Biochemical Oxygen Demand (BOD)

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INDICATOR BACKGROUND AND RATIONAL FOR INCLUSION

The open nature of most marine finfish production systems results in the discharge of uneaten feed and fish wastes directly into the marine environment. The impacts of nutrient loading on water quality as well as on the sea floor environment are well documented (Ackefors and Enell 1990; Barg 1992; Gowen et al. 1991; Wu 1995).

While expert opinion was sought during the development of all indicators, a dedicated expert workshop was convened to address the particular challenges surrounding the formulation of a “waste” indicator. A number of metrics can be used to assess farm-derived organic pollution. In this case, workshop participants concluded that the most tractable method for estimating the direct ecological effect of waste discharges is to estimate the biochemical oxygen demand (BOD) required to process the waste materials. BOD is a measure of the relative oxygen-depletion effect of waste contaminants (uneaten feed and feces) and is defined as the amount of oxygen required to oxidize organic carbon (C) and nitrogen (N) from feed inputs that is not recovered in the biomass at harvest. The development of the metric was led by Dr. Claude Boyd, and the underlying calculations are available in Boyd 2009.

Other possible measures of “waste” impacts such as Reduction (Redox) potential and nitrogen discharges, which often vary greatly even over short distances, were deemed inappropriate for a country-scale analysis such as GAPI. Further, all other indicators that were considered required a baseline measure to define the “normal” profile of the area against which the magnitude of farm impact could be calibrated. The GAPI BOD indicator is empirically derived and invariant; the oxygen required to oxidize a unit of organic carbon or nitrogen is the same everywhere and therefore is a powerful standard against which to measure organic pollution performance. The bio-physical environment in some areas may be better able than that in others to mitigate organic outputs. However, the GAPI BOD indicator assumes performance decreases as pollution load increases, irrespective of location.

INDICATOR FORMULA

Formula

BOD = BOD(mT O2) x Area of Overlap (km²)

BOD = (Total Nitrogen in Feed - Total Nitrogen in Fish) x 4.57 + (Total Carbon in Feed - Total Carbon in Fish) x 2.67
Area of Overlap (km²) = The sum of the area of overlap between farms using a 3 kilometer radius on each location.

Units: mT O2 x km²

Sample Calculation

Sample Normalised Calculation: Altantic salmon in Canada, Chile, Norway, UK,, 2007

BOD

BOD is calculated by assessing the total carbon and nitrogen per unit of feed. According to Boyd (2009), the amounts of molecular oxygen necessary to oxidize 1 kg of organic carbon and 1 kg of ammonia nitrogen are 2.67 kg and 4.57 kg, respectively. These relationships allow us to estimate the biochemical oxygen demand of feed as demonstrated in the BOD equation above.

Area of Overlap

GAPI assumes that nutrient loading spread across a large area will, on average, have a lesser impact than the same loading concentrated in a small area. In order to establish the area of impact, GAPI identified farm locations using Google Earth images (Google Inc. 2009) and calculated the area of impact using the geographic information system, ArcGIS 9.2.(For more detailed information on the Area of Overlap calculation, visit the FAQ page)

ADDRESSING DATA GAPS

GAPI employed the following decision rules to treat gaps in BOD related data: