Red Snapper: Ecology and Fisheries in the U.S. Gulf of Mexico

Modeling the Dependence of Batch Fecundity on Size and Age for Use in Stock Assessments of Red Snapper in U.S. Gulf of Mexico Waters

Clay E. Porch, Gary R. Fitzhugh, Michelle S. Duncan, L. A. Collins, and Melissa W. Jackson

doi: https://doi.org/10.47886/9781888569971.ch15

Abstract.—The red snapper Lutjanus campechanus exhibits rapid growth early in life, yet is a relatively long-lived species with an indeterminate spawning pattern. Batch fecundity (BF), a principal determinant of the reproductive potential of indeterminate spawners, appears to increase geometrically with length and asymptotically with age based upon combined data from National Marine Fisheries Service (NMFS) and Marine Fisheries Initiative (MARFIN) reproductive sampling programs. As the life history pattern sets up a potential disconnection between size and age, we modeled their interaction as explanatory variables of BF. Visual analysis suggests a dome-shaped relationship between BF and age within a given size-class. To test this, a general log-linear model and a new “standardized-age” model (where the effect of age depends explicitly on the size of the fish) were fitted to data. These analyses suggest that the effect of age, while statistically significant, was relatively small (the models with age and length terms explaining only slightly more of the variation in BF than a simple power function of length). The age effect seems to be most pronounced for fish that are exceptionally small or exceptionally large given their age, which constitute a small fraction of the sample and presumably also a small fraction of the population at large. Hence, it seems unlikely that including this interaction would have any important ramifications for stock assessments of red snapper. Nevertheless, the effect of age on BF remains open to the degree that old fish (i.e., greater than age 13) were rare in the combined data set and tended to exhibit lower BF values than predicted. Age effects of other reproductive determinants remain to be evaluated but we found hydrated females to be of greater size-at-age than nonhydrated red snapper suggesting an age-length interaction on batch frequency.