Diet partitioning in sympatric Atlantic salmon and brown trout in streams with contrasting riparian vegetation

Prey intake by Atlantic salmon Salmo salar and brown trout Salmo trutta was measured across different riparian vegetation types: grassland, open canopy deciduous and closed canopy deciduous, in upland streams in County Mayo, Western Ireland. Fishes were collected by electrofishing while invertebrates were sampled from the benthos using a Surber sampler and drifting invertebrates collected in drift traps. Aquatic invertebrates dominated prey numbers in the diets of 0+ year Atlantic salmon and brown trout and 1+ year Atlantic salmon, whereas terrestrial invertebrates were of greater importance for diets of 1+ and 2+ year brown trout. Terrestrial prey biomass was generally greater than aquatic prey for 1+ and 2+ year brown trout across seasons and riparian types. Prey intake was greatest in spring and summer and least in autumn apart from 2+ year brown trout that sustained feeding into autumn. Total prey numbers captured tended to be greater for all age classes in streams with deciduous riparian canopy. Atlantic salmon consumed more aquatic prey and brown trout more terrestrial prey with an ontogenetic increase in prey species richness and diversity. Atlantic salmon and brown trout diets were most similar in summer. Terrestrial invertebrates provided an important energy subsidy particularly for brown trout. In grassland streams, each fish age class was strongly associated with aquatic, mainly benthic invertebrates. In streams with deciduous riparian canopy cover, diet composition partitioned between conspecifics with older brown trout associated with surface drifting terrestrial invertebrates and older Atlantic salmon associated with aquatic invertebrates with a high drift propensity in the water column and 0+ year fish feeding on benthic aquatic invertebrates. Deciduous riparian canopy cover may therefore facilitate vertical partitioning of feeding position within the water column between sympatric Atlantic salmon and brown trout. Implications for riparian management are discussed.     

The relative importance of shading and nutrients on algal production in subtropical streams

1. We used artificial substrata in forested and open streams in South-East Queensland, Australia, to determine the relative importance of shading from riparian vegetation and of nutrients on periphyton growth, and whether nitrogen and/or phosphorus limited algal productivity.
2. Nutrient-diffusing substrata consisting of agar enriched with N, P and N + P, and controls without nutrients, were deployed in duplicate at 15 sites in headwater streams with riparian canopy cover ranging from 0 to 88%.
3. Shading was the over-riding factor controlling periphyton biomass accrual on the artificial substrata, with nutrients playing a relatively minor role. Microscopic examination of periphyton scrapings taken after 7 weeks revealed that diatoms dominated on the artificial substrata in shaded streams, whereas filamentous green algae dominated the algal assemblage in the more open canopy streams.
4. Whilst nutrients had little effect on the accrual of algal biomass compared with riparian shading, there was evidence that nitrogen, and not phosphorus, stimulated periphyton production in streams with sufficient light.     

Environmentally induced spatio-temporal variations in the fecundity of brown trout Salmo trutta L.: trade-offs between egg size and number

1. Resident brown trout Salmo trutta in the Esva River basin (north Spain) live in a patchy environment with tracts of riparian forest or meadow along stream banks. This study assessed whether the reproductive traits of brown trout from four contrasting sites reflected site-specific factors.
2. Length at maturity (10.5–11 cm of 1 + individuals) was the same in the four sites examined but slowest growers in slow-growing sub-populations delayed maturity for 1 year relative to fast-growing fish. The analysis of monthly variations in egg size and number suggest that two 'decisions' in two consecutive years are required to complete spawning. The first concerns the number of eggs, determined when trout are still 0 +, and the second concerns egg size.
3. At three sites, egg size and number did not differ significantly between years but highly significant interannual variations were apparent at another site. Fish length was the major determinant of egg size and number at all sites but for any given length, brown trout at sites where the fish exhibited higher growth rates spawned more, but smaller, eggs than those at slow-growing sites. This spatial pattern was identical to the temporal pattern exhibited by trout at another site. The combination of temporal (year-to-year) and spatial (between rivers) variations in egg size and number showed a significant negative correlation, supporting the operation of a trade-off between these two traits.
4. The trade-off between egg size and number seems to be determined by site-specific factors, with slow-growing trout at sites which are fully covered by canopy spawning fewer, but larger, eggs than fast-growers in unshaded sites.     

Exotic riparian vegetation lowers fungal diversity but not leaf decomposition in Portuguese streams

1. We characterised the fungal communities of eight streams in Portugal, four bordered by native deciduous forest and four bordered by pure stands of Eucalyptus globulus .
2. Aquatic hyphomycete species richness and evenness, but not numbers of water-borne conidia, of aquatic hyphomycetes were significantly lower in eucalypt bordered streams.
3. Multivariate analyses subdivided the fungal communities into two distinct groups corresponding to riparian vegetation.
4. Despite these differences in the dominant decomposer community, decay rates of eucalypt leaves (accounting for ≥98% of naturally occurring leaves in eucalypt bordered streams, absent in native forest) and chestnut leaves (occurring naturally in native forests) did not differ between the two groups of streams.     

Variation in growth of the detritivore-shredder Sericostoma personatum (Trichoptera)

1. We investigated the growth of the detritivore-shredder Sericostoma personatum by feeding groups of larvae on a fresh macrophyte, Potamogeton perfoliatus , conditioned sitka spruce needles, Picea sitchensis , conditioned alder leaves, Alnus glutinosa , conditioned ash leaves, Fraxinus excelsior , and a fresh filamentous green alga, Microspora sp. A sixth treatment group of larvae was fasted.
2. The nitrogen content of the food items ranged from 1.4% dry weight (DW) for Picea needles to 4.4% DW for Microspora filaments. Consumption of the various food items by Sericostoma differed significantly, being highest for Picea , and lowest for Potamogeton and Microspora .
3. The instantaneous growth rate ranged from −1.3% DW day⁻¹ in the fasting group to 0.75% DW day⁻¹ in the Alnus group. The growth rate of larvae fed on Alnus , Fraxinus and Microspora was similar, and significantly higher than that of the other three groups. An increase in the size of larval cases followed the same pattern as larval growth, although with less variation between food items.
4. Larvae fed on Microspora exhibited the greatest increase in fat content, while that of the larvae fed on Picea and those in the fasting group decreased. The gross growth efficiency (G/I%) of Sericostoma (larva + case) ranged from 2% when fed on Picea to 34% when fed on Microspora . Consumption was significantly correlated with the nutrient content of food items, whereas G/I% and growth rate were only weakly related to food quality.
5. The growth of Sericostoma varies with diet and the food items sustaining highest growth rate are not necessarily the 'typical' shredder food resources.     

The effect of temperature and fish size on growth and feed efficiency ratio of juvenile spotted wolffish Anarhichas minor

The growth properties of juvenile spotted wolffish Anarhichas minor reared at 4, 6, 8 and 12° C, and a group reared under 'temperature steps', (T‐step) i.e . with temperature reduced successively from 12 to 9 and 6° C were investigated. Growth rate and feed efficiency ration was significantly influenced by temperature and fish size. Overall growth rate was highest at 6° C (0·68% day⁻¹) and lowest at 12° C (0·48% day⁻¹), while the 4 and 8° C, and the T‐step groups had similar overall growth rates, i.e . 0·59, 0·62 and 0·51% day⁻¹ respectively. Optimal temperature for growth ( T _{opt G} ) and feed efficiency ratio (T_{opt FCE}) decreased as fish size increased, indicating an ontogenetic reduction in T _{opt G} and T _{opt FCE}. The results suggest a T _{opt G} of juvenile spotted wolffish in the size range 135–380 g, dropping from 7·9° C for 130–135 g to 6·6° C for 360–380 g juveniles. The T _{opt FCE} dropped from 7·4° C for 120–150 g to 6·5° C for 300–380 g juveniles. A wider parabolic regression curve between growth, feed efficiency ratio and temperature as fish size increased, may indicate increased temperature tolerance with size. Individual growth rates varied greatly at all time periods within the experimental temperatures, but at the same time significant size rank correlations were maintained and this may indicate stable size hierarchies in juvenile spotted wolffish.     

Comparison of field-based and indirect estimates of daily food consumption in larval perch and zander

Since bioenergetics models for 0+ fish have seldom been validated by field consumption estimates, field-based and indirectly estimated daily food rations were compared in larval perch Perca fluviatilis and zander Stizostedion lucioperca. Field-based estimates were calculated with linear and exponential evacuation rates based on gut fullness data during a 24-h cycle, with hourly field samplings instead of the normally recommended 3-h intervals. Indirect calculations used bioenergetics modelling with variable activity multipliers ( A ). Field-based estimates of daily rations ranged between 0·21 and 0·27 g g⁻¹ day⁻¹ in perch (mean L _T 13·1 mm) and 0·31–0·40 g g⁻¹ day⁻¹ in zander (mean L _T 10·6 mm). The higher values were calculated by using the exponential model. Daily rations calculated by bioenergetics modelling with A = 1 were only slightly higher than direct estimates in both species. However, if A values >1 were used, calculated daily rations were substantially higher than direct estimates. Estimates of daily ration based only on every third value ranged between 41 and 72% compared with 1-h intervals, mainly because of lower estimates of evacuation rate.     

Local and ecoregion effects on fish assemblage structure in tributaries of the Rio Paraíba do Sul, Brazil

1.  We examined the effects of physical and chemical habitat variables and ecoregions on species occurrence and fish assemblage structure in streams of the Paraíba do Sul basin, in southeast Brazil.
2.  Fish and environmental data were collected from 42 sites on 26 first to fourth order streams (1 : 50 000 map scale) in three ecoregions. The sites occurred in one valley and two plateau ecoregions at altitudes of 40–1080 m and distances of 0.1–188 km from the main channel of the Rio Paraíba do Sul. Physical habitat (substratum, riparian cover, habitat types) and water quality (dissolved oxygen, pH, temperature and conductivity) variables were measured at each sampling site.
3.  A total of 2684 individuals in 16 families and 59 species were recorded.
4.  Ecoregion was a better predictor of the fish assemblage than the other environmental variables, according to the differences between the mean within-class and mean between-class similarities in assemblage data.
5.  Differing landscape characteristics were associated with differing local variables and thereby with differing fish assemblage structures. Riffles, shrub, grass, dissolved oxygen, conductivity and temperature were closely related to fish assemblage structure.
6.  Fish assemblages in sites far from the main river and at higher altitudes also differed from those near the Paraíba do Sul main channel, presumably as a result of differences in connectivity, covarying environmental factors and anthropogenic influence.
7.  These results reinforce the importance of understanding how stream communities are influenced by processes and patterns operating at local and regional scales, which will aid water resource managers to target those factors in their management and rehabilitation efforts.     

Grazing management influences the subsidy of terrestrial prey to trout in central Rocky Mountain streams (USA)

1. Research in forest and grassland ecosystems indicates that terrestrial invertebrates that fall into streams can be an important prey resource for fish, providing about 50% of their annual energy and having strong effects on growth and abundance. However, the indirect effects of land uses like cattle grazing on this important prey subsidy for stream salmonids are unclear. 2. During summer 2007, we compared the effects of three commonly used grazing systems on terrestrial invertebrate inputs to streams in northern Colorado and their use by trout. Cattle graze individual pastures for about 120 days under traditional season‐long grazing (SLG), about 35–45 days under simple rotational grazing and 10–20 days under intensive rotational grazing in this region. We also compared these effects to a fourth group of sites grazed only by wildlife (i.e. no livestock use). 3. Overall, rotational grazing management (either simple or intensive), resulted in more riparian vegetation, greater inputs of terrestrial invertebrates, greater biomass of terrestrial invertebrate prey in trout diets, a higher input compared to trout metabolic demand and more trout biomass than SLG. However, these differences were frequently not statistically significant owing to high variability, especially for trout diets and biomass. 4. Despite the inherent variability, riparian vegetation and terrestrial invertebrates entering streams and in trout diets at sites managed for rotational grazing were similar to sites managed for wildlife grazing only. 5. These results indicate that rotational grazing systems can be effective for maintaining levels of terrestrial invertebrate subsidies to streams necessary to support robust trout populations. However, factors influencing the effect of riparian grazing on stream subsidies are both spatially variable and complex, owing to differences in microclimate, invertebrate and plant populations and the efforts of ranchers to tailor grazing systems to specific riparian pastures.     

Influence of different rations and water temperatures on the growth rates of shortfinned eels and longfinned eels

Juvenile (12–152 g) shortfinned eels Anguilla australis and longfinned eels A. dieffenbachia caught in New Zealand streams were fed squid mantle Nototodarus spp. 4 days per week in laboratory experiments. A linear multiple regression equation showed the amount of food eaten (0–2·7% w day⁻¹) explained 77·7% of the variation in specific growth rates (–0·60 to +1·07% w day⁻¹) among individual eels, while previous growth rates, water temperature (10·0–20·6°C), and eel weight (12–152 g) explained a further 5·6, 1·4 and 0·8%, respectively. Growth in length ranged from –0·3 to +0·9 mm day⁻¹. Eels which were starved and then given high rations grew substantially faster than expected. Once growth rates were adjusted for differences in ration and other factors, there were no significant differences in growth rates between species or individual fish. Growth of shortfinned eels fed maximum rations of commercial eel food depended on fish size and water temperatures and ceased below 9·0°C. Growth rates in the wild were substantially less than the maximum possible, after seasonal changes in water temperatures were taken into account, indicating that food supplies and not low water temperatures were controlling growth rates in the wild.     

Aquatic and terrestrial invertebrate drift in southern Appalachian Mountain streams: implications for trout food resources

1. We characterised aquatic and terrestrial invertebrate drift in six south‐western North Carolina streams and their implications for trout production. Streams of this region typically have low standing stock and production of trout because of low benthic productivity. However, little is known about the contribution of terrestrial invertebrates entering drift, the factors that affect these inputs (including season, diel period and riparian cover type), or the energetic contribution of drift to trout. 2. Eight sites were sampled in streams with four riparian cover types. Drift samples were collected at sunrise, midday and sunset; and in spring, early summer, late summer and autumn. The importance of drift for trout production was assessed using literature estimates of annual benthic production in the southern Appalachians, ecotrophic coefficients and food conversion efficiencies. 3. Abundance and biomass of terrestrial invertebrate inputs and drifting aquatic larvae were typically highest in spring and early summer. Aquatic larval abundances were greater than terrestrial invertebrates during these seasons and terrestrial invertebrate biomass was greater than aquatic larval biomass in the autumn. Drift rates of aquatic larval abundance and biomass were greatest at sunset. Inputs of terrestrial invertebrate biomass were greater than aquatic larvae at midday. Terrestrial invertebrate abundances were highest in streams with open canopies and streams adjacent to pasture with limited forest canopy. 4. We estimate the combination of benthic invertebrate production and terrestrial invertebrate inputs can support 3.3–18.2 g (wet weight) m⁻² year⁻¹ of trout, which is generally lower than values considered productive [10.0–30.0 g (wet weight) m⁻² year⁻¹]. 5. Our results indicate terrestrial invertebrates can be an important energy source for trout in these streams, but trout production is still low. Any management activities that attempt to increase trout production should assess trout food resources and ensure their availability.     

The effect of ration level and social rank on the development of fin damage in juvenile rainbow trout

The development and severity of fin damage was examined in groups of juvenile rainbow trout Oncorhynchus mykiss of different strength of feeding hierarchies. The development of dorsal and caudal fin damage over time was compared between four groups fed different ration levels (0·25, 0·5, 1·0 and 1·5% body weight day⁻¹) and between individuals of different feeding rank within each group. Dominance hierarchies were assessed from repeated daily measurements of food consumption of individuals using radiography. The feeding and growth data indicated that the strength of the social hierarchy weakened with increasing ration. Caudal fin damage developed with time in all groups whereas dorsal fin damage developed only under limited rations. The severity of both dorsal and caudal fin damage was significantly dependent on the ration size fed to the group, with lower ration groups sustaining more fin damage. The severity of dorsal fin erosion was greater than for the caudal fin. Within the two lower ration groups, subordinate fish suffered the most dorsal fin damage. The results suggested that the severity of dorsal fin damage within groups of juvenile rainbow trout can be used as an indicator of hierarchy Strength.     

Digestion rate, gut passage time and absorption efficiency in the Antarctic spiny plunderfish

The absolute gut evacuation rate (GER) (g day⁻¹) of Harpagifer antarcticus increased with increasing ration mass, fish mass only influenced the absolute GER at a daily ration level of 0·3% wet fish mass (approximately a maintenance ration). The relative GER (% of meal fed day⁻¹) was also affected differently by fish and ration mass depending on the relative ration level being fed; at rations of 0·7% wet fish mass or above the relative GER decreased with increasing fish or ration mass (in such a way that the absolute GER remained constant and unaffected by fish mass). At maintenance (0·3% wet fish mass) rations the relative GER was not affected by fish size or ration mass. Thus, there appears to be a ration threshold above which the digestion physiology alters. Mass-specific GER (% g fish⁻¹ day⁻¹) decreased with increasing fish mass. Within a set relative ration level (% wet fish mass) an increase in fish mass decreased the mass-specific GER. At a fixed ration mass, an increase in fish mass (i.e. a reduction in the ration expressed as % fish mass) resulted in a decrease in mass-specific GER. Gut evaluation time (GET) decreased and absorption efficiency (A) increased with increasing absolute GER. The effect of ration and fish mass on the absolute and relative GER followed the same pattern irrespective of the diet, however the A and GER (% day⁻¹ and g day⁻¹) were higher and the GET shorter when the fish were fed shelled krill rather than amphipods.     

Food intake and growth in the blue-spotted trevally, Caranx bucculentus Alleyne and Macleay 1877, with reference to predation on penaeid prawns

Blue-spotted trevally, Caranx bucculentus , were fed different rations of pilchard and prawn in order to investigate feeding and growth relationships. Maintenance rations at 25.5° C amounted to 3.7% B.W. day⁻¹ and 2.7% B.W. day⁻¹ for prawns and pilchards, respectively. Additional feeding experiments at 28.9° C yielded a maintenance ration of prawns of 3.8% B.W. day⁻¹, suggesting there is very little if any temperature effect on the feeding-growth relationship over the range studied. Fish fed twice or more each day consumed about 7.3 ± 1.4% B.W. day⁻¹.
Given the biomass of this trevally in Albatross Bay, Gulf of Carpentaria, and the contribution of prawns to its diet, we estimate consumption of commercial prawns at 25 ± 5 g.ha⁻¹ day⁻¹ or 11 g kg⁻¹ day⁻¹.     

The effects of diet composition and ration on biotransformation enzymes and stress parameters in rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout (Oncorhynchus mykiss) were fed one of three isoenergetic diets varying in protein (35-55%) and lipid content (8-18%), at full satiation levels or half rations for 6 weeks in order to investigate the effects of diet on baseline stress parameters and biotransformation enzyme activity. Growth was greatest in fish fed to satiation on a low protein and high lipid diet. Stress parameters, including plasma lactate, glucose and cortisol concentrations were not significantly affected by dietary treatment or ration. Basal biotransformation enzymes, including glutathione S-transferase (GST) and ethoxyresorufin O-deethylase (EROD) activity, were also unaffected by dietary treatment. Fish exposed to the biotransformation enzyme inducer beta-naphthoflavone did not exhibit an alteration in stress indicators or GST activity; however, EROD activity was increased (87- to 210-fold) in fish receiving all diets and rations. The results of the present study indicate that, unlike mammals, fish may be more recalcitrant to different levels of ingestion of isoenergetic diets varying in protein and lipid concentration with respect to stress responses and the maintenance of basal titers of biotransformation enzymes and their induction.     

Importance of terrestrial subsidies for native brook trout in Appalachian intermittent streams

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RNA-DNA ratio as an index to growth in salmonid fishes in the laboratory and in streams contaminated by carbaryl

Free nucleotides, RNA, and DNA were extracted from fish white muscle and analysed by a two-wavelength ultraviolet (UV) absorbance technique to determine their relation to growth rates in brook trout, Salvelinus fontinalis , and Atlantic salmon, Salmo salar . In the laboratory, fish given ample feed had higher growth rates than fish on restricted rations, and these growth rates were significantly correlated with RNA-DNA ratios. Wild brook trout from natural streams had higher average RNA-DNA ratios than those in the laboratory. Exposure of brook trout in one of the streams to low levels of the insecticide carbaryl (Sevin) that drifted from the spraying of nearby forests did not reduce RNA-DNA ratios.     

Seasonal influence of brook trout and mottled sculpin on lower trophic levels in an Appalachian stream

1. In some situations fish have strong top‐down effects in stream communities while in others they seem to be relatively unimportant. Differences in the impact of fish may depend on a variety of factors including the foraging mode of the fish, interactions among fish species and temporal variation in environmental conditions and species interactions. 2. We investigated the effect of brook trout (Salvelinus fontinalis) and mottled sculpin (Cottus bairdi) on lower trophic levels in Appalachian streams and whether or not interactions between these fish changed their influence. Mesocosms were placed in a headwater stream in a randomized complete block design. Within blocks, mesocosms were randomly assigned to one of the following treatments: (i) no fish; (ii) sculpin only; (iii) trout only and (iv) both sculpin and trout. Fish biomass was the same in all three fish treatments. Invertebrate density and algal biomass in mesocosms were determined after 3 weeks. We repeated the experiment in the autumn, spring and summer to test for seasonality of fish effects. 3. The effect of fish on invertebrate assemblages was seasonal and depended on prey identity. Sculpin strongly suppressed grazer abundance in spring while trout had little effect on grazers in any season. The influence of both fish on insect predators was similar and relatively constant across seasons. We found little evidence of an interaction between sculpin and trout that strongly influenced their effect on prey across seasons. 4. None of the fish treatments influenced algal biomass during any of the seasons. Algal growth was also seasonal, with a two‐ to four‐fold increase in algal biomass in spring compared to autumn and summer. 5. Our results indicate that benthic and drift feeding fish differ in their effects on some, but not all prey. Furthermore, fish effects on prey were strongly seasonal for some, but not all prey types. While the temporal context is not commonly considered, our results indicate seasonality can be an important component of predator–prey interactions in streams.     

Forest canopy cover determines invertebrate diversity and ecosystem process rates in depositional zones of headwater streams

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