Red snapper demographic structure in the northern Gulf of Mexico based on spatial patterns in growth rates and morphometrics

Fishery Bulletin, Oct, 2004 by Andrew J. Fischer, M. Scott Baker, Jr., Charles A. Wilson

Our von Bertalanffy growth models on FL at age showed that red snapper from all three states exhibit a pattern of rapid, linear growth to approximately 10 years, after which maximum theoretical (asymptotic) FL is soon reached and growth in length becomes negligible. This pattern of rapid growth was similar to that reported in previous studies (Szedlmayer and Shipp, 1994; Manooch and Potts, 1997; Patterson, 1999; Wilson and Nieland, 2001). However, our models predicted smaller L and higher values of k. Because of the minimum size limits on the recreational fishery, very few fish under age 2 years (>300 mm FL) were included in our sample populations. We forced our models through [t.sub.0]=0 to more accurately predict juvenile growth, which in turn increased our estimates of k. In addition, we had a much larger sample population that included more older, larger fish than most of the previously cited studies. These larger fish pulled the curve down, driving the lesser estimations of [L.sub.[infinity]]. The lack of significant differences in growth parameters between the Alabama and Louisiana models supports the findings of previous research, which indicates that Alabama and Louisiana red snapper grow at similar rates and reach comparable sizes (Patterson et al., 2001). However, values of [L.sub.[infinity]] for Texas red snapper were significantly smaller than parameters predicted for Alabama and Louisiana red snapper. Interestingly, Texas had a value of k that was significantly larger then that for Alabama and Louisiana and this would indicate that Texas fish obtain a smaller maximum theoretical FL but reach it at a faster rate then fish from Alabama and Louisiana.

Von Bertalanffy growth models of mean weight at age produced similar results, indicating that Texas red snapper obtain significantly smaller maximum theoretical TW than fish from Alabama and Louisiana. Fish sampled from tournaments were excluded from all growth models to more accurately reflect growth of a random population. Tournament anglers target large fish, possibly the fastest growing individuals at a given age, and their catches may bias growth estimates (Ottera, 1992; Vaughan and Burton, 1993; Goodyear, 1995). Without these tournament fish, however, the Alabama red snapper TW model did not reach an asymptote. Therefore the growth parameters for that model were poorly estimated. Notwithstanding, Alabama and Louisiana models did not differ significantly. Estimates of [W.sub.[infinity]] and k predicted for Louisiana red snapper were slightly larger than previously reported for fish from the Louisiana commercial and recreational catches (Render, 1995). Although the Texas model predicted a value of [W.sub.[infinity]] that was significantly less than those for both Alabama and Louisiana red snapper, Texas had a growth coefficient (k) that was larger then that for Alabama. It appears that, as in the length models, Texas fish reach a smaller theoretical maximum weight but at a faster rate than Alabama fish. Louisiana fish attained maximum weight at a faster rate than Alabama or Texas red snapper. Our growth models indicate that although Texas red snapper grow in mass at a faster rate than Alabama fish, Texas red snapper are consistently smaller at age and reach smaller maximum sizes than those from Alabama and Louisiana and that there is a veritable difference in size at age and growth rates among regions. Similar demographic variations in growth rates among populations have been previously noted for other marine fish species of the South Atlantic and GOM, such as gray snapper (Johnson et al., 1994; Burton 2001), and king mackerel (DeVries et al, 1990; DeVries and Grimes, 1997).