ESR 3:229-237 (2007)  -  DOI: https://doi.org/10.3354/esr00033

ABSTRACT: We used our extensive data on freshwater mussels (Family: Unionidae), collected from 1984 to 2005 at multiple sites in the upper Mississippi River (UMR), USA, to examine the status of the endangered Higgins eye pearlymussel Lampsilis higginsii (Lea, 1857). Historically, this species inhabited an 850 km reach of the Mississippi River from Prescott, Wisconsin to Louisiana, Missouri, plus 9 UMR tributaries. After 1965 its range in the UMR declined by 50% and it was found in only 2 tributaries, the St. Croix and Wisconsin rivers. In river reaches unaffected by zebra mussels Dreissena polymorpha, L. higginsii mean density ranged from 0.0 to 1.4 m^–2 (average = 0.25 m^–2) and it comprised less than 2% of the unionid fauna. Recent distribution and abundance data indicate that the range of L. higginsii populations was misrepresented on historical maps and suggest that populations were in fact either absent or very uncommon both at the periphery of their historical range and in small tributaries where they were reported historically. Although this species has always been rare, it can usually be found in appropriate habitats within its current range. It was listed as endangered before there were data on its density, recruitment, and relative abundance. Although it was nearly extirpated by D. polymorpha in the late 1990s, L. higginsii appears to be resilient to zebra mussel infestations. A multi-agency conservation plan is now being implemented to reintroduce this species into small and medium-sized rivers within and outside its historical range. Our data indicate that this species is not in imminent danger of extinction, has always been rare, and is not adapted to small rivers. It would be more realistic, and beneficial to L. higginsii, to implement strategies that protect all unionid species and the habitats upon which they depend.

KEY WORDS: Endangered Species Act · Upper Mississippi River · Lampsilis higginsii ·Zebra mussels · Freshwater mussels · Unionidae