Age,Growth and Maturation of Largehead Hairtail(Trichiurus japonicus) in the East China Sea

CHENG Shuo, SUN Peng, *, LIU Yang, CHEN Qi, SHI Zhengtong, and SUN Runlong

Age,Growth and Maturation of Largehead Hairtail() in the East China Sea

CHENG Shuo1), SUN Peng1), *, LIU Yang1), CHEN Qi2), SHI Zhengtong1), and SUN Runlong1)

1),,266003,2),,266003,

Largehead hairtail ()is a commercially important fish speciesin China Seas, characterized by wide distribution, seasonal migration, spawning all year round. Under long-term high fishing pressure, the biological traits of largehead hair- tail have changed, such asminiaturization, smaller size-at-age and earlier age-at-maturation.To explore its current biological charac- ters, we analyzed age, growth and maturation of largehead hairtail in the East China Seaby analyzing the otolith microstructure and gonads. The results showed that largehead hairtail is mainly composed of 1 year and 2 years old individuals. The estimated von Ber- talanffy growth function (VBGF) showed significant differences between sexes, and asymptotic pre-anal length of male is longer than that of female. Two spawning seasons were observed in spring and autumn by monthly gonadosomatic index ().Compared to historic survey data, the average pre-anal length at age decreased and size-at-maturation became smaller in the East China Sea. This study updates life history parameters of largehead hairtail, and provides a reference for the present status of largehead hairtail, which can contribute to largehead haritail stock assessment and sustainable utilization in the China Seas.

;East China Sea;age; growth; maturation

1 Introduction

Largehead hairtail () is a commer- cially important fish species, and widely distributed in the China Seas (Ling., 2005; Liu., 2009). As it has been extensively exploited since the 1950s, the catch shows an increasing trend and maintains more than 100×104t in recent years, accounting for 80% of largehead hairtail catch in the world (Ling., 2005; Wang., 2011; Xu and Chen, 2015).The East China Sea is primary spawning and feeding ground for largehead hairtail, and largehead hair- tail mostly inhabits the coastal area off Zhejiang Province. With the strengthening of Taiwan Warm Current and the in- creasing of sea temperature, largehead hairtail migrates from northeast to west, reaching to Zhoushan Fishing Groundin March and April, and finishes spawning migration in this period (Xu and Chen, 2015). Based on the hatching datedata, largehead hairtail spawns almost year round with two dominant spawning periods in the East China Sea (Li, 1982; Shih., 2011; Sun., 2020). The females spawn mul- tiple times in each spawning season, which contributes to the new born ?sh almost every month (Kwok and Ni, 1999). The year-round spawning seasonality of largehead hairtail can allow this species to thrive under long-term fishing pres- sure with sustained high yield (Sun., 2020).

The East China Sea is one of the typical over-exploited ecosystems in the world where most of fish populations are overfished under long-term sustained fishing pressure. Meanwhile, thebiological traits of exploited fish popula- tions have changed, such as miniaturization, smaller size- at-age and earlier age-at-maturation (Deng and Zhao, 1991;Yan and Chen, 2000; Lin., 2004; Li., 2015). Some evidences support the changes of biological traits in the exploited populations. The average length of Japanese Spanish mackerel () decreasedabout 6cm in recent 30 years, and the small yellow croaker () showed the miniaturization and early-maturation in the East China Sea (Deng and Zhao, 1991; Lin., 2004). Analyses of long-term data of large- head hairtail reveal that the biological traits of largehead hairtail have changed under high fishing pressure since 1960s, and these changes are primarily reflected in minia- turization and early maturation (Mi, 1997; Zhou., 2002; Ji., 2019). Long-term fishing pressure also has result- ed in variations of species diversity and community struc- ture in the regional ecosystem (Cheng and Yu, 2004; Cheng., 2006). Changes of body size and related life history traits can have important repercussions for species and community ecology, management reference points, popu- lation productivity, and fisheries yield (Law and Grey, 1989; Hutchings and Fraser, 2008; Heino., 2013).Meanwhile, many researches have addressed important in- formation of largehead hairtail including age and growth,mortality parameters, catch and effort data of fishery, re-sources and stock structure, reproductive habitat and feed- ing habitat around the East China Sea in the past (Wu.,1985; Mi, 1997; Zhou., 2002; Yan., 2005; Wang., 2011; Chen., 2013).Therefore, an updated as- sessment of life history parameters for largehead hairtailis necessary for management and sustainable utilization of largehead hairtail.

To update the assessment of life history parameters and stock statusoflargehead hairtail, we obtain biological data about length and age oflargeheadhairtail collected by bot- tom trawls in the East China Sea. Demographics (size and age structures) and life history parameters (growth, matu- ration) are analyzed to identify appropriate management units for largehead hairtail. The purposes of this paper are to 1) estimate updated life history parameters oflargehead hairtail in the East China Sea; 2) compare these parameters with previous studies to understand the changes of biolo- gical traits under long-term fishing pressure; and 3) provide recommendations for developing appropriate fisheries ma-nagement strategies.

2 Materials and Methods

2.1 Sample Collection

Samples were collected by bottom trawls from the Zhou-shan fishing ground (29?30?–31?00?N, 120?30?–125?00?E) in the East China Sea (Fig.1).The height of net mouth of the bottom trawl was 7.5m, and the horizontal spreading ofnet mouth was about 15.0m. The mesh size of codend was25mm. Subsamples were randomly selected and taken back to the laboratory. A total of 598 largehead hairtail were captured from January 2017 to December 2018 (Table 1).

Fig.1 Sampling area (the dashed box, Zhoushan fishing ground)of largehead hairtailin the East China Sea.

Table 1 Sampling information of largehead hairtailcapturedat the Zhoushan fishing ground

Pre-anal length, total weight, gonad weight, sex, and se- xual maturity for individuals were measured according to ‘Specifications for Oceanographic Survey-Part 6: Marine Biological Survey (GB/T 12763.6-2007)’in the laboratory.Sex and sexual maturity stages were determined using ma-croscopic observation index of male and female gonads (Su,1982; Wu, 1984). The individuals with visible gonads were classified as mature fish.The mature fish with eggs in the gonads were classified as females, otherwise the fish were regarded as males. Individuals with no visible gonads were classified as immature and of unknown sex. In this study, pre-anal length (PL) data were used to conduct the analy- sis.

2.2 Age Estimation

We used the Sturge Method (Chen., 2008) to esti- mate an appropriate length frequency interval,which wasdetermined to be 20mm in this study. Then our survey sam- ples were partitionedinto length intervals for the purpose of random selection of aging samples. Totally 84 otoliths were selected for section by random sampling of length in- tervals. Otoliths were embedded individually in epoxy re- sin (Spurr, 1969; Neuman., 2001).A transverse sec- tion (0.5mm thick) was cut through the otolith core usinga low-speed sawwith high precision (Buehler IsoMet witha diamond edged wafering blade) (Wu., 1985). The section was mounted on a microscope slide and grounded with waterproof abrasive paper until annual marks were clearly visible. The digital image of each section was cap-tured using a Nikon microscope fitted with a digital ca- mera (Fig.2). Under transmitted light, otolith sections dis-played a clear pattern of opaque and translucent zones.The opaque zones were considered annual rings, and ring num- ber was adopted as an indicator of age in years (Wu., 1985). The first annual ring tended to be located at or be- yond the half distance from the core to the edge (Wang and Heino, 2018). Age was read with asoftware-Image J which provided the increment width. Each sagittal otolith section was read twice with no knowledge of fish size or time of capture by the same reader. When both counts were identical, the value was taken as the final age estimate. In case the two readings are inconsistent, a third reading was taken to determine a final age estimate.

Fig.2 Sagittal otolith section of a largehead hairtail with 4 years old. The image displays black dots on the four translucent zones plus one red dot on the core.

2.3 Length-Weight Relationship

The length-weight relationship was estimated by the pow- er function model:

whereis the total weight (g),is the pre-anal length (mm),is the condition factor andis the allometric growth factor (Froese, 2006; Froese., 2011). The length-weight relationships for females and males were estimated by least square linear regressions after log transformation and2(coefficient of determination) (Froese, 2006; Froese., 2014). All analyses were carried out using Micro- soft Excel 2016.

2.4 Growth Equation

Growth parameters were estimated for population usingthe von Bertalanffy growth equation (von Bertalanffy, 1938).Because thegrowth showed significant difference betweendifferent sexes (Shih., 2011), we fitted this model se- parately for males and females. The von Bertalanffy growth model in terms of length is typically expressed as:

whereis the age(years) of sample,0is the hypothetical age at length 0,Lis length at age,∞is asymptotic length (mm), andis the Brody growth coefficient. And the growth rate for length was calculated by the equation:

The growth equation was fitted using least-squares es- timation procedures embedded in the SOLVER routine thatwas contained in the Microsoft Excel software (Haddon, 2001).Residual sum of squares (ARSS) was used to sta- tistically compare fitted growth curves between sexes (Chen., 1992; Haddon, 2001). All analyses were carried out using Origin 2018 and Microsoft Excel 2016.

2.5 Sexual Maturity

Gonads were collected from 243 females,and sexual maturity stages were determined with Wu’s 6-staged ma- turity scale(1984):

Stage I (immature): Gonads are very small, ova invisi- ble to the naked eye.

Stage II (immature): Gonads are small, dully transparent and pinkish-whitish.

Stage III (ripening): Gonads are enlarged. Ovary is or- ange-yellow with granular appearance.

Stage IV (ripening): Gonads are enlarged. Ovary is pale yellow with large transparent ova and small white ova.

Stage V (ripe): Gonads are considerably enlarged. Ovary is orange-pink with conspicuous superficial blood vessels. Ripe ova are visible and large and transparent.

Stage VI (spent): Gonads are shortened, walls loose, flab- by, empty, dark red with traces of ova.

2.6 GonadosomaticIndex (GSI)

Gonads were collected from 243 females, thewas calculated by the equation (Deng and Zhao, 1991):

Thefor females showed no significant variation in the same month in different years based on ANOVA (>0.05).Thus we analyzedmonthly changes from Ja- nuary 2017 to December 2018.

3 Results

A total of 598 largehead hairtail are analyzed in the la- boratory, including 243 femalesand 355 males. The sex- ratio (males/females) is approximately 1.46:1. Otoliths are selected for section by random sampling of length inter- vals.

3.1 Length Frequency Distribution and Age Composition

The pre-anal length ranges from 127 to 437mm in fe-males and from 124 to 343mm in males (Fig.3a). The scopes of distribution on pre-anal length are 331mm for females and 219mm for males. Pre-anal length distribution in fe- males (225.6mm) is wider than that in males (207.2mm). The distributions of pre-anal length are significantly dif- ferent between males and females. The dominant pre-anal length intervals of females (200–240mm, 50.62%) and males (180–220mm, 52.11%) both show a single peak.

Fig.3 Frequency distribution of the pre-anal length (a) and age composition (b) of largehead hairtail (Trichiurus japo- nicus).

The age-at-capture of 84 samples is determined by count- ing opaque zones of otolith sections (Fig.2). Among them, 43 otoliths show that the age distribution of females va- ries from 1 to 6, and the other 41otolith samples show the age distribution ranges from 2 to 5 for males (Fig.3b). Do- minant age-at-capture both is age-2 (2-year-old) for bothfemales (32.56%) and males (36.59%).

3.2 Length-Weight Relationship

The relationships between(mm) and(g) for fe- males and males are as follows (Fig.4):

Fig.4 Length-weight relationship of largehead hairtail(Tri- chiurus japonicus).

3.3 Growth Equation

The growth between sexes shows significant difference(ARSS,2.08,0.05) (Fig.5; Table 2). The growth equation isL=442.9(1–e–0.2154(t+1.003)) for females, andL=491.5(1–e–0.1434(t+1.356)) for males. The asymptotic length for males is 491.5mm, greater than females of 442.9mm (Table 2). Before six years old, the pre-anal length and growth rates of females are greater than those of malesat the same age (Fig.5). The asymptotic length and theaverage pre-anal length in our study are smaller than his- toric survey data (Table 2; Table 3).

Fig.5 The relationship between pre-anal length and age of largehead hairtail (Trichiurus japonicus).

Table2 Parameters of von Bertalanffy growth function (VBGF) and growth rate (dL/dt) of largehead hairtail (Trichiurus japonicus) in four different time periods

Notes:L, the pre-anal length (PL) in mm; F, female; M,male; Age-2, 2-year-old; Age-5, 5-year-old.?, this study.

Table3 Comparisons of average pre-anal length in four different time periods

Notes:?, this study; –, not available.

3.4 Gonadosomatic Index (GSI)

shows two peaks in Fig.6.reaches the first peak in March and April, followed by decreases until August, then the second peak is in October.

Fig.6 Monthly variation in gonadosomatic index (GSI) for females of largehead hairtail (Trichiurus japonicus).

3.5 Maturation

Comparing to historic survey data, the minimum pre- anal length at maturation for females decline in the past 60 years. In the 1960s, the minimum pre-anal length at matu- ration of largehead hairtail was 200–210mm. It decreasedto 135–140mm in the 2000s, and the minimum pre-anal length at first maturation of largehead hairtail is found to be 127–135mm for females in this study. Therefore, it is estimated that the minimum pre-anal length at first matu- rity of largehead hairtail females has decreased over 65mmsince 1960s (Fig.7).

Fig.7 Comparisons of minimum pre-anal length at first ma- turation of largehead hairtail (Trichiurus japonicus) in dif- ferent time periods (Historical data source: Luo et al., 1983;Wu, 1984; Xu et al., 2003; Lin et al., 2008).

4 Discussion

This study provides new information to the life history traits of largehead hairtail in the East China Sea. These re-sults have demonstrated that the biological traits of large- head hairtail have been changed under long-term fishing pressure, and the principal changes include miniaturization, smaller size-at-age and earlier age-at-maturation. These changes are helpful to understand the stock status under over exploitation in East China Sea, and support recom- mendations for fishery management.

The growth of largehead hairtail show difference between female to male (Fig.3). The age distribution of largehead hairtail was 0–7 years old in 1960s while that was 0–4 years old in 2000s (Wu, 1985; Yan., 2005). The primary age structure of largehead hairtail is composed of 2 years old individuals. The VBGF differ significantly between sexes, while females have a greater growth rate and the asympto-ticpre-anal length for males is greater than females (Table 2; Fig.5). This result is consistent with the result of Shih. (2011). Males attain larger asymptotic pre-length than females in this study, indicating that females might expend more energy on reproduction than males (Love, 1980). Theasymptotic length and the average pre-anal length at age in our study are smaller than historic survey data (Table 2; Table 3), which supports the theory that growing trends to miniaturization and smaller size-at-age under long-term high fishing pressure. Meanwhile, the growth rate of length is also lower than historic survey data (Table 2).The rea- son might be an error in the fitting of the growth equation or samples differences. It is therefore imperative to con- duct more studies to better compare the change in growth rate.

Based on the monthly variation infor females, we obtain information on maturation of largehead hairtail (Fig.6). Previous studies on the gonad of females gave the evidence that largehead hairtail may spawn multiple times within each spawning season (Kwok and Ni, 1999). Theshows two peaks within one year,while the activity seasons spring and autumn are same to the main spawn- ing cohorts according to the data of largehead hairtail hat- ching date (Li, 1982; Shih., 2011; Sun., 2020). Multiple spawning a year of largehead hairtail may be the reason why this species can sustain high yield even under long-term fishing pressure, and also can explain the increa- sing catch of largehead hairtail in recent years. Meanwhile, the minimum pre-anal length at first maturation of large- head hairtail females is over 65mm shorter than that of 1960s (Fig.7). These results demonstrated that the biolo- gical traits of largehead hairtail have been changed, inclu- ding miniaturization, smaller size-at-age and earlier age- at-maturation. Some evidences showed that distribution of the body length-at-age of largehead hairtail were irrever- sibly changed under long-term trawl fishing in the East China Sea (Marty., 2015;Sun., 2015). The pre- dicted results of largehead hairtail are identical with our results response to over exploitation sea area. Moreover, en-vironmental pressure may also be the reason for the changesof biological traits.Ectotherms exposed to increasing tem- peratures often display smaller size at maturity and reduc- tion in maximal body size, known as the ‘temperature-sizerule’ (TSR) (Atkinson, 1994). Climate warming will result in smaller body size, higher mortality and earlier matura- tion (Wang., 2020). Wang. (2020) showed po- sitive effects of temperature on body growth, reproduc- tive rates, and natural mortality for 332 species of Indo- Pacific fishes, with the effect strength varying among habi- tat-related species groups. In addition, the environmental variables contributed to growth of some fish species, espe- cially in the early growth pattern and their response to en- vironmental variables (Koenker., 2018; Sun., 2020). However, this study has certain limitations, such asdifferences in the number of samples between months, small number of samples in some months, and combined analysisof spring- and autumn-spawned cohort, which should col- lect more samples in the further research.

Based on the change of biological traits, scientific andsuitable fishery management approach need to be proposed to protect largehead hairtail resource. Seasonal fishing mo-ratorium, marine protected areas and individual quotas canserve as effective tools for the fishery management and conservation of over exploited marine species. For exam- ple, northern Atlantic cod () showed signs of recovery under the several decades moratorium, which was a good case study for fishery resources management (Ku-parinen and Uusi-Heikkil?, 2020). In addition, a straight- forward measure is to carefully control fishing mortality to keep it ecologically sustainable and economically optimal (Eikeset., 2013; J?rgensen and Zimmermann, 2015). More reasonable and effective approaches should be de- veloped in order to ensure the sustainable management andutilization of largehead hairtail resources in the over ex- ploited China Seas.

Acknowledgements

The research is supportedbythe National Key R&DProgram of China (No. 2018YFD0900902), and the Na- tional Natural Science Foundation of China (No. 418611 34037). We appreciate the assistance provided by the Institute of Oceanography in National Taiwan University and Department of Marine Biotechnology and Resources in National Sun Yat-sen University.

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