Grazing catfish,fishing birds,and attached algae in a Panamanian stream

Synopsis In streams where algivorous fishes abound, striking variation of attached algae often develops along depth gradients, with bands of high standing crops in shallow water (<20 cm) and sparse standing crops on deeper substrates. Experimental results from a stream in central Panama support the hypothesis that vertical variation in algal standing crops arises when grazing fishes avoid predators in shallow water by forgoing food resources that accumulate there. When 38 rocks bearing algae in a stream in central Panama were transferred from shallow (<20 cm) to deeper (>20 cm) water, algae were rapidly consumed by grazing catfish. Catfish were removed from three stream pools and left in place in three control pools. Ten days after catfish removal, algal standing crops in deep and shallow areas of removal pools were similar, while algal standing crops were higher in shallow than in deep areas of control pools. Catfish were exposed to fishing birds in open-topped enclosures. In one of three series of these pens, most catfish in shallow pens (10 and 20 cm) disappeared after 14 days, while catfish in deeper pens (30 and 50 cm) did not. Other groups of catfish which were caged 8 days showed differences in behavior depending on whether they had been fed or starved. After their release into their home pool, starved catfish spent more time feeding than did fed catfish. Despite their apparently increased hunger levels, starved catfish did not venture into shallow water to obtain algae. These results support the view that predator induced avoidance by grazers of certain areas can produce spatial pattern in the flora of flowing water communities.     

The effects of intra- and inter-specific competition on the distribution of stocked juvenile Atlantic salmon, Salmo salar L., in relation to depth and gradient in an upland trout, Salmo trutta L., stream

The relative effects of inter- and intra-specific competition on the distribution of stocked salmon in relation to depth and gradient were investigated in an upland stream during two summer sampling periods. The stream was divided into two areas by an impassable fish barrier, and trout were removed from the upstream section prior to 2 years of salmon stocking. A small amount of trout re-immigration to the cleared area occurred. Under sympatric conditions in the downstream section the fry of both species were significantly more abundant in shallow water, whereas the yearling and older fish tended to inhabit the deeper, slow-flowing areas. Under near allopatric conditions in the cleared section salmon fry changed their distribution to include deeper areas, with shallow, fast-flowing water becoming the least preferred habitat. This trend for a wide distribution of salmon fry over all the available habitat in the upstream section was maintained in the second year when parr were present but trout remained at very low densities. It was therefore concluded that high inter-specific competition from trout was responsible for restricting the distribution of salmon fry to shallow habitat in the control area. Intra-specific competition from older salmon apparently only affected the growth and survival of salmon fry. The regulating mechanisms involved in these inter- and intra-specific effects are discussed in terms of competition for stream resources and predation. Recommendations based on the findings are made for stocking and habitat management of salmonid waters.     

Swimming ability and behaviour of post-larvae of a temperate marine fish re-entrained in the pelagic environment

The degree to which behaviour, vertical movement and horizontal transport, in relation to local hydrodynamics, may facilitate secondary dispersal in the water column was studied in post-larval Sillaginodes punctata in Port Phillip Bay, Australia. S. punctata were captured in shallow seagrass beds and released at the surface in three depth zones (1.5, 3 and 7 m) off-shore at each of two sites to mimic the re-entrainment of fish. The behaviour, depth and position of S. punctata were recorded through time. The direction and speed of local currents were described using an S4 current meter and the movement of drogues. Regardless of site, fish immediately oriented toward the bottom, and into the current after release. In shallow water (1.5 m), 86% of fish swam to the bottom within 2 min of release. At one site, the net horizontal displacement of fish was largely unrelated to the speed and direction of local currents; at a second site, fish could not maintain their position against the current, and the net horizontal displacement was related to the speed and direction of currents. In the intermediate depth zone, wide variability in depths of individual fish through time led to an average depth reached by fish that was between the shallow and deep zones. Based on daily increments in the otoliths, however, this variability was not related significantly to the time since entry of fish into Port Phillip Bay. In the deepest depth zone, 81% of fish remained within 1 m of the surface and their horizontal displacement was significantly related to the direction and speed of currents. Secondary dispersal of post-larval fish in the water column may be facilitated by the behaviour and vertical movements of fish, but only if fish reach deeper water, where their displacement (direction and distance) closely resembles local hydrodynamic regimes. In shallow water, fish behaviour and vertical migration actually reduce the potential for secondary dispersal.     

Homing behaviour of stream-dwelling brook charr following experimental displacement

Precise homing of 24 brook charr Salvelinus fontinalis (157–215 mm L_F), displaced from their site of capture in a natural stream, was rare both in fish displaced upstream and those displaced downstream. Most fish settled in preferred habitats (pool or glide) away from their home site and showed restricted movement up to the pre-spawning period. Partial return responses, however, were stronger in fish displaced downstream than in fish displaced upstream. Most fish displaced downstream showed directional upstream movement soon after displacement, whereas fish displaced upstream did not show directional upstream movement, suggesting that it is primarily olfactory cues from upstream locations, and not positive rheotaxis, which mediate the return response. Homing success was unrelated to distance of displacement, body size, or population density in the home site, but daily mobility increased with body size for fish displaced downstream. There was no evidence of switching between specific areas between night and day, nor of differences between daytime and night-time mobility, suggesting that diel movement cycles did not bias the evaluation of homing success. Site attachment in stream-dwelling brook charr may be characterized as persistent (undisturbed fish remain stationary over long periods), but readily disrupted by involuntary displacement.     

Social status and growth rates as determinants of site attachment in juvenile Atlantic salmon

This paper describes an experimental study of the effects of food supply, growth rates and social interactions on homing by juvenile Atlantic salmon Salmo salar in response to displacement. Groups of five fish were housed in a section of an artificial stream and given either rations allowing maximum growth (the rich condition) or 0·1 of this amount (the poor condition); daily specific growth rates were significantly higher in the rich condition. After a 6-day settlement period, the fish were captured, displaced downstream and their movements recorded over the next 3 h. Prior to displacement, the fish showed a high degree of site fidelity and high levels of aggression. Dominant fish and those with stronger site attachment grew faster prior to displacement, these effects being independent. Following displacement, 24% of all fish returned to their previously favoured site and stayed there, 23% returned home initially, but subsequently moved on, 5% settled in a new site and 49% failed to move. The distribution of responses was identical for the rich and poor conditions, but fish that homed were dominant and had grown faster during the pre-displacement period.     

Effects of a predatory fish (Galaxias olidus) on the structure of intermittent stream pool communities in southeast Australia

Abstract The impact of predation over a 3 week period by the small (mean length 53 mm) salmoniform fish, Galaxias olidus, on the invertebrate communities in the still summer pools of an intermittent stream in southeastern Australia was tested using enclosures that incorporated both deep and shallow habitat areas. Twenty G. olidus, a key generalist predator in the system studied, were enclosed for 3 weeks in 1.5 × 1.7 m enclosures. Galaxias olidus was found to reduce significantly the distribution and abundance of air-breathing nektonic species. In contrast, the abundance of non-air-breathing nektonic species increased in the presence of fish in the deep areas of the enclosures. There was no significant impact offish predation on species richness, total abundance, epibenthic or interstitial species. The most likely reason for the general lack of response to the presence of fish by epibenthic and interstitial species is the availability of abundant spatial refugia from predation within the complex substrate of the stream. In contrast, air-breathing nektonic species are vulnerable to predation by fish due to the lack of refuges in the open water. Increases in the abundance of non-air-breathing nektonic species in the presence of fish may be related to reductions in the abundance of predatory dytiscid beetles. Significant differences between deep and shallow habitats were observed in total abundance and species richness, and in the abundances of air-breathing nektonic and epibenthic species, suggesting that physicochemical factors play a key role in determining invertebrate distribution within stream pools.     

Short-term effects of highway construction on the limnology of a small stream in southern Ontario*

SUMMARY. A limnological investigation was carried out to document the effects of constructing a modern highway across a small stream in southern Ontario. During construction, suspended solids increased to as high as 1390 mg/l but later returned to pre-construction levels of <5 mg/l. Similarly, sediment deposition increased ten-fold to 0. 61 g dry wt/cm². day directly below the construction site during stream rechannelization after completion of the culvert. Decreased proportion of organic matter in sediments indicated that they came from the construction site. Sediments were readily removed by spates and apparently settled out in downstream ponds. There was no change in water chemistry. Standing crop of fish was reduced from 24 to 10 kg/ha immediately below the site. This decrease did not occur further downstream and fish populations at the affected site returned to original levels after construction. No change in numbers of riffle macroinvertebrates was observed during or after construction. However, there was a noticeable shift in species composition. Invertebrates present during construction activities may have remained in sheltered areas avoiding sedimentation effects. Evidence from invertebrate sampling in denuded areas around the site strongly suggests that organisms which may have been removed during construction were replaced quickly by drift.     

The distribution of salmonids in upland streams in relation to depth and gradient

The distribution of various age classes of salmon and trout was assessed in upland streams by electrofishing. Water depths and site gradients were measured and correlated to fish densities. The fry of both species were significantly more abundant in shallow water; up to 75·3% of salmon fry and 72·2% of trout fry were captured in sites of mean depth < 20 cm. Older trout were found mainly in the deeper areas, with a maximum of 7·4% captured in sites < 20 cm mean depth. Yearling fish were found in all the depth-ranges sampled, but with a tendency for higher numbers in mid-range depths. There were similar correlations in the abundance of each age class with the actual areas of shallow, mid-range and deep water habitat available within sites. Correlations of fish density with gradient indicated that trout were limited in their distribution to areas of lower flow, whereas salmon were not. Since depth and gradient were significantly negatively correlated, there was an apparent preference of trout for slightly deeper habitats than the equivalent year classes of salmon. The observed habitat segregation is discussed in terms of competition and selection.     

A sedentary fish on the move: effects of displacement on long-snouted seahorse (Hippocampus guttulatus Cuvier) movement and habitat use

To understand how a sedentary fish responds to displacement and identify key habitat characteristics for that fish, we translocated long-snouted seahorses (Hippocampus guttulatus) within their natural habitat but far beyond their home range. After displacement, we located these small fish using acoustic technology and collected environmental data where we captured, released, and relocated them. Displaced fish (n?=?9) moved much greater distances (max: 150?m; mean daily: 6.4?C48.2?m) than expected from known home range movements and there was no evidence of homing. Seahorses varied in the specific environments they moved towards but tended to move towards environments more similar in both depth and water current speed to their original capture locations than their release locations.     

The impact of bird herbivory on macrophytes and the resilience of the clear-water state in shallow lakes: a model study

Land-use change associated with human development can alter aquatic habitat and imperil aquatic species. Fish are challenged when urban streams are altered, for example for stormwater conveyance, but little is known about how such activities influence the space use of individual fish. Electronic tagging and experimental displacement of fish can be used to explore site fidelity and homing behaviour of fish and can therefore be useful for testing hypotheses about space use and habitat selection. In this study, we used experimental displacement to determine how longnose dace (LND, Rhinichthys cataractae) utilize reaches within a watershed that have varying degrees of degradation. LND were tagged using passive integrated transponders (PIT tags), transported upstream, and released either into the natural stream reach, impaired stormwater drain reach, or at their confluence. Fixed PIT antennas were used to monitor movement of the PIT-tagged fish among the three reaches for a period of 3 weeks. LND exhibited dramatic and rapid selection against the stormwater drain. No LND moved into the drain and 97% of fish transported to the drain left within 24 h. LND were actively avoiding the stormwater drain, emphasizing the need for enhancement work to improve the biological connectivity of the system.     

Within-population heterogeneity of habitat use by lake trout Salvelinus namaycush

To learn more about the degree of individual variation in resource use by lake trout Salvelinus namaycush , ultrasonic telemetry was used to study their habitat use in a lake without pelagic schooling fish prey. Individuals spent most of their time within the metalimnion in favourable water temperatures. They also made frequent excursions, however, into lake temperatures exceeding their optimum for physiological performance at all temporal scales considered. Their frequent use of nearshore habitats suggested that feeding in littoral areas may be common. Habitat use was highly variable among individuals, but spatial habitat use by individuals showed remarkable consistency between years. In particular, some lake trout exhibited high site fidelity to shallow, nearshore areas, whereas others used deep areas extensively. This level of between-individual variation indicated niche partitioning by depth and the possibility of alternative foraging strategies.     

A preliminary examination of depth associated spatial variation in fish assemblages on small artificial reefs

Small concrete artificial reef modules (hemisphere-shaped, approximately 1.3 m diameter, 1 m high) were placed at two sites, eight modules per site. The sites were in 7 m and 21 m of water, 1.6 km apart, off the coast of southeast Florida, USA. The reefs were censused monthly for fish over a 19-month period. Species, number of fish, and estimated total length of each individual were recorded by divers using SCUBA. After the monthly census, all fish were removed from the reefs with a piscicide. A total of 88 species were recorded in the study, with significantly greater diversity on the deep reefs (monthly mean of 7.6 versus 3.0 shallow, P < 0.001, anova ). There were also significantly more biomass (calculated from length), and more large fish (>5 cm) on the reefs at 21 m than at 7 m (P < 0.001). There were more small fish at the shallow site (P < 0.05). Although it is not clear what variable(s) associated with the two depths is responsible for the differences, these results highlight the potential differences in artificial reef and ambient environment interactions within a localized area.     

Hydrologic Pathways during Snowmelt in First-order Stream Basins at the Turkey Lakes Watershed

The chemical composition of stream and soil water collected from two first-order stream basins in the Turkey Lakes Watershed (TLW) during the spring melt periods of 1992–1996 was examined to determine the flowpaths of snowmelt to the stream channel. Soil water was intensively sampled from within the soil organic layers as well as above (shallow soil water) and within (deep soil water) a compact basal till. Stream SiO₂ concentrations of the high-elevation basin 47 were the same as the levels found in shallow soil water, and forest-floor percolate SiO₂ concentrations were elevated to these levels during intense melting periods. The SiO₂ concentrations from the stream and the shallow and deep soil water were similar at the low-elevation basin 31. With the exception of deep soil water, water collected from the soil and stream at basin 47 had higher H⁺ and Al and lower base cation concentrations than basin 31. Stream Al concentrations were significantly correlated with forest-floor percolate Al concentrations at the high-elevation basin, whereas stream Al concentrations were correlated with mineral soil water Al concentrations at the low-elevation site. There were significant positive correlations between stream and shallow soil water H⁺ at both basins. Shallow soil water pathways, therefore, were an important contributor to streamflow, and influenced stream chemical response during the spring snowmelt at TLW. Received 5 October 1999; accepted 16 August 2000.     

Aspects of ecology and conservation of sympatric,prosobranch snails in a large river

We examined factors influencing distribution of prosobranch snails with field surveys, lab substrate-choice and movement experiments, and outdoor artificial stream experiments manipulating current, substrate and temperature. Restriction of Lithasia obovata to shallow, isolated rock patches and the more continuous distribution of Pleurocera canaliculatum on diverse substrates (shallow to deep water) seem related to food availability, predator vulnerability, wave resistance and dispersal abilities. Shallow, rocky habitats favored by the relatively fragile-shelled Lithasia maximize the more nutritious, epilithic microalgal food while minimizing wave displacement (firmer substrate) and exposure to molluscivorous fish inhabiting deeper waters. The broader distribution of the heavily-armored Pleurocera is attributed mostly to greater dispersal abilities (e.g. via deep water, gravel pathways), prevalence in more substrate types, and resistance to fish predators, but their relatively small foot may account for low densities in very shallow, wave-swept habitats. Lithasia is vulnerable because of its restriction to isolated, shallow rocky habitats, inability to reach deeper gravel dispersal pathways, and greater susceptibility to cold temperatures (slowing colonization of other habitats). Impoundments increase this threat by blocking inter-reach dispersal and hindering intra-reach dispersal through siltation. Pleurocera's survival is threatened by biofouling from zebra mussels which are more prevalent in depths inhabited by this species.     

Influences of low-head dams on the fish assemblages in the headwater streams of the Qingyi watershed, China

The influences of low-head dams on the fish assemblages were examined in this study, using fish data collected in six treatment and five reference sites at three low-head dams in the headwater streams of the Qingyi watershed, China. Comparing with those in the reference sites, local habitat variables were significantly altered by low-head dams in the treatment sites, involving wider channel (only in the impoundment area), deeper water and slower flow. Fish species richness varied significantly across seasons, not across site categories, suggesting that these low-head dams did not alter species richness. However, significant decreases in fish abundance and density were observed in the impoundment areas immediately upstream of dams, but not in the plunge areas downstream. Fish assemblage structures kept relative stability across seasons, and their significant difference between-site was only observed between the impoundment areas and the sites far from dams upstream. This variation in assemblage structures was due to the differing relative abundance of some co-occurring species; more lentic but less lotic fish was observed in the impoundment areas. The spatial and temporal patterns of fish assemblages were correlated with local habitat in this study area. Wetted width had negative correlation with fish species richness, abundance and density, respectively. Water temperature also positively affected species richness. In addition, wetted width, water depth, current velocity and substrate were the important habitat variables influencing assemblage structures. Our results suggested that, by modifying local habitat characteristics, low-head dams altered fish abundance and density in the impoundment areas immediately upstream of dam, not in the plunge areas immediately downstream, and thereby influenced fish assemblage structures in these stream segments.     

Differential behavioural responses of mayflies from streams with and without fish to trout odour

1. In streams, mayflies (Order Ephemeroptera) are at risk from fish feeding visually in the water column. The effect of fish odour on the behaviour of Baetis bicaudatus from a fishless stream and a trout stream was investigated in four large oval tanks supplied with water from the fishless stream.
2. For each mayfly population, mayfly positioning on the substratum and movement in the water column (drift) were measured during the day and night, over 3 days. Brook trout ( Salvelinus fontinalis ) odour was added to two tanks to test the effect of a threat from fish.
3. Throughout the experiment more mayflies from the trout stream were observed on the substratum surface and in the water column during the night than the day, but the magnitude of night drift was less in tanks with fish odour.
4. Baetis from the fishless stream also displayed a nocturnal periodicity in drift and positioning, but their night-time drift was not affected by the presence of fish odour. On the first day of the experiment, however, more mayflies were observed on the substratum surface and drifting in tanks without fish odour during the day.
5. Sensitivity to fish odour may enable mayflies to alter their behaviour according to the risk of predation from fish.     

Differential behavioural responses of mayflies from streams with and without fish to trout odour

1. In streams, mayflies (Order Ephemeroptera) are at risk from fish feeding visually in the water column. The effect of fish odour on the behaviour of Baetis bicaudatus from a fishless stream and a trout stream was investigated in four large oval tanks supplied with water from the fishless stream.
2. For each mayfly population, mayfly positioning on the substratum and movement in the water column (drift) were measured during the day and night, over 3 days. Brook trout ( Salvelinus fontinalis ) odour was added to two tanks to test the effect of a threat from fish.
3. Throughout the experiment more mayflies from the trout stream were observed on the substratum surface and in the water column during the night than the day, but the magnitude of night drift was less in tanks with fish odour.
4. Baetis from the fishless stream also displayed a nocturnal periodicity in drift and positioning, but their night-time drift was not affected by the presence of fish odour. On the first day of the experiment, however, more mayflies were observed on the substratum surface and drifting in tanks without fish odour during the day.
5. Sensitivity to fish odour may enable mayflies to alter their behaviour according to the risk of predation from fish.     

Short-term cues used by foraging trout in a California stream

Stream salmonids choose foraging locations to maximize the energy benefit of foraging within the constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use a search process to monitor changing habitat conditions as a means of locating potentially-better foraging locations. I explored the cues used by the cutthroat trout, Oncorhynchus clarki clarki, when searching for food at the pool scale by artificially increasing prey availability at different locations by using special feeders and by manipulating pool velocities. Behavior of individually marked fish was monitored from stream bank platforms under unmanipulated control conditions and under seven experimental sets of conditions involving different combinations of feeder location and velocity manipulation. Under natural conditions fish elected to forage in the deepest (>50 cm), fastest (0.10–0.25 m s⁻¹) locations and within 1 m of structure cover, but would readily move to shallower (<30 cm) water away from cover if velocities were manipulated to be highest there. Although fish did not locate feeders unless they were placed in high-velocity areas, when high velocity was provided fish would move into very shallow water (<20 cm) if prey were delivered there. Responses of individual trout to manipulations indicated that water velocity was the main physical cue used by fish to decide where to forage, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicated fish were not “perfect searchers” that could quickly locate new food resources over short time scales, even when the new resources were within a few meters of the fish’s normal foraging location. When given the correct cues, however, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the ten-day observation period was common, consistent with the idea that trout used movement as a means of exploring and learning about habitat conditions at the reach scale.     

Spatial Heterogeneity of Denitrification in Semi-Arid Floodplains

Riparian ecosystems are recognized as sinks for inorganic nitrogen (N). Denitrification, a heterotrophic microbial process, often accounts for a significant fraction of the N removed. Characteristics of both riparian soils and hydrologic vectors may constrain the locations where denitrification can occur within riparian ecosystems by influencing the distribution of substrates, water, and suitable redox conditions. We employed spatially explicit methods to quantify heterogeneity of soil characteristics and potential rate of denitrification in semi-arid riparian ecosystems. These results allow us to evaluate the relative contributions of hydrologic vectors (water courses that convey materials) and soil resources (materials required by biota) to spatial heterogeneity of denitrification. During dry and monsoon seasons we contrasted a mesic site, characterized by shallow groundwater and annual inundation by floods, with a xeric site that is inundated less often and has a deeper water table. Potential denitrification was detected throughout the mesic floodplain and the average rate of denitrification was greater at the mesic site than at the xeric site, indicating the influence of water availability on denitrification. At the xeric reach, sharp declines in pools of soil resources and rate of denitrification occurred away from the stream, demonstrating the importance of the stream in determining spatial patterns. Using geographically weighted regression analysis, we determined that soil organic matter and soil nitrate were significant predictors of denitrification at the xeric site, but that factors influencing denitrification varied spatially. Spatial heterogeneity of carbon (C) and N substrates in soils likely influenced spatial patterns of denitrification, but distribution of C and N substrates was ultimately organized by hydrologic vectors. Droughts will increase the abundance of reaches with hydrogeomorphic templates similar to the xeric reach studied here. Consequences of such a transition may include a reduced rate of denitrification and patchy distribution of denitrification in floodplain soils, which will decrease the contribution of riparian ecosystems to N removal. TKH designed and completed the study and wrote the paper; EAW contributed methods and edited the paper; NBG designed the study and edited the paper.     

Spatial Dispersion and Nonmigratory Spawning in the Bluehead Wrasse (Thalassoma bifasciatum)

Earlier studies of the behavior of the bluehead wrasse Thalassoma bifasciatum have shown it to be a migratory spawner with large terminal-phase males defending temporary spawning territories. We describe a variant social structure where fish occupy permanent home ranges, spawn within or near these home ranges, and are not territorial. Movements of identified terminal-phase and intermediate males and females were mapped in the backreef areas of three coral reefs in Puerto Rico. Locations of spawns were then compared with these home-range maps. All fish used most or all of their morning home range during the afternoon spawning period. Both terminal-phase males and females spawned within or near their home ranges. Males were relatively tolerant of other males during the afternoon spawning period: only one in five encounters between large males resulted in aggressive chases. The location of chases bore no relation to spawning sites, areas of morning foraging, or to home-range borders. A Monte Carlo computer simulation was used to measure home-range dispersion of terminal-phase and intermediate males at our main site. If males were territorial, we should expect their home ranges to be significantly overdispersed within the cumulative area they occupied. During the first year male home ranges were overdispersed but during the second year they were randomly dispersed, even though the same number of terminal-phase males occupied the same study site both years. Differences in social behavior between backreef areas and other areas reported in the literature, and found in other portions of our reefs, may be related to differences in feeding ecology between populations and between different portions of the same reef.