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Recovery of fish populations in protected areas can take decades

The populations of some fishes in the Medes islands (Spain) have recovered after more than two decades of protection, according to a study of the CSIC and the University of Barcelona. This study collates valuable information for the governments to design fish recovery strategies. Scientists warn: “Without protection, the destruction of these fish populations is a matter of days”.

Zebra sea bram. Picture: Enrique Ballesteros (CEAB-CSIC)It is possible to almost totally recover the fish populations in protected areas although it can take decades. A study by scientists at the CSIC’s Centro de Estudios Avanzados de Blanes and at the University of Barcelona (UB) shows that after more than 20 years of protection in the Medes Islands, populations of Dusky Grouper (Epinephelus marginatus), Zebra seabream (Diplodus cervinus), and European sea bass (Dicentrarchus labrax) have almost totally recovered. These populations are now, according to scientists, very near to the so-called carrying capacity, which is the maximum population that can live in one area.

The populations of Common dentex (Dentex dentex) and the Brown meager (Sciaena umbra) are still growing. On the contrary, the population of Gilthead seabream (Sparus aurata) not only has not recovered but is decreasing in numbers compared to 20 years ago, although the size of the animals is bigger inside the protected area. Scientists think that giltheads go to spawn outside the borders of the protected area, where they can be illegally caught.

All these species are long-lived, can reach a considerable size and are prized for their culinary qualities, making them especially vulnerable to fishing. To obtain data on fish populations, scientists have made visual inventories during systematic dives for 21 years.

The authors of the study, which is published in PLOS ONE, are Antoni Garcia-Rubies, at the Centro de Estudios Avanzados de Blanes of the CSIC, and Bernat Hereu y Mikel Zabala, at the Universidad de Barcelona.

Protected areas with high carrying capacity

Marine protected areas can reach high fish biomass if conditions are suitable. But the biomass that a zone can host has a limit: it is the so-called “carrying capacity” of the system.

Antoni Garcia-Rubies explains: “in a previous study (Coll et al., 2012) we found out which are the most relevant factors to have a high fish biomass: exposure to winds and marine currents, complex rockery bottoms and a near bottom drop which would offer shelter for fishes. The Medes Islands have these environmental requirements.

Common dentex. Picture: Antoni Garcia-Rubies (CEAB-CSIC)

The present work shows that, with these good conditions, the total fish recovery can take decades, as the most important fish biomass fall on long-lived species, which are sedentary and have slow development.

On the contrary, there are marine protected areas where environmental conditions are not so suitable to get high fish biomass. These zones can reach a minor carrying capacity quickly, with short-lived species that have a quicker development.

García-Rubies points out: “the clear message of the present study is that the species which are most sensitive to fishing need long time of total protection to recover. Without protection, its destruction is a matter of days”.  When protected populations are compared to non protected, he adds, “it is clear to what point these species are over-fished”.

Choosing the best zones to be protected

All these data should help the Administration to choose the best marine zones to be protected. “There are many protected areas with low fish biomasses, but other zones which have high potential are not protected”.
Theoretically, when a protected zone reaches their carrying capacity, part of the fish population could move to the surrounding zones. Therefore this would help to maintain the fish populations in the exploited zones: it is what the scientists call “spillover”.

The ideal solution would be a network of protected areas, placed in the best places, quite close to each other

Nevertheless, in the case of the Medes Islands, scientists have seen that these benefits don’t reach the surrounding zones or they do poorly. “At least for the species we have studied, which are very sedentary, the sand strip that separates the Medes Islands from the coast prevents sedentary species, such as the Dusky Grouper and the Zebra seabream, to leave the protected area.”

In order to make the spillover work, another protected area should exist along the coast, without gaps in the habitats between the protected zones, which would allow a fluent movement of fishes between zones.  This would improve the fishing activities. Still it would not guarantee the total protection of species, as one part of the population would be exploited.

According to the scientists, the ideal solution would be a network of protected areas, placed in the best places, quite close to each other, and combining isolated reserves, like the Medes Islands, with protected areas along the coast. In this way the survival and the sustainable exploitation of these species could be ensured.

García-Rubies A, Hereu B, Zabala M (2013) Long-Term Recovery Patterns and Limited Spillover of Large Predatory Fish in a Mediterranean MPA. PLoS ONE 8(9): e73922. doi:10.1371/journal.pone.0073922

Coll J, Garcia-Rubies A, Morey G, Grau AM (2012) The carrying capacity and the effects of protection level in three MPAs in the Balearic Islands (NW Mediterranean). Scientia Marina 76: 809–826. doi: 10.3989/scimar.03531.02h.