Specifying divergent migrations in the concept of stock: the contingent hypothesis

Abstract

The current concept of stock does not sufficiently accommodate the effects of migration behavior on population dynamics. New insight on the prevalence and ecological significance of divergent migrations within stocks has arisen through advancing technologies in tracking fishes. In particular, otolith elemental analysis may provide unique perspective on divergent migrations within stocks. Using otolith elemental fingerprints of Hudson River striped bass ( Morone saxatilis), I evaluated Clark’s (Clark, J.,1968. Trans. Am. Fish. Soc. 97, 320–343.) contingent hypothesis of intrapopulation migratory groups. Similar to the results of Clark’s earlier tagging study, I observed three spatially discrete cohorts of Hudson River striped bass. An intriguing question is, if such contingents exist, then how are they regulated? Using frameworks for understanding smoltification in salmonids and ontogenetic niche shifts, I hypothesized that early life decisions regarding energy allocation lead to later divergence in habitat shifts (migration). This mechanism results in the maintenance of “retentive” and “exploratory” behaviors and contributes to features of population regulation including persistence, range contraction and expansion, and colonization. Support for the contingent hypothesis, i.e. divergent migration tactics, exists for a diverse array of fish taxa. Assignment of contingents should allow improved specification of factors which affect a population’s accessibility to human impacts. Consideration of stock structure should incorporate a hierarchy of biological levels which evaluate both lineage (sub-population–population–meta-population–sub-species–species) and accessibility (individual–brood–school/shoal–year class–contingent).