594 



Fishery Bulletin 102(4) 



32 



30 



28 



26 



24 



22 



96 



94° 



92° 



90 



88° 86° 



84 



82 = 



80 



Figure 1 



Map of the northern Gulf of Mexico showing the three red snapper iLutjanus campecha- 

 nus) sampling locations. 



An underlying assumption crucial to a fishery man- 

 agement 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 north- 

 ern 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 pos- 

 sible 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 nonmix- 

 ing populations. Should separate red snapper stocks 

 exist, management plans would have to be enacted 

 for each defined stock in order to follow federal guide- 

 lines. 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 har- 

 vests of 1999, 2000, and 2001 from the northern GOM 

 at Dauphin Island, Alabama, at Port Fourchon, Louisi- 

 ana, 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 I fork length 

 [FL] in mm, total weight [TW] in kg, and eviscerated 

 body weight |BW| in kg), sex was determined by macro- 

 scopic 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. 



A transverse thin section (containing the core) was 

 taken from the left sagittal otolith of each individual. 

 Sections were made with the Hillquist model 800 thin- 

 sectioning machine equipped with a diamond embedded 

 wafering blade and precision grinder (Cowan et al., 

 1995). When the left otolith was unavailable, the right 

 otolith was sectioned. Examinations of otolith sections 

 were made with a dissecting microscope with transmit- 

 ted light and polarized light filter at 20x to 64x mag- 

 nification Opaque annulus counts were made along the 

 ventral side of the sulcus acousticus from the core to 

 the proximal edge (Wilson and Nieland. 2001). Annulus 

 counts were performed by two independent readers (AJF 



