Food and feeding of the hooded seal (Cystophora cristata) in Newfoundland

Ross, Sue-Anne (1992) Food and feeding of the hooded seal (Cystophora cristata) in Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Knowledge of diet is fundamental to studies of seal ecology and especially to any understanding of their role as predators in the northwest Atlantic marine ecosystem. Hooded seals (Cyostophora cristata ) are one of the largest of the northern phocids and are abundant in the north Atlantic and Arctic seas, but very little quantitative data is available on their feeding behaviour and dietary preferences. The main objective of this study was to determine the diet of hooded seals in Newfoundland waters. -- Otoliths of six common prey species, determined from a previous study on diet of hooded seals, were used to determine fish length / weight - otolith length / height regressions. For all species except Greenland halibut, the strongest correlations between otolith size and fish length / weight were established using maximum otolith length. For Greenland halibut, maximum otolith height gave a better correlation with fish length and weight. Least squares linear equations were used to derive fish length for Arctic cod, Atlantic herring and capelin, while second order polynomial models were used for Greenland halibut, redfish spp., and Atlantic cod. Fish weight estimates were derived using power (log - log) models for all six fish species. -- Stomach contents of 67 hooded seals collected from inshore and offshore waters off Newfoundland were examined to determine the types of prey eaten by hooded seals. The majority of stomachs (73%) came from the nearshore region along the northeast coast of Newfoundland and were taken in April. Over half of the samples collected were female (64.2%). -- Fourteen prey groups were identified in stomachs (10 fish, 4 invertebrate). The relative importance of prey, expressed as the percent total wet weight of prey recovered, indicated that Greenland halibut (Reinhardtius hippoglossoides) was the most important species, followed in order of importance by redfish (Sebastes spp.), Arctic cod (Boreogadus saida), Atlantic herring (Clupea harengus ), squid (Gonatus spp.), Atlantic cod (Gadus morhua ) and capelin (Mallotus villosus). -- Using otoliths from a previous study and from this study, lengths and weights of fish were estimated from a total of 72 stomachs. Hooded seals fed mainly on fish of 25 - 35 cm length for the larger species, and 15 - 25 cm for the smaller ones. Fish consumed by seals caught incidentally from offshore trawlers were larger than those taken elsewhere. The estimated lengths, and proportions (% weight) of fish found per stomach did not differ significantly between male and female hooded seals. -- The proportions (percent weight) of redfish and Atlantic herring consumed by hooded seals were significantly larger in the summer months, whereas a higher proportion of Arctic cod was consumed in the winter months. No seasonal differences were found in actual lengths of fish eaten. -- Total energetic values for fish eaten were calculated from estimated wet weights and energy densitites (kJ/g). Greenland halibut, Atlantic herring and Atlantic cod had the highest average energetic values (kJ), followed by redfish, Arctic cod, capelin and squid. Relative contributions of prey, expressed as the percent total energy of prey recovered, showed that Greenland halibut contributed approximately 53% of the total energy consumed, followed by redfish, Atlantic herring, Arctic cod, squid, Atlantic cod and capelin. These proportions corresponded with those obtained from percent total wet weight of prey recovered. -- The relative abundance of the fish species has a strong influence on the composition of the food in the diet of seals. Hooded seals spend some time in areas that are exploited by commercial fisheries, and consume commercial fish species of commercial size. However, in order to evaluate the impact of local predation on individual fish stocks, more information on behavioural and physiological characteristics of seals and fish are needed.

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