A Pre-Restoration Fishery Resource Utilization Assessment of an Estuarine Borrow Pit in Mobile Bay, Alabama
ENGINEER RESEARCH AND DEVELOPMENT CENTER VICKSBURG MS ENVIRONMENTAL LAB
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Many open-water habitats that provide Essential Fish Habitat functions are also thought to be particularly susceptible to dredging project impacts. Evidence exists, however, that placement of dredged material in open-water environments can result in viable- even enhanced- habitat attributes and functions for fish, shellfish and benthic invertebrates. For example offshore dredged material placement sites are often used extensively as recreational fishing areas. Dredged material can also be used to restore degraded fish habitat, such as to fill artificial pits holes, and depressions that are scattered throughout a majority of estuaries and coastal embayments. Studies dating back to the 1960s have shown that these artificial pits periodically or chronically have poor water quality conditions and consequently represent degraded fish habitat. Several factors contributing to the degraded habitat within the pits include both low dissolved oxygen and high hydrogen sulfide concentrations, particularly during summer months. A major contributing factor to potentially poor water quality in dredged holes is hypoxia or anoxia resulting from accumulation of organic material, poor tidal flushing, and water column stratification. In 2011, the Engineer Research and Development Center ERDC conducted a joint study with the U.S. Army Corps of Engineers, Mobile District to assess habitat quality of two dredged holes designated Brookley and Airport Holes, located in upper Mobile Bay, Alabama. Seasonal conditions in both borrow pits were assessed in terms of water quality, benthic invertebrate community structure, and fishery resource assemblage composition and borrow pit utilization patterns. Water quality data indicated hypoxicanoxic conditions were present during spring and summer with dissolved oxygen DO concentrations less than 2.5 mgL 30 saturation at depths of 4 m, falling to near 0.0 mgL at depths greater than 5 m.
- Biological Oceanography
- Civil Engineering