Attached find a copy of the presentation I gave for the Walleye Searchers Club in Rochester MN earlier this week. I have added relatively extensive notes to each slide to try and provide the information I gave at the original presentation that may not be included in the slides themselves. Hopefully folks will find this interesting and informative. As always feel free to contact me if there are any questions.
Thanks,
Nick
Thanks for taking the time to post. Very informative.
Nick,
Can you please explain what is meant by 86 F being the incipient lethal temperature at which 50% mortality is expected based on thermal stress? Does this figure refer to hooking mortality? In other words, 50% of all walleyes caught in water with a temperature of 86 F or greater will die after being released? If so, it seems like catch-and-keep may be the best approach when fishing such warm water, and to limit the number of fish caught under such conditions. I suspect larger walleyes are much more susceptible to thermal stress than smaller fish as it related to hooking mortality.
Thanks for the great work.
Boone
Sure Boone.
Upper incipient lethal temperature is defined as follows:
“The upper and lower incipient lethal temperatures represent the temperature values beyond which 50 percent of the population can no longer live given an indefinite period of time (Giattinna and Garton 1982). A standard seven-day week is used as the lethal test exposure time (McCauley 1981). The incipient lethal levels define the upper and lower boundaries to the ”zone of thermal tolerance” within which there is no mortality from temperature (Figure 1).”
This is a pretty technical document, but lays out the thermal optimums and limits for many of the Great Lake s species both gamefish and others. (see attached)
So what it means in general terms is that if a fish is forced to remain in water that warm for an extended period (in the case of these studies a 7 day week) 50% of the fish are expected to die. Not from hooking mortality, but just from being forced to experience those temperatures. When putting together the presentation I believe I used the average of the results from a number of studies reported in the report cited below and attached as a pdf the highest reported values were from a study in Wheeler Reservoir Tennessee where the incipient lethal temperature for adult Walleye was 93°F. As with most things in biology this temperature is different in different systems and I do not know exactly where Lake Pepin stands on this gradient.
That being said while Pool 4 is relatively well mixed there is some thermal refuge in deeper water and water near incoming trout streams that can moderate the exposure to warm water, any additional stress from hooking or handling adds to a fish’s stress load and increases the likely hood of mortality. In 2012 for instance a hot summer likely led to fairly high mortality among large Walleyes on Lake Pepin. This was not necessarily angling related, but Walleye caught angling during this period no doubt suffered higher hooking mortality rates than those caught in the cold water temperatures of spring.
The attached pdf graph represents the hooking mortality calculated for the Nov 2011-Oct 2012 creel period on Pool 4. During July of 2012 we experienced surface water temps on much of Pepin in the low 90s. Though the average water temperature in the graph never reached that high it is taken at LD 3 and significant warming likely occurred during its time in Lake Pepin. The highest peak in hooking mortality corresponded with this peak in temperature despite the fact that there were approximately ten times more Walleye caught and handled in the March-May period than in July.
Citation (attached): Wismer, D.A. and A.E. Christie. 1987. Temperature Relationships of Great Lakes Fishes: A Data Compilation. Great Lakes Fish. Comm. Spec. Pub. 87-3. 165 p.
Nick,
Thanks. Keep up the great work.
Boone
You must be logged in to reply to this topic.
