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.

Red snapper management in the Gulf of Mexico: science- or faith-based?

“…Once past the life stage where G = M, a cohort or year class will increase in biomass (G [ M) to a point later in life where G = M again, followed by another period where G \ M when cohort biomass again declines as old members of the cohort grow slowly and die. Red snapper most certainly follow this same pattern and year class success is likely determined for red snapper by subtle changes in M during the first year of life before they recruit to artificial reefs (Strelcheck et al 2005;Wells et al 2008b;Gazey et al 2008), which is entirely consistent with what is known about other fishes with similar life-history strategies (Houde 1987(Houde , 1989a(Houde , b, 1996(Houde , 2008.…”

“…Once small juveniles settle to the bottom, there appears to be a spike in rates immediately after settlement for many species (see Able et al 2006 for review), followed by rapid decline in rates thereafter. While no estimates of mortality rates for planktonic red snapper exist, recent work on young juveniles show that Ms of age-0 snapper are high (M = 0.98-3.7 year -1 ) during and following settlement on sand and mud substrates, and may increase when the density of red snapper recruits increases (Rooker et al 2004;Szedlmayer 2007;Brooks and Powers 2007;Wells et al 2008b;Gazey et al 2008;Gallaway et al 2009;SEDAR 2009; M = 2.0 year -1 was used for model projections in the 2009 red snapper assessment update). At some point cohort biomass ceases to decline, when increasing biomass via growth is equivalent to biomass lost via mortality (G = M).…”

“…Furthermore, the attraction of a diverse suite of predators (e.g., jacks, mackerels, groupers, sharks, cobia, bluefish, other snappers, etc. ; VERSAR 2009) by deploying artificial reefs into shallower red snapper nursery habitats, where population size is much more likely to be regulated (Wells et al 2008b;Gazey et al 2008), could actually have a negative impact on stock productivity by increasing predation (Hixon and Beets 1993;Hixon and Webster 2002;Walters et al 2008b). Therefore, we infer that red snapper life history is inconsistent with the notion that habitat limitation (Gallaway et al 2009;Shipp and Bortone 2009) is a strong actor in regulating population size in this species.…”

Development of a Kemp's Ridley Sea Turtle Stock Assessment Model

“…Following Gazey et al (2008), a robust version of the negative log-likelihood was used (excluding all constant values), i.e.,…”

The Artificial Habitat as an Accessory for Improving Estimates of Juvenile Reef Fish Abundance in Fishery Management

“…Here, the pattern of early recruitment to structured habitat was consistent over many years (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009), indicating that previous estimates of abundance, movements, mortalities, and habitat value based on fish sampled using trawl surveys are inadequate. This study suggests that previous trawl studies were only comparing the leftover or remaining surplus production of fish that were unable to compete for the structured habitat and would not ordinarily survive due to increased predation and reduced prey resources (Piko and Szedlmayer, 2007;Gazey et al, 2008;Caddy, 2008;Redman and Szedlmayer 2009). …”