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Latin american journal of aquatic research

versão On-line ISSN 0718-560X

Lat. Am. J. Aquat. Res. vol.40 no.SpecIssue Valparaíso set. 2012

http://dx.doi.org/103856/vol40-issue3-fulltext-13 

Lat. Am. J. Aquat. Res., 40(3): 649-656, 2012

Research Article

 

Age and growth changes and population dynamics of the black pomfret (Parastromateus niger) and the frigate tuna (Auxis thazard thazard), in the Taiwan Strait

Cambios en la edad y el crecimiento y dinámica poblacional de la palometa negra (Parastromateus niger) y del atún fragata (Auxis thazard thazard), en el estrecho de Taiwán

 

Yu Tao1, Chen Mingru1, Du Jianguo2, Lu Zhenbin3 & Yang Shengyun1

1Department of Oceanography, Xiamen University, Xiamen 361005, China.
2The Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China.
3Fisheries Research Institute of Fujian, Xiamen 361012, China
.

Corresponding to:


ABSTRACT. The age and growth and population dynamics of the black pomfret, Parastromateus niger and the frigate tuna, Auxis thazard thazard were studied from specimens collected monthly in the Taiwan Strait from April 2005 to March 2006. With five age groups of each, the dominant black pomfret was one year, while frigate tuna was two years. The total mortality coefficient (Z) and the fishing mortality (F) of the black pomfret were at high levels, the exploitation ratio (E) was over 0.5, and the number of captured juvenile fishes showed that the stock of black pomfret in this area was overexploited and improper fishing gear was utilized. Compared with previous studies, the maximum and mean fork length, body weight and age of the two fishes declined gradually, the populations were younger in age and smaller in size. The growth coefficient k increased and age at the inflexion point of weight tr was younger than before. The smaller change of the fork length at first maturity of the two fishes has indicated a more vulnerable fishery ecosystem in this area. Therefore, in addition to the traditional fishery management approach, such as taking the minimum length into consideration, ecosystem approach to fisheries management (EAF) should be taken to improve the practical fishing management, including conserving the small pelagic fishery population in order to achieve the sustainable fishing of the stocks.

Keywords: Parastromateus niger, Auxis thazard thazard, age, growth, population dynamics, management, Taiwan Strait, China.


RESUMEN. La edad y crecimiento de la palometa negra, Parastromateus niger y del atún fragata o melva, Auxis thazard thazard se estudiaron a partir de especímenes capturados mensualmente en el estrecho de Taiwan, entre abril 2005 y marzo 2006. Se determinaron cinco grupos de edad, siendo dominante en la palometa negra los ejemplares de un ano, mientras que en el atún fragata los de dos anos. El coeficiente de mortalidad total (Z) y la mortalidad por pesca (F) de la palometa negra se encontraron en niveles altos. La tasa de explotación E resultó sobre 0,5 y el número de peces juveniles capturados demuestra que la población está sobreexplotada en la región y los equipos de pesca utilizados resultan inadecuados. En comparación con estudios previos, la longitud horquilla máxima y media, el peso corporal y la edad en ambas especies ha disminuido en forma gradual, presentando las poblaciones peces de menor edad y tamano. El coeficiente de crecimiento k aumentó y la edad en el punto de inflexión en peso tr se obtuvo en ejemplares más jóvenes que en anos previos. El pequeno cambio de la longitud horquilla determinado en la primer madurez en las dos especies indica que en esta zona, el ecosistema pesquero se encuentra más vulnerable. Por eso, además del enfoque tradicional en la gestión de la pesca, como la adopción de una longitud mínima, se requiere adoptar el enfoque ecosistémico a la ordenación de la pesca (EEP) para mejorar la conservación de la población de pequenos pelágicos y lograr así el desarrollo sostenible de los recursos pesqueros.

Palabras claves: Parastromateus niger, Auxi thazard thazard, edad, crecimiento, dinámica poblacional, manejo pesquero, Estrecho de Taiwán, China.


 

INTRODUCTION

The black pomfret, Parastromateus niger (Bloch, 1795), is a small pelagic fish that inhabit the shallower inshore waters of the Indian Ocean to the western Pacific Ocean, it is distributed off East Africa to southern Japan and Australia (Liu, 2008). The frigate tuna, Auxis thazard thazard (Lacepède, 1800), is a small pelagic tuna-like member of the family Scombridae which includes tunas, mackerels and bonitos. It is a highly migratory species with a worldwide distribution throughout all tropical and subtropical seas (Liu, 2008). In China, the black pomfret and frigate tuna are distributed in the East China Sea and South China Sea, they are more abundant in the Taiwan Strait, and are important for the lighting purse seine fishery in Taiwan, Fujian and Guangdong Province (Lu & Yan, 1985; Lu et al., 1991) . Studies on their age and growth and population dynamics are very important for analyzing the changing trend in fish populations and evaluating the level of fish being exploited.

The age and growth determination, biology and fishery of the P. niger have been studied in China (Lu & Yan, 1985; Lu et al., 1986; Yang et al., 2009), Philippines (Hernando, 1981), India (Khan et al., 1992) and Malaysia (Maran et al., 2009), and a few studies have reported on A. thazard in China (Lu et al., 1991; Lu et al., 1992; Zhang et al., 2006; Yan et al., 2009), India (Jones, 1957; Muthiah, 1985; Jude et al., 2002), Guinea (Voegborlo et al., 2007), USA (Uchida, 1981) and Australia (Hoese et al., 2006).

However, little information on the age and growth characteristics and population dynamics of these species is available, accordingly, adequate fishery assessment and management of the two fishes in the Taiwan Strait are impossible. This paper studies the age and growth characteristics of black pomfret and frigate tuna in the Taiwan Strait, and discusses the changes in population dynamics by comparison with previous studies.

MATERIALS AND METHODS

Sampling area

The black pomfret and frigate tuna were sampled by lighting purse seine monthly in the Taiwan Strait from April 2005 to March 2006 (Fig. 1). The Taiwan Strait is subtropical and it is an important interface between the East China Sea and the South China Sea, which is impacted by a branch of the Kuroshio Current and a South China Sea Warm Water current, the appropriate water temperature and salinity make this area a good habitat for the black pomfret and frigate tuna.


Age determination

The age of black pomfret was determined by their scales, as each scale contains concentric growth band pairs, including translucent and opaque bands. The scales were observed under microscope directly after rinsed and dried. While the age of frigate tuna was determined by their vertebrae, and the 4-5th vertebrae had the same band pair counts as those located in other locations and had the largest and exhibited sizes, these vertebrae were used for age determination in this study. Vertebrae were rinsed in 1% (winter) or 2% (summer) KOH for 1 day to remove connective tissue, washed in running water for 24 h, and then rinsed in glycerol for one day. A narrow band and a broad band appeared alternately on the vertebra surface. Radius of each centrum was measured to be 0.1 mm. The growth band pairs were examined under a microscope (60x). Growth band pairs were counted and measured without prior knowledge of the sex and length of the specimen. Counts were accepted only if both counts by two readers were in agreement. The distance from the focus to the outer margin of the translucent band of growth band pairs and the scale/vertebrae radius were measured as a straight line from the focus to the edge of the scale/vertebrae.

Biological parameters

The fork length (FL, in mm) and body weight (W, in g) of all the specimens were measured following the Specifications for Oceanographic Survey (Part 6: Marine Biological Survey, GB/T 12763.6-1991).

The mean values of gonadosomatic index (GSI) were used for the gonadal maturity analysis and first mature fork length determination of the two fishes.

where Wg is the gonad weight (g), Wp is the pure body weight (g), that is body weight without viscera weight.

The relationship between the fork length and the scale radius (R) was estimated by linear regression analysis. The length-weight relationship was determined by the following allometric equation (Sparre & Venema, 1992):

 

Then, the minimum length-limit (LR) can be calculated though the allometric equation stated above (Sparre & Venema, 1992).

RESULTS

TW-FL relationships

The total of 587 and 584 individuals were collected for black pomfret and frigate tuna, respectively. All the samples were measured in fork length and body weight. (Table 1). Both black pomfret and frigate tuna consisted of five year groups, the dominant group of black pomfret was one year, while frigate tuna was two years. The relationship between total weight and fork length is described as follows:

black pomfret:


The fork length and body weight growth rate curves of the two fishes could be analyzed by calculating the first derivative of von Bertalanffy growth equation. The fork length growth rates are curves decreasing with age until the curve approaches zero, while the body weight growth rates are parabolas first increasing with age until the age reaches tr (the age at the inflexion point of body weight), then decreasing with age (Fig. 2). The tr of black pomfret and frigate tuna are 1.67 and 2.00 years, respectively (Figs. 2 and 3).


The fork length and body weight acceleration curves of the two fishes could be analyzed by calculating the second derivative of growth functions. The body weight acceleration is positive before the age reaches tr. The body weight growth rate reaches the maximum value when the age reaches tr. The body weight acceleration is negative after the age has reached the tr when the body weight growth rate decreases (Fig. 3).


Mortality characteristics

The fishing mortality F of the black pomfret was much higher than that of the frigate tuna, and the exploitation ratio E of the black pomfret was more than 0.5, indicating that the black pomfret was exploited in the Taiwan Strait. Based on E, of the two fishes, the minimum length-limit of black pomfret and frigate tuna were calculated as 158.91 mm and 226.82 mm, respectively (Table 2).


The gonadial maturity and minimum mature fork length of the two fishes were also analyzed (Table 2). About 16.87% (black pomfret) and 19.65% (frigate tuna) landed from the lighting purse seine fisheries were lower than the minimum mature fork length, indicating that a large number of juvenile and young fishes died due to being caught.

DISCUSSION

Compared with the population structure of the black pomfret and the frigate tuna studied in the Taiwan Strait in the past years (Lu & Yan, 1985; Lu et al., 1991), the maximum and mean fork length, body weight and age of the two fishes decreased, the populations were younger in age and smaller in size (Table 1). For example, compared to that of 19821984, the mean fork length, body weight and age of Age jyears) frigate tuna have decreased from 349.0 mm, 662.9 g and 1.88 a to 321.5 mm, 622.3 g and 1.79 a respectively, indicating that the fisheries ecosystem has turned to be vulnerable.

The exploitation ratio E expressed the proportion of a given population that ultimately dies due to fishing under an existing exploitation pressure. Suitable E was related with natural mortality, fecundity and other factors, and different species have different suitable E. The suitable E was suggested to be between 0.308-0.500 or no more than 0.5, the balance of recruitment and catch would be disturbed if E was over 0.5 (Cushing, 1968; Gulland, 1971; Zhan, 2000). For black pomfret in the Taiwan Strait, F was at a high level, and E was over 0.5 (Table 2), and 16.87% of the catches were smaller than the minimum mature fork length, which indicated that the stock of black pomfret in this area was overexploited and the fishing gear was irrational.

The growth characteristics of the two fishes in the Taiwan Strait has also changed by comparison with previous studies (Lu & Yan, 1985; Lu et al., 1986, 1991, 1992). The growth coefficient k increased, and the age at the inflexion point of weight (tr) was younger than before (Table 2). The changes of the ecological characteristics were impacted by human disturbance, environmental changes and their biological characteristics, but the main factor was the overfishing (Luo et al., 1993; Lu et al., 1995). The overfishing of fish has taken much more of the recruitment, and resulted in significant declines of the population, especially the older members. This has moderated the feeding competition and made the younger ones grow faster.

Frigate tuna grows to almost 600 mm fork length and a weight of nearly 4,000 g, but they are more commonly 250-400 mm and 300-1,300 g in Australia (NSW, 2007), similar with that in the Taiwan Strait. The length at first maturity of frigate tuna in Thoothukkudi coast (southeast coast of India) was 328 mm and 308 mm for females and males respectively (Jude et al. , 2002), a little longer than that in the Taiwan Strait, which indicated that the frigate tuna in the Taiwan Strait and in the Thoothukkudi coast may be different populations due to geographic isolation, however, there is not much evidence to prove that, and further studies need to be carried out.

Black pomfret and frigate tuna in the Taiwan Strait are in the middle layer of the food web as the middle carnivores playing an important role in the food web of fishes in the Taiwan Strait (Zhang et al., 1981). The whole marine ecosystem structure may be subjected to impacts if the ecological characteristics and population dynamics of black pomfret and frigate tuna deteriorates. So, the protection and management of the resources should be intensified. As many juvenile and young fishes were captured in this area, the first capture length should be implemented. Based on this study, the suggested minimum length-limit of black pomfret and frigate tuna are 158.91 mm and 226.82 mm respectively (Table 2).

The ecosystem approach to fisheries (EAF) assessment and management has led to global acceptance and awareness of the limitation of traditional fisheries management (FAO, 2008). EAF is a new concept in fisheries management, which emphasizes integrated fisheries management from the perspective of the whole marine ecosystem, and the implementation of EAF may improve the fisheries management in China (Zhang & Mu, 2006; Tang & Zou, 2009; Chu, 2010). The spawning season of the black pomfret in the Northern Taiwan Strait is earlier than that in the Southern Taiwan Strait: May-June, June-July and July-August in Northern, Middle and Southern Taiwan Strait, respectively. In addition, the juvenile and young fishes are broadly distributed in shallower inshore waters, most of the black pomfret in August to October are smaller than the minimum length-limit, and they were often fished by the stake nets and small fisher (Lu et al., 1985). So, the spawning ground should be protected at the right time of the year and the fishing gears and methods should be improved too.

Frigate tuna in Taiwan Strait is carnivorous, its trophic level is 3.3 as it feeds mainly on fishes like Bregmacerotidae and Clupeidae (trophic level: 2-3), and in a minor level on Macrura (trophic level: 1.8) (Zhang et al., 1981). The small pelagic fishes occupy a large proportion in the diet. So, the whole fishery ecosystem, especially the small pelagic fishes should be protected first based on EAF. Meanwhile, there are many fish species that compete for food with frigate tuna, and many of them are non-economic species like Echenis naucrata. So it is necessary to increase the fishing intensity on the non-economic species and to reduce the feeding competitors of frigate tuna, a potential benefits for the sustainable use of frigate tuna stock.

ACKNOWLEDGEMENTS

The authors are grateful to Prof. Zhou Qiulin in the Third Institute of Oceanography, State Oceanic Administration for his valuable suggestions of this paper, and Dr. Zhu Kewei and Prof. Joel Thompson for their help on Spanish and English of this manuscript. And we thank all anonymous reviewers for their constructive comments which greatly improved this paper. Financial support of this study was from National Natural Science Foundation of China under contract N°31101902, The Scientific Research Foundation of Third Institute of Oceanography, SOA under contract N°2011006, Marine Science Foundation for Young Scientist under contract N°2011143, Marine Public Welfare Project of China under contract N° 201005012, 200905019-6 and 201105015.

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Received: 24 November; 2010 Accepted: 15 May 2012.

Corresponding author: Chen Mingru (mrchen@xmu.edu.cn) and Du Jianguo (dujianguo999@gmail.com)

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