An international investigation involving the Biodiversity and Conservation (BIOCON) group of the University Institute ECOAQUA, belonging to the University of Las Palmas de Gran Canarias (ULPGC), through David Jiménez Alvarado, postdoctoral researcher of the institute, reveals that female angel sharks (Squatina squatina), a critically endangered species, are changing their normal mating routines in increasingly warm oceans, as they prioritize staying cool instead of visiting breeding areas when it is too hot.
According to scientists, these changes are creating a possible mismatch in mating behaviors between the sexes of the angel shark that could have serious consequences for the future of the species.
A team of marine scientists, co-led by researchers from the University of Lancaster (United Kingdom) and the Angel Shark Project: Canary Islands (the result of a collaboration between the Leibniz Institute for the Analysis of Biodiversity Change -Germany-, the University of Las Palmas de Gran Canaria (ULPGC), through ECOAQUA, and ZSL -United Kingdom-), used acoustic trackers to discover that the prolonged warming of the seas surrounding the Canary Islands is altering the reproductive behaviors of female angel sharks.
During a period of unusually high and extreme sea temperatures in 2022, researchers discovered that female angel sharks were largely absent from the species' traditional mating areas in the La Graciosa Marine Reserve, located off the coast of Lanzarote and the largest marine reserve in Spain.
That year, sea surface temperatures in the study area rose to over 23.8 °C and remained above 22.5 °C for almost three times longer than in previous years. These high temperatures persisted throughout the angel shark mating season, which traditionally begins in late autumn, when the seas should be cooler.
However, although the warmer sea seemed to deter female angel sharks, the males were not discouraged and returned to these places in November, as usual, in search of mating opportunities.
Ocean wildfires
Dr. David Jacoby, Professor of Zoology at the University of Lancaster and principal investigator of the study, says: "These more frequent and extreme heat waves are potentially the wildfires of the ocean and are having incalculable effects on marine species."
"In the case of angel sharks, we observed significant behavioral differences between the sexes: females were disproportionately affected by the increase in sea temperature, which caused them to be absent from their traditional mating sites during this prolonged warming period," the researcher highlights.
"The males were much more resistant to temperature increases and stuck to their regular arrival and departure patterns, apparently prioritizing mating despite these extreme temperatures," Jacoby points out.
The Canary Islands are located at the southern end of the angel shark's distribution and the Archipelago is an exceptionally large stronghold for the species, which is listed as 'Critically Endangered' by the International Union for Conservation of Nature (IUCN). Adults and juveniles are regularly sighted and are an emblematic species for the local diving industry.
Between 2018 and 2023, the research team was able to monitor the movement and distribution of more than 100 individual angel sharks using acoustic tracking and link this data to environmental conditions.
Prior to 2022, the presence of angel sharks, both male and female, in the marine reserve systematically peaked in November and December each year.
However, in 2022 the number of males reached its usual peak, but the number of females remained low throughout the year. The study showed that the presence of female angel sharks is closely related to temperature, and that 22.5°C, approximately, may be an upper thermal limit for females.
During the five-year monitoring period, scientists observed a general upward trend in maximum sea surface temperatures and an increase in the number of days with temperatures above 22.5 °C, from 30 days in 2019 to 85 days in 2022.
Crucially, sea surface temperatures in 2022 remained above 22.5 °C until the end of November, only falling a month later than observed the previous year. The unusually high temperatures persisted throughout the autumn and winter mating season.
More sensitive to temperature
Scientists believe that the reason why female angel sharks prioritize staying cool over mating is probably due to their biology. Due to their reproductive biology, female angel sharks have a more energetically demanding lifestyle and, consequently, are more sensitive to temperature, as they need to regulate metabolic processes and energy expenditure.
Researchers are concerned that these alterations in the reproductive behaviors of angel sharks could have serious consequences for the future of the species.
"The fact that extreme environmental conditions cause differences in the arrival times of males and females at coastal mating sites is especially worrying for this critically endangered species," says the lead author of the study, Dr. Lucy Mead, researcher at the ZSL Institute of Zoology and the University of Lancaster.
In her opinion, "angel sharks, like most sharks, are ectothermic, which means that their body temperature depends directly on the temperatures of the surrounding water. It seems that male angel sharks prioritize mating even when conditions are unfavorable, while females prioritize staying within their preferred temperature range."
Mead states that "the Canary Islands are already at the warmer end of tolerable temperatures for the angel shark, and with significant warming predicted here, our concern is that key areas may become inhospitable for females."
"These findings have important implications for the conservation of this highly threatened species in a warming world. The effects of climate change on the ocean tend to go unnoticed, but just as heat waves and forest fires are becoming more frequent on land, so are extreme phenomena in the oceans," she said.
Eva Meyers, co-director of the Angel Shark Project and researcher at the Leibniz Institute for the Analysis of Biodiversity Change (LIB), declares that "these findings remind us how climate extremes are already reshaping the behavior of threatened marine species. This study underlines exactly why long-term monitoring of critical habitats in the Canary Islands must be a fundamental pillar of any effective marine biodiversity recovery plan."
"The Canary Islands are one of the main strongholds of this species. Safeguarding these waters is more urgent than ever," she highlights.
Conservation Strategies
Dr. David Jiménez Alvarado, co-director of the Ángel Shark Project and postdoctoral researcher at the ECOAQUA Institute, assures that "it is important to understand how changes in the physicochemical conditions of the ocean can affect the species that inhabit coastal ecosystems. These environmental changes can alter the habitat use and behavior of key predatory species that play a crucial role in maintaining trophic balance."
Jiménez Alvarado asserts that "since many of these species are already threatened worldwide, studying how climate-related stressors affect their ecological function is essential to develop effective conservation and management strategies."
The research is described in the article 'Rapid ocean warming drives sexually divergent habitat use in a threatened predatory marine ectotherm', which has recently been published in Global Change Biology.
The research has been supported by Shark Conservation Fund, Oceanário de Lisboa, Government of the Canary Islands, Loro Parque Foundation, Save Our Seas Foundation, Ocean Tracking Network, WWF Netherlands, Deutsche Elasmobranchier Gesellschaft, Queen Mary University of London, ZSL and Natural Environment Research Council.
