Recruitment of fishes into the spawning population is thought to depend upon the number of fish surviving from the larval to the juvenile stage. Survivorship of larval fishes in the wild is typically very low and is partially controlled by their nutritional condition. Inadequate diets may decrease survival directly through starvation or indirectly by reducing growth rates, prolonging the period during which larvae are most vulnerable to predation. The ability to delineate the nutritional condition of larvae in the field would provide us with an estimate of recent growth rates and increase our understanding of recruitment variability. We are currently evaluating the usefulness of a variety of biochemical, morphological, and histological measures as indicators of nutritional condition in field collected red drum larvae. For example, we have found that the ratio of RNA to DNA varies with the nutritional history of laboratory reared red drum larvae. By comparing the RNA:DNA ratios of wild caught larvae to those raised in the lab, we are able to assess the nutritional condition of larvae living in different habitats. For more information on the field component of this study, go to our web site detailing current field projects.
Salinity Tolerance Studies |
Juvenile spotted seatrout |
Spotted seatrout (Cynoscion nebulosus) are an important recreational sport fish found along the Atlantic and Gulf of Mexico coastlines. These fish live and spawn in estuaries and bays where eggs, larvae, and adults are likely to experience fluctuations in salinity due to variations in freshwater inflow, evaporation, etc. Adult spotted seatrout are able to tolerate a broad range of salinities. In fact, adult fish have been found in salinities ranging from .2 to 75 ppt. However, young fishes often show a reduced level of tolerance to salinity extremes. Therefore, large fluctuations in salinity may affect the recruitment of estuarine fishes, such as the spotted seatrout, into the natural spawning population by increasing the natural mortality of larvae. We are investigating the relationship between short- and long-term changes in salinity on the survival of seatrout larvae. Adult seatrout spawning in the lab are being held at three different salinities of 20, 30, and 40 ppt. Measurements of survival, growth, and condition are estimated for the eggs and larvae produced by these fish. We have found that changing the salinity of adults brought into the lab during the spawning season has detrimental effects on eggs and larvae produced during this time. This reduction is most prevalent at first feeding, which occurs three days after hatching. Several hypotheses emerge that could explain the spotted seatrout adaptation regime and will be investigated over the course of this project. One hypothesis is that long-term acclimation by adults to salinity regulates the pattern of salinity tolerance found in young larvae. Alternatively, non-genetic, possibly irreversible adaptation of the embryo or larva may regulate salinity tolerance at the population level.
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