Effects of Saline Water on Growth and Survival of Mottled Duck Ducklings in Louisiana

Abstract

We studied the effects of saline drinking water on the growth and survival of penned mottled duck (Anas fulvigula) ducklings at Rockefeller Wildlife Refuge in southwestern Louisiana in 1989. Duckling mortality at different salinities was 100% at 18 ppt, 90% at 15 ppt, and 10% at 12 ppt. No ducklings died within treatments of <12 ppt, but the growth rate of ducklings in the 12-ppt treatment was lower (P _ 0.05) than all lower salinity treatments. When ducklings were given 12-ppt salinity water and exercised, mortality would have exceeded 70%. Initial lethargy shown by ducklings in the 9-ppt treatment, combined with potentially high mortality of ducklings given 12-ppt salinity water and exercised, suggested that the upper salinity threshold tolerated by wild mottled duck broods lies within this range. We advocate creation and management of semi-impoundments in coastal Louisiana and Texas to prevent deterioration and/or loss of marshes of <9-ppt salinity to provide quality habitat for mottled duck broods and other species of flora and fauna dependent on these marshes. J. WILDL. MANAGE. 55(3):471-476 The mottled duck is a nonmigratory species occupying a restricted range in North and Central America. Isolated populations occur in Florida, central Mexico, and the coastal marshes from Mississippi westward to northern Veracruz, Mexico (Bellrose 1980). However, the greatest number (98%) of mottled ducks occurs in coastal marshes of Louisiana and Texas where there is an estimated breeding population of 110,000 (Stutzenbaker 1988). There is concern about the mottled duck because annual surveys conducted in Louisiana have indicated a 44% decline from 1971 to 1983 (Stutzenbaker 1988). Stutzenbaker (1988) hypothesized that the loss and deterioration of remaining habitat in both Texas and Louisiana might be responsible. Saltwater intrusion, in particular, has caused significant habitat changes in Louisiana marshes. From 1968 to 1978, for example, freshwater marsh vegetation decreased and saline vegetation increased, especially in areas transected by large canals (Chabreck and Linscombe 1982). Furthermore, saline drinking water may affect the condition of both adult and juvenile waterfowl (Windingstad et al. 1987, Tiejte and Teer 1988). Ducklings are even less tolerant of salt water than adult birds because their salt glands are poorly developed, especially at <6 days of age (Ellis et al. 1963, Schmidt-Nielsen and Kim 1964, Riggert 1977). Mallard ducklings, for instance, suffered mortality, reduced growth, and a variety of sublethal physiological abnormalities when exposed to high concentrations of sodium sulfate, magnesium sulfate, and saline water occurring in natural wetlands in Saskatchewan (Mitcham and Wobeser 1988a,b). In North Dakota, wild ducklings concentrated and fed around freshwater seepages on saline lakes (Swanson et al. 1984). Female mottled ducks often bypass less productive areas of marsh close to the nest while leading ducklings to more productive broodrearing areas (Baker 1983). Therefore, ducklings traveling through, or remaining in, marshes affected by saltwater intrusion could suffer decreased survivorship. Our study determined the effects of saline water on the growth and survival of mottled duck ducklings. We hypothesized that survivorship, growth rates, and levels of body mass and carcass components would decrease as salinity levels increased. Financial support was provided by private corporations and landowners concerned with wetland management in the Louisiana coastal zone. We are extremely grateful to R. G. Vincent, Jr., from the Miami Corp., B. N. Kirkpatrick from the J. G. Gray Estate, W. P. Hardeman from the Amoco Production Co., J. P. Donohue from the Vermilion Corp., T. Wright from the Sweetlake Land and Oil Co., and J. Woddard from the FINA Oil and Chemical Co. J. T. Joanen and W. G. Perry provided assistance and ideas throughout the study. W. M. Shields and L. W. VanDruff reviewed the manuscript, and