Food of anadromous brown trout at sea

The food of migratory Salmo trutta in coastal marine waters along the Norwegian Skagerrak coast varied significantly with age, season and habitat of the fish. The main prey categories in terms of frequency of occurrence were fishes followed by crustaceans, surface insects and polychaetes. Seasonal variation in diet and within habitats was found, supporting the view that brown trout is an opportunistic feeder. An ontogenetic niche shift was observed with post-smolts feeding on inshore and shallow water prey communities, while larger brown trout fed mainly on pelagic fishes.     

VIS – A database on the distribution of fishes in inland and estuarine waters in Flanders,Belgium

The Research Institute for Nature and Forest (INBO) has been performing standardized fish stock assessments in Flanders, Belgium. This Flemish Fish Monitoring Network aims to assess fish populations in public waters at regular time intervals in both inland waters and estuaries. This monitoring was set up in support of the Water Framework Directive, the Habitat Directive, the Eel Regulation, the Red List of fishes, fish stock management, biodiversity research, and to assess the colonization and spreading of non-native fish species. The collected data are consolidated in the Fish Information System or VIS. From VIS, the occurrence data are now published at the INBO IPT as two datasets: ‘VIS - Fishes in inland waters in Flanders, Belgium’ and ‘VIS - Fishes in estuarine waters in Flanders, Belgium’. Together these datasets represent a complete overview of the distribution and abundance of fish species pertaining in Flanders from late 1992 to the end of 2012. This data paper discusses both datasets together, as both have a similar methodology and structure. The inland waters dataset contains over 350,000 fish observations, sampled between 1992 and 2012 from over 2,000 locations in inland rivers, streams, canals, and enclosed waters in Flanders. The dataset includes 64 fish species, as well as a number of non-target species (mainly crustaceans). The estuarine waters dataset contains over 44,000 fish observations, sampled between 1995 and 2012 from almost 50 locations in the estuaries of the rivers Yser and Scheldt (“Zeeschelde”), including two sampling sites in the Netherlands. The dataset includes 69 fish species and a number of non-target crustacean species. To foster broad and collaborative use, the data are dedicated to the public domain under a Creative Commons Zero waiver and reference the INBO norms for data use.     

An Analysis of Fish Species Richness in Natural Lakes

There is a growing recognition of the need to conserve biodiversity that has been conceptualised in the Convention of Biological Diversity. Maintenance of fish species richness is particularly important, because habitat degradation in inland waters continues to accelerate on a global scale. Here we develop empirical models for predicting fish species richness in natural lakes in various geographical regions of the world. In tropical lakes where fish biodiversity is richer than in temperate lakes, fish species richness can be predicted by a few variables such as lake area and altitude. Low fish species richness in most temperate lakes might be due to the effect of glaciation on colonisation and speciation of fishes. In US, Canadian and northern European lakes, lake acidification is one of the important factors influencing fish species richness. Although limnological characteristics influence fish species richness in temperate lakes, lake area and altitude have greater predictive power. This is in contrast to fish species richness in rivers, which can be reliably predicted by basin area. In the power curves, which describe the relationship between fish species richness and habitat size in lakes and rivers, the exponent is always greater in tropical regions than in temperate regions. Because fish biodiversity is greater in the tropics threats to fish biodiversity through habitat degradation are greater than those in temperate inland waters.     

Nearshore fish habitat utilization and species associations in Lake Opinicon (Ontario,Canada)

Synopsis The bulk of Lake Opinicon fish biomass is concentrated in the physically diverse inshore areas. Quantitative analysis of the community compositions of the various inshore habitat types (weedy inlets, sandy shallows, rock shelf, gravel, etc.) showed that each supported a characteristic assemblage of fishes with a relatively constant species and year class composition. This was maintained throughout the season despite a drop in fish biomass in late summer when there was a progressive movement into the offshore waters.Weedbed areas supported the highest biomass, and greatest species and year class diversity. Thereafter, in declining order of richness, were rocky and sandy areas. A few fish species were restricted to single habitat types, e.g.Notropis heterodon to inshore weedbeds.Lepomis macrochirus, the commonest species in the lake was, by contrast, versatile and occurred in all inshore habitats. Commonly the numbers of a species in a habitat differed between day and night. Significant diel movements between habitats characterized two nocturnal feedersPomoxis nigromaculatus andIctalurus nebulosus.     

Food and feeding ecology of piscivorous fishes at Lake St Lucia, Zululand

The food and feeding ecology of piscivorous fish in Lake St Lucia was monitored for two years. Piscivorous fishes feed predominantly on the planktivorous Gilchristella aestuarius and Thryssa vitrirostris but a wide range of prey species was recorded. Numbers of the predominant piscivores, Argyrosomus hololepidotus and Elops machnata , in an area appear to be related to the densities of their major prey, T. vitrirostris and G. aestuarius . Large piscivorous fishes are restricted to the deeper portions of the lake, whereas small piscivores such as Johnius belengerii and Terapon jarbua feed predominantly on small fishes in the littoral zone. The highly significant correlation between the composition of prey fish species in the lake and prey fish species in the diet of piscivorous fishes, indicates that piscivores are feeding in a density dependent manner. However, factors such as habitat, fish size and swimming speed of prey species are shown to be important in prey selection. Juvenile fish of species such as Sarotherodon mossambicus, Liza macrolepis and Acanthopagrus berda remain in shallow marginal areas, thus avoiding large piscivorous fishes. However by frequenting shallow areas these species become vulnerable to bird predators, especially egrets and herons.     

Why are there so few freshwater fish species in most estuaries?

The freshwater fish assemblage in most estuaries is not as species rich as the marine assemblage in the same systems. Coupled with this differential richness is an apparent inability by most freshwater fish species to penetrate estuarine zones that are mesohaline (salinity: 5·0–17·9), polyhaline (salinity: 18·0–29·9) or euhaline (salinity: 30·0–39·9). The reason why mesohaline waters are avoided by most freshwater fishes is difficult to explain from a physiological perspective as many of these species would be isosmotic within this salinity range. Perhaps, a key to the poor penetration of estuarine waters by freshwater taxa is an inability to develop chloride cells in gill filament epithelia, as well as a lack of other osmoregulatory adaptations present in euryhaline fishes. Only a few freshwater fish species, especially some of those belonging to the family Cichlidae, have become fully euryhaline and have successfully occupied a wide range of estuaries, sometimes even dominating in hyperhaline systems (salinity 40+). Indeed, this review found that there are few fish species that can be termed holohaline (i.e. capable of occupying waters with a salinity range of 0–100+) and, of these taxa, there is a disproportionally high number of freshwater species (e.g. Cyprinodon variegatus, Oreochromis mossambicus and Sarotherodon melanotheron). Factors such as increased competition for food and higher predation rates by piscivorous fishes and birds may also play an important role in the low species richness and abundance of freshwater taxa in estuaries. Added to this is the relatively low species richness of freshwater fishes in river catchments when compared with the normally higher diversity of marine fish species for potential estuarine colonization from the adjacent coastal waters. The almost complete absence of freshwater fish larvae from the estuarine ichthyoplankton further reinforces the poor representation of this guild within these systems. An explanation as to why more freshwater fish species have not become euryhaline and occupied a wide range of estuaries similar to their marine counterparts is probably due to a combination of the above described factors, with physiological restrictions pertaining to limited salinity tolerances probably playing the most important role.     

Human development modifies the functional composition of lake littoral invertebrate communities

Free-floating plants are important components of aquatic ecosystems in tropical climates, playing a key role in the structure and spatial distribution of fish communities. This study aims at elucidating the potential effects of free-floating vegetation on fish community structure in a tropical floodplain lake, using an experimental approach based on natural and artificial devices (Eichhornia crassipes), in high and low (LT) turbidity waters. A total of 32 fish species were found, richness, abundance, biomass, mean fresh body weight, and standard length were all significantly higher in the LT regions. Although no significant differences of community traits were found between artificial and natural substrates, regardless of water turbidity, fish composition differences between devices were observed in clear waters. Benthivorous fishes were the most widespread trophic group, with higher abundance and biomass in LT, while no differences were found among plant types. The results confirmed the structural role played by free-floating plants in the fish community by offering a refuge to smaller bodied fish species and younger specimens of larger species, independently of turbidity conditions. However, the effect was stronger in clear waters. The evidence also supported the hypothesis that the fish community forages within the plant beds. Turbidity spatial gradients or turbid regimes in tropical shallow lakes, as well as important floating macrophyte coverage could have strong impacts on the fish community structure.     

The fish populations in inshore waters of the west coast of Scotland. The food and feeding of the whiting (Merlangius merlangus L,.)

The food and feeding of the whiting ( Merlangius merlangus ) of the inshore waters and sea lochs of the west of Scotland were studied from 1969–1973. Fish and Crustacea were the main food items and their occurrence in the stomachs was clearly related to the size of the whiting and also to their seasonal abundance. The diurnal changes in feeding were also investigated and the results indicated that the size of fish was again an important factor which could affect the interpretation of the seasonal data.     

An analysis of the distribution of fish species in a large prairie lake

The distribution of fishes in Dauphin Lake, a large, shallow, turbid, prairie lake in west-central Manitoba, was examined using gillnets. Catch per unit effort was used to assess differences in distribution thai may be attributed lo year, season, lake zone (inshore, offshore), substratum, water depth, water temperature, turbidity and presence of other fish species. Although 13 species were caught, only the distributions of the six most abundant was analysed. Variation in number of species caught was attributed mainly to lake zone, water depth and water temperature. White sucker (Catastomus commersoni) distribution can be explained by zone and temperature. Shorthead redhorse (Moxostoma macrolepidotum) catches were related to depth and temperature. Cisco (Coregonus artedii) catches varied with year, zone and temperature. Variances in northern pike (Esox Indus) distribution reflected year and zone. Walleye (Stizostedkm vitreum) varied with year, depth and temperature and differences in the catches of yellow perch (Pertaflurescens) were attributed to year, depth, temperature and turbidity. They were also positively correlated to the numbers of northern pike caught.     

Recherches hydrobiologiques au lac Tumba (Congo Belge,Province de l'Equateur)

Summary Lake Tumba is a shallow lake situated in the Congo basin near the Equator. Its area is about 191.250 acres. It gets its waters from different rivers and the surplus is shed into the Congo River, through the Irebu Channel, a little south-west from Coquilhatville. The maximum depth is about 8 meters and the mean depth is 3 to 5 meters (see bathymetric map).The waters are very rich in humic substances, owing to their origin in the forest soils around the lake. Its acid and brown waters are poor in true phyto-and zooplancton. The bottom is hard, made of kaolin and sand, without mud and as a matter of consequence, the bottom fauna is very poor. There is oxygen in the water down to the bottom at any time.The coasts are covered with a thick forest and dead leaves, branches and tree trunks contribute much foodstuff to the waters. This makes it possible for this lake to be rather rich in fish. Sixty six species of fishes are listed for the lake itself and ten species for the swamps or the flooded forest which, at high waters, communicate with the lake. Their food relations are described.As mature fish is very rare in the lake, the hypothesis has been made that, at high waters, the fishes migrate into the flooded forests and swamps around the lake when maturity comes. Breeding would happen in the inundated areas and the fish would return afterwards to the lake.A discussion is given about the origin of lake Tumba. Morphology of the lake, Chemistry of the waters and nearly complete lack of endemic species and of pelagic life point to a recent origin.The hypothesis is advanced that lake Tumba would be the result of the damming of an affluent of the Congo River and not the residue of the slow evaporation of an ancient central african lake.     

Clay-Turbid Interactions May Not Cascade—A Reminder for Lake Managers

Food web management is a frequently used lake restoration method, which aims to reduce phytoplankton biomass by strengthening herbivorous zooplankton through reduction of planktivorous fish. However, in clay‐turbid lakes several factors may reduce the effectivity of food web management. Increasing turbidity reduces the effectivity of fish predation and weakens the link between zooplankton and phytoplankton. Therefore, the effects of fish stock manipulations may not cascade to lower trophic levels as expected. Additionally, in clay‐turbid conditions invertebrate predators may coexist in high densities with planktivorous fish and negate the effects of fish reductions. For instance, in the stratifying regions of the clay‐turbid Lake Hiidenvesi, Chaoborus flavicans is the main regulator of cladocerans and occupies the water column throughout the day, although planktivorous Osmerus eperlanus is very abundant. The coexistence of chaoborids and fish is facilitated by a metalimnetic turbidity peak, which prevents efficient predation by fish. In the shallow parts of the lake, chaoborids are absent despite high water turbidity. We suggest that, generally, the importance of invertebrate predators in relation to vertebrate predators may change along turbidity and depth gradients. The importance of fish predation is highest in shallow waters with low turbidity. When water depth increases, the importance of fish in the top‐down regulation of zooplankton declines, whereas that of chaoborids increases, the change along the depth gradient being moderate in clear‐water lakes and steep in highly turbid lakes. Thus, especially deep clay‐turbid lakes may be problematic for implementing food web management as a restoration tool.     

A review on the early life history and ecology of Japanese sea bass and implication for recruitment

Recruitment in marine fishes is regulated largely by the demographic changes that occur during the early life stages; therefore, a thorough understanding of early life stages is essential for predicting recruitment variability in fishes. Japanese sea bass (JSB), Lateolabrax japonicus, is a coastal marine fish distributed in East Asian coastal waters, and is regarded as highly important for commercial and recreational fisheries, for marine and brackish water aquaculture as well as for stock enhancement. JSB is a typical estuarine dependent temperate fish, which spawns in shelf areas and coastal embayments and the larvae and juveniles are dispersed and transported into shallow nearshore habitats and estuaries where they spend the early life. In this paper, we provide insight into the early life history and ecology of JSB through a revision of the available information and using the data we obtained from a relatively long-term research. We review and discuss the distribution and habitat use, food and feeding, age and growth, mortality and recruitment of larval and juvenile JSB in coastal waters around Japan. We extend our discussions in all available dimensions: habitat-specific, ontogenetic, and spatio-temporal, and highlight the importance of nursery habitats. We also discuss the implications of early life history for recruitment of JSB as well as the possible effects of climate change. At the end, we point out potential areas for future research.     

The food of Bagrus docmac (Forsk.) (Pisces: Siluriformes) and its relationship with Haplochromis Hilgendorf (Pisces: Cichlidae), in Lake Victoria, East Africa

The diet of the siluriform fish Bagrus docmac (Forsk.) is described in relation to various localities in Lake Victoria, East Africa. Invertebrates which are the principal food of young fish, but generally of little significance in adults, are particularly poorly represented in fishes from Tanzanian waters and most important in those from the Kavirondo Gulf where the ontogenetic changes in diet differ from those found elsewhere in the Lake. B. docmac starts feeding piscivorously at about 10 cm standard length with the transition from a principally invertebrate to a primarily piscivorous diet occurring at 15 cm S.L. In adult B. docmac fish are found in over 95 % of individuals with food. Haplochromis is the all important food type, and Tilapia , the most valuable commercial fish in the lake, is not preyed upon at all. Bathymetric changes with respect to the invertebrate food are noted, and it is shown that piscivorous feeding occurs mainly by day and invertebrate feeding by night. Quantitative estimates of consumption of Haplochromis by B, docmac are computed from the results of a series of 24 h fishing surveys and found to exceed published data from elsewhere. The formula for the computation of a daily ration is given as Log R =2.973 Log L —3.36089 and it is calculated that the mean annual consumption of Haplochromis by B. docmac is approximately 75% of the mean standing stock of the prey fish, as compared with a commercial catch of about 5% of the mean standing stock.     

Changes in rocky reef fish assemblages throughout an estuary with a restricted inlet

Rocky reef habitat is common in many estuaries, yet its role as a habitat for fishes is poorly understood. There is also limited understanding of how access of coastal species into estuaries and habitat quality can affect the distribution of rocky reef fishes within estuaries. This study used baited remote underwater video stations to determine spatial patterns in fish assemblages associated with rocky reef habitat throughout a barrier estuary with a permanently open but restricted inlet. Estuarine rocky reefs provided habitat for a diverse assemblage of fishes, many of which were large juveniles and subadults. In the absence of a pronounced salinity or temperature gradient, a clear transition in fish assemblages occurred from coastal waters, through the inlet channel, to the central estuary, and into the inner estuary. The inlet channel, notably its narrowness and length, limits tidal input into this estuary, which acts as a significant impediment to the dispersal of many coastal fishes, and insufficient habitat excludes many coastal rocky reef species from the inner estuary. This study highlights the need to recognise estuarine rocky reefs as providing habitat for diverse fish assemblages and the role inlets play in restricting access of coastal species.     

Comparative feeding strategies of yellowfin tuna around St Helena and adjacent seamounts of the South Atlantic Ocean

Yellowfin tuna are the mainstay of the traditional tuna fisheries in St Helena waters, but there is limited knowledge of their ecology and feeding behaviour in the area. In this study yellowfin tuna stomach contents were used to assess spatio-temporal changes in feeding strategy and consider the role of tuna in the local ecosystem. Comparisons of the feeding spectra of yellowfin tuna between inshore regions of St Helena and oceanic seamounts demonstrated that in both areas the species was largely piscivorous. In inshore waters yellowfin consumed more neritic fauna, including significant numbers of crab megalopa, whereas around seamounts the diet included a greater diversity of epi- and mesopelagic fish and squids. The most important fish prey species in inshore waters was the St Helena butterflyfish Chaetodon sanctahelenae, and around seamounts was the pufferfish Lagocephalus lagocephalus. Results indicate that the diet spectrum of yellowfin tuna in St Helena waters is relatively similar to those of conspecifics living in waters with relatively low productivity, with strategies indicative of food-poor ecosystems. The availability of coastal fauna may make areas around islands and seamounts more attractive for feeding aggregations of yellowfin tuna, compared to the open ocean. The relatively unselective feeding of yellowfin tuna means that stomachs can provide valuable data on the species diversity, particularly in remote areas with limited opportunities for dedicated research expeditions.     

Are distributions of fishes in tropical estuaries influenced by turbidity over small spatial scales?

This study found no evidence that turbidity influenced the distribution of fishes in four tropical estuaries in north-eastern Queensland. When both more turbid and less turbid environments were immediately available to Ambassis vachelli and Leiognathus equulus there was no difference in abundance between the more and less turbid environments and no correlated change in abundance as the difference in turbidity increased. A similar outcome was evident for an additional six species of fish. Furthermore, the distribution of fishes was not correlated with the distribution of an important prey species, juvenile banana prawns Penaeus merguiensis , which exhibited a clear response to differences in turbidity level with higher abundance in the more turbid samples, however, the magnitude of the difference did not alter with changes in the magnitude of the differences in turbidity.     

The influence of turbidity on juvenile marine fishes in estuaries. part 1. field studies at Lake St. Lucia on the southeastern coast of Africa

Lake St. Lucia, the largest estuarine system in Africa (325 km²), was chosen as the field study area for a 3.5-yr ((1980)–83) investigation into relationships between water turbidity and estuarine fish distribution. The variety of habitats, from clear water, open sandy shores to shallow muddy substrata and turbid waters, together with high species diversity (108 species) rendered the area suitable for this study. The relationships between fish distribution and environmental factors were monitored by monthly seine netting of fishes at seven sites representative of the range of conditions in St. Lucia. Simultaneously, water turbidity, salinity, and temperature were recorded.The possible influences of substratum type and food availability were also investigated by using recently published data on invertebrate benthos and Zooplankton distributions. Published data were also used to determine the diet of the common fish species. The results showed that the distribution of juveniles of the 20 commonest fish species were statistically correlated only with water turbidity, water temperature, and food availability. The correlation with temperature was related to seasonal not spatial temperature patterns.Turbidity and food type influences were difficult to separate but exceptions were the anchovyThryssa vitrirostris (Gilchrist and Thompson) and the soleSolea bleekeri Boulenger which occurred only in turbid water despite the widespread occurrence of their prey, andGenes acinaces Bleeker,G. rappi (Barnard), andG. fllamentosus Cuvier, all of which occurred only in clear water although the greatest densities of their bivalve prey were in turbid waters. Similarly, the sparidsRhabdosargus holubi (Steindachner) andR. sarba (Forsskal) were distributed according to turbidity and not their preferred foods.Principal component analysis with a minimum spanning tree plot and a canonical correlation test showed that the fish fauna could be divided into five groups according to their occurrence in various turbidities. These were: clear water species (e.g. Gerreidae) in < 10 NTU, clear to partially turbid species (e.g.Liza dumerilii (Steindachner) andL. macrolepis (Smith)) in < 50 NTU, intermediate turbidity species (e.g.Valamugil cunnesius (Valenciennes) andLeiognathus equula (Forsskal)) in 10–80 NTU, turbid-water species (e.g.Elops machnata (Forsskal) andThryssa vitrirostris) in > 50 NTU, and species indifferent to turbidity (e.g.Acanthopagrus berda (Forsskal) andTeraponjarbua (Forsskal). It is, therefore, suggested that turbidity plays a significant roˆle, either singly, or in combination with other variables in determining the distribution of juvenile marine fishes in estuaries.     

Comparison of inshore and offshore fish larval assemblages within the wider Yangtze River estuary (China)

It is hypothesized that fish larvae undertake vertical movements in estuarine waters. To test this hypothesis, a sampling period was selected during times when many spring‐spawning fish reach maximum abundance in coastal waters so that spring and summer variations in larval fish assemblages could be determined in the Yangtze River estuary. Six oceanographic surveys were conducted across the salinity gradient of an inshore (freshwater) and offshore area (brackish intersection) during spring (May) and summer (August) between 2010 and 2012. The fish larval community was dominated by species of Engraulidae, Gobiidae, Champsodontidae and Mugilidae. The pre‐flexion and flexion larval stages of euryhaline marine species, which are dependent on estuaries as nursery areas, were common. The brackish and marine larval assemblage was the most abundant with taxa such as Coilia mystus and Engraulis japonicus accounting for more than 57.3% of the total catch. Spatial differences in the taxonomic composition of larval fish assemblages were evident between the inshore and offshore areas. Additionally, the Yangtze River runoff regulatory functions as affected by the Three Gorges Reservoir operational mode (hydrological alternating operations) showed weak influences on fish and habitat environments. Low salinities from high freshwater inflow limited bay anchovy production in the inshore area.     

THE SCIENCE: LIMNOLOGY

SUMMARY

Lake Chilwa, which has a fishery of economic importance to Malawi, declined in water level from 1964–68 and dried up completely for a few months in 1968. It then regained its former depth of 2–3 m in one wet season of five months. In the pre-drying period, zoo-plankters produced different diapause forms; in the drying period, zooplankton appeared to die out; in the refilling period rotifers predominated in the first year and crustacean populations were low; in the post-filling period crustaceans predominated. The same species occurred both before and after dryness. The conditions that possibly determined these responses are considered. After a lag of one year numbers of zooplankters were high, but decreased annually when the three dominant, largely planktivorous species of fishes recolonised the lake in succession over three years.

A main detritus food web is postulated and evidence for it is discussed. A practical method for fish conservation in times of low lake level, using artificial ‘lagoons’ in the swamp, and boreholes, is suggested.     

Managing conflicts arising from fisheries enhancements based on non‐native fishes in southern Africa

Southern Africa has a long history of non‐native fish introductions for the enhancement of recreational and commercial fisheries, due to a perceived lack of suitable native species. This has resulted in some important inland fisheries being based on non‐native fishes. Regionally, these introductions are predominantly not benign, and non‐native fishes are considered one of the main threats to aquatic biodiversity because they affect native biota through predation, competition, habitat alteration, disease transfer and hybridization. To achieve national policy objectives of economic development, food security and poverty eradication, countries are increasingly looking towards inland fisheries as vehicles for development. As a result, conflicts have developed between economic and conservation objectives. In South Africa, as is the case for other invasive biota, the control and management of non‐native fishes is included in the National Environmental Management: Biodiversity Act. Implementation measures include import and movement controls and, more recently, non‐native fish eradication in conservation priority areas. Management actions are, however, complicated because many non‐native fishes are important components in recreational and subsistence fisheries that contribute towards regional economies and food security. In other southern African countries, little attention has focussed on issues and management of non‐native fishes, and this is cause for concern. This paper provides an overview of introductions, impacts and fisheries in southern Africa with emphasis on existing and evolving legislation, conflicts, implementation strategies and the sometimes innovative approaches that have been used to prioritize conservation areas and manage non‐native fishes.