Some observations on the reactions of sardine (Sardina pilchardus Walb.) to light under experimental conditions

Abstract

      The observations covered the effects on sardine behaviour of the total and nearly total darkness as well as radical changes in the intensity of white light in both directions. They have been done on groups of sardines. The fish used were in very good condition and did not show signs of sexual activity. The average lengths for samples taken from the catches used amounted to 14.3, 15.0 and 15.6 cm. The sardines were kept in rectangular tanks with running water, which were 107-120 cm. long, 42-57 cm. wide and 39-50 cm. deep. A large rectangular tank with the internal dimensions 295x119x120 cm. was also used. The experiments have been carried out mainly at temperature ranging from 19 to 24° C.

       In total or nearly total darkness sardines kept previously in low daylight illumination slowed down their movement, remained more scattered throughout the tank and lost equilibrium. A similar response of the fish was recorded after a sudden change of the white light intensity (tungsten filament bulb) from 100 m.c. to values too low for the means of measurement. When sardines were schooling before, as observed in the large tank, the school completely broke up (Figures 1 and 2).

       The sudden change of the white light intensity from very low values to 100 m.c. was followed by an immediate sinking of sardines near the bottom of the tank and some subsequent rise. Such a change troubled the fish and made their reactions to stimuli more violent, but did not hinder them from feeding.

        The critical lower light intensity, i.e. the light intensity below which no feeding took place, was very low as shown by the observations in the small tanks. In those tanks sardines took food at a light intensity below 0.2 m.c. The feeding then seemed to be, however, less intense when the white light prior to the experiment was maintained at 100 m.c. instead of 10 m.c. or at low daylight illumination.