Retrospective analysis of dispersive and retentive behaviors of young-of-the-year fishes: Are fish otoliths environmental chronometers

University Of Maryland Center For Environmental Sciences, Cambridge MD

Investigators

Abstract

Spatial behaviors in marine fishes serve as a principal ecological response to climate change, exploitation patterns, and environmental degradation. Yet there exist few quantitative and conceptual tools to develop hypotheses related to intra?population variations in migration and habitat use patterns. Contingent structure (intra?population modalities in dispersive and retentive behaviors) has been proposed as a useful means to measure and categorize variation in spatial behaviors, and to evaluate the consequences of variation at the population and metapopulation level. Contingent structure has been demonstrated in diverse estuarine and coastal fishes using strontium, multi?element, and isotopic tracers in fish otoliths. This research project will test several factors that lead to contingent structure and evaluate population?level consequences of contingent structure for estuarine?dependent populations of white perch, Morone americana, within the Chesapeake Bay. Past research supported by NSF uncovered the existence of two discrete juvenile contingents classified as retentive (freshwater) or dispersive (brackish water) individuals. Further, in a large sample of adults, juvenile contingent behaviors were observed to structure growth and recruitment rates at the population level. Contingent structuring in Chesapeake Bay white perch represents a unique opportunity to evaluate important hypotheses that relate contingent structure (spatial behaviors) to growth, recruitment, abundance, life history, and environmental circumstance. Hypothesized mechanisms for the maintenance of contingent structure draw heavily upon literature for resident and migratory salmon ecomorphs, which emphasizes early growth rate and metabolic differences between retentive and dispersive contingents. For this research project, intensive ichthyplankton field studies and larval cohort analyses will be conducted to link larval growth, mortality and environmental conditions to subsequent juvenile contingent structure. Juvenile contingents will be sampled in the field and in the laboratory, bioenergetic studies will be conducted to test whether dispersive juveniles allocate proportionately more energy toward feeding and less toward routine metabolism. For a large sample of adults collected from seven major Chesapeake Bay sub?estuaries, juvenile dispersive behaviors will be reconstructed, and contingent structuring investigated as a mechanism shaping recruitment and resiliency within populations, and possibly connectivity among sub?populations. Climate effects on contingent structuring will also be investigated. Otolith isotope tracers, which are expected to be more reliable tracers of nursery habitats than strontium in otoliths, will be further developed. Protection of Essential Fish Habitat (EFH) is now mandated in the management of marine fishes, yet concepts of EFH remain poorly defined and many of the approaches to rank habitats lack rigor or are untenable. Other researchers have proposed systems that rank nursery habitat according to how many juveniles from a set of nursery habitats recruit to the adult stock. Past NSF?supported research on the contribution of white perch juvenile contingents to the adult stock represents an initial effort to evaluate nursery habitats in this manner. Ranking systems deserve additional evaluation because 1) the concept is testable with tools developed in past NSF research and elsewhere, but 2) the concept may need to be broadened. In temperate systems, the consequences of juvenile habitat use may largely depend upon climate. For instance, preliminary results for Chesapeake Bay white perch indicate that in high flow years, brackish water habitats contributed a majority of adult recruits and thereby are most important for white perch recruitment and population growth. Still, during low flow years, freshwater nursery habitats made disproportionately important contributions to the adult stock and thereby may confer resiliency to the population during decadal cycles of drought. A broader impact of this research is the comprehensive evaluation of the EFH concept through simultaneous analysis of many populations across relevant climatic cycles.