Conservation Fisheries Human impacts Sharkbites Saturday

Sharks vs. Fishing Vessels: The fatal overlap


Global spatial risk assessment of sharks under the footprint of fisheries. Nuno Queiroz, Nicolas E. Humphries, et al. Nature (2019).



Over the past 50 years, world fisheries production has increased significantly. Many studies have shown fishing impacts on ocean health. Beyond the direct effects of fish removal, fishing causes an indirect effect through incidental capture of nontarget species. This is known as bycatch, and it is used to define all unwanted or discarded species. Bycatch species include sharks, that after being caught, are released back into the ocean lifeless. In some countries there are targeted shark fisheries, however most sharks casualties in the world are a result of accidental bycatch. 


Blue Shark. By Mark Conlin/NMFS

Pelagic sharks, such as tigers, hammerhead and blue sharks, live in the open waters of seas and oceans. They usually perform annual migrations and long-term trans-ocean movements. Blue sharks, for example are among the world’s most highly migratory fish species. They travel from the Caribbean Sea, along the coast of United States, east to Europe, south to the African Coast, and back to the Caribbean. Blue sharks are targeted for their fins but are also caught as bycatch. These animals are likely the most frequently  large shark in the world’s oceans suffering from the bycatch effect.


The migrating behaviors of some species of sharks overlap with different fisheries operating from coastal areas to the deep ocean. Fisheries bycatch data can be difficult to collect. An accurate data set highlighting the areas of bycatch would benefit greatly from a clearer understanding of the spatial relationships between the habitats of sharks and active fishing zones. The collection of this data is an important step for the conservation actions of many shark species.

Combining different technologies for conservation purposes

A recent study combining the efforts of more than 20 scientists around the world, focused on providing a global estimate of overlap areas between sharks and industrial fisheries. They estimated the overlap extension based on analysis of the movements of pelagic sharks tagged with satellite transmitters in the Atlantic, Indian and Pacific oceans, together with the movements of monitored fishing vessels by automatic identification system (AIS). AIS transmitters track the location of a vessel and are normally used for safety purposes and as anti-collision systems. However, in this study the AIS systems were used to track where fishing occurred.

The study focuses on 23 species of large pelagic sharks that, according to the International Union for Conservation of Nature (IUCN) Red List, are facing different levels of vulnerability by high fishing pressures. Scientists demonstrated that, globally, industrial fishing activity overlaps important areas for threatened pelagic sharks. The authors of the study believe that the overlap happens because fishers work in areas where sea conditions are attractive for both sharks and their preys. These overlapping areas increase fisherman’s chances of making lucrative catches and in turn endanger sharks due to fishing malpractices.

What does the overlap indicate?

Although sharks are generally considered to be of less commercial value to fishers, according to the study, there is a high chance for these animals to be caught from fishing vessel’s longlines in most tracked regions. Longline-fishing efforts are generally located on ecologically important spots for oceanic sharks, which limits areas that can be used as their refuge or sanctuary.

Hotspots of spatial overlap of sharks population and fishing activities (red shaded areas) and location where target sharks where caught by commercial fisheries (blue circles).

This study is important to understand the difficulties of implementing shark conservation actions. The maps also show important potential future marine protected areas for sharks, especially those where fishing activity is relatively low.

Protecting the environment through the protection of sharks

Sharks are a fundamental link in marine environment. As the top predators in marine ecosystems, they keep the oceanic food chain in balance and promote biodiversity among prey species by feeding. Sharks also play an important role in the cycling of nutrients, especially nitrogen and carbon. According to Neil Hammerschlag, some very mobile shark species can redistribute nutrients by feeding in one location and egest in another. The Grey reef shark, for example, supports coral health by egesting an estimated 95 kg of nitrogen daily onto the reef.

The decline in the abundance of large ocean predators has led to major changes in the ocean`s health. Assessing bycatch patterns across ocean regions is essential in conducting ocean conservation efforts as well as properly managing ocean conservation resources. Unfortunately, these efforts are limited since there is a lack of data available for ocean researchers. There is an urgent need for coordinated international actions to effectively reduce bycatch of megafauna species (i.e. sharks and whales) both in high seas and coastal waters. Some international and regional agencies in the world have already taken the first step by creating bycatch reduction measures. There is a pressing need for these measures to be adopted worldwide in order to reduce the constant ecological human footprint in our oceans. By reducing bycatch, fisheries activities can be more effective and sustainable. Promoting sustainability and managing human action in high-seas areas is a challenge. However, technological innovation will be the key to save top predators and maitain the balance of marine ecosystems. 

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