Differences between fishery-dependent and fishery-independent estimates of single-species and mixed-species dolphin schools: Implications for single-species stock assessments

Since 1979, fishery-independent data and fishery-dependent data have been used to estimate trends in the abundance of the northeastern stock of spotted dolphins in the eastern tropical Pacific. Data collected aboard tuna vessels have been used to estimate trends in relative abundance, while data collected from fishery-independent research vessels have been used to estimate trends in actual abundance. One of the largest discrepancies between the two data sources is that tuna vessels tend to report dolphin schools that are 400%-500% larger than schools observed by research vessels. After comparing research vessel and tuna vessel observations overlapping in space and time, it appears that either measurement error or selective reporting of large schools is the most likely explanation for the disparity. Comparing single-species schools against portions of mixed-species schools introduced bias. Revised fishery-dependent abundance estimates were generated using only single-species dolphin schools, resulting in estimates that appear more similar to recent fishery-independent abundance estimates. Biases associated with mixed-species schools did not appear to be correctable in fishery-dependent data, suggesting that data on mixed-species schools should not be considered if fishery-dependent data are to be combined with fishery-independent data for stock assessment purposes.