Parasites and salinity: costly tradeoffs in a threatened species

Parasites and environmental conditions can have direct and indirect effects on individuals. We explore the relationship between salinity and parasites in an endemic New Mexico State threatened fish, the White Sands pupfish (Cyprinodon tularosa). Spatial variation in salinity limits the distribution of the endemic springsnail (Juturnia tularosae) within Salt Creek, a small desert stream. The springsnail is the presumed intermediate host for trematodes that infect the White Sands pupfish, and trematode prevalence and intensity in pupfish are positively associated with the springsnail. Salinity and parasites both have negative impacts on pupfish, but in areas of high salinity, pupfish can effectively escape parasites. Pupfish trematodes were absent from sites lacking snails. At the upstream site, the absence of parasites and lower variance in salinity were correlated with larger pupfish that were in better condition than pupfish at either the middle or lower sites. Springsnails were present in the middle section, an area with moderate salinity, and all pupfish had trematodes (median abundance 847 trematodes/fish). Lipid levels and condition were lowest in fish from the middle site. Additionally, fewer older fish indicated an increased mortality rate. At the lower site, springsnails were absent due to high salinity; pupfish trematode abundance was much lower (six trematodes/fish), and fish condition was intermediate. An additional experiment revealed that snail activity and survival were significantly reduced at high salinities commonly present at the lower site. Although both high salinity and parasites significantly affect pupfish, parasites might be more detrimental.     

Summer movements of desert pupfish among habitats at the Salton Sea

Summer movement behavior of native desert pupfish (Cyprinodon macularius Baird and Girard) was evaluated among various habitats around the Salton Sea, located in southern California. Agricultural drains, shoreline pools, and Salt Creek were sampled six times between June 28 and September 16, 1999. Collected pupfish were marked using fluorescent elastomer implants. Unique marks were used at each site. Movements were detected from locations of recaptured pupfish. Desert pupfish were found in 10 of 12 sites sampled. Of 3239 pupfish captured during the study, 278 were recaptures, including 27 recaptures at areas different from where they were initially marked. The best evidence of pupfish movements was in the southwestern area of the Salton Sea between a drain and a connected shoreline pool. Movements were also observed from lower Salt Creek into a shoreline pool at the mouth of the creek as the water level dropped. The use of the Salton Sea as a migration corridor between habitats was not documented during this short study. The marking technique was successful and showed promise for future mark and recapture studies of desert pupfish.     

Distribution,habitat and conservation status of desert pupfish (Cyprinodon macularius) in the Lower Colorado River Basin,Mexico

In order to better determine the currentstatus of desert pupfish populations (Cyprinodon macularius macularius) in the LowerColorado River Basin of Mexico, bimonthly fishsampling and habitat evaluations were carriedout from September 1996 to August 1997 in sixlocalities of Baja California and Sonora.Desert pupfish were collected using minnowseines and traps. Four sampling sites are inSonora, in shallow marginal habitats of theCiénega Santa Clara (Hunters' Camp, Outletof the Welton-Mohawk channel, El Doctor andFlor del Desierto) and two are in BajaCalifornia (Cerro Prieto and streams south ofCerro Prieto). The most abundant fish speciessampled was native desert pupfish (C.macularius, 59%) followed by sailfin molly(Poecilia latipinna, 19%), redbellytilapia (Tilapia cf. zilli, 10%) and western mosquito fish (Gambusiaaffinis, 7%). Significant temporalfluctuations in distribution and abundance ofdesert pupfish populations, as previouslyreported for these populations, was againdocumented. The main anthropogenic factorsaffecting distribution and abundance of desertpupfish populations in the Mexican portion ofthe Lower Colorado River Basin are progressivealteration of aquatic and riparian habitats, aswell as presence of exotic fishes that havecaused the displacement or elimination ofnative fish populations. Both habitat andpresence of the desert pupfish populations inthe study area are controlled by theperiodicity, quality and volume of dischargesinto the Mexican portion of the basin from theUnited States.     

PHOSPHOGLUCONATE DEHYDROGENASE POLYMORPHISM AND SALINITY IN THE WHITE SANDS PUPFISH

The phosphogluconate dehydrogenase (Pgdh) locus is the only polymorphic allozyme locus observed among 37 loci examined in all four populations of a New Mexico state Endangered species, the White Sands pupfish (Cyprinodon tularosa). We report evidence suggesting that this polymorphism may be associated with salinity. Salinity levels vary widely within and between habitats occupied by White Sands pupfish. The frequency of the Pgdh¹⁰⁰ allozyme was correlated with salinity but not with temperature. Frequency of Pgdh¹⁰⁰ differed between low (3.76 parts per thousand (ppt)) and high (9.23 ppt) salinity sites at Malpais Spring despite no obvious barriers to fish movement. Frequencies of Pgdh¹⁰⁰ in two introduced populations differed from that of the presumptive founding stock and correlated with salinity in the current habitats.     

Phenotypic plasticity and contemporary evolution in introduced populations: evidence from translocated populations of white sands pupfish (<Emphasis Type="Italic">Cyrpinodon tularosa</Emphasis>)

Contemporary evolution has been shown in a few studies to be an important component of colonization ability, but seldom have researchers considered whether phenotypic plasticity facilitates directional evolution from the invasion event. In the current study, we evaluated body shape divergence of the New Mexico State-threatened White Sands pupfish (Cyprinodon tularosa) that were introduced to brackish, lacustrine habitats at two different time in the recent past (approximately 30 years and 1 year previously) from the same source population (saline river environment). Pupfish body shape is correlated with environmental salinity: fish from saline habitats are characterized by slender body shapes, whereas fish from fresher, yet brackish springs are deep-bodied. In this study, lacustrine populations consisted of an approximately 30-year old population and several 1-year old populations, all introduced from the same source. The body shape divergence of the 30-year old population was significant and greater than any of the divergences of the 1-year old populations (which were for the most part not significant). Nonetheless, all body shape changes exhibited body deepening in less saline environments. We conclude that phenotypic plasticity potentially facilitates directional evolution of body deepening for introduced pupfish populations.     

Genetic signatures of translocations and habitat fragmentation for two evolutionarily significant units of a protected fish species

Genetic population structure was evaluated for the White Sands pupfish (Cyprinodon tularosa), a protected fish species comprised of two Evolutionarily Significant Units (ESUs); the Malpais Spring ESU and the Salt Creek ESU. The Malpais Spring ESU is restricted to Malpais Spring, whereas the Salt Creek ESU includes the native Salt Creek population and two Salt Creek-derived populations at Mound Spring and Lost River; all three of these habitats are physically fragmented. We sampled the upper and lower reaches of the four populations, examining 13 DNA microsatellite loci from 40 individuals per population. As expected, significant genetic structure was observed between the two ESUs; Malpais Spring and Salt Creek. Substantial genetic drift was observed for the introduced Lost River population, with modest genetic drift for the introduced Mound Spring population. Taken together with ecological data, neither of the introduced populations successfully replicates the Salt Creek population. We also report significant reductions in genetic diversity for the upper reaches of both Salt Creek and Lost River, indicating that recent habitat changes have altered the genetic structure of these two populations. We consider these findings along with previously reported ecological data to develop guidelines for managing C. tularosa.     

Tidal and diurnal variations in larval fish abundance in an estuarine inlet in Ariake Bay,Japan: implication for selective tidal stream transport

We have conducted a preliminary study of tidal and diurnal variations in the distribution of dominant larval and juvenile fishes in the Chikugo River inlet (Ariake Bay, Kyushu, Japan) to determine whether selective tidal stream transport (STST) occurs. Larval and juvenile fish were collected from the mesohaline zone of the Chikugo River inlet during spring 2002. Temperature, salinity, depth, and current velocity were measured. Larval and juvenile abundance were compared among four tidal conditions, flooding tide, high tide, ebbing tide, and low tide, and between day and night. A total of 12 families, 15 species, and 5,577 individuals were collected. Temperature did not vary significantly with tidal conditions whereas salinity, depth, and current velocity varied significantly. Salinity also was correlated significantly and positively with depth. The abundance of most of the fishes was correlated positively and significantly with salinity and depth. Lateolabrax japonicus, Trachidermus fasciatus, Acanthogobius hasta, and other gobiid larvae (Gobiidae spp.) were significantly more abundant during high tide; in contrast, Coilia nasus and Neosalanx reganius were most abundant during low tide. The abundance of most of the fishes was higher during high tides at night than during the day, indicating the existence of STST, which may be strategically associated with ascending progress to upstream nursery areas.     

Environmental drivers of diurnal visits by transient predatory fishes to Caribbean patch reefs

Video cameras recorded the diurnal visitation rates of transient (large home range) piscivorous fishes to coral patch reefs in The Bahamas and identified 11 species. Visits by bar jack Caranx ruber, mutton snapper Lutjanus analis, yellowtail snapper Ocyurus chrysurus, barracuda Sphyraena barracuda and cero Scomberomorus regalis were sufficiently frequent to correlate with a range of biophysical factors. Patch‐reef visitation rates and fish abundances varied with distance from shore and all species except S. regalis were seen more frequently inshore. This pattern is likely to be caused by factors including close proximity to additional foraging areas in mangroves and on fore‐reefs and higher abundances close to inshore nursery habitats. Visitation rates and abundances of C. ruber, L. analis, O. chrysurus and S. regalis also varied seasonally (spring v. winter), possibly as fishes responded to temperature changes or undertook spawning migrations. The abundance of each transient predator species on the patch reefs generally exhibited limited diurnal variability, but L. analis was seen more frequently towards dusk. This study demonstrates that the distribution of transient predators is correlated spatially and temporally with a range of factors, even within a single lagoon, and these drivers are species specific. Transient predators are considered an important source of mortality shaping reef‐fish assemblages and their abundance, in combination with the biomass of resident predators, was negatively correlated with the density of prey fishes. Furthermore, transient predators are often targeted by fishers and understanding how they utilize seascapes is critical for protecting them within reserves.     

Structure of two macrolepidopteran assemblages on Salix nigra (Marsh) and Acer negundo L.: abundance,diversity, richness,and persistence of scarce species

1. Most insect species occur at low abundance but a greater research effort has been devoted to so‐called outbreak species and little research is available on scarce (low abundance) species that are typical of most insect species. 2. Larval free‐feeding macrolepidoptera of two riparian trees Salix nigra (Marsh) (black willow) and Acer negundo L. (box elder) were sampled and sorted by species and abundance. 3. Data collected established that the majority of species in the assemblages in each tree species occurred at low abundance in each of the 5 years when larvae were sampled. 4. Species in the Noctuidae and Geometridae dominated both assemblages. 5. On both trees, assemblages were dominated numerically by relatively few species, a pattern that has been observed for insect assemblages on plants in managed and unmanaged habitats.     

An assessment of fish species assemblages in rice fields in West Bengal,India: implications for management

Wetlands such as rice fields and associated canals are congenial habitats for different fishes and provide the foundation of rice‐fish culture. The selection of suitable fish species determines the success for this linkage of aquaculture and agriculture. An assessment of fish assemblages in selected rice fields and associated canals in West Bengal, India documented the candidate species for culture and conservation in rice fields. Indigenous minnow traps made of bamboo were employed in the rice fields and canals for a period of 3 months, to collect and record the fish abundance. A total of 531 fishes representing 19 different species were caught from the rice fields in 30 sampling efforts. From the canals, 13 different fish species totaling 676 fishes were collected in 80 sampling efforts. The fish collected per trap in the rice fields (mean 17.7 ± 1.97 SE) remained significantly different (t = 4.461, df = 112; P < 0.001) from those of the canals (mean 8.05 ± 1.09 SE). Diversity indices ranged between 1.91 and 2.01, with a 53% similarity between the rice fields and canals. Among the fish species collected, Badis badis, Colisa fasciatus and C. lalia exhibited high abundance; the two species Heteropneustes fossilis and Mystus vittatus established as threatened fish categories, showed low abundance. The fish species collected were important as food resources and the ornamental fish trade, and many of them bear potential in regulating mosquitoes in the rice fields. These fish assemblages in the rice fields and associated canals suggest a rice‐fish culture potential for integrating food and health.     

Assessment of Potential Impacts of Exotic Species on Populations of a Threatened Species, White Sands Pupfish, Cyprinodon tularosa

The potential impact of introduced species on rare taxa is of particular concern to conservation biologists. We evaluate the impacts of western mosquitofish (Gambusia affinis) and virile crayfish (Orconectes virilis) on experimental populations of a threatened species, the White Sands pupfish (Cyprinodon tularosa). Forty experimental pupfish populations were exposed to one of four treatments; (a) 1 crayfish, (b) 4 crayfish, (c) 5 adult mosquitofish and (d) control. Pupfish population size and biomass was monitored over the duration of one breeding season. A repeated measure multiple analysis of covariance revealed a significant effect of treatments on response variables (population size and biomass) (P<0.0001). Mosquitofish had a significant effect on population size and biomass (P=0.0330). The effect of one crayfish was not significant (P=0. 0683). However, 4 crayfish had a significant effect (P<0.0001) on population size. We use these data, along with information on environmental tolerances of crayfish and mosquitofish, to evaluate risks for specific pupfish populations. An erratum to this article is available at .     

Are native cyprinids or introduced salmonids stronger regulators of benthic invertebrates in South African headwater streams?

The introduction of nonnative salmonids in the Southern Hemisphere generally leads to a reduction in invertebrate abundance and changes in assemblage composition. In the Cape Floristic Region of South Africa, introduced rainbow trout Oncorhynchus mykiss is the dominant predator in many headwater streams, where they have replaced small‐bodied native fishes such as Breede River redfin Pseudobarbus burchelli. To examine the consequences of this species replacement on food web structure, we used a month‐long field experiment to compare the top‐down effects of Breede River redfin and rainbow trout on benthic invertebrate assemblages (abundance and composition) and basal resources (periphyton and particulate organic matter) in 1 × 1.5 m of plastic cages. Benthic invertebrate abundance was more strongly depleted in the cages with redfin than in the cages with trout, and redfin and trout had distinct effects on invertebrate assemblage composition. On the other hand, neither redfin nor trout had a significant influence over standing stocks of periphyton or organic matter, implying that their differential effects on benthic invertebrates did not cascade down to the base of the stream food web in our experiment. Gut content analysis showed that aquatic invertebrates contributed more to the diet of redfin, while terrestrial invertebrates contributed more to the diet of trout, which may be responsible for the relatively weak effect of trout on aquatic invertebrates. This pattern contrasts with nonnative salmonid impacts elsewhere in the Southern Hemisphere. That trout can strongly alter the structure of benthic invertebrate assemblages, in addition to severely depleting native fish abundance, in Cape Floristic Region headwater streams should be weighed into management decisions, and our findings highlight the need for a detailed understanding of species‐specific top‐down effects where native predators are replaced by invasive predators.     

Structure of rotifer assemblages in shallow waterbodies of semi-arid northwest Iran differing in salinity and vegetation cover

Rotifers are important components of freshwater ecosystems and sensitive indicators of environmental changes. This study was carried out to test the hypothesis that, among environmental variables, salinity and aquatic vegetation have significant effects on rotifer diversity and abundance. We analyzed rotifer assemblages in the littoral zone of 22 hydromorphologically different shallow waterbodies in West Azarbaijan, Iran. Rotifer diversity and abundance were not significantly associated with basin morphology, but were positively correlated with the percentage of vegetation cover. Salinity and electroconductivity positively influenced rotifer abundance, while they had significantly negative effects on rotifer diversity. Halobiont species from the genera Brachionus, Hexarthra, Synchaeta, and Notholca reached their highest abundances in the waterbodies with pronouncedly higher salinities. Our findings are in agreement with recent records showing that distinct rotifer assemblages occur in saline and non-saline waterbodies. The role of salinity and aquatic vegetation as the most important environmental drivers in shaping rotifer communities is confirmed. The results of this study suggested that environmental changes could be significant on the micro-biogeographical level, and that the interaction of salinity and observed human impact, i.e., trophic level, promote rotifer abundance as sensitive indicators of environmental changes.     

Habitat Restoration as a Means of Controlling Non-Native Fish in a Mojave Desert Oasis

Non‐native fish generally cause native fish decline, and once non‐natives are established, control or elimination is usually problematic. Because non‐native fish colonization has been greatest in anthropogenically altered habitats, restoring habitat similar to predisturbance conditions may offer a viable means of non‐native fish control. In this investigation we identified habitats favoring native over non‐native fish in a Mojave Desert oasis (Ash Meadows) and used this information to restore one of its major warm water spring systems (Kings Pool Spring). Prior to restoration, native fishes predominated in warm water (25–32°C) stream and spring‐pool habitat, whereas non‐natives predominated in cool water (≤23°C) spring‐pool and marsh/slack water habitat. Native Amargosa pupfish (Cyprinodon nevadensis) and Ash Meadows speckled dace (Rhinichthys osculus nevadensis) inhabited significantly faster mean water column velocities (MWCV) and greater total depth (TD) than non‐native Sailfin molly (Poecilia latipinna) and Mosquitofish (Gambusia affinis) in warm water stream habitat, and Ash Meadows speckled dace inhabited significantly faster water than non‐natives in cool water stream habitat. Modification of the outflow of Kings Pool Spring from marsh to warm water stream, with MWCV, TD, and temperature favoring native fish, changed the fish composition from predominantly non‐native Sailfin molly and Mosquitofish to predominantly Ash Meadows pupfish. This result supports the hypothesis that restoring spring systems to a semblance of predisturbance conditions would promote recolonization of native fishes and deter non‐native fish invasion and proliferation.     

Changes in pelagic Polychaete assemblages along the California current system

The composition, abundance, species richness and structural changes of the planktonic polychaete assemblages were analysed along a latitudinal transect in the California Current System (California, U.S.A. and Baja California, Mexico). The biological (species and abundance) and physical (temperature and salinity) data were analysed using Principal Component Analysis (PCA). The principal water masses in the survey area were determined. Twenty-four holoplanktonic species belonging to families Alciopidae, Iospilidae, Lopadorhynchidae, Tomopteridae and Typhloscolecidae were identified. Three clear species assemblages were discerned in the PCA results: 1. A `north group' (from Oregon-California border to San Francisco), with relatively high species richness (11) and the highest mean abundance (121 ind. per 500 m<sup>3</sup>) was characterised by Tomopteris septentrionalis, T. planktonis, Plotohelmis tenuis, and Travisiopis lobifera. California Current Water and Subtropical Central Water were present in the area occupied by this assemblage. 2. A `south group' (from off Bahía Magdalena to Cabo San Lucas), with the highest species richness (16), but low mean abundance (37.8 ind. per 500 m³); it included tropical affinity species, such Lopadorhynchus henseni, Tomopteris nationalis, and Travisiopsis dubia. In concordance Surface Equatorial Water was identified in this region. 3. A `transition group' (between the north and south regions) recorded the lowest mean abundance (2.3 ind. per 500 m³) and species richness (9). Only the California Current Water was detected in this area. The spatial pattern of species richness found along of this transect, was at least, partially due to the planktonic productivity distribution in the epipelagic region and the influence of several water masses coming from different directions.     

Spatially variable response of native fish assemblages to discharge,predators and habitat characteristics in an arid‐land river

1. Fish assemblages and habitats were sampled annually at fixed sites in three tributaries of the Gila River catchment over a 21‐year span that included prolonged low‐ and high‐flow periods. Model selection was used to evaluate responses of seven native fishes with variable ecological traits (four small‐bodied cyprinids, one large‐bodied cyprinid, and two large‐bodied catostomids) to mean annual discharge and predacious non‐native fishes across the three sites. We also compared habitat use and overlap of native and non‐native fishes to identify potential for negative interactions among species. 2. Assemblage structure (species abundance and richness) and recruitment of native species was strongly and primarily affected by mean annual discharge and secondarily by location and densities of non‐native predators (mainly the centrarchid Micropterus dolomieui). 3. Densities of age‐0 catostomids and small‐bodied cyprinids were positively associated with discharge, and this pattern was strongest in the tributary with the lowest densities of non‐native predators. Absence or extreme low abundance of natives during low‐flow years was most pronounced at the sites where non‐native predators were comparatively common. Densities of adults of large‐bodied native species also varied by site, but often were positively associated with densities of non‐native predators. 4. Spatially variable responses of native fish assemblages indicated that the persistence of native fishes could be jeopardized if key habitats were lost or flow regimes unnaturally altered, particularly during low‐flow conditions when recruitment of native fishes is low and predation by non‐natives is high. Large‐bodied species may be less vulnerable to multiple years of poor conditions because adults are able to avoid predation by non‐natives and thus can rely on occasional high discharge years for successful recruitment. 5. As in other arid‐land streams, native fish assemblages of the Gila River Basin continue to decline. Our results indicate that conservation requires specific knowledge and consideration of physical influences as well as life‐history attributes of native and non‐native fishes.     

Effect of simulated macroalgae on the fish assemblage associated with a temperate reef system

Increased habitat complexity is supposed to promote increased diversity, abundance and biomass. This study tested the effect of the macroalgal cover on temperate reef fishes by mimicking macroalgae on artificial reefs in NW Sicily (Mediterranean Sea). Macroalgal cover affected reef fishes in different ways and independently of intrinsic temporal trends. The fish assemblages of manipulated and control artificial reef units differed in the relative abundances of the associated species, but little in species composition. In line with studies in seagrass habitats, fishes were most abundant in reefs covered by artificial macroalgae. Three species (Boops boops, Serranus scriba and Symphodus ocellatus) exhibited consistently greater abundance on vegetated reef units than on control reef units. The total number of species and the abundance of three particular species (S. scriba, S. ocellatus and Thalassoma pavo) displayed temporal trends which were independent on short and large temporal scales. Only fish total biomass and one species (Spicara flexuosa) displayed strong effects of interaction among the experimental factors. Mechanisms to explain these findings are discussed from observational evidence on habitat use and interactions among multiple species. This study highlights that manipulative experiments involving repeated sampling of fish in artificial habitats appear to be a valid approach to study fish-habitat relationships in fluctuating environments. It is also concluded that macroalgae mimics may serve as a tool for restoring lost marine vegetated habitats when current human-induced conditions prevent the recovery of pristine macroalgal stands.     

Spatiotemporal Variation in Shallow-Water Freshwater Fish Distribution and Abundance in a Large Subtropical Coastal Lagoon

Patos Lagoon is located off the southern Brazilian coast and represents one of the largest coastal lagoons in the world. We estimated hydrological and physicochemical conditions associated with spatial variation in the abundance and diversity of freshwater fishes along the lagoon, and inter-annual variability in abundances of freshwater fishes occurring in its estuarine zone. During our study, the region experienced two periods of average rainfall and two periods with above-average rainfall. The characids Astyanax eigenmaniorum and Oligosarcus jenynsii and the siluriform Parapimelodus nigribarbis were the most abundant freshwater fishes in the estuary during wet periods when water levels were higher and salinity was lower. Increases in abundance of these species in the estuarine area, all of which members of primary-division freshwater families, apparently were associated with pulses of reproduction and passive transport from freshwater habitats located near middle and upper lagoon reaches. Abundance of species from secondary freshwater families, such as poeciliids and cichlids, were less correlated with hydrological conditions, and their patterns of occurrence in the estuary suggest active migration from nearby freshwater habitats draining into this area. Findings indicate that freshwater discharge in the basin and expansion/retraction of freshwaters in the middle-upper lagoon determined patterns of freshwater fish abundance and species richness in the estuarine zone.     

Alien Fishes in Natural Streams: Fish Distribution, Assemblage Structure, and Conservation in the Cosumnes River, California, U.S.A.

The Cosumnes River is the largest stream without a major dam on its mainstem in the Sacramento–San Joaquin drainage, central California, U.S.A. We studied its fishes over a 3-year period to answer the following questions: (1) Was the native fish fauna still present? (2) Why were alien fishes so abundant in a river system with a 'natural' flow regime, which elsewhere has been shown to favor native fishes? (3) Were there assemblages of fishes that reflected environmental differences created by the underlying geology? (4) Were there features of the watershed that consistently favored native fishes or that could be managed to favor native fishes? Of the 25 species collected, 17 were alien species; 14 species (five native) were abundant or widely distributed enough to use in detailed analyses. Of the native species, only rainbow trout, Oncorhynchus mykiss, still occupied much of its native range in headwater streams. Other native species have been extirpated or persisted mainly above barriers to alien invasions. The most widely distributed alien species was redeye bass, Micropterus coosae, previously unknown from the river, whose abundance was associated with low-numbers of native species. Other aliens were found primarily in low-land habitats on the valley floor or foothills. Canonical Correspondence Analysis indicated that both native and alien species located on environmental gradients determined largely by elevation, temperature, flow, and emergent vegetation, but the associations with these variables were not strong. While most alien fishes were found in lowland sections of river flowing through agricultural regions, the general relationships between species abundance and landscape-level variables were weak. Assemblages of fishes were poorly defined mixtures of native and alien species. The strikingly distinct geological regions of the basin no longer supported distinct fish assemblages. Species distributions were highly individualistic, reflecting dynamic patterns of introductions, invasions, and local extinctions, as well as physiological tolerances and life history patterns. Most native fishes are likely to persist in the Cosumnes River only if summer flows are increased and if populations above natural barriers are protected from further invasions by alien species, especially redeye bass. General conclusions from this study include: (1) altered habitats can support native species under some circumstances; (2) new fish assemblages with characteristics of 'natural' communities are likely to develop in invaded systems; (3) restoring flow regimes to favor native fishes may require restoring minimum summer flows as well as high channel-forming flows. However, reversing or even reducing, the impact of the predatory redeye bass, pre-adapted for California streams, is probably not possible.     

马鞍列岛多种生境中鱼类群聚的昼夜变化

为了解岛礁水域鱼类群聚的昼夜变化特征,以便更全面地设计采样方法和掌握采样的时间尺度,于2009年9月对马鞍列岛7种生境进行了共计24网次的刺网昼夜采样,结合排序和聚类方法,从种类组成、相对生物量和丰度、种类丰富度、多样性和相似性等方面对研究海域鱼类群聚特征的昼夜变化作了探讨.在采获的55种鱼类中,昼夜出现的分别为41和46种,数量差别不大,但其昼夜组成却随栖息水层的变化而不同,底层鱼类更趋向于夜间在硬相生境集群活动;近底层鱼类的昼夜集群随生境变化而变化,在同一生境中既有偏向白天也有趋向夜间的;中上层鱼类更多地出现在白天的人工生境(AH).AH白天的丰度渔获率显著大于晚上,而天然生境(NH)昼夜差别不大;生物量渔获率无论NH还是AH皆无显著昼夜差异.具体到种类,仅有小黄鱼Larimichthys polyactis和赤鼻棱鳗Thryssa kammalensis等少数种类的数量在AH有显著的昼夜差别,其他多数种类虽然昼夜的出现率大多有别,但渔获率昼夜差异皆不明显.多样性差异更多的表现在不同生境之间,而同一生境的昼夜差异往往不甚显著.各个生境中鱼类的昼夜种类交替现象非常明显,形成了以褐菖(鲐)Sebastiscus marmoratus和鳗鲇Plotosus anguillaris为代表的夜间优势类群为主的硬相生境群聚格局、以丝背细鳞鲀Stephanolepis cirrhifer和细刺鱼Microcanthus strigatus为代表的白天优势类群为主的硬相生境群聚格局以及缺乏底层优势类群、以石首鱼科鱼类为代表的近底层鱼类为绝对优势类群的软相生境群聚格局.因此,采用被动性渔具在近岸典型生境进行鱼类等相关生物调查时,应使采样时间覆盖昼夜两个时段,且至少保证24h.