Red snapper demographic structure in the northern Gulf of Mexico based on spatial patterns in growth rates and morphometrics

Fishery Bulletin, Oct, 2004 by Andrew J. Fischer, M. Scott Baker, Jr., Charles A. Wilson

Abstract--Red snapper (Lutjanus campechanus) in the United States waters of the Gulf of Mexico (GOM) has been considered a single unit stock since management of the species began in 1991. The validity of this assumption is essential to management decisions because measures of growth can differ for nonmixing populations. We examined growth rates, size-at-age, and length and weight information of red snapper collected from the recreational harvests of Alabama (n=2010), Louisiana (n=1905), and Texas (n=1277) from 1999 to 2001. Ages were obtained from 5035 otolith sections and ranged from one to 45 years. Fork length, total weight, and age-frequency distributions differed significantly among all states; Texas, however, had a much higher proportion of smaller, younger fish. All red snapper showed rapid growth until about age 10 years, after which growth slowed considerably. Von Bertalanffy growth models of both mean fork length and mean total weight-at-age predicted significantly smaller fish at age from Texas, whereas no differences were found between Alabama and Louisiana models. Texas red snapper were also shown to differ significantly from both Alabama and Louisiana red snapper in regressions of mean weight at age. Demographic variation in growth rates may indicate the existence of separate management units of red snapper in the GOM. Our data indicate that the red snapper inhabiting the waters off Texas are reaching smaller maximum sizes at a faster rate and have a consistently smaller total weight at age than those collected from Louisiana and Alabama waters. Whether these differences are environmentally induced or are the result of genetic divergence remains to be determined, but they should be considered for future management regulations.

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Red snapper (Lutjanus campechanus) in the United States waters of the Gulf of Mexico (GOM) are heavily exploited by both recreational and commercial fishermen (Wilson and Nieland, 2001; Shirripa and Legault (1)). Harvest, however, has not proceeded without detrimental affects on the population. Commercial landings have declined substantially from 6048 metric tons (t) in 1964 to 1207 t in 1990; recreational landings exhibited similar declines from 1937 t in 1981 to 481 t in 1990 (NMFS (2)). In 1991, harvest restrictions including reef fish permits, seasonal fishing, fish quotas, creel limits, and minimum size limits were placed upon the red snapper fishermen by the Gulf of Mexico Fishery Management Council (GMFMC (3)) to increase the spawning potential ratio to 20%, which is indicative of recovery. These regulations have also been adopted for state waters in Alabama, Louisiana, and Texas. Despite the management actions, GOM red snapper remain overfished (Goodyear (4); Schirripa and Legault (1)).

An underlying assumption crucial to a fishery management plan is that the fish species being managed is a unit stock (Gulland, 1965). A stock is defined as the part of a fish population that is under consideration as an actual or potential resource (Ricker, 1975). Since management began in 1991, red snapper in the northern GOM have been considered a unit stock. Genetic studies to date have shown that there is little evidence to dispute this assumption (Camper et al., 1993; Gold et al., 1997, Heist and Gold, 2000). On the other hand, tag-recapture studies indicate that red snapper have the capacity to move great distances, making it possible for separate stocks to develop (Patterson et al., 2001).

The validity of an assumption of a single stock of red snapper is essential to management decisions because measures of growth, natural mortality, reproductive capacity, and recruitment can differ among nonmixing populations. Should separate red snapper stocks exist, management plans would have to be enacted for each defined stock in order to follow federal guidelines. Even if a single large red snapper stock exists, management should be sensitive to both the diversity of habitats and user groups within the species area of occurrence. Because red snapper are arguably the most important recreational and commercial offshore fishery from Florida to southern Texas, every effort should be undertaken to develop the most effective and productive management plan.

The objective of this study was to evaluate the stock structure of GOM red snapper based on growth rates and size-at-age information. We hypothesized that red snapper sampled from across the northern GOM would be indistinguishable in their growth rates and size at age--a uniformity indicative of a single unit stock.

Methods and materials

Red snapper were collected from the recreational harvests of 1999, 2000, and 2001 from the northern GOM at Dauphin Island, Alabama, at Port Fourchon, Louisiana, and at Port Aransas, Texas (Fig. 1). A maximum of 75 fish were randomly selected and sampled from the daily catch of each charter boat or head boat while the captains and deck hands cleaned fish. These fish were not selected by size. Larger individuals (>6.8 kg) were opportunistically sampled from spear fishing and hook-and-line fishing tournaments in Alabama and Louisiana. In addition, a number of smaller fish (<406 mm, <457 mm during summer 1999) were randomly sampled during red snapper tagging cruises in Alabama. Morphometric measurements were recorded (fork length [FL] in mm, total weight [TW] in kg, and eviscerated body weight [BW] in kg), sex was determined by macroscopic examination of gonads, and both sagittal otoliths were removed, rinsed, and stored in coin envelopes until processed. Fish weights were not recorded for 1999 Texas samples.