CA^ APIAN FISHERIES EXPEDITION, 19Vrlo 37 



The yield of cod and mackerel eggs at this station gives the following numbers: — 



Cod. Mackerel. 



At surface. 3 eggs. 172 eggs, 14 larvae. 



*• 30-0 metres 12 " 8 " 3 " 



" CO-0 " 25 " 1 " 



Mackerel eggs and young are by far most numerous at the surface (in the horizon- 

 tal hauls) the cod eggs, on the other hand, being found in greatest numbers farther 

 down (vertical hauls). Naturally, however, this cannot be taken as indicating that 

 the mackerel eggs were spawned near the surface, and the cod eggs lower down. The 

 vertical position of fish ova in the layers is determined by the specific gravity of the 

 former as compared with that of the ^vater, and they will, under normal conditions, be 

 found in water of specific gravity equal to their own. The specific gravity of ova, 

 however, is not a constant magnitude; that of cod eggs, for instance, which normally 

 varies but very slightly, may be affected by a low degree of salinity in the water, which 

 has the effect of gradually reducing the weight of the eggs. If exposed to strong sun- 

 light, again, or to comparatively high temperature, they will sink, even in water of 

 high salinity; and the same applies to newly hatched young. This last is a very well- 

 known phenomenon in marine hatcheries; I am, how'ever, unable to give any definite 

 figures, having had no opportunity this season of making experiments in this direction. 



The specific gravity — and variation of same — in cod eggs at Flodevigen will be 

 seen from the following experiments (Dahl & Dannevig; Undersokelser over nytten 

 av Torskutlsekningi Ostlandske fjorde, Bil. I, p. 28). 



The roe and young were placed in three separate receptacles in order to determine 

 their approximate specific gravity. 



I. Sp. gr. 1019 at + 5-4° (sp. gr. in situ, 101796). 



Ova, about one-third floating, others suspended, the remainder at the bottom. 

 1 oung, most at bottom, some suspended and at surface. 



II. Sp. gr. 1-020 at + 5-4:° (sp. gr. m situ, 1-01897). 



Ova, more than half floating, the remainder at the bottom or suspended. 

 1 ou7ig, about half floating, the rest at bottom or in suspension. 



III. Sp. gr. 1-022 at + 5-4° (sp. gr. in situ, 1-020965). 

 Ova, practically all floating. 

 Young, practically all floating. 



Station 30, above-mentioned, the specific gravity of the water near tlie surface 

 oscillates about the value for specific gravity of cod egg^ as noted in Norway : we 

 might therefore have expected to find a greater number of ova at the surface. That 

 this was not the case should possibly be ascribed to the influence of the high tempera- 

 tures there prevailing. 



^Ve find, then, that the simultaneous spawning of cod and mackerel in one and 

 the same water may be explained as due to conditions of temperature. And conse- 

 quently, we cannot, in Canadian waters, distinguish between winter and spring-spaw- 

 ning fish, as is generally done in Europe; if any such biological distinction is to be 

 made, it must be based upon the limits of temperature between which spawning takes 

 place, a method which would of course be equally justifiable on the eastern side of the 

 Atlantic. 



The question arises, however, whether the temperature of the water really does 

 exert such influence upon the spawning of the fish; whether other causes might not 

 be imagined, and the explanation above suggested be found insufficient. 



In this connection, it will be natural to recall the annual periodicity in the 

 development of the sexual organs, which might be supposed to proceed independently 

 of all extf^rnal factors. It has, hovever, repeatedly been found, by investigation in 



