Ontogeny of swimming speed in larvae of pelagic-spawning, tropical, marine fishes

During the pelagic larval phase of teleost fishes, the larvae are subject to dispersal by currents. Dispersal trajectories can be substantially modified if the larvae have sufficient swimming abilities, so it is important to document how swimming ability develops during the pelagic larval phase. We used reared larvae (4 to 29 mm standard length) from commercial aquaculture farms in Taiwan to measure the development of swimming ability (critical speed, Ucrit) in larvae of 9 species (from 7 families) of Indo-Pacific coral reef and coastal fishes that hatch from pelagic eggs: Trachino- tus blochii (Carangidae - jacks), Chanos chanos (Chanidae - milkfish), Platax teira (Ephippidae - batfishes), Leiognathus equulus (Leiognathidae - ponyfishes), Lutjanus malabaricus (Lutjanidae - snappers), Eleutheronema tetradactylum (Polynemidae - threadfins), and Epinephelus coioides, E. fuscoguttatus and E. malabaricus (Serranidae - groupers). Mean critical swimming speeds increased from <5 cm s -1 in the smallest larvae to a maximum of 47 cm s -1 in settlement stage larvae, with the increase in speed by the time of settlement ranging from 6- to 100-fold. Increase in swim- ming speed was more strongly correlated with size of larvae (R 2 = 0.38 to 0.93, p < 0.005) than with age (correlation with age was absent in 3 species and explained 10 to 43% less variation than did size in the others). The relationship between speed and size was linear. In 6 species (T. blochii, C. chanos, L. malabaricus and the 3 Epinephelus species) speed increased at a rate of 2.1 to 2.6 cm s -1 for each 1 mm increase in size. Three species (P. teira, L. equulus and E. tetradactylum) had a significantly slower rate of increase of 1.3 to 1.7 cm s -1 for each 1 mm increase in size. On average, the best per- formers in each 1 mm size increment were 1.5 to 7.3 cm s -1 faster than mean performers, depending on species. Throughout development the vast majority of mean length-specific speeds were 10 to 20 body lengths (BL) s -1 , and length-specific speed increased significantly with size in 6 species. Maxi- mum length-specific speeds for each species reached 18 to 31 BL s -1 . Although the ontogeny of swim- ming speed varies among species of tropical marine fishes, over similar size ranges, larvae that hatch from pelagic eggs have swimming abilities similar to those reported for larvae that hatch from demersal eggs.