Juvenile White Shrimp Growth, Mortality, and Emigration in Weired and Unweired Louisiana Marsh Ponds

Abstract

To study effects of marsh weirs on shrimp ecology, over 6,000 juvenile white shrimp Penaeus setiferus were marked with fluorescent pigments and released into two coastal Louisiana marsh ponds, one with a fixed-crest weir at its only opening, the other without. Marked white shrimp were recaptured as they emigrated toward the Gulf of Mexico from each pond. White shrimp growth was about 0.3–1.4 mm/d and 0.4–2.5 mm/d in 1983 and 0.3–0.4 mm/d and 0.5–0.7 mm/d in 1984 in the unweired and weired ponds, respectively. Growth was significantly greater in the weired pond. Mean time from release to emigration ranged from 31.5 to 47.4 d in the unweired pond and from 30.2 to 59.9 d in the pond with a weir. White shrimp emigrated significantly sooner from the unweired pond than from the weired pond in most experiments, but whether the weir actually delayed emigration was unclear. Peaks in emigration were associated with the occurrence of cold fronts. Instantaneous monthly mortality rates (Z ) from release to recapture were 0.76–1.13 and 1.45–2.08 in 1983 and 1.46–1.72 and 0.86–1.54 in 1984 in the unweired and weired ponds, respectively. The increased growth observed in this study did not compensate for reduced white shrimp numbers and biomass in the weired pond. If weirs and other structures must be installed in coastal marshes, managers could maximize white shrimp production by ascertaining optimum white shrimp densities, managing water control structures for immigration sufficient to achieve those densities, and ensuring safe, rapid emigration from the managed areas with the passage of cold fronts.