FISHERY BULLETIN: VOL. 85, NO. 1 



are separated among these zones as well (Table 6; 

 Fig. 4d). Diaphus dumerilii (300 m MDO) and Noto- 

 lychnus valdiviae and Benthosema suborbitale (400 

 m MDO) are upper mesopelagic, while Myctophum 

 affine appears to be middle mesopelagic (based also 

 on recent unpublished data of J. V. Gartner). Cerato- 

 scopelus warmingii inhabits lower mesopelagic 

 depths during the day, and while both Lampanyc- 

 tus alatus and Lepidophanes guentheri have upper 

 mesopelagic MDO's, both are most abundant in the 

 lower mesopelagic zone. 



Nighttime patterns show that many species fre- 

 quently range shallower than their zones of max- 

 imum abundance (Table 6) and that all but one 

 species (excluding Taaningichthys minimus and T. 

 paurolychnus) vertically migrate. Species from four 

 genera (Myctophum, 4 spp.; Hygophum, 4 spp.; Cen- 

 trobranchus nigroocellatus; Gonichthys cocco) are 

 regularly found in the surface waters (0-5 m) at 

 night. However, none of the Hygophum species have 

 their highest abundances in this stratum. 



Diaphus species show several groupings at night 

 though most occur below 100 m. The Lampanyctus 

 species are all most abundant below 75 m, although 

 individuals of L. nobilis are taken as shallow as 50 

 m. Taaningichthys bathyphilus, the only Taaning- 

 ichthys species for which we have day and night data 

 characteristically shows no evidence of vertical 

 migration (see also Clarke 1973; Backus et al. 1977). 

 This deep night group also includes nonmigratory 

 individuals of many species. 



Of the seven abundant species, four have night- 

 time MDO's at 75 m which coincide with their zones 

 of maximum abundance (Table 6; Fig. 4d). A fifth 

 species. A'', valdiviae, while also most abundant 

 below 75 m, occurs as shallow as 50 m. Benthosema 

 suborbitale is also captured as shallow as 50 m, but 

 shows no particular zones of abundance at night. 

 Myctophum affine is most abundant at the surface. 



Vertical partitioning and species associations have 

 also been noted by Karnella (1983). Factorial anal- 

 ysis of abundant species collected from the Bermuda 

 "Ocean Acre" project resulted in from five to eight 

 daytime and six to eight nighttime discrete species 

 associations, depending on season. The Ocean Acre 

 location is in a subtropical region (Backus et al. 1977) 

 and 45 of the 63 species reported by Karnella (1983) 

 also occur in the Gulf. Despite differences in species 

 abundances and general faunal structure and the 

 fact that factorial analysis was inapplicable to the 

 present data set, there are similarities in species 

 associations both day and night between our studies. 

 Karnella (1983) also showed generally shallow day- 

 time distributions for Diaphus species, Lobianchia 



gemellarii and Notolychnus valdiviae (the only 

 species which is abundant for both studies), and deep 

 (>700 m) daytime distributions for most Lampanyc- 

 tus species and Ceratoscopelus warmingii. At night, 

 his surface group included Myctophum and Hygo- 

 phum species, Centrobranchus nigroocellatus, and 

 Gonichthys cocco. Middle groups (30 and 50 m MDO) 

 included Benthosema suborbitale, Ceratoscopelus 

 warmingii, and Hygophum species, while most 

 Lampanyctus species were in the deeper (>100 or 

 200 m, depending on season) groups. The abundant 

 species also tended to be divided among several 

 depth groups both day and night. Our findings are 

 also in general agreement with the distribution and 

 abundance ranges reported by other authors for the 

 same species (Clarke 1973; Badcock and Merrett 

 1976; Nafpaktitis et al. 1977). 



Although vertical stacking of species groups is ap- 

 parent, it is also obvious that the nighttime overlap 

 of peak abundances among most of the abundant 

 species is quite pronounced (Fig. 4d). It may be that 

 vertical partitioning is on a much finer scale than 

 the present data can resolve or that temporal par- 

 titioning of the same depth stratum may occur. It 

 does not seem that day-night MDO's are linked to 

 the extent of vertical migration, since some species 

 which live relatively deep by day (e.g., Ceratoscope- 

 lus warmigii) are found at the same or shallower 

 depths than those species inhabiting relatively 

 shallow daytime depths (e.g., Diaphus dumerilii). 



A number of factors including light (e.g., Clarke 

 and Backus 1956, 1964; Paxton 1967; Badcock 1970; 

 Marshall 1980), temperature (Paxton 1967; Robison 

 1972), and feeding migrations (Marshall 1960) have 

 been suggested as control mechanisms limiting the 

 vertical distribution range of myctophids and other 

 mesopelagic animals. The relationships between 

 these factors and myctophid vertical distributions 

 in the eastern Gulf, however, are not readily ap- 

 parent. Clarke (1973) determined that lunar period 

 at night was an important factor in limiting the up- 

 ward extent of vertical migration for many species, 

 finding that the upper depth of migration was de- 

 pressed by 50 to 125 m during a full moon period 

 for a number of myctophid species (although Hygo- 

 phum species apparently did the opposite, migrating 

 shallower during full moon). In the Gulf, only a 

 single species, Lepidophanes guentheri, showed a 

 markedly shallower depth of capture during a new 

 moon period (10 m vs. 75 m during new and full 

 moons, respectively). All other species tended to 

 show the same upper depths of capture regardless 

 of the lunar phase. In fact, individuals of the 10 Gulf 

 species with nighttime surface captures were reg- 



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