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

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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 during the day. Katsuwonus pelamis, 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. maccoyii and K. pelamis were captured at night, indicating marked differences in net avoidance between day and night. No evidence of increased avoidance during the day was found in T. alalunga. The relative abundance of T. maccoyii larvae could be predicted from surface tows; however, the relationship between surface and oblique tows showed regional and diel variability. To reduce bias and increase precision, estimates of tuna larvae abundance should be based on oblique tows made at night from the surface to just below the mixed layer.

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