complex on the planet. The largest predators, such as sharks, dolphins, and seals, eat large fish, which in turn feed on smaller fish and shellfish. These smaller fish and other organisms feed on still smaller organisms, all the way down to plankton and phytoplankton. Although this may seem simple, it is not! For example, whales are on the same trophic level as sardines and other small filter feeding fish: they both eat krill and plankton. On the other hand, sharks don’t necessarily just eat fish on the trophic level below them. They will eat basically anything that is edible, including other sharks and top order predators.
While the problem of overfishing high up in the food chain is well known – overfishing of tuna, shark finning, whaling etc. – people don’t realise that overfishing can also occur towards the bottom of the food web. Many small species which have little or no food value are often caught and used as a food source for fish farms. Since it takes four kilograms of fish meal or small fish to produce one kilogram of farmed fish, this is exploiting the ocean at a much faster rate than simple fishing would².
What consequences are there from fishing down the food web?
Unfortunately, many people have the misconception that the ocean is invincible. Since the consequences of overfishing and oceanic overexploitation are not as visible as they would be if it was happening on land, people don’t care as much. However the truth is there to punch you in the face if you let it.
At least 32% of the world’s fisheries are overexploited, and another 53% are fully exploited. This leaves just 15% of world fisheries which have room for development – not many when you consider how rapidly the world’s demand for seafood is growing³.
Unfortunately, since most of these overexploited fisheries are focused on species high in the food web and the unexploited fisheries occur low on the food web, the development of these would cause serious damage. If the overexploited fisheries are to recover and survive, then they must have access to a food source: namely, small fish and other organisms⁴.
When you remove these smaller species – the species down the food web – you not only begin to exploit their populations, you also destroy any hope that there may have been for the recovery of predator species. This is extremely environmentally destructive, and can even cause a spiralling effect where the removal of one species causes the death of another species, which causes the death of another species, and so on.
One of the best examples of this occurred in the European North Sea⁵.
- Norwegian cod were overfished to the point of population collapse, which caused the local fisherman to shift their focus to pout.
- However, the removal of large numbers of pout caused an explosion in the krill population, as krill are pout’s largest food source.
- Krill eat copepods, which means that the copepod number declined with the pout numbers.
- Now comes the big problem: juvenile cod rely on copepods as a food source as well.
- The removal of pout led to lower amounts of available food for young cod, which in turn reduced their population recovery.
There is one main lesson to be learned from this: be careful about what you eat! Although it may seem more environmentally friendly to consume smaller species which come from lower on the food web, it isn’t always! Know what you are eating, and look for the mark of approval of the Marine Conservation Society.