A HISTORY OF PROPOSALS FOR SUBPOPULATION STRUCTURE IN THE PACIFIC SARDINE (SARDINOPS SAGAX) POPULATION OFF WESTERN NORTH AMERICA

Recent work has examined the structure of stocks, races, or subpopulations in the recovering Pacific sardine (Sardinops sagax) biomass of the northeast Pacific Ocean. Individual fish characteristics do not clearly indicate the geographic origin of birth, but collections of sardines from different areas show some heterogeneity in growth rate, time of birth and recruitment, blood type, and number of vertebrae. Even when heterogeneity is in question, precautionary management principles, which reduce the risk of overfishing, should support management of stocks of fish in different areas as independent stocks. The sardines of the northeast Pacific have been estimated to have up to three subpopulations based on tagging, size-at-age, isolated spawning centers, blood groups, vertebral column counts, estimated natural mortality rate, or bimodal seasons of recruitment. Spawning centers are thought to occur off the Gulf of California (GOCAL), Baja California Sur inshore (BSI) and Central California offshore (CCO). Cursory genetic examinations of sardines from these areas neither support nor refute these divisions. Genetic analysis of S. sagax from four far-flung sardine habitat sectors of the Pacific basin can be described as “shallow,” meaning the separation of all the species in the Pacific is relatively recent. However, on the time scale of fisheries management, decades, the separation of the two stocks on the Pacific coast, BSI and CCO, appears chronic in that the collapse of the northern stock did not stimulate an apparent replacement from the southern stock in decades. Therefore, it would be prudent to institute separate management measures that define the boundary between GOCAL and BSI and the boundary between BSI and CCO regardless of the genetic or habitat basis for stock separation. Three datarich bases for describing the two stocks’ modern isolation should be used to design careful studies of representative samples of sardines at the boundaries between the stocks for devising the most practical method for allocating catches among the neighboring stocks. This paper reviews existing data to see which might be applied to a precautionary approach to managing the revived Pacific sardine fisheries. It also examines what advances in our knowledge of these stocks and the methods used to assess them may be required to ensure an adequate spawning biomass and yield of the sardine fisheries for the northeast Pacific stock(s).