The Visible Implant Alphanumeric tag (VI Alpha) is a small bio-compatible, relatively rigid label, implanted beneath transparent tissue, and designed to identify individual specimens. Although many fishes have transparent tissue (adipose eyelids, fin membranes, clear boney tissue, etc.), tag retention varies by species. For example, the adipose eyelids of salmonids have generally proven to be suitable locations for these VI Alpha tags, but implants into larger masses of similar appearing tissue in mullet (Mugulidae) have been rapidly shed, (further discussion of adipose eyelid tissue). Size of the tagged specimens is also important: shedding rates from adipose eyelids of salmonids less than 150 mm total length have been excessive while retention in larger fish often exceeds 90%.
Tagging sites in other body locations may also be used successfully. Various sites on the head appear to be particularly useful ( e.g., mandible in walleye Stizostedion vitreum [Larscheid 1995], and elsewhere on heads as reported by Buckley [1994]). Dorsal, anal, and adipose fins also provide potential targets (Tipping and Heinricher 1993, Oven and Blankenship 1993, Wenburg and George 1995). Body undulation combined with the relative stiffness of these tags appears to limit usefulness in more posterior locations.
Figure 3. VI Alpha tag in the adipose eyelid of a rainbow/steelhead trout Oncorhynchus mykiss.
Advantages of VI Alpha tags: