Comparison of Light Traps and Plankton Tows for Sampling Brachyuran Crab Larvae in an Alaskan Fjord

We compared the effectiveness of light traps and plankton tows for sampling brachyuran crab larvae in Bartlett Cove, Glacier Bay, Alaska, U.S.A. during three nights each in July and August 2001 and June, August, and September 2002. Proportions of species and stages were used to compare larvae caught by light traps and plankton tows. Absolute numbers of larvae are difficult to compare because of the difference in sampling volume and duration of each method. Both methods captured early and late-stage larvae of four brachyuran families, Cancridae, Grapsidae, Pinnotheridae, and Majidae. However, light traps collected many late-stage (megalopae) cancrid larvae, which were rarely collected by plankton tows, suggesting that late-stage larvae may be undersampled by plankton tows. In contrast, plankton tows collected many early stage (zoeae) pinnotherid larvae, which may be undersampled by light traps. These results follow the same pattern as that found for tropical fish larvae, that light traps are a useful sampling device for settlement-size larvae. Light traps provide a simple method to collect high resolution temporal data on late-stage larval abundance, and their use has improved our ability to discern recruitment dynamics for decapod crustaceans.