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  • Writer's pictureNew Wave Fishing Academy

Why Do Muskie Porpoise?

Updated: Oct 29, 2023

What is the weirdest thing you've seen a fish in the wild do? For us it has got to be when muskie porpoise. They seem to come to the surface with no apparent reason just to momentarily breach the edge with their back and tail only to go back down into the water. It's as if they want you to know that they are around and you still can't catch them!

This past weekend we saw not one, not two, not three, but 8 cases of this in a day on the water. While we have witnessed it in the past it was commonly a single incident rather than multiple in one outing. This got us thinking; what reason could they possibly have for doing this? After some quick research we reached the conclusion; nobody knows! There are a few theories out there in the angling community but no real backup for them or full breakdown on how the theory was developed. Due to this we thought it would be fun to do a root cause analysis on this behaviour and try to come up with our own theory!

To start off we need to identify the problem or in this case the behaviour:

"Muskies witnessed coming to the surface and diving back down. Not a jump out of the water or swimming with head out of water but a more of a roll on the surface as if to show you its there. Back and tails were spotted"

Next we need to identify the main factors which can impact fish behaviour to start trying to find some reasons which could be causing or leading to this behaviour. The main factors which impact fish behaviour are:

  1. Forage

  2. Oxygen

  3. Water Temperature

  4. Barometric Pressure

Now we will look at each category to brainstorm reasons that could cause the behaviour witnessed. This is a brainstorming part and we don't want to let any preconceived notions influence our conclusions so it is important to think of all possible reasons; we will assess and determine probability later.

1 a. Forage is on or near surface and muskie are feeding on them

1 b. Muskie are struggling with swallowing a large meal and end up on surface in process

1 c. Muskie are chasing forage near surface and breach is result of chasing

2 a. Muskie are trying to gulp air due to low dissolved oxygen content

2 b. Muskie are floating on surface to "catch their breath" by getting gills in oxygen rich layer (Bonus if current or wind)

2 c. Muskie are trying to burp out gases or improve CO2 exchange in gills

3 a. Trying to get in warmest water possible for digestion

3 b. Hot stagnant water causing low DO

3 c. Water is coolest on top (cooling trend) and this is more comfortable

4 a. Barometric pressure decreases rapidly and so more buoyant and forced to surface until adjust swim bladder

4 b. Fish just spent time in deep water and returned to shallows. Swim bladder very buoyant

4 c. Fish want to increase their buoyancy by gulping air (reason unknown)

4 d. Fish want to decrease buoyancy by expelling gases

Before we start evaluating some of these options it's important to understand what the environment was when the behaviour was witnessed. The more detail that can be recalled the better! You never know what detail may be the missing link to a new discovery.

"On our day we were in June under pre-storm front conditions, it was uncharacteristically hot (2 days) for this time of year with extreme humidity, little wind all season so far, higher than normal water temperatures(74-74F), weeds in areas from years past had not yet grown or were much smaller than normal, fog like smoke in the air from ongoing forest fires, and witnessed more in mid-day hours than morning. We did not catch or have any action from these surfacing fish. Later in the day bait was marked on sonar in the 20-25 ft zone yet fish were seen surfacing in 0-7 ft"

Some quick research on this topic leads us to believe that this behaviour is witnessed mostly in hot weather and that they are extremely difficult to catch when behaving like this. Others noted that in sections of river where there is a cool water inlet porpoising happens more often.

Now on to the fun part; let's try to make sense of the factors that could be playing a role in this behaviour.

1 a. Possible, Not Probable - During our outing the mayfly hatch was on and so there was a ton of forage on the surface. In years past this time of the season usually leads to a good topwater bite for smallmouth and both pike / smallmouth caught have bugs in their mouths. This is not as much of a Georgian Bay pattern but one for smaller inland lakes. While the mayfly hatch was on if these fish were actively feeding on the larvae or carcasses one would suspect they would be willing to bite a bucktail or topwater bait making a lot of commotion in their area. Despite seeing multiple surfacing muskie none of them even followed our baits. Perhaps we din't have the right presentation but some follows were expected if the fish had been actively feeding.

1 b. Possible, Not Probable - We have seen this behaviour before and the muskie tend to spend more time on the surface trying to swallow bait than the short roll we witnessed. In this case the fish just made a quick splash on the surface rather than swimming or sitting with the meal in its mouth.

1 c. Possible, not Probable - Despite seeing multiple surfacing muskie in an area none of them even followed our baits. Perhaps we din't have the right presentation but some follows are expected if the fish are actively feeding. In other outings we have witnessed muskie chasing bait or bass on the end of our lines but they would actively chase lures and bite once presented!

2 a. Possible & Probable - Muskies are physotomous fish which means they have a direct connection from their swim bladder to the outside world. This gives them the ability to regulate their swim bladder through their mouth. Salmon, Lake trout, whitefish, pike, gar, & catfish also have this ability. Bass, Walleye (Yellow Pickerel), perch, and crappie do not have this ability which can lead to barotrauma when removed from deep water too quickly.

Given the hot stagnant conditions during our day as well as underdeveloped weed growth and unseasonably warm temperatures there could have been serious potential for low dissolved oxygen conditions and the need for ski's to gulp some air! Higher dissolved oxygen content is available in nearby deeper water however not all fish will respond to stressors the same way.

2 b. Possible, Not Probable - Given the hot stagnant conditions during our day as well as underdeveloped weed growth and unseasonably warm temperatures there could have been serious potential for low dissolved oxygen conditions. However, If the fish were trying to obtain oxygen from topmost layer through gills they would have remained on the surface longer or continued swimming along the surface rather than an only surfacing briefly. Alternatively higher dissolved oxygen content is available in nearby deeper water.

2 c. Possible, Not Probable - Just like humans muskie breathe in oxygen and excrete carbon dioxide. Not releasing carbon dioxide or inability to release effectively can lead to carbon dioxide poisoning which is the same as suffocating. Rolling above the surface may allow a quick release of CO2 by shocking the gills to enter into a CO2 free environment promoting a high rate of excretion across the gill membrane. While this may be a reason for rolling on surface it is unlikely that the excretion into the water is insufficient unless trapped in a small amount like a fish tank or small pond in extreme heat.

3 a. Possible & Probable - Muskies are warm water fish. Their metabolism peaks in the heat of the summer and this allows them to digest food more quickly. Quicker digestion means there is more time they can spend feeding and building energy reserves. It is possible that the fish we witnessed were merely sitting on the surface soaking in the sun to aid in digestion but spooked once we got close (or started casting nearby) leading to the surfacing we witnessed. The top layer was warming considerably throughout the day.

3 b. Possible, Not Probable - As water temperature increases the amount of dissolved oxygen it can hold decreases. Given the rapid increase in temperature and lack of wind over the past month there could be some potential for low dissolved oxygen levels however on such a large body of water this seems unlikely. Despite weeds being less grown than normal there was still substantial green vegetation in the area producing oxygen. None of the vegetation we found was brown or decaying.

3 c. Not Possible - Given the current weather this option is completely ruled out.

4 a. Possible, Probable - Given the pre-thunderstorm conditions dropping barometric pressure was expected. Looking back at weather data from 7:30 AM until 3:00 PM the barometer dropped from 101.14 to 100.82 kPa. As the barometric pressure decreases the volume of the swim bladder expands and thus the density and buoyancy of the fish change. If this happens more quickly than the fish can regulate it will become positively buoyant and float towards the surface. Given all these fish were in shallow water it wouldn't take much upset for them to float towards the surface before they burp our excess air and return to bottom or depth they were occupying.

For those that are interested in the physics behind this we will oversimply it below. The buoyancy is the force created by the difference in density of water and an item. It is calculated by equation buoyant force = density water * volume object. If this force is greater than the mass of the object times the force of gravity the object will float upwards. If they are equal it will suspend, and if the force is less the object will sink. In muskie the swim bladder runs along the spine and can be as wide as the fish. While the mass of the fish will stay the same the volume will change based on how much the swim bladder inflates or deflates. Using some ideal gas laws a 0.32 kPa change in atmospheric pressure (for fish 10 ft deep) can lead to a volume change of 1% in swim bladder. The shallower the fish are sitting the more impactful this change is. For instance if the fish was in 3 ft the volume change would be 3.7%. It may not seem like much but a 1% volume change leads to a buoyant force increase of 1%. For a fish that was stable at a given depth this now means it will start to float if it cannot keep up with the rate of gas excretion needed to maintain buoyancy. More active fish may be able to regulate immediately but those in slumber or digesting food will react much more slowly.

4 b. Possible, Probable - Whenever there is a large change in pressure the volume of confined gases will change (container allowing). The swim bladder on fish is a gas chamber that expands and contracts to maintain a desired pressure by fish to rise, sink, or suspend. When a fish is in deep water it is under a lot of pressure. If it has been there for a while it is likely regulated to suspend at the depth it has chosen. If this fish quickly comes back to shallow water the pressure around it decreases considerably and the swim bladder will expand. This increases the volume of the fish and leads to larger buoyant force. In the event ski's are feeding in deep water and then quickly return to the shallows this increased buoyant force could cause them to float to the surface before they burp or excrete the air needed to suspend in the warm water shallows for digestion.

4 c. No Benefit, Not Probable - There is no scenario that we can think of where muskie would want to increase its buoyancy by trying to quickly fill its air bladder that would lead to an advantage for the fish.

4 d. Possible, Not Probable - Muskie do not need to wait until they are on the surface to expel gases. Expelling the contents of their swim bladder through a burp can occur while in the water however it may be less energy efficient than doing so on the surface due to the added back pressure of the water. The event described in 4a is likely the only scenario when a fish would try to expel these gases quickly and use the added buoyancy to float to the surface rather than swim before expelling.

Based on the above assessment it appears the most likely cause for the behaviour we noticed is due to either the quickly dropping barometer, the fish returning to the shallows quickly after feeding out deep, fish sitting on the surface in warm water, or a combination of all three. In fact is seems like the fish may have been sitting near the surface soaking in the heat trying to digest while the barometer was dropping making them more buoyant and easily susceptible to floating a few inches up before regulating or swimming back under; but this is just out theory.

Regardless of the true factor 2 of these 3 reasons are based on the muskies having eaten already before we saw them and preparing for digestion. This would make sense with why the fish are so difficult to catch as they are already full and looking to digest only. In either case the fish are unlikely to bite but we now know where these fish are and can return later in the day to try and catch these fish before they head back out to deeper water to feed. We may also want to consider targeting the deeper areas during peak feeding windows as the fish are likely not feeding where they were porpoising.

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