The spatial distribution patterns of Caligus rogercresseyi and C. elongatus on Atlantic salmon hosts (Salmo salar)

The location of attached and mobile stages of Caligus rogercresseyi and C. elongatus on Atlantic salmon Salmo salar hosts was compared with that of the distribution patterns of Lepeophtheirus salmonis. The distribution of C. rogercresseyi was also compared in experimental and natural infections, and there were no significant differences for either attached or mobile stages between infection routes. Most C. rogercresseyi chalimi were located on the abdomen and post anal areas. Although the distribution of mobile stages was more homogenous there was a significantly higher percentage on the post anal area, 35% of all mobiles, compared with the salmon surface in the post anal region of only 3%. Significantly more attached stages, from 70 to 75%, of both Caligus species were located on the fins compared with mobiles. The mobile stages of both Caligus species had a predilection for the abdominal body. A higher percentage of attached stages of C. elongatus was located on the ventral fins and tail compared with C. rogercresseyi and, in contrast,significantly more were present on the body in C. rogercresseyi. However, there was no difference in the distribution of mobile stages of the two Caligus species with a significantly higher percentage located on the abdomen. In contrast, mobile L. salmonis were predominantly located on the back and head. Significantly more attached stages of L. salmonis were present on the dorsal fins and adjoining basal epidermis, 30% comparedwith < 2% in the Caligus species. These results suggest that C. rogercresseyi and C. elongatus show similar preferences for the host ventral body and fin locations and there is no direct competition for host substrate between C. elongatus and L. salmonis.