Page 860 - The Toxicology of Fishes

840 The Toxicology of Fishes

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Extract Dose–Response in Lake Trout
LD50 = 108 pg TEQ per g egg
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Fry Mortality (%) 60

40

20

0.1 1 10 100 1000
Dose (pg TEQ per g egg ww)

FIGURE 21.5 Embryo/fry mortality caused by graded doses of an extract of lake trout collected in Lake Michigan in
1988 injected into freshly fertilized, hatchery-derived lake trout eggs. Doses of the extract (TEQs) were based on the
measured concentrations of HAHs and an additive model of toxicity. (Data are from Tillitt, D.E. and Wright, P.J., Organo-
halogen Comp., 34, 221–225, 1997.)

rainbow trout-specific REPs)—were 1.8- and 1.3-fold greater, respectively, than those for TCDD (200
pg TCDD per g rainbow trout egg and 74 pg TCDD per g lake trout egg). Although these results suggest
a less than additive interaction for the TCDD-like PCB, PCDD, and PCDF congeners in the mixture,
the TEQ LD and TCDD LD values for each species were only 30 to 80% different (Walker et al.,
50
50
1996). Considering all of the potential variation in the model variables, this is a small amount of difference
among the predicted and actual toxicity; consequently, additivity appears to be the appropriate model
for HAHs in early life stages of salmonines. Furthermore, when the lake-trout-specific REP for PCB
126, 0.003 (Zabel 1995c), is used instead of the slightly more potent rainbow trout-specific REP for
PCB 126, 0.005, to calculate the TEQ concentration of the mixture, then the TEQ LD and TCDD LD 50
50
values for early-life-stage mortality in lake trout are no longer significantly different (Cook et al., 1997).

Symptoms of TCDD-Induced Early-Life-Stage Toxicity in Salmonines
Time Course

Lake trout embryos exposed to TCDD as fertilized eggs show stage-specific periods of sensitivity at
hatch and later during the sac fry stage (Spitsbergen et al., 1991). The stage when toxicity occurs is
determined by the egg TCDD concentration. At TCDD concentrations in eggs that are at or exceed the
LD , toxicity is generally manifested at the time of hatching and is characterized by a high incidence
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of mortality. The affected lake trout embryos are typically incompletely hatched and have significant
yolk sac edema. On the other hand, at lake trout egg concentrations of TCDD below the LD embryos
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hatch successfully and signs of toxicity are delayed until later during the sac fry stage. Mortality generally
occurs from middle to end of the sac fry stage. Once the yolk sac has been absorbed and the lake trout
fry begin feeding, TCDD begins to be more rapidly eliminated. Mortality during the fry stage is typically
very low. Thus, the critical period for early-life-stage mortality in lake trout is from about one week
prior to hatching until the end of the sac fry stage. There is typically no mortality in TCDD-exposed
lake trout eggs before about one week prior to hatch. This does not include induction of CYP1A, which