| The Pacific Spadenose Shark Scoliodon macrorhynchos(Bleeker,1852)(Carcharhinidae,Carcharhiniformes)is a small placental viviparous shark which distributes in western Pacific Ocean coastal waters.This shark has abundant resources in the southern Taiwan Strait and also is an important commercial fish.Sharks are 'K'selected life history strategists which can be characterized by low fecundity,slow growth,relatively large body size and bear large offspring.These biological characteristics render shark population particularly vulnerable to high motality.Studies have revealed severe depletions of some populations of elasmobranchs in different areas of the world since 1950s due to the overexploitation through targeted fisheries and incidental catches,habitat loss,and climate change.This study addressed age,growth and reproductive biology of Pacific Spadenose Shark in order to provide sicentific basis for adequate management and conservation measures.(1)246 males and 197 females,including 64 pregnant individuals,were used for reproductive biology research.The relationship between 'total length and body mass was estimated as BW = 3.7385 × 10-7 × TL3.3469 for female,and BW = 2.5182 ×10-6 × TL3.0330 for male.Size at 50%sexual maturity were 464 mm and 429 mm TL for males and females respectively.It has a well-defined annual reproductive cycle.Based on the monthly changes of GSI of males and the presence of females with fresh bites on their body surface and with spermozeugmata in their uteri,mating occurs from August to December.According to the monthly changes of embryo size,the pregnancy activity is carried out from January to August,starting from January,the near-term fetuses were delivered from June to August after 5-6 months gestation.The total length at birth is about 190mm.Litter size ranges 1 to 20(mean=9.41)and increases with maternal total length.Sex ratio of embryos was 1:0.86(n=195),not significantly different from 1:1(x2=2.262,p>0.05).Latitudinal difference in maturity length and maximum was found compared with relative researches.(2)191 males and 154 females were used for age and growth research.Vertebral centra were embeded and sectioned for band counting.Under transmitted light sections showed a bright and dark alternating pattern.Centra Edge Analysis indicated that the band-pair formed once a year,and the translucent bands were formed between June and September.Ages of shrarks ranged from 0 to 5 for males and 0 to 6 years for females.The AIC(Akaike's information criterion)indicated that the von Bertalanffy growth equition was the best fit on the data.The parameters of von Bertalanffy growth equation based on age and observed length were estimated as L?=823.53 mm,k=0.24yr-1,Lt = 823.527(1-e-0.244(t+3.239))for males,L?=866.74mm,k=0.25yr-1,Lt=866.744(1-e-0.253(t+2.840))for females.By comparing with the previous study of the same research area,the increasing growth rate and the miniaturization of body size of S.macrorhynchos were found,indicating its adaptive changes under high-intensity fishing. |
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