Protection reveals density-dependent dynamics in fish populations: A case study in the central Mediterranean

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.

Fig 1. Study area.

Torre Guaceto marine protected area (light grey) and sampling sites at unprotected locations (EXT/N and EXT/S). Map based on public domain shapefiles released by the Italian National Institute of Statistics (www.istat.it).

Fig 2. Population density and body size of Diplodus sargus sargus inside and outside Torre Guaceto marine protected area.

Population density (a) and body size (b) were measured inside the protected area (MPA) and at two unprotected locations outside (EXT/N and EXT/S) via underwater visual census between 2004 and 2015. In (a), dots and vertical bars show mean and interquartile range, respectively, deriving from data variability among transects at the same location. In (b), boxes show the interquartile range (with the horizontal crossing line indicating the median) and whiskers encompass 90% CI.

Fig 3. Relationship between population density of Diplodus sargus sargus in two subsequent years.

Dots indicate data collected inside the protected area (MPA) and at two unprotected locations outside (EXT/N and EXT/S); colored lines and shaded areas are median and interquartile range, respectively, of multi-model predictions (weighted mean of the predictions obtained with the three best models; see text for details). Intersections between the 45° identity line and the colored lines identify equilibrium densities for the deterministic component of the model.

Fig 4. Results of the Population Viability Analysis inside and outside Torre Guaceto marine protected area.

Depletion probability (a) and expected time to depletion (b) inside the protected area (MPA) and at two unprotected locations outside (EXT/N and EXT/S) are shown as functions of the depletion threshold and were estimated via Monte Carlo simulation (10⁵ iterations) and multi-model inference (see text for details).