Gobiomorphus Breviceps Research Articles

Impacts of river flow and thermal regimes on fish growth dynamics during early life history

Growth during early life stages is critical to fish population persistence. Few studies explore how river discharge and thermal regimes impact growth dynamics of young-of-the-year (YOY) fishes. We used otolith biochronologies and generalised additive mixed models to partition the effects of age, individual, river, temperature, discharge, and variance in discharge on daily growth rates of YOY fishes with an opportunistic life-history strategy: Galaxias vulgaris and Gobiomorphus breviceps. Growth of both species increased with temperature. Galaxias growth decreased with discharge, while Gobiomorphus growth increased with lowered discharge but only to a point (ca. 1/3 of median discharge), after which growth plateaued. Floods had a strong negative effect on YOY growth. Our study systems and species had different characteristics to those that formed the basis of the Riverscape Recruitment Synthesis Model (RRSM) and may explain why our results were only partially consistent with flow predictions for opportunistic species of the RRSM. Further, Galaxias growth was a negative function of high-frequency, daily variation in flow, and we present testable hypotheses to explain this result.

The potential role of biotic interactions in stream restoration

Biotic interactions perform an important role in structuring freshwater communities; however, these are rarely considered during stream restoration. Degraded stream communities are often dominated by organisms with shell or case protections, such as snails, which are less vulnerable to predation than desired organisms, such as mayflies. Unprotected organisms may be preferentially eaten, limiting biotic restoration success after the physical restoration of degraded stream communities. We investigated whether restoration success depended on differences in vulnerability to predation by testing whether consumption‐based interaction strengths (IS) between vulnerable colonizing Deleatidium spp. mayflies and upland bullies (Gobiomorphus breviceps) changed with different densities of protected Potamopyrgus antipodarum snails and whether snails affected Deleatidium behavior. Deleatidium and fish IS were stronger with fewer Deleatidium present, a destabilizing effect on vulnerable prey populations, and less total prey biomass was consumed with higher densities of protected relative to vulnerable prey, potentially having long‐term negative effects on predators. Furthermore, mayflies appeared less on surfaces with high snail densities, potentially due to altered resource use. The combination of these biotic interactions could form a barrier to successful biotic restoration by preventing colonization of desired vulnerable prey, through priority effects within the established community. Therefore, considering biotic interactions in stream restoration is important to overcome biotic restoration barriers.

Individual diet specialization drives population trophic niche responses to environmental change in a predator fish population

Isotopic niche indices are increasingly used for assessing food web responses to ecological stress. Understanding the trophic mechanisms that result in ‘smaller’ or ‘larger’ isotopic niche space limits the utility of isotopic niche-based metrics. In this study, individual-level diet patterns were quantified for a meso-predatory fish (Gobiomorphus breviceps) in two contrasting lakes, one with stable macrophyte beds and the other a filamentous green algae dominated system. Metabarcoding of gut contents and stable isotope data from 48 fish were compared to abundance and isotopic compositions of macroinvertebrate prey communities. We examined whether: i) niche contraction occurs in response to reduced basal resource breadth and, ii) trophic niche contraction is caused by individual fish transitioning from diet-specialization to diet-generalism. Isotope niche contraction within the meso-predatory fish diet reflected altered macroinvertebrate food web structure within the low-macrophyte lake and additional population-level diet patterns. Niche contraction was associated with increased diet generalism; fish had more taxa (sequenced taxa) in their diet resulting in population-level diet homogenization. Isotopic and diet taxa composition dissimilarity were positively related indicating the isotopic niche reflects diet diversity between individuals. Resource partitioning analysis within populations demonstrated that the meso-predator population diet composition exhibited a non-random distribution of prey. Diet-specialization between individuals, or lack of, determined population trophic structure suggesting that food web structure was driven by diet compartmentalization. These findings illuminate food webs respond to disturbances such as macrophyte loss and suggest observed attributes of food web structure may be scale-independent emergent properties occurred-occurring at lower levels of trophic organization.

Cross-ecosystem bottlenecks alter reciprocal subsidies within meta-ecosystems.

Reciprocal subsidies link ecosystems into meta-ecosystems, but energy transfer to organisms that do not cross boundaries may create sinks, reducing reciprocal subsidy transfer. We investigated how the type of subsidy and top predator presence influenced reciprocal flows of energy, by manipulating the addition of terrestrial leaf and terrestrial insect subsidies to experimental freshwater pond mesocosms with and without predatory fish. Over 18 months, fortnightly addition of subsidies (terrestrial beetle larvae) to top-predators was crossed with monthly addition of subsidies (willow leaves) to primary consumers in mesocosms with and without top predators (upland bullies) in a 2 × 2 × 2 factorial design in four replicate blocks. Terrestrial insect subsidies increased reciprocal flows, measured as the emergence of aquatic insects out of mesocosms, but leaf subsidies dampened those effects. However, the presence of fish and snails, consumers with no terrestrial life stage, usurped and retained the energy within in the aquatic ecosystem, creating a cross-ecosystem bottleneck to energy flow. Thus, changes in species composition of donor or recipient food webs within a meta-ecosystems can alter reciprocal subsidies through cross-ecosystem bottlenecks.

Phylogeography of the Cran’s bullyGobiomorphus basalis(Gobiiformes: Eleotridae) and an analysis of species boundaries within the New Zealand radiation ofGobiomorphus

AbstractNew Zealand has a complex recent history of climatic and tectonic change that has left variable signatures in the geographic distribution and genetic structure of the region’s flora and fauna. To identify concordant patterns, a broad range of taxa must be examined and compared. In New Zealand’s North Island, a consensus is forming as to the dominant biogeographic barriers in the region although obligate freshwater taxa have not been considered in this framework. We use single-nucleotide polymorphisms to investigate phylogeography in the widespread obligate freshwater fish Gobiomorphus basalis on the North Island. Phylogeographic patterns within G. basalis reveal biogeographic disjunctions that are in some ways consistent and in other ways at odds with established patterns, providing insight into the processes that have shaped the islands’ biogeography. We also use phylogeography to delineate species boundaries within the entire New Zealand radiation of Gobiomorphus and find that it contains several morphologically cryptic species. We resolve two clades within G. basalis that correspond to areas north and south of the Taupo Volcanic Zone. We confirm the distinctiveness of Gobiomorphus alpinus relative to Gobiomorphus cotidianus, as well as the presence of two lineages within Gobiomorphus breviceps that were previously identified based on mitochondrial data.

Biogeography of parasitism in freshwater fish: spatial patterns in hot spots of infection

Contrary to species occurrence, little is known about the determinants of spatial patterns of intraspecific variation in abundance, particularly for parasitic organisms. In this study, we provide a multi‐faceted overview of spatial patterns in parasite abundance and examine several potential underlying processes. We first tested for a latitudinal gradient in local abundance of the regionally most common parasite species and whether these species achieve higher abundances at the same localities (shared hot spots of infection). Secondly, we tested whether intraspecific similarity in local abundance between sites follows a spatial distance decay pattern or is better explained by variation in extrinsic biotic and abiotic factors between localities related to local parasite transmission success. We examined the infection landscape of a model fish host system (common and upland bullies, genusGobiomorphus: Eleotridae) across its entire distributional range. We applied general linear models to test the effect of latitude on each species local abundance independently, including the abundance of each co‐infecting species as another predictor. We computed multiple regressions on distance matrices among localities based on abundance of each of the four most common trematode species, as well as for geographic distance, biotic and abiotic distinctness of the localities. Our results showed that the most widely distributed parasites of bullies also achieve the highest mean local abundances, following the abundance – occupancy relationship. Variation in local abundance of any focal parasite species was independent of latitude, the abundance of co‐occurring species and spatial distance or disparity in biotic attributes between localities. For only one parasite species, similarity of abundance between sites covaried with the extent of abiotic differences between sites. The lack of association between hot spots of infection for co‐occurring species reinforces the geographic mosaic scenario in which hosts and parasites coevolve by suggesting non‐deterministic, species‐specific variation in parasite abundance across space.

Responses of stream fish populations to farming intensity and water abstraction in an agricultural catchment

Summary Agriculture affects streams worldwide, and multiple stressors are usually at work. The effects of farming intensification in the catchment are likely to interact with flow reduction due to abstraction for irrigation, but their combined effects on fish communities are unknown. In a low‐rainfall, agricultural catchment, we sampled instream physicochemistry and fish populations (non‐native brown trout, Salmo trutta, and native upland bullies, Gobiomorphus breviceps, dominated the fish communities) at 36 stream sites chosen to cover wide gradients of % Farming Intensity (%FI) (% catchment in ‘high‐producing exotic grassland’) and % Water Abstraction (%WA) (estimated from a published hydrological model). These landscape variables were not well correlated, allowing us to unravel their individual and combined effects. Presence of trout was best described by an additive multiple‐stressor model consisting of a unimodal response to %FI and a negative response to %WA. Trout density only showed a negative response to %FI. Upland bullies were unrelated to either landscape variable. When populations were modelled using instream variables, trout presence was negatively related to fine sediment depth, while density was negatively related to both sediment depth and total nitrogen (themselves closely related to %FI). Upland bully presence and density showed unimodal responses to just total nitrogen. Ammonium concentration was the only measured instream variable related to %WA. The final models for instream stressors explained more of the variation in fish density, whereas the final models for landscape stressors explained more of the variation in fish presence. Overall, farming intensity showed stronger negative relationships with fish populations than water abstraction, and fish were absent from stream reaches whose upstream catchments contained more than 40% high‐producing exotic grassland. Resource managers considering intensifying water abstraction or agriculture in low‐rainfall river catchments should be aware of the interplay between these two agricultural stressors.

Individual and combined effects of fish predation and bed disturbance on stream benthic communities: a streamside channel experiment

Summary1. The composition and spatiotemporal dynamics of biological communities are influenced by biotic processes, such as predation and competition, but also by physical disturbances, such as floods in running waters. However, the interplay of disturbance with predation is still poorly understood, especially in frequently disturbed streams. Further, different predator species can affect prey communities in different ways depending on their feeding mode and efficiency.2. We investigated the individual and combined effects of flood‐induced bed disturbance and fish predation on the benthos for 4 weeks in 18 streamside channels fed by a flood‐prone New Zealand river. Bed movements caused by floods were simulated by tumbling the substratum in half the channels. Six channels each were stocked with introduced brown trout (Salmo trutta) or native upland bully (Gobiomorphus breviceps) or had fish excluded. We studied algal biomass and both invertebrate density and daytime activity on surface stones on several dates after the disturbance, invertebrate community composition in the substrata of the entire channels on day 28 and leaf decomposition rates over the 28‐day period.3. Disturbance affected algal biomass and density, richness and activity of surface stone invertebrates, and overall density and richness of channel invertebrates. Presence or absence of fish, by contrast, did not influence overall invertebrate standing stocks when subsurface substrata were included but did affect invertebrate densities on surface stones in 45% of all analysed cases and invertebrate activity on surface stones in all cases. Leaf decomposition rates were not influenced at all by the experimental manipulations.4. Native upland bullies featured more often than exotic brown trout in causing invertebrate density changes and equally often in causing changes to grazer behaviour. Overall, our results imply that fish predation can have strong effects on the benthic invertebrate community in frequently disturbed streams, especially via behavioural changes.

Diel variation in stream fish habitat suitability criteria: implications for instream flow assessment

Abstract Habitat suitability criteria that fail to incorporate temporal variability in habitat preferences of stream fish may mis‐represent critical habitat requirements and lead to setting of inappropriate flow targets when used in instream flow assessments. Developing suitability criteria from daytime observations alone relies on the assumption that habitat preferences are constant over the diel cycle. Few studies have tested these assumptions, particularly for small‐bodied, cryptic, benthic species. During summer in two gravel‐bed rivers, bluegill bullies (Gobiomorphus hubbsi), torrentfish (Cheimarrichthys fosteri) and upland bullies (Gobiomorphus breviceps) exhibited strong preferences with respect to water depth, velocity and substratum size. All three species underwent a diel shift in microhabitat preference for at least two of these variables. Microhabitat preferences were generally weaker when fish were active at night; bluegill bullies, upland bullies and especially torrentfish were observed over a broader range of depths, velocities and substratum sizes at night than during the day. Observations of fish in a stream simulator confirmed that bluegill bullies and torrentfish showed a preference for runs at dusk and return to riffles before dawn, but habitat preferences of upland bullies remained static across the diel cycle. Diel microhabitat shifts affected the assessment of flow requirements. Instream habitat analysis of the Waipara River using separate day and night suitability criteria predicted differing amounts of habitat available at a given flow, and the relationships between fish abundance, fish density and flow. The presence of diel microhabitat shifts in stream fishes suggests that instream habitat analyses will produce more effective and defensible flow recommendations when patterns of nocturnal microhabitat preferences are known and critical habitat bottlenecks can be identified. Copyright © 2011 John Wiley & Sons, Ltd.

Has the introduction of brown trout altered disease patterns in native New Zealand fish?

Summary1. It is well recognised that non‐indigenous species (NIS) can affect native communities via the ‘spillover’ of introduced parasites. However, two other potentially important processes, the ‘spillback’ of native parasites from a competent NIS host, where the latter acts as a reservoir leading to amplified infection in native hosts, and the ‘dilution’ of parasitism by a NIS host acting as a sink for native parasites, have either not been tested or largely overlooked.2. We surveyed the helminth parasite fauna of native New Zealand fish in Otago streams that varied in the abundance of introduced brown troutSalmo trutta, to look for evidence of spillback and/or dilution. Spillover is not an issue in this system, with trout introduced as parasite‐free eggs.3. Seven native parasite species were present across 12 sites; significant inverse relationships with an index of trout abundance (i.e. dilution) were documented for three species infecting the native upland bullyGobiomorphus breviceps, and one species infecting the native roundhead galaxiasGalaxias anomalus.4. An inverse relationship between bully energy status and infection intensity of one parasite species suggests that parasite dilution could have positive effects on bully populations. Our failure to detect similar relationships for the other parasites does not preclude the possibility that dilution is beneficial to native fish, since parasites may have subtle or unmeasured impacts.5. The parasite dilution patterns reported are compelling in that they occurred across several native host and parasite species; as such they have important implications for invasion ecology, providing an interesting contrast to the largely negative impacts reported for NIS. Mechanisms potentially responsible for the patterns observed are discussed.

Fish community responses to drying disturbances in an intermittent stream: a landscape perspective

Summary1. Refugia are critical to the persistence of individuals, populations and communities in disturbed environments, yet few studies have considered how the position of refugia within the landscape interacts with the behavioural responses of component species to determine the influence of disturbance events on mobile animals.2. An 18‐month quantitative electrofishing survey was undertaken on the Selwyn River, a stream that is intermittent in its middle reaches, to determine how the direction and distance to refugia affect the response of fish populations to drying, and how landscape context interacts with flow permanence to produce spatial patterns in communities.3. Overall, the propensity of fish to take refuge in perennial reaches during drying episodes, and the rate and extent of recolonization from these refugia upon rewetting, depended upon the direction and distance to refugia and the behaviour of component species.4. In the upper river, Canterbury galaxias (Galaxias vulgaris), upland bullies (Gobiomorphus breviceps) and brown trout (Salmo trutta) migrated upstream to permanent water as the stream dried from the bottom up, but frequent drying and slow recolonization by most species combined to produce a fish community in intermittent reaches that was quantitatively and qualitatively different to that in neighbouring perennial reaches.5. In the lower river, fish did not appear to migrate downstream to permanent water as the stream dried from the top down, but a lower frequency of drying episodes and faster recolonization by upland bullies and eels (Anguilla spp.) from downstream refugia allowed the fish community in intermittent reaches to converge with that in neighbouring perennial reaches during prolonged wetted periods.6. Longitudinal patterns of increasing fish density and species richness with flow permanence are interpreted as the product of species‐specific responses to drying events and the spatial position of refugia within the riverscape.

Geological Dates and Molecular Rates: Rapid Divergence of Rivers and Their Biotas

We highlight a novel molecular clock calibration system based on geologically dated river reversal and river capture events. Changes in drainage pattern may effect vicariant isolation of freshwater taxa, and thus provide a predictive framework for associated phylogeographic study. As a case in point, New Zealand's Pelorus and Kaituna rivers became geologically isolated from the larger Wairau River system 70 to 130 kyr BP. We conducted mitochondrial DNA phylogeographic analyses of two unrelated freshwater-limited fish taxa native to these river systems (Gobiomorphus breviceps, n = 63; Galaxias divergens, n = 95). Phylogenetic analysis of combined control region and cytochrome b sequences yielded reciprocally monophyletic clades of Pelorus-Kaituna and Wairau haplotypes for each species. Calibrated rates of molecular change based on this freshwater vicariant event are substantially faster than traditionally accepted rates for fishes but consistent with other recent inferences based on geologically young calibration points. A survey of freshwater phylogeographic literature reveals numerous examples in which the ages of recent evolutionary events may have been substantially overestimated through the use of "accepted" calibrations. We recommend that--wherever possible--biologists should start to reassess the conclusions of such studies by using more appropriate molecular calibrations derived from recent geological events.

Do toxic baits containing sodium fluroacetate (1080) affect fish and invertebrate communities when they fall into streams?

Large‐scale control of Australian possums throughout New Zealand uses toxic cereal baits containing 1080 (sodium fluoroacetate). These baits are often aerially applied over rough terrain where ground application is impractical. Many small streams flow in these areas, so 1080 baits can potentially fall into them. The ecological effect of 1080 leaching from baits was assessed on freshwater fish and invertebrate communities in four streams using a BACI experimental design. Four sites were selected in each stream: 10m and 100m below and 10m and 100m above where 1080 baits were placed. All sites were monitored 4 days and 1 day before and after baits were added, respectively. Separate experiments were conducted to assess impacts of 1080 on native fish and invertebrate communities. Baits were added to each stream to achieve bait densities 10 × greater than found after normal control operations. Three species of native fish, longfin eels (Anguilla dieffenbachii), koaro (Galaxias brevipinnis), and upland bullies (Gobiomorphus breviceps) were placed into separate cages at each site in each stream, and mortality monitored during the experiment. Analysis of water samples collected during the fish experiment showed that 1080 was detected only for 12h, and at low concentrations (c. 0.2 ìg litre‐1), despite the large number of baits placed in each stream. No fish died after addition of 1080 baits, suggesting that all three species were tolerant to dissolved 1080 at concentrations observed in this study. Invertebrate communities were quantified by sampling 10 replicate rocks at each site. Caddisflies (Helicopsyche, Pycnocentrodes, and Pycnocentria), orthoclad midges, and the mayfly Deleatidium dominated the community. 1080 had no detectable effects on the invertebrate community. These results suggest that 1080 leaching from submerged baits in small streams has no demonstrable biological impacts. Based on this finding, the need to maintain buffer zones around large waterways as some councils require is questioned.

Refuge‐use strategies of stream fishes in response to extreme low flows

Low flows were simulated in an artificial stream to determine the refuge‐use strategies of two benthic New Zealand fish species, Canterbury galaxias Galaxias vulgaris and upland bullies Gobiomorphus breviceps, and to investigate how refuge availability and the rate of flow recession affect the type and incidence of refuge use. When riffles dewatered, upland bullies tended to move to runs, whereas Canterbury galaxias showed a stronger propensity to burrow into the substratum. Both species showed a strong and consistent tendency to move upstream when emigrating from riffles. Burrowing was more frequent on coarse substrata and during rapid flow recessions. The incidence of surface stranding increased with the rate of flow recession, but only when interstitial refugia were unavailable, and was higher on gravel than on cobble substrata. The effect of rate of flow recession and substratum size on the probability of stranding depended upon the type of refuge‐use strategy adopted by the fishes. Rate of flow recession affected upland bullies more than Canterbury galaxias, whereas substratum size affected Canterbury galaxias more than upland bullies. These results suggest that the impact of disturbance is contingent upon species‐specific refuge‐use strategies, which result from interplay between refuge availability, the nature of the disturbance and species’ behaviours and morphologies. When component species adopt contrasting refuge‐use strategies, disturbance events may not consistently favour one species over another but rather inflict species‐specific mortality that varies both temporally and spatially.

RIVER CAPTURE, RANGE EXPANSION, AND CLADOGENESIS: THE GENETIC SIGNATURE OF FRESHWATER VICARIANCE

River capture is potentially a key geomorphological driver of range expansion and cladogenesis in freshwater-limited taxa. While previous studies of freshwater fish, in particular, have indicated strong relationships between historical river connections and phylogeographic pattern, their analyses have been restricted to single taxa and geological hypotheses were typically constructed a posteriori. Here we assess the broader significance of river capture among taxa by testing multiple species for the genetic signature of a recent river capture event in New Zealand. During the Quaternary an upper tributary of the Clarence River system was diverted into the headwaters of the Wairau River catchment. Mitochondrial DNA (control region and cytochrome b) sequencing of two native galaxiid fishes (Galaxias vulgaris and Galaxias divergens) supports headwater exchange: populations from the Clarence and Wairau Rivers are closely related sister-groups, whereas samples from the geographically intermediate Awatere River are genetically divergent. The upland bully Gobiomorphus breviceps (Eleotridae), in contrast, lacks a genetic signature of the capture event. We hypothesize that there is an increased likelihood of observing genetic signatures from river capture events when they facilitate range expansion, as is inferred for the two galaxiid taxa studied here. When river capture merely translocates genetic lineages among established populations, by contrast, we suggest that the genetic signature of capture is less likely to be retained, as might be inferred for G. breviceps. Rates of molecular evolution calibrated against this recent event were elevated relative to traditional estimates, consistent with the contribution of polymorphisms to branch lengths at shallow phylogenetic levels prior to fixation by purifying selection and drift.

RIVER CAPTURE, RANGE EXPANSION, AND CLADOGENESIS: THE GENETIC SIGNATURE OF FRESHWATER VICARIANCE

River capture is potentially a key geomorphological driver of range expansion and cladogenesis in freshwater-limited taxa. While previous studies of freshwater fish, in particular, have indicated strong relationships between historical river connections and phylogeographic pattern, their analyses have been restricted to single taxa and geological hypotheses were typically constructed a posteriori. Here we assess the broader significance of river capture among taxa by testing multiple species for the genetic signature of a recent river capture event in New Zealand. During the Quaternary an upper tributary of the Clarence River system was diverted into the headwaters of the Wairau River catchment. Mitochondrial DNA (control region and cytochrome b) sequencing of two native galaxiid fishes (Galaxias vulgaris and Galaxias divergens) supports headwater exchange: populations from the Clarence and Wairau Rivers are closely related sister-groups, whereas samples from the geographically intermediate Awatere River are genetically divergent. The upland bully Gobiomorphus breviceps (Eleotridae), in contrast, lacks a genetic signature of the capture event. We hypothesize that there is an increased likelihood of observing genetic signatures from river capture events when they facilitate range expansion, as is inferred for the two galaxiid taxa studied here. When river capture merely translocates genetic lineages among established populations, by contrast, we suggest that the genetic signature of capture is less likely to be retained, as might be inferred for G. breviceps. Rates of molecular evolution calibrated against this recent event were elevated relative to traditional estimates, consistent with the contribution of polymorphisms to branch lengths at shallow phylogenetic levels prior to fixation by purifying selection and drift.

Extensive genetic differentiation in Gobiomorphus breviceps from New Zealand

Partial mitochondrial DNA sequences for parts of the cytochrome b gene and control region were obtained for 89 upland bullies Gobiomorphus breviceps from 19 catchments in New Zealand. There were two highly distinctive mtDNA clades: a northern clade corresponding to the North Island, northern South Island and west coast South Island, and a south‐east clade, in the southern and eastern South Island. Within these major clades there were further distinct clades that correlated with geographic sub‐regions and catchments. The marked genetic differentiation has occurred in the absence of obvious morphological divergence. Based on cytochrome b sequence divergences and the molecular clock hypothesis, the northern and southeastern clades correspond with the uplift of the Southern Alps during the Pliocene, while populations in the North Island and northern South Island were estimated to have diverged during the Pleistocene. The widescale geographic divergences were similar to those observed in the galaxiids, Galaxias vulgaris and Galaxias divergens, but biogeographic management boundaries may not be the same, reflecting different evolutionary histories for non‐diadromous species occupying the same areas.

Effects of substrate and sedimentation on the abundance of upland bullies (Gobiomorphus breviceps)

Most indigenous New Zealand freshwater fish are benthic, and use substrate elements for shelter, cover, and nesting sites. Sedimentation can reduce the amount of interstitial space provided by the substrate, thus reducing the amount of available cover and potentially influencing fish abundance. To test the effects of sedimentation on fish density, upland bullies (Gobiomorphus breviceps Stokell) were introduced into experimental channels with different substrate treatments. After an acclimatisation period, they were allowed to emigrate. The maximum fish density and relative habitat suitability was then determined by the number of fish remaining after 6 days. Bully numbers declined by over 50% after 6 days in the trials with no cover, whereas with 20 cobbles providing cover for 40 bullies, numbers reduced by only 10%. When sufficient fine sediment was added to fill the spaces under the cobbles, bully numbers reduced by more than 60%, regardless of the number of cobbles. However, when cobbles were raised above the fine sediment, the number of bullies remaining in the channels was similar to the number remaining with cobbles and no sediment. The overall effect of high levels of sedimentation on bully density was similar to that of providing no cover, with the effect diminishing as the level of sedimentation decreased.

Female preference and male nuptial colouration in the freshwater fish Gobiomorphus breviceps: geographic variation among populations

Geographic variation in the strength or direction of sexual selection acting on different populations has been documented before but has not been the subject of intense study. We examine covariation among sexually selected traits and environmental factors for five stream populations of a small New Zealand freshwater fish, the upland bully, Gobiomorphus breviceps (Eleotridae). Levels of infection by a trematode parasite and some aspects of female behaviour and male colouration varied significantly among populations. Among individual fish we found that female mate choice behaviours and male colouration did not correlate similarly with other traits, or with parasite load, in the different populations studied. We also determined whether average population values correlated among the different stream populations. Whereas environmental factors such as water clarity and substrate colouration appeared to be unimportant, average parasite load correlated with some components of male colouration, and female preferences appeared to be correlated with male colouration. These results illustrate both the variability among populations that prevents results obtained from one population from being generalized to the entire species, and the plasticity of sexually selected traits in relation to local conditions.

A test for competition between two Gobiomorphus species (Teleostei: Eleotridae) in artificial channels

Abstract– The occurrence of competition between common (Gobiomorphus cotidianus) and upland (G. breviceps) bullies was tested in summer in artificial channels. Although these species have different life histories, they are morphologically and ecologically similar, and frequently cooccur in New Zealand's fresh waters. Mortality during the test period averaged 11.3±2.4% and 17.7±1.9% for the common bully in triplicate control and experimental channels, respectively; in contrast, upland bully mortality was 2.3±1.9% in the control and zero in the experimental channels. Common bullies showed no significant differences in mean length, weight, condition, and specific growth between the control and experimental tests. For upland bullies, all of the above parameters were significantly less in the controls than the experimentals, an outcome that was indicative of intraspecific competition. Moreover, there was a significant linear trend in the distribution of upland bullies, with their numbers being highest at the upstream end of the channels in both the control and experimental tests. Common bullies showed no such trend, although significantly higher proportions of fish were present towards the central positions in the experimental channels, suggesting there was some displacement of them by upland bullies. It is concluded that interspecific competition is not of major importance between populations of these two eleotrids in small streams, although the abundance and upstream penetration of common bullies may be affected to some extent by interaction with upland bullies.