MEPS 137:59-70 (1996)  -  doi:10.3354/meps137059

Age-structure analysis provides information on important aspects of the population biology of exploited marine species. The pandalid shrimp Heterocarpus reedi inhabits the benthic environment of the continental shelf off central Chile, and is one the main and oldest Chilean crustacean fisheries. We analysed 10 years of length-frequency data (LFD) from sampling of the commercial catch, covering a period of 27 yr, to examine the age structure, annual average growth, and variance of size-at-age. In the absence of actual age evidence, we classified year classes into age classes using a conceptual model based on 2 assumptions: first, individuals grow as they age, and second, the population has an annual cycle of recruitment. The procedure can be applied to a collection of LFD from several years. Two components of variance of size-at-age are calculated: the within-year-class variance, due to pre-settlement processes, and the among-year-classes variance, due to post-settlement processes. An across-ages coefficient of variation of size-at-age for each variance term is provided as a single quantity useful for interspecific comparisons. The age structure of the exploited fraction of H. reedi is made up of 5 age classes, with age class IV as the most represented. Annual average growth follows a von Bertalanffy growth function with significantly different parameterization for both sexes. Females are smaller and lighter than males at early ages but larger and heavier than males at late ages. The within-year-class variance proved to be a conservative quantity that does not change with age. Total variance of size-at-age for all ages and both sexes is mostly due to pre-settlement processes.

Population biology . Age structure . Growth . Size variance . Shrimp