Abstract
Challenges associated with species identification, live collection and laboratory maintenance of billfish larvae have hindered research on their physiology and behavior. In the present study, short-duration neuston net tows in the Straits of Florida yielded 19 live istiophorid billfish larvae, which were immediately placed in a shipboard vertical swimming chamber to measure their vertical movement rates. After swimming trials, larvae were transferred to the laboratory where they were identified to species, classified as to flexion stage and measured for length. Mean vertical swimming speeds of captive larval sailfish (Istiophorus platypterus) and blue marlin (Makaira nigricans) ranged from 1.0 to 7.0 cm s-1 or 1.6 to 5.6 body lengths s-1. These rates exceed most larval fish sinking rates reported for other species and are comparable to mean larval "cruising" speeds reported for several temperate freshwater and marine fishes; however, they appear far lower than most swimming speed estimates for reef fish larvae.