Pacific Cod in the Salish Sea: Population Connectivity and Implications for Conservation

Patterns of population genetic differentiation can provide insights into both contemporary and historical gene flow, which may be useful for the management of exploited species.Isolation-by-distance (IBD) patterns may evolve relatively rapidly if dispersal is limited, while sharp breaks may reflect historical population isolation and secondary contact. Here, we describe both these patterns in Pacific cod (Gadus macrocephalus) and describe their management implications. F_ST estimates from 11 microsatellites showed a clear IBD pattern among coastal samples in the northeast Pacific Ocean, with higher than expected differentiation between coastal and Salish Sea cod. The average dispersal distance estimated from the IBD relationship was 26 km per generation. Although these patterns do not reveal clear population boundaries, they show limited dispersal that can inform stock management decisions. Cod from central Puget Sound and the Strait of Georgia represented a distinct mitochondrial lineage, sharing only nine of 39 observed haplotypes with coastal cod from Washington State. Planktonic larvae may be retained locally by limited water exchange among the fjord basins of the Salish Sea and juvenile settlement in shallow, nearshore environments may limit dispersal. Both populations once supported commercial fisheries but have been declining for several decades. The Strait of Georgia trans-boundary stock is one of four recognized for management purposes in British Columbia, Canada and Salish Sea Pacific cod have recently been added to the NMFS Species of Concern List. Our results underscore the utility of genetic studies to resolve appropriate spatial scales for management and conservation actions.