Diel vertical migrations of marine fishes: an obligate or facultative process?

Publisher Summary Diel vertical migrations are cyclic changes in the position of aquatic organisms in the water column that occur with 24-h periodicity. Such movements occur at various stages of the development of teleosts, although they are often more evident during the first year of life. Despite frequent study, fundamental questions concerning the nature of diel vertical migrations remain. For example, most life-history traits exhibit phenotypic plasticity in response to environmental factors during the development. From the fisheries viewpoint, an understanding of diel vertical migration is critical for surveys of abundance at all life-history stages. Such surveys of exploited populations of adult fish have long been employed during the stock assessment process and are becoming increasingly significant, particularly, in light of the difficulties associated with obtaining reliable commercial fishery statistics. This chapter presents a review of the studies of diel vertical migration of marine fish species. The extent to which fish exhibit plasticity in their patterns of diel migrations is examined, both with respect to varying environmental conditions and ontogeny. Most importantly, the chapter also assesses whether the patterns of diel vertical migration fits the criteria for endogenous circadian rhythms. Finally, it attempts to address whether the observed diel periodicity in depth occupied is a result of the endogenous effects whereby the periodicity is derived from the fishes' biochemistry or biophysics, rather than more direct effects of environmental stimuli, such as changes in light and temperature providing a cue for certain behavior.