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

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

We assigned 1 May and 1 June as the birth dates for T. lepturus and T. nanhaiensis, respectively (Kwok and Ni, 1999). Relative ages derived from aging were then converted to absolute ages. Von Bertalanffy growth curves were fitted by nonlinear regression on age and preanal length data (SPSS vers. 7.5). The von Bertalanffy growth equation for length is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

where [PL.sub.[infinity]] = the asymptotic length;

k = growth coefficient; and

[t.sub.0] = the hypothetical age at zero length.

Plots of residuals from regression models were used to check the assumption of normality. ANCOVA was used to compare log-transformed age-at-length regressions between sexes and species.

Results

The preanal length (mm) and gutted weight (g) regression models were significantly different between sexes (ANCOVA: T. lepturus: [F.sub.2,932]=4.00, P [is less than] 0.05; T. nanhaiensis: [F.sub.2,530]=3.34, P [is less than] 0.05) and species (ANCOVA: [F.sub.2,1466]=83.76, P [is less than] 0.001). The regression models were

T. lepturus

males:

W = 1.513 x [10.sup.-4] [PL.sup.2.571] (n=212, [r.sup.2]=0.9777, P [is less than] 0.001);

females:

W = 1.748 x [10.sup.-4] [PL.sup.2.549] (n=724, [r.sup.2]=0.9761, P [is less than] 0.001);

sexes combined:

W = 1.624 x [10.sup.-4] [PL.sup.2.561] (n=936, [r.sup.2]=0.9771, P [is less than] 0.001);

T. nanhaiensis

males:

W = 3.363 x [10.sup.-5] [PL.sup.2.846] (n=282, [r.sup.2]=0.8506, P [is less than] 0.001);

females:

W = 6.553 x [10.sup.-5] [PL.sup.2.729] (n=252, [r.sup.2]=0.9299, P [is less than] 0.001);

sexes combined:

W = 5.672 x [10.sup.-5] [PL.sup.2.755] (n=534, [r.sup.2]=0.8968, P [is less than] 0.001).

Sectioned sagittae of both species had an opaque nucleus located above the sulcal groove toward the dorsum. The nucleus was surrounded by a pattern of alternating wide, translucent zones and thin, opaque zones; the latter were considered annuli (Fig. 3). Annuli were distinct on the dorsum of the sections but were usually indecipherable on the ventral side. A total of 757 and 534 otoliths were embedded in resin and sectioned for T. lepturus and T. nanhaiensis, respectively. Of these, 33 (4.3%) and 9 (1.7%) were unreadable, and the percentage agreement between the two readings for each species was 95.4% and 92.7%, respectively.

[Figure 3 ILLUSTRATION OMITTED]

The least marginal increment values (Fig. 4) occurred in February for both species, suggesting that one growth ring (annulus) formed each year. Only specimens age 1-4 were included in the analyses because older fish were rare in our collections. The mean otolith annular radius (MOAR) of the first annulus (ANOVA: [F.sub.5,582]=3.046, P [is less than] 0.05) and third annulus (ANOVA: [F.sub.3,80]=4.024, P [is less than] 0.05) of T. lepturus were significantly different among different age groups (Fig. 5); MOARs increased slightly with older age groups. However, no particular trend was found with regard to the MOARs of T. nanhaiensis (Fig. 5). Lee's phenomenon was not evident for either species, although reverse Lee's phenomenon was possible for T. lepturus.