Habitat Equivalency Analysis (HEA): A Tool for Assessing Marine Impacts and Mitigation Requirements – the Sitka Airport Example

Although several widely used approaches exist to assess function (quality) of freshwater wetlands, no such approach is available in the marine environment.  A method was needed to assess ecological functions of marine habitats impacted by planned safety expansion of the Sitka Airport.  Project fill would eliminate 8.9 acres of marine habitat and alter the nature of 9.4 additional acres.  We worked with permitting agencies to generate consensus to use an innovative application of the Habitat Equivalency Analysis (HEA) model to define project marine impacts and to evaluate alternative mitigation actions.  The HEA model was developed by NOAA to assess the mitigation required to replace marine ecological functions and services lost due to an injury or impact. 

We began with an inventory of habitat types that would be impacted and developed a matrix to rate (0 to 10) the degree to which each habitat type provides a suite of ecological functions.  This “relative habitat function” for each habitat, times its area, was used to calculate project losses and gains in units of Habitat Functional Acres (HFA).  Net present value of habitat in units of Discounted Functional Acre Years (DFAYs) was then calculated using actuarial principles of economic theory. 

The DFAY “debit” calculated for the project reflected temporal losses in habitat function, and the time to achieve full function of altered habitats.  DFAYs lost to project construction (and gained from recolonization of newly placed rock surfaces) determined the net amount of mitigation needed to compensate for lost ecological functions of impacted marine habitat.  After calculating DFAYs lost to airport expansion, we identified a suite of potential habitat protection, restoration, and enhancement actions within Sitka Sound that would compensate for some or all of the losses of marine habitat.  Finally, we used HEA to determine the effectiveness of the selected mitigation action in compensating for (replacing or protecting) those functions that would be lost.  The objective of compensatory mitigation was met when the number of DFAYs provided by the mitigation action(s) equaled or exceeded the number of DFAYs lost.