Reproductive Biology Of Skipjack Tuna, Katsuwonus Pelamis, In The Western And Central Pacific Ocean

Skipjack tuna (Katsuwonus pelamis) is the main targeting specie of tuna fishery and makes greatcontribution to tuna fishery worldwide, especially in the Western and Central Pacific Ocean. As theincreasing amount of tuna purse seine fishing vessels in the recent years, skipjack tuna resourceconservation has become an important topic. Study on reproductive and biological characteristicsdirectly provides reference for the forecast of population growth level and stock abundance.Affiliation study among the gonad development level, migration and early development history wasmade to determine fisheries management measures such as fishing season closure, area closure andcatchable size; those are important guideline in the resource management and protection.Investigation in this paper provides a reliable data base. Fisheries management measures can bemade after analysis of the association between development and growth characteristics.The ovary and testis samples were collected from October2007to January2008and fromFebruary2010to May2011in the Western and Central Pacific Ocean. Histological analysis on thesamples was made to observe skipjack gonads in order that we can have a detailed description of thegonad development characteristics for skipjack tuna. Statistical analysis methods was used to studythe first sexual maturity fork length,50%sexual maturity, breeding season, the gonad maturationlevel, and sex ratio. The main conclusions are as follows:(1) According to one-way ANOVA test, there is a significant difference in the body lengthcomposing between male and female skipjack tuna(F=35.997, P＜0.01). The range body length forfemale is from27.9cm to72.8cm, the dominant body length range is41.5~54.0cm. The range bodylength for male is from33.2cm to74.6cm, the dominant body length is44.0~56.5cm.(2) According to one-way ANOVA test, there is also a significant differences in body weightcomposition (F=20.6, P＜0.01), The range body weight for female skipjack is from0.40kg to9.91kg, with the average of1.88kg±0.09kg (confidence interval is95%), the dominant body weightgroup is0.78~2.70kg(n=414). The range body weight for male skipjack tuna is from0.56kg to9.17kg, with a average of2.23kg±0.13kg (confidence interval is95%), the dominant body weight is1.26~3.18kg(n=296).(3)Based on ANCOVA (analysis of covariance) test, it showed that relationship between forklength(cm) and body weight(kg) of skipjack tuna in the Western and Central Pacific Ocean could bedescribed as follows: W=5.513×10~6L3.307(r~2=0.9593,n=954).There are no significantdifferences between female and male skipjack tuna (F=0.027, P>0.05).(4) Histologically, the development process of skipjack tuna oocyte is divided into six phases.in phaseⅠ, mitosis occurs actively, the number of cells increased rapidly; from phase Ⅱ to Ⅳ, theaccumulation of cytoplasmic substances happens gradually, yolk and fat bubble increase slowly; inthe Ⅴ phase, nuclear moves to the animal pole of the mature oocyte, little fat vesicles fuse into abig one. Not all the mature oocytes spawned, those did not spawn are degraded, cytoplasm wasre-absorbed, gradually shrinking. (5)According to the distribution of dominant oocytes, ovary development process is dividedinto six periods. Ovary in phase Ⅰis too small to distinguish sex when observed by naked eyes. Inthe Ⅳ stage oocytes can be seen clearly,in stage Ⅴ, if abdomen was squeezed slightly, fluid willoutflow. In each maturation level of skipjack tuna ovary, those modality indexes, such as body length,body weight, gonad weight, and other macroeconomic indicators have a large range, and they havesignificant dispersion. So all the indexes including oocyte cell growth conditions, gonad stage indexand morphology characteristic should be considered when we make judgment about the maturationlevel.(6) Skipjack tuna is a type of multiple spawning species. Its characteristics are: varied oocytephases can be observed in one ovary at the same time, especially in stage Ⅴ and VI; Interval timebetween the spawning peek during continuous breeding time is short.(7) Skipjack tuna testis is a type of tubular (also known as radiation type), regularly alignedtubular structures was radiating. Testicular development process is divided into six periods. In stageⅤ, if abdomen was squeezed slightly, semen fluid will outflow.(8)There are many differences when contrasting visual method with histological analysis forjudgement of skipjack ovarian development status, especially when the ovary is in phase Ⅵ.When the ovary is in Phase Ⅰ, gonad look like slender, naked eye can not distinguish sex, onlythrough tissue sections experimental observation, cell development status can be determined.(9)During sampling time, in the investigation waters there are at least two peak spawningperiod, the end of November (early December) and the end of December (early January). FromFebruary to May, the maturation level is in a low level, with no obvious peaks and troughs.Spawning period for male skipjack tuna show no difference with female.(10)Based on the combination of visual observation and analysis of tissue sections for samplesfrom Western and Central Pacific, the smallest mature body length of skipjack tuna is30.8cm formale, and46.7cm for femal,50%maturity body length is42.4cm for male, and58.2cm for female.(11) In Western and Central Pacific Ocean, batch fecundity range of skipjack tuna is from140,648.65to906857cfu with a average of (739,278.53±159,433.03) cfu (confidence interval is95%), the relative fecundity range is from29.56to172.10cfu/g, with a average of (189.40cfu/g±28.2454)(confidence interval is95%). According to the Pearson Correlation Test, the relationbetween batch fecundity and length is positively correlated (correlation coefficient is0.7778), andthe relation between batch fecundity and weight is also positively correlated (correlation coefficientis0.8126), the relation between relative fecundity and body length also has a positive correlation(correlation coefficient is0.4190), for weight is also positively correlated (correlation coefficient is0.4024).(12) When the body length is from33to41cm, sex ratio is significantly different (p <0.05),when the body length is between41and61cm, there is no significant difference among sex ratio (p>0.05). As the body length is small, female skipjack tuna account for dominant amount, with the bodylength increased, male individuals has a dominance significant.