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The age‐specific sensitivity of topsmelt (Atherinops affinis) larvae to copper was assessed. A series of 7‐d growth and survival experiments were conducted using cohorts of larval fish isolated into different age groups of 0, 1, 3, 5, 7, 9, 11, 15, and 20 d post‐hatch. Fish aged 0, 3, and 5 d were less sensitive to copper chloride than fish ≥7 d old. The median lethal concentration (LC50) for copper ranged from 365 μg L−1 in 0‐d larvae, to 137 μg L−1 in 20‐d larvae. NOECs remained relatively constant for all ages: 180 μg L−1 for 1‐ and 3‐d‐old fish, 100 μg L−1 for all other cohorts. Regression analysis indicated a significant negative correlation between LC50 and gill surface area (GSA; R2 = —0.793) and cutaneous surface area (CSA; R2 = —0.760. Although these correlations were expected because both morphometrics increase with age, the relationships between increasing respiratory surface area and LC50 may indicate that the increase in sensitivity with larval age is related to an increase in copper uptake, either cutaneously or branchially. GSA increased more than sevenfold between hatch and 20 d, whereas CSA increased only threefold throughout the same period.

Abstract-A 7-d growth and survival toxicity test using the west coast marine mysid crustacean Holmesimysis costata (Holmes) was evaluated to determine test precision and sensitivity. The intralaboratory coefficient of variation (CV) among six reference toxicant test median lethal concentrations (LC50s) was 25%, whereas the mean intralaboratory CV for side-by-side effluent test LC50s was 7%. The mean intralaboratory CV for the concentration at which growth was inhibited by 25% (IC25) was 19%. Interlaboratory CVs for effluent LC50s averaged 14%, whereas variation among growth IC25s averaged 15%. Test precision and sensitivity compared favorably with literature values for a number of commonly used toxicity tests and chemical methods. Toxicity increased slightly with increased mysid exposure from 4 to 7 d (mean effluent LC50s of 9.9% for 4 d of exposure and 7.5% for 7 d), and more significantly from 7 to 24 d (zinc LC50 values were 50 g/L and 7.8 g/L for concurrent 7-d and 24-d tests). Although growth was a less sensitive endpoint than survival in tests with individual chemicals (zinc and sodium azide), growth was the more sensitive endpoint in seven of nine tests with complex effluents. Seventy-five percent of tests conducted at all participating laboratories met protocol test acceptability criteria (n ϭ 40).