An analysis of prey detection in cutthroat trout (Salmo clarki clarki) and Dolly Varden charr (Salvelinus malma)

Laboratory feeding experiments showed that sympatric Dolly Varden charr (Salvelinus malma) were able to locate and consume prey targets at lower irradiance levels than sympatric cutthroat trout (Salmo clarki clarki) but their acuity was poorer. Reaction distance (RD) of both species to artificial and natural prey targets increased as irradiance level increased from a visual irradiance threshold (VIT), an irradiance level below which prey targets were not detected visually (3.0 x 10¹⁴ photons m⁻²s⁻¹ for charr and 3.0 x 10¹⁵ photons m⁻² s⁻¹ for trout) to a saturation irradiance level (SIL) which produced the maximum RD (3.0 x 10¹⁶ photons m⁻² s⁻¹ for charr and 6.6 x 10¹⁸ photons m⁻² s⁻¹ for trout). The VIT and SIL were independent of prey type and prey characteristics in both species. At all irradiance levels greater than the VIT of trout, the RD of trout exceeded that of charr for the same prey type. At a given irradiance level the largest RD in both species occurred in the presence of red irradiance followed in decreasing order by green, yellow and blue irradiance. Reaction distance of trout and charr increased with increases in prey size, movement and contrast. The percent increase in RD was greatest in trout. Between the VIT and the SIL the percent increase in RD was greater at higher irradiance levels in both species. At or above the SIL the percent increase in RD was constant. The central portion of retinas of trout and charr from sympatric and allopatric populations were examined by light microscopy. The two trout populations had a similar density of cone cells and higher than either charr population. Sympatric charr had the lowest cone cell density and allopatric charr intermediate density. Rod cell density showed the opposite trend, being highest in sympatric charr, lowest in sympatric and allopatric trout and intermediate in allopatric charr. The cone cell mosaic, consisting of a regular array of double and single cones, was the same in all four populations. Both cone types were smallest in the two trout populations, largest in the sympatric charr population and intermediate in allopatric charr. The degree of retinal summation was greatest in sympatric charr, least in the trout populations and intermediate in allopatric charr. In summary, histological studies indicated that sympatric and allopatric trout possessed the highest level of visual acuity while sympatric charr were the most sensitive to low irradiance conditions. Foraging velocity of sympatric trout and charr increased as irradiance increased reaching a maximum at the SIL of each species. From foraging velocity, RD and information on the diel irradiance regime in Loon Lake I estimated the volume of water searched visually for two natural prey types by trout and charr on a mid-summer day. Below the VIT, only charr were able to locate and consume buried prey targets, presumably by employing their chemosensory system. Differences in visual and non-visual feeding behaviour in sympatric trout and charr and in retinal structures of sympatric and allopatric trout and charr are generally as expected based on field studies of their vertical distribution and feeding habits.Science, Faculty ofZoology, Department ofGraduat