GARTNER ET AL.: LANTERNFISHES 



ularly taken at the surface under a full moon with 

 the deck lights turned on. The effect of temperature 

 in the eastern Gulf is also unclear since the night 

 distributions of almost all species extend through 

 the base of the thermocline and many species enter 

 the mixed layer which is generally isothermal (Fig. 

 2). Thus, many species encounter the highest tem- 

 peratures found in Gulf surface waters. 



If, as Marshall (1960) and later researchers sug- 

 gested, nighttime vertical migrations of myctophids 

 and other midwater animals are feeding migrations, 

 a third possible control of depth range would be prey 

 density. In this case, it would be reasonable to 

 assume that zones of maximum potential prey and 

 predator densities would be closely correlated. Anal- 

 ysis of zooplankton catches taken concurrently with 

 our fish trawls show that this is not the case in the 

 eastern Gulf (see Hopkins 1982). Rather, maximum 

 zooplankton biomass of potential forage size organ- 

 isms occurs in the upper 30 m at night in the east- 

 ern Gulf (Hopkins 1982), which is well above the 

 MDO's of all species except surface dwelling Af^cto- 

 phum species, C. nigroocellatus, G. cocco, and occa- 

 sional individuals of Hygophum species and L. 

 guentheri. 



Size Structure 



The trend of increasing body size or advancing 

 ontogenetic stage with increasing depth has been 

 demonstrated among myctophids by many workers 

 (Badcock 1970; Gibbs et al. 1971; Clarke 1973; Bad- 

 cock and Merrett 1976; Willis and Pearcy 1980; 

 Hulley 1981; Robison et al.^). Our data, which are 

 confined to the abundant Gulf species, are in general 

 agreement with these earlier findings for all species 

 at night and for most during the day as well. 



Many myctophid populations have individuals 

 which do not migrate on a daily basis, and these non- 

 migrators are usually small juveniles (Gibbs et al. 

 1971; Clarke 1973; Badcock and Merrett 1976; Willis 

 and Pearcy 1980). Our data also show this in that 

 at least 19 of the 49 Gulf species had individuals cap- 

 tured at or below daytime depths (Table 3). In com- 

 parison with published accounts of identical species, 

 our data on nonmigratory individuals supports the 

 findings of Clarke (1973) for Benthosema suborbitale, 

 Bolinichthys supralateralis, Ceratoscopelus war- 

 mingii, Lampadena luminosa, and Lampanyctus 



nobilis off Hawaii, and of Badcock and Merrett 

 (1976) for B. suborbitale, Diaphus rafinesquii, and 

 Hygophum reinhardtii in the eastern North Atlan- 

 tic. Badcock and Merrett also captured C. war- 

 mingii but did not observe nonmigration, possibly 

 because they took no individuals of the deep non- 

 migratory size range. Both Clarke (1973) and Bad- 

 cock and Merrett (1976) reported that Notolychnus 

 valdiviae had a significant nonmigratory fraction of 

 the population, whereas in the Gulf the entire 

 population apparently migrated. 



Comparison of the size ranges of our abundant 

 species with published sizes of the same species from 

 other tropical-subtropical areas (Clarke 1973; Hulley 

 1981) show distinctly smaller sizes of adult in- 

 dividuals in the Gulf (Table 7). With a few excep- 

 tions, none of the Gulf species approaches maximum 

 recorded sizes. This may have to do with sampling 

 mechanics (e.g., net mouth area, towing speed, and 

 net avoidance); however, the fact that we have made 

 many additional net hauls since 1977 (20 cruises, ca. 

 600 discrete depth and oblique samples from to 

 1,000 m) with a variety of gear and have not sig- 

 nificantly increased the upper size limit of the abun- 

 dant species suggests that this is not the case. A 

 second possibility, which is supported by research 

 on a variety of inshore and offshore species, is that 

 fish species in the Gulf tend to grow faster, with 

 given developmental stages being smaller, and reach 

 maturity at smaller sizes than the same species 

 found outside the Gulf (e.g., Cynoscion nebulosus, 

 Tabb 1961; Micropogonias cromis, White and Chit- 

 tenden 1977; Mycteroperca microlepis, Manooch and 

 Haimovici 1978; Mycteroperca phenax, Godcharles 

 and Bullock 1984; adult Sciaenops ocellatus, Mur- 



Table 7.— Size range comparisons of dominant eastern Gulf myc- 

 tophid species witii the same species from other tropical- 

 subtropical regions. 



^Robison, B. H., T. L. Hopkins, and J. J. Torres. Ecology, phys- 

 iology and nutrient energy dynamics of the Southern Ocean myc- 

 tophid £'/ecfro«a awiardi'ca. Manuscr. in prep. Marine Science 

 Center, University of California at Santa Barbara, Santa Barbara, 

 CA 93106. 



^Based on 1 specimen. 

 2Based on 13 specimens. 



93 



