Hierarchical scales of spatial variation in the smaller surface and near-surface macrobenthos of a subtropical intertidal seagrass system in Moreton Bay,Queensland

Core samples were taken along a 4 km stretch of intertidal seagrass on North Stradbroke Island, eastern Australia, at nested scales of 1 m (stations), 150 m (sites), and 2 km (localities) to investigate the extent to which abundance, diversity, and assemblage composition of the dominant smaller members (<10 mm) of the intertidal seagrass macrobenthos vary spatially and over what scales. Gastropods and polychaetes dominated both the 91 species present and, together with decapods, also the numbers of individuals. Abundance was low (mean < 2000 individuals m⁻²) but species diversity was high (overall Simpson’s index of diversity 0.91), with 44% of species occurring only as one or two individuals, and with only two species contributing >10% to the total numbers (the microgastropod Calopia imitata and crab Enigmaplax littoralis, both little known, rarely recorded endemics). On average, a species only occurred at 6% of stations and only four occurred at >25%. Assemblages at the three localities did not vary significantly in gross ecological features (levels of species richness, faunal abundance and species diversity per component site) (ANOVA P ≫ 0.05), but did vary markedly in their composition at all spatial scales (PERMANOVA P < 0.05). Variance partitioning showed that components of total variance were least at the largest spatial scale (locality 15.9%) and greatest at the smallest scale (station 59.3%). The commoner individual species all showed random distributions at small spatial scales but clumped distributions at large spatial scales.     

Body size, sex ratio and diet composition of Arctogadus glacialis (Peters, 1874) (Pisces: Gadidae) in the Northeast Water Polynya (Greenland)

Little is known about the biology and feeding ecology of Arctogadus glacialis (Peters, 1874), an endemic gadid fish species occurring circumpolarly in both ice-free and ice-covered Arctic seas. In this study we analysed specimens sampled from bottom trawl and Agassiz trawl catches conducted at eight stations in water depths from 115 to 490 m in the Northeast Water Polynya off Greenland in August 1990. Size composition and sex ratios were assessed for 585 fish with standard lengths ranging from 7.8 to 34.5 cm. Median fish sizes at the stations ranged between 8.6 and 18.8 cm. Quantitative stomach content analyses of a subsample of 175 specimens revealed that in the Northeast Water A. glacialis fed almost exclusively on pelagic prey. Truly benthic organisms were not found in the stomachs, and sympagic species were rarely recorded. Overall, calanoid copepods were the major prey in terms of numbers. However, stomach contents varied considerably between fish size groups. In terms of prey biomass, copepods dominated the diet of small fish (<12 cm), primarily occurring at shallow stations (115–250 m), whereas amphipods and mysids were relatively more important for larger fish (>16 cm), which dominated the catches at deep stations (360–480 m). The broad variety of diet composition indicated an opportunistic pelagic feeding pattern. Received: 6 November 1997 / Accepted: 11 May 1998     

Structure of fish assemblages along spatial gradients in a deep subtropical reservoir (Itaipu Reservoir,Brazil-Paraguay border)

Synopsis We assessed the structure of the fish assemblages in the Itaipu Reservoir (Paraná River, Brazil-Paraguay) along a longitudinal or river-dam gradient (composed of riverine, transitional and lacustrine zones) and transversal or upstream–downstream gradients of the tributaries (composed of lotic and lentic stretches of tributaries and reservoir shores). We sampled stations distributed along the environmental gradients quarterly during two years. We caught mostly piscivorous, detritivorous and insectivorous fishes. The structure of the whole fish assemblage both at the regional spatial scale and at the zones of each gradient was significantly non-random according to null models and ordination analysis. When the assemblage was broken into groups, the piscivores and detritivores as well as the predators and prey showed significant non-random structure whereas the insectivores, omnivores, herbivores, benthophages and mud-eaters showed random structure. The first groups are those that had more species and the last mentioned are those with fewer species. The tributaries varied more in composition among themselves than the reservoir shores, showing a high heterogeneity in the transversal gradient. The most widespread species was the introduced piscivore Plagioscion squamosissimus. The loricariids were most abundant in the lotic and lentic stretches of the tributaries. We also observed temporal variability in species composition, mainly in the reservoir shores possibly due to the influence of the upstream floodplain on the migratory fish that periodically use the reservoir as feeding habitat. In the tributaries, the temporal changes were less predictable. A combination of habitat preferences and interactions among species, especially predator–prey relations, might be responsible for the observed structure patterns.     

Effect of urban and industrial wastes on species diversity of the diatom community in a tropical river,Malaysia

Monthly species diversity and other components of diversity of diatom samples collected over a period of one year at eight stations in the Linggi River Basin, Malaysia were analysed. Species diversity ranged between 0.52–3.62 bits individua^–1. Among the four stations located in the Linggi River (sensu stricto), highest diversity values were recorded at a station polluted by rubber effluent, followed by the stations unpolluted from point sources. Lowest diversity was recorded at a station polluted with urban sewage. At Kundor River, highest diversity was recorded at stations located downstream of rubber and palm oil waste discharges. On the whole, diversity values at unpolluted stations were always lower than at mildly polluted stations. Severe organic loadings caused low diversity by reducing the number of species (species richness) but did not increase the evenness (equitability) as expected when compared with unpolluted or mildly polluted stations. The changes in species diversity can be related to changes in diatom community structure and thereby changes in water quality, but cannot be used as an index of water quality.     

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.     

Anthropogenic influences on population sizes, age and growth of naturalized rainbow trout, Oncorhynchus mykiss, in Kenya

Riverine fishery in Kenya has witnessed profound changes since the 1950s, especially after independence (in 1963) when watersheds on the south‐eastern slopes of Mt Kenya were opened up for settlement. In particular, rainbow trout populations have declined essentially resulting from anthropogenic changes through over‐exploitation and/or degraded habitats. While there still are self‐sustaining rainbow trout populations in this stream among others they are in decline an indication that changes that have occurred on the watershed have altered their sizes, age and growth. The study was carried out in the Sagana, a third‐order stream, which rises at about 4000 m altitude on the south‐eastern slope of Mt Kenya, to obtain and document information on population sizes, age and growth of rainbow trout populations and to relate them with those recorded in the 1950s when trout streams were pristine (with little human influence). Stations were fished bi‐monthly from 1996 to 1998 and later for 6 months in 2002. Information on age and growth was obtained by use of annual marks, tagging fish of known age, and by validating their age using captive fish of known age. The minimum mean back‐calculated length at age for age one rainbow trout was 13.09 cm in upstream station and 15.10 cm for downstream stations. However, there was no significant difference in mean back‐calculated lengths at age for all years between fish in upstream and downstream stations (t‐test, t = −0.01, P = 0.99). Although female fish showed higher mean annual back‐calculated length increments than males, there was no significant difference in mean back‐calculated lengths at age between sexes (t‐test, t = −0.27, P = 0.80). The rate of growth in length was rapid for 1‐year‐old fish and declined in the second and third years. This study observed that most of the fish were small with only a few reaching more than 2 years of age because of overfishing. There are good reasons for optimism about the future of trout populations in this stream but concerted efforts are required to rehabilitate them. If trout populations are to increase, a management strategy is required to reduce fishing pressure and to maintain stream fishery against competing needs for resources in the catchments.     

Effects of temperature gradients resulting from reservoir discharge on Dorosoma cepedianum spawning in the Savannah River

Synopsis The timing and spatial pattern of gizzard shad, Dorosoma cepedianum, spawning were markedly affected by a temperature gradient caused by the release of hypolimnetic water from an upstream reservoir into the middle reaches of the Savannah River. During 1983 and 1984, a distinct thermal gradient occurred with the warmest temperatures at the downstream end of the 257 km study reach and the coolest temperatures at the upstream end. The occurrence of gizzard shad larvae indicated that spawning began at the sample stations farthest downstream and progressed upstream, peaking at each sample station when it warmed to approximately 19° C. The estimated difference in date of peak density of larvae between the upstream and downstream ends of the gradient was 23 days in 1983 and 35 days in 1984. Because of this pattern, densities of larvae were highest at the downstream end of the study area early in the spawning season and highest at the upstream end of the study area late in the spawning season.Photoperiod and daylength appeared unrelated to the patterns under study. Far-reaching effects of anthropogenic temperature changes on fish spawning and the distribution of larvae are probably common in rivers that receive hypolimnetic discharge.     

Diel,semi‐lunar and seasonal patterns in the fish community of an intertidal zone of the Yangtze estuary

The structure and temporal variations of the fish community in the intertidal estuarine zone of shallow mud areas have been poorly studied in China. This paper analyses the diel, semi‐lunar and seasonal patterns of fish assemblages in the Yangtze estuary in 2006. Fish were collected by consecutive day and night samplings using tide‐stow‐nets deployed parallel to each other in three stations. A total of 56 fish species belonging to 21 families was caught during the study period. The family Cyprinidae dominated with 25 species. Freshwater fish species were the important dominant commercial fishery species and well represented with five species (sharpbelly Hemiculter bleekeri, goldfish Carassius auratus, bream Parabramis pekinensis, likely‐bream Pseudobrama simony, and glossy yellow catfish Pelteobagrus nitidus) in the three stations. Juvenile fishes dominated the fish community, comprising 93.9% in station 1 and 96.6% in station 2 of the total abundance. The number of fish species in day tides was slightly lower than those in night tides in spring and summer, but the opposite in other seasons. In neap tides, the numbers and abundance of fish species were both lower than those in the spring tides. Fish abundance was lowest in winter, increasing during spring and summer (March–September) in both stations 1 and 2, with obviously large fluctuations in each season. The pattern of habitat selection of fishes could effectively decrease the food competition of intraspecies or interspecies and favour the growth and nursing of young fishes. These findings indicate that the intertidal zones in the estuary may serve as important nursery areas for fish communities.     

The temporal variation of intertidal nematodes in the Westerschelde I. The importance of an estuarine gradient

Biannual meiobenthic sampling (Spring and Autumn) was carried out in 1983–1989 at two fine sandy intertidal stations in the Westerschelde estuary. Both stations are exposed daily for more than one hour and are situated in the polyhaline and the mesohaline zone of the estuary. Average density data of non-selective deposit-feeders > predators > epigrowth-feeders > selective deposit-feeders for both stations are presented in spite of different nematode species composition. No difference between Spring and Autumn nor trend over 7 years could be detected. Higher total nematode densities are found at the polyhaline station (average 3200 ind. 10 cm^?2) in comparison with the mesohaline station (average 2300 ind. 10 cm^?2), a difference mainly due to higher non-selective deposit-feeders and predators densities in the polyhaline station. Each year, heterogeneous variance is found for all feeding types at the mesohaline station, but only for epigrowth-feeders and predators at the polyhaline station. The higher nematode density at the polyhaline station is probably caused by the more stable nematode structure. An unstable nematode temporal pattern at the mesohaline station is suggested to be combined with the detritus food chain system in the mesohaline zone. The unstable estuarine habitats are mainly caused by their upstream effects: the River Schelde, which clearly influences the stability of the nematode communities.     

Limestone Treatment of Whetstone Brook, Massachusetts. II. Changes in the Brown Trout (Salmo trutta)and Brook Trout (Salvelinus fontinalis)Fishery

Density, age structure, and growth rates of wild brook trout (Salvelinus fontinalis)and brown trout (Salmo trutta)in Whetstone Brook in northcentral Massachusetts were monitored for 4 years before and 3 years during limestone treatment to mitigate acidic conditions. The population density of brook trout increased significantly during treatment. Liming did not have any significant effects on the growth rates of brook trout or brown trout. Actual survival rates of brook trout and brown trout were not calculated due to the low density of both species, but more older individuals of both species were captured during the treatment period. Fulton condition factors (an index of fish condition) increased significantly for both brook trout and brown trout during treatment. Seven-day in situ bioassays of brown trout and rainbow trout demonstrated that liming improved the chemical environment for fish in Whetstone Brook. During a pretreatment bioassay in 1987, 100% rainbow trout mortality was observed at both the control and treatment stations in Whetstone Brook. Brown trout mortality was 67% in the control station and 70% in the treatment station. The pH during the 1987 bioassay averaged 4.90 in the control station and 4.99 in the treated station. During a bioassay conducted in 1990 after treatment began, rainbow trout mortality was 100% in the control station and 0% in the treatment station. Brown trout mortality was 17% in the control station and 0% in the treatment station. The pH during the 1990 bioassay averaged 5.23 in the control station and 6.60 in the treatment station. Analysis of total aluminum in the gills of fish from the 1990 bioassay revealed higher levels in fish from the control station than in those from the treatment station.     

Zooplankton composition, dominance and abundance as indicators of environmental compartmentalization in Jurumirim Reservoir (Paranapanema River), São Paulo, Brazil

Spatial and temporal patterns in the distribution of the zooplankton in a large tropical reservoir were investigated for a year. The zooplankton was sampled at 10 limnetic stations. Rotifera were richest in number of species and individuals, especially in transitional river–lake zones. They were dominant during the summer in nine sampling stations, and decreased in spring. The main species were Polyarthra vulgaris, Keratella americana, K. cochlearis and Conochilus unicornis. Polyarthra vulgaris was widely distributed. Keratella was more abundant at upstream stations, and a dense population of C. unicornis was observed in a lateral, sheltered compartment. Among copepods, Calanoida were more abundant in spring and Cyclopoida in autumn. Longitudinal gradients in the Calanoida/Cyclopoida relation were observed, with the predominance of Cyclopoida at upstream sampling stations and Calanoida in more lacustrine zones towards the dam. Notodiaptomus iheringi, Thermocyclops minutus and T. decipiens were the main species. Diaphanosoma birgei, the most abundant cladoceran, mainly occurred in lacustrine zones, while Moina minuta was more abundant at riverine sampling stations, generally in association with Bosminopsis deitersi. Peaks of tintinnid protozoans were observed in upstream zones during summer and spring.     

Influences of dams with different levels of river connectivity on the fish community structure along a tropical river in Southeastern Brazil

The influence of three types of dams with different degrees of river connectivity on the structure of fish communities along the Paraíba do Sul River was studied: total blockage (Dam 1); partial blockage allowed by a permanent lateral channel (Dam 2); and total blockage, but with a mechanism operating in the summer for fish passage (Dam 3). The tested hypothesis is that the degree of river connectivity influences the fish community. The largest differences in fish fauna were expected between the reservoir and downriver stretch in Dam 1 with total blockage; an intermediate difference in Dam 3 with total blockage but with a fish passage; and the least difference in Dam 2 with partial blockage. Fish were caught by gill nets between January and March 2010 and between January and February 2011 (wet seasons) in two zones: the reservoir and the downriver stretch from the dam. A total of 43 fish species was recorded, including eight non‐native and two marine species. The 13 most abundant species (n > 100; frequency of occurrence >20%) occurred in all three stretches of the river. The community structure changed significantly between the reservoir and associated downriver stretch, with higher richness downriver compared to the reservoir zone. A trend for higher occurrence of migratory fishes (e.g. Pimelodus maculatus, Pimelodus fur, Leporinus copelandii and Prochilodus lineatus) was found in the downriver zone, suggesting the influence of dams on their upriver migration. The predictions were not fully matched. Although the most significant difference in the fish community structure between the reservoir and the downriver was found for Dam 1, the partial blockage in Dam 2 showed broader differences in the fish fauna than the total blockage in Dam 3 with the fish ladder, which may indicate that the latter is not a guarantee that species and genetic flux will be interchanged, since the water velocity may be a constraint to upriver fish migration.     

Population dynamics, distribution, and growth rate of tilapia (Oreochromis mossambicus) in the Salton Sea, California, with notes on bairdiella (Bairdiella icistia) and orangemouth corvina (Cynoscion xanthulus)

The Salton Sea is a highly saline lake that has long supported sportfishery and large populations of fish-eating birds. A study was initiated in 1999 to assess the status of orangemouth corvina (Cynoscion xanthulus), bairdiella (Bairdiella icistia) and tilapia (Oreochromis mossambicus × O. urolepis). Multimesh (50 × 2 m) gillnets were set at nine stations in 1999, ten stations in 2000 and six stations in 2002. These stations were sampled every two months in 1999, every three months in 2000 and once in 2002. O. mossambicus was the most abundant of the four species, with a maximum mean catch per unit effort (CPUE) 13.8 kg net⁻¹ h⁻¹ or 29.9 fish net⁻¹ h⁻¹ being observed at the river mouth stations in August 1999. From spring to summer, tilapia CPUE increased at nearshore and river mouth stations and decreased at pelagic stations, apparently reflecting migration away from midlake areas in response to anoxia or hypoxia caused by periodic springtime overturn events in deep waters. Tilapia catches in nearshore, river mouth and pelagic habitats were 83 and 60% males in 1999 and 2000, respectively. Tilapia catches in rivers in August 1999 averaged only 6% male. During 1999–2000, the tilapia population consisted essentially of only the 1995 and 2000 year classes. Harsh conditions at the Salton Sea have led to erratic reproduction and survival rates and unstable age structures for its resident fishes. Massive parasite infestations of fry and physiological stressors such as anoxia, high sulfide levels, high salinity and high and low temperatures are potential causes of the irregular recruitment and periodic dieoffs of tilapia. The abundance of all fish species declined over the years of study. Between 1999 and 2002, the late summer mean CPUEs for tilapia, bairdiella and orangemouth corvina at four nearshore stations dropped from 16 fish to 0.02 fish, from 4.7 fish net to 0.23 fish, and from 0.08 fish to 0.02 fish, respectively. During 2000–2003, parallel declines occurred in estimated numbers of adult fish involved in mass mortality events at the Sea. The boom-and-bust dynamics of tilapia and other fish populations in the Sea have major consequences for fish-eating bird populations, for other components of the ecosystem, and for the recreational value of the lake. Guest Editor: John M. Melack Saline Waters and their Biota     

Longitudinal and Vertical Spatial Gradients in the Distribution of Fish within a Canyon‐shaped Reservoir

The large‐scale spatial distribution of fish was investigated within a morphometrically simple canyon‐shaped reservoir with a single major tributary and a longitudinal trophic gradient (Římov Reservoir, Czech Republic). Samples of fish were taken by Nordic survey gill nets (several mesh sizes from 8 to 70 mm knot to knot) installed as surface nets at several offshore areas located along the longitudinal axis of the reservoir. Surveys were carried out in late summer during 1999–2003. An obvious distribution gradient of fish was revealed along the longitudinal axis of the Římov Reservoir. The total relative fish abundance and biomass (catch per unit effort) decreased considerably from the upstream end of the reservoir toward the dam. Roach (Rutilus rutilus), bleak (Alburnus alburnus) and bream (Abramis brama) comprised the bulk of catches at all areas. Enhanced dominance of bream was observed in the fish assemblage at the uppermost, more eutrophic area of the reservoir. The highest number of fish species and the highest abundance of young‐of‐the‐year fish were also observed in the tributary area. In the downstream part of the reservoir, gill net surveys along the vertical depth profiles indicated that offshore fish occupied mostly the epilimnion. Extreme flood events affected the Římov Reservoir, however, it seemed they had no significant impact on the gradients described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Water quality changes in relation to Diptera community patterns and diversity measured at an organic effluent impacted stream in the Niger Delta,Nigeria

Impact of abattoir effluents (characterized by intestinal and stomach contents of slaughtered animals, ashes from roasted animals and blood stains) on water quality, distribution and abundance of Diptera were investigated in an urban stream, River Orogodo, Southern Nigeria, from July 2003 to June 2004. Water quality changes indicated significant differences (p < 0.05) in conductivity, dissolved oxygen, BOD₅, COD, total hardness, nitrate-nitrogen and phosphate-phosphorus between the three stations sampled. Higher values of these parameters were observed at the impacted station. The abundance and community structure of Diptera patterns, especially Chironomidae, Culicidae and Syrphidae families (all indicative of poor water quality) showed strong evidence of impact from the abattoir effluents. Comparisons of abundance values demonstrated high significance (p < 0.05) between the impacted station and the upstream (station 1) and downstream station (station 3). Shannon index and Berger–Parker dominance were greater at the impacted station (station 2). Analysis of faunal similarities showed that upstream station 1(unpolluted site) was significantly different from stations 2 and 3. The distinct taxa found in station 2 (the impacted station) suggest that the organic input from the abattoir favoured their abundance as most of them were opportunistic species.     

Aquatic pollution assessment based on attached diatom communities in the Pinang River Basin,Malaysia

The diatom communities collected from artificial substrates (glass slides) at 12 sampling sites along Pinang River Basin were analysed. Species composition was highly variable among sampling sites, and the highest diversity value (mean H = 3.56 bit/individual) and species richness (mean S = 17.32) was recorded at a polluted station in an estuary, which received salt-water intrusion during high tide. The downstream sampling stations were heavily organically polluted with Saprobic Index values ranging between 2.81 and 3.10, while upstream stations were relatively clean with Saprobic Index values recorded between 1.40 and 1.96. The diatom community structure and the specific sensitivity of certain diatom species can be related to the degree of water quality in Pinang River Basin. The abundance of certain diatom species could be used as biological indicators to measure impacts of river pollution.     

Species composition of benthic fish fauna in the Sea of Marmara,Turkey

In order to determine the present benthic fish fauna in the Sea of Marmara, sampling was conducted at a total of 10 stations for 2 weeks in August 2009, using a commercial twin beam trawling vessel with 18 and 36 mm stretched mesh sizes at the cod-end. In addition, some physicochemical parameters that play important roles, such as temperature, salinity, dissolved oxygen, and pH, were measured for each station. As a result, 31 fish species from 23 families were found at a total of 10 stations in depths of 30–1000 m. At each station, the Shannon–Weaver diversity index, Simpson dominance index, species richness index, and evenness index were estimated. This study reports on species composition of the demersal fish catches in the Sea of Marmara for a preliminary assessment of the status of these fish communities.     

Postprandial changes in pH and enzyme activity from the stomach and intestines of Tilapia rendalli (Boulenger, 1897), Oreochromis mossambicus (Peters, 1852) and Clarias gariepinus (Burchell, 1822)

The ability to utilise dietary components differs among fish species. Digestive enzymes may be used to determine the efficiency of the digestive process. In this study, the activities of the digestive enzymes in Tilapia rendalli, Oreochromis mossambicus and Clarias gariepinus were explored. Protease, amylase, lipase and cellulase activities were measured in different parts of the digestive tract of the three fish species. The pH dynamics along the digestive tract were monitored. In all fish species, the presence of food led to a reduction in stomach pH, whereby pH values of 1.54, 1.58 and 2.01 were recorded 12 h after feeding in O. mossambicus, T. rendalli and C. gariepinus, respectively. Protease and amylase activities were significantly higher (P < 0.05, anova ) in the tilapias than in C. gariepinus. The tilapias may be pre‐adapted to produce more protease and amylase to digest plant material, which is more difficult to digest than animal matter. In all species amylase activity was significantly higher in the proximal intestine than in the other parts of the digestive tract (P < 0.05, anova ). The highest protease activity was recorded in the distal intestines. This is because of the alkaline pH recorded in the proximal and distal intestines, which favours amylase and protease activity, respectively. Lipase activities were significantly higher (P > 0.05) in C. gariepinus than in both tilapias. Marginal cellulase activities were recorded in all species. It is inferred here that phylogeny and not diet may be the main factor influencing enzyme activities, as all fish were fed a similar diet.     

Fish composition and abundance in the wetlands of Cross River, Nigeria

Environmental parameters and ichthyofauna were investigated over a 2 years period in three regions along the 200 km length of Cross River. The objective of the study was to quantify the relative importance of local environmental conditions prevailing within sampling sites and the composition and abundance of the principal fish species in the upper, middle and lower reaches of the river. Vegetation cover, size of river, flow velocity, water level, temperature, transparency, and food availability explained the observed seasonal and spatial changes in fish abundance. Forty-six species and 28 genera of fish belonging to 16 families were recorded among the 14,466 fish caught. Three fish families (Cichlidae, Bagridae, and Clariidae) yielded highest number (41.3%) of species while Denticeptidae, Protopteridae, and Osteoglossidae had the least. Oreochromis niloticus, Chrysichthys nigrodigitatus, and Clarias anguillaris numerically dominated (46.4%) catch composition. Species richness was higher for the river stretch in forest area than in savanna, and it was correlated significantly with width of the reach, water transparency, depth, and flow velocity (P < 0.001) of the river. Wet season samples were more diverse (>0.6) and had higher richness (>9.7) than those for dry season. Wet season and forest regions were therefore critical in maintaining fish stock of Cross River.     

Heavy Metals in Eight Edible Fish Species from Two Polluted Tributaries (Aik and Palkhu) of the River Chenab,Pakistan

Concentration of heavy metals (lead (Pb), cadmium (Cd), chromium (Cr), and copper (Cu)) was determined in the liver, gills, kidneys, and muscles of eight edible fish species (Channa punctata, Cirrhinus reba, Labeo rohita, Heteropneustes fossilis, Mystus cavasius, Oreochromis niloticus, Puntius sophore, and Wallago attu) from upstream and downstream zones of the Nullah Aik and Palkhu tributaries of the River Chenab located in the Sialkot district known for its tanning industries worldwide. The pattern of metal accumulation in studied organs was in the order: Cr > Pb > Cu > Cd. Liver showed greater metal accumulation, followed by gills, kidneys, and muscles. Accumulation of Pb and Cr was significantly different in organs between upstream and downstream zones. Accumulation was greater in fish species dwelling downstream, indicating impairment of ambient stream water due to untreated discharge of industrial and municipal effluents into studied streams. Highest concentration of Pb and Cr and lowest of Cd was detected in H. fossilis whereas Cu showed higher concentration and Cr lowest concentration in P. sophore. In contrast, lower concentration of Pb and Cu was recorded in M. cavasius, O. niloticus, and W. attu. Mean concentrations of Cd, Cr, and Cu were higher in pre-monsoon compared to post-monsoon season. Measured concentrations of Pb, Cd, and Cr in muscles of species such as C. punctata, W. attu, L. rohita, P. sophore, and O. niloticus were above permissible limits of heavy metals for human consumption, indicating potential health risks. Therefore, these fish species from studied locations should be avoided for human diet.