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Cataracts, growth and histopathology in juvenile chinook salmon (Oncoryhnchus tshawytscha) as influenced by dietary calcium, phosphorus, zinc and sodium phytate

University of British Columbia

In 1981 a severe cataract outbreak occurred in chinook and coho salmon stocks at several British Columbia and Washington State hatcheries. It was hypothesized that the cataracts resulted from a zinc deficiency due to high levels of dietary calcium and phosphorus in relation to zinc. This thesis, consisting of three experiments, was designed to investigate this possibility in juvenile chinook salmon. Experiments I (preliminary study) and III (comprehensive study) investigated the influence of wide variations in dietary levels of calcium, phosphorus, zinc and phytic acid (as sodium phytate) on growth, cataract incidence, appetite, food conversion, protein efficiency, health and histopathology in chinook salmon. Experiment II was conducted to determine whether chinook salmon were more susceptible to cataract formation at different times in their early life history. The test diets for all studies were casein-gelatin based. Levels of calcium, zinc and phytic acid (g/kg diet) ranged from 4.4-53, 0.034-0.400 and 0.06-25.8, respectively. Diets were formulated to have a calcium to phosphorus ratio of close to unity when disregarding the phytate-phosphorus contribution. In experiments I and III, high phytic acid content (21.1-25.8 g/kg diet) depressed growth, food and protein conversion, increased mortality and induced bilateral cataracts (zinc at 0.05-0.100 g/kg). Moreover, high dietary levels of calcium and phosphorus exacerbated the effects of high phytate and low zinc on cataract incidence. Cataracts were prevented however, when dietary zinc levels were ≥0.27 g/kg diet. In experiment II, cataract formation was noted to be dependent upon the time of exposure to a cataractogenic diet. For example, opacities did not appear until day 126 in fish which were fed the cataract diet between days 42 and 84. This knowledge of an apparent delay in cataract formation may be useful to hatchery managers when investigating the cause of a cataract problem. Diets containing ≥11.6 g phytic acid and 29.1 g calcium/kg induced anomalies in pyloric caeca! structure. Furthermore, the ingestion of diets containing a minimum of 25 g Ca/kg produced nephrocalcinosis in chinook salmon. Plasma, blood, liver and kidney zinc levels were directly related to dietary zinc concentration and, in the case of plasma and blood, inversely related to dietary phytic acid level. It is concluded that high dietary levels of phytic acid are detrimental to the health and performance of juvenile chinook salmon and that zinc is essential for normal eye development. Further, under the conditions of this study, cataracts could not be induced in chinook salmon fed high dietary ratios of calcium (or phosphorus) to zinc unless a strong mineral (zinc)-binding agent was also present.

Text

2010-06-30

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http://hdl.handle.net/2429/26061

Animal Science

University of British Columbia

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