FISHERY BULLETIN: VOL. 87, NO. 1 



numerically dominant lanternfish species in the east- 

 ern Gulf of Mexico (Gartner et al. 1987). All nets 

 were fished at 1.5 to 2.5 knots with a total fishing 

 duration for each sample of approximately 1 hour. 

 Sampling usually began about 1 hour after sunset 

 and ended 1 hour before sunrise. The bottom trawl 

 bar was weighted to incline the net mouth about 30° 

 from the vertical at these towdng speeds (determined 

 from observations using scuba). Two trawl con- 

 figurations were used: a 5.3 m^ (effective fishing 

 area) net of 4 mm bar mesh in the body and 1 mm 

 mesh in the funnel, and a 2.6 m' net of 1.6 mm 

 mesh. Both nets had cod ends lined with 505 i^m 

 mesh. 



Trawl depths were recorded by mechanical time- 

 depth recorder (TDR) and monitored with an elec- 

 tronic deck readout linked to a transducer mounted 

 on the trawl frame. The volume of water filtered 

 during each net haul was calculated from flow 

 meters mounted on the trawl frame. 



Myctophids were fixed in 10% (v:v) formalin and 

 preserved in 50% isopropanol. During all cruises ex- 

 cept January and March 1987, a large number of 

 postlarval specimens from the dominant species 

 were removed from the catches for use in life history 

 studies. These were blotted to remove excess mois- 

 ture and measured to the nearest millimeter stan- 

 dard length (mm SL). The remaining myctophids 

 were measured in the lab after preservation. Be- 

 cause of shrinkage of preserved specimens, the 

 lengths of freshly measured individuals were de- 

 creased by 12% (shrinkage factor determined from 

 Gartner, unpub. data; K. J. Sulak pers. commun.^). 

 All myctophids were identified to the lowest possi- 

 ble taxon, with species identifications made using 

 Nafpaktitis et al. (1977). 



The effect of using nets of differing mouth areas 

 was minimized by calculating the abundance of in- 

 dividuals per 10"* m^ for each net over the entire 

 size range, which was then divided into 5 mm SL 

 size classes. Kolmogorov-Smirnov (K-S) two-sample 

 tests (Siege! 1956) were used for overall internet 

 comparisons of capture over the size range by size 

 classes and by net mesh for size groups smaller than 

 30 mm SL and larger than 30 mm SL. The K-S tests 

 were appHed to similar comparisons for each of the 

 ranking myctophid species. Except where noted, the 

 significance level for all tests was P < 0.01. Rank- 

 ing species for all cruises were defined as the most 

 abundant species which combined comprised 75% 



or more of the total number of specimens captured 

 (Gartner et al. 1987). 



To evaluate if escapement was related to body 

 morphology as well as size, measurements of the 

 greatest cross-sectional dimensions were made on 

 a series of preserved specimens of each of the rank- 

 ing species. Measurements were made to the near- 

 est 0.01 mm using dial calipers on a series of ran- 

 domly selected individuals which encompassed the 

 postlarval size range of each species. Assuming that 

 myctophids are elliptical in cross section, areas were 

 calculated for each specimen using the formula nab, 

 where a and b are the radii of the short and long 

 axes of the ellipse. Cross-sectional areas were re- 

 gressed against the square of length and tested for 

 significance (P < 0.01) among species using a Stu- 

 dent's i-test (Sokal and Rohlf 1981). 



RESULTS 



Collection Data 



The 4 mm mesh net was used at 78 stations, from 

 which 7,861 myctophids were collected with a total 

 volume filtered of 1.65 x 10« m^ (Table 1). The 1.6 

 mm mesh net was also used at 78 stations, with 

 totals of 7,494 individuals captured and 8.97 x 10^ 

 m^ filtered (Table 1). The mean ratio of volume 

 filtered for the larger to smaller nets was 1.84:1 

 (range for all cruises was 1.72:1 to 2.07:1). 



Table 1 .—Collection data. 



'K. J. Sulak, Atlantic Reference Centre, Huntsman Marine Lab- 

 oratory, St. Andrews, New Brunswicl<, Canada EOG 2X0, pers. 

 commun. May 1988. 



Abundances by Size Class 



The numbers of individuals collected per 10^ m^ 

 for both nets are shown in Figure 1. Data for fishes 

 larger than 80 mm SL were not included because 

 only 16 specimens were collected. Catch differences 

 were highly significant between the two mesh sizes 

 (P < 0.001). In both nets, the 16 to 20 mm SL size 

 class was most abundant, but the 1.6 mm mesh net 



214 



