Shining the light on threats to life in the dark

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We know more about space than we do the deep sea. 

We are only just discovering and learning about deep sea elasmobranchs. However, they face growing threats from climate change and overfishing; these anthropogenic (man-made) impacts highlight the need for a better understanding of how we can conserve deep-sea communities.

When you think of Elasmobranchs, you typically think of the charismatic manta rays, hammerhead sharks, whale sharks, and other fish of the blue. Deep-sea Elasmobranchs, however, are far from typical. These creatures live below 1000 m in what’s called the ‘midnight zone’, the area of the ocean where light doesn’t penetrate at all. This environment encompasses the largest ecosystems on the planet, and is home to over half of all shark species.

The Portugese Dogfish (Centrophorus squamosus) is the deepest living elasmobranch, reaching depths of up 4,000 m below sea level. It is well-adapted to the deep: it has better vision than most sharks thanks to its large pupil and lens and a high concentration of ganglion cells (cells in the eye that enable it to detect motion with high sensitivity). It can also detect bioluminescence (ability to glow in the dark) in their prey, which includes squid and bony fishes (Guallart et al., 2015).

Other deep-sea elasmobranchs, such as frilled shark, blackmouth catshark, and Greenland shark are also well-equipped with other useful adaptations such as sharp defensive fin spines, an acute sense of smell, large oily livers for buoyancy and low metabolisms to save energy (Serena et al., 2009).

Above: Portuguese Dogfish. IUCN red list: Endangered.

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Above: Blackmouth Catshark (Galeus melastomus). IUCN red list: Least Concern. (Image: Serena et al., 2009).

Historically, deep-sea elasmobranchs and the ecosystems they inhabit have been considered to be too isolated and stable to be affected by anthropogenic threats. However, in recent years, we are learning that climate change, pollution and overfishing can all pose a significant threat.

Overfishing

Fisheries are moving deeper and deeper into our oceans, targeting deep-sea elasmobranchs for their liver oil (Morato et al. 2006) which is used in a range of cosmetic and medical products. Many species are also caught as bycatch of longlines targeting bony fishes and trawlers targeting crustaceans (Cohelo and Erzini, 2008).

Like other elasmobranchs, deep-sea sharks have life-history traits that make them vulnerable to overexploitation by fisheries. They are generally slow-growing, are late to mature, have low fecundity and have long life-spans. These life-history traits mean that they are at the lower end of the fish productivity scale and have little ‘intrinsic rebound potential’, i.e. they take long periods of time or have little capacity to recover from overfishing (Simpfendorfer and Kyne, 2009).

This low recovery potential means that deep-sea elasmobranchs are at high risk of extinction. When we look at the fishing mortality necessary to drive chondrichthyans to extinction, deep-sea species require 42% fewer deaths than continental shelf species to become extinct. In fact, many species are already threatened; the Portuguese dogfish, for example, has declined by up to 90% in the past 3 years alone (Guallart et al., 2015).

Pollution 

Human impacts like the Deepwater Horizon oil spill highlight the devastating effect pollution can have on deep-sea communities. This 2010 oil spill is widely recognised as one of the worst environmental disasters in US history, releasing around 5 million barrels of oil into the environment.

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Although the impacts are still being studied, research showed the oil was toxic to a wide range of organisms; including plankton, invertebrates, fish, birds, and sea mammals. Effects included increased mortality, deformities, reduced growth and reduced feeding (Langangen et al., 2017). These wide-reaching effects meant that even if sharks didn’t come into contact directly with the oil, because they are top-predators, their food supply was likely affected, potentially leading to population declines (Peterson et al., 2003).

Being top predators also means that deep-sea elasmobranchs are vulnerable to pollution because of bioaccumulation. This is when predators accumulate all the pollutants and toxins from their prey, which become more ‘concentrated’ up the food chain. For example, zooplankton, a component of many marine animals diets, ‘absorb’ toxins, which are then passed onto predators through the food chain.

These chemical toxins, including pesticides and pharmaceuticals, have been found in shark species all over the world; from sharks in areas of high human activity like Florida to Greenland sharks that prefer the isolated colder waters of the Arctic. Pollution, therefore, is a global problem.

Oil and chemicals, however, aren’t the only pollutants. Discarded plastic, nets, fishing gear and other rubbish found on the ocean floor poses a serious threat to deep-sea elasmobranchs that can become entangled or ingest rubbish and die.

Climate change 

Deep-sea communities are likely to be some of the worst affected by climate change; projections suggest deep-sea temperatures could increase by 1°C over the next 84 years, pH could reduce by 0.8 units (ocean acidification) and oxygen concentration could reduce by up to 3.7% or more (warmer water can hold less gas).

Elasmobranchs are likely to be affected directly by these changes as they reach their tolerance limits to temperature, pH and oxygen concentration. This will mean that many species could experience range shifts and range restrictions, as the areas they are able to inhabit become smaller. This is already predicted for many shallower species; for example, whale sharks are predicted to shift their habitat towards the poles in both the Atlantic and Indian Oceans as temperatures increase (Sequeira et al., 2013).

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Above: Whale shark (Rhincodon typus). IUCN red list: Endangered.

Indirectly, elasmobranchs may be negatively impacted by changes further down the food web: reduced primary production, changes to prey species compositions and reduced biodiversity. Unfortunately, the consequences of this are, in many cases, poorly understood. However, it is almost certain that climate change will cause significant changes to overall ecosystem function and composition (Sweetman et al., 2017).

Although some animals may be able to adapt in some capacity to climate change, for example, by accelerating their growth and reaching maturity at earlier ages, the slow population growth rates and long generation times of deep-sea elasmobranchs ultimately limits their adaptability. Hence, we are at a danger of losing many of these ‘midnight’ species before we have had a chance to learn about them.

Journal articles:

Coelho and Erzini., 2008. Effects of fishing methods on deep water shark species caught as by-catch off southern Portugal. Hydrobiologia606, 187193

Guallart et al., 2015. Centrophorus squamosus. The IUCN Red List of Threatened Species 2015: e.T41871A48954989. Downloaded on 09 July 2018.

Langangen et al., 2017. The effects of oil spills on marine fish: Implications of spatial variation in natural mortality. Marine Pollution Bulletin. 119(1); 102-109.

Morato et al., 2006. Fishing down the deep. Fish and Fisheries 7: 24–34

Peterson, et al., 2003. Long-term ecosystem response to the Exxon Valdez oil spill. Science, 302(5653), 2082-2086. doi: 10.1126/science.1084282.

Serena et al., 2009. Galeus melastomus. The IUCN Red List of Threatened Species 2009: e.T161398A5414850. http://dx.doi.org/10.2305/IUCN.UK.2009-2.RLTS.T161398A5414850.en. Downloaded on 09 July 2018.

Sequeira et al., 2013. Predicting current and future global distributions of whale sharks. Global Change Biology. 20(3).

Simpfendorfer and Kyne, 2009. Limited potential to recover from overfishing raises concerns for deep-sea sharks, rays and chimaeras. Environmental Conservation 36 (2): 97–103

Sweetman et al., 2017. Major impacts of climate change on deep-sea benthic ecosystems. Elem Sci Anth, 5: 4, DOI: https://doi.org/10.1525/elementa.203

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