An Engineering Approach To Trout Fishing

• UNDER THE TITLE "Busman's Holiday" we mentioned in our September, 1945, issue the measurements made by Mr. Robert F. Field of the General Radio engineering staff on water depth and temperature in Lake Winnepesau-

kee. Apparently there are a few engineers who like to spend their leisure hours in the highly optimistic pursuit known as angling, and some interest has been expressed by them in a more detailed statement of Mr. Field's investigation.

The project had two underlying motives, one economic, the other scientific. From the economic viewpoint it was desirable to find where the trout were in summer in order to increase the protein content of the family food supply; the scientific objective was to establish a relationship between water depth and temperature, and to find the deepest spot in the lake. Both objectives, we are happy to report, were attained.

That excellent fish, the lake trout, abhors warm water. Consequently, when summer comes and the water at the lake surface and in shallow areas reaches temperatures of 70 to 80 degrees Fahrenheit, he beats a hasty retreat to the deep water, where more comfortable temperatures can be found. At any lake, the old timers eagerly point out where the deep spots are located. Unfortunately, their stories seldom agree closely enough to permit the fisherman to troll through the exact spot and pull in the trout. At times, the stories follow a pattern and are obviously folklore. At others, a comparison of data from different sources produces more than a suspicion of organized conspiracy to suppress the true facts. An objective investigation, however, will invariablv settle the matter.

The necessary equipment consisted of a boat, a minimum-reading thermometer and a line marked at 10-foot intervals.

In the accompanying plot are shown the results of the investigation. For the first 30 feet of depth, the temperature depends upon the time of day and upon such factors as the weather for the previous several days. The two upper curves show the difference between late afternoon and the following morning, and the lowest curve is the residt of several days of cool, rainy weather. Below these surface differences, the ultimate depth does not affect the temperature-depth relationship until a 40-foot depth is reached. At greater depths, the data follow two well-defined curves. The upper curve is for ultimate depths of about 100 feet or less and a minimum temperature of about 50 degrees. When the ultimate depth is around 150 feet, the temperature below 90 feet drops to 41 degrees and stays constant thereafter. The 41-degree water is where the lake trout are found. This temperature is close to the theoretical value of 39.2 degrees where water has i Ls maximum density.

From the plot, it is evident that, if the temperature readings fall along the upper curve, no 41-degree water (and no trout) will be found at that spot. If the readings follow the lower curve, how-

Temperature of water as a function of depth. Near the surface, the temperature depends upon the time of day and the weather for the preceding several hours, and the three branches ofthe curve illustrate the differences encountered. At depths below about 40 feet the curve has two brar' "V, one for ultimate dep. jf about 100 feet, the other for ultimate depths of 160 feet or more.

ever, a deep area at depths up to 150 feel or more is indicated. It should be noted that 41-degree water is possible at ultimate depths less than 150 feel if the spot is spring fed. From the fisherman's viewpoint, this is quite satisfactory, since temperature, rather than depth, is his primary concern.

In summary, it can be said that the results were eminently satisfactory. In the deep spots located, lake trouL were plentiful, and a 159-foot depth was determined to be the maximum in that part of the lake covered by the survey.

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