Migration is an important component of many animals’ life cycles. In addition to benefiting the animal’s reproduction, feeding, and survival, migration also transports resources that connect ecosystems.
A classic example of this is migratory fish, who migrate between water bodies for spawning. As fish migrate from oceans and lakes into rivers, they store nutrients in their body tissues and release them through excretions and carcass decomposition. These inputs from migratory fish can be important for introducing nutrients into river ecosystems, but little is known about how and to what extent these nutrient inputs impact river ecosystems overall.
Kurasawa, A., Onishi, Y., Keisuke, K., Fukushima, K., Hiromi, U. 2024. Sequential migrations of diverse fish community provide seasonally prolonged and stable nutrient inputs to a river. Science Advances 10: eadq0945.
Recent research examines the role of fish migration for nutrient inputs in the Chinai River in southern Japan, where fish migrate into the river from Lake Biwa. Researchers conducted regular surveys of fish biomass in the Chinai River to track the migration timing of six fish species. They estimated the total ammonium and phosphorus inputs from fish excretions and carcasses in the river and looked at whether nutrient fluxes in the river matched migration patterns. They also tracked how ammonium from fish was taken up by the food web by tracking isotopes in river macroinvertebrates.
The study found that fish migration caused a significant increase in nutrient inputs in the river. For individual fish species, migration periods only extended about 1-4 months, meaning that their individual nutrient subsidies were short term. However, because the six fish species migrated at different times of the year, nutrient subsidies extended over the entire 8-month period of fish migration. Migration increased nutrient subsidies in ecosystems, and the diversity in migratory fish increased the temporal stability of these subsidies.
The nutrients provided by migratory fish were taken up into the river food web, indicating that nutrient subsidies from migration have ecosystem-wide effects. Nutrient subsidies from migrating fish were taken up by algae, increasing algae biomass. Benthic macroinvertebrates, such as mayfly and caddisfly larvae, subsequently consumed algae. These nutrient enrichments from migration had cascading effects for multiple trophic levels in the river ecosystems, and these enrichments extended across the entire migratory season.
These findings highlight the critical role of migration in connecting ecosystems. Migratory fish are nutrient transporters, carrying essential resources upstream and contributing to the productivity of food webs. However, fish migration is vulnerable to barriers like dams that fragment rivers, preventing fish from migrating to spawning areas and reducing nutrient subsidies from migration. Prioritizing conservation that preserves and restores migratory pathways will protect both migratory species and their contributions to nutrient cycling.
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