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

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We recognized that the larger red snappers from Louisiana might bias the data; therefore we compared growth for fish from 2 to 10 years of age--a time period when red snapper have demonstrated rapid linear growth (Szedlmayer and Shipp, 1994; Patterson et al., 2001; Wilson and Nieland, 2001). Linear regressions of mean FL at age for all individuals 2 to 10 years (Fig. 6A) were compared among states. We found no significant differences among states (ANCOVA test of homogeneity of slopes, [F.sub.2;28]=2.7; P=0.08; ANCOVA test for equal intercepts, [F.sub.2;28]=0.52, P=0.6).

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Mean TW at age was also examined among states for red snapper 2 to 10 years in age as above (Fig. 6B). No significant differences were found between Alabama and Louisiana (ANCOVA test of homogeneity of slopes, [F.sub.1;17]= 0.1; P=0.75; ANCOVA test for equal intercepts, [F.sub.1;17]= 0.26; P=0.66 for intercepts). However, a significant difference between slopes was detected when comparing Alabama and Texas red snapper (ANCOVA test of homogeneity of slopes, [F.sub.1;16]=19.68; P<0.0007; ANCOVA test for equal intercepts, [F.sub.1;16]=2.74; P<0.12). The same was found when comparing slopes for Louisiana and Texas red snapper (ANCOVA test of homogeneity of slopes, [F.sub.1, 16]=9.62; P<0.008) but not when comparing intercepts ([F.sub.1, 16]=0.64; P<0.44).

Discussion

Demographic variations in growth rates and in size-frequency distributions may indicate the existence of isolated management units of red snapper in the northern GOM. The recreational harvests of Alabama and Louisiana red snapper were dominated by individuals ranging from 375 to 425 mm FL, whereas the majority of Texas fish (69%) were 375 mm FL or less. It was within this size range (375-400 mm FL) that the significant differences in red snapper among states were detected. The FL distribution of red snapper sampled in Texas also differed from those for Alabama and Louisiana; there were very few large fish represented in the Texas sample population, partly because fishing tournaments (where larger individuals are targeted) were not sampled in Texas. Significant differences in TW frequencies among states were also detected at approximately 1 kg (the approximate weight of a red snapper 375-400 mm FL); 86% of Texas fish weighed 1 kg or less, compared to only 27% of Alabama fish and 28% of Louisiana fish in this size range.

One factor possibly contributing to the modal size class difference was the type of fishing vessel used to catch the fish. The majority of Texas specimens (~95%) were sampled from headboats; whereas Louisiana and Alabama fish were obtained almost exclusively from charterboats. This is not to say that charterboats were purposely excluded from the Texas survey. On the contrary, red snapper were sampled from any and all available recreational fishing parties at the three individual sampling locations. Differences in modal size and number of red snapper caught per person onboard charterboats versus headboats may be inconsequential considering that both trip types used similar gear and targeted similar or the same fishing locations. It should be noted however that in the Texas study area, charterboats routinely frequented a wider array of fishing spots (rigs, hardbottom, wrecks, etc.) than did headboats, which typically return to the same few rigs and large structures over and over again (Tolan (5)).