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Annual rates of marine mammal strandings have increased in the last 30 years, across all species of dolphins, whales and porpoise native to Scottish waters.

The concerning new finding comes from a study led by researchers at the University of Glasgow, and marks the first time scientists have been able to quantify the scale of the increase in marine strandings around Scotland's coastline.

The study, which looks at all cetacean species seen in Scottish waters—baleen whales, short-beaked common dolphins, deep divers, harbor porpoises and pelagic dolphins—is in Scientific Reports.

The research team used a 30-year dataset collected by the Scottish Marine Animal Stranding Scheme (SMASS) between 1992 and 2022 to examine the distribution and trends in marine mammal strandings. While overall, the data showed increasing stranding rates across all cetacean species, some species showed steep increases in their stranding rates.

Two species, common dolphins and baleen whales, showed an exponential increase in strandings, suggesting these species are facing unprecedented pressures in Scottish waters. While this trend may partly reflect increased record effort in recent years, the scale and consistency of the rise indicates a genuine cause for concern. Experts say, based on the findings of this new study, common dolphins and baleen whales should now be prioritized for conservation efforts.

A total of 5,147 cetaceans were included in this study, having stranded in Scotland between 1992 and 2022. Harbor porpoises accounted for more than half of all the strandings included in the study (2,676, 52%), followed by pelagic dolphins (1,217, 24%), common dolphins (494, 10%), baleen whales (479, 9%), and deep divers (281, 5%).

Stranding rates for baleen whales and common dolphins remained consistently low during the first two decades of the study but reports rose sharply from 2010, followed by an exponential increase. The data also revealed a disproportionate rise in strandings among juveniles of both species, indicating that younger animals may be particularly vulnerable.

In contrast, deep divers and pelagic dolphins showed a steadier increase in strandings, while harbor porpoises had an oscillating pattern, but with an overall increase in strandings. There were no sex differences in annual stranding rates, although the research team were able to pinpoint distinct seasonal trends for each group.

Overall, stranding events were widespread across Scotland, but with strandings of different species clustering in certain areas. While almost all species strand on the northwest coast, porpoises predominantly strand along the east coast around the Inner Moray Firth, the Outer Moray Firth and Forth and Tay, and the southeast in the Clyde.

Dr. Andrew Brownlow, Reader and Director of the SMASS, said, "This research shows how reports of stranded marine animals—many made by members of the public—can offer important clues about the overall health of our seas. These animals act as sentinels of the ocean, and rising numbers of strandings may be an early warning that something is changing in the marine environment.

"This research demonstrates the exceptional value of Scotland's long-term ecological datasets. While some of the findings raise important concerns, the study also highlights that different regions face unique threats, each requiring tailored mitigation strategies. By identifying where and when species are most at risk, we can target monitoring and conservation efforts at the critical times and locations needed to best safeguard the health these ecosystems."

Rachel Lennon, lead author and Ph.D. researcher at the University of Glasgow, said, "To conserve and manage marine species effectively, we need to understand how their populations are changing over time, but for many marine mammal species, that information is incredibly hard to obtain. The inaccessibility of the deep ocean makes it exceedingly difficult to monitor, or even detect, population trends in many species.

"While effort-based surveys of live cetaceans provide valuable insights, their high financial and logistical costs often limit how frequently they can be conducted. That's where stranding schemes like SMASS play a critical role, as they offer a cost-effective and continuous source of biological and mortality data, making them an essential complement to traditional monitoring approaches."

While exact causes are difficult to pinpoint, scientists agree that human activities are steadily intensifying within ocean ecosystems, and that this is likely impacting wildlife populations. Predominant threats include declining fish stocks, increased bycatch and entanglement rates, noise pollution, habitat change from warming sea temperatures, and cascading impacts from chemicals and plastics.

Lennon added, "Fisheries interactions are recognized as a critical threat to marine mammals, especially to baleen whales in Scotland. As populations begin to recover from the impacts of historic whaling, reports of entanglement have increased.

"While determining the exact causes of strandings can be complex, identifying these emerging trends provides a vital starting point for targeting future research. It also underscores the value of long-term stranding programs as practical monitoring tools that can help guide conservation strategies and policy interventions."