Science Serving Maryland's Coasts
Edward Houde, Ph.D.
Chesapeake Biological Laboratory
Spatial Variability in Larval White Perch, Morone americana, Diet and Growth in the Upper Chesapeake Bay
Recruitment variability causes fluctuations in fish populations and is mostly attributed to variable survival rates during early life. High mortality rates in egg and larval stages prevent dramatic increases in numbers, but mortality rates significantly higher than the average cause poor recruitments of year classes. Factors that control feeding and growth at the larval stage are important in determining survival and thus recruitment success or failure. In this study, feeding success and growth rates of white perch larvae in designated areas of the upper Chesapeake Bay were evaluated based upon ichthyoplankton surveys conducted during the May 2002 spawning season. A particular objective was to determine if the salt-fresh transition zone, i.e., the Estuarine Turbidity Maximum (ETM), was an area of enhanced feeding or larval growth. Comparisons of larval feeding (based on gut analysis) and growth rate (based on otolith increment analysis) encompassed four broad areas in the upper Bay, including the 1) shoal and 2) channel regions 3) above and 4) within the ETM. Planktonic copepods were the most common prey in larval white perch guts in all areas. The mean number of copepods per gut was significantly higher in larvae from channel stations within the ETM compared to channel stations up-estuary of the ETM, supporting the hypothesis that the ETM serves as an advantageous larval nursery. However, feeding incidence (percent of larvae with prey in guts) and mean number of total prey per larval gut did not differ among areas. Larvae >4.5mm in length had a higher feeding incidence and mean number of prey per gut than first-feeding larvae <4.5mm. The low feeding incidence and mean number of prey per larval gut in the smaller larvae suggested poor feeding conditions for the smaller larvae, which may have contributed to poor recruitment of white perch in 2002. No significant differences in growth rates were detected among locations. Larvae in the channel, on the shoal, above the ETM, or below the ETM were growing at similar rates (0.20 to 0.30 mm/d). In addition, no significant differences were detected in recent (past 48 hours) growth rates. One interpretation is that there is little variability in larval habitat quality among the areas evaluated in this study. However, high variability in growth rates among individual larvae suggests that small sample sizes may have reduced the power of statistical inference to detect differences. A potential confounding factor is the possibility that undernourished or starving larvae, which had been feeding and growing poorly, were dying at above average rates and were poorly sampled, making them unavailable for inclusion in the growth-rate analysis. A future study focusing on spatial variability in abundance of microzooplankton that serve as prey for white perch larvae would be important to better define relative habitat quality of areas occupied by larval white perch.
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