The influence of habitat availability and adult density on non-diadromous galaxiid fry settlement in New Zealand

This study investigated factors controlling settlement and initial density of two species, Galaxias vulgaris and Galaxias paucispondylus (Galaxiidae), in New Zealand streams. Factors affecting maximum fry abundance were assessed at 10 sites (eight streams) differing in habitat characteristics and the presence or absence of introduced brown Salmo trutta and rainbow trout Oncorhynchus mykiss . Regression analysis indicated that conspecific adult numbers, extent of backwater habitat and brown and rainbow trout density were the most important factors determining initial fry density. Experimental additions of backwater habitat increased fry settlement tenfold compared with control reaches, demonstrating that the area available for fry settlement had a substantial effect on fry abundance. Results from this study indicate that fry settlement may be a critical limiting phase for some non-diadromous galaxiid populations. Increasing settlement area to enhance fry retention offers a practical conservation tool for fisheries managers that may aid conservation of threatened fishes like galaxiids.     

Recruitment variation in a stream galaxiid fish: multiple influences on fry dynamics in a heterogeneous environment

1. The relative importance of density‐dependent and density‐independent processes in explaining fluctuations in natural populations has been widely debated. In particular, the importance of larval supply and whether it may control the type of regulatory processes a population experiences has proved contentious. 2. Using surveys and field experiments conducted in streams in Canterbury, New Zealand, we investigated how variation in the survival of non‐migratory Galaxias vulgaris fry was affected by density‐dependent and density‐independent processes and how this variation influenced recruitment dynamics. 3. Fry populations with high settlement densities experienced a 70–80% reduction in population size from density‐related mortality during the first fourteen days after peak settlement but thereafter the influence of density‐dependent processes on fry was weak. The impact of environmental conditions on fry populations was dependent on fry size and the magnitude of the perturbation, such that flooding effects on fry survival were most severe when fry were small. 4. In streams not affected by flooding, the size and density of introduced trout (Salmo trutta and Oncorhynchus mykiss) were the most significant factors determining the abundance of eventual recruits. A field experiment manipulating brown trout access to fry populations revealed that trout as small as 110 mm may be capable of greatly reducing and possibly preventing galaxiid recruitment. 5. Overall, the results indicated density‐dependent population regulation was only possible at sites with high native fish densities because trout were likely to be suppressing the number of potential recruits at sites with low native fish numbers. Whilst density‐dependent processes had a strong effect on fry survival following the period of peak fry abundance, density‐independent processes associated with flow and predatory trout influences on fry survival largely determined recruitment variability among galaxiid populations. Focusing conservation efforts on improving habitat to increase fry retention and reducing the impacts of trout on galaxiids would ensure more native fish populations reached their potential abundance.     

Competition for space between introduced brown trout (Salmo trutta L.) and a native galaxiid (Galaxias vulgaris Stokell) in a New Zealand stream

Increasing circumstantial evidence indicates that the introduction of brown trout ( Salmo trutta L.) to New Zealand has caused a widespread decline in native fish populations but few of the underlying mechanisms have been investigated. The possibility of spatial competition was investigated by comparing the microhabitat used by native Galaxias vulgaris Stokell (Family Galaxiidae) that were sympatric and allopatric with brown trout. A range of microhabitat variables was measured from random locations where G. vulgaris were present in the Shag River during the day. G. vulgaris preferred coarse substrates, using them as resting places, but showed no other microhabitat preferences. This pattern of microhabitat use did not change in the presence of brown trout although galaxiid densities were considerably lower. Experiments in in situ stream channels confirmed that competition for space does not occur during the day even at high galaxiid densities. This situation changed dramatically at night, however, with G. vulgaris spending significantly more time in slower areas when trout were present. G. vulgaris feeds on drifting invertebrates, so brown trout could affect the galaxiids deleteriously by forcing them to occupy less profitable feeding positions. Interspecific competition for space, perhaps combined with competition for food and predation by trout, could explain declines in G. vulgaris populations.     

Are impacts of an exotic predator on a stream food web influenced by disturbance history?

Predatory species have been introduced to habitats spanning a wide range of environmental conditions. To better understand the consequences of predation in natural communities we need to examine how variations in abiotic factors modify the influence of predation. The effects of introduced predators may vary amongst habitats if natural disturbance affects the abundance and taxonomic composition of consumers and their resources, or the predator alters recolonisation after disturbance. We tested whether a bed-moving disturbance altered subsequent interactions involving native and introduced predatory fish, invertebrate grazers and algae in experimental channels within a New Zealand stream. Disturbance reduced the abundance of invertebrates by 84%, and induced mortality of Conoesucidae caddisflies. However, the relative abundance of taxa changed little immediately following the disturbance. Invertebrate communities recovered following disturbance in fishless channels and those with native galaxiids (Galaxias vulgaris), and were almost indistinguishable from undisturbed fishless controls after 2 weeks. Invertebrate abundance declined and algal abundance increased in channels with exotic brown trout (Salmo trutta) and their effect was strongest in previously disturbed channels. However, predators and disturbance only had interactive effects on grazer emigration rates. Trout affected grazers through direct consumption (e.g. Conoesucidae caddisflies), and induced higher emigration rates of grazers from channels via drift (e.g. the mayfly Deleatidium). The effects of predatory trout and galaxiids combined differed in disturbed and stable channels. The observed combined effects of predatory trout and galaxiids on invertebrate grazers were lower than expected in stable channels partly due to low emigration rates of Conoesucidae, whereas emigration of grazers was higher than expected in the disturbed channels. The biomass of algae was higher than expected in disturbed channels with both predators. Collectively, our results indicate that predator substitutability and the non-lethal effects of introduced predators varied depending on disturbance history, but their effects on the biomass of grazers and algae did not.     

The impact of an introduced predator on a threatened galaxiid fish is reduced by the availability of complex habitats

1. The availability of complex habitats such as macrophytes may be vital in determining the outcomes of interactions between introduced predators and native prey. Introduced brown trout (Salmo trutta) have impacted numerous small native freshwater fishes in the southern hemisphere, but the potential role of complex habitats in determining the direct outcomes of brown trout – native fish interactions has not been experimentally evaluated. 2. An in‐lake enclosure experiment was used to evaluate the importance of structurally complex habitats in affecting the direct impacts of brown trout on a threatened galaxiid fish. Five Galaxias auratus and a single brown trout were added to enclosures containing one of three different habitat types (artificial macrophytes, rocks and bare silt substrate). The experiment also had control enclosures without brown trout. Habitat‐dependence of predation risk was assessed by analysis of G. auratus losses to predation, and stomach contents of remaining fish were analysed to determine if brown trout directly affect the feeding of G. auratus and whether this is also habitat‐dependent. 3. Predation risk of G. auratus differed significantly between habitat types, with the highest mortality in enclosures with only bare silt substrate and the lowest in enclosures containing artificial macrophytes. This result highlights the importance of availability of complex habitats for trout – native fish interactions and suggests that increasing habitat degradation and loss in fresh waters may exacerbate the direct impacts of introduced predators. 4. Stomach contents analyses were restricted to fish in enclosures with artificial macrophytes and rocks, as most fish were consumed in enclosures with brown trout and only bare silt substrate. These analyses suggest that brown trout do not directly affect the feeding of G. auratus in complex habitats, but it is still unknown whether its feeding is reduced if complex habitats are unavailable.     

Changes in the distribution of native fishes in response to introduced species and other anthropogenic effects

Aim Globally, one of the major threats to the integrity of native faunas is the loss of biodiversity that can result from the introduction of exotics. Here we document recent changes in the distribution of five common fish species that are linked to introductions in Chile. Location Chile from 28° S to 54° S. Methods We assess the extent of changes in distribution of galaxiid species by comparing their historical and current distributions based on the results of the most extensive survey of freshwater fishes in Chile to date, a range that encompasses the full latitudinal and elevational range of the Galaxiidae in Chile. We test for relationships of the distributions and abundances of native fishes with the incidence of introduced species. Results The latitudinal range of Galaxias maculatus has declined by 26%, and most of this reduction has occurred in the northern part of its range. Aplochiton taeniatus and Brachygalaxias bullocki have experienced reductions (8–17% loss) in total drainage area occupied, and they have disappeared from, or are now extremely difficult to find, in latitudes 36° to 41° S, coincidently with areas of urban growth and intense economic activities. The distribution of Galaxias platei has, instead, increased considerably. In northern basins, G. maculatus has apparently been replaced by an introduced poeciliid Gambusia sp. High‐elevation systems remain dominated by native Galaxias platei, whereas systems at intermediate elevations, especially rivers, are now dominated by introduced salmonids. Within drainages, native galaxiids remain abundant where exotic salmonid abundance is low. Main conclusions We suggest that negative interactions between introduced and native fish are responsible for some of the range reductions among Galaxiidae in Chile. The severity of the impacts varies with latitude and altitude and is probably related to temperature. The effects of Gambusia are restricted to warmer systems. Native fish also appear to have found temperature refugia from salmonids; impacts are low in the warmer northern and coastal systems, as well as in high‐altitude relatively cold systems. Native fish also appear less vulnerable to salmonids in lakes than in rivers. This study identifies watersheds critical for the conservation of biodiversity within the Galaxiidae.     

Facilitation and interference among three predators affect their consumption of a stream-dwelling mayfly

1. We experimentally tested if a multiplicative risk model accurately predicted the consumption of a common mayfly at risk of predation from three predator species in New Zealand streams. Deviations between model predictions and experimental observations were interpreted as indicators of ecologically important interactions between predators. 2. The predators included a drift‐feeding fish [brown trout (T), Salmo trutta], a benthivorous fish [galaxiid (G), koaro, Galaxias brevipennis] and a benthic predatory stonefly (S; Stenoperla sp.) with Deleatidium sp. mayflies as prey. Eight treatments with all predator species combinations and a predator‐free control were used. Experiments were performed in aquaria with cobbles as predator refuges for mayflies and we measured the proportion of prey consumed after 6 h for both day and night trials. 3. Trout consumed a higher proportion of prey than other predators. For the two predator treatments we found less than expected prey consumption in the galaxiid + trout treatment (G + T) for both day and night trials, whereas a higher than expected proportion of prey was consumed during night time in the stonefly + trout (S + T) treatment. 4. The results indicate interference (G + T) and facilitation (S + T) between predators depending on predator identity and time of day. Thus, to make accurate predictions of interspecific interactions, it is necessary to consider the ecology of individual species and how differences influence the direction and magnitude of interactions.     

Location of demographic sources affects the distributions of a vulnerable native fish in invaded river networks

1. Invasive predators negatively affect native prey to varying degrees across landscapes, and spatial configuration of invader‐free refugia may affect prey distributions across invaded river networks. In New Zealand, introduced trout (Salmo trutta and Oncorhynchus mykiss) create source‐sink dynamics in native Galaxias vulgaris populations, and their co‐occurrence with trout may be enhanced by immigration from trout‐free reaches. 2. We investigated how network configuration of trout‐free demographic sources affected the distribution of G. vulgaris across trout‐invaded riverscapes. Using quantitative biomass surveys and spatially extensive presence–absence surveys, the interaction between habitat variability and location relative to sources in limiting distributions of G. vulgaris in trout‐invaded reaches was assessed. 3. Galaxias vulgaris biomass at invaded sink sites decreased with increasing network distance to the nearest trout barrier. The maximum distance to barriers at which G. vulgaris occurred in the riverscape was limited, so that galaxiids were excluded from small and stable streams far from sources. 4. Large predatory trout (i.e. >150 mm fork length) occurred in high densities at stable sites all year round and were seasonally excluded from sites disturbed by flooding. Large streams probably provide increased refugia for galaxiids to avoid predation from trout, but narrowness and habitat stability may act synergistically to extirpate G. vulgaris from sites that are too far from sources to receive regular immigrants. 5. The interaction between immigration and habitat configuration in mediating effects of trout on G. vulgaris distributions indicates the spatial dependency of predator–prey interactions in river networks. 6. These results indicate creating new invader‐free source habitat should enhance co‐occurrence in nearby invader‐occupied reaches. Moreover, adding source habitat in sink streams far from existing sources, and ensuring barriers prevent future invasion, will also allow native fish dispersal between sources and sinks and will maximise the conservation gains from management across invaded riverscapes.     

Tropical and temperate freshwater amphidromy: a comparison between life history characteristics of Sicydiinae,ayu, sculpins and galaxiids

Amphidromy is a distinctive form of diadromy, but differences in the life histories of tropical and temperate amphidromous fishes suggest that there are two types of freshwater amphidromy. The life histories of Sicydiinae gobies, ayu (Plecoglossus altivelis), Japanese sculpins (Cottus) and galaxiids (Galaxiidae), suggest that the Sicydiinae are representatives of tropical freshwater amphidromy, whereas ayu, sculpins and galaxiids are representatives of temperate freshwater amphidromy. The Sicydiine larval stage may be required to occur in the ocean for all species, but ayu, sculpins and galaxiids have landlocked or fluvial forms with larvae that do not need to enter the ocean for larval feeding and growth. This suggests that Sicydiine larvae have a high oceanic dependency whereas ayu, sculpins and galaxiid larvae have a low oceanic dependency. Freshwater amphidromous fish in tropical and temperate zones appear to have developed two different strategies in the evolution of their life histories. It is likely that the evolutionary direction of the larval stage of tropical amphidromy is to remain in the sea and that of temperate amphidromy is towards having the ability to remain in freshwater if needed. Tropical and temperate amphidromy appear to be biologically informative categories and evaluations of this hypothesis will facilitate better understanding of the various forms of amphidromy in the future.     

Genetic variation and diadromy in some native New Zealand galaxiids (Teleostei: Galaxiidae)

We examined genetic variation at 13 isozyme loci in three diadromous and two non-diadromous species of galaxiid fish from streams in South and Stewart Islands of New Zealand. Analysis of allele frequency data revealed higher F_s7 and Nei's D values among conspecific populations of the non-diadromous species. We propose that the genetic structurings of these species are affected by their migratory behaviours. We suggest that the derived condition of non-diadromous behaviour may increase scope for speciation, but also the probability of extinction. This is exacerbated by population fragmentation over the last century.     

Distribution of native and introduced fish in the Seven Creeks River system,Victoria

The distribution offish within the Seven Creeks River system, a tributary of the Goulburn River in the Murray–Darling basin, was determined primarily by a survey carried out during the summer of 1975–76. Information on the past occurrence of fish in the system was obtained from historical records. Seventeen species offish, eleven native and six exotic, have been recorded from the system. Habitat characteristics, cohabiting species and food habits are presented for each species recorded during the survey, and factors affecting present distributions are discussed. Siltation appears to have had adverse effects on native Murray cod, Maccullochella peeli, and Macquarie perch, Macquaria australasica, both of which were once common in the lower reaches of the system. The presence of Macquarie perch and the rare native trout cod, Maccullochella macquariensis, in the upper reaches of the system is traced to fish released in 1921 and 1922. A nother native species, the western carp gudgeon, Hypseleotris klunzingeri, was first observed in the system only after it had been introduced into farm dams in the area in the mid 1960s. Relationships between native and introduced fish are complex. Although the food requirements of some native and introduced species overlap and some native fish have been found in the stomachs of introduced species, the only evidence of a substantial effect of an introduced species on a native species is the apparent fragmentation of the range of common mountain galaxiids, Galaxias olidus, by brown trout, Salmo trutta, whose numbers in the system were, until recently, augmented by continual releases of hatchery-reared fish.     

Effects of prey dispersal on predator–prey interactions in streams

1. We studied the effect of mesh size (6 and 3 mm) on interactions between brown trout ( Salmo trutta ) and benthic invertebrates in enclosures placed in a stream in southern Sweden. We also compared how different prey exchange rates affected interactions between trout and invertebrates.
2. Trout had strong impacts on some benthic taxa, and different mesh sizes produced different patterns. Trout affected the abundance of 10 of the 21 taxa examined, six in enclosures with 3 mm mesh and six in enclosures with 6 mm mesh. The abundance of nine of the prey taxa was lower in the presence of trout, only leptocerids were more numerous in the presence of trout.
3. Our measurements of prey immigration/emigration, together with trout diet data, suggest that direct consumption by trout, rather than avoidance behaviour by prey, explains most decreases in prey abundance. There was avoidance behaviour by only two of the twenty-one prey taxa, with trout inducing emigration of the mayflies Baetis rhodani and Paraleptophlebia sp.
4. Trout indirectly increased periphyton biomass in both 3 and 6 mm enclosures. The effect of trout on periphyton was probably due to strong effects of trout on the grazer, Baetis rhodani , Heptagenia sp. and Paralepthoplebia sp.
5. Our results suggest that mesh size, through its effects on exchange rates of prey, may affect interactions between predators and prey in running waters, but that the effects of dispersal and predation on invertebrates are taxon specific.     

When galaxiid and salmonid fishes meet–a family reunion in New Zealand

New Zealand's small freshwater fish fauna has been augmented by introductions of exotic species, primarily salmonids. Brown and rainbow trout have been successful and the chinook salmon has established anadromous populations. Although few explicit data are available, it appears that addition of large salmonids has had harmful impacts on some indigenous species, particularly several galaxiids; these appear to result from predation and competitive exclusion. Interactions may be chronic or catastrophic. It is difficult to determine causes of chronic interactions, many instances of which are likely to be occurring and escaping notice. Further, it is difficult to separate the effects of human-induced habitat deterioration from inter-specific interactions. Catastrophic interactions are much easier to observe but even here determining the nature of interactions and ascribing causes is fraught with difficulties.     

Temperature,invaders and patchy habitat interact to limit the distribution of a vulnerable freshwater fish

Interacting global‐change drivers such as invasive species and climate warming are likely to have major and potentially unexpected influences on aquatic ecosystems. In river networks, modified water temperature combined with patchy physical conditions will likely cause shifts in the amount and distribution of suitable habitat, with influential invasive species further altering habitat availability. We examined how distributions of a thermally sensitive galaxiid fish native to the alpine rivers of New Zealand, Galaxias paucispondylus, were influenced by these drivers using spatially extensive presence–absence electrofishing surveys of 46 sites spread over four subcatchments. A unimodal response to water temperature and an interaction with substratum size meant G. paucispondylus were limited to streams with average summer water temperatures between 10.6 and 13.8 °C and were absent when average substratum sizes were <36 mm, regardless of temperature. In addition, non‐native trout >150 mm long excluded G. paucispondylus, but were only found in streams with average summer water temperatures <10.6 °C. These influences of trout likely strengthened the unimodal temperature response of G. paucispondylus and led to a very small G. paucispondylus realized niche. When predicted temperature increases were applied to catchment models, G. paucispondylus distributions were patchy and variable across subcatchments. Moreover, local physical characteristics of river networks were particularly important because of the non‐linear and interactive influences of temperature and substratum size on the outcome of species interactions. Therefore, substratum sizes, water temperature and a non‐native predator combined to influence the distribution of this thermally sensitive fish, illustrating how the effects of climate warming will likely be strongly context‐dependent and interactive.     

The impact of non-native rainbow trout within Afro-montane streams in eastern Zimbabwe

Non-native trout species have been associated with many negative effects in receiving ecosystems. The first aim of this study was to determine the impact of non-native rainbow trout Oncorhynchus mykiss on distribution and abundance of native mountain catfish Amphilius uranoscopus within Afro-montane streams in Nyanga Mountains, eastern Zimbabwe. The second aim was to compare macro-invertebrate community responses to the presence of the trout and the catfish. We examined trout impact on catfish’s habitat associations, whereas macro-invertebrate composition was compared using open fish and fish exclosure experiments in habitats with and without trout. Trout influenced both the distribution and abundance of the catfish that occupied shallow reaches possibly to avoid predation from trout that occurred in the deeper habitats. Within trout invaded reaches, most macro-invertebrate taxa were more abundant in exclosure than open treatments. By contrast, within trout-free reaches, most macro-invertebrates either did not differ between treatments or were generally more abundant in open than exclosure treatments. This suggests that the macro-invertebrate communities responded differently within invaded and non-invaded reaches. By influencing distribution and abundance of native biota, non-native rainbow trout may have wider ecological effects, such as influencing trophic interrelationships within invaded habitats.     

Stocked trout have minimal effects on littoral invertebrate assemblages of productive fish‐bearing lakes: a whole‐lake BACI study

1. Rainbow Trout (Oncorhynchus mykiss [Walbaum]) is commonly stocked as a sport fish throughout the world but can have serious negative effects on native species, especially in headwater systems. Productive fish‐bearing lakes represent a frequently stocked yet infrequently studied system, and effects of trout in these systems may differ from those in headwater lakes. 2. We used a Before‐After Control‐Impact (BACI) design to determine how stocked trout affected assemblage‐level and taxon‐level biomass, abundance and average length of littoral invertebrates in a stocked lake relative to three unstocked control lakes in the boreal foothills of Alberta, Canada. Lakes were studied 1 year before and for 2 years after stocking. Because characteristics of productive fish‐bearing lakes should buffer impacts of introduced fish, we predicted that trout would not affect assemblage‐level structure of littoral invertebrates but might reduce the abundance or average length of large‐bodied taxa frequently consumed by trout. 3. Relative to the unstocked control lakes, biomass, but not abundance, of the littoral invertebrate assemblage was affected indirectly by trout through increases of some taxa after trout stocking. At the individual taxon‐level, trout stocking did not affect most (23 of the 27) taxa, with four taxa increasing in abundance or biomass after stocking. Only one taxon, Chironomidae, showed evidence of size‐selective predation by trout, being consumed frequently by trout and decreasing significantly in average length after stocking. 4. Our results contrast with the strong negative effects of trout stocking on invertebrate assemblages commonly reported from headwater lakes. A combination of factors, including large and robust native populations of forage fish, the generalised diet of trout, overwinter aeration, relatively high productivity and dense macrophyte beds, likely works in concert to reduce potentially negative effects of stocked trout in these systems. As such, productive, fish‐bearing lakes may represent a suitable system for trout stocking, especially where native sport fish populations are lacking.     

Reciprocal trophic niche shifts in native and invasive fish: salmonids and galaxiids in Patagonian lakes

1. Rainbow (Oncorhynchus mykiss) and brown trout (Salmo trutta) are widespread and invasive salmonids with important lethal effects as predators, although indirect effects are also possible. We used stable isotope analyses (δ¹⁵N, δ¹³C) to explore how the density of invasive trout in 25 Patagonian lakes alters the trophic niche (TN) of a widespread native fish, Galaxias platei (Galaxiidae). We also explored how the density of the galaxiid influences the TN of invasive trout. 2. We quantified two aspects of the TN: (i) the proportion of littoral carbon (PL) and (ii) trophic height (TH) (i.e. the ‘height’ at which the fish feeds in the food web). We related these measures of TN in a given species to the density of other species (as estimated by catch‐per‐unit‐effort). 3. As G. platei body size increased, their PL increased (increasing littoral feeding) in several lakes. However, none of the fish species investigated showed changes in PL with increasing density of the other fish species. TH increased with body size in all three species. In addition, the TH of large G. platei declined with increasing trout density and, reciprocally, the TH of large S. trutta decreased with decreasing G. platei density. 4. The reciprocal effects of native and the invasive fish on TH were as large as a shift of one trophic level. This pattern is consistent with an exhaustion of galaxiid prey for both piscivorous G. platei and S. trutta in lakes with high trout density. 5. These finding support the suggested management strategy of culling trout from overpopulated lakes, which should simultaneously protect native fish and enhance a lucrative sport fishery for large trout.     

Movements of brown trout, Salmo trutta, and juvenile Atlantic salmon, Salmo salar, in a coastal stream in northern Norway

Movements of resident brown trout (age 2+ to 9+ years) and young Atlantic salmon (age 1+), stocked as fry, were studied in July, August and September in a coastal stream in northern Norway. Between 85 and 89% of the brown trout were recaptured in the study area (346m, 1326m²) within 45m of their release point throughout the investigation period. Most specimens had moved less than 150m. Trout movements were related to local variation in density. Trout occupying those sections of stream with the lowest fish densities (5.3–10.9 fish 100m^?2) had a significantly lower movement rate than fish from sections with densities between 13.7 and 31.5 fish 100m^?2. Trout that moved from their marking section were significantly larger than specimens that did not leave their original site. There was a significant correlation between permanence of station each month and the mean water level that month. The majority of the trout (47%) were caught at undercut stream banks or at sites in the proximity of these. A decrease in water level during the study period resulted in a high loss (36%) of such habitat, probably forcing some individuals to move. The recapture data indicate that the trout population consists of one small (c. 15–20%) mobile, and one large sedentary component. Young salmon displayed high station permanence in July and August (93% and 96%). However, in the autumn they exhibited a significant downstream movement, and only 73% were recaptured within their release section. This movement was significantly higher for larger specimens, and is thought to occur because of a pre-winter change in habitat, initiated by a decline in stream temperature. In contrast to trout, salmon in sections containing the lowest densities (22.0–25.0 fish 100m^?2) did not show significantly lower movement rates when compared with salmon at higher densities (32.2–46.3 and 51.8–60.6 fish 100m^?2). The spatial distribution of young salmon indicated the formation of territorial mosaics over the stream bed, which are thought to reduce intraspecific competition.     

Extensive diel fish migrations in a deep ultraoligotrophic lake of Patagonia Argentina

Andean Patagonian lakes are ultraoligotrophic, highly transparent water bodies with very low densities of organisms at the different levels of their trophic food webs. Little is known about spatial distribution and habitat use by fish in these lakes. Hydroacoustic and active sampling techniques were used to study the distribution, composition, and displacement of organisms in an Andean lake throughout the diel cycle on two different moon phase days. Sound scattering layers (SSLs) were found both in the near-shore and pelagic habitats at different times of the day. These SSLs, formed by galaxiid larvae and adults, underwent displacement during the time periods of dawn and dusk, giving rise to the redistribution of organisms in the different habitats. The organisms show high sensitivity to light intensity, displaying different behaviors depending on moon phases. Extensive diel vertical migrations show that the deep pelagic habitats provide diurnal refuge for native galaxiids in Andean Patagonian lakes. Due to the high densities of galaxiid larvae and adults, it is likely that diel migrations generate an active flow of energy and matter between habitats, which could have a profound influence on whole lake dynamics.     

Proliferative activity and motoneurone recruitment persist at the spinal cord central canal during larval and some postlarval stages in the rainbow trout (Oncorhynchus mykiss)

We previously found a linear relationship between the cross sectional myotomal area and the motoneurone number in the growing trout during postlarval stages. These neurones increased in number until a fish length of 150 mm, which prompted us to examine how motor neurones are recruited afterwards to meet the growth of their target myotomal muscle. Young adult (260 mm in length), fingerlings (F, 120-170 mm), fry (Fr, 70 mm) and eleutherembryos (Es, 20-30 mm) of rainbow trout (Oncorhyncus mykiss) were employed in this study. PCNA immunohistochemistry was used for monitoring the proliferative activity in the epithelium of the spinal cord central canal. This activity was quantified as the number of PCNA labelled cells for each spinal cord section. In Es and Fry, a mean value of 3-5 labelled cells for each section was found with a sharp decrease in young F (120 mm long). After this fish length, it was not possible to quantitatively evaluate the proliferative activity at the central canal. However, labelled cells were seldom found in the spinal cord sections until a fish length of 260 mm. From these data it is possible to conclude that motoneurone recruitment in the trout spinal cord is down-regulated at the F stage. Afterwards, we found that motoneurones increase in size to meet the growth of their target myotomal muscle.