1901.] Spurge (Euphorbia hiberna, L.) on Salmonoicl Fishes. 65 



trout in its earliest post-larval stages is comparatively immune from 

 the action of both Spurge- extract and tannic acid. Very strong solu- 

 tions of both had to be employed before fatal effects were produced, 

 and even then the time taken was much greater than for older speci- 

 mens in weaker solutions. At later stages the larvae gradually lost 

 this power of resisting the action of the Spurge and tannin. 



The time taken by the Spurge to produce fatal effects varies, of 

 course, with the strength of solution employed. In the 'strongest 

 possible extract death would occur in a few seconds. In a 20 per cent, 

 solution of the fresh extract prepared as previously described death 

 takes place within five minutes. As the solutions grow weaker the 

 time taken becomes longer until with a solution of 0*01 per cent 

 4 hours elapse ere death occurs. The time varies, it should be said, 

 with different specimens, but these represent the averages. The 

 0*01 per cent, solution is the weakest that need be considered for all 

 practical purposes. This is 1 part in 10,000, and shows that a pool 

 100 feet long by 20 feet broad and 5 feet deep would prove fatal to all 

 fishes within 4 hours if 1 cubic foot or 6 gallons of the fresh extract 

 were poured into it. If, however, the plants are cut up, pounded, and 

 placed directly into the pool, then a much smaller quantity would 

 suffice. For, as has been previously stated, the " fresh extract " does 

 not exhaust the possibilities of the plant, and several hours may elapse 

 ere the poison is entirely dissolved out of it. The calculation is 

 somewhat rough, but if we take an entire plant when fully de- 

 veloped to weigh about 400 grammes, then about a hundred plants are 

 needed to poison a pool of the dimensions given. If the plants are 

 pounded and placed in the pool, however, probably a fifth of this 

 number would suffice. 



If such a pool were almost stagnant, with little water passing 

 through, it would be fatal to fishes, not merely for hours but days. 



Eeference to Table I will show that the Spurge-extract retains its 

 deadly qualities for several days, and if the quantity thrown into the 

 pool be not less than that quoted above, the fish there have little 

 chance of escaping death. 'If the water be running in and out of 

 the pool, the same effects will result from placing the plants at its 

 upper end, for their presence there will keep the pool in a poisoned 

 state for many hours and even a whole day. It is evident therefore 

 that the salmon which lie in quiet pools waiting their opportunity to 

 get up the rivers, are especially liable to be killed in this way. If 

 they are swimming about in a broad reach of water they will be 

 unaffected by any small current containing the Spurge-extract — unless 

 it be in enormous quantity. But the peculiarity of the poison, causing 

 as it. does a loss of sensibility and consequent stupidity, affects the 

 resting fish in such a way that it comes under the influence of the more 

 fatal component of the Spurge before it is able to detect anything 



F 2 



