Fall Shocking Surveys

[ozark trout fisher]

I've always wondered why the MDC decides to electroshock most of the trout streams in the late summer/early fall...When the water temperatures are about at their highest and dissolved oxygen levels at their lowest. Electroshocking would have to be a pretty stressful process for the fish, so why do they choose to do it at the time of year when trout are most likely to be stressed out anyway? Do you think this causes excessive mortality in the sampling process?

I am no fisheries biologist,but don't you think they'd be better off to do their surveys some time of year when the dissolved oxygen levels are usually higher? I understand this isn't a particularly vital issue, I'm just curious what you think.

[jdmidwest]

The fish roll to the top better this time of year...

In reality, this is the most sensible time of the year to do it, they should be at their prime. Any later, then they would be in breeding season. After that, it would increase the mortality of the young fry. Winter time, bring a fish out of 50 something degree water into air temps in the 20's and take measurements, there will be stress. Early spring and summer time, the shockers would have to compete with canoes and fish hooks. Not to mention that it is a time they need to replenish fat reserves after a long winter.

There will be some stress and mortality any time of year, no matter what. I never made it to the status of fishery biologist, but I stayed in a Holiday Inn one time. And I did a graduate level study of a local water using the shocking method to sample back in the early 80's. We sampled several stations each season of the year for a full year.

[Gavin]

Why?...because they have it marked on the calendar........bout the same time, same sample stretches........ been on that trip....I'm not sure what the delayed mortality is....but I helped measure about 5-600 fish & clip fins over the course of two days....lots of minnows & a bunch of suckers died in the process....they have big scales and get zapped a bit harder than the trout. Saw one dead trout in the net pens..most seemed pretty healthy after we measured em.

[stlfisher]

Just a guess but it probably has to do with water level and angler pressure more than anything. Late summer and early fall usually bring shallower water and easier for the shocking to take place. Probably less traffic on the streams/lakes so it has the least effect on fisherman. Winter could be dangerous since the freezing cold might be worse than blazing heat. Again just a bunch of guesses.

I would be interested to know how effective the shocking is at getting a true measure of the fish counts and size. I would thinks some fish that hold deep may escape the sampling...probably more of a possibility in deep lakes than the shallow streams.

[jdmidwest]

It is a "sampling", not a true head count of every fish in the stream. From that they extrapolate a figure to come up with fish per mile of stream.

[Kayser]

It's called a "capture, recapture" study. The basically shock it once, mark all that were captured, then come back to that stretch and sample again. By looking at the percentage of fish that were recaptures, you can determine the overall population using cross multiplication. (#Recapture/day2 capture)=(day1 captured/total population). Basically, the recapture tells you what percentage of the fish in the stream you are capturing, and then you just multiply to get 100%.

Rob

[Tim Smith]

I've always wondered why the MDC decides to electroshock most of the trout streams in the late summer/early fall...When the water temperatures are about at their highest and dissolved oxygen levels at their lowest. Electroshocking would have to be a pretty stressful process for the fish, so why do they choose to do it at the time of year when trout are most likely to be stressed out anyway? Do you think this causes excessive mortality in the sampling process?

I am no fisheries biologist,but don't you think they'd be better off to do their surveys some time of year when the dissolved oxygen levels are usually higher? I understand this isn't a particularly vital issue, I'm just curious what you think.

I have less experience with cold water fish, but warm and cool water fish are also at their peak condition during fall, with the highest amount of fat reserves. If memory serves the pattern is the same for trout. Should probably check. Also, once the water cools the DO is usually higher, not lower. DO levels might be low right now, but when the weather turns they will be reliably high enough to avoid stressing the fish. Fall is the time fish handle stress better than any other time of year.

Also, if they're doing growth analysis, their scales stop growing about now so if they collect scales now they can back-calculate fish size and see how much they grew during the year (although they might also just compare size distributions from year to year which is cheaper and faster...if less accurate).

[Tim Smith]

It's called a "capture, recapture" study. The basically shock it once, mark all that were captured, then come back to that stretch and sample again. By looking at the percentage of fish that were recaptures, you can determine the overall population using cross multiplication. (#Recapture/day2 capture)=(day1 captured/total population). Basically, the recapture tells you what percentage of the fish in the stream you are capturing, and then you just multiply to get 100%.

Rob

Yeah, that's how you can get an actual head count, and it's the preferred way to sample. Unfortunately, it takes a long time plus extra equipment to mark and hold the fish. To avoid those extra labor costs and cover more ground they'll just count how many fish they catch over a designated time period (catch per unit effort or CPUE). CPUE is less accurate but it gives a qualitative measure from year to year of the relative numbers of fish in the stream in a given stretch.

[Gavin]

MDC marks & recaptures.....

Norm Crisp has a thorough explaination of the process on his website.

http://streamsideadventures.com/features/current-river-shocking.html

[ozark trout fisher]

I have less experience with cold water fish, but warm and cool water fish are also at their peak condition during fall, with the highest amount of fat reserves. If memory serves the pattern is the same for trout. Should probably check. Also, once the water cools the DO is usually higher, not lower. DO levels might be low right now, but when the weather turns they will be reliably high enough to avoid stressing the fish. Fall is the time fish handle stress better than any other time of year.

Also, if they're doing growth analysis, their scales stop growing about now so if they collect scales now they can back-calculate fish size and see how much they grew during the year (although they might also just compare size distributions from year to year which is cheaper and faster...if less accurate).

I understand that the DO levels increase as water temperatures decrease...And vice versa. But to it can still be pretty hot by the end of September, which is when they shock the Current I know. And, to be honest I'm not sure where I heard this, doesn't the amount of flow in a stream also have some effect on the DO levels? I mean, it would make some sense. The lower the flows the less intense whitewater there is in the riffles and rapids, and seemingly, that would mean less oxygen making it's way into the stream. And water levels are just about always as low as they get towards the end of September. All pretty unscientific I know, which is why I asked this question in the first place.

In any case I really appreciate the explanations you all have given.