Hypoxia affects Fish

New findings about how marine resources are affected in Oregon’s Dead Zone

Since the 2002 hypoxia event in Oregon’s coastal waters that caused die-offs of crabs, fish, and other species, researchers have been studying the causes and consequences of this seasonally-occurring phenomenon. Hypoxic (low oxygen) zones are often termed "dead zones" because extreme hypoxic conditions can cause death of organisms. Milder hypoxic conditions occur far more often than "dead zones" but still can drastically affect marine life, impacting resources such as commercial fishery yields, and the dynamics and resilience of coastal ecosystems. Despite clear indications that species are affected within Oregon’s hypoxic areas, the exact nature of the effects to economically and ecologically important species haven’t yet been quantified.

Recent findings, from collaborative research in Oregon between the National Oceanographic and Atmospheric Administration’s (NOAA) Northwest Fisheries Science Center and PISCO, shed new light on how low-oxygen conditions affect economically and ecologically important fish and invertebrate species.

 

Research Vessel Elakha

  As the physical and chemical conditions in the ocean change with the Earth’s climate and increased pollution from the land, the prevalence of hypoxia along our coasts is of increasing concern for fishers and marine resource managers (Diaz and Rosenberg, 1995). Since 2002, PISCO researchers at Oregon State University have been studying the hypoxic zone that has seasonally been forming since 2002, over the inner shelf along the Oregon coast. Using advanced monitoring technology for ecosystem observations; vessel-based surveys, advanced sensors on fixed moorings and autonomous ocean gliders, the PISCO team gathers information on the formation and spatial extent of these hypoxic zones.

The recent 2007 study combined these focused PISCO field efforts with the annual NOAA National Marine Fisheries Service (NMFS) West Coast Groundfish Bottom Trawl Survey. The researchers’ objective was to determine how the presence of hypoxic waters affects bottom-dwelling, mobile marine species that are of economic and ecological importance. By mounting oxygen sensors on the trawl nets, the scientists were able to correlate the “catch per unit effort” (CPUE) for specific species to bottom oxygen levels. The average CPUE fell in hypoxic zones (meaning there were less animals in these low-oxygen conditions and harder to catch). Oxygen levels at the sea-floor explained up to 84% of the observed changes in biomass for individual species. The researchers outline three possible hypotheses to explain the observed pattern between fish biomass and oxygen concentrations:

  1. Bottom dwelling fish and invertebrate populations respond to low oxygen levels by moving to areas with higher oxygen, potentially settling for less suitable habitat in the process.
  2. Bottom organisms in lower-oxygen areas die at a higher rate than those in higher-oxygen areas, with no movement necessary to explain the biomass pattern.
  3. Some fish and invertebrates move out of the lower oxygen areas and there is higher mortality in the lower-oxygen areas (combination of 1 and 2).

Future work looking at species abundance and oxygen levels at a single location should provide further insights into these hypotheses.

Find more about Hypoxia off the Pacific north west coast

Results from this study, were recently published in the Fisheries Oceanography journal (v. 19, pp. 76-87, 2010). NOAA National Marine Fisheries researchers are Aimee Keller, Victor Simon, W.W. Wakefield, M.E. Clarke, Dan Kamikawa and Erica Fruh. PISCO/OSU researchers are Jack Barth, Francis Chan. The National Oceanographic and Atmospheric Administration National Marine Fisheries Service is a collaborative partner.

R.J. Diaz and R. Rosenberg, 2008. Spreading dead zones and consequences for marine ecosystems, Science 321: 926–929.

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