The maritime environment in which shipping operates is fragile and needs to be understood and it needs to be protected.

Part of the marine environment is the inter-tidal zone (see the diagram above), an area where humans have greatest contact with the sea. It is here that many swim, surf, fish and have other contact with the sea and marine creatures such as shell fish, crustaceans and other small marine creatures. It is also an area of particularly interesting marine growth such as the kelp beds (with their own ecosystems) along the west and south-west coasts of South Africa.

It is the part of the marine environment where much marine “rubbish” is found – waste from ships and other sources, oil pollution (especially after a marine accident or where illegal oil discharge has occurred.) These products of marine operations or of other human activities along rivers that flow into the sea can be harmful to marine creatures, but also spoil the attractiveness of the coastline.

The Heads at Knysna in the Western Cape. This is the mouth of a large tidal lagoon into which millions of cubic metres of water flow as the tide comes in, and then flow out as the tide ebbs. A wide range of marine creatures – ranging from microscopic organisms to large fish move between the sea and the lagoon.  Photograph : Brian Ingpen

When the tide rises, seawater flows into the estuaries, and drains out when the tide goes out. (We say that the tide ebbs.) Because they are exposed to rise and fall of the tides, river estuaries also form part of the marine environment. Depending on the depth of water at the mouth of the river, there is much movement of marine creatures from the sea to the estuary and back to the sea. These creatures range in size from microscopic organisms and smaller fish to creatures such as quite large sharks in rivers where the water is deeper at the mouth. Quite large sea creatures, including seals and Zambezi sharks, have been found more than 25 kilometres upstream in the Breede River (Western Cape); riverine creatures such as otters also move between the estuary and the seashore, preying on crabs and shellfish. The interchange of marine life with the estuaries at Sodwana Bay and elsewhere along the northern coast of KwaZulu-Natal is well documented.

Coastal marine pollution can affect aquatic life in the estuaries. An example of this was the aftermath of the collision between the tankers Venoil and Venpet off the southern coast in December 1977. Venoil was laden and two of her tanks were holed, resulting in a major oil spill. In the Agulhas Current, the resulting oil slick drifted towards the south-west, threatening estuarine life in rivers between Plettenberg Bay and Mossel Bay. Booms were placed across some river mouths, preventing the oil from entering the estuaries.