ALCOM Field Document No. 39

THE USE OF MULTI MESH GILLNETS FOR SAMPLING FISH STOCKS IN RESERVOIRS IN SOUTHERN AFRICA Contents

Henk van der Mheen
Aquaculturist
ALCOM

Funding Agencies:

SWEDISH INTERNATIONAL DEVELOPMENT AUTHORITY BELGIAN ADMINISTRATION FOR DEVELOPMENT CO-OPERATION

Executing Agency:

FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
Harare, Zimbabwe, December 1995

Preface

The ALCOM action programme “Utilization of Small Water Bodies” aims at increasing the fish production of reservoirs in southern Africa. In order to identify possibilities for improvement and for monitoring the effects of management strategies the programme used a fish stock sampling method with multi-mesh gillnets.

This document analyses the experiences of the programme with these nets. The objective of this analysis was to determine the suitability of the method to sample fish stocks in reservoirs in southern Africa, to monitor the effects of changes in management, and to come with recommendations to adapt the sampling to the particular needs of the programme.

ALCOM, the Aquaculture for Local Community Development Programme, is a regional aquaculture and fisheries programme of the FAO (Food and Agriculture Organization of the United Nations). Based in Harare, Zimbabwe, it covers the member-countries of SADC: Angola, Botswana, Lesotho, Malawi, Mauritius, Mozambique, Namibia, South Africa, Swaziland, Tanzania, Zambia, and Zimbabwe.

ALCOM's aim is to assist member countries improve the living standards of rural populations through the practice of aquaculture. Towards this end, pilot activities are conducted in selected member-countries to demonstrate new techniques, technologies or methodologies. Successes achieved, ideas derived, and lessons learnt are disseminated for use in the region. ALCOM began its work in 1986 and is funded by Sweden and Belgium, with contributions to pilot projects from member countries.

ALCOM Address:

Abstract

In order improve the management of a fishery it is necessary to determine which species are present and in what quantities. Sampling of the fish stock is thus needed. For monitoring the effects of certain management strategies, it will also be necessary to gather information concerning the fish stocks. Gillnets are often used for sampling fish stocks because they are easy to use, relatively cheap, catch a wide variety of species, and their use can be standardized. Gillnets are however, selective towards the species they catch, the size of fish they catch and the amounts of fish they catch. Also outside factors influence the effectiveness of the nets. ALCOM used multimesh gillnets in 34 reservoirs in 5 countries in southern Africa over a period of 5 years to collect information on the fish stocks of these reservoirs. The analysis of the results of these samplings showed that the gears were useful to collect general information of the fish stocks, but also demonstrated that the catch per unit of effort could not be used as a reliable indication of abundance. The reasons for this were that the fluctuation in the catches were too great, not all species could be sampled with the required precision and, although a wide range of mesh sizes were used, the gears were still size selective. Furthermore it was agrued that the effects on the fish population of different management strategies would not be detected by sampling with gillnets alone. The effects of outside factors, and especially of the water level fluctuation of the reservoirs, would easily confound the effects of the fisheries management on the reservoirs. It was recommended to concentrate the studies on the target species, to determine the effects of the water fluctuatiuon of fish production, and to use the existing fishery in the collection of data.

The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning its legal status.

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Contents

1.   INTRODUCTION

2.   MATERIALS AND METHODS

3.   RESULTS

3.1   Species Composition
3.2   Catch per Unit of Effort

3.2.1   Pooled Catches
3.2.2   Catches per Species
3.2.3   Precision and Accuracy of the Data

3.3   Size Selectivity of the Gear
3.4   Data Transformation

4.   DISCUSSION AND CONCLUSIONS

4.1   Fish Species
4.2   CPUE and Abundance
4.3   Size Selectivity
4.4   Abiotic Factors influencing Catchability
4.5   Further Activities

REFERENCES