With care, it is possible to tag small fish with Visible Implant Elastomer (VIE) tags, and have little negative effect on the animal under study. Frederick (1997) reported 100% tag visibility and retention of VIE tags in small (8-56 mm SL) Hawaiian coral reef fishes for observation periods up to 76 days. Dewey and Zigler (1996) tagged juvenile bluegills (34-55 mm TL) with VIE and reported 100% retention for 6 months in three tag sites (the dorsal fin, the point of insertion of the dorsal fin, and the caudal peduncle). Haines and Modde (1996) tagged age-0 Colorado pikeminnow (mean = 50 mm TL) with VIE, tattoo ink, and fin clips. After 142 days, retention of VIE tags was considerably higher (87%) than tattoo ink (26%) and fin clips (34%). We have come to appreciate that VIE tags can be applied to a wide variety of small fish without compromising their growth, survival or behavior.
Handle with care!
Very small fish, especially juvenile stages, tend to be difficult to hold and susceptible to handling mortality. With careful attention to fish welfare, some practice, and an unhurried schedule, it is usually possible to tag with little or no mortality. It is nearly always necessary to anesthetize the fish before tagging. The fish need to be completely immobile, but still strongly gilling – pay close attention to this part of the process. Tagging small fish tends to take much longer than tagging larger fish, so be careful to anesthetize only the number of fish you can process safely.
Small fish can be difficult to grasp and hold firmly for tagging. If this is the case with your fish, consider laying them on soft wet sponge or cloth for tagging. Looking through a microscope may also be helpful in guiding the needle. After tagging, place the fish gently into fresh water and monitor them carefully. If they are not recovering well, make some immediate changes to your tagging procedure.
Select safe tag locations
When you tag small fish, the options are limited by body size. An excellent tag location in a larger specimen may be too small to even accept the needle in a younger fish, or the proximity to vital organs may make tagging risky. This means you will can not generate as broad a coding scheme for small fish as you might for larger fish. The most common solution for tagging small fish is to place the VIE tags into the fish’s body. One of the best tag injection sites in small fish is parallel to the base of the dorsal fin. Another excellent location is just behind the head, on the dorsal side. If fish had necks, this location would be the nape of their neck. The base of the anal fin and the caudal peduncle are often suitable. Tags can be placed in the clear adipose tissue behind the eye of some species, including most salmonids, provided the fish exceed about 50 mm TL.
Long term visibility
Obviously, you can only place a small tag in a small fish. If you tag juvenile stages that grow substantially after tagging, the tag may not remain visible for the life of the fish. As the fish grows, the tag can become obscured by tissue and pigmentation growing over it. For example, if you tag a 30 mm salmon weighing about a gram, it is unlikely that you will be able to find that tag when the fish is 900 mm long and weighs 20 lbs. On the other hand, if you are tagging a small species that won’t grow very much after tagging, Visible Implant Elastomer Tags are generally retained and visible for long periods.
Hundreds of published papers describe how to tag a variety of animals with VIE. NMT’s biologists are also available to help you. Please contact us at +1 (360) 764-8850 or email firstname.lastname@example.org .