Patterns of horizontal distribution of the larvae of southern bluefin (Thunnus maccoyii) and other tuna in the Indian Ocean

The fine- to coarse-scale distribution patterns of tuna larvaein the east Indian Ocean were investigated by a combinationof continuous transect sampling using surface tows and randomsampling using double oblique tows. Thunnus maccoyii was themost abundant species, reaching densities near patch centresof 22 m^–1 in surface tows, which is 25 times greater thanthe highest previous records for tuna larvae. Patches of T.maccoyiilarvae in areas of high abundance appeared to 5.2 for T.maccoyii.There was no change in the index when tow distance was doubledto 1200 m, older larvae. Lloyd's index of patchiness was consistentlyhigh for all tuna species, ranging from 3.0 to 5.2 for T.maccoyyii.There was no change in the index when tow distance was doubledto 1200 m, which suggests that the dominant patch size was somewhatlarger than the larger sampling interval. Sampling larvae atthe same site 4 days apart resulted in estimates of abundancethat differed by an order of magnitude. Abundance estimatedfrom a single station would depend largely on what day the stationwas occupied and where the sample was taken in relation to apatch. ¹Present address: Victoria Institute of Marine Sciences andDepartment of Zoology, University of Melbourne, Parkville, Victoria3052, Australia