Annual and supra-annual patterns: migrations
Many fishes engage in periodic long-distance movements. A vast literature exists on various aspects of migratory behavior (e.g., Harden-Jones 1968; Leggett 1977; Baker 1978; Northcote 1978; McCleave et al. 1984; McKeown 1984; Dodson 1997; Lucas & Baras 2001). Our focus will be on species that undergo fairly large-scale migratory cycles with an annual or greater period, either in the ocean or between the ocean and fresh water, with lesser treatment of the so-called potamodromous fishes that undergo reproductive migrations within fresh water (see Lucas & Baras 2001; Welcomme 2003).
Migrations take several general forms. Reproductive migrations take animals from a feeding locale to a spawning locale, moving the animal from a habitat that is optimal for adult survival to one that is better for larval or juvenile survival. Fish that spawn several times in their lives (the iteroparous condition) may undergo this migration more than once (e.g., Atlantic Sturgeon, American Shad, Atlantic Salmon, and the world’s largest salmon, the Taimen Salmon of Siberia, Hucho taimen, which may weigh 70 kg). Semelparous fishes, those that spawn once and die, undergo the migration only once (e.g., sea lampreys, anguillid eels, Pacific salmons, some galaxiids).
Inherent in reproductively migrating species is the complementary migration that juveniles take to juvenile and adult feeding areas. In some species, nonspawning juveniles and adults also migrate between feeding and spawning areas along with reproductively active individuals (e.g., sturgeon). Reproductive migrations may involve movement between lakes and tributary streams or between different parts of a river system, as occurs in large tropical characins and catfishes. Adults of the prochilodontid Coporo, Prochilodus mariae, in the Orinoco region migrate from Andean piedmont tributary rivers to wet-season spawning and feeding habitats in lowland floodplains, returning to tributaries as river levels fall. All such species are decimated by dam construction that blocks these extensive migrations (Barbarino-Duque et al. 1998; Lucas & Baras 2001). Other reproductive migrations involve fishes that move between the sea and fresh water (diadromy, see below), or may entail movements within ocean basins in a roughly circular or back-and-forth pattern (Bluefish, tunas). Additional species engage in transoceanic, seasonal migrations that do not appear linked directly to reproduction, but instead probably place adult fish in optimal locales to intercept seasonally available food sources (pelagic sharks, billfishes) or may move individuals away from climatically unfavorable areas to regions that are less harsh (e.g., Summer and Winter flounder).