Utilization of shoalgrass resources and nutritional ecology of wintering redheads in the Laguna Madre of Texas

The reduction of shoalgrass (Halodule wrightii) rhizomal biomass, attributable to redhead (Aythya americana) grazing, and redhead nutritional ecology was examined in the Laguna Madre of Texas during the winters of 2000--01, 2001--02, and 2002--03. Comparison of biomass trends across winter within exclosures where redheads were not allowed to forage and along transects where redheads may have foraged indicated that there were no significant (P<0.05) declines in biomass between exclosed and unexclosed areas in year 1. However, these results should be cautiously reviewed due to an improper sampling design in this year. In years 2 and 3 of the study, some sampling zones within the Laguna Madre showed biomass decline suggesting that redheads reduced biomass within these localized regions. In fact redhead foraging accounted for 33% and 26% of the overall shoalgrass rhizome reduction in years 2 and 3, respectively. Initial rhizome biomass was significantly different (P<0.0001) between sampling zones in all years with a mean of 49.96 +/- 7.02 g/m 2 in exclosures and 41.94 +/- 4.11 g/m2 along transects. Shoalgrass biomass, however, appeared to recover during the growing season from previous winter foraging, as biomass recorded in October of each year was significantly higher than biomass from the previous March. Redheads were distributed throughout the Laguna Madre and were recorded in all sampling zones, however, some zones received considerably more use than others and these areas were generally located on the eastern side of the Gulf Intracoastal Waterway, where water levels are shallower and shoalgrass density is higher. Body condition indices indicated that redheads were obtaining sufficient resources to maintain endogenous reserves throughout winter. Lipid reserves increased in all years for all sex and age cohorts however lean body mass decreased for all cohorts in year 1 and for juvenile females in year 2. This change in body mass was explained by a decrease in carcass protein. This study indicates that redheads reduce shoalgrass rhizome biomass locally; however, shoalgrass within the Laguna Madre is energetically capable of sustaining current redhead population levels. Management recommendations derived from this study are to continue conservation and enhancement of shoalgrass distribution throughout the Laguna Madre as declines of shoalgrass biomass could have detrimental effects on the continental redhead population.