Fishing the Scum Line

 

 

Scum may produce visions of filth, decay, perhaps disease, for most people but mention scum to the Lake Michigan fisherman and an entirely different mental picture is created.  Here we see placid conditions, constant action with Steelhead trout leaping many feet clear of the water on the end of their fishing lines.  The scum line is one of Great Lakes fishermen’s fondest dreams.  So just what is the scum line?  How is it formed and how can the fisherman find this perhaps fondest of his dreams?

 

Let’s go back to the  previous discussion on the thermal properties of water and envision Lake Michigan at the coldest  part of the winter.  Its not frozen over, but there is ice floating around in it, being blown about by the winter winds.  Some Ice has accumulated along the shores, grounded by the winds,.  One can see that there is a lot of cold water there, but just how cold.  The winter air temperatures  have cooled the waters and we know from above that the densest water (~37 deg F) has accumulated on the bottom of the lake.  How much of the water is 37 deg?  Well we know that not all of it is 37 deg, because if it were the Lake would be frozen over, but there is an awful lot of that 37 deg water out there.  I can remember fishing on July 4^th weekend one year after a long(many days), hard east blow.  Guess what, all the lighter, warmer water had been blown west and it was 37 deg from top to bottom out past 100 feet of water.  If that can happen in the middle of the summer, you know that there is a lot of that 37 deg water in the lake at the end of the winter.  To simplify this discussion however, let’s assume that the entire lake is filled with 37 deg water at winter’s end.  Then what?

 

As the lake is warmed by the Spring breezes, the warmer water, being less dense (lighter) accumulates above the colder more dense 37 deg water.  The prevailing westerlies pile this water onto the eastern shore.  A sudden temperature difference is noted as one travels from the eastern shore towards the west.  This temperature difference and the ecology of the lake create the scum line.

 

The scum line is an elusive beast.  At one time stream fishermen, who considered the Steelhead trout their exclusive property,  feared  scum line fishing populated not only by the Steelhead but also by huge crowds of lake fishermen equipped with every fishing device short of gill nets, would quickly depopulate lake Michigan of its Steelheads.  The scum line certainly creates a fantastic fishing opportunity, but it has proved to be very hard to locate and seldom a miles long phenomenon crowded with hungry steelhead.  But every once in a while …………………

 

How does the scum line form and why is the fishing so good there?  As the prevailing westerly winds blow across lake Michigan in the spring, the warmer lighter water is pushed against the eastern shore of the lake.  As this process occurs, a moving line of temperature difference is formed between the warmer water on the eastern shore and that great mass of 38 deg water which has filled the lake during the winter.  Why should this make a difference?  For starters there are  the  currents formed in the water by the temperature differences at the interface between the two masses of water.  The other factor is that life in these northern lakes has evolved to flourish in these cold wintry waters.

 

First let’s look at the currents.  At the interface between the warm and the cold, water is moving, cold water is becoming warm and rising.  Warm water is becoming cold and sinking.  This causes rising currents on the cold side and sinking currents on the warm side.  In between is a calm zone, the scum line.  In my experience I have seen a ten degree difference across only a few feet of surface.  In this area the currents were vigorous enough for me to easily observe the churning water around my boat.  Typically though, for the scum line, there may be a temperature difference of perhaps 5 deg or so, but across hundreds of feet of water.  Thus the currents are not seen but let us imagine them anyway. 

 

The stream fishermen considers Steelhead Trout to be his sole possession.  He looks at  the scum line, where the Steelhead gather far out in the lake from him with great trepidation.  In the Spring, the stream fisherman sits on shore far from the Steelhead trout who he imagines to be collecting in great numbers,  at the scum line, many miles from him, far out in the middle of the lake.  Soon, lake fisherman will also be arriving in great numbers,  To console the Steelheader, let us discuss the stability of the scum line.  For the line to form, those westerly winds must blow warm water across the cold 38 deg  lake water.  This, of course, is the typical situation found in the spring in the Great Lakes region.  So does this mean that the scum line can be found at any time during the Spring months.  Well….  While these winds are blowing, the movement of the warm water across the cold 38 deg mass of water supersedes any currents that may form at the interface.  This means that the scum line can only form on relatively windless days.  Even on these days, the scum line may not form everywhere since there may be enough local wind to blow it away.  Thus a real solid scum line that lasts even for a day is a relatively uncommon occurrence on the windy Great Lakes .  That’s the good news or bad news for you depending whether you’re a lake or stream fishermen.

 

So now we have our scum line, but is it only water moving around.  I have often said that fishing on the Great Lakes is like the Bedouin tribesmen wandering across the great wastelands of the middle east.  Every once in a while he finds an oasis which keeps him going.  The Great lakes  are like a fish desert.  The fisherman wanders around and every so often finds an oasis of fish which keeps him going.  What is it about the scum line oasis which keeps us going back for more.

 

The rest of the story is also related to the temperature/density behavior of water in the winter lakes of the north.  Life in these lakes and other cold waters has evolved to fit in with this behavior.  When I started fishing the Great Lakes I was introduced to a magic temperature of 52 deg F.  This was the comfort zone for the salmon and trout population in the water.  There is some variation from species to species, but in fact fishermen today use 52 deg as that magic number when fishing.  The most notable thing about this number is that it’s a significant amount distant from the aforementioned number of the 38 deg.  This means that the fishes of the Great Lakes don’t feel very comfortable during the winter.  In fact, being reptiles they need temperature variation in order to be active at all.  So when virtually the entire lake is at 38 deg they are inactive.  This inactivity is true of the rest of the food chain as well.  The plankton, the bait fish, the predator fish, etc, spend the winter in a relatively inactive state.

 

So what has this inactivity have to do with the scum line?  The water on one side of the scum line is cold, it may approach the 38 deg temperature of the most dense water in the early spring.  In any event it is usually well below that other magic number of 52 deg, the comfort zone for predator fish in the Great lakes.  This cold water contains forms of life.  Plankton and bug eggs are floating around in it.  In the cold water they are inactive.  But the currents at the scum line carry plankton and bug eggs from the cold water on one side of the scum line and into the warmer  water on the other side.  Here life proliferates.  Bugs hatch, Plankton becomes active and multiplies, the currents carry them towards the scum line.  The next step occurs when bait fish which feed on plankton are attracted to the area.  Steelhead trout which feed on bugs as well as bait fish and the other salmonids which feed mainly on bait fish are attracted to the dinner table.  The final step in this happy process occurs when the fisherman arrives to harvest the predators.

 

It is easy to see that  scum line fishing  is not a simple process.  It’s success depends on many variables.  On Lake Michigan, a steady west wind to pile the warm water on the eastern coast  begins the process.  This must then be followed by a period of relative calm, so that the currents created at the interface of the two temperatures can carry the plankton and bug eggs from the cold inactive water to the warm water where life can take over hatching bugs and multiplying the plankton.  Next the bait fish arrive, then the predators.  There is a lot of timing involved and all of the players need to be at the scum line before the final curtain can be rung.

 

The timing is the hard part .  The fisherman (that’s me and buddies) leaves port on the eastern shore of Lake Michigan. We travel west, keeping a close eye on the surface temperature.  In the Spring it is usually in the fifties or sixties, depending on how early it is or how long that  west wind has been blowing the warm water towards the east.  As we proceed to the west we are looking for a temperature break of about 5 deg or more.  If this break occurs gradually, then the scum line will not form.  However, if it occurs over a distance of a few hundred feet, this is the situation we are looking for.  The scum line in its self-descriptive glory may be see as it is approached.  A slick area is seen, with rippling water on both sides stretching from south to north.  The feeding fish are usually found on the surface in the scum line or on the warm side of it.

 

Sometimes the temperature break is found, but there is no scum line.  If the weather is calm, it means that the scum line has not yet formed.  This is bad news because the whole scum line process is just starting.  If the weather stays calm there will be good fishing here, but tomorrow.  If there is a little wind, it means one of two situations.  First, the bad news.  There is too much wind for the scum line to form, or to have formed, and we are out of luck.  The good news would be that there once was a scum line in the area, but the wind has destroyed it.  This is good because it means that there once was a concentration of fish and they may still be around.  Trolling back towards shore towards a disappeared scum line of yesterday may reveal a concentration of fish attracted by the remaining bait.

For the weekend fisherman, fishing around an old dissipated scum line happens much more often than finding a scum line in all its glory.

 

But, let us say that finally, all of the various conditions have occurred.  The temperature break, the oily looking scum area with bugs floating in it, predator fish may even be seen rolling on the surface.  Wonder of wonders, we have found the elusive scum.   Believe it or not, we can finally start fishing.  To Great Lakes fishermen who are used to doing most of their fishing at depths, the fishing is strange indeed.  One can just drag a bait along the surface and catch fish.  However, the back end of your boat can become crowded with fishing rods and tangles can proliferate.  Thus methods for spreading the lines out behind the boat have developed.  The one I like the most is planer boards.  These are contrivances made of parallel boards tethered to the boat and which  run far out to the side.  The lines are attached to releases which are attached to the tether lines.  When a fish hits the line is released from the tether.  The second type is called yellowbirds.  Here the boards are smaller, directly attached to the fishing line and held by a release in a geometry so that they run out the side of the boat.  When the fish hits, the boards  are released from this geometry and are dragged along as the fish is played.  Both of these methods allow many surface lines to be trolled simultaneously.  In contrast, the old standby, outriggers usually allow for one rod for each outrigger.

 

The baits used are the usual spoons and plugs.  Dodger and fly are not often used because the fish are hungry and don’t need a lot of action to entice them.  Since Steelhead is the main target, we find orange and gold colors to be most effective.  Surface fishing is enhanced by using plugs that dig in a little, like Rapalas and Hottentots.  A little lead can help to get the spoons down enough for the right action.  Having found the scum line, with feeding fish on it, the only question remaining is whether you have the fishing skill to land Steelhead, those leaping gymnasts of the fishing world.

 

A final word, this about getting back.  The scum line can form any time in the Spring as the water in  the Great Lakes warm up.  Its all about the wind.  The west wind is no problem, since it is the prevailing wind.  It’s the other winds and their strength which present a problem.  In the early Spring in Lake Michigan the winds are too strong and often tend to be from the south.  This is not conducive to the scum line.  Later in the Spring and early Summer  when the scum line tends to be a dozen miles or more from shore, wind conditions are better and  the scum line is more stable.  I have traveled 30 or more miles to find it in June and July.  Be sure the wind conditions are stable and you have a buddy with you on these long trips.  I can remember one time when I had to tow a a friend’s boat 20 miles back to shore .

 

So now you know about scum line fishing.  Though it is an elusive target, when you find those ideal conditions you will find some of the worlds best and most exciting fishing.