Delayed Mortality

The July 2010 weather pattern has led to a keen interest in hot lake water and how it might affect the incidence of delayed mortality on muskies being caught and released. The short answer is that the odds for delayed mortality are higher because of the water temperatures and the only way to completely avoid it is not to fish until conditions improve. The fact that State and private fish hatcheries, as a practice, do not deliver and stock even channel catfish in 80-degree water temperatures because of increased fish mortalities serves as a real world example. Can muskie anglers still fish and use methods that reduce the odds for delayed mortality? Absolutely. I have written and presented techniques at seminars for many years on how to accomplish this. Fast water releases are the key technique. However, this post is aimed at what effect higher lake temperatures have on our ability to release healthy muskies that continue to grow bigger and be caught another day not just to see them eventually “go-down”. It’s important to note that delayed mortality may take days and be hardly noticed but frequently it occurs by the next morning after the release.

Delayed mortality may occur from the combination of stressors a fish experiences during a catch and release fishing event along with any associated tissue damage and blood loss. Our lake conditions since the Fourth of July have become a major factor because the sustained hot weather conditions have warmed the surface waters above 80-degrees (morning temperature) and caused that temperature to reach all the way down to the thermocline. The dissolved oxygen at the thermocline depth drops below 4 ppm. This is a key point because once the coolest water available to a “cool-water” species like muskies goes above the 80-degree range or below 4-6 ppm oxygen, the fish reach a stressful situation. That’s why the odds for delayed mortality go up under these conditions. The fish are already at the point of stress before being caught. A muskie stressed by warm water or low oxygen will continue to feed to a point, making them vulnerable to angling. In fact, being cold-blooded means their metabolism is greater at higher temperatures, but eventually the stress shuts their feeding down and they start to lose body condition.

For example, both Alum Creek and Caesar’s Creek Lakes currently have conditions where the 80-degree water has reached all the way down to the 4 ppm oxygen levels at the thermocline. With continued hot weather conditions, it will only become worse. The oxygen in warm upper layer the (epilimnion) that doesn’t mix with the colder water below it with little or no oxygen (hypolimnion) will continue to reduce in depth, making the thermocline shallower and forcing the muskies into hotter water and even more stressful conditions.

It’s no coincidence that some of our best muskie lakes have cool or coldwater sources, whether from springs, ground water, or cold feeder streams. The cold water helps provide summer refuge from extreme water conditions. A side effect of summer stress is reduced growth. The good news is that we commonly start to experience better conditions by the end of August. Alum Creek, for example, starts to de-stratify by the end of August in most years. We were fortunate last year to experience an abnormally cool summer with water temperatures that stayed below 78-degrees in August.

There are a few other facts to consider while fishing during summer if you choose to do so. Years ago delayed mortality was not an issue because most everyone kept the muskies they caught. Anglers quit fishing if they reached their legal limit. As Muskie Inc. members, most of us strive to obtain quality releases and our hopes are to avoid delayed mortality. However, we could be causing more mortality by fishing at the wrong time or with bad releases than someone whose goal is to keep muskies. How many of us stop fishing after just one release?

Immediate, or near immediate, mortality fish are easy to spot because they float belly up in the surrounding area of the attempted release. Fish caught from deep water that can’t be released immediately will experience air-bladder expansion which also causes them to float belly-up. If they are too weak to swim back down to neutral buoyancy, they will die in the hot surface waters. Some of you may have seen my muskie presentation where I show a technique that is a last ditch effort to save a muskie that experiences air bladder expansion. I use my anchor rope marked for the thermocline depth and clip the muskie by the front of its jaw using a very short 4 lb test line and a #10 hook and then lower it back down to just above the thermocline. When the muskie regains its strength, it will pop itself free and not float back up. Some key points if you need to try this: you must know the thermocline depth or at least the depth you caught your muskie at. Below this depth there may be no oxygen plus the fish will only reach neutral buoyancy at this depth. Choose a release site that is just shallower than the thermocline so the fish can rest on the bottom. If one releases a fish that is not strong enough to maintain swimming after pulling free from the release hook over open water, the fish can sink like a rock if water deeper than its original depth is present. Water pressure is what causes this effect. More compressed oxygen in the air bladder is needed to remain neutrally buoyant the deeper a fish goes. A fish that drops below the summer thermocline after a bad release will suffocate and will not float. They lay in the cold un-oxygenated “anoxic” water and are slowly digested by anaerobic bacteria.