The slow growth rate and low reproductive potential of sharks is a cause for concern in today's fishing reality

08/07/2021

Neither their sharp rows of teeth nor their perfect hydrodynamic shape have allowed them to escape from fishermen throughout history. It is believed that sharks have been consumed in Mexico since pre-Hispanic times and that at the end of the last century a certain amount of fins were already being exported from La Paz, in Baja California Sur. However, the shark continued to be the object of an eventual fishery for many years until 1930, when its capture began to increase in the coastal states of Sonora and Sinaloa.

The development of this fishery had, of course, an economic purpose. In the 1940s, shark liver oil, rich in vitamin A, was a product in great demand in the world market. Ten years later, synthetic vitamin A gave sharks a break, although not for long, since in the 1970s interest began to awaken in the Western world in the consumption of shark fins, used since ancient times by the Chinese to prepare exquisite dishes.

Although not always fully exploited, the shark has multiple uses. Its meat is consumed fresh, or salted and dried as a substitute for cod; its skin, which has served as sandpaper, now fetches a good price as it is used to make leather goods; its teeth are used in ornaments, weapons, or rituals; the liver and fins are used for the aforementioned purposes, and squalene (a substance used in the cosmetics industry and pharmacology) is also extracted from the liver, and the cornea has been used experimentally for human transplants. Finally, some people attribute healing properties to the cartilaginous skeleton.

On the subject of shark cartilage, there are strong controversies. There is talk of the powder made from the cartilage as a remedy against cancer, and it is also attributed with properties to combat rheumatoid arthritis, lupus, and osteoporosis. Opinions on the subject are divergent; some doctors claim to have improved the quality of life of cancer patients by administering this product, others say they are disappointed by such a preparation. On the other hand, all the physicians interviewed agree that they have obtained very favorable results in the treatment of rheumatoid arthritis.

It seems that there are few published studies on the clinical experiences of the use of cartilage in medicine, so it would be advisable to expand this field of research, especially in the areas where better results have been obtained. However, the powder is already produced and consumed in several countries, including Mexico.

Of the 350 species of shark that exist in the world, approximately 100 live in Mexican waters, and of these about 40 are used commercially, although some authors assure that only thirteen are caught. Among them are the flatnose shark, the hammerhead shark, the dogfish, and the tiger shark. Some species such as the night shark and the dusky shark are also fished in certain seasons when they are found in greater abundance near the coast.

Some shark species give birth to their young alive, while others lay eggs. The eggs are, depending on the species, of different shape and size, some of them look like amber plastic bags where the tiny shark in development is transparent.

When one considers the slow growth rate and low reproductive potential of sharks, their indiscriminate and abundant capture is worrisome, even though, according to some experts, no shark species in Mexico is in danger of extinction. Many sharks are caught without having had the opportunity to reproduce; moreover, often the reproduction areas coincide with the capture areas since when some of the shark species come close to give birth to their young or lay their eggs, fishermen take advantage of the opportunity to catch hundreds of sharks with their fishing gear (nets or longlines). Pregnant females that are taken out on deck abort their young at that time.

Small sharks require that certain stages of their development take place close to the coast, and during that time they are highly exposed to predators and environmental changes. Similar dangers are faced by the eggs of oviparous females, which can fall victim to industrial discharges or pollution from human settlements. For this reason, it is urgent to carry out studies on the biological characteristics and reproductive behavior of each species, which are absolutely necessary to recommend adequate measures to ensure their perpetuation. For the time being, perhaps it would be opportune to dictate some closures in order to protect the reproductive stages of the species whose habits are already known, as well as to take care of their reproductive areas.

Although shark fishing in Mexico is mainly coastal, there is also pressure on these animals offshore, especially from fin hunters. Many sharks are still alive when they are taken on deck, their fins are immediately cut off, which is known as finning, and then they are immediately thrown back into the sea, where they die because they cannot swim. In other words, the unheard-of cruelty of the procedure is compounded by the loss of tons of meat, skin, and cartilage, since fishermen consider it more profitable to continue their work after the fins that fetch a high price on the market than to waste time processing and storing the rest of the sharks already caught.

In both the Pacific Ocean and the Gulf of Mexico, sharks are fished only during certain times of the year, because they are a resource that moves according to their migratory habits. In the Pacific, the main shark landing ports are Puerto Madero (Chiapas), Guaymas (Sonora), San Francisquito (Baja California) and Mazatlán (Sinaloa); and in the Gulf of Mexico, Alvarado (Veracruz), Ciudad del Carmen (Campeche) and Progreso (Yucatán). The Gulf of California and the waters of Campeche are the most traditional shark fishing sites.

It is very important that each shark caught is exploited to the maximum, i.e. its meat, fins, skin, cartilage, liver.

In spite of being an artisanal fishery carried out with small boats -the so-called "pangas" of less than 10 meters in length- shark catches have been increasing. Shark fishing represents an important source of employment and food for Mexico, which has apparently placed Mexico in fourth place in the world in shark catches. Although shark meat is in great demand in Mexico, other products such as fins and skin are much more important in terms of exports. According to some reports, fins and raw skin have been exported to Asian markets and to the United States. Another interesting issue is the export to the United States of cowboy style boots, made with shark skin in some Mexican border towns.

It is very important that each shark caught is fully utilized, although this requires facilitating the commercialization of the elements that are not easy for the fishing communities to sell, such as sometimes the skin or even the cartilage itself. It is also essential to set a quota for fishermen on the number of adult sharks they can catch, always taking into account that this fishing must take place after the reproductive season.

In Mexico, dogfish meat is preferred over shark meat, but any shark measuring less than 1.5 m is called dogfish. This misclassification includes not only adult sharks of species that do not exceed that size, but also juvenile forms of larger sharks. In the latter case, by fishing "hunters" of species that have not yet reached their full growth and reproductive age, the natural balance of the populations is being jeopardized.

Sharks, elasmobranchs with a cartilaginous skeleton, are animals with an evolutionary past of 400 million years. Their wide distribution in all the world's seas, as well as the value of their meat, oils, fins, etc., have made them the target of fishermen, especially during the 20th century.

Sharks, as the top organisms in the marine trophic pyramid, play an important role in maintaining control over the species they feed on. If these predators are eliminated in an ecosystem, competition may arise between prey that have lost their predator, leading to the elimination of the less competitive "prey" species. It follows that the effects of an excessive shark fishery would affect the structure of its ecosystem.

Shark studies, especially those related to commercial species, should therefore be supported in order to understand the impact of fisheries and other human pressures. Adequate laws that guarantee the good use of this resource in the present will allow sharks to continue their magnificent evolutionary history in the future.

Species of high commercial value

Carcharhinus acronotus - Dogfish
Carcharhinus brevipinna - Beaked dogfish
Carcharhinus fa/ciformis - Silky shark
Carcharhinus /eucas - Flatnose shark, bull shark, xmoa shark
Carcharhinus /imbatus - Flying shark
Ga/eocerdo cuvier - Tiger shark, blue shark, striped shark
Ging/ymostoma cirratum - Catshark
Muste/us canis - Sucker shark
Rhizoprionodon /ongurio - Bironche shark
Rhizoprionodon terranovae - Billfish
Sphyrna /ewini - Hammerhead shark
Sphyrna mokarran - Hammerhead shark, big hammerhead
Sphyrna tiburo - Spadehead dogfish, chub mackerel

By Emma Romeu, Source: CONABIO, Biodiversitas (2)