Introduction 3 estimates of the sizes of newly recruiting year

8.1.1 Introduction 3 estimates of the sizes of newly recruiting year

classes;

An essential task in fish stock assessment is the

4 some assumptions concerning future overall preparation of short-term forecasts of catch and levels of fishing mortality. biomass. Whenever management is based on Total

Note that the first three ingredients are all Allowable Catches (TACs) and quotas, such calcu- actual estimates of the current or future situation, lations are a necessary part of the provision of whereas the last is just a set of assumptions of what management advice. Indeed, so important had this may possibly occur, rather than an estimate or pre- task become in certain areas, such as the North diction of what will actually occur. These assump- Atlantic, that it had until recently almost over- tions are usually used to generate a range of catch shadowed the other essential ingredients of options, from which managers are expected to an assessment – and regrettably had received choose an appropriate course of action. In doing higher priority than the provision of long-term so it is to be hoped that they will also have some advice, such as that based on yield-per-recruit idea of the long-term strategy which they are at- and biomass-per-recruit analysis, and the stock– tempting to implement, and some guidance on recruitment relationship, which is discussed later the probable consequences, in both the short- and in this chapter in Section 8.2.

longer-term, of the options presented. In some

A number of methods are used for the prepara- parts of the world the options are more constrained tion of short-term catch forecasts, depending on by management policy and strategic decisions the data available, and the type of assessment that which have already been taken, so that it may be has been carried out. The ‘classical’ dynamic pool necessary to carry out calculations for only one method, which is based on estimates of population option, for example, that corresponding to the

and fishing mortality for each age group, is the sub- F 0.1 level of fishing mortality. The nature of the ject of this chapter. Methods based on simpler mul- calculation is, however, the same. tiplicative models and on length compositions are

The distinction between ingredients 2 and 4 is also possible, but are not treated here (see Pitcher, not very precise, because the future exploitation Chapter 9, this volume). Methods based on stock- pattern may sometimes be expected to differ from production models are discussed by Schnute and that of the recent past, if one is about to implement Richards (Chapter 6, this volume).

a mesh change, for example. We have separated

165 them in order to stress that the exploitation pat-

Dynamic Pool Models II: Forecasting

The summation extends over all ages, from that tern can be estimated from recent data, and often of first recruitment to the fishery, up to and includ- can reasonably be assumed to remain the same for ing that of the greatest age (g), which is usually a the immediate future. The future level of overall plus group. This means that it includes fish of age

fishing mortality is, however, not only uncertain, g and older simply so that no fish are ‘lost’ from and not necessarily similar to recent levels – it is, the calculation. A modified form of equation (8.2) moreover, just this which man-agement action is thus required to take account of the plus group. normally seeks to control. At the stage of preparing This is just catch option calculations, it must therefore be treated as unknown, and can only be fixed by as-

Py ( + 1 , g )=( Pyg , exp ) [ -( Zyg , ) ] sumption, not from the data.

+ Pyg ( , - 1 ) exp [ - Zyg ( , - 1 ) ] . (8.4)