Seasonal Mass Migration of Water Boatmen (Hemiptera: Corixidae) as a Wetland–River Linkage and Dietary Subsidy to Riverine Fish

Cross-boundary movements of organisms can act as important ecosystem linkages by subsidizing food webs. We investigated the magnitude and implications of a little understood food web subsidy in the form of migrating aquatic insects, corixids (Hemiptera: Corixidae), that fly from geographically isolated wetlands into large rivers in the Prairie Pothole Region (PPR) of North America every fall, to overwinter. We found that these migrations can lead to drastically increased riverine corixid densities as high as ~ 3,000 individuals/m2 within areas of standing or slow-moving water, with ~ 500 g of corixid material entering every meter of water immediately adjacent to the banks of rivers, where landings are concentrated. This movement shifts the species assemblage in rivers to one dominated by wetland-breeding species, namely Callicorixa audeni, Sigara bicoloripennis, and Sigara decoratella. Stomach content analyses of fish reveal that goldeye (Hiodon alosoides), mooneye (Hiodon tergisus), longnose sucker (Catostomus catostomus), and white sucker (Catostomus commersoni) make heavy use of this forage subsidy, with corixids occurring in 97% to 100% of these fishes and accounting for 38% to 97% of stomach contents by weight during the corixid migration period in fall. We estimate that seasonal migrations could result in ~ 1500 metric tons of corixids entering the North and South Saskatchewan rivers within Saskatchewan, and ~ 12,000 tons of biomass moving between wetlands and rivers across the entire PPR. Our study has demonstrated an extensive cross-boundary flux that occurs between spatially separated wetland and river ecosystems, highlighting a need for conservation to ensure that this connection is maintained.