The effects of grey seal predation and commercial fishing on the recovery of a depleted cod stock

Cook, Robin and Trijoulet, Vanessa (2016) The effects of grey seal predation and commercial fishing on the recovery of a depleted cod stock. Canadian Journal of Fisheries and Aquatic Sciences, 73 (9). pp. 1319-1329. ISSN 1205-7533 (https://doi.org/10.1139/cjfas-2015-0423)

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Abstract

Cod (Gadus morhua) are preyed upon by grey seals (Halichoerus grypus) and there is debate over the impact this has had on the decline of stocks and their prospects for recovery. We analysed a depleted stock to the West of Scotland and show that seal predation rate is consistent with a type II functional response. Forward projections of a model including the functional response under varying levels of fishing and seal population size suggest that stock recovery is possible under current conditions but there is a modest probability that the stock will decline further in both the short and long term. The potential recovery is fragile and sensitive to relatively small increases in either fishing or seal predation. Forward projection models that exclude the functional response estimate a lower probability of stock decline and may underestimate the risk to the stock. At low stock sizes and high fishing mortality rates functional response models project slower recovery but the opposite is true at low fishing mortality.

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