It is truly horrifying and upsetting to witness... Dozens or even hundreds of beautiful marine mammals lying helpless on the sand, baking in the sun. As kind volunteers rush in to help the prone animals back into the water and scientists flock to try to understand it, these stranding events often garner a lot of attention. The startling rise in "mass strandings" of whales and dolphins on our beaches in recent decades, has lead to such great concerns, that global initiatives have been implemented to study (and hopefully prevent) this baffling phenomenon. But what about other animals that experience strandings? You would not be alone if you have never even heard of a shark stranding. Yet mass shark strandings and ongoing individual strandings of sharks have been recorded for decades. So, when were these great standing events? Where do sharks get stranded? And why on Earth does this happen?
Sharks get Stranded Too!
Strandings - when an animal that normally lives in the ocean winds up beached on the shoreline, unable to get back into the water - have been documented thoroughly for whale and dolphins, and even turtles. Despite single strandings of sharks being recorded with some regularity since the late 1880s, poor record keeping and minimal investigations mean there are huge gaps in our knowledge about sharks strandings and many questions about why and how they happen are still to be answered (Wosnick, 2022).
These questions are never more important than following a "mass stranding" event - when multple individuals of one species or from several different species, all strand in the same place, at the same time. The largest of which was recorded in 2017, when approximately 1000 sharks stranded all at once (Wosnick, 2022).
Which Types of Sharks get Stranded?
Varying in their sizes and natural habitats, strandings seem to affect a very wide range of different shark species. In a review of all the available stranding reports, scientists discovered a total of nine different species of sharks belonging to 22 taxonomic families had been involved in stranding events over the past 140 years (Wosnick, 2022).
Strandings seem to be most common in coastal species. However, there have even been reports of strandings of very deep sea species of sharks, such as the velvet belly lanternshark (Etmopterus spinax), the velvet dogfish (Zameus squmulosus) and the angular roughshark (Oxynotus centrina) (Wosnick, 2022).
Both single strandings and mass strandings seem to be most common for smaller species of sharks. For example, the species that seems to be consistently worst affected by both single and mass strandings is the leopard shark (Triakis semifasciata). Mass stranding events also seem to be fairly common for brown smoothhounds (Mustelus henlei) and have been recorded for dusky smoothhounds (Mustelus canis). Scientists think the phenomenon affects these species in particular, as they often travel in large schools (Wosnick, 2022).
However, this does not mean that large sharks are immune to strandings. On the contrary, the salmon shark (Lamna ditropis) is one of the species that suffers some of the highest rates of single strandings. Along the west coast of the American continent, there have been 403 strandings of salmon sharks since 1880! There have also been reports of mass strandings of blue sharks (Prionace glauca) in South Africa. Reports show that occasionally big sharks like the great white (Carcharodon carcharias) and whale sharks (Rhincodon typus) sometimes get stranded too, but these tend to be single rather than mass events (Wosnick, 2022).
Where do Sharks get Stranded?
Shark strandings happen all over the world. Scientists that performed a review found that stranding of sharks had been reported from 47 different countries since 1880 - almost 32% of all countries with a coastline. So it is a very common phenomenon (Wosnick, 2022).
The United States has by far had the highest number of shark strandings reported - 2462 over the last 140 years or so. New Zealand, the UK and South Africa also report high numbers of strandings (Wosnick, 2022).
Why do Sharks get Stranded?... Is it the Natural Order?
The most obvious question we ask ourselves when a stranding occurs is, 'why on Earth did this happen!?'. Why would a marine animal suddenly choose to beach itself and not return to the water. This behaviour is especially bizarre for sharks as, unlike marine mammals, they need to be submerged in water to be able to breathe!
In some cases, strandings are just a part of the natural cycle. For example, in smaller coastal species of sharks that hunt and breed in shallow regions, strandings can happen when the tide goes out and they become trapped in rock pools. Leopard sharks and Port Jackson sharks (Heterodontus portusjacksoni) for example, are prone to this problem (Wosnick, 2022).
The confusion and disorientation caused by certain diseases can also cause strandings, sometimes even mass stranding events. Illness caused by Carnobacterium, Streptococcus bacteria or by the protozoan Miamiensis avidus have been associated with shark strandings, but by far the most common is meningoencephalitis. This disease leads to confusion and disorientation, and casues sharks to swim erratically before they strand. Salmon sharks are most regularly affected by this, and leopard sharks have also suffered multiple mass strandings in California thanks to meningoencephalitis - in 1967, 2006, 2011 and 2017 (Wosnick, 2022).
Environmental disturbances also play a major role in shark strandings. For example, sudden changes in temperature or oxygen levels in the water have been linked to mass strandings of blue sharks and Chilean catsharks (Schroederichthys chilensis) (Wosnick, 2022).
In 2008 a massive mass stranding event in Chile, which included 26 different types of sharks, was linked to the influx of winds which created "hypoxic" conditions in waters of the bay. This means the dissolved oxygen levels in the water were so low that sharks experienced physiological stress and ended up stranded (Wosnick, 2022).
Extreme changes in temperature have also been linked to strandings of young salmon sharks. Scientists modelling the environmental conditions preceding their stranding events, found that young-of-the-year salmon sharks were much more likely to become stranded if they experienced cold shock. It is thought the stress of cold itself or the reduced body condition is causes may make them more susceptible to infections like meningoencephalitis, leading to stranding (Wosnick, 2022).
Why Do Sharks get Stranded?... Is it Due to Human Disruption?
Whilst scientists have found that environmental stresses can lead to shark strandings, there are also many incidents where standings are caused by human activities...
"Capture stress" is thought to be a common reason that sharks get stranded. If a shark has been caught in fishing gear, even if they are carefully released, they can be seriously injured or affected by the stress. This can cause them to become disoriented and end up stranded. Capture stress has been linked to strandings of bluntnose sixgills (Hexanchus griseus), broadnose sevengills (Notorynchus cepedianus), thresher sharks (Alopias vulpinus) and porbeagle sharks (Lamna nasus) (Wosnick, 2022).
Being injured thanks to collisions with boats has also be shown to cause shark strandings. Scalloped hammerheads (Sphyrna lewini) and shortfin mako sharks (Isurus oxyrinchus) are known to end up stranded after such traumas (Wosnick, 2022).
There is also evidence that marine debris and other types of pollution can cause strandings. Pollution is thought to cause whale sharks to strand and the largest mass stranding of brown banded smoothhounds (Mustelus henlei) in 1967, is thought to have been cased by a chemical pollutant that caused the sharks to behave strangely and swim eratically (Wosnick, 2022).
So whilst there are certainly incidents where sharks become stranded thanks to unusual, yet perfectly natural events, there are many situations in which our actions may be making strandings more regular or happen on a larger scale. It is also possible that the causes are more complex and strandings happen when several different variables are at play - both natural and human. We still have so much left to learn about these mysterious events and as so many species of sharks are now threatened with extinction, it will be vital to learn as much as we can about them, if we have any hope of reducing shark strandings in the future.
What to do if you find a stranded shark.
First off, NEVER approach a wild animal, especially if it poses a threat to your safety! Call the authorities for assistance! If you feel comfortable to help a small stranded shark, make sure you handle them carefully to avoid injury to both yourself and the shark:
Approach slowly and calmly, so as not to startle the shark
Place the flat of your hand down on the back of the neck to pin the head down, this will mean they cannot swing round and nip you
Pick the shark up by holding it around the back of the neck and the base of the tail
Do not pick the shark up by the tail alone, as this can be painful
Do not wrestle the animal if it is becoming distressed
If you cannot pick the shark up, you can pull them gently across the sand by the tail - take care the shark does not swing round to bite you
Do not pull the shark through the water backwards by their tail, as this can flood their gills and drown them
Pop them back into the ocean as quickly as possible
Revel in your awesomeness!
References
Wosnick N, Leite RD, Giareta EP, Morick D & Musyl M (2022). Global assessment of shark strandings. Fish and Fisheries, 00, 1–14. Access online.
By Sophie A. Maycock for SharkSpeak
