OHI+ Kenya_Technical Report_2018

Ocean Health Index - Kenya 2018 35
The trend is given a higher weight (β) than the pressure/resilience component. This value was chosen
based on the assumption that trend is a better indicator of the near future condition than indirect
measures of pressure and resilience (Halpern et al., 2012). All pressure and resilience values are scaled
from 0 to 1 and trend is constrained to -1.0 to +1.0.
Pressures and Resilience
Cumulative pressures acting on each goal are calculated as the sum of ecological (P E ) and social pressures
(P S ):
Where γ is the relative weight for ecological vs. social pressures, set as equal (0.5). Each pressure layer is
assigned to an ecological or social category, with ecological pressures further assigned to one of five
subcategories (pollution, alien species, habitat destruction, fishing pressure, and climate change). Each
ecological pressure layer is then applied to relevant goals and assigned a rank sensitivity or the weight of
the pressure on that goal from 1 to 3 with 1 being low impact, 2 being medium impact, and 3 being high
impact. Ecological pressures are then calculated as the weighted average of the pressure scores for each
category for each goal (see Halpern et al. 2012). Social pressures are unweighted, and the social pressure
exerted on a goal is calculated as the average of the all the social pressures applied to that goal (Halpern
et al. 2012).
Resilience layers are assigned to three categories: ecological integrity, regulatory efforts, and social
integrity. Ecological integrity and regulatory effort resilience scores address ecological pressures and
therefore are averaged together and added to the social integrity resilience scores for each goal (Halpern
et al. 2012).
The pressure and resilience layers help capture the complex interactions social and ecological interactions
that are exerted on each goal.
The OHI Toolbox
The OHI Toolbox is the engine that powers all OHI assessments. By utilizing R scripts and GitHub
repositories, it combines prepared data, goal models, and pressures and resilience layers to calculate OHI
scores.