Sewage and Waste
Untreated sewage from boats can spread gastroenteritis; contaminate shell fish beds and mussel ropes and use up vital oxygen in the water. Human waste also contains phosphorous and nitrogen which increase the levels of algae and reduce water clarity. Chemicals such as chlorine, formaldehyde, ammonium and zinc compounds used as disinfectants, breakdown and deodorise waste are toxic to marine life.
Discharge of boat sewage to coastal waters is regulated under MARPOL Annex 4 and whilst this only applies to recreational craft carrying 15 or more passengers, you only need 3 boats (with 5 crew in each) in a rally to cause a similar amount of damage. So it is certainly worth considering using your holding tank when out and about.
Sewage discharge from vessels in rivers, canals and lakes is regulated by many authorities and byelaws and although there may be varying local byelaws it is generally prudent to assume that discharges are not permitted.
Since 2006 the Recreational Craft Directive, applied to newly built vessels, has required provision to be made for a holding tank to be fitted. If buying a new boat consider having the holding tank installed by the manufacturer.
Grey water discharge from sinks, showers and washing machines can be very damaging to sensitive aquatic life. Most washing detergents contain phosphates which encourage rapid algal growth and eventual oxygen depletion when the algae die and start to break down. This eutrophic effect can cause fish and other aquatic life to suffocate. Added to this, the degreasers found in washing up liquids and soaps strip the natural oils from fish gills making it difficult for them to breathe.
On the inland waterways, it is a requirement under British Waterways licences that only phosphate free detergents may be used.
Litter and Waste
Marine litter has a major impact on wildlife, the main issues being entanglement and ingestion. It is estimated that over 50,000 marine mammals die every year from becoming tangled in or eating marine litter. Plastics are the most prevalent beach litter material. Certain types of plastics are known to absorb chemicals from the surrounding environment, such as PCB's and heavy metals at concentrations up to 1 million times higher than in waters around them. The ingestion of these toxins can have life threatening impacts upon marine wildlife. Research shows that little of the litter entering the marine environment comes from marine boating; the 2007 MCS beach watch survey identified the main contributers as beach visitors (35.3%) and the fishing industry (13.7%). However boaters are likely to be more aware of litter than other groups and should still do their part to prevent any litter from entering the water.