MESH SIZE REGULATION AND ITS ROLE IN FISHERIES MANAGMENT

R. Jones
Marine Laboratory
Aberdeen, Scotland

Abstract

This paper is concerned with the principles of mesh regulation, its role in fisheries management and difficulties encountered in its implementation.

The theoretical basis of mesh regulation is based on the concept of an “optimum harvesting strategy”, i.e. the fact that, for many commercially important fish species, recruitment is unrelated to spawning stock size over a wide range of stock sizes. This makes it possible to conceive of an optimum exploitation pattern for exploiting each year class throughout its life, irrespective of its absolute magnitude. Within this context, theoretical considerations are concerned with the probable age at which a fish is likely to be captured. If, for example, fish are caught when they are very young, the catch will typically consist of large numbers of very small individuals. This will provide a numerically very large catch but not necessarily a large weight of fish. Conversely, if fish are not caught until they are very old, the catch will tend to consist of a relatively small number of relatively large fish. Again the aggregate weight of catch may not necessarily be large. The theoretical approach is to determine the best average age or size at which to capture so as to maximise the sustainable yield in weight per year class entering the fishery.

There are various ways of achieving this, and one of these is by regulating the age (or size) of first capture directly. This may be done by regulating the mesh size, in a demersal drag-net fishery, or the hook size as in a hook and line fishery. One object of mesh regulations therefore is to influence the sustainable yield in the long-term. Other objectives are to protect juvenile fish from capture, and to try to ensure that sufficient fish survive to maturity. This may be especially important for stocks which are exploited at so high a level that there is a danger of recruitment failure due to diminished stock size.

There are a number of difficulties with mesh regulations, one of which is that it is not usually possible to demonstrate that an actual change in mesh size has had the expected effect on catches. This is because natural fluctuations in stock size (and catch) tend to be much larger than the expected effects of the changes in mesh size that have been implemented in practice. This does not necessarily mean that mesh regulations have no effect. It does mean, however, that the effects of mesh regulations cannot be demonstrated, in practice. Because of this, and because the short-term effect of a mesh increase is likely to be a reduction in catch, it may be difficult to convince industry that such restrictive measures are really necessary. Other difficulties include:

How to persuade industry to accept regulatory measures if different sectors of the industry are likely to be affected to different extents (for example, fisheries for fish meal frequently require smaller-meshed nets than fisheries for fish for human consumption
How to determine the best mesh size for a multispecies fishery such as a demersal trawl fishery
How to ensure enforcement in situations where it may be possible to corrupt the enforcement officers
How to prevent evasion of regulations as might be done, for example, by fitting one codend inside another as to reduce the effective mesh size when in use
How to compromise between the spirit and the letter of the law; for example, to comply with the intention of a regulation, it may be appropriate to require the average mesh size to be of a certain size, but for legal purposes it may be necessary to require that no meshes are below a certain size.

In conclusion, it must be accepted that there are a number of difficulties associated with mesh regulations, but equally, it should be pointed out that none of them need be insuperable.

1. INTRODUCTION AND GENERAL MANAGEMENT CONSIDERATIONS

The general history of fisheries throughout the world shows that there has been a gradual increase in exploitation, partly due to an increase in the rate of exploitation stocks close to the land and partly due to an expansion of fishing effort to more distant fishing grounds. Associated with this increase in effort, there has been a decline in the catch rates on certain grounds and in many cases an increase in the quantities of small fish, including the juveniles of economically important species taken in coastal fisheries. Because of this, as well as the growing recognition that stocks may not be inexhaustible, it is generally recognised that stocks should be managed, even if only to prevent further declines.

There are a number of possible management objectives, not all of which are necessarily compatible one with another. For example, one important objective is to exploit a stock so as to obtain an “optimum” return or yield from the fishery. Another is simply to ensure that it is never exploited so heavily that its continued existence is threatened. In some parts of the world the principle objectives are socio-economic: for example, to provide employment to fishermen.

If the objective is to optimise something, there may be different things that one may wish to optimize, or maximize. For example, one may wish to maximise the total yield that can be removed annually from the stock. This may be a desirable national objective. An alternative is to maximize the total profit from the industry. This is likely to be an objective if a fishery were exploited by a single owner.

In practice, there are usually many owners (often individual fisherman) who operate more or less in competition with one another. From the viewpoint of any one fisherman, a suitable objective therefore would be to maximize the catch rate per boat. In practice, however, so long as it is profitable to do so, the tendency is for more and more boats to be built until the catch per boat drops to a level at which it is no longer profitable to build more boats. Theoretical considerations and experience show that under these conditions the total fishing effort is likely to be considerably higher than it would be if the objective were to maximise either total yield or total profit.

Thus, so long as it is uncontrolled, fishing is likely to increase to the point where it is no longer profitable to build more boats. The danger then, is that the spawning stock will be reduced to the point where there is a possibility of a stock collapse. This may be particularly so for those stocks that are influenced by large natural fluctuations in stock size. Under favourable natural conditions, for example, a stock may be able to withstand a relatively high level of fishing, whereas, under other and less favourable natural conditions, the same level of fishing effort may be far too high and the stock may be driven to a dangerously low level.

Fish stocks may be managed by controlling fishing effort directly, by controlling catch, through quotas, by controlling the age and size at which the fish are first exploited, by restricting fishing either in certain areas, or at certain times of the year, or by measures designed to prevent habitat deterioration. Of these, it is regulation of the size at first capture that is discussed below, since it is this measure that is concerned with mesh size regulation.

Control of the size at first capture can be an important measure for regulating fisheries in which the amount of fishing is not otherwise controlled. By introducing an appropriate minimum size of mesh, for example, it is possible to protect juveniles so as to give them time to grow to an economically useful size before they are harvested. If necessary, individuals can also be protected from exploitation until they are large enough to spawn, and in this way the size of the spawning stock can be protected.

2. OBJECTIVES OF MESH SIZE REGULATION

There are basically two reasons for regulating mesh size. One is to conserve the spawning stock and the other is to increase the long-term sustainable yield.

2.1 Conservation of the spawning stock

In situations where fishing effort is unregulated, there is the possibility that effort may become so high that there is a danger of a stock collapse, due to a depleted spawning stock and a resultant recruitment failure. If this danger exists, and it is not practical to regulate fishing effort directly, an increase in mesh size may be a useful alternative means of conserving the spawning stock. A suitable choice of mesh size should reduce the rate of capture of juveniles, and make it more likely that an individual will survive to the size of first maturity and have an opportunity of spawning at least once.

2.2 Regulation of long-term yield

Regulation of long-term yield is based on the concept of an “optimum harvesting strategy”. For many commercially important fish species, recruitment is unrelated to spawning stock size over a wide range of stock sizes. It is realistic therefore to conceive of an optimum exploitation pattern, or harvesting strategy, which leads to the optimum yield per year class. The basic theory is summarised below, and considered in greater detail in Section 4.

Regulation of the age (or size) of first capture, ensures that the full effects of exploitation are felt only by the older (or larger) individuals. Theory is based on the assumption that changes in the age (or size) of first capture will cause stock size to change from one equilibrium level to some other level. Theoretical computations are largely concerned with the effects of changing from one age (or size) of first capture to another. Changes may be immediate, long-term, or transitional.

The immediate effect of an increase in mesh size, is to lose some animals that would otherwise have been captured. The immediate effect therefore is a loss in catch and earnings for all the vessels that increase their mesh size.

Some of the fish that escape through the larger meshes will be captured eventually and by then they will have had time to grow older and larger. Other individuals will die of natural causes however, and not be caught at all. This means that, out of a given number of recruits, the long-term effect of an increase in mesh size will be a decrease in the number caught and an increase in the average size of individuals caught. Provided the gain in weight, due to the growth of the individuals that happen to be caught, is greater than the loss in weight due to those that are not caught at all, there should be a long-term benefit.

Even if the long-term effect of an increase in mesh size is beneficial, the immediate effect will involve a loss. There will therefore be a transition period before the full long-term benefit is experienced. The length of the transition period will depend on the length of time required for the youngest fish in the catch to pass completely through the fishery under the new conditions of mesh size. In general the transition period will depend approximately on the number of years for which an individual is subject to exploitation. In heavily exploited stocks, the bulk of the catch consists of fish that have been subject to exploitation for only a few years. In these stocks, therefore, possibly as much as 70–80 percent of the long-term benefit might be expected after a transition period of only two to three years. In the case of a lightly exploited stock, individuals will tend to be exploited for a longer period and the transition period will therefore be longer also.

In summary, a change of mesh size can usually be regarded as beneficial if it causes catches, in the long-term, to be greater than they otherwise would have been. A mesh regulation does not necessarily lead to an increase in the absolute level of catches, however, since these will continue to be influenced largely by natural variations in the level of recruitment.

3. PRACTICAL DIFFICULTIES AND MANAGEMENT PROBLEMS

In spite of its basic theoretical simplicity, the determination of the “optimum” mesh size can be associated with a number of practical difficulties and management problems.

3.1 Demonstrating the effect of a change in mesh size

Perhaps the greatest difficulty with a mesh regulation is that it is usually impossible to demonstrate that a change in mesh size has had the effect that is predicted by theory. This is a consequence of recruitment fluctuations of an irregular nature in the short-term and of periodic variations in recruitment in the long-term. As a result, there tend to be natural fluctuations in stock size that are considerably larger than the mesh-change effect one may be expecting.

All that can be said, in practice, is that the effect of a change in mesh size is to cause catches, in the long-term, to be a certain percentage higher or lower than they otherwise would have been. Because, in practice, natural variation is likely to be so much larger than an expected effect, the effect of a mesh regulation is usually statistically undetectable.

The does not necessarily mean that mesh regulation have no effect. It does mean, however, that the effects of mesh regulations usually cannot be demonstrated and this may make it difficult to convince industry that a regulation is likely to be beneficial (see next section).

3.2 Convincing industry of the need for restrictive measures

Conservation measures are almost invariably restrictive in the short-term. For example, there can be the need to restrict fishing effort, the need to increase mesh size or the need to interfere with activities that might cause habitat deterioration. An objection to an increase in mesh size, therefore, is that there is likely to be an immediate loss and that it could be several years before long-term benefits are likely to be experienced. Because of this, fishermen, and sometimes administrators, may need to be convinced of the need for such measures, and this may be difficult to achieve in practice because of the natural variability of recruitment. Even stocks which are believed to be a very low level can sometimes produce good year classes and lead to temporary increases in catches. This may make it difficult to convince fishermen that a stock is being over-exploited and that restrictive measures are really necessary.

Uncertainty can also arise because of periodic fluctuations in stock size of a fairly long period, i.e. perhaps 50 years or more). Thus, until sufficient data have been accumulated to quantify these cycles, management may find itself confronted with a declining stock but without any means of knowing for certain whether this is due to exploitation or to natural causes.

One way of minimizing these objections might be to increase the mesh size gradually or to increase the mesh size at a time when the stock happened to be increasing due to natural causes, such as when there is a strong recruit year class. However, whatever course of action may seem appropriate, little is likely to be achieved without the understanding, and acceptance of the necessity for it, on the part of industry.

3.3 Determining the best mesh size for a fishery

Determining the best mesh size for any one species can be a straightforward business. Determining the best mesh size for a fishery in which more than one species is caught by the same gear, however, is likely to be much less straightforward.

In the North Atlantic, where demersal fisheries are based on a relatively small number of species of value, this problem is not too difficult. In tropical demersal trawl fisheries, however, where there may be several hundred commercially valuable species, the mesh size has to be a compromise between the best mesh sizes for the individual species. Some species (e.g. Trichiurus) are long and thin, whereas others (e.g. Apogon) are short and fat. Others may be small-bodied when fully grown, so that the best compromise mesh size for a tropical fishery may be anything but the best mesh size for many of the species individually.

3.4 Meshing

A problem that can occur if a mesh size is too large is that of “meshing”, i.e. the occurrence of fish that have become lodged in the meshes of a net while trying to escape. Meshing, if it occurs on a large scale, is a nuisance to fishermen because of the time needed to remove meshed fish individually from a net.

3.5 Socio-economic problems

An increase in mesh size is less likely to be accepted if benefits are not experienced by all sections of the industry. Thus, if an increase in mesh size favours one section of the industry at the possible expense of another, there is likely to be a conflict of interests. Possible sources of conflict involve inshore subsistence fishermen who may want to use relatively small mesh nets, and offshore fishermen who may be prepared to use larger nets with larger mesh sizes. Alternatively, there may be a conflict between fishermen who wish to use small meshes to catch large quantities of small-bodied fish for processing into fish meal, and fishermen who are prepared to use large meshes to catch larger fish for human consumption. So long as there are conflicting aims, enforcement is likely to be difficult to achieve in practice.

3.6 Quality of enforcement

To be effective, some way of enforcing a mesh regulation is essential, which means that the quality of enforcement must be reasonably high. If, for example, the enforcement officers happen to be people with very low salaries, who may be tempted by bribes to relax their vigilance, then enforcement is not likely to be effective.

3.7 Chafers and double codends

To be effective, a mesh regulation must ensure that the mesh size, when the net is fishing, is not less than some specified amount. There is, then, the danger that the mesh size when the net is fishing is less than the measured mesh size. This could arise if, for example, double codends (i.e. one codend inside another) were used. Similarly, if pieces of canvas or netting (known as chafers) are attached to a codend for strengthening purposes, the effective mesh size may be smaller than the apparent mesh size. Chafers are usually sub-divided into bottom chafers and top-side chafers, these terms referring to the part of the codend to which the chafer is attached.

Because of the possibility of using a double codend, it may not be sufficient to attempt to regulate the mesh size by prescribing a minimum mesh size for factory-made codends. This might appear to be a useful measure but, whether it is employed or not, it is likely to be necessary to take measures also to permit inspection of codends at sea. To take account of double codends and chafers, it may be necessary to word regulations to take account of the following points:

That no device shall be used by means of which the mesh size is, in effect, diminished.
That inspectors may be authorized to inspect nets at sea as well as ashore.
That, if exceptions are to be permitted, the regulations should clearly specify what form these exceptions may be permitted to take; in the Northeast Atlantic, for example, consideration has been given to:
- A chafer of netting, having in all its parts a mesh twice the size of the mesh in the codend when measured wet, fastened along the fore and lateral edges only, and that is at least 1½ times the breadth of the codend.
- A “multi-flap” chafer, i.e. a top-side chafer made up of numerous flaps, each fastened along the fore and lateral edges only.

If possible, the use of chafers and covers should be avoided and this may be possible if the codend is made of strong enough material.

3.8 Minimum landing sizes

In the North Atlantic, minimum landing size have been specified for the principal species in conjunction with existing mesh regulations. The object is to aid enforcement by discouraging fishermen from using very small-mesh nets and then having to reject a relatively high proportion of their catch. This can be done by making the minimum landing size related, in an appropriate way, to the 50 percent length of the legal mesh size. To be effective, some under-sized fish will unavoidably be caught and have to be rejected. The loss, or waste, of these fish can then be regarded as the cost of adopting this particular enforcement measure.

If there is to be a minimum landing size, the regulations should specify that under-sized fish should not be sold, or landed, but should be returned to the sea.

In practice, minimum landing sizes may be really effective only in regions where there is relatively little market demand for small fish. In regions where small fish are in demand for duck food, for feeding pond fish or for fish meal, for example, the introduction of a minimum landing size may only lead to an illegal market in undersized fish.

Without minimum landing sizes, however, there may be an even greater economic incentive to fish with a mesh size smaller than that specified in a regulation.

3.9 Exemptions from mesh regulations

When a mesh size is to be increased, there may be pressure, if only for a transition period, to permit exemptions to mesh regulations for certain types of gear or for certain regions.

A problem with exemptions is that legal loopholes tend to be created and these may enable people to escape conviction.

3.10 Mesh differentials

Codends made of different materials can have different selective properties even if the mesh size is the same.

To allow for this, it may be necessary to:

Frame a mesh size regulation with reference to a particular cod-end material.
Specify “differentials” (i.e. conversion factors) so that the mesh size for one material can be converted to an equivalent in terms of the mesh size for some other material.
Specify whether a net is to be measured wet or dry.

3.11 Standardization of mesh-measuring methods for enforcement purposes

A difficulty with all mesh measurements is the specification of the amount of tension to put on the netting when the measurements are made. When measuring a mesh size, for example, different individuals may stretch a piece of netting by different amounts. The tension can also be influenced by the number of meshes measured at any one time. Alternatively, when measuring individual meshes with a wedge-shaped gauge, there is the difficulty of deciding with what pressure to insert the gauge into the netting.

Because of this difficulty, it may be necessary to treat separately the problem of measuring the size of a particular mesh from the problem of deciding if the meshes of a particular codend are under-size for legal purposes.

For measuring individual meshes with a standard tension between knots, two methods have been employed:

For scientific purposes in the North Atlantic, a special gauge has been constructed so that the internal stretched mesh of a single mesh can be measured with a predetermined amount of tension between the knots.
An alternative method is to construct a wedge-shaped gauge with a specified taper and to standardize the tension by hanging a specified weight on it after it has been inserted through a mesh.

For legal purposes, because of the statistical variation in mesh sizes under natural conditions and because of the difficulty of standardizing methods of measurement, it may not be enough to measure a number of meshes to decide whether or not a particular cod-end contravenes a regulation.

In the Northeast Atlantic, to avoid legal loopholes, it has been found appropriate to state that:

No meshes shall be smaller than the minimum size.
The minimum size shall be such that when the mesh is stretched diagonally lengthwise of the net, a flat gauge 2 mm thick of the appropriate width shall pass through it easily when wet.

The appropriate width of gauge in relation to any type of net in any type of the convention area is then stated in each case.

An implication of the wording is that the average mesh size in use will have to be larger than the minimum mesh size specified in the regulation, and this should be taken into account when determining the minimum mesh size.

4. THE THEORETICAL BASIS OF MESH SIZE REGULATION

Experience has shown that, when a stock is moderately exploited, it does not necessarily dwindle away and collapse after a few years. Instead, the effect of exploitation is more likely simply to be an initial decline in the stock, associated with a decline in the catch rate. Provided there is no further increase in effort, the stock then is likely to stabilize itself at a level lower than it otherwise would have done before but not to decline indefinitely. In general, experience suggests that stocks are able to maintain themselves under even moderately high rates of exploitation.

A simple model is depicted in Figure 1. This shows the factors affecting the numbers in a stock. Each year the number in the stock is augmented by an influx of young fish commonly known as “recruits”. At the same time, there is a steady rate of removal of fish, partly due to fishing and partly due to natural causes. If a stock maintains itself under these conditions, it may be supposed that in the long term the following equilibrium equation ought to hold:

Recruits = natural losses + number caught

Figure 2 shows the same considerations applied to the weight of fish in the stock. Here the weight of individuals is augmented partly by the weight of recruits produced each year and partly by the growth of the individuals in the population. At the same time, there will be a loss in weight comprising the weight of fish removed. In the long-term, provided the stock does not collapse, the following equilibrium equation ought to hold:

Recruits + growth = natural losses (in weight units) + yield in weight

Re-arrangement of these terms gives an equation for expressing the annual catch in weight from a stock in equilibrium. This is:

Yield in weight = weight of recruits + growth - natural losses (in weight units)

This simple analysis shows that yield is partly dependent on recruitment and partly on the difference between growth and natural mortality in the exploited part of the stock.

4.1 The concept of an optimum harvesting strategy

The determination of the effect on yield of, for example, a change in size is usually done on the basis of principles described by Beverton and Holt (1957). These authors noted that, for very many commercially-exploited fish species, recruitment is highly variable and apparently unrelated to the size of the spawning stock, at least over a relatively wide range of stock size. For this reason, the contribution of recruitment to yield can be treated separately from the contribution due to growth and natural mortality. This approach leads to the concept of an optimum harvesting or yield-per-recruit strategy, that is providing a stock is not reduced to a level at which recruitment failure might occur, the problem reduces to that of determining the optimum combination of mesh size and fishing effort for harvesting each year class, whatever its absolute numerical strength.

4.2 Sub-division of the stock into age groups

To further understand the principles on which this approach is based, it is not sufficient to sub-divide the stock into two parts (spawning and non-spawning) as in Figure 3. Instead, it is necessary to appreciate that a stock is made up of a number of age groups in different proportions.

Figure 4A shows the sub-division of a stock into age groups at the beginning of a year. It is supposed that there is a certain number of one-year old fish, a certain number of two-year olds, so many three-year olds, and so on. As the year progresses, the numbers of fish in each group gradually decline as a result of a loss of fish partly due to fishing and partly due to natural causes. The situation by the middle of the year, for example, might be shown in Figure 4B. By the end of the year (Figure 4C), corresponding to the beginning of the following year, all the individuals in age group 1 will have declined in number and become transformed into age group 2. Similarly the 2's will have become 3's and the 3's will have become 4's, and so on. At the same time, eggs will have been produced and a new year class formed to provide a fresh lot of recruits. Under theoretically ideal conditions of stability (i.e. constant recruitment and mortality from age group to age group), the situation at the beginning of one year should be the same as that at the beginning of the preceding year. The stock will have successfully reproduced itself and may be said to be in a state of dynamic equilibrium. It may be thought of as being dynamic in the sense that the numbers in each age group are continuously declining and changing from one age group to the next. At the same time, it can be regarded as being in equilibrium provided that the overall age composition reproduces itself from year to year. In this situation, the age composition is referred to as “a stable age composition”.

4.3 The biological “model”

If the concepts depicted in Figures 3 and 4 are combined, the result is a biological “model” that can be used for inferring the effect of changes in fishing effort, or age of first capture, on a stock.

The first step is shown in Figure 5. It depicts the numbers in each group in the stock. It represents either:

(a) The decline in numbers in a single year class throughout the course of its life,

or, (b) The stable age distribution in the stock at any moment, on the assumption that the stock is in a state of dynamic equilibrium as described above and in Figure 4

Figure 6 introduces the concept of deaths. While, for example, the one-year olds decline until they become two-year olds, fish are dying and being removed. The number of deaths will then be equivalent to the hatched part of the histogram in the figure. The catch from each age group will therefore be generated as a percentage of the total deaths in each age group as shown in Figure 7.

At a certain age, the fish will be large enough to mature and produce eggs. Figure 8 shows that part of the stock is mature and able to produce eggs. These eggs will hatch into larvae and the survivors will eventually become recruits, as shown by the dotted line.

Figure 8 also shows the extension of the simple concepts in Figures 1 to 3, to take account of the fact that a stock is made up of a number of age groups (or year classes). To allow for weight as well as numbers, it is necessary to take account of the fact that, as fish age, they increase in weight due to growth. A given number of old fish will be heavier than the same number of young fish and this factor has to be taken into account when computations are done.

4.4 The effect of exploitation on the size of the spawning stock

Figure 9 shows the effect of different levels of exploitation on the proportion of the stock that may attain maturity. Thus, Figure 9A shows the situation in which the rate of exploitation is relatively very low. As a result, the rate of decline from one age group to the next is relatively small, so that a relatively large proportion of the total stock consists of fish of mature age.

By comparison, Figure 9B shows the situation where the rate of exploitation is relatively high. Here, even if the annual recruitment is assumed to remain unchanged, the rate of decline from one year to the next will be very much higher, so that the proportion of the total stock able to reach maturity will be very much lower than before.

Figure 9 shows how the egg production from a given number of recruits can be expected to decline as the rate of exploitation increases.

It is believed that, in the virgin, unexploited stock, total egg production is greatly in excess of that required to produce the number of recruits that normally survive. As a result, it is generally accepted that, up to a point, exploitation - although causing a decline in the number of spawners - need not necessarily lead to a decline in the annual recruitment to the stock. It is only when the rate of exploitation reaches some critical level that the spawning stock becomes so low that recruitment begins to decline and the stock starts to collapse. Good management aims to prevent the level of exploitation from ever reaching this point.

4.5 The effect on the spawning stock of the age of first capture

The proportion of spawners in the stock is influenced not only by the rate of exploitation but also by the age at which the fish are first subject to exploitation. Figure 10A, for example, shows a typical situation in a stock subjected to a high rate of exploitation from a relatively early age. Numbers decline rapidly from age group to age group and the percentage of spawners in the stock is relatively very low.

Figure 10B shows the situation for the same stock after an increase in mesh size so that the age at first capture is postponed. Up to the new age of first capture, the decline in numbers is relatively slow because it is due to natural causes only. It is only after the age of first exploitation that the decline is as rapid as it was before. Comparison of Figures 10A and 10B shows that, with a higher age of first exploitation, the proportion of spawners for a given number of recruits should be higher than before.

4.6 Mesh regulation as a means of conserving the spawning stock

One of the objectives of good management is to ensure that a spawning stock does not become so small that there is danger of a recruitment failure. Figure 9 shows that one way of doing this is by limiting the amount of exploitation. Figure 10 shows that alternatively it may be done by adjusting the age of first capture. In the case of a demersal trawl fishery, for example, this can be done by regulating the mesh size at an appropriate value.

4.7 Mesh regulation as a means of increasing the yield per recruit

The problem remains of determining the effect on the catch of an increase in mesh size. In this connection, it is important to separate immediate, transitional and long-term effects.

The immediate effect of an increase in mesh size is to lose some fish that would otherwise have been captured. The immediate effect, therefore, is a loss in catch and earnings for all vessels that increase their mesh size.

Some of the fish that escape through the larger meshes will be captured eventually after they have had time to grow older and larger. Not all will be caught in this way, however, since some will die from natural causes. Therefore, out of a given number of recruits, the long-term effect of an increase in mesh size will be a decrease in the number caught. Nevertheless, because those fish that are caught will be larger, there is the possibility that the gain in weight due to growth will be greater than the loss in number due to natural causes. Provided this is so, the increase in mesh size will have resulted in an increased yield in weight in the long term.

Even if the long-term effect of an increase in mesh size is beneficial, the immediate effect will involve a loss. There will therefore be a transition period before the full long-term benefit is experienced. The length of the transition period will depend on the length of time required for the youngest fish in the catch to pass completely through the fishery under the new conditions of mesh size. In general the transition period will depend approximately on the number of years for which an individual is subject to exploitation. In heavily-exploited stocks, the bulk of the catch consists of fish that have been subject to exploitation for only a few years. In these stocks, therefore, much of the long-term benefit, possibly as much as 70–80 percent, might be expected after a transition period of only two to three years. In the case of a lightly-exploited stock, individuals will tend to be exploited for a longer period, and the transition period will therefore be longer also.

The situation is depicted in further detail in Figure 11. This shows histograms of numbers in the sea exactly as shown in Figure 10. In addition, the numbers caught have been included, both as part of each histogram and also repeated separately to the right of each diagram.

Figure 11A shows the catch composition expected from a fishery subject to a relatively high rate of exploitation and a low age of first capture. The catch consists primarily of young fish.

Figure 11B shows the same fishery but with the age of first capture increased. Comparison with Figure 11A shows that some of the very smallest fish would no longer appear in the catch but that more old (and hence more large) fish should be captured.

There are two important differences between the catch compositions shown in Figure 11. The first is that, with the larger mesh, the average size of the fish in the catch would be greater. The second is that, with the larger mesh, fewer fish would be caught. It follows that, as the mesh size is increased, the number caught declines and the average weight increases. Calculations show that it should be advantageous to increase the mesh size, provided that the weight gains due to the growth of the fish are sufficient to offset the numerical losses due to natural causes. Theoretical considerations show that, for any given species subject to any given rate of exploitation, there should be an optimum mesh size at which the potential yield per recruit is at a maximum.

4.8 Relating age or size of first capture to mesh size

Once an optimum age of first capture has been determined, it is necessary to determine the corresponding mesh size. For any one species and fishing gear, this is not particularly difficult and can be determined experimentally and in several stages.

The first step is to convert age of first capture to length at first capture. If the species can be aged, this can be done directly from an age/length curve for the species concerned. If it cannot be aged, the optimum length of first capture should be determined by using one of the methods of handling length composition data directly (Gulland, 1961; Jones, 1981).

A second step necessitates experiments to determine a selection curve or “ogive”. This is simply a curve-relating length of fish to proportion retained, for a given net or gear. From such a curve it is possible to determine the 50 percent length, i.e. the length of fish at which 50 percent are retained and 50 percent released.

Finally, by preparing a number of selection curves, for different mesh sizes, it is possible to arrive at a relationship between mesh size and 50 percent length. It is this relationship that is used for relating mesh size to fish length.

5. REFERENCES

Beverton, R.J.H. and S.J. Holt, 1957 On the dynamics of exploited fish populations. Fish.Invest.Minist. Agric.Fish.Food G.B.(2 Sea Fish.), 19:533 p.

Gulland, J.A., 1961 The estimation of the effect on catches of changes in gear selectivity. J.Cons.CIEM, 26(2):204–14

Jones, R., 1981 The use of length composition data in fish stock assessments (with notes on VPA and cohort analysis). FAO Fish.Circ., (734):60 p. Issued also in French and Spanish