First-Year Survival of Northern Fur Seals (Callorhinus ursinus) Can Be Explained by Pollock (Gadus chalcogrammus) Catches in the Eastern Bering Sea

Jeffrey W Short,Lowell W Fritz,Jonathan J Warrenchuk,Harold J Geiger

doi:10.3390/jmse9090975

Jeffrey W Short, Lowell W Fritz + Show 2 more

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https://doi.org/10.3390/jmse9090975

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Abstract

The Pribilof northern fur seal (Callorhinus ursinus) herd in the eastern Bering Sea has declined by ~70% since the 1970s, for elusive reasons. Competition for pollock (Gadus chalcogramma) with the commercial fishery has been suspected as a contributing factor, but no correlative relationship between fishing activity and fur seal population declines has heretofore been demonstrated. Here, we present evidence for a moderately strong inverse relationship between fishery catches of pollock and first-year survival of fur seals, based on three different approaches to evaluation. We suspect this relationship results from the dependence of lactating female fur seals on locating dense and extensive schools of pollock near the Pribilof Islands to efficiently provide nutrition for their pups, because the pollock fishery also targets these same schools, and when fished, the remnants of these schools are fragmented and dispersed, making them more difficult for fur seals to locate and exploit. Inadequately fed pups are less likely to survive their initial independent residence at sea as they migrate south from the Pribilof Islands in the fall. Our results imply that pollock catches above ~1,000,000 t within ~300 km of the Pribilof Islands may continue to suppress first-year survival of Pribilof fur seals below the estimated equilibrium survival value of 0.50, leading to continued decline of the population.

Highlights

Published: 7 September 2021The Pribilof Island breeding population of northern fur seals (Callorhinus ursinus) has been in long-term decline since the mid-1950s, after fully recovering from severe over-harvesting in the late 19th and early 20th centuries [1]
Our objective here is to evaluate the strength of any relationship between catches of pollock by the EBS commercial fishery and estimates of Pribilof fur seal pup births after explicitly accounting for changes in fur seal vital rates. We evaluate this relationship in three ways: (1) empirical comparisons of cross-correlations between annual EBS pollock catches and total pollock biomass against the estimated number of pup births after lags of one to 15 years; (2) a simple modification of a Leslie matrix population projection of seal pup births that includes functions of pollock catch or biomass which modify the first-year survival probability in the equilibrium Leslie matrix for fur seals; and (3) a search for Bayesian posterior probabilities of first-year pup survival when EBS pollock catches are zero, low, medium or high
Model 1: We model the effect of pollock catches on first-year survival of St

Summary

Introduction

The Pribilof Island breeding population of northern fur seals (Callorhinus ursinus) has been in long-term decline since the mid-1950s, after fully recovering from severe over-harvesting in the late 19th and early 20th centuries [1]. The population has continued to decline and was estimated to number less than a half million in 2018 [3,4]. An effort to increase population fecundity by deliberately killing adult females, both on land from 1956 to 1968, and at sea from 1958 to 1974, accounts for most of the decline through the 1970s [5,6]. Isolation from most terrestrial predators and proximity to the highly productive waters of the EBS has historically provided exceptionally favorable conditions for fur seal growth and survival, for adult females nurturing new-born pups. Consuming up to ~1 million t of fish and squid each

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