712 



Fishery Bulletin 90(4|, 1992 



and mortality rates to environ- 

 mental parameters and food 

 availability. 



Materials and methods 



Sampling procedure 



Atlantic bumper larvae were col- 

 lected during five cruises off the 

 Louisiana-Mississippi barrier 

 islands in the Gulf of Mexico 

 (Chandeleur Is., Ship I., and 

 Horn I.; 29°50'-30°15'N and 

 88°40'-89°00'W; Fig. 1). Three 

 cruises were completed in 1986 

 (5-7 Aug., 8-9 Sept., and 22-24 

 Sept.) and two in 1987 (8-10 and 

 24-26 Sept.). Adverse weather 

 conditions canceled the sched- 

 uled August 1987 cruise. 



The sampling design consisted 

 of a 4x4 grid of stations (N 16) 

 randomly sampled on two con- 

 secutive nights, and a 3 x 3 grid 

 of stations (A'^ 9) randomly sam- 

 pled during daylight, starting 12 

 hours after the initiation of the 

 first nocturnal sampling. The 

 sampling grid had a fi.xed com- 

 pass orientation with respect to 

 three windowshade, subsurface 

 current drogues (five drogues were used in 1987) which 

 were released at the beginning of each cruise (Shaw 

 et al. 1988). The change to five drogues in 1987 allowed 

 for a more defined sampling grid. Surface-water 

 temperature and salinity, as well as water depth, were 

 recorded for each ichthyoplankton tow. 



Three-minute surface tows were taken at ~1.0m/s 

 using a 60cm "bongo-type" plankton sampler fitted 

 with a flowmeter (General Oceanics model 2030). In 

 1986, samples were collected using a 202 jjm mesh net, 

 while in 1987 a 333^m mesh net was used. During the 

 two cruises in September 1987, the bongo sampler was 

 fitted with one 202 ^.(m mesh net and one 333 /urn mesh 

 net for comparisons of daytime collections. Atlantic 

 bumper collected using the two mesh sizes were placed 

 into 1 mm size-classes and tested for differences using 

 a Median test (a 0.05; SAS Inst. 1985). Ichthyoplank- 

 ton samples used for age determination were preserved 

 with 95% ethanol, stored in ice water, and later trans- 

 ferred to 70% ethanol in the lab. 



Live larval and juvenile Atlantic bumper were col- 

 lected for an age-validation experiment and length- 

 weight measurement analysis by dipnetting the jelly- 



V. .""'-"S^. ;i<. I. 



Do,,pV!fiJjB^<Si 



Dou^JJ^^ 



STUDY AREA 



2^0- 



Figure I 



Location and dates of ichthyoplankton cruises during 1986-87 in which Atlantic bumper 

 Chloroscombrus chrysurus were collected for age determination. Shaded areas repre- 

 sent trajectory of the water mass followed by current drogues during 1986 cruises, and 

 diagonally-lined areas represent 1987 cruises. 



fish Aurelia aurita with which the fish are often 

 associated (Reid 1954, Franks 1970). Fish were then 

 transferred to a cooler containing lOOppm calcein 

 (2,4-bis-[N,N'-di(carbomethyl)aminomethyl]fluorescein) 

 in 13 L of aerated ambient seawater to create a fluores- 

 cent mark in their otoliths using the method described 

 by Wilson et al. (1987). Fish were held between 6 and 

 12 h in the seawater-calcein solution and then trans- 

 ferred into a 127 L aquaria. Fish were held under a 

 12h/12h photoperiod in 23°C and 25ppt water and fed 

 ad lihidium on brine shrimp. Fish were sacrificed 2, 

 7, and 10 days after marking. 



Lab analysis 



Ichthyoplankton samples from the bongo net collections 

 were split once with a Folsom plankton splitter (Van 

 Guelpin et al. 1982). Chloroscombrus chrysurus larvae 

 were sorted, counted, and measured to the nearest 

 0.1mm standard length (SL). Preflexion larvae were 

 measured to the end of their notochord, otherwise 

 larvae were measured to the posterior tip of the 

 hypural plate. When more than 52 fish were present, 



