Age and growth of the swordfish in the waters around Taiwan determined from anal-fin rays - Xiphias gladius L

Fishery Bulletin, Oct, 2002 by Chi-Lu Sun, Sheng-Ping Wang, Su-Zan Yeh

Abstract--Age and growth of the swordfish (Xiphias gladius) in Taiwan waters was studied from counts of growth bands on cross sections of the second ray of the first anal fin. Data on lower jaw fork length and weight, and samples of the anal fin of male and female swordfish were collected from three offshore and coastal tuna longline fishing ports on a monthly basis between September 1997 and March 1999. In total, 685 anal fins were collected and 627 of them (293 males and 334 females) were aged successfully. The lower jaw fork lengths of the aged individuals ranged from 83.4 to 246.6 cm for the females and from 83.3 to 206 cm for the males.

The radii of the fin rays and growth bands on the cross sections were measured under a dissecting microscope equipped with an image analysis system. Trends in the monthly marginal increment ratio indicated that growth bands formed once a year. Thus, the age of each fish was determined from the number of visible growth bands. Two methods were used to estimate and compare the standard and the generalized von Bertalanffy growth parameters for both males and females. The nonlinear least square estimates of the generalized von Bertalanffy growth parameters in method II, in which a power function was used to describe the relationship between ray radius and LJFL, were recommended as most acceptable. There were significant differences in growth parameters between males and females. The growth parameters estimated for females were the following: asymptotic length ([L.sub.[infinity]) = 300.66 cm, growth coefficient (K) = 0.040/yr, age at zero length (to) = -0.75 yr, and the fitted fourth parameter (m) = -0.785. The growth parameters estimated for males were the following: asymptotic length (L) = 213.05 cm, growth coefficient (K) = 0.086/yr, age at zero length (to) = -0.626 yr, and the fitted fourth parameter (m) = -0.768.

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The swordfish (Xiphias gladius L.) is a cosmopolitan species found in the tropical, subtropical, and temperate waters of the world's oceans and adjacent seas (Sakagawa, 1989). In the Pacific Ocean, swordfish is generally distributed from Asia to the Americas between 50[degrees]N and 50[degrees]S (Bartoo and Coan, 1989). In the waters of Taiwan, the swordfish is an incidental bycatch of the offshore tuna longline and harpoon fisheries. Both fisheries contributed an estimated 1528 metric tons (99%) to the total swordfish landings from Taiwan waters in 1999.

Information on age and growth of fishes is a central element in fishery management (Brothers, 1983). Measurement of the age of the fish provides the key variable of time needed to estimate life history and biology factors, such as mortality and growth. Mortality and growth-rate models provide quantitative information on the status of fish stocks and at the same time may be used in more sophisticated models, such as yield-per-recruit analyses and cohort analyses (Powers, 1983), which will directly contribute to the rational exploitation of fish resources, as well as to the development of proper management plans.

Most age determination studies of swordfish have dealt with Atlantic populations and have used different hard parts, such as anal-fin rays (Berkeley and Houde, 1983; Wilson and Dean 1983; Prince et al., 1988; Ehrhardt, 1992; Esteves et al., 1995; Ehrhardt et al., 1996), otolith (Radtke and Hurley, 1983; Wilson and Dean, 1983; Esteves et al., 1995), and vertebrae (Esteves et al., 1995). In contrast, only a few attempts have been made to determine the age of swordfish in the Pacific Ocean. Yabe et al. (1959) estimated the growth of swordfish caught in the western North Pacific (140[degrees]-160[degrees]E) by longline during the period from 1948 to 1956 using the modal analysis of length frequencies. Castro-Longoria and Sosa-Nishizaki (1998) compared the age estimates of swordfish caught by drift gillnet vessels off Baja California from 1992 to 1993 based on otolith microstructure and cross sections of the second ray from the first anal fin, and highly recommended the use of cross sections of the second ray to determine the ages of swordfish in the Pacific Ocean. Uchiyama et al. (1998) evaluated various hard parts (including rays of the first dorsal and first anal fins, vertebrae, and sagittae) for aging swordfish in the central North Pacific by Hawaii longline fishery from 1991 to 1993, and provided preliminary estimates of length-at-age.

The objectives of our study were to estimate the age and growth of swordfish by counting the growth rings on the cross sections of the second ray of the first anal fin and to compare the generalized growth function proposed by Richards (1959) with the standard von Bertalanffy model for representing the best growth model of swordfish around Taiwan waters. The information is crucial because it will allow the age composition of the catch to be determined, which in turn will allow the status of the swordfish stock in the waters around Taiwan to be assessed by using yield-per-recruit and sequential population analyses.