The Fishconnect project (2011-2014) aims at predicting fish connectivity among marine protected areas under climate change scenarios and studying the implications of connectivity for conservation and fishery management. It is funded by the Total Foundation and the Fondation pour la recherche sur la biodiversité.
Estimating larval connectivity among Mediterranean marine protected areas (MPAs) and larval export from MPAs to fished areas
The Mediterranean Sea harbors more than 600 fish species with a high fraction of endemism and experiences unprecedented levels of human pressure from fishing, exotic species, and pollution. The Mediterranean Sea benefits from the presence of approximately a hundred MPAs, mainly concentrated in its Northern coastal areas. Previous studies showed that fishing restrictions in Mediterranean MPAs have positively affected the density, size, biomass, and diversity of exploited species. However, the connectivity among MPAs and their ability to provide recruitment benefits beyond their boundaries remain under scrutiny. This knowledge is crucial since MPA networks need to ensure the persistence of target species and to maintain fisheries yield over large scales.
Map of Mediterranean MPAs
We implemented a biophysical model to estimate recruitment within Mediterranean MPAs, connectivity among them and larval export to fished areas for an overexploited species: the dusky grouper Epinephelus marginatus. Due to its slow growth and late maturation, E. marginatus should be considered as a “conservation-dependent” species, a species for which the role of MPAs is critical for maintaining a viable population. By considering that MPAs concentrate E. marginatus adults in the Mediterranean Sea, we assessed the patterns of recruitment and connectivity among MPAs and whether larval export from MPAs benefits exploited areas over the entire continental shelf of the Mediterranean Sea.
The biophysical model predicted a mean larval dispersal distance of 120 km, in front of a mean distance between MPAs of about 1000 km. We also evaluated connection among MPAs.
Map of larval connections among MPAs
The system of Mediterranean MPAs is not a fully connected network. Out of 13,225 possible connections between MPAs, only 637 were realized. This means a connectance = 4.8 %. In addition, when MPAs were connected, the connection probability was always very low (median 0.00028, interquartile range 0.0017).
Map of larval abundance at the end of larval transport
Since MPAs can supply fished areas with larvae, we evaluated the distribution of larval abundance at the end of larval transport. Larval export to the continental shelf was limited: the percentage of area of the continental shelf seeded with larvae from MPAs was 78%. The percentage of larvae retained on the continental shelf was 44%.
Andrello M, Mouillot D, Beuvier J, Albouy C, Thuiller W, and Manel S. 2013. Low Connectivity between Mediterranean Marine Protected Areas: A Biophysical Modeling Approach for the Dusky Grouper Epinephelus marginatus. PLoS ONE 8(7): e68564. doi:10.1371/journal.pone.0068564. (URL)
Evaluating the impact of climate change on larval connectivity and larval export in the Mediterranean MPA network
The results of the biophysical simulations of larval dispersal can be summarized by four variables:
Larval dispersal distance. Median dispersal distance of larvae is 120 km
Connectance. Fraction of nonzero connections out of the total number of potential connections: 4.8%
Seeded area. The percentage of area of the continental shelf seeded with larvae from MPAs is 78%
Retention fraction. The percentage of larvae retained on the continental shelf is 44%
We now ask: how will these four variables be affected by cimate change ? Climate change can alter larval connectivity in many ways, by modifying adult reproductive success, reproductive timing, larval dispersal routes, planktonic larval duration, larval survival and larval behaviour. Using biophysical simulations forced with an emission-driven regional climate change scenario for the Mediterranean Sea, we explored the combined effects of changes in hydrodynamics, adult reproductive timing and larval dispersal on the four connectivity variables (larval dispersal distance, connectance, seeded area and retention fraction) to see whether climate change would affect connectivity among MPAs and their ability to seed fished areas with larvae.
The simulations showed that larval dispersal distances would decrease by 10%, the continental shelf area seeded with larvae would decrease by 3% and the larval retention fraction would increase by 5%, resulting in higher concentration of larvae in smaller areas of the continental shelf. However, connectance within the MPA network would increase by 5% since more northern MPAs would become suitable for reproduction with increasing temperatures.
Andrello M, Mouillot D, Somot S, Thuiller W, Manel S. 2015. Additive effects of climate change on connectivity among marine protected areas and larval supply to fished areas. Diversity and Distributions,21(2): 139-150. (URL)
Optimizing the future network of MPAs in the Mediterranean by integrating larval connectivity and larval export in conservation planning
Connectivity is essential to ensure population persistence of protected species and should be an explicit objective in the creation of new protected areas. The surface of Mediterranean coastal protected areas is targeted to increase from the actual ~2% to 10% by 2020, therefore, many MPAs are likely to be created in the next years. It is important that connectivity will be taken into account in the designation of future protected areas, and several methods have been developed to include connectivity into MPA design algorithms. These algorithms aim at finding the best sites among a collection of sites available for conservation according to some criteria. Nilsson Jacobi and Jonsson (Ecol Appl, 2011) developed an algorithm based on metapopulation theory to rank sites according to their contribution to population growth rate. In this algorithm, connected sites contribute more than unconnected sites to population growth rate, so it is possible to maximize connectivity by maximizing the population growth rate. This method offers also a way to focus the optimization on the consequences of connectivity rather than on connectivity per se. The population growth rate is calculated as the highest eigenvalue of the connectivity matrix while the contributions of individual sites are calculated based on the eigenvalue perturbation theory (EPT).
We modified the original algorithm by taking into account the possibility that some sites are already protected and the increased productivity of already pretected sites. We then applied the modified algorithm to the system of Mediterranean MPAs to extend the existing network of MPAs. We ranked sites according to their contribution to population growth rate and took the top 10% to produce a list of unprotected sites that are to be prioritized for protection on the basis of connectivity
Map of existing protected areas (blue) and unprotected sites prioritized for protection on the basis of connectivity (red)
Andrello M, Nilsson Jacobi M, Manel S, Thuiller W, Mouillot D. 2015. Extending networks of protected areas to optimize connectivity and population growth rate. Ecography, 38: 273-282. (URL)
Genetic connectivity: a simulation model
Simulation models are extremely useful tools to study population genetic processes and patterns, including genetic connectivity. There are dozens of models available for simulation of population genetics, which differ in the type of processes that can be simulated (e.g. discrete vs. overlapping generations, spatially explicit vs. non-spatial models, etc.). All forward-time models are individual-based, which makes it possible to incorporate directly demographic stochasticity and genetic drift in the simulations. Accounting for stochasticity and drift is the main reason for preferring simulation models to analytical model, whose solution is often limited by the complexity of the stochastic processes.
The individual-based technique however limits the number of individuals simulated, dictated by computing power and memory size. We have therefore developed a new forward-time simulation model, MetaPopGen, where the number of individuals is not a limiting factor, because the individual-based approach is abandoned in favor of a probabilistic approach that accounts for demographic stochasticity and genetic drift. This is achieved by using random number generators from probability distributions describing the demographic and genetic processes. MetaPopGen is therefore adapted to simulate large populations, or combinations of large and small populations, a typical scenario for the marine species. Features included in the model are age-structure, monoeciuous and dioecious (or separate sexes) life-cycles, mutation, dispersal, selection. The model is limited to one locus. MetaPopGen is an R library and is free.
We applied MetaPopGen to Mediterranean marine protected areas, using the larval dispersal probabilities obtained with the biophysical model.
Matrix of pairwise genetic differentiation (FST) predicted by MetaPopGen
on the basis of the larval connectivity matrix obtained with the biophysical model