Diversity in young shark habitats provides the potential for portfolio effects

The young (neonates, young-of-the-year and juveniles) of many shark species occupy a diverse range of habitats and areas. However, the contribution of individual nurseries or habitats to an adult population is difficult to quantify. In addition, little attention has been paid to the potential importance of ‘non-nursery’ young shark habitats to the long-term sustainability of shark populations. Portfolio theory predicts that contributions from a diverse range of young shark habitats may reduce variability in the overall production of adults, and maintain population resilience. This review examines case studies of portfolio effects in teleost fish and evaluates the relevance and potential implications of these processes for shark populations. Environmental heterogeneity in young shark habitats can result in locally adapted habitat-use patterns and life history traits. Therefore, young shark habitats may be differentially impacted by anthropogenic disturbance or environmental change, with different habitats performing well at different times. In addition, increased stability in production may be achieved when the effects of localised disturbance in one area are buffered by production in others. However, the behavioural and life history characteristics of some shark species may limit portfolio effects. These include the repeated use of a narrow range of habitats or areas for reproduction, and the production of relatively stable numbers of offspring. This description of the relevance of portfolio theory to shark populations highlights the importance of maintaining habitat diversity.