Allochthonous and autochthonous carbon sources for fish in floodplain lagoons of an Australian dryland river

Dryland rivers associated with arid and semi-arid land areas offer an opportunity to explore food web concepts and models of energy sources in systems that experience unpredictable flooding and long dry spells. This study investigated the sources of energy supporting three species of fish feeding at different trophic levels within floodplain lagoons of the Macintyre River in the headwaters of the Murray-Darling river system, Australia. Stable isotope analyses revealed that fish consumers derived, on average, 46.9% of their biomass from zooplankton, 38.1% from Coarse Particulate Organic Matter (CPOM) and 24.0% from algae. Ambassis agassizii derived on average 57.6% of its biomass carbon from zooplankton and 20.4–27.8% from algae or CPOM. Leiopotherapon unicolor derived most of its carbon from zooplankton and CPOM (38.3–39.5%), with relatively high contributions from algae compared to the other species (33.3%). An average of 48.4% of the biomass of Nematalosa erebi was derived from zooplankton, with CPOM contributing another 38.1%. Zooplankton was the most important source of organic carbon supporting all three fish species in floodplain lagoons. Phytoplankton, and possibly, particulate organic matter in the seston, are the most likely energy sources for the planktonic suspension feeders (zooplankton) and, consequently, the fish that feed on them. These results indicate a stronger dependence of consumers on autochthonous sources and on locally produced organic matter from the riparian zone (i.e., the Riverine Productivity Model), than on other resources.     

Fish diet composition in floodplain lagoons of an Australian dryland river in relation to an extended dry period following flooding

Floodplain rivers worldwide are threatened by loss of connectivity to their floodplains and hence reduced benefits from floodplain energy subsidies. Dryland rivers with ‘boom and bust’ ecological responses to flooding and extended dry periods may be particularly vulnerable. This paper describes variations in dietary composition of three fish species of contrasting trophic position in dryland floodplain lagoons with variable flood inundation and drying histories. The study species were Ambassis agassizii - a microphagic carnivore, Leiopotherapon unicolor - a carnivore/omnivore, and Nematalosa erebi - an algivore/detritivor. Despite the range of food items recorded in fish guts, each species fed mostly on relatively few food categories and few food items within each category. Most of the spatial (i.e. among lagoons) and temporal dietary variation was associated with different proportional contributions of these food items. Given the absence or low magnitude of flooding during the study period, temporal changes in diets of the three species are probably the result of successional changes in composition of invertebrate prey as the dry season progressed. The focus of each fish species on relatively few food categories and a few reliable food items within each category may be the most profitable foraging strategy when food resources are limiting in progressively drying floodplain lagoons.     

Emergence of epizootic ulcerative syndrome in native fish of the Murray-Darling River System, Australia: hosts, distribution and possible vectors

Epizootic ulcerative syndrome (EUS) is a fish disease of international significance and reportable to the Office International des Epizootics. In June 2010, bony herring Nematalosa erebi, golden perch Macquaria ambigua, Murray cod Maccullochella peelii and spangled perch Leiopotherapon unicolor with severe ulcers were sampled from the Murray-Darling River System (MDRS) between Bourke and Brewarrina, New South Wales Australia. Histopathology and polymerase chain reaction identified the fungus-like oomycete Aphanomyces invadans, the causative agent of EUS. Apart from one previous record in N. erebi, EUS has been recorded in the wild only from coastal drainages in Australia. This study is the first published account of A. invadans in the wild fish populations of the MDRS, and is the first confirmed record of EUS in M. ambigua, M. peelii and L. unicolor. Ulcerated carp Cyprinus carpio collected at the time of the same epizootic were not found to be infected by EUS, supporting previous accounts of resistance against the disease by this species. The lack of previous clinical evidence, the large number of new hosts (n = 3), the geographic extent (200 km) of this epizootic, the severity of ulceration and apparent high pathogenicity suggest a relatively recent invasion by A. invadans. The epizootic and associated environmental factors are documented and discussed within the context of possible vectors for its entry into the MDRS and recommendations regarding continued surveillance, research and biosecurity are made.     

A Large-scale, Hierarchical Approach for Assessing Habitat Associations of Fish Assemblages in Large Dryland Rivers

Multiple-scale assessments of fish-habitat associations are limited despite the fact that riverine fish assemblages are influenced by factors operating over a range of spatial scales. A method for assessing fish-habitat assemblages at multiple scales is proposed and tested in a large Australian dryland river, the Barwon–Darling River. Six discrete mesohabitat types (large wood, smooth bank, irregular bank, matted bank, mid-channel and deep pool) nested within 10 km long river reaches were sampled. Individual reaches were, in turn, nested within four larger geomorphological zones, previously identified along the river. Fish assemblages varied significantly between mesohabitat types and at different spatial scales. Golden perch (Macquaria ambigua), Murray cod (Maccullochella peelii peelii) and common carp (Cyprinus carpio) were strongly associated with large wood, but golden perch and Murray cod exhibited higher habitat specificity than carp. Bony herring (Nematalosa erebi) were more common in shallow edgewater habitats. At the river-scale, regional differences in the fish assemblage occurred at scales closely corresponding to geomorphological zones and these differences were associated with changes in the relative abundance of species rather than the addition or replacement of species. The proposed hierarchical framework improves the efficiency of fish surveys in large rivers by viewing meso-scale fish-habitat associations in the context of larger-scale geomorphological processes.     

Effects of temperature and dissolved oxygen on heart rate,ventilation rate and oxygen consumption of spangled perch,Leiopotherapon unicolor (Günther 1859), (Percoidei,Teraponidae)

Summary Heart, ventilation and oxygen consumption rates ofLeiopotherapon unicolor were studied at temperatures ranging from 5 to 35°C, and during progressive hypoxia from 100% to 5% oxygen saturation. Biopotentials recorded from the water surrounding the fish corresponded to ventilation movements, and are thought to originate from the ventilatory musculature. Cardio-respiratory responses to temperature and dissolved oxygen follow the typical teleost pattern, with bradycardia, increased ventilation rate and reduced oxygen consumption occurring during hypoxia. However, ventilation rate did not increase at 15°C and below. Ventilation rate showed a slower response to increasing temperature (normoxic Q₁₀=1.39) than heart rate and oxygen consumption (normoxic Q₁₀=2.85 and 2.38).L. unicolor is unable to survive prolonged hypoxia by utilising anaerobic metabolism, but has a large gill surface area which presumably facilitates oxygen uptake in hypoxic environments. Periodic ventilation during normoxia in restingL. unicolor may improve ventilation efficiency by increasing the oxygen diffusion gradient across the gills.Abbreviations EBG electrobranchiogram - ECG electrocardiogram     

Feeding behavior of the cichlid,Sarotherodon galilaeum: selective predation on Lake Kinneret zooplankton

In laboratory feeding trials, we analyzed the feeding behavior and selectivity of the cichlid, Sarotherodon galilaeum, for zooplankton prey from Lake Kinneret, Israel. The feeding behavior was dependent on fish size. Fish less than 20 mm SL fed on zooplankton as obligate particulate feeders. Fish from 20 to 42 mm SL fed either as particulate feeders or as filter feeders. Fish larger than 62 mm SL fed as obligate filter feeders. Particulate-feeding fish were size selective and had highest feeding electivities for large-sized zooplankton species. Filter-feeding fish had highest feeding electivities for zooplankton species with poor escape ability. In general, S. galilaeum predation pressure would be greatest on Ceriodaphnia reticulata, a large-bodied and easily captured species which is selected by both particulate-feeding and filter-feeding fish.     

浙江饮用水源地浮游动物群落特征及环境响应

2010年1月至2011年10月,对浙江省16个饮用水源地(H1—H8为河网型,K1—K8为水库型)的浮游动物群落进行季节调查。共记录浮游动物优势种(属)21种(轮虫8种、枝角类5种、剑水蚤5种和哲水蚤3种),各类群第一优势种分别为针簇多肢轮虫(Polyarthra trigla)、长额象鼻溞(Bosmina longirostris)、粗壮温剑水蚤(Thermocyclops dybowskii)和汤匙华哲水蚤(Sinocalanus dorrii)。2年间,河网和水库浮游动物平均密度分别为345.2 L-1和199.4 L-1,生物量分别为0.667 mg/L和0.421mg/L。各类群密度百分比例均以轮虫和桡足类无节幼体为主,甲壳动物以剑水蚤为主。经逐步回归分析表明,浮游动物群落密度(生物量)与河网水质因子(P0.01)之间相关性比水库(P0.05)更密切,总磷和氨氮分别入选了河网和水库所有有效的回归方程中。通径分析和决策系统分析表明,河网的总磷和叶绿素a含量对浮游动物群落变动具有正效应,溶解氧具有负效应;总磷含量是影响河网群落变动的最重要因子,叶绿素a含量则是影响群落增长最主要的限制因子。河网剑水蚤、无节幼体和轮虫群落的密度(生物量)与水体综合营养指数TLIc密度(TLIc生物量)之间有显著的线性回归关系(P0.001),无节幼体密度构成了TLIc密度变动的限制因子,轮虫生物量成为TLIc生物量变动的限制因子,而剑水蚤是一类最重要、稳定的水质指示群落,这对于筛选浮游动物群落的一些拓展性监测指标具有重要的参考作用。     

Temporal Variations in Fish Catch Using Pop Nets in Mangrove and Saltmarsh Flats at Towra Point,NSW, Australia

Fish assemblages utilising saltmarsh and mangrove during spring tides were surveyed over a 12-month period using buoyant pop nets. A total of 48 net releases in the saltmarsh identified 16 species, at a density of 0.56 fish m⁻², with six species being of commercial importance. The same number of releases within the mangrove collected a total of 23 species at a density of 0.76 fish m⁻². However, fish density was higher within the saltmarsh than the adjacent mangrove when corrected for water volume. Multidimensional scaling revealed different assemblages of fish in the two habitats, with higher numbers of Ambassis jacksoniensis and Pseudomugil sp. in the saltmarsh and higher numbers of Mugilogobius sp. and Acanthopagrus australis in the mangrove. The result suggests a potentially significant role for saltmarsh as a fish habitat in the estuaries of southeast Australia.     

DNA barcoding of freshwater fishes and the development of a quantitative qPCR assay for the species‐specific detection and quantification of fish larvae from plankton samples

The barcoding of mitochondrial cytochrome c oxidase subunit 1 (coI) gene was amplified and sequenced from 16 species of freshwater fishes found in Lake Wivenhoe (south‐eastern Queensland, Australia) to support monitoring of reservoir fish populations, ecosystem function and water health. In this study, 630–650 bp sequences of the coI barcoding gene from 100 specimens representing 15 genera, 13 families and two subclasses of fishes allowed 14 of the 16 species to be identified and differentiated. The mean ± s.e . Kimura 2 parameter divergence within and between species was 0·52 ± 0·10 and 23·8 ± 2·20% respectively, indicating that barcodes can be used to discriminate most of the fish species accurately. The two terapontids, Amniataba percoides and Leiopotherapon unicolor, however, shared coI DNA sequences and could not be differentiated using this gene. A barcoding database was established and a qPCR assay was developed using coI sequences to identify and quantify proportional abundances of fish species in ichthyoplankton samples from Lake Wivenhoe. These methods provide a viable alternative to the time‐consuming process of manually enumerating and identifying ichthyoplankton samples.     

Ontogenetic shift in dietary preference and low dietary overlap in rohu (<Emphasis Type="Italic">Labeo rohita</Emphasis>) and common carp (<Emphasis Type="Italic">Cyprinus carpio</Emphasis>) in semi-intensive polyculture ponds

In order to investigate ontogenetic changes in diet and diet overlap between rohu (Labeo rohita) and common carp (Cyprinus carpio) in polyculture ponds, food preferences of different size classes of these fishes were quantified. Rohu diet consisted of both phytoplankton and zooplankton, and there was a distinct ontogenetic shift in the relative importance of these food items. Zooplankton was the dominant food for rohu up to 20.6 cm total length (TL) and then gradually decreased in importance as fish grew. Phytoplankton was initially a minor component of rohu diet but gradually increased in importance and became the dominant food for rohu at 24.2 cm TL. Phytoplankton biovolume in rohu guts was positively correlated with fish size (TL). Chesson’s α indicated that rohu of all sizes preferentially selected Cladocera and avoided Cyanophyceae and Euglenophyceae. Young rohu initially preferred Rotifera and Copepoda but gradually switched to Bacillariophyceae and Chlorophyceae. Common carp diet consisted of phytoplankton, zooplankton, and benthic macroinvertebrates, but was dominated by benthic macroinvertebrates (63–92% of total diet). As common carp grew, the proportion of zooplankton ingested decreased and the proportion of benthic macroinvertebrates increased. Benthic macroinvertebrate biovolume in common carp guts was positively correlated with fish size. Common carp of up to 15.4 cm TL preferentially selected zooplankton, but common carp larger than 18.9 cm TL avoided this food item. Common carp of all sizes avoided phytoplankton. A low dietary overlap was found between rohu and common carp (Schoener overlap index: 0.08–0.35), probably due to ingestion of smaller quantities of zooplankton by the latter. Dietary overlap also decreased with increasing rohu and common carp size because of divergent ontogenetic shifts in dietary preferences of the two species.     

Distribution and abundance of fish larvae in the northern Ionian Sea (Eastern Mediterranean)

The purpose of this paper was to study the spatial distribution, abundance and composition of fish larvae in the northern Ionian Sea. Samples were collected to the 600 m depth with an electronic multinet BIONESS during the “INTERREG Italia-Grecia” oceanographic cruise carried out in March 2000 off the Apulian Italian coast. A total of 46 species of teleost early stages were collected, belonging to 38 genera and 22 families. Over 52% of the larvae identified were mesopelagic species, almost 27% were demersal and about 21% pelagic. A total of 307 myctophids, 69 clupeids and 61 gadid post-larvae dominated the community. Benthosema glaciale (mean 6.1 mm SL) was the most abundant species (21.6%), the most frequent in the samples (28.8%), and dominant in the whole study area (mean 1.4 ind/100 m³). Particular attention was given to the horizontal and vertical distribution and abundance of the three dominant post-larval species: Benthosema glaciale, Sprattus sprattus sprattus and Notoscopelus elongatus. The Pearson coefficient (R = 0.734) showed a high correlation between total zooplankton and fish larval assemblages in terms of spatial distribution abundance values. Regarding the vertical distribution of fish larvae, Sorensen’s index (S = 0.69) showed that fish larvae and total zooplankton abundance peaks co-occurred along the water column.     

Composition,size, and biomass of zooplankton in large productive Florida lakes

Crustacean zooplankton data were compiled from long-term observational studies at seven large shallow Florida lakes, to determine whether there are general characteristics in regard to species composition, body size, and biomass. In particular, we examined whether patterns in body size and species richness fit empirical models developed by Stanley Dodson. The lakes included range in size from 125 to 1730 km² and encompass mesotrophic to hyper-eutrophic conditions. We found that zooplankton biomass was strongly dominated by one species of calanoid copepod—Arctodiaptomus dorsalis. Large daphnids were absent, and Cladocera assemblages were dominated by small taxa such as Ceriodaphnia, Chydorus, and Eubosmina. The total number of species of pelagic cladocerans (8–12) was consistent with Dodson’s predictions based on lake area. The average size of crustacean zooplankton in Florida lakes is small in comparison with temperate communities. A. dorsalis is the smallest calanoid copepod in North America, and the mean length of Cladocera (0.6 mm) is consistent with Dodson’s results that size decreases from temperate to tropical zones. Total biomass of crustacean zooplankton was very low, ratios of zooplankton to phytoplankton biomass (0.01–0.1) are among the lowest reported in the literature, and the zooplankton displayed short-lasting early spring peaks in biomass. Cladocera were almost entirely absent in spring and summer. Factors known to occur in Florida lakes, which appear to explain these characteristics of biomass, include intense fish predation and high summer water temperature.     

Feeding ecology of two sillaginid fishes and trophic interrelations with other co-existing species in the southern part of South China Sea

Feeding ecology of Sillago sihama and Sillago ingenuua and interrelationship with other 15 fish species, including Stolephorus commersonii, Escualosa thoracata, Lutjanus russelli, Terapon puta, Ambassis kopsii, Ambassis vachellii, Dendrophysa russelli, Gerres filamentosus, Leiognathus decorus, Leiognathus splendens, Leiognathus equulus, Thryssa hamiltonii, Carangoides praeustus, Lethrinus lentjan, and Pomadasys maculatum, utilizing coastal waters off the southern part of the South China Sea were investigated. Dietary contents from 4308 guts of those species collected during July 2003–May 2005 were examined. They were significantly grouped, based on similarity coefficient, into five feeding clusters (P < 0.01). Of these, most of the diets were dominated by calanoid copepods, shrimps, polychaetes and gammarid amphipods. Comparative study on two sillaginid species showed that both Sillago sihama and Sillago ingenuua were carnivores fed mainly on polychaetes and other benthic organisms. Dietary ontogenetic change of Sillago sihama was very distinctive where small sized classes started feeding mainly on calanoid copepods before changing almost completely to polychaetes. The two species applied both sharing and partitioning strategies for food, depending on size classes, to live with each other.     

Tidal variation in bacteria,phytoplankton, zooplankton,mysids, fish and suspended particulate matter in the turbidity zone of the Elbe estuary; Interrelationships and causes

In June 1992, an extensive investigation programme was carried out in the turbidity zone of the Elbe river. Special attention was paid to salinity and suspended particulate matter (SPM) and their influence on bacteria, phytoplankton, zooplankton, mysids and fish. SPM was separated into three fractions of different settling velocities. Mean settling velocity (w_s) was 0.05 cm s⁻¹. The major part of SPM belonged to the slow settling fraction (w_s<0.02 cm s⁻¹). Bacterial exoenzymatic activity showed a positive correlation with SPM and chlorophyll-a content, and also to total dissolved free amino acids. Phytoplankton biomass reached maximum values of 5.7 μg chlorophyll-a l⁻¹ at ebb tide. Chlorophyll-a correlated negatively with salinity, indicating riverine input of phytoplankton. A positive correlation was found between chlorophyll-a and dissolved oxygen. Abundance of zooplankton species and their developmental stages varied over the tidal cycles; abundance of cirriped larvae and copepodite stages of the dominant speciesEurytemora affinis (Copepoda, Crustacea) was positively correlated with salinity. Individual filtering rates (IFR) ofEurytemora affinis were negatively affected by the SPM content of the water. Maximum IFR for adults was 7.2 ml h⁻¹. Community grazing reached maximum rates of 30.3 ml l⁻¹ h⁻¹ (i.e. 72.7% d⁻¹). The dominant mysidNeomysis integer showed maximum abundance at night, possibly resulting from diel vertical migration. Abundance ofN. integer was positively correlated with SPM content. The fish community, consisting of 17 fish species, was characterised by high densities of smelt (Osmerus eperlanus). A positive correlation was found between salinity and abundance of typical marine fish species, such as sprat (Sprattus sprattus). Similar temporal variation of abundance of smelt, sprat, andEurytemora copepodites indicates processes of habitat preference of these planktivorous fish in relation to optimal food supply. SPM was the factor controlling both distribution of organisms and the turnover of nutrients. Salinity only was important for the distribution of organisms.     

Effects of nutrient recycling by zooplankton and fish on phytoplankton communities

In laboratory experiments we tested the hypothesis that nutrients supplied by fish and zooplankton affect the structure and dynamics of phytoplankton communities. As expected from their body size differences, fish released nutrients at lower mass-specific rates than Daphnia. On average, these consumers released nutrients at similar N:P ratios, although the ratios released by Daphnia were more variable than those released by fish. Nutrient supply by both fish and Daphnia reduced species richness and diversity of phytoplankton communities and increased algal biomass and dominance. However, nutrient recycling by fish supported a more diverse phytoplankton community than nutrient recycling by Daphnia. We conclude that nutrient recycling by zooplankton and fish have different effects on phytoplankton community structure due to differences in the quality of nutrients released. Received: 21 December 1998 / Accepted: 31 May 1999     

Trophic interrelationships amongst cichlid fishes in a tropical African reservoir (Lake Chivero,Zimbabwe)

The diet of seven cichlid species of Lake Chivero, Zimbabwe, was investigated from October 2004 to April 2005 to determine the degree of dietary overlap and interspecific competition. These fish could be separated into four groups. Two microphages, Oreochromis niloticus and O. macrochir, fed mostly on blue-green algae and their diets, in all size classes, overlapped almost completely. This suggested a high degree of competition between the two species and may account for the fact that O. niloticus, introduced into the lake in the 1980s is now the dominant species. There was also a significant dietary overlap between the two macrophages Tilapia sparrmanii and T. rendalli, which were macrophages with macrophytes as a significant component of their diet. Interspecific competition between these two species may account for the increase in the abundance of T. sparrmanii and a decrease in the numbers of T. rendalli over the last two decades. The diet of two small omnivores Pharyngochromis acuticeps and Pseudocrenilabrus philander consisted of algae, zooplankton, plant material, fish and detritus and also overlapped extensively, but these species probably avoid competition by living in different habitats; P. philander is usually associated with vegetation while P. acuticeps tends to live in more open waters. The last species, Serranochromis robustus, was largely piscivorous. There were no ontogenetic variations in diet, apart from P. acuticeps where small fish fed mainly on zooplankton becoming omnivorous in larger ones, and S. robustus where the diet shifted from zooplankton in specimens <50 mm SL to fish in larger size classes. Data available from the 1960s and 1970s indicate that the diets of some species have changed with invertebrates such as chironomid larvae being previously more important in all diets, while most species now take a higher proportion of algae. This reflects changes in the lake, which is eutrophic and has experienced severe algal blooms since 1960. Handling editor: C. Strumbauer     

Mesozooplankton community in the Bay of Bengal (India): spatial variability during the summer monsoon

This study addresses the spatial variability in mesozooplankton biomass and composition in the Central and Western Bay of Bengal (India) during the summer monsoon season of 2001. Perennially warmer sea surface temperatures (>28°C), stratified top layer (sea surface salinity, 28–33 psu), high turbidity, and low nutrient concentrations due to weak/null upwelling and light limitation make the Bay of Bengal a region of low primary productivity. Despite this, mesozooplankton biomass values, i.e. 2.9–104 mg C m⁻³ in the Central Bay and 1.3–31 mg C m⁻³ in the Western Bay, observed in the mixed layer (2–51 m) during the summer monsoon were in the same range as reported from the more productive Arabian Sea. Mesozooplankton biomass was five times and density 18 times greater at stations with signatures of cold-core eddies, causing a higher spatial heterogeneity in zooplankton distribution. Among the 27 taxonomic groups recorded during the season, Copepoda was the most abundant group in all samples followed by Chaetognatha. The dominant order of Copepoda, Calanoida, was represented by 132 species in a total of 163 species recorded. Oncaea venusta was the key copepod species in the Bay. In the Central Bay, the predominant copepod species were carnivorous/omnivorous vis-a-vis mostly herbivores in the Western Bay. Pleuromamma indica increased to its maximum abundance at 18°N in the Central Bay, coinciding with the lowest dissolved oxygen concentrations. The Central Bay had higher mesozooplankton biomass, copepod species richness and diversity than in the Western Bay. Although zooplankton biomass and densities were greater at the eddy stations, correlation between zooplankton and chl a was not statistically significant. It appears that the grazer mesozooplankton rapidly utilize the enhanced phytoplankton production in cold-core eddies.     

Planktonic and symbiotic organisms in nutrition of coralobiont fish

Nutrition of coral reef fish closely associated with the colonies of scleractinian corals Acropora spp. (South China Sea, Gulf of Nha Trang) was investigated. Parabionts Dascyllus reticulatus and Chromis caeruleus (Pomacentridae) feed mainly on zooplankton in the water column above the hosts’ colony, and inbionts Gobiodon quinquestrgatus (Gobiidae) rarely leave a host colony and feed generally on coral reef invertebrates. A high part of secondary nutrition objects (on average 19–29%) that indicates deficiency in main food were noted in both categories of fish. Food of all three fish species contains parasites (from 19% in C. caeruleus and up to 25% in G. quinquestrigatus) whose significant part is represented by parasites of coral (approximately 7% in D. reticulatus and C. caeruleus and 15% in G. quinquestrigatus). In addition, larvae of parasites of Isopoda and Facetotecta that are fed by fish in the water column were found. Mucus from the surface of host coral which was recognized on the basis of a high number of nematocysts in fish stomachs was found in food of G. quinquestrigatus for the first time. Positive and negative effects of symbiosis of fish and host-coral were discussed. A commensal character of symbiosis was supposed.     

On non-Eltonian methods of hunting Cladocera,or impacts of the introduction of planktivorous fish on zooplankton composition and clear-water phase occurrence in a Mediterranean reservoir

Among the topics covered by Hutchinson’s Santa Rosalia article, the question of the shortening and lengthening of food webs occupies a central role. As Hutchinson realized, at the time scales of ecological studies, the impact of invader species on established food webs is the fastest shortcut to the shortening or lengthening of the food webs. The construction of thousands of dams in Spain during the last century has offered ecologists a good opportunity to test the effects of invader fish species on the plankton dynamics of these systems. In this article, a series of data related to the food web structure of Sau Reservoir is analyzed for the period 1997–2005. Parameters such as Secchi depth and chlorophyll concentration, as well as abundance and size structure of zooplankton, have been matched to the zooplankton dynamics in the reservoir. Most of the changes detected within this period are attributed to the introduction of zooplanktivorous fish in the reservoir. The Secchi depth measurements have showed a progressive diminution in the clear-water phase during recent years. These changes have been related to the decrease in the abundance of Daphnia and to the reduction of the size of zooplankton, which help to explain concomitant increases in the chlorophyll concentration in the same period. Other observed changes in the composition of the zooplankton community have been the substitution of Daphnia by Bosmina and the increase in the abundance of rotifers. Thus, the annual average abundance of Bosmina in 1997 was 70% of cladocerans, while in 2005 it reached 98%. In parallel, the percentage occurrence of individual rotifers was 40% of total zooplankton numbers but had risen to 85% at the end of the period. All these changes are attributed to the artificial expansion of the food web through stocking of the reservoir with zooplanktivorous fish (Rutilus rutilus and Alburnus alburnus). This study improves our understanding of the trophic relationships in the food web prior to the introduction of the fish.     

Ontogenetic habitat shifts by Galaxias gracilis (Galaxiidae) between the littoral and limnetic zones of Lake Kanono, New Zealand

Synopsis Diel and spatial differences in distribution were determined for the larvae, juveniles, and adults of Galaxias gracilis (Galaxiidae) in a New Zealand dune lake during summer months. Larvae (mostly 10–25 mm TL) and juveniles (25–40 mm TL) inhabited shallow (0–3 m) waters of the limnetic zone and fed predominantly on two limnetic zooplankton species; Bosmina meridionalis and a calanoid copepod. At about 40 mm TL, fish moved from the limnetic to the littoral zone and expanded dietary breadth from two to over seven main prey species, including five species of littoral invertebrates. After reaching a size of about 60 mm TL, most fish moved back offshore to the deeper waters (5–15 m) of the limnetic zone during the day, moving back to the littoral zone at night to feed on invertebrates. The selection of different intra-lacustrine habitats by the various size groups of G. gracilis, and the movements between them, are interpreted as adaptive responses to the interaction between ontogenetic changes in feeding requirements and predation risk.