External marks may be defined as a mark visible on the outside of the fish to identify individual fish or groups of fish without any information regarding reporting format.
Visible modification. (Fin clipping; Fin pinching, Punching operculum)
Pigments, dyes and stains -(Paint, Elastomers, Tattooing)
Brands-(Cold branding, Hot branding Laser)
Meristics and Morphometrics - (Scale patterns, numbers of vertibrae , number of fin rays, gill-rakers, truss-patterns, shape of scales or otoliths) See Internal external marks.
Pros and Cons
Fin clipping. including spines in a fin. This is probably the most simple form of marking fish. One or a combination of clips can be used to identify small groups of individuals or different groups. Cutting the adipose fin in often combined with CWT’s in salmonoids.
Fin pinching. A variant of fin-clipping is pinching of one or several fins in different patterns. Could be used for short-term markings on minor groups of fish.
Punching operculum. This could be used for mass marking of two different groups of fish, i.e. punching on reverse sides of the fish. The durability of the mark may be species dependant.
Pigments, dyes and stains
Paint - possibly florescent spraying (including colours in food). Marking by painting fish is used for mass marking of large cohorts. The colour, often florescent under UV-light, could be sprayed on the fish. The durability is species dependant, i.e. the scale structure needs to be rough to keep the colour fragments for longer periods.
Elastomers - coloured latex. Florescent coloured latex can be injected in the operculum of the fish, around the jaw or between fin rays, depending on the species in question. By injecting in different patterns, minor groups can be marked individually. This methods can also be used to separate two or more bigger groups of fish. Durable methods but after a certain period of time mostly visible under UV-light.
Tattooing - Panjet, chemical marks. Tattoeing by injection of coloured fluids can be used to separate groups of fish, i.e. to identify migration time of fish in rivers. Relatively short durability and applicability is species dependant.
Cold branding is applied by pressing a freezing (liquid Nitrogen) metal bar of a certain shape against the fish. Different patterns can be used to separate groups. Durability is species dependant but is applicable for short-term experiments. Used in aquaculture to identify individual male and females in broodstock and reported in some species to be visible for several years. Complicated patterns may become difficult to read after a certain time period due to partial regeneration.
Hot branding is applied by pressing a hot metal bar of a certain shape against the fish. Less effective than cold branding and presumably with shorter durability.
Laser this section is not complete
External marks are mostly used to identify a small number of individual fish or to distinguish between larger groups of fish and the techniques are suitable in field studies in relatively confined areas where recoveries of the marked animals are done by the institute which has carried out marking. External marking of individual fish has been done mainly in limited numbers. They are often simple and quick to apply but carry limited information. Several different external marking methods have been evaluated by Coombs et al (1990), Laufle et al (1990), Knight (1990), Nielsen (1988) and Moffet et al (1997). External marks have also been used as a means of calling attention to the presence of internal tags.
Pros and Cons