Mortality of Largemouth Bass (Micropterus salmoides) and Steelhead Trout (Oncorhynchus mykiss) in Densely Vegetated Littoral Areas Tested Using in situ Bioassay

ABSTRACT

Concentrations of dissolved oxygen (DO) in dense beds of aquatic macrophytes in two western Washington lakes were routinely measured below reported lethal limits (≤1 mg L⁻¹) for largemouth bass (Micropterus salmoides) and steel head trout (Oncorhynchus mykiss). The impact of the observed low DO concentrations on resident fish was examined both by field observation and by conducting 72 hour in situ cage bioassays. Replicate cages were placed in two Western Washington lakes at the surface, 1 m, and 2 m deep, in dense patches of aquatic macrophytes and open water. No steelhead trout mortalities occurred over 72 hours in the open water or surface water in dense patches of Myriophyllum spicatumia Lake Washington where DO concentrations were consistently > 4.0 mg L⁻¹. No significant mortality of steelhead trout occurred in the surface cages of the floating leaf Nymphaea odorata, but all fish were dead within 12 hr at 1 m, where concentrations of DO were consistently < 4 mg L⁻¹. No significant mortalities of largemouth bass occurred in the open surface water of Keevies Lake, or in the upper 1 m of water in patches of Potamogeton natans where concentrations of DO were consistently > 2 mg L⁻¹. Significant largemouth bass mortalities occurred at both the surface and at 1 m depth in dense beds of the floating leafed macrophyte Brasenia schreberi, and in the bottom water (<2.0 m) of both floating leafed and submersed species, where DO concentrations were < 2 mg mg L⁻¹. The observed lethal DO concentrations and high mortalities in the bioassay tests indicate that at high densities aquatic macrophytes can have significant detrimental localized effects on fish.

Key Words:

• aquatic macrophytes
• dissolved oxygen
• bioassay
• fish mortality