VFD simplified.

Embryos of grayling (Thymallus thymallus) were exposed to different concentrations of methylmercury (0.16, 0.8, 4.0 and 20 micrograms Hg l-1) during the first 10 days of development. The exposure resulted in body concentrations in the newly hatched fry of 0.09, 0.27, 0.63 and 3.80 micrograms Hg g-1 wet wt., respectively. A control group had a body concentration of 0.01 microgram Hg g-1. Morphological disturbances were only found in the highest exposure group. Three years later, at a size of 13.8 +/- 0.8 cm, the different groups were tested for sublethal toxicant effects on foraging behavior. In the first series of experiments we tested the foraging efficiency of the fish when kept alone for 5 min in small flow-through aquariums. In the second series of experiments we tested the competitive ability of eight individuals from an exposed group vs. eight individuals from a control group when kept together for 30 min in a 300-1 aquarium. In both experiments live Dapnia magna were used as prey. We found impaired feeding efficiencies and reduced competitive abilities in grayling from the exposed groups which as yolk-fry had Hg concentrations of 0.27 microgram g-1 or more. In the foraging efficiency experiments these groups were 15-24% less efficient as compared to the control group. In the competitive ability experiments the control group caught two to six times as many preys as these exposed groups. Such harmful body concentrations of Hg (> 0.27 microgram g-1) may be found in eggs from piscivorous fishes in lakes receiving diffuse atmospheric depositions of mercury. We suggest such concentrations may have ecological consequences by reducing the fitness of the affected populations.

Animal Feed↗

PubMed1998

Cataracts in farmed Atlantic salmon (Salmo salar) in Ireland, Norway and Scotland from 1995 to 1997.

From 1995 to 1997 cataracts were observed in Atlantic salmon (Salmo salar) in Ireland, Norway and Scotland at around the time of smoltification (when freshwater fish become adapted to the hypertonic seawater) in both fresh- and seawater fish. Over 38,000 fish were screened for the presence of cataracts. Posterior cortical cataracts were the earliest and most consistent change, followed by perinuclear, equatorial and anterior cortical cataracts. On histological examination vacuolation of the lens fibres was consistently present in the posterior cortex. The pattern of the outbreak suggested that a nutritional factor was involved although the variable incidence and severity of the condition indicated that a number of modifying factors may have been involved in the expression of the condition.

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