Threats to Cetaceans - Chemical Pollution

Pollution by chemical substances can be a threat to the health of cetaceans. A huge range of chemicals find their way into the sea from the air, rivers, domestic waste, industrial discharges and run off from the land. Many of these chemicals have harmful properties. Very little work has been carried out to investigate these impacts and their effects on whales and dolphins. One of the main problems is that we know very little about how these chemicals behave in the marine environment what combined effects they may have.

Cetaceans are at the top of the food chain and are at risk from persistent compounds which accumulate up the food web. Cetaceans are mobile animals and are therefore exposed to potentially a wide range of pollutants. Coastal species such as harbour porpoise and bottlenose dolphin may be at increased risk due to their proximity to main centres of population and discharge outfalls. There is only limited information about the effects of pollution on whales and dolphins in the west of Scotland. The paragraphs below outline the main groups of chemical pollutants and the threats they present.

Organochlorines

Organochlorines is the name for a group of synthetic chemicals which are very stable in the environment. Some of the most infamous organochlorines include DDT, PCB's, and many pesticides and insecticides. They hate water and love fat and therefore accumulate in fatty tissues, such as blubber, and bioaccumulate very effectively up the food chain. It is believed that increased levels of organochlorines in mammals can cause a decrease in their ability to reproduce and also suppresses the immune system. Because of the long life span of cetaceans, they are particularly at risk from contamination over the long term. It is found that baleen whales often have a lower organochlorine burden than toothed whales. This is because they feed on prey which are lower down the food chain and therefore do not contain such high levels of chemicals.

Analysis of blubber in stranded animals in the Hebrides has revealed that many cetaceans in Scotland carry organochlorine burdens of a magnitude equal to those where reproductive and immune system disorders have been observed in species of small cetaceans.

Heavy metals

Heavy metals such as mercury, cadmium and zinc are byproducts of many industrial processes and once in the environment tend to accumulate in protein rich tissues such as muscle and liver. High levels of heavy metals have been observed to cause lesions, degeneration of fatty tissue and a poor nutritional state in cetaceans. As with organochlorines, these metals can build up over time from consumption of prey species. Analysis of liver and kidney tissue of stranded animals in the Hebrides has revealed that the highest concentrations of heavy metals occur in striped dolphins and long finned pilot whales which feed predominantly on squid.

High levels of mercury in otters has also been reported from West Scotland, which suggests that other aquatic species occupying the same coastal and sea loch habitats, such as harbour porpoises and bottlenose dolphins, may also be exposed to relatively higher mercury levels.

Hydrocarbons

Hydrocarbons exist in many forms from the very inert paraffin to the very toxic Polycyclic aromatic hydrocarbons (PAHs). They are released into the environment from many sources including accidental discharges, oil spills, sluicing out the tanks of oil tankers, offshore oil and gas production.

The main threats to cetaceans are from the ingestion, inhalation and physical contact with oils. Oil-related pollution is generally thought of in terms of crude oil slicks, however there are less apparent forms of oil-related pollutants. For example, within the oil industry, use is made of drilling lubricants which may contain high concentrations of hydrocarbons as well as toxic heavy metals.

The most toxic group of hydrocarbons are polycyclic aromatic hydrocarbons (PAHs). These are a cancer-forming group of pollutants which come from oil-related industries and the burning of fossil fuels. A harbour porpoise calf stranded on the Isle of Islay, is the only cetacean from the west coast of Scotland which has been examined for the presence of PAHs . The porpoise had detectable levels of PAHs and it could be construed that adults would be expected to build up higher concentrations of this pollutant.

Butyltins

One class of toxic pollutants which has been highlighted as being a risk to the marine biota are the butyltins (BTs), the most famous of which is tributlytin (TBT). Butyltins are extremely toxic and can cause growth retardation and reproductive abnormalities in marine organisms at low concentrations. They are also known to disrupt the immune system of mammals This class of pollutant has been described from several species of whale and dolphin in recent years and there is concern about its toxic effects on cetacean populations.

Historically, butyltins were primarily used as anti-fouling treatments upon ship hulls and marine structures, such as fish farm cages. Studies in the late 80's reported that BT contamination was high in sea lochs, as a result of contamination from fish farms. In 1986, the use of TBT on boats less than 25m was banned in Scotland . Since then the use of TBT has also been banned on fish farms.  It is hoped that due to these laws the amount of TBT entering the system is now much reduced.

However, BT's are very stable and bio-accumulate through the ecosystem.  There is concern that the historical inputs of TBT will bio-accumulate up the food chain to cetaceans. It would be expected that coastal species inhabiting sea lochs, such as bottlenose dolphins and harbour porpoises would be most likely to be exposed to these elevated levels of BT contamination. So far there have been no studies upon the level of BT contamination in whales and dolphins from West Scotland . Although, it is hoped that levels of BT in the environment will not increase, the toxic effects of BTs on cetaceans from past inputs is still of concern.

Fish farm related pollutants

Fish farms produce a variety of pollutants, the monitoring of which is the responsibility of the fish farm operators and overseen by SEPA. These pollutants have included butyltins (see above), which were used as anti-foulants on cages.  Farmed fish are also treated with various chemicals such as treatments to kill fish lice populations. The cultured fish are also fed with hormonal growth promoters and antibiotics to improve fish yields and reduce disease.  Laboratory risk assessments are carried on these compounds before use; however, the impacts of them once they enter the marine environment are little understood.