Research Product

The influence of food quantity on the effects of chlorpyrifos was determined in early life-stage (ELS) toxicity tests with estuarine sheepshead minnows (Cyprinodon variegatus). Three 28-day ELS tests were conducted simultaneously, each with a different feeding rate: approximately 20, 110 or 550 Artemia nauplii/fish per feeding. In the first group of three tests, growth was reduced significantly (p <= to 0.001) at nearly all feeding rates and chlorpyrifos concentrations tested (3.1 to 52 µg/L). Therefore, a second group of three tests was conducted at lower chlorpyrifos concentrations (0.4 to 6.8 µg/L) and the same feeding rates used in the first series. Chlorpyrifos concentrations that significantly decreased fish growth were greater than or equal to 3.0 µ g/L, regardless of feeding rate. Weights of fish at the end of all tests were directly associated with concentration and food. Fish receiving the greatest amount of food weighed 10 times more than those receiving the least and were three times heavier than those in the intermediate feeding rate. In treatments where growth was affected, mean percentage survival ranged from 67% at 52 µg/L to 99% at 3.0 µg/L. The standard deviations for this survival varied from 14 at the lowest feeding rate for fish exposed to 52 µ/L to 2.8 for fish fed 550 Artemia per cup in 3.0 µg/L. Bioconcentration factors (amount of chlorpyrifos in tissue divided by average measured water concentrations) and chlorpyrifos in whole fish at exposure concentrations >=3.0 ug/L generally increased with increasing feeding rates and increased chlorpyrifos concentrations. Within the feeding range tested, the quantity of available food was not an important factor controlling differences in growth of Cyprinodon variegatus exposed to chlorpyrifos. However, when food quantity restricted growth, survival of sheepshead minnows was not as reproducible, and variability (standard deviation) increased with decreased food.