Sharks play an essential rol in the health of our oceans

Information, News, Research

What is actually threatening the sharks?

By Dorien Schröder, marine biologist

To this day a lot of people would answer that question with shark finning. Even a lot of conservation organisations will use the number of 73 million sharks being killed for their fins each year as a reason for you to support them. But as it turns out, the international criticism, combined with stricter rules on trade and changing attitudes in Asian countries, has been super effective in reducing shark fin consumption. Unfortunately, that does not mean our work is done. Other factors have now surpassed shark finning as threats to the shark populations. Below, we will discuss the two main threats.

Overfishing and bycatch

With the threat of shark finning becoming much less, one might think fishing is not a problem anymore. However, sharks are still being fished. Not just for their fins, but for their meat up to 100 million sharks are caught each year.

Spanish and Portuguese fishermen that are longlining in the North Atlantic often encounter blue and mako sharks on their hooks, that are sold on the markets for their meat. Queiroz et al. (2016) found that blue and mako sharks are often using the same waters as longlining vessels (20 and 12 days per month respectively), creating a high chance of them being caught.
According to Oceana (2013) up to 24 countries are catching sharks in the Atlantic and Mediterranean without reporting the catches. This makes it difficult to manage the shark species and their fisheries.
For some fishing vessels the sharks are bycatch that has to be landed according to regulations (for instance in the European Union). So even though the market for shark meat (and other bycatch species) might not be that big, they are not allowed to throw the bycatch back. This is a shame, because some species can survive catch and release quite well.

According to the FAO (2018) Spain is by far the biggest shark fishing country (77% of total shark fishing in the European Union), followed by Portugal, France and the UK. Not all European countries have a big market for shark meat. Italy, France, Belgium and Switzerland use a lot of shark meat. Some of that is sold under a different name, or used in cosmetics and supplements, and because of this people might not realise that they are consuming sharks. In the United Kingdom, shark is often used in the famous ‘fish and chips’. 
Shark meat itself is not sold for a high price in the markets. However, by selling the cartilage for cosmetics and supplements, and the fins to the Asian market, a shark can still bring in a lot of money.

 

Often shark meat is sold as other, more expensive fish, like swordfish steak, or tuna. In some countries, 60% of all shark meat is mislabelled. 

 

Shortfin mako (Isurus oxyrinchus), steaks on fish market, labelled ‘Plamtu’, Atlantic Bonito

 

Climate change

We have all heard about climate change. The temperatures are rising (1° in the past 100 years) because of the increased carbon dioxide (CO2) concentration (40% in the past 100 years) in the atmosphere adding to the greenhouse effect. With the air temperature rising, so does the temperature in the oceans. And with the CO2 concentration in the atmosphere rising, so does that of the ocean. The CO2 added to the oceans is also causing ocean acidification because of the chemical reactions between water and carbon dioxide.

So how is all of this affecting the sharks? Several studies have been done recently to find out what the effect of these changing conditions will be on sharks. One study (Dixson et al., 2015) found that under the CO2 concentrations projected for the end of this century, sharks were less responsive to odour cues and showed reduced attack behaviour. This implies that the change in CO2 concentration can cause changes in feeding behaviour, and with that potential starvation in sharks.
Another study (Di Santo, 2015) showed that both the increased temperature and ocean acidification are negatively affecting skate embryo development, survival, activity, metabolic rates and body condition. Johnson et al. (2016) had similar results with shark embryos reared in increased CO2 concentrations. Some studies (Pistevos et al., 2017) show some positive effects of increased temperatures, however when this was combined with inceased CO2, the positive effects were eliminated. This implies that climate change can reduce the reproductive success of sharks and skates.
Besides these negative effects on both reproduction and the ability to find and catch food, other effects of climate change are much more difficult to predict. Prey species might move with the changing water conditions, and so will some shark species. This will change ocean food webs and the effects of could be catastrophic.

It’s time for us to stop focussing just on the fins of sharks, and start looking at entire populations!

 

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