¹School of Freshwater Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53204

²Michigan Department of Natural Resources, Marquette, MI

July 2012

ABSTRACT:

Very little is known about the spawning habitat of lake trout in deep water. Because siscowet lake trout are a top predator, hence a likely keystone species, and very abundant in Lake Superior, a basic understanding is essential for management of Lake Superior’s fisheries. The little that is known about deepwater spawning in the Great Lakes is primarily based on work on Lake Michigan’s Mid-Lake Reef Complex (MLRC, three summits at 40-50 m) where we have documented lake trout spawning (Janssen et al. 2006). The MLRC lake trout are stocked as part of a lake trout restoration program. Whether spawning by such stocked populations is useful for understanding deepwater spawning by siscowets is not guaranteed. Given significant logistic success at studying spawning in deep water by stocked fish we applied the MLRC approach to two deepwater reefs in Lake Superior.

The approach used at the MLRC and applied to Lake Superior is to (1) conduct high resolution multibeam bathymetry mapping to locate potential reefs and habitat, (2) conduct a bioacoustical assessment during spawning season, and (3) deploy a Remotely Operated Vehicle (ROV) equipped with suction sampler and electroshocker to sample for lake trout eggs and egg predators such as sculpins. The general area for surveying was near Marquette, Michigan and it was chosen because siscowet lake trout gametes from there had been collected, eggs fertilized, and fry raised previously. Hence the approximate area and timing of spawning were well known.

Bathymetry mapping via multibeam sonar was conducted in 2009 as part of a Minnesota Sea Grant project by Nigel Wattrus. The survey revealed two potential reefs, a priority reef (here referred to as Sitar Shoal) with a summit at about 90 meters and close to where ripe siscowets had been artificially spawned and a second reef (Shot Point Reef) with a shallower summit.

The bioacoustic survey was conducted in fall 2011 focused around Sitar Shoal. The survey did not demonstrate any siscowet aggregations associated with any obvious physical features, as distinct as those at the MLRC (see Warner et al. 2009), either at the priority reef or in the adjacent area.

At the priority reef, in two days of ROV survey, we found scattered patches of cobble, but the rocks were embedded in sand that, based on sand ripples, frequently drifts. We did collect two slimy sculpins and a rainbow smelt, none of which had consumed lake trout eggs.

The secondary reef, Shot Point Reef, was surveyed on a third day. The reef was a solid mass of granite showing glacial grooves and no likely cavities for eggs to incubate. However, at its base there were significant areas of cobble likely derived from the mass. We did not collect any eggs during sampling. Three isolated mature lake trout were seen but no aggregations typical at spawning sites were seen. We did collect potential egg predators: 17 slimy sculpin and one burbot. None had lake trout eggs in their stomachs.

Our observations of substrate at the two reefs suggest that clean cobble for spawning depends on strong currents. At the priority reef the sand apparently drifts, but does not get removed from the summit. At the secondary reef we think that strong currents at the base keep the cobble clean.

This first attempt at finding a siscowet spawning reef failed at actually finding eggs, but all logistical aspects worked well. The likely key to eventual success may be either to use a means other than bioacoustics for locating siscowet aggregations or generate bathymetry maps that estimate substrate type via seabed classification and also to map/monitor currents in the vicinity of reefs. We also think we need to evaluate the physical conditions that generate potential spawning habitat.