odorless human respiratory irritant often present in or near mortality 

 areas where droplets of sea water become air-borne as a result of 

 wind, wave action, etc. (Galtsoff, 194-8; Gunter et al., 1948; Woodcock, 

 1948; Ingle, 1954; et al.). 



Failure to find either one or both of the mentioned diagnostic 

 characteristics in an isolated area of dead or dying fish would not 

 necessarily eliminate G. breyis as the cause. There are at least 

 fovic possible reasons for this statement, (l) The conditions or 

 agents required to make droplets of sea water air-borne may be 

 absent. (2) Dead fish may drift or be carried into an area either 

 iinsuitable for the survival of G, brevis or removed from the "bloom". 

 (3) An isolated mass of water in which G. brevis is "blooming" may 

 suddenly become unsuitable for this organism and yet not lose its 

 toxicity to fish until sometime later. There is experimental 

 evidence to support this suggestion since the removal of living 

 G. brevis from cultures by millipore filtration or killing them 

 with gentle heat did not inactivate the toxic substance. A specific 

 diagnostic test for the toxic substance(s) produced by this organism 

 would be helpful in diagnosing the cause of mortality in such cases. 

 G. brevis is so delicate that under adverse conditions it may die 

 within a matter of minutes leaving only fragmentary remains which 

 are not readily identifiable. (4-) The fish may contact the toxic 

 substance in a "bloom" of G, brevis in one area and yet not succumb 

 until moving into an area where the organism is not flourishing. 

 This possibility is based on limited observations that fish exposed 

 to G, brevis cultures for short periods and removed before showing 

 distress, died after being placed in sea water. The "death times" 

 after removal to sea water appear to decrease as the exposure time 

 is increased. 



Results of our studies with both heated and filtered G. brevis 

 cultures, tests with other dinoflagellate cultures, and oxygen 

 analyses of G. brevis cultiires emphasize the existence of a toxic 

 substance(s) . The results of these studies support Galtsoff 's 

 (1948) conclusion that fish are not killed by clogging of the gill 

 filaments by masses of G. brevis. 



The available evidence makes untenable the view that fish 

 suffocate as a result of mechanical occlusion of gill surfaces by 

 the mere presence of large numbers of organisms „ G. brevis cult'ureb' 

 heated to 35 and 45° C. did not lose their toxicity although the 

 organisms were disrupted. Likewise, the removal of this organism 

 (both unialgal and bacteria-free) by millipore filtration did not 

 detoxicate the cultures. Unialgal cultures of G. splendens and 

 prorocentrum sp. were nontoxic in spite of the fact that the number 

 of organisms compared with and even, in some cases, exceeded the 

 concentrations in the toxic G. brevis cultures. 



43 



