At the other end of the scale, the North Sea demersal fishery consists of about 20 species caught by some 12 countries using a variety of fishing gears. It is possible to disaggregate this major fishery into sub-fisheries, by country, species or area or fishing gear, but none of these would be a cohesive fishery unit because other sub-fisheries would impact it. This amalgam of sub-fisheries makes these fisheries very difficult to manage. The range of different fishing methods generates conflicts among fishers, often with the most efficient gear blamed for overfishing and/or damage to the environment.
Given these two different situations, it is easy to classify up to about 70 percent of the world's catch into around 200 fisheries. However, the remaining 30 percent is simply classified as the North Sea fishery (an Area classification) broken down into species and/or countries.
This range of fishing methods is, in many cases, a measure of the fishery's maturity given that, as each more efficient method has been introduced, a remnant of the previous prevailing fishing method still co-exists. The wide range of target species gives rise to one of the most intractable problems of fisheries management: that of the optimum fishing effort on a multispecies fishery. Each species has a different optimum fishing effort and the management decision on the fleet's optimum size will be greater than the optimum for any one species. This leads to overfishing on some species and a less than optimum catch for others. Attempts to correct this maladjustment by imposing catch quotas cause overquota fish to be discarded in order to keep more of the underquota species - thus optimizing the boat's revenue when the overall catch quota is close to being reached.
As a general rule, industrial fisheries are confined to the continental shelf where fish are more abundant, although there are exceptions. The fisheries for large pelagics such as tuna and swordfish are ocean-ranging using purse seines and longlines. However, the global total catch of these vessels would be only around five percent of the those vessels fishing on the continental shelf. Another group of fishing vessels presently attracting much attention target Patagonian toothfish and similar species, but again their numbers are minimal compared to the continental shelf or near coastal fisheries. One issue that is causing concern is bottom trawling in very deep water on species that are very long-lived and therefore vulnerable to overfishing. Another problem is the possible damage to sensitive areas that might prove to be irreversible.
The most spectacular industrial fisheries were the fleets of factory vessels and factory trawlers built in the Soviet bloc between 1960-1980. These very large vessels fished all around the world and were supported by a fleet supplying fuel and provisions and transporting the catch. The largest vessels were about 8 000 GRT and more than 120 metres in length. The extension of the EEZs in the early 1980s restricted the fishing grounds that were freely available to these vessels but in many cases fishing agreements between the flag State and the coastal State allowed the vessels to continue fishing for some years.
As coastal countries developed the ability to harvest their own resources, such agreements were more difficult to negotiate and, around 1990, the building programme for these very large vessels came to an end. Although there are still some very large vessels currently being built for fishing in the North Atlantic and North Pacific, these are exceptions and the numbers are in single figures compared to the hundreds built in the 1970s and 1980s.