Dam Removal and Fish Passage Improvement Influence Fish Assemblages in the Penobscot River, Maine

Abstract

Dams and their impoundments disrupt river habitat connectivity to the detriment of migratory fishes. Removal of dams improves riverine connectivity and lotic habitat, which benefits not only these fishes but also resident fluvial specialist species. Restoration efforts on the Penobscot River, Maine, are among the largest recently completed in the United States and include the removal of the two lowermost dams and improvements to fish passage at several remaining barriers. We assessed fish assemblages in the main‐stem river and several major tributaries before (2010–2012) and after (2014–2016) dam removal using boat electrofishing surveys and a stratified random sampling design. In total, we sampled 303 km of shoreline and captured 107,335 individual fish representing 39 species. Similarity indices and rarefaction curves indicated that significant changes in fish assemblage composition occurred in reaches that underwent both habitat and connectivity changes (i.e., directly above removed dams). The newly connected reaches became more similar in fish assemblage composition, as demonstrated by an average increase of 31% in similarity scores. The changes in similarity score in these reaches were driven by increasing access for anadromous fishes and decreasing abundances of slow‐water specialist species. For example, we observed a marked reduction in lacustrine species in former impoundments. These assemblage shifts were further illustrated by nonmetric multidimensional scaling in which sites directly above former dams exhibited the largest ordinal shifts immediately following dam removal. We also found all anadromous species in greatest abundance below the lowermost dam during each respective sampling period, though we did find some anadromous species above the lowermost dam during postremoval sampling. Our results demonstrate the potential for large dam removal projects to restore both fluvial and anadromous fish assemblages.

Using hydroacoustics to relate fluctuations in fish abundance to river restoration efforts and environmental conditions in the Penobscot River, Maine

“…Parallel studies investigating fish abundance changes in the Penobscot River after dam removal have also found an increase in fish abundance and species richness after dam removal. Kiraly, Coghlan, Zydlewski, and Hayes (2015) and Watson (2017) conducted electrofishing surveys from 2010 to 2012 (before dam removal) and 2014 to 2015 (after dam removal) to study fish assemblages in the main stem and tributaries. They found that river reaches that underwent habitat and connectivity changes after dam removal displayed a 31% average increase in Morisita-Horn's species similarity index that was attributed to increased habitat access for diadromous fishes (Watson, 2017).…”

“…Kiraly, Coghlan, Zydlewski, and Hayes (2015) and Watson (2017) conducted electrofishing surveys from 2010 to 2012 (before dam removal) and 2014 to 2015 (after dam removal) to study fish assemblages in the main stem and tributaries. They found that river reaches that underwent habitat and connectivity changes after dam removal displayed a 31% average increase in Morisita-Horn's species similarity index that was attributed to increased habitat access for diadromous fishes (Watson, 2017). Furthermore, an increase in total abundance for alewife, American shad, American eel, sea lamprey, and striped bass was observed at the respective lowermost dam in both predam and postdam removal years (Veazie Dam 2010-2013and Milford Dam 2015-2016Simpson, Mitchell, Maine Department of Marine Resources, unpublished data).…”

“…Most traditional approaches to document variation in fish abundance are spatially and temporally limited (e.g., fyke nets; Gewant & Bollens, 2012) and focus on individual fish rather than total abundance. Tagging techniques, trawling, and electrofishing are all sampling techniques that yield sporadic fish abundance and species estimates, providing good spatial coverage (Kiraly et al, 2015;Watson, 2017) but poor temporal resolution. High temporal resolution in this study enabled us to record peak fish abundance during only 0.02-2.3% (140 of 7,518 hr and 113 of 4,817 hr at Pen A before and after dam removal) of the total number of hours sampled.…”

Toward process‐based conservation prioritizations for freshwater ecosystems

“…Habitat restoration for threatened species (e.g. for coho salmon; Wright & Souder, ) and landscape restoration measures for whole‐ecosystem recovery, as in the Penobscot River (Watson, Coghlan, Zydlewski, Hayes, & Kiraly, ), have a long tradition in North America (Bernhardt et al, ) and globally (Wohl, Lane, & Wilcox, ). Optimization‐based systematic approaches to restoration planning are not as established as their counterparts in conservation science, though.…”

Opening the tap: Increased riverine connectivity strengthens marine food web pathways

“…Current interest in the status of alewives and the success of dam removal and improved fish passage in increasing alewife abundance, particularly in Maine [26,27], encouraged us to test, via ecosystem modeling, the impacts of increasing anadromous forage fish populations on marine food webs. First, we estimated potential alewife production in three Maine watersheds (Androscoggin, Kennebec, and Penobscot) based on the spawning habitat potentially available to them.…”