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.
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...
The full article was published in – and can be read in – The Marine Professional, a publication of the Institute of Marine Engineering, Science & Technology (IMarEST).
Image: 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)