Type of Document Master's Thesis Author Harmon, David J URN etd-0604102-203723 Title The Effects of Fall Coldfront Passages on the Nekton Community in a Tidal Creek in Port Fourchon, LA, as Monitored by Hydroacoustics Degree Master of Science (M.S.) Department Oceanography and Coastal Sciences Advisory Committee
Advisor Name Title Charles A. Wilson Committee Chair Conrad Lamon Committee Member Richard Shaw Committee Member Keywords
- coldfront passages
- tidal creek
- nekton community
Date of Defense 2001-07-20 Availability unrestricted AbstractSplit-beam hydroacoustic sampling with two, 420 KHz, 2x6 degree elliptical transducers was evaluated as a tool to quantify nekton movements in a Louisiana estuary. I measured the effects of atmospheric cold front passages on nekton in a tidal creek in Port Fourchon, LA during the fall of 2000. Six “fronts” were sampled between October 6 and December 18, 2000. Nekton density ranged from 0 to 24.4 fish/m3 with an overall mean of 2.44 fish/m3. Net samples were taken to supplement the acoustic data with species composition data. Species composition changed from October to December. A higher number of smaller targets were found with the acoustic gear than with the nets, evidence of gear bias.
Hydroacoustic based density was generally greater during falling tide (3.17 fish/m3) than rising tide (1.81 fish/m3). Fish Energy, a proxy for biomass, ranged from 1.76*10-7 units to 2.06*10-3 units, with an overall mean of 5.96*10-5 units. Fish Energy was higher on rising tides (6.10*10-5) than falling tides (5.79*10-5). Biomass and density were highly variable and similar to previous studies. Barometric pressure combined with tide affected nekton movement. The range of barometric pressures during a sampled ‘front” had a significant effect on biomass. When interacted with tidal stage, the range of barometric pressure significantly affected biomass and nekton density (α=0.05).
Hydroacoustics were valuable, although sampling during high densities confounded data acquisition. Higher biomass on incoming tides and higher density on outgoing tides suggest an emigration of small nekton during fronts. Thus, judging by total biomass measured over the study, a front may not be as significant a cue in moving estuarine biomass to emigrate as had been hypothesized.
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