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
By combining periodic X-radiographic estimations of feed intake with the use
of tagged fish in growth studies, it is possible both to estimate feed intake and
measure growth of individual fish. This enables feed-growth relationships to be
examined in detail. There may, however, be large day-to-day variations in feed
consumption by individual fish, therefore it follows that daily feed intake should be
estimated on several occasions during the course of a growth study if a realistic
assessment of an individual's feed consumption is to be obtained (Jobling et al.,
1989).
The application of the X-radiographic method for- the investigation of feed-
growth relationships is demonstrated using data collected during a study designed
to examine the effects of prolonged exercise on feed intake and growth of juvenile
Arctic chary of 20 g initial weight (Christiansen and Jobling, 1990). Groups of fish,
comprising 50 individually tagged fish per group and 2 replicates per treatment,
were forced to swim at different speeds (between 0 and 2.0 body lengths per
second) for a period of 9 weeks. Individual feed intake was estimated on four
separate occasions during the course of the experiment and the average feed intake
of each fish was calculated. Specific growth rates, in terms of weight, were calculated
for each fish, and the data were then used in the investigation of feed-growth
relationships (Christiansen and Jobling, 1990).
Ill. PREREQUISITES FOR APPLICATION OF THE
METHOD
If the X-radiographic method is to be used to estimate feed intake, the marker must
be retained in the gastrointestinal tract for some time and not be rapidly passed
through the gut and defecated. Any defecation of marker particles in the period
between the time that a feed intake measurement started and the time that the fish
were X-ray photographed would lead to an underestimation of feed intake. In
studies with chary in which feed intake has been measured for periods of up to 24
h, the vast majority of the X-ray dense marker particles was always seen in the
stomach and the foremost part of the intestine. Very few fish having marker particles
in the hindgut are ever encountered. By contrast, when experiments were conducted
with Atlantic salmon held under the same conditions, marker particles were recorded
in the hind part of the gut of several fish (J~rgensen and Jobling, 1992). This suggests
that the marker particles may be passed through the gut of the salmon more rapidly
than through the gut of the chary, increasing the risk of defecation of particles, and
consequent underestimation of feed intake in experiments carried out on salmon.
Thus, direct counts of the numbers of particles in the gut may not give an accurate
estimate of feed intake in Atlantic salmon fed for extended periods, e.g., 24 h
(J~rgensen and Jobling, 1992). Thus, either the duration of feeding should be
restricted, or modifications should be introduced into the method of estimation in
order to correct for the evacuation of marker particles (Thorpe et al., 1990).
When repeated measurements are to be made on individuals, or groups of fish,
a problem may arise because several days are required for all the X-ray dense marker
consumed in one feeding bout to be completely evacuated from the gastrointestinal
tract. Obviously, all particles consumed by individual fish during the course of
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