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Multispecies interactions Capelin are good for cod: can the northern stock rebuild without them?
Abstract
The stock of northern cod (Gadus morhua )o ff Newfoundland and Labrador is at its lowest level in recorded history, with no rebuilding of northern spawning aggregations since the fishing moratorium in 1992. Cod diet was historically dominated by capelin (Mallotus villosus), which have been scarce in the northern areas since 1990. Using the study areas Hawke Channel and Trinity Bay within the historical northern cod range, and Placentia Bay (south coast), we examine growth, condition, and reproductive potential with respect to capelin diet in 18 000 cod sampled primarily in January and June of the years 1996-2000. Overall diet weights differed among areas and seasons (Placentia=Hawke>Trinity in January; Placentia>Trinity=Hawke in June). However, just 7 of 3383 cod stomachs (0.2%) from Hawke contained capelin (PFI=0). In contrast, 10% of cod of ages 3-7 in Placentia and Trinity Bays preyed on capelin and overall capelin partial fullness indices (PFI) were 0.2-0.4. Capelin PFIs were correlated with an index of availability (potential contact of capelin within 40 km of cod) calculated from acoustic surveys. Seasonally adjusted cod liver condition was strongly associated with capelin availability (pTrinityHawke). 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
... Capelin is an energetically profitable, or high-quality prey type, with twice the energy density and four times the percent lipid content compared to northern shrimp (Pandalus borealis; Lawson et al. 1998), the primary alternative prey type of cod (Sherwood et al. 2007). Rose and O'Driscoll (2002) first proposed that a shortage of high-quality prey (i.e. capelin) has contributed to the lack of recovery of Atlantic cod, a suggestion later supported by many studies (e.g., Sherwood et al. 2007;Mullowney and Rose 2014). ...
... Inter-annual variation in the timing of capelin spawning results in dietary variation of coastal marine predators during the summer (e.g., Gulka et al. 2017. As capelin consumption during the main summer growing season of Northern cod has been estimated to comprise around half of their energy budget annually (Templeman 1966;Lilly 1987;Rose and O'Driscoll 2002), this variation in timing of capelin spawning may lead to cod dietary shifts during the summer, which may result in reduced body condition and reproductive potential (Sherwood et al. 2007;Link and Sherwood 2019), thereby limiting the recovery of Northern cod (Mullowney and Rose 2014). The objective of this study was to evaluate cod dietary shifts, with a focus on capelin, in inshore waters during the summer under varying capelin availability on the northeast Newfoundland coast (Notre Dame Bay; Fig. 1). ...
... Although including a variety of prey types in the diet allows cod to acquire energy under lower capelin availability, consuming lower-quality prey types during their main summer growing season may negatively influence growth, body condition and reproductive potential (Mehl and Sunnanå 1991;Marshall et al. 1999;Yaragina and Marshall 2000;Sherwood et al. 2007;Mullowney and Rose 2014). Indeed, Northern cod are estimated to acquire around half of their energy budget annually from consuming capelin during the summer (Templeman 1966;Lilly 1987;Rose and O'Driscoll 2002). Although our findings are limited to one inshore region during a portion of the main growing season for cod, body condition indices of cod in offshore waters during 2018 in the fall did appear to be higher than 2017 (DFO unpubl. ...
In the early 1990s, the Atlantic cod (Gadusmorhua) population on the Newfoundland and Labrador shelf declined dramatically, leading to a fishing moratorium in 1992. The Northern cod population has not fully recovered, possibly due to the simultaneous population collapse of an important prey type, capelin (Mallotus villosus), which migrates from the continental shelf into coastal Newfoundland to spawn every summer. Our objective was to test whether capelin availability influenced short-term dietary shifts of cod while inshore on the northeast Newfoundland coast. We quantify the dietary composition and relative abundance and biomass of prey types in cod stomachs sampled weekly or bi-weekly throughout July-August 2017 and 2018. Survey-based acoustic estimates of capelin biomass and the timing of spawning in the study area revealed that peak capelin biomass was five times higher (0.239 g/m2) and two weeks earlier (July 28) during 2018 relative to 2017 (0.048 g/m2, August 9). Cod stomachs were ~ 10 times more likely to contain capelin during 2018 relative to 2017. As capelin availability increased throughout each summer, cod shifted from a high diversity diet of lower-quality invertebrate prey (e.g., shrimp, crab) to a low diversity, capelin-dominated diet, whereby capelin presence, abundance and biomass increased. These findings indicate that capelin remains a primary prey type of cod in inshore waters during their summer growing season, but dietary proportions of capelin vary with their availability. As low dietary proportions of capelin are associated with reduced body condition and reproductive potential in cod, findings support the need for an integrated capelin-cod management approach.
... Atlantic cod often undergo seasonal migrations from offshore spawning areas to inshore feeding areas (Robichaud and Rose 2004). These seasonal migrations around Newfoundland are primarily expected to be driven by cod predation on capelin (Rose and O'Driscoll 2002); however, there is evidence that these migrations also result in increased feeding on zooplankton and other fish species (e.g., sand lance) for 3Ps cod (Sherwood et al. 2007). This increase in feeding inshore is likely due to increased spatial overlap between cod and lipid rich prey (Schwalme and Chouinard 1999). ...
... Furthermore, environmental changes can affect ecosystem productivity and, therefore, indirectly impact fish condition and growth. For instance, there is evidence of oscillations and declines in cod condition through time from other cod stocks around Newfoundland, and these trends appear to match trends in capelin availability (Rose and O'Driscoll 2002;Morgan et al. 2018;Regular et al. 2022). Here, we identified a significant positive correlation between the temporal change in weight-at-length with sand lance availability. ...
The relationship between the length and weight of fish is used to assess their growth and condition. This relationship is often assumed to be the same spatially and temporally. However, variability in the weight–length relationship can occur, which provides important information about stock productivity. We developed a spatiotemporal model for the weight–length relationship that is useful for predictions in un-sampled areas. We applied the model to survey data for Atlantic cod off the southern coast of Newfoundland, Canada. We found that weight-at-length was higher inshore, oscillated over time, was below average in recent years, declined during late-January to early-June especially for intermediate sized cod, and that the temporal oscillations were correlated with several local environmental time series. Finally, the model estimated a decrease in the allometric coefficient for intermediate sized cod (40–80 cm), indicating that those cod may be experiencing additional feeding deficiencies. Spatiotemporal variation in the weight-at-length relationship should be accounted for in the stock assessment process when fishery catch numbers are derived from tonnes landed and when estimating stock and fishery weights-at-age.
... [58][59][60][61]. All of these diets have been reported to have changed along with the large-scale ecosystem changes that were observed in this region in the early 1990s [57,61,62]. ...
... This pattern tends to became more pronounced with increasing fish size, suggesting a balance between the consumption of pelagic and benthic productivity. The balanced acquisition of pelagic and demersal productivity has recently been hypothesized to explain the dominance of large benthic demersal fishes over large pelagic fishes in boreal and temperate regions [98], while other comparative studies in this region have focused largely on the dynamics of single prey species [62,99]. Whether this energy acquisition hypothesis also influences ecosystem recovery is an important question with potential implications for differential ecosystem recovery pathways. ...
Spatial heterogeneity in food web structure and interactions may reconcile spatial variation in population and community dynamics in large marine ecosystems. In order to assess food web contributions to the different community recovery dynamics along the Newfoundland and Labrador shelf ecosystem, we quantified species interactions using stable isotope mixing models and food web metrics within three sub-regions. Representative samples of each species caught in trawls and plankton tows were analyzed for stomach contents and stable isotope ratios (δ¹⁵N and δ¹³C) to parameterize isotope mixing models. Regional variation, highlighted by the diets of three economically important species, was observed such that the southern region demonstrated a variety of trophic pathways of nutrient flow into the higher food web while the diets of fish in the northern regions were typically dominated by one or two pathways via dominant prey species, specifically shrimp (Pandalus sp.) and hyperiids. Food web metrics indicated that the low-diversity northern regions had higher connectance and shorter food chain lengths. This observed regional variation contributes to our understanding of the role of specific forage species to the ecosystem which is an essential contribution towards ecosystem-based management decisions.
... The drivers of groundfish M in the Northwest Atlantic are difficult to establish and vary to some degree by species and ecosystem (Wiedenmann and Legault 2022 ). For instance, elevated M for adult Atlantic cod ( Gadus morhua ) off Newfoundland and Labrador may be linked to capelin ( Mallotus villosus ) availability (Rose andO'Driscoll 2002 , Regular et al. 2022 ). Strong spatiotemporal correlation between groundfish M and seal abundance has led many to hypothesize that increased predation by recovering pinniped populations is a driver of elevated groundfish M in other ecosystems (Mohn and Bowen 1996, Fu et al. 2001, Chouinard et al. 2005, Chassot et al. 2009 ). ...
The recovery of many groundfish stocks in the Northwest Atlantic has been impeded by elevated mortality among older/larger individuals. The sources of groundfish mortality are generally uncertain, though predation by growing grey seal herds and unreported fishing are thought to be dri ver s of mortality for Atlantic cod (Gadus morhua) on the Western Scotian Shelf and Bay of Fundy (4X5Y), as well as in nearby ecosystems. We developed a statistical catch-at-age model for 4X5Y cod that accounted for grey seal predation and bycatch/discards and tested the model across a broad range of consumption and discard scenarios. We found that most cod mortality was unexplained by seal predation and fishing when cod comprised a small to moderate portion of the grey seal diet. Predation and fishing accounted for slightly more than half of recent cod mortality when cod was highly prevalent in the seal diet (45% by weight) but explained little historical mortality. Bycatch was a minor source of mortality prior to 2019 and accounted for 0.%1-8.1% of recent mortality. Increased sampling of seal diets and monitoring of emerging seal colonies is necessary for reducing uncertainties around seal predation effects in the Northwest Atlantic.
... For instance, in the northern distribution range of cod, off the coast of Newfoundland and Labrador, capelin has migrated southward due to ocean cooling, resulting in reduced prey availability for cod. This has led to diminishing cod populations as a result of poor body conditions and lower reproductive success (Rose & O'Driscoll, 2002;Vilhjálmsson, 1997). Long-term, but also occasional range expansions of highly mobile predators with high dispersal potential such as tuna, may lead to changes in their abundance and distribution and will impact spawning and migratory behavior, regional food webs, and fisheries (Muhling et al., 2017), also potentially in the Fram Strait. ...
The Arctic Ocean is home to a unique fauna that is disproportionately affected by global warming but that remains under‐studied. Due to their high mobility and responsiveness to global warming, cephalopods and fishes are good indicators of the reshuffling of Arctic communities. Here, we established a nekton biodiversity baseline for the Fram Strait, the only deep connection between the North Atlantic and Arctic Ocean. Using universal primers for fishes (12S) and cephalopods (18S), we amplified environmental DNA (eDNA) from seawater (50–2700 m) and deep‐sea sediment samples collected at the LTER HAUSGARTEN observatory. We detected 12 cephalopod and 31 fish taxa in the seawater and seven cephalopod and 28 fish taxa in the sediment, including the elusive Greenland shark (Somniosus microcephalus). Our data suggest three fish (Mallotus villosus, Thunnus sp., and Micromesistius poutassou) and one squid (Histioteuthis sp.) range expansions. The detection of eDNA of pelagic origin in the sediment also suggests that M. villosus, Arctozenus risso, and M. poutassou as well as gonatid squids are potential contributors to the carbon flux. Continuous nekton monitoring is needed to understand the ecosystem impacts of rapid warming in the Arctic and eDNA proves to be a suitable tool for this endeavor.
... Although we do not have information about local capelin abundance in East Greenland, this might have contributed to lower energy reserves at site 2. Nevertheless, our results show that consumption of capelin was associated with lowest EPA:ARA ratios, which are linked to high egg production and a high number of viable eggs (Røjbek et al., 2014). As consumption of capelin was not linked to highest levels of most commonly used body condition indicators K and HSI, this beneficial fatty acid composition might further explain, why capelin is the most important prey resource for body condition and population replenishment in all Arcto-boreal cod stocks (Kjesbu et al., 1998;Link and Sherwood, 2019;Rose and O'driscoll, 2002). ...
Body condition and energy reserves are important indicators of organism health, habitat suitability and pre-dictors for the reproductive success in fish. In Greenland waters, Atlantic cod (Gadus morhua) inhabits different habitats in shallow and deep parts on top of the shelf and along slopes, where diet composition differs. We investigated the influence of habitat heterogeneity and trophic niche on body condition using organosomatic indicators, e.g. the morphometric index K, hepatosomatic and gonadosomatic indices, and biochemical indicators , e.g. lipid content and fatty acid compositions, of mature female cod. Body condition differed between sites and peaked in northeastern regions in depths below 300 m towards the slope of the northern Irminger Sea. Trophic niches as indicated by stable isotope values and stomach composition data varied between sites, which was likely related to depth and differences between benthic and pelagic feeding regimes. Total lipid content and fatty acid profiles important for reproduction were associated with a pelagic diet comprised of mesopelagic fish and crustaceans. Interestingly, consumption of capelin was not linked to highest energy reserves as indicated by traditional body condition indices, such as K and the hepatosomatic index, but lowest ratios of eicosapentaenoic acid to arachidonic acid in gonads, which are known to be beneficial for high egg production. This shows that body condition on biochemical level can differ from conventionally used indices, which emphasizes the need to take fatty acid composition into account, when investigating condition and reproductive potential in cod. Our results emphasize that the factor habitat cannot be ignored for population replenishment of the cod stocks in Greenland waters and indicate that capelin as part of an Arcto-boreal diet benefits reproductive success in cod.
... It should not be surprising, then, that loss of forage species is associated with marine ecosystem decline. Deficient quantity and quality of the forage base have been linked to apex predator's poor physical condition, low productivity, and the failure of population recovery after depletion events [10,11]. Along with global warming, spatiotemporal mismatch with lipid-rich prey may reduce even more the productivity in highly valuable fished populations, such as the Gulf of Maine's Atlantic cod stocks (Gadus morhua), exacerbating their decline, or impairing their recovery [12]. ...
Reduction of ecosystem connectivity has long-lasting impacts on food webs. Anadromous fish, which migrate from marine to freshwater ecosystems to complete reproduction, have seen their historically larger ecosystem role undercut by widespread riverine habitat fragmentation and other impacts mainly derived from anthropogenic sources. The result has been extensive extirpations and increased susceptibility to a suite of environmental factors that currently impede recovery. Under this present-day context of reduced productivity and connectivity, aggressive management actions and enforcement of catch limits including bycatch caps and complete moratoria on harvest have followed. What remains less understood are the implications of changes to food webs that co-occurred. What benefits restoration could provide in terms of ecosystem functioning in relation to economic costs associated with dam removal and remediation is unknown and can limit the scope and value of restoration activities. Here we employ, historical landscape-based biomass estimates of anadromous alosine for the first time in an ecosystem modeling of the Northeast US large marine ecosystem (LME), to evaluate the value of improving connectivity by measuring the increase in energy flow and population productivity. We compared a restored alosine model to a contemporary model, analyzing the impacts of the potential increase of connectivity between riverine and oceanic systems. There was the potential for a moderate biomass increase of piscivorous species with high economic value, including Atlantic cod, and for a major increase for species of conservation concern such as pelagic sharks, seabirds and marine mammals. Our study highlights the benefits of increased connectivity between freshwater and ocean ecosystems. We demonstrate the significant role anadromous forage fish could play in improving specific fisheries and overall ecosystem functioning, mainly through the diversification of species capable of transferring primary production to upper trophic levels, adding to benefits associated with their restoration.
... Our conclusion is thus consistent with that of numerous past studies that concluded that the Div. 2J3KL capelin stock suffered an order of magnitude decline in the early 1990s (DFO 1994, 2008, 2010, 2015, Miller 1994, 1997, Rose & O'Driscoll 2002, Davoren & Montevecchi 2003, Buren et al. 2014a, Mullowney & Rose 2014, Murphy et al. 2018. ...
Capelin are a focal forage species in the Northwest Atlantic ecosystem as they act as an energy conduit from lower to higher trophic levels. Fisheries and Oceans Canada determined that the Newfoundland capelin stock (Northwest Atlantic Fisheries Organization Divisions 2J3KL) suffered an order of magnitude decline in biomass in 1990-91. This collapse was concomitant with drastic changes observed in the ecosystem during the late 1980s and early 1990s. While the results of more than a dozen studies have supported the capelin stock collapse hypothesis, an alternative non-collapse hypothesis proposed that rather than collapsing in 1990-91, the capelin stock either (1) changed its migratory patterns while the timing of the spring capelin acoustic survey remained constant, leading to a spatio-temporal mismatch between the spring acoustic survey and the stock, or (2) became less migratory and remained inshore year round, therefore being largely underestimated by the offshore spring and fall acoustic surveys. The collapse and non-collapse hypotheses were tested using multiple independent data sets, which included both fishery-dependent (inshore commercial catch) and fishery-independent (spring and fall acoustic and fall bottom trawl surveys, capelin larval indices, aerial surveys, predator diet and behavior) data, and diverse statistical methods. The weight of evidence approach led us to reject the non-collapse hypothesis and conclude that the Newfoundland capelin stock did collapse in 1990-91 with minimal recovery over the subsequent three decades.
... Following the collapse of the Newfoundland capelin stocks, the common murre (Uria aalge), an important avian predator on capelin, delayed breeding and delivered smaller and lower quality capelin to their chicks, which resulted in poor condition of the chicks (Davoren and Montevecchi 2003). Recently, the recovery of cod stocks in the same areas has been linked to the parallel recovery of the capelin stock (Rose and O'Driscoll 2002;Rose and Rowe 2015). Hence, there is good evidence that changes to the capelin feeding ecology has an impact far beyond the capelin itself and may affect the population dynamics of top-predators. ...
Capelin (Mallotus villosus) is an important trophic node in many Arctic and sub-Arctic ecosystems. In Godthåbsfjord, West Greenland, the zooplankton community has been shown to change significantly from the inner part of the fjord, which is impacted by several glaciers to the shelf outside the fjord. To what extent this gradient in zooplankton composition influences capelin diet during their summer feeding in the fjord is yet unknown. To investigate this, we analysed stomach content of capelin (8–14 cm) sampled using a pelagic trawl at three stations in outer (GF3), mid (GF7) and inner (GF10) part of Godthåbsfjord in May and August 2013. In May, the copepod nauplii numerically dominated the diets, but euphausiids contributed > 92% by carbon mass at all stations. In August, calanoid copepods were the most important prey numerically and by carbon mass. Smaller copepod species became more important towards the inner stations, whereas the large Calanus species dominated in the outer stations. There was also a trend in decreasing stomach carbon content towards the inner stations, and on the individual level, variation in stomach content was strongly negatively related to the proportion of small copepods in the diet. This suggests that the inclusion of small copepods in the diet cannot compensate for the absence of larger euphausiids and copepods. Therefore, any change in the ecosystems that favours these at the expense of larger zooplankton and euphausiids is likely to impact capelin feeding negatively with consequences for the whole ecosystem.
... The compositional rearrangement is consistent with the theory that the cod recovery was slowed through reduced forage fish abundance [22], and by competition and juvenile predation from species that became dominant post-collapse [21]. While invertebrate abundances (primarily crab and shrimp) increased substantially following the groundfish collapse [19,59] and could have provided an alternative food source for cod and other predatory fish [60], these species do not have the same nutritional value for groundfish and may not have provided a sufficient food source to rebuild stock abundances [61]. The recovery of groundfish stocks may also be contributing to the observed declines in invertebrate stocks through increased predation rates. ...
The Northwest Atlantic cod stocks collapsed in the early 1990s and have yet to recover, despite the subsequent establishment of a continuing fishing moratorium. Efforts to understand the collapse and lack of recovery have so far focused mainly on the dynamics of commercially harvested species. Here, we use data from a 33-year scientific trawl survey to determine to which degree the signatures of the collapse and recovery of the cod are apparent in the spatial and temporal dynamics of the broader groundfish community. Over this 33-year period, the groundfish community experienced four phases of change: (i) a period of rapid, synchronous biomass collapse in most species, (ii) followed by a regime shift in community composition with a concomitant loss of functional diversity, (iii) followed in turn by periods of slow compositional recovery, and (iv) slow biomass growth. Our results demonstrate how a community-wide perspective can reveal new aspects of the dynamics of collapse and recovery unavailable from the analysis of individual species or a combination of a small number of species. Overall, we found evidence that such community-level signals should be useful for designing more effective management strategies to ensure the persistence of exploited marine ecosystems.
... Close linkage has been described between shrimp and Atlantic cod (Gadus morhua Linnaeus, 1758), and cod predation in particular can have strong effects on the development of shrimp stocks (Worm & Myers 2003). Numerous studies have demonstrated the importance of shrimp in the diet of cod, but they highlight the fact that the proportion of shrimp can vary greatly between areas (Rose & O'Driscoll 2002;Jónsdóttir et al. 2012), seasons (Pálsson & Björnsson 2011), years (Mehl 1989;Pálsson & Björnsson 2011;Jónsdóttir et al. 2012) and size of cod (Albers & Anderson 1985;Berenboim et al. 2000). The consumption of shrimp by cod can sometimes be equal to (Hvingel & Kingsley 2006) or of greater importance than commercial fishing pressure (Parsons 2005;Savenkoff et al. 2007). ...
Northern shrimp (Pandalus borealis) is targeted by commercial fisheries but is also an important prey in the marine food web. In this study, stomach content data were used to study predation on shrimp by three gadoid species, cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus), in six inshore areas around Iceland. The results showed that shrimp was more important in the diet of cod compared to the other two predators. However, the overall predation pressure was similar for cod and haddock, due to the high abundance of haddock. Therefore, even though shrimp is not the primary source of food for haddock, this species can have a substantial effect on shrimp stocks when haddock abundance is high. In addition, while cod and haddock did not select for any particular size of shrimp, whiting seemed to preferentially target juvenile shrimp. The results suggest that due to the overall effects of these three predators on shrimp stocks, gadoids need to be included in management of shrimp stocks as predation is one of the major drivers in the development of this important prey stock prey stock.
... In seasonal marine environments, large emphasis has been put on the role of dietary lipids. This also applies to cod, where the size and productivity of cod stocks have been linked to lipid-rich forage fish (Rose and O'Driscoll 2002), and liver lipid levels in late fall have been proposed as a predictor of reproductive (recruitment) success at the population level (Marshall et al. 1999). Cod store lipids in the liver and proteins in muscle tissue when food is abundant, and mobilize these resources for later reproduction or during periods of food shortage (Schwalme and Chouinard 1999). ...
Accumulating research argues that migrants influence the functioning and productivity of local habitats and ecosystems along migration routes and potentially drive cross-system energy fluxes of considerable magnitude, yet empirical documentation of local ecological effects and descriptions of the underlying mechanisms are surprisingly rare. In this study, we discovered migrant–resident interactions and substantial cross-system lipid transportation in the transition zone between the Baltic Sea and the North Sea where a resident cod population (predators) was found to interact with a herring population (prey) on a seasonal basis. We traced the lipids, using fatty acid trophic markers (FATM), from the herring feeding grounds in the North Sea to the cod livers in the Western Baltic Sea. Time series analysis of population dynamics indicated that population-level production of cod is positively affected by the herring subsidies. However, the underlying mechanisms were more complicated than anticipated. During the herring season, large cod received most of its dietary lipids from the herring, whereas smaller cod were prevented from accessing the lipid pool due to a mismatch in predator–prey size ratio. Furthermore, while the herring were extremely rich in bulk energy, they were surprisingly poor in a specific functional fatty acid. Hence, our study was the first to illustrate how the magnitude cross-system fluxes of subsidies in migrant–resident systems are potentially constrained by the size structure of the resident predator population and the nutritional quality of the migrants. © 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
... Capelin (Mallotus villosus) is the most important forage species in the waters off Newfoundland and Labrador (NL) for many species of finfish, seabirds, and marine mammals (i.e. Montevecchi and Piatt, 1984;Rose and O'Driscoll, 2002;Davoren et al., 2007). A thirty-fold reduction in capelin abundance occurred in the Northwest Atlantic Fisheries Organization (NAFO) Divisions 2J3KL during 1991 (hereafter the northern capelin, Fig. 1) (Bakanev, 1992). ...
In May 2012–2015, 90 large (85–117 cm) Northern cod (Gadus morhua) with pop-up satellite archival tags (PSATs) set to release 1 year later were released from offshore spawning aggregations on the northeast Newfoundland shelf. Forty-nine (54%) transmitted or were recovered; 82% migrated inshore and 15% remained offshore (balance predated). Geo-locations were based on release, capture, and surfacing positions, and at tag-recorded depths <50 m, on Kalman-predicted light-based longitudes matched with coastal bathymetry. Migrating fish moved southwestward on average 5° longitude (350 km) and 2° latitude (222 km) to the northeast coast of Newfoundland, southern Labrador, and northern Grand Bank, as historically from similar release points. Migration onset through the Labrador Current (<0 °C) and arrival inshore were correlated with capelin (Mallotus villosus) peak spawning, inshore duration with capelin, and cod biomass. Both species were delayed a month from historical norms. The probability of a cod remaining inshore after 1 December was 50%, which could impact time-dependent surveys. Fishing exploitation was 6% and predation 5%. Northern cod have re-established historical spatial migration patterns, with capelin a key influence on timing and duration inshore.
Temperature fluctuations impact the distribution of various marine fish species and typically result in distributional changes, seeing either a geographical expansion or, possibly, a retraction. Survival and fitness in the new habitat depend, however, on the species’ adaptability capacity. Here, we describe changes in geographical distribution of Icelandic haddock (Melanogrammus aeglefinus) based on annual survey data for a 38-year period, 1985–2022. Most of the stock is in the warm waters off the south and west coasts of Iceland (south area), but a small proportion in colder water masses off the north and east coasts (north area). Due to high recruitment, density increased greatly in both areas in the years 2001–2005. Haddock density was usually higher in the south area in 1985–2004, in the north area in 2005–2014, but similar in both areas in 2015–2022. Haddock condition was found to be influenced by both temperature and haddock density. Density-dependent factors influenced haddock mean weight, which dropped considerable during times of high haddock density. Capelin was of great importance as a diet, especially in the south, where stomach fullness decreased considerably when capelin was not a part of the diet. Food availability seems to be highly variable between the two areas but more stable in the north, where fluctuations in capelin fullness were small. Even though favourable environmental conditions may result in increased haddock density in the colder waters in the north, lower condition indicates that the northern area may be less suitable for this species.
Introduction
On the Newfoundland-Labrador Shelf, Canada, capelin (Mallotus villosus) is a key forage fish that migrates annually from offshore to spawn within coastal embayments. Although capelin are thought to primarily spawn on beaches in this region, they also spawn subtidally in deeper water (5–40 m), where their eggs remain throughout incubation. The spatial extent of subtidal (i.e. “deep-water”) spawning habitat in coastal Newfoundland is unknown and is a research priority for fishers and management.
Methods
We collaborated with capelin fishers to identify putative deep-water spawning sites as a first step in determining the contribution of deep-water spawning to capelin recruitment. Given limited fine-scale coastal bathymetry and seabed habitat type data, which impeded spatial modeling to determine suitable capelin spawning habitat, this science-industry research collaboration was key to addressing this knowledge gap.
Results
Through two years of multi-bay fisher interviews, 84% of interviewed fishers (56 interviewees) reported having observed deep-water spawning and identified a broad distribution of putative spawning sites throughout coastal Newfoundland. The majority of fishers indicated inter-annual variation in beach and deep-water spawning habitat use, and most interviewees linked this variation to temperature and capelin abundance. Further collaborations with fishers during boat-based surveys, we sampled 136 unique sites within 12 search areas in eastern Placentia Bay and 26 unique sites within six search areas in Bonavista Bay. Underwater video surveys combined with sediment sampling revealed seven previously undocumented deep-water spawning sites.
Conclusion
The deep-water spawning areas derived from these fisher interviews can now be used to build a time series for monitoring capelin spawning habitat use alongside citizen-based beach monitoring data, as a general capelin stock health indicator in a weight of evidence approach for the science advisory process.
Forage fish form a critical trophic link in marine ecosystems, and yet, for many species, there is limited information available. As nations move from single species stock assessments to ecosystem approaches to fisheries management (EAFMs), more information on forage fish will be required. In this study, 50 years of scientific literature were systematically mapped for 11 forage fish species in Canada's Arctic, Atlantic, and Pacific oceans. The objectives were to identify (1) knowledge clusters and gaps and (2) the pressures studied in relation to forage fish outcomes. Of the 2897 articles mapped, the majority studied adults and the distribution, productivity, growth, and life history of commercially fished species. Knowledge gaps were identified for forage fish: (1) that were noncommercially exploited; (2) egg and larval life history stages of most species and juveniles of noncommercial species; (3) diets of most species; (4) migration and performance for all species and survival of noncommercial species; and (5) the effects of some pressures (e.g., large-scale climate pressures). Addressing these knowledge gaps would improve the application of EAFMs.
Natural mortality (M) is difficult to measure directly in exploited fish stocks. Therefore, in stock assessments, M has usually been assumed to be constant for age classes of fishable size. However, M is likely to vary annually due to temporal changes in food availability, predation, fish size at age, and other biological and environmental factors. We suggest that M of fish at age and year is the sum of length-related mortality and condition-related mortality (M = ML + MC). We propose that MC is directly proportional to PKH, the percentage of fish in the stock with condition factor (K) and liver condition (H) below critical values. This relationship is used to demonstrate likely patterns in annual variation in MC for ages 2–9+ in Icelandic cod (Gadus morhua) based on fish measurements in groundfish surveys in spring and autumn. There were annual fluctuations and temporal trends observed in MC for different age classes attributable to variation in food availability. The described method is suitable for age-structured stock assessments of cod and other fish stocks. Our results indicate that basic stock assessments may be improved by including estimates of variable MC.
Fisheries management requires spatially congruent production and assessment areas. Canada’s Northern cod (Gadus morhua), initially considered a stock complex distributed from northern Labrador to the northern Grand Bank, had its northern boundary reduced to southern Labrador in the early 1970s. Spatial incongruence has resulted in spawning stock biomass (SSB) and recruitment (R) between historical and recent eras. To investigate temporal changes, four stanzas of SSB and R were derived from statistical Perron breaks. In stanza 1, the 1960s, spawning off northern Labrador coincided with higher SSB and R than in following stanzas from the 1970s onward. SSB–R relationships that include 1960s data do not represent potential production from a more southerly distributed stock. Loglinear models of R (density-dependent models did not improve fit) indicated SSB had greatest effect and, with indices of climate and south–north distribution, explained 86% of variance. Lack of density dependence suggests long-standing recruitment overfishing, making reference points problematic. SSB growth is suggested as an alternative management target. Rebuilding the Northern cod to historical abundance requires a full “portfolio” of spawning from northern Labrador to the Grand Bank (a remanaged 2+3KL stock). With a contracted range, lower production should be anticipated.
In ecological sciences, animal diets are often simplified to ‘resources' or ‘caloric quantities'. However, in the present study, we investigated the optimal foraging strategy of Atlantic cod Gadus morhua when both macro- and micro-nutritional requirements are accounted for. Proteins cannot be synthesized from fatty acids, so the proteins for gonad development must come from other dietary sources. In addition, micronutrients are required in smaller quantities. For example, for cod, arachidonic acid (ARA) acts as a micronutrient precursor for prostaglandins, which is important for reproduction. We formulated a dynamic state-dependent model to make predictions about optimal diet choice and foraging behavior. We applied the model to a case study in the strait between Denmark and Sweden. The model predicted that energy acquired from dietary protein should be twice that acquired from lipids, with a small increase in the lipid requirements when gonads are growing. The model also predicted that the ‘energy sparing effect of lipids' made it beneficial to engage in risky foraging activity to supplement a lean diet with a little bit of fat. When we constructed the model to also optimize ARA uptake, the cod consumed relatively more ARA-rich crabs in the months prior to spawning, despite the otherwise poor energetic value of this prey. In support of the model predictions, field observations indicated that lipid stores reached a peak shortly after the arrival of the lipid-rich migrating herring and the fatty acid signal of these herring were evident in the liver of nearly all cod. Three month later, only half of the cod contained the herring-derived fatty acid signal, supporting the predicted shift in prey type prior to spawning. From these model predictions and field observations, we conclude that, also in the wild, nutritional requirements can be at least as important as pure energy acquisition.
Starvation is a ubiquitous process in nature as all animals depend on finite resources to survive. Limited food resources can lead to starvation-induced mortality and, depending on the scale of the limitation, population-level consequences may emerge. However, attempts to model starvation-induced mortality are rare as it is difficult to isolate the effects of specific sources of natural mortality. Using commonly collected data on body condition, we develop a statistical approach to index starvation-induced mortality by estimating the proportion of individuals experiencing severe emaciation in a fish population. This approach accounts for seasonal patterns in body condition, allowing data collected at any time of year to be included. Using Canada’s Northern cod (Gadus morhua) stock as a case study, we found a positive association between rates of natural mortality estimated by an integrated population model and the index of mortality derived from proportions of cod in poor condition. This index is also associated with the availability of two key prey species, capelin (Mallotus villosus) and Northern shrimp (Pandalus borealis). Taken together, our results indicate that starvation-induced mortality represents an important component of the natural mortality experienced by Northern cod, supporting the idea that insufficient food contributed to the collapse of the stock in the early 1990s and, subsequently, limited its recovery. These implications highlight the practical relevance of explicitly considering prey availability (e.g., through body condition or prey abundance indices) to improve stock assessment modeling and aid the development of actionable ecosystem-based fisheries management strategies.
The fishery for Northern Atlantic cod (Gadus morhua) off Newfoundland and Labrador, Eastern Canada, presents the most spectacular case of an exploited stock crashed in a few decades by an industrial bottom trawl fishery under a seemingly sophisticated management regime after half a millennium of sustainable fishing. The fishery, which had generated annual catches of 100000 to 200000 tonnes from the beginning of the 16th century to the 1950s, peaked in 1968 at 810000 tonnes, followed by a devastating collapse and closure 24 years later. Since then, stock recovery may have been hindered by premature openings, with vessels targeting the remains of the cod population. Previous research paid little attention towards using multicentury time series to inform sustainable catches and recovery plans. Here, we show that a simple stock assessment model can be used to model the cod population trajectory for the entire period from 1508 to 2019 for which catch estimates are available. The model suggests that if fishing effort and mortality had been stabilized in the 1980s, precautionary annual yields of about 200000 tonnes could have been sustained. Our analysis demonstrates the value of incorporating prior knowledge to counteract shifting baseline effects on reference points and contemporary perceptions of historical stock status.
The population of Atlantic cod significantly contributes to the prosperity of fishery production in the world. In this paper, we quantitatively investigate the global abundance variation in Atlantic cod from 1919 to 2016, in favor of spatiotemporal interactions over manifold impact factors at local observation sites, and propose to explore the predictive mechanism with the help of its periodicity, time–frequency co-movement, and lead-lag effects, via long short-term memory (LSTM). We first integrate evidences yielded from wavelet coefficients, to suggest that the abundance variation potentially follows a 36-year major cycle and 24-year secondary cycle at the time scales of 55 years and 37 years. We further evaluate the responses of Atlantic cod abundance to the external impact factors, including sea surface temperature (SST), catches, prey biomass, and sea surface salinity (SSS), in aid of the wavelet coherence and phase difference, which allows us to identify the dominantly correlative factors and capture the leading roles along the time domain and then divide the responses around the recent 60 years into three stages: before 1985, 1985–1995, and after 1995. At the first stage, the reason for the decline in abundance could be mainly attributed to the rapid rise of fish catches. At the second stage, the impact of SST and SSS also provides significant indices, besides overfishing; meanwhile, the mortality of primary producers and forced migration of fish species indirectly cause the decline. At the third stage, warming SST and growing SSS directly led to the decrease of abundance. Finally, we establish one ensemble of LSTM-SAE architecture to comprehensively reflect the predictive patterns at each stage. It has been demonstrated from experimental results that the models behaved better when intentionally feeding with the dominantly correlative multivariate inputs, instead of either all factors or only the abundance. The proposed scheme provides opportunities to symmetrically identify the underlying predictive attributes of Atlantic cod abundance and potentially perform as the quantitative references in reasonably making fishing decision. With the rapid development in deep learning capabilities, it is hopeful to expect better predictions of the responses to global changes, not only for Atlantic cod but also for other fish species and the ecosystem as a whole.
The Atlantic cod (Gadus morhua) stocks in the Newfoundland-Labrador Shelves (NL) and Barents Sea (BS) ecosystems have shown divergent trajectories over the last 40 years. Both stocks experienced either an important decline (BS) or a collapse (NL) in the mid-1980s and early 1990s, respectively. After these population reductions, the BS stock quickly rebounded and it is currently at record high levels, while the NL stock, despite showing some improvement since the mid-2000s, remains at low levels. Fishing and environmental conditions are known to be important drivers of cod dynamics in both ecosystems, especially the availability of high energy prey like capelin (Mallotus villosus), however, the question of how different or similar these two stocks truly are remains. Could, for example, the NL cod stock rebuild if presented to conditions like the ones experienced by BS cod? To explore such questions, we developed a simple biomass dynamic model for cod using a bioenergetic-allometric approach. This model includes fisheries catches and capelin availability as external drivers and was implemented for both ecosystems. Despite the contrasting trends, the model produced very good fits, and showed some remarkably similar estimated parameters in both systems. We explored these similarities by (a) performing the thought experiment of transferring cod stocks between ecosystems by switching estimated key parameters between models and comparing the output, and (b) implementing an integrated model architecture which allowed fitting common parameters for both stocks to evaluate the similarity of key vital rates. Our results indicate that cod trajectories in NL and BS can be reliably described using simple bioenergetic-allometric arguments, fishery catches, and capelin availability. Model parameters that encapsulate intrinsic vital rates were not significantly different between stocks. This indicates that NL and BS cod stocks are biologically similar, and that the differences in their trajectories are driven by the ecosystem context in which these stocks are embedded, and suggests that the NL stock would be expected to rebuild if enough capelin were available. This also indicates that capelin status and trend should be an important consideration for effective management of these cod stocks.
This chapter focuses on the epipelagic zone, which comprises 71% of the planetary surface, where biological production, biogeochemical cycles, and marine food webs are maintained by the inhabiting planktonic organisms. The case studies highlighted in this chapter collectively indicate that there is substantial observational evidence that many pelagic ecosystems, both physically and biologically, are responding to changes in regional climate caused predominately by the warming of sea surface temperature, ocean current changes, and, to a lesser extent, the modification of precipitation regimes and wind patterns. The biological manifestations of climatic variability have rapidly taken the form of biogeographical, phenological, biodiversity, physiological, species abundance changes, community structural shifts, and whole ecological regime shifts. Summarizing the observed case studies, of note, is the sensitivity and rapidity of the pelagic and planktonic response, be it biogeographically or phenologically, to climate warming and global change compared with their terrestrial counterparts.
Climate change is altering the distribution and biomass of marine species in Arctic and sub-Arctic waters. In this study, we investigate the influence of sea temperature on the annual distribution and biomass of pelagic fishes in the eastern Bering Sea during late summer, 2002-2018. The distribution (easting, northing, and area occupied) and biomass of capelin, Pacific herring, juvenile chum salmon, juvenile pink salmon, juvenile sockeye salmon, and age-0 walleye pollock collected by surface trawl were estimated using a standardized geostatistical delta-generalized linear mixed modeling approach. Species showed varied responses to warming on a temporal scale. Warming corresponded with more northerly distributions for capelin and juvenile sockeye salmon, a more westerly distribution of juvenile sockeye salmon, and range expansions for juvenile chum and sockeye salmon. Warming corresponded to a decrease in the annual biomass of capelin and an increase in the biomass of herring, age-0 pollock, and juvenile sockeye salmon. The spatio-temporal covariation in sea temperature and the distribution was nonlinear for juvenile pink salmon and age-0 pollock, positive for juvenile chum salmon and juvenile pink salmon, and negative for capelin indicating different responses of the distribution of pelagic fishes to warming in the eastern Bering Sea during late summer. In warmer areas, we found that the catch rates were higher for juvenile pink salmon, lower for capelin, and not significantly different for juvenile chum salmon, herring, age-0 pollock, and juvenile sockeye salmon. Juvenile sockeye salmon, a southerly distributed species in the survey area, appeared most responsive to warming. In this study, sockeye salmon and pollock are the most commercially important species while chum salmon are important for subsistence fishing. These temperature related changes during early life history stages for survival may have impacts on the numbers of these fishes recruiting to the fisheries.
Seasonal prey bursts are important for the life cycles and energy budgets of many predators. This study documents the diet and, especially, the importance of the ephemeral occurrence of capelin as prey for Atlantic cod (Gadus morhua) in Godthaabsfjord, west Greenland, over an annual cycle. The cod showed clear differences in diet composition on the 11 sampling dates resulting in a spring–summer, late summer–autumn and winter cluster. Moreover, a single sampling date, 12 May, was defined by cod gorge feeding on spawning capelin, which led to average stomach contents 4.3 times higher than the average for the remaining sampling dates. Changes in nitrogen stable isotope values from 22 April to 7 July in cod liver and muscle tissue were used to calculate the consumption of capelin. Based on this, the consumption of capelin varied between 538 and 658 g wet weight for a 1.3 kg cod. Using published consumption/biomass estimates and observed growth rates, the capelin intake corresponds to 10.1%–33.3% of the annual food consumption and accounts for 28.1%–34.5% of the annual growth of the cod. The present study documents the omnivorous feeding mode of Atlantic cod but highlights the utilization and importance of ephemeral prey bursts for the annual energy budget of the cod. It is hypothesized that access to capelin is critical for the postspawning recovery of Godthaabsfjord cod.
We investigated the pelagic habitat requirements of Arctic larval fish and the effects of interannual variability of ocean conditions on their distribution. We examined the distribution of larval Arctic cod, Bering flounder, yellowfin sole and capelin in the Chukchi and northern Bering seas during two years with different oceanographic conditions. We found that despite marked changes in water mass distribution, the distributions of larval fishes were not significantly different between the two years. In both years, Arctic cod and Bering flounder were found in cold, high salinity shelf waters advected from the south and influenced by winter cooling (Chukchi Winter Water and Anadyr Water mix). Yellowfin sole and capelin distributions were also similar from year-to-year but they were only found in warm, low salinity Alaska Coastal Water. The cold, high salinity water masses had elevated large copepod biomass, and the Alaska Coastal Water had elevated small copepod biomass. Thus, we propose that these water masses provided different but nonetheless potentially profitable foraging habitat for the four species of larval fishes. We conclude by suggesting that the timing and location of spawning of these species has evolved such that larval offspring are distributed in suitable foraging habitat despite interannual variability in ocean conditions. This study provides a baseline of Arctic larval fish distribution and insight into the degree of climate variability that might be expected to impact early life history stages of larval fish. Our results also increase the knowledge of the mechanistic links between oceanography and the early life history of fish. Because growth and survival of early life stages of fish often drives population change, our results contribute to the understanding of the impacts of climate change on Arctic fish populations.
Species- and size-selective overexploitation often have ecosystem-wide impacts that are evident in community size spectra. To both derive potential ecosystem targets for community rebuilding and assess contemporary indicators relative to these targets, we constructed theoretical size spectra to predict pristine biomass densities using a combination of species- and size-specific nitrogen stable isotope signatures and a range of trophic efficiencies and primary productivity estimates within and among 3 sub-regions of the Newfoundland and Labrador shelf. Theoretical size spectra were compared to empirically derived size spectra using trawl survey data. The descending slopes of the empirically determined size spectra were between 1.25 and 2.42 times steeper than the theoretical slopes. The percentage of the theoretical distribution represented by the empirical size structure ranged between 1.1 and 29.4%, with the closest and furthest estimates associated with the smallest- and largest-sized fishes, respectively, and strongly influenced by estimates of trophic efficiency and primary productivity. Regional variation was also observed, with southern regions reaching 1.3-32.3% of the theoretical biomass density and the northernmost region reaching only 0-8.3%. Importantly, the descending slopes varied depending on fish size, with biomass density of larger sizes decreasing faster than that of smaller sizes. Variations among sub-regions and fish guilds were also observed. These analyses provide a means to derive potential ecosystem targets and indicators through which recovery of fish communities can be monitored and assessed.
Canada’s Northern cod stock (NAFO Divisions 2J3KL), once one of the world’s most important stocks, declined rapidly in the early 1990s and remains at historically low biomass despite a recent comeback. In contrast to many studies, recent stock assessments based on a new state-space model (hereafter Canada Department of Fisheries and Oceans, or NCAM model) concluded that natural (M) and not fishing mortality (F) was the main cause of stock depletion and has kept it at low levels since. Here we offer a second opinion on those conclusions, using the same catch and survey data, but based on a VPA model that allows for variability in M and unreported catch. This model posits that the biomass, both total and spawning from the NCAM model, is overestimated (35%), that fishing mortality was a major contributor to the early 1990s decline and slow rebuilding, and that the decline began in the late 1980s and was not as knife-edged as suggested by the NCAM. In agreement with the NCAM results, M has been variable and episodically elevated, but much less than suggested by the NCAM. VPA using a non-variable M did not fit well. In addition, the stock recruitment relationship suggests an asymptote at near 10⁶ t, similar but slightly higher than the current limit reference point for spawning biomass. Of importance, the present analyses suggest that fishing has impacted rebuilding and will slow achievement of a spawning biomass that avoids recruitment overfishing.
Climatic drivers of the size and body condition of forage fish in the North Pacific are poorly known. We hypothesized that length and condition of forage fish in the Gulf of Alaska (GoA) vary in relation to ocean temperature on multiple scales. To test this hypothesis, we analyzed morphometric data for capelin (Mallotus catervarius) and Pacific sand lance (PSL; Ammodytes personatus) sampled by a seabird (Cerorhinca monocerata) in two regions of the GoA, 1993–2016. Based on previous studies, we predicted that capelin length and body condition (Fulton's K) would be negatively related to the Pacific Decadal Oscillation (PDO) and sea surface temperature (SST), whereas PSL length and condition would be positively related. Interannual variation in length and body condition was evaluated relative to seasonal values of ocean climate using regression. Forage fish length and condition varied interannually, between sampling regions, and were dependent on the size/age class of the fish sampled. As predicted, length and body condition of capelin (mostly age 1+) were negatively related to the PDO and SST. Relationships with ocean climate for PSL varied by size/age class: positive for putative age‐0 fish and negative for putative age‐1+ fish. We conclude that our hypothesis was supported for capelin and partially supported for PSL. This study demonstrates that ocean climate determines key morphometric characteristics of forage fish that may relate to interannual variation in the energetic value of prey, and provides an example of how seabirds can be used to obtain specimens for evaluations of potential prey quality.
This study assesses spatiotemporal and sex‐specific growth of Atlantic cod Gadus morhua in Icelandic waters. We use a Bayesian approach which lends itself to fitting and comparing nested models such as these. We then compare fitted parameters of these models to potential explanatory variables using a redundancy analysis (RDA) to look for drivers of growth in G. morhua. Results indicate that models that incorporate differences in growth among time, space and sex are the best‐fitting models according to deviance information criterion (DIC). Results from RDA indicate that capelin Mallotus villosus recruitment and biomass is highly correlated with deviations in the von Bertalannfy growth parameter k and that L∞ is correlated with G. morhua landings in the model that uses year to account for time‐varying growth and estimated G. morhua recruitment in the model that uses cohort to account for time‐varying growth.
Exploitation and changing ocean conditions have resulted in altered species interactions and varied population dynamics within marine fish communities off northeast Newfoundland and southern Labrador, Canada. To understand contemporary species interactions, we quantified the isotopic niches, niche overlap, and ontogenetic niche change among seven dominant fish species using stable isotope analyses. Analyses used fishes from three regions differing in fish and prey diversities. Differences in fish and diet composition diversity among regions were found using Simpson’s inverse diversity index. The regions of lowest diversities had higher instances of niche overlap and higher percentage of niche overlap area. The region of highest diversity had the widest spread of niches with greater distances from the community centroid. Ontogenetic shifts were observed such that larger individuals shifted towards the community centroid with the exception of Atlantic cod. Atlantic cod in particular was found to consistently be the top predator of the analyzed species. Our results reveal: (a) overlap in isotopic niches and spread within niche space was correlated with fish and diet diversity; (b) ontogenetic shifts are important when considering a species’ niche and quantifying spatial variation in community niche profiles.
The study of the diet and feeding of cod, the methods and strategies by which cod find food, how such food is utilized, and how such resulting cod growth impacts other components of marine ecosystems is valuable. Cod have a long history as linchpins in the trophic ecology of marine ecosystems. This chapter evaluates the trophodynamics, growth, and role of cod in the ecosystem from a bioenergetics perspective. There are two basic requirements for growth estimation in fish: age and size. Novel techniques may minimize some of the subjectivity associated with age determination in stunted fish. The chapter briefly provides the methodologies, magnitudes of estimates, and simple summaries of how much cod eat. Cod dynamics can be influenced by interactions with predators, prey, and/or competitors, as well as influencing the dynamics of those other interacting species.
Understanding population fluctuations is a major goal of population ecology. In unpredictable marine environments, population variation is thought to be caused primarily by varying survival rates through a critical early life‐history stage. However, there is increasing evidence that somatic growth variation is common and causes population fluctuations.
We examine the relative effects of empirically validated variability in somatic growth and recruitment on two response metrics across eight different life‐history archetypes of marine fish. We evaluate how much variation is propagated into mature biomass (MB), a proxy for population resilience, and population production, a measure of population rebuilding capacity. Production is defined as the biomass produced by the stock above what is needed to sustain the population at a constant level.
We used empirical estimates of reproductive success and somatic growth rate, coupled with a population model, to evaluate the relative role of both types of variation in population fluctuations. The effects of this variation on population production and MB were examined across three variation scenarios, in which somatic growth only, reproduction only or both processes varied temporally. We also examined three levels of age truncation to explore whether modified population age structure altered these dynamics.
The contribution of somatic growth to biomass variability exceeded that of recruitment for some species (2/8), while in others (5/8 species), recruitment variation was more influential. When population production was examined, somatic growth variation contributed more to population variation for three species. The relative importance of the two processes was not clearly correlated with key life‐history traits (i.e., growth and mortality rates), but instead was determined by time‐series characteristics of growth and recruitment variation. Increasing age truncation slightly increased the relative effect of recruitment variation on MB variation for three species.
These results suggest somatic growth variation can be as important as early life‐history survival in driving population fluctuations in some marine fish species. This analysis provides a counterexample to the commonly held assumption of many marine population dynamics models: That population variability is induced primarily through variation in reproductive success.
The trophic link between cod (Gadus sp.) and capelin (Mallotus sp.) is important in many panarctic ecosystems. Since the early 2000s, the Northeast Arctic cod stock (G. morhua) in the Barents Sea has increased greatly, and the sea has been exceptionally warm. Such changes have potentially large effects on species distributions and overlap, which in turn could affect the strength of species interactions. Due to its high latitude location, the Barents Sea has strong seasonal variation in physical conditions and interactions. To study drivers of variation in cod-capelin overlap, we use data from two annual surveys run in winter and in autumn of 2004–2015. We first model winter and autumn spatial distributions of mature and immature cod and capelin. We then calculate overlap from model predictions on a grid with similar spatial resolution as the survey data. Our approach allowed us to interpret changes in overlap as species-specific effects of stock size and temperature, while accounting for sampling variation due to sampling time and depth. We found that during winter both species expanded their distribution in response to increased stock sizes, but how strongly and where the expansion occurred varied. The effect of temperature on distributions varied in space, and differed for cod and capelin and for different components of the two species. The results for autumn were clearer and more consistent. Both species expanded their distribution areas as their stock sizes increased. A positive effect of temperature was found in the north-eastern Barents Sea, where temperatures were lowest at the start of the study. Overlap increased and shifted north-eastwards during the study period and remained high despite a decline in the capelin stock. The increased overlap during autumn could mainly be attributed to the shift in cod distribution with increased cod stock biomass.
A version of multispecies virtual population analysis (MSVPA‐X) had been the sole means of evaluating predation mortality by Striped Bass Morone saxatilis, Bluefish Pomatomus saltatrix, and Weakfish Cynoscion regalis on Atlantic Menhaden Brevoortia tyrannus during 1982–2008. Simplified models may offer a different view of predation, so we developed alternative estimates of predation mortality on Atlantic Menhaden with a simpler biomass dynamic predator–prey (Steele–Henderson or S–H) model. Strongly contrasting predictions of the importance of predation on age‐1+ Atlantic Menhaden, particularly Striped Bass predation, emerged between the MSVPA‐X and the S–H model. Striped Bass selected mostly age‐0 and age‐1 Atlantic Menhaden when modeled by MSVPA‐X, even though large Striped Bass (>40% of biomass after the mid‐1990s) were capable of consuming all ages. Consumption of Atlantic Menhaden by Striped Bass in the S–H model was drawn from aggregated age‐1+ biomass. Combined predation mortality from Striped Bass, Bluefish, and Weakfish was well below fishing mortality for all but the earliest years in the MSVPA‐X but was greater than or equivalent to fishing mortality throughout most of the S–H model time series. Striped Bass were a relatively minor source of Atlantic Menhaden predation mortality with the MSVPA‐X but were the major source since the late 1990s in the S–H model. Annual estimates of Atlantic Menhaden biomass consumed per unit of Striped Bass biomass from the S–H model were roughly five times higher than MSVPA‐X estimates and more consistent with estimates from bioenergetics models and trends of Striped Bass well‐being indicators (tag‐based natural mortality rates and weight at age). Use of an alternative S–H model provided a more complete picture of the potential range of predation pressure.
Atlantic cod (Gadus morhua) in the Gulf of Maine (GOM) is an iconic marine fishery stock that has experienced a substantial distributional shift since the mid-1990s. A geostatistical delta-generalized linear mixed model was utilized to hindcast yearly season-specific distributions of GOM cod. These distributions were calculated using the spring and fall bottom trawl survey data for the stock, along with cell-based bathymetry and bottom temperature data for the study area for the years 1982-2013. The centre of stock distribution (the centre of gravity), spatial extent in latitude and longitude, area occupied and median habitat temperature were estimated annually to quantify changes in the spatial dynamics of GOM cod. Time series of these distributional metrics were then used to evaluate the influences of climate change and densitydependent habitat selection on GOM cod's distribution. Results showed that the rapid southwestward shift in the stock distribution after the late 1990s could not simply be attributed to decreasing stock abundance or warming bottom temperatures. The observed shift in cod distribution requires further investigation on whether it is possibly a result of other factors, like fluctuating productivity among subpopulations. © 2017 International Council for the Exploration of the Sea. All rights reserved.
Growth and condition in fishes have been taken as indicating levels of energy available for survival and reproduction, major components of a population's productivity. After a rapid collapse in population size, northern (NAFO Division 2J3KL) cod (Gadus morhua) remained at a very low level of abundance for 20 years. We investigated the potential for poor growth and condition to have played a role in the collapse and lack of recovery of northern cod. Juveniles and adult males and females all showed similar patterns. Perceptions about the importance of growth and condition to population status depended on the metrics and area examined. When the northern cod population was declining, the northern areas of the population clearly had reduced growth and condition, while these metrics improved in the south. Results were equivocal as to the potential role of growth and condition in the continued low abundance of northern cod and, to some extent, depended on the nature of the metric being examined. Indices of condition, which included lipid storage in the form of liver weight, were generally lower in the north while the stock remained at a low level. Metrics associated with longer-term protein storage returned to precollapse levels quickly following the period of collapse. An index of food availability was more closely related to growth and condition than was temperature. These results point to the need both for studies of growth and condition in a population to have a comprehensive time-series of data covering the entire range of the population and the need for a better understanding of the causes and implications of changes in different metrics of condition.
Life history theory predicts selection for higher reproductive investment in response to increased mortality among mature individuals. We tested this prediction over the period from 1978 to 2013 for three populations of Atlantic cod (Gadus morhua) off Newfoundland. These populations were heavily fished for a long period. We considered changes in standardized gonad weight as a proxy for changes in gonadal investment. We accounted for the allometry between gonad and body weight, individual body condition, water temperature, and potential spatial and density-dependent effects. Males display significant temporal trends in gonadal investment in all populations; in agreement with theoretical predictions, these trends show increased gonadal investments during the earlier part of the time series when mortality was high, with the trends leveling off or reversing after the later imposition of fishing moratoria. In contrast, females display patterns that are less consistent and expected; significant trends are detected only when accounting for density-dependent effects, with females in two populations unexpectedly showing a long-term decline in gonadal investment. Our results support the hypothesis that fisheries-induced evolution has occurred in gonadal investment in males, but not in females, and suggest that gonadal investment is more important for male reproductive success than expected in this lekking species.
In Greenland waters, Atlantic cod (Gadus morhua) is showing signs of recovery following a collapse in the early 1990s, and quantitative diet studies are needed to address predation on highly commercial species such as northern shrimp (Pandalus borealis). We analyzed 2483 Atlantic cod stomachs from Greenland offshore waters and the correlation between Atlantic cod and northern shrimp abundance. Fish and crustacean prey accounted for 96% of the prey by weight, with the relative importance shifting from crustaceans to fish with increasing cod size. Spatial differences were distinct and northern shrimp dominated the diet in Northwest Greenland (>50% by weight), but declined in importance in Southwest Greenland and was absent from the diet in East Greenland. Instead, other crustacean preys such as krill were important in Southwest Greenland while fish prey was most important in East Greenland. Southwest Greenland was sampled in both summer and autumn, and there was a significant seasonal effect on most prey groups, but most pronounced for Atlantic cod and krill both of which increased in importance. Extensive cannibalism was limited to the largest cod (]70–100 cm]) and mainly in Southwest Greenland. In Southwest Greenland, Atlantic cod and northern shrimp biomass were significantly negatively correlated, while no significant trends were found in Northwest or East Greenland.
The assessment of Eastern Georges Bank cod with the assumption of a constant natural mortality of 0.2 has over-estimated stock productivity, resulting in a severe retrospective pattern since the late 2000s. Comparing relative exploitation rate (ratio of fishery catch at age to survey abundance indices at age) with total mortality calculated from the age distribution in surveys indicated a conflict when constant natural mortality was assumed. This inconsistency implies an increase in natural mortality since the mid-1990s. In this paper, natural mortality estimated by Virtual Population Analysis (VPA) indicated that natural mortality for this stock has increased to 0.8 since the mid-1990s for ages 6+. Potential factors contributing to this elevated natural mortality, including poor fish conditions and increased losses due to seal predation were discussed.
The food supply of the Black Sea sprat Sprattus sprattus phalericus off the Crimea coast in the feeding periods of 2006–2013 was studied. The level of accumulation of storage lipids in the fish body and the content of essential polyunsaturated fatty acids (eicosapentaenoic and docosahexaenoic) served as criteria of food supply. It is shown that the average annual content of sprat total lipids varies over a wide range (4.5–9.5% of wet body weight) due to changes in the storage lipid content (58–71% of total lipids). The content of eicosapentaenoic and docosahexaenoic acids (101–451 and 123–622 mg/100 g wet weight, respectively) is positively correlated with the level of storage lipid accumulation in the body. The interannual variability in the content of lipids and fatty acids is determined not only by the quantity but also the qualitative composition of consumed food (namely, the proportion of planktonic organisms of the cold-water and warm-water complexes). The possible influence of the interannual variability in the composition of forage mesoplankton on the condition of the Black Sea sprat off the Crimea coast is discussed.
Major groundfish populations in the western Atlantic north of Georges Bank/Western Scotian Shelf experienced near-Total population collapses beginning in the late 1980s-early 1990s. Explosive increases in the abundance of their pelagic fish and invertebrate prey (notably capelin, sand lance, herring, shrimp and snow crab) generally followed. However, several investigators have concluded, based on the results of annual acoustic surveys, that the lightly exploited capelin stock in the Newfoundland/Labrador (NL) region experienced a sudden 6.8 Mt (98%) decline in biomass from 1990 to 1991 coincident with the groundfish collapse. Given the enormity, duration and unexpected nature of this decline, it was characterized as a collapse. The contrasting response of this capelin stock to the groundfish collapse relative to other North Atlantic (NA) ecosystems prompted us to undertake systematic analyses of the NL ecosystem. The reported capelin collapse in NL should have induced significant responses of trophically linked species, as have been reported in other NA ecosystems. We found little evidence of changes in the population abundance, diet and condition of marine mammals, cod, seabirds and zooplankton, in indexes of the areal occupancy and spawning biomass of capelin, or of environmental effects. These results strongly support the hypothesis that the reported collapse did not occur. Profound seasonal distribution changes, a fixed in time and space acoustic survey in a restricted portion of the distributional range of capelin, and the possibility that NL capelin stock has become less migratory provide a more robust and consistent explanation of the observations.
The northwest Atlantic has had a rich history of living marine resource exploitation. As stocks have undergone sequential depletion with some dramatic instances of stock declines in this region, there have been calls for evaluating and improving approaches to managing our use of these resources. As part of these calls and along with recognition that there are broader issues to consider when managing a fishery, the need for more holistic ecosystem-based approaches to manage living marine resources (LMRs) have been increasingly recognized. Here we present a history of LMR management for both Canadian and US waters. We also provide contextual information for the major ecosystems in this region, with synopses and descriptions of major biophysical features in the northwest Atlantic. After noting the main data sets in this region, we discuss some of the major ecosystem models produced by our institutions. Finally, we discuss current efforts to implement ecosystem-based marine fisheries management in the northwest Atlantic. Introduction: There have been numerous prescriptions and admonitions to implement an ecosystem-based approach to the management of fisheries (EBMF; Larkin 1996, Link 2002a, 2002b, Garcia et al. 2003, Browman and Stergiou 2004, 2005). There have been relatively few instances where such an approach has been implemented to any significant extent (Pitcher et al. 2009), but the number is growing as fisheries scientists, managers, and stakeholders grapple with the specific details of how to do EBMF. As a discipline, and as a practice, we are now moving beyond the “whys” and “whats” of EBMF (Murawski 2007) and squarely facing the “hows”.
The great “northern” cod (Gadus morhua) stock, formerly among the world’s largest and the icon for depletion and supposed nonrecovery of marine fishes, is making a major comeback after nearly two decades of attrition and fishery moratorium. Using acoustic-trawl surveys of the main prespawning and spawning components of the stock, we show that biomass has increased from tens of thousands of tonnes to >200 thousand tonnes within the last decade. The increase was signalled by massive schooling behaviour in late winter first observed in 2008 in the southern range of the stock (Bonavista Corridor) after an absence for 15 years, perhaps spurred by immigration. Increases in size composition and fish condition and apparent declines in mortality followed, leading to growth rates approaching 30% per annum. In the spring of 2015, large increases in cod abundance and size composition were observed for the first time since the moratorium in the more northerly spawning groups of this stock complex. The cod rebound has paralleled increases in the abundance of capelin (Mallotus villosus), whose abundance declined rapidly in the cold early 1990s but has recently increased during a period of warm ocean temperatures. With continued growth in the capelin stock and frugal management (low fishing mortality), this stock could rebuild, perhaps within less than a decade, to historical levels of sustainable yield. More generally, if this stock can recover, the potential exists for recovery of many other depleted stocks worldwide.
Total landings of cod in the Newfoundland region (NAFO Divisions 2GHJ,
3KLMNO, and Subdivision 3Ps) increased from ~1 x 105 metric tonnes (t) in 1800 to
~4 x 1051 in 1950. Landings then increased to >1 x 1061 in 1968 and then declined to
<2 x 1051 by 1978. Landings again increased to 3.5 x 1051 in the 1980s and after 1988
dropped to <1 x 10s t. By 1993 the cod fishery throughout the majority of the region
was closed. Northern cod (2J+3KL) spawner population estimates follow a temporal
pattern similar to landings; current levels are at a record minimum of <1 x 10 and
have been paralleled by a collapse in the size and age structure of the stock. Under the
prevailing levels of fishing mortality, recruitment per spawner was below replacement
from 1983 onwards, paralleling the stock decline. Recruitment determination is
consistent with a parent stock-recruitment relationship. Widespread distributions of
age 5+ cod, typically seen in the 1980s, became more aggregated toward the offshore,
culminating in very few major aggregations along the shelf break in 1992. Winterspring
spawning throughout the region is protracted (3-4 months) and may be more
frequent in inshore regions and on banks than has been previously perceived. The
overall physiological condition of northern cod in more northerly regions has been
decreasing since 1988, and only in the most recent years have capelin been virtually
absent from the stomachs of cod in northerly regions. Lengths at 50% maturity have
been declining since the early 1980s, particularly in northerly regions. Fishing
mortality for northern cod was consistently >0.4 in the mid-1970s. The reduction of F
to <0.4 during 1977 to 1980 was paralleled and followed by increases in the abundance
of the spawner population. Slowly increasing fishing mortalities after 1980 accelerated
after 1987. The combined effects of fishing mortality on spawning population size and
reduction in length and age reduced fecundity contributions of older individuals. A
recent examination of a salinity-recruitment relationship shows that with the addition
of spawning-stock biomass estimates, recruitment estimates can be well modelled. A
series of years characterized by better than average environmental conditions will
likely be required to provide the good recruitment necessary to rebuild the stock size
and length/age structure.
We compared the effects of the inshore trap and the offshore trawl fisheries on the population dynamics of the northern cod (Gadus morhua) stock using data analyses and simulation modelling. We first statistically characterized the catch versus stock biomass relationships for the two fisheries (1977-1986). We found a significant (P < 0.0001) relationship between the trawl catch at time t and the stock biomass at time t - 2. No temporal lag was evident in the trap catch versus stock biomass relationship. The variability in these two relationships was similar. We then modelled the catch and stock biomass dynamics of the two fisheries in parallel, incorporating the observed catch versus stock biomass relationships, but assuming equal mean catches, to examine the effects on cod population dynamics of the temporal lag and variability in the catch versus stock biomass relationships. The results suggest that, for the same amount of fish taken, a quota-based trawl fishery presents a much greater risk of collapse to the cod stock than does an inshore trap fishery. Current management methods overestimate the "safe" catch for the trawl fishery because they do not incorporate the consequences of the lag in the relationship between stock biomass and trawl catch.
Using catch data collected by fishery observers from the otter trawl fleet, we describe changes in the winter distribution of cod on the Newfoundland-Labrador Shelf between 1980 and 1999. We determined the relative abundance of cod in three areas of winter aggregation: south-east of Hamilton Bank, south-east of Belie Isle Bank and south-east of Funk Island Bank. The principal finding was that the cod, during their reproductive period, exhibited significant changes in distribution and behaviour, well before the stock showed signs of collapse. There were three indicators of these changes. First, there was a progressive disappearance of fish from the north with time. Most of the biomass was located in the Hamilton Bank area before 1984. By 1989, most of the fish were found at the Belie Isle Bank and south-east of Funk Island Bank locations and by 1991, only to the south (Funk Island Bank). Second, movements of the prespawning/spawning schools became more limited. Compared with the extensive along-shelf migrations, often covering distances of about 100 km, observed in earlier years, the fish schools after 1989 exhibited less movement to the north. Density of fish in the schools was highest in 1990. By 1991, the schools remained nearly stationary around 49 degrees 30'N, near the southern end of their distribution. Third, the schools spread over an increasingly greater range of depths. The greatest density of fish as reflected by high catch rates was generally observed no deeper than 600 m between 1980 and 1988. By 1989-1990, substantial catch rates were observed as deep as 1100 m, particularly to the north although catches continued to be considerable at the shallower depths. These changes in distribution and behaviour were generally coincident with the occurrence of anomalously cold ocean temperatures and heavy ice conditions, but no direct link between these changes and the environment was evident. (C) 1995 International Council for the Exploration of the Sea
The extent of energy depletion was assessed in Atlantic cod (Gadus morhua) in spring and early summer (1993-1995) to assess relationships between poor condition and natural mortality. Several indices of condition were compared in wild fish in the northern Gulf of St. Lawrence and in fish exposed to a prolonged period of starvation in laboratory experiments. Discriminant analyses classified only a small fraction of the wild fish as similar to cod that did not survive and a much larger fraction as similar to cod that survived starvation. This percentage increased from April to May and peaked in June 1993 and 1994. Condition factor and muscle somatic index allowed a clear distinction between live and dead fish. Muscle lactate dehydrogenase activity suggested that cod had experienced a period of negative growth early in 1993, 1994, and 1995. Fish classified as similar to starved individuals were characterized by a higher gonad to liver mass ratio than others. Reproduction may have a negative impact on survival not only in spring but also later into summer, as some individuals were found not to have recovered by late summer. This study shows that natural mortality from poor condition contributed to lower production in the early 1990s.
Research on northern cod (Gadus morhua) from 1983 to 1994 indicated that a southward shift in distribution in the early 1990s was real and not an artifact of sequentially fishing down local populations. In the early 1990s, seasonal fishery and survey data showed distribution changes where there was no fishery, and large tonnage and densities (450 000 t, densities fourfold higher than 1980s levels) appeared in the south concurrent with declines in the north. All fishery, acoustic, and trawl survey indices increased in the south, while the stock declined. Southern-caught cod in the early 1990s exhibited northern characteristics: (i) antifreeze production capacities above historical norms and equivalent to those of northern fish, (ii) vertebral counts above historic norms and equalling northern counts, and (iii) declines in size-at-age to levels associated with northern fish. The cause of the shift is thought to be a combination of abiotic (climate) and biotic (capelin (Mallotus villosus)) environmental changes and cumulative long-term fisheries effects on cod behavior. The shifted distributions increased vulnerability to Canadian and foreign fisheries and led to a rapid decline in abundance, both before and after the moratorium on fishing in Canadian waters in 1992. Rebuilding will occur in three steps: environmental restoration, recolonization by adults, and enhanced recruitment across the shelf.
Analysis of simulated data showed that potential contact statistics could be used to describe spatial pattern in sample density data. Potential contact is a new method, analogous to Ripley's K function for mapped point pattern analysis. Potential contact can be used to describe spatial pattern and association over a range of scales without grouping data and is robust against the presence of zeros. The statistical output is ecologically interpretable, as a measure of the degree of contact between individuals. This new technique was applied to examine changes in the spatial distribution of Atlantic cod (Gadus morhua) off Newfoundland, Canada, from 1985 to 1994, a period that encompassed a collapse of the cod stock. Sample data from bottom-trawl surveys indicated that cod were aggregated in patches with dimensions of 100-250 km. During the period of cod decline in the 1990s, spatial structure changed in three ways: the number of patches decreased, patch size shrank, and contact with conspecifics at small (10-20 km) scales fell. Cod were broadly associated with capelin (Mallotus villosus), a major prey species. Spatial distribution of capelin changed over the same time period as changes in cod distribution, and there was no evidence that contact between cod and capelin decreased.
Changes in the wet weights and lipid contents of muscle, liver and gonad were determined in male and female Mallotus villosus in Balsfjorden, northern Norway, in 1981, from January, when gonadal development becomes noticeable, until May when the fish are spawning. Fatty acid compositions of tissue lipids were also determined. Over 4 mo prior to spawning, the weight of muscle in female capelin decreased by 32% while the weight of the ovary increased exponentially by 830%. In males the weight of the muscle remained constant and that of the testis decreased slightly. The lipid contents of the muscle of both males and females decreased by 76% over the period and an inverse relationship existed between the water and lipid contents of muscle in both sexes. Male liver weight remained constant over the period of study whereas female liver weight increased transiently by 300% between January and March. 38% of the lipid lost from female muscle was accounted for by lipid deposited in ovary whereas negligible amounts of the lipid lost from male muscle was accounted for by lipid in the testis. Gonadal lipid was always richer in polyunsaturated fatty acids than muscle lipid and, immediately prior to spawning, 42% of the fatty acids in ovarian lipid were polyunsaturated. Muscle lipid of males and females showed a progressive increase in the percentage of the long-chain monoenes 20:1 and 22:1 between January and May. It is concluded that male capelin catabolise more of their muscle lipid reserves than females in the 4 mo prior to spawning and that most of the lipid catabolism in males is associated with physical activity. Conversely, females deposit much more of their muscle lipid in gonads than males, although considerable selectivity occurs in the mobilisation of fatty acids from muscle lipid into ovarian lipid. Additionally, biosynthesis of gonadal constituents accounts for a considerable proportion of the lipid catabolised in females.
1.1. Lipid levels of capelin are highest in late fall and lowest during the summer spawning season; protein levels are constant at 13–14% body wt throughout the year.2.2. Ovid females contained significantly more lipid and protein and less water and had higher energy densities than males and spent females.3.3. Surgically-removed egg masses made up 34.2 ± 10.3% female body wt and were very similar in composition and energy density to gravid females, differing from spent females and males in similar respects. Owing to the ovarian development of females, sexes differ in energy density only during the spawning season.4.4. Sexes were similar in amino acid composition. Analysis of capelin and three other seabird forage species revealed that isoleucine levels were lower than minimum avian maintenance and growth requirements.5.5. Implications for the foraging behaviour and food preferences of diving seabird predators (murres, puffins) are discussed.
With the collapse of fisheries in many parts of the world causing widespread economic harm, attention is focused on a possible cause and remedy of fishery collapse. Economic theory for managing a renewable resource, such as a fishery, leads to an ecologically unstable equilibrium as difficult to maintain as balancing a marble on top of a dome. A fishery should be managed for ecological stability instead--in the analogy, as easy to maintain as keeping a marble near the base of a bowl. The goal of ecological stability is achieved if the target stock is above that producing maximum sustainable yield and harvested at less than the maximum sustainable yield. The cost of managing for ecological stability, termed "natural insurance," is low if the fishery is sufficiently productive. This cost is shown to pay for itself over the long term in a variable and uncertain environment. An ecologically stable target stock may be attained either with annually variable quotas following current practice or, preferably, through a market mechanism whereby fish are taxed at dockside if caught when the stock was below target and are untaxed otherwise. In this regulatory environment, the goal of maximizing short-term revenue coincides with the goal of ecological stability, thereby also maximizing long-term revenue. This new approach to fishery management is illustrated with the recently collapsed Newfoundland fishing industry. The Newfoundland cod fishery is expected to rebuild to an ecologically stable level in about 9 years and thereafter support an annual harvest of about 75% of the 1981-1990 average.
Catches of cod (Gadus morhua) during random depth-stratified bottom-trawl surveys,
and the contents of the stomachs of cod sampled during those surveys, were used to
study the distribution of cod and the distribution of feeding by cod on capelin (Mallorus
villosus) on the continental shelf off southern Labrador and eastern Newfoundland
(NAFO Divisions 2J, 3K, and 3L) during the autumns of 1978-1992. In 1989 and
subsequent years, there occurred a substantial decline in abundance of cod and a
reduction in cod distribution. During the same period, the biomass of capelin, as
estimated from hydroacoustic surveys in Divisions 2J and 3K, declined to very low
levels. Despite the apparent severe decline in capelin biomass, many cod had a
relatively high content of capelin in their stomachs, in part because the capelin
changed their distribution and occupied the general area where the remaining cod
were concentrated. Inspection of the data on a finer spatial scale revealed that in some
years cod sampled from large catches had few or no capelin in their stomachs, whereas
cod from small catches nearby often had relatively large quantities of capelin in their
stomachs. An examination of bottom temperatures during 1978-1992 does not support
the postulate that the disappearance of cod from the north and west of Division 2J+3K
in 1989-1992 was caused by avoidance of cold water. It remains unclear whether the
change in cod distribution was related to the change in capelin distribution.
Statistical analysis of large-scale (1-1,000 km) predator-prey associations between cod (Gadus morhua) and capelin (Mallotus villosus) is difficult because the spatial distributions of both species are heterogeneous and temporally dynamic. Statistics based on Ripley's K-function were used to describe the spatial association between cod and capelin off Newfound-land. The number of capelin prey around cod predators (potential con-tact) was computed from acoustic survey data for a range of possible cod ambits, from 5 to 100 km. Potential contact between cod and capelin var-ied seasonally in Placentia Bay, southeastern Newfoundland in 1998, be-ing highest in June and lowest in January. This seasonal difference was largely attributable to an increase in the spatial association of cod and capelin at scales of 10-50 km in spring. A similar pattern was observed off the northeast Newfoundland shelf in 1991-1994 where postspawning cod migrated inshore in spring and encountered groups of capelin. As a con-sequence of this migration pattern, spatial association and potential con-tact between cod and capelin were dependent on survey timing relative to the timing of cod spawning.
Indices of abundance for capelin in the Newfoundland area collected since the early 1980s include biomass estimates from offshore acoustic surveys, school surface area near spawning beaches from aerial surveys, and catch rates from the inshore commercial fishery. During the 1990s, the offshore acoustic estimates declined dramatically but the inshore in-dices remained stable. This divergence was coincident with below nor-mal water temperatures and changes in capelin biology. The biological changes included large-scale changes in distribution and later spawning, both of which have been linked to the colder water temperatures, and smaller fish size. The dichotomy in the trends of abundance indices has never been fully reconciled. However, based on data from several sources, it appears that the acoustic surveys were underestimating the true population abundance of capelin offshore. It seems likely that the severe environmental conditions during the early 1990s were the major cause of the changes in biology and behavior of capelin, and these changes had their greatest impact on the acoustic estimates.
Two new surveys have been conducted in recent years to assess the status of the northern cod (Gadus morhua) stock (2J3KL). An offshore acoustic survey in spring sampled concentrations of spawning adults and estimated potential fecundity. A pelagic juvenile fish survey carried out in late summer measured cod year-class strength prior to settlement. Spawning biomass of offshore cod decreased from 11 700 t in 1994 to 2000 t in 1995 to 900 t in 1996 with a corresponding decrease in eggs spawned. This decline was mirrored in the distribution and abundance of pelagic juvenile cod. Natural mortality estimated between the egg and pelagic juvenile stages increased from –0.068·day–1 in 1994 to –0.084·day–1 in 1995 to –0.112·day–1 in 1996. The decline in year-class strength of cod appeared to result primarily from a reduction in the number of offshore spawners compounded by an increase in natural mortality during the egg and larval stages. The decline in spawners occurred 2 years after the implementation of a fishing moratorium and cannot be linked to fishing. The rapid decline in abundance of adult fish combined with increased natural mortality during the egg and larval drift is consistent with depensation.Deux nouveaux inventaires ont été menés ces dernières années pour évaluer l'état du stock nordique (2J3KL) de la morue franche (Gadus morhua). Un inventaire acoustique au large au printemps a permis d'échantillonner les concentrations des adultes en fraye et d'estimer la fécondité potentielle. Un inventaire des jeunes poissons pélagiques en fin d'été a mesuré la force de la classe d'âge avant l'établissement au fond. La biomasse des géniteurs au large a décru de 11 700 t en 1994 à 2 000 t en 1995 et à 900 t en 1996 et le nombre d'oeufs pondus a baissé en conséquence. Ce déclin s'est reflété dans la répartition et l'abondance des jeunes morues pélagiques. La mortalité naturelle estimée entre le stade oeuf et le stade jeune poisson pélagique a augmenté de –0,068·jour–1 en 1994 à –0,084·jour–1 en 1995 et à –0,112·jour–1 en 1996. La réduction de l'importance des classes d'âges de morue semble être due principalement à une diminution du nombre de géniteurs au large, à laquelle s'ajoute une mortalité naturelle accrue durant les stades embryonnaire et larvaire. Le déclin des géniteurs s'est produit 2 ans après la mise en place d'un moratoire sur la pêche et ne peut donc être relié à la pêche. La diminution rapide de l'abondance des poissons adultes ainsi qu'une mortalité naturelle accrue durant la dérive des oeufs et des larves laissent croire à l'existence d'un effet Allee («depensation»).[Traduit par la Rédaction]
A stochastic, age-structured life history model was used to examine how age at maturity (θ), pre- (Zimm) and postreproductive (Zmat) mortality, and postreproductive growth rate can affect maximum reproductive rates of fish at low population size. Simulations suggest that annual (r) and per-generation (R0) metrics of population growth for Newfoundland's northern Grand Bank Atlantic cod, Gadus morhua, are primarily influenced by changes to mortality prior to and following reproduction. At observed weights at age and Zmat =0 .2,r ranged between 0.135 and 0.164 for cod maturing at between 4 and 7 years. Incremental increases in either Zimm or Zmat of 0.1 were associated with 0.03- 0.05 reductions in r. To effect similar reductions, individual growth rate would have to decline by approximately one half. At observed weights at age, increases in Zmat from 0.20 to 0.45 increased the probability of negative per- generation growth from 3 to 26% for cod maturing at 4 years and from 6 to 46% for cod maturing at 7 years. Thus, even in the absence of fishing mortality, little or no population growth by Atlantic cod may not be unexpected in the presence of environmental stochasticity, particularly when accompanied by increases in mortality and declining individual growth.
Acoustic target strengths (TS) of Atlantic cod (Gadus morhua L.) of lengths 12-65 cm were measured ex situ at 38 and 120 kHz (dual beam) under continuous video monitoring. Each fish swam within an acoustically inert monofilament mesh bag secured 10 m beneath fixed location transducers. Overall ex situ target strength-length (L, cm) relationships were TS=20 log L (cm)-66 at 38 kHz and TS=20 log L (cm)-65 at 120 kHz (n=16). Cod TS peaked at aspects of 5-10 degrees off horizontal (head-down). Full rotation directivities spanned 20dB. Stationary variations of low amplitude (<3 dB) correlated with swimming movements. A linear model was used to study in situ TS (38 kHz) from 49 acoustic-trawl experiments targeted at cod and associated species at depths from 200-375 m. Significant factors were species composition (40% of variance), depth distribution (20%), and time of day (5%; total r(2)=0.65). Modelled TS (standardized to cod) was 1.5 dB higher at night when cod were up to 140 m off bottom. Night-time TS did not differ from the ex situ TS-daytime TS was significantly lower by 1.5 dB. Our findings are consistent with cod being neutrally buoyant off bottom (night) and negatively buoyant nearer bottom in the day-time. Pooled neutrally buoyant in situ and ex situ TS (38 kHz) indicated the relationship (TS=20 log L-66, r(2)=0.80, n=39). (C) 1996 International Council for the Exploration of the Sea
Prey energy density values are crucial inputs to bioenergetic consumption models. Vertebrate predators in the northwest Atlantic consume a variety of prey species, but the proximate composition (PC; proportions of lipid, protein, ash and water) and energy density (ED; kJ g(-1)) of prey, and their variability, are known poorly. In this study, key prey species from Newfoundland and Labrador were studied: Atlantic cod Gadus morhua, American plaice Hippoglossoides platessoides, sand lance Ammodytes dubius, Arctic cod Boreogadus saida, northern shrimp Pandalus borealis, redfish Sebastes spp., Greenland halibut Reinhardtius hippoglossoides, squid Illex illecebrosus and Gonatus fabricii, capelin Mallotus villosus, Atlantic herring Clupea harengus and daubed shanny Lumpenus maculatus. PC and ED varied greatly among species and were influenced by size, season, geography and year. Herring, capelin and G. fabricii had the highest ED, whereas Atlantic cod, plaice, sand lance and shrimp had the lowest. Halibut and I. illecebrosus increased in ED with size. EDs of capelin and redfish varied seasonally; that of plaice and sand lance did not. Herring and halibut had higher ED in the early 1990s than in recent years. Such variation in prey ED has important implications for digestive efficiency, foraging energetics, and dietary preferences of vertebrate predators.
A dispersed, monospecific aggregation of juvenile (0+) capelin was detected acoustically in shallow (20–70 m) water in Bonavista
Bay, northeastern Newfoundland in January 2000. This provided a rare opportunity to measure acoustic target strength (TS)
of very small (mean length=51 mm) capelin in situ. Mean observed TS at 38 kHz was −61.0 dB. Observed TS was similar to TS predicted by the Norwegian-Icelandic capelin TS-length
relationship (TS=19.1 log L (cm)−74), but ∼2 dB lower than predicted by the existing TS-length relationship for capelin in
Newfoundland waters at 38 and 49 kHz (TS=20 log L (cm)−73.1). Combining present data with previous 38 kHz data indicates the
relationship TS=23.3 log L (cm)−77.1 (r2=0.95, n=6) for capelin of lengths 5–14 cm in Newfoundland waters.
Sexually mature Arcto-Norwegian female cod, Gadus morhua, were sampled off northern Norway either during spawning migration (Vesterålen) or at spawning sites (Lofoten) from 1986 to 1996. This period comprised a dramatic, nearly cyclical change in the Barents Sea ecosystem. The stock of the main food item, viz. the Barents Sea capelin Mallotus villosus villosus, changed from a low (1986), to a high (1991) and again to a low (1994) level of abundance while the climate changed from a cold (≤1989) to a warm regime. The relative annual potential fecundity (i.e. number of vitellogenic oocytes per g prespawning fish) increased by approximately 40% from 1987 to 1991. However, information from a back-calculation technique calibrated in the laboratory using spawning fish indicated that this change might have been as high as 80 to 90%. Ovaries were analysed by the gravimetric, the automated particle counting and the stereometric method (modified to use with ovaries too large to section whole). All three methods gave similar fecundity estimates. The latter method was applied to quantify atresia of developing oocytes in the good-condition year of 1991. Atresia was rare, occurring in only 30% of the ovaries and where it was present in only 1 to 4% of the vitellogenic oocytes. Spawning females sampled from 1991 to 1996 gradually produced fewer eggs and demonstrated clear interannual variations in vitellogenic oocyte mean size and distribution thought to reflect a delicate reproductive tactic to minimise negative nutritional effects on egg size and egg quality. Estimates of annual potential fecundity for the duration of the study were significantly positively correlated with environmental temperature and the availability of capelin during vitellogenesis.
We examine the population to population variability of intrinsic rate of natural increase, r(m), of Atlantic cod, Gadus morhua. The intrinsic rate of increase is positively related to temperature, contrary to the expectation that r(m) might increase as the high and low temperature limits of habitability for cod are approached. For the parameter regime considered, r(m) has a simple dependence on age-at-maturity and the number of replacements each spawner can produce at low population densities (ᾱ). It is shown that ᾱ has no significant temperature dependence, and thus the covariation of r(m) and temperature arises from the influence of temperature on age-at-maturity. We demonstrate that our estimates of r(m) are robust and thus may be of use in estimating the recovery time of depleted populations.
This study examined whether simple indicators of condition in individual Atlantic cod (Gadus morhua) are related to biochemical composition and energy content for the whole range of conditions potentially observed in natural populations. The energy content of muscle was largely associated with its protein content, while liver energy content mainly consisted of lipids. Accurate predictions of muscle and liver energy content can also be made from the measurement of their water content. Relationships between energy content, water content, and simple measures such as condition factor and hepatosomatic index provide a fast and accurate assessment of seasonal changes in total available energy reserves in cod. These indices could advantageously be used to monitor the status and recovery of cod stocks and to ensure improved management of that resource.
We demonstrate a direct relationship between the amount of prey (capelin, Mallotus villosus) in its predators' stomachs (Atlantic cod, Gadus morhua) and the biomass of the prey in the same localities. The relationship at a local scale implies that predator stomach contents can be used to estimate prey distribution. There is no evidence for either (1) a negative effect of local predator abundance on stomach contents (expected if local prey depletion and/or feeding interference occurs when predator density is high) or (2) a negative effect of the amount of other food in cod stomachs on the amount of capelin in the stomachs (expected if prey switching occurs). There is a negative effect of bottom depth on amount of capelin in cod stomachs as expected, since capelin are pelagic and cod are primarily bottom-dwelling, resulting in less vertical overlap between the species in deep water. This type of analysis may be useful in other predator–prey systems. Simultaneous stomach samples and independent biomass estimates of the prey are required, but once the model is built, it permits estimation of prey distribution in places and times when no direct survey of the prey can be done.
Two new surveys have been conducted in recent years to assess the status of the northern cod (Gadus morhua) stock (2J3KL). An offshore acoustic survey in spring sampled concentrations of spawning adults and estimated potential fecundity. A pelagic juvenile fish survey carried out in late summer measured cod year-class strength prior to settlement. Spawning biomass of offshore cod decreased from 11 700 t in 1994 to 2000 t in 1995 to 900 t in 1996 with a corresponding decrease in eggs spawned. This decline was mirrored in the distribution and abundance of pelagic juvenile cod. Natural mortality estimated between the egg and pelagic juvenile stages increased from 0.068·day1 in 1994 to 0.084·day1 in 1995 to 0.112·day1 in 1996. The decline in year-class strength of cod appeared to result primarily from a reduction in the number of offshore spawners compounded by an increase in natural mortality during the egg and larval stages. The decline in spawners occurred 2 years after the implementation of a fishing moratorium and cannot be linked to fishing. The rapid decline in abundance of adult fish combined with increased natural mortality during the egg and larval drift is consistent with depensation.
Capelin (Mallotus villosus), an important forage and commercial fish in the Northwest Atlantic, has exhibited dramatic changes in its biology during the 1990s, coincident with extreme oceanographic conditions and the collapse of major groundfish stocks. Commercial exploitation has not been a serious factor influencing the population biology of capelin in the area. The overall consumption of capelin has declined as predator stock abundances have changed. Data on plankton are sparse, but there appears to have been a decline in zooplankton abundance during the 1990s, and at the same time, a phytoplankton index increased. The impact of the changes in the physical environment has been the subject of previous studies and these are reviewed. The relative impacts of four factors, commercial exploitation, predation, food availability, and the physical environment, on the changes in capelin biology are discussed in the context of capelin as a single species and in the context of the ecosystem. The overall patterns suggest the existence of a "trophic cascade" within the distributional range of capelin in the Northwest Atlantic during the 1990s primarily driven by declines in major finfish predators.
The influence of a lower condition on reproductive investment, somatic energy losses, and postspawning condition of Atlantic cod (Gadus morhua) was examined under the hypothesis that females, in response to lower available energy reserves, would reduce reproductive investment in order to limit somatic energy losses. Laboratory experiments revealed that female cod with high prespawning condition factors ended reproduction in better condition than females with low prespawning condition factors. Fecundity and total egg dry weight were significantly lower in poor-condition females. The loss in somatic mass and energy in these poor-condition females was nevertheless higher, in relative terms, than the losses experienced by females in good condition. Consequently, energy reserves invested in reproduction by poor-condition females increase their risk of mortality. In the northern Gulf of St. Lawrence during the early 1990s, reproductive females had lower fecundities and were in worse prespawning and postspawning condition. The condition of spent females suggested a greater impact of changes in environmental conditions on adult than on immature cod. Reproductive potential and possibly recruitment may have suffered from that situation and could have contributed to the failure of that stock to recover despite the moratorium on commercial fishing.
The indeterminate relationship between the total biomass of mature fish (spawner biomass) and the number of offspring produced (recruitment) has puzzled population dynamicists and impeded fisheries management. The relationship assumes that spawner biomass (in tonnes) is proportional to the total number of eggs produced (TEP) by the stock, an assumption under increasing challenge. Most stocks require proxies for TEP because contemporary and/or historical fecundity data are lacking. Here we show a positive association between recruitment and the liver weights of spawners in the Barents Sea cod stock which suggests that recruitment is constrained by the amount of lipid energy stored in the liver. This stimulated our interest in estimating total lipid energy (TLE; in kilojoules) for mature females in the stock. We examined the suitability of TLE as a proxy through correlation and simulation analyses. The results indicate that TLE is proportional to TEP and exhibits a similar response to varying food abundance. Replacing spawner biomass with more accurate measures of reproductive potential is essential to developing a rational basis for stock conservation. Correctly specifying the first-order maternal effect on TEP is a prerequisite to detecting environmental and ecological effects on recruitment.
FIVE hundred years of fishing and fifty years of research have produced only vague accounts of the spawning and migrations of Atlantic cod (Gadus morhua) off Newfoundland in the north-west Atlantic1. Here I report the discovery of 'spawning columns' and a 'highway' used by cod to traverse the northeastern Newfoundland Shelf during annual springtime feeding migrations. Sea research using echosounders2,3 showed that cod spawned in dense shoals (to one fish per m3) that featured midwater spawning columns comprised of pairs of fish. Immature joined mature post-spawning cod to migrate in large (scales of tens of kilometres and hundreds of millions of fish) size-structured aggregations led by larger 'scouts'. Cod traversed the cold waters of the shelf along a deep highway of warm oceanic water (2–2.5 °C). During migration, fish spacing appeared to maximize search volumes while maintaining visual contact. Aggregations fragmented when prey (capelin Mallotus villosus; shrimp, Pandalus spp.) were encountered.
Sequential acoustic surveys and mark-recapture experiments were used to examine seasonal movements and distribution of coastal cod in Placentia Bay, Newfoundland (NAFO stock sub-division 3Ps). Cod were located in Placentia Bay in all seasons of 1997 and early 1998. Acoustically-measured densities and abundances were highest during spring-time spawning (>1 fish m−2). The degree of aggregation was highest in spring and fall (99% of fish in 8–12% of surveyed area), and low in summer (17–19%). Cod moved shallower after the spawning period, and occupied an increasingly narrow range of depths from spring to fall. In contrast, the range of temperatures occupied broadened over this period, as did available temperatures. Stomach contents indicated that cod were feeding on pelagic zooplankters (hyperiid amphipods) in late summer and fall. We conclude that local movements of cod are more related to depth and feeding behaviour than to temperature. Cod tagged primarily on spawning grounds in Placentia Bay were recaptured mostly within the bay (87%), where fishing effort was high. Recaptures of marked cod per unit weight of fishery catch indicated that 10–30% may move in spring and summer into the adjacent stock area (3L). The majority of tagged cod recaptured during the following spring was caught within Placentia Bay, suggesting a return migration. Almost all cod recaptured at long distances (>100 km, and up to 600 km) from tagging sites were large (>50 cm). Smaller fish were recaptured mostly within the bay, and closer to tagging sites. Our results are consistent with the hypothesis of an ontogenetic threshold to long-distance migratory behaviour in cod. Evidence of repeat spawning, year-round residence, and return migrations from the bay, all suggest the existence of a Placentia Bay coastal cod stock.
It is well established that the lipid content of the herring Clupea harengus from the waters around the British Isles and Ireland undergoes a marked seasonal cycle, being highest during the summer months of active feeding and lowest after a winter fast (Wood, 1958; lies & Wood, 1965; Lovern & Wood, 1937; Molloy & Cullen, 1981; Wallace, 1986). In the case of spring-spawning herring, the minimum lipid content coincides with the spawning period and the depletion of lipid over winter corresponds with the development of gonads (Lovern & Wood, 1937).
Atlantic cod Gadus morhua were collected from Smith Sound, Newfoundland in January 1999. Visual examination of females (n=150) and males (n=126) revealed that some large fish (42–79 cm) had underdeveloped gonads. Histological examination of underdeveloped ovaries indicated that the majority of these females were undergoing mass resorption of oocytes and would not have spawned in 1999. Fish in this condition included females that were aborting their first attempt at maturation and females that had spawned the previous year but were failing to re-ripen. Somatic and liver condition were significantly lower (P<0·05) for fish undergoing mass oocyte resorption than ripening females, suggesting that the interruption in the maturation cycle may have been related to insufficient nutrient storage. In males, testes of some adult fish were considered to be non-reproductive as they showed no signs of ripening and probably would not have spawned in 1999. Liver condition was significantly higher (P<0·05) for non-reproductive males than those that were ripening. Disruptions in male and female reproductive cycles may also have been related to water temperatures that were too cold (0–0·5° C) for successful gamete development. Immature gametes (perinucleolar oocytes in females; spermatogonia in males) showed no signs of breakdown in non-reproductive individuals, suggesting that they retained the potential to develop and spawn gametes in 2000.
Acoustic target strengths (TSα) of dispersed capelin (Mallotus villosus) were assessed in situ in six experiments conducted in June from 1992 to 1996 in Newfoundland waters. Calibrated dual-beam
(120, 49, and 38 kHz) and split-beam (38 kHz) echo-sounders were used. A general linear model indicated that method (dual-
or split-beam), fish density, and time of day relative to light conditions had no effect on TSα (p's>0.05). TSα at 120 kHz was 1 dB less than at 38 and 49 kHz (p<0.05). Capelin length and weight were the major determinants of TSα (p's<0.05). A length based model for 38 and 49 kHz was TSα=20 Log L−73.1 (−73.8 to −72.4; R2=0.69). However, fish in better condition had lower TSα independent of length. A model including the effects of length and weight was TSα=68.6 log L (cm)−15.2 log W (kg)−157.3 (R2=0.80). The effect of condition was greater on TSα assessed by weight: TSαkg−1=69.7 log L−25.5 log W (kg)−159.0 (R2=0.71; p's<0.05). These results are compared to previous work on TSα of capelin and similar species.
In 1995 and 1996, groups of female cod (Gadus morhua) of varying nutritional condition and maturing at different temperatures were used to investigate relationships between female
condition, egg production, egg characteristics, and viability. Over the two years, egg samples from every batch spawned by
34 females were analyzed for egg diameter, dry mass, and energy content. In addition, 51 egg batches from 24 females (ca.
2 egg batches female−1 yr−1) were used to monitor development rate and hatching success. The weighted mean egg dry mass was positively correlated (p<0.05)
to the females' pre-spawning condition in 1996. In 1995 this positive correlation was not observed for females at ∼2°C during
maturation. For both years, the decline in the mean egg dry mass per batch throughout spawning was correlated (p<0.05) with
the females' post-spawning energy reserves. However, no trend in the total egg dry mass per batch during the sequence of spawning
was observed. Thus, the total energy investment per batch remained stable throughout spawning and was not influenced by the
depletion of the females' energy reserves. In 1996, high hatching successes were observed only for egg batches from females
in high pre-spawning condition. However, egg size and dry mass showed no correlation with survival rate or hatching success
in either year.
Acoustic surveys for marine fish in coastal waters typically involve identification of several species groups. Incorrect classification can limit the usefulness of both distribution and biomass estimates. Fishing catch data can assist in identification but are rarely spatially comparable to acoustic data and are typically biased by gear type. We have developed analytical tools to enable identification of Atlantic cod (Gadus morhua) and capelin (Mallotus villosus) using high-resolution echograms. The approach is to assess and analyze various features of the acoustic returns from shoals and individuals using image processing techniques, then to use these features in a learning mode to develop algorithms that discriminate among species. A Mahalanobis distance classifier, which uses the covariance matrix for each species in its distance measurement between species, has been implemented and tested. We demonstrate these techniques using the software “FASIT”, developed for that purpose, in the analysis of inshore fisheries data from Placentia Bay, Newfoundland using data from a 38 kHz digital echo sounder.
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