It can vary by days, but in Maine the middle of April always ushers in the annual smelt runs from coldwater lakes up nearby brooks and streams. As every salmon angler knows, these small rainbow smelts are the key food source for landlocked salmon.

Rule of thumb: no smelt, no salmon.

Back in the 1970s, my family and I experienced unprecedented, thrilling salmon angling at a remote northern Maine lake. It was an angler’s feast for three or four years. Then it shut down all together. It was like nature threw a switch and the salmon just disappeared.

What caused the salmon famine? “Smelt crash” was on the lips of many disappointed fishermen, who sought to understand the cycle, which was not unique in the annals of salmon angling.

What causes a smelt crash? What are the scientific and biological nuances that impact smelt numbers and, hence, salmon survival rates?

Greenville fisheries biologist Tim O’Brey and his staff have been working diligently to find answers to these questions in ongoing studies at Moosehead Lake, Maine’s most expansive sport fishery.

Put simply, they inventory smelt fry populations by netting them at Lily Bay Brook. From there, through “head counts,” they are able to extrapolate the gross number of smelt fry that head back into the big lake. Some years the runs have been 7 million, and some years as high as 9 million. Late in the summer they conduct more population surveys of the smelt survival rates on the lake.

O’Brey and his team also study plankton prevalence. Plankton is the main food source for the smelts.

O’Brey, in his words:

“Recently, we began work on a new project that we hope will shed some light on the biology of smelt. This work is not just relevant to Moosehead Lake, but could be useful for other big lakes in Maine. The concept was to develop a study to provide an index of abundance for young smelt and to help understand the natural variation that often occurs in smelt populations. There have been a few studies that indicate young, newly hatched smelt can be subject to high mortality if the correct food source (plankton) is not present during their initial two-week window after hatching and dropping out of the stream.”

This biological dynamic and interdependence among the salmon, the smelt and the plankton explains why game fish management is such a complex and fickle discipline.

As O’Brey explains in the following paragraph, angler agitation for increased salmon stocking on a lake is the wrong way to go if smelt numbers are on the margins. You might call it “protecting the seed corn.”

O’Brey explains:

“Finally, we hope to combine the information from the summer smelt tows with the plankton abundance, and the spring hatch numbers to detect any changes in abundance that could be related to lack of food. If we determine that the smelt are in low abundance, we could possibly reduce our salmon stocking to protect the growth of our game fish. Or if everything is peachy, we could increase our stocking to improve the catch while maintaining the good growth rates.”

Maine is, indeed, fortunate — not only to have a crown jewel like Moosehead Lake as a gamefish repositor, but to have seasoned, dedicated and knowledgeable fisheries stewards like Tim O’Brey.

V. Paul Reynolds is editor of the Northwoods Sporting Journal. He is also a Maine guide and host of a weekly radio program, “Maine Outdoors,” heard at 7 p.m. Sundays on The Voice of Maine News-Talk Network. He has authored three books; online purchase information is available at www.maineoutdoorpublications.net.