Gametes and excretion from iteroparous migratory suckers subsidize spawning streams

Background/Question/Methods

Transfer of materials by migratory species can drive nutrient availability and productivity in recipient ecosystems. The study of material transfers by migratory fish has focused largely on semelparous Pacific salmon. Salmon carcasses were traditionally thought to be the major source of material contributions, but recent evidence suggests that excretion by live salmon may be more important to nutrient dynamics than carcasses. We examined the relative importance of carcasses, gametes, and excretion of iteroparous catostomids (suckers) migrating into Great Lakes tributaries. Suckers migrate in large numbers in early spring, delivering nutrients that are incorporated by the resident biota. Three key differences between salmon and sucker migrations exist: 1) salmon die after spawning, whereas suckers do not; 2) salmon bury their eggs causing a major disturbance, whereas suckers are broadcast spawners; 3) sucker larvae emigrate from breeding streams at small size and prior to feeding, whereas salmon reside in their natal habitat for months to years.  These differences mean that the relative importance of carcasses, gametes, and eggs to overall nutrient contributions is likely to differ. To examine this hypothesis, we measured excretion rates, ovary mass, fish residence time, and migration size and calculated the potential nutrient input from each subsidy pathway.

Results/Conclusions

Sucker eggs represented the largest contribution of N and P to their spawning streams and dominated the potential subsidy of limiting nutrients during the study. The quantity of egg N and P inputs was an order of magnitude greater than that of excretion and carcasses. However, temporal dynamics of nutrient chemistry were associated with both live fish and egg densities in the stream. We demonstrate that the large inputs of eggs by migrating iteroparous fish can play a major role in driving ecosystem processes and can constitute a substantial proportion of the annual inorganic nutrient budgets of spawning streams. Thus, spawning fish migrations may be broadly important in continental stream ecosystems, and management lessons derived from research on Pacific salmon may apply to other large migrations.