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Sturgeon Spawning Update from Ron Bruch on May 9, 2011

May 10, 2011

Looks like I missed this update–sorry I’m posting it a little late!  This update has been posted with permission from Ron Bruch and the Wisconsin DNR:

“Water temperatures continue to rise and more and more lake sturgeon have come in to spawn at the site below the Shawano dam on the Wolf River. The “Shawano Above” camera has been adjusted so you can look right down on the large group of spawning lake sturgeon 24 hours a day. There is a spotlight at this site so you can easily see fish at night on the “Above” cam, and on the underwater cam as well Looks like this 3rd run at Shawano is definitely the “big one”. I don’t expect this to last to much longer though – in a day or two it could be over.

This is a late spawning run in a season, but not the latest we have on record. In 1979 we had fish still spawning on May 9 and 10 like this year; in 2000 we had a second run on May 16-18; and in 2002 we had fish still spawning on May 18-20 of that year.

I initially addressed a question in an earlier e-mail about the impact of cold water during the runs 3 weeks ago on the incubating eggs and embryos. Fred Binkowski, of the University of Wisconsin-Milwaukee Great Lakes WATER Institute has done research which showed that lake sturgeon eggs incubate and embryos develop better at 59 degrees F (15 C) compared to 50 F (10 C) and 68 F (15 C). Almost all of the spawning prior to this last big run at Shawano took place at water temperatures at or below 50 F (10C) so we undoubtedly had 100s of millions of eggs incubating as sub-optimal temperatures this spring. But then – as I also mentioned in an earlier e-mail, we see 2 or 3 “runs” of sturgeon every year as well; so odds are that almost every year our lake sturgeon stock will conduct at least one of its spawning “runs” at a time that the eggs can incubate and embryos can develop at or close to optimal temperatures. The sturgeon apparently figured this game out a long time ago.

Once the 2.7 mm diameter (1/9th of an inch) eggs are broadcast by the female and fertilized in mid-broadcast by the males, they settle and stick on to the rocky substrate – the luckiest ones settle and stick deep into the interstitial spaces between the rocks so predators can not get at them, and where fungus may be less likely to infect and kill them. Water flow, in addition to optimal temperature, is critical to incubating eggs. Eggs down in the interstitial spaces between the rocks have additional protection from being eaten by predators (suckers, redhorse, male lake sturgeon, carp, and crayfish). The predators though, by consuming eggs on the surface of the rocks (which at some sites can be 2 or more inches thick), may provide a benefit to the eggs down in the rocks by keeping the substrate clean and allowing water to more readily flow through the spaces between the rocks.

After about 10 days or so the sturgeon larvae, with their yolk sac, hatch out (at about 0.5 inches or 12 mm long) and immediately burrow down between the rocks where they stay for another 10 days or so until their yolk sac is used up and to the point when they need to start feeding on their own. At this point they begin moving downstream at night under the cover of darkness to avoid predation and settle on to shallow sand and pea gravel bars 200 yards (~200 meters) to perhaps 5 miles (8 km) or more below the spawning site where they live for their first summer feeding on benthic aquatic insects. At the end of their first summer the young fingerling sturgeon average about 8-10 inches long. Modeling I’ve done estimates that by the end of the growing season after spawning, each spawning adult female produces on average 12 fingerlings from the 500,000 eggs she laid 5 months earlier.

We don’t have a good way to assess how big the hatch was of any particular year in that year. We know how to capture migrating larvae at night; we can capture fingerlings in the river during their first summer; but the young sturgeon are pretty spread out in the river and are very difficult to capture in a consistent enough way for us to set up any kind of index stations to accurately measure year class strength. The best way for us at this time to get an idea of the size of a hatch is to wait until fish from a particular year class are recruited into the spear fishery 10-15 years after they are hatched.

Till my next report……….

Ronald M. Bruch, PhD
Upper Fox-Wolf Fisheries Work Unit Supervisor, Wisconsin Department of Natural Resources”

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