Eyes on the Seafloor : Using Video Tools to Monitor Benthic Communities at Dredged Material Disposal Sites at the Mouth of the Columbia River Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8910jz22k

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  • The mouth of the Columbia River (MCR) is an intersection of more than a river and ocean, but also multiple economically valuable activities including dredging to maintain a deep draft shipping channel and an active Dungeness crab (Metacarcinus magister) fishery. More than three million cubic meters of sediment are annually dredged from the MCR and disposed at designated ocean disposal sites both near and offshore. These designated sites are composed of mostly flat, soft-bottom habitat populated by demersal fish and benthic invertebrates, including Dungeness crab. Despite years of site monitoring, concerns remain about the resilience of the benthic community to dredged material disposal as no direct observations of behavioral response to dumping have been made. This research addresses these concerns with new video-based approaches to monitoring at a deep and a shallow disposal site. During the summers of 2014-2015, CamPods (baited video landers) were used to record the behavior of Dungeness crab in response to dredge disposal events at a shallow disposal site and video sleds to monitor benthic communities at both disposal sites before, during, and after disposal seasons. Past and current MCR benthic monitoring used trawling and crab pots, but video tools may be effective alternatives that provide direct visual observations of organisms in their associated habitat. The goals of this research were twofold: to determine if dredged material disposal affects benthic epifauna distributions and behavior at disposal sites, and to compare the video survey impact assessments to those of traditional monitoring tools. CamPods provided a ‘crab’s-eye’ view of a disposal event at the shallow site, documenting Dungeness being engulfed in the sediment plume and allowing for before and after relative abundance comparisons. The impact of the disposal plume appeared to be localized and temporary, with Dungeness displaced by the plume, but returning to the impact area shortly after a disposal event. Crab pots did not detect any differences in abundance between impact or control areas. Video sled footage revealed unique species associations which would not be discernible from a pile in a trawl net. At the community scale, no differences between disposal and reference sites were detected at either disposal site. Rather differences in communities were detected on a temporal scale. Trawl surveys at the deep site also detected temporally distinct communities.
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