An invasive and a native macrophyte species provide similar feeding habitat for fish

Invasive macrophytes usually increase the homogenisation of habitats and can affect local communities with negative effects on the food chain. We evaluated experimentally the composition, density, richness and diversity of the invertebrates in the invasive macrophyte Hydrilla verticillata and assessed the potential effects of this macrophyte on the diet and foraging efficiency of small native fishes. The native macrophyte Egeria najas was used as control, since it has very similar physical structure and physical complexity to H. verticillata. Macrophytes of both species were maintained separately in aquaria with the fishes, and the invertebrates richness and abundance associated with plants and within fish stomach were evaluated. No significant difference was found in the richness and diversity of invertebrates attached to macrophytes; however, the two macrophytes differed in regard to invertebrate composition, and the density was significantly higher in H. verticillata. The fish foraging efficiency was high in both macrophytes, and the difference between them was not significant. The fish diet did not differ significantly between the native and the invasive macrophyte in respect to invertebrate composition, abundance and richness, and trophic niche breadth. We found no evidence that H. verticillata negatively affects the density of associated invertebrates, and as a consequence, fish diet and foraging efficiency. Our results indicate that the replacement of E. najas by a similar biomass of H. verticillata does not affect the feeding behaviour of associated fish species.     

Fish feeding in changing habitats: effects of invasive macrophyte control and habitat complexity

Abstract –  Many freshwater lakes have been invaded by non-native aquatic plants. Although managing the invasions is necessary to restore native macrophytes, little is known about the effects of invasive macrophyte eradication on fish foraging. This study was designed to determine whether fish feeding was affected by large-scale changes in plant composition after selective control of invasive Eurasian watermilfoil, Myriophyllum spicatum , and whether feeding was correlated with habitat complexity or abundance of invasive plants on a smaller scale. Popnets were used to collect plants, invertebrates and fishes in the littoral zone of four Minnesota lakes twice a year for 4 years. The degree of stomach fullness in fishes was not correlated with the proportion of invasive plants or habitat complexity. Bluegills, the most abundant fish species, exhibited high selectivity for Diptera and their preferences varied little in different plant habitats. Changes in niche width of bluegills were not consistent with invasive plant control. Overall, invasive macrophyte control with timely restoration of the native plant community did not affect the characteristics of fish feeding we investigated. This study also contributes to our understanding of fish–macroinvertebrate interactions as a function of habitat complexity.     

Coexisting invasive gobies reveal no evidence for temporal and trophic niche differentiation in the sublittoral habitat of Lake Erhai,China

Niche differentiation facilitates the coexistence of species within a community through avoidance of competition via mechanisms involving spatial, temporal and/or trophic dimensions. Where invasive species coexist in their non‐native range, niche differentiation could allow their persistence at higher levels of abundance. Here, we tested whether there was temporal and/or trophic niche differentiation between two congeneric invasive goby species (Rhinogobius cliffordpopei and Rhinogobius giurinus) in the sublittoral habitat of Lake Erhai (south‐west China) through quantifying their diel and seasonal rhythm of locomotion activities, feeding activities and diet composition. Both species displayed two similar diel peaks in activity and two main feeding periods (6:00–10:00 and 18:00–22:00), with rhythms of locomotion and feeding activity not differing significantly between the species in each season. Their diets had a high degree of overlap, being primarily composed of macrozooplankton, aquatic insects and shrimp larvae, with no diel changes across the seasons. Thus, in this habitat, there was no clear temporal or trophic niche differentiation between the invasive congeners, indicating their coexistence with high temporal and trophic overlap. In conjunction with data from the littoral and profundal habitats, the gobies revealed different strategies across the habitats (e.g. spatial segregation, trophic niche differentiation) that minimised their competitive interactions and promoted their coexistence. This suggests that the interactions of invasive fishes during the integration into native communities can be context dependent, varying according to factors including habitat and the availability of food resources.     

Assemblage structure and habitat use of fishes in a Central European submontane stream: a patch-based approach

Abstract –  We examined macrohabitat patch level assemblage composition and habitat use patterns of fishes over four seasons in a second-order submontane stream (Danube drainage, Hungary). Rainfall data indicated that our study included both high- and low-water periods, and hence our results may be applicable to broader time scales. Principal component analysis of physical habitat data from 13 macrohabitat patches indicated that these patches represented a riffle-pool continuum. Correspondence analysis of fish assemblage structure data from these patches identified a continuum in assemblage composition that was positively correlated with the habitat continuum. The riffle fauna was dominated by stone loach ( Barbatula barbatula ), whereas chub ( Leuciscus cephalus ) were most abundant in pool patches. We detected little evidence of seasonality in either fish assemblage structure or habitat use. Fish density did not differ significantly among macrohabitat patches in two (summer and autumn 1999) of three seasonal samples, although riffle patches displayed significantly higher fish abundance in late spring 2000. This difference primarily was due to higher abundance of juvenile stone loach in riffles. Two species, stone loach and minnow ( Phoxinus phoxinus ), displayed generalized habitat use patterns, whereas chub and rare species (i.e., gudgeon, Gobio gobio ; dace, Leuciscus leuciscus ; Barbus petenyi ; and burbot, Lota lota ) were significantly over-represented in pool habitats. We hypothesized that pool specialists (i.e., chub and rare species) were responding primarily to the increased depth of these habitats. Nevertheless, our data did not demonstrate the presence of separate pool and riffle habitat guilds. In conclusion, we believe that our understanding of stream fish ecology will be greatly facilitated by use of a 'patch-based approach'.     

Growth and efficiency of feed usage by Atlantic salmon (Salmo salar) fed diets with different dietary protein: Energy ratios at two feeding levels

The effect of dietary digestible protein/digestible energy (DP/DE) ratios and feeding level on growth, feed efficiency, nutrient and energy usage by Atlantic salmon ( Salmo salar ; initial body weight, 7.0 g/fish) at 15°C was investigated in a 16-week feeding trial. Three diets, differing in their DP and DE contents, namely 37/18 (regular diet, RD), 37/21 (high fat diet, HF) and 44/ 22 (high nutrient-dense diet, HND) g/MJ of dry feed were formulated. DP/DE ratios were 20, 18 and 20 g/MJ for the RD, HF and HND diets, respectively. Salmon were hand-fed three times a day at either 100% or 85% of the feed requirement estimated by a bioenergetics model. At each feeding level, DE intake (kJ/fish) was similar for all three diets. Diet composition did not affect growth rate. However, increasing the digestible energy density from 18 to 22 MJ/kg of dry feed resulted in a significant increase ( P  < 0.05) in feed efficiency. Restricting feed intake significantly decreased live body weight gains for all diets. However, feed efficiency was not affected by feeding level. Diet composition and feeding level did not affect carcass composition and nutrient and energy usage, with the exception of a higher ( P  < 0.05) carcass lipid of fish fed the HF100 diet compared with the fish fed the RD and HND diets and a higher ( P  < 0.05) lipid gain (g/fish) of fish fed the HF100 diet compared with fish fed all the diets at the restricted feeding level. Restricting feeding resulted in significantly lower ( P  < 0.05) energy gain (kJ/fish) compared with fish fed at 100%. Increasing the DE and nutrient density of the diet had no effect on growth but improved feed efficiency and lowered solid wastes (g of solid wastes per kg of fish produced) while dissolved wastes were not affected by dietary ormulation.     

Directed movements and diel burrow fidelity patterns of red tilefish <Emphasis Type="Italic">Branchiostegus japonicus</Emphasis> determined using ultrasonic telemetry

ABSTRACT:   Wild red tilefish Branchiostegus japonicus were tracked and/or monitored for up to nearly 3 months in Western Wakasa Bay and Maizuru Bay, Japan, using ultrasonic telemetry to examine their movements and burrow fidelity patterns. In Western Wakasa Bay, it is guessed that fish released in relatively shallow waters, about 30 m deep, actively moved into more suitable deeper habitats after their release. In Maizuru Bay, the fish did not undertake large-scale movements and showed burrow fidelity about 1 month after their release. That is, the fish primarily remained in the burrow, which was constructed in the sea-bottom, during the night and moved outside the burrow during the day. The marked circadian rhythm from Fourier analysis indicated a clear diel movement pattern of the red tilefish.     

Diel and seasonal abundance of fishes in the Platte River, Nebraska, USA

ABSTRACT:   The objective of the present study was to determine diel and seasonal patterns of fluvial fish composition and abundance. Electrofishing grids were used to collect fish in the Platte River at North Bend, Nebraska, USA. Sampling was started at 12.00 h and continued every 3 h for a 24-h period. More species were taken during the day than at night in the spring. In contrast, more species were taken at night than during the day in summer. In the fall, equal numbers of species were caught during the day and night. Comparisons of fish densities showed significant differences among the three seasons. Major taxa were significantly more abundant in spring, but river shiner Notropis blennius and western silvery minnow Hybognathus argyritis were more abundant in the fall. The greater nocturnal abundance of fish in summer and fall than in the spring may be due to changes in the physical and chemical characteristics during the study periods. The significance of seasonal difference in abundance of fish assemblages may indicate a response to changes in available habitats.     

Habitat utilization and diel behavior of juvenile bull trout (Salvelinus confluentus) at the onset of winter

Abstract— Underwater observations were used to describe habitat use and diel behavior of juvenile bull trout ( Salvelinus confluentus ) at the onset of winter (0.8°C) in a second-order central Idaho stream. All fish observed during daytime counts were concealed beneath "home stones" (mean dimensions 32.3 by 21.9 cm) in primarily pool and run habitats. Focal point depths and velocities averaged 57.2 cm and 4.7 cm/s (home stone removed). Fish maintained stations over substrate with low percentages of fine (< 2 mm, mean= 5.2%) sediment. Concealed bull trout were not evenly distributed across the stream channel. Significantly more home stones were distributed near the midline of the channel, compared to the channel margins. Fish size was positively correlated with home stone size. At night, bull trout exhibited a diel behavioral shift, some fish moved out of daytime concealment cover into the water column. At night, bull trout were observed feeding and resting, primarily in pool and run habitats. During both day and night, bull trout used riffles significantly less frequently than those habitats were available.     

Distribution and feeding of juvenile fish on invertebrates in littoral reed (Phragmites) stands

Abstract – Complex interactions between fish predators and their prey have been found in structurally complex habitats built by submerged macrophytes. In contrast, the role of comparably structured littoral reed stands in shaping biotic interactions has not been investigated. We hypothesised that reed stands may be a valuable feeding habitat for juvenile fish, and that perch and roach may segregate along the spatial and dietary niche dimensions between reed and open water habitats. In contrast, the protection effect of reed against predators was assumed to be rather low because of the lower plant volume infested in reed when compared with submerged macrophytes. We analysed biomass and growth of juvenile (age 0 and age 1) perch and roach in littoral reed habitats and in open water habitats in front of the reed in the shallow Lake Müggelsee over 4 months in 2000. Sampling was conducted by point-abundance electrofishing over the full diel cycle (day, dusk, night, dawn). Zooplankton and benthos biomasses were determined in both habitats as well, and habitat-specific diet of fish was assessed during day and night. Roach were more frequent than perch in both habitats. Food of roach included a higher proportion of zooplankton, whereas perch fed more on macroinvertebrates. Overall, diet overlap between the fish groups was high. Diel distribution of fish did not follow the expectations of habitat segregation between perch and roach. Instead, the function of reed as refuge habitat against littoral piscivores (mainly birds) may have caused the strong daytime preference for reed in almost all fish groups, which was partly upset by roach at night. The higher behavioural plasticity of roach may explain their good performance even under the conditions of high structural complexity.     

Foraging behaviour and invasiveness: do invasive Gambusia exhibit higher feeding rates and broader diets than their noninvasive relatives?

Abstract –  Serious impacts by nonindigenous species often occur via predation. The magnitude of impact is expected to be closely tied to the invading species niche breadth. For predatory impacts, diet breadth should be particularly important. We examined the relationship between a species foraging behaviour and its invasiveness and impact by comparing the feeding behaviour of four Gambusia species, two invasive and of high impact and two noninvasive. Individual feeding rates, feeding preferences, and diet breadths were tested across three prey items in a sequence of four laboratory feeding trials. Invasive Gambusia consistently fed at higher rates, but no species differences were found in feeding preferences or diet breadth. All Gambusia preferred Daphnia , avoided Lirceus , and consumed Drosophila in proportion to their availability. Female size affected most feeding variables. Larger fish consumed more prey per unit time and were able to incorporate larger prey items into their diets, thus increasing diet breadth.