Fisheries impact habitats and ecosystems

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Fishing gear is designed to maximise yields of target species and minimise cost of effort but they also trap non-target species and damage the marine environment and habitats. Nontarget organisms affected include benthos, birds, marine mammals, marine reptiles (turtles), plants and non-target fish. The effects on non-target species can either be direct (for example, accidental entrapment) or indirect (for example, through the alteration of energy transfers through trophic levels ⁽¹⁾ thus reducing abundance and/or modifying relative size composition).

Capture fisheries tend to target the more valuable larger fish that are at higher trophic levels such as species that eat other fish. However, as overfishing reduces the populations of these fish, the landings of fish lower down the food web such as those species that eat zooplankton make up a larger proportion of the overall catch. This means a change in the ecosystem composition from fish eating species to plankton eating species. This is generally indicative of a negative impact on the whole ecosystem caused by fishing and has been called 'fishing down marine food webs'. For example, Figure 2.15 shows that the mean trophic levels in both the north-east Atlantic and Mediterranean and Black Sea fishing areas have declined since 1950. It seems that fundamental changes in the structure of these marine ecosystems have occurred and it is likely that this is due to fishing. Fishing at lower trophic levels may suggest exploitation at unsustainable rates. It is also reported from the Baltic Sea that commercial fisheries are responsible for altered food web dynamics (Helcom, 2002).

There is particular concern about the impact of fishing on marine mammals, turtles and birds. Cetaceans (whales, dolphins and porpoises) are accidentally caught in drift nets and are in competition with fishermen for small pelagic resources. Drift nets and pelagic long lines are the major threats to birds and marine turtles. Even though there are no comparable datasets to properly assess the extent of the problem across Europe, there has been some efforts nationally or regionally to monitor the bycatch of mammals, birds and turtles. For example, Figure 2.16 shows that between about 20 and 55 % of all cetacean strandings (on which post mortems were undertaken) in England and Wales can be attributed to bycatch.

⁽¹⁾ The level in the food web at which a group of organisms occurs. One way to detect ecosystem changes is to study the ratio of landings of predatory fish (piscivores) to landings of fish that feed on plankton (planktivores). As predatory fish are removed from the population, the proportion of plankton feeders in catches may grow.