Research Document - 2011/078

Estimating the Impact of Grey Seals on the Eastern Scotian Shelf Cod Population to 2009

By M.K. Trzcinski, R. Mohn, and W.D. Bowen

Abstract

The cod-seal, predator-prey model of Trzcinski et al. (2006) was used to estimate the total consumption by grey seals on the Eastern Scotia Shelf (ESS, NAFO: 4VsW) and the impact of grey seals on cod recovery up to 2010. New data included the 2010 estimate of Gulf of St. Lawrence and Sable Island pup production and cod catch-at-age data to 2009. Data from satellite tracking was updated. There was little change in the estimate of the proportion of grey seals using the ESS. There was no new data on seal diet using fatty acid signature analysis. New data and analysis of otoliths collected from scat were used as the basis for an alternative diet scenario. As in Trzcinski et al. (2009), a model constraint was used so that cod removals-at-age could not exceed the estimate of cod abundance at age.

On the ESS, cod comprised on average less than 2% of a grey seals diet. The updated model produced lower estimates of natural mortality than Trzcinski et al. (2006). Four scenarios of seal diet were run: 1) 7% cod in seal diet and age distribution of cod eaten estimated from scats, 2) 7% cod in seal diet and age distribution of cod eaten  shifted to older cod using RV survey data, 3) 2% cod in seal diet from fatty acids and age distribution of cod eaten estimated from scats, and 4) 2% cod in seal diet from fatty acids and age distribution of cod eaten shifted to older cod using RV survey data. These scenarios were selected to explore the consequences of uncertainty in the proportion of cod in the diet and potential bias caused by regional foraging by seals in areas of small / young cod. These scenarios were selected to explore the consequences of uncertainty in the proportion of cod in the diet and potential bias caused by regional foraging by seals in areas of small / young cod. The updated model showed that in 2009 grey seals imposed a low level of instantaneous mortality under the four scenarios (0.013, 0.021, 0.087, 0.166), which ranges from approximately 2 to 22% of the total mortality (0.75).

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