Stock‐Specific Size and Migration of Juvenile Coho Salmon in British Columbia and Southeast Alaska Waters

Terry D Beacham,Strahan Tucker,Richard J Beamish,John R Candy,Marc Trudel,Chrys M Neville,Colin Wallace

doi:10.1080/19425120.2016.1161683

Abstract

AbstractThe variation at 17 microsatellites was analyzed for 5,270 juvenile Coho Salmon Oncorhynchus kisutch obtained from coastal British Columbia and Gulf of Alaska surveys during 1998–2012. A 270‐population baseline was used to determine the individual identifications of the fish sampled, with individuals being identified to 22 stocks of origin. Columbia River and Washington juveniles were consistently larger than those from British Columbia and Alaska. During June, the larger individuals within a stock were observed in more northerly locations. There was a relationship between the timing of northward migration and juvenile body size, with larger individuals migrating earlier than smaller individuals from the same stocks. Stock composition was more diverse in the northern sampling regions than in those in southern British Columbia. There was only a modest change in stock composition between fall and winter samples in both the Strait of Georgia and west coast of Vancouver Island sampling regions, indicating that juvenile migration had largely been completed by the fall. There was a wide divergence among stocks in juvenile size and dispersion among sampling locations.Received December 9, 2015; accepted February 9, 2016

Highlights

Coho Salmon Oncorhynchus kisutch juveniles in the southern portion of the species’ distribution usually reside in freshwater for a year or more, some juveniles may rear in estuaries for a portion of their first summer and move back upstream to overwinter (Miller and Sadro 2003) or migrate to the ocean during their first fall and early winter (Roni et al 2012; Bennett et al 2015)
Body Size The earliest month of sample collection after juveniles migrated to the ocean was May, but sampling during this period was limited and Strait of Georgia (SOG) samples were derived from a regional purse seine survey (Table 1)
The generalized linear model (GLM) indicated that in June the mean Fork length (FL) within reporting groups was significantly lower for juvenile Coho Salmon caught in the SOG (t = –6.3, P < 0.0001) and off west coast of Vancouver Island (WCVI) (t = –2.1, P < 0.05) and significantly higher for those caught in SEAK outside (t = 4.2, P < 0.0001)

Summary

Introduction

Coho Salmon Oncorhynchus kisutch juveniles in the southern portion of the species’ distribution usually reside in freshwater for a year or more, some juveniles may rear in estuaries for a portion of their first summer and move back upstream to overwinter (Miller and Sadro 2003) or migrate to the ocean during their first fall and early winter (Roni et al 2012; Bennett et al 2015). The greater presence of larger juveniles in more northern sampling regions may reflect differences in juvenile size at the time of smolting and ocean entry, with larger individuals quickly commencing a northward migration while smaller ones rear for a period of time in local waters (Beacham et al 2014a; Freshwater et al, in press) Even so, this does not preclude the possibilities that larger smolts swim faster or that juveniles encounter better growing conditions as they move farther north (Tucker et al 2009). Identification of the relative seasonal abundance of specific stocks in specific marine habitats will aid in evaluating the importance of mortality agents in shaping cohort abundance

Methods

Results

Conclusion