Standardization of the catch per swept area (CPUA) for direct stock assessment cruises of nylon shrimp (Heterocarpus reedi) (1998-2006)

Abstract

This work analyzes the operational data of direct stock assessment cruises carried out on nylon shrimp (Heterocarpus reedi) off the central coast of Chile between 1998 and 2006. These data were modeled using the product of the expected CPUA (catch per unit area) for hauls with catch and the probability of catch (that is, catch greater than zero) to estimate the CPUA. A generalized linear model was applied; the model used four effects as factors: year, zone, depth layer, and year-zone interaction. The results showed that the year was the most relevant effect, followed by the year-zone interaction for each model analyzed. The effects of the year of the assessment and the probability of catch showed sustained growth during the study period. The interaction effect showed growth in the shrimp population mainly from Valparaiso southward. Moreover, we found that shrimp abundance was related positively to the probability of catch and inversely to the area of aggregation of the population. Finally, the discrepancy found between the analyzed abundance indexes and the biomass (particularly north of 32°S) due to the geomorphological characteristics of the sea floor and the diverse methodological criteria used in the "swept-area" assessment of this species leads us to we recommend CPUA modeling as the best option for obtaining a relative abundance index comparable over time and space.