172 OUR PUBLIC AQUARIA. 



that during these night hours they were more than 

 ordinarily alert for food, dashing vigorously at any 

 entomostracan or other minute organism that passed 

 through the water. This circumstance would seem to 

 explain why " drift-net " fishing for herrings can only 

 be carried on successfully at night, that being the 

 time when fish rise to the surface of the water to feed 

 on the innumerable organisms which abound there. 

 They are, in fact, so ardent at such times in pursuit 

 of their food that they needlessly strike into the 

 meshes of the net and get caught, just as the 

 individuals under artificial conditions dash against 

 the rockwork of their tank if sufficient light is not 

 provided for their avoidance. This plan of dimly 

 illuminating the whitebait tank was practised with 

 equal benefit in association with other species that 

 exhibited a tendency to injure themselves during the 

 dark hours of the night, such species again being 

 usually free rangers of the open sea. The picked dog- 

 fish (Acanthias vulgaris) is one of these, and is a 

 variety so given to rendering itself an unsightly object 

 by knocking its head against the boundaries of its 

 tank, till it lays its cartilage bare, that it is frequently 

 refused admittance in aquaria. Observations made 

 at the Manchester aquarium, however, revealed that 

 this self mutilation was invariably effected during the 

 night ; and a small light, enabling the fish to see and 

 avoid the rocks, was found an effectual preventive 

 remedy ! 



