. Diet samples are needed to determine whether grey seals are feeding on the fish groups that distinguish foraging from non-foraging areas or alternatively on other prey that occur in the same areas.
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The nearshore diet of northwest Atlantic harp seals (Phoca groenlandica) was determined by reconstructing the contents of 1167 prey-containing stomachs (78.3% of 1490) collected from 1990 to 1993. Although harp seals consumed at least 62 species, 6 accounted for most of the mass consumed and their relative importance varied by area. Based on percent wet mass, sculpins (Cottidae) and Arctic cod (Boreogadus saida) were the main components of the diet of older seals (>1 year old) off Labrador, whereas Arctic cod and Atlantic herring (Clupea harengus) were the main prey of seals from northeastern Newfoundland. A more diverse diet was observed in seals taken off the west coast of Newfoundland, where capelin (Mallotus villosus), herring, Atlantic cod (Gadus morhua), redfish (Sebastes spp.), and Arctic cod were the main species. Redfish and Atlantic cod were important to seals along the south coast of Newfoundland. Eighty percent of fish consumed were less than 18 cm long, smaller than those taken by commercial fisheries. Pups (less than 1 year old) consumed fewer and smaller prey of a less varied assortment. Annual and seasonal variation in the diets was observed in the collection from northeastern Newfoundland. Arctic cod was the major prey consumed throughout the year by seals of all ages, although the relative importance of herring, capelin, and squid (Teuthoidea) increased during the summer. Invertebrates and capelin made up a greater proportion of the diet in 1992, owing to a decline in consumption of Arctic cod. This finding was associated with a decrease in the mass of stomach contents. Diet diversity did not change significantly over the study period.
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The distribution of animals is the result of habitat selection according to sex, reproductive status and resource availability. Little is known about how marine predators investigate their 3-dimensional space along both the horizontal and vertical axes and how temporal variation affects space use. In this study, we assessed the spatio-temporal movement of a sexually dimorphic marine mammal, the grey seal Halichoerus grypus by 1) determining seasonal home range size, 2) testing whether space use of seals was affected by water depth, and 3) investigating the vertical movement of seals according to the maximum depth of each dive. Between 1993 and 2005, we fitted 49 grey seals in the Gulf of St. Lawrence with satellite transmitters. We estimated seasonal 95% fixed-kernel home ranges for each individual. For each seal, we tested for selectivity and preference for 4 water depth classes at the home range scale and within the home range. We also evaluated the proportional number of dives made in each water depth classes according to the maximum depth of each dive. Home ranges were 10 times larger in winter than in summer. Seals generally selected habitats Document Type: Research Article DOI: http://dx.doi.org/10.1111/j.0906-7590.2008.05218.x Publication date: June 1, 2008 $(document).ready(function() { var shortdescription = $(".originaldescription").text().replace(/\\&/g, '&').replace(/\\, '<').replace(/\\>/g, '>').replace(/\\t/g, ' ').replace(/\\n/g, ''); if (shortdescription.length > 350){ shortdescription = "" + shortdescription.substring(0,250) + "... more"; } $(".descriptionitem").prepend(shortdescription); $(".shortdescription a").click(function() { $(".shortdescription").hide(); $(".originaldescription").slideDown(); return false; }); }); Related content In this: publication By this: publisher In this Subject: Biology , Ecology By this author: Harvey, Valérie ; Côté, Steeve D. ; Hammill, Mike O. GA_googleFillSlot("Horizontal_banner_bottom");
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Lundström, K., Hjerne, O., Lunneryd, S-G., and Karlsson, O. 2010. Understanding the diet composition of marine mammals: grey seals (Halichoerus grypus) in the Baltic Sea. – ICES Journal of Marine Science, 67: 1230–1239. Dietary studies are important in understanding the ecological role of marine mammals and in formulating appropriate management plans in terms of their interactions with fisheries. The validity of such studies has, however, often been compromised by unrepresentative sampling procedures, resulting in false weight being given to external factors seeming to influence diet composition. The bias caused by non-random sampling was examined, using canonical correspondence analysis to assess how the prey species composition in digestive tract samples of Baltic grey seals (Halichoerus grypus) was related to spatial, temporal, and demographic factors and to whether the samples were collected in association with fishing gear or not (“sampling condition”). Geographic region explained the largest fraction of the observed variation, followed by sampling condition, age group, and year. Season and gender were not statistically significant. Segregation of the two age categories “pups” and “juveniles–adults”, and the two geographic categories “Baltic proper” and “Gulf of Bothnia” are proposed to estimate the diet and fish consumption of the Baltic grey seal population as a whole. Atlantic herring was the most commonly recovered prey item in all areas and age groups, followed by European sprat in the south, and common whitefish in the north. Pups had eaten relatively more small non-commercial species than older seals.
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Thirteen captive phocid seals (10 grey seals (Halichoerus grypus), 2 harp seals (Phoca groenlandica), and 1 ringed seal (P. hispida)) were fed Atlantic herring (Clupea harengus harengus) in experiments designed to examine the use of otoliths recovered in stomach contents to interpret food consumption of wild seals. The percentage of ingested otoliths recovered in the stomach contents decreased with the time elapsed after feeding; 100% of otoliths were recovered between 0 and 3 h after feeding and 0% were recovered by 12.9 h after feeding. Absence of otoliths in the large intestine of seals having fed 3 to 6 h previous indicated that unrecovered otoliths had been digested (i.e., a complete disappearance of whole otoliths) while in the stomach. A significant relationship was also evident between the state of digestion of a seal's stomach contents, as measured by the proportion of otoliths remaining in skull cases (skull-recovered otoliths), and the duration of time since it had fed. These relationships have potential application in the estimation of daily fish consumption of seals feeding in the wild.
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This study compared diet reconstructed from different compartments of the digestive tract of harp seals (Pagophilus groenlandicus (Erxleben, 1777)) with the diet estimated using stable carbon and stable nitrogen isotope ratios in mixing models. Diet composition in 18 feeding harp seals (mean age = 2.4 years, SD = 1.8 years, range = 0–6 years) was determined using traditional methods of hard-part identification and reconstruction, and stable carbon and stable nitrogen isotope ratios. Diet composition consisted of 68.8% (SD = 8.7%) and 69.6% (SD = 11.6%) by mass of invertebrates or 65.0% (SD = 8.8%) and 66.5% (SD = 11.8%) by energy of invertebrates for the stomach and small-intestine compartments, respectively. Reconstructed diets using material recovered from the large-intestine contents suggested a diet of 43.1% (SD = 12.2%) and 38.0% (SD = 11.9%) invertebrates using mass and energy, respectively. Stable carbon and stable nitrogen isotope ratios determined for the same individual harp seals suggested a diet consisting of approximately 66.1% (SD = 117.4) invertebrates, indicating that diet reconstructions based on hard parts from stomachs are likely to be more representative than reconstructions from large-intestine contents. In species that feed on a combination of vertebrates and invertebrates, the use of faecal material to reconstruct diet composition will likely underestimate the importance of invertebrates in the diet.
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Atlantic cod were food-deprived for a period of 84 days at three temperatures (2, 6, 10� C), and changes in the liver, gonads and somatic weights, and muscle and liver water contents were monitored and compared with changes observed in wild cod over winter in the northern Gulf of St. Lawrence. Total lack of food during the period January-April would have caused condition to decline to a level at which very high mortality takes place. Actual changes in condition in wild cod were less than predicted from the laboratory experiments except during the period April-May at the onset of spawning. Thus, wild cod were able to meet part of the metabolic costs during winter through occasional feeding, as confirmed by stomach content data. We conclude that previous estimates of natural mortality associated with poor condition in spring were not biased by the selective mortality of poor-condition fish in winter. Crown Copyright 2003 Published by Elsevier Ltd on behalf of International Council for the Exploration
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We compared habitat associations of southern Gulf of St. Lawrence Atlantic cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) between the summer feeding season on the Magdalen Shallows and the overwintering period in the Cabot Strait. Data were from bottom trawl surveys conducted in September 1993, 1994, and 1995 and January 1994, 1995, and 1996. Both species occupied much deeper, warmer water in winter than in summer. The effect of cod age on temperature distribution reversed between the two seasons, with younger cod occupying warmer water than older cod in summer and colder water in winter. Selection of both depth and temperature by cod tended to be more significant in September than in January. The reduced statistical significance of habitat selection by cod in winter was associated with a more aggregated distribution in this season. The contrast between seasons in habitat associations was particularly strong for plaice. The median habitats occupied by plaice were 58-67 m and -0.1 to 0.3°C in September and 374-426 m and 5.2-5.4°C in January. Habitat selection by plaice was significant in both seasons, but significance tended to be greater in January. Degree of aggregation in plaice distribution was similar between the two seasons. Female plaice occupied significantly warmer water than males in September but not in January. The ecological and practical implications of this striking seasonal variation in habitat associations are discussed.
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We describe dramatic shifts in the species composition of the marine fish community of the southern Gulf of St. Lawrence using a 35-year time series of catch rates in an annual bottom-trawl survey. We attempt to understand the causes of these changes using a traits-based approach that relates the similarity among species in their abundance trends to similarities in their ecological traits. We selected traits based on a priori beliefs of how each should reflect susceptibility to changes in a different external factor potentially affecting the community. We found evidence for an effect of ocean climate and top-down effects of fishing and seal predation, but not for bottom-up effects of prey availability on adult fishes. Mean body length in the community decreased dramatically in the 1990s. This reflected the removal of large-bodied fishes by fishing and sharp increases in the abundance of small fishes. The biomass of small fish was inversely correlated with an index of predation on those fish by larger fish, suggesting strong predator control of the abundance of small-bodied fishes. Our results suggest that changes in ocean climate combined with direct and indirect effects of harvesting can dramatically and rapidly alter the composition of marine fish communities.
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Application of digestion correction factors to measurements and counts of fish otoliths and cephalopod beaks recovered from seal scats is required before the size or quantity of prey consumed can be accurately estimated. We carried out 86 feeding trials with seven grey seals (Halichoerus grypus) and 18 prey species to derive estimates of digestion coefficients (to account for partial digestion), recovery rates (to account for complete digestion), and passage rates (to estimate the time between consumption and excretion of an item). Mean digestion coefficients were greatest for sandeel (Ammodytes marinus) and then less for large gadoid, flatfish, and Trisopterus spp. otoliths; and finally squid (Loligo forbesii) beaks. Recovery rates were greatest for squid beaks and then less for large gadoid, Trisopterits spp., flatfish, and sandeel otoliths. Greater than 95% of otoliths and beaks recovered were passed within 4 days (similar to 88 h) of consumption. The large differences in partial and complete digestion rates found among prey species reinforce the importance of obtaining robust estimates of these quantities. Results from this study are the most comprehensive and systematically obtained for any species of pinniped and will allow accurate and precise estimation of the number and size of fish represented by otoliths recovered from grey seal scat samples collected in the wild.
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Most of the exploited fish stocks in the North Sea are also used as a food supply by a number of seal species; the same is true for some fish and invertebrate stocks in the Antarctic—although the fisheries there are, at present, much smaller than those in the North Sea. The information needed for a critical assessment of such interactions is reviewed. Using existing techniques it is possible to estimate the quantity and size-classes of each fish or invertebrate species consumed by seals and to compare this with the commercial catch. If fishing mortality is known, these estimates can be used to calculate the level of mortality imposed by the seals. However, a realistic evaluation requires information on the distribution and movements of the fish, the seals' feeding effort, and the fisheries effort in time and space. At present it is difficult or impossible to obtain this information, but recent technological developments in telemetry equipment will soon make it feasible. To assess the economic effects of changes in seal numbers on the fishery, or the ecological effects of changes in fisheries effort on seal populations, requires additional information on the responses of the fishery and the seals to changes in fish abundance, and of the commercial market to changes in the supply of fish.
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The distribution of animals is the result of habitat selection according to sex, reproductive status and resource availability. Little is known about how marine predators investigate their 3-dimensional space along both the horizontal and vertical axes and how temporal variation affects space use. In this study, we assessed the spatio-temporal movement of a sexually dimorphic marine mammal, the grey seal Halichoerus grypus by 1) determining seasonal home range size, 2) testing whether space use of seals was affected by water depth, and 3) investigating the vertical movement of seals according to the maximum depth of each dive. Between 1993 and 2005, we fitted 49 grey seals in the Gulf of St. Lawrence with satellite transmitters. We estimated seasonal 95% fixed-kernel home ranges for each individual. For each seal, we tested for selectivity and preference for 4 water depth classes at the home range scale and within the home range. We also evaluated the proportional number of dives made in each water depth classes according to the maximum depth of each dive. Home ranges were 10 times larger in winter than in summer. Seals generally selected habitats <50 m deep. They also mainly dove to depths of 40 m or less. At both scales of selection, preference for shallow areas decreased in winter. We also observed that adults used shallow habitats more than juveniles to establish their home range. A spatial segregation based on sex also occurred at the finer scale of selection where females were more concentrated in the shallowest parts of their home range than males. Segregation in space use according to age and sex classes occurred at both the horizontal and vertical scales. Our results emphasise the importance of studying habitat selection of marine predators in 3-dimensional space, in addition to the temporal scale.