Research Document - 2011/041

Estimated consumption of southern Gulf of St. Lawrence cod by grey seals: bias, uncertainty and two proposed approaches

By H.P. Benoît, M.O. Hammill, and D.P. Swain

Abstract

A Zonal Assessment Process on the potential impacts of grey seals (Halichoerus grypus) on fish populations in eastern Canada was held October 4-8, 2010. The terms of reference for that meeting included providing stock/area-specific estimates of grey seal diet and prey consumption, and assessing the possible sources of bias and uncertainty in estimates of grey seal diets. This research document was prepared to address these questions as it relates to grey seals feeding in the southern Gulf of St. Lawrence (sGSL) and neighboring areas, with an emphasis on the consumption of sGSL cod (Gadus morhua) by grey seals. The diet of grey seals in the sGSL has been inferred using prey hard parts recovered from seal stomachs and intestines. Based on an analysis of grey seal movements using satellite telemetry data, the median stomach and intestine sample generally reflects prey that were consumed within 5 and 12 km of the diet sampling location, respectively. Taking into consideration the geographic locations of grey seal diet samples and the spatial distribution of different sizes of cod, the size composition of cod in the diet suggests that grey seals selectively prey on larger cod. Furthermore, based on the geographic distribution of diet sampling sites, we conclude that the diet of grey seals remains unknown in many areas where they occur. In particular, during the spring and summer, there are no diet estimates from areas where there would be a high probability of sampling seals that foraged offshore. This precludes directly estimating a representative and reliable grey seal diet for the sGSL based on the existing data. We therefore used two different approaches to estimate possible consumption of two size classes of cod (<35 cm and 35+ cm) by seals in the chosen focal year of 2005. The first approach was based on fine scale spatio-temporal overlap between cod and seals, with assumptions concerning the diets of seals where they overlap with cod. Using this approach seals were estimated to consume on average approximately 1800-2300 t of cod <35 cm and 1600-2000 t of 35+ cm cod, depending on assumptions. The second approach was based on spatio-temporal overlap at the scale of Northwest Atlantic Fisheries Organization areas, with assumptions concerning the average diet of grey seals in the areas. Using this second approach, seals were estimated to consume on average approximately 8200-9800 t of cod <35 cm and 7600-8900 t of 35+ cm. Both approaches for estimating diet are sensitive to unverified assumptions, and their reliability cannot be assessed. The estimates presented should therefore be interpreted as scenarios that indicate possible consumption given particular assumptions for filling data gaps, rather than reliable estimates.

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