Ecomare

Optimal fishing limits itself to that amount which is hauled in yearly without bringing the spawning stock in danger (the maximum sustainable yield = MSY). When overfishing is avoided, one can be assured of good fish yields for years to come. In the graph above, one can see that the spawning stock maintains itself partially because there is sufficient young fish (recruits) that can grow to adulthood.
There are various forms of overfishing. When overfishing happens because fish are caught before they reach the optimal weight and size, one speaks of 'growth overfishing'. Recruitment overfishing happens when too many fish are caught that have not as yet reproduced. A fish stock can also totally collapse, and then one speaks of an exhausted situation.

The spawning stock of a species that is not fished is high. There are enough recruitment (juvenile fish that provide offspring) and the fish stock remains the same.
In the North Sea, there are only a few species that are absolutely not fished. These are small fish such as pipefish, gobies and solenette. They are only caught as undesired by-catch in the nets. Lesser sandeel and greater sandeel were barely caught before 1970, however they now belong to the most caught fish in the North Sea. Even other species in the North Sea, which were traditionally speaking commercially uninteresting, such as squid are getting more attention.

With optimal fishing, approximately one fourth of the total population (spawning stock and recruitment together) are caught. The spawning stock declines slightly, but there are enough recruits produced to keep the fish population stable. The fishery efforts and the catch are in a positive proportion: the costs per kilogram caught fish is low with regard to intensive fisheries. The largest amount of fish which can be caught while still maintaining a healthy fish stock is called the maximum sustainable yield.
The North Sea plaice were optimally fished between 1957 and 1976. However since 1977, the fishery pressure has been increasing to a point of overfishing the plaice. After years of serious overfishing followed by extremely strict regulations, the herring fisheries in the North Sea turned into an optimal situation between 2002 and 2005. The same is true for mackerel and scad.

With growth overfishing, a too large section of the spawning stock and recruitment stock are fished. Therefore, there are too few offspring to maintain the stock and the size of the catch. The stock slowly declines, however it could happen more quickly when factors other than the fisheries negatively influence the stock. The catches could still be relatively high. The relationship between the fishery effort and the catch grow unfavorable.
Plaice and sole have been at this stage of overfishing for years. Specific fishery policy is necessary to maintain the spawning stock and the present profitable fisheries.

With overfishing of the recruits, the fishery effort is so great that even the juvenile animals decline in number. Therefore, the population of the fish species eventually on consists of young and immature fish. The stock is extremely productive because small fish grow much more quicky than older fish. Nevertheless, the fisheries must put in a lot of effort to catch anything.
A small and young spawning stock and an even smaller group of recruits make a fish population very vulnerable for natural fluctuations. Fish species that are overfished this much have a large risk of being totally wiped out.
The phenomenon of a too low or continually declining spawning stock is occurring in 2007 with sole and whiting. Strict fishery policy with low quota for these species is necessary, in order for them to survive.

In extreme cases of overfishing, the entire stock collapses. The spawing stock has been reduced to below the safe minimum and produces too few offspring. The great the fishery effort is, there is no profitable catch anymore.
After a collapse, extreme protective measures are necessary to rebuild the fish stock to a certain strength. Extermination is not in question since the fishery will have collapsed beforehand. A fish species can totally disappear locally, which happened to the thornback ray in the Wadden Sea and the coastal zone of the North Sea.
For the fishermen and the thereby related industry, a collapse of the fish stock is often disastrous. Something of the sort has often led to the pauperizing of a fishing fleet. A classical example is the Pacific sardines which used to be found by the coast of Monterey in California. Within a few years since 1950, there was nothing more to fish while before then, 200,000 to 300,000 tons per year were landed. Other examples are the Peruvian anchovies, the cod by the coast of Newfoundland and the North Sea herring. In none of these cases was the fisheries the only cause of the collapse, however an extremely high fishery pressure together with weak year classes lead to a collapse in the stock. Often times, the precise share of the various factors is not possible to register and the blame is thrown in all directions. Fishermen, fishery biologists, politicians, climate or whales and seals each are blamed in turn.
In the North Sea, the cod stock has been on the edge of collapse since 1998. Causes are the high fishery pressure on the species itself, the large amount of cod caught as by-catch in flatfish nets and perhaps also the change in temperature of the seawater: cod is a cold water species that migrates to the north when the water in the North Sea warms up.
Even when a collapse happens, the fish stock can still recover when one lowers the pressure. At the end of the 1980s, the herring were once again in high numbers. Despite past experiences, the herring fishery is again fishing rather intensively.