Abstract

Based on field observations of unexplained mass mortalities of the Louisiana red swamp crawfish in both North American and European culture systems, laboratory studies were performed to determine the effect of water temperature on pesticide toxicity. Five different water temperature regimes were established by the use of water-bath tanks with thermostat controlled heaters and coolers. The thermal regimes were 34, 27, 20, 14, and 7 degrees Celsius. Each thermal regime had 36 control and 72 experimental animals in separate, oxygenated glass containers. Two field grade pesticides (one herbicide 'Propanil' and one insecticide 'Malathion') were used to produce solutions which were at the industry-accepted LD/50/96. Experimental animals were acclimated to control water and then subjected for 96 hours to these pesticide concentrations at the indicated thermal regimes. Results indicate abrupt and extreme differences in toxicity due to the various thermal regimes. Malathion mortalities were far in excess of anticipated results at the higher temperatures while Propanil produced a step-wise increase inmortality in response to additional heat. Control group deaths due to acute thermal stress endorse the results of the pesticide mortality. Present crawfish industry guidelines on pesticide toxicity levels are routinely performed near 20 degrees Celsius. Field conditions in culture systems often exceed these temperatures and may influence the elevated mortalities found under those situations.

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