Predator and prey events at the entrance of a surface‐oriented fish collector at North Fork Dam, Oregon

Abstract

Quantifiable estimates of predator–prey interactions and relationships in aquatic habitats are difficult to obtain and rare, especially when individuals cannot be readily observed. To overcome this observational impediment, imaging sonar was used to assess the cooccurrence of predator‐size fish and juvenile salmonids, Oncorhynchus spp., at the entrance to a floating surface collector (FSC) in the forebay of North Fork Dam on the Clackamas River, Oregon (USA). Imaging sonar can be used to transform active sound waves into visual data, making it possible to obtain continuous underwater observations on the presence and interspecific interactions between predator‐size fish and prey (juvenile salmonids). Hourly counts of smolt‐size fish tracks, diel phase, water clarity and river discharge were used as covariates within a zero‐inflated Poisson model to determine how these factors may influence the number of predators in front of the FSC. Both the number of smolt‐size fish tracks and diel phase had the strongest effects on the number of predator‐size fish tracks, with more predator‐size fish tracks observed during the daytime, and as the number of smolt‐size fish tracks increased. Additionally, the presence of predator‐size fish may affect the abundance and direction of travel of juvenile salmonids, as fewer smolt‐size fish were observed when predators were present, and a greater proportion of smolt‐size fish were observed travelling away from the FSC when predator‐size fish were present. This study provides estimates of predator and prey fish abundance in the vicinity of surface collection systems at moderate‐sized hydropower projects and could help resource managers better understand mechanisms that can influence the survival and passage behaviour of juvenile salmonids using surface collection structures at dams.

Impact of anthropogenic infrastructure on aquatic and avian predator–prey interactions in a modified lowland river

“…Indeed, a scarcity of upstream habitat availability and tendency for dispersed prey populations to form large shoals over winter combine to speculate that prey fish in this study were paradoxically dependent on hazardous anthropogenic structure for refuge, and aquatic and avian predators have learnt to exploit this, similar to that proposed by Smith et al. (2020). This refuge provided the greatest level of protection for prey, but it is likely that resource quality is low i.e., access to food, space, and light (Donelan et al., 2017), which could counterintuitively exert stronger non‐consumptive effects of predators (Orrock et al., 2013), potentially imposing individual (growth, foraging) and population level (reproduction) fitness costs.…”

“…Whilst fish in healthy ecosystems move between refuges to avoid predators, prey fish in the modified lowland setting studied here were probably confined to living with predators in hazardous habitats. Indeed, a scarcity of upstream habitat availability and tendency for dispersed prey populations to form large shoals over winter combine to speculate that prey fish in this study were paradoxically dependent on hazardous anthropogenic structure for refuge, and aquatic and avian predators have learnt to exploit this, similar to that proposed by Smith et al (2020). This ref-…”

“…Simultaneous observation and quantification of predators and prey, as presented here, is rare due to the methodological and financial challenges of studying such interactions in the wild using passive methods (see Becker & Suthers, 2014; Campanella et al., 2019; Smith et al., 2020). The overall predation rates presented in this paper should be considered minimum estimates; the real‐world predation rate is presumably higher since the PREs were subsampled from four discrete time periods, and data were limited by the width of the sonar field‐of‐view.…”

“…Additionally, many of these structures operate infrequently (e.g., once in 5 years), meaning large aggregations of fish accumulate over time and especially during winter. However, these refuges come with several caveats; when pumps operate they become hazardous to prey fish, with a high likelihood of mortality (Rytwinski et al., 2017) and because they are often situated at terminal points (e.g., confluences), they can concentrate and confine fish (e.g., Smith et al., 2020). The latter exacerbates density‐dependant predation (Lannin & Hovel, 2011) and may counterintuitively enhance the non‐consumptive effects of predators (e.g., Donelan et al., 2017).…”

A jump-driven self-exciting stochastic fish migration model and its fisheries applications

“…There are several complex and mutually interacting factors, that can trigger fish migration, including but are not limited to, the water flow discharge [12], water temperature [13], nutrient limitation [14], environmental heterogeneity or genetic predisposition [15], and social interactions [16]. It is infeasible to fully describe the fish migration mechanism due to a number of random biological phenomena that can affect both inter-and intra-migration events, as suggested in the reported data for diverse case studies [17][18][19][20][21][22]. A concise -but nonetheless sufficiently realistic -stochastic process model is therefore essential for the analysis of fish migration.…”