Mangrove loss: a risk for fishing in Quintana Roo

There are fish that are iconic because of their size. Witnessing their presence at sea is always a reason for emotion. If they are commercially important species, finding them is also a stroke of luck for anglers. The cherna, as the species Polyprion americanus is known in the Mexican Caribbean, is an example of this: a greyish fish with brown shadows, with a prominent jaw, which reaches two meters in length and can weigh up to just over 100 kg.

Unfortunately, “it's getting rarer to see them,” says Dr. Felipe Eloy Sosa Cordero, a researcher at the Colegio de la Frontera Sur and a specialist in the study of fisheries in the Yucatan Peninsula. “Now, when a fisherman finds a shark, it's like winning the lottery,” he adds, both because of how difficult it is to find a specimen and because of the profits involved in the catch. A single cherna can fetch prices of around ten thousand Mexican pesos, he tells me. He takes advantage of everything: his head, steak and even the crab, that is, his swim bladder —a highly valued snack in the Caribbean—, adds Sosa Cordero.

The fact that these marine giants are less and less present in the waters of the peninsula is due to different reasons. The main one is attributed to their overfishing. However, there is another phenomenon that is happening on the coasts of southeastern Mexico that affects not only chernas, but many other fish and invertebrates, important products of the region's fishing: the loss of mangroves, those coastal ecosystems inhabited by woody trees that tolerate high amounts of salt. Fishes such as the cherna spend the early stages of their lives among the roots of these trees. Without this kind of coastal forest, the existence of many marine species is threatened, in addition to the loss of biodiversity and connectivity with other ecosystems that this implies. The destruction of mangroves has and will have a severe impact on fish production in the region.

Marine childhoods under the shade of the mangrove

The care of young in the marine world is not similar to what we are used to seeing among the animals with which we live the most, such as humans and other mammals. Males and females of coral fish, for example, seek mates and reproduce on reefs, but after releasing the result of their Caribbean frenzy—their eggs—they invest little time in their young, at least compared to what some terrestrial animals do (although, as biologists say, it depends on the species).

“[Adult] fish come and spawn in mangroves,” says Dariel Andrei Correa, a master in coastal zone management and a doctoral student at the Technological Institute of Chetumal. The eggs of some of these fish species hatch and the larvae remain part of their life cycle there, feeding and protecting themselves from predators. It's until they grow when they “migrate from mangroves to coral reefs,” he adds. There they will spend most of their adult lives and will look for a partner to reproduce and repeat the cycle.

Thus, fish larvae, still vulnerable, have to face the vicissitudes of their environment: searching for food and protecting themselves from predators that are notoriously larger than them. In this context, the niches that interweave the vegetation of seagrasses and mangroves are essential for their development. These coastal ecosystems are even known as nurseries or breeding areas because they are home to many marine animals during the early stages of their lives.

The three-dimensional structure of mangroves acts, in part, as a refuge: the complex framework of their roots serves as a hiding place for small larvae and juveniles and does not give easy access to larger fish that seek to hunt them. The fact that sunlight does not penetrate easily and that the waters surrounding these ecosystems are a bit murky, which complicates the work of aquatic predators, also contributes to this protective role of mangroves, says coastal zone management teacher Chloe Brynie Ulanie Rosas, who is currently studying her doctorate at the Technological Institute of Chetumal.

For all these reasons, the fact that fish have access to a mangrove increases their survival rate, especially for the youngest; although there are also some that are highly dependent on mangroves even in their adult state. A study carried out on the Glover Reef, in Belize —not far from the Quintana Roan coast—, compared the biomass of coral fish, that is, the total weight of fish, in corals surrounded by mangrove areas with that of fish without access to these “nurseries”. ¹

The absence of mangroves in some areas of this reef is mainly the result of deforestation. The conclusion of the analyses suggests that mangroves are essential for many species of coral fish, many of them of commercial importance. The disappearance of mangroves has even led to the local extinction of the largest herbivorous fish in the Atlantic, Scarus guacamaia —from the parrotfish family—, according to this work.

Mangroves don't just serve as shelter. There, larvae, juveniles, and even adult fish will find food. Mangrove leaves, when they fall into the water, are not digestible for these organisms, but the bacteria that live at the foot of the trees degrade them. This decomposition process is essential for food to finally be available for fish, crabs and other animals. Thus, in addition to protection, mangroves are spaces where a great diversity of animals can be nourished.

Fishing in Quintana Roo and its economic value

Mangroves are not only essential to the life cycle of fish, but also to many invertebrates. The lobster, for example, spends a large part of its life there, anchored to the roots of mangroves, explains Ulanie Rosas. This crustacean is also of great commercial value for the area. It is a “product that sells and is very expensive” and, we know, “it is highly dependent on mangroves,” he tells me.

Quintana Roo ranks last — among the states of Mexico that have a coastline — in fish production at the national level. However, “when you do the analysis of the value of what you catch, how much the kilograms that each state extracted are worth” things change, says Sosa Cordero.

In this southeastern Mexican state, “there is a tendency for fishing to be around resources of high economic value” and that many of these resources are sold at better prices, starting with lobster, says Sosa Cordero. He explains that, for example, Quintana Roo produces almost the same as Yucatán in terms of lobster, but in the former, it has a better price. Tourism could be one of the explanations behind this price variation. He tells me that “the biggest buyer of lobster is in Cancun”, which is the one that sells to many hotels and large chains in the region.

In addition to lobster, other products of significant economic value in the state are shrimp and fish such as groupers and snappers, which are highly valued in the peninsula. The cherna, that sea giant that can almost reach 10,000 Mexican pesos, belongs precisely to the group of groupers.

Many other fish, such as bass and saws, also play a fundamental role in terms of social and food security, says Sosa Cordero. And a large number of them, especially those that inhabit the region's corals, are also dependent on mangroves during the early stages of their lives.

Mangrove loss: fishing in danger

However, the loss of mangrove areas around the world—including Quintana Roo—, in addition to having multiple other consequences, threatens the life of marine species, and therefore the region's fisheries. During the last decades of the 20th century, the rate of mangrove deforestation was unprecedented. According to data from the National Commission for the Knowledge and Use of Biodiversity (Conabio), Mexico lost more than 80,000 hectares of mangroves between 1981 and 2005. ² Although recent years have seen a decreasing rate of deforestation, mangrove destruction continues to occur today.

In Quintana Roo, specifically, logging for land use is the main cause of mangrove loss, Ulanie Rosas and Andrei Correa explain to me. The construction of houses and hotels in the state's coastal area is a constant threat to these ecosystems.

In 2016, for example, municipal and state authorities destroyed 59 hectares of mangrove forest in Tajamar, on the coast of Cancun, for the construction of a real estate project that would include a shopping mall — a project canceled in 2019. “Overnight,” herons, crocodiles and seabirds were exposed, says Ulanie Rosas. ³ And just a few months ago, at the end of 2021, citizens and local organizations on Isla Chica de Holbox —which has become a famous tourist destination—reported illegal logging of mangroves for the construction of ⁴ buildings, many of them for tourist purposes. ⁵

In addition to the multiple consequences of the loss of these ecosystems — for example, coastlines that are much more vulnerable to storms, hurricanes, floods and loss of biodiversity — the disappearance of mangroves will surely have and will have a severe impact on fish production in the region.

Unfortunately, until today, there are no studies that quantify it in the Yucatan Peninsula. However, Sosa Cordero, Ulanie Rosas and Andrei Correa agree that it is possible to extrapolate studies in other regions of the world to speculate that the fisheries of Quintana Roo are also affected by the disappearance of mangroves. In the first stay, we saw that studies such as the Glover Reef in Belize indicate a clear relationship between mangrove cover and the abundance of fish stocks, many of them of commercial importance.

The study of the economic impact, specifically, has been little evaluated, but the limited studies that exist indicate that mangrove deforestation can be a problem for fish production. Research in the Gulf of California, published in 2008, provides evidence in this regard. ⁶

In this region, the destruction of mangroves has been mainly due to tourism development and the transformation of mangrove areas into shrimp farms. When the authors quantified fish production and mangrove cover in different segments of northwestern Mexican states between 2001 and 2005, they found that fish catches benefit significantly from local mangrove abundance. In addition, based only on the long-term contribution of these areas to the local production of fish products, the authors speculate that, over a period of 30 years, the transformation of each hectare of mangrove forest in that region would cost local economies around 605,000 US dollars.

Despite being an understudied and under-discussed topic, evidence that mangrove destruction has a severe economic impact on fish production is accumulating. In Quintana Roo, which is the state with the largest mangrove area in Mexico, this will have social and economic consequences that we cannot yet quantify. Linking these ecosystems to the social benefits they provide could, hopefully, reduce destructive practices towards them, for example, by some branches of the tourism sector, which even if they don't see it, benefit greatly from them.

To learn more about ecosystems, ecosystem services and citizen participation, visit www.serviciosecosistemicos.mx

This article was produced as part of the Conservation and Sustainable Use of Marine Biodiversity in the Mexican Caribbean project of the National Commission for Protected Natural Areas and the German Agency for International Cooperation GIZ, in collaboration with the Mexican Network of Science Journalists.