Ontogenetic niche shifts and resource partitioning in a subarctic piscivore fish guild

The feeding ecology of three piscivorous fish species (perch (Perca fluviatilis), pike (Esox lucius) and burbot (Lota lota)), was studied in the subarctic Pasvik watercourse (69 °N), northern Norway and Russia. All three species primarily occupied the benthic habitats in the watercourse. Perch and burbot exhibited distinct ontogenetic niche shifts in food resource use, perch changing from a dominance of zooplankton to zoobenthos to fish, and burbot from zoobenthos to fish. Fish prey dominated the diet of all the investigated size-classes of pike, but small-sized pike (<20 cm) were not represented in the sample. Fish prey size was positively related to predator size in all three species. Whitefish (Coregonus lavaretus) was the dominant prey of pike and large-sized burbot and perch. Nine-spined sticklebacks (Pungitius pungitius) was also an important prey and appeared to be a dietary stepping-stone enhancing the transition from invertebrate feeding to consumption of large-sized whitefish prey for all three predators. A cluster analysis separated the different size groups of the three predator species into five functional feeding groups, most of them containing two or all three species. Within these feeding groups, and especially among the piscivorous size-classes, there was a strong and significant interspecific overlap in prey selection, and the dietary similarities between the species were in general much larger than the intraspecific similarities between ontogenetic stages. All three piscivorous species are important top predators in the aquatic food web of the watercourse, and their ontogenetic diet shifts and resource partitioning patterns generate a substantial food web complexity in this subarctic ecosystem.     

Behavioural variation between piscivore and insectivore rainbow trout Oncorhynchus mykiss

A proactive-reactive continuum integrating multiple (i.e., 3+) dimensions of animal behaviour has been reported as a major axis of behavioural differentiation, but its stability along a biological hierarchy from individuals to populations remains speculative. Piscivore and insectivore rainbow trout (Oncorhynchus mykiss) represent closely related ecotypes with strong ecological divergence driven by selection for a large-bodied piscivorous lifestyle with fast juvenile growth vs. selection for smaller adult body size and lower growth associated with an insectivorous diet. To evaluate whether differences in behaviour between ecotypes are consistent with a proactive-reactive axis and consistent along a biological hierarchy, the authors examined variation in emergence time from a shelter, exploration, activity and predator inspection among individuals, populations and ecotypes of juvenile piscivore and insectivore rainbow trout O. mykiss. As expected, the faster-growing piscivore ecotype was more proactive (i.e., shorter emergence time, exploration and predator inspection) than the more reactive insectivore ecotype. This behavioural contrast was partly maintained across populations, although activity differences were most pronounced among populations, rather than emergence time. Insectivore fry showed substantial variation in behavioural expression among individuals within populations; by contrast, piscivores showed highly similar proactive behaviours with significantly lower inter-individual variation in behavioural expression, suggesting intense selection on behaviour supporting their faster growth. This work suggests that piscivore and insectivore O. mykiss broadly differ in behaviour along a proactive vs. reactive continuum, and highlights the greater multidimensionality of behavioural expression within the insectivore ecotype. Contrasting behaviours between ecotypes may result from differential selection for slow vs. fast juvenile growth and associated metabolism, and may contribute to adult trophic specialization.     

Daily ration and prey size of juvenile piscivore Japanese Spanish mackerel

Daily ration of juvenile Japanese Spanish mackerel Scomberomorus niphonius (32·1–33·1 mm standard length, L _S) was estimated at three temperatures (18·6, 19·5 and 21·2° C) in the laboratory. Gastric evacuation rate ranged between 0·398 (18·6° C) and 0·431 (21·2° C). Diel change in instantaneous consumption rate indicated that juvenile Japanese Spanish mackerel are daylight feeders. The estimated values of the daily ration ranged between 66·1%(18·6° C) and 82·6%(21·2° C) of body mass. These per cent values of daily ration were converted to daily consumption in mg (28 mg at 18·6° C to 34 mg at 21·2° C) using the mean dry body mass of juvenile Japanese Spanish mackerel of 30 mm (42·1 mg). Stomach content analysis of wild specimens collected in the Seto Inland Sea, south‐western Japan, revealed that the majority of wild Japanese Spanish mackerel larvae and juveniles ingested fish larvae with a body size >50% of the predator L _S. Based on the predator‐prey size relationship, the daily consumption of a Japanese Spanish mackerel juvenile of 30 mm was equivalent to 5·1 (18·6° C) to 6·4 (21·2° C) Japanese anchovy Engraulis japonicus larvae which was the dominant prey organism in stomachs of the wild Japanese Spanish mackerel.     

Migration patterns of a potamodromous piscivore,asp (Leuciscus aspius), in a river–lake system

Potamodromous fishes require safe migration routes between spawning, feeding and wintering habitats to complete their life cycle. As knowledge on asp migrations is restricted, this work investigated the movements of adult asp tagged with acoustic transmitters for 3 years in the large Peipsi-Emajõgi-Võrtsjärv lake–river system, Estonia, which is free of migration barriers. Asp showed complex migration patterns, moving between and within different waterbodies (lakes, river, tributaries) in all seasons, but with a tendency to repeat habitat use patterns between years. Lakes were mainly used for feeding during spring and summer (after spawning 65% of the fish migrated to Lake Peipsi), and more so by large fish. The majority (80–96%) of the fish spent the winter in the rivers, mostly close to their subsequent spawning area. Spawning areas were in swift-flowing waters in tributaries and the main river. The results indicate that asp may benefit from an extensive and diverse complex of habitats, and any migration barrier during any season may restrict the natural habitat use by asp. Maintenance and restoration of habitat heterogeneity and connectivity is critical to protect behaviourally diverse fish populations and increase resilience in rivers negatively impacted by various human activities.     

The piscivore feeding guild of fishes in small freshwater ecosystems

Synopsis Analysis of the piscivore guild in fish species-rich lake and stream systems in eastern Ontario showed the co-occurrence of three types: (1) specialists that became piscivorous at the age of a few weeks (Esox, Micropterus); (2) ‘secondary’ piscivores that are fish-eaters only later in life (Perca, Ambloplites); and (3) species in which fish consumption is limited to taking some larvae (Lepomis macrochirus). In the first group the basic series of dietary shifts that characterize many long-lived fish (i.e. zooplankton followed by small invertebrates then large invertebrates and finally fish); is greatly accelerated. Prey size increases with growth. Overall prey selection was on the basis of body size and abundance. Most piscivores took a range of fish prey. There was little evidence of specialization at the species level. Esox and Micropterus spawn some weeks ahead of their major prey species. This is seen as adaptive. Their young harvest the larvae of the latter. The ensuing predator/prey association with growth is highly advantageous to the piscivore as prey of optimum body size are thus continually available.     

Pikeperch Sander lucioperca trapped between niches: foraging performance and prey selection in a piscivore on a planktivore diet

The foraging behaviour of planktivorous pikeperch Sander lucioperca during their first growing season was analysed. Field data showed that S. lucioperca feed on extremely rare prey at the end of the summer, suggesting the presence of a bottleneck. In experiments, foraging ability of planktivorous S. lucioperca was determined when fish were feeding on different prey types (Daphnia magna or Chaoborus spp.) and sizes (D. magna of lengths 1 or 2·5 mm) when they occurred alone. From these results, the minimum density requirement of each prey type was analysed. The energy gain for three different foraging strategies was estimated; a specialized diet based on either large D. magna or Chaoborus spp. or a generalist diet combining both prey types. Prey value estimates showed that Chaoborus spp. should be the preferred prey, assuming an energy maximizing principle. In prey choice experiments, S. lucioperca largely followed this principle, including D. magna in the diet only when the density of the Chaoborus spp. was below a threshold value. Splitting the foraging bout into different sequences, however, resulted in a somewhat different pattern. During an initial phase, S. lucioperca captured both prey as encountered and then switched to Chaoborus spp. if prey density was above the threshold level. The prey selection observed was mainly explained by sampling behaviour and incomplete information about environmental quality, whereas satiation only had marginal effects. It was concluded that the observed diet based on rare prey items was in accordance with an optimal foraging strategy and may generate positive growth in the absence of prey fish in suitable sizes.     

Effects of prey abundance,distribution, visual contrast and morphology on selection by a pelagic piscivore

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Species-specific responses of planktivorous fish to the introduction of a new piscivore: implications for prey fitness

1. Antipredator behaviour by the facultative planktivorous fish species roach (Rutilus rutilus), perch (Perca fluviatilis) and rudd (Scardinius erythrophthalmus) was studied in a multi‐year whole‐lake experiment to evaluate species‐specific behavioural and numerical responses to the stocking of pikeperch (Sander lucioperca), a predator with different foraging behaviour than the resident predators large perch (P. fluviatilis) and pike (Esox lucius). 2. Behavioural responses to pikeperch varied greatly during the night, ranging from reduced activity (roach and small perch) and a shift in habitat (roach), to no change in the habitat use and activity of rudd. The differing responses of the different planktivorous prey species highlight the potential variation in behavioural response to predation risk from species of similar vulnerability. 3. These differences had profound effects on fitness; the density of species that exhibited an antipredator response declined only slightly (roach) or even increased (small perch), whereas the density of the species that did not exhibit an antipredator response (rudd) decreased dramatically (by more than 80%). 4. The maladaptive behaviour of rudd can be explained by a ‘behavioural syndrome’, i.e. the interdependence of behaviours expressed in different contexts (feeding activity, antipredator) across different situations (different densities of predators). 5. Our study extends previous studies, that have typically been limited to more controlled situations, by illustrating the variability in intensity of phenotypic responses to predators, and the consequences for population density, in a large whole‐lake setting.     

Evidence for ram suspension feeding by the piscivore, Seriola dumerili (Carangidae)

Synopsis We have quantitatively analyzed a videotape of Seriola dumerili (Carangidae) displaying ram suspension-feeding behavior and ram ventilation in the field. This is the first report of facultative suspension feeding by a piscivorous carangid. The intraoral morphology of S. dumerili is not typical of ram suspension-feeding fishes in that closely-spaced, long gill rakers are lacking. While the mechanism of particle retention is not known for any ram suspension-feeding fish species, scanning electron microscopy revealed denticles on the branchial surfaces of S. dumerili that could play a role in particle entrapment.     

Seasonal variation in the functional response of a coral-reef piscivore alters the inverse density-dependent mortality of its prey

Density-dependent processes are critical for regulating species’ populations, and piscivory of coral-reef fishes is frequently density dependent. However, the mechanism driving this density-dependent mortality is poorly understood, but may be caused by changes in a predator’s feeding rate at different prey densities (its functional response). An aquarium experiment replicated in winter and summer examined the functional response after 22 and 47 h of Cephalopholis cruentata feeding on Halichoeres pictus. With the exception of summer data after 47 h (density-independent mortality), mortality was inversely density dependent across all prey densities and increased with higher summer temperatures. The absence of an asymptotic pattern of inverse density-dependent mortality was caused by type II (summer) or dome-shaped type IV (winter) functional responses, with the benefits of schooling likely to cause the low mortality rates at higher prey densities. Predators’ functional responses may underlie the inverse density-dependent mortality reported in field studies of aggregating fishes.     

Temporal variation in the diet of a coral reef piscivore (Pisces: Serranidae) was not seasonal

Seasonal and smaller scale temporal variations in the diet of the large predatory grouper Plectropomus leopardus were examined on the central Great Barrier Reef, Australia. In 5 months over a 2-year period, 742 P. leopardus (₅ cm standard length) were collected for stomach content analysis. Despite intensive sampling, identified prey were spread over 20 families and much of the dietary variation was considered to be stochastic. Most aspects of the diet of P. leopardus did not vary seasonally. Frequency, rates of feeding, and proportion of fish consumed were consistent throughout the study. Size of fish prey did not vary significantly between seasons. Pomacentridae, Caesionidae, Scaridae, and Labridae were four of the five most important prey families consumed and did not vary seasonally in the diet. Prey in the other family, the small-sized schooling Clupeidae, were consumed sporadically among months with no seasonal patterns. One less common prey family variedseasonally in the diet; Synodontidae, which are small voracious predators of fish recruits, were consumed by P. leopardus more in summer than winter. Although the diet of P. leopardus is not influenced by the seasonal influx of small fishes recruiting to reefs, these large piscivores may indirectly improve recruit survival by consuming small and abundant predators of new recruits.     

Rapid proliferation of an endemic galaxiid following eradication of an alien piscivore (Perca fluviatilis) from a reservoir

Following the complete eradication of the alien piscivorous perch Perca fluviatilis from a potable reservoir, the abundance of the endemic western minnow Galaxias occidentalis, which was previously undetectable prior to the initial eradication event, increased dramatically. The study reveals the potential of reservoirs to act as ecological refuges and has implications for understanding the relative effects of alien fishes v. habitat alteration on native freshwater fishes.     

Effect of diet on otolith composition in Pomatomus saltatrix, an estuarine piscivore

To test the hypothesis that elemental composition of otoliths (sagittae) could be influenced by differences in natural prey type, young‐of‐the‐year bluefish Pomatomus saltatrix were captured immediately after their migration from oceanic waters into mid‐Atlantic Bight estuaries and fed either shrimp, Crangon septemspinosa and Palaemonetes spp. or fish Menidia menidia under similar temperature and salinity regimes in two separate 60 day experiments. Unlimited rations of fish and shrimp prey were provided in the first experiment which led to differences in bluefish growth rate between the two prey treatments; fish prey was limited in the second experiment to ensure that growth rates of bluefish in the two prey treatments were similar. Concentrations of seven elements in bluefish otoliths were determined using solution‐based inductively coupled plasma mass spectrometry (ICPMS). There was no significant effect of diet on five of the seven elements examined (Na, Mg, K, Ca and Mn). The levels of Sr and Ba in the otoliths of shrimp‐fed bluefish, however, were significantly higher than fish‐fed bluefish in both experiments. Concentrations of Ba in shrimp‐fed bluefish otoliths were double that found in fish‐fed bluefish. The results suggest that diet can explain some of the variation in otolith chemistry previously attributed to physical and chemical properties of the water.     

Total phosphorus and piscivore mass as drivers of food web characteristics in shallow lakes

We assessed the relative influence of total phosphorus and piscivore biomass on the abundance of benthivores, soft‐rayed planktivores, spiny‐rayed planktivores, zooplankton and phytoplankton in 69 shallow lakes in the prairie and parkland areas of Minnesota, USA. Piscivore biomass was the best predictor for three of these response variables, exhibiting a negative relationship with soft‐rayed planktivores, a positive relationship with benthivores, and a weaker positive relationship with large‐bodied cladocerans. Total phosphorus and piscivores comprised the best model for predicting spiny‐rayed planktivores, while neither variable showed any strong relationship to small‐bodied cladocerans. Total phosphorus was positively related to phytoplankton abundance, and was the best predictor among all candidate models. Moreover, contrary to predictions of trophic cascade theory, the relationship between chlorophyll a and total phosphorus did not differ between lakes with and without piscivores. Our results indicated top‐down influences of piscivores extended through parts of two trophic levels, but failed to influence zooplankton – phytoplankton interactions, leaving phytoplankton abundance constrained largely by total phosphorus. Lack of a relationship between piscivores and phytoplankton was likely due to high densities of larval planktivores less susceptible to piscivory, as well as positive influences of spiny‐rayed planktivores and benthivores on algal abundance. These results support the idea that top–down influences of piscivores on phytoplankton abundance may be reduced in more diverse fish communities where some prey species are less susceptible to piscivory.     

Lake restoration by biomanipulation using piscivore and Daphnia stocking; results of the biomanipulation in Japan

Biomanipulation has been employed in numerous locations throughout the world as a means for reducing phytoplankton biomass; however, it has not been employed very often in Japan. A common approach involves the introduction of piscivorous fish to reduce the abundance of planktivorous fish. In our study, to first apply biomanipulation, we stocked Lake Shirakaba (a high-altitude, protected area in a park) in central Japan with rainbow trout fingerlings and cladoceran Daphnia (Daphnia galeata) in 2000. A “pre-biomanipulation” data set (1997–1999) and “a post-biomanipulation” data set (2000–2006) allowed us to evaluate the lake's response to biomanipulation. After the biomanipulation, zoo-planktivorous pond smelt disappeared and a large population of Daphnia had been established, which substantially reduced the number of the previously dominant small cladocerans and rotifers. Water transparency increased from about 2 m (before biomanipulation) to more than 4 m (after biomanipulation). Reductions in algal biomass and increased transparency led to expansion of the submerged macrophyte Elodea nuttallii. Total phosphorus concentrations declined as well over this time period. Based on these results, we concluded that biomanipulation using piscivore and Daphnia stocking succeeded in improving lake water quality by reducing algal abundance and providing favorable conditions for the establishment of rooted plants.     

Rapid and unexpected effects of piscivore introduction on trophic position and diet of perch (Perca flavescens) in lakes recovering from acidification and metal contamination

1. Yellow perch (Perca flavescens) are often the only surviving fish species in acidified lakes. We studied four lakes along a gradient of recovery from acidification and that had different food web complexities. All had abundant yellow perch, two had low piscivore abundance, one had a well‐established piscivore population and one was manipulated by introducing piscivorous smallmouth bass (Micropterus dolomieu). We hypothesised that there would be strong effects on perch abundance, behaviour and diet induced by the presence of piscivores. 2. In the manipulated lake, the bass reduced yellow perch abundance by 75% over a 2‐year period. Concomitantly, perch use of the pelagic habitat fell from 48 to 40%. 3. In contrast to findings from less disturbed systems, yellow perch in the littoral zone of the manipulated lake did not strongly shift from zooplankton to benthic food sources after the arrival of piscivores. Diet analysis using stable carbon isotopes revealed a strong continued reliance on zooplankton in all lakes, independent of the degree of piscivory. The failure to switch to benthos in the refuge area of the littoral zone is most likely related to the depauperate benthos communities in these formerly acidified lakes. 4. Yellow perch in lakes recovering from acidification face a considerable ecological challenge as the necessary switch to benthic diet is hindered by a low abundance of benthos. The arrival of piscivores in these recovering lakes imposes further restrictions on perch access to food items. We infer that future recovery of perch populations (and higher trophic levels) will have to be preceded by the re‐establishment of diverse benthic macroinvertebrate communities in these lakes.