Seasonal changes in fish catch and environmental variables in a large Tropical Lake,Volta, Ghana

Fish species assemblage and selected environmental variables were monitored in Lake Volta from September 2014 to August 2016 to determine seasonal variability in species composition, catch and environmental variables that determine the structure of the fish community. A total of 1,557 individual fish belonging to 41 species, 25 genera and 13 families were recorded. The important fish species with respect to frequency of occurrence, abundance and weight, respectively, were as follows: Chrysichthys (100%; 43.03%; 17.93%), tilapias (100%; 28.97%; 17.86%), Alestes (100%; 14.13%; 32.10%) and Bagrus (91.67%; 5.65%; 12.80%) in that order. The composition of fish species and their temporal variation in experimental catches were similar to that of commercial catches. There were no significant differences (p > 0.05) in species abundance, weight and diversity indices between the dry and wet seasons. The modal class of length frequency distribution of the dominant species in the catch, Chrysichthys, reduced from 125 mm in 2006 to 95 mm in the current study indicating overfishing. Environmental variables considered showed little variation and within optimal ranges for fish survival and did not differ significantly between seasons. Canonical Correspondence Analysis showed that environmental variables explained 43.30% of the variation in species abundance with Lake water level, nitrite‐nitrogen and total dissolved solids being the main environmental factors influencing the structure of the fish community.     

Relationships between the spread of Caulerpa filiformis and fish communities on temperate rocky reefs

The previously sub‐dominant native marine macrophyte Caulerpa filiformis is now dominant on many sub‐tidal rocky reefs in New South Wales (NSW), Australia and is expanding its distribution. As C. filiformis is highly chemically defended and structurally different to co‐occurring habitat‐forming macrophytes, two key attributes that govern fish assemblages, we hypothesized that fish assemblages, particularly herbivorous fishes, would be different at sites where C. filiformis occurred from where it was previously absent and within sites, fish community structure would be correlated to the cover of C. filiformis. We investigated these hypotheses by determining reef‐associated fish assemblage attributes (assemblage structure, species richness, total abundance, Shannon‐Weiner diversity, abundance of herbivorous species) along transects within sites where C. filiformis was present and absent. Surprisingly, despite large patches and very high densities of C. filiformis on the reefs we sampled, at larger spatial scales (i.e., among sites) no fish assemblage metrics differed between sites with large stands of C. filiformis and sites without the alga. Moreover the abundance of one dominant herbivore, the rock cale Aplodactylus lophodon, was greater at sites within large beds of C. filiformis. At smaller spatial scales, however, i.e. within sites where C. filiformis was present, fish assemblages did vary as a function of C. filiformis cover along transects, although this was not consistent across sampling times. Overall, our results suggest that the potential effects of the spread of this alga on faunal communities warrants further investigation.     

Spatial and temporal variations and assemblage structure of fish species in Beymelek Lagoon,Turkey

Spatial and temporal variations and assemblage structure of fish species were investigated in Beymelek Lagoon, on the south‐western Mediterranean coast of Turkey. A total of 3956 fish, mainly juveniles, from 24 species representing 11 families were sampled by gillnets and trammel nets from February 2006 to January 2007. Twelve of the species were marine straddlers, 11 were marine‐estuarine‐dependent and one was catadromous. Numerical contribution of marine‐estuarine‐dependent species to the total fish abundance was 86.2%, while marine straddlers contributed only 13.8%. Despite the higher number of species, the numerical abundance of marine straddlers was lower than the abundance of marine‐estuarine‐dependent species. The contribution of catadromous species by number was only 0.02%. The assemblage was taxonomically dominated by Sparidae (seven species, 51.3%), Mugilidae (five species, 36.0%), and Clupeidae (one species, 10.3%). Among sparids, Sparus aurata, Diplodus sargus and Lithognathus mormyrus contributed 38.0, 7.2 and 4.6% to the total catch, respectively. The most abundant mugilid species was Liza saliens with 25.2%, followed by Chelon labrosus 5.2%, and Liza aurata 4.0%. Contribution by the other mugilid species to total catch was quite low. The Clupeidae (10.3%) were represented by Sardinella maderensis with 9.1% of the total catch. While the upper reaches of Beymelek Lagoon were dominated by Sparus aurata and Liza saliens, dominant in the lower reaches were Sardinella maderensis, Sparus aurata, Diplodus sargus, Lithognathus mormyrus and Liza saliens. Sparids were generally caught from mid‐summer to mid‐winter while mugilids were caught throughout the year. Clupeids occurred mainly from autumn to spring.     

Shifts in the composition and distribution of Pacific Arctic larval fish assemblages in response to rapid ecosystem change

The Pacific Arctic marine ecosystem has undergone rapid changes in recent years due to ocean warming, sea ice loss, and increased northward transport of Pacific-origin waters into the Arctic. These climate-mediated changes have been linked to range shifts of juvenile and adult subarctic (boreal) and Arctic fish populations, though it is unclear whether distributional changes are also occurring during the early life stages. We analyzed larval fish abundance and distribution data sampled in late summer from 2010 to 2019 in two interconnected Pacific Arctic ecosystems: the northern Bering Sea and Chukchi Sea, to determine whether recent warming and loss of sea ice has restricted habitat for Arctic species and altered larval fish assemblage composition from Arctic- to boreal-associated taxa. Multivariate analyses revealed the presence of three distinct multi-species assemblages across all years: (1) a boreal assemblage dominated by yellowfin sole (Limanda aspera), capelin (Mallotus catervarius), and walleye pollock (Gadus chalcogrammus); (2) an Arctic assemblage composed of Arctic cod (Boreogadus saida) and other common Arctic species; and (3) a mixed assemblage composed of the dominant species from the other two assemblages. We found that the wind- and current-driven northward advection of warmer, subarctic waters and the unprecedented low-ice conditions observed in the northern Bering and Chukchi seas beginning in 2017 and persisting into 2018 and 2019 have precipitated community-wide shifts, with the boreal larval fish assemblage expanding northward and offshore and the Arctic assemblage retreating poleward. We conclude that Arctic warming is most significantly driving changes in abundance at the leading and trailing edges of the Chukchi Sea larval fish community as boreal species increase in abundance and Arctic species decline. Our analyses document how quickly larval fish assemblages respond to environmental change and reveal that the impacts of Arctic borealization on fish community composition spans multiple life stages over large spatial scales.     

Spatial variation in relative abundance of a widespread, numerically dominant fish species and its effect on fish assemblage structure

Collections of fish assemblages from streams in the midwestern United States were used to examine assemblage-level effects of spatial variation in relative abundance of the red shiner, Cyprinella lutrensis, a widespread and highly abundant minnow species. This species has been widely introduced outside its native range and is suspected to have impacted local assemblages where it has become established. Given its overall dominance of midwest fish assemblages, and its suspected impact on assemblage structure, we asked if structure of the residual fish assemblages (red shiners excluded) was a function of the relative abundance of red shiners throughout the native range of C. lutrensis in the USA. Although red shiner ranked first in abundance in half of the assemblages and numerically dominated 28% of the assemblages, red shiner relative abundance in an assemblage had no detectable effect on richness, diversity, evenness, or complexity of other (residual) species in the assemblage. Relative abundance of red shiners did have a positive effect on the abundance of benthic minnows in the residual assemblage, but not on water column minnows that are ecologically most like red shiners. Environmental factors did not explain a significant amount of the variation in relative abundance of red shiners, but did explain some variation in residual assemblage structure. Although widespread and numerically dominant at many localities, red shiners do not appear to have a strong impact on local fish assemblage structure within their native range. This is in sharp contrast to the reported negative effects of red shiners on fish assemblages where they have been introduced outside their native range.     

Temporal and spatial patterns in littoral-zone fish assemblages of a reservoir (Lake Texoma,Oklahoma-Texas,U.S.A.)

Synopsis We sampled the littoral-zone fish fauna of Lake Texoma reservoir by electrofishing from January through December 1986 to examine species abundance, species associations and assemblage structure. Although total fish abundance differed significantly across seasons, only one common species (Dorosoma cepedianum) exhibited significant seasonal movement into or out of the littoral zone. Overall littoral-zone assemblage structure (based on rank order of species abundance) was concordant across seasons and habitat types, (vegetation, wood, open). However, within individual seasons and habitat types, assemblage structure was likely influenced by temporal and spatial differences in habitat availability and physicochemical conditions. Associations characteristic of species in natural aquatic environments were not well developed among species in this partly artificial, evolutionarily short-lived reservoir assemblage. Conditions related to water-level fluctuation appeared to deter the formation of persistent species associations and assemblage structure, especially in vegetation and open littoral zone habitats of this multi-purpose reservoir.Senior author     

Seasonal dynamics of fish assemblage structures in a surf zone on an exposed sandy beach in Japan

Seasonal dynamics in fish assemblage structure, comprising postlarval to early adult stages, in the surf zone on an exposed sandy beach on the Kashimanada coast were studied over a 2-year period. A total of 32 species and two higher group taxa were found. In terms of abundance, Salangichthys ishikawae, Mugil cephalus cephalus, Plecoglossus altivelis altivelis, and Engraulis japonicus were the four most dominant species. The results of cluster analysis and canonical correspondence analysis using 12 dominant species in terms of abundance revealed that water temperature and wind factor (wind speed × wind direction) had significant independent associations on species occurrence patterns for 11 species; that is, occurrence of 11 species was significantly related to water temperature and occurrence of 7 species was related to wind factor. Two species (S. ishikawae and P. altivelis altivelis) showed clear monthly increases in size, consequently suggesting their use of the surf zone as a nursery area. In contrast, M. cephalus cephalus and E. japonicus showed few increases in standard length, indicating their use of the surf zone only in their postlarval and/or juvenile stage. It is suggested that numerous fish species use the surf zone as shelter and/or a nursery area, even in the harsh conditions of an exposed sandy beach with great wave action.     

Partitioning the diversity of riverine fish: the roles of habitat types and non-native species

1. Quantifying how biological diversity is distributed in the landscape is one of the central themes of conservation ecology. For this purpose, landscape classifications are being intensively used in conservation planning and biodiversity management, although there is still little information about their efficacy. 2. I used data from 158 running water sites in Hungary to examine the contribution of six a priori established habitat types to regional level diversity of fish assemblages. Three community measures [species richness, diversity (Shannon, Simpson indices), assemblage composition] were examined at two assemblage levels (entire assemblage, the native assemblage). The relative role of non‐native species was quantified to examine their contribution to patterns in diversity in this strongly human influenced landscape. 3. Additive diversity partitioning revealed the primary importance of beta diversity (i.e. among‐site factors) to patterns in species richness. Landscape‐scale patterns in species richness were best explained by between‐habitat type (beta₂: 41.2%), followed by within‐habitat type (beta₁: 37.7%) and finally within‐site (alpha: 21.1%) diversity. Diversity indices showed patterns different from species richness, indicating the importance of relative abundance distributions on the results. Exclusion of non‐natives from the analysis gave similar results to the entire‐assemblage level analysis. 4. Canonical analysis of principal coordinates, complemented with indicator species analysis justified the separation of fish assemblages among the habitat types, although classification error was high. Multivariate dispersion, a measure of compositional beta diversity, showed significant differences among the habitat types. Contrary to species diversity (i.e. richness, diversity indices), patterns in compositional diversity were strongly influenced by the exclusion of non‐natives from the analyses. 5. This study is the first to quantify how running water habitat types contribute to fish diversity at the landscape scale and how non‐native species influence this pattern. These results on riverine fish assemblages support the hypothesis that environmental variability (i.e. the diversity of habitat types) is an indication of biodiversity and can be used in large‐scale conservation designs. The study emphasises the joint application of additive diversity partitioning and multivariate statistics when exploring the contribution of landscape components to the overall biodiversity of the landscape mosaic.     

Reef habitats and associated sessile-benthic and fish assemblages across a euphotic–mesophotic depth gradient in Isla Desecheo, Puerto Rico

Quantitative surveys of sessile benthos and fish populations associated with reef habitats across a 15–50 m depth gradient were performed by direct diver observations using rebreathers at Isla Desecheo, Puerto Rico. Statistically significant differences between depths were found for total live coral, total coral species, total benthic algae, total sponges and abiotic cover. Live coral cover was higher at the mid-shelf (20 m) and shelf-edge (25 m) stations, whereas benthic algae and sponges were the dominant sessile-benthic assemblage at mesophotic stations below 25 m. Marked shifts in the community structure of corals and benthic algae were observed across the depth gradient. A total of 119 diurnal, non-cryptic fish species were observed across the depth gradient, including 80 species distributed among 7,841 individuals counted within belt-transects. Fish species richness was positively correlated with live coral cover. However, the relationship between total fish abundance and live coral was weak. Abundance of several numerically dominant fish species varied independently from live coral cover and appeared to be more influenced by depth and/or habitat type. Statistically significant differences in the rank order of abundance of fish species at euphotic vs mesophotic stations were detected. A small assemblage of reef fishes that included the cherubfish, Centropyge argi, sunshine chromis, Chromis insolata, greenblotch parrotfish, Sparisoma atomarium, yellowcheek wrasse, Halichoeres cyanocephalus, sargassum triggerfish, Xanthichthys ringens, and the longsnout butterflyfish, Chaetodon aculeatus was most abundant or only present from stations deeper than 30 m, and thus appear to be indicator species of mesophotic habitats.     

The spatial scale of homogenisation and differentiation in Chihuahuan Desert fish assemblages

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Within‐lake distribution patterns of fish assemblages: the relative roles of spatial,temporal and random environmental factors in assessing fish assemblages using gillnets in a large and shallow temperate lake

In this study, the relative role of spatio‐temporal factors and associated environmental variables (water transparency and temperature) were quantified in relation to gillnet samples of fishes in a large and shallow lake (Lake Balaton, Hungary). Most of the variance (56·1%) in the relative abundance data (%) was related to the vertical segregation of fishes. This gradient substantially affected the catch per unit effort (CPUE) by number of the dominant species, the surface‐oriented bleak Alburnus alburnus and the benthic common bream Abramis brama. It also influenced total CPUE, mean fish mass and species richness and diversity. At the lake level, horizontal habitat heterogeneity (i.e. littoral v. offshore) accounted for only 8·3% of the total variance in relative abundance data, but was important in structuring the CPUE of the ruffe Gymnocephalus cernua and the pikeperch Sander lucioperca. The longitudinal environmental gradient (i.e. lake basin), year and season of sampling, water transparency and temperature had significant effects on relative abundance only at the habitat level, but were also important components of variability of CPUE in some species at the lake level. As sampling schemes need to consider the main gradients in fish assemblage distributions, the use of surface and pelagic gillnets should be more intensively incorporated in the study and monitoring of fish assemblages in shallow lakes and lake habitats.     

Fish assemblage structure in an estuary of the Atlantic Forest biodiversity hotspot (southern Brazil)

We described the fish assemblage in the estuary of the Guaraguaçu River (one of the largest tributaries of the Paranaguá Bay Estuary, located within Brazil’s Atlantic Forest Biosphere Reserve) from June 2005 to May 2006, and assessed the seasonal and spatial effects of abiotic environmental attributes on the fish assemblage structure. Despite some oscillations in salinity, the upper and lower estuaries had year-round persistent oligohaline and polyhaline conditions, respectively. Despite high species richness (55 species), the Guaraguaçu River Estuary fish community contains a few dominant taxa; 11% of the richness accounts for >60% of its density and biomass. The most abundant species (in terms of both biomass and density) was Atherinella brasiliensis. Species whose densities were most strongly associated with the upper estuary were Centropomus parallelus, Ctenogobius schufeldti, Eucinostomus melanopterus, Platanichthys platana, Trinectes paulistanus, and Eugerres brasilianus. Those whose densities were most strongly associated with the lower estuary were A. brasiliensis, Sphoeroides greeleyi, Eucinostomus argenteus, Sphoeroides testudineus, Diapterus rhombeus, and Harengula clupeola. Throughout the year, canonical correspondence analysis identified: (1) the pattern of horizontal stratification of salinity along the river as being the most important variable for explaining most of the fish fauna structure; and (2) a strong relationship between the fish fauna and the salinity gradient along the estuary. Analysis of similarity further confirmed that each estuarine zone supports a year-round persistent and relatively homogeneous fish species assemblage. Total mean density and biomass remained constant over time in each estuarine habitat, but density shifted in the most abundant species, which appears related to recruitment patterns. Such species and abundance persistence likely occurs because seasonal rainfall-induced changes in river discharge are not sufficient to significantly shift runoff and salinity and thus fish assemblage structure (species composition, density and biomass) along the estuary. Such a lack of seasonal fish fauna movement as a response to changes in river flow contrasts with other estuarine systems around the world.     

Temporal and spatial variability of pelagic wild fish assemblages around Atlantic bluefin tuna Thunnus thynnus farms in the eastern Adriatic Sea

The abundance and size structure of wild fishes aggregated around the sea‐cages of two commercial Thunnus thynnus farms, including control locations, were assessed and compared over a 1 year period. The T. thynnus farms were located in the eastern Adriatic Sea, offshore of the islands of Ugljan and Bra?. Fish assemblages were evaluated through visual census using scuba at 2 month intervals at two sites within each farm. The data suggest that wild fish assemblages at the study sites differed greatly; 20 species occurred at the Ugljan farm and 17 at the Bra? farm, while only seven species were observed at the control locations. The abundance and diversity of wild fish assemblages were greater at the farms in comparison to control locations. The most abundant families were Sparidae and Belonidae (>80% of aggregated fishes). At both farms, the abundance and diversity of wild fishes were highest during summer, while diversity was lowest in winter and was mainly characterized by schools of bogue Boops boops and garfish Belone belone. Variability was also detected in spatial assemblages between farms; B. boops and B. belone were the most abundant species for the overall study at the Bra? farm, while B. belone and saddled bream Oblada melanura were the most abundant at the Ugljan farm. The settlement also played a significant role in farm‐associated fish assemblages, as both juveniles and advanced juveniles were common residents at farms. The majority of species which settled at the farms belonged to the sparids. Results indicate that aggregations of wild fishes at T. thynnus farms are persistent year‐round, though the assemblage compositions and size structures of dominant species vary in respect to location and season.     

The seasonality of zooplanktivorous fish in an African reservoir (Gebel Aulia Reservoir,White Nile,Sudan)

The fish fauna of Gebel Aulia reservoir (White Nile) consists of 52 species withAlestes baremoze Joannis andBrycinus nurse Rüppell dominating in numbers and biomass. The seasonality and condition ofA. baremoze, A. dentex (L.) andB. nurse are described in relation to various limnological factors in the reservoir, close to the dam. The zooplankton assemblage is small-sized, consists mainly of small Cladocera and shows striking seasonality. The hydrological regime, temperature and the grazing effect of fish are the major factors influencing its development.The three species belong to a eurytopic but basically riverine fish guild. They show a seasonal cycle of abundance with high numbers at periods of water storage (October–April) and low numbers during riverine conditions. This seasonal pattern of occurrence, the size structure of the population and the maturity indices give strong evidence that no resident population is present at the study site, but upstream migrations of the immature fish occur at low water level (May–September). Return migrations of juvenile fish into the area at the dam proper take place from October until April.

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Zusammenfassung In der Fischfauna des Gebel Aulia Stausees am Weißen Nil im Sudan dominieren Arten der Gatung Alestes. Die quantitative Bedeutung und das jahreszeitliche Auftreten vonA. baremoze Joannis,A. dentex Cuvier andB. nurse Rüppell wurde im Zusammenhang mit dem saisonalen Zyklus der hydrologischen und limnologischen Bedingungen untersucht. Die Häufigkeit der Fische im Staubereich ist in der Stauphase (Oktober–April) hoch und in den Monaten, in denen kein Rückstau erfolgt (Mai–September), gering.Das saisonale Muster des Auftretens, die größenstruktur der Fischpopulation and die Maturitätsfaktoren im eigentlichen Staubereich keine seßhafte Population vorhanden ist. Die Fische unternehmen kurz vor der Geschlechtsreife anadrome Wanderungen, die Jungfische wandern katadrom.

     

Year-to-year variation in larval fish assemblages of the Southern North Sea

In order to test the temporal stability within and the reproducibility of larval fish assemblages between years, the larval fish assemblage at Helgoland Roads, North Sea (NE Atlantic) was quantitatively sampled almost daily from January 2003 to December 2005. The survey resulted in a total of 462 samples containing 50,632 larval fish of at least 42 taxa. In winter the larval fish assemblage was mainly dominated by larvae emerging from demersal eggs. This changed gradually to larvae hatching from pelagic eggs. Larvae from pelagic eggs dominated the ichthyoplankton assemblage in summer. A remarkably stable seasonality in terms of dominance patterns with recurring, season-specific fish assemblages was observed over the 3 years, despite substantial variation in environmental conditions such as a temperature difference of almost 20°C between summer and winter. The lesser sandeel (Ammodytes marinus), was the only species which showed significant fluctuations in abundance between the years. After removal of this species from the analysis, the dominance patterns of the remaining fish species were almost identical between years.     

Imprint of monsoonal patterns on the fish assemblage in coastal waters of south‐east India: a case study

The composition, species richness and diversity of a coastal fish assemblage from the Kalpakkam coast of south‐east India are described along with temporal distribution patterns related to seasonal fluctuations in dissolved oxygen, salinity, pH, chlorophyll‐a, phytoplankton and zooplankton species richness and density. A total of 244 fish species belonging to 21 orders, 87 families and 163 genera were recorded. The fish assemblage was dominated by reef‐associated species, followed by demersal species. The majority of the species (63%) are widely distributed in the western Indo‐Pacific as well as in the central Indo‐Pacific. Jaccard's coefficient analysis showed three distinct seasonal patterns of fish occurrence: pre‐monsoon (PrM), monsoon (M) and post‐monsoon (PoM). The maximum number of species was during the PrM period, followed by the PoM and M periods. Species occurrence analysis showed Sardinella longiceps to be dominant during PrM and M periods, Leiognathus dussumieri during the M period and Secutor insidiator and Secutor ruconius during the M and PoM periods. Canonical correspondence analysis indicated that salinity and rainfall were the two most influential environmental factors strongly correlated with temporal variation in the fish assemblage. The physico‐chemical conditions, in combination with factors such as greater food availability and shelter, might control the seasonal local distribution of the ichthyofauna in these Indian coastal waters.     

Fish biology and fisheries ecology of the Salton Sea, California

Studies of the fisheries ecology and fish biology of the Salton Sea, California, were conducted in 1999 and 2000 using 50 m gill nets in river, nearshore, pelagic, and estuarine areas. Total lengths and weights were measured for all fish captured, and sub-samples were dissected for gonad weights and aging. Ten fish species were captured of which a hybrid tilapia (Oreochromis mossambicusx O. urolepis hornorum) was dominant by number and weight. Nearshore and estuarine areas had highest catch rates (over 11 kg h^–1 net^–1 for tilapia). Rivers were richest in the number of species (6 of 10 species were exclusively riverine), but lowest in fish abundance. Orangemouth corvina (Cynoscion xanthulus), bairdiella (Bairdiella icistia), sargo (Anisotremus davidsoni), and tilapia grew faster, but had shorter life spans than conspecifics elsewhere and Salton Sea conspecifics of 50 years ago. Reproduction occurred mostly in the nearshore and estuarine areas. Onset of reproduction of bairdiella and sargo was in the spring and extended through the beginning of summer. Reproduction of orangemouth corvina started in the summer and of tilapia in the spring. Reproduction of orangemouth corvina and tilapia extended through the fall. Gender ratios of tilapia were skewed toward males in all areas, except the rivers, where females predominated. All four species aggregated along the nearshore and estuarine areas in the summer when dissolved oxygen in the pelagic area was limited. Any restoration alternative for the Salton Sea should consider areas close to shore as primary areas for fish reproduction and survival.     

Temporal dynamics of gastropod fauna on subtidal sandy sediments of the Ensenada de Baiona (NW Iberian Peninsula)

The temporal variation of the gastropod fauna inhabiting sandy sediments of the Ensenada de Baiona (Galicia, Spain) was studied at three subtidal sites from February 1996 to February 1997 by means of quantitative sampling. A total of 5,463 individuals representing 51 gastropod species and 22 families were found. The family Pyramidellidae was the most diverse in number of species (11 species), followed by Rissoidae and Trochidae (4 species each). The dogwhelk, Nassarius reticulatus, and the rissoid snail, Rissoa parva, were the numerically dominant species at the three studied sites; those and other abundant species showed their greatest densities by the end of summer and the beginning of autumn. In general, univariate measures of the assemblage (number of species, abundance, diversity and evenness) showed variations through time; greater values were recorded between summer and autumn depending on the site. Multivariate analyses done on abundance data showed certain seasonality in the evolution of the assemblage as expected for shallow subtidal sandy sediments at temperate latitudes; those seasonal changes were mostly related to variations in abundance of numerically dominant species. Although the measured sedimentary variables did not show significant correlations with faunal univariate parameters, sediment heterogeneity due to the presence of mats of Zostera marina L. and shells of dead bivalves might explain the differences in composition of the gastropod assemblage among sampling sites.     

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.     

Seasonal and interannual variation in fish assemblages of northern temperate rocky subtidal habitats

Fish assemblages on two inshore rocky subtidal sites on the west coast of Scotland, were studied using diver visual surveys on a monthly basis between September 1995 and December 1999. A total of 17 689 fishes and 26 species were recorded from the two sites, Saulmore Point (056°27'N; 005°24'W) near Oban and Davy's Rock (055°46'N; 004°53'W) on the Isle of Great Cumbrae. The gobiid Thorogobius eppiphiatus , dominated the Saulmore Point site; six fish species accounted for >93% of total abundance at that site. At Davy's Rock four species contributed at least 93% of total fish abundance, and the dominant species was the labrid Ctenolabrus rupestris . Total abundance of the dominant species displayed a clear seasonal trend, and this was significantly related to recorded daily average seawater temperature. A maximum abundance of 4.9 fishes m⁻² was recorded in November 1998 at Davy's Rock and 2.5 fishes m⁻² at Saulmore Point in October 1998. Multivariate analysis indicated a degree of variation in assemblage structure between winter and summer at both sites. A number of species showed some degree of interannual variation, in particular the gobiid Gobiusculus flavescens whose abundance increased by over 300 times over a 5 month period in 1998. Correlation analysis showed that variation in annual winter seawater temperature could act as an indicator of interannual variation in abundance of some of the dominant species utilizing rocky subtidal habitats.