Two methods of sampling fish larvae over reefs: a comparison from the Gulf of California

I compared the sampling properties of two methods for collecting fish larvae over reefs: nighttime collecting with a light trap, and daytime collecting with a small plankton net that could be steered by a diver. Samples were collected in the Gulf of California during summer, 1989 and 1990. The 90 light-trap samples yielded 9406 larvae from 31 families, while the 75 plankton-net samples yielded 17852 larvae from 43 families plus unidentified anguilliforms. Four families were collected only in the light trap, and 16 families plus the anguilliforms were collected only with the plankton net. With one exception, the families that were collected by only one method were rare. Twenty-seven families were collected by both methods, but only 13 were collected at least five times by each. The average catch per sample differed significantly between methods for 9 of these 13 families. In each case, the plankton net yielded more larvae per sample. The distribution of larvae among families was less equitable in light-trap samples than in planktonnet collections, primarily because clupeids were so dominant in the former. However, the taxonomic composition of light-trap and plankton-net collections was broadly similar. Seven families were shared among the ten most abundant families for each method, and the relative abundances of taxa (47 families plus anguilliforms) were strongly correlated between methods. A comparison of larval size-distributions for 12 families indicated that the size structure of catches usually differed between collecting methods. In four families there was little overlap in the size classes collected, in five families the distributions overlapped broadly but had different shapes, and in three families the size distributions were similar. Although the light trap collected larger larvae on average, its catches were not limited to settlement-stage or transition larvae. Larvae of at least ten families were present over reefs in all size classes, but the combination of both sampling methods was usually required to detect this. Based on their abundance and wide size distribution over reefs, at least some larvae from these ten families may remain over reefs throughout development. However, additional data are required to determine the importance of water over reefs as a larval habitat.