FAO FISHERIES TECHNICAL PAPER   397

Cover
Manual on estimation of selectivity for gillnet and longline gears in abundance surveys

CONTENTS


by
Holger Hovgård
Danish Institute for Fisheries Research
Charlottenlund, Denmark
Hans Lassen
International Council for the Exploration of the Sea
Copenhagen, Denmark


The designations employed and the presentation of the 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 the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.


ISBN 92-5-104461-9


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Preparation of this document

During many training courses and workshops organized by the FAO/DANIDA Projects “Training in Fish Stock Assessment and Fishery Research Planning” under the symbols GCP/INT/392/DEN and GCP/INT/575/DEN the processing of data from fisheries with highly selective gears such as gillnets and hooks, always posed major problems.

Towards the end of the Project Holger Hovgård was asked to assemble material on this topic that would be useful for scientists who have to deal with these gears. He sought and obtained assistance from his colleague Hans Lassen. The document they produced was edited by Paul Medley.

The present document should be considered as one of the final outputs of the FAO/DANIDA projects.

Siebren C. Venema
Project Manager

Hovgård, H.; Lassen, H.

Manual on estimation of selectivity for gillnet and longline gears in abundance surveys.

FAO Fisheries Technical Paper. No. 397. Rome, FAO. 2000. 84p.

ABSTRACT

This manual defines methods for estimating selectivity of gillnet and longline gears, with particular reference to the use of these gears in abundance surveys. The manual covers how gear design, the basic concepts of size selection and fish morphometric data might be used to model selectivity. Statistical estimation methods are described in detail, and examples given of their application in spreadsheets. The efficient use of these gears in abundance and research surveys is also discussed. Although the manual concentrates on gillnets and longlines, the methods given are general enough to be relevant to any research involving gear selectivity.



Distribution:
DANIDA
NORWAY/NORAD
All FAO Members and Associate Members
All FAO Offices
FAO Fisheries Department
Other interested Nations, International Organizations and NGOs

Foreword

The objective of the present manual is to provide a description of the estimation of gear selectivity and the use of gillnet and longline gears in abundance surveys. The manual is written for the professional fisheries biologist, who works with gillnets or longlines. We have attempted to cover the main theories and concepts but emphasis has been placed on the more practical aspects of choosing appropriate gears and on the evaluation of experimental results.

After a short introduction (Chapter 1) the design of both gear types is briefly described in Chapter 2. The basic concepts of size selection are presented in Chapter 3. The importance of morphometric information as an aid to describe the size selection is discussed in Chapter 4. Estimation methods are presented in Chapter 5. Estimation methods based on rigorous statistical principles may appear complicated to some fisheries biologists. However, methods have been limited only to those that can be applied in spreadsheets (MS Excel), and when necessary, instructions are supplied on how to arrange the calculations. A number of examples have been included to allow the reader to gain a practical feeling for the various calculations. Chapter 6 discusses abundance surveys with emphasis on the problems and restrictions associated with using gillnets and longlines. The importance of various gear parameters is summarised in Chapter 7 with the objective of optimising the gear rigging for research work. We provide a number of references in Chapter 8, which may be useful for the reader who wishes to look into the issues in more detail.

Food and Agriculture Organization of the United Nations
Rome, 2000 © FAO


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CONTENTS

1   INTRODUCTION

2   A SHORT DESCRIPTION OF GILLNET AND LONGLINE GEARS

2.1   The Basic Design of Gillnets

2.2   The Basic Design of Longlines

3   THE SELECTION CURVE

3.1   Baranov's Principle of Geometric Similarity

3.2   Relating Selection to Catch Processes

3.2.1   Gillnets

3.2.2   Longlines

3.3   Shape of Selection Curves

4   COLLECTING AND USING MORPHOMETRIC INFORMATION

Measurements of Girth Information

4.2   Estimating Gillnet Selection from Morphological Measurements

5   ESTIMATING LONGLINE AND GILLNET SELECTION CURVES FROM SELECTION EXPERIMENTS

5.1   Using Length Frequencies of Catches as the Selection Curve

5.2   Direct Selection Estimates: The Population Available to the Gear is Known

5.3   Indirect Selection Estimations: The Population Available to the Gear is Unknown

5.3.1   Methods Based on Catch Relative to Best Catch Methods

5.3.2   Holt's Method

5.3.3   Graphical Methods Using Baranov's Principle of Geometric Similarity

5.4   Computer based statistical methods

5.4.1   Maximum Likelihood Methods for the Estimation of Selection Curves

5.4.2   Regression Framework

5.5   Estimating Fishing Power

5.6   Size Selectivity of Hooks

5.6.1   The Effect of Hook and Bait Size

5.6.2   Estimation of the Size Selectivity of Hooks

6   USING GILLNETS AND LONGLINES IN ABUNDANCE SURVEYS

6.1   Choice of Survey Gears

6.1.1   Gillnets

6.1.2   Longlines

6.2   Absolute Stock Estimates: Relating Catchability to Area Measures

6.3   Relative Stock Measures - Use of CPUE Indices

6.4   Use of CPUE Indices from Longline and Gillnet Surveys

6.4.1   Gillnets

6.4.2   Longline

6.5   Station Allocation in Surveys Using Passive Gears

7   OPTIMISING RESEARCH GEARS

7.1   The Major Gear Parameters of Gillnets

7.1.1   Colour of Netting

7.1.2   Dimensions of Netting Material

7.1.3   Types of Netting Material

7.1.4   Hanging Ratio

7.1.5   Trammel Nets

7.1.6   Design of Net Series to Be Used for Research

7.2   Major Gear Parameters of Longlines

7.2.1   Hooks

7.2.2   Bait

7.2.3   Line Materials

7.2.4   Gangion Attachment

7.2.5   Designing Longlines for Research Use

8   REFERENCES

9   APPENDIX 1: DERIVATION OF THE ESTIMATION EQUATIONS

9.1   Maximum Likelihood Estimation

9.1.1   Poisson Errors

9.1.2   Log-normal Distributed Errors

9.2   Power Transformation and the Regression Framework