Variability in reproductive investment of skipjack tuna (Katsuwonus pelamis) in relation to the ocean–climate dynamics in the tropical eastern Indian Ocean

Abstract

The Indian Ocean is characterised by significant climatic and oceanographic variability such as the seasonal monsoon and the decadal and inter-annual oscillation of the Indian Ocean Dipole (IOD). Understanding the effects of ocean–climate variability on marine top predators is important for both fisheries and ecosystem management. To test the hypothesis that variability in surface and subsurface temperature affects the annual reproductive cycles of skipjack tuna (Katsuwonus pelamis) in the tropical eastern Indian Ocean, we examined the seasonal and inter-annual variations of the gonadosomatic index (GSI) and determined hatch date by using otolith microstructure analysis. The GSI tended to be higher in January–February during the north-eastern monsoon when seawater cooling at the subsurface layer is induced by the seasonal inflow of the South Equatorial Countercurrent (SECC). The mode of the hatch-month distribution was also detected in that season. The generalised linear model (GLM) resulted in a dome-shaped quadratic relationship, with a peak GSI between 24°C and 26°C occurring at a depth of 50 m during austral summer. These findings indicate that seasonal monsoons and associated changes in oceanographic conditions strongly affect the energy-allocation pattern of skipjack tunas.