Dynamic Population Structure of Pacific Herring in Puget Sound

Studies of population structure in marine fishes are usually snapshots representing present day conditions without much consideration of temporal and spatial dynamics. Although metapopulation concepts are widely accepted for marine fish, there is little empirical evidence. Here, we present microsatellite data of Pacific herring in Puget Sound, which is known to consist of three genetically differentiated populations among 19 spawning aggregations:  a spring spawning population (Cherry Point), an isolated winter spawning population in the South basin (Squaxin Pass) and the remainder of all spawning aggregations in a single winter spawning population. A newly established aggregation spawning in spring along the Seattle waterfront was widely assumed to be an offshoot of the rapidly declining Cherry Point population.  Genetic data from Seattle waterfront eggs, however, indicated that these spring spawners most likely derived from proximate winter spawners. In addition, we found that another population in the South Basin belonged to the larger winter spawner complex, not the geographically proximate Squaxin Pass stock. Our data challenge accepted hypotheses of mechanisms of temporal and spatial isolation between herring spawning stocks, and support a dynamic metapopulation structure that needs to be considered in management and conservation.