Estimating the Relative Contribution of Spring- and Summer-Spawned Cohorts to the Atlantic Coast Bluefish Stock

Abstract

The timing of recruitment for age-0 bluefish Pomatomus saltatrix along the southern and mid Atlantic coast of the USA is bimodal, consisting of spring-spawned and summer-spawned cohorts. We present evidence from surveys of age-0 fish that the recent precipitous decline in the Atlantic bluefish stock over the past decade has been associated with a switch in the relative production of the two cohorts from predominance by spring-spawned fish throughout the 1970s and 1980s to predominance by summer-spawned fish in most years from 1992 to 2002. Given this evidence of a shifting recruitment pattern, we reexamined the contribution of the two cohorts to the adult portion of the population and compared our results with similar earlier assessments. Cohort origin of adults was identified based on back-calculated length at age 1. In four year-classes in which age-0, summer-spawned bluefish predominated, there was little evidence of summer-spawned fish among harvested adults from the same year-classes captured one or more years later. Most adults examined in this study (N = 976) displayed lengths at age 1 greater than 20 cm, as predicted for spring-spawned bluefish. This contrasts with year-classes of bluefish from the late 1950s, when back-calculated size at age 1 was clearly bimodal, consisting of both spring-spawned and summer-spawned cohorts. Low numbers of summer-spawned bluefish in our samples did not appear to be an artifact of inaccuracies in aging or back-calculation. The recent decline in the abundance of Atlantic coast bluefish appears to result from poor age-0 recruitment of the spring-spawned cohort and failure of the summer-spawned cohort to contribute substantially to the adult population.