NO. 2 (1917) FISH MORTALITY 6I 



were observed by fishermen in the Bay of Agu . . . As this 

 'red tide' (sic) is said to have been in former times highly 

 destructive to the beds of the pearl oysters which form one of 

 the principal productions of the Bay, great alarm was felt for the 

 molluscs. Fortunately on 28th of the month, a heavy storm arose 

 and cleared the waters of the bay so that very little damage was 

 actually done." When Nishikawa. went to Agu to investigate, the 

 discolouration of the water had almost disappeared and in conse- 

 quence his enquiry was incomplete. He was able to show however 

 that the discoloured water was due to the presence of a superabun- 

 dance of a Peridinian, Gonyaulax polygramma Stein, so numerous 

 that he estimated the Peridinian population of a drop of this water 

 at from 800 to 3,000 at the densest area. Nishikawa remarks that 

 " usually the appearance of discoloured water is accompanied by a 

 great mortality of fishes, molluscs, and shrimps. According to the 

 observation of a pearl-oyster culturist, in the latter part of August 

 1899, large streaks and patches of yellowish-red water floated 

 about with the tide in the Bay of Toba. Fishes which were kept 

 in baskets floating on the surface of the sea were damaged by 

 them. Fishermen easily caught the littoral fishes by spearing, for 

 the fishes had become very sluggish in the discoloured water. 

 Even Haliotis seemed to suffer," 



Owing to the incompleteness of his observations Nishikawa was 

 uncertain whether the presence of the peridinians per se was the 

 immediate cause of the mortality. He noted as significant that 

 other forms of plankton abundant in neighbouring uncontaminated 

 water were practically absent from the discoloured water, a fact 

 similar to that which I noticed in the euglenid water off Malabar. 

 He inferred that water fitted for the propagation of peridinians 

 and unsuitable for the existence of the usual plankton is probably 

 also unsuitable for other fish life, or else the dead bodies of enor- 

 mous numbers of peridinians sinking to the bottom and putrefying 

 there, may eventually become injurious to other organisms. 



Mr. Nishikawa, whom I had the pleasure of meeting when in 

 Japan in 1907, informed me further that when "red water " runs 

 into a bay fishes float to the surface stupefied. He added that in 

 Omura Bay, near Nagasaki, the farmers of the district welcome the 

 appearance of this poison water as they can then catch quantities 

 of fish with ease. 



He mentioned that truly red or pink water is also found in 

 Japan, also yellow water. The former is due to a superabundance 

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