Mass strandings involving as many as 150 whales can occur — and individual whales can also come ashore, as happened on two occasions in Kerry in the past week.
It’s always an event when these magnificent creatures appear on a beach — something that evokes a warm, human response as well-meaning people do their utmost to keep the creatures alive and return them to the sea.
But why should whales come ashore?
Irish Whale and Dolphin Group (IWDG) co-ordinator Dr Simon Berrow says it happens for many reasons. A member of a pod could be sick, diseased and already dying, for instance, or they could do so because of navigational mistakes.
“If a key member of a group of whales is dying, others may come ashore with it because of strong social bonds,” he said.
North Kerry and north Mayo are ‘hotspots’ for strandings, mainly for pilot whales, white-sided dolphins and striped dolphins, with Dr Berrow, a marine biologist, suggesting magnetic factors may be at work in Killala Bay.
“Species that are normally found well offshore can come onto gently sloping, sandy beaches. This might be due to navigational failure. Species that are accustomed to the open sea can obviously run into difficulty when they meet land,” Dr Berrow, who’s based in Kilrush, Co Clare, remarked.
This view is borne out by an American expert, Dr David Williams, who’s been studying the phenomenon for over 25 years. Records show it’s nearly always toothed whales, which feed consistently in deep water offshore, which mass strand themselves.
“More than 90% of live mass strandings are pilot whales or false killer whales, which are not known to frequent shallow water and are therefore not familiar with the coastline,” Dr Williams said. “Species known to spend a lot of time in shallow water become familiar with the beach and seldom strand alive.”
Species known to mass-strand live in tight social groups, he added. Individuals require close social contact and are seldom observed more than several hundred meters from the centre of the pod.
“On the other hand, species that form loose pods, in which two or three individuals are often found several miles away from the pod, never mass strand,” Dr Williams observed.
Live-stranded whales may appear outwardly healthy, but post-mortem examinations reveal dehydration, starvation and a multitude of bacterial, viral and parasitic infections.
“It is clearly obvious from medical examinations that all the adult members of the pod are ill, not just one or two lead members,” he said. “The nursing young are usually in far better health than the adults, and can often be rehabilitated.
Mass strandings occur repeatedly along certain shores, while other similar shores seldom record such events.
“Mass strandings most often occur at locations where large geographical land masses extend out to sea, opposing the flow of current,” Dr Williams said.
Sloping beaches are the most frequent locations for strandings, rather than rocky shores, mud flats or areas where sand isn’t accumulating unless the stranding occurs in backwater areas inside an inlet.
“Obviously whales are not going to strand where the water is not shallow enough to stop their forward progress, so it could be said that a gentle shelving beach is the only area that would naturally trap whales,” Dr Williams observed.
Another observation is that when refloated, animals often return to shore or are found re-stranded further downstream.
Noises, explosions and signals at sea that whales aren’t familiar with can interfere with their navigational powers and even damage their sinuses and ear drums, some experts believe.
US government scientists have claimed, for example, the US Navy’s use of sonar during maritime exercises may have contributed to the mass stranding of more than 150 whales in Hawaii’s Hanalei Bay two years ago. As a result, the Navy was asked to reduce sonar power during exercises in Hawaiian waters.
Sonar technology’s used to detect threats and to navigate. Some wildlife advocates believe the sound waves hurt whales, possibly by damaging their hearing or causing them to rise to the surface too quickly and get decompression sickness.
The day before the whales entered Hanalei Bay, six US and Japanese vessels steamed north from the island of Oahu toward Kauai, intermittently using active sonar signals.
Scientists concluded the whales, which usually inhabit only deep water, may have heard these signals and headed into the shallow water.
“It adds to a long and growing list of strandings that have been associated with the Navy’s use of sonar,” said Michael Jasny, senior consultant with the Natural Resources Defence Council, in Los Angeles.