Age Composition, Growth, and Density-Dependent Mortality in Juvenile Red Snapper Estimated from Observer Data from the Gulf of Mexico Penaeid Shrimp Fishery

Abstract

Beginning in May 1998, the National Marine Fisheries Service has required that bycatch reduction devices be installed in penaeid shrimp trawl gear in the Gulf of Mexico. Changes in observer protocols were introduced, one of which was that all of the red snapper Lutjanus campechanus collected would be enumerated and measured when possible. This change has yielded catch and length information from the Gulf of Mexico shrimp fishery. To date, however, an integrated approach to determining the age composition, growth, and mortality of juvenile red snapper has not been attempted. To address this problem, we constructed a length-based, age-structured model to objectively estimate the growth and mortality parameters and age composition of the shrimp trawl bycatch of red snapper in the western Gulf of Mexico from 81 monthly length frequency data sets (a total of 239,521 fish were measured) from July 1999 to February 2007. Our modeling strategy was to make simple and straightforward representations of the deviations in growth from the mean trend, partial recruitment, density-dependent mortality, and population dynamics. Bayesian parameter estimation was accomplished by calculating the mode of the posterior distribution. Important findings of this paper include (1) the shrimp trawl bycatch during the first and third trimesters is dominated (86–87%) by age-0 fish, (2) the growth patterns of age-0 and age-1 red snapper suggest that the fish form an opaque annulus in winter months, (3) the natural mortality of age-0 fish appears to be about double (a minimum 70% increase) the value used in the last stock assessment, and (4) the evidence for density-dependent juvenile mortality is overwhelming. Inclusion of these findings in the red snapper stock assessment has the potential to substantially alter management practices.