SPAWNING BIOMASS AND EARLY LIFE OF NORTHERN ANCHOVY; ENGRAULIS MORDAX, IN THE NORTHERN SUBPOPULATION OFF OREGON AND WASHINGTON

ABSTRACf A major spawning center for the northern subpopulation of northern anchovy, Engraulis morda:t, is documented off the Oregon-Washington coast beyond the continental shelf, based on collections of planktonic eggs in July 1975 and July 1976. Biomass estimates of northern anchovy in this spawning concentration ranged from 262,506 to 769,511 metric tons in 1975 and 144,654 to 1,005,263 metric tons in 1976, based on egg and larva surveys. Spawning biomass was estimated to be 800,000 metric tons in 1977, based on an acoustic survey of adults. The most prohable biomass may be more than 100,000 but less than 1,000,000 metric tons. Potential yield estimates ranged from 86,792 to 633,316 metric tons, but realizable yields may be considerably lower ifmanagementstrategies applied to northern anchovy in the central subpopulation are implemented for the northern subpopulation. Spawning appears to be associated with waters of the Columbia River plume which may provide favorable conditions, in terms of stability and productivity, for survival of first feeding northern anchovy larvae. Evidence oflarval transport south away from the spawning center leads to questions about return mechanisms to explain the occurrence ofjuveniles in Oregon bays and rivers later in the season. Additional spawning centers within the range of the northern subpopulation have not been documented although some evidence from the literature indicates another spawning center may occur in the Strait of Georgia, British Columbia, around the Fraser River plume. Conditions related to spawningdiffer between the northern and central subpopulations. OffOregon, spawningoccurs from mid-June to mid-August, when current flow to the south is at a maXimum,water temperatures are reaching maximum levels for the year, coastal upwelling is at a maximum, and day length is at or near maximum duration. Off California, peak spawning occurs from January through April when southward current flow is minimal, watertemperatures are reaching minimal levels for the year, upwelling is minimal, and day length is at minimum duration. These factors are indicative of some degree of reproductive isolation as well as differing reproductive strategies between the two subpopulations.