Stock assessment for venus clam, Chione californiensis (Broderip, 1835) in Ensenada de La Paz, Baja California Sur, Mexico

摘要

The stock assessment of the venus clam Chione californiensis from Ensenada de La Paz, Baja California Sur, Mexico was made based on survey and commercial catch data. Stock is exploited twice a year, February to early April, and September and October. Elefan I was used to estimate growth parameters of the von Bertalanffy equation. Age structure was computed from a length-age key probability matrix to assign age to length. Natural mortality varying with age and time was estimated using an algorithm based on the Leslie transition matrix and data from unexploited periods. Catchability-at-age was also estimated through an analogous algorithm, but using data for exploited periods. Both algorithms have a numerical solution which uses a least square as fitting criterion. A modified age-structured virtual population analysis (VPA), accounting fishing-effort data, was used to estimate fishing mortality and stock size. Higher values for harvest rate (HR) and fishing mortality (F) occurred during the secondfishing period, reaching values of HR = 70 percent. This is particularly important because it coincides with spawning and recruitment seasons such that exploitation is strongly impacting stock size. Our results also offer an explanation about why clam stocks have been depleted or collapsed.