Predicting stock productivity when fecundity is indeterminate

Most stock assessments are data poor when it comes to detailed information on spawning characteristics.  As such, assessments commonly treat stock productivity as a function of spawning biomass.  This approach assumes that fecundity relates linearly to body weight, an assumption supported by empirical work.  However, if fecundity is indeterminate, use of spawning biomass additionally assumes constant spawning frequency and constant spawning duration.  We surveyed existing literature and found that these assumptions do not survive the test of data.  Our own analyses of four stocks in the southeast United States (gag, scamp, vermilion snapper, and gray triggerfish) showed nonlinear relationships between body length and spawning characteristics.  We examined the effects of those relationships on stock assessment results, along with the consequences of incorrectly assuming they are constant.  Our results underscore the importance of collecting species-specific information on spawning characteristics, but in the absence of such information, it may be time to reconsider some default assumptions common in stock assessment.