Lake Trout Spawning

Shallow reefs in Lake Champlain may provide ideal habitat for spawning lake trout. Dr. John Fitzsimons of the Great Lakes Laboratory of Fisheries and Aquatic Sciences in Canada and Dr. Ellen Marsden of the Rubenstein School of Environment and Natural Resources of the University of Vermont conducted a study factors that affect survival of lake trout eggs in Lake Champlain. Efforts to restore stocks of lake trout may benefit from a greater understanding of ideal habitat for spawning. Shallow, artificial reefs provide variable currents that may be ideal for lake trout spawning. The eggs deposited on these reefs can be at risk of damage and displacement due to high current velocity. However, these same currents may help the eggs survive by providing oxygenated water and reducing deposition of fine sediment, both important for egg survival. The researchers hypothesized that lake trout strategically balance mortality risk with factors that may improve egg survival, choosing spawning habitat that maximizes survivability over a variety of conditions. The researchers tested this hypothesis by measuring

• current velocity using an acoustic Doppler current profiler located off the reef and plaster of Paris blocks, to measure dissolution, a proxy for currents, located on the reef
• egg displacement using a known number of artificial eggs

These measurements were used for predicting that

• egg density and egg displacement are related to current velocity,
• density of predators is inversely related to current velocity,
• egg mortality in interstitial spaces is unrelated to current velocity.

Egg deposition and survival were measured, as well as artificial egg displacement, on a breakwall near the Grand Isle Ferry in Lake Champlain. Spawning regularly occurs there, with lake trout depositing thousands of eggs. These eggs have a high survival rate, although hatched fry do not appear to survive after they leave the breakwall. Most spawning occurred within a single week, following a large upwelling event that resulted in a temperature decrease. Spawning may have been synchronized with this drop in temperature. Upwelling of water on the reef due to variable currents can cause egg displacement, as demonstrated by the high loss of artificial eggs located where the highest currents were estimated on the reef. However, the viability of the eggs that remained on the reef was not reduced due to current velocity. The variability of currents on reefs changes from year to year, so an ideal depth for egg survival one year may not be the same depth the following year. Selection of a reef with variable localized current velocities may prove beneficial for the survival of eggs, despite losses due to displacement. By spawning in areas with variable currents, the lake trout are hedging their bets that in a given year, there will be an ideal location for egg survival in one portion of the reef, while other locations may be more prone to displacement of eggs.