A simple length-structured model based on life history ratios and incorporating size-dependent selectivity: application to spawning potential ratios for data-poor stocks

Selectivity in fish is often size-dependent, which results in differential fishing mortality rates across fish of the same age; an effect known as “Lee’s Phenomenon”. We extend previous work on using length composition to estimate the spawning potential ratio (SPR) for data-limited stocks by developing a computationally efficient length-structured per-recruit model that splits the population into a number of sub-cohorts, or growth-type-groups, to account for size-dependent fishing mortality rates. Two simple recursive equations, using the life-history ratio of the natural mortality rate to the von Bertalanffy growth parameter (M/K), were developed to generate length composition data, reducing the complexity of the previous approach. Using simulated and empirical data we demonstrate that ignoring Lee’s Phenomenon results in over-estimates of fishing mortality and negatively biased estimates of SPR. We also explored the behavior of the model under various scenarios, including alternative life-history strategies, and the presence of size-dependent natural mortality. The model developed in this paper may be a useful tool to estimate the spawning potential ratio for data-limited stock where it is not possible to apply more conventional methods.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author