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HARVEY, V., M.O. HAMMILL, 2010. Variations on spatial distribution on fish abundance in eastern Scotian shelf over the past four decades ; Variations de la distribution spatiale de labondance des poissons sur le plateau néo-écossais au cours des quatre dernières décennies. DFO, Canadian Science Advisory Secretariat, Research Document ; MPO, Secrétariat canadien de consultation scientifique, Document de recherche, 2010/132, 16 p .
Over the last four decades there have been major changes in demersal fish abundance. During the same period grey seal abundance has increased dramatically to the highest levels seen in the last 5 decades. Synoptic bottom trawl surveys conducted on the Scotian shelf (NAFO fishing zone 4VsW), were examined to determine if there have been changes in the abundance of fish with respect to distance from Sable Island. Survey data were combined into 10 years blocks (1970-1979, 1980-1980, 1990-1999, 2000-2009) which corresponded to periods of low, high, declining and low cod abundance for each decadal period respectively. Fish abundance was examined in relation to distance classes of 0-50 km, 50-100 km, 100-150 km and 150-200km from Sable Island. A significant shift in distances of high abundance away from the Island was observed for cod, hake, herring and haddock when the 2000s were compared with the 1970s as would be expected as a response to minimize predation mortality from grey seals.
HARVEY, V., M. HAMMILL, D. SWAIN, 2010. Summer overlap between a centralplace forager and its prey in the Southern Gulf of St. Lawrence ; Chevauchement estival entre un prédateur salimentant depuis un point central et ses proies dans le sud du golfe du Saint-Laurent. DFO, Canadian Science Advisory Secretariat, Research Document ; MPO, Secrétariat canadien de consultation scientifique, Document de recherche, 2010/131, 52 p .
Predators affect prey populations by consuming individuals and inducing spatial changes in their behaviour. For the prey population, indirect effects of predation may be more important than predation itself by inducing reduced survival, growth or reproduction. The biomass of large demersal fishes of northwest Atlantic collapsed in early 1990s and failed to recover whereas the abundance of grey seals (Halichoerus grypus) is currently at historically high levels which could amplify their predation pressure on the fish stock. We followed 14 grey seals equipped with satellite transmitters in the southern Gulf of St. Lawrence during the summer period. Overall, seals were concentrated at <50 km from haulout place with a decreasing probability of foraging in areas away from haulout sites. Small white hake, small herring, and winter flounder were more abundant at distances of less than 50km from the haulout sites , whereas cunner, capelin, herring were more abundant further offshore. In August, seals overlap more with small cod near the haulout-sites compared to the overlap observed at 50-100km offshore, whereas the reverse was observed for sandlance. During September, the probability of having a foraging patch in areas of high density of cod was highest in between 50-100 km from the haulout site than in closer areas. The distribution of fish near haulout sites suggests that they try to minimize exposure to predation from grey seals, which could on a longer term affect recovery.
HARVEY, V., M.O. HAMMILL, D.P. SWAIN, G.A. BREED, C. LYDERSEN, K.M. KOVACS, 2010. Winter foraging by a top predator, the grey seal, in relation to the distribution of prey ; Alimentation hivernale du phoque gris en relation avec la distribution des proies. DFO, Canadian Science Advisory Secretariat, Research Document ; MPO, Secrétariat canadien de consultation scientifique, Document de recherche, 2010/124, 45 p .
Identifying areas of restricted search (ARS) of an individual and correlating them with resource abundance may allow a better understanding of predator-prey relationships. Using satellite telemetry data from a large marine predator, the grey seal, collected between 1993 and 2005 and trawl survey data on the winter distribution of ground-fish prey species in 1994-1997, we examined whether seal movements were associated with overwintering concentrations of several commercially important fish species in the Cabot Strait area of Atlantic Canada. The distribution of ARS sites differed between males and females. Male foraging zones were distinguished from female foraging zones by higher densities of herring and medium and large cod. The distribution of searching effort of males varied throughout the winter. In early winter, males were more likely to use habitats around St. Pauls Island where their ARSs were positively related to the abundances of medium and large cod. In late winter, they were found to the southeast of this area and their ARS were negatively associated with large turbot, medium cod and small witch flounder. Females ARSs were also more likely to occur in this southern region. They were negatively related to high abundance of small witch flounder while the probability of having an ARS along their path increased with the abundance of medium sized redfish and decreased with large redfish and white hake. Spatiotemporal differences in distribution and fish communities targeted by male and female ARS may reflect sexual differences in energetic requirements for reproduction. By concentrating their activity in areas of high abundance of large fishes targeted by the fishery in early winter, male grey seals may have an impact on fishes of commercial concern.
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