I’ve been a Lake Michigan “coho” fisherman since 1979 and a charter captain on Lake Michigan since 1998. I hold a degree in wildlife biology and worked for the Indiana Div. Fish and Wildlife for 38 years. Being old and battle scarred doesn’t make a person an expert, but even a career custodian at MIT probably knows a bit more physics than the average guy on the street.
Lake Michigan cohos have been migrating to the south end of Lake Michigan since 1968 (since they were first stocked in 1966). At the time the lake was swarming with both alewife and Diporea so they didn’t migrate to the south end due to food; rather, to seek out comfortable temperatures.
Also, they don’t move to the southern end in the spring (or late winter). They actually move to the southern basin in late autumn - migrating ever southward as the lake cools from north to south, stopping when they meet rapidly cooling waters from the shore and then, with no other place to go, they overwinter basically in the area just north of where IN, IL and MI come together. I’ve caught them out there - we called them “next year’s coho” - in December, back when I was young, foolish and obsessed.
As the shallows warm nearshore in late February a portion of these fish move from offshore to nearshore areas - almost entirely restricted to Indiana shoreline areas - often concentrated in warm water areas - industrial discharges and stream outflows.
These fish are not yet “fish eaters” for some reason. I’ve caught hundreds if not thousands of them in warm water areas in the late winter and in shallow water areas from the IL line to the Michigan line in early spring (say the first 10 days of April) and I can count the number of minnows or other small fish I've found in their bellies - even in warm water areas where shad, shiners and other small fish are present. If there is anything in the coho at this time of year, it’s tiny unidentifiable black bugs. I truly believe for many of these cohos, the first fish they ever tried to eat in their life was the lure at the end of my line.
When alewives were abundant in the lake (and thus here in Indiana) the cohos came to shore in late winter to warm up, the alewives showed up to spawn in mid-April, the cohos learned to eat alewives and hung around until early May and when the alewives left (and the nearshore waters warmed into the mid-50s) then both the alewives and cohos headed north to cooler waters and to follow the alewife spawn on north.
Not all of the cohos migrate into the Indiana shallows in late winter. A portion simply stayed out in the offshore areas, and ate whatever bugs or shrimp or diporea they could find. Witness the years when the nearshore waters warmed early, there were no baitfish nearshore and the cohos left, often in the second half of April. If you wanted to catch a coho it was necessary to head offshore to the areas where the cohos had wintered and fish for the fish which hadn’t migrated to nearshore areas.
You could easily differentiate between fish from the two “schools.” By the time the nearshore fish deserted the shallows they’d grown from 16-inchers, barely over the one pound mark to three or four pounders. Once they switched to an alewife diet, you could almost see them grow day to day, certainly week to week. Offshore, however, the fish which didn’t migrate to shore didn’t have alewives to eat, didn’t grow rapidly and a two-pounder was a good size.
When the alewife decline was at its worst, the cohos still came to warm up, quickly deserted the Indiana shallows (often by the third week of April) and presumably just rejoined their brethren offshore where, when the offshore waters finally started to warm, the possum shrimp and other zooplankton started to bloom and for lack of anything better to eat (like alewives) that’s what they ate.
Last spring alewife numbers in the south end of the lake in April and May were suddenly back to historic numbers. The cohos didn’t desert our end and in fact, most of them stayed here longer than normal to the detriment of anglers up north of Chicago and into Wisconsin.
I had some customers who fished with me a couple days in late May, caught plenty of cohos the first day, a few more the second day and then wanted to catch some trout. So we pulled lines and headed offshore where there were plenty of lakers and we still caught a few cohos, but the offshore salmon were little two-pounders. Again, I would guess the first fish they tried to eat was my lure.
I don’t know what “ocean” cohos eat, but I surmise there’s a time in their life cycle they make the switch from eating bugs and krill to herring or whatever other bait fish are around. In the Great Lakes they eat bugs and shrimp then switch to alewives. Those offshore cohos gorged on possum shrimp aren’t eating them because they like them or even flourish on them. They are alewife-starved.
