Where will the sharks go when it gets too hot?

Predicting changes to shark ranges under different climate change scenarios

It is well documented that the emission of greenhouse gases in our atmosphere is causing the temperature of our oceans to rise and that this has impacted where and how marine species live. But when considering the future, how do we go about trying to predict how species will further respond to continually warming waters? And how, with action pledges like those made at COP26, can we predict the outcomes if we successfully meet, or unsuccessfully miss, our global climate objectives?

Sharks are a key species in ecosystem functioning as they help to maintain healthy fish populations. They also migrate over long ocean stretches, connecting distant ecosystems, another important role. However, they are already facing severe threats, largely from overfishing, which has resulted in many species being threatened with extinction. Another threatening factor is that most shark species are ectothermic, meaning that their body temperatures change as the temperature around them changes, with higher temperatures increasing their metabolism and oxygen demand, requiring them to eat more food to survive. Because of this, they could be significantly affected by rising ocean temperatures.

Given this, scientists have been using climate models to predict the areas that will become suitable to pelagic sharks (those that live in the open ocean) with increasing temperatures in Australian waters, which are warming at almost four times the global average. They published their findings in Frontiers in Marine Science in 2020 where they focussed on two shark families; mackerel (which includes the shortfin mako) and requiem (which includes the oceanic whitetip). Within the models, they used shark occurrence data taken from fishing catch records, a total of 3973 individuals, as well as ocean temperatures. Two climate scenarios were modelled; one which represents a stabilisation scenario assuming less use of fossil fuel, and one which represents the worst-case scenario of ‘business-as-usual’ for fossil fuel use with increasing populations and high energy demands.

Shortfin mako and oceanic whitetip sharks. Credits to Jonathan Lavan (upspurdog from iNaturalist) and Lindensea (from iNaturalist).

Their results showed shifts of suitable habitat for both shark families. By the end of the century, habitable areas decreased off the south-western and south-eastern Australian coasts, as waters exceeded the temperatures that these shark families can tolerate. In other areas, the habitat for the mackerel sharks increased as some species are endothermic (meaning they are able to somewhat regulate their own temperature) so are able to acclimatise to rising temperatures. However, this acclimatisation comes at an energetic cost which might impair how they grow, reproduce and feed. It is likely that the physiological impacts from warming temperatures will be great for these active pelagic shark species as they may face starvation if increases in metabolic rates are not met with higher food intake. Their prey availability might also be impacted as the Australian coastline has limited continental shelf area to the south, meaning there is limited space for marine organisms, including shark prey species, to move south to avoid higher water temperatures. However, species cannot only migrate latitudinally but also vertically, meaning that they can move deeper to occupy more favourable temperatures. This might be the answer to allow individuals to remain within waters of a suitable temperature.

This kind of research gets us thinking here at ELMO, on the other side of the Indian ocean: could studies like this be applied to our shark species in South African waters? For our endemic species, what might the impacts of rising ocean temperatures be by the end of the century? This highlights to us the importance of our citizen science data on shark sightings that you help us to collect, as it is providing us with the tools to potentially undertake valuable research such as this.

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Reference:

Birkmanis, C.A., Freer, J.J., Simmons, L.W., Partridge, J.C. and Sequeira, A.M.M. (2020). Future Distribution of Suitable Habitat for Pelagic Sharks in Australia Under Climate Change Models. Frontiers in Marine Science, Volume 7, Article 570