Two approaches for trapping were used in the initial phase of this study: removal trapping and "standardized surveys". Traps set for removal of rusty crayfish were concentrated in areas of the lake to maximize catch rates. In 2001, removals began on 14 August 2001 and traps were emptied daily during the last 2 weeks of August. From 2002 on, crayfish are trapped and removed from mid to late June through late August. Traps are wire mesh minnow traps with openings widened to 3.5-cm diameter. (In 2001, other traps and trapping methods were also evaluated.) Traps are baited with 4- 5 dead smelt.
Removal traps were set in arrays of 10 at 10-m intervals along the 1-m depth contour, and were emptied daily during during 2001 - 2003 and every 1 to 4 days starting in 2004. Removal traps were concentrated on the southern and western shorelines of the lake where catch rates are highest from 2001-2004. From 2005-2008, traps were set around the entire perimeter of the lake. From 2001 to 2004 the sex of each crayfish in a trap was recorded, and a randomly selected subsample of the daily crayfish catch was used to estimate mean size. From 2005-2008, the number of crayfish in each trap was recorded, and a randomly selected subsample of 50 individuals was measured and their sex was determined.
To assess the environmental predictors of rusty crayfish catch rates, "standardized surveys" were conducted prior to harvest in 2001 through 2006. Standardized surveys were comprised of perimeter trapping and depth trapping. Although perimeter trapping occurred every year, depth trapping only took place in 2001 and 2003. For perimeter trapping, 43 traps were baited with 120 g of beef liver and set for 24 hours at 1-m depths at 100-m intervals along the shoreline. Perimeter traps were set on 6 dates in June through August. Three days after the June and July perimeter trapping events, 14 depth transects were set around the perimeter of the lake. Depth transects were spaced 300-m apart and along the transect, traps were set at 0.5, 3, 5, 8 and 12-m depths. Perimeter trapping at the 43 sites, but not the associated depth transect trapping, was done on four dates in 2002 and continued to be done for three to five dates annually through 2006.
Trap_id: During removal trapping, from 2001-2004 10 traps were set at each of the standard trapping sites. For years 2001- 2004, the trap_id of removal traps includes additional information about the spatial location of the trap. The first number of the trap_id indicates the trap site (1 to 43) and the number after the dash identifies which trap of 10 was pulled from the site as you move clockwise around the lake. For example, trap 12-1 is at the flagpost of site 12, trap 12-5 is approximately halfway between sites 12 and 13, and trap 12-10 is just before you arrive at site 13.
Starting in 2005, the removal traps are distributed equally around the lake starting at trap site 1 and proceeding in a clockwise direction. These traps are given trap_id's of sequential numbers as they are lifted. These trap_id's do not relate directly to the trap site. However, you can calculate the approximate trap site for each trap by knowing the total number of traps set over the 43 standard trap sites. In 2005, a total of 277 traps were initially set. In 2006, 220 traps were set over the 43 sites. During the initial retrieval in 2006, data were grouped for each of the 22 sets of 10 traps. To make these data comparable to the rest of the removal trap data, the crayfish represented in the grouped data were assigned randomly to individual traps within the 10 trap set. In 2007, the maximim trap_id was 269. In 2008, the maximum trap_id was 289.
Removal: Traps set annually at 43 sites around the lake and fished through the trapping season. In 2002, additional single traps were set near logs and other likely places which were not in close proximity to other traps. These traps have "-MIN" appended to the trap number in TRAP field.
Perimeter: Traps set annually (through 2006) on standard arrays at 43 sites around the lake at 1 m deep for a limited number of days. The last year perimeter traps were used was 2006.
Depth Transect: Traps set on standard arrays that ranged from 0.5 to 12 m deep. Depth transects were set in 2001 and 2003 only.
Lead: Traps were set at the ends of a “lead” made of aluminum flashing and staked to the bottom of the lake in 2001 only. Experiment was to see if the flashing would be a barrier to the crayfish, and would lead crayfish into small minnow traps. Traps were set at different depths. Leads were set at survey sites: 7, 15, and 26. (Site is indicated in the TRAP field for these traps). Traps were set at each end of the lead and along the middle, as indicated by the depth they were set.
Minnow: Minnow traps set in 2001.
Commercial: Experimental large box traps used only in 2001.
Wik: Traps designed by Don Wik and used in 2002 only. These were square traps with trapezoid-shaped ends, and an entrance on the top of the trap.
Hein, Catherine L., Brian M. Roth, Anthony R. Ives, and M. Jake Vander Zanden. 2006. Fish predation and trapping for rusty crayfish (Orconectes rusticus) control: a whole lake experiment. Canadian Journal of Fisheries and Aquatic Sciences: 63 383-393
Hein, Catherine L., M. J. Vander Zanden, John J. Magnuson. 2007. Invasive trapping and increased fish predation cause massive population decline of an invasive crayfish. Freshwater Biology:
Crayfish trap sites in Sparkling Lake