ome of the more well-documented fishing activities that can have potentially negative impacts on the environment include:
Overfishing and excessive fishing can reduce the spawning biomass of a fishery below desired levels such as maximum sustainable or economic yields.
When there is sustained overfishing, changes in species composition and biodiversity can occur with progressive reduction of large, long-lived, and high value predator species and the increase in small, short-lived, and lower value pelagic and demersal prey species, a process described as 'fishing down the food chain'. Important macroscopic changes have been observed in many ecosystems such as the North Sea, Yellow Sea, North Atlantic (e.g. George's Bank and Barents Sea), Gulf of Thailand, and southeastern Australia. Intensive fishing can also reduce genetic diversity of wild populations (e.g. rapidly depressing the proportion of fast growing and late spawning individuals) and changes in species composition or dominance can also be provoked through competition for food between fisheries and marine apical predators.
Non-selective fishing gear that is not modified to exclude or otherwise deter the entanglement of fish, turtles, or seabirds, and as a result, may take a significant bycatch of juvenile fish, benthic animals, marine mammals, marine birds, vulnerable or endangered species, etc. that are often discarded dead. While bycatch and discard problems are usually measured in the potential loss of human food, the increased risk of depletion for particularly vulnerable or endangered species (e.g. small cetaceans, turtles) can be significant. In the North Sea, for example, the impact of discarded fish on the food chain and species composition is consequential because the discards can represent up to 30% of what some birds' would otherwise consume.
Ghost fishing can occur when certain gear such as pots or gillnets have either been lost or abandoned at sea and, although untended, continue to catch and kill fish until the gear falls apart.
Impacts on the bottom can result from the intense use of trawls and other mobile bottom gear (e.g. dredges) can change bottom structure, microhabitats, and benthic fauna. The effect is particularly obvious when these gears are used in sensitive environments where there are sea grass and algal beds, coral reefs, sponges, and tube worms. Where fishers work the same area year after year much like a farmer's fields, the long-term impacts of such repeated activities are less obvious on soft bottoms, although the scraping or ploughing the bottom to depths of as much as 30 cm can seriously disturb the substratum habitat and productivity.
Fishing entailing the use of dynamite and poisons can have severe and broad-reaching impacts, particularly on coral reefs.