Evaluating Compositional Fidelity And Sources Of Mortality Of Shoreline Fish Carcasses In A Freshwater Lake Community In Western Canada

Taxonomic comparisons of death assemblages with extant communities continue to provide insight for ecological studies but whether demographic aspects such as age structure, sex ratio, and causes of mortality arc accurately captured by the former remains little studied. At a 110 ha dystropic lake in western Canada, seasonal shoreline accumulations of fish carcasses were collected annually for 12 years (N=4499) during 95 full lake-circuits and analyzed for taxa, body size frequencies, sex ratio relative to live-captures and sources of mortality. Rank order of the four fish species (Gasterosteus aculeatus, Oncorhynchus clarki, O. keta, Salvelinus mabna) was similar for live-captures and carcasses. Of the dominant species (G. aculeatus), modal adult body length (similar to 80 mm SL) was the same for carcasses and live-captures and shifted by about 10% over 30 years with parallel trends between the groups for both sexes. Age-specific body size was about 5% larger (P<0.001) in carcasses than live-captures. Carcasses were significantly female-biased (2:1) each year relative to a population sex ratio of 1:1 in the lake. There was a complete absence of juvenile fish (<30 mm) among carcasses but these constituted 70% of the live population. Estimated relative contributions to mortality for the carcasses include starvation (<1%), parasitism (3%), senescence (4%) and lethal injuries from predator attack (70-80%). If these carcasses are representative of a fossil series in freshwater lake sediments, then several demographic parameters including age-specific body size, age-class frequencies and sex ratio depart substantially from the live population. As well, the virtual absence of avian piscivores in the carcass assemblage, the major source of predation on the fish population, warrants additional attention in paleoecological studies.