GARTNER ET AL.: FISH ESCAPEMENTS FROM MIDWATER TRAWLS 



the two mesh sizes, e.g., Notolychnus valdiviae, 

 while others, e.g., Lampanyctus alatics, show very 

 different catch rates depending on size group and 

 mesh size (Fig. 2). 



Overall biomass values for myctophids from the 

 two mesh sizes are very similar, 19.61 g/10'* m^ (1.6 

 mm mesh) and 19.22 g/lO* m'' (4.0 mm mesh). This 

 suggests that the larger mesh sizes provide a fairly 

 accurate estimate of overall standing stock of myc- 

 tophids and allow for interregional data comparisons 

 (Maynard et al. 1975; Hopkins and Lancraft 1984). 

 However, estimates of standing stock for certain 

 size classes would be erroneous, especially for juve- 

 niles smaller than 30 mm SL (Fig. 4). 



Net escapement by small size classes can also bias 

 quantitative determinations of relative species abun- 

 dance, spawning period, juvenile recruitment, and 

 trophodynamic impact. Benthosema suborbitale, 

 Ceratoscopelus townsendi and Notolychmis valdiviae 

 are pan-oceanic in tropical-subtropical latitudes and 

 are among the most abundant species throughout 

 their zoogeographic range (Gartner et al. 1987). 

 Based on our calculations, it is likely that B. sub- 

 orbitale and A'', valdiviae are of much greater nu- 

 merical importance than previous data sets have 

 suggested (e.g., Clarke 1973; Backus et al. 1977; 

 Hulley 1981; Gartner et al. 1987). 



Net escapement can also affect assessment of 

 spawning period. Abundance comparisons for the 

 two nets by cruise for D. duvxmlii show that if one 

 were to attempt to determine periods of larval re- 

 cruitment for this species using length frequencies 

 from the 4.0 mm mesh net, there are only sugges- 



tions of a spring-early summer spawning period 

 (May 1986, July 1985), whereas the 1.6 mm net 

 catches clearly indicate an early spring through fall 

 influx of newly metamorphosed juveniles (Fig. 5). 

 Using the small mesh net, the birthdays of juveniles 

 for age and growth studies can more readily be 

 fixed. 



In trophodynamic studies that have considered the 

 impact of midwater fishes on zooplankton prey, con- 

 siderable predation pressure has been shown on cer- 

 tain size classes and taxa of zooplankters (Gjosaeter 

 1973b; Merrett and Roe 1974; Clarke 1978, 1980; 

 Hopkins and Baird 1981, 1985; T. M. Lancraft pers. 

 commun^; Hopkins and Gartner unpub. data). It is 

 well documented that ontogenetic changes in prey 

 taxa and size selection occur among myctophids. Our 

 data suggest that predation pressure would be much 

 higher from small size classes or species of mycto- 

 phids <30 mm SL than could be calculated from 

 studies using larger mesh collection gear. 



CONCLUSIONS 



As Harrisson (1967) remarked, no single midwater 

 net will adequately sample all species or size ranges. 

 At high latitudes and in the lower mesopelagic zone 

 where many midwater fish species may attain sizes 

 >100 mm SL, it has been observed that myctophids 

 readily avoid even large midwater trawls (Pearcy 



'T. M. Lancraft, Department of Marine Science, University of 

 South Florida, 140 Seventh Avenue S.E., St. Petersburg, FL 

 33701, pers. commun. June 1988. 



O 

 O 

 O 



1- nj 



LU 



C/5 UJ 



o 



CD 



1.6mm MESH 

 4.0mm MESH 



SIZE GROUP (mmSL) 



Figure 4.— Overall biomass of myctophids per 10^ m^ water filtered for the 1.6 

 mm mesh and 4.0 mm mesh nets. 



219 



