Diel patterns of vertical distribution in larvae of southern bluefin Thunnus maccoyii, and other tuna in the East Indian Ocean

Abstract

Vertical distributions of larvae of 3 tuna species were investigated in the East Indian Ocean. In the presence of a strong pycnocline, the entire depth range of tuna larvae could be covered by sampling the mixed layer by both day and night. A small proportion of tuna larvae were found below the mixed layer when the pycnocline was weak. Thunnus maccoyii and T. alalunga larvae moved into the surface layers durlng the day. Katsuwonuspelamis, however, moved into deeper water during the day. All species of tuna were more evenly dispersed in the mixed layer at night. Greater numbers and larger larvae of T rnaccoyli and K pelanijs were captured at night, indicating marked differences in net avoidance between day and night. No evldence of increased avoidance during the day was found in T alalunga. The relative abundance of T m a c c o j ~~i larvae could be predicted from surface tows; however, the relationship between surface and obllque tows showed regional and diel variability. To reduce blas and increase preclslon, estimates of tuna larvae abundance should be based on oblique tows made at night from the surface to just below the mixed layer.