Comyns et al.: Spatial and temporal variability in growth and mortality of fish larvae in the Gulf of Mexico 



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Figure 1 



Station locations (•) of plankton collections in the northcentral Gulf of Mexico, September 1990 to 1993. 



Pepin (1991) questioned this conclusion because he found 

 no net effect of temperature on postlarval stage-specific 

 mortality rates, although his study was based mainly on 

 interspecific variation in mortality (Francis, 1994). 



The objectives of our study were to determine if growth 

 rates of Atlantic bumper (Chloroscombrus chiysurus, 

 Carangidae) and vermilion snapper (RhombopUtes auro- 

 riibens, Lutjanidae) varied over small spatial scales in the 

 northcentral Gulf of Mexico (GOM); determine the magni- 

 tude and variability of cruise estimates of larval mortal- 

 ity; and determine the potential influence of variability 

 in these vital rates on cohort survivorship in a region 

 where summer water temperatures approach 30°C and 

 larval stage durations are as short as two to three weeks. 

 Vermilion snapper is the most abundant species of snap- 

 per in the northern GOM (Goodyear and Schirripa'), and 

 Atlantic bumper is the most abundant carangid. 



Materials and methods 



Sampling location and shipboard procedures 



Seven, three-day cruises were conducted in inner-shelf 

 waters of east Louisiana, Mississippi, and Alabama during 



' Goodyear, C. P., and M. J. Schirripa. 1991. A biological profile 

 for vermilion snapper with a description of the fishery in the 

 Gulf of Mexico. Unpublished report CRD 87/88-16, 53 p. " South- 

 east Fisheries Science Center, National Marine Fisheries Ser- 

 vice, 75 Virginia Beach Drive, Miami FL 33149. 



September 1990-93 (Fig. 1). Cruise estimates of larval 

 mortality were determined by using data from all cruises 

 during the four-year period. Specimens used for age and 

 growth analyses were collected during 14-16 September 

 1991 when larvae of both vermilion snapper and Atlantic 

 bumper were abundant. 



Larvae were collected with a 1 m x 1.4 m Tucker trawl 

 fitted with a 333-pm mesh nitex net and a mechanical 

 flowmeter. Oblique tows were taken from the surface to 

 within a few meters of the bottom and back to the surface 

 at a speed of approximately two knots (1.0 m/s). Samples 

 were concentrated and stored in 95'7f ethanol. At each 

 sampling location surface, midwater and bottom measure- 

 ments of temperature and salinity were obtained with 

 water-bottle casts. 



Laboratory procedures 



Lengths of larvae were measured to the nearest 0.1 mm 

 by using a stereomicroscope (12x or 25x) fitted with an 

 ocular micrometer and the larvae were sorted into 0.5-mm 

 size classes. Measurements were taken from the tip of 

 the snout to the end of the notochord in preflexion larvae 

 (notochord length), and from the tip of the snout to the 

 end of the urostyle or hypural plate (whichever was more 

 distal) in flexion or postflexion larvae (standard length). 

 Larval shrinkage was not accounted for because between- 

 station and between-cruise comparisons of growth rates 

 were made with larvae that were preserved in the same 

 concentration of ethanol and stored for approximately 

 the same length of time. Shrinkage of ethanol-preserved 



