HISTORICAL EVIDENCE FOR HABITAT CONVERSION AND LOCAL POPULATION DECLINE IN A NEW ZEALAND FJORD

Abstract

Although direct links between habitat conversion and human activities are often apparent, their causal mechanisms may be less obvious. We identified historic salinity change, attributed to freshwater input from a hydroelectric power plant, as a potential mechanism of intertidal habitat conversion and severe local decline of the New Zealand littleneck clam (Austrovenus stutchburyi) in Doubtful Sound, New Zealand. Live clams were abundant at sites distant from the power plant with relatively unaltered salinity regimes, and were absent from the inner fjord near the power plant outflow. Remnant shells indicate past habitation by A. stutchburyi at inner fjord sites. Historic salinity conditions inferred from oxygen isotope ratios in shell carbonates confirm that salinity previously reached higher values in the inner fjord, and that A. stutchburyi probably has not inhabited that area since salinity was at pre‐power‐plant levels. A laboratory experiment showed that A. stutchburyi from Doubtful Sound have a high tolerance for low salinity conditions, but that hyposaline conditions become lethal after extended exposure (37‐day LC50 = 2.5). In situ transplant experiments demonstrated that present salinity conditions in the inner reaches of Doubtful Sound are lethal to this species. Our findings suggest that the increased freshwater input maintained by the power plant has converted the habitat in the inner reaches of Doubtful Sound to one with sustained low salinity unsuitable for the survival of A. stutchburyi.