The Seasonal Variation in Size-Spectrum Slopes within Lakes and the Capacity to Detect Environmental Change

The mid-point heights, y-intercepts and slopes of size-spectra linear relationships between log-scaled fish abundance and body size have been studied in primarily marine fisheries as potentially useful indicators of community change arising from fishing. As larger individuals are harvested the high end of the size-spectrum is diminished, and as prey species are released from predation higher abundances increase at the low end of the spectrum, thus steepening slopes and increasing y-intercepts. Here we investigate three potentially confounding factors to this scenario including: 1) shifts in prey sizes following the release from predation, 2) temporal variation from the seasonal and decadal dynamics of fast-growing species, and 3) the generally high coefficients of variation associated with monitoring abundances and sizes in fisheries. The relevance of these factors are presented using time-series data of the fish community in the pelagic zone of Lake Opeongo from gill netting, trawls and hydroacoustic surveys spanning over a 40 year period, over which time the Lake Trout population experienced overfishing, and experienced decreasing body sizes in both the Lake Trout and their prey, Cisco. Last, we compare Lake Opeongo’s size spectra with over 30 hydroacoustic lake surveys from across Canadian lakes containing Lake Trout.