A New Sampling Technique for Assessing Lake Trout Egg Density in Deep Water

Lake trout (Salvelinus namaycush) restoration efforts at shallow sites have yielded minimal success in the lower four Great Lakes, and management agencies are examining offshore, deep-water reefs as possible sites for restoration. Studies have estimated egg density thresholds that should be adequate for recruitment; therefore, development of methods to assess and quantify the reproduction of lake trout at deep offshore reefs is needed and has been identified as a research priority for lake trout restoration. Previous methods developed for passively collecting lake trout eggs are limited to shallow water and are not viable for use on offshore, deep reefs. A deepwater egg trap was developed and tested in Lake Champlain, where heavily used lake trout spawning sites were readily accessible.  Deepwater egg traps were paired with buried egg bags to compare lake trout egg collections between the two types of gear at two shallow sites in two years. Only 3 of 55 deepwater egg traps were found to be nonfunctional upon retrieval in Lake Champlain and the remaining traps fished effectively on shallow reefs, which are much more susceptible to wave energy than deep reefs. There was a significant positive correlation between estimates of egg densities in the paired deep-water traps and buried egg bags and there was no significant difference between egg densities in the two types of gear. The coefficients of variation of egg densities in both types of gear were similarly high. After testing, the new egg traps were deployed in Lake Michigan to quantify lake trout egg deposition at multiple sites within the Mid-Lake Reef Complex.  Eggs were successfully collected at depths > 35 m; the egg densities within the Mid-Lake Reef Complex were above or comparable to lake trout egg densities from egg bags used at shallow sites in Lake Michigan. There was a significant positive correlation between eggs per trap and eggs per ROV suction sample at several reefs in the Mid-Lake Reef Complex. The deepwater traps were also used to successfully document lake trout and lake whitefish spawning on a shallow reef in Lake Huron.  The deepwater egg trap is an effective quantitative method for sampling lake trout egg density in deep and shallow water and may improve evaluation of the contribution of deep reefs to lake trout restoration in the Great Lakes.