Tide gates form physical and ecological obstacles to river herring (Alosa spp.) spawning migrations

River herring (Alosa spp.) are anadromous fish that enter North American Atlantic coastal rivers and lakes each spring to spawn. Anthropogenic structures such as dams and tide gates serve as physical obstacles that limit river herring access to spawning habitat. This study examined the physical and ecological components affecting herring passage through a tide gate by applying a time-to-event analysis framework to multiple movement behaviors derived from telemetry data. Herring had higher passage success early in the season (78%) than later on (16%). Key behaviors that govern passage varied with diel period, tide, and flow direction through the gates. Furthermore, these behaviors shifted as the season progressed, consistent with the hypothesis that predator avoidance may be driving passage failure late in the spawning season.

Wait and snap: eastern snapping turtles ( Chelydra serpentina ) prey on migratory fish at road-stream crossing culverts

There is growing evidence that culverts at road-stream crossings can increase fish density by reducing stream width and fish movement rates, making these passageways ideal predator ambush locations. In this study, we used a combination of videography and δ 13 C stable isotope analyses to investigate predator–prey interactions at a road-stream crossing culvert. Eastern snapping turtles ( Chelydra serpentina ) were found to regularly reside within the culvert to ambush migratory river herring ( Alosa spp.). Resident fish species displayed avoidance of the snapping turtles, resulting in zero attempted attacks on these fish. In contrast, river herring did not display avoidance and were attacked by a snapping turtle on 79% of approaches with a 15% capture rate. Stable isotope analyses identified an apparent shift in turtle diet to consumption of river herring in turtles from culvert sites that was not observed in individuals from non-culvert sites. These findings suggest that anthropogenic barriers like culverts that are designed to allow passage may create predation opportunities by serving as a bottleneck to resident and migrant fish movement.

Evidence for temperature-dependent shifts in spawning times of anadromous alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis)

We analyzed four decades of presence–absence data from a fishery-independent survey to characterize the long-term phenology of river herring (alewife, Alosa pseudoharengus; and blueback herring, Alosa aestivalis) spawning migrations in their southern distribution. We used logistic generalized additive models to characterize the average ingress, peak, and egress timing of spawning. In the 2010s, alewife arrived to spawning habitat 16 days earlier and egressed 27 days earlier (peak 12 days earlier) relative to the 1970s. Blueback herring arrived 5 days earlier and egressed 23 days earlier (peak 13 days earlier) in the 2010s relative to the 1980s. The changes in ingress and egress timing have shortened the occurrence in spawning systems by 11 days for alewife over four decades and 18 days for blueback herring over three decades. We found that the rate of vernal warming was faster during 2001–2016 relative to 1973–1988 and is the most parsimonious explanation for changes in spawning phenology. The influence of a shortened spawning season on river herring population dynamics warrants further investigation.