Age and growth of cutlassfishes, Trichiurus spp., from the South China Sea

Fishery Bulletin, Oct, 2000 by Kai Yin Kwok, I-Hsun Ni

[Figures 4-5 ILLUSTRATION OMITTED]

Otolith weight accounted for 68.7% and 68.9% (Table 2) of the variability in age for T. lepturus and T. nanhaiensis, respectively. A negligible amount of the remaining variability was explained by considering otolith length in addition to otolith weight. The otolith weight-age regression was improved by fitting the untreated variables (otolith weight and age) with simple linear regression models:

[TABULAR DATA 2 NOT REPRODUCIBLE IN ASCII]

T. lepturus:

OW = 6.3533 5.2913Age (n=718, [r.sup.2]=0.7168, P [is less than] 0.001);

T. nanhaiensis:

OW = 6.3921 3.6850Age (n=515, [r.sup.2]=0.7561, P [is less than] 0.001).

These regression results suggest a linear relationship between otolith weight and age (Fig. 6). The regression models were significantly different between the two species (ANCOVA: [F.sub.2,1229]=224.17, P [is less than] 0.001). Normal probability and residual plots showed that the regressions complied with the assumptions of normality and homogeneous variance.

[Figure 6 ILLUSTRATION OMITTED]

Von Bertalanffy growth equations for both species were

T. lepturus

males:

PL = 755.2 {1 - [e.sup.[-0.116 (t 2.737)]]} (n=146, [r.sup.2]=0.684, P [is less than] 0.001);

females:

PL = 601.4 {1 - [e.sup.[-0.158 (t 2.850)]]} (n=578, [r.sup.2]=0.765, P is less than] 0.001);

sexes combined:

PL = 589.1 {1 - [e.sup.[0.168 (t 2.682)]]} (n=724, [r.sup.2]=0.749, P is less than] 0.001);

T. nanhaiensis

males:

PL = 501.7 {1 - [e.sup.[-0.306 (t 1.673)]]} (n=281, [r.sup.2]=0.682, P [is less than] 0.001);

females:

PL = 612.6 {1 - [e.sup.[-0.220 (t 1.792)]]} (n=244, [r.sup.2]=0.726, P is less than] 0.001);

sexes combined:

PL = 602.1 {1 - [e.sup.[-0.207 (t 2.044)]]} (n=525, [r.sup.2]=0.699, P [is less than] 0.001).

Von Bertalanffy growth curves for sexes combined are depicted in Figure 7. No systematic trend was found in the residual plots for all regressions. The [t.sub.max] (age at 95% of asymptotic length) of T. lepturus and T. nanhaiensis were 15.1 and 12.4 years, respectively. The [W.sub.[infinity]] (asymptotic weight: estimated by substituting [PL.sub.[inifinity]] to the preanal length-weight equations) of T. lepturus and T. nanhaiensis were 2025 g and 2585 g, respectively. Log-transformed age-at-length regressions were significantly different between sexes (ANCOVA: T. lepturus: [F.sub.2,720]=4.39, P [is less than] 0.05; T. nanhaiensis: [F.sub.2,521]=23.78, P [is less than] 0.001) and species (ANCOVA: [F.sub.2,1245]=169.69, P [is less than] 0.001).

[Figure 7 ILLUSTRATION OMITTED]

Discussion

Our aging study of cutlass fishes from the South China Sea was successful in that we 1) found distinct growth rings on sectional sagittal otolith, 2) had excellent precision in independent ring counts, and 3) used marginal increment analyses to validate our aging method. In general, cutlassfishes from the northern seas of China (Misu, 1958, 1964; Hamada, 1971; Sakamoto, 1976; Hong, 1980; Wu et al., 1985a; Du et al., 1988; Hanabuchi, 1989; El-Haweet and Ozawa, 1996) and the South China Sea (our study) deposit annuli in late winter or early spring, suggesting that ring formation likely occurs in response to reduced water temperatures and is not correlated with peak spawning as indicated in Chen and Lee (1982). Summer is the peak spawning period of T. lepturus and T. nanhaiensis in the South China Sea (Kwok and Ni, 1999).