GROWTH AND MORTALITY OF LARVAL SUNFISH IN BACKWATERS OF THE UPPER MISSISSIPPI RIVER

We estimated the growth and mortality of larval sunfish Lepomis spp. i n backwater habitats of the upper Mississippi River with an otolith-ba sed method and a length-based method. Fish were sampled with plankton nets at one station in Navigation Pools 8 and 14 in 1989 and at two st ations in Pool 8 in 1990. For both methods, growth was modeled with an exponential equation, and instantaneous mortality was estimated by re gressing the natural logarithm of fish catch for each 1-mm size-group against the estimated age of the group, which was derived from the gro wth equations. At two of the stations, the otolith-based method provid ed more precise estimates of sunfish growth than the length-based meth od. We were able to compare length-based and otolith-based estimates o f sunfish mortality only at the two stations where we caught the large st numbers of sunfish. Estimates of mortality were similar for both me thods in Pool 14, where catches were higher, but the length-based meth od gave significantly higher estimates in Pool 8, where the catches we re lower. The otolith-based method required more laboratory analysis, but provided better estimates of the growth and mortality than the len gth-based method when catches were low. However, the length-based meth od was more cost-effective for estimating growth and mortality when ca tches were large.