Fish assemblages in Tanzanian mangrove creek systems influenced by solar salt farm constructions

Deforestation of mangrove forests is common occurrence worldwide. We examined fish assemblage composition in three mangrove creek systems in Tanzania (East Africa), including two creeks where the upper parts were partly clear-cut of mangrove forest due to the construction of solar salt farms, and one creek with undisturbed mangrove forest. Fish were caught monthly for one year using a seine net (each haul covering 170 m²) within three locations in each creek, i.e. at the upper, intermediate and lower reaches. Density, biomass and species number of fish were lower in the upper deforested sites compared to the mangrove-fringed sites at the intermediate and lower parts in the two creeks affected by deforestation, whereas there were no differences among the three sites in the undisturbed mangrove creek system. In addition, multivariate analyses showed that the structure of fish assemblages varied between forested and clear-cut sites within the two disturbed creeks, but not within the undisturbed creek. Across the season, we found no significant differences except for a tendency of a minor increase in fish densities during the rainy season. At least 75% of the fishes were juveniles and of commercial interest for coastal fisheries and/or aquaculture. Mugil cephalus, Gerres oyena and Chanos chanos were the most abundant species in the forested sites. The dominant species in the clear-cut areas were M. cephalus and Elops machnata, which were both found in relatively low abundances compared to the undisturbed areas. The conversion of mangrove forests into solar salt farms not only altered fish assemblage composition, but also water and sediment conditions. In comparison with undisturbed areas, the clear-cut sites showed higher salinity, water temperature as well as organic matter and chlorophyll a in the sediments. Our results suggest that mangrove habitat loss and changes in environmental conditions caused by salt farm developments will decrease fish densities, biomass and species numbers as well as alter the overall fish assemblage composition in the salt farm area but not downstream in the creek.     

Day–night variations in the demersal nekton assemblage on the Mediterranean shelf-break

The composition of the demersal nekton assemblage inhabiting the Mediterranean shelf-break of west-central Italy was examined to investigate the diel variation in species composition and abundance. Fish were obtained during four 24-h sampling periods that represented the four annual seasons. Thirty-two hauls (eight in each season) were conducted in a shelf-break area at 140–160 m depth. The density of fish, crustaceans, and decapods exhibited differences on short- and long-term time scales: multidimensional scaling suggested that season and light intensity may be important factors influencing fish assemblage composition.At the diel scale two main assemblages separating nocturnal and diurnal hauls were identified. The typical assemblage structure of shelf-break bottoms was represented during daylight by species such as Merluccius merluccius, Capros aper, Serranus hepatus, Macroramphosus scolopax and Trisopterus minutus capelanus. At night, the assemblage showed a drastic change due to the increase of small nektobenthic species, Argentina sphyraena and Glossanodon leioglossus, together with cave-dwelling fish (Gnathophis mystax, Chlopsis bicolor, Nettastoma melanurum) and nektobenthic crustaceans (i.e., Parapenaeus longirostris and Solenocera membranacea).The above-mentioned differences in species composition may be related to changes in vertical distribution for trophic needs (M. merluccius) and/or in the shoal structure (A. sphyraena and G. leioglossus). The lack of data on the pattern of trawl catchability of Mediterranean species during the day may lead to an inaccurate estimation of species abundance when only diurnal trawl survey data are used.     

Spatial pattern of coastal fish assemblages in different habitats in North‐western Mediterranean

Variability of fish assemblages across habitat structures can depend on spatial scales. A hierarchical sampling design was used to assess the spatial variability of temperate fish assemblages in different habitats and at multiple scales. Underwater visual censuses were carried out along the coasts of Elba Island (NW Mediterranean) on Posidonia oceanica beds, rocky algal reefs and sandy habitat at three spatial scales, namely tens of metres (individual replicates), hundreds of metres (sites) and tens of kilometres (locations). At the assemblage level, there was a clear relationship between fish and habitat type and the observed habitat‐related differences were largely dependent on species identity. Fish assemblages on P. oceanica beds and rocky reefs shared a high number of species, whereas overlap with sandy assemblages was negligible. Multivariate analyses revealed significant differences in fish assemblages among habitats, although there was also a significant habitat × site interaction. These differences relied mainly upon assemblage composition and species richness. Assemblages on rocky reefs and P. oceanica meadows usually harboured a higher number of species and individuals compared with sandy assemblages. Nevertheless, the patterns of habitat‐related differences in species richness and, especially, in the total number of fish, changed significantly from site to site. Eight species showed significant differences over habitats, but they were not consistent due to the interaction of habitat with site. Predictability of fish at both assemblage and population levels decreased with the scale of observation, and the spatial pattern of fish observed at the smallest scale was likely dependent on factors other than habitat type.     

Do Mediterranean fish assemblages associated with marine caves and rocky cliffs differ?

Fish assemblages associated with marine caves and rocky cliffs were investigated in the Salento Peninsula (SE Italy, Mediterranean Sea) by using visual census methods. Sampling was done at three sites, each of which included 4 habitat types: the external and the internal portions of caves, and shallow and deep rocky cliffs. 10 and 13 species were found exclusively inside cave habitats (e.g. Corcyrogobius lichtensteini, Thorogobius ephippiatus and Grammonus ater) or in rocky cliffs (e.g. Diplodus annularis, Sarpa salpa, Sparisoma cretense, Spondyliosoma cantharus), respectively. The four habitat types shared 10 species, and the external portions of the caves shared the most species (both with the internal cave portions and the external rocky cliffs). As a general rule, dissimilarity in the fish assemblage structure between habitats was far greater than dissimilarity between sites. Apogon imberbis (mostly associated with caves) and Chromis chromis (typifying rocky cliffs, mainly the deep ones) mostly contributed to dissimilarities between caves and rocky cliffs. Apogon imberbis (mostly associated with internal caves) and Coris julis (mainly associated to external cave portions) contributed strongly to dissimilarities between internal and external cave portions, while C. chromis, Symphodus mediterraneus and C. julis (associated with the deeper cliffs) and Thalassoma pavo (mostly present in shallow cliffs) differentiated deep and shallow cliffs. Diplodus vulgaris, Oblada melanura and Mullus surmuletus showed a marked increase in density during the cold season in the caves. These results show that fish assemblages associated with rocky reefs rich in marine caves (in terms of relative densities, species composition, species richness, exclusive species and presence of juveniles of some valuable species) may be affected by the peculiar ecological conditions within caves, which could provide additional resources for fishes (e.g. food availability, refuge against predators, sand patches within a rocky matrix) compared to rocky reefs without caves. These results suggest that stretches of rocky coasts rich in marine caves should be considered within management/conservation programs (e.g. when establishing Marine Protected Areas).     

Temporal and spatial distributions of larval fish assemblages in the Lima estuary (Portugal)

The Lima estuary (NW Portugal) is at the end of an international watershed, whose potential role as a spawning and nursery habitat for local fish populations has not been previously examined. To address this knowledge gap, fortnightly plankton surveys were conducted between April 2002 and April 2004. A total of 12,903 larvae, belonging to 20 families and 50 taxa were collected, with a mean abundance of 8 individuals per 100 m³. Gobiidae was the most abundant family comprising 71% of the total catch, followed by Clupeidae with 12% of the total. The top six abundant taxa (Pomatoschistus spp., Sardina pilchardus, Ammodytes tobianus, unidentified Clupeidae, Symphodus melops and Solea senegalensis) represented 91% of the total catch. Fish larvae showed a seasonal trend with abundances increasing during spring and summer. Diversity was generally low (H′ = 0.65) with high dominance of very few taxa. Near the ocean, the larval fish assemblage was more diverse due to the presence of marine species. In the lower estuary Channel zone, abundance was lower than in the two upstream salt marsh zones (North and South zones) and no statistical differences in abundance or diversity values were found within the latter zones. ANOSIM results demonstrated seasonal differences in the species composition, mainly during the second winter period which was typified by a pelagic species A. tobianus. The community in the Channel zone was more diverse in comparison with the other zones, which were highly dominated by the most abundant species. The spatial and temporal trends of the most abundant species were typical for Iberian estuaries, with the exception of the low abundance of anchovy larvae and the unusually high numbers and frequency of S. pilchardus, usually mentioned as accidental in estuarine systems. Overall results suggest that the Lima estuary larval fish assemblage has a strong seasonality and affinity to the salt marsh zones. It seems that spawning seasonality controlled the presence of temporary estuarine residents, while environmental aspects controlled the general abundance trend of the resident species.     

Diel and seasonal variability in the fish community structure of a mud‐bottom estuarine habitat in the Gulf of Mexico

We hypothesized that temporal variation in fish species composition and community structure in a low complexity habitat in the Pueblo Viejo Lagoon, Mexico, is influenced by diel light/dark cycles and tidal stage, and by seasonal changes in salinity and temperature. We collected a total of 17,661 individuals during 2‐h interval sampling over six bi‐monthly 24‐h sampling cycles representing 53 species, of which 11 (～20%) were previously unknown in the system. Diel variation indicated that significantly higher numbers of individuals and species were caught at night, whereas diversity and evenness were higher during the day. Species richness was significantly higher in July and January, whereas diversity and evenness peaked around May; both were correlated with temperature. Diel variation in species composition was influenced primarily by the light/dark cycle. Cluster analyses of each diel cycle separated fish assemblages from midday samples from those of nocturnal samples, separated by an extended wide transition period as fish moved at dawn and during the late afternoon/dusk. Significant shifts (as determined by MANOVA) in assemblage structure occurred between months. Canonical correspondence analysis showed that temperature and day/night effects were the most important environmental variables structuring the fish community. This constrained ordination also defined species with specific habitat preferences as follows: (i) diurnal, warm temperature species (mainly planktivores) (Brevoortia gunteri, Cetengraulis edentulus, Diapterus auratus, and Membras martinica); (ii) nocturnal, warm temperature species (mainly predators) (Citharichthys spilopterus, Cathorops melanopus, and Bairdiella spp.); and (iii) low temperature, diurnal species (Brevoortia patronus and Mugil curema) or those with twilight and nocturnal distributions (Anchoa mitchilli, the most numerically abundant species). Our results indicate that diel and seasonal changes in fish community structure were mainly related to day/night cycles and temperature regimes.     

Influence of summer conditions on the larval fish assemblage in the eastern coast of Tunisia (Ionian Sea,Southern Mediterranean)

The structure of the summer larval fish assemblage off the eastern coast of Tunisia and its relation to environmental conditions was studied, from ichthyoplankton samples taken during a survey conducted between 23rd June and 9th July 2008. A total of 68 larval fish taxa were identified, 52 to species level. The taxonomic composition and abundance of the larval fish assemblage showed high spatial heterogeneity. Mesoscale hydrographic features, such as eddies, seem to play an important role in the spatial distribution of fish larvae in the area, enhancing concentration and retention. The larval fish assemblage was dominated by the small pelagic species Sardinella aurita (26.6% of the total larval fish abundance), followed by Engraulis encrasicolus (22.6%), Spicara spp. (8.6%) and Mullus barbatus (6.8%). Shannon–Weaver index (H′) ranged between 0 and 2.62. The highest values were found offshore, at 95 miles east of Sousse, over depths around 250 m. The diversity was higher in this region as a result of transport by currents and retention by eddies. It has also been shown that the eastern coast of Tunisia is a spawning ground for the tuna species Auxis rochei, Thunnus thynnus and Thunnus alalunga. Larvae of mesopelagic fishes represented 5.46% of the total abundance, with Cyclothone braueri, Ceratoscopelus maderensis and Lampanyctus crocodilus being the most important species. Canonical correspondence analysis (CCA) indicated that depth was the most important environmental factor in explaining species distribution.     

Alongshore distribution and abundance of fish larvae off the coast of Kenya

Knowledge is limited on the fish larval assemblage in shallow lagoonal reefs along the Kenyan coast. Fish larvae from five lagoons, spanning 120 km on the Kenyan coast, were sampled in March 2007 and April 2008 to compare interannual spatial variations in species composition, abundance and diversity along the coast. In all, 2 644 fish larvae were sampled, comprising 26 families and 37 species in 2007 and 43 families and 73 species in 2008. The larval assemblage was dominated by Gobiidae, Blenniidae, Pomacentridae and Gerreidae during both years. Larvae hatched from non-pelagic mode of spawning constituting 92% of total numbers. Mean larval abundance (no. 100 m^?3 ± SE) along the coast ranged from 5.0 ± 1.0 to 414 ± 226, with highest densities occurring on the northern sites of Watamu (414 ± 226) and Malindi (31 ± 10). Interannual variation in larval abundance between 2007 (2.17 ± 0.3) and 2008 (2.16 ± 0.1) was not significant (p > 0.05). Shannon-Wiener species diversities between sites ranged from 1.2 ± 0.4 to 2.3 ± 0.3, with highest diversities occurring in Mombasa (2.2 ± 0.5) and Nyali (2.3 ± 0.3). In 2007, the occurrence of preflexion larvae increased northwards from Mombasa (18.2%) to Watamu (86.4%), whereas in 2008, the reverse was the case with the incidence of preflexion larvae reducing northwards from Mombasa Marine Park (76%) to Watamu Marine Park (2%). These trends indicate interannual variation in larval source sites for fish species. Correspondence analysis revealed distinct larval assemblages at sites along the coast, which varied between years.     

Trawls and cooling-water intakes as estuarine fish sampling tools: Comparisons of catch composition,trends in relative abundance,and length selectivity

Fish populations in estuaries are often monitored with traditional sampling gears such as trawls. Trawling is relatively expensive and may be hindered by environmental conditions such as tides and substrates. Power station cooling-water intake screens have been effectively used as estuarine fish sampling devices for many years, but very few quantitative comparisons of intake fish-catch characteristics with samples from other collection methods have been made. Fish collected at the cooling-water intake of a large power station in the lower Forth estuary, UK, were more similar in assemblage composition to fish caught by nearby pelagic trawling than to fish caught by Agassiz (demersal) trawling, mostly because the intake and pelagic-trawl catches were largely composed of clupeids (Sprattus sprattus and Clupea harengus). The intake catch was typified by pelagic, demersal, and benthic species, however, and was less variable than the catches made by the two trawls. Monthly trends in relative abundance correlated reasonably well between the intake and trawl samples. Fish collected at the intake tended to be significantly smaller than those collected by trawling, which was probably attributable to the intake's smaller mesh size. The study highlighted the utility of a cooling-water intake as an efficient, low-cost fish sampling device, which should be considered as an alternative to trawling as the cost of the latter increases into the future.     

Feeding ecology and ontogenic diet shifts of juvenile fish species in an inverse estuary: The Sine-Saloum,Senegal

We examined the diet of juvenile fish of four species – Eucinostomus melanopterus (Gerreidae), Ethmalosa fimbriata (Clupeidae), Monodactylus sebae (Monodactylidae), and Sarotherodon melanotheron (Cichlidae) – between their appearance on the nursery ground and the end of the recruitment season, when they began to shift to deeper parts of the estuary. Fish were collected from the Sine-Saloum, an inverse estuary in Senegal, West Africa. Sampling was conducted at six sites located along the salinity gradient. The study was conducted in the context of an inverse estuary where increasing salinity leads to the disappearance of mangroves.     

Temporal and cross-shelf distribution of ichthyoplankton in the central Cantabrian Sea

Environmental variables have been measured and sampling for ichthyoplankton has been conducted monthly, since April 2001, at three stations, located at the inner (1), middle (2) and outer (3) shelf of the central Cantabrian Sea. This paper presents the results of the study of the ichthyoplankton collected from July 2001 to June 2004. Fish larvae from 99 species, belonging to 37 families, were identified. Families with higher number of species were Gadidae, Sparidae and Labridae. The larval fish assemblage was dominated by pelagic fish species, with Sardina pilchardus, as the most abundant. There was a pronounced spring peak in larval abundance, dominated by S. pilchardus. A smaller peak, dominated by S. pilchardus and Micromesistius poutassou, was recorded in late winter at Stns 2 and 3. This pattern was evident for the three-year study. Results also indicate that this study was limited to the coastal larval fish assemblage inhabiting the central Cantabrian Sea shelf. This assemblage was temporally structured into other three assemblages: winter, late winter–spring and summer–autumn. Temperature was apparently a key factor in larval fish assemblage succession. In a scenario of global warming, this study constitutes a basis to evaluating the ongoing changes in the pelagic coastal ecosystem of the central Cantabrian Sea.     

Fish assemblages in coastal lagoons in land-uplift succession: The relative importance of local and regional environmental gradients

The assemblages of young-of-the-year fish were studied in coastal lagoons in an archipelago with post-glacial land-uplift, which affects environmental gradients at local and regional scale, i.e. lagoon habitat isolation and archipelago position, respectively. The categorisation of 40 undisturbed lagoons into nine habitat types based on habitat isolation and archipelago position was supported by clear relationships with spring temperature and total fish abundance. Rutilus rutilus, breams (Abramis/Blicca sp.) and Perca fluviatilis were the most abundant and frequently occurring species. The fish assemblage differed among the nine habitat types. Rutilus rutilus, P. fluviatilis and breams were discriminating species in the majority of habitat types with low physical harshness, whereas Alburnus alburnus and Gasterosteus aculeatus increased their contributions in habitat types with high physical harshness. Rutilus rutilus and breams were thus common in lagoons with high habitat isolation situated in the inner archipelago. These lagoons were characterised by warm water and high vegetation coverage. Gasterosteus aculeatus was restricted to lagoons with low habitat isolation and exposure and low vegetation coverage, situated in the outer archipelago. Perca fluviatilis had the widest distribution of all species. The coverage of two macrophytes, Potamogeton perfoliatus and Zannichellia palustris, and salinity matched best the distance among habitat types. These habitat characteristics, as well as the fish abundances and assemblages differed most across the habitat types in the outer and mid archipelago zones and in the lowest habitat isolation. These patterns suggest that the structuring effect of habitat isolation increases along the archipelago gradient as differences between local and regional conditions increase. In the inner archipelago, overall low physical harshness induces homogeneous conditions and the habitat isolation is less important here than in the other zones. We suggest that this difference in the relative importance of the two gradients depending on the level of respective gradient ultimately forms these heterogeneous coastal habitats in a successional landscape. Rutilus rutilus and P. fluviatilis were responsible for large parts of the assemblage patterns. Although sympatric due to similar habitat requirements, differences in dispersal capability, competitive ability and predation vulnerability may add explanation to detected differences in distribution and abundance in these two species in an open system. Our results also stress the structuring role of vegetation in terms of total coverage and species composition, as these two aspects of macrophyte diversity may act as complementary habitat modifiers across gradients of physical harshness.     

Structural aspects of the surf-zone fish assemblage at King's Beach,Algoa Bay,South Africa: Long-term fluctuations

Regular collections of fish were obtained from the surf-zone at King's Beach, Algoa Bay. A total of 3970 fish, representing 50 species was caught with a coarse net and 16 857 fish, representing 37 species, were caught with a fine net. Predominant species were the blacktail, Diplodus sargus; the sand steenbras, Lithognathus mormyrus; the mullet, Liza richardsoni; the gorrie, Pomadasys olivaceum; the white stumpnose, Rhabdosargus globiceps; the sandshark, Rhinobatos annulatus; and the streepie, Sarpa salpa. No seasonal trends were discernible in the overall abundance or species diversity. The species composition of the dominant component of the fish assemblage varied considerably. This indicated instability in the community structure and cast doubts on the applicability of a classic community concept and the use of diversity indices. Neither classification nor correspondence analysis were of any use in identifying a characteristic species component. Multiple regression analysis indicated that short-term variations in wind conditions might be a primary determinant of fluctuations in abundance. The lack of seasonality in the community parameters may reflect the fact that short-term variability masks seasonal perturbations.     

Structural aspects of the surf-zone fish assemblage at King's Beach,Algoa Bay,South Africa: Short-term fluctuations

Short-term changes in the King's Beach surf-zone fish assemblage were investigated by means of two 24 h studies (January and August 1979), and by seine netting on three consecutive days at both high and low tide. They revealed considerable variation in the fish assemblage over a 24 h period. Four species: the gorrie, Pomadasys olivaceum; the mullet, Liza richardsoni; the streepie, Sarpa salpa; and the sand steenbras, Lithognathus mormyrus, dominated the catches of both 24 h studies. The number of species caught reached a peak just after twilight. Trends in other community parameters were not consistent for these two sets of data. Considerable variation in dominant species, numbers and species diversity indices was observed from day to as well as between high and low tide. Shortterm fluctuations in various community parameters often exceeded the long-term variability observed in a seasonal study of this fish assemblage.     

The introduction of Sargassum muticum modifies epifaunal patterns in a Moroccan seagrass meadow

Marine macrophytes sustain valuable epiphytic biodiversity. Nonindigenous macroalgae may induce changes in composition and structure of epifaunal assemblages and therefore support different assemblages from those associated with native species. In this study, differences in faunal community structure between the introduced fucoid Sargassum muticum and the native seagrass Cymodocea nodosa were tested over a year on an intertidal shallow sandy bottom at the southern introduction front, the El Jadida coastline (NW Morocco). Epifaunal community structure consistently differed between macrophytes through seasons, with more species‐rich assemblages associated with S. muticum than C. nodosa despite comparable abundances. The significantly greater epifauna diversity on S. muticum may be related to its structural complexity. However, the species contributing most to differences in assemblages between both macrophytes, such as Steromphala umbilicalis and S. pennanti, were found on both habitats with temporally varying abundances. Some species‐specific affinities were detected (Stenosoma cf. acuminatum, Elasmopus vachoni, Chauvetia brunnea). Nitrogen, dissolved oxygen, suspended matter and temperature were identified as the best explanatory variables contributing to the observed macroepifaunal patterns. This study provides evidence that S. muticum acts as a favourable and additional habitat for epifaunal species and supports a more diverse epifaunal assemblage in this Moroccan seagrass meadow.     

海草床鱼类群落结构的时空变化及其在热带海草床中的育幼

本文以印尼北苏拉威西东部的潟湖为对象,研究了热带海草床鱼类群落的种类组成和时空变化。研究发现当地鱼类的香农-威纳指数在1.57-3.69之间多样性相对较高。在科水平,天竺鲷科的丰度（8.27 ind./（100 m²））和生物量（28.49 g/（100 m²））最高。在物种水平,侧带天竺鲷（Apogon lateralis）的丰度最高,环纹圆天竺鲷（Sphaeramia orbicularis）的生物量最大。对于物种的空间变化,潟湖湖顶、湖中和湖口首先聚类在一起,这可能是由于基质类型引起的。干季和湿季的鱼类种类、丰度和生物量均比转换季高,这可能是由较强的季风造成的,强季风为鱼类提供了更适宜的环境和食物。93.1%的鱼类的最大体长小于它们的性成熟体长,暗示了海草床是很多鱼类的育幼场。因此,保护海草床对渔业和资源的可持续利用至关重要。     

Deep-sea reef fish assemblage patterns on the Blake Plateau(Western North Atlantic Ocean)

Deep‐water coral habitats are scattered throughout slope depths (360–800 m) off the Southeastern United States (SEUS, Cape Lookout, North Carolina, to Cape Canaveral, Florida), contributing substantial structure and diversity to bottom habitats. In some areas (e.g. off North Carolina) deep corals form nearly monotypic (Lophelia pertusa) high profile mounds, and in other areas (e.g. off Florida) many species may colonize hard substrata. Deep coral and hard substrata ecosystems off the SEUS support a unique fish assemblage. Using the Johnson‐Sea‐Link submersible (in 2000–2005, 65 dives), and a remotely operated vehicle (in 2003, five dives), fishes were surveyed in nine deep reef study areas along the SEUS slope. Forty‐two benthic reef fish species occurred in deep reef habitats in these study areas. Species richness was greatest on the two coral banks off Cape Lookout, North Carolina (n = 23 and 27 species) and lowest on the two sites off Cape Canaveral, Florida (n = 7 and 8 species). Fish assemblages exhibited significantly (ANOSIM, Global R = 0.69, P = 0.001) different patterns among sites. Stations sampled off North Carolina (three study areas) formed a distinct group that differed from all dives conducted to the south. Although several species defined the fish assemblages at the North Carolina sites, Laemonema barbatulum, Laemonema melanurum, and Helicolenus dactylopterus generally had the most influence on the definition of the North Carolina group. Fish assemblages at three sites within the central survey area on the Blake Plateau were also similar to each other, and were dominated by Nezumia sclerorhynchus and L. melanurum. Synaphobranchus spp. and Neaumia sclerorhynchus differentiated the two southern sites off Cape Canaveral, Florida, from the other station groups. Combinations of depth and habitat type had the most influence on these station groups; however, explicit mechanisms contributing to the organization of these assemblages remain unclear.     

Sediment macrobenthos off eastern Waiheke Island,Hauraki Gulf,New Zealand

Aspects of sea-bed structure and benthic-macroinvertebrate species composition, distribution, richness and diversity in coastal waters off eastern Waiheke Island, Hauraki Gulf, are reported. In contrast to the sole historical account of sea-bed community structure from this same region, no widely distributed assemblages of species are recognised throughout it; no two sites share the exact same complement of species; and almost all sites are less than 80% similar in their taxonomic composition, most considerably so. Species richness and diversity are reported to vary with substratum type and depth, and spatially; species occurring within muds are the least diverse and species rich, followed by those of muddy gravels, and then gravels; many taxa prove common to the three substratum types; and dominance of taxa is recognised to decrease with an increase in substratum complexity, from muds to gravels, and species richness. With the exception of invasive marine species, apparent changes in the composition of assemblages throughout this region over the eight-decade period that data span are considered artefacts of the way in which such assemblages were historically defined. We recommend historical accounts of sea-bed community distributions throughout Hauraki Gulf be interpreted with caution, especially when attempting to use such schematic depictions to determine whether changes have occurred in assemblage composition.     

Short‐term dynamics in fish assemblage structure on a sheltered sandy beach in Guanabara Bay,Southeastern Brazil

Short‐term dynamics in juvenile fish assemblage structure were studied to test whether the most abundant species show temporal segregation, in order to assess whether selected environmental variables could predict species groupings, and to examine the stability of sunset–day–sunrise–night differences. Samplings were collected at 3‐h intervals over 48 h on a seasonal basis between spring 2005 and winter 2006. Fish species richness and abundance were higher in spring, and the lowest values occurred in winter. Harengula clupeola occurred mainly in spring, whereas Atherinella brasiliensis peaked in summer and autumn. On the other hand, Trachinotus carolinus, Umbrina coroides and Mugil liza were abundant in winter. Although temperature, salinity and dissolved oxygen were not found to have a strong effect on the abundance patterns of most species, they did appear to have a significant influence on assemblage groupings, according to canonical correspondence analysis and Spearman rank correlation. There is no consistency of diel usage patterns by a given species across seasons. The relative abundance differed between the time of day, which differed among the seasons; this further complicates an understanding of the dynamics of an assemblage. Studies of diel changes that pooled the sampling period as day or night can miss important changes that occur in a short time scale, such as a 3‐h period.