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.

Effects of Slotted Water Control Structures on Nekton Movement within Salt Marshes

“…Water velocities at the WCS slots were too low (<0.2 m/s) to limit the migration of most nekton and only exceeded the swimming capabilities of larvae and small (e.g., <3 cm) juveniles (e.g., Mitchell 1989;Luckenbach and Orth 1992;Faria et al 2009). Individuals migrating out of managed marshes have been reported to be larger than those leaving unmanaged marshes (e.g., penaeid shrimps; Knudsen et al 1989Knudsen et al , 1996, ostensibly due to emigration impedance (resulting in long retention times), reduced predation, or decreased competition in managed marshes (Hoese and Konikoff 1995). Although we observed a similar pattern at another salt marsh (Kimball et al 2010), some emigrating individuals may have exited the marsh undetected by passing through large flap gates (open only during ebb tide) that could not be monitored.…”

“…Most studies comparing managed and unmanaged salt marshes show that transient species are less abundant in managed areas (Knudsen et al 1989;McGovern and Wenner 1990;Herke et al 1992Herke et al , 1996Rozas and Minello 1999) and suggest that nekton movement into managed areas is restricted by WCSs. Few studies, however, have directly examined saltmarsh nekton passage through WCSs of any type because collecting unbiased data at these structures is a challenge (Rogers et al 1994;Hoese and Konikoff 1995).…”

“…Exceptions occur for emigrating life history stages of some nekton species (e.g., penaeid shrimps; Knudsen et al 1985Knudsen et al , 1989Knudsen et al , 1996 and for individuals migrating in response to environmental cues (e.g., cold front passage; Herke and Rogers 1984). Observations of such phenomena are more likely to occur with approaches that involve near-continuous (e.g., daily) sampling efforts or that target a single species during critical life history periods (e.g., offshore spawning migration) rather than with efforts focusing on the entire nekton assemblage during specific seasons (e.g., Kimball et al 2010) or tide stages (present study).…”

Bioenergetic and landscape considerations for scaling expectations of nekton production from intertidal marshes

“…However, this does not account for losses to predators such as red drum and other sciaenids, which were abundant in the impoundment. Knudsen et al (1989) had earlier estimated mortality rates of brown shrimp in these same impoundments and, in the situation without weirs, estimated mean monthly instantaneous mortality rates of between 0.877 and 1.27. Assuming an average mortality rate of 1.07, net production would have to account for at least 2.9 times (e 1.07 ) the standing stock to maintain a constant value for shrimp biomass in the ponds.…”

Coastal Habitats of the Gulf of Mexico

“…Several factors may affect the accessibility of coastal wetlands to nekton including the frequency and duration of tidal or storm-driven inundation of intertidal habitats (e.g., marshes, mangroves, tidal flats) and the presence of structural landscape features (e.g., passes, creek channels, and ditches) that facilitate nekton movements (Saucier and Baltz 1993;Raynie and Shaw 1994) among otherwise isolated aquatic elements (e.g., lagoons, ponds, and impoundments) embedded within coastal landscapes (Knudsen et al 1989;Herke 1995). Unlike most coasts, which experience semidiurnal tides (i.e., two high and two low tides daily), much of the GoM experiences diurnal tides (i.e., 1 high and 1 low tide daily) as illustrated in Figure 6.16.…”