A method for determining stomach fullness for planktivorous fishes


Mean stomach fullness provides a useful index for quantifying fish diets. However, estimating stomach fullness for planktivorous fishes can be time-consuming and prone to error because of small prey and unidentifiable remains. In this study we developed a predictive equation for estimating the stomach volume of yellow perch Perca flavescens as a function of total length (TL). We then used an optical plankton counter (OPC) to estimate the biovolume of invertebrate prey consumed. The OPC quickly estimated the digital size and abundance of zooplankton prey, which can then be converted to estimates of prey biovolume. Stomach volume (V [mm3]) for yellow perch (113-279 mm TL) was significantly related to body size (L [mm]) and was estimated as V = 3 × 10-7·L2.96. Using the OPC, yellow perch stomach contents (99%. Daphnia pulex) were converted to prey biovolume (mm 3) and then divided by stomach volume (mm3) to estimate stomach fullness (%). This approach provided reasonable estimates of stomach fullness ranging from 3% to 85% (mean = 21%). Although the initial cost for the OPC equipment is relatively high, this method provided substantial time and labor savings compared with traditional approaches for quantifying zooplankton abundance and biomass (e.g., microscopic identification and enumeration and length-mass conversions). Similarly, the OPC can be used to estimate the abundance and biomass of freshwater zooplankton, thus reducing the time and costs associated with traditional plankton analyses. The approach is limited, however, in cases where very small prey (<250 μm) are a dominant proportion of the sample because of the potential errors involved in detecting and estimating the biovolumes of small particles.

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North American Journal of Fisheries Management