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

Because of a lack of smaller individuals in all sample populations, no y-intercepts for [t.sub.0] were specified and models were forced through 0. Larger individuals and juveniles selectively sampled by size were excluded from the models to more accurately reflect a random sample. Likelihood ratio tests (Cerrato, 1990) were used to test for differences among states in models and in growth parameter estimates. Differential growth was evaluated for red snapper in the first 10 years of life when somatic growth is most rapid (Szedlmayer and Shipp, 1994; Patterson et al., 2001; Wilson and Nieland, 2001). Linear regressions of mean FL and mean TW at age for fishes aged 1 to 10 years were compared among states with analysis of covariance (ANCOVA) and tested for homogeneity of slopes.

Results

During the three-year study period, 5192 red snapper were sampled from the recreational harvest of the northern GOM (Table 1): 642 individuals from fishing tournaments, 71 undersize fish from tagging cruises, and 4479 random samples from recreational catches. The samples included 2502 males, 2568 females, and 122 individuals of undetermined sex. The resultant male-to-female ratios were 0.96:1 for Alabama, 1:0.99 for Louisiana, 0.94:1 for Texas, and 0.97:1 for all states combined. A chi-square test indicated no significant difference in the number of males to females ([chi square]=0.78, P=0.38). Fork lengths ranged from 237 to 916 mm (Fig. 2A). Specimens from Alabama ranged from 237 to 916 mm FL, Louisiana specimens ranged from 282 to 913 mm FL, and Texas specimens ranged from 266 to 846 mm FL. The FL frequency distributions of the random samples were different among all states (AL and LA, maximum difference (MD)=5.26; AL and TX, MD=51.86; LA and TX, MD=51.77) (Fig. 2A).

[FIGURE 2 OMITTED]

Total weights of all fish sampled ranged from 0.11 to 17.35 kg (Fig. 2B). Specimens from Alabama ranged from 0.22 to 15.42 kg TW, Louisiana specimens were 0.42 to 17.35 kg TW, and Texas specimens ranged from 0.33 to 9.42 kg TW. Total weight-frequency distributions (in 0.5 kg increments) differed significantly between all states (AL and LA, MD=5.37; AL and TX, MD=53.68; and LA TX, MD=52.28)(Fig. 2B). Significant differences in red snapper FL-TW regression models were detected among states (ANCOVA test of homogeneity of slopes, [F.sub.5, 4522]=23.36; P<0.001; [r.sup.2]=0.98; ANCOVA test for equal intercepts, [F.sub.5, 4522]=22.77, P<0.001, [r.sub.2]=0.98); therefore, separate models were fitted for each state. The resultant equations were

AL TW = 1.51 x [10.sup.-5] (F[L.sup.3.03]) ([F.sub.1;1965]=102740; P<0.0001; [r.sub.2]=0.98);

LA TW = 1.02 x [10.sup.-5] (F[L.sup.3.09]) ([F.sub.1;1856]=77981; P<0.0001; [r.sup.2]=0.98);

TX TW = 2.88 x [10.sup.-5] (F[L.sup.2.92]) ([F.sub.1;699]=13345; P<0.0001; [r.sup.2]=0.95).

Ages were obtained from 5035 transverse otolith sections. Thirty fish had otolith sections deemed unreadable by both readers. The age estimates determined by the two readers were evaluated for reader agreement, precision, and average percent error for first and second readings of otolith sections by sample year. Table 2 gives APE, CV, D, percentage agreement (O), and percentages of differences in age estimates ([ or -] 1, 2, and 3 years). The readers agreed on age estimates for 4053 otoliths (80.5%) after the initial reading. Re-examination of the 982 otolith sections for which annulus counts differed produced agreement for 5007 individuals.