A number of biological factors can directly affect the design and performance of an instream PIT-tag monitoring system. Some of the following questions may be appropriate:
For example, is information needed on all life stages year–round, or only on seasonal or migrational movements? Answers to these questions will affect system design. Minimum size of the study fish is another important consideration: a large fish can be tagged with a small tag, but the reverse is not true because of potential effects on growth, behavior, survival, and tag retention (Knudsen et al. 2009).
Tag size is directly related to reading range, with larger tags yielding greater ranges for any manufacturer or tag type (FDX or HDX). Greater reading range in turn allows use of larger antennas. However, if several tag sizes are used, antenna size will be limited by the maximum reading distance of the smallest tag.
For most fisheries applications, researchers use PIT tags that range 8–32 mm in length, with 12 mm being the most commonly used size. In the Columbia River Basin, an acceptable minimum size for study fish is 60–mm (fork length) for a 12–mm tag or 100–110 mm for a 23–mm tag.
Fish behavior may also affect the study design. Release of study fish near the interrogation site increases the chance of tag–collision, which occurs when 2 or more tagged fish are within the antenna energizing field at the same time. If both tags modulate the field simultaneously, neither tag is likely to be decoded correctly by the transceiver.
Releasing fish near the interrogation site also increases the probability that at least one fish will linger near the antenna. This can result in multiple detections of the same fish over an extended period. It also increases the potential for tag collision, since other tagged fish will likely enter the detection field while the resident continues to linger.
Repeated readings of a single tag can overload buffer memory, and for projects that rely exclusively on the transceiver buffer to store data, this can result in lost data (unless data is downloaded before being overwritten).
To decrease the impact of resident fish on the buffer, some transceivers have a delayed "unique" code function, which requires a preset time to elapse before a new record of the same tag is stored. However, activating this function will not eliminate potential tag collisions. The fact remains that if tagged fish are released near an interrogation antenna, there will be an increased chance of lost data.