146 C. E. Lucas 



this general heading of organic matter are included, sometimes in very minute 

 quantities, substances whose effects within the community may not unreasonably 

 be compared with those of endocrine metabolites within the body. It now seems 

 necessary to believe that such substances play a considerable part in the growth of 

 aquatic communities of bacteria, algae and fungi (we still have much to learn of the 

 activities of the last in natural waters), so that the success of various organisms, and 

 consequently their ecological succession, may be largely determined in this way. The 

 next point is that we now have a number of instances in which such free metabolites 

 affect not only the lives of protozoans (whose communal life may be expected to be 

 very similar to that of the micro-plants, except in so far as their need for external 

 metabolites can be expected to be much greater), but also those of higher animals 

 such as worms, echinoderms and molluscs and probably crustaceans. We can 

 confidently expect this list to be extended, probably even to include fish. 



For example, Hasler and Wisby (1951) have obtained most interesting evidence 

 of the influence of different natural waters upon the movements of fish. They found 

 that minnows were able to discriminate between the waters of two Wisconsin creeks 

 after as little as two months conditioning, while cautery of their olfactory epithelia 

 rendered them unresponsive to conditioning. It appeared, therefore, that olfaction 

 was the principal, if not the sole, means of discrimination. They further demonstrated 

 that the chemical response was not to carbon-dioxide in the creek waters, for example, 

 but that the significant fraction was probably organic, in the usual sense, in that the 

 minnows reacted to the distillate rather than the residue (vacuum distillation at 25° C), 

 so that the existence of a volatile substance in the water can be anticipated. Here then 

 is the essence of one reaction system on which " homing ", and perhaps other aspects 

 of migration, might reasonably be based. Indeed, Hasler's preliminary tests suggest 

 that salmon can detect such " odours " of streams and discriminate between them. 

 In that instance, it would be necessary to imagine the salmon being conditioned during 

 its early fresh water life to the " odours " of the " home " tributary. Hasler has also 

 been able to demonstrate that minnows could respond to such odours after " forgetting 

 periods " which were longer in fishes trained when young than in old ones. It may not 

 therefore be so unreasonable to think of such reactions as even applying to the move- 

 ments of other and wholly marine fish. 



Lastly, when mentioning this particular possibility as one of the possible uses to 

 which such fundamental research might ultimately be put in a fisheries laboratory, 

 Johnston (1955) also referred to the possibility that " fertilization " of natural waters, 

 normally by enrichment with phosphates and nitrates, might come to include the 

 addition of minute amounts of critical metabolites. Ideally, at least, these would be 

 selected as those likely to mediate the succession of '' desirable " algae in relation 

 to the favoured crop. For example, it has seemed at times that the risks of fertilization 

 being followed by blooms of " undesirable " algae are considerable, but it does not 

 seem unreasonable to imagine that such a succession could, to some extent, be con- 

 trolled if the fertilizer included not only normal manures but a specific metabolite 

 antagonistic to these algae and preferably favouring other forms. Indeed, it may not 

 be too speculative to suggest that some natural waters, normally producing little in 

 the way of a desirable crop, might be induced to undergo a complete ecological change 

 by the addition of a critical metabolite alone. The apparent scarcity of nutrients in 

 some of these waters may not be so significant as it may seem. Not only may there 



