Estimating connectivity in marine fish populations: what works best?

Measuring connectivity in marine populations is a key, yet poorly understood issue. The degree of connectivity determines the spatial scales over which population dynamics operate, the spatial scales over which fisheries should be managed and how marine protected areas should be designed and implemented. Thus, much research is directed towards a better understanding of connectivity. The tools currently used to measure connectivity broadly include larval distribution, numerical models (physical or biophysical), genetic techniques and otolith chemistry. However, few studies use more than one of these approaches, and users of connectivity estimates are frequently unclear about what any approach actually measures or whether it is the best approach for their purposes. That is, does the tool provide information on evolutionary connectivity or ecological connectivity, can it forecast connectivity, what do the results of such studies mean in that context, and what are the limitations of the tool? This review provides an overview of these four approaches to estimating connectivity and how they work, examines what each actually measures, outlines the spatial and temporal scales over which each is appropriate and details the pros and cons of each. A historical approach is used to describe the development of these four approaches, focusing on examples using reef fishes. However, the key points are broadly applicable to marine and estuarine fishes and some invertebrates. Finally, we discuss the advantages of using multiple methods to elucidate a more complete understanding of marine connectivity.