Functional response and size‐dependent foraging on aquatic and terrestrial prey by brown trout (Salmo trutta L.)

Abstract

Terrestrial invertebrate subsidies are believed to be important energy sources for drift-feeding salmonids. Despite this, size-specific use of and efficiency in procuring this resource have not been studied to any great extent. Therefore, we measured the functional responses of three size classes of wild brown trout Salmo trutta (0+, 1+ and ‡2+) when fed either benthic-(Gammarus sp.) or surface-drifting prey (Musca domestica) in laboratory experiments. To test for size-specific prey preferences, both benthic and surface prey were presented simultaneously by presenting the fish with a constant density of benthic prey and a variable density of surface prey. The results showed that the functional response of 0+ trout differed significantly from the larger size classes, with 0+ fish having the lowest capture rates. Capture rates did not differ significantly between prey types. In experiments when both prey items were presented simultaneously, capture rate differed significantly between size classes, with larger trout having higher capture rates than smaller trout. However, capture rates within each size class did not change with prey density or prey composition. The two-prey experiments also showed that 1+ trout ate significantly more surface-drifting prey than 0+ trout. In contrast, there was no difference between 0+ and ‡2+ trout. Analyses of the vertical position of the fish in the water column corroborated size-specific foraging results: larger trout remained in the upper part of the water column between attacks on surface prey more often than smaller trout, which tended to seek refuge at the bottom between attacks. These size-specific differences in foraging and vertical position suggest that larger trout may be able to use surface-drifting prey to a greater extent than smaller conspecifics.

Causes and consequences of ontogenetic dietary shifts: a global synthesis using fish models

“…A number of laboratory and field studies have identified size‐dependent effects on the foraging modes of fishes and, ultimately, on ODSs (e.g. Nakano et al, ; Persson & Brönmark, , ; Gustafsson et al, ; Sánchez‐Hernández & Cobo, ). Gustafsson et al .…”

Effects of temperature on the foraging and growth rate of juvenile common carp, Cyprinus carpio

“…the feeding rate of a forager as a function of prey density (Soloman, 1949;Holling, 1959), with this important in providing insights into the dynamics of consumer-prey systems (Nilsson, 2001). Functional responses have been determined for a number of fishes, including brown trout Salmo trutta (Gustafsson et al, 2010), pink salmon Oncorhynchus gorbuscha and chum salmon Oncorhynchus keta (Moss and Beauchamp, 2007), lake trout Salvelinus namaycush (Barnhisel and Kerfoot, 2004) and walleye Sander vitreus (Galarowicz and Wahl, 2005). Feeding rates have been used in an invasion context to reveal superior food intake rates of invasive mosquitofish Gambusia holbrooki over native fishes in Iberia that assisted their invasion and resulted in declines of some native toothcarps (Caiola and de Sostoa, 2005).…”

The role of temperature in the prey capture probability of drift‐feeding juvenile brown trout (Salmo trutta)

“…A foraging trial began after we had observed the fish starting to feed actively and the trials were videotaped on mini DV cassettes. Prey were randomly assigned to the five funnels, and fed at random times, at intervals between 8 and 16 s. The intervals were designed to be long enough for handling times not to play a significant role (Gustafsson et al 2010). The tubes were attached at a distance of 0.10 m from one another to the front steel mesh, with the middle tube centred (Fig.…”