Protective measures have allowed the stocks of various tuna species to recover. Even swordfish populations seem to be doing better with sustainable fishing. Shark populations, on the other hand, are a concern for marine researchers.
Stocks of tuna and some other large predatory fish have recovered in recent years thanks to sustainable fishing measures. However, many shark species are still threatened with extinction, reports an international research team in the journal Science. The results reaffirmed the value of conservation measures, especially for species in high commercial demand, while also showing that measures need to be extended to sharks.
With the growth of the economy and world population, ecosystems have been exploited by humans over the past hundred years and often severely impaired or even destroyed. It is estimated that about half of all commercial fish and invertebrate stocks in the oceans were overfished in the 20th century. The stocks of larger predatory fish such as swordfish and sharks also declined.
The extent of the damage to the oceanic ecosystems and how effective the protection and management measures taken are often unclear, write the researchers led by Maria José Juan-Jordá from the marine research institute AZTI in their article. They have now developed an adjusted Red List Index which – based on classifications and criteria in the International Union for Conservation of Nature Red List – indicates a species’ risk of extinction for each consecutive year.
In this way, they determined the endangerment status of seven tuna species, six other species of large predatory fish such as swordfish – and five shark species between the years 1950 and 2019. The Red List classifies species into one of eight categories – from “extinct” to “endangered”. “. In their study, the researchers focused on predatory fish, partly because they are high up in the food chain and play a critical role in ecosystems.
The authors define a species as “sustainably managed” when its fishing-related mortality rate is below the mortality rate that can ensure maximum yield for the fishery. Thereafter, the condition of tuna populations has steadily improved in the 1990s and since the late 2000s. Swordfish populations only started to recover in the last decade. On the other hand, the status of shark populations has steadily deteriorated: their mortality rate is well above the maximum possible fishery yield, which means that they are increasingly threatened with extinction.
The scientists attribute the different levels of danger to the stricter management measures of the fishing industry, which apply to tuna and many predatory fish, but not to sharks. For example, the “International Commission for the Conservation of Atlantic Tuna” has set catch limits for tuna and monitors compliance with them.
Matthew Burgess and Sarah Becker from the University of Colorado Boulder (USA) write in a commentary on the study that other biological factors such as fishing-related factors also contribute to the higher risk of extinction of sharks: The five shark species studied therefore all show slow population growth . In addition, they are often fished out of the sea as bycatch by fisheries that actually specialize in tuna or swordfish. Since these shark populations only increase slowly, they are also very vulnerable if numerous individuals of their species end up in the net as bycatch.
While some progress has been made in the sustainable management of shark stocks, the lucrative shark fin trade and associated illegal fishing threaten these achievements. While some large predatory fish such as marlin are often fished as by-catch and are therefore highly endangered, tuna in particular are specifically fished. According to the authors, their short generation time and relatively low market price are the main factors for the better conservation status of tuna species – even more so than controlled fishing.
Stocks of endangered species can improve through improved management measures in fisheries, which in the long term will also benefit the fishing industry. The protection of the particularly endangered by-catch and non-target species is likely to be more difficult because fisheries have to invest in better fishing gear and fishing methods or reduce fishing effort without directly benefiting from these changes. “The trade-offs between fishing benefits and ecosystem impacts will require difficult negotiations and compromises between stakeholders.”
