Previous Page Table of Contents Next Page


5.1 How To Determine Levels of Sustainable Fishing Using Fish Population Models

In studying trends of exploited fish stocks in stable and unstable ecological systems two types of modelling should be considered:

  1. One type deals with natural history of a fish species, its growth, feeding, reproduction, genetics, physiology and behaviour - as these relate to migration, inter-and intra-specific competition, recruitment and fish yields.

  2. The other type is concerned with the dynamics of exploited populations and has its foundation in watching depletion of many fish stocks as fish harvesting methods become more intense and efficient. This aspect of fishery research has developed a cohesive body of observation and theory. The modern theory of fishing has resulted in a rather narrow class of mathematical models intended to describe and predict the response of a single species to various rates of exploitation. It should be noted that the great Lakes are more characterized by multiple species and single species situation only feature in small unstable lakes with harsh environment.

A relationship exists between fish population models and ecological models applicable to the evaluation of stocks management of fisheries. Hence, resource managers can use relationships to manipulate ecosystem for the long-range nutritional benefit of mankind and conservation of stocks, providing the food-web links are not disrupted. Most model taxonomies consider details of mathematical structure or assumptions employed versus non-linear. Attention should be focussed on abstract, qualitative features of fishery models, asking whether they fit the scheme of maximizing realism, generality and precision. The major ecological issue is whether we have an equilibrium centred view or are we concerned with the transient behaviour of lake systems that are away from equilibrium state and for which the probability of extinction of e.g. haplochromines, Schilbe and Labeo (in Lake Victoria). Oreochromis macrochir (Lake Mweru), Citharinus or Mpoi (Lake Mobutu Sese Seko) and the indigenous tilapia of Lake Baringo (O. niloticus baringoensis). Two basic approaches have evolved to modelling the dynamic response of a fish population to harvesting regimes. They are not conceptual alternatives, but rather different ways of solving the same general model; they result from different types of data and simplifying assumptions. The general model may be constructed by expressing the relative rate of change in biomass of an exploited population as the sum of input rates (growth and recruitment) and loss rates (mortality) to the biomass.

The first approach focusses attention on the steady state, in which the fish biomass is not changing, the catch can be expressed in terms of recruitment, growth and mortality, especially fishing mortality which is somewhat under the direct control of effort. This “analytical” approach which was first developed by Baranov (1918) is frequently referred to as the Beverton-Holt Model (1957).

The other approach, which was more completely developed, and first applied to fishery management problems by Schaefer (1954), combines the elemental rates of recruitment, growth and natural mortality, and expresses the rate of population increase as a single function of population size. This could be a very comforting approach to resources managers around Lake Victoria who appear to record ever-increasing catches of Nile perch notwithstanding a reported somewhat stable fishing effort. If the catch and effort data collecting system is adequate and reliable around Lake Victoria and if there is no double-counting because of porous borders, then scientists with experience in the region should start testing the validity of theories on primary production, food-web links and carrying capacity.

5.2 The Role of Ecological Models

Ecological models have been used primarily to describe distribution of species assemblages in space and time. Attempts have also been made to describe mathematical diversity in biological communities and the spatial theory of biogeography as described by MacArthur and Wilson (1967).

Fishery biology has traditionally been more ecosystem oriented than most other areas of renewable resources management, perhaps because the emphasis in commercial fisheries even in the Great Lakes is on biomass, and hence on the actual fish production process. Ecological theory has always played an important role in the development of fishery biology, but ecological models could also provide valuable input to ecosystem theory, which safeguards against manipulating the aquatic systems.

Fish landings alone are not very valuable data, but when a fishery is intensive and it can be assumed that fishing contributes a large proportion of total mortality (Z), then catches give reasonable notion of productivity of a lake. If we can couple landings with effort data, age or size composition, then we can use models to estimate fishing mortality and natural mortality; and if values of changing primary productivity and corresponding fish yields are known, then we can build a food web based on the main constituent historical species groups fished. This might be very difficult in the case of Lake Victoria where ecological stress from the Nile perch is to be enhanced by the invasion of black bass (Micropterus salmoinedes).

5.3 The Problems of Overfishing

Some exploited fish stocks of the African lakes show signs of overfishing because of increased fishing effort by the artisanal fishermen. There may be both biological and economic factors affecting overfishing.

Biological overfishing is associated with decreasing growth in size and recruitment success but other factors have also to be considered. For example, the total catch of a certain species may drop as a result of changes in target species in response to new market conditions and fishing methods. There may also be seasonal and/or non-density dependent factors affecting the behaviour of stocks, namely: changes in feeding grounds, migration patterns which prevent fish from encountering gears.

Economic overfishing occurs essentially when redundant inputs are used, leading to depletion of any rents (i.e., total revenues minus total costs) which could be produced.

Economic overfishing does not necessarily imply biological overfishing. However under open-access conditions, a fishery will experience both biological and economic overfishing for as catches decline, the price of fish rises and the cost per unit of catch increases.

5.4 Causes of Overfishing

There are four main factors leading to overfishing in the African Great Lakes, viz.

Profit maximizing behaviour need not be the only driving force. In traditional or subsistence fisheries where no formal markets for fish exist, excessive effort is usually a consequence of large population size and lack of alternative sources of food and employment.

Under free and open access (or absence of ownership), severe economic and biological overfishing is likely to take place whenever cost-price ratios are very favourable (i.e., sizable rent is extracted) and usually this situation is not self-correcting. It needs control and enforcement measures which are not yet effectively implemented in East/Central/Southern Africa.

5.5 Problems of Exploited Fish Stocks and Effects of Overfishing

Excessive effort may have several important economic consequences of which the most evident is the misallocation of resources generated by the use of excessive and unnecessary effort to harvest a given amount of fish. Other important consequences are:

Rational development and management of exploited fish stocks should be based on reliable biostatistical data, monitoring of exploited stocks and fisheries research.

5.6 Rational Use of Fishery Biostatistical Data on Lakes of Eastern/Central/Southern Africa

Promotion and improvement of fishery biostatistical systems for the lakes in Africa would require the following exercises:

  1. dissemination of information on the status of exploited stocks and the rate of exploitation of various fisheries, particularly for shared lakes;

  2. feasibility studies, description of existing fisheries and harmonization of data collection approaches;

  3. study of fishing gear and provision of advise on suitable types and mesh sizes for various African lakes;

  4. provision of advice on development and unified management of shared fishery resources such as for Lakes Mobutu Sese Seko, Malawi and Turkana.

In addition to obtaining information from the fisheries research institutes and statistics units of Fisheries Departments, we can obtain data on the state of fisheries from the following sources:

5.7 Strengthening Fisheries Research in Eastern/Central/Southern Africa and Manpower Training for the Region

Rational development of fishery research in Eastern/Central/Southern Africa could be achieved in the following ways:

  1. assess the availability of data and their suitability for stock assessment purposes and feasibility studies; determine the reliability of data and the main weaknesses;

  2. assess the need for fishery research institutes and equipment for limnological and biological studies;

  3. assess the need for fishery research vessels in the region, particularly larger Lakes Tanganyika, Malawi, Victoria;

  4. assist in setting up and supervising data collection systems for catch and effort, including data processing;

  5. set up a reference collection of fish species and seek advice on the validity of species from taxonomists; and assist in preparing field guides for determination of exploited commercial species;

  6. assess the need for expertise and training in fisheries biology and related sciences like statistics, biomathematics and limnology;

  7. teach nationals various aspects of fisheries science and extension services;

  8. support the dissemination of relevant scientific papers to scientists and policy makers in the region: and

  9. improve fisheries libraries and fishery information systems, particularly for shared lakes.

Previous Page Top of Page Next Page