Fishing intensity keeps increasing throughout the world, which has negative impacts on nontarget species, including marine mammals. Thus bycatches have become a critical issue for marine mammal populations.

Every year fishing industry removes about 80–90 million tons of fish and other marine organisms from the world’s oceans. Another 7–8 million tons of unwanted animal biomass is caught and discarded. Such levels of fishing activity have significant effects on marine ecosystems, especially on top predators. Mortality due to interactions with fisheries is likely to be the most serious, immediate anthropogenic threat to most marine mammal species.

Many coastal fishermen consider dolphins as serious competitors. Common dolphins (Delphinus delphis) have often been perceived as a threat to fisheries in and, consequentally, have been deliberately caught. Reasons for declines in populations are thus direct and incidental catches by fisheries.

The issue fishermen see about dolphins is that dolphins collect fish from the nets and tear them in doing so. Bottlenose dolphins, killer whales, false killer whales and pilot whales are notorious fish stealers, there are reports of gear damage by these species. Cetaceans may feed directly on a fishery’s target species or prey on fish confined in mariculture enclosures (i.e., fish farms).

Marine mammals generally reproduce slowly and their populations are not able to withstand additional non-natural mortality. The removal of just 1 % of the population per year may be more than a population can sustain in the longer term. Long-term effects of extreme population depletion include reduced genetic diversity and associated health and reproductive problems.

Excessive fishing consequnces

• fish numbers decline –> existential dolphin food decline;
• destruction of underwater habitats –> depletion of marine resources;
• entaglement in fishing nets;
• bycatch: one of the most important sources of anthropogenic mortality among many species of marine mammals;
• dolphins used to be, in some parts of the world still are, hunted for meat, skin, bones, oil and delphinariums. In Japan dolphins and porpoises are taken by drives, hand harpoon and small-type whaling.
• Prey depletion: considered of primary or secondary importance in causing habitat degradation and loss of small cetacean species.

The most widespread threat to populations of small cetaceans is bycatch. Synthetic gillnets were widely introduced into the world’s fisheries as a durable and inexpensive fishing gear. Since the 1960s national governments have provided gillnets to fishermen and promoted their use, radically changing the nature of fishing in coastal and freshwater ecosystems around the world. Synthetic nets are less expensive and easier to maintain than nets made of cotton and hemp. In promoting their use, agencies have not given adequate consideration to the impacts of unintentional mortality of non-target species, such as small cetaceans, sea turtles, and other vulnerable megafauna. Bycatch in gillnets became the primary driver of population declines in many species of marine mega fauna, including elasmobranchs, sea turtles, seabirds, and marine mammals. Gillnets are responsible for the deaths of hundreds of thousands of cetaceans each year. Stronger, better-enforced conservation measures are called for, including a complete ban on all fishing operations using gillnets and other gear known to kill and injure dolphins.

Tuna fishing

One of the very important interactions and a serious concern for the conservation of marine mammals was the large-scale capture of pelagic delphinids, mainly Stenella and Delphinus species in tuna fishing. Tuna boat skippers learned they could catch large tuna by herding dolphin schools with speedboats and then surrounding them with long, deep nets. Fishermen learned that in some areas, large yellowfin tuna Thunnus albacares will school under and follow dolphin schools. Pelagic delphinids seem to find it difficult to escape from an enclosed net situation, and many become trapped and die. This fishing technique begun in the 1950s, but was not recognized as a potential problem until the 1970s, when a monitoring program was established. A variety of efforts were made to reduce the kill, but tens of thousands of dolphins were still being killed annually. Largely as a result of public pressure and the introduction of dolphin safe tuna retailing, this practice has been reduced. New techniques have been devised to ensure that a high proportion of dolphins »used« this way to catch tuna are encouraged to escape from the nets before the fish are removed. The populations have not yet shown strong signs of recovery. In areas where dolphin abundance is high, some fishermen use alarms on nets. Despite such efforts, there is still incidental capture of cetaceans. So note, that even when tuna meat is labelled as dolphin safe, there were still a number of dolphins killed for the purpose.

Dolphin and fishery cooperations

In a few places in the world; west waters of Africa and Brazil, fishermen have learned to collaborate with dolphins. Fishermen spread the nets, dolphins force the fish into the nets, fishermen wait for dolphins to feed, then take the rest.

There are reports of associations between dolphins and native fishermen at the region of Morenton Bay. The natives would hit the water with their spears and the dolphins would come close herding the fish. There are mutual advantages.

There are complex behavioral sequences, ritualized and demanding a precise coordination between both parts. The association is always initiated by the dolphins, while the fishermen await for the right moment. The behavioral patterns of T. truncatus change according to environmental factors and the distribution of preys. Both the dolphins and the fishermen take profit in such association. The fishermen act as an active barrier, thus striking the prey back. The advantages brought by the cooperation became evident through the increase in the efficiency of capture when compared to independent fishery. The preys captured during the interaction were significantly larger than those in independent fishery, showing size selectivity.

Bycatch is recognized as one of the most important sources of anthropogenic mortality among many species of marine mammals. For this reason, many nations now legislate to protect marine mammal populations from deliberate or accidental exploitation and there are several international agreements with the same aim. Fisheries are dynamic and are changing activities that importantly affect the environment, thus they must be monitored continuously.

So next time you think about chosing marine fish, specially tuna, for a meal, know that every single one of your/our actions has cansequences. All it takes to make a change for the better, are some thoughtful decisions and actions.

References:

• Encyclopedia of marine mammals
• http://www.scielo.br/pdf/rbzool/v15n3/v15n3a16.pdf
• https://www.int-res.com/articles/esr2019/40/n040p285.pdf