Annual variations in the visual pigments of brown trout inhibiting lochs providing different light environments.

Visual pigments were extracted at regular intervals over the year from trout (Salmo trutta) inhabiting three Scottish lochs. Measurements of the spectral quality of the light in the lochs were also made. In all cases only A2-based pigment was found in the winter, with A1-based pigment appearing as well in summer. Fish from Loch Turret had significantly less A1-based pigment than fish from the other two lochs. Loch Turret differs from the other two lochs in being dystrophic, as opposed to eutrophic, and the light penetrating into it has more long wavelength energy. Possible correlations between this environmental difference and the visual pigments of the three trout populations are discussed.