Research Document - 2009/052

Estimating the Impact of Grey Seals on the Eastern Scotian Shelf and Western Scotian Shelf Cod Populations

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 Western Scotian Shelf (WSS, NAFO: 4X) and the impact of grey seals on cod recovery up to 2007. New data included the 2007 estimate of Gulf of St. Lawrence and Sable Island pup production and cod catch-at-age data to 2007. Data on hunting removals on Sable Island in the 1960s were incorporated, which put a lower bound on the early population size and increased our estimate of seal abundance for that period. Data from satellite tracking was updated and we provide the first estimate of the number of Sable Island grey seals foraging on the WSS. There was little change in the estimate of the proportion of grey seals using the ESS. The proportion of cod-at-age consumed by grey seals was recalculated, which resulted in more 1 year olds being eaten. Fatty acid estimates of grey seal diets, collected in 2003 and 2004, were also incorporated in the model. A model constraint was added so that cod removals-at-age could not exceed the estimate of cod abundance at age.

In the ESS, cod comprised on average less than 2% of a grey seals diet. The updated model produced significantly lower estimates of natural mortality than Trzcinski et al. (2006). The updated model showed that in 2007 grey seals imposed a low level of instantaneous mortality (0.08), which is approximately 11% of the mortality due to all sources except fishing (0.74) (Figure 8).

On the WSS, our catch-at-age stock assessment model showed that cod have decreased 50% since 1988. As there is no data on the diet of grey seals on the WSS, we assumed that grey seals feeding on the WSS had a similar diet as those on the ESS; i.e., that cod comprises approximately 2% of the diet. Since cod grow faster on the WSS, the proportion of age-1 cod in the diet was estimated to be larger. The proportion of the Sable Island and Gulf of St. Lawrence herd, which foraged on the WSS, was estimated by quarter using satellite tagging data. A higher proportion of males foraged on the WSS than females. The number of seals foraging on the WSS varied by season and was estimated to average 13,322 (± 2,235 SE) in 2007. Our consumption model estimated that grey seals consumed 21,954 t (± 7,897 SE) of prey. Mortality of cod caused by grey seals increased with the increase in the seal population and averaged 0.0012 which is estimated to be approximately 0.2 % of total natural mortality. Fishing (bycatch) mortality of cod (F=0.16) and natural mortality due to sources other than seal predation (Mn –Mseals ~ 0.66) are high and are causing the stock to decline.

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