Producing Fluorescent Marks on Atlantic Salmon Fin Rays and Scales with Calcein via Osmotic Induction

Calcified tissues of Atlantic salmon in both larval and postscale developmental life stages became labeled with the fluorochrome dye known as calcein in 7 min by sequential immersion in salt and calcein solutions. I introduce the term “osmotic induction” to describe this process. At 47 d posttreatment, fish that had been exposed to osmotic induction of calcein as nonfeeding larvae exhibited mortality and growth equivalent to that of nonexposed fish (P < 0.05). Atlantic salmon parr (at 5 months posthatch) exposed to osmotic induction of calcein exhibited no mortality during a 120-d posttreatment period. In addition to inducing a calcein mark on fin rays of all larvae and parr, the procedure also produced a distinct calcein mark on scales of parr. A second exposure of parr to osmotic induction 90 d after the first produced a conspicuous 2-band pattern on scales which corresponded to the two separate marking events. Calcein marks are invisible to the unaided eye but were observed without sacrificing the experimental fish by employing a calcein detection device (patent applied for) and fluorescence microscopy. Osmotic induction of calcein marks is a potentially useful technique for batch-marking fish because it greatly reduces the amount of contact time between the fish and marking solution and produces an easily identified mark; this may enable salmon fishery managers and researchers to perform a variety of nonlethal fisheries evaluations not practical or possible using previous marking and tagging techniques.