Lacking a sex drive
I read with interest the answer from Ronnie Glass (“Trout Surgery”) regarding less aggression in today’s rainbow trout. The reasons he gives for the less aggressive nature of rainbows may be true, but I believe there is a more genetic answer. I remember many years ago discussing the merits or not of triploid rainbow trout with Doctor Ted Wise when he ran Coldingham Loch. Ted had introduced triploids and was interested to see how they fared compared to the diploid rainbows that had been stocked previously. As I recall, he noticed the percentage of triploids being caught shortly after stocking decreased more rapidly than diploid rainbows. The speculation was that they were less aggressive and consequently did not chase lures for as long after being stocked as diploids. Male and female trout are probably more aggressive due to being sexually mature whereas a sexless trout probably does not have the same level of drive and aggression. Although the physical reasons for having less sex drive and aggression are different, look no further than geldings in the horse world or bullocks in the cattle world to see what the lack of a sex drive does: they are more docile. As the world has moved on, most fisheries stock triploid rainbows exclusively and there are few if any sexed rainbows in our stillwaters. If today’s triploids are less aggressive than those of previous decades then it is no wonder that heaving the Viva is less effective.
Colin Mcisaac, Edinburgh