Waters & words

Adjusting stocking rate for size of Trout

Stocking Trout is such a fickle thing.

If one researches the stocking rates recommended on the internet, as I have done, the answers are as varied as the size of the flies in your fly box.

Generally one stocks more if you are putting in little fish (fry to perhaps 3 inches), and fewer if you are stocking larger ones (say above 5 inches). This rate of stocking for different sizes is of course on a continuum from “fry” (being something that has only just absorbed its yolk sac) to fish of 10 inches or so.

By this we take it that larger fish have a higher survival rate. This assumption should possibly be questioned, or at least explored in more detail.

IMG_2998

We know from rearing fish in a hatchery, that we continually “clean”, meaning that we remove dead fish from the tank or raceway, and that the number we have to remove diminishes as the Trout grow larger. However we stock twice as many 2 inch fish as we do say 6 inch fish, despite the fact that we know that we don’t lose twice as many 2 inch fish as we do 6 inch fish in the hatchery. So by implication we are adjusting further for predation in the natural environment (post stocking). And we are assuming that smaller fish are more prone to predation. However, this is a rough and ready assumption, since for example we know that a large cormorant is not interested in a 2 inch fish, but it loves to eat 9 or 10 inch fish that we stock. And 9 inch fish always seem to shoal as you stock them, but this is not always true of the smaller ones.

For those who are interested, have a look at the excel file on this blog (Trout stocking formula theory), found to the “Box.com” file location in the ribbon to the right. This is a model that makes an attempt at estimating predation and other factors, per fish size at time of stocking.

stocking theory

We can model the expected results, and we can test this model against our fishing success, but we are now entering a realm in which chance, and fishing skill enter the formula to further blur the crisp lines of science!

As a friend recently remarked, stocking cattle is a little easier, since you can see them grazing on the veld. With Trout beneath the surface across several hectares, we will never know.

We stock at similar rates per acre or hectare in several stillwaters in our local fishing club, but have vastly varying fishing experiences from venue to venue. And this changes from season to season. Same input. Different output.

So we soldier on and remain hungry for slithers of evidence that will allow us to perfect stocking rates.

One response

  1. Interesting questions… I wish you could chat about this with my Dad – he was National Parks fisheries research officer for Lake Kariba and Lake Kyle from 1968 to 1976 and Curator of Ichthyology at the Natural History Museum for many years in Bulawayo… He still occasionally consults to fish farmers about stocking and catch rates in Southern Africa. No surprise that different lakes have different yields because each one is its own unique ecosystem. In different seasons, the same lake will behave differently depending on water levels, biomass and predator:prey ratios. Please continuing your ongoing endeavours because all research adds value to understanding. Best regards – metiefly

    Like

    April 16, 2014 at 10:46 pm

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