Throughout my career I have been involved with many worldwide marine mammal rescue and rehabilitation programs. Consequently, I have witnessed countless cases where marine mammals have become stranded, most of which have had direct or indirect links to human impacts. Stranded animals are defined as ‘any dead marine mammal on shore, any live dolphin or whale cast ashore or unable to return to its natural habitat, or any live seal that cannot leave shore due to injury or poor health’ (NOAA). Conservation medicine is emerging to better understand the causations of these complex and multifactorial stranding events, resulting in policy makers being able to implement changes in national and international mitigation measures to protect the marine environment. Marine mammals are widely considered as ‘charismatic mega fauna’ and are able to create special public appeal, drawing attention to the plight of ecosystems.
Stranded marine mammals are a great source of information about the ocean environment. They can be sampled to quantify contaminant levels in tissues and alert researchers to diseases that are present in the more inaccessible wild animals. A number of infectious agents in marine mammals were first identified in stranded animals, after which their presence in the free-ranging population was confirmed. These include, for example, the phocine distemper virus (PDV), which caused the death of over 18,000 European harbour seals in 1988, the phocine herpes virus (PhHV1) isolated from stranded harbour seals in 1985, and Brucella in a variety of other species.
Stranded animals in rehabilitation centres can also offer an excellent opportunity to monitor clinical signs that may result from changes in ocean health. For example, over the past 20 years a thorough examination of stranded California sea lions detected domoic acid a marine biotoxin produced by the diatom Pseudonitzschia australis, which has since been linked to human biowaste and agricultural runoff. The sea lions had consumed toxin-laden anchovies, and the domoic acid concentrated in the tissues caused muscle tremors, seizures, and death. In this case, the findings warned against human consumption of the anchovies, and increased monitoring of other seafood in the area.
The hearing and sound production capabilities of cetaceans have been well studied and many species can hear well outside the range of human hearing. However, most of what we know about the hearing ability of cetaceans has been inferred from studies conducted on dolphins housed at marine mammal facilities. Much less is known about hearing in other cetacean species, such as large baleen whales and some of the larger toothed whales.
Over the past 20 years, intense sounds from naval vessels and seismic testing have been implicated in several stranding events at various locations. Studies are currently under way to investigate the effects of anthropogenic noise on cetaceans. Many of the cetaceans I have personally worked with in rehabilitation almost all showed signs of hearing impairment, with one dolphin being completely deaf. Whether these cases can be connected with anthropogenic noise requires further investigation resulting in understanding the effects of intense noise and contributing to the development of mitigation strategies. Ultimately, the aim is to find a balance between the basic needs of marine mammals and the important role the ocean plays in travel, commerce, natural resources, exploration, and maritime defence.
Environmental stressors can impact upon animal life and cause changes in behaviour, habitat use and prey availability. Moreover, human activities can produce chemical pollution thus distorting marine processes and resulting in disease outbreaks. We need to constantly consider our impacts and how we can reduce them.
The long-term consequences of environmental change on aquatic ecosystems are not well understood but are likely to include aspects of disease emergence in aquatic plants and animals. Teams of veterinarians and conservation biologists are in the midst of a global effort to understand the “ecology of disease.” As the effects of climate change and environmental degradation are debated, worldwide concern is being raised about the health of our aquatic ecosystems. Emerging diseases have themselves become new drivers of environmental change since they can cause extinction of endangered species, alter the ratios of predators-prey-competitors-recyclers necessary for a balanced ecosystem, and alter habitats already threatened by human expansion and climate change.
By Niru Neil Dorrian
Images N Dorrian
For more information on marine mammal rescue and stranding research organisations that Niru has worked with:
SOS Dolfijn - www.sosdolfijn.nl
Channel Islands Marine & Wildlife Institute - www.CIMWI.org
Mote Marine Laboratory Whale & Dolphin Hospital - www.mote.org
The Marine Mammal Center - www.marinemammalcenter.org
SeaWorld Rescue - www.seaworldparks.com
British Divers Marine Life Rescue - www.BDMLR.org