Multi-mesh gillnets to estimate species composition and catch per unit effort of fish in a small water body in Zambia

A standard method comprising multi-mesh monofilament nylon gillnets and depth stratification, the Drottningholm method, was employed in test fishing of a small Zambian reservoir (32 ha) on eight occasions over 1 year. The catch per unit effort (cpue) from the 0–3 and 3–6 m zone was timated with benthic and pelagic nets, while in the 6–12 m zone only benthic nets were used. Results include species caught and cpue with coefficients of variation and confidence intervals, for net types and depth zones. A total of 11 species were caught, of which Burbus paludinosus , B. marequensis , Laheo cylindricus , and Oreochromis sp. dominated. Coefficients of variation did not indicate a preferable season for test fishing. During the cold season some species were not caught. The applicability of the method in estimation of species composition and cpue is discussed.     

The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brown Salmo trutta was assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989–1994) in 15 lakes located at altitudes between 230–715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4·93–5·51) and calcium concentrations (0·19–0·44 mg 1⁻¹), but varied more with respect to concentrations of inorganic, monomeric aluminium (7·0–41·0 μg l⁻¹). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6·56, calcium levels of up to 0·74mg 1⁻¹, and inorganic aluminium levels as low as l·0 μg 1⁻¹. The cpue was inversely correlated with lake altitude ( r ²=0·50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid-neutralizing capacity, had the highest predictive power of the variability in cpue ( r ² = 0·49).The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue ( r ² = 0·27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes.     

Fish assemblages of Cais do Carvão Bay (Madeira Island) determined by the visual census technique

Fish assemblages in Cais do Carvão Bay, Madeira Island, a proposed marine protected area (MPA), were determined from a diver visual census. A total of 32 transect counts were performed. Habitats sampled included sandy bottom, rocky boulders, vertical walls and rocky outcrops. Species richness, diversity, density, trophic structure, size and spatial organization were documented for the fish assemblages. Forty‐four species from 23 families were encountered; 32% belonged to Sparidae (10) and Labridae (four). The greatest species richness (25) was observed in rocky boulder habitat at 10–15 m depth, while the lowest (five) occurred over a deeper sand habitat. The greatest density (760·5 individuals per 100 m²) was recorded over rocky outcropping (20–25 m deep), and the lowest of 11·6 individuals per 100 m² was over a sand bottom at 10–15 m depth. Thalassoma pavo , Abudefduf luridus and Chromis limbata had higher densities on hard bottoms, while Heteroconger longissimus was the most abundant species in sand bottom habitats. No significant differences were detected for all indices calculated among depth intervals for sand and rocky boulder stations. Sand and rock boulder substratum, however, differed significantly for the 10–15 m depth stratum.     

Depth distribution and biological characteristics of the European eel Anguilla anguilla in Lough Ennell, Ireland

Using a longline survey, a total of 196 European eels Anguilla anguilla were collected at different depths in Lough Ennell (maximum depth 30 m), central Ireland. The catch per unit of effort of A. anguilla that were caught from 1 to 25 m depths was lowest at 0·5–5·0 m and greatest at the deepest depth range (22·5–25·0 m). Sub-samples of A. anguilla from depths of <15 m showed little or no difference in size, sex ratio, age, growth rate, condition factor, length–mass relationship, gonado-somatic index, fin index or eye index with fish from depths of >15 m. All fish examined were female yellow-phase A. anguilla that had ages from 7 to 20 years (mean ± s . d . = 10·3 ± 2·9 years), with growth rates from 24·0–60·8 mm year⁻¹ (mean ± s . d . = 40·7 ± 8·5 mm year⁻¹). Variations in the growth rates were greater in the shallow group than that of the deep group. This study suggested that deeper regions are important feeding habitats for A. anguilla and that fish in this lake were growing moderately fast compared to similar habitats and areas in the species' range.     

A summer-winter comparison of zooplankton in the oceanic area Around South Georgia

Summary Zooplankton was sampled with RMT (1+8) gear on a synoptic grid of stations centred on South Georgia during the austral summer (November/December 1981) and winter (July/August 1983). This initial paper compares zooplankton biomass, vertical distribution and species composition from RMT 1 catches in the oceanic portion of the grid (water depth greater than 2000 m) during the two surveys. In the winter survey, mean zooplankton biomass within the top 1000 m of the water column was 68% of its summer level. This drop was largely due to a decrease in abundance of krill (Euphausia superba), although biomass of copepods and remaining zooplankton also decreased. Copepods averaged 48% of total biomass in summer and winter, but outnumbered all other taxa put together by a factor of 10. Antarctic epipelagic species predominated around the island in the summer survey but tended to be replaced by sub-Antarctic or cosmopolitan species during the winter survey. The majority of zooplankton also showed a downwards seasonal migration out of the top 250 m layer in winter. However, several epipelagic species, including E. superba, did not migrate, and these tended to have the largest summer-winter differences in overall abundance. These trends were attributed to variation in the position of the Polar Front, which lay north of the island during the summer survey but lay across the survey area in winter, resulting in a greater influence of sub-Antarctic water and the displacement of Antarctic species.     

Seasonal changes in the habitat use and movements of adult European grayling in a large subarctic river

In late summer (13 August–13 September 1998), at water temperatures of 12·0–15·7° C, grayling ( n =14) stayed mainly in the riffle-section where they were captured in a large regulated river in northern Finland, moving little between consecutive days. In autumn (2–30 October 1998), at 1·7–6·7° C, the fish ( n =16) migrated to potential overwintering sites 0–14 km up- or downstream by mid October, moving mainly short distances thereafter. The daily movement rates, and the total ranges covered by the fish in late summer and autumn were 54±32 m (mean± s.d ) and 1053±1636 m, and 190±168 m and 3135±1850 m, respectively. In autumn the fish used deeper habitats (most suitable range 150–400 cm) with lower current velocities (20–80 cm s⁻¹) and finer bottom substrata (mainly sand) than in late summer (depth 100–325 cm, velocity 30–110 cm s⁻¹, and cobble-boulder substrata).     

Comparison of fish communities between above‐ and below‐dam sections of small streams; barrier effect to diadromous fishes

To analyse the effects of small‐scale dams on fish communities, species diversity, population density, biomass, migration over dams and trophic relationships were investigated in the above‐ and below‐dam sections of three Japanese streams. The dams were 1·5–3·9 m high, constructed within 400 m of the sea, and had no impoundment or fishway. Fish communities were mainly composed of diadromous fishes. Of the 17 species sampled, only a few species of gobies, Rhinogobius spp. and Gymnogobius spp. were able to climb and migrate upstream over the dams. Consequently the number of species, and total density and biomass of fishes below the dams was high, but low above the dams. One of the most dominant fish below dams, Plecoglossus altivelis altivelis , predominantly fed on benthic algae, whereas other fishes preyed on benthic invertebrates. Trophic relationships above dams were very simple, the few species of gobies preying on chironomid larvae and other aquatic invertebrates.     

黄海中南部不同断面鱼类群落结构及其多样性

基于2006年9月和12月、2007年3月和5月对黄海中南部不同断面的底拖网调查数据,对黄海中南部不同断面的鱼类群落结构、多样性、单位时间的渔获量分布及其与环境因子的关系进行了分析。结果表明,调查中共捕获鱼类109种,其中鲈形目种类最多(45种,占40.18%),其次是鲉形目(12种,占10.71%)、鲱形目(10种,占8.93%)和鲽形目(10种,占8.93%),主要由暖温种和暖水种组成,并且暖水种和暖温种在各断面渔获量中所占的比例随时间不同而有所差异。各断面主要鱼种除小黄鱼Larimichthys polyactis、带鱼Trichiurus lepturus、银鲳Pampus argenteus等种类外,以小型中上层鱼类(鳀Engraulis japonicus、黄鲫Setipinna taty等)和底层经济价值较低的鱼类(黄鮟鱇Lophius litulon、细纹狮子鱼Liparis tanakai等)为主,各断面优势种渔获量均占其总渔获量的50%以上,主要鱼种渔获量均占其总渔获量85%以上。渔获物的营养级主要分布在4.2—4.5、3.0—3.3和3.6—3.9,各营养级渔获量随时间和断面的不同有所变化。鱼类长度谱主要集中在3—24 cm,由南向北长度谱逐渐增大。各断面鱼类群落多样性指数差异不大,与底层温度和深度关系密切。     

Biology and ecology of Nezumia liolepis and N. stelgidolepis from the west coast of North America

Nezumia liolepis and Nezumia stelgidolepis were collected during a National Marine Fisheries Service 1997 groundfish survey along the Pacific coast of North America. Bottom trawling was conducted from 188 to 1260 m depths. Nezumia liolepis were collected from 581 to 1247 m (3·4–5·7°C) and N. stelgidolepis from 285 to 555 m (6·0–8·4°C). The two species had distinct depth and temperature distributions and the majority of the specimens for each species came from narrow depth ranges. Nezumia stelgidolepis attains a larger pre-anal fin length (117 mm) than N. liolepis (83 mm), and age estimates from otolith ring counts indicate ages from 3 to 9 years for N. liolepis and 7 and 13 years for N. stelgidolepis from specimens collected. The gonads of N. liolepis were not reproductively active, and a single N. stelgidolepis possessed eggs of various sizes suggesting batch spawning. N. liolepis and N. stelgidolepis fed benthically, primarily on crustaceans such as amphipods, shrimp, mysids and polychaete worms. Although diet overlap was high, these two species appear to limit competition for resources by habitat separation.     

Composition, structure and distribution of the ichthyoplankton in a Mediterranean coastal lagoon

The species composition and assemblage structure of the ichthyoplankton from the Mar Menor Lagoon in south‐east Spain are given. The fish larvae were sorted from zooplankton samples collected at 20 stations with a plankton net (50 cm mouth diameter and 500 μm mesh) during 36 surveys between February and December 1997. A total of 39 575 fish larvae representing 14 families, 22 genera and 36 species were identified. Gobiidae was the most dominant family (77·0%) followed by Blenniidae (19·4%) and Atherinidae (1·3%). The most abundant species were Gobius niger and Gobius paganellus , which accounted for 42·7 and 19·3% of the total respectively. These species were followed in order of relative abundance by Pomatoschistus marmoratus (13·9%), Parablennius pilicornis (9·4%), Lipophrys pavo (7·7%), Atherina boyeri (1·3%) and Parablennius tentacularis (1·3%). The high species diversity (2·0–2·8 bits individual⁻¹ for the annual diversity spectra at each sampling station) reflected a diverse assemblage of species. The main commercial species in the lagoon (Sparidae and Mugilidae) were poorly represented among the ichthyoplankton and they probably enter the lagoon on the bottom as recruits. Chlorophyll a concentration in the water column was the main factor explaining the seasonal variation in larval abundance. Spatial distribution of larvae was related to hydrographical circulation patterns in the lagoon and the movement of marine‐spawned larvae through the channels connecting the lagoon with the Mediterranean Sea.     

Behaviour and diet of sea trout post-smolts in a Norwegian fjord system

In order to study the behaviour of sea trout Salmo trutta L., in the early post-smolt phase, reared smolts were tagged with acoustic transmitters and released in the estuary of the River Aurland (Western Norway) and observed for 31–137·5 h. The study was conducted in May-June 1991 and 1995 on a total of seven tagged smolts. The fish moved back and forth along the littoral zone close to the surface and migrated net distances of 100–8000 m. In 1991 the tagged fish were usually observed in schools averaging 24–55 fish, while the tendency to school was less pronounced in 1995. Post-smolts of wild and reared origin were caught by 25 m long gillnets in the littoral zone. In 1991 the mean distance to shore at capture was 4·3 m and the mean depth 0·6 m, and the corresponding numbers in 1995 were 8·0 and 0·8 m. The distribution of the post-smolts is explained by a general preference for shallow water caused by osmoregulatory problems in high-salinity deep water, together with a preference for the sheltered littoral zone and a strong predation pressure in the open sea from fast swimming pelagic fish predators and gulls Larus sp. Schooling seemed to restrict predation by cod Gadus morhua L., in the littoral zone in 1991, while no cod were observed in this habitat in 1995. In both years the stomach contents of the netted fish consisted mainly of terrestrial insects. Reared fish chose prey items similar to those taken by wild fish, but consumed less food.     

Summer-winter differences in copepod distribution around South Georgia

Zooplankton was sampled on a synoptic grid of stations centered on South Georgia during the austral summer of 1981/82 and winter 1983. Within the top 1000 m layer at oceanic stations, copepods averaged 48% of the total biomass in summer and winter, but outnumbered all other taxa combined by a factor of 10. In winter the mean zooplankton biomass within the top 1000 m was 68% of its summer level. Copepod biomass was 77% of its summer level. During both surveys, the large and abundant Calanoides acutus and Rhincalanus gigas dominated the copepod biomass and, with several other species, showed a marked downwards seasonal migration out of the top 250 m layer in winter. Antarctic epipelagic species predominated around the island during summer but tended to be replaced by sub-Antarctic and cosmopolitan species during the winter. Factors likely to influence our estimates of overall copepods abundance and changes in species composition include seasonality of reproduction, net mesh selection and differences in water mass distribution. The observed trends are attributed mainly to variation in the position of the Polar Front which lay north of the island during the summer survey yet lay across the survey area in winter. This resulted in a greater influence of sub-Antarctic water around South Georgia in winter and the displacement of Antarctic species.     

Reproductive biology of the endangered Oxleyan pygmy perch Nannoperca oxleyana Whitley

The reproductive biology of the Oxleyan pygmy perch Nannoperca oxleyana is described from simultaneous studies of wild populations in north-eastern New South Wales and mature fish held in aquaria. In the wild, 50% of males and females matured at total lengths of 24·0–25·9 and 28·0–29·9 mm, respectively. The species displays sexual dichromatism during the spawning season, with males developing more intense red and brown fin and body colouration, and black pelvic fins. Captive male broodfish displayed territoriality during the breeding season, closely guarding sites within artificial, plant-like substrata in which pairs of fish spawned adhesive eggs. Protracted serial spawning of wild and captive fish occurred from September to April and May at mean water temperatures ≥16·6° C and day length ≥10·7 h. Captive broodfish spawned on an average of 57% of days during the 256 day spawning period. Gonado-somatic indices averaged 0·7% for all ripe males and 4·1–4·2% for all ripe females collected. Mean total and batch fecundities of captive females were 1323 eggs per fish and 7·8 eggs per fish per day, respectively, and relative fecundity was 587 eggs g⁻¹ of body mass. Batch fecundity of wild females was estimated at 7·8 eggs per fish. The adaptive significance of this reproductive strategy in a harsh, variable environment is discussed.     

Depth equilibration by two predatory cichlid fish from Lake Malawi

The cichlid fish, Haplochromis polystigma and Haplochromis livingstonii are piscivorous ambush predators endemic to Lake Malawi, Africa. Experimentally it was found that large adult H. livingstonii could equilibrate to 38 m, but that juveniles were restricted to shallower depths. Mean depth attained by 8 H. polystigma was 45 m but two individuals exceeded the maximum simulated depth (59 m). H. livingstonii had a mean equilibration rate of 4–7 m day⁻¹ for the first 10 m declining to 2·4 m day⁻¹ thereafter. The mean rate of descent for H. polystigma was 3·8 m day⁻¹ for the first 10 m and 2·8 m day⁻¹ thereafter. The fish were decompressed at a rate of a 20% decrease every 12 h. Although cichlid fishes occur down to 200 m depth in Lake Malawi, experimental and field data indicate that the depth distribution of individual species is restricted and that the restriction is largely related to swimbladder physiology. All cichlid species so far tested equilibrate slowly to depth (2 to 5 m a day) and are not capable of rapid decompression.     

Reproductive biology of the threatened golden galaxias Galaxias auratus Johnston and the influence of lake hydrology

Golden galaxias Galaxias auratus (31–235 mm fork length, L _F) were collected monthly from littoral habitats in Lakes Crescent and Sorell, Tasmania, Australia, between July 2000 and December 2002. Spawning habitats were identified and monitored in both lakes, and surveyed in Lake Crescent. Trends in gonado-somatic indices and reproductive stages of development indicated that gonad development in both sexes begins in midsummer and peaks in late autumn to early winter. Males mature at smaller sizes (50% at 52 mm L _F) than females (50% at 76 mm L _F), larger individuals are predominately females (95% of fish ≥138 mm L _F), and overall male to female ratios are female biased ( c . 1:2). Spawning occurs late autumn to early spring (water temperatures = 1·4–9·7° C) with peaks in spawning activity in winter (mean water temperatures <5° C). Demersal adhesive eggs ( c. 1·5 mm diameter) were found on cobble substrata ( c. 20–250 mm diameter) in littoral areas ( c. 0·2–0·6 m deep) and fecundity of fish 71–181 mm L _F ranged from 619 to 14 478 eggs. The rate of change in water level over the 20 days prior to monthly sampling was important in explaining the occurrence of spent fish and this accounted for temporal differences in spawning between the populations. Lake hydrology influences the reproductive cycle of G. auratus by possibly providing a stimulus for spawning and it controls the availability of spawning habitat in Lake Crescent. Seasonal hydrological cycles ( i.e. rises during late autumn to winter) and a minimum water level of 802·20 m Australian Height Datum in Lake Crescent during autumn (above which littoral areas of cobble substratum are inundated) are critical to G. auratus populations.     

广东省大中型供水水库营养现状及浮游生物的响应

于2000年调查了广东省18座大中型供水水库的水质现状并探讨了浮游生物对营养水平的响应。总氮、总磷、透明度和叶绿素a分别为0．15～7.15mg／L、0．003～0．387mg／L、0．4～6．3m和0．6～32．3ug／L。总氮、总磷、透明度均与叶绿素a呈较高的相关性。根据这4个因子的综合加权营养状态指数为23．7～55．1,季节差异不大,大多数水库处于中营养状态。金藻在中-富及富营养型水库中没有分布,而蓝藻、绿藻、硅藻和甲藻在调查水库中均有比较广的营养生态位,但它们的密度及相对优势度在各营养型水库中有一定的差异。高营养水平水库有较高的细胞密度和叶绿素a含量。营养水平较低的水库浮游植物以硅藻-甲藻、硅藻-绿藻或金藻-硅藻为主;营养水平较高的水库以蓝藻-硅藻或蓝藻-绿藻为主,并有较高的裸藻密度。浮游动物基本上以桡足类为优势种群,但在中-富营养和富营养型水库中,哲水蚤种类比低营养型水库中少。枝角类优势种类在各营养型水库差别不大。轮虫对水体营养水平的响应相对比较显著。低营养水平水库的轮虫以广营养型、中营养型或寡中营养型种类为主,种类数目比较少;富营养和中-富营养型水库的轮虫以喜在中营养到富营养条件下生长的种类为主,且轮虫种类数目比较多。     

Effects of Changes in Acidity on Aquatic Insects in Rocky Littoral Habitats of Lakes Near Sudbur y, Ontario

Benthic aquatic insects were collected from rocky nearshore areas (< 1 m deep) of 17 lakes near Sudbury, Ontario, with a pH range of 4.7–7.3 and a size range of less than 10 ha to over 10,000 ha. These insect communities were composed of taxa common to lake soft-sediments and streams. Direct and indirect effects of lake acidity appeared to be major controls on the structure of these communities, implying that several factors may be involved in restructuring during acidification or recovery. Declines in abundances of several taxa of Ephemeroptera at pH below 5.5 were attributable to acid toxicity, while increases in the abundances of Odonata and Diptera at pH below 5.5 were associated with the absence of fish predators and other indirect effects of acidity. The communities of two experimentally neutralized lakes restructured rapidly within 5 years, approaching but not achieving community structures typical of our near-neutral survey lakes. Neutralization led to recolonization or increased abundance of the acid-sensitive mayfly, Stenacron interpunctatum , and the dragonfly, Boyeria grafiana ; however, recolonization by other taxa expected to be present in near-neutral lakes ( Stenonema femoratum, Eurylophella, and Basiaeshna janata ) was not observed. Consistent with results for the survey lakes, declines in the abundances of the dragonflies Aeshna interrupta, Aeshna eremita , and Leucorrhinia glacialis in the neutralized lakes were associated with reintroductions of Salvelinus fontinalis (aurora trout) and increased fish predation pressure, while reduced abundances of the dipterans Ceratopogonidae, Psectrocladius , and Stackelbergina may be related to indirect effects of acidity other than fish predation. Although community composition varied greatly across the acidity gradient, total species richness and abundance were not correlated with lake chemistry or number of fish species.     

Analysis of the demersal community of fish and cephalopods on the Agulhas Bank, South Africa

The demersal fish and cephalopod communities of the continental shelf and upper slope from 17 to 395m deep were studied during five annual cruises between Cape Agulhas and Port Alfred, South Africa. The cruises showed a consistent pattern of an inshore community (<100m), a shelf community ( c . 90–190m) and a shelf-edge/upper slope fauna (>200m). These groups were identified by dendrograms and multidimensional scaling cluster analysis, which supported on-board observations of catch variation with depth. Although the boundaries are not clearly defined, examination of physical features at the clustered stations suggests that depth, temperature and, to a lesser extent, oxygen concentration are important in the grouping. Occasional, apparently anomalous associations of inshore stations suggested that water temperature and oxygen may over-ride the normal depth distributions of the species groups. This intimates that patterns offish and cephalopod distribution may be dynamic and in part related to the physical parameters of the water body.     

Diel distribution patterns of fishes in a temperate large lowland river

During the day, juveniles and typical inshore species in the lower Oder River, Germany, inhabited the littoral (0·05–1·50 m deep) while potamal species inhabited the mid‐channel (40–100 m from the banks). During the night, two behavioural groups were distinguished: 1) potamal species and large individuals of other species, moved from offshore to the littoral, and 2) nocturnal foraging species, became active and increased in inshore and offshore habitats. Inshore, the most significant differences at night was the increasing density of silver bream Blicca bjoerkna increasing both total biomass and total length of fishes caught. Offshore, the increasing density of whitefin gudgeon Gobio albipinnatus at night was most significant. Additional night electrofishing improved the assessment of the abundance, age and size structure of typical potamal fish species. It should be incorporated in large river fish monitoring programmes.