THE PHYSICS AND BIOLOGY OF VERTICAL MIGRATIONS 89 



vertical migrations were confined to these individuals that were living by day above the threshold of 

 light. However, there is a distinct possibility that the deeper living fishes may be migrating upwards 

 to the mid-water concentration of plankton, which is centred at levels between 500 and 1000 m. (see 

 p. 87). Considering the plankton maximum at 800 m. in the western North Atlantic, this could 

 mean that the diurnal movements of the fishes living below this level were taking place without the 

 cues provided by submarine sunlight. However, it was in this area and at a depth of 900 m. that 

 Clarke (1958) found a maximum in the frequency of luminescent flashes from deep-sea animals. 

 Furthermore, Kampa and Boden (1957) have found a diurnal rhythm in the mean frequency of 

 flashes from animals in a sound scattering layer in the San Diego trough. The frequency was least at 

 midday, greatest during the twilight migration of the layer and maintained at an intermediate level 

 during the night. If this thythm proves to be common to all diurnally migrating, deep-scattering 

 layers (and these occur down to at least 800 m.), it might well be the cue for the upward migrations of 

 the deeper living fishes. Zenkevitch and Birstein (1956) visualize a ladder of migration extending to 

 very deep levels down which organic matter produced at the surface is conveyed to the greatest depths. 

 Perhaps luminescent light will prove to be as important as sunlight in maintaining the daily move- 

 ments up and down the ladder. Recently, Nichol (1958) has estimated the maximal distance at which 

 the light of various marine animals can be seen in seawater by eyes that can just perceive light of 

 i-6 x io -10 ^ w/cm 2 . In very clear water the distances vary from 6 to 170 m., and luminescent flashes 

 have been detected down to a depth of 3750 m. (Clarke, 1958). 



PHYSICAL PROBLEMS 



Against this background of their physical and biological environment, we may now turn to the 

 physical stresses that face a fish moving up and down in the water column. Clearly, these will depend 

 on the extent of its vertical displacement. In considering the migrations of fishes with swimbladders, 

 they may be divided as before into thermocline crossers, those that pass through the upper thermo- 

 cline into the surface mixed layer, and partial migrators, those that remain below the thermocline. 



Thermocline-crossers 

 Gas secretion 

 While fishes are diving to their daytime levels, the pressure exerted on the swimbladder gases will 

 increase by 1 atmosphere for every 10 m. of the descent. An adult of one of the larger myctophids, 

 e.g. {Myctophum punctatum) weighs about 5 g. and the capacity of its swimbladder will be about 

 0-25 ml. If it moves from 10 to 300 m. and produces no gas during the descent, the volume of the 

 swimbladder will be compressed to 0-017 mL To inflate the sac to the requisite capacity it must 

 produce about 0-23 ml. gas at a pressure of 30 atmospheres or 6-9 ml. at one atmosphere, the pressure 

 of dissolved gases in seawater. Supposing the fish to have an oxygen consumption of 1 ml./hr., the 

 physical problem of secretion seems insuperable. 



Using the energy of compression equation as a basis for their calculations, Kanwisher and Ebling 

 (1957) have considered the physiological effort needed. They take a 10-g. fish with a 0-4 ml. swim- 

 bladder and an oxygen consumption of 0-4 ml./hr. Assuming that one-third of the blood circulation 

 is available for the swimbladder and the efficiency of the secretion process is 25 per cent, they com- 

 pute that 33 hr. secretion would be needed to restore buoyancy after a migration from the surface 

 to a depth of 400 m. But judging from the movements of deep-scattering layers, the time taken for 

 the downward migrations would be not much more than 1 hr. and (presumably) the fish will ascend 

 again in about 12 hr. 



