First Ever Review On Global Shark Strandings Uncovers Interesting Trends

The whirring of the drone hovering over the shore is drowned out by the crashing of the waves against the massive black bodies slapping weakly against the white sand. “How many can you see?” a volunteer asks the drone pilot, who climbs higher to get a better idea of ​​how many of these sea creatures have chosen to strand here. There are hundreds and counting…

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The sun is barely rising, 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 is no exact definition as it can sometimes depend on the group of animals, but it is generally accepted that these animals can be found dead (either on land or swimming) or alive but cannot return to the sea due to injury or disease. Why this happens is not 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 the 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 critically endangered), their strandings have historically been neglected and little information is currently available on their numbers and subsequent trends.

While overfishing is undoubtedly the main factor behind observed population declines in these animals, their strandings may also shed light on additional stressors that could affect survival. To fill this knowledge gap, researcher Natascha Wosnick from the Programa de Pós-Graduação em Zoologia at the Universidade Federal do Paraná in Brazil conducted a systematic review of 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 come mainly from scientific publications and abstract conferences. Video and photo recordings on multimedia platforms (e.g., Facebook, YouTube, Instagram, and Twitter) and open-access database reports from rescue program websites and Facebook stranding monitoring groups were also included, and all iNaturalist reports were considered a citizen science source . The entire process uncovered 89 shark species from 22 families with single (or “small”) and mass strandings. Species ranged from the coast to the ocean and even included some that are known to prefer the deep. “There have even been some reports for deep sea sharks (e.g. velvet belly lanternshark, Etmopterus spinax; velvet shark, Zameus squamulosusand the angular rough shark, Oxynotus centrina),” according to the authors.

Both large and small species were represented; The largest recorded individual was a 9.45 m whale shark (Rhincodon type) and the smallest was a 20 cm Chilean dogfish (Schroederichthys chilensis). Of the 148 countries that have a coastline, 47 had reported some form of shark stranding. The United States was the location with the most strandings, with a total of 2,462 reports, mainly 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). Surprisingly, the second highest location was New Zealand (n=121), followed by the United Kingdom (n=118) and South Africa (n=114). Reports of shark strandings were more common (69% of all reports) in 2011-2021 than in the previous decade (2000-2010), and were less available before 1999. “Aside from two mass strandings in 1967 (n = 505), only 164 strandings were reported in open databases between 1880 and 1999,” the authors revealed. “Of these, 70 were reported in 1982, the most representative year in terms of individual and small stranding reports.”

Among the ten most stranded species, the leopard shark had the most strandings (n=1,153), followed by brown smoothhound (n=531) and salmon shark (n=403). Interestingly, the reports of leopard shark and brown smoothdog strandings occurred in the United States, and most of the salmon shark strandings (22 out of 41) occurred there. Makes sense considering the species are known to populate the waters around the North American country. Age didn’t seem to make much of a difference, with juveniles and adults being affected in equal proportions. But sex? That definitely seemed to make a difference! For most of the records considered (n=1837; 61.5%), the sex of the animal was not identified. The remainder of the reports (n=1,149; 38.5%) showed 848 (73.8%) stranded females compared to 301 (26.2%) individuals identified as males. Survival rates were extremely low for sharks of all ages and both sexes, suggesting that stranded sharks are more vulnerable than other taxonomic groups, which have higher chances of making it out of these highly stressful events alive. The exact reasons why sharks strand are not explicitly known. Some proposed explanations include environmental issues, disease (such as meningoencephalitis), fisheries, collisions, and marine debris, to name a few.

“Our results show that shark strandings, while occurring at lower rates compared to mammals, are neglected and urgent measurements are needed to better understand, document, and respond appropriately to these events,” the authors explain. “Although small species predominate, medium and large sharks are also affected by this phenomenon. However, mass strandings (aside from isolated incidents such as a case of blue sharks in South Africa) seem to affect small sharks more. It’s possible that this pattern is related to species-specific behaviors rather than shark size, including a propensity to travel together in schools.” It’s likely that the number of strandings is much higher given the study’s limitations, and the authors point out Recalls the “urgent need to establish rigorous shark stranding monitoring programs worldwide that record, preserve and share comprehensive morphometric data and coordinate full diagnostic investigations of shark stranding events to facilitate better study and understanding.”

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