Fish migration

Biology and range

Many types of fish migrate on a regular basis.^[1] The bull shark is a euryhaline species that moves at will from fresh to salt water.^[2] Many marine fish make a diel vertical migration, rising to the surface to feed at night and sinking to lower layers by day.^[2] Anadromous fish are those whose adults inhabit saltwater but travel to freshwater for spawning, while catadromous fish are the opposite, with adults living in freshwater and moving to saltwater to reproduce.^[3]

Striped bass and salmon serve as classic examples of anadromous species, whereas freshwater eels exemplify catadromous fish undertaking extensive journeys.^[2] Tuna are among the fish that shift northward and southward seasonally, tracking changes in water temperature.^[2] The dourada catfish holds the record for the longest freshwater migration, ascending the Amazon River for 5500 km.^[2] Freshwater fish often move upstream to reproduce, but dam construction is increasingly blocking these traditional routes.^[4]

A given stock's schools typically follow a triangular route linking these three types of grounds.^[4]

What the specialist sources record

The Pacific Fishery Management Council notes that Pacific sardine are small schooling fish that have at times been the most abundant fish species off the West Coast.^[5] The northern subpopulation of Pacific sardine is most important to U.S. commercial fisheries.^[5] Jack mackerel are a schooling fish that range widely throughout the northeastern Pacific, with much of their range lying outside the 200-mile U.S. Exclusive Economic Zone.^[5] Older, larger jack mackerel range from Baja California to the Gulf of Alaska.^[5]

A 2020 experiment by researchers from Hungary and Spain showed that fish eggs can survive passage through a bird's digestive tract and subsequently hatch.^[6] The researchers fed eight mallard ducks 500 fish eggs each from Prussian carp and common carp, and retrieved eggs from feces after an average of one hour.^[6]