Nutrient Subsidies from Iteroparous Fish Migrations Can Enhance Stream Productivity

Abstract

Migratory animals often transfer nutrients betweenecosystems, enhancing productivity in the subsi-dized system. Most research on nutrient subsidiesby migratory fishes has focused on Pacific salmon,whose semelparous life history is unusual amongmigratory fishes. To test whether iteroparous spe-cies can provide ecologically important nutrientinputs to stream ecosystems, we experimentallyblocked the migration of suckers (Catostomidae)midway up an oligotrophic tributary of LakeMichigan. Comparing reaches upstream of thebarrier to downstream reaches containing thou-sands of breeding fish, we found that suckers ele-vated phosphorus and nitrogen concentrationsthree- to five-fold. Algal accrual was doubled andcaddisflies grew 12% larger in subsidized reachesrelative to reference reaches. An enclosure experi-ment demonstrated that caddisflies with access to afish carcass rapidly became enriched in15N and13C, and experimental carcass additions were rap-idly colonized by high densities of caddisflies.However, under natural conditions below theexperimental barrier, caddisflies became enrichedin15N but not13C, indicating that fish-derivednutrients entered the stream food web primarilythrough indirect pathways rather than direct con-sumption of carcasses or gametes. At pupation, anaverage of 18% of caddisfly tissue N below thebarrier was sucker-derived. In comparison to ourfocal stream, a reference stream with few suckersshowed no seasonal or longitudinal patterns innutrients and stable isotopes. These results dem-onstrate that iteroparous fish migrations can spurproductivity via nutrient subsidies, despite lowmortality rates. Thus, concerns about negativeecosystem-level consequences of blocking migra-tions of semelparous fishes are also applicable toiteroparous species when migrations are large.