Size Discrimination in the Brook, Brown, and Rainbow Trout

Abstract

FOR MANY YEARS, AQUATIC BIOLO-GISTSAND LAYMEN ALIKE have noted behavioral differences between brook, brown, and rainbow trout. However, experimental studies designed to determine the basic mechanisms which may contribute to or produce these variations have been sadly lacking. Therefore, conclusions pertaining to interspecies behavioral differences seem to be based upon anecdotal evidence for the most part.Some theorists believe that we may obtain species comparisons by determining the fish's ability to learn a sensory discrimination, i.e., to learn to perform a specified response to one stimulus and to inhibit a response to another similar stimulus.Reeves (1/•19), for example, demonstrated rudimentary color discrimination in fish by a modification of the discrimination learning paradigm. The present authors also hold that the ability of fishes to learn a sensory discrimination offers an adequate criterion by which behavioral differences may be studied. By the use of such methodologies it should then be possible to determine the relative psychological position of different species of fish. The purpose of the present study was to compare brook, brown, and rainbow trout in their ability to learn a size dis-Psychological Research Services, Inc., is under contract to the Michigan Department of Conservation. crimination. In addition, the effect of learning one size discrimination •pon the learning of a subsequent finer discrimination was determined for the three species. Subjects Group I consisted of 8 brook trout, group II of 8 rainbow trout, and group HI of 8 brown trout. The mean length of the fish in all groups was ?. 5 inches. All the trout had lived under almost identical conditions throughout their lives; all had been reared and hatched at Wolf Lake Hatchery, Mattaws. u, Michigan. This site also was utilized as the experimental site. Apparatus The apparatus used was a screen box 16 inches long, 9 inches wide, and 11 inches deep. The front of the box was constructed of brown fiberboard, ih which were cut windows 5 inches high and 3 inches wide. These windows were placed I inch from the bottom and sides of the box and I inch apart. Latches (not visible from the interior of the .box) were fixed above each window, on the exterior of the box.When they were down, the clear plate-glass slides containing the discrimination stimuli remained firm. ly in place; and when up, they could be readily pushed out by the fish.The discrimination box was suspended in a large aquarium with an independent water supply.The original water source and temperature were 2 6 JANUARY 19 5 6