Using elemental chemistry of fish otoliths to determine connectivity between estuarine and coastal habitats

Connectivity between estuarine and coastal populations is poorly understood but fundamental to the study of population dynamics, as well as the design of effective conservation and management strategies. Naturally occurring elemental signatures (or composition) in the otoliths of fish are an ideal natural tag due to the metabolic inertness of otoliths, continuous growth including daily increments and incorporation of elements that are influenced by environmental variables. In this paper, I review the use of otolith chemistry to determine connectivity between estuarine and coastal habitats focusing on assumptions of using elemental signatures as natural tags, consequences of violating these assumptions and finally providing examples of the application of natural tags to determine connectivity. As a first step to determining connectivity, it is important to determine whether fish residing in different estuaries do in fact have different elemental signatures. Spatial differences must be determined for each species of interest since variability has been found between species collected from the same location. For retrospective determination of origins of fish, it is also necessary to ensure either that there is no temporal variation at both the small (e.g. among months within a year) and large scale (e.g. among years) or to match adults to the appropriate year class of juveniles. In addition, fish should be collected from all possible source groups contributing to the group mixture. The majority of studies that show connectivity between estuarine and open coastal populations using otolith chemistry have determined the contribution of different habitats or estuaries to the adult population, although natal homing has also been examined