Fish Assemblages in the Penobscot River: A Decade after Dam Removal

IntroductionEcological restoration through dam removals receives increasing attention from scientists, environmental managers and policy makers. However, most dam removal projects focus on small structures (< 10 m in height) and on few ecosystem compartments at a time (e.g. river morphology, reservoir sedimentation, aquatic invertebrates, terrestrial vegetation), but rarely aquatic and riparian ecosystems simultaneously.MethodsWe explored the joint taxonomic recovery (temporal patterns in α- and β-diversity) of three communities after the removal of the Vezins dam (36 m in height; Sélune River, France): aquatic benthic invertebrates, riparian invertebrates, and the riparian vegetation. These communities were monitored yearly, during three years after the dam removal, on sites located within (n = 3; restored sites) and outside (n = 3; two upstream and one downstream; non-impounded sites) the former reservoir.ResultsResults showed a fast recovery of the three ecological communities, as patterns in α-diversity between restored and non-impounded sites were not necessarily different from each other 1.5 years after complete reservoir dewatering. The mean number of species or morphospecies (± standard deviation) reached up to 28.2 ± 5.2, 17.0 ± 2.3 and 77.5 ± 11.2 for the aquatic invertebrates, and the riparian vegetation and invertebrates, respectively. Relative to the sampled area, the riparian invertebrates were the most diversified of all ecological communities with > 500 taxa (i.e., pooling all sites and years). In addition, in some restored sites, α-diversity kept increasing over time while species turnover (β-diversity) remained high after three years for all ecological communities suggesting a transient recovery (i.e., still facing temporal changes in species diversity and composition). This recovery was mediated by the identity of the ecological community as inter-annual changes in α- and β-diversity of the riparian vegetation were less pronounced compared to those of aquatic and terrestrial invertebrates. This recovery depended also on site-specific features as the most distant restored site from the former dam had more time for recovery following the slow reservoir dewatering and through increased sedimentation in the downstream site.DiscussionDifferential patterns of recovery in α- and β-diversity found in this study are discussed in light of species functional traits and ecosystem functioning.

Section: Introductionmentioning

confidence: 97%

Fast but transient recovery of aquatic and terrestrial communities after a large dam removal

Dézerald,

Roussel,

Lanoë

et al. 2023

“…The removals of main-stem dams as part of the PRRP has revealed the direct relation between dams and restricted access to habitat. The PRRP and associated efforts have dramatically increased the abundance of diadromous fishes within the Penobscot River (Scherelis et al, 2020;DMR, 2022;Whittum et al, 2023). Before dam removal, the American shad population was of unknown size and limited to habitat downstream of Veazie Dam.…”

Section: First: Impaired Access To Habitatmentioning

confidence: 99%

Seven dam challenges for migratory fish: insights from the Penobscot River

Zydlewski,

Coghlan,

Dillingham

et al. 2023

More than a century of impoundments in the Penobscot River, Maine, USA, has contributed to population declines in migratory fish in the system. A decade of change, research, and monitoring has revealed direct and indirect ways that dams have influenced the river habitat, connectivity for migratory fish, and the food web. The removal of two main-stem dams (in 2012 and 2013) and bolstering of fish passage have been part of coordinated restoration efforts in the watershed. Integral to this undertaking was support for short- and long-term monitoring and research that included physical habitat, fish passage, and broad scale ecological assessments. Herein we discuss the seven interconnected and complex ways that dams have affected the Penobscot River ecosystem, particularly for migratory fish. These include familiar influences ascribed to dams: i) impaired access to habitat, ii) injury and mortality, and iii) delays of migration. Other ecological influences are less studied and more subtle: iv) facilitation of predation, v) community shifts, and vi) demographic shifts. Lastly, dams result in vii) a loss of ecosystem services that would otherwise be intact in an unimpounded system. We draw on both direct examples from the Penobscot River and broader information to characterize how impoundments have transformed this ecosystem for more than a century. Recent dam removals and mitigation efforts have reestablished some of these ecological functions.

“…Those that have, primarily described changes in the river community using perhaps more basic metrics such as community composition, species richness, abundance or density (e.g., Catalano et al, 2007;Stanley et al, 2007;Maloney et al, 2008;Burroughs et al, 2010;Kornis et al, 2015;Magilligan et al, 2016;Poulos and Chernoff, 2017;Bubb et al, 2021). Furthermore, such studies often used methods that have provided information regarding only a snap-shot in time and/or space such as electrofishing, hydroacoustics, snorkeling or eDNA (e.g., Dorobek et al, 2015;Hogg et al, 2015;Duda et al, 2020;Scherelis et al, 2020;Huang et al, 2023;Jones et al, 2023;Kiffney et al, 2023;Whittum et al, 2023) and were often narrowly-scoped temporally, spatially or both (Bellmore et al, 2017;Foley et al, 2017;Whittum et al, 2023). These methods may not fully reflect fish responses, and do not reveal any direct information about fish movement and timing.…”

Section: Introductionmentioning

confidence: 99%

“…These methods may not fully reflect fish responses, and do not reveal any direct information about fish movement and timing. In the Penobscot River in Maine, for example, where two dams were removed and fish passage structures at remaining dams were improved, annual counts of migratory species at the new lowermost dam (Milford Dam) postremoval far exceed fishway passage counts at the former lowermost dam (Veazie Dam) pre-removal, while electrofishing indicated only limited movement upstream of Milford Dam post lower dam removals (Whittum et al, 2023). While some tagging studies (PIT, radio, hydroacoustic) have been performed in this system, post-dam removal work has focussed on evaluating the new fish passage structure at Milford Dam (Izzo et al, 2016), with no information regarding movement rates with respect to just the dam removals.…”

Section: Introductionmentioning

confidence: 99%

Adfluvial migration and passage of Steelhead before and after dam removal at a major Great Lakes tributary

Bunt,

Jacobson

2024

Despite the importance of Great Lakes fisheries and the increasing popularity of dam removal as a method to restore river connectivity and increase fish passage, the adfluvial migration of Steelhead (Oncorhynchus mykiss) has been drastically understudied and only relatively few published studies have examined the impacts of dam removal on fish movement and timing. To help fill these knowledge gaps, spawning migrations of Great Lakes adfluvial Steelhead tagged in spring and fall were monitored for two years before and three years after removal of a dam that partially blocked upstream movement for 100 years. Removal of the dam not only reduced downstream delay and increased passage at the site of the dam removal itself, but increased travel speeds and increased passage at remaining upstream dams for both spring and fall run fish, underscoring the cumulative impact of successive dam passage on fish migration. Fall fish were most impacted by the dam removal and were able to pass not only the dam footprint, but, for the first time, were also able to pass both remaining upstream dams, allowing them to overwinter at locations closer to known spawning areas. For these fish, delay at the footprint was reduced from the order of 100+ days to < 1 hour and the number of days passage occurred compared to the number of days fish were present and blocked increased from 2% to 82%. The benefits of dam removal should ultimately equate to increases in fish production, as more critical habitat becomes reconnected and more fish are able to pass dams and arrive at spawning beds. The observation that fish are likely able to remain in better condition and retain more energy for continued migration and reproduction warrants further study.