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Larval connectivity for effective MPAs networks: a multidisciplinary approach

Final Report Summary - MAPACO (Larval connectivity for effective MPAs networks: a multidisciplinary approach)

The marine environment provides important food resources for many populations all around the world. However the excessive and uncontrolled fishing activities led to the present alarming state in which a third of the worldwide stocks are overexploited. Marine protected areas (MPAs) have been proposed as a valuable tool for the conservation of marine resources and marine biodiversity. Furthermore, their integration in a network of MPAs is more and more considered as the key for enhancing their performance. Thus, the design of these MPAs networks is one of the most important factors in determining the achievement of the MPAs objectives, but increasing this knowledge is necessary to maximize such benefits. Connectivity among fish populations, achieved through larval dispersal, has then a pivotal role.
The Mediterranean Sea, the largest and deepest enclosed sea on earth, is a hotspot of marine biodiversity and around 185MPAs exist at this time in the Mediterranean basin, most of them situated in its north-western part. In this context we have developed the 2-years project MaPACo, with the aims to: (1) evaluate the relationship between several coastal areas in the Western Mediterranean for assessing the spatial scale of fish population connectivity; (2) evaluate the part of the local self-recruitment after the larval dispersal period, and its consequences on the recruitment efficiency from surroundings areas and (3) evaluate the benefits of the larval export from protected areas to the surrounding small-scale fisheries, as a help for sustaining these artisanal activities. The achievement of these objectives was successful and here below we present the main results obtained from MaPACo.
The MaPACo project was developed using two complementary tools for evaluating fish connectivity: population genetics and larval dispersal modelling. These analyses were done on the striped red mullet Mullus surmuletus, due to its large distribution in the whole Mediterranean Sea, its strong socio-economic value, and hence for being heavily fished.
The genetic analyses were obtained using 10 microsatellite markers developed for M. barbatus and commonly used with M. surmuletus. We analysed a total of more than 900 fish samples coming from 9 areas evenly located in the Western Mediterranean Sea: southern France, Balearic Islands, Murcia, Gibraltar, Algeria, northern coast of Tunisia, Sicily, south of Sardinia, and north of Italy. Moreover, in each of these areas we sub-sampled 3 sites separated by 40 to 60 km, in order to integrate the variability at finer spacial scale. In each of these sites, we sampled 30 to 40 individuals, an effort which is considered sufficient for representing the genetic diversity of a natural population. We first highlighted that 2 microsatellites were not adapted to characterise the genetic diversity of M. surmuletus, since they amplified only for M. barbatus, but are an effective way to discriminate the two species, very similar during they early-life stages. Moreover, we found that the high number of samples we used (more than 100 per area) gave an excellent picture of the genetic diversity, representing more than 97% of the total allelic diversity. The genetic distance was very low between the 9 areas (even if it was slightly higher between Gibraltar compared to the other sites), indicating that the population of M. surmuletus is not structured in the Western Mediterranean Sea, and is highly homogeneous. This is the consequence of a great level of gene exchanges between areas, achieved by larval dispersal. However, we noticed that in various areas, the observed heterozygosity is lower than the expected one, maybe due to local retention or higher levels of self-recruitment than expected in a natural open population. This could be the result of some circular currents retaining the fish larvae in their spawning area while limiting the arrival of external larvae.
The larval dispersal models, by combining a biological part and an oceanographic one, are more and more precise and complex for getting closer to the phenomenon they simulate, through the integration of an increasing number of parameters. However, when working on natural populations, a great part of these parameters are unknown or badly known, and their value is difficult to obtain. Thus, in the MaPACo project, we investigated the impacts of the input parameters uncertainty on the models results while proposing several scenarios. We saw that some parameters – such as the precision in the identification of the spawning areas, or the existence or not of the larval diel vertical migrations, are highly impacting the results of the model. However, we were able to counteract this variability by processing the whole scenarios results with a GIS tool, offering the possibility to evaluate the highest probabilities of larvae arrival sites. The results of the models confirm a strong temporal variability, in which dispersal patterns can highly differ between months and years. However, the main strong currents are still drifting the larvae. We found that the distance they travel during the 30 days of larval dispersal is of more than 200 km from they spawning area, settling far away.
As a conclusion, the genetic and modelling results support the fact that the M. surmuletus population in the Western Mediterranean Sea is a single one and that the level of exchanges of individuals and genes among sites is high. This has strong consequences for the future management of this resource, since the Mediterranean Sea is one block of fluent water, drifting fish larvae from a country to another, without taking into account political borders. But the management of the fish stocks around the Mediterranean Sea is very local. Consequently, a spatial gap in the management of this resource will not impact precisely this same area but the ones receiving the larvae. Therefore, the existing lack of MPAs in the southern part of the Mediterranean Sea is a big concern for such a widely connected population. These results are of particular interest for the MPAs and marine resources managers, and for the politics: such results should convince them to work together and to take common decisions at the same scale than the one of the resource they consider. Moreover, this as also of interest for the fishermen around the Mediterranean Sea, since their catches will impact in a way their future ones, but overall, they will impact the catches of the “connected”-fishermen. Without being aware of that, the fishermen in the Mediterranean sea are as connected as the fish they catch.
All this makes the MaPACo project a successful initiative since it moreover allowed the acquisition of new knowledge, competencies and skills constituting a bridge to reach a common objective to better understand the stocks dynamics for a better management of our common resource.

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