Male-male competition and female choice in brown trout

In some salmonid species, the females have been assumed to choose their mat es on the size of the male's adipose fin. This hypothesis was tested in a s tream water aquarium, in which 19 brown trout, Salmo trutta, females were a llowed to choose between two males of the same body size but with different adipose fin sizes. The two males were separated from each other in cages. After the female had started to prepare her nest close to one of them, the males were released and allowed to fight each other for the opportunity to spawn. Out of 19 females, 14 prepared a nest closest to the male with the l arger adipose fin. However, only six of the 14 females spawned with this ma le. Males that spawned were more dominant (i.e. were more likely to win fig hts). When the female spawned with the male she chose, he was less aggressi ve towards her than when she spawned with the other male. There were no sig nificant differences in the plasma levels of testosterone (T) and 11-ketote stosterone (11-KT) between the chosen males and those not chosen. However, the dominant males had significantly higher plasma levels of T and 11-KT bo th before and after the experiment. The results support the view that femal e brown trout exhibit mate choice, but their choice is overruled by male-ma le competition. (C) 1999 The Association for the Study of Animal Behaviour.