Challenges for Diadromous Fishes in a Dynamic Global Environment

The Origin of Fish Migration: The Random Escapement Hypothesis

Katsumi Tsukamoto, Michael J. Miller, Aya Kotake, Jun Aoyama, and Kazuo Uchida

doi: https://doi.org/10.47886/9781934874080.ch4

Abstract.—The remarkable migrations of some fishes are still the subject of active research, but there has been much less attention on the evolutionary or behavioral origins of diadromous migrations. Diadromy likely evolved as a result of the adaptive advantage of using higher productivity habitats for growth, but the reason why the first individuals left their habitat to enter a new one is unknown. The objective of this paper is to examine the possible evolutionary and behavioral origin of diadromous fish migration and to explore the possible factors that could have led to the development of migratory behavior, using a behavioral model for a triggering mechanism of fish migration. Eels likely evolved in the tropics from a marine ancestor, and salmon evolved in temperate regions from a freshwater ancestor. Evidence of the ancestral life history states of these fishes can be seen in geographic clines of occurrence of nonmigratory residents of the sympatric Japanese eel Anguilla japonica, a catadromous species, and masu salmon Oncorhynchus masou, an anadromous species, with a higher proportion of resident type “sea eels” at higher latitudes, whereas masu salmon have more river residents at lower latitudes. The amphidromous ayu Plecoglossus altivelis was used as a model species for exploring the behavioral origins of migration. The triggering mechanism of upstream migration of ayu could be explained by a three-step model of necessary conditions for starting migration that include age and body size, endocrinological condition, and psychological processes. A behavioral model of drive was proposed to explain the last step that is influenced by exogenous/endogenous factors such as water temperature, fish density, and hunger level. The drive of upstream migration behavior appears to be a psychological repulsion that occurs in stressed fish when the behavioral rule of optimum interindividual distance is broken, since fish with stronger repulsion and a larger interindividual distance showed more active jumping behavior and stronger upstream migration. A vacuum activity of jumping behavior of ayu contained in a limited space occurred in random directions without any behavioral stimuli when water temperature increased. This vacuum activity can be interpreted as an escapement behavior of fish that bolt out of unfavorable environments, and this escapement might be a behavioral origin of fish migration that may also help to understand the evolutionary origin of migration (random escapement hypothesis).