FISH CUIvTURE 1325 



which ma}', from their appearance, be regarded as the breeders in the pre- 

 vious outward migrations, who return to the lake after spawning. The 

 Mugil thus present the characteristic migratory type which the writer has 

 termed " thalassotoccic " ; they normally live in an en\'ironment of brack- 

 ish or almost fresh water, pass out into the sea water for reproduction, 

 then after spawning return to their normal habitat. 



In their outward migration the individuals pass from a less salt into a 

 more salt environment, from an environment wdth higher to one with lower 

 temperature ; the opposite is the case however (or, as regards temperature, 

 at any rate there is equality between the two environments) at the time of 

 the inward migration. Apparently, therefore, the differences in relation 

 to saline character and temperature play no predominating part in the phe- 

 nomenon of migration, since the displacements take place both in the posi- 

 tive and negative direction in reference to these conditions. 



This is no longer found to be the case however with regard to dissolved 

 oxygen. The researches of the writer on the outward migration have 

 shown that the sea water of the coast at that time is always richer in ox^^gen 

 than the water of the lake. In the same way, at the time of the inward 

 migration the saltish v/ater of the lake is richer in dissolved oxygen than 

 that of the sea. In both cases, the two inverse displacements have the com- 

 mon condition of being directed from a less oxygenated en\dronment to- 

 wards an envirormient better pro\dded with dissolved oxygen. 



The displacement of the migratory fish in both directions does not take 

 place at haphazard, but is connected with the presence and duration of the 

 currents existing between the sea and the lake. At the time of the outward 

 migration the majority of the migrating fish only go to the sea when the 

 sea water currents flow towards the lake, and they pass up these currents 

 swimming against the stream. In the same way, at the time of the inward 

 migration, the principal displacement towards the lake is effected against 

 the current when the waters of the lake are passing to the sea. The whole 

 thing takes place just as if the migratory fish did not decide to travel and 

 accomplish their migration until after they had been touched by waters 

 dissimilar fiom those in which they had hitherto been, and as if such mi- 

 gration only consisted in their maintaining themselves in this new environ- 

 ment, passing along it by gradual stages until the place from which it has 

 its origin is reached. 



These considerations lead to several conclusions bearing on the migra- 

 tion of fish : 



i) The Mugil- of the lake of Thau exhibit a simplified type of repro- 

 ductive or genetic migration, as they need only travel a few miles to accom- 

 plish spawning. This type, however, is complete ; the indications it provides 

 for simplification are therefore of great importance, as accessory cir- 

 cumstances to which otherwise one may be tempted to attach great import- 

 ance did not here come into operation. 



2) This reproductory migration, in its two inverse directions, pos- 

 sesses an external determining cause, due to the direct action of the sur- 

 rounding medium. In order that it may take place, it is essential that the 



