Some information on the dead fish reports

[djkimmel]

I talked MDNR fisheries research biologist Mike Thomas on May 3rd about the fish kills being reported from various lakes - especially St. Clair. Here's more information and thoughts on the situation.

First of all, small fish kills are fairly common, especially during periods of stress such as season changes. This happens all over the country, not just in the north. I've read numerous times that 50 to 60% of the fish die each year.

Just like successful spawns, various factors effect fish stress and die offs. Some years, die offs are barely noticed. Some years, the die offs can seem fairly large such as this year. It seems like several factors are creating that affect including the number of larger fish such as muskies on St. Clair.

Many fish die under the ice or during the cold of winter and lay on the bottom. When the water warms, it is less dense, so many fish that previously died over an extended time of weeks or months may float up about the same time making it seem like a lot of fish died around the same time. Fish in cold water may not look as bad after being dead for a while unlike the warm water periods when they can deteriorate very fast.

Some of the dead fish on St. Clair have been dead for a while. That makes it hard to determine exactly what killed those fish as previously reported. When you add everything up though, it starts to look like Mother Nature is once again the most likely culprit with some possible extenuating circumstances in a few places.

On St. Clair, things have been confusing because we are dealing with two probably unrelated issues right now. One is the die off of muskies. We already know that St. Clair muskies are having issues with a bacterial infection that could be adding to stress. The MDNR has been out surveying multiple times this year and most of the fish have been dead too long to test, but biologist Gary Towns did find a freshly dead musky that has been sent to Lansing for testing. That may tell something, but for now, the die off appears to be tapering off.

Last Wednesday, MDNR biologists saw 7 or 8 dead muskies from Harley down to the Detroit River, and then a few more in the Detroit River. On the day I talked to Mike, they had run down the lake again from Harley Ensign to Peche Island, and didn?t find any dead muskies on that trip.

The last substantial die off of muskies happened in 2003. During that period, all of the dead muskies observed were over 42 inches. This year, a wider range of sizes has been seen ? from 28 to 50 inches. It could be a combination of the normal winter/spring stress along with bacterial infection, but that is just a possibility. The MDNR will try to get as much information as they can.

The second issue is the die offs occurring in many of the canals and river/streams off St. Clair. This is probably stress-related and unrelated to what is happening to the muskies ? most likely stress exacerbated by the large water temperature shifts over short periods in shallow, protected water. I?ve read plenty of studies that state that bass, for example, are highly stressed, possibly to a fatal level, by quick water temperature changes of 10 or more degrees Fahrenheit. Add that to the normal winter/spring stress and the low water, and you can lose some fish.

In the canals this year water temps went up very fast and then back down both over short times. The die offs were probably not oxygen-level related because many dead carp and bowfin were observed too. We all know these are fish that can live with low oxygen.

Winter/spring stress and shock are most likely the culprits because the MDNR saw plenty of long dead fish, recently dead fish, and very stressed, but alive fish that were swimming weakly along the surface. They could actually go up and easily pick up the fish from the water.

Our mild winter had several warm-ups that could have been hard on weak fish. I have heard of fish kills on several other lakes, while plenty of lakes are not showing any significant die offs. The type of water has some impact on susceptibility along with other factors (weeds, water clarity, average depth, ice thickness, snow cover, current, etc.).

Because the fish observed died over a wide range of time and on scattered lakes (St. Clair, North shore of Erie, Macatawa, South shore of Erie/Sandusky Bay ?  die off of large drum was reported last weekend in Sandusky Bay and the South shore ? not a common occurrence), that pretty much rules out things like lawn treatment run off or other man-made things being significant causes. The types of fish and scattered locations seem to point to Mother Nature.

Probably our main concern should be if the die offs are a significant percentage of the fish population. That doesn?t seem to be the case at this time. We should also watch to see how the spring goes as the smallmouth bass move shallow in larger numbers. I?m hoping the water is stabilizing enough that most fish will do fine from this point on and we won?t have any more die offs of any consequence.

For St. Clair, I?m sure we?ll see some dead smallmouth bass through the spawn, but hopefully it will be no more than what we see every spring out there. Just do your best in getting any bass you catch in quickly, handle them carefully and return them immediately to the water. Lures, with single hooks and a quick hookset will help minimize stress on the bass. Some anglers are choosing to use barbless hooks. I like to flatten my barbs down when I know I?m going to be releasing the fish anyway.