Acoustically derived fish size spectra within a lake and the statistical power to detect environmental change

Fisheries acoustic surveys are increasingly being used to monitor the abundance of fish stocks, yet their adoption as a tool to monitor changes in community size spectra has not been well explored. In this study, we use a series of historical acoustic surveys of the pelagic zones of three arms of Lake Opeongo to determine if acoustically derived size-spectra indicators (slope and height) can be effectively measured and used as a monitoring tool. Acoustic size-spectra indicators were successfully measured for every survey and resembled the same indicators found in netting surveys. From 2005 to 2009, the slope of the size spectra became shallower, likely due to a decrease in abundance in schooling prey fish. Estimates of sources of survey variation, including fish size estimates and interbasin differences, were low (<10% coefficients of variation), suggesting that monitoring programs would detect annual changes in the size-spectra slopes ranging from 2% to 15% within 10 years. Size-spectra heights did not change very much over the time series of surveys. Using lake trout (Salvelinus namaycush) fishery data from Lake Opeongo, we estimate that sources of natural variation in size spectra at the population level could be much higher and potentially require longer monitoring periods. We noted from our study that standardized surveys and data analyses are critical if acoustically derived size spectra are to be adopted as a monitoring tool. We also suggest that size spectra be calculated through echo-integration methods rather than echo-counting methods because, in this study, echo counting led to lower estimates of abundance and size-spectra indicators, which are likely underestimates of the true fish community's abundance.