Aspects of the early life history of billfish off Kona, Hawaii

Publication Type:
Thesis
Issue Date:
2004
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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- Of the six billfish that frequent the Hawaiian waters (black marlin, blue marlin, striped marlin, sailfish, short-billed spearfish and swordfish), larvae from only blue marlin, shortbilled spearfish and swordfish have been found in the region. Published keys were evaluated in the identification of 200 larval blue marlin (Makaira nigricans) and shortbill spearfish (Tetrapturus angustirostris). These were compared to genetically identified (PCR) specimens. Several new features emerged as possible diagnostic characters. A multi-character program was written incorporating 1 0 different diagnostic characters. Larval blue marlin, spearfish and swordfish are present in the waters off Kona year round, with the exception of swordfish, which showed a strongly delineated spawning season. Temporal abundance of billfish larvae was closely associated with peaks in adult abundance. A peak in adult spearfish abundance was closely related to the yearly temperature low (23.5°C), while adult blue marlin abundance was associated with warmer water temperatures (approx. 26 °C). Recently spawned eggs of all three species {never previously identified for blue marlin and spearfish) were collected within 2 km of the coast. This is the first evidence of these species spawning so close to land. Larval billfish are attracted to surface slicks, probably due to the high concentration of prey within them. Larval billfish use a wide range of threat and defense postures, and are extremely aggressive to threat or predation risk. Swimming speeds of larval istiophorids ranged from 1 to 53 body lengths per second (BUS), while swordfish swimming speeds ranged from 1 to 10 BUS. All larval billfish observed were solitary, with little intra or inter-specific interactions observed. This is the first study of larval billfish behaviour in situ. lstiophorid larvae off the Kona coast of Hawaii showed relatively fast growth rates when compared to other pelagic fish. Water temperature appeared to have minimal effect on increment width. Five uniform ontogenic growth "pulses" were observed in otolith increments corresponding to changes in larval behaviour, morphological development, and dietary shifts. The feeding and diet of 138 larval blue marlin and shortbill spearfish were analysed and compared with conspecific larvae caught thoughout the Pacific and Indian oceans. From 3.8-7 mmTL, Hawaiian istiophorid larvae feed primarily on copepods, and from 7-12 mmTL the diet became a combination of zooplankton and fish larvae. Over 12 mm TL the larvae feed almost exclusively fish larvae. lstiophorid larvae are voracious feeders and were recorded consuming fish larvae (including conspecifics) up to 95% of their total length.
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