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its fins, an instinct which is exercised at the least cause of alarm, as the fish is 

 exceedingly shy and timid. All the other flat-fishes have the same habit, but of those 

 observed in our aquaria, namely, the plaice, dab, flounder, and turbot, none remain 

 concealed so persistently as the sole, at least in the daytime. In the night soles 

 behave quite differently, they then emerge from beneath the ground and move actively 

 about in search of food. In the daytime it is extremely difficult to discover how 

 many soles there are in a given tank even by driving them out of the gravel with a 

 stick : but on going to the same tank in the dark with a lighted taper one may count 

 twenty or thirty where only five or six were expected. But they soon disappear if the 

 light is held over the surface of the water. 



The sole then is a nocturnal fish in the aquarium, and therefore doubtless also at the 

 bottom of the sea. This agrees with the belief of the majority of trawlermen that 

 more soles are caught in the trawl by night than by day. I have met one or two 

 fishermen who deny this and assert that they have sometimes taken a good number of 

 soles in a daylight haul and scarcely any in the following night. But of course 

 exceptional cases may well occur, and are probably to be explained by the fact that 

 soles were abundant in the track passed over by the trawl in the daytime, and very 

 scarce in the ground swept at night. Of course some soles are taken in daylight, and 

 it will be easily understood when the mode in which the trawl works is considered, 

 that it depends on the position and behaviour of each individual sole whether it passes 

 over the foot-rope into the net or not. If the foot -rope is heavy and the trawl going 

 at a moderate speed it may disturb the ground deep enough to cause most of the soles 

 in its path to rise from the bottom, in which case they will most likely pass over the 

 foot-rope and be swept into the net : if on the other hand the foot-rope passes more 

 lightly over the surface, or if the soles bury themselves more deeply instead of rising 

 in alarm the rope will pass above them and they will escape capture. 



There can be no doubt that at 30 to 40 fathoms depth beneath the surface of the 

 sea there is in the daytime a good deal of light, but still much less than at the surface 

 or in an aquarium tank. It has been ascertained that light disappears altogether at 

 200 fathoms, and if we assume that the absorption is proportional to the depth there 

 must be at 50 fathoms three-quarters of the quantity of light that exists at the surface. 

 Now the quantity of light in our aquarium is a great deal less than the quantity 

 outside, as a great part of the windows are obscured. It follows, therefore, that the 

 tanks in the aquarium are not very much more illuminated than the sea bottom at a 

 depth of thirty fathoms, and it may justly be concluded that soles in their natural 

 condition at that depth will for the most part remain buried during the day and emerge 

 from the sand to seek food at night : the sole therefore is a nocturnal fish in its natural 

 state. 



