So long, and thanks for all the fish: Fishing effort, sustainable yield and standing stock.

If a fishery exploited at a constant level of fishing effort over a number of years does not show any overall change in catch rate it implies that the resource biomass has stabilized and that the fishery is being harvested on a sustainable basis.

However, as demonstrated in our previous articles, fishing on a sustainable basis, despite the considerable hype around this term, is not necessarily of great value and is in fact the norm in a number of fisheries worldwide. We take for example the North African European hake (Merluccius merluccius) fishery, in the Eastern Central Atlantic (CECAF) management area. At present three nations, Morocco, Spain and Portugal, are involved in this fishery. Fishing fleets are composed of artisanal coastal trawlers, trawlers/sardine-seiners and longliners.

A large proportion of the fleet is made up of small artisanal vessels (about 54 GRT) using codends with a small mesh size (40 mm) which allows them to fish for a mix of species (e.g. hake and deep water shrimp). Although the fishery is relatively stable at annual catches of 10,000 tons, the fishery is using very high effort levels. The MSY at the prevailing mesh size of 40 mm is estimated to be as much as 15,800 tons. This figure could be increased 3 or 4 fold if larger mesh sizes were used. Indeed the mesh size is very small in comparison to mesh sizes in use in other hake fisheries.

The problem is that the Moroccan fleet, which is primarily artisanal, cannot cope with the inevitable short term reduction in catch, both hake and shrimp, that would result from the introduction of a larger mesh size, even if this is still quite small, say 50 mm. In other words the Moroccan hake fishery is being exploited at a sustainable yield which is perhaps 3 or 4 times smaller than the potential MSY because the gear harvests fish before they reach a larger size at which the overall yield-per-recruit is better. The other problem, of course, is that the biomass of hake upon which the present sustainable harvest of 10 000 tons is based is well below the biomass required to generate the MSY. This means that in order to build the fishery to a more productive level it will be necessary to significantly reduce catches for a number of years.

The end result is a sustainable but unproductive high effort (= high cost) fishery with few options for improvement which do not involve paying a considerable socio-economic price. The inability to endure, even on a temporarily basis, the economic implications of more restricted fishing regulations is a characteristic of many artisanal and other fisheries around the world.

In order to understand why many fisheries in the world are presently trapped in a high effort, low production fishing regime we first need to understand the relationship between sustainable yield and long term fishing effort. Figure 1 is a schematic involving flows of water which attempts to demonstrate the relevant relationship. When very little fishing effort is applied over a very long time, then the resource eventually stabilises at a large population size generating a fairly small sustainable yield (1a). As before, there is some effort level which will result in the resource generating MSY (1b). However, when the population size drops very low (1c), it becomes much more difficult to harvest the sustainable yield, and the amount of fishing effort required to do this is larger than in part (a) or (b). The overall result is that the relationship between sustainable yield and fishing effort is as shown in the x-y plots which accompany the water diagrams.

In article 6 of our column (October issue of FNI), we presented a very similar looking diagram giving the plot of sustainable yield versus biomass instead of effort versus biomass. The diligent reader may wish to refer to this earlier diagram in order to understand the following explanation. The difference between the two illustrations is that when fishing effort is zero, sustainable yield will be zero, and the resource biomass will be at its carrying capacity – this is roughly equal to the average biomass that was present before the resource was ever exploitated.

In general, as fishing effort increases, one moves to the right on the graph of sustainable yield versus fishing effort, and one moves to the left on the graph of sustainable yield versus resource biomass. The links between fishing effort, sustainable yield and standing stock set out above can be used to interpret the development characteristics of fisheries in precise biological and economic terms. This leads to a clear picture of how one should manage a fish resource so as to avoid overfishing. The following phases are commonly described:

Phase I: Since catch rate (i.e. CPUE) is an index of resource biomass, the reduction in catch rate that occurs during the development of a fishery is due to a reduction in the standing stock of the resource. In the diagram published in the October issue of FNI, this is depicted as a decline in the water level in the container. During this period of the fishery, the annual catches must be larger than the sustainable yield in the resource, for the reduction in the resource biomass to persist.

Phase II: As the resource biomass gets smaller, so the fishing effort and the cost of harvesting will increase. A point will be reached where the increase in catch is too small to justify the cost of additional fishing effort. Fishing effort and catches will level off.

Phase III: However, because there is a strong chance that the catch level that has been reached exceeds the MSY of the resource, the resource biomass will continue to decline. Although this decline must stabilise, in uncontrolled fisheries this point of stabilisation is often economically non-viable.

With reference to these concepts, two types of fish resources are generally recognised:

• i) Productive fisheries: These are fisheries where the recruitment and the natural mortality streams are quite large compared to the resource biomass. This leads to a curve of sustainable yield versus resource biomass that is quite distinctly humped, which means that the MSY is quite large compared to the carrying capacity.
• ii) Unproductive fisheries: These are those in which the recruitment and natural mortality streams are very small compared to the resource biomass. This leads to a much flatter relationship between sustainable yield and resource biomass.

Comparison of these two fishery examples illustrates a fundamental problem in fisheries management. During the development phase of a fishery, when catch rates are declining, it is very difficult to determine whether the resource is being "mined out", with catches grossly in excess of sustainable yields, or whether catches only modestly exceed sustainable yield levels. In the case of an unproductive resource which is being mined out, the future of the fishery is at risk, and economic expectations about the future performance of the fishery are completely unrealistic. In the case of a highly productive resource, the fishery can be placed on the correct development path with minimal economic dislocation.

A typical example of an unproductive fishery which was mined out and which generated unrealistic expectations is the rock lobster stock along the northern reaches of the South African west coast. This fishery experienced peak annual catches of about 2 000 metric tons, however recent calculations suggest that the maximum sustainable yield of this resource may be less than 100 metric tons. An example of a productive fishery which has been placed on a corrective development path is the Namibian hake which has an estimated MSY level of in excess of 300 000 metric tons. In this resource, catches peaked at about 850 000 tons in 1972, and persistent catches in excess of MSY eventually reduced the resource below its MSY biomass level. Catches are presently kept below the MSY (around 200 000 tons) in an attempt to rebuild the stock to an economically and biologically desirable level.

The risk associated with the development of new resources whose productivity is unknown is now recognized by all modern fisheries managers. Nevertheless, recent experience suggests that despite this knowledge new resources have been rapidly reduced to very low biomass and productivity levels as demonstrated by a number of orange roughy and Patagonian toothfish fisheries. The reason for this is that there are economic forces driving the commercial development of these fisheries which do not favour sustainable resource exploitation practice. For them the desire to cover the high risk of their investment in a very short time (boom and bust approach) overrides the desire to sustain fisheries at their optimum productive levels. In addition high seas fisheries where it is almost impossible to limit access creates another problem commonly referred to as the “tragedy of the commons” which will be discussed in future articles.