Abstract-The reproductive biology of Yellowfin Tuna (Thunnus albacares) in the western Indian Ocean was investigated from samples collected in 2009 and 2010. In our study, 1012 female Yellowfin Tuna were sampled: 320 fish on board a purse seiner and 692 fish at a Seychelles cannery. We assessed the main biological parameters that describe reproductive potential: maturity, spawning seasonality, fish condition, and fecundity. The length at which 50% of the female Yellowfin Tuna population matures (L 50 ) was estimated at 75 cm in fork length (FL) when the maturity threshold was established at the cortical alveolar stage of oocyte development. To enable comparison with previous studies, L 50 also was estimated with maturity set at the vitellogenic stage of oocyte development; this assessment resulted in a higher value of L 50 at 102 cm FL. The main spawning season, during which asynchrony in reproductive timing among sizes was observed, was November-February and a second peak occurred in June. Smaller females (<100 cm FL) had shorter spawning periods (December to February) than those (November to February and June) of large individuals, and signs of skipspawning periods were observed among small females. The Yellowfin Tuna followed a "capital-income" breeder strategy during ovarian development, by mobilizing accumulated energy while using incoming energy from feeding. The mean batch fecundity for females 79-147 cm FL was estimated at 3.1 million oocytes, and the mean relative batch fecundity was 74.4 oocytes per gram of gonad-free weight. Our results, obtained with techniques defined more precisely than techniques used in previous studies in this region, provide an improved understanding of the reproductive cycle of Yellowfin Tuna in the western Indian Ocean.Knowledge of reproductive traits is important for understanding population dynamics, including a population's resilience to fi shing (Schaefer, 2001;Murua and Motos, 2006;Morgan et al., 2009). As alternatives to the traditional spawning stock biomass (SSB), reproductive potential indices have been proposed in which basic reproductive parameters are included as important factors that affect population productivity (Trippel, 1999;Morgan et al., 2009). These parameters include sex-ratio, the age and size of females, maturation ogive, fecundity, fi sh condition, and reproductive history. Inclusion of these biological parameters allows integration of fl uctuations in a population's reproductive success into the assessment and management processes, in addition to estimation of spawning stock biomass (SSB) Murua et al., 2010). Hence, to improve the assessment and management of stocks, it is necessary to increase the quality and quantity of the basic reproductive data used to estimate these reproductive parameters .Yellowfin Tuna (Thunnus albacares) is one of the major target species of the tuna fi shery in the Indian Ocean. Total annual catch of Yellowfi n Tuna in the Indian Ocean has increased signifi cantly, since the early 1980s, with the advent of the purseseine...

Total lipid content and lipid class composition were analyzed in gonads, liver and white muscle of yellowfin tuna (Thunnus albacares) throughout ovary development to understand its reproductive allocation strategy and to assess the relation between female condition and reproduction. A total of 112 females were collected onboard purse-seiner in the Western Indian Ocean from January to March 2009, from June to July 2009, and from April to May 2010. Gonads were characterized by highly variable total lipid contents ranging from 5 to 27 μg mg −1 of wet weight (ww) with a predominance of neutral lipids, mainly triacylglycerols (TAG) and sterol-and wax-esters. The different lipid classes in gonads described an accumulative pattern through the maturity process from immature to hydration phase. Total lipid content in liver varied from 10 to 21 μg mg −1 ww, and serves as fuel for yellowfin tuna reproduction. TAG and phospholipid deposits became depleted as the ovary developed, suggesting a transfer of lipids directly from liver to the oocytes during vitellogenesis. In contrast, muscle total lipid content was low and constant throughout ovarian development (2.5-6 μg mg −1 ww). Hence, yellowfin tuna can be defined as an income-capital breeder species for which the cost of reproduction depends mainly on concurrent energy income from feeding and only little on stored lipids. Besides, no significant relationship between gonad lipid composition and fecundity was found in females able to spawn. Finally, the influence of yellowfin tuna aggregation behaviour on reproductive female condition has been investigated: gonad total lipid contents were higher in females caught in free-swimming schools than in females caught under fish aggregating devices (FADs). However, these results did not clarify whether the influence of FADs on associated yellowfin tuna affects their reproductive capacity.

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