Last week I participated in a marine mammal rescue training workshop in Byron Bay, Australia, through ORRCA (the Organization for the Rescue and Research of Cetaceans in Australia) and am now registered to assist in marine mammal stranding incidents. It was interesting to learn about the biology and identification of whales, dolphins, seals and dugongs, and the reasons why these animals strand. Using cetaceans as the prime example, we will discuss the natural and human causes of marine mammal strandings, and suggest the potential actions we can take to eliminate or reduce the impact we have, by reducing our consumption.
Marine mammal strandings have occurred for hundreds of years, although science has only recently begun to record and document them. Cetacean strandings were first recorded in Victoria, Australia in 1862 and have been recorded on a regular basis ever since, providing substantial documented evidence of stranding incidents for scientific analysis. Stranding incidents occur irregularly in the Southern Hemisphere, but at higher frequencies on the southernmost areas of land masses. Historically, in Victoria, Australia, there are between four and seven incidents of cetacean strandings annually.
Marine mammal strandings occur as either a single or group event, where animals become trapped in shallow water or onshore. Single strandings involve one or two animals (perhaps a mother-calf pair), multiple strandings involve three to six animals, and mass strandings involve seven or more animals (sometimes hundreds of animals, or entire pods). Cetaceans are the only marine mammals known to strand ‘en masse’.
Most stranding incidents involve cetacean species usually found in deep sea (pelagic) waters. In Australia, cetacean species more likely to strand use sonar (echolocation) for navigation – dolphins, sperm whales and pilot whales. Some single strandings are the result of an animal that has died at sea and been washed ashore. Seals do not strand, but ‘haul out’, which is considered normal behaviour.
Scientists do not fully understand why marine mammals strand, although a number of theories have been proposed. Some causes are due to natural factors, while others are due to human impact and interaction.
Natural causes of marine mammal strandings
Whales and dolphins are highly social creatures with complex group social structure. Individuals form strong bonds and depend on the safety of the pod for survival, and will stay together, even if a member is sick, injured or disoriented. If an individual enters shallow water or strands, the remaining healthy pod will often follow, responding to distress calls from stranded, ill or injured animals. Mass strandings may result.
Smaller whales are sometimes forced closer to shore when trying to avoid larger predators, such as sharks or killer whales.
The topography or shape of the coastline and beach may be a contributing factor. Wide and gently sloping shorelines may be undetectable by echolocation. Beaches with sloping sandy or muddy substrates are a common factor in mass strandings. Marine mammals may panic from fear of being trapped in bays with narrow mouths.
Extreme weather, rough seas and high winds may create storm surges, allowing marine mammals to venture farther inshore than normal, increasing the likelihood of becoming stuck when the tide recedes.
Human causes of marine mammal strandings
The cause of greatest concern, and the one we may have direct control over, is human impact. Marine mammals may become entangled in fishing equipment and marine debris (including plastic waste), and larger whales may suffer injuries from being struck by cargo ships. Cetacean species using echolocation for navigation may become disoriented and potentially strand due to underwater acoustic disturbance.
Whales and dolphins face risk of entanglement from marine debris and fishing equipment, including plastic garbage, bags and bottles, ropes, floating material lost or discarded from ships, netting and fishing lines, usually of human origin.
Many cetacean species are vulnerable to ship strikes, although large whales are most often affected. Collisions between vessels and smaller cetacean species may often go unnoticed. Ship strikes can have devastating, often fatal effects on whales. The most effective solution for ship strikes is to reduce the risk of collision between vessels and whales. Reduction strategies have already been successfully applied around the world.
Acoustic disturbance in the ocean results from extremely loud noises produced by the thousands of ships and vessels using the oceans, military navy sonar and seismic exploration. Sound travels approximately 4.4 times faster in seawater than in air, and 100 times further underwater than in air. Many of these man-made sounds are louder than a cetacean’s pain threshold. There are definite links between human-produced acoustic disturbance and mass strandings of cetaceans. In 2000, 16 cetaceans stranded in The Bahamas over a period of 36 hours on March 15 and 16, while the U.S. Navy was conducting a military exercise involving sonar systems on five ships. Some of the cetaceans suffered structural damage and haemorrhage (ruptured eardrums).
Have marine mammal strandings increased?
Marine mammal strandings appear to be occurring more frequently, involving larger numbers of individuals than previously documented, however the reasons for this increase are unclear. Scientists don’t know if the increase is due to human impact or to an increase in reporting these incidents. If the increase is due to human impact, we need to modify the actions and change the behaviours that have led to this increase. Regardless, we should still be doing everything we can to ensure the impact we have on the Earth and the ocean is eliminated or greatly reduced.
What can we do?
I always stress how important it is for us to reduce our consumption and our impact on the Earth. Nowhere is this more important than when it comes to keeping the oceans and the marine environment safe and healthy, not just for marine mammals, but for all of us.
Ensure that most of our diet consists of fruit and vegetables obtained from locally grown sources, not from overseas. Avoid purchasing imported fruits and vegetables and buy seasonal produce from farmer’s markets.
Eliminate or reduce consumption of items that must be transported long distances by cargo ship. Find alternatives that are made locally with materials from regional sources.
Reduce your use of plastic items and recycle through a local government or council scheme if one is available in your region. Ensure plastic does not enter the marine environment. Reduce your consumption of oil and gas by driving less.
We need to create a quiet, safe and clean environment for marine mammals and all marine organisms to ensure the long-term health of the ocean. We share the Earth with many other forms of life and it’s important to consider how our actions and our behaviour may impact non-human life.