The effects of seaweed harvesting on fishes: a response

References Rangeley (1994) has argued that sampling biases, errors in experimental design and low statistical power make the conclusion by Black & Miller (1991), that there was no large impact of an experimental harvest of the seaweedAscophyllum nodosum on fishes, unmerited and misleading. Our study was conducted mainly at two sites close to Lower Argyle, southern Nova Scotia, Canada where harvesting ofAscophyllum nodosum has been carried out continuously for 30 years along-side intense and productive shellfish and finfish fisheries. In southern Nova Scotia, on-going removal ofAscophyllum nodosum with mechanical harvesters is about 17% (Sharpe & Semple 1991); planned annual removal in New Brunswick where Rangeley has worked will be only about 5% (Anon. 1993). The context of our investigation was a small-scale (400 m²) but extreme, experimental removal (100%) of the seaweed. Our focus was on fishes that moved into the intertidal zone as the tide rose in the evening and early morning and were caught in trammel nets, and on the fishes that were retained in a fixed seine that was set at high tide in early morning and the fishes removed at the next low tide. Thus, subject to the constraint on the size of the fishes imposed by the nature of our fishing gear with its stretch mesh of 25 mm, we were concerned with the identity and abundance of fishes that moved in and out of the intertidal zone and their behavioural response to the 400 m² patches of habitat that formed our experimental units. The most striking result was the unexpected, low abundance of fishes caught in our gear.     

Comparison between community structures of fishes in <Emphasis Type="Italic">Enhalus acoroides</Emphasis>- and <Emphasis Type="Italic">Thalassia hemprichii</Emphasis>-dominated seagrass beds on fringing coral reefs in the Ryukyu Islands,Japan

To clarify differences in community structures and habitat utilization patterns of fishes in Enhalus acoroides- and Thalassia hemprichii-dominated seagrass beds on fringing coral reefs, visual censuses were conducted at Iriomote and Ishigaki islands, southern Japan. The numbers of fish species and individuals were significantly higher in the E. acoroides bed than in the T. hemprichii bed, although the 15 most dominant fishes in each seagrass bed were similar. Cluster and ordination analyses based on the number of individuals of each fish species also demonstrated that fish community structures were similar in the two seagrass beds. Species and individual numbers of coral reef fishes which utilized the seagrass beds numbered less than about 15% of whole coral reef fish numbers, although they comprised about half of the seagrass bed fishes. Of the 15 most dominant species, 5 occurred only in the two seagrass beds, including seagrass feeders. Ten other species were reef species, their habitat utilization patterns not differing greatly between the two seagrass beds. Some reef species, such as Lethrinus atkinsoni and L. obsoletus, showed ontogenetic habitat shifts with growth, from the seagrass beds to the coral areas. These results indicate that community structures and habitat utilization patterns of fishes were similar between E. acoroides- and T. hemprichii-dominated seagrass beds, whereas many coral reef fishes hardly utilized the seagrass beds.     

Diet,food preference,and algal availability for fishes and crabs on intertidal reef communities in southern California

Synopsis Herbivory by wide-ranging fishes is common over tropical reefs, but rare in temperate latitudes where the effects of herbivorous fishes are thought to be minimal. Along the west coast of North America, herbivory by fishes on nearshore reefs is largely restricted to a few members of the Kyphosidae, distributed south of Pt. Conception. This paper presents information on natural diets and results from feeding choice experiments for two abundant kyphosids from intertidal habitats in San Diego, California —Girella nigricans andHermosilla azurea, and similar data for the lined shore crab,Pachygrapsus crassipes, which also forages over intertidal reefs. These results are compared with the availability of algae in intertidal habitats measured during summer and winter, on both disturbed and undisturbed habitats. The diets of juveniles ofG. nigricans andH. azurea collected from nearshore habitats were dominated by animal prey (mainly amphipods), but adults of these fishes, andP. crassipes, consumed algae nearly exclusively, with 26, 10, and 14 taxa of algae identified fromG. nigricans, H. azurea, andP. crassipes, respectively. Algae with sheet-like morphologies (e.g.Ulva sp.,Enteromorpha sp., members of the Delesseriaceae) were the principal algae in the diets of the fishes, and calcareous algae (e.g.Corallina sp.,Lithothrix aspergillum) and sheet-like algae (Enteromorpha sp.) comprised the greatest identifiable portion of the shore crab's diet. Feeding choice experiments indicated that the fishes preferred filamentous algae (e.g.Centroceras clavulatum, Polysiphonia sp.,Chondria californica) and sheet-like algae (e.g.Enteromorpha sp.,Ulva sp.,Cryptopleura crispa) over other algal morphologies, whereas the shore crab chose jointed calcareous algae (e.g.Lithothrix aspergillum, Corallina vancouveriensis, Jania sp.) most frequently. The diets and preferences for algae by the fishes were generally most similar to the assemblage of algae available in early successional (disturbed) habitats during summer when sheet-like and filamentous algae are abundant. The shore crab exhibited the opposite trend with a diet more similar to late successional (undisturbed) habitats.     

Long-term stability of the seasonal succession of different zooplankton species in a brackish water lagoon (southern Baltic Sea)

While it has been widely suggested that freshwater fishes from East Asia invaded the western Palaearctic, details about this process are largely unknown. Here, using the cytochrome b gene, we evaluated the phylogenetic relationships of a small group of Eurasian primary freshwater fishes (Cobitidae), which are widely distributed and species rich in East Asia and Europe, with the purpose of inferring their invasion process of Europe from East Asia. Though phylogenetic relationships of cobitids were not well resolved, our analysis could identify three sister groups formed by the European and East Asian cobitids, which brought new insights into the biogeography of the genera Cobitis, Misgurnus, and Sabanejewia. The present results support the view that Asian cobitid fishes may have invaded Europe at least five times independently, and once reverse colonization of European cobitids to East Asia could also be found. Ancestral Sabanejewia might have been the first cobitids to cross Siberia and invade the EMZS (Euro-Mediterranean zoogeographic subregion) about 33.54 million years ago (MYA). One lineage of Cobitis and the ancestor of Misgurnus fossilis (Linnaeus) almost in the same time invaded the Europe, responding to 16.71 MYA and 16.59 MYA, respectively. Three different lineages of Cobitis were found to have invaded the EMZS from East Asia, and once reverse invasion to East Asia occurred to one subclade of European Cobitis. And our data also suggest that the diversity of East Asian cobitid fishes, especially of the genus Cobitis, is greatly underestimated. Handling editor: C. Sturmbauer     

Is there a reproductive basis to solitary living versus pair-formation in coral reef fishes?

Many species of coral reef fishes form pairs. While it is assumed that pairs represent the breeding unit of these species, the reproductive status of paired versus solitary individuals, and changes in status associated with pair-formation have seldom been investigated. In order to assess whether pairing is related to reproduction we examined whether the ontogenetic timing of pair formation coincided with the onset of maturation in four species of fishes: Chaetodon lunulatus and Chaetodon melannotus (family Chaetodontidae), and Valenciennea muralis and Valenciennea strigata (family Gobiidae). 65–78% of all fishes occurred in pairs. In C. lunulatus and V. muralis, pair-formation coincided with maturation, suggesting that these species form pairs for breeding. Further, C. lunulatus and V. muralis exhibited significant positive size-assortative pairing, which is often associated with monogamous mating. In contrast, pair formation in C. melannotus and V. strigata did not coincide with maturation. In both these species many solitary individuals were reproductive, and same sex pairs were common. While reproduction may be the basis for pairing in some species, both solitary and paired individuals are capable of breeding in others. We propose that non-reproductive mechanisms, such as predator vigilance, may explain pair-formation in coral reef fishes with non-monogamous breeding systems.     

In-stream behaviour of threatened fishes and their food organisms based on remote video monitoring

Can remote underwater video be used to investigate the in-stream behaviour of small fishes and decapods? Diel activity of two threatened freshwater fishes (Macquaria australasica and Gadopsis bispinosus), a palaemonid prawn and an atyid shrimp, was established from remote underwater video in a pool of an upland stream in the current study. Decapods and large fishes (>5 cm TL) were nocturnal, whereas, small fishes (<5 cm TL) were diurnal. The suitability of using underwater video to quantify short-term (seconds) behavioural interaction among individuals was also demonstrated, with early-juvenile-phase Gadopsis bispinosus exhibiting interference competition on 35% of occasions when two or more individuals were observed. This study demonstrates that remote underwater video is useful for observing the in-stream behaviour of threatened freshwater fauna where other techniques are not viable, and presents sub-sampling of video as a means of reducing video processing time in assessing fish diel activity patterns.     

Genetic identification of larvae and juveniles reveals the difference in the spawning site among Cyprininae fish species/subspecies in Lake Biwa

To understand the differences in the spawning sites among Cyprininae fishes in Lake Biwa, we conducted periodic sampling of larvae and juveniles at three sites (irrigation ditch, St. 1; river, St. 2; and satellite lake, St. 3). On the basis of species/subspecies identification by using RAPD analysis, we examined the species composition of the larvae and juveniles at these three sampling sites. The number of specimens was 616, 68, and 117 at St. 1, St. 2, and St. 3, respectively. Based on morphological and genetic identification, the specimens were found to include nine fish species/subspecies, namely, Carassius auratus grandoculis, Carassius cuvieri, Carassius auratus langsdorfii, Cyprinus carpio, Sarcocheilichthys sp., Silurus asotus, Oryzias latipes, Odontobutis obscura obscura, and Rhinogobius sp. The species composition at the three sites also differed. Among the Cyprininae fishes, C. auratus grandoculis, C. auratus langsdorfii, and Cyprinus carpio were found in abundance at St. 1; C. cuvieri was not collected from St. 1 but was found at the other two sites, particularly St. 3. Among the other fishes, Rhinogobius sp. was collected at St. 1 and St. 3, whereas the other four occurred only at St. 1. These results suggest that the selection of spawning sites by C. cuvieri differs to a certain extent from that of the other Cyprininae fishes, and the irrigation ditch in the lake is an important habitat for the larvae and juveniles of native fish species.     

Trophic relationships and seasonal utilization of salt-marsh creeks by zooplanktivorous fishes

Synopsis In a high salinity estuary at North Inlet, South Carolina, co-occurrence and possible competition among adults of four dominant zooplanktivorous fishes were minimized by seasonal adjustments in lateral and vertical distributions as well as in dietary preferences. In winter, Atlantic silversides, Menidia menidia, occupied the entire water column while other planktivores were rare or absent from the estuary, and they consumed large prey such as mysid shrimps and fish larvae. An immigration of bay anchovies, Anchoa mitchilli, in the spring resulted in a redistribution of species with Atlantic silversides shifting to the surface waters and bay anchovies dominating the lower half of the water column. Both fishes consumed mostly copepods in the spring, but each favored a different species. There was little similarity in the large prey items consumed by the two fishes. Striped anchovies, Anchoa hepsetus, arrived in mid-summer and were most abundant at the surface while bay anchovies continued to dominate the bottom waters. Atlantic silversides were rare in all summer collections. The diets of the two anchovies were similar, but vertical separation during the period of maximum zooplankton abundance probably minimized competition. Rough silversides, Membras martinica, which were obligate surface dwellers, shared the upper water column with striped anchovies, but the two species had very different diets during their period of co-occurrence. Although seasonal changes in fish diets reflected shifts in zooplankton composition and all fishes consumed a variety of prey types, preferences for some prey taxa and total avoidance of others were indicated. Electivity indices indicated an especially strong selection for fiddler crab megalopae by all fishes in the summer and fall. All fishes, except rough silversides, which fed almost exclusively on copepods and crab zoeae, consumed large prey items when they were available. Fine scale partitioning of the food resources was apparent in the selection of different copepod and insect species by the fishes. Spatial and temporal separation in the distribution and/or dietary preferences of the zooplanktivores fishes probably reduces the potential for resource competition. Given the high abundances and selectivity of the planktivores, significant impacts on some zooplankton populations probably result.     

The effects of seaweed harvesting on fishes: a critique

Synopsis Black & Miller (1991) concluded that there was no large impact of an experimental harvest of rockweed,Ascophyllum nodosum, on fishes. A critique of their study demonstrates that this conclusion cannot be substantiated by their data because of sampling biases, errors in experimental design and low statistical power. Further, evidence is presented which supports the hypothesis that rockweed may provide an important nursery habitat for juvenile fishes.     

Sponge-dwelling Fishes of Northeastern Brazil

In this first attempt to survey the Brazilian sponge-dwelling fishes we present a list of collected fishes, with notes on their distribution, abundance and habitat preferences. Risor ruber, an obligate sponge-dwelling goby, and Scorpaenodes tredecimspinosus, never before collected in association with sponges, are recorded for the first time in the western South Atlantic based on collections made in localities on the northeastern Brazilian coast. Previous work on Risor ruber indicated that it preferred massive sponges, but in our study the majority of the specimens were found in the lumen of tubular sponges, Aplysina lacunosa, together with other fishes and invertebrates, mostly crustaceans. Elacatinus figaro, originally described as a cleaner goby, is also a sponge-dweller that occurs in waters much deeper than previously thought. The sponge community off northeastern Brazil represents the only shelter for several species of fishes in a desert of rubble and flat rocky bottoms, perhaps functioning as habitat oases. We also found four Brazilian endemic species of fishes associated with sponges in depths greater than 50m, which contradicts a previous hypothesis suggesting that endemic fishes in Brazilian coastal waters are restricted to depths less than 50m.     

Importance of Shallow Water Habitats for Demersal Fishes and Decapod Crustaceans in Penobscot Bay, Maine

We characterized demersal fish and decapod crustacean habitats in 14 shallow (< 12m) areas in Penobscot Bay, Maine, by mapping the distribution of subtidal substrata with an acoustic sea bottom profiler. We identified the aquatic vegetation and the fishes and decapods associated with these habitats. Samples of fishes and decapods in each habitat were collected with a small beam trawl. The seabed at most of the stations sampled was composed of a mosaic of substrata. More species of fishes tow^-1 and higher abundances of fishes and decapods were found in stations with vegetation present, particularly eelgrass, Zostera marina. Decapod species richness tow^-1 was similar between vegetated and unvegetated habitats. Shallow habitats in Penobscot Bay were dominated by juvenile fishes and function as nursery areas. The greater species diversity and higher abundances of epibenthic fishes and decapod crustaceans observed in vegetated habitats, particularly beds of Zostera marina, compared with unvegetated areas in Penobscot Bay conform to the hypothesis that increased habitat complexity results in increased species richness and abundance.The first author is also senior author     

Morphological differences between larvae and in vitro-cultured adults of Anisakis simplex (sensu stricto) and Anisakis pegreffii (Nematoda: Anisakidae)

Proper identification of Anisakis species infecting host fishes is very important to both human health and fish disease diagnosis. The foremost problem in the identification of Anisakis larvae in fishes is that L3 larvae cannot be easily differentiated morphologically, especially between A. simplex (sensu stricto) (s.s.) (Rudolphi, 1809) and A. pegreffii Campana-Rouget et Biocca, 1955. Instead, molecular means such as allozyme, mitochondrial DNA (mtDNA) cox2 region and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analyses had been successfully used. In this study, morphological differences of L3 larvae collected from fishes and in vitro-cultured L4 larvae and adult A. simplex (s.s.) and A. pegreffii were evaluated. Anisakis larvae were collected from 7 different host fishes within Japan. Undamaged A. simplex (s.s.) and A. pegreffii collected from Oncorhynchus keta (Walbaum) and Scomber japonicus Houttuyn, respectively, were used for in vitro-culture in order to obtain L4 and adult stages. Species identification was confirmed by PCR-RFLP analysis of the ITS region (ITS1-5.8S-ITS2) of ribosomal DNA and by mtDNA cox2 gene sequencing. Results revealed that L3, L4 and adult stages of A. simplex (s.s.) and A. pegreffii are morphologically distinguishable based on ventriculus length, wherein the former has longer ventriculus (0.90–1.50 mm) than the latter (0.50–0.78 mm). For oesophagus/ventriculus ratio, these two species are distinguishable only during L4 and adult stages. Also, adult male A. simplex (s.s.) and A. pegreffii were found to be distinguishable by differences in the distribution pattern of the caudal papillae, particularly the 3rd pair of distal papillae.     

Grazing by the intertidal fish Anoplarchus purpurescens upon a distasteful polychaete worm

Synopsis Cirratulid worms are common inhabitants of the central California rocky intertidal zone and appear to offer a potentially rich source of food for intertidal fishes. However, analyses of stomach contents revealed that they do not appear in the diets of the commonest intertidal fishes. Apparently only one species, the eel blenny Anoplarchus purpurescens, feeds on cirratulids in significant amounts.Feeding experiments employing local intertidal fishes showed that the common intertidal cirratulid, Cirriformia luxuriosa, is distasteful to most of the fishes. Only two species ate its tentacles, and only A. purpurescens consistently ate large quantities. We propose that C. luxuriosa possesses a predation-deterring chemical similar to that reported in C. spirabrancha, although A. purpurescens apparently has been able to circumvent this anti-predator mechanism.The ability of A. purpurescens to eat Cirriformia tentacles allows it to tap a seemingly little-used food source, and thus may decrease competition between A. purpurescens and other intertidal fishes. This feeding relationship also represents a possible example of coevolution between a predator and its prey.     

Seaweed-herbivore-predator interactions: host-plant specialization reduces predation on small herbivores

Summary Because feeding specialization among marine herbivores is rare, marine communities provide a simplified system for identifying factors selecting for specialization. On Australia's Great Barrier Reef, we investigated interactions among the chemically-defended seaweed Chlorodesmis fastigiata, herbivores specialized on this alga, and potential predators of these herbivores. Chlorodesmis is a low preference food for reef fishes but appears to be the only food of the crab Caphyra rotundifrons and the ascoglossan gastropods Elysia sp. and Cyerce nigricans. The crab is found only in patches of Chlorodesmis, feeds solely on the alga, and selectively shelters in it in laboratory choice experiments. Crab grazing on the red seaweed Acanthophora spicifera was stimulated when this alga was coated with increasing concentrations of the cytotoxic diterpenoid chlorodesmin, the major secondary metabolite of Chlorodesmis. Crabs did not sequester Chlorodesmis metabolites but avoided predators by sheltering in the unpalatable alga. All crabs tethered on the reef without access to Chlorodesmis patches were rapidly eaten; those with access to Chlorodesmis patches were much less susceptible to predation. The cryptic ascoglossan Elysia sp. was found exclusively in patches of Chlorodesmis and sequestered metabolites from the alga. Living Elysia were unpalatable to the common wrasse Thalassoma lunare in laboratory assays, but the crude organic extract of Elysia did not significantly deter feeding by Thalassoma. Elysia sequestered chlorodesmin, which deterred feeding by reef fishes in field assays but was ineffective against Thalassoma in laboratory assays at 5% food dry mass. Unlike Elysia, the aposematically colored ascoglossan Cyerce nigricans sequestered Chlorodesmis metabolites in relatively small amounts, but produced larger amounts of unrelated polypropionate compounds. Cyerce were never attacked by fishes and the crude organic extract of this slug strongly deterred feeding by wrasses in laboratory assays. The dorid nudibranch Gymnodoris sp. was found only in Chlorodesmis patches and appeared to be a specialized predator on Elysia; it would not prey on Cyerce. Data from this and other recent investigations demonstrate that some small marine herbivores feed selectively or exclusively on seaweeds that are chemically defended from fishes. This association reduces predation on the herbivores and suggests that escape from and deterrence of predation may be a dominant factor selecting for specialization among these herbivores.     

Spatial variation in density of stream benthic fishes in northern Hokkaido, Japan: does riparian vegetation affect fish density via food availability?

The densities of two benthic fishes, the Siberian stone loach (Noemacheilus barbatulus) and the wrinklehead sculpin (Cottus nozawae), and the biomass of their food resources (i.e., periphyton and benthic invertebrates) were compared between forest and grassland streams in northern Hokkaido, Japan, to examine whether riparian deforestation had positive effects on the benthic fishes via enhancement of food availability. The comparisons indicated that riparian vegetation had little influence on periphyton, invertebrates, or fishes. Regression analysis indicated that spatial variations in loach and sculpin densities were explained more by substrate heterogeneity, competitor abundance, or both, rather than by food abundance. However, when the two species were combined as benthic insectivores, a strong correlation was found between total benthic fish density and invertebrate biomass. Our results suggest that, although total benthic fish abundance was food limited, riparian vegetation had no positive effects via food availability on the benthic fishes in our streams.     

Molecular characterization of larval anisakid nematodes from marine fishes off the Moroccan and Mauritanian coasts

A total of 242 larval forms of Anisakis collected from marine fishes at different sites off the Moroccan and Mauritanian coasts, recognised as belonging to Type I and Type II larvae, were identified by PCR-RFLP (Polymerase Chain Reaction-Restriction Fragment Length Polymorphisms) of the ITS (Internal Transcribed Spacers) region (ITS-1, 5.8 subunit rRNA gene and ITS-2), using a previously established molecular key. The Type I larvae were found with a frequency of 98.34% and were identified as belonging to the following species: A. simplex s.str., A. pegreffii, A. simplex s.str/A. pegreffii heterozygote genotypes, A. typica, A. ziphidarum and Anisakis sp. A. The Type II larvae were found to belong to A. physeteris, with the frequency of 1.65%. The results reported in the present study provide further epizootiological and biological data on the Anisakis spp. in marine fishes off the Moroccan and Mauritanian coasts, improving the picture of the occurrence of these species in the central Atlantic coasts.     

Stable Isotope Analysis of Amphidromous Hawaiian Gobies Suggests Their Larvae Spend a Substantial Period of Time in Freshwater River Plumes

Synopsis We employed stable isotope analysis (δ¹³C, δ¹⁵N) to evaluate the sources of nutrients used by amphidromous gobiid fishes (Lentipes concolor, Sicyopterus stimpsoni, Awaous guamensis) caught migrating into and living in Hakalau Stream, Hawaii. Although considerable variation amongst the stable isotope values of stream items was noted across all 4 years of our study, the relationships between the fishes were relatively constant. Stable isotope values of recruiting gobies were consistently closer to those of both inshore plankton and freshwater adults than those of offshore plankton, suggesting that the larvae of these species derive much of their nutrition from inshore environments influenced by fresh water. Small differences between the stable values of these species further suggested that their larvae come from different inshore locations. After entering fresh water all species appear to swim rapidly upstream without feeding. Finally, once well upstream, adult L. concolor and A. guamensis appear to assume an omnivorous diet while adult S. stimpsoni rely upon autochthonous production within streams. We propose that freshwater food webs play an integral yet complex role in the lives of both larval and adult amphidromous Hawaiian fishes.     

The Distribution of Nearshore Fishes in Kelp and Eelgrass Communities in Prince William Sound, Alaska: Associations with Vegetation and Physical Habitat Characteristics

The nearshore (less than 20m depth) demersal fish community in Prince William Sound, Alaska, is dominated by Pacific cod, Gadus macrocephalus, pricklebacks (mostly Arctic shanny Stichaeus punctatus), gunnels (mostly crescent gunnels Pholis laeta), a variety of greenlings (Hexagrammidae) and sculpins (Cottidae). During summer, the spatial distribution of fishes, over scales of 100's of m to 10's of km, varied by habitats characterized by different vegetation types. Juvenile Pacific cod and greenlings were numerically dominant in eelgrass, Zostera marina, beds. Pricklebacks and sculpins were dominant in areas with an understory of the kelps Agarum cribrosum and Laminaria saccharina. Greenlings and sculpins were the most abundant demersal fishes in more exposed sites with a canopy of Nereocystis luetkeana and an understory of L. bongardiana. Measured habitat variables, including vegetation type, slope, vegetation biomass, and substratum type, explained a significant proportion of the variation in the presence or absence of most fishes. The relative importance of different habitat characteristics varied between taxonomic groups of fishes. Vegetation type explained a significant proportion of variation for cod, rockfishes, and ronquils. Juvenile cod were closely associated with eelgrass, while rockfish and ronquils were associated with kelps. Pricklebacks and rockfishes were more frequently observed on steeply sloped shorelines, while ronquils were more often found at sites with higher biomass of vegetation. Within A. cribrosum habitats, more greenlings and sculpins were present at sites where algal biomass was higher. Also, sculpins were more abundant in deeper water and gunnels were more abundant in shallow water within this habitat. These associations may not have been causative. However, evidence suggests that some differences between fish communities in eelgrass and Agarum beds may have been causally related to vegetation characteristics. The possible roles of different vegetation types as refugia from predators or as sources of prey are discussed.     

Fish assemblage of a traditional fishery and the seasonal variations in diet of its most abundant species Wallago attu (Siluriformes: Siluridae) from a tropical floodplain

The fish assemblage of a traditional Kata fishery and the stomach contents of Wallago attu were studied over a period of one year from a large floodplain system in Bangladesh. A total of 19 species of fish and 2 species of shrimp were recorded. W. attu (9.7–17.8%), Mystus vittatus (6.0–11.3%), M. aor (5.0–12.1%), Amblypharyngodon mola (4.4–9.3%) and Mastacembelus armatus (3.5–10.5%) dominated the catch. W.␣attu occupied the top position in abundance throughout the year, followed by M. vittatus and M. aor. Bagridae, Siluridae and Cyprinidae were three major families that contributed respectively 21.0, 17.4 and 16.1% of the total catch. Considerable seasonality was observed in the abundance of different fishes. A total of 14 different prey items were recorded, belonging to three major groups (fish, prawn and plant matters). The fish fed on at least eight species of small fishes (A. mola, M. vittatus, M. cavasius, Puntius stigma, P. ticto, Puntius sp., Glossogobius guris, and Heteropneustes fossilis) and some other unidentified small fishes. Other major prey items were small prawn, fish and prawn remains, and macroalgae. A. mola was the most important food item, contributing 23.7% of the total amount of diet by weight and 19.9% by frequency of occurrence. A. mola was followed by unidentified small prawn (13.7%), M. vittatus (13.1%), and unidentified small fishes (8.8%) by weight and by unidentified small fishes (15.9%), fish remains (12.5%), and M.␣vittatus (12.3%) by occurrence. Of the major diet categories, fish contributed 74.3% of the total diet by weight and 80.9% by occurrence, prawn contributed 18.5% by weight and 11.0% by occurrence and plants contributed 7.2% by weight and 8.1% by occurrence. A. mola and small shrimps were positively selected by W. attu. We concluded that W. attu is a piscivorous predator with potential impacts on prey fish communities; we also hypothesized that a specialized food-web based on the Kata fishery exists in and around the Katas which is of particular ecological significance.     

Introduction and cage culture of exotic carps and their impact on fish harvested in Lake Begnas,Nepal

The suitability of exotic carps namely Aristichthys nobilis (Bighead carp), Hypophthalmichthys molitrix (Silver carp), Ctenopharyngodon idellus (Grass carp), Cyprinus carpio (Common carp) and Labeo rohita (Rohu) in a sub-tropical lake was evaluated. The impact of their introduction on native fishes was also studied. After the introduction and cage culture of exotic carps the total harvest reached 92 kg·ha^–1; an increase of 266% within eight years. The planktivorous bighead and silver carps were most successful. The harvest of the other three species was poor. Since the introduction of exotic carp the harvest of indigenous fishes declined by 42%. Considering the food habits of these fish, further stocking should be limited to bighead and silver carps to limit the adverse effects on the indigenous species.