Methods

Adult rusty crayfish were trapped in August of 2001 and from June-August of 2002-2011 using Gee-style minnow traps modified by widening the openings and baited with beef liver or frozen fish following (Capelli and Magnuson 1983). Between 30-313 (mean=149) traps were set at 43 sites around the perimeter of the lake at the 1 m depth contour, with higher concentrations of traps in locations of higher crayfish abundance (Hein et al. 2007). Traps were emptied daily from 2001-2003 and every 1-4 days from 2004-2011 as catch rates declined. From 2001-2008 all trapped rusty crayfish were removed, and all native virile crayfish (Orconectes virilis) were released. From 2009-2011 trapping continued but all crayfish were released. Catch per unit effort (CPUE) of each crayfish species is used as an index of adult crayfish abundance, and was calculated daily (total crayfish caught/total trap days).
Capelli G.M. and Magnuson J.J. (1983) Morphoedaphic and Biogeographic Analysis of Crayfish Distribution in Northern Wisconsin. J. Crustacean Biol., 3, 548-564
Hein C.L., Vander Zanden M.J. and Magnuson J.J. (2007) Intensive trapping and increased fish predation cause massive population decline of an invasive crayfish. Freshwater Biol., 52, 1134-1146

Methods

To control for sampling methodology and allow comparisons among native and invasive species, we only included data where both invasive and native species from a taxonomic group were sampled using the same methods across multiple sites. Exceptions were made to include rusty crayfish (Orconectes rusticus) in its native range and zebra mussel (Dreissena polymorpha) data. Native rusty crayfish data were obtained from (Jezerinac 1982). Zebra mussel data were mainly obtained from a meta-analysis (Naddafi et al. 2011) which compiled data from 55 European and 13 North American sites from 1959-2004. Additional densities from North America were compiled from multiple primary literature sources (Table S3). All zebra mussel records were presented as number per m2 and are from their invaded range; we did not include native mussel data.Crayfish data were obtained from multiple sources. Crayfish were collected in Wisconsin, USA during summers of 2002-2010 from lakes in the Northern Highlands Lake District following their protocol for crayfish collection. Crayfish were sampled in Wisconsin streams tributary to Lake Michigan from 2007-2010 using 10 gee-style minnow traps per site baited with chicken livers and set overnight. Swedish crayfish were sampled using 30 minnow traps baited with frozen fish in lakes and streams of southern Sweden from 2001-2003 as described in (Nystrom et al. 2006). Washington crayfish were collected from 100 lakes in the Puget Sound Lowlands region of Washington State, USA between 2007 and 2009 from mid-June to early October of each year. At each lake, the investigators set 20 minnow traps baited with fish-based dog food. Traps were deployed in four clusters of five traps each and recovered the following day. All crayfish densities are presented as number per trap per day, with the exception of native range rusty crayfish data, which were reported as number per site (Jezerinac 1982) and excluded from all comparisons that depend on sampling units.Wisconsin fish data were collected from streams throughout the state from 2005-2010 using either a backpack or towboat electrofisher with pulsed DC current in wadeable (less than1m depth) streams for a minimum of 15 minutes. For Wisconsin trout species, locations sampled within 10 years following a stocking event of that species were excluded. Lamprey data were collected from 2008-2010 from Great Lakes tributaries using backpack electrofishers following standardized methods as a part of the sea lamprey assessment program of the United States Fish and Wildlife Service and Department of Fisheries and Oceans, Canada. North American fish densities are presented as number per minute of sampling. Swedish fish data were collected using backpack electrofishing between 1980 and 2010 from streams in Vasterbotten county, northern Sweden, and were obtained from the Swedish Electrofishing REgister (SERS), www.fiskeriverket.se, and are reported as number per 100 m of stream.Snail data were collected in 2006 from lakes in the Northern Highlands Lake District in Wisconsin as described by (Solomon et al. 2010), and densities are presented as number per two m2. Aquatic plant data were collected using a systematic grid-based point-intercept sampling methodology to record macrophyte frequency of occurrence in 242 Wisconsin lakes from 2005-2008. Aquatic plant presence absence was recorded from a boat using a double-sided rake sampler at each point on a sampling grid as described in (Mikulyuk et al. 2010). Density data are presented as proportion of sites within lake littoral zone where a species was present.For all data, if multiple records existed from the same location, we used the most recent record. If replicate samples existed within the same site on the same sampling date, the mean value was used.

Methods

CRAYFISH AND MINNOW TRAPS There have been introductions of exotic crayfish species in recent years into many north temperate lakes. Monitoring yearly abundances of crayfish species is important in determining the status and extent of the invasions.Crayfish traps are set on all lakes except the bog lakes (Crystal Bog and Trout Bog). Minnow traps are set only on the bog lakes. Prior to 1998, five traps were set at each fyke net site. Starting in 1998, three traps are set per site. Thus, prior to 1998, thirty traps were set on each lake (covering 6 sites.) As of 1998, 18 traps are set on each lake.Minnow traps and crayfish traps are set in shallow water (approx 1 m), 2 traps on one side, and 1 trap on the other side of the fyke net lead. Minnow traps are baited with 1 slice of bread per trap to attract minnows inhabiting the bogs. Crayfish traps are baited with 120 g of liver. Traps are fished for approximately 24 hours . Crayfish are identified to species and returned to the lake, except 2001 - 2008 when in Sparkling Lake Rusty Crayfish (Orconectes rusticus) was removed. Minnows caught in either the crayfish or minnow traps are identified to species, measured for total length.Minnow traps used are galvanized steel two piece traps, 44.5 cm long by 30.5 cm maximum diameter with 2.5 cm diameter openings at the ends. The mesh size is 6.4 mm on a side. Crayfish traps are identical, but the opening hole of both sides of the trap has been forced to 5 to 7 cm.