Fischer et al.: Demographic structure of Lut/anus campechanus in the northern Gulf of Mexico 



601 



In addition to increased food availability off of the 

 north-central GOM, the amount and condition of pre- 

 ferred habitat may have some effect on the observed 

 differences in growth rates for Texas and those for 

 Louisiana and Alabama. Approximately 95"% of all 

 Louisiana fishes sampled in this study were harvested 

 from waters surrounding nearshore (<50 km) oil and 

 gas platforms. Similarly, about 95% of all Alabama 

 fishes sampled were caught over artificial reef sites. 

 The fact that there was no detectable difference in size 

 at age and overall growth rates between Louisiana and 

 Alabama red snapper therefore is not surprising, given 

 the similarity in the habitats sampled and the proxim- 

 ity of both locations to the Mississippi River discharge 

 plume. Texas was the only area in which samples were 

 routinely obtained from natural hard bottoms (40%), 

 as well as from oil and gas platforms and artificial 

 reefs (60%). Given that more than half of the Texas 

 specimens were captured in the waters immediately 

 surrounding artificial structures (i.e., oil and gas plat- 

 forms), we can assume that habitat type is not be the 

 sole source for the observed differences in growth rates 

 among regions. 



Despite the current acceptance of a unit stock hypoth- 

 esis for GOM red snapper, the species is not, and to our 

 knowledge never has been, uniformly distributed across 

 the northern GOM. The fishery for red snapper began 

 in northwest Florida approximately 20 years before the 

 Civil War (Collins, 1887) and during that time period 

 was centered between Mobile, AL, and Fort Walton, FL 

 (Camber, 1955). One hundred years of landings data 

 indicate that the fishery, and possibly the population, 

 has undergone a major shift from the natural outcrop- 

 pings of the West Florida Shelf to oil and gas platforms 

 of the north-central portion of the GOM (Shirripa and 

 Legault 1 ). Fishery-dependent data indicate that cur- 

 rently there is a center of abundance of red snapper off 

 southwest Louisiana and a second, smaller center off 

 Alabama (Patterson et al., 2001; Goodyear 4 ; Shirripa 

 and Legault 1 ). Patterson et al. (2001) stated that Loui- 

 siana and Alabama accounted for 32.6% and 11.4%, 

 respectively, of the combined recreational and commer- 

 cial GOM landings from 1981 to 1998. This is especially 

 surprising for Alabama, considering that its coastline 

 accounts for only 3% of the GOM coastline from the 

 Texas-Mexico border to the southern tip if Florida (Pat- 

 terson et al., 2001). 



Red snapper have never been reported to be plentiful 

 in Texas waters, despite the availability of suitable hab- 

 itat in the form of natural hard bottom and the cur- 

 rent high concentration of oil and gas platforms. In a 

 historical report on red snapper fishing in the GOM, 

 Camber ( 1995 ) reported that although a few red snap- 

 per were taken from the "Galveston Lumps" or the 

 "Western" fishing grounds off Texas, the fishery never 

 fully developed in this region during the latter part of 

 the nineteenth century. Commercial landings for red 

 snapper from the GOM indicated that Texas accounted 

 for approximately only 18% of the total catch during 

 the time period 1981-95 (Goodyear 6 ). In a recent fish- 



ery-dependent survey of recreational headboat discards 

 and landings in Texas coastal waters, red snapper less 

 than the minimum legal size (15 inches) made up 64% 

 of the catch (Dorf. 2000). In the latter study, Galveston, 

 Port Aransas, and Port Isabel were surveyed to canvas 

 a large portion of the Texas coast. Discard-to-landing 

 ratios were as high as 211:1 in the waters off Galveston 

 and were possibly indicative of the paucity of legal-size 

 red snapper in Texas waters. Of the three sampling 

 locations. Port Aransas had the lowest discard-to-land- 

 ing ratio (5.2:1) and the largest mean fish length and 

 weight (387 mm, 0.9 kg) — length and weight data that 

 are consistent with a 3-yr-old fish from our Texas (Port 

 Aransas) specimens. The majority of Texas fish (63%) 

 were aged at 3 years or less. Age distribution, along 

 with FL and TW distributions, may indicate that red 

 snapper are being harvested from Texas waters just 

 as they reach legal size. Given the vast differences 

 in historical landings data between the northern and 

 southwest GOM, the highly disproportionate discard- 

 to-landing ratio reported for headboats in Texas wa- 

 ters (Dorf, 2000), and the large number of young fish 

 sampled in Texas, it is not inconceivable to speculate 

 that there are fewer red snapper available for harvest 

 in Texas waters. 



Demographic variation in growth rates may indicate 

 the existence of separate management units of red snap- 

 per in the GOM. Our data indicate that the red snapper 

 inhabiting the waters off Texas are reaching smaller 

 maximum sizes at a faster rate, but are consistently 

 smaller (TW) at age than those collected from Louisi- 

 ana and Alabama waters. Whether these differences are 

 environmentally induced or result from genetic diver- 

 gence remains to be determined. The more productive, 

 nutrient-rich waters of the Mississippi River and north- 

 central GOM off Louisiana and Alabama may be more 

 conducive to faster growth than the less fertile waters 

 off Texas. Fishing pressure and its effects on population 

 size may also be leading to the observed differences in 

 growth rates. Fishery-dependent landing data and dis- 

 proportionate discard-to-landing ratios in Texas waters 

 loosely support the concept that fewer red snapper are 

 available for harvest in the southwest GOM. Regardless 

 of the cause, the existence of demonstrable demographic 

 differences argues for the delineation of multiple red 

 snapper management units in the GOM. 



Acknowledgments 



Funding for this research was provided by the U.S. 

 Department of Commerce Marine Fisheries Initiative 

 (MARFIN) program (grant number NA87FF0424). We 



6 Goodyear, C. P. 1996. An update of red snapper harvest 

 in U.S. waters of the Gulf of Mexico. Report MIA-95/96- 

 60, 21 p. Miami Laboratory, Southeast Fisheries Center, 

 National Marine Fisheries Service. 75 Virginia Beach Dr. 

 Miami, FL., 33149-1099. [Not available from NTIS]. 



