Szedlmayer and Conti: Nursery habitat, growths rates, and seasonality of age-0 Lul/anus campechanus 



627 



Figure 1 



Map of station locations in the northeast Gulf of Mexico. 



abundance; and identify multiple cohorts and primary 

 nursery areas for this species. 



Materials and methods 



Age-0 red snapper were collected 0-26 km south of 

 Mobile Bay Alabama. Fish were sampled by trawl 

 (7.62-m head rope, 2.54-cm mesh, 2-mm codend 

 mesh ). Five 10-min tows were taken every two weeks, 

 at five fixed stations, from June 1994 through Janu- 

 ary 1995, and from June 1995 through January 1996 

 (Fig. 1). Sampling periods were sometimes longer 

 owing to poor weather. 



All trawl tows were made with a 6:1 ratio of tow- 

 ing cable length to depth, and tow speeds of 40 to 80 

 m/min. Bottom dissolved oxygen, salinity, and tem- 

 perature were recorded with a Hydrolab Surveyor 

 II. We did not sample in May but were able to plot 

 temperature data measured at 1 m depth, for May 

 through December, from a NOAA buoy moored 70 

 km west of the sites. Sediment samples were collected 

 by SCUBA divers and examined by grain size analy- 

 sis (Holme and Mclntyre, 1971). 



All age-0 red snapper were stored on ice in the field 

 and later frozen in the laboratory Fish were thawed, 

 weighed to the nearest 0.01 g with an Ohaus bal- 

 ance, and measured with a caliper to the nearest 0.1 

 mm standard length (SL). 



Sagittal otoliths were removed from all age-0 red 

 snapper and mounted in epoxy (Spurr, 1969). Left 

 and right sagittal otoliths were assumed to be iden- 

 tical; only one otolith was used to estimate age from 

 a particular fish. Otoliths mounted in epoxy were first 



cut with a Buehler diamond blade saw, then mounted 

 on glass slides with Crystalbond thermoplastic ce- 

 ment, and ground on both sides in the transverse 

 plane to the primordium, on 400 and 600 grit wet- 

 dry sand paper. Type A alumina powder (0.3 |im) and 

 Buehler polishing cloth were used for final otolith 

 preparations (Secor et al., 1991). 



Microincrements of otoliths were counted from the 

 primordium to the margin, along the sulcus ridge, in 

 the transverse plane. We made all counts at 750x 

 magnification, using a Sony GVM video monitor, a 

 Sony CCD video camera, and an Olympus BH2 light 

 microscope. 



Two independent blind counts were made on each 

 otolith. If the two independent counts were within 

 10*^ of each other, their mean value was used to es- 

 timate age. If the two counts differed by >10%, a third 

 blind count was made. The means of the two closest 

 counts were then used to estimate ages. If counts 

 still differed by >10 '^ after three counts, the otolith 

 was rejected. 



Age of each fish was subtracted from date of cap- 

 ture for hatching-date estimations. Hatching-date 

 histograms were plotted after applying a three-day 

 moving average for each year, and local minima were 

 used to separate cohorts (Szedlmayer et al., 1991 ). A 

 significance level of <0.05 was used for all statistical 

 analysis. Catch per unit of effort (CPUE=mean num- 

 ber/10-min tow) was estimated for each sample date 

 and station. Prior to analysis CPUEs were log(.r-i-l) 

 base-10 transformed. Two-way analysis of variance 

 (AN OVA) was used to test for significant differences 

 in mean CPUEs and mean SLs among dates and sta- 

 tions. Waller-Duncan's multiple comparison test was 

 used to show specific differences detected by the 

 ANOVAs, 



Growth rates were estimated by linear regression 

 of SL on collection date, and SL on age from otolith 

 microincrement counts. Analysis of covariance 

 (ANCOVAl, Student's t-test, and Tukey's multiple 

 comparison test were used to compare estimated 

 growth rates (Zar, 1984). 



Results 



Salinity ranged from 30.5 to 35.4 ppt, dissolved oxy- 

 gen from 3.7 to 7.9 ppm in 1994. Temperature in- 

 creased from 22°C in June to a peak of 29°C in Au- 

 gust, then decreased to 17°C by January 1995. The 

 temperature (26°C) in early June 1995 was higher 

 compared with the previous year, stayed near this 

 level through July, increased to 30°C in September, 

 then decreased in the fall (Fig. 2). Temperature data 

 from the NOAA buoy indicated that temperatures 



