284 THE COD FAMILY. 



THE ROCKLINGS. (Onos mustela, 0. cimbrius and 0. tricirratus.) 



These little fishes have a very close relationship to the 

 gadoids, and by some observers they have even been classified 

 under the same genus, but it is usual now to place them 

 together in one genus, known as Onos or Motella. They cannot 

 strictly be termed food-fishes in that their diminutive size as a 

 rule precludes their being utilised as food: nevertheless their 

 importance in connection with fishery questions can hardly 

 be over-estimated, for both in the young stages and later they 

 form a considerable source of nourishment to the larger food- 

 fishes, while their pelagic eggs are found throughout the 

 greater part of the year. 



At certain seasons of the year young rocklings are found 

 in vast shoals frequenting the open seas, thus having a free- 

 swimming or pelagic habitat : they are popularly known as 

 ' mackerel-midges ' and have been at various times described by 

 naturalists as distinct species. 



Disporting themselves in the surface waters, it may be amidst 

 the gaily tinted jelly-fishes and salps, they are often "thrown 

 on board vessels with the spray, or drawn into fishing-boats 

 with the nets" (Day). The name mackerel- midge is derived 

 from the fact that they are preyed upon by vast numbers of 

 mackerel, and the movements of the latter in the spring are 

 probably sometimes dependent upon the presence of shoals of 

 ' midges.' Later, when the pelagic ' midges ' have grown into 

 'rocklings' with, at any rate in some, a change of habit to 

 shallow rocky shores, they still fulfil the function for which 

 they seem to be destined by nature and serve as food to many 

 of the large edible fishes. These facts being taken into con- 

 sideration, a detailed knowledge of their life-history and habits 

 becomes a desideratum. 



The rocklings fall naturally into three 'forms' which pro- 

 bably have the morphological value of species, as three distinct 

 eggs of rocklings are found on the east coast : the adults 

 seem to differ mainly in the number of barbels or feelers 

 which protrude from the region of the head. These appear 



