Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin to early life stage brook trout (Salvelinus fontinalis) following parental dietary exposure

The toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the early life stages of F-1 generation brook trout (Salvelinus fontinalis) was determined, when dosed by maternal transfer. Effects were compared across six treatments, including a control. The experimental groups, based on TCDD concentrations in freshly spawned eggs (control, 41, 84, 156, 285, and 517 pg TCDD per gram egg [wet weight]), were established by treating 1.5-year-old brook trout with TCDD-dosed food throughout the period beginning with final gonadal maturation and continuing until just prior to spawning. Measurement endpoints assessed at various times throughout the study, included (1) TCDD and lipid concentrations in ovaries, eggs, alevins, and juvenile fish, (2) mortality, (3) fertility indices, (4) growth, (5) juvenile sex ratios, (6) pathology, and (7) cytochrome P4501A1 mRNA (CYPIA1) levels in embryos. The cumulative lethality dose response was determined just prior to hatch, at swim-up, and at the end of the study. The concentration in eggs causing 50% mortality (LC(egg)50) at swim-up and at the end of the study were 138 and 127 pg TCDD per gram egg, respectively. No treatment effects were observed on fertility, growth, or juvenile sex ratios. Pathologies were observed that have been previously reported in TCDD-exposed salmonid embryos, including edema in free embryos from all TCDD-exposed treatment groups at hatch. At the end of the test, juvenile fish from all treatments exhibited an increased prevalence of exophthalmia compared to the controls. The EC(egg)50 for this effect was 117 pg TCDD per gram egg. Histological analysis of the end-of-test juveniles, in gonad, nerve, vertebrae, neuromasts, liver, kidney, gut, gas bladder, skin, and blood vessels, did not indicate any significant pathology below the LC(egg)50. P4501A1 mRNA levels in whole 91-d postspawn free embryos from the 84 pg TCDD per gram egg treatment were 2.8 times higher compared with control or 41-pg TCDD per gram egg treatments. Results from this study were consistent with other published results of TCDD effects on salmonids, in which the exposures occurred after fertilization.