First Ever Review On Global Shark Strandings Uncovers Interesting Trends

The whirring of the drone hovering above the shore is drowned out by crashing of the waves against the massive black bodies thrashing weakly against the white sand. “How many can you see?” a volunteer asks the drone pilot, who is climbing higher to get a better picture of just how many of these marine beasts have decided to beach themselves here. There are hundred and counting…

The sun is barely up, but these volunteers have been here for hours trying to keep the giants alive. Everyone is soaked and tired, the air a mixture of death and salty brine. Strandings. There doesn’t exist an exact definition, since it sometimes can depend on the group of animals, but it is generally accepted that these animals may be found dead (either on land or floating) or alive but unable to return to sea due to injury or illness. Why they happen isn’t clear, although mass strandings are usually associated with disease outbreaks, climatic events or as a result of anthropogenic disturbances. A global phenomenon, for some taxonomic groups these events are well monitored and documented. Marine mammals, turtles, and seabirds are most commonly reported in stranding reports. And although sharks are among the most threatened vertebrates (i.e. 31.2% of currently described species are now threatened with extinction), their strandings have historically been neglected, with little information currently available on their numbers and subsequent trends.

While overfishing is undoubtedly the main contributing factor for observed population declines in these animals, their strandings may also shed light on additional stressors that could impact survival. To address this knowledge gap, researcher Natascha Wosnick from the Programa de Pós-Graduação em Zoologia at the Universidade Federal do Paraná in Brazil led a systematic review on scientific publications from indexed databases, multimedia material and citizen science databases. A total of 3,150 reports were recovered by the team, with strandings dating back to 1880!

The reports primarily came from scientific publications and abstract conferences. Video and photographic records on multimedia platforms (e.g Facebook, YouTube, Instagram, and Twitter) and open-access database reports from rescuing programs websites and stranding monitoring Facebook groups were also included, and any reports from iNaturalist were considered as a source of citizen science. The whole process uncovered 89 species of sharks from 22 families with single (or “small-scale”) and mass stranding events. The species ranged from coastal to oceanic, and even included some known to prefer the deep. “There were even some reports for deep-sea sharks (e.g. velvet belly lanternshark, Etmopterus spinax; velvet dogfish, Zameus squamulosus, and the angular roughshark, Oxynotus centrina),” said the authors.

Both large and small-sized species were represented; the largest individual reported was a 9.45-m whale shark (Rhincodon typus) and the smallest was a 20-cm Chilean catshark (Schroederichthys chilensis). Of the 148 countries that have a coastline, 47 had reported some sort of shark stranding. The United States was the location with the highest number of strandings, with a total of 2,462 reports primarily in the states of California (58.9%; n= 1,419) and Oregon (6%; n = 146) followed by Florida (n = 54), Texas (n = 49), and North (n = 28) and South Carolina (n = 17). The next highest location, surprisingly, was New Zealand (n = 121), followed by the United Kingdom (n = 118) and South Africa (n = 114). Shark stranding reports were more common in the years 2011–2021 (69% of all reports) compared to the decade (2000–2010) before, and were less commonly available before 1999. “Apart from two mass stranding events in 1967 (n = 505), only 164 strandings were reported in free-access databases between 1880 and 1999,” revealed the authors. “Of these, 70 were reported in 1982, being the most representative year in terms of single and small-scale stranding reports.”

Among the ten most commonly stranded species, the leopard shark had the majority of the strandings (n = 1,153), followed by brown smoothhound (n = 531) and salmon shark (n = 403). Interestingly enough, the leopard shark and brown smoothhound stranding reports occurred in the United States, and the majority of salmon shark strandings (22 of the 41) there. Makes sense when you take into account the species are known to frequent the waters around the North American country. Age didn’t seem to make much of a difference as juveniles and adults were affected in the same proportion. But sex? That definitely seemed to make a difference! For most considered records (n = 1,837; 61.5%), the animal’s sex was not identified. The rest of the reports (n = 1,149; 38.5%) showed 848 (73.8%) females stranded compared to 301 (26.2%) individuals that were identified as males. For sharks of all ages and both sexes, survival rates were extremely low, indicating that stranded sharks are more vulnerable that other taxonomic groups who have higher chances of making it out alive from these highly stressful events. The exact reasons why sharks strand are not explicitly known. Some explanations proposed include environmental issues, disease (like meningoencephalitis), fisheries, collisions and marine litter to name a few.

“Our results indicate that although they occur to a lesser extent when compared to mammals, shark strandings are neglected, and urgent measurements are necessary to better understand, document and to properly respond to these events,” explain the authors. “Although there is a predominance of small-sized species, medium and large-sized sharks are also affected by this phenomenon. However, mass strandings (apart from singular events such as a case of the blue sharks in South Africa) appear to affect small sharks more significantly. It is possible that this pattern is more related to species-specific behaviours rather than shark size, including propensity to travel together in schools.” It is likely that the number of strandings is much higher, given the limitations of the study, and the authors point to the “urgent need to establish worldwide rigorous shark stranding monitoring programs that will record, preserve and share comprehensive morphometric data and coordinate complete diagnostic investigations of shark stranding events to permit for improved study and understanding.”