Age‐specific toxicity of copper to larval topsmelt Ather1Nops Affinis

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Abstract
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.