Thursday, 9 October 2014

Brown trout ready to spawn

I tend to think of this as the ‘otter bridge’. After all, for a while this was the place to come and watch for the Thetford otters. I’ve not seen the otters here for some time, although I occasionally stumble across them elsewhere on the rivers that skirt the margins of town. Perhaps that’s a good thing now that several sizeable brown trout have stationed themselves upstream of the bridge, where a bed of gravel lips a deeper pool.

It seems likely that the trout will use the gravel bed for spawning and one individual in particular appears to have already claimed his patch. From the bridge you can watch him as he lunges at those tempted to trespass. It is perhaps a little early for the trout to have started spawning but it will not be long before the gravel hosts many hundreds of eggs, each just a few millimetres in diameter. Peak spawning in UK waters falls during late October or early November but, since it is influenced by the weather and by water conditions, it may continue well into the New Year. Depending on when the eggs were deposited and the temperature of the water, the eggs are likely to hatch from mid-March. Since the proportion of eggs successfully fertilised tends to be high, large numbers of fry emerge, most of which will fall to predators.

All of our native populations are descended from colonisations that occurred soon after the end of the last glaciation and most of our rivers and streams host brown trout populations. This is a pattern repeated across much of Europe and western Asia. Some of those trout that inhabit our lakes make an autumn migration into feeder rivers and streams, seeking out pure, fast-flowing water running over the deep gravel beds used for egg-laying. It is the female trout who makes one or more scrapes within these gravel beds, each of which is known as a ‘redd’. Spawning is a stressful time for trout and many will be taken by predators, die from exhaustion or succumb to the effects of disease. It would be nice to see the otters but I suspect that the trout would make easy pickings while they remain so focused on spawning.

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