Trends and Factors Influencing the Length, Compensatory Growth, and Size‐Selective Mortality of Juvenile Bristol Bay, Alaska, Sockeye Salmon at Sea

Ellen M Yasumiishi,Gregory T Ruggerone,Beverly A Agler,Lorna I Wilson,Ed V Farley

doi:10.1080/19425120.2016.1167793

Abstract

AbstractThe productivity of Bristol Bay, Alaska, Sockeye Salmon Oncorhynchus nerka increased during the mid‐1970s. This increase is believed to be partially due to an increase in early marine growth associated with the 1976–1977 cool‐to‐warm shift in summer sea surface temperature (SST). The body size of juvenile salmon during their first year at sea is believed to regulate their ability to survive over winter. The back‐calculated smolt length, first‐year ocean growth, and total juvenile length of Sockeye Salmon from five Bristol Bay river systems (Egegik, Kvichak, Naknek, Ugashik, and Wood) and two smolt ages were used to examine trends and factors influencing total juvenile length, compensatory growth, and size‐selective mortality in the first year in the ocean from 1962 to 2007. Juvenile length increased in relation to summer sea temperature, the 1977–2001 and 2002–2007 warm temperature regimes, smolt length, and compensatory growth. Compensatory growth—an inverse relationship between first‐year ocean growth and smolt size—increased over time as well as after the 1976–1977 climate regime shift, was more common in age‐1.0 fish than in age‐2.0 juveniles, and was important in determining the length of juvenile Sockeye Salmon from the Wood River (the shorter fish among rivers and smolt ages). The coefficient of variation in length did not change with SST, suggesting that size‐selective mortality occurred prior to the end of the first year at sea for all 10 fish groups. The predictor variables that were significant in the models varied among river systems and smolt ages. This study demonstrated that the frequency of compensatory growth and the total lengths of juvenile Sockeye Salmon during their first year at sea increased with summer SST (range, 7.5–10.5°C) in the eastern Bering Sea, a possible mechanism for the increased productivity of Bristol Bay Sockeye Salmon associated with warmer sea temperatures.Received June 2, 2015; accepted February 23, 2016

Highlights

This study demonstrated that the frequency of compensatory growth and the total lengths of juvenile Sockeye Salmon during their first year at sea increased with summer sea surface temperature (SST) in the eastern Bering Sea, a possible mechanism for the increased productivity of Bristol Bay Sockeye Salmon associated with warmer sea temperatures
Evaluation of Parameters Juvenile length.—To evaluate the factors influencing the juvenile length of survivors, we modeled mean juvenile length at the end of the first year at sea as a function of climate regime, summer SST, mean smolt length, juvenile abundance, compensatory growth (CG) slope index, and underlying cycles in total juvenile length using the following time series regression model: L1i;a;t 1⁄4 β0 þ β1regime1976À1977 þ β2regime2001À2002 þ β3SSTt þ β4FWi;a;t þ β5abundancei;a;t þ β6CGi;a;t þ þφi;a;tÀxL1i;a;tÀx þ εi;a;t: (5)
Factors Influencing Juvenile Length For the Sockeye Salmon from the five Bristol Bay river systems analyzed in this study, juvenile length was influenced by climate regime, summer sea temperature, smolt length, and compensatory growth

Summary

Introduction

Following the 1976–1977 cool-to-warm climate regime change, the productivity of Bristol Bay Sockeye Salmon increased and was generally higher during the warm climate regime, possibly due to higher growth rate potential as juveniles (during the first year at sea) and greater available of prey in the ocean (Farley et al 2009). Juvenile Sockeye Salmon collected at sea were more abundant, more widely distributed on the eastern Bering Sea shelf, and had higher growth rate potential during warm years (2002–2005) than in cold years (2000, 2001, and 2006) (Farley and Trudel 2009) The density of their prey was positively correlated with mean May SST in the eastern Bering Sea (P = 0.02), indicating a bottom-up control on growth (Farley and Trudel 2009). We hypothesized that the length of juvenile Sockeye Salmon would increase with ocean warming

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