Research Document - 2012/156

Estimating consumption of prey by Harp Seals (Pagophilus groenlandicus) in NAFO Divisions 2J3KL

By G.B. Stenson

Abstract

Consumption of Atlantic Cod (Gadus morhua) by Harp Seals (Pagophilus groenlandicus) off the east coast of Newfoundland in NAFO Divisions (Div.) 2J3KL was estimated by integrating information on the numbers at age, age-specific energy requirements, seasonal distribution, and diet of Harp Seals in the Newfoundland and Labrador area. Abundance was estimated using a population model integrating pup production between the late 1970s and 2004, annual estimates of reproductive rates from 1954 to 1998, and data on age-specific removals from 1952 to 2008. Energy requirements of the population were estimated using a simple allometric model based on body mass obtained from monthly sex-specific growth curves. The proportion of energy obtained in Div. 2J3KL was estimated using data obtained from satellite telemetry and traditional tagging studies. The diet of Harp Seals in nearshore and offshore waters during winter (October – March) and spring (April – September) was determined by reconstructing the wet weight and energy content of prey in stomachs collected in 1982 and 1986 to 2007. The impact of different diet determination methods was explored by estimating consumption using a multinomial regression approach and fatty acid signatures from Harp Seal samples. Uncertainty in the consumption estimates was approximated by incorporating the uncertainty in the numbers at ages, diets, energy requirements, and seasonal distribution. Total prey consumption by Harp Seals in Div. 2J3KL during 2008 was estimated to be approximately 4.2 million metric tons. However, this estimate was imprecise with a 95% confidence interval (C.I.) being 3.2 million – 5.4 million tons. Consumption of individual prey species varied greatly depending upon the assumed diet.

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