Four causes of nursery function degradation and their consequences for declining marine coastal- and estuarine-dependent species

Many fish species inhabit shallow coastal habitats as juveniles before migrating offshore as adults. In recent decades, densities of these marine coastal- and estuarine-dependent species have declined dramatically in many coastal habitats. The causes of these declines remain unknown. In this study, we model four scenarios that may explain the decline of fish in nurseries: reduced food availability in the nursery, increased mortality in the nursery, increased mortality in the offshore habitat, and reduced connectivity from the offshore habitat to the nursery. We analyse these scenarios using a physiologically structured population model, which accounts for the full life cycle of individual fish. Our model includes three habitats, namely a focal nursery, an offshore habitat, and alternative nurseries. Our results show that three of the four scenarios can lead to the disappearance of fish populations from nurseries, namely reduced food availability in the focal nursery, increased mortality in the focal nursery, and increased mortality in the offshore habitat. Loss of connectivity to the nurseries may amplify the negative effects of these three scenarios. In addition, we find that the consequences for size distribution and individual growth are different for each scenario. Specifically, reduced food availability in the focal nursery leads to slow juvenile growth, whereas increased mortality in the focal nursery leads to fast juvenile growth. We show that including recruitment from an uncoupled system does not prevent extinction in case of deteriorating conditions in the focal nursery, while it weakens the negative effect of offshore mortality. Furthermore, we show that high-quality alternative nurseries can act as a population refuge and may prevent population extinction when the focal nursery is heavily disturbed. These findings highlight the importance of protecting coastal nurseries for fish population conservation.