Structural complexity and fish body size interactively affect habitat optimality

Predator–prey interactions are strongly influenced by habitat structure, particularly in coastal marine habitats such as seagrasses in which structural complexity (SC) may vary over small spatial scales. For seagrass mesopredators such as juvenile fishes, optimality models predict that fitness will be maximized at levels of SC that enhance foraging but minimize predation risk, both of which are functions of body size. We tested the hypothesis that in eelgrass (Zostera marina) habitat, optimal SC for juvenile giant kelpfish (Heterostichus rostratus), an abundant eelgrass mesopredator in southern California, changes through ontogeny. To do this, we quantified eelgrass SC effects on habitat associations, relative predation risk, and foraging efficiency for three size classes of juvenile giant kelpfish. We found that habitat selection differed with fish size: small fish selected dense eelgrass, whereas larger fish selected sparse eelgrass. Small kelpfish experienced the lowest relative predation risk in dense eelgrass but also had higher foraging efficiency in dense eelgrass, suggesting that dense eelgrass is selected by these fish because it minimizes risk and maximizes potential for growth. Surprisingly, larger kelpfish did not experience lower predation risk than small kelpfish. However, larger kelpfish experienced higher foraging efficiency in sparse eelgrass vs. dense eelgrass, suggesting that they select sparse eelgrass to maximize foraging efficiency. Our study highlights that trade-offs between predation risk and foraging can occur within a single habitat type, that studies should consider how habitat value changes through ontogeny, and that seagrass habitat value may be maximal when within-patch variability in SC is high.     

The costs of becoming nocturnal: feeding efficiency in relation to light intensity in juvenile Atlantic Salmon

1. Most animals are active by day or by night, but not both; juvenile salmonids are unusual in that they switch from being predominantly diurnal for most of the year to being nocturnal in winter. They are visual foragers, and adaptations for high visual acuity at daytime light intensities are generally incompatible with sensitive night vision. Here we test whether juvenile Atlantic Salmon Salmo salar are able to maintain their efficiency of prey capture when switching between diurnal and nocturnal foraging.
2. By testing the ability of the fish to acquire drifting food items under a range of manipulated light intensities, we show that the foraging efficiency of juvenile salmon is high at light intensities down to those equivalent to dawn or dusk, but drops markedly at lower levels of illumination: even under the best night condition (full moon and clear sky), the feeding efficiency is only 35% of their diurnal efficiency, and fish will usually be feeding at less than 10% (whenever the moon is not full, skies are overcast or when in the shade of bankside trees). Fish were unable to feed on drifting prey when in complete darkness.
3. The ability of juvenile salmon to detect prey under different light intensities is similar to that of other planktivorous or drift-feeding species of fish; they thus appear to have no special adaptations for nocturnal foraging.
4. While winter drift abundance is slightly higher by night than by day, the difference is not enough to compensate for the loss in foraging efficiency. We suggest that juvenile salmon can nonetheless switch to nocturnal foraging in winter because their food requirements are low, many individuals adopting a strategy in which intake is suppressed to the minimum that ensures survival.     

Reduction of Reactive Distance and Foraging Success in Smallmouth Bass, Micropterus dolomieu, Exposed to Elevated Turbidity Levels

We determined how turbidity affected the reactive distance and foraging success of smallmouth bass, Micropterus dolomieu. Smallmouth bass reactive distance decreased exponentially with increasing turbidity, from 65cm in clear water to 10cm at the highest turbidity. Turbidity significantly decreased the probability of a fish reacting to a prey item, but did not influence foraging success following reaction to the prey. Elevated turbidity may reduce stream fish foraging efficiency and decrease prey consumption.     

Effects of turbidity on the spontaneous and prey-searching activity of juvenile Atlantic cod (Gadus morhua)

Increasing turbidity in coastal waters in the North Atlantic and adjacent seas has raised concerns about impacts on Atlantic cod (Gadus morhua) using these areas as nurseries. A previous experiment (Meager et al. 2005 Can. J. Fish. Aquat. Sci. 62, 1978-1984) has shown that turbidity (up to 28 beam attenuation m-1) had little effect on the foraging rate of juvenile cod. Although this was attributed to cod using chemoreception in conjunction with vision to locate prey, foraging rates may also be maintained by increased activity. Higher activity, however, is energetically costly and may offset benefits from increased foraging return. We examined the effects of turbidity on prey searching and spontaneous activity of juvenile cod in the laboratory, by measuring activity with and without prey cues. Activity of juvenile cod was nonlinearly affected by turbidity and was lower at intermediate turbidity, regardless of the presence of prey odour. Activity increased over time when prey odour was present and decreased when absent, but the effects of prey odour were similar across all turbidity levels. Position in the tank was unaffected by turbidity or prey odour. Reduced activity at intermediate turbidities is likely to offset longer prey-search times. At high turbidity (greater than 17m-1), both longer prey-search times and higher activity indicate that increased energetic costs are probable.     

Glacial clay affects foraging performance in a Patagonian fish and cladoceran

Climate change is altering temperatures and precipitation patterns all over the world. In Patagonia, Argentina, predicted increase in precipitation together with rapidly melting glaciers increase the surface runoff, and thereby the transport of suspended solids to recipient lakes. Suspended solids affect the visual conditions in the water which in turn restricts visual foraging. The native fish Aplochiton zebra Jenyns, and its filter-feeding cladoceran prey, Daphnia commutata Ekman, were subjected to foraging experiments at three turbidity levels. A. zebra foraging rate was substantially reduced at naturally occurring turbidity levels and the filtering rate of D. commutata was reduced at the highest turbidity level. This indicates that Daphnia may be partly released from predation from A. zebra at the same time as it can maintain relatively high feeding rates as turbidity increases. Lower foraging rates at the same time as the metabolic demand increases, through increased temperatures, may result in larger effects on A. zebra than could be expected from increases in turbidity or temperature alone. Turbidity may, as an indirect effect of climate change, decrease planktivore foraging rates and thereby alter the interaction strength between trophic levels.     

Effect of turbidity on habitat preference of juvenile Atlantic cod, Gadus morhua

We examined the effects of turbidity on habitat preference of juvenile Atlantic cod in the laboratory, using a shuttle box where fish could select between two different habitats. In the first experiment, we compared three turbidity levels of kaolin (3, 8 and 21 beam attenuation m⁻¹). In the second experiment, we looked at the effect of turbidity media (kaolin versus algae), after controlling for spectral differences between turbidity media. Although cod preferred an intermediate turbidity of kaolin over low turbidity water, comparisons between low and high turbidity, and intermediate and high turbidity did not significantly influence habitat preference. Algae did not influence habitat preference by cod. Although other studies have found that turbidity affects both foraging and antipredator behaviour of juvenile cod, this study has shown that gradients in turbidity per se do not have a strong effect on their habitat preference.     

Age and hunting success in the brown pelican: influences of skill and patch choice on foraging efficiency

Summary Age-related differences in the foraging efficiency of piscivorous birds may be the results of differences in foraging skill, patch usage, or both. Brown pelicans were observed while foraging around a small Caribbean island. Areas where the birds fed were subdivided into small, homogeneous subunits (patches), and the bird's foraging success and patch use were noted and analyzed using multivariate techniques. Adult birds were found to be better at capturing prey under all conditions than were juveniles, but the differences were small in some patches. The density of prey and the birds' foraging success influenced the foraging efforts of adult and juvenile pelicans to similar degrees. Both age groups utilized local enhancement in their foraging, but such behavior augmented the foraging success only of juvenile birds. Both age groups often fed in patches where their foraging success was quite low. Such behaviour was much more costly for juvenile than for adult pelicans.     

The effects of turbidity and vegetation on the risk of juvenile salmonids, Oncorhynchus spp., to predation by adult cutthroat trout, O. clarkii

Synopsis During their seaward migration, juvenile salmonids encounter structural and visual cover which varies between and within watersheds. In this study, the effects of two types of cover (turbidity and artificial vegetation) on the predation mortality of juvenile salmonids exposed to fish piscivores was investigated in outdoor concrete ponds. During experiments, adult coastal cutthroat trout, Oncorhynchus clarkii clarkii, were allowed to feed on juvenile salmonid prey — chinook salmon, O. tshawytscha, chum salmon, O. keta, sockeye salmon, O. nerka, and cutthroat trout — in separate trials. Daily instantaneous per capita predation rate was determined for each turbidity and vegetation treatment, within each trial. Mean predation rates varied between 1% and 76% daily. In the presence of cover, mean daily predation rates were 10–75% lower than those in controls (no vegetation and clear water), depending on prey species. Predation rates were significantly lower in the presence of vegetation cover and did not covary with prey size or species. The effects of turbidity were generally not significant and were not additive with the effects of vegetation. However, turbidity appeared to significantly reduce the effectiveness of vegetation as cover for juvenile chinook and sockeye salmon. We suggest that these two forms of cover do not affect risk of predation by fish piscivores to juvenile salmonids via the same mechanism.     

Experimental analyses of patch selection by foraging black surfperch (Embiotoca jacksoni Agazzi)

The dynamics of microhabitat use by foraging adult and juvenile black surfperch (Embiotocajacksoni Agazzi) were explored. Detailed observations of black surfperch feeding at Santa Catalina Island, California, revealed that adults and young-of-year juveniles co-occurred in the same habitat but used different algal substrata as foraging sites. Juveniles selected invertebrate prey almost exclusively from the surface of foliose algae. The occurrence of young E. jacksoni was highly correlated with that of foliose algae. Adults tended to bite most frequently from turf, a low-growing matrix of plants, colonial animals, and debris covering the rocky substratum. The abundance of adults was negatively correlated with the occurrence of foliose algae. Adults and juveniles showed marked, but different, preferences in their utilization of taxa of algae as foraging substrata. Certain algae (e.g., Zonaria farlowii Setchell & Gardner) were preferred while other taxa (e.g., Sargassum palmeri Grun) were avoided by both age groups. However, most types of algae were preferred by one group but not the other. To test the hypothesis that knowledge of algal substratum composition allows prediction of fish occurrence and foraging behavior in a patch, algal cover on 2 × 2 m² areas of bottom was manipulated creating plots dominated by turf, Zonaria farlowii, or Sargassum palmeri. Fish occurrence could be accurately predicted on the basis of abundance of foliose algae, but foraging activity of fish was highly dependent on the algal taxon that dominated the patch. Differential prey availabilities among foraging substrata provided some insight into the patterns of foraging patch preferences displayed by adult and juvenile Embiotoca jacksoni.     

Visual Detection of Speckles in the Fish Xenotoca variata by the Predatory Snake Thamnophis melanogaster in Water of Different Turbidity

Semi-aquatic snakes integrate visual and chemical stimuli, and prey detection and capture success are therefore linked to the display of visual predatory behavior. The snake Thamnophis melanogaster responds preferentially to individuals of the fish Xenotoca variata with a greater number of bright, colorful spots (lateral speckles) compared with those with a smaller number; however, water turbidity can reduce underwater visibility and effect the vulnerability of fish. In this study, we tested whether the presence of iridescent speckles on the flanks of male X. variata interacted with water turbidity to modify the predatory behavior displayed by the snake T. melanogaster. We predicted that in an experimental laboratory test, the snakes would increase the frequency of their predatory behavior to the extent that the water turbidity decreases. The snakes were tested at six different levels of water turbidity, in combination with three categories of male fish (with few, a median number of, or many speckles). The results showed that in a pool with high or zero turbidity, the number of speckles is not a determining factor in the deployment of the predatory behavior of the snake T. melanogaster toward X. variata. Our findings suggest that snakes can view the fish at intermediate percentages of turbidity, but the number of speckles in male X. variata is irrelevant as an interspecific visual signal in environments with insufficient luminosity. The successful capture of aquatic prey is influenced by integration between chemical and visual signals, according to environmental factors that may influence the recognition of individual traits.     

Impact of black bullhead (Ameiurus melas) on turbidity in a diked wetland

Benthivorous fish in shallow, aquatic systems have been correlated with increased turbidity and declines in macrophyte production and wildlife use. Bullheads have been credited with increasing turbidity, but this has been seldom tested and has not been studied in a diked marsh. To assess the relationships of black bullhead (Ameiurus melas) and turbidity, we assembled mesocosmsin the Show Pool Management Unit of The Ottawa National Wildlife Refuge, OH, U.S.A. We stocked treatment enclosures with different biomasses of black bullhead at weekly intervals. Mean turbidity within treatment enclosures was significantly higher than within controls but remained lower than that of the open marsh. Both surface and bottom turbidity increased with adult and juvenile black bullhead biomass. Turbidity increased with fine sand concentration only in the presence of juvenile fish. Wind speed and direction were significant influences on the turbidity of the open marsh, but not within control enclosures. That treatment turbidity – even at extreme biomasses – remained significantly lower than the turbidity of the open marsh implicates fetch in having a greater influence on a marsh's turbidity than the presence of black bullhead. The greater impact of benthivorous fish on turbidity within shallow systems may be an indirect one through the destruction of macrophytes and subsequent destabilization of unconsolidated substrates.     

Turbidity amplifies the non‐lethal effects of predation and affects the foraging success of characid fish shoals

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Mechanisms of drift-feeding behavior in juvenile Chinook salmon and the role of inedible debris in a clear-water Alaskan stream

Drift-feeding fish are challenged to discriminate between prey and similar-sized particles of debris, which are ubiquitous even in clear-water streams. Spending time and energy pursuing debris mistaken as prey could affect fish growth and the fitness potential of different foraging strategies. Our goal was to determine the extent to which debris influences drift-feeding fish in clear water under low-flow conditions when the distracting effect of debris should be at a minimum. We used high-definition video to measure the reactions of drift-feeding juvenile Chinook salmon (Oncorhynchus tshawytscha) to natural debris and prey in situ in the Chena River, Alaska. Among all potential food items fish pursued, 52 % were captured and quickly expelled from the mouth, 39 % were visually inspected but not captured, and only 9 % were ingested. Foraging attempt rate was only moderately correlated with ingestion rate (Kendall’s τ?=?0.55), raising concerns about the common use of foraging attempts as a presumed index of foraging success. The total time fish spent handling debris increased linearly with foraging attempt rate and ranged between 4 and 25 % of total foraging time among observed groups. Our results help motivate a revised theoretical view of drift feeding that emphasizes prey detection and discrimination, incorporating ideas from signal detection theory and the study of visual attention in cognitive ecology. We discuss how these ideas could lead to better explanations and predictions of the spatial behavior, prey selection, and energy intake of drift-feeding fish.     

浅水湖泊人工繁育鳜放养规格对其成活、生长和产量的影响

文章采用标志放流回捕方法评估了人工繁育鳜放流规格对其成活、生长和产量的影响。2012年5—6月选取3种不同大小规格的人工繁育鳜鱼苗各800尾, 中等规格鳜苗种和大规格鳜苗种分别采用剪尾鳍和编码金属标标记后放入试验湖泊小赛湖, 2012年12月试验湖泊逐渐降低水位, 回捕收集放流鳜并鉴定身份。结果显示3种规格放流鳜的总成活率为40.5%, 其中大规格放流鳜的成活率为63%, 明显高于中规格放流鳜(39.1%)和小规格放流鳜(19.5%)。放流鳜的终末体长和体重均随放流规格的增加而增加, 且3种规格放流鳜的终末体长和体重均呈现显著差异。放流鳜的产量和净收益在大规格处理组最大, 在小规格处理组最小。结果表明鳜放流规格对鳜放流成功与否有重要影响, 建议长江中游湖泊鳜的最佳放流规格应不小于50 mm。     

Foraging success of largemouth bass at different light intensities: implications for time and depth of feeding

Laboratory feeding trials were conducted to determine how light intensity affects foraging success by the visual piscivore, the largemouth bass ( Micropterus salmoides ). Foraging success was greater than 95% at light levels ranging from low intensity daylight (2.43 × 10² lx) to moonlight (3 × 10⁻³ lx), but declined significantly to 62% at starlight (2 × 10⁻⁴ lx) and near 0% in total darkness. Over a range of low to high water clarities (0.5, 2.0, and 4.0 m Secchi depth), estimated depth limits for feeding during the day ranged between 5.5 to 44 m and from 1.6 to 13 m at night during a full moon. At starlight, light intensity rapidly attenuated to a level below the feeding threshold within 0.5 m of the surface at all water clarities. The depth of the water column available for feeding in low clarity water (0.5 m Secchi) was 67 and 75% less than at moderate (2.0 m Secchi) and high (4.0 m Secchi) water clarities. The findings illustrate how differences in the light environment can have important ramifications for predator-prey interactions.     

Effects of ice cover on the diel behaviour and ventilation rate of juvenile brown trout

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Abundance and species richness as a function of food resources and vegetation structure: juvenile fish assemblages in rivers

Availability of food and habitat complexity are two factors generally invoked to explain the high density of fish in vegetated habitats. The role of food resources (zooplankton) and habitat complexity (expressed by a vegetation structural index) in determining juvenile fish abundance and fish species richness in three morphologically contrasted macrophyte types (Sagittaria, Ceratophyllum and Nuphar) was studied for a large, lowland river.
Our results showed that fish abundance increased with food availability, and was maximal for intermediate values of vegetation complexity. Food resources and vegetation complexity did not, however, explain the higher juvenile fish abundance observed in Sagittaria beds. We suggested that plant growth form, acting on fish foraging success and risk of predation, might influence patterns of juvenile fish distribution.
Species‐abundance relationships were similar among the three macrophyte types, but relationships between number of fish species (fish richness) and number of samples differed. Fish richness in terms of total number of fish species found at each sampling point showed the same pattern as for fish abundance: it increased with food availability and was highest at intermediate vegetation complexities. However, since both fish abundance and fish richness responded in the same manner to food availability and vegetation complexity, we were not able to distinguish statistically any effect for the specific fish richness formulated by the number of fish species per unit fish abundance. The current paradigm that structural complexity of vegetation provides a wider range of niches, increasing species diversity, was thus not verified. This finding indicates a simple species‐abundance relationship (the passive sampling hypothesis), and suggests that no special mechanism acts directly on fish species richness.     

Characteristics of reaction field and the reactive distance of a planktivore, Pseudorasbora parva (Cyprinidae), in various environmental conditions

The characteristics of the reaction field and the reactive distance of the Stone moroko (Pseudorasbora parva) were studied under three environmental conditions (structural complexity, light intensity and turbidity) and three prey sizes. In optimal experimental conditions, under no structural complexity, light intensity of 200 lux and turbidity less than 1 NTU (Nephelometric Turbidity Units), the cross-section of the reaction field was found to be elliptic with a bearing angle larger than the elevation angle, but both angles changed slightly depending on environmental conditions. The reactive distance was large, and the fish frequently attacked prey that was located within 15–60 degrees to each side from the frontal direction of a fish (i.e., ± 15 degrees from the axis of the fish body) horizontally. In the light intensity below 50 lux or turbidity above 10 NTU, however, the attack frequency and the reactive distance in the frontal direction of a fish did not differ from other horizontal directions in the reaction field. The average reactive distance increased proportionally with increasing strand distance, but it gradually reached a constant value for strand distances greater than about 3.6 times the fish body length. The average reactive distance increased in the light intensity range of 10–200 lux and decreased negatively with turbidity increasing. The average reactive distance increased with larger prey size, but the rate of increase of the reactive distance gradually decreased.     

Visual performance impaired by elevated sedimentary and algal turbidity in walleye Sander vitreus and emerald shiner Notropis atherinoides

The objectives of this study were to determine the effects of different forms of elevated turbidity on the visual acuity of two native Lake Erie fishes and to assess the response of fishes from different trophic levels to elevated turbidity. Additionally, the role of visual morphology (e.g., eye and optic lobe size) on visual acuity was evaluated across visual environments. Reaction distance, a behavioural proxy for measures of visual acuity, was measured for a top predator, walleye Sander vitreus and a forage fish, emerald shiner Notropis atherinoides. In both S. vitreus (n = 27) and N. atherinoides (n = 40) reaction distance across all types of turbidity (sedimentary, algal, sedimentary + algal; 20 NTU) was approximately 50% lower relative to the clear treatment. Reaction distance was further reduced in algal compared with sedimentary turbidity for wild-caught S. vitreus. Eye and brain morphology also influenced reaction distance across turbidity treatments, such that larger relative eye and brain metrics were positively correlated with reaction distance. This study provides evidence for disrupted visual acuity as a potential mechanism underlying fish responses, such as decreased foraging efficiency, to increased turbidity and further indicates that algal turbidity will probably be more detrimental to visual processes than sedimentary turbidity. With the increasing occurrence and severity of harmful algal blooms due to cultural eutrophication globally, this could have significant implications for predator–prey relationships in aquatic systems.     

Guiding lights: Foraging responses of juvenile nocturnal orb‐web spiders to the presence of artificial light at night

The reach of artificial light at night (ALAN) is growing rapidly around the globe, including the increasing use of energy‐efficient LED lights. Many studies document the physiological costs of light at night, but far fewer have focused on the potential benefits for nocturnal insectivores and the likely ecological consequences of shifts in predator–prey relationships. We investigated the effects of ALAN on the foraging behaviour and prey capture success in juvenile Australian garden orb‐web spiders (Eriophora biapicata). Laboratory experiments demonstrated that juvenile spiders were attracted to LED lights when choosing foraging sites, but prey availability was a stronger cue for remaining in a foraging site. Field experiments revealed a significant increase in prey capture rates for webs placed near LED lights. This suggests that any physiological costs of light at night may be offset by the foraging benefits, perhaps partially explaining recently observed increases in the size, fecundity and abundance of some orb‐web spider species in urban environments. Our results highlight the potential long‐term consequences of night lighting in urban ecosystems, through the impact of orb‐web spiders on insect populations.