Predator swamping reduces predation risk during nocturnal migration of juvenile salmon in a high‐mortality landscape

Abstract

Animal migrations are costly and are often characterized by high predation risk for individuals. Three of the most oft-assumed mechanisms for reducing risk for migrants are swamping predators with high densities, specific timing of migrations and increased body size. Assessing the relative importance of these mechanisms in reducing predation risk particularly for migrants is generally lacking due to the difficulties in tracking the fate of individuals and population-level characteristics simultaneously. We used acoustic telemetry to track migration behaviour and survival of juvenile sockeye salmon (Oncorhynchus nerka) smolts released over a wide range of conspecific outmigration densities in a river associated with poor survival. The landscape was indeed high risk; smolt survival was poor (˜68%) over 13·5 km of river examined even though migration was rapid (generally <48 h). Our results demonstrate that smolts largely employ swamping of predators to reduce predation risk. Increased densities of co-migrant conspecifics dramatically improved survival of smolts. The strong propensity for nocturnal migration resulted in smolts pausing downstream movements until the next nightfall, greatly increasing relative migration durations for smolts that could not traverse the study area in a single night. Smolt size did not appear to impact predation risk, potentially due to unique characteristics of the system or our inability to tag the entire size range of outmigrants. Movement behaviours were important in traversing this high-risk landscape and provide rare evidence for swamping to effectively reduce individual predation risk.

Post‐release predation mortality of age‐0 hatchery‐reared Chinook salmon from non‐native smallmouth bass in the Snake River

“…Nocturnal releases could reduce foraging efficiency of visual-based predators near release sites and give fish more time to disperse downstream (Furey, Hinch, Bass, et al, 2016;Ruggerone & Rogers, 1983). First, studies assessing seasonal consumption of hatchery prey by predators need to account for short-term, intensive predator feeding that could bias consumption and loss estimates if sampling missed such a behavioural event.…”

“…The temporal and spatial variability among releases and release locations, as found in this study, illustrates the importance of making evaluations on a per-release basis. Thus, releasing all hatchery fish on one day, if possible, instead of every other day as occurred at Hells Canyon Dam, could potentially reduce predation by swamping predators with prey (Fritts & Pearsons, 2008;Furey, Hinch, Bass, et al, 2016). Nocturnal releases could reduce foraging efficiency of visual-based predators near release sites and give fish more time to disperse downstream (Furey, Hinch, Bass, et al, 2016;Ruggerone & Rogers, 1983).…”

The Economic Impacts of Humpback Whale Depredation on Hatchery‐Released Juvenile Pacific Salmon in Southeast Alaska

“…That strategy is supported by studies in freshwater, where juvenile salmon have been shown to survive better when migrating in large numbers, satiating place‐based predators (Furey et al. ). However, filter‐feeding baleen whales, like humpback whales, rely on dense aggregations of prey to forage efficiently (Piatt and Methven ; Deméré et al.…”

Ecological implications of changing hatchery practices for Chinook salmon in the Salish Sea

“…Numerical responses by avian (mergansers, Mergus merganser) and piscivorous fish species (spiny dogfish, Squalus suckleyi) have been documented in the Salish Sea in response to high densities of hatchery salmonids (Wood 1987, Beamish et al 1992. Alternatively, high densities of migrating juvenile salmonids have been shown to produce predator swamping effects, which result in lower mortality rates (Furey et al 2016). Alternatively, high densities of migrating juvenile salmonids have been shown to produce predator swamping effects, which result in lower mortality rates (Furey et al 2016).…”