Where to now? Using stock assessments for the job of managing fish stocks.

In our previous articles we exposed you to some of the mathematical models used for fish stock assessments. This is a complex and murky world, apparently involving black magic, but which is really just a very unwieldy scientific discipline where objectivity can easily be lost, and where a single assumption can have a decisive impact on estimates of stock size and productivity, with enormous social and economic consequences. If you survived this journey with your sanity intact, and you still want to find out more, then you are ready for the next step. This step tackles the question “We’ve done all this weird and wonderful modeling work, now what?”

Or have we in fact become so absorbed in chopping wood and sharpening our axe that we have lost sight of the forest! It’s worth reminding ourselves that what we’re really trying to do is to manage fish stocks on a sustainable basis for maximum economic benefit.

So how does one take all the mathematical mumbo jumbo of stock assessments and come up with something that is useful in a management context. Well, we need to go back to basics and remind ourselves what the stock assessments give us. They give us an estimate of the productivity of the resource, how this productivity is related to the resource at different biomass levels, and, critically, an estimate of the present resource biomass level.

What happens in practice is that when stock assessment results are submitted to a management forum, and people realize what their actual implications might be, then all manner of objections are raised, assumptions and data are questioned, and the mathematical modelers are asked to go back to check their assumptions, to check the data, and to check their calculations. A number of iterations of this process then takes place. This is an unavoidable socio-economic dimension of stock assessments, which is seldom ever fully resolved to everybody’s satisfaction.

Quite apart from this is the added complication that the mathematical models do not and in fact cannot provide an answer to the basic management questions: “What is the best way to manage the resource? What is the optimal management approach?”. In a perfect world in which the mathematical models are true and perfect, it is possible to estimate the “optimal harvesting strategy”.

This might be a strategy which maximizes the catch or profit or rent from a stock over a period of 10 or 20 or 30 years, or even over an indefinite period of time. These optimal harvesting strategies are unfortunately woefully impractical. They invariably involve extreme and unfeasible types of harvesting. They may include, for example, wild and socially unacceptable swings in catches from one year to the next, or completely impossible selectivities, such as the harvest of a proportion of fish at one size, and the harvest of all fish at another larger size.

It is therefore generally accepted that the management process cannot be purely scientific or purely mathematical, but that there must be an overall objective within which the investigation of management approaches takes place. Fisheries management has therefore left the purely mathematical ideas behind and tried to focus on choosing between a number of different practical approaches.

The first step in the management process is to identify the objective of resource management for the resource under consideration. From a biological point of view the main issue is the size of the resource biomass. Is it too low, in which case the resource needs to be rebuilt? Is it too high and should it be depleted in order to realize maximum productivity? Is the resource biomass adequate implying that there is no need to rebuild or deplete it but rather an attempt should be made to try to stabilize the resource biomass at its present level?

North Sea cod is an example of a stock that needs to be rebuilt. Rebuilding strategies can be economically devastating, because large reductions in catches are often required to achieve even a moderate and slow recovery in the resource.

Economic issues are often more diverse. The simplistic view is that short term economic considerations always favour an increase in the total catch. However, in mature fisheries which have stable rights allocation regimes, fishers generally appreciate the need for a long term view and they understand that they cannot rely solely on an increase in the total volume of fish to enhance profitability. This is particularly true of capital intensive fisheries where the costs of fishing is high and margins are low. In these fisheries the CPUE has an added economic dimension – small average changes in the CPUE can have big economic effects.

Biologists and fisheries scientists are beginning to appreciate that markets are complex and sensitive. Subtle changes in fishing strategy can have serious economic consequences. Finfish processing and marketing is geared to particular size distributions of the catch. Changes may require retooling and the development of new products. Fishers and processors are therefore very sensitive to stability in the catch and its size structure. Stability is an important economic objective in resource management.

Once an overall objective or set of objectives has been identified for a fishery, the next step is to devise a harvesting strategy which is able, at least in concept, to achieve the stated objectives. Almost by definition, a long term view has to be adopted for the development of these harvesting strategies. Whatever harvesting strategy has been adopted however is of no use unless adequate management controls are in place. The efforts of fisheries scientists and enforcement agencies really need to be commensurate. For example, it’s no use developing a sophisticated harvesting strategy for an abalone resource if poaching is rampant.

An important component of any fisheries management system is therefore to ascertain for a given fishery what kinds of management measures are practical and useful?. Management measures can be divided into two kinds: input controls and output controls. The simplest definition of input controls is those that are concerned with fishing effort and whatever goes with it (fish gear). Ouput controls involve the catch itself and the size mix of the catch. Under the heading of input controls is included the following:

• Closed seasons, either fixed or variable depending on the progress of annual catches
• Gear regulations (e.g. escape gaps on lobster pots)
• Closed areas
• Effort allocations, e.g. permissible number of fishing days per operator
Under output controls one finds
• TAC – total allowable catch
• Individual quotas
• Minimum and/or maximum size restrictions
• Prohibition on the harvest of vulnerable biological stages, e.g. lobster in berry, or soft shell lobsters.

Debates on whether to use input or output controls are often linked to a country’s broader political traditions. Some countries emphasise output controls, others emphasise input controls.

Assuming that there are sufficient controls in place, then in years gone by a number of standard harvesting strategies were considered, almost as rules of thumb, for the achievement of long term management objectives. These harvesting strategies went by names like fMSY or f0.1. These two strategies correspond to a rule for setting the annual catch as a proportion of the estimated available resource biomass each year, and over time each one drives the resource towards different theoretical target biomass levels, the first to the MSY point, and the second to a biomass larger than the MSY point. These strategies have been modified in different ways as their shortcomings have been realized.

For example, a constant proportion harvesting rule has a significant shortcoming that the harvest can suddenly be very big, exceeding even the MSY of the resource, if for some reason the estimated resource biomass is very large. This shortcoming has led researchers to propose harvest strategies where the catch cannot exceed the MSY of the resource. As additional bits and pieces are added to the harvesting strategy it soon loses its identity as a simple formula and becomes just a set of rules.

The modern approach is to take any harvesting strategy whose rules can be captured on a computer and study it using computer techniques. This approach is relatively new in fisheries management, and falls within a general area of study known as ‘Management procedures’. Management procedures are just sets of rules for managing fisheries. One can use the model of the resource developed from stock assessments as the basis of a computer representation of the population for studying the long term effects of different management procedures, by running the computer model 10 or 20 years into the future many times over for different recruitment scenarios and other key quantities.

In theory at least, once a management procedure which suitably compromises biological and economic management objectives has been identified in the virtual computer world, one can apply it in the year to year management of real fish stocks.

As one may imagine, management procedures have considerable appeal for bureaucracies, and fisheries scientists are also drawn to their underlying philosophy. But are they really of significant value? We are compelled to return to some of the concerns expressed at the start of this article.

Are we getting back to chopping wood and sharpening our axe, or have we in fact advanced fisheries management with this new mathematical marvel? As will be seen in the next article, management procedures are not without their difficulties and shortcomings, and one needs to keep an open mind and a critical eye on affairs at all times.