Size structure and reproductive biology of skipjack tuna Katsuwonus pelamis and kawakawa Euthynnus affinis in the western Indian Ocean

Abstract

Skipjack tuna Katsuwonus pelamis and kawakawa Euthynnus affinis are economically important tuna species targeted by artisanal fishers in the western Indian Ocean. Both species are currently considered sustainably fished despite substantial catches. Even so, ongoing management is crucial to maintain healthy populations. A comprehensive understanding of the size structure and reproductive biology of these tuna species across a broad spatial scale is essential for accurate stock assessments. This study investigated spatial and gear-based seasonal variations in the size structure of landed catches and describes the reproductive traits of the two species, including spawning season, size at maturity and sex ratio. Data were collected from the artisanal fishery in Kenya, Tanzania and Mozambique, and the recreational fishery in South Africa. The results revealed seasonal variations in size structure for both species, potentially influenced by fishing methods and local environmental conditions. The landings were biased towards male fish for both species, with sex ratios of 58% and 53% males for skipjack and kawakawa, respectively. Size at maturity (L ₅₀) was estimated at 42.0 cm fork length (FL) for skipjack females and 47.0 cm FL for males, and kawakawa at 44.0 cm FL for females and 45.3 cm FL for males. At all study sites, spawning occurred year-round, with peak spawning activity from October to March, which coincides with the northeast monsoon. The findings provide updated information on the reproductive demographics of skipjack and kawakawa, which is crucial for assessing stock status and implementing sustainable fishery management practices in the region.