Predation probabilities and functional responses: How piscivorous waterbirds respond to pulses in fish abundance

Abstract

How predators respond to changes in prey abundance (i.e., functional responses) is foundational to consumer-resource interactions, predator-prey dynamics, and the stability of predator-prey systems. Predation by piscivorous waterbirds on out-migrating juvenile steelhead trout (Oncorhynchus mykiss) is considered a factor affecting the recovery of multiple Endangered Species Act-listed steelhead populations in the Columbia River basin. Waterbird functional responses, however, may vary by predator species and location, with important implications to predator management strategies. We used a 13-year dataset on waterbird abundance across seven breeding colonies (three Caspian tern [Hydroprogne caspia], two double-crested cormorant [Nannopterum auritum], and two California and ring-billed gull [Larus californicus and Larus delawarensis] colonies) and steelhead tag-recovery data (>645,000 tagged and >32,000 recovered steelhead) to quantify weekly predation probabilities and functional responses across waterbird species, colonies, and years. Weekly predation probabilities were highly variable, ranging from 0.01 to 0.30 at tern colonies, 0.01 to 0.20 at cormorant colonies, and 0.03 to 0.13 at gull colonies.Per capita predation probabilities were an order of magnitude higher at inland tern and cormorant colonies relative to estuary colonies of the same species.Terns displayed Type II functional responses across colonies and years, where predation probabilities peaked at low steelhead abundances and declined as steelhead abundance increased (i.e., predator swamping). Cormorants nesting at the large estuary colony (several thousand birds) displayed a Type III functional response, but cormorants nesting at the smaller inland colony (several hundred birds) displayed a Type II response. Consumption probabilities of steelhead by gulls remained consistent across a large range of steelhead availability, suggesting a Type I or a Type III functional response, but a lack of colony abundance data prevented quantifying functional responses. The level of tern predation combined with Type II functional responses indicate possible

A Review of Factors Affecting the Susceptibility of Juvenile Salmonids to Avian Predation

“…Multiple studies identified associations between environmental factors and smolt susceptibility to avian predation (Table 3). Per capita (i.e., per bird) predation rates on salmonids were higher at breeding colonies located in freshwater systems relative to colonies in estuaries, a result supported by mark-recovery (Evans et al 2012;Hostetter et al 2022) and diet composition studies (Collis et al 2002;Roby et al 2002;Lyons 2010). Similarly, susceptibility of salmonid smolts to both tern and cormorant predation in the Columbia River estuary were related to large-scale climatic indices (North Pacific Gyre Oscillation and spring upwelling), likely due to associations with availability of marine forage fish (alternative prey) in the estuary (Table 3; Lyons et al 2014;Evans et al 2016a).…”

“…Prey Density, Predator Density, and Migration Timing Multiple studies identified relationships between juvenile salmonid predation susceptibility and prey density, predator density, or migration timing (Table 4). Susceptibility to tern and cormorant predation increased as colony size increased (Table 4; Evans et al 2016a;Hostetter et al 2022). For example, colony-specific tern predation rates increased as the number of terns counted increased within (weekly) and across (annual) breeding seasons.…”

“…Predator-specific functional responses may best explain the relationships between prey abundance and smolt susceptibility to avian predation (Table 4; Evans et al 2016a;Roby et al 2017b;Hostetter et al 2022). Tern predation on steelhead smolts followed a type II functional response, whereby individual smolt susceptibility to tern predation declined as smolt abundance increased (Hostetter et al 2012(Hostetter et al , 2022Roby et al 2015Roby et al , 2017aRoby et al , 2017bEvans et al 2016c).…”

“…Predator-specific functional responses may best explain the relationships between prey abundance and smolt susceptibility to avian predation (Table 4; Evans et al 2016a;Roby et al 2017b;Hostetter et al 2022). Tern predation on steelhead smolts followed a type II functional response, whereby individual smolt susceptibility to tern predation declined as smolt abundance increased (Hostetter et al 2012(Hostetter et al , 2022Roby et al 2015Roby et al , 2017aRoby et al , 2017bEvans et al 2016c). Conversely, cormorants displayed type II and type III functional responses depending on colony location (Hostetter et al 2022), implying that the response of cormorants to changes in salmonid abundance may vary in freshwater versus estuarine environments.…”

“…Tern predation on steelhead smolts followed a type II functional response, whereby individual smolt susceptibility to tern predation declined as smolt abundance increased (Hostetter et al 2012(Hostetter et al , 2022Roby et al 2015Roby et al , 2017aRoby et al , 2017bEvans et al 2016c). Conversely, cormorants displayed type II and type III functional responses depending on colony location (Hostetter et al 2022), implying that the response of cormorants to changes in salmonid abundance may vary in freshwater versus estuarine environments. Whether this is driven by predator attraction, prey switching, predator swamping, or other seasonal factors remains unknown (Table 4; Hostetter et al 2012Hostetter et al , 2022Roby et al 2015Roby et al , 2017aRoby et al , 2017bEvans et al 2016c).…”