What Comes Down Must Go Up: The Migration Cycle of Juvenile-Return Anadromous Taxa
Kim N. I. Bell
Abstract.—The migration cycle of juvenile-return anadromous (a.k.a. amphidromous) taxa is overviewed using the goby Sicydium punctatum as representative. The first migration, a physiologically obligate drift to the sea, has a cost of extreme mortality, frequently exceeding 0.5/h, effectively a 1-h half-life. Stream drift may be the most hazardous known natural habitat. A nest sited 5 km upstream reduces larval drift survival to 0.55 = 0.0313 or is 97% lower than a downstream nest. The conservation implication is strong: the lower reaches are disproportionately important for population maintenance. Mortality implies two new metrics for conservation and for understanding evolution of habitat choice: the mainstay habitat and point of expatriation. The second migration is oceanic, of which little is known beyond its duration until return to freshwater (recruitment), but salinity choice experiments demonstrated a requirement for intermediate salinities. Consistent with that and with others’ stable isotope work, observations suggest that the combination of rain and onshore wind creates intermediate-salinity habitats on windward coasts of high islands. The third migration is the upstream migration of juveniles and adults. Drift (first migration) mortality, however, has strong implications for adult habitat choice: it challenges the notion that this goby can be successful by migrating far upstream because that entails a large reproductive penalty. The assumption that populations can be maintained through conservation of adults at high altitude or far from the sea is challenged.