8.1 The need for stock assessment
8.2 The concepts of sustainable yield and potential yield
8.3 The unit stock
The previous section deals with problems of estimating the important parameters (growth, mortality, etc.) of fish populations which are of scientific interest, but not of direct interest to others concerned with fish stocks (members of the industry, administrators, etc.). It is, for instance, of no direct relevance to the practical problems of the industry that the present fishing mortality coefficient has a value of, e.g. 0.50. This section of the manual is concerned with the use of these estimates of the parameters to determine the total catch, and how the catch and the catch per unit effort (catch per boat, or catch of the individual fisherman) varies with changes in the pattern of fishing. These results are of direct practical interest - for instance, analysis showing that any increase in fishing above the present level (giving a fishing mortality of 0.50) would give no sustained increase in total catch is of vital importance for plans of possible expansion of fishing.
The problems of stock assessment occur at all stages of the development of a fishery, with the need for accuracy and precision increasing as the fishery develops. In an undeveloped fishery all that is generally required is a rough measure, say to within a factor of two or three, of the magnitude of the resource and the potential yield from it and also a measure of the decrease in catch per unit effort likely to be caused by future increases in fishing effort. These estimates may be used as bases for determining the possibilities of expansion. Later, better estimates will be required to determine when the fishery is approaching the level beyond which further expansion of fishing will give little increase in total catch, so that further expansion can be discouraged, or some form of regulation considered. Finally, in a very intense fishery, very accurate estimates may be necessary to ensure proper regulation, e.g. in setting precise catch quotas, etc.
At each stage, the scientist may be called upon to come to conclusions and give advice on the basis of data which might be considered inadequate for scientific conclusions. However, it is often essential that advice should be given, with explicit statements of its reliability where necessary, especially when a fishery is developing rapidly. For instance, in such a fishery a further expansion of say 30 percent may be actively considered. Scientific advice is required on whether the stock is fully exploited, so that such an increase would give no increase in yield, or, at the other extreme, is very lightly exploited, so that the 30 percent increase in fishing would produce very nearly the full 30 percent increase in total catch, or somewhere between. In the absence of advice, the development might take place in the expectation that the increase in fishing will produce a proportional increase in catch, so that the action taken is effectively the same as if the scientific advice were that the resource was very lightly exploited. If, therefore, there is scientific evidence that the degree of exploitation of the stock is appreciable, this must be stated even if there is no certainty whether the increase in catch from a 30 percent increase in fishing is 5 percent, 10 percent, or 20 percent.
In many discussions of fishery management problems, the terms "sustainable yield" and "maximum sustainable yield" are used. These are particularly, but not exclusively, used in relation to the simple population models, such as those of Schaefer (see section 10.2). These terms can often be useful in explaining the results of population analysis to administration; thus a catch equal to the sustainable yield will, on the assumptions of the simple model, leave the situation as it is. The attainment of the maximum sustainable yield is, at least apparently, a reasonable and definable objective of management, so that if the stock abundance is less than that corresponding to the maximum sustainable yield, it should be allowed to increase, i.e. less than the sustainable yield should be taken. Conversely, if the abundance is above this level, more than the sustainable yield should, on this criterion, be taken. Though a detailed discussion of the objectives of fishery management is outside the scope of this manual, it is very doubtful if the attainment of the maximum sustainable yield from any one stock of fish should be the objective of management except in exceptional circumstances. When there is a value of fishing effort which will in the long run give an average catch greater than the average catch from any other level of effort, i.e. there is a maximum in the yield/effort curve, then over some range of effort near this maximum the curve will be rather flat. Within this range a reduction of fishing effort from that giving the maximum catch will result in a decrease in catch that is much less than proportional to the reduction in effort. For economic reasons it will usually be more desirable to fish at this lower level of effort than at that giving precisely the maximum catch. Even when the objective is to harvest as much food as possible, this is most likely to be done by diverting effort to some other less heavily exploited stock.
The term sustainable yield also can be misleading, as it rather implies a degree of constancy in the situation - other than changes due to fishing - which often do not exist. In a fluctuating fishery, e.g. one with big changes in year-class strength, it may be difficult, if not impossible, to adjust the amount caught to maintain the stock abundance at the end of the year the same as at the beginning, and when possible the catch taken would fluctuate very widely.
A more useful concept is that of the potential yield. This is the greatest average annual yield that can be taken over a period or under average environmental, non-fishery-induced, conditions, with any pattern of fishing. The actual average yield over a period will be generally less than the potential because it is impossible or undesirable (e.g. due to high costs) to employ the pattern of fishing giving precisely the potential yield. In any one year the yield may be greater or less than the potential yield, depending on such factors as year-class strength, etc.
Before population theory can be applied to a particular situation it is necessary to determine to what extent the fish population and the fishery based on it can be treated as a unit system. There is no simple objective definition of what constitutes a unit stock, or a unit fishery, and the definition may be made purely on practical grounds. A group of fish can be treated as a unit stock if the results of assessments and other population studies in which it is treated as a unit stock do not diverge significantly from the real situation. This means, on one hand, that happenings external to the unit stock, e.g. fishing in other areas, do not have a significant effect, and on the other hand, that there are no subgroups within the unit stock with significantly different population characteristics.
One corollary of this definition is that, as the precision of the population studies increases, the choice of unit stock may also have to change. A group of fish may have sufficiently uniform characteristics to be treated as a single stock when these characteristics are known only roughly, but may have to be treated as two or more unit stocks, perhaps with certain mixing rates, each with different characteristics, as these characteristics become measured with greater precision.
A unit stock, as used here, does not necessarily correspond to a biological, or genetic, unit. Thus, if two species in an area have the same growth pattern and mortality rates and are fished on the same grounds by the same gears, then, at least in the early stages of analysis when data are scarce, it may be permissible to treat them as a single unit stock. Conversely, male and female plaice in the North sea differ so much in growth, and in fishing and natural mortality rates, that for detailed analysis they should be treated as different stocks, i.e. yield curves, etc. should be calculated for the male and female populations separately, and the results for the plaice population as a whole obtained by addition.
While most of this course is concerned with the simple situation of a unit stock, or unit fishery, it is worth enumerating here some of the situations which cannot be treated as a unit stock or unit fishery.
(a) Distinct fish stocks which intermingle for some part of the year, and are fished while mixed together. Even if the separation of fish between the stocks at the end of the mixing period is perfect, the fishing mortality of the two stocks has an element in common, and neither can be treated as a unit unless the mixed catch can be separated into the elements from each stock (? Bank and Downs herring in the North sea, ? Southern bight and German bight plaice).(b) Distinct stocks, fished separately, but with a significant amount of interchange between them (e.g. Iceland and Greenland cod).
© A species inhabiting a large area with no obvious dividing features, so that any two groups of fish in adjacent areas mix freely together, but the mixing is slow enough for the fish at opposite ends of the area to be effectively independent (? many coastal species,? North sea whiting).
A determination of what can be treated as a unit stock will be made from a wide range of information, e.g.
(a) Distribution of the species. This will give an upper limit to the extent of the unit stock, while the presence of more or less effective barriers, e.g. a deep water channel for a strictly demersal species, may suggest subdivisions which could behave as unit stocks.(b) Spawning areas. A single compact spawning area suggests that the population is a biological unit at least so far as reproduction is concerned. Two or more distinct spawning areas suggest distinct spawning groups, though these may well mix as in types (a) and (b) above.
(c) Catch and effort data. Common features in records of catch and effort data for adjacent fisheries over a period may indicate that they are acting on the same stock and are parts of the same unit fishery.
(d) Age-composition data. Comparisons of the relative strength of year-classes in different areas may indicate the extent to which the fish in different areas are recruiting from the same source. Mortality rates, especially when combined with effort data, can be compared as in ©.
(e) Tagging. This may not always give reliable results (incomplete returns from some areas, etc.), but under favourable conditions it is a valuable technique, being especially valuable as giving a mixing rate (when mixing takes place) which can be expressed in quantitative terms.
(f) Morphological or physiological characteristics. Genetic differences are good evidence of groups of fish being independent stocks, but the converse is not true. Characters which may be environmentally determined (e.g. vertebral counts) are less satisfactory, since differences can exist among groups of fish which are mixing but not mixing completely.
