Multiple nutrient subsidy pathways from a spawning migration of iteroparous fish

Migratory animals can enhance ecosystem productivity through the delivery of material subsidies. Among fish, Pacific salmon are well known to deliver large quantities of nutrients to streams as they die after spawning, but the input pathways by which iteroparous species provide nutrient subsidies have not been resolved. Our objective was to determine the importance of excretion, eggs and carcasses as nutrient sources from a large migration of longnose suckers into a stream draining a moderately agricultural catchment. Additionally, we evaluated nutrient limitation in the stream using nutrient-diffusing substrates and determined the timing of nutrient releases during egg decomposition using a microcosm experiment. Eggs were the largest component of the nitrogen (N, 57%) and phosphorus (P, 76%) inputs from the migration, followed by excretion by live adults (40% N, 16% P). Carcasses were a minor component of inputs. Estimated P inputs from fish were over three times larger than the observed export of dissolved P in this P-limited stream during the 66-day sampling period. In contrast, sucker N inputs were <2% of dissolved N export, which was dominated by NO3. However, the dynamics of NH4 concentrations through the course of the migration were closely associated with estimated NH4 inputs from excretion and eggs. Eggs and excretion constitute significant nutrient inputs during migrations, even in catchments with elevated nutrient loads from agriculture. Mass mortality is not required for migratory fish to enhance nutrient availability in their spawning habitats. Given that sucker excretion rates and female reproductive investment are typical for freshwater fish, our results suggest that spawning migrations of iteroparous species in rivers around the world may deliver important nutrient subsidies when migrations are large.