358 FISHES 



obtained from the recapture of marked specimens in the Baltic 

 indicates that the journey from the Baltic to the spawning 

 grounds in the deep Atlantic occupies about six or seven 

 months, so that spawning probably takes place in the April or 

 May following the migration. 



Diseases. — Diseases are best known in fresh-water fishes and, 

 as in man, are usually due to microscopic parasites. The 

 salmon disease has already been mentioned. Parasitic protozoa 

 cause a disease in flounders ; the symptom is the presence of 

 little white tubercles on the skin of the fins and body ; these 

 tubercles contain multitudes of microscopic spores. A similar 

 disease has been observed in fishes of the carp family 

 (Cyprinidse) and may be epidemic. Larger parasites of various 

 kinds both external and internal abound in fishes, both marine 

 and fresh-water, but many of them are constantly present, and 

 unless in exceptional numbers, can scarcely be said to produce 

 disease. So many fishes end their lives by being devoured by 

 other fishes and other animals, that cases of natural death, es- 

 pecially in the sea, are comparatively rare ; a few cases such as 

 that of the tile-fish and the salmon, the eel and the conger, 

 have been mentioned, but of old age and natural death in the 

 majority of species we know nothing. 



