Warming seas scare away female angel sharks - with potentially dramatic consequences

Acoustic signals show behavioural change

The study was carried out by researchers from Lancaster University and the Angel Shark Project: Canary Islands - a collaboration between the Leibniz Institute for the Analysis of Biodiversity Change (LIB), the University of Las Palmas de Gran Canaria and the Zoological Society of London.
The team used acoustic transmitters to study the behaviour of angel sharks(Squatina squatina) around the Canary Islands and found that the prolonged warming of the seas in the region is upsetting the reproductive behaviour of female animals.

Females react differently to males

This became particularly clear in 2022, when exceptionally high sea temperatures prevailed. The researchers observed that female angel sharks almost completely disappeared from the La Graciosa marine reserve - the largest in Spain - during this time. Normally, this area off Lanzarote is a central mating area for the species. The water temperatures at the time were over 23.8 °C and exceeded 22.5 °C for almost three times as long as in previous years - and this throughout the mating season in autumn and winter, which is usually characterised by cooler conditions.
So while the females avoided the heat, the males returned as usual in November to breed.

Like forest fires under water

"These increasingly frequent and extreme heatwaves are like underwater forest fires - with huge, as yet unpredictable impacts on marine life," explains Dr David Jacoby, zoologist at Lancaster University and lead author of the study. "We see clear gender differences: Female angel sharks are significantly more sensitive to temperature and avoid their traditional mating areas when the water is too warm. The males, on the other hand, stick to their usual behaviour - even in extreme temperatures."
The Canary Islands mark the southernmost part of the natural range of angel sharks. The population there is considered particularly important for the survival of the species, which is classified as critically endangered according to the Red List of the International Union for Conservation of Nature (IUCN). The animals - both adults and juveniles - are regularly sighted and have become a figurehead for regional diving tourism.

Critical temperature exceeded

Between 2018 and 2023, the scientists tracked over 100 individual animals using acoustic localisation and linked their locations to environmental parameters. Prior to 2022, there were regular peaks in the presence of both sexes in November and December. However, in 2022, the number of female sharks remained unusually low throughout the year. The data suggest that 22.5 °C may represent a critical upper temperature limit for females.

Reproduction requires energy

During the five-year observation period, the maximum sea temperatures rose continuously. The number of days with temperatures above 22.5 °C rose from 30 days (2019) to 85 days (2022). This year, the warmth lasted well into late autumn and exceeded the critical limit until November - in the middle of the mating season.
The reason for the change in the females' behaviour lies in their physiology: reproduction is particularly energy-intensive for female angel sharks. In order to keep their metabolic processes in balance, they are dependent on certain temperature ranges.

In many species, females are more temperature-dependent

Dr Lucy Mead, lead author of the study and researcher at the Zoological Society of London and Lancaster University, emphasises:
"The fact that environmental changes are now leading to a divergence in the timing of male and female arrival at breeding sites is extremely worrying. Even under unfavourable conditions, male angel sharks seem to prioritise mating, while females react more strongly to temperature stimuli. This makes perfect biological sense - as with many cold-blooded species, females are more temperature-dependent.
However, with the expected further warming, important habitats could become uninhabitable for females. This has serious consequences for the protection of this endangered species."

How important it is to protect the oceans

Eva Meyers, co-initiator of the Angel Shark Project and researcher at the LIB, also warns: "These results show once again how strongly climatic extremes are already changing the behaviour of sensitive marine species.
Long-term monitoring of particularly sensitive habitats - such as around the Canary Islands - is a central element of any effective strategy for the recovery of marine biodiversity.
The Canary Islands are one of the last refuges of this species - the protection of these waters is more urgent than ever."

For Dennis Rödder, private lecturer and working group leader at the LIB, the study is important evidence of the effects of climate change on the oceans: "Anthropogenic climate change not only has a serious impact on terrestrial systems, but also influences biotic communities in our oceans. These can be very diverse: from coral mortality to demographic changes in populations"

The study entitled
"Rapid ocean warming drives sexually divergent habitat use in a threatened predatory marine ectotherm"
was published in the journal Global Change Biology:https://doi.org/10.1111/gcb.70331

The research was financially supported by:
Shark Conservation Fund, Oceanário de Lisboa, Gobierno de Canarias, Loro Parque Fundación, Save Our Seas Foundation, Ocean Tracking Network, WWF Netherlands, German Elasmobranch Society, Queen Mary University of London, Institute of Zoology (ZSL) and the Natural Environment Research Council.