Acute Toxicity of Methoxychlor to Larval White Sucker (Catostomus commersoni) as Modified by Age, Food Availability, and Egg Preexposure

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Abstract
Methoxychlor is currently pulsed into several western Canadian rivers at 0.30 mg∙L−1 to control the emergence of blackflies (Diptera: Simuliidae). A 2-h exposure of white sucker (Catostomus commersoni)) eggs to 4.83 mg methoxychlor∙L−1 at 1, 3, or 6 d postfertilization increased the incidence of deformities in both 1-d-old and pooled 3- and 6-d-old eggs (2.9 and 3.1%, respectively) compared with control eggs(1.1%). Larvae from exposed eggs showed a 2-d delay in the onset of exogenous feeding. Exposure as eggs increased methoxychlor tolerance. Respective 168-h PE LC50s (the 2-h pulse-exposure concentration causing 50% mortality over 168 h) for 2-d-old larvae from control eggs and eggs exposed for 2 h to methoxychlor at 1, 3, or 6 d postfertilization were 0.36, 1.04, 2.05, and 2.67 mg∙L−1 Methoxychlor toxicity in control larvae was modified by age; 10-d-old larvae were less tolerant than 2-, 18-, and 26-d-old larve, with respective PE LC50s of 0.04, 0.36, 0.34, and 0.20 mg∙L−1. After the onset of feeding, food availability modified toxicity. The96-h PE LC50sfor 18-d-oldfedand unfed larvae and 26-d-old fed and unfed larvae were 0.34, 0.032, 0.20, and 0.038 mg∙L−1, respectively. Swimbladder development was arrested in 2-d-old larvae exposed for 2 h to 0.10 mg methoxychlor∙L−1 at 9 d of age. White sucker survival may be at risk in methoxychlor-treated rivers.