AT A GLANCE...

BBC Wildlife Magazine - - Agenda Opinion -

WHAT ARE STRAND­INGS?

Ev­ery year, thou­sands of whales, dol­phins and por­poises strand on coast­lines all over the world. They may be alive or dead, alone or in groups, and healthy or un­well. Strand­ings of dead an­i­mals are rel­a­tively easy to ex­plain – they sim­ply die at sea and are washed ashore. Live an­i­mals may strand be­cause they are sick or in­jured, con­fused by strong cur­rents or high tides, or be­cause they get lost while chas­ing prey or evad­ing preda­tors. But mass strand­ings of oth­er­wise healthy an­i­mals are still one of the great un­solved mys­ter­ies of the an­i­mal king­dom.

WHAT DO WE KNOW ABOUT MASS STRAND­INGS?

Mass strand­ings can be nat­u­ral – there is ev­i­dence of them in the fos­sil record – or caused by hu­man ac­tiv­i­ties. Many weird and won­der­ful the­o­ries have been pro­posed, rang­ing from un­der­wa­ter vol­canic erup­tions and par­a­sites (quite pos­si­ble) to mass sui­cide bids (highly un­likely). But don’t be­lieve the many news sto­ries claim­ing that we’ve fi­nally cracked it – be­cause we haven’t.

WHAT ARE THE MOST LIKELY EX­PLA­NA­TIONS?

In re­al­ity, there are likely to be dif­fer­ent ex­pla­na­tions for dif­fer­ent strand­ings. One pos­si­bil­ity is that cetaceans use the earth’s mag­netic field for nav­i­ga­tion and their in­ter­nal com­passes can be dis­rupted by mag­netic dis­tur­bances – caused by so­lar storms, for ex­am­ple. This may have been why 29 sperm whales stranded on beaches in the North Sea early in 2016. Coastal to­pog­ra­phy is another likely cause: New Zealand has a par­tic­u­lar hotspot called Farewell Spit – a thin 5km arc of sand at the top of South Is­land – where strand­ings oc­cur on a reg­u­lar ba­sis. There are also more sin­is­ter rea­sons: deaf­en­ingly loud noises from mil­i­tary sonar and oil and gas ex­plo­ration, as well as pile driv­ing to build wind farms, are all be­lieved to cause mass strand­ings. Col­li­sions with boats and en­tan­gle­ment in fish­ing gear are likely to be the cause of many in­di­vid­ual strand­ings.

DO SOME SPECIES MASS STRAND MORE THAN OTH­ERS?

Yes. In­di­vid­u­als of most species have stranded at one time or another, but most mass strand­ings tend to be of toothed whales, species that form tightly knit groups, and those more com­monly found in deep wa­ter. There are many ex­cep­tions, though. An as­ton­ish­ing 343 baleen whales (mostly sei whales) stranded in Chile in 2015, for ex­am­ple, in the big­gest strand­ing of baleen whales ever known. In terms of sheer num­bers, pi­lot whales suf­fer the most. The largest strand­ing on record was of 1,000 long-finned pi­lot whales in the Chatham Is­lands in the South Pa­cific in 1918, and ex­actly a cen­tury later more than 150 short-finned pi­lot whales stranded south of Perth in Western Aus­tralia.

WHAT CAN BE LEARNED FROM STRAND­INGS?

Much of our knowl­edge of many species – es­pe­cially elu­sive beaked whales – comes from dead an­i­mals washed ashore. Even a rot­ting car­cass can yield in­valu­able data on ev­ery­thing from anatomy and ge­net­ics to feed­ing ecol­ogy and pol­lu­tants. Mass strand­ings pro­vide fur­ther in­for­ma­tion on sex ra­tios, age struc­ture, re­lat­ed­ness within a group, and so on.

WHAT SHOULD YOU DO IF YOU FIND A STRANDED WHALE OR DOL­PHIN?

At­tempt­ing to res­cue stranded cetaceans can be dan­ger­ous and, in many coun­tries, is il­le­gal. Ex­pert help is es­sen­tial to re­turn them to the sea, eu­thanise them, or trans­port them to a care fa­cil­ity for re­ha­bil­i­ta­tion. In New Zealand, for ex­am­ple, Project Jonah has more than 3,500 trained vol­un­teers on per­ma­nent standby, and deals with mass strand­ings like a mil­i­tary op­er­a­tion. In the UK, Bri­tish Divers Marine Life Res­cue runs one-day Marine Mam­mal Medic Train­ing Cour­ses – a great way to be­come qual­i­fied to at­tend rescues any­where in the coun­try.

MASS STRAND­INGS OF OTH­ER­WISE HEALTHY AN­I­MALS ARE STILL ONE OF THE GREAT UN­SOLVED MYS­TER­IES.”

Vol­un­teers guide pi­lot whales back out to sea after a mass strand­ing at Farewell Spit.

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