9334

• UCREFRP Catalog Number: 9334
• Author: Stickney, R. R.
• Title: Editor
• USFW Year: Series
• USFW - Doc Type: 1993
• Copyright Material: YES

Volume 1 (Issue #3), 1993 X-RADIOGRAPHY IN FEEDING AND GROWTH STUDIES
feeding devices. When feed is supplied either from on-demand feeders, or in excess,
the estimation of feed intake requires that a labeled feed technique, such as the
X-radiographic method, be used (Talbot, 1985).
The estimation of daily feed intake by fish of different sizes at different water
temperatures provides the type of information required for the compilation of feed
tables (Jobling et al., 1991; Palsson et al., 1992), and such information can be
collected easily using the X-radiographic method. It is, however, well known that
there may be large day-to-day variations in feed intake by fish, especially if they have
been subjected to disturbance or environmental stressors. From this it follows that
the information from a single feed intake estimate should not be used uncritically.
If the aim of a study is the compilation of feed tables, repeated measurements must
be available for a number of groups of fish, the groups should cover a wide range
of fish sizes, and fish used in the feed intake studies exhibit normal growth rates.
The appetite of fish may be depressed for a few days following handling and
sorting procedures, or if the diet is changed abruptly. Because fish of several species
can be trained to operate on-demand feeding devices, the use of on-demand feeders
'enables changes in levels of feeding activity to be studied (Landless, 1976; Alanara,
1992a,b; Boujard et al., 1992). However, even though records from on-demand
feeding systems may give an accurate reflection of feeding activity, it is not possible
to say which or how many, of the fish within the group have been operating the
system. In addition, the records do not give any information either about the
numbers of fish that feed or about how much of the feed provided has been
consumed. The X-radiographic method, on the other hand, provides a suitable
means for investigating the recovery of appetite following exposure to changes in
a range of biotic and abiotic factors, and for obtaining records on the responses of
individual fish (Usher et al., 1991; Christiansen et al., 1992; Jtargensen et al., 1993).
For example, the abrupt transfer of groups of chary from freshwater to waters
of increased salinity led to a depression of feeding, with a decrease in feed intake
being recorded when measurements were made 3 days following the time of transfer
(Figure 1). The proportion of nonfeeding fish increased with increasing salinity
(Figure 1). When measurements of feed intake were repeated 30 days after the time
of transfer, no significant differences were recorded among the groups exposed to
the different salinity treatments (Figure 1). Thus, exposure to increased salinity
appeared to have had an acute, but short-lived, depressive effect on feed intake in
the Arctic chary (Arnesen et al., 1993). These results for chary differ from those
reported for Atlantic salmon smolts, in which appetite and growth were suppressed
for a period of at least 30 days following transfer from fresh- to seawater (Usher et
al., 1991). When studies are carried out using tagged fish, there is the possibility to
perform more in-depth examinations of the effects of changes in rearing environ-
ment on feed intake and growth. For example, although exposure to full-strength
seawater may have an acute depressive effect on appetite in the majority of the fish
within a group, pronounced and persistent reductions in feed intake and growth.
may be exhibited by certain individuals. Use of the X-radiographic method highlights
this variability, and enables these interindividual differences to be examined with
respect to the morphological and physiological characteristics exhibited by indi-
vidual fish.
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