1. Introduction
Artificially-induced triploidy can be used to produce sterile salmonid fishes for Ž
. aquaculture and fisheries management Pepper, 1991; Benfey, 1996 . An important
physiological consequence of induced triploidy is the resultant increase in cell size and decrease in cell number observed for a variety of cell types, including circulating
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leucocytes Benfey, 1999 . Leucocytes are a critical part of the fish’s immune system, but the effects of changes in leucocyte size and number on immunocompetence in
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triploids has not been investigated. Small and Benfey 1987 and Yamamoto and Iida Ž
. 1995
suggested that the larger size of triploid leucocytes may confer increased phagocytic activity per cell, but any such effect is apparently offset by reduced leucocyte
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numbers Yamamoto and Iida, 1994, 1995 . Although it is well known that stress affects leucocyte numbers and reduces immuno-
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competence in diploid fishes Ellis, 1981; Barton and Iwama, 1991; Pickering, 1992 , virtually no information is available on the effects of stress on any aspect of triploid
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physiology. A previous study with triploid brook trout SalÕelinus fontinalis showed their stress response to be essentially identical to that of diploids, with respect to
changes in cortisol, glucose and hematocrit, when subjected to an acute handling and Ž
. confinement stress
Biron and Benfey, 1994 . Cortisol is the major corticosteroid produced by teleosts during stress-induced activation of the hypothalamic–pituitary–in-
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terrenal HPI axis, and is considered a principal component of the primary stress Ž
. response Mazeaud et al., 1977; Donaldson, 1981 . Increases in plasma glucose levels
and hematocrit are secondary responses to stress, associated with corticosteroid and Ž
. catecholamine release during the primary stress response Barton and Iwama, 1991 .
The purpose of this study was to investigate the effects of an acute handling and confinement stress on leucocyte numbers in triploid brook trout and rainbow trout
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Oncorhynchus mykiss , relative to diploid controls. In addition, various other hemato- logical and physiological measurements were made to verify and expand upon our
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earlier results Biron and Benfey, 1994 , namely plasma cortisol, glucose, and chloride levels, hematocrit, total blood hemoglobin levels, and erythrocyte numbers.
2. Materials and methods
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