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

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

Abstract--Age and growth of two species of cutlassfishes, Trichiurus spp. (Trichiuridae), from the South China Sea were examined. Between December 1996 and November 1997, 1495 specimens were collected from coastal waters near Hong Kong. Two species, Trichiurus lepturus and T. nanhaiensis, were harvested and ages of specimens were estimated by using transverse sections of the sagittal otoliths. Opaque growth rings were verified to have formed annually during February. Lee's phenomenon was not observed for either species, although T. lepturus tended to display reverse Lee's phenomenon. Otolith weight was linearly related to age, and accounted for about 72% and 76% of the variation in age (t) for T. lepturus and T. nanhaiensis, respectively, comparable to the von Bertalanffy growth models in preanal length (PL). For older fish, otolith weight provided a more precise estimate of age than preanal length. Preanal length and age were fitted to the von Bertalanffy growth model by nonlinear regression, resulting in

PL (mm) = 589{1 - [e.sup.[-0.168 (t + 2.682)]]}

(T. lepturus);

PL (mm) = 602{1 - [e.sup.[-0.207 (t + 2.044)]]}

(T. nanhaiensis).

Growth in length of the two species was significantly different (ANCOVA, [F.sub.2,1245]=169.69, P [is less than] 0.001).

The cutlassfish, Trichiurus lepturus Linnaeus 1758, occurs throughout tropical and temperate waters of the world, between latitude 60 [degrees] and 45 [degrees] S (Froese and Pauly, 1997). World harvests are approximately 750,000 tonnes annually and China lands about 80% (600,000 tonnes) (Claus, 1995). In terms of weight, cutlassfish is the most important commercial marine fish species in China (Luo, 1991) and has accounted for about 10% to 20% of the total marine fish catch. It is caught in all Chinese seas, the Bo Hai, the Yellow Sea, the East China Sea, and the South China Sea (Jiang et al., 1991), and about 15% of the catch comes from the South China Sea (Fig. 1) (Liu, 1996). Cutlassfish is used as food fish and is caught mainly by bottom trawling (Luo, 1991) and in lower amounts by longline, hand line, gill net, drift net and purse seine (Chen and Liu, 1982).

[Figure 1 ILLUSTRATION OMITTED]

Age and growth studies and their derived growth parameters are indispensable in determining stock dynamics (Brouard et al., 1984). Numerous age and growth studies of T. lepturus have been conducted over the past few decades (Table 1); however, most research has focused on northern populations in the East China Sea, the Yellow Sea, and the Bo Hai. Similar work on populations in the South China Sea has not been available.

[TABULAR DATA 1 NOT REPRODUCIBLE IN ASCII]

Three species of trichiurids occur in the South China Sea, T. lepturus, T. nanhaiensis (Wang and Xu, 1992, in Wang et al., 1992), and T. brevis (Wang and You, 1992, in Wang et al., 1992), whereas only one species, T. lepturus, occurs in the waters of China farther north (Wang et al., 1992, 1993, 1994). Populations of T. lepturus in the Bo Hai, the Yellow Sea, and the East China Sea suffer from overfishing (Lin, 1985; Du et al., 1988; Ma and Xu, 1989; Luo, 1991; Ye and Rosenberg, 1991; Xu et al., 1994). It is harvested only in the Bo Hai and the Yellow Sea as bycatch in other fisheries (Lin, 1985). The condition of trichiurid stocks in the South China Sea remains unclear.

Numerous methods have been used to age trichiurids. The length-frequency method has proven useful in India (Narasimham, 1976; Chakraborty, 1990) and the Philippines (Ingles and Pauly, 1984). Yet, hard parts such as whole or sectioned otoliths and vertebral centra are most frequently used to age cutlassfish (Table 1). Measuring otolith weight or otolith size may be a cost-effective method for aging some fishes (Barbieri et al., 1994; Ferreira and Russ, 1994; Worthington et al., 1995). Although sectioned otoliths are reliable for aging fish, the method is time consuming and expensive (Beckman et al., 1991).

The aims of our study were 1) to validate age estimates by using transverse sections of sagittal otoliths; 2) to verify Lee's phenomenon; 3) to evaluate the potential of using otolith size and weight to estimate age; 4) to fit the age-length data to the von Bertalanffy growth model; and 5) to provide age-growth information for management of cutlassfish resources from the South China Sea.

Materials and methods

Between December 1996 and November 1997, 960 specimens (preanal length [PL] range:138-468 mm; PL=the tip of the lower jaw with the mouth closed to the middle of anus) of T. lepturus, and 535 specimens (PL range:253-551 mm) of T. nanhaiensis were obtained from commercial catches in the coastal waters of Hong Kong. Commercial gears included longlines, purse seines and bottom trawls. Fresh specimens were placed on ice, transported to the laboratory, and identified by using the diagnostic key of Wang et al. (1992, 1993): if the frontal bone split laterally, specimens were identified as T. lepturus, otherwise T. nanhaiensis. Preanal lengths were measured to the nearest mm. Specimens were blotted dry and weighed (whole and gutted) to the nearest 0.01 g. To estimate the relationship between PL and gutted weight (W), the variables were log-transformed to meet the assumptions of normality and homogeneous variance. A linear version of the power function: W (g) = a [PL.sup.b] (mm) was fitted to the data.