MarBEF examples of fishery effects

Effects on ecosystem structure and functioning

Fishing affects fish populations in many ways. Many fish populations have been reduced to low numbers due to long-term impacts of fishing (e.g., cod in Baltic Sea) and some populations may be approaching collapses (e.g., bluefin tuna in the North East Atlantic and the Mediterranean Sea).

Fishing and fish collapses

Historical studies by MarFISH have shown that cod in the eastern Baltic Sea were more abundant 400 years ago than in the late 20th century. This result is surprising, because the Baltic Sea 400 years ago was not very “cod-friendly”: It was much less productive than today (i.e., before the increase in nutrients and primary production of the 20th century) and marine mammal predators of cod (seals) were more abundant. The higher abundance of cod in the 1500s was probably related to the overall lower level of exploitation in the 1500s.

Bluefin tuna were abundant in northern European waters such as the North and Norwegian Seas until the late 1960s and the early 1970s, when they disappeared; they have not yet returned. The reasons for their disappearance are not clear. However, since the 1970s, the overall biomass in the entire North East Atlantic and Mediterranean has declined and landings have been too high for too many years to allow recovery. Legitimate fishing quotas are exceeded by illegal landings and catches of undersized fish. As a result, the population is at risk of collapse and has been disappearing from other areas of its range including the Black Sea and parts of the Mediterranean.

Predator-prey interactions

Heavy exploitation of a fish population can also have consequences for other species in the ecosystem. These consequences include effects on abundances of prey species, and how predators and prey interact (e.g., the structure and functioning of ecosystems). It can cause “cascading” effects in which abundances of prey species increase in response to decreases in abundances of predators. The increase in the prey species then has an effect the next trophic level (its food), and so on.

An early example of this ecological cascade occurred in Denmark, in the early 1800s, when heavy fishing pressure contributed to the collapse of a local herring population. This caused a dominance of jellyfish including Aurelia aurita. The ecosystem became so dominated by jellyfish that fishermen were complaining that they could not haul their nets. This example seems to have been repeated in other areas around the world where fishing has removed large quantities of zooplanktivorous fish, such as herring, sardines or anchovy, and jellyfish subsequently became abundant.