Examining genetic diversity, outbreeding depression, and local adaptation with slimy sculpin re-introductions in the southeast Minnesota Driftless Region, USA

Summary

The slimy sculpin (Cottus cognatus), extirpated from parts of Minnesota's Driftless Region by poor land-use practices, was reintroduced to nine restored streams between 2003 and 2005 by a University of California researcher working with the Minnesota Department of Natural Resources, explicitly to test whether mixing genetically distinct source populations helps or harms establishment in human-altered habitat. Roughly 150 sculpins were translocated annually in near-equal proportions from three donor streams, verified disease-free over three years and surveyed to confirm removals would not harm source populations, with fin-clipping to distinguish stocked from naturally reproduced fish and individual tagging to track growth and movement. Monitored annually through 2009, sculpins became established at all nine recipient sites, with 2009 population estimates ranging from 200 to 3,100 fish and range expansion varying from limited to basin-wide depending on site. Genetic analysis found re-introduced mixed-source populations had slightly higher heterozygosity and allelic richness than any single source population, but pure-strain descendants from one source persisted disproportionately well, while second-generation hybrid descendants grew more slowly than pure-strain or first-generation fish, suggesting a degree of outbreeding depression. Difficulties included securing funding for a habitat-restoration project not centered on a threatened species and developing new microsatellite genetic markers from scratch. The project rates itself Successful, and recommends future reintroductions favor single, locally similar source populations given the observed hybrid fitness costs.