Estimating Natural Mortality Rates of Juvenile White Shrimp Litopenaeus setiferus

Comparisons of natural mortality rates can be used to identify essential habitat and nursery areas for fishery species. We estimated and compared natural mortality rates of juvenile white shrimp Litopenaeus setiferus using length-frequency and mark-recapture data and attempted to identify factors that may affect these mortality rates. Daily instantaneous natural mortality rates (95 % confidence interval (CI)) obtained from length-frequency data by following individual cohorts were 0.043 (0.031-0.054) and 0.014 (0.0-0.039). Combining all length-frequency data, converting to age-frequency data, and using two types of catch-curve analyses yielded estimates of 0.069 (0.042-0.095) and 0.060 (0.046-0.073). Mark-recapture estimates obtained in a separate study from two ponds were 0.129 (0.054-0.203) and 0.014 (-0.048-0.076). These estimates are comparable to previously reported values for this species, but we are the first to report a measure of precision with our estimates. In the mark-recapture study, mortality rates appeared to be related to predator abundance in ponds and flooding patterns of the surrounding marsh. The only mortality rate significantly different from any of the other estimates was the lower of the two length-frequency estimates, but this result should be interpreted with caution because of the uncertainty in that estimate, relative imprecision of our estimates, and confounding factors between the methods we used to estimate mortality. Despite this caveat, the results from our study can be used to improve population models for L. setiferus and our understanding of the role of marsh habitats as nursery areas.