With recent threads on the impact made by cormorants and goosanders, I would be grateful for views about the theory that grayling numbers and the average sizes of fish seem to naturally go through cycles. I have heard this from a number of people and indeed I have seen it more than once on this forum.

The suggestion is that a fishery develops a decent head of grayling of a decent average size, but then over a season or two numbers dwindle and then anglers start catching shedloads of grayling but all of a uniform "fingerling" size. Then over time the avergae sizes start going up and then the cycle repeats itself.

Have any of you found this on your fisheries? Are there any theories as to why this happens? Does the demographic of the trout population correspond in any way during these cycles?

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strikealight


Dynamite fishing is always effective.

I have heard exactly the same thing mentioned for the Taff and anecdotally it seems to hold true. At the moment you can fill your boots with small grayling but there seems to be far fewer big grayling than you would expect.

Dont really understand it though, my little understanding of population biology would lead me to understand that the main cause of this sort of thing is a predator prey relationship where the predator would also be expected to cycle. No idea what the predator would be though.

Dan

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http://eclecticangler.blogspot.com/

This is common to most fish species and usually reflects a very good spawning year and good river conditions during the crucial first year. Funnily enough the conditions that WE consider to be virtually disastrous - ie drought and low flows are in fact ideal for many riverine fish. Warm weather encourages spawning and creates the ideal conditions for the food items for juvenile fish so there is more food to go round. Growth rates are good. Low flows mean that the fry will not be washed away in floods and survival through the first winter is good as a result plus the fact that there was plenty of food around for the young fish to aid their survival.

Way back in 1975/76 we had two very warm summers plus a low flow winter. The dace spawned very well in both years on the Kennet and grew really well. The result was that we had an enormous cohort of dace that reached specimen size over the next dozen years or so. During the 1988/89 period I was catching 1lb+ dace every time I went out, and the best I encountered though not on rod & line went 1lb 6oz - well above the record. By the early 1990's those fish were gone. These things do die naturally which is something the angling fraternity tend to forget. We never saw the like of them again, though there are a few 12oz dace around now and I have hopes that some of those might go on a bit.

I was coarse fishing on the Kennet yesterday. You know, that horrid way of fishing using maggots & worms! For the third time this winter I had a really good net of roach and by far the biggest group in my catch were fish in the 5-7 ounce size range. I reckon these fish are probably 3+ years of age. I was also catching roach in two other sizes - 2-3" and about 4-5". So we have three strong year classes of roach in the river.

So what? Well the odd thing is that up until this winter catching roach of ANY size in that part of the Kennet was a rare thing. You'd hear tales of occasional decent catches on one day, then nothing.

Catches like those described above are really encouraging for the future. The larger fish will be spawning for the first time come mid-May and I will be out looking for them on the spawning shallows then.

In about 3-4 years time those fish will be pushing 1lb in weight so there should be some fabulous roaching then.

What does it prove? It proves we know very little about fish populations, and what we do know is mostly speculation based on experience and generally sketchy survey statistics.

I understand this phenomenon to be related to the relatively short lifespan of the grayling. They typically live as little as 3-4 years and with such compression of the demographic range, it stands to reason that particularly good or poor spawning seasons can have an exaggeratedly marked effect on the subsequent populations, presence/absence of particular size classes etc.

Matt

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The sun pushed dark spokes of melt and sparkle
Across the fields of hoar. And the river steamed -
Flint-olive.


http://northcountryangler.blogspot.com

Matt has a good point regarding the fairly short life span of grayling. A 5+ grayling is a very old fish indeed. Compare that to roach, chub and dace that commonly reach into double figures; to bream and tench that can go well into their teens and carp......? Who knows?

So it follows that a couple of poor years in any given four year cycle is going to have a much greater impact on grayling than on most other fish species.

Out of interest it was very noticeable that when we surveyed the River Dun - a Kennet tributary - in March 2010 we turned up hundreds of juvenile grayling in a 100 metre survey section. So 2009 was obviously a very good recruitment year. It remains to be seen how that translates into lots of adult grayling this year and so on.