Species Wars: Sharks are now significantly smaller and rarer in areas closer to human population

The body size and the number of sharks and other marine predators fall by 5% and 20% respectively at distances less than 1,250 km from human activity


                            Species Wars: Sharks are now significantly smaller and rarer in areas closer to human population

The average body size of marine predators such as sharks, which are vital to maintaining healthy ocean ecosystems, are falling dramatically in habitats close to human population and fish markets, a new study has warned.

A research team, led by the Zoological Society of London (ZSL), has discovered that there are 20% fewer sharks at distances less than 1,250 km from human activity, in areas where sharks are caught and killed intensively for their meat and fins.

This, says the study published in the journal PLOS Biology, shows that human activity is now the biggest influence on sharks’ distribution, overtaking every other ecological factor. 

The study defines marine predators as animals that actively prey on other individuals, including top predators at the apex of the food web, such as billfish and sharks, which have few natural enemies as adults.

According to Dr. Tom Letessier from the Zoological Society of London, the findings are important as large marine predators - and sharks in particular - play a “unique and irreplaceable role” in the ocean ecosystem by controlling populations of prey species, keeping those populations healthy by removing sick or injured animals, and transporting nutrients between loosely connected habitats over vast distances. 

“Human activity is now the biggest influence on sharks’ distribution, overtaking every other ecological factor. Just 13% of the world’s oceans can be considered ‘wilderness,’ but sharks and other predators are much more common and significantly larger at distances greater than 1,250 kilometers from people. This suggests that large marine predators are generally unable to thrive near to people and is another clear example of the impact of human overexploitation on our seas,” Dr. Letessier told MEA WorldWide (MEAWW).

He further said: “Regarding the body size, distance to fishing markets account for a 5% decline in the body size of the fish and marine predator community. This might not sound like a lot, but it is when you consider that this is for all animals - that is, wildlife is, on average, 5% smaller in areas that are fished.”

An earlier study by an international team of over 150 scientists from 26 countries had concluded that shark hotspots are under worldwide threat from overfishing.

Their analysis showed that major high seas fishing activities are currently centered on ecologically important shark hotspots globally.

The study, published in Nature, found that 24% of the space used by sharks on an average month falls under the footprint of longline fishing.

For commercially exploited sharks such as blue and shortfin makos sharks in the North Atlantic, the overlap was much higher, with on average 76% and 62% of their space use, respectively, overlapping with longlines each month.

The researchers found that even internationally protected species, such as great whites, had over 50% overlap with longline fleets.

For the current research, the team examined the video footage taken at 1,041 sites across the Indian and Pacific Oceans, selected to test the biggest possible range of conditions and habitats.

Sites varied in proximity to fish markets and human populations, with some close to cities and others up to 1,500 km away.

Sharks - and other free-swimming predators - were studied using cameras attached to canisters filled with bait.

Overall, the team recorded 23,200 animals representing 109 species - 22,334 individuals from 85 teleost fish species, 841 individual sharks from 19 different species, 23 individuals from three reptile species, two marine mammal species (two individuals), and two ray species (five individuals). 

A blue shark swims past bait canister. (Dr. Tom Letessier, ZSL)

The analysis shows that both the average body size and the number of sharks and other marine predators fell significantly in proximity to cities with more than 10,000 people and associated fishing fleets.

“The minimum distance from people and fishing, which had no measurable effect, was 1,250 kilometers. This is far further than previous studies have suggested and probably reflects the increased distances fishing boats can now travel. As a result, sharks were only observed at 12% of sites monitored,” the findings state.

The team found that sea surface temperature also has a strong influence on the average body size of predators, with a marked decrease at more than 28°C.

“While this is consistent with normal biogeographic patterns - it is known that many smaller species live in tropical waters, for example - it could become a problem as global temperatures continue to rise,” says the study.

The researchers recommend immediate conservation action to prevent further declines.

According to the findings, shallower water habitats, of depths less than 500 meters, are important for marine predator diversity.

“Our study also found that shallower water habitats, of depths less than 500 meters, were vital for marine predator diversity. We, therefore, need to identify sites that are both shallow and remote and prioritize them for conservation. However, there are still numerous shallow hotspots in the vicinity of human markets that are not appropriately protected, and this must change. Existing large, no-take marine-protected areas need to be better enforced and extended to focus on the last refuges where these extraordinary animals remain abundant,” Dr. Letessier told MEAWW. 

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