40
3 An ERA Acute Model Overview
3.3 Surface Compartment Calculations
The VEC unit (N) in the sea surface compartment is a population characterized by (1)
population density, (2) population distribution and (3) population size. The values in
the cells are fractions (relative abundances) of the population. Seabirds, marine (or
aquatic) mammals and sea turtles are assigned to different wildlife groups depending
on the species characteristics related to their individual vulnerability to oiling (physiological sensitivity to oil) and population vulnerability (factors affecting the potential
rate of growth and long-term population size) (Bjørgesæter and Damsgaard Jensen
2015).
3.3.1 Impact Modelling
The main impact to surface VECs is through physical contact with surface oil with
subsequent effect on feather structure, insulation and buoyancy, ingestion of oil, aspiration and absorption of oil components (e.g. Deepwater Horizon Natural Resource
Damage Assessment Trustees 2016; National Research Council (US) 2003). The
proposed threshold levels for lethal oil film thickness (2 μm for seabirds and 10 μm
for marine mammals/turtles) are derived from existing literature (Hughes et al. 1990;
Jenssen and Ekker (1989, 1991a, b), Jenssen 1994; Koops et al. 2004; O’Hara and
Morandin 2010; Peakall et al. 1985; Scholten et al. 1996; Stephenson 1997), different
environmental risk analyses methods (French-McCay 2004, 2009; NOROG 2007;
Spikkerud et al. 2006) and peer group discussions. In their comparative risk assessments as input to a relative risk methodology, Bock et al. (2018) used a lower threshold
of 10 μm and an upper threshold of 100 μm.
The impact for surface VECs in a cell is proportional to the fraction of the cell
covered with oil above the threshold thickness of oil and the period with harmful oil
in the cell, adjusted by two individual species/species group-specific vulnerability
factors (behavioral and physiological factor s); p beh and p phy (See Supplementary
Information, Tables 1 and 2). The factors represent the likelihood of being oiled and
the likelihood of lethal effect given exposure, respectively and are derived for 13
wildlife groups and 58 species based on different oil vulnerability indexes (OVI).
The fraction of VEC impacted (denoted N let ) is calculated for the relative abundance
of a defined population (N) in a grid cell i and the calculations are summarized as
follows (Eq. 3.2).
N let =
n
i=1
N i −
1 − p beh × Cov TH × p phy
T exp TH × N i
(3.2)
where:
• TH is oil film thickness threshold level
• Cov is the fraction of a cell covered with oil thicker than TH
3 An ERA Acute Model Overview
3.3 Surface Compartment Calculations
The VEC unit (N) in the sea surface compartment is a population characterized by (1)
population density, (2) population distribution and (3) population size. The values in
the cells are fractions (relative abundances) of the population. Seabirds, marine (or
aquatic) mammals and sea turtles are assigned to different wildlife groups depending
on the species characteristics related to their individual vulnerability to oiling (physiological sensitivity to oil) and population vulnerability (factors affecting the potential
rate of growth and long-term population size) (Bjørgesæter and Damsgaard Jensen
2015).
3.3.1 Impact Modelling
The main impact to surface VECs is through physical contact with surface oil with
subsequent effect on feather structure, insulation and buoyancy, ingestion of oil, aspiration and absorption of oil components (e.g. Deepwater Horizon Natural Resource
Damage Assessment Trustees 2016; National Research Council (US) 2003). The
proposed threshold levels for lethal oil film thickness (2 μm for seabirds and 10 μm
for marine mammals/turtles) are derived from existing literature (Hughes et al. 1990;
Jenssen and Ekker (1989, 1991a, b), Jenssen 1994; Koops et al. 2004; O’Hara and
Morandin 2010; Peakall et al. 1985; Scholten et al. 1996; Stephenson 1997), different
environmental risk analyses methods (French-McCay 2004, 2009; NOROG 2007;
Spikkerud et al. 2006) and peer group discussions. In their comparative risk assessments as input to a relative risk methodology, Bock et al. (2018) used a lower threshold
of 10 μm and an upper threshold of 100 μm.
The impact for surface VECs in a cell is proportional to the fraction of the cell
covered with oil above the threshold thickness of oil and the period with harmful oil
in the cell, adjusted by two individual species/species group-specific vulnerability
factors (behavioral and physiological factor s); p beh and p phy (See Supplementary
Information, Tables 1 and 2). The factors represent the likelihood of being oiled and
the likelihood of lethal effect given exposure, respectively and are derived for 13
wildlife groups and 58 species based on different oil vulnerability indexes (OVI).
The fraction of VEC impacted (denoted N let ) is calculated for the relative abundance
of a defined population (N) in a grid cell i and the calculations are summarized as
follows (Eq. 3.2).
N let =
n
i=1
N i −
1 − p beh × Cov TH × p phy
T exp TH × N i
(3.2)
where:
• TH is oil film thickness threshold level
• Cov is the fraction of a cell covered with oil thicker than TH