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Comparisons of swimming ability measurement methods for larval marine fishes: Which method is best for studies of larval dispersal?

Jeffrey M Leis

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ABSTRACT: Larval perciform fishes are able for much of their pelagic larval dispersal stage (PLD) to directly influence their dispersal by horizontal swimming, but it is unclear which means of measuring swimming is most appropriate for modelling dispersal and studying demographic and genetic connectivity. Most studies use Critical Speed (Ucrit), a laboratory flume measure derived by increasing flow until larvae cannot maintain position. Most swimming ability data on fish larvae are Ucrit, usually for larvae nearing the end of PLD. Recognizing a forced laboratory measure is inappropriate for dispersal, researchers have used decreased Ucrit values, usually by 50%, and argued Ucrit is strongly correlated with more relevant swimming measures. Examined here was suitability of Ucrit versus in situ speed (ISS), wherein speed of larvae is measured by divers following them in the ocean with a flow meter. Considerations of dispersal require inclusion of swimming ontogeny. Swimming speed regressions of speed on size of 10 species of 8 families show Ucrit and ISS were not well correlated. Ucrit-SL slope was greater than ISS-SL slope in six species, and not different in four. No overall metric, e.g. X%Ucrit = ISS, was appropriate for conversion of Ucrit to ISS. Conversion of Ucrit to ISS is not straightforward. Ucrit measures swimming potential, not what larvae do in the ocean, whereas, ISS directly measures larvae swimming in the ocean. Ucrit ontogeny is less variable, but ISS ontogeny is more relevant to dispersal. Ucrit may be useful for other purposes.