Friday February 13, 2015 was no ordinary day for residents in New Zealand’s north-western part of South Island. Over the next few days, 300+ people headed to Farewell Spit in the hope of rescuing 198 pilot whales that had become stranded. By Saturday, only 67 of the pilot whales made it back out into the ocean alive. Their longer term survival… unknown. Farewell spit, a long narrow sand bar that stretches out 25 kilometres from the mainland, is no stranger to mass strandings. In recent years, the area has seen 70 pilot whales stranded in January 2014. November 2011 through to January 2012 saw three mass strandings, all of pilot whales.
Farewell spit is not the only stranding ‘hotspot’, nor are pilot whales the only species to strand, nor is it a recent phenomenon. Perhaps earliest known stranding was reported by Nick Pyenson, from the Smithsonian’s National Museum of Natural History in his 2014 paper, detailing the discovery of over 40 cetacean skeletons (primarily baleen whales) found next to what is now the Pan-American Highway in northern Chile. Forming four distinct layers, Pyenson and colleagues believe that it was the site of four separate mass strandings occurring over a period of approximately 10,000 years, 6-9 million years ago.
A stranded Atlantic White-Sided Dolphin. The foam padding helps reduce the stress experienced by stranded dolphins. Credit IFAW/Flickr (CC BY-NC 2.0)
There appears to be many potential causes of cetacean strandings, but the mechanisms behind them remain poorly understood. Pyenson and the team suggest that harmful algal blooms, whose toxic effects can cause organ failure in marine mammals, may have been the culprit of the repeated multi-species stranding events. Harmful algal blooms are the only known natural cause of a multi-species stranding events, but there are other environmental factors that can result in strandings. For example, research published in 2005 and lead by Karen Evans (now based at CSIRO, Australia) has indicated that, for south-east Australia/Tasmania at least, strandings between 1902 and 2002 peaked every 11 – 13 years. These peaks correlated with changes in the wind patterns, which likely pushed nutrient-rich waters (and the cetacean’s prey) closer to the coast than they normally would be, increasing the likelihood of traditionally deeper-water cetaceans becoming stuck in shallow (and arguably unknown) waters.
Such conditions may help to explain temporal peaks in strandings, but not necessarily stranding ‘hotspots’, where Geology may be a more important factor. The curved, shallow-shelves of Farewell spit cause two problems for pilot whales – an oceanic species largely unfamiliar with coastlines and tides. First, the shallow shelves are not easily detectable by the whale’s sonar, meaning they may be unaware of their increasingly shallow location situation. Second, the curved shape of the bay, forming a single easterly exit point from Golden Bay, limits exit options.
Not all natural events that cause strandings are ‘large scale’. Common dolphin strandings in the Black Sea over a three month period during 1994 was linked to cetacean morbillivirus infections. Generally speaking, cetaceans that strand en-masse are pelagic odontocetes – the toothed whales (including dolphins), that live in highly socialised pods. Whilst no conclusive evidence exists, this may explain why several seemingly healthy members of a pod will strand without apparent cause. If one member of the pod – particularly a leader – becomes sick, injured, or disorientated - and ends up in shallow waters, the others follow – and find themselves in a similar sticky situation.
Equally, pods may wait in ‘risky areas’ close to the location of a stranded member of their pod. This behaviour appears somewhat common in orca (killer whales), such as witnessed in 2013 at Australia’s Fraser Island, as well as the pilot whales recently stranded at Farewell Spit. Escape from predation (and indeed predation injuries) has also been implicated in a number of strandings. A 2007 paper by John Ford (Fisheries and Oceans Canada) and Randall Reeves (Okapi Wildlife Associates) notes that in some cases individuals may intentionally beach themselves to avoid predation, whilst in others they flee unsuspectingly into shallow, confined spaces.
Just as other marine animals can cause trauma sufficient to result in stranding, so too can humans. Individuals struck by ships may wash up on the coast after death, but there are also reports of live strandings where ship strike was obvious. One such incident was documented by Koen van Waerebeek (Peruvian Centre for Cetacean Research) and colleagues, who recount an incident of an injured humpback whale discovered on a beach at Assini Mafia, eastern Ivory Coast in 2007. Substantial injuries to the individuals head and back, apparently caused by a propeller, proved fatal within a day of the whale being discovered. In this example, tying down the likely reason for the stranding is somewhat obvious, but in most cases uncovering anthropogenic causes of strandings is tricky.
Nevertheless, just like natural events there are a number of patterns emerging which indicate some activities are proving harmful. In particular, sonar and human-induced noise are areas of concern. June 9, 2008 saw the UK’s largest mass stranding event of short-beaked dolphins. Research lead by Paul Jepson (Zoological Society of London) into the incident (including necropsies on the 26 individuals that could not be returned to the sea) ruled out a range of natural and human-induced causes. Certainly, the animals appeared to be in perfect health until the stranding. What they did note was that the presence of international naval exercises, which included the use of mid-frequency sonar) undertaken in the days preceding -as well as the day of - the stranding. Whilst correlation does not provide evidence for causation, this is not the first such incident being reported, nor is sonar the only naval activity to have occurred in the vicinity of a stranding or indeed direct mortality. For example, work by Kerri Danil (NOAA) reports mortality of long-beaked common dolphins as a result of ‘mine counter-measure training’ near San Diego, USA in 2011. From the pod in the area, four deceased individuals were found within minutes of the blast, with the survival of the remaining dolphins being unclear.
Over the years, global stranding reports have increased. Whether that is because there are more strandings, or simply more human eyes on the coast - and greater ability to report strandings - is not clear. Historically strandings may have proved beneficial to some coastal communities, by bringing food, fat, and oil within easy reach. Today in many societies, strandings are no longer seen as a bonanza, instead being a cause of distress. Rescue attempts are huge undertakings, and not necessarily effective. Beached animals, especially for large whales, can suffer mortal injuries from beaching, such as having their own body weight – normally supported by water – crush their internal organs. Dehydration, stress, and infection are also a huge concern. For many individuals, euthanasia seems the kindest option. For those that are successfully returned to the sea, survival is largely unknown.
There is little we can do about natural causes of strandings –and indeed questions over whether we should do anything about natural causes. Whilst we are still unravelling the human-causes of strandings, many suggest we should take a precautionary approach to our human activities to reduce the likelihood of stranding animals. The long-beaked common dolphins impacted by the naval blast exercise, for example, were spotted in the area, but it was decided to continue with the exercise anyway.
Thankfully there are examples where people are considering cetaceans in their activities. Vessel speed limits are either imposed or requested voluntarily in various locations to reduce the risk of ship strikes. Following the conclusion that naval sonar exercises had causes haemorrhages, and the subsequent fleeing of beaked whales onto beaches in the Bahamas in 2000, researcher Ken Balcomb, who had previously worked in the US Navy on submarine-detecting sonar, was able to pressure the US navy into considering cetaceans when planning their exercises. How much they will do so remains to be seen.
This story was written for The Marine Professional, a publication of the Institute of Marine Engineering, Science & Technology (IMarEST).