Challenges for Diadromous Fishes in a Dynamic Global Environment

Evaluating Small Barrier Removal and Passage Improvement Scenarios to Enhance Diadromous Fish Restoration in the Penobscot Basin, Maine

Wesley S. Patrick and Rory Saunders

doi: https://doi.org/10.47886/9781934874080.ch76

The Penobscot River Restoration Project (PRRP) offers some unique opportunities to reconnect 12 native diadromous species with historic habitats and restore associated ecological functions. In particular, alewives Alosa pseudoharengus are absent from the drainage with the exception of two tributaries of the Penobscot estuary: the Souadabscook Stream and the Orland River. While the PRRP will substantially improve passage conditions for diadromous fish in the Penobscot Basin, it is in no way a “silver bullet” for ecosystem restoration of the Penobscot Basin. There are roughly 100 small dams and other fish passage barriers in the Penobscot Basin that will prevent some diadromous species from gaining access to historic habitats after the implementation of the PRRP. Many other passage impediments will need to be addressed to realize the full potential of the PRRP. We assessed the restoration potential for alewives in the major subwatersheds using a novel, yet simple, geographic information system-based approach to help prioritize restoration efforts in the Penobscot basin.

We mapped the historical range of alewives in the Penobscot Basin (Figure 1) with ArcGIS 9.2 software and 1:24,000 scale data layers using a variety of sources, most notably Houston et al. (2007). We used four parameters to select streams that were likely to have supported historical alewife runs in subwatersheds, but for which no historical accounts were available: (1) presence of an adjacent stream that supported an alewife run, (2) presence of lake habitat, (3) river width leading to the lake habitat was greater than 50 ft, and (4) absence of a significant change in elevation that could create impassable falls within the tributary. We considered streams and lakes as being historical alewife habitat if these four conditions were met.

Subwatersheds were prioritized based on the acreage of available lake habitat that could have supported alewife spawning. We used a sensitivity analysis to determine the relative importance of each subwatershed given varying downstream impediments and associated passage efficiencies. The sensitivity analysis reduces the productivity of each unit of spawning habitat based on the passage efficiency. We simulated passage efficiency rates using a high (100%), medium (75%), and low (50%) range for each impediment. For example, given a “medium” passage scenario in a 1,000-acre lake upstream of five dams, the 1,000 acres would be reduced to 237 acres (i.e., 1,000 acres × 0.75⁵ = 237.3 acres). The revised estimates of accessible lake habitat were then ranked from highest to lowest in terms of restoration potential, or the number of alewives that could be supported by the lake habitat.