A review of length-based approaches to assessing fish stocks


J.A. Gulland
A.A. Rosenberg
Renewable Resources Assessment Group
Imperial College
8 Princes Gardens
London SW7 1NA, UK

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ISBN 92-5-103121-5

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This document was prepared as part of the Regular Programme activities of the FAO Fisheries Department in an attempt to present in a single document a comprehensive overview of the several approaches on the use of fish length-frequency data for stock assessment purposes which have been emerging in fisheries literature. The use of length-based stock assessment methods is spreading rapidly, particularly with the extended use and the increased availability of personal computers and user friendly computer programs which simplify computation procedures and the handling of data. This document aims at providing a useful complement to the several manuals, documents and software available, and some general guidance on how to go about using and applying the different methods available. As a general review, this document cannot cover all the different aspects, potentialities, constraints, and limitations of the methods and approaches discussed here, and readers are therefore encouraged to consult the abundant literature listed at the end of the document.

* Present address: National Marine Fisheries Service,
Woods Hole, MA, USA

Gulland, J.A.; Rosenberg, A.A.*
A review of length-based approaches to assessing fish stocks.
FAO Fisheries Technical Paper. No. 323. Rome, FAO. 1992. 100p.
This document reviews the several fish stock assessment methods based on length-frequency analysis. Emphasis is placed on sampling and collection of length-frequency data, to the estimation of population parameters such as growth, mortality and recruitment and the estimation of catch selectivity. Attention is given to the conversion of length to age using age-length keys, slicing length-frequency composition, and using modes. It reviews long-and short-term effects of changes in selection and fishing mortality, and discusses methods for long-term assessments and for short-term projections. The manual gives several examples of the most common methods used and stresses the advantages of using personal computers and the most recent software for data processing and analysis.


FAO Fisheries Department
FAO Regional Fisheries Officers
Directors of Fisheries
Marine Science (general)

After an outstanding career, John Alan Gulland, FRS, died in Cambridge, England, on 24 June 1990. One of the most prominent fisheries scientists of the last 50 years, he leaves behind an immense production, a legacy of dedicated and inspiring work.

The value and extent of his gift to fisheries are evidenced by more than 150 publications. And, never to be forgotten, for those who knew him personally, are the recollections of his lively and original interventions at the many formal and less formal technical meetings he attended, occasions on which he was frequently called upon to act as rapporteur thanks to his particular ability for summarizing and expressing in clear and simple words his own and others' comments and ideas.

John Gulland was born in Andover, England, on 26 September 1926. He started his work in fisheries science shortly after graduating in mathematics and statistics at the University of Cambridge in 1951, when he was recruited by the Fisheries Laboratory, Lowestoft. At the Fisheries Laboratory he joined the fish populations dynamics team, and soon began to make important contributions to the development of quantitative methods in fisheries research, first in Lowestoft, and then internationally. As a member of the British delegation, John Gulland started to participate and to take an influential part in fish stock assessment groups and specialized meetings of international commissions, such as the International Council for the Exploration of the Sea (ICES) and the International Council for the Northwest Atlantic Fisheries (ICNAF).

In 1966 he joined what is now the Fisheries Department of FAO, and expanded his area of interest to include fishery resources and fisheries all over the world. While in FAO he played a key role in the evaluation and assessment of world fisheries, and in the development and diffusion of quantitative methods for the assessment and management of fisheries, particularly those applicable in developing countries. As Chief of the Marine Resources Service, John Gulland contributed to building up and maintaining a well recognized capability of assisting both developed and developing countries in the assessment and management of their fishery resources. He was fast in identifying and understanding new problems, but reluctant to suggest sophisticated methods. As one of his senior staff members, FAO benefited from his sharp competence and critical spirit until 1984, when he took early retirement and settled in Cambridge. He then joined the newly-created Renewable Resources Assessment Group (RRAG) at Imperial College, London.

At Imperial College John Gulland continued to be active as ever in the field of international fisheries research. Although retired from FAO, he maintained a close interest in FAO's work and still collaborated with the Organization in its efforts to support and strengthen the research, assessment and management of world fisheries.

This document is a fine example of this continued collaboration, a very valuable one, and regretfully the last. The ultimate efforts of the professional career of John Gulland were devoted to finalizing this contribution, and he succeeded. Once again, he left no ends untied.

Hyperlinks to non-FAO Internet sites do not imply any official endorsement of or responsibility for the opinions, ideas, data or products presented at these locations, or guarantee the validity of the information provided. The sole purpose of links to non-FAO sites is to indicate further information available on related topics.




2.1 Background
2.2 The mechanics of length sampling
2.3 Operational considerations
2.4 Processing of data

2.4.1 Market Categories

2.5 Statistical design

2.5.1 How much sampling?
2.5.2 How should samples be distributed?


3.1 Estimation of growth

3.1.1 General considerations
3.1.2 Identification of modes
3.1.3 Fitting growth-curves to modes
3.1.4 Other methods of fitting growth-curves

3.2 Estimation of mortality

3.2.1 General considerations
3.2.2 Use of mean length
3.2.3 Length-converted catch-curves
3.2.4 Cohort analysis and Virtual PopulationAnalysis
3.2.5 Estimation of growth and mortalitysimultaneously

3.3 Selection and recruitment
3.4 Conversion of length to age

3.4.1 Age-length keys
3.4.2 Slicing length-compositions
3.4.3 Use of modes


4.1 General considerations
4.2 Long-term assessments
4.3 Changes in selection
4.4 Changes in fishing mortality
4.5 Short-term projections
4.6 Other uses of length-data


5.1 Sensitivity and confidence limits
5.2 What method to use
5.3 Area differences; migration, stock separation, etc.