Retrospective Determination of Atlantic Croacker Migration Through Laser-Based Elemental Analysis of Otoliths

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Year to year variations in the number of fish which survive the first year of life (termed recruitment or year class strength) are often sufficient to control the success or failure of the fishery a few years later. Despite the scientific and economic benefits which would result if year class strength could be estimated before fish reached a marketable size, recruitment prediction has proved to be an elusive goal. Likely mechanisms contributing to year-to-year variations in the year class strength of Atlantic croaker include variations in the location and timing of spawning, the age and date of subsequent advection to the estuarine nursery area, the duration and relative growth rate while in the estuary, and the age and date of migration of the estuary as an adult. In conjunction with age-structured measures of growth and survival based on otolith daily growth rings, this project will test these hypotheses using a new laser- based elemental analysis technique (LS-ICPMS) to retroactively determine the hatching site and estimate the age and date of subsequent migrations in individual fish. The technique is based on the acellular nature of the fish otolith (earstone), which forms through the concentric daily additions of mineralized tissue around a central nucleus. Given the fact that the elemental composition of a given daily ring reflects the elemental composition of the water which passed through the fish's gills on that same day, migration between water masses at some age and/or date will be recorded as a change in the elemental composition of the otolith at the appropriate daily increment. By corollary, water mass differences at the time and place of hatching will be reflected in elemental composition differences in the otolith nucleus. At present there are few (if any) markers of marine fish populations, let alone indicators of past movement and water mass association. Successful completion of this project has implications not only for recruitment predication of Atlantic croaker, but for studies of recruitment, stock structure, migration and fisheries management around the world.