30 A. E. HEFFORD. 



almost over. It is a fact of common knowledge to the Plymouth 

 fishermen that the whiting ( G. merlangus), bib {G. minutus), pout 

 (G. luscus), and pollack (G. pollachius) are to be found nearer the shore 

 in summer and autumn than in winter and early spring, when the 

 breeding season occurs. This habit of migrating to deeper water for the 

 colder months of the year they have in common with the other import- 

 ant food fishes of the Plymouth district, sucli as the Pleuronectidae and 

 gurnards. Very many more observations, both physical and biological, 

 are necessary before definite conclusions can be made as to the real 

 causes of these phenomena. The off-shore migration in winter and the 

 corresponding approach to shallow water in summer may, in some cases, 

 follow the seasonal distribution of food, but I do not think this is at 

 most more than a partial explanation. The fact that the temperature 

 of the water in the deeper parts of the Channel is appreciably higher 

 than that of the more inshore parts of the Channel in the coldest 

 months of the year, may be taken as a sufficient reason for the majority 

 of fishes preferring to seek the outer grounds at this time. That 

 conditions directly related to the phenomenon of spawning are involved 

 in this migration (which certainly coincides with the ripening and 

 liberation of the sexual products of most of the species) may be con- 

 cluded from analogy with the cases already worked out under less 

 complicated conditions, e.g. the plaice and cod, by Jobs. Schmidt (21c), 

 who has shown that these and other species show a special sensitive- 

 ness to external conditions, especially of temperature, in relation to 

 spawning, and therefore make special and well-marked migrations. 



So far I have not been able to obtain direct proof of extensive 

 spawning of our four common Plymouth species of Gadus on the oft- 

 shore grounds because winter samples of plankton from such regions 

 have not been collected, but the general fact may be taken for granted. 

 With due precautions one may accept the occurrence of pelagic post- 

 larvae, such as were captured in the young-fish trawl in April and 

 subsequent months, as evidence giving more or less quantitative in- 

 formation as to the spawning times and the relative extent of the 

 reproduction of the various Gadus species in this neighbourhood. As 

 far as can be judged from our takings of the small fry — and evidence 

 from the fisheries points to the same conclusion for the adults — 

 Gadus merlanfjiis is the most abundant, very many post-larvae of this 

 species having been taken, especially in May and June. Next in 

 abundance comes G. minutus, which has an almost similar period of 

 occurrence, if anything earlier than the whiting. The early pelagic 

 post-larval stages of the pollack have always in my experience been 

 less common than the two foregoing, but they are very abundant, at a 



