Fisheries science and management is founded upon the Beverton-Holt theory of fish stock demography. The theory uses age as the structuring variable, however, there are several reasons to use body size as the structuring variable: most processes that affect a fish are determined by its body size rather than its age: consumption, mortality, maturation, fecundity, fish gear selectivity, etc.., and measurements of body size are easy, accurate, and abundant. Here I review size-based theory of a fish stock and compare it to classic age-based theory. I show that size- and age-based demography are equivalent representations of demography. However, size-based theory is axiomatic, which leads to a deeper theory with two advantages: predictions need fewer parameters than age-based theory and the theory connects directly to life-history traits. The connection with traits makes size-based theory particularly useful for data-poor application and facilitates evolutionary calculations. I compare age- and size-based theory for fisheries impact and stock assessments, and provide a perspective on the challenges and future of single-stock theory.
- Size spectrum