Sharks - like almost all living creatures - need oxygen for their energy supply. Again, climate change is making it harder for great white sharks and many other large pelagic species to survive.
Warm water has a reduced oxygen content due to a lower storage capacity. Thanks to their high metabolic rate, the large, charismatic species are particularly sensitive to a drop in oxygen levels . In some coastal and marine regions around the globe, oxygen-rich layers are becoming shallower, driving shark species closer to the surface. There, they are more vulnerable to fishing gear, end up more as bycatch or get caught in ghost nets.
A bad start in life - Epaulette Shark © C. Gstöttner
Queensland epaulette sharks (Hemiscyllium ocellatum) are a species of small bottom-dwelling sharks from the bamboo shark family.
They are found mainly off the coasts of Australia and in the South Pacific. Like 40 per cent of all shark species, they lay eggs that take three to four months to hatch. During this time, the developing embryo is completely on its own, as there is no brood care. It is completely at the mercy of the elements - and thus the rising water temperatures.
The thermal stress exerts a strong impact on epaulet shark eggs, with drastic consequences . Studies show that juveniles raised under artificially high temperatures hatch earlier and are thus born weaker than conspecifics in cooler regions. Juveniles are often smaller and have fewer markings, making them more vulnerable to predation.
If temperatures rise above a certain threshold, embryos may not even survive to hatch. Since sharks have a low reproductive potential anyway, this would have devastating consequences for steadily declining population numbers.
Acidic oceans = Hungry sharks
The increase in atmospheric CO2 concentration not only leads to rising temperatures, but also affects the chemical composition of seawater. The uptake of additional CO2 from the atmosphere causes the pH of the ocean to drop - they become more acidic .
The effects of this acidification are mainly studied in the examples of calcifying organisms such as corals and mussels, but sharks also feel the consequences. Studies from Cape Town on puff adder catsharks (Haploblepharus edwardsii) clearly show that the composition, formation and strength of denticles suffer under the changed conditions . Since shark teeth and denticles have a similar structure, the effect on teeth is expected to be analogous.
Pelagic species, which rely on efficient swimming and sudden acceleration, will be particularly affected. Shortfin mako sharks (Isurus oxyrinchus) reach top speeds of up to 74 km per hour, with hydrodynamically shaped denticles playing a major role . Altered surface structure will lead to increased energy expenditure, a definite disadvantage for many open ocean inhabitants. This, plus weakened and brittle teeth, paints a sad picture for the sharks of the future.
But bottom-dwelling sharks will also feel the change. Here, the shape of the denticles does not create a speed advantage, but serves as protection against predators . A reduction in armour will be problematic, especially for young animals.
The increasingly obvious effects of climate change are crystallising into one of the most significant problems of our time. We need every ally and resource possible to save our future - and that of our planet. Sharks are on our side!
Robust shark populations contribute significantly to maintaining healthy reefs & seagrass beds. Both absorb large amounts of atmospheric CO2 and provide us with what we desperately need - oxygen.
In just under three months, the period for supporting the EU citizens' initiative "Stop Finning - Stop the Trade" will end. We can't do it without you - your voice for shark protection: https://eci.ec.europa.eu/012/public/#/screen/home
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Text Sarah Russwurm
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