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that marker evaluation studies probably are biased toward those showing
effects because results indicating no effects might be published less often. This
implies that our sample of the literature overestimates the occurrence of
marker effects in evaluation studies. However, the objective of our review is
not to determine how often marker effects occur, but rather to provide exam-
ples of the range and diversity of negative effects among marker types, species,
and sex, and thus encourage biologists to consider seriously the effects of
marking animals. Our review begins with these examples, presented by taxo-
nomic group in the following sections and associated tables.
Fish
TAGGING Marking has been used widely in fish population estimation;
accordingly, the earliest tests evaluating marker effects in vertebrates occurred
in fish. Historically, most evaluations of marking effects were anecdotal
(Mellas and Haynes 1985), but by the 1940s researchers were suspicious of the
potential effects of markers and thus began evaluating their merit in the field.
Early fish research often involved the use of commercially made plastic or
metal tags, and fish tagging was considered an effective marking system
because tags were inexpensive, easily applied and seen, and rarely lost by tagged
fish. However, studies evaluating potential effects of tags often found that tags
altered aspects of fish biology (table 2.1). For example, several field studies
used mark-recapture techniques and concluded that tags reduced survival and
growth of fish. In some situations (DeRoche 1963), negative effects persisted
throughout the life of a fish, whereas in others (Carline and Brynildson 1972),
the effects seemed to be short-lived. Tagged fish were found to experience
reduced swimming ability because of increasing drag (Clancy 1963), but not
all effects of tagging can be attributed directly to the tags themselves. For
instance, choice of tag placement on the fish's body can elicit marker effects
(Bardach and LeCren 1948; Stroud 1953; Kelly and Barker 1963; Rawstron
1973; Rawstron and Pelzam 1978), and it is generally considered that tags
placed in and around the mouth may interfere with feeding. It is notable that
not all tag evaluation studies have shown negative effects of tagging (table 2.1),
and with additional study some tags will be shown to be more appropriate
than others.
The recent development of passive integrated transponder (PIT) tags has
allowed researchers to mark fish and other vertebrates with smaller tags than
those used previously. PIT tags are electromagnetically charged microchips
implanted either subcutaneously or intraabdominally, and are read remotely
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