Detection and Retention of Postocular Visible Implant Elastomer in Fingerling Rainbow Trout

Short‐term (195‐d) detectability and retention of both green and yellow visible implant elastomers (VIE) in the postocular adipose tissue of fingerling rainbow trout Oncorhynchus mykiss were examined. Both colors gradually became undetectable over 186 d of observation when a blue‐filtered flashlight and amber glasses were used to aid in mark detection. However, with the use of ultraviolet light, one mark was detectable in 87% of a 60‐fish sample after 195 d, and the other mark was visible in 57%. Marking skill apparently varied between personnel and probably led to the difference in detection rates between colors. The mean total length of fish with two detectable marks was 231 mm, compared with 220 mm for those with one or no detectable marks, suggesting that detectability increased with size of fish at marking.