Annual rates of marine mammal strandings have risen over the past 30 decades, with all native species of dolphins, whales and porpoise affected.
For the first time scientists have been able to quantify the scale of the increase, in a study led by University of Glasgow researchers. Using 30 years of data collected by the Scottish Marine Animal Stranding Scheme (SMASS) between 1992 and 2022, they analysed distribution and trends in the strandings of the 5,147 cetaceans recorded.
Of these, 52% were harbour porpoises (2,676). A total of 1,217 pelagic dolphins accounted for 24%, followed by 494 common dolphins (10%), 479 baleen whales (9%) and 281 deep divers such as beaked and sperm whales (5%).
Some species showed particularly steep increases in stranding rates over the period, including common dolphins and baleen whales. The rates had remained low during the first two decades but then reports of strandings started rising sharply from 2010 onwards.
A disproportionate increase in strandings among juveniles of both species indicated that younger animals might be particularly vulnerable.
Cause for concern
The finding might reflect better recording of incidents but the scale and consistency of the rise gives cause for concern, say the scientists, and indicates that common dolphins and baleen whales should be made a conservation priority.
Deep divers and pelagic dolphins showed a steadier increase in strandings, while the trend in harbour porpoises oscillated, albeit with an overall increase.
There were no gender differences in annual stranding rates, but the researchers were able to pinpoint distinct seasonal trends for each group.
Geographically, most species strand on Scotland’s north-west coast but some tend to cluster in other areas. Porpoises, for example, strand mainly along the east coast around the Inner and Outer Moray Firth and the rivers Forth and Tay, as well as the Clyde in the south-west.
“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,” comments SMASS director Dr Andrew Brownlow.
“This research demonstrates the exceptional value of Scotland’s long-term ecological datasets. 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 of these ecosystems.”
Critical role
“The inaccessibility of the deep ocean makes it exceedingly difficult to monitor, or even detect, population trends in many species,” says PhD researcher and lead author Rachel Lennon.
“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.”
The study does not pinpoint causes for the strandings but indicates that declining fish stocks, increased bycatch and entanglement rates, noise pollution, habitat change from warming seas and chemical and plastics pollution all play a part.
”Fisheries interactions are recognised as a critical threat for marine mammals, especially for baleen whales in Scotland,” says Lennon. “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 programmes as practical monitoring tools that can help guide conservation strategies and policy interventions.” The study is published in Scientific Reports.
