When the big ones land in the net,the small ones benefit
Overfishing has also altered the habitat in the waters off Nova Scotia on the east coast of Canada. For years cod and other bottom-living (demersal) predators such as coalfish have been heavily fished here. The stocks collapsed in the early 1990s. More than 40,000 fishermen lost their jobs. Although a ban on fishing was imposed relatively quickly, the stocks did not rebound even after many years. There is much concern that the habitat has been irreversibly altered.
The cod is a predatory fish at a high trophic level that hunts small planktivorous species, plankton eaters such as the capelin and herring. As the cod disappeared the small planktivorous species became more abundant. Unfortunately, both the planktivorous fish and the larvae of the larger predators feed on zooplankton, which makes them competitors. In addition, the planktivores eat cod roe and larvae, which further increases pressure on the predators. The number of planktivores increased by a factor of nine, while predator stocks remained small.
- The food fish therefore have a strong influence on their predators. Specialists use the term “predator-prey feedback”. Because of this feedback, the stocks of cod, coalfish and other large predators off Nova Scotia have been slow to rebound. The planktivorous fish were thus able to predominate over the predators for a period of 20 years. But now the stocks of planktivores are declining. This is attributed to the fact that the capacity of this region is exhausted: there are so many planktivores that their food supply has become scarce. But a poorly nourished population produces fewer offspring, so the total biomass of the planktivorous fish stocks decreases. The predation pressure on the early life stages of the large predator fish off Nova Scotia has thus declined. As a result the stocks of some predators, for example the coalfish, have recovered. The warning status for cod stocks, however, cannot yet be lifted.
- Similar interdependencies between predator and planktivorous fish are also known from other marine regions. In the Baltic Sea researchers refer to the “cod-sprat swing”. After the general conditions for cod roe and larvae had deteriorated due to low salinity and oxygen deficiency, the cod stocks declined drastically. Because the cod fishery did not adjust to the situation and decrease the catch amounts rapidly enough, the stocks decreased even more. Thus the stocks of their prey, the planktivorous sprat, increased. Because the sprat diet also includes cod roe, pressure on the cod population was further increased. But in this case, temperature also had a crucial impact on the success of the population: slightly increased water temperatures enhanced development of the eggs and larvae of the sprat.
Now the “cod-sprat swing” is sweeping back because the fishery was adjusted: a reduction in cod fishing and interim increase in sprat fishing led to a moderate recovery of the cod stocks.
1.4 > “Predator-prey feedback”: In the mid-1980s the stocks of the northwest Atlantic cod off Canada drastically declined (left figure). As a result the biomass of the smaller food fish increased (right). In recent years this trend seems to be turning around again.
There is evidence that not only the planktivorous fish, but also algae benefit from the disappearance of large fish. Planktivorous fish feed on zooplankton, which, in turn, feed on the small free-floating algae, the phytoplankton. Increased numbers of planktivorous fish produce a drop in the amount of zooplankton, and phytoplankton can flourish. This can cause a problem, especially in the nutrient-rich coastal waters where phytoplankton can grow practically unchecked. The result is known as an algal bloom. When the algae die they sink to the bottom. There they are broken down by bacteria, which consume oxygen.
The formation of algal blooms is complex. It seems that a number of favourable conditions must be present at once. In addition to a sufficient supply of nutrients, moderate water temperatures are necessary. By adding the factor of overfishing of large predators, the problem is apparently exacerbated.
- Greater amounts of algae sinking to greater depths results in increased bacterial activity there, and ultimately leads to a shortage of oxygen. Thus, oxygen-deficient dead zones develop in the ocean where neither fish, crustaceans, nor mussels can survive. Many scientists are therefore now urging fishery management to expand their focus from only the species being fished to consideration of the entire habitat. By recognizing the interdependencies among different species and the trophic levels, this ecosystem-based management should prevent the continued damage or drastic alteration of entire ocean regions caused by intensive fishing and consideration or monitoring of single species. >
- 1.5 > Copepods are usually only a few hundred micrometres to a few millimetres in size. They are an important food staple for fish and for other crustaceans, and make up the largest share of the marine zooplankton.