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Sandeels - The elusive species
... The ICES Working Group on Norway Pout and Sandeel (Anon., 1978) reported length and weight-at-age data from the northern and southern assessment areas (i.e. north and south of 56830 0 N) from 1978, based on data sets collected from Danish landings (Anon., 1978), and there are similar data available from Shetland from the same year (Warburton, 1982). Scattered information from other parts of the range of A. marinus suggest that length-at-age can vary considerably (Reay, 1970). ...
... also for the Danish fleet, we assume that many samples originated from that area. The Shetland data were the same as those presented by Warburton (1982). By the late 1970, all these areas had been explored and were fished on an annual basis. ...
... The plots of mean length and weight-at-age for the entire data sets (Figs. 2 and 3) illustrated the population trajectory, the seasonal nature of the growth in both length and weight, and the considerable individual variation at a given age. In our age-specific analyses of length, weight, and condition based on the data from November-January and March, we found no differences between the sexes in mean length, weight or condition, and as others (Macer, 1966;Anon., 1978;Warburton, 1982), we pooled data for males and females. We could not pool fish from different year-classes, however, because on the ECG there were significant differences between yearclasses for many age groups. ...
The somatic growth of lesser sandeel, Ammodytes marinus, was highly seasonal with the strongest growth in weight and length occurring in the period March–June. The condition factor also showed a pronounced seasonal pattern. In the latter half of the year, the observed mean length-at-age declined. Mean length and weight of the 0-group increased from first appearance in near-bottom catches in June until at least October.Sandeel on fishing grounds of the northern North Sea usually had greater sizes-at-age than sandeels on an unexploited coastal bank off southwestern Norway. In the North Sea, mean length, weight, and condition-at-age differed between year-classes. The abundant 1996 year-class had comparatively low length, weight, and condition-at-age, i.e. not different from that of the coastal sandeel.Comparisons of new and historical growth data from the North Sea showed that both spatial and temporal variation in growth could be substantial. No evidence for growth compensation due to enhanced mortality from commercial exploitation was found but sandeel on the main fishing areas of the northern North Sea and the southwestern Dogger Bank may grow faster than those in coastal waters. We hypothesise that temporal and spatial variation in food availability within the North Sea is important for modulating the growth of sandeel. The reduced mean size-at-age for the 1996 year-class suggests that density-dependent processes may affect growth of exceptionally abundant year-classes.
... Growth ceases altogether during winter (Cameron, 1958;Macer, 1966). Marked differences in sandeel growth exist across the North Sea (Boulcott et al., 2007), with fastest growth in Southern United Kingdom waters and off the Norwegian coast (Wright and Bailey, 1993;Bergstad et al., 2002;Boulcott et al., 2007) and slowest growth off Shetland (Warburton, 1982). Many studies have analysed temporal and spatial variation in length at age (Macer, 1966;Baistrocchi, 2003;Boulcott et al., 2007;van Deurs et al., 2014), however, relatively few studies have analysed year-to-year changes in sandeel growth over a lifespan (Bergstad et al., 2002;Rindorf et al., 2016). ...
Sandeels Ammodytes marinus are a crucial forage fish species in the North Sea, transferring zooplankton energy to higher trophic levels. However, there has been a sustained decline in sandeel abundance in the northwestern North Sea since 2000. Here we use field data to analyze year-to-year changes in A. marinus growth rate between 1997 and 2009 and assess whether variation in growth rate corresponded with variation in abundance. The signature of the reduction in abundance between 2000 and 2009 was a decline in age 1 sandeels, while no other age class declined. Analysis of age-length data showed that the decline in abundance coincided with a period of low growth. Growth performance indexes were correlated with zooplankton and phytoplankton biomass but not temperature. Further, we observed a significant correlation between larval growth rate and 0-group sandeel length during a period when hatch dates were relatively fixed; suggesting recent changes in length were influenced by food availability.
... No such effects were evident in Shetland. However, during the pre-breeding period, adults on Mousa fed on sandeels (Arruriodytidae) in addition to a variety of benthic fishes, and it is possible that sandeel activity is affected more by current strength than tidal state per se (Warburton 1983). Even in windy conditions most breeders regularly attended nest sites in the pre-breeding period, suggesting that daily variation in maximum attendance may be largely due to non-breeders. ...
The diurnal rhythm of colony attendance by Black Guillemots varied seasonally. No tidal effects were detected but in April attendance was lower when winds exceeded force 4. Numbers were most stable during 05.00–08.00 GMT in the pre-breeding period. Counts at this time and in winds of force 4 or less provide the best estimate of adult populations. Between late March and mid-May, counts of adults associated with breeding habitat account for most birds occupying suitable nest sites and censuses in northern Britain should be conducted in this period. As the proportion of non-breeding adults varies between colonies, no universal correction factor can be used to estimate the number of breeding attempts from counts of birds. Counts aimed at detecting maximum attendance provide greater accuracy than single counts made by flushing all birds on to the sea but take longer. Monitoring units should be stretches o-f coastline that include a number of colonies of different sizes.
... Relatively high densities of juvenile monkfish are known to occur on some of the sandeel (Ammodytidae species) grounds around the Shetland Islands during the summer months and diet analysis has shown that those fish feed predominantly on sandeels (Laurenson, 2003). As sandeels hibernate in the sediment from the autumn until spring (Warburton, 1982), they will be mostly unavailable as a food source during that period. This could lead to dispersal from those grounds when prey availability decreases. ...
Movement and growth of Lophius piscatorius were investigated around the Shetland Islands, UK, in a tagging study when a total of 1768 trawl caught fish was tagged using conventional dart tags and released on inshore fishing grounds during the autumn of 2000 and the summer of 2001. This is the first tagging study to have been undertaken on the species in UK waters and for the first time provides actual growth data for the northern monkfish stock. Monkfish of between 25 and 83 cm total length were tagged. The overall recapture rate was 4.5% and times at liberty ranged from 5 to 1078 days. Recapture positions of 35% of individuals were less than 25 km from the release positions, with some recaptures close to release positions after periods of more than 1 year. Recapture locations of some individuals indicated movements between inshore grounds. During the late autumn and winter recapture locations suggested that a general offshore migration towards the shelf edge had occurred. The largest displacement recorded was 876 km with movement being from the release location at Shetland to a fishing ground at the south east of Iceland, another recapture occurred close to Faroe. This suggests that these individuals had crossed the Faroe–Shetland Channel. This is of particular importance as it may indicate a wider mixing of stocks. Growth of recaptured fish averaged 9.4 cm year−1.
... Sandeels are relatively short-lived fish ( generally up to eight years) compared to most exploited North Sea fish species and become mature at two years (Macer, 1966; Gauld and Hutcheon, 1990). Adults are thought not to undertake extensive spawning migrations and during spawning attach their eggs to sand grains on the seabed (Reay, 1970; Warburton, 1982). At Shetland, larvae emerge during the early spring, when the North Sea circulation is largely wind-driven. ...
... Sandeels are relatively short-lived fish ( generally up to eight years) compared to most exploited North Sea fish species and become mature at two years (Macer, 1966; Gauld and Hutcheon, 1990). Adults are thought not to undertake extensive spawning migrations and during spawning attach their eggs to sand grains on the seabed (Reay, 1970; Warburton, 1982). At Shetland, larvae emerge during the early spring, when the North Sea circulation is largely wind-driven. ...
Sandeels represent a major component in the diet of fish, bird, and mammal predators as well as supporting a large industrial
fishery. The availability of young sandeels in coastal waters around Shetland is generally considered a key factor influencing
the breeding success of many seabird species in the area, but the risk to the populations as a direct consequence of the fishery
is unknown. Low exploitation rates coupled with high natural mortality rates make assessment of the Shetland sandeel stock
problematic and safe biological limits have not yet been defined. We use stochastic models to evaluate the likely effect of
varying fishing mortality on kittiwake breeding success. The models consider some main sources of uncertainty about natural
processes, such as recruitment and natural mortality, which may affect the design of robust management strategies. The type
of model tested had a stronger influence on sandeel recruitment than the level of fishing pressure. Even with low exploitation
rates, poor years for seabird breeding were inevitable.
... Given the relative spatial isolation of sandeel aggregations in the North Sea, it might be reasonable to expect that the great variability in abundance of zooplankton, on both temporal and spatial scales (Fransz et al., 1991), could lead to spatial differences in growth rate. Indeed, significant spatial differences in the growth of sandeels have been described, although the studies upon which such findings are based tend to have relied on data collected in different years or restricted to specific areas of the North Sea (Macer, 1966; Warburton, 1982; Wright, 1996; Bergstad et al., 2001 Bergstad et al., , 2002). Differences in size-at-age north and south of 56830 0 N reported in 1978 were, until 1995, used as a basis for dividing the North Sea into two regions for the purpose of sandeel assessment (ICES, 2005). ...
Boulcott, P., Wright, P. J., Gibb, F. M., Jensen, H., and Gibb, I. M. 2007. Regional variation in maturation of sandeels in the North Sea. – ICES Journal of Marine Science, 64: 369–376.
The current assessment of lesser sandeel (Ammodytes marinus) in the North Sea assumes a single stock and a knife-edge maturity ogive. However, there is evidence that the North Sea stock consists of several reproductively isolated components, raising the possibility of demographic differences among regional aggregations. We examine regional variation in size- and age-at-maturity in four components of the North Sea stock. Surveys in 1999 indicated pronounced regional differences in length- and weight-at-age, implying a disparity in growth rate across the North Sea. Logistic regression revealed that the onset of maturity was significantly related to regional distribution, in addition to length and age, with a tendency for fish off the UK's northeast coast to mature later and smaller than elsewhere. No significant effect of year on either growth or length-at-maturity was revealed from a comparison with other data collected in 2004. The results show that important regional phenotypic variation not currently represented in stock assessments could have implications for the local sustainability of sandeel aggregations.
Large numbers of Gannets were observed moving towards a small feeding area north of the Dogger Bank on 10 May 1994. Feeding occurred in close association with feeding white-beaked and white-sided dolphins. From the direction of the movements and the number of birds involved it was concluded that the Gannets originated from the Bass Rock, some 300 km away from the feeding area. Previous surveys around the Bass Rock and in the North Sea suggested that distant feeding is highly unusual during the breeding season. The Gannets occurred in small numbers at trawlers but were highly successful as scavengers compared with other species. Trawlers occurred in very low densities in this offshore area but did not attract many birds. Most Gannets ignored the fishing vessels completely.
The diet of black guillemots Cepphus grylle (L.) was studied in Shetland between 1982 and 1985, both during and outside the breeding season. Full-grown birds consumed a wide variety of fish and invertebrate species in summer, broadly reflecting the diversity of the inshore benthic fauna. In winter a greater diversity of invertebrates was taken, which assumed increased importance in the diet as the availability of benthic fish probably declined. Chicks were fed a diet of fish, with 70-80% being sandeels Ammodytes marinus and butterfish Pholis gunnellus on the island of Mousa. Sandeels were delivered more frequently in the early morning, coincident with a peak in their feeding activity. Older chicks were fed significantly longer fish. The composition of chick diet varied significantly with date, not chick age. A seasonal decline in the proportion of sandeels in the chick diet may have reflected changes in zooplankton abundance, but the possibility that intensive industrial fishing of sandeels had adversely affected local stocks can not be ruled out.
Sandeels, especiallyAmmodytes marinus are a major component of the diet of many predatory fish, seabirds and seals. The industrial fishery for sandeels is now the largest of the North Sea fisheries. A sandeel fishery in the Shetland area began in 1974 but has recently declined. This change was accompanied by dramatic declines in the breeding success of certain seabirds, particularly Arctic terns and kittiwakes. Current information on seabirds and sandeels in the Shetland area is reviewed and areas where further research is needed, highlighted. The Shetland problem illustrates the difficulties of reconciling conservation and exploitation when fundamental ecological and behavioural knowledge is lacking, and also the need to obtain further information on the ecological impact of industrial fisheries.
(1) This study compares the foraging performance, adult body condition and breeding success of arctic terns (Sterna paradisaea Pontoppidan) in a successful (Coquet Island) and an unsuccessful (Shetland) breeding colony. (2) During courtship feeding, males in Shetland brought larger fish to their mates than did the birds on Coquet. However, the latter brought more energy-rich fish, made more foraging trips per hour and their rate of energy transfer to their mates was higher. (3) There was no difference between the two colonies in clutch or egg size, or in hatching success. (4) During chick-rearing, birds in both areas concentrated on sandeels as prey. In contrast to Coquet, the Shetland birds brought their young a very high proportion of small fish, and lost a high proportion of their large prey to conspecific kleptoparasites; they also foraged at a higher rate and brought less energy per chick per hour. (5) The chicks in the Shetland colony grew at a slower rate than those on Coquet and the majority died in the first week of life. Breeding success on Coquet was good. (6) In both areas, adults lost weight during the breeding period. The birds on Shetland were lighter than those on Coquet, but their subsequent rate of weight loss was less. During the chick-rearing period on Coquet, birds fledging two young remained heavier than those where one young died of starvation in the first week after hatching. (7) The data suggest that the birds in Shetland had difficulty in finding sandeels of the 4-8 cm size-class to feed their young, and that the adults themselves were in poor condition prior to the chicks hatching. Adult arctic terns appear to adjust their breeding effort in response to body condition. These findings are discussed in relation to the life-history strategy of arctic terns and factors influencing their food availability.
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