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.

Extending full protection inside existing marine protected areas, or reducing fishing effort outside, can reconcile conservation and fisheries goals

“…Although results are affected by manifold sources of uncertainty (including the intrinsic variability of environmental and ecological processes driving the dynamics of the stocks across time and space, as well as the scarcity of data available to build a comprehensive picture), we are confident that our results are robust, at least with regard to the general patterns emerging from the analysis. As a further proof of the robustness of our results, it is interesting to note that the reserve effect emerging from our data on D. sargus (see Figures and ) is fully consistent with what observed by Melià, Casagrandi, Di Franco, Guidetti, and Gatto (2020) for the same species inside and outside another Mediterranean MPA (Torre Guaceto, southern Adriatic Sea).…”

Defining Cost-Effective Solutions in Designing Marine Protected Areas, Using Systematic Conservation Planning

“…Thus, the declaration of a no-take area would represent a negative impact on the fishing activity, and should encourage the call for reallocation of marine resource access privileges (Carter, 2003;Coccoli et al, 2018). The hypothesis is that relationships exist between the biodiversity and delivery of ecosystem services (Melià et al, 2020). The protection of ecosystem components will maintain in good ecological condition, with positive effects on the delivery of ecosystem services (Potts et al, 2014;Arkema et al, 2015), from which fishers would be one of the direct beneficiaries.…”

Salmon of the Southeastern U.S.: Sucker migrations deliver resource subsidies to oligotrophic stream

“…To estimate the number of spawning adults in Citico Creek on days of maximum density for each wave, we sampled with replacement from the observed US and DS density distributions (Allgeier et al, 2020; Melia et al, 2020), with the number of draws from the distribution defined by the total area (m 2 ) occupied by buffalo on that day. For example, if buffalo occupied an area of 5000 m 2 on a given day, we drew 2500 density values from both the DS and US density distributions and summed across the 5000 draws to estimate the abundance of individuals within the entire system on that day.…”