Feeding ecology and habitat preferences of the catfish genus Trichomycterus in low‐order streams of the Colombian Andes

Six lower order streams in the south‐west of Colombia were sampled on a monthly basis in order to determine the status of Trichomycterus species in relation to their food supply (macroinvertebrates), and physico‐chemical habitat. Fishes were sampled by electrofishing and samples taken for dietary analysis. Macroinvertebrates were collected using a Surber sampler. Trichomycterus spp. populations were present in all streams in habitats ranging from soft to hard substrata and from relatively stagnant waters to fast flowing streams. The average density of the fishes ranged from 0·06 to 1·14 individuals m⁻² with peaks in population corresponding to recruitment of individuals of <1 g. Trichomycterus spp. were found to be benthic carnivores with a diet mostly of aquatic insects. The index of relative importance for the dietary items was 35·0, 10·5, 1·9 and 7·8% for Chironomidae, Trichoptera, Ephemeroptera and Oligochaeta respectively. Combined with physical habitat data, ordinations of fishes and macroinvertebrate data suggested that differences in abundance among sites were driven by stream size and flow regime, indicating dominance of the physical habitat, although a high correlation between Trichomycterus spp. and Chironomidae abundances ( r  = 0·81, P  = 0·049) was also recorded.     

A new technique to gather 3‐D spatial information using a single camera

A new technique using a single camera and shadows to determine 3‐D spatial positions of fishes in the laboratory is described. The apparatus consisted of a large aquarium (2·0 × 1·5 × 0·4 m), a wide‐angle camera mounted above and two light sources to cast shadows to either side of the fish. Using image analysis and vector mathematics, aquarium objects were plotted within 1·5 cm of their actual location along the x ‐, y ‐ and z ‐axis. The technique was also successful in quantifying changes in 3‐D spatial pattern of juvenile fish, Atlantic cod Gadus morhua (7·4–8·6 cm standard length, L _S) and cohabitant piscivorous shorthorned sculpin Myoxocephalus scorpinus (12·0–25·8 cm L _S), at these same viewing fields. The new technique should have a wide application, largely because it is potentially less expensive, laborious and invasive than alternative methods for determining 3‐D positions of fishes.     

Ultrastructural study of skin and eye of UV‐B irradiated ayu Plecoglossus altivelis

A scanning electron microscopic study of the skin and eye of UV‐B radiated ayu Plecoglossus altivelis (age 30 days, mean ±  s . e . total length: 16·25 ± 0·11 mm) under laboratory condition showed marked changes when compared with the control fish without UV‐B radiation. The exposure of the fish to the radiation resulted in the destruction of microridges in the epidermis and exposed neuromast cells of the skin. Domed protrusions were also more common in the skin of UV‐B radiated fish than in the control fish. The appearance of mucus in both groups was different. In the control skin the mucus was spread over a wide area whereas in the treated fish the mucus was concentrated in a small area. The anastomozing structures of the microridges of the eyes were lost in UV‐B radiated fish and the microridges themselves were fewer in number, fragmented, and aggregated. Mucus cells, prominently visible in the control fish, were distorted in the treated fish. Cell contours were irregular in UV‐B radiated fish and cell to cell contacts had been lost in this group.     

Short‐term freshwater exposure benefits sea lice‐infected Atlantic salmon

Atlantic salmon Salmo salar were infected with sea lice Lepeophtheirus salmonis (0·08 ± 0·007 sea lice g⁻¹) over a period of 4 h. Both infected and non‐infected fish were swim tested in sea water (SW) and fresh water (FW). The ventral aorta of each fish was fitted with a Doppler cuff in order to measure cardiac output, stroke volume and heart rate during swim testing. Blood samples were taken at rest and after exercise. Critical swimming speed of infected fish in SW (2·14 ± 0·08 body lengths, bl s⁻¹) was significantly lower ( P  < 0·05) than infected fish switched to FW (2·81 ± 0·08 bl s⁻¹) and non‐infected fish in SW (2·42 ± 0·04 bl s⁻¹) and FW (2·61 ± 0·08 bl s⁻¹). Cardiac and blood results indicated infected fish exposed to FW did experience stress, but relief from osmotic and ionic distress probably reduces energy expenditure, allowing the increase in performance. As the performance of sea lice‐infected fish improved upon transfer to FW, it is likely that heavily infected salmonids do return to FW to restore compromised osmotic and ionic balance, and remove sea lice in the process.     

Inter‐individual differences in rates of routine energy loss and growth in young‐of‐the‐year juvenile Atlantic cod

Inter‐individual differences in rates of routine (non‐feeding) metabolism and growth were evaluated in young‐of‐the‐year (YOY) juvenile Atlantic cod Gadus morhua . Rates of O₂ consumption, CO₂ production and ammonia (TAN) excretion were measured in 64, 25–43 mm standard length ( L _S) YOY growing at different rates (0·27–0·47 mm day⁻¹) in a common rearing tank. Parameter rates ( y ) increased allometrically ( y = a·M^b ) with increasing body mass ( M ) with b ‐values for O₂ production, CO₂ consumption and TAN excretion equal to 0·81, 0·89 and 0·56, respectively. In some cases, residuals from these regressions were significantly negatively correlated to fish growth rate. In no cases did residuals of parameter rates increase with increasing growth rate. These data suggest that, during unfed periods, relatively fast‐growing fish were more metabolically efficient than slower‐growing fish from the same cohort. The fish condition factor, derived from     , also significantly decreased with increasing growth rate. Results indicated differences in both the rates of routine energy loss and the patterns of growth allocation among YOY Atlantic cod. Since these physiological attributes were positively correlated with growth rate, they may be indicative of 'survivors' in field populations.     

Indirect effects of the algivorous fish Plecoglossus altivelis altivelis on the growth of two insectivorous benthic fish

1. Animals exploiting different resources may nevertheless interact if one species indirectly alters the abundance and distribution of the food of the other. To analyse this indirect effect, we conducted experiments in artificial pools and in the field to investigate the influence of the algivorous fish Plecoglossus altivelis altivelis (known as the ayu) on two species of insectivorous benthic fish, Pseudogobio esocinus esocinus and the goby Gymnogobius petschiliensis .
2. In the pool experiments, algal biomass was not correlated with the number of ayu, but the percentage of blue-green bacteria rose as the number increased. The number of aquatic macroinvertebrates on the upper surface of ceramic tiles placed in the pool bed decreased as the number of ayu increased.
3. Although ayu and the benthic species did not interact directly, the reduction in invertebrate abundance on the upper surface of tiles in the pool reduced the growth rate of the benthic insectivores.
4. In field experiments, the introduction of ayu into habitats with P. esocinus esocinus or G. petschiliensis reduced the growth rate of these benthic fish. In the field experiment that was carried out over 5 years in the G. petschiliensis habitat, the population density of the goby decreased when ayu were stocked.
5. The ayu is a strong interactor or bioengineer in streams, affecting not only benthic algae but also aquatic invertebrates and fishes. We conclude that to predict the outcome of interspecific interactions amongst fishes in streams with high algal production, possible indirect effects must be considered alongside better known direct effects.     

Spatial patterns of the biological traits of freshwater fish communities in south‐west France

Spatial patterns in the combinations of biological traits of fish communities were studied in the Garonne River system (57 000 km², south‐west France). Fish species assemblages were recorded at 554 sampling sites, and the biological traits of species were described using a fuzzy‐coding method. A co‐inertia analysis of species distributions and biological traits identified some spatial patterns of species trait combinations. Fish species richness progressively increased from up‐ to downstream sections, and the longitudinal patterns of fish assemblages partitioned the river into clear biogeographic areas, such as the brown trout Salmo trutta (headwater streams), the grayling Thymallus thymallus , the barbel Barbus barbus and the bream Abramis brama zones (most downstream sections), which fitted with Huet's well‐known zonation for western European rivers. Only a few biological traits, chiefly related to life‐history attributes, significantly influenced the observed fish distributions. Fecundity, potential size, maximum age and reproductive factor increased from headwater to plain reaches. As a theoretical framework for assessing and predicting the functional organization of stream fish communities, spatial variations in species traits can be related to habitat conditions, thus providing explicit spatial schemes that may be useful to the design of both scientific studies and river management.     

Correlations between type‐indicator fish species and lake productivity in German lowland lakes

Morphotypes for 67 lakes in the German lowlands were derived, based on maximum depth and mixis type. A threshold of 11 m maximum depth was identified to be the best level to discriminate shallow from deep lake morphotypes. The fish communities in these two morphotypes were significantly different. Indicator species analyses based on fish biomasses found vendace Coregonus albula in deep lakes and ruffe Gymnocephalus cernuus , bream Abramis brama , white bream Abramis bjoerkna , roach Rutilus rutilus , pikeperch Sander lucioperca and small perch Perca fluviatilis in shallow lakes to be the most representative species of their communities. Lake productivity was closely related to biomass and in part abundance of the type‐indicator species, with vendace declining with increasing chlorophyll a concentration in the deep lakes, whereas biomass of pikeperch, bream, white bream and ruffe increased and biomass of small perch decreased with increasing chlorophyll a . These results indicate that assessment of ecological integrity of lakes by their fish fauna is generally possible, if lakes are initially separated according to a depth‐related morphotype before the assessment, and if eutrophication is considered to be the main anthropogenic degradation.     

Intraguild indirect effects through trophic cascades between stream-dwelling fishes

1. To examine the strength and role of indirect effects through trophic cascades by omnivorous fish on algal biomass in streams, we introduced one of four fish species (ayu Plecoglossus altivelis altivelis, pike gudgeon Pseudogobio esocinus esocinus, Japanese dace Tribolodon hakonensis and pale chub Zacco platypus) in experimental pools. We also investigated the indirect effects of gudgeon, dace and chub on the growth of ayu. 2. We chose the four fish species across a continuum of omnivory. Ayu fed mainly on benthic algae, and gudgeon predominantly on invertebrates. Dace and chub fed on both algae and invertebrates, but dace preyed on invertebrates more than chub. 3. The presence of gudgeon, dace and chub reduced the number of algal-feeding invertebrates and increased the algal biomass through trophic cascades. Consequently, ayu growth rate over the experimental period in pools with one of the three fish species was 25.9-44.1 times greater than the growth rate in pools with only ayu. 4. The positive indirect effect on ayu growth was large for gudgeon and dace and small for chub, whereas the addition of ayu reduced ayu growth considerably due to intraspecific competition. 5. The relative intensity of indirect effects on ayu growth through trophic cascades was predictable from the food overlap between ayu and the other fishes. However, the similar strength of indirect effects by gudgeon and dace that fed differently on algae and invertebrates suggests that feeding behaviour, prey preference and trait-mediated indirect interactions were also important in the prediction.     

Hatch date‐dependent differences in early growth and development recorded in the otolith microstructure of Trachurus japonicus

Otolith microstructure of 308 juvenile jack mackerel Trachurus japonicus sampled in the southern Sea of Japan between June and September 2002 were examined by light microscopy. Hatch‐date distribution indicated a protracted spawning season from 16 January to 30 May 2002. The first secondary primordium (SP) was formed at ages 19–54 days, with a mean ±  s . d . of 30·4 ± 6·1. The number of SP in an otolith ranged from two to 15, with a mean of 6·1 ± 1·7. Age of the first SP formation, number of SP and increment width varied with hatch date. In general, late‐hatched fish were younger when the first SP formed, and had a higher number of SP and wider increments, than early‐hatched fish. As increment width is a linear function of somatic growth and formation of the first SP is associated with metamorphosis in jack mackerel, these variations in otolith microstructure indicated that rates of growth (as inferred from wider increments) and development (as inferred by younger age of first SP formation) were higher in late‐hatched fish relative to early‐hatched fish. Higher temperature experienced by late‐hatched fish during early life stages may contribute critically to their higher growth and development rates compared to early‐hatched fish.     

Temporal and spatial variation in potential and realized growth rates of age‐0 year northern rock sole

The possibility of prey limitations on the growth performance of age‐0 year northern rock sole Lepidopsetta polyxystra was evaluated at three sites along the north‐east coast of Kodiak Island, Alaska, U.S.A., by comparison of observed to potential growth rates. Growth potential was measured in the laboratory across the range of temperatures encountered by this species during the first summer of life. Growth potential ( g _L, mm day⁻¹) increased with water temperature (T) between 2 and 13° C, according to: g _L = 0·0151 + 0·3673·log₁₀(T). There were significant differences in growth rate between the three field sites such that Holiday Beach fish were 7·1 mm longer than Shakmanof Beach fish by mid‐September, with Pillar Creek Cove fish of intermediate size. Temperature differences between sites accounted for less than half of this variation. The remainder may have been related to differences in prey availability among the sites in association with observed differences in sediment characteristics. In addition to the spatial variability, there was significant monthly variation in growth performance. Realized growth rates between July and August were in excess of 85% of potential. Between August and September, however, realized growth fell to 43–71% of potential indicating a decline in conditions for growth. The spatial variation in growth rates was not density‐dependent as the site with the highest fish densities (Holiday Beach) also supported the highest growth rates. The available data indicates that for this subtidal species, interannual variation in growth may be more important than site variation.     

Feeding preference and deterrence in rabbitfish Siganus fuscescens for the cyanobacterium Lyngbya majuscula in Moreton Bay, south‐east Queensland, Australia

Rabbitfish Siganus fuscescens preferences for Lyngbya majuscula collected from three bloom locations in Moreton Bay, Queensland, Australia, were tested along with a range of local plant species in the laboratory. Consumption of L. majuscula by fish did not differ between wild and captive‐bred fish ( P  = 0·152) but did differ between bloom location ( P  = 0·039). No relationship was found between consumption rates and lyngbyatoxin‐a concentration ( r ² = 0·035, P  = 0·814). No correlation existed between C : N and proportion of food consumed when all food types were analysed statistically, whereas a clear correlation was observed when L. majuscula was removed from the calculations. In simulated bloom conditions, fish avoided ingestion of L. majuscula by feeding through gaps in the L. majuscula coverage. Both wild and captive‐bred S. fuscescens showed a distinct feeding pattern in 10 day no‐choice feeding assays, with less L. majuscula being consumed than the preferred red alga Acanthophora spicifera . Lyngbya majuscula however, was consumed in equal quantities to A. spicifera by wild S. fuscescens when lyngbyatoxin‐a was not detectable. Wild fish probably do not preferentially feed on L. majuscula when secondary metabolites are present and are not severely impacted by large L. majuscula blooms in Moreton Bay. Furthermore, poor feeding performance in both captive‐bred and wild S. fuscescens suggests that they would exert little pressure as a top‐down control agent of toxic L. majuscula blooms within Moreton Bay.     

Diurnal bottom feeding of predator fish strengthens trophic cascades to benthic algae in experimental flow-through pools

Mechanisms that determine the strength of trophic cascades from fish to benthic algae via algivorous invertebrates in stream communities have not been clarified. Using seven fish species, we tested the hypothesis that the interspecific variation of predatory behavior of fishes affects the strength of trophic cascades in experimental streams. One or two species of fish were introduced into flow-through pools of 2.5 m² and the abundances of benthic invertebrates and algae were monitored. Pike gudgeon, a diurnal benthic feeder, triggered a strong trophic cascade but masu salmon, a diurnal drift feeder, did not have a cascading effect. Japanese dace, which is both a diurnal benthic and drift feeder, increased the algal biomass, but the nocturnal benthic feeder cut-tailed bullhead had little cascading effect. The diurnal benthic feeder silver crucian carp also had a cascading effect, but no trophic cascade was triggered either by Asian pond loach or by Japanese common catfish, both of which are nocturnal benthic feeders. Thus, diurnal benthic fish exerted a stronger cascading effect than diurnal drift feeders or nocturnal fish. The combination of two fish species enhanced the per-capita strength of trophic cascades, probably because one of the two species, the benthic feeder, preyed on more invertebrates than in the single-species pools.     

Spatial variation in density of stream benthic fishes in northern Hokkaido, Japan: does riparian vegetation affect fish density via food availability?

The densities of two benthic fishes, the Siberian stone loach (Noemacheilus barbatulus) and the wrinklehead sculpin (Cottus nozawae), and the biomass of their food resources (i.e., periphyton and benthic invertebrates) were compared between forest and grassland streams in northern Hokkaido, Japan, to examine whether riparian deforestation had positive effects on the benthic fishes via enhancement of food availability. The comparisons indicated that riparian vegetation had little influence on periphyton, invertebrates, or fishes. Regression analysis indicated that spatial variations in loach and sculpin densities were explained more by substrate heterogeneity, competitor abundance, or both, rather than by food abundance. However, when the two species were combined as benthic insectivores, a strong correlation was found between total benthic fish density and invertebrate biomass. Our results suggest that, although total benthic fish abundance was food limited, riparian vegetation had no positive effects via food availability on the benthic fishes in our streams.     

Heat stress‐induced alterations of antioxidants in the freshwater fish Channa punctata Bloch

The effect of elevated temperature on the antioxidants in the freshwater fish Channa punctata was investigated. Fish stressed with an elevated temperature of 12° C, range ± 1° C over the ambient temperature for 3 h showed a significant ( P  < 0·05–0·01) reduction in the levels of antioxidants: reduced glutathione (GSH) and non‐protein thiols. Activity of glutathione reductase was also reduced in all the tissues (liver, kidney and gills) after 3 h of heat stress and 24 h recovery. Catalase (CAT) showed enhanced activity in liver in both the conditions while gills and kidney showed a decreased CAT activity. Glutathione S‐transferase (GST) activity in kidney and liver decreased significantly ( P  < 0·05–0·01) after 3 h of heat stress. At 24 h GST activity showed a tendency to normalize in all the tissues along with a concomitant increase in the GSH level in the kidney. Total and protein thiols in heat stressed fish when matched with controls, showed significant ( P  < 0·05) reduction in the kidney only with a transient increase in liver and gills. Heat shock also induced lipid peroxidation in 3 h heat‐treated and recovery groups when compared with controls. Elevated temperature therefore resulted in tissue specific and time‐dependent alterations of antioxidants in the fish. It also induced lipid peroxidation in various tissues.     

Diel distribution patterns of fishes in a temperate large lowland river

During the day, juveniles and typical inshore species in the lower Oder River, Germany, inhabited the littoral (0·05–1·50 m deep) while potamal species inhabited the mid‐channel (40–100 m from the banks). During the night, two behavioural groups were distinguished: 1) potamal species and large individuals of other species, moved from offshore to the littoral, and 2) nocturnal foraging species, became active and increased in inshore and offshore habitats. Inshore, the most significant differences at night was the increasing density of silver bream Blicca bjoerkna increasing both total biomass and total length of fishes caught. Offshore, the increasing density of whitefin gudgeon Gobio albipinnatus at night was most significant. Additional night electrofishing improved the assessment of the abundance, age and size structure of typical potamal fish species. It should be incorporated in large river fish monitoring programmes.     

Predator‐induced hyperventilation in wild and hatchery Atlantic salmon fry

Following exposure to a predator stimulus (a brown trout Salmo trutta ), the opercular rate of Atlantic salmon Salmo salar fry increased by 35·3 ± 11·0%(mean ± 95% CI). The time taken for opercular rate to decline to baseline levels depended upon the occurrence of any associated locomotory activity. Opercular rates of fish that dashed when exposed remained elevated for 38·2 ± 20·6 min, whereas those of individuals that did not move ('freezers') recovered within 7·2 ± 2·9 min. The duration that opercular rate remained elevated was positively correlated with the magnitude of the elevation, which was higher in 'dashers' than freezers. The maximum opercular rate in 'freezers' was similar between wild fry and hatchery‐reared fry (from wild parents). There was a significant delay, however, in hatchery compared with wild fry in the time until peak ventilatory response and onset in the decline phase. This difference in opercular response suggests that hatchery fish were slower to realize fully the potential danger from the predator. Any delay in response could be directly attributed to the effect of hatchery‐rearing environment, rather than domestication or hatchery selection effects.     

Fish drift in a Danube sidearm‐system: I. Site‐, inter‐ and intraspecific patterns

A total of 10 649 larval and juvenile fishes of 24 species were caught in the drift at Marchfeldkanal, a man‐made side branch of the Danube River near Vienna, Austria, with tubenose goby Proterorhinus marmoratus being the dominant species. Distinct differences in inter‐ and intraspecific drift patterns among different sampling stations along the course of the channel were found. The percentage of the rheophilic and rheoparous ecological guild was highest at the inlet of the channel, where it is directly fed with water from the Danube. For individual species, significant differences in drift densities among sites were found for tubenose goby, barbel Barbus barbus , ide Leuciscus idus , roach Rutilus rutilus and for the two most abundant percids, the pike‐perch Sander lucioperca and zingel Zingel zingel combined. The occurrence of larval and juvenile fishes in the drift was related to certain developmental stages and differed between species and sites. Most species (common bream Abramis brama , bleak Alburnus alburnus , gudgeon species Gobio spp., chub Leuciscus cephalus , ide and roach) occurred with highest densities at the earliest developmental larval stage, but some species ( e.g . common bream and roach) were also found abundantly in drift at later developmental stages. Application of Ivlev's index of electivity as a drift index describing the propensity of the different species to drift, yielded the highest indices for the gudgeon species, common bream and bleak and the lowest for perch Perca fluviatilis , Prussian carp Carassius auratus gibelio and rudd Scardinius erythrophthalmus .     

Food selection of Coregonus lavaretus in a brackish water ecosystem

This study examined how variability in the abundance and biomass structure of benthic invertebrates affected the feeding choice of the whitefish Coregonus lavaretus on a hard bottom habitat of the brackish Baltic Sea. In general, crustaceans such as Idotea balthica and Gammarus spp. were preferred over molluscs. Although being the most numerous taxon in the invertebrate samples, Mytilus trossulus was the lowest ranking in C. lavaretus food preference. The availability of benthic invertebrate prey set the dietary range of fish but the selectivity largely described fish feeding within this range. There was no clear link between fish predation and the dominance structure of benthic invertebrate communities, suggesting that species composition, abundance and biomass of invertebrate species had no impact on the feeding selectivity of the fish. Thus, while fish predation may not affect the dominant species within a benthic community, due to strong selectivity fish may impose strong pressure on some rarer but highly preferred invertebrate prey species.     

Effects of the freshwater aquarium trade on wild fish populations in differentially‐fished areas of the Peruvian Amazon

Base‐line data were collected to examine the possible effects of the aquarium trade on Amazon fish populations in differentially‐fished locations in the Departmento of Loreto, Peru. Fish abundance, species diversity (richness) and biomass were quantified at three locations with differential fishing pressure, including the Rio Nanay (high pressure), Rio Apayacu and Rio Arambassa (medium) and Rio Yanayacu –Pacaya‐Samiria National Reserve (low). Seining results indicated that the location with the highest fishing pressure had reduced fish abundance, species diversity and biomass compared to the other locations. A similar trend was seen using minnow traps. There was no significant difference in abundance, diversity and biomass between the medium and low fishing pressure locations. Habitat differences (pH and conductivity) among the three locations accounted for <13% of the observed variation, and thus it seemed possible that some of the decline in the Rio Nanay location could be tentatively attributed to increased fishing pressure. Although effects of pollution and habitat alteration could not be ruled out, this is one of the first studies to provide quantitative data on the effect of the freshwater aquarium trade on wild fish populations. While results suggest success in the protection of fishes for the aquarium trade in Pacaya‐Samiria National Reserve, it also may be sustainable to establish limited levels of fishing, as was found in the medium fishing pressure locations, in order to promote economic opportunities and incentives for habitat preservation for indigenous communities.