Geology topics

Shawn Snyder

Publications and source records attributed to Shawn Snyder.

3 recordsLinked to original sources

USGS2026

Impacts of dams on river herring populations through their native range: What’s left?

Introduction: River herring (alewife, Alosa pseudoharengus , and blueback herring, Alosa aestivalis ) are native along the East Coast of North America. These fish are culturally important and provide critical ecosystem linkages between marine and freshwater habitats. Due to overfishing, and habitat loss (through degradation and damming), river herring populations are at historical lows across their native range. In the last 200 years, dams and other impoundments have reduced access to spawning habitat for these and other anadromous fish. Methods: To assess the theoretical coast-wide spawning potential for river herring, we quantified historically accessible spawning habitat (pre-dam) in coastal freshwater rivers using physical river characteristics (width and gradient). To assess the impact of dams on spawning habitat, we used a dams database to segment river reaches. This allowed us to characterize spawning habitat upstream and downstream of dams. River-specific population models were then used to estimate the number of potential spawners for each species, based on habitat estimates and life history parameters. We investigated three scenarios: no dams, favorable dam passage, and no dam passage. We use these simulations to compare the theoretical spawning potential for river herring prior to and after dam construction coastwide. Results: We estimated that 52% of alewife and 51% of blueback herring habitat is located upstream of dams throughout the East Coast of North America. This results in a theoretical loss of ~880 million alewife and ~100 million blueback herring potential spawners due to dams, reducing both the ecological connection and fishery potential of these species. Discussion: Even with the best-case “current” scenario for fish passage, we observed marginal increases in spawner abundance. These results highlight the relatively small theoretical influence current passage structures may have on restoring river herring to their historical abundances compared to dam removal. Our sensitivity analyses demonstrate that upstream passage to access additional habitat is only advantageous when effective juvenile and adult downstream passage are in place.

East Coast of North America↗

USGS2026

Working group on American Eel (WGAMEEL; outputs from 2024 meeting)

The Working Group on American Eel (WGAMEEL) met virtually three times in 2022-2024 to address the five Terms of Reference (ToRs) of its three-year term. The first two ToRs tasked WGAMEEL with listing and evaluating data on American eel landings, abundance indices, and spatial and habitat data and also to describe assessment methods used in Canada and the US. Canada-wide American eel trajectory was estimated using 12 fishery-independent datasets. Generally, longer datasets had more negative trends than shorter ones. Limiting data to the post-2000 years produced fewer negative trends that did not differ from zero, suggesting the observed declines occurred pre-2000. Spatial modelling for American eel requires knowledge and mapping that covers the breadth of habitat types occupied by the species, including freshwater, estuarine, and marine environments. In recent years there has been an expansion of online databases with data from the aquatic environment, particularly in freshwater, with estuarine and marine data less consistently documented. This report broadly compiles abiotic data series of relevance to American eel. A larger challenge for spatial modelling will be acquiring enough high quality, georeferenced biological data sets with suitable observations to assess occurrence, abundance, and trends over time in a spatial framework. The third ToR considered Indigenous Knowledge Systems for American eel. A survey reaching First Nations representatives from four Canadian provinces confirmed the cultural importance of eels in Indigenous communities, and that Indigenous knowledge possessed by the participant groups are place-based and contextual, especially regarding threats impacting eels. The final two ToRs focused on identifying stock assessment modelling approaches applicable to American and European eel, and assessing whether any of these approaches might be appropriate for American eel management moving forward. WGAMEEL evaluated the various approaches for assessing American eel or providing management advice. Two approaches that could be completed in the next few years because of their minimal data needs are index-based methods and catch-only method. A suite of approaches considered by WGAMEEL that would take more time and data were spatial or habitat models, management strategy evaluation, and spawner-per-recruit (SPR) models potentially paired with meta-population models.

ICES Scientific Reports↗

USGS2025

Assessing American eel (Anguilla rostrata) distribution in a heavily dammed watershed using eDNA : The Penobscot River watershed, Maine, USA

Catadromous American eel ( Anguilla rostrata ) are native to Maine's Penobscot River watershed and historically have migrated through many of its tributaries prior to extensive damming. Recent restoration efforts, including dam removals, have improved connectivity in the lower reaches of the Penobscot River. Characterizing the extent of the American eel's distribution is important to inform restoration and identify extant barriers to migrations within the watershed. In the summer of 2023, we conducted eDNA surveys throughout the Penobscot River watershed to estimate the current distribution of the American eel and identify barriers to inland waters. Water samples were collected from 70 sites representing 37 rivers and streams; the presence or absence of American eel genetic markers within those samples was assessed using qPCR. We have shown that American eel are present in virtually the full extent of the area surveyed (68/70 sites). The results suggest that the majority of the main-stem dams may be passed by American eels at some level, with eel DNA being confirmed upstream of six dams. We confirmed the presence of American eels throughout the lower watershed with just 1 week of eDNA sampling and have highlighted this method for determining the species' access to habitat upstream of dams. The use of eDNA to sample locally (or regionally) for American eel may provide cost-effective information in data deficient areas and help assess the permeability of dam structures to diadromous species.

Maine↗