When Lengths Are Better Than Ages: The Complex Case of Bocaccio

When Lengths Are Better Than Ages: The Complex Case of Bocaccio

S. Ralston and J.N. Ianelli

When Lengths Are Better Than Ages: The Complex Case of BocaccioThis is part of Fishery Stock Assessment Models
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Bocaccio (Sebastes paucispinis) has historically been the most important rockfish harvested in the California groundfish fishery. A stock assessment of bocaccio in the Eureka-Monterey-Conception area indicated that in 1996 spawning biomass was 5-10% of that present in 1970. This finding was based on the application of the Stock Synthesis model to a split-sex population, assuming length-dependent gear selectivities for four distinct fisheries. A variety of fishery-dependent and fishery-independent data sources were used to model population biomass, including (1) landings from the trawl, setnet, hook-and-line, and recreational fisheries, (2) trawl catch-at-age data for the period 1980-1985 using surface otolith ages, (3) trawl catch-at-age data for 1988, 1991, and 1994 using break-and-burn otolith ages, (4) a probability transition matrix for conversion of age types, (5) length composition data from each fishery over the period 1980-1994, (6) an effort index in the recreational fishery, (7) triennial shelf trawl survey CPUE and length-frequency data, (8) a spawning biomass index derived from larval abundance in CalCOFI surveys, and (9) an index of year-class strength from a midwater trawl survey of young-of-the-year pelagic juvenile abundance. An evaluation of these diverse sets of information indicated that the age composition data were in fundamental disagreement with all other data sources. This discrepancy was apparently due to bias and imprecision in bocaccio ages, which resulted in uninformative age composition data that were incapable of resolving a highly variable pattern of recruitment to the fishery. For this purpose, length composition data were much more useful, especially including those from the trawl and recreational fisheries.

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