Last week I ended my discussion in this column with the topic of predator-prey relationships. It is an ages-old process of the transfer of energy from one critter through many other critters so that the resources of one life type endures to sustain the lives of many others.

I had not been to Sand Lake in a while. I was there last month to inspect the ice conditions from shore.

Some open water shoreline areas and a few internal open water areas gave full notice that the overwintering ice was rotten and melting fairly fast. Then came a weather system a few days later with very strong winds.

Wave action from splashing open water accelerates the decay and break up of ice. Wind finished the job of ice removal. Soon all of Sand Lake’s ice would be history for another year.

Fast forward a few weeks. Warmer weather enticed some members of the public to take a walk at Sand Lake, just for the exercise and a reason to get out of the house for fresh air and enjoyment of whatever bird life may be offered.

That is how a large number of bald eagles were observed, perched in nearby tree tops or circling high overhead. Yes, the eagles were part of the spring migration story of these raptors preparing for northward travels.

In addition, a large flock, several hundreds to be more precise, of gulls were resting on the shoreline or sand deposits at Sand Lake. More than one nature observer called me to let me know about a large number of eagles and gulls.

Plus the callers added another observation to the list, namely a lot of dead fish along the shoreline of the northeast pool of Sand Lake. They said the fish were shad.

Bingo! For this biologist’s inquiring mind, this was a simple math setting where two plus two equals four.

Dead fish are a food source for gulls. Dead fish are a food source for eagles. It was time to investigate Sand Lake for myself while I toted my long lens camera to record what I might find.

I did find a lot of small to medium sized gizzard shad dead and floating across the pool surface. Along the shoreline wind action had accumulated many more dead fish in long windrows. Shoreline mud tracks indicated raccoons had been exploring the area well before I came upon the scene.

Out in the open water, there were many gulls. Binocular views brought details of all the birds floating on the water or circling overhead. A diagnostic marking on the bird’s bill was a black marking near the bill tip. This gull with a wingspan of about 48 inches was identified as a Ring-billed Gull, (Larus delewarensis). Okay, that ID was completed.

Gulls of this species like to take opportunities to dine upon food sources that are easy pickings. Dead gizzard shad fit the circumstances perfectly.

Many gulls were partaking in the food frenzy of floating dead fish. And a few gulls were busy doing a gull thing of stealing fish from another gull, a habit they like to use among their neighbors.

Several of these Ring-billed Gulls were flying from the pool westward to a larger resting flock. As the birds passed overhead, I used my long lens camera to attempt to capture a bird in flight. I did that, and subsequent details from the camera’s digital recording told me more about life cycles.

If you look closely at the inflight image of the gull, it has a fish in its beak with the back half of the fish sticking out. It was a take-out dinner for that gull.

There are other gull species that have and will be found at Sand Lake. The common American Herring Gull (Larus argentatus) is also a large bird.

It has a lot of similarity markings to the Ring-billed. However, the herring has a red spot near the tip of its bill, not black. The herring has a wingspan of up to 58 inches. Many gulls have feeding habitats that use scavenging as one large component.

Dead fish are easy to find and easy to break apart to eat chunks of meat. Gulls do eat many other food types from insects, rodents, earthworms and more. Fish living or dead are main menu items.

Free fish dinners were not lost on bald eagles. I observed at a minimum 15 eagles perched on surrounding tall trees. There were more high in the sky, and I am sure my ground based point of view could not see all the other eagles that may have been somewhere near or along the Iowa River both upstream and downstream.

As I stood on the river bank taking in the views of meandering river water, big sand bars and trees with swelling bud tips on their branches, an eagle flew downstream past me.

I focused my camera on the big bird. It was an immature bald eagle, all dark feathered with a nice yellow beak. I am sure some of the dead fish were collected by eagles.

I found some interesting facts about gizzard shad, the small fish of the Mooney family. This prolific fish species last fall found the deep pool of water at Sand Lake a good place to hang out for the winter. This pool was not connected to the Iowa River’s main channel a few years ago.

Now, due to natural erosion of a thin line of soils that once separated the river course from the old quarry, the pit pool has washed away. A few years ago during high flood water river flows, that thin line of bank soils gave away.

A huge rush of river water entered the northeast pool area. Fish could and did explore the new water, both from the river to the once landlocked pool, and from the pool out into the river. Mixing of fish species did take place.

Gizzard shad are very sensitive to water temperature changes of only a few degrees. Their tolerance ability can be compromised when outside air temperatures have a wide spread of very cold to mild and back to very cold.

That is what happened this winter. We humans endured a week of intense cold air. I found and read a science paper by Ichthyologist researchers concerning gizzard shad and cold stressing weather.

The paper found low mortality of shad if the water temperature stayed relatively stable above 46 degrees fahrenheit. If water temperatures decreased to 39 degrees or less as a result of very cold air over the water body, the fish were subject to high mortality.

A prolonged cold air weather event was just the ticket to kill Sand Lake gizzard shad. This fish species cannot acclimate to severe cold front air masses that affect the water temperatures to become even lower.

Gizzard shad are also near the northern edges of their natural range in mid Iowa latitudes. Gulls and eagles benefited from a die off of gizzard shad.

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The season of spring officially happened a few days ago on March 20. Hurray!

If we humans need a reminder of who is really in charge of the weather, just look back at the past week. Very warm air for a few days was followed by cold air, rain and even snowy conditions.

March gives up its hold on winter reluctantly. There is absolutely nothing new about weather gyrations during March.

Hang on for the ride on this roller coaster give and take. Enjoy.

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Garry Brandenburg is the retired director of the Marshall County Conservation Board. He is a graduate of Iowa State University with a BS degree in Fish & Wildlife Biology.

Contact him at:

P.O. Box 96

Albion, IA 50005