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Update from Ron Bruch on Feb. 20, 2011

February 20, 2011

It was a slow day on the lake today as expected with the snowy weather and degrading ice conditions.  Yesterday the DNR said there were approximately 1,800 shacks left on the ice, but I’m sure the numbers have decreased since that count.  As Ron states in his update below, spearing will end tomorrow on the Upriver Lakes, but will continue on Lake Winnebago.  Good luck to all remaining spearers!

This update has been posted with permission from the Wisconsin DNR and Ron Bruch:

“Winnebago Sturgeon Spearing Report – Sunday February 20

See Sturgeon Vignette below: “How do you know how many sturgeon are in Winnebago and how do you set those harvest caps?”

A few dedicated spearers braved the ice and snow today on the Winnebago Lakes bringing in a total of 26 fish, 23 from Lake Winnebago and 3 from the Upriver Lakes – Yes – finally – we have hit the 90% closure trigger for the Upriver Lakes. The 2011 sturgeon spearing season on the Upriver Lakes will end tomorrow Monday February 21 at 12:30 PM; spearers have until 1:30 PM to register their fish. On Lake Winnebago we need 201 adult females to hit the 90% closure trigger there and, given the impact the snowstorm today will have on reducing effort to near zero over the next couple of days, we expect the season on Winnebago may last through the full 16 days allowed by current rule.

Due to anticipated reduced effort we will be closing the following registration stations until further notice: Calumet Harbor, Quinney, and Harrison Town Hall. Spearers on the east shore of Lake Winnebago are asked to take their fish to Stockbridge to register them. We will watch to see if spearing pressure increases going into next weekend, and if needed, we will re-open needed stations at that time.

Largest fish of the day – a 106.5 lb, 69.5” F2 female from Lake Winnebago registered at our Quinney Station in Oshkosh by Caleb Kempen of Neenah.

List of totals by station and the largest fish registered at each station today is in the attached daily report for February 20th.

Totals by lake area and harvest category:

Lake Winnebago: 4 Juvenile Females
10 Adult Females
9 Males
23 Lake Winnebago Total for the Day – 988 total for season

Upriver Lakes: 0 Juvenile Females
1 Adult Females
2 Males
3 Upriver Lakes Total for the Day – 333 total for season

System-Wide: 4 Juvenile Females
11 Adult Females
11 Males
26 System-wide Total for the Day

1321 System-wide Total for the Season

“How do you know how many sturgeon are in Winnebago and how do you set those harvest caps?”

There are a number of favorite questions people ask when our crews are out tagging sturgeon during the spring run or registering sturgeon during the winter spearing season. One of the all time favorites is “What’s the biggest one you got today?” Generally it’s not too hard to remember the really big fish of the day on each crew, but sometimes when we don’t have a standout large fish, we have to think a bit to answer the question. Another favorite two question set is “How many sturgeon are in the Winnebago System? followed by “And how do you know how many can be safely harvested each year?”.

The portions of the sturgeon stock that we annually develop population estimates for are the adults – figuring separate estimates for adults females and males. The adult females are the most vulnerable to spear harvest because of differences between adult female and male migration patterns and spawning cycles which results in a disproportionate amount of females in Lake Winnebago during the winter (remember that although most of the adults spawning this spring, were already out of Lake Winnebago before the spearing season started, more adult females will be in the big lake since only about 25% of them spawn each year compared to about 70% of the males that spawn each year).

To estimate the number of adult fish in the population each year we capture as many males and females we can during the spring using dip nets while the fish are crowded into their spawning sites. Typically we capture and tag between 1000 and 1500 fish each spring out of which 100 to 150 are females. All the fish are spawning adults so determining the sex is fairly straight forward (this is the only time you can definitively sex a sturgeon by just examining it without cutting it open). We then watch for these tagged fish in subsequent harvests for the next ten years. We account for tagged fish removed from the population by spearers through our harvest assessment, and also subtract 5.4% of the tags each year from each tagged group to account for natural mortality (fish dying from disease, boats, etc). I use three numbers then to estimate the adult population of males and females: 1) the total number of fish from each tagged group of spawners over the last ten years at large during the spearing season, 2) the total number recaptures in the spearing season from fish tagged over the last ten years, and 3) the total number of adult males and females in the harvest. I run this calculation separately for adult females and males to come up with an estimate for each, and also calculate 95% confidence intervals which tell us how good the estimates are. The estimates for 2010 were 15847 adult females and 31748 adult males.

I use an average of these estimates over the last 5 years to follow trends in the size of the adult female and male populations, and to set the harvest caps for the upcoming spearing season. I set harvest caps at 5% of the estimated number of adult female and males, and set juvenile females at 50% of the adult female cap. Estimates of juvenile stock are very difficult to develop as you can not sex the fish without looking inside them at that age and size, and we don’t capture many juveniles in our standard surveys. To ensure we do not overharvest juveniles and to protect the stability of the future adult stock, we set the cap at 50% of the adult females which has worked well.

Finally – where do we get the 5% maximum exploitation rate from?? This safe harvest rate was initially set by Gordon Priegel in the early 1960s based on research on the Winnebago sturgeon population and harvest done in the 1950s. This number has withstood the test of time and recently I was able to validate it through some mathematical modeling. Our exploitation limit of 5% is viewed by many around the world as a solid standard for managing sturgeon fisheries, and is an important factor contributing to the success of our Winnebago sturgeon program.

Till tomorrow,

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

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