Hi, all...

Here's a great post from the WSC forum, written by our friend Smalma...I hope he doesn't mind me pirating it over here.

It's in response to a question that I asked him regarding how to avoid hatchery smolt predation upon wild chinook fry...

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Todd -
I think it is obvious to all us fisherman that large fish eat small fish. Since steelhead generally are the largest of the smolts then if the goal is to have no wild smolts eaten by hatchery steelhead there is only one way to accomplish that -PLANTED NO HATCHERY FISH!

However if the objective is to minimize the impacts to some acceptable (again we are talking willingness to accept some impact or risk) then there are likely modifications that would be helpful. The place to start is to examine the information available about hatchery steelhead predation. To determine the mangnitude of the problem (amount of predation) one needs to know the rate at which steelhead predate on wild chinook fry and the lenght of time they have access to the fry (exposure). Then the question can be asked can either the rate or exposure be reduced?

Unfortunately there are few studies that have looked at the predation rates on a large scale. One of the better studies is form the Salmon River in Idaho where in 1992 they looked at the stomachs of 6,762 hatchery steelhead and found 10 chinook. They found that "most steelhead smolts did not start feeding extensively until about a week after release". If this predation rate is typically then it is a simple to estimate the total predation = (predation rate) times (the number of smolts) times (the number of days in the river).

Additional background information.
Fish planted in late April to mid-May leave the system quckly.

Fish that remain in the system more than a few weeks (residuals)have twice or more predation rates.

The percentage of the smolts that residualize increases if the smolts are planted too small, too large, too early or too late. Target size should be about 6 fish/#

In large rivers and/high flows smolts move downstream about 20 miles a day - slower at low flows and in smaller rivers (as little as a mile or two a day).

Hatchery smolts are hatchery fish that have been fed pellets all their lives and need to learn on to feed on natural foods (shortly after release twigs, fir needles etc are common food items.

Predators generally eat the weak, sick and injuried. All fry consumed by smolts or other fish are not destined to become all fish.

The vulunerability of the fry to predation is in part a function of their habtiat - those streams with complex habitats and channels (natural streams) have lower exposure than those that have been altered by man.

Lets look at a hyporthical example - a large North Puget Sound stream that recieves a plant 500,000 smolts fifty miles above the mouth of the river. "Best Hatchery Practice" might be planting the smolts at 6/# in early May with direct releases from the hatchery into main stem areas. On the North Sound rivers we can expect that the spring run-off will have started by May thus helping to speed the smolts to sea. In that example we would expect most of the smolts to leave the system (low % of residuals) in just a days. The high spring flows would further flush many of the potential of the residuals from the system). Using half of the 20 miles/day would expect them to be out of the system in 5 days.

500,000 smolts time 0.00148 fry/smolt (from the Idaho study above) times 5 days yields a predation mortality of 3,700 fry.

Typical fry to adult survival for chinook would be well less than 1%. In North Puget Sound rivers where chum and pink fry substantially outnumber chinook they would likely outnumber chinook as preferred items (In May they would be smaller and less mobile than chinook - my own sampling at that time of year found the chum but no chinook in 150 stomachs).

If all the fry eaten were chinook (I feel that only a small % would be) then we are looking at impacts equal to less than 37 adult chinook. The actual details in each system would vary depending a variety of factors. In this hypothical example it would be for each of us to decide whether those impacts are too great for the benefits (steelhead fishing). Washington Trout clearly thinks it is -do you?

Tight lines
Smalma

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This represents some of the first "facts" that anyone has so far added to the conversation, at least useful ones

Fish on...

Todd.

P.S. Here's the link to the entire thread...

WT Thread on WSC Forum