LIFE-HISTORIES OF THE COD AND WHITING. 5 



young cod are found in the neighbourhood of the shore. Their numbers per haul arc 

 seen to increase up to a length of about 33 mm., and then to steadily decrease till 

 the curve is carried off the table. An inspection of Table I. will show that the 

 littoral forms continue to occur with less and less frequency, up to 134 mm., before 

 which size the tow-net is no longer an appropriate method of capture. 



One other curve has to be noticed, namely, that of the bottom tow- net. This 

 is at all times very small, although the bottom tow-net greatly exceeds the surface 

 net in size of aperture, at least in the case of those captured in St Andrews Bay. 

 The curve does not approach that of the midwater net in height, and is mainly carried 

 on by one's and two's. Secondly, it will be found that its curve is almost exactly 

 parallel to that of the combined curve of surface and midwater. For these reasons 

 one is, I think, justified in regarding this curve as due to "incidental" specimens. 

 These may occur in two ways. Firstly, a certain small proportion of the surface and 

 midwater forms may exceptionally and precociously move to the bottom ; and, secondly, 

 the captured specimens may never have been upon the bottom at all. The bottom 

 tow-net is a large trawl-like piece of apparatus, and, after its journey along the bottom, 

 it is hauled up with open mouth through the midwater and surface. The parallel 

 course of this curve to that of the combined surface and midwater curves would favour 

 this view, i.e., that it is clue to a series of samples of the surface and midwater regions, 

 taken by the bottom tow-net acting as a vertical net upon being hauled up. 



It is quite possible that both the factors above referred to conduce to cause the 

 occurrence of young cod in the bottom net, but in either case they would fall under the 

 incidental category. They cease to occur after 19 mm., just before the midwater forms 

 become littoral. 



In tracing out the ontogenetic migration from these data, we have the two special 

 relations of the migration to deal with. 



In a horizontal direction we have the pelagic eggs, as a starting-point, floating in 

 the offshore spawning areas. A study of the distribution of these eggs leads us to 

 obtain evidence that a large proportion of them drift in towards the shore, and in 

 addition, as all the specimens in Table I. have been caught in the districts between the 

 spawning grounds and the shore, there is plenty of positive evidence that young cod 

 do migrate in the course of development, from the offshore spawning areas to the 

 littoral district of the East Coast. Whether others in any quantity migrate seawards,, 

 and if they do so, whether they survive and come to maturity, has yet to be proved. 



As regards the vertical migration, the surface curve does not exactly merge into 

 that of the midwater net, but from 4 mm. onwards the post-larval cod evidently commence 

 to leave the surface water and move downwards into the midwater. The two curves cross 

 at about 8 mm., and at about 9 mm. the young fishes occur in greatest numbers per haul. 



From this point onwards to about 18 mm. the surface and midwater forms both 

 decrease in number, so that it is improbable that the former owe their decrease mainly 

 to a migration to midwater, except in so far as the surface curve decreases more rapidly 



