3. 9. Types of data

freeze-brand

Freeze branding is an efficient way to mark a large group of fish with the same identification code (Mighell, 1969). A metal branding tool is cooled with liquid nitrogen, and the fish are pressed against the tool. This method does not distinguish among individuals, but release groups can be distinguished. This allows for the determination of release site and release time of recaptured fish. In general, the data acquired from freeze brand fish are the number of fish collected during discrete collection periods.

PIT tag

PIT (passive integrated transponder) tags are used to monitor individual fish. The tag, 12 mm long, is inserted in the fish's body cavity and contains a microchip that is programmed to contain individual fish identification codes (Prentice, et al., 1990). At monitoring sites the tag emits a signal in response to excitation from an interrogation system. The signal is decoded to yield information about instantaneous passage times of individuals. The tags do not seem to adversely affect the fish in terms of survival or swimming performance (Prentice, et al. 1990).

radio-tracking

Radio-tracking has been used successfully in the past to monitor movements of migrating juvenile salmon. Atlantic salmon (Salmo salar) have been radio-tagged and monitored in the northeastern United States (Stasko, 1975), Canada (Brawn, 1982), Norway (Holm, et al., 1982) and Scotland (Tytler, et al., 1978). Several studies have also been performed on juvenile salmonids in the Columbia River Basin (Giorgi et al., 1985; Stuehrenberg et al., 1986). Two qualitatively different types of studies have been performed. In the first type (e.g., Giorgi et al., 1985), individual fish are followed with their position being noted at relatively frequent time increments to create a radio track. This allows one to analyze individual behavior on a relatively fine scale. The other type of study (e.g., Stuehrenberg et al., 1986) involves releasing a group of fish and recording the arrival time of individuals at receivers located at fixed sites downstream. Many more fish can be included in this type of study, with information about the distributions of groups of fish being obtained.

The question of whether internal radio tags affect the migratory behavior of juvenile salmon has been addressed in at least two studies (McCleave and Stred, 1975; Stuehrenberg et al., 1986). Both studies determined that there is no effect of internal radio tags on the swimming stamina of juvenile salmon, although buoyancy may be affected. Also, the latter study made qualitative observations of swimming behavior and concluded that there was no difference in the behavior of fish with dummy tags and control fish.

hydroacoustics

Several studies have employed fixed location hydroacoustic transducers to monitor the abundance of juvenile salmonids (Johnson, et al., 1985; Dawson, et al., 1984b). The procedure is to attach transducers at the base of a dam and to point it upward through the forebay. The instruments yield an estimate of the density of juvenile salmonids migrating at specific depths (Dawson, et al., 1984a). The equipment cannot distinguish among species.