230 Discussion 



perforated netting cage inside a two-litre jar, it grows at a rate not 

 much less than if it had the full swimming space. I do not think 

 restriction of movement affects the growth of these fish. There is a 

 very different effect when you put more than one fish into a tank. 

 There is a social effect on size, which is partly behavioural and partly 

 chemical. Where the fish have been kept in separate perspex com- 

 partments and water comes rapidly through from a large tank, they 

 seem to grow at a not much lower rate than they would have grown 

 if they had been loose in the tank. 



Gerking: Many fish are very aggressive to others around them. 

 A fish in company with others will fight or nip in an attempt to 

 maintain territory. The social hierarchy is similar to that which has 

 been described for the chicken and many other animals. These 

 social factors play an important role in the rate of growth offish kept 

 together in aquaria. 



Danielli: Has the experiment been tried of taking fish out of a 

 tank in which a "pecking" order has been established, and putting 

 them into a mirrored tank? 



Comfort: No, but if you remove the largest fish from a tank in 

 which a size hierarchy has been established, for no good reason 

 everybody "moves up" one place. 



Danielli: The fish in the mirrored tank would see one exactly the 

 same size as itself. 



Comfort: You might try an enlarging mirror! This should be done 

 with other fish and not with guppies, because guppies are extremely 

 unaggressive. I have been very fortunate in having guppies which 

 have never shown any sign of eating their young, and the counts 

 from trap tanks are no higher than without traps. This is not true 

 of all breeds of guppy, and it would be a perpetual reservation on all 

 this work if the fish fight and eat each other's young. 



