Variability in Estimating Abundance of Postlarval Brown Shrimp, Farfantepenaeus aztecus (Ives), Migrating into Galveston Bay, Texas

Three sets of monitoring data were used to examine the variability associated with abundance estimation of postlarval brown shrimp, Farfantepenaeus aztecus (Ives) in Bolivar Roads, Texas—the main connection between the Gulf of Mexico and Galveston Bay. Abundance of postlarvae (PL) caught with Renfro beam trawl varied greatly in different years on the same dates. A “spring peak” of brown shrimp PL migrating into Galveston Bay was found for 2 April with a quadratic regression fit to 6-day moving averages of daily mean abundance from 22 yrs of monitoring data: Ln(PL+1) = 0.8736 + 0.09037Day 0.0004934Day2 (adj-R2 = 0.83, n = 159), where Day is Julian day. Abundance varied by four orders of magnitude (0 to 24,616 PL/tow) in just 4 d during a four-week intensive monitoring of PL during the 1987 spring peak. Abundance also varied by three orders of magnitude between the North and South Jetty sites during the same collection time. During a third study, PL abundance varied by two orders of magnitude along 360 m of the beach in < 4 hr. These investigations demonstrate that detecting significant differences in PL shrimp abundance in a pass requires substantial sampling that may not be logistically possible. However, best estimates could be obtained by including as many dates as possible, followed by including more sites, and finally by collecting during both day and night. Conclusions drawn from abundance studies of PL shrimp, fish, and crab immigrants through estuarine passes that are based on only a few samples should be reviewed. Geoffrey A. Matthews NOAA Fisheries, Galveston Laboratory, 4700 Ave U, Galveston, Texas 77551, e-mail: geoffrey.matthews@noaa.gov IntroductIon The brown shrimp, Farfantepenaeus aztecus (Ives), is a key commercial species in the shrimp fishery of the northwestern Gulf of Mexico (GOM). Most adults inhabit water depths of 20-65 m (Darnell et al. 1983, Neal et al. 1983) and spawning and larval development occur in these waters. Postlarvae (PL) migrate into the bay where they grow for about three months in salt marshes (Zimmerman and Minello 1984). Then, as advanced juveniles or sub-adults, they migrate back through the bays to the GOM, during which time they recruit to the bait and bay shrimp fisheries. All shrimp fisheries are valuable, are managed based on age-0 individuals (J. Nance and F. Patella, pers. comm., NMFS, Galveston, TX), and are characterized by large variability in annual catches (Klima et al. 1986). It is beneficial to commercial shrimp fishers and resource managers to have a forecast of the upcoming harvest, and the abundance of immigrating PL is a potential indicator of shrimp harvest (Baxter 1963, Berry and Baxter 1969, Baxter and Sullivan 1986). Various attempts to establish an early forecast using PL abundance have been unsuccessful (Williams and Deubler 1968, Berry and Baxter 1969, Sutter and Christmas 1982, DeLancey et al. 1994). These forecasting models have relied upon three important assumptions: (1) mortality rates for young brown shrimp in the estuary are either constant or vary in a regular manner seasonally from year to year; (2) the majority of recruitment of PL shrimp to estuarine nurseries occurs during the same months each year; and (3) accurate estimates of PL immigration to bays and estuaries have been obtained. Mortality rates of juvenile shrimp can be highly variable on a weekly or annual basis, but few measurements of this mortality are available (Minello et al. 1989). Accurate estimates of the influx of PL might not be possible; even the precision of such estimates has been studied only to a limited degree (Berry and Baxter 1969, Caillouet et al. 1968, 1970, Lochmann 1990). Only about 60% of the age-0 shrimp recruit to the fishery during the early summer, the rest recruit mostly during the next four months. The PL for the summer recruitment enter the estuaries in late winter and early spring, and Berry and Baxter (1969) hypothesized that the magnitude of the spring peak immigration might control fishery recruitment for that year. However, during winter and spring Arctic frontal passages, when the water is chilled and blown out of the estuaries by north winds (i.e. during a “blue norther”), the immigration of PL is delayed (Wenner et al. 1998, Blanton et al. 1999, Benfield and Downer 2001). These events weaken temporally-dependent models, increase the variability in the rate of PL immigration, and increase the variability in estimated density obtained by sampling. Brown shrimp larvae grow and develop as plankton in shelf waters of the GOM, and many factors lead to a patchy distribution as the PL migrate towards shore and immigrate through passes into bays. As meroplankton, their distribution is governed by seasonal circulation patterns, shelf gyres, wind-driven coastal and tidal currents (Temple and Fischer 1965, 1967, Temple and Martin 1979), and by wind and temperature controlled upwelling and downwelling (Wenner et