The previous sections have described only the management of one stock, treated in isolation. In reality any regulation will have some effect, direct or indirect, on a number of stocks.
Two situations may be distinguished - several stocks of the same species, inhabiting several areas, and stocks of different species inhabiting the same area, and liable to be caught by the same type of fishing gear. Each can be sub - divided into situations where all stocks are too heavily exploited, and are in need of management, and those where some of the stocks are under-exploited. A full management regime for a region should be concerned with all the potentially exploitable stocks in the region, and in particular the encouragement of fishing on under - exploited stocks is as important as the limitation of fishing on the too heavily exploited stocks.
If several stocks are too heavily fished, the best long-term results would be obtained if each is managed separately, to ensure the optimum fishing mortality on each stock. This may be very difficult to achieve in practice. The numerous cod stocks in the North Atlantic differ in the natural parameters of growth, and probably also natural mortality, and will therefore require different fishing mortality rates. However, with the existing long-range vessels fishing in the North Atlantic, which may exploit several stocks in the same voyage, enforcement of separate controls for each stock would be very difficult. This problem was discussed by the ICNAF Bio-economics Working Group (ICNAF 1968), which concluded that since virtually all the cod (and also haddock) stocks were too heavily exploited, a combined catch quota, for all cod and haddock stocks in the North Atlantic would be the most feasible management action, and one that would be much better than the only other practicable alternative - to do nothing.
Some ways in which a single overall quota could be modified to produce a better balance between the amount of fishing on different stocks were examined by the group. In an appendix to the ICNAF report, Gulland described the use of closed seasons to divert effort, within a limited total, from the most heavily fished stocks to those that were only slightly over-exploited. Modifications of the closed season were suggested to allow for ships that are not mobile, and cannot move from area to area. These could receive a share of the overall quota, but specified for a particular stock; within this stock quota fishing by short-range vessels would be unaffected by a closed season.
Another way to differentiate between stocks, particularly if there is an allocation of quotas to individual enterprises, is some form of weighting system. The best known of these is the Blue Whale Unit, used lay the International Whaling Commission. Under this, the catch limit was expressed in BWUs, equal to 1 blue whale, 2 fin whales, 2½ humpback whales, or 6 sei whales. These ratios were based on the relative oil yields of the individual species. This system did not prevent successive over-exploitation of first the blue and humpback whales, and then the fin whales, while the other species were initially under-exploited. So long as the quota remained unallocated, as it was between 1946 and 1960, any other weighting system, e.g. for 3 fin whales to be counted as equal to 1 blue whale, would have changed the date of the close of the season (which might have a very marginal effect on the balance between species), but not the tactics and species preference of an individual gunner, or manager of an expedition. If quotas are allocated, then a gunner may prefer to kill a blue whale (if he can), in preference to two fin whales, but prefer three fin whales to one blue whale. In 1955, blue and humpback whales were heavily exploited, the fin whale stocks were at about their optimum level, and sei whales were untouched. If, at that time, quotas had been allocated, and expressed in Whale Regulation Units, 1 unit being equal to ½ blue whale, 2 fin whales, 1 humpback and 10 sei whales, the effort would have been directed away from the over - exploited stocks towards the fin whales, and especially the sei whales. That is, a better distribution of effort would be achieved if the weights given to each species depart from the relative economic values and give more weight to those stocks which are over-exploited.
The same approach seems possible, though has not yet been adopted, for fish stocks, either for similar species, or for stocks of the same species in different areas. There would be an enforcement problem in the latter case, if say cod from West Greenland counted as 1.2 times cod from Labrador, since a fisherman would try to claim that all his catch from West Greenland.
If some stocks are still under-exploited, e.g. skipjack tuna in the East Pacific, the major short-term problems concern the catches of the species being managed, e.g. yellowfin tuna, made by ships fishing for skipjack when they are not allowed to fish for yellowfin - outside the season, or because they have no licence. The simplest solution, adopted by the International North Pacific Halibut Commission, is to ban the landing of such incidental catches. This removes any incentive for the fishermen to fish where halibut are abundant. It has, however, two big disadvantages. First, some incidental catches are often inevitable; the fisherman's first responsibility is to his gear and to his marketable fish, so that when the protected and unmarketable fish are returned to the sea, even the toughest is likely to be dead. These incidental catches are therefore a complete waste. Second, even if the incidental catches are small, they may still make the difference between a good trip and a bad trip; a ban on landing them may inhibit the development of the fishery on the under-exploited stocks. Thus the ban on landing trawl-caught halibut probably slowed down the development of the trawl fishery in the northwest United States, while the allowance of 15% yellowfin in skipjack landings outside the yellowfin season has helped the development of the skipjack fishery.
The tuna regulations of I-ATTC are an example of many under which the problem has been resolved by allowing a certain percentage of the protected species in the landings. However, as explained before, it seems that in this fishery the higher value of yellowfin causes the fisherman to deliberately seek yellowfin during the early part of their trip, thus ensuring their allowed 15% and if more yellowfin are caught later (while looking for skipjack) these have to be discarded and wasted. In other fisheries, however, the percentage allowance may eliminate this form of waste.