Jonas Dahl

Effects of a benthivorous and a drift-feeding fish on a benthic stream assemblage

Abstract I assessed the impact of both drift-feeding (Salmo trutta, brown trout) and benthic-feeding (Cottus gobio, bullhead) fish on a benthic assemblage during a 1-month-long field experiment. I used enclosures containing cobble/gravel substrata with 6-mm mesh net that allowed invertebrates to drift freely in and out of enclosures. Four treatments, arranged in a factorial design, were tested: a predator-free control, bullheads only (2.67 bullheads/m2, two per enclosure), brown trout only (2.67 brown trout/m2, two per enclosure), and bullheads and brown trout together (2.67 fish/m2, one of each). Bullheads reduced the densities of seven invertebrate taxa (Gammarus pulex amphipods, Baetis rhodani mayfly nymphs, Leuctra spp. stonefly nymphs, Polycentropus spp. caddis larvae, Pacifastacus leniusculus signal crayfishes, Simuliidae, blackfly larvae, and Limnephilidae, caddis larvae) whereas brown trout only affected one taxon (B. rhodani). The weaker effect of brown trout on benthic prey was probably related to its heavy reliance on terrestrial prey. Dietary analyses showed that more than 80% of prey consumed by brown trout were terrestrial animals, whereas bullhead only consumed benthic prey. Neither bullhead nor brown trout affected the absolute number of immigrating or emigrating invertebrates in enclosures, but bullhead affected the per capita emigration rates of G. pulex. Direct predation by bullhead was more important than avoidance behaviour (drift) in determining densities of six of the seven taxa; only G. pulex densities were equally affected by avoidance behaviour and direct predation. Direct predation by brown trout was also more important in determining densities of B. rhodani. The presence of bullhead raised periphyton biomass, presumably mediated via their consumption of grazers. Brown trout had no effect on periphyton biomass and these results indicate that the presence of alternative prey, in this case terrestrial animals, may have repercussions for fish-benthic macroinvertebrate-periphyton interactions and may potentially explain some of the variable impacts of fish on benthic macroinvertebrates that have been reported in the literature.

PREDATOR CHEMICALS INDUCE CHANGES IN MAYFLY LIFE HISTORY TRAITS: A WHOLE-STREAM MANIPULATION

In high-elevation streams of western Colorado, mayflies ( Baetis bicaudatus) develop faster, but mature at a smaller size where trout are present compared to streams where fish are absent. These life history traits reduce the time of larval exposure to trout predation, but cost reduced fecundity. We designed a field experiment involving manipu- lation of whole streams to determine whether these changes were caused by the presence of brook trout, and specifically, whether they could be triggered by trout chemical cues. In 1999 and 2000, we introduced water from containers with brook trout (Salvelinus fon- tinalis) into five naturally fishless streams, and fishless stream water into five adjacent control streams, to determine whether these cues alone could induce the mayfly life history traits we have observed in natural trout streams. As in previous small-scale experiments, the size at which mayflies matured declined significantly in streams with added trout chem- icals but did not change in streams with fishless water only. Thus, life history traits similar to those observed in the field were induced within the natural variability inherent in streams. These results demonstrate the strength of this predator-prey interaction and indicate that brook trout are an important agent of natural selection on mayfly life history traits.

Impact on stream benthic prey by benthic vs drift feeding predators : a meta-analysis

The impact that predators have on invertebrate prey density in running waters is the subject of debate. Some studies show strong effects of fish predation on benthic prey density whereas others show little or no effects. Recent reviews discuss different factors that affect interactions between predators and invertebrate prey. These include differences in enclosure size, mesh size, predator densities, small sample sizes and the role of invertebrate predators. We suggest the complementary hypothesis that vertebrate predators used in experiments differ in foraging methods. Some predators feed primarily on benthic prey (e.g. sculpins, blacknose dace, creek chub, darters) whereas other predators feed on both drift and benthic prey (predominantly salmonids). Potentially, if the amount of drifting terrestrial animals is large, the impact of drift-feeding fish on the benthic community should be low. We conducted a meta-analysis on the results of field studies that manipulated these two types of predators and found that benthic feeding predators have significantly larger impacts on benthic prey than drift feeding predators. Four methodological variables, mesh size, size of enclosures, duration of experiments, and predator density were analysed to determine whether these could explain the differences between studies involving benthic and drift feeding predators. No correlations existed between any of these four methodological variables and predator impact, indicating that the difference was not due to methodological differences between experiments.

The impact of vertebrate and invertebrate predators on a stream benthic community

Abstract I assessed the impact of both vertebrate and invertebrate predators on a lotic benthic community in a 1-month-long experiment, using enclosures containing cobble/gravel bottoms, with large-mesh netting that allowed invertebrates to drift freely. Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) were used as predators and four treatments were tested: a predator-free control, leeches only, trout only, and leeches and trout together. A density of 26.7 leeches/m2 (20 leeches/enclosure) and 1.3 trout/m2 (one trout per enclosure) was stocked into the enclosures. The total biomass of invertebrate prey was significantly lower in the trout and trout plus leech treatments than in the leech and control treatments, which were due to strong negative effects of trout on Gammarus. On the individual prey taxon level, both trout and leeches affected the abundance of Asellus, Baetis and Ephemerella, whereas the abundance of Gammarus was only affected by trout, and the abundance of Orthocladiinae and Limnephilidae was only affected by leeches. In the treatment with trout and leeches together, the abundance of Ephemerella and Baetis was higher than when trout or leeches were alone, which was probably due to predator interactions. Leeches and trout had no effects on prey immigration but did affect per capita emigration rates. Both trout and leeches indirectly increased periphyton biomass in enclosures, probably due to their strong effects on grazers. Both trout and leeches were size-selective predators, with trout selecting large prey, and leeches selecting small prey. Size-selective predation by trout and leeches affected the size structure of five commonly consumed prey taxa. Trout produced prey populations of small sizes owing to consumption of large prey as well as increased emigration out of enclosures by these large prey. Leech predation produced prey assemblages of larger size owing to consumption and increased emigration of small prey. These results suggest that in lotic habits, predatory invertebrates can be as strong interactors as vertebrate predators.

Foraging rates of a vertebrate and an invertebrate predator in stream enclosures

We compared prey consumption by brown trout (Salmo trutta) and leeches (Erpobdella octoculata) when they occurred together and alone in stream enclosures. One density of trout was compared with three densities of leeches during 24-h-long experiments. Separate experiments were conducted with two prey taxa, Gammarus pulex and Baetis rhodani, using two types of substrate, fine gravel and fine gravel plus cobbles. We found that trout had a higher foraging rate over fine gravel bottoms than over cobble bottoms, whereas leeches were more effective over cobble bottoms. Prey type also affected foraging rates, with trout consuming more Baetis than Gammarus, and leeches consuming more Gammarus than Baetis. A direct comparison of foraging rates of trout and leeches when alone showed that trout consumed more prey than the highest density of leeches, except when Gammarus occurred over cobble bottoms; then trout consumed prey at the same rate as the lowest leech density. When leeches and trout were together, trout foraging was unaffected by leeches, but leech foraging was affected by trout when feeding on Baetis but not Gammarus. These results show that one leech consumes much less than one trout, but at representative densities, leeches can have similar impacts on prey per area of stream, suggesting that the role of invertebrates in structuring benthic communities can be similar to that of fish.

Complex trophic interactions in freshwater benthic food chains

Complex trophic interactions play an important role in structuring benthic communities in lakes and streams. In this review of freshwater, benthic food chains, a variety of different types of complex interactions were identified, including trophic cascades, interaction modifications and indirect commensalisms. The strength of these interactions varied greatly among studies, depending on habitat complexity, the type and intensity of environmental stresses, the ways in which prey defended themselves from predators, and the degree of trophic omnivory in the food chain. Areas that deserve further study include the effect of trophic omnivory on interaction strength and indirect effects of piscivores in benthic communities. Future studies should include whole-system, long-term manipulations to deepen our understanding of the role of complex interactions in freshwater communities.

Effects of fish predation and habitat type on stream benthic communities

The influence of habitat on interactions between a fish predator (brown trout Salmo trutta) and a benthic invertebrate community was studied in nine field enclosures (8 ×3 m) in a creek in southern Sweden. Three habitat treatments were tested, a shallow sandy habitat, a deep habitat containing a mixture of large and small cobbles and a moderately deep habitat with large cobbles. The one month-long experiment showed that there were no major differences in the abundance and biomass of the benthic macroinvertebrate fauna among these habitats as no functional groups of invertebrates and only a few taxa differed between treatments. Invertebrate drift rates decreased over time, which was probably related to seasonal changes in invertebrate life cycles or to effects of predation independent of habitat type, as there was no difference between treatments.

Indirect behavioral effects of a piscivore on trophic interactions in stream enclosures

Abstract:The indirect behavioral effect of piscivorous pike (Esox lucius) on the growth and diet of brown trout (Salmo trutta) as well as the consequences of the pike-trout interaction for benthic macroinvertebrates and periphytic algae were studied in nine 2 × 3 m field stream enclosures. The indirect effect of pike on trout behavior was studied by holding trout density relatively constant, which was done by replacing consumed trout during the course of the experiment. Three treatments were established, each replicated three times: fishless controls and two fish treatments, one with 15 brown trout (2.5 trout m-2) and one with 15 trout and 1 northern pike (0.17 pike m-2). Growth of brown trout was lower in the presence of pike than in the absence of pike. Pike affected the biomass of the leech, Erpobdella, whose biomass was greater in the presence of pike than in its absence. The biomass of grazing heptageniids was lower in the trout only treatment than in the other two treatments. Moreover, the number of grazing Baetis observed over algal-cultured ceramic tiles was lower in the two different treatments with trout than in the fishless controls. Periphytic chlorophyll-a biomass reflected trouts effect on Baetis as chlorophyll-a biomass was similar in all enclosures with trout (with and without pike), and these enclosures had higher biomasses of chlorophyll-a than the fishless controls.