Standing Crops

by Jerry Burton, NMT President

Ever wonder how many trout are in a stream? To a fisheries biologist that number is known as the standing crop that is measured in pounds per surface area of stream. So that the standing crop of trout in a stream has meaning, the America Fisheries Society developed a standard procedure on how the data should be gathered. While the techniques they developed can only be used on smaller streams, it will provide data that can be compared to data collected from many streams.To gather the data a reach of typical habitat on a stream is selected. The length of the reach is measured and several measurements of the width of the stream are taken. The upper and lower ends of the reach are blocked with seines and using an electro-shocker, the reach is shocked from the lower end to the upper end three times. All trout captured are placed alive in buckets and removed from the stream and are weighed and measured. With these data it is then possible to calculate what percent of an acre of stream was sampled and how many pounds of trout would have been captured if a whole surface acre of stream had been sampled. Of course the sample only represents a snap-shot of the trout population at the time the sample was taken. It would no doubt vary during the year and from year to year.

I often read in the popular literature that a stream, and especially a river, contains a certain number of trout per mile. While this type of information sounds good to the angler, it is at best a good educated guess on behalf of the biologist who conducted the study. The reason it is a good guess is that trout in large rivers are difficult to sample. While mark and recapture data may provide a general idea of the kinds and sizes of trout in river, it provides scant data on the standing crop of trout.

The chart above shows some of the standing crops of brown trout that were present in various New Mexico streams.

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