? for Mike G (or anyone).: weed spraying and disolved oxygen

[Scott E.]

They (MWD) have been aggressively spraying the millfoil and coontail up here in Big Bear Lake the last several years and I've noticed the disolved oxygen levels this summer have been the lowest I've seen, consistently less than 5 ppm (the benchmark for good fish survival from what I've read) for a good portion of the summer and the last limno reports are from 3.7 avg. in the west end by the dam to 4.5 in the east end and somewhere in between for the rest of the lake. Does the aggressive use of herbacides and subsequent decaying of weeds significantly reduce the d/o levels? If so, would the installation of more aeration units significantly increase the d/o to higher levels. Also, how damaging are these current low oxygen levels to the bass, panfish and trout populations? We had a big problem with die off of LMB's a couple years ago and I'm wondering if we're in for a repeat? Thanks, Scott

[some guy]

anytime you lose veg your DO levels are going to drop. Aeration systems are really good for boosting DO levels. They had the same problem at Elsinore and look at it now. Low DO throws off the whole food chain.. the smaller species are going to affected first (emaciation/suffocation) then with less forage thats when the game fish start to be affected. No bait for the bass to feed on and basically they are swimming around with a wet pillow case over their head. Its takes a toll.

for example... if you over fill your bait tank with bait what happens? Not enough flow to go around and the bait fish dies. This is the same thing as that but on a grander scale.

[some guy]

cue Old School to clean up what i said in a scientific manner.

[Mike Giusti]

Big Bear is a relatively shallow lake so it does take much to drive the D.O. down. Yes decaying vegetation will cause D.O. to drop. Aeration units can be of some help but they also can cause and increase in water temperature. If the water get to hot the trout will die. Also, the units need to be turned on before the lake stratifies and the D.O. near the bottom begins to drop. The level you have reported should not adversely affect the warmwater fish populations. My guess is that these are surface D.O. levels and I would need to know what the profiles through the water column look like to make any real definate statements. I am not sure what caused your die off as it seemed that it was just bass and the trout are much more sensitive to low D.O. than the bass. My guess right now is the D.O. is good in a large portion of the lake and most of the fish have moved into that area.

Hope this helps.

Mike

[Scott E.]

Mike, thank you for the response. Here is the limnos link http://www.bbmwd.org/, after you click on it you can look on the left side of the page for "this weeks limnos" and click it. It lists 5 different sights in the lake as well as d/o levels and water temps from surface temps and different levels all the way to the bottom. The lake manager said the bass got stressed and developed a parasite (which caused severe redness around gills and tails) along with a slimy cottony looking coating on em and lots of em were swimming on the surface very slowly. Thanks again, Scott

[Oldschool]

Big bear lake has had an ongoing battle with weed abatement that dates back into the early 50's. The lake is over 7 miles long and averages over 50 feet deep for the 3 1/2 miles from the dam to the middle area of the lake. The upper half and major bays are about 20 to 25 feet deep, where the weed growth has always been a problem for the lake shore areas.
The fact that chemical weed abatement kills the weeds and therefor lowers the DO levels in those areas, doesn't mean the entire lake is low in DO levels. Big Bear is subject to afternoon wind that aerates the main lake basin, plus there is a aeration system near the dam. The thermocline at Big Bear tends to be around 15 to 20 feet during the summer warm water period.
You will find the smallmouth bass in the lower 1/3rd of big bear and the largemouth throughout in the main lake located on isolated gravel bars, humps and spring springs throughout the lake.
Like most lakes, Big Bear goes through cycles where bass and crappie populations are down. The large carp population, IMO, is one reason for poor spawn recruitement at Big Bear.
Tom

[Scott E.]

Tom, take a look at the limnos (2 posts up from this one) by clicking the link. The D/O levels ARE very low throughout the lake and water column. Scott

[Oldschool]

The data isn't consistant with known factors. The water temperatures are listed the same from top to bottom and that is questionable at best. The surface water at Big Bear has always been warmer than the water about 3' to 5' down and the water under 30 feet is very cold, usually around 55 degrees or less. There is no evidence of a thermocline in the data. I haven't been in the water at BBL for several years, but the climate hasn't changed that much.
Rainbow trout need DO levels in the upper 6 mg/L and I can't tell want the numbers are mg/L or something else. The trout are doing good at BBL, the bass are more tolerant to water temps and DO levels then trout. With the values listed everything should be rolling over except the carp and catfish.
Thank you for the link, interesting data.

Tom

[Scott E.]

The lake in years past has stratified in the summer in the past umptine years with the exception of 2 of the last 4 (if I remember correctly) and turns over in the fall, normally. They sprayed aggressively 4 and 5 years ago and kinda gave it a rest for 2 years then went at it pretty good this year. I'm pretty sure the guys take the readings on mondays or tuesdays EARLY in the day so that probably explains the surface temp being cool as our nights have been in the 40's most of the summer. If you notice, there is a slight deviation in d/o and temps at most reading sights, but slight non the less (and it's been like this more or less for the whole summer). Most years the thermo/oxyline varies from 15 to 25 feet, give or take throughout the summer until fall turnover. That's why I'm concerned about what's going on and possible causes.

The only thing that has changed over the last 4 or 5 years is they are spraying the weeds instead of using the harvesters, (aside from water levels fluctuating as they have done up here for decades so that could be the culprit in my best estimation). Since they started spraying (well, actually the year following the spraying of the entire lake), is when the numbers of bass and panfish started dropping like a stone. I attribute the decline in numbers to lack of habitat (no usable cover since dead weed areas are very low in oxygen) which enabled a very large population of pelicans and cormorants to go to town for a couple years, they didn't even leave in the winter for 2 years (many smaller bass I caught had beak marks either across their backs or through their sides for 2 years after the first sprayings), above normal predation of fry from other fish (due to the same lack of cover) and the die off 2 years ago. But now I'm wondering if low d/o content should be added to my list (including a possible culprit of the stress induced parasite).

Also on a side note, The trout fishing in the early spring for the last few years has not produced the consistent quality sized rainbows it has in years past. I'm wondering if alot of the trout aren't making it either or if their feed of small shrimps and larvae that live in the milfoil and coontail is severly diminished due to dead weeds? We've always had millions of carp since I moved up here in '81 and there was never a noticable decline in bass and panfish populations until the spraying started. On the up side, the MWD has left a few areas of invasive weed species untreated and is trying to let native weeds grow back. Also they stocked several hundred smallmouth last fall and 65,000 largemouth (about 5 or 6 inches from what I've observed) this summer and about 15,000 or more panfish in the last couple/few years. Additionally MWD is purchasing a shock boat and will be shocking, scooping and emulsifying carp over the years to come to help lower their numbers. Hopefully the lake will be on an upswing over the next several years. Take care Tom, Scott

[Mike Giusti]

Scott, I agree with alot of what oldshcool posted. I would expect a thremocline in the deeper part of the lake unless they have aerations system going. If the D.O. levels listed are mg/l they are marginal for trout survival and not necessary the best for bass either. It would seem to me that with D.O. level this low we would see a lot of fish in trouble as there is no where it good oxygen for them to go to. I am guessing it is going to get worse as the weed start to die and decay.

The carp removal might help in the long term but it seems that is just one of many problems at the lake.

Mike

[Scott E.]

The oxygen levels are parts per million and yes, the aerator down by the dam has been on full time for a couple months. Thanks for response