Previous Page Table of Contents Next Page



The importance of fisheries in a country cannot only be measured by the contribution to the GDP, but one must also take into consideration that fisheries resources and products are fundamental components of human feeding and employment.

Another aspect that makes fisheries resources important is the self renewable character. Unlike mineral resources, if the fishery resources or any other biological resources are well managed, their duration is pratically unlimited.

An important conclusion is that the fundamental basis for the conservation and management of fisheries resources stems from the biological characteristics. (This does not mean that social, economic or any other effects are not important for management).

In Portugal, the fisheries contribution to the GDP is less than 1.5 percent. However, with regard to food, the annual consumption value of 60 kg of fish per person, is very high. Only countries like Iceland, Japan and some small insular nations reach a higher value. We still have to consider that of the total amount of protein necessary in our food consumption, 40 percent comes from fisheries. This corresponds to 15 percent of the total amount spent on food by the Portuguese population.

From a social point of view, we estimate that there are, at present, 34 000 fishermen in Portugal. Assuming that each job at sea generates 4 or 5 jobs on land (canning, freezing and fish meal industry, commercialization, administration, research and training, etc.) one can estimate that about 150 000 Portuguese work in the several sectors of fisheries. Consequently, taking a minimum of 3 people per family, it is not unreasonable to say that about half a million Portuguese people depend on fisheries activities for their livelihoods.


Sætersdal (1984) defined a general principle of fisheries management as:

"to obtain the BEST POSSIBLE utilization of the resource for the benefit of the COMMUNITY"

It will be necessary to define, in each particular case, what best, possible and community mean.

In fact, best can be taken as:

Community may also be taken as:


reminds us that we cannot forget the self renewable character of fisheries resources and consequently, that the conservation of the fishery resource must be guaranteed in order to allow the application of the general principle for a long period of time. This statement means that conservation of an ecosystem does not imply that one should attribute the same importance to all its components.


Figure 1.1 shows that the research on fisheries resources covers several sectors of the fishing activity.The assessment models are the main concern of this manual. Among the several works and books on fish stock assessment, the books and/or manuals by Beverton & Holt (1956), Ricker (1958, 1975) and Gulland (1969, 1983) are historical standing references.

Figure 1.1 The several sectors of fishing activity


The following are necessary to assess a fish stock:

The different steps to assess a stock can be summarized as follows:

a) To define the objectives of the assessment according to the development phase of the fisheries and the available information.

b) To promote the collection of information:

c) To analyse the stocks

The knowledge gained about the resource and the available basic data, determine the type of models that should be used and consequently the type of analyses that can be done. As an illustration, let us look at some general situations:

Fishery resource with little information


Analyses using particular methods to estimate biomasses and potential yields.

Fishery resource with data on catches and catch per effort (CPUE) or stock abundance indices during several years


Analyses using production models in order to make projections of yield and catch per effort.

Fishery resource with information collected over several years on:

  • Biological distribution of the catches by species, by length, by ages, etc.
  • Commercial catches
  • Fishing effort or CPUE
  • Research cruises (distribution of the stock by areas, by length, by ages, etc.)


Historical analysis of the stock (VPA)
Long and short-term projections with different conditions (scenarios)


1. The lack of information may prevent certain projections, but allows other types of analyses.

2. The Precautionary Principle forces one to estimate and to project catches and biomasses, even if they are not very precise. This will be discussed later.

Previous Page Top of Page Next Page