Physical and biological evolution of constructed reefs – long-term assessment and lessons learned

Abstract

Artificial reefs are widely used to restore lost or damaged habitat, particularly spawning habitat. However, reefs are often designed without consideration for testing design features and constructed without clear objectives or a long-term assessment plan to evaluate changes in reef characteristics. To mitigate for degradation of lake trout spawning habitat in Thunder Bay, Lake Huron, we constructed 29 cobble reefs in 2010–11. Reef design incorporated height, orientation, and size as replicated factors. In addition to assessment of reef use by lake trout, we evaluated the biotic and abiotic changes in the reefs over five years post-construction and compared them to two adjacent degraded reefs and a semi-natural reef. Each reef was mapped annually by side-scan sonar to examine changes in the physical structure, such as slumping. Divers surveyed a subset of the reefs each year, and sampled substrate to quantify benthic macroinvertebrates, including zebra mussels. Algal colonization and round gobies were evaluated qualitatively. Temperature, dissolved oxygen, and sediment accumulation were measured over the winter at the vertical center and base of two reefs each year. Little physical change or sedimentation occurred. Qualitative abundance of coregonine eggs, presence of other fishes, and colonization by algae and macroinvertebrates increased throughout the monitoring period, indicating that the reefs have long-term value for a diversity of species.