THE PHYSICS AND BIOLOGY OF VERTICAL MIGRATIONS 85 



THE PHYSICS AND BIOLOGY OF VERTICAL MIGRATIONS 

 After reviewing the evidence for the daily changes in the vertical disposition of bathypelagic fishes, 

 some appreciation of their physical and biological environments will be given. In moving up and 

 down, these fishes are following inner urges, and in so doing are exposing themselves to changing 

 physical and biological conditions. Discussion of these aspects forms the third part of this section 

 and leads to the final one, which centres around the vertical distribution of pelagic and benthic 

 deep-sea fishes with a swimbladder. 



The evidence for vertical migrations 

 Study of the deep-sea fishes taken by the ' Challenger' (1872-6) led Giinther (1887) to regard species 

 with well-developed luminous organs and eyes as nocturnal surface-swimmers, which during the 

 daytime, withdraw to the darkness of the deeper waters. The 'Valdivia' (1898-9) and 'Gauss' 

 (1901-3) took numerous luminous fishes in surface-nets at night, and both Brauer (1906) and Pappen- 

 heim (19 14) concluded that many deep-sea fishes regularly seek the surface-waters during the hours 

 of darkness. The fishes they listed include several species of myctophids, Astronesthes niger, Idia- 

 canthus fasciola, Stomias affirm and certain melanostomiatids. 



Murray and Hjort (19 12) also found that Astronesthes niger and Idiacanthus could be taken at the 

 surface during the night. Moreover, the hauls of the ' Michael Sars ' enabled Hjort to be more precise 

 concerning the vertical migrations of Gonostoma elongatum and Photostomias guerni. During the day 

 both species were only to be found in nets fished at 500 m. and below, but at night were taken between 

 150 and 500 m. Jespersen (191 5) was also able to analyse the diurnal changes in distribution of 

 Argyropelecus hemigymnus in the Mediterranean. Daytime hauls revealed that the populations were 

 centred below 500 m., whereas at night the main concentrations were between about 150 and 500 m. 



The Dana Expeditions fished many nocturnal nets, and scrutiny of the papers concerning the fishes 

 (Regan and Trewavas, 1929, 1930; Ege, 1934, 1948) shows that numerous stomiatoids were taken in 

 the upper 50 m. during the night. However, many species were not caught in these near surface-nets. 

 Considering only the former and the commoner records, the following are evidently nocturnal surface 

 fishes: Astronesthidae ; Astronesthes niger, A. indicus, A. filifer, Melanostomiatidae: Eustomias 

 obscurus, E. brevibarbatus, E. macrurus, Bathophihis metallicus, B. pawneei and Melanostomias 

 spilorhynchus; Stomiatidae, all species, except Stomias nebulosus and S. colubrinits ; Idiacanthidae, 

 Idiacanthus fasciola; Chauliodontidae, Chauliodus danae, Malacosteidae, Photostomias guerni and 

 Aristostomias polydactylus. 



However, by far the commonest fishes in the surface-waters at night are the Myctophidae. Many, 

 but seemingly not all, species can be netted at the surface after sunset. In the eastern tropical Pacific, 

 Beebe and Vander Pyl (1944) studied the migration of various species, particularly Gonichthys coccoi. 

 By day, the schools were centred about 400 m., but just after dark the fishes appeared at the surface, 

 where they swarmed between 7.0 and 10.0 p.m. Thereafter, they appeared to disperse or withdraw 

 from the surface, but were still caught there until 6.30 a.m., when they withdrew to their daytime 

 levels. 



The other species that massed near the surface after dark included Lampanyctus macropterus, 

 L. omostigma, Myctophum affine, M. aureolatematum, M. evermanni, M. laternatum, M. reinhardti, 

 Notolychnus valdiviae. But the catches of certain species, notably Lampanyctus mexicanus, came from 

 deeper-lying waters at all times of day. Other evidence for diurnal movements in Myctophidae may 

 be found in the papers by Taning (1918), Tucker, 1951, Grey (1956) and Barham (1957). 



WOODS 

 HOLP 



Mass' 



