Abstract
Casting light on how the interaction between protection and density dependence affects fish population dynamics is critical for understanding the effectiveness of marine protected areas (MPAs). We developed a framework based on nonparametric statistics, model selection and multi-model inference to contrast alternative hypotheses about the effect of density dependence on demographic dynamics under protected and unprotected conditions. We trialed it using a 12-year long time series of white seabream (Diplodus sargus sargus) population density within the no-take zone of Torre Guaceto MPA (Italy) and at two nearby unprotected locations. Then, we showed how the demographic models obtained can be used to assess the consequences of protection on population viability. Population dynamics were significantly influenced by fish density within the MPA and at one of the unprotected locations, where demography is possibly driven by directional recruitment subsidy from the MPA. The comparison of population growth rates within and outside the MPA suggests that in unprotected conditions the fishery may remove a fraction between 40 and 70% of the population each year. The population viability analysis pointed out that, while the probability that the population becomes depleted (i.e. undergoes a local, temporary quasi-extinction) is high in unprotected locations, it is negligible within the no-take zone of the MPA.
Highlights
Density dependence has important consequences on how fish populations respond to the establishment of marine protected areas (MPAs)
Alternative hypotheses can be contrasted in terms of evidence strength by using Akaike weights produced via the model selection procedure, which can be used to assess whether specific dynamics are shared across protected and unprotected populations [37,38]
Population density within the MPA seems to vary, albeit with remarkable fluctuations, around a value determined by density-dependent mechanisms
Summary
Density dependence has important consequences on how fish populations respond to the establishment of marine protected areas (MPAs). Density-dependent mortality determined by predation and competition is a key compensatory mechanism damping down inter-annual variability of recruitment [1]. While population density in exploited conditions may be too low to provide evidence for resource limitation, competition for food or space may become important in protected populations, affecting life history traits such as life span, body. Protection reveals density-dependent dynamics in fish populations (project "FishMPABlue2" co-financed by the European Regional Development Fund – ERDF). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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