Age-based and size-based illustrations

Size-based theory for fisheries advice

Monday 16 Nov 20


Ken Haste Andersen
Professor, Head of Section
DTU Aqua
+45 35 88 33 99
Today’s advice for fisheries management uses the venerable “Beverton-Holt” theory of fish demography. The theory describes the abundance of fish of different ages and how fishing distorts this age-distribution. In the “Center for Ocean Life” we have developed a modern alternative theory, which describes the abundance of fish at different sizes instead of different ages. And there are good reasons for doing so.

Most processes affecting a fish is determined by its body size rather than its age: consumption, mortality, maturation, fecundity, fish gear selectivity, etc. Moreover, measurements of body size are easy, accurate, and abundant, while measurements of age are costly and prone to error. In this work we make a systematic comparison of age- and size-based theory, and analyses strengths and weaknesses of either approach. Because size-based theory is axiomatic it is deeper and provides a unified framework for calculating the fisheries reference points, data-poor stock assessments, and fisheries induced evolution. This “Food for Thought” article in ICES Journal of Marine Science provides an introduction to size-based theory for modern fisheries advice.

K H Andersen (2020) Size-based theory for fisheries advice, ICES Journal of Marine Science.

The theory is demonstrated in our web-apps: and
22 APRIL 2021