Magnitude and Causes of Smolt Mortality in Rotary Screw Traps: An Atlantic Salmon Case Study

Abstract

AbstractRotary screw traps (RSTs) are commonly used for collecting and holding fish swimming downstream. We used these traps to collect Atlantic salmon Salmo salar smolts from multiple rivers draining to the Gulf of Maine from 1996 to 2008. Limited prior evaluations suggest that RSTs cause only minimal mortality, but detailed evaluation of the potential for negative impacts specific to Atlantic salmon and other smolts has not occurred. There are concerns that RSTs contribute to smolt mortality via physical injury during collection or via exposure to stressful holding conditions within RST live‐cars. We evaluated the magnitude and causes of mortality associated with our use of RSTs and recorded the water temperatures and velocities present within the live‐cars at the times smolts were moving downstream. Of the 24,675 Atlantic salmon smolts collected, 48 (0.2%) were classified as trap‐caused mortalities. The most frequent causes of death were physical injuries associated with the clogging of traps with debris and overcrowding of the live‐cars. Water velocities within the live‐cars ranged from 0.09 to 0.57 m/s, and mean velocities never exceeded the published tolerance limits for Atlantic salmon smolts. The seasonal water temperatures within the live‐cars (range, 4.2–20.8°C) were not significantly different from those of the water columns (4.3–21.0°C) adjacent to the traps. Our analysis suggests that, under typical operations, RSTs represent a minimal threat to Atlantic salmon smolts, and we present methods to further reduce the risk.