John Mullican, Fisheries Biologist
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Posted on September 5, 2013 | Permalink
Observations from the Upper Potomac
Location: Upper Potomac
Final sites were completed this month for the annual Potomac River summer seining survey. The survey is used to monitor annual smallmouth bass yearclass strength, as well as the abundance of nongame forage species. A total of 120 hauls were completed on the Potomac at sites from Cumberland downstream to below Great Falls. Additional sites were surveyed on the two largest Potomac tributaries, Conococheague Creek and the Monocacy River. The geometric mean number of young smallmouth bass collected per haul is used as an index of yearclass strength. Smallmouth recruitment was expected to be low this year because of flooding that occurred during early May while the bass were spawning. Strong river flow and colder-than-average water temperature increase mortality of bass eggs and bass fry by displacing them from the nest, covering the eggs or sac fry with sediment, or requiring fry to use more energy to swim and feed in the heavier currents. This year’s index fell below the long term median values throughout the mainstem Potomac (1.2 bass per haul, median 1.7) and the Monocacy River (1.2 bass per haul, median 1.9). Yearclass strength was slightly higher in Conococheague Creek (2.2 bass per haul, median 2.4) and the North Branch Potomac River between Cumberland and Oldtown (1.7 bass per haul, median 1.6). It is common for yearclass strength to vary from year to year.
While conducting the seining survey other fascinating animals are often encountered. This year is was quite common to find freshwater shrimp Palaemonetes paludosus, frequently referred to as ghost or grass shrimp, in the seine. Grass shrimp are nocturnal, hiding in vegetation during the day and feeding on phytoplankton during the night. In turn, they are preyed upon by many birds and fish. The abundance of aquatic vegetation in the Potomac this year is providing ideal habitat for the shrimp…and the hungry fish.
As water temperatures begin to cool this fall the large beds of aquatic vegetation will begin to die and decompose. As aquatic plants and autumn leaves decay, the process releases organic substances called fatty acids that reduce the surface tension of the water allowing air to mix with the water producing bubbles and foam. The foam will be more evident downstream of dams and rapids where the water is more easily mixed with air. Although a large amount of foam can indicate pollution, it is more commonly the result of decaying organic matter and the mixing of air and water.