Effects of a Fixed-Crest Weir on Brown Shrimp Penaeus aztecus Growth, Mortality, and Emigration in a Louisiana Coastal Marsh

Abstract

We investigated the effects of water-control weirs on growth, emigration, and mortality of brown shrimp. Juvenile brown shrimp were captured, marked, and released in two shallowwater marsh ponds. One pond had a weir at its only exit. All surviving marked brown shrimp were recaptured as they emigrated from each pond. Shrimp remained in the weired pond 12–18 d longer after marking than in the unweired pond. Growth of brown shrimp was best described by quadratic functions and was significantly faster in the weired pond than in the unweired one although differences in growth may have been confounded by delayed emigration from the weired pond. Estimated monthly instantaneous mortality of juvenile brown shrimp, from marking to recapture, ranged from 0.550 to 1.582 in the weired pond and 0.877 to 1.270 in the unweired pond and apparently varied between years. Even though brown shrimp remained in the weired pond longer, peaks in emigration from both ponds coincided with new and full moons. Mean total lengths of marked brown shrimp at emigration ranged from 83 to 106 mm for the weired pond and from 66 to 84 mm for the unweired pond. Size of shrimp at emigration and time of emigration varied between years. Although brown shrimp emigrating from the weired pond were larger than those from the unweired pond, probably due to faster growth and delayed emigration, conclusions from concurrent research were that total biomass of brown shrimp emigrating from the unweired pond was more than double the biomass from the weired pond.