AGE STRUCTURE, ANNUAL GROWTH, AND VARIANCE OF SIZE-AT-AGE OF THE SHRIMP HETEROCARPUS-REEDI

Age-structure analysis provides information on important aspects of th e 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 crust acean fisheries. We analysed 10 years of length-frequency data (LFD) f rom sampling of the commercial catch, covering a period of 27 yr, to e xamine the age structure, annual average growth, and variance of size- at-age. In the absence of actual age evidence, we classified year clas ses 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 collec tion of LFD from several years. Two components of variance of size-at- age are calculated: the within-year-class variance, due to pre-settlem ent processes, and the among-year-classes variance, due to post-settle ment processes. An across-ages coefficient of variation oi size-at-age for each variance term is provided as a single quantity useful for in terspecific comparisons. The age structure of the exploited fraction o f 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 fu nction with significantly different parameterization for both sexes. F emales are smaller and lighter than males at early ages but larger and heavier than males at late ages. The within-year-class variance prove d to be a conservative quantity that does not change with age. Total v ariance of size-at-age for all ages and both sexes is mostly due to pr e-settlement processes.