Selectivity by planktivorous fish at different prey densities, heterogeneities, and spatial scales

Abstract

Studies in artificially confined, small‐scale experimental systems with a homogeneous prey distribution suggest that selectivity in planktivorous fish for more conspicuous and rewarding prey (such as adult Daphnia with eggs in the brood cavities) is stronger at high than at low prey population density. We examined whether prey selectivity is altered when fish are allowed to feed in larger scale systems and on more heterogeneously distributed prey. The experimental system was comprised of 1000 liter tanks (one, four, or eight), interconnected to allow free movement of roach (Rutilus rutilus, eight per tank), with different densities of Daphnia prey, distributed with varying levels of homogeneity. As reported previously, the selectivity of roach increased at higher Daphnia densities, but only in small‐scale systems where prey distribution was homogeneous. However, the opposite was found in large‐scale systems with heterogeneous prey distribution, i.e., greater selectivity at lower prey densities. The reversed slope of the selectivity‐on‐prey‐density regression was linked to increased swimming speed at low prey density, where the fish, less constrained by the limited size of the experimental system, accelerate, possibly in an effort to compensate for low encounter rate and to more rapidly locate patches of prey. More selective feeding on scarce and patchy prey may result from the decreased relative visibility of smaller prey and from the increased capture costs due to the need to expend more energy for postcapture accelerations to regain a higher speed.

Top-down control of invertebrates by Ninespine Stickleback in Arctic ponds

“…Predation pressure on specific taxa led to shifts in invertebrate community structure (similar to findings by Bendell and McNicol 1987, Herbst et al 2009, Winkelmann et al 2011. Fish diet breadth is hypothesized to be narrower and selection of larger prey to be stronger when prey exist at high densities Hall 1974, Maszczyk andGliwicz 2014). Ninespine Stickleback exhibited this type of prey selectivity early in the experiment, when 50 to 70% of their diet consisted of 1 or 2 relatively abundant taxa, Daphniidae and Baetidae.…”

Interference competition in a planktivorous fish (Rutilus rutilus) at different prey densities and temperatures

“…The results did not reveal any difference in the strength of interference at different prey density levels, even though it might be expected that interference would be stronger at low prey densities when fish swim faster in an attempt to compensate for a lower prey encounter rate (Munk andKiørboe 1985, Maszczyk andGliwicz 2014). The effect of higher swimming speed could be even more pronounced in a spatially large-scale system with heterogeneously distributed prey, where fish are able to swim with appropriate speed.…”

Bridge under troubled water: Turbulence and niche partitioning in fish foraging

“…Because the turbulence in the ponds was not strong enough to affect the swimming of fish, but strong enough to affect zooplankton distribution, encounter rates between roach and Bosmina likely increased. Roach selectivity decreases with increasing densities of zooplankton (Maszczyk & Gliwicz, ), which together with the increased encounter rate led to increased consumption of Bosmina . Moreover, as turbulence disturbs evasive responses in copepods (Härkönen et al., ; Saiz & Alcaraz, ), roach foraging on copepods was enhanced in turbulent water.…”

“…Turbulence can, for instance, disperse planktonic animals and thereby affect encounter rates between planktivores and their prey (Baranyai, G.‐Toth, Vári, & Homonnay, ; Joensuu, Pekcan‐Hekim, Hellèn, & Horppila, ; MacKenzie, Miller, Cyr, & Leggett, ; Rothschild & Osborn, ). As prey selection by fish is often density‐dependent and species‐specific (e.g., Maszczyk & Gliwicz, ; Werner & Hall, ), turbulence may affect encounter rates, prey selectivity, and thereby competitive interactions between fish species. Moreover, turbulence could affect encounter rates differently between foraging strategies of competitors (e.g., cruising predators vs. pause‐travel predators; MacKenzie & Kiørboe, ), with consequences for interaction strengths between coexisting competitors.…”