Fish Communities In Lakes Research Articles

Development and evaluation of the Finnish fish-based lake classification method

The Finnish fish-based lake assessment system EQR4 was updated using more extensive lake data with tighter reference criteria. The updated EQR4 (EQR4new) had significant and tighter pressure (eutrophication) relationship compared to the previous version (EQR4old). The EQR4new separated impacted and reference lakes with lower misclassification risk compared to the EQR4old. All the variables within the multi-metric index EQR4new (total biomass, total number of individuals, cyprinid biomass %, and indicator species) were found to be relevant. The classification output in relation to eutrophication was higher in the northern part of the country compared to the southern lakes which was probably due to the climate-dependent reduction in productivity. Despite the clear pressure response and the ability to separate reference and impacted lakes, the accuracy of the EQR4new values is relatively modest. Including regional patterns in the composition and variability of lake fish communities in Finland would increase the accuracy of the assessment method and, moreover, contribute to the excellence of fish community ecology of boreal lakes.

Analysis of Prey Selection by Double‐Crested Cormorants: A 15‐Year Diet Study in Oneida Lake, New York

AbstractPiscivorous birds, in particular cormorants Phalacrocorax spp., have been reported to cause declines in some fish populations in both Europe and North America, but not in others. This difference may be due to prey selection by cormorants that is further dependent on the composition of the fish assemblage present. We present 15 years of diet data collected from Oneida Lake, New York, where we have previously documented negative effects of double‐crested cormorants Phalacrocorax auritus on two fish populations valued by anglers: Walleye Sander vitreus and Yellow Perch Perca flavescens. The Oneida Lake fish community changed through the study period and this change was reflected in cormorant diet samples. Diet samples were variable based on season and year with Emerald Shiner Notropis atherinoides, Gizzard Shad Dorosoma cepedianum, Lepomis spp., Logperch Percina caprodes, Walleye, and Yellow Perch having the highest overall relative importance. In years when age‐0 Gizzard Shad were abundant they dominated double‐crested cormorant diets in the fall after the shad reached a length of 45 mm. Consumption of Emerald Shiner and Gizzard Shad was positively related to each species’ abundance, but no significant correlation between availability and consumption was found for Walleye, White Perch Morone americana, or Yellow Perch. Double‐crested cormorants may be displaying prey‐switching behavior and selecting for smaller, soft‐rayed, prey species. We conclude that variation in fish recruitment influences double‐crested cormorant food selection habits and highlight the importance of continued monitoring with changing fish communities to reliably assess potential impacts of cormorants on a fish community over time.

Impaired littoral energy pathways cause a shift to pelagic resources by fish in recovering lake food webs

AbstractFish communities may increase in biomass and productivity due to energy subsidies from the littoral invertebrate community. In lakes recovering from acidification and metal contamination, such as those in Sudbury, Ontario, Canada, impaired benthic invertebrate communities (i.e., low diversity with higher abundance of small‐bodied taxa) allowed a critical test of the role of these littoral pathways on fish diet. We compared fish abundance, diversity, diet and biomass in eight recovering and eight reference lakes and related availability of the main littoral and pelagic invertebrate groups to fish diet regime using stable isotope analysis. A Bayesian mixing model (MixSIR) was used to estimate diet likelihood, and convex hull analysis was used to estimate trophic niche space of fish communities. Fish biomass did not differ between impaired and reference lakes despite substantial differences in potential diet. Fish depended strongly on littoral benthos in the reference lakes but consumed more pelagic food in the impaired lakes. The trophic niche of the focal, most common fish species (i.e., yellow perch, smallmouth bass, pumpkinseed and brown bullhead) was larger in the impaired lakes. We attributed these differences to low diversity at the highest trophic levels of fish communities in the impaired lakes as well as to depauperate benthic invertebrate communities. In contrast to the food webs of most temperate lakes, fish in impaired lakes preyed less on littoral invertebrates yet still managed to maintain a reference lake level of biomass standing crop by relying more on pelagic resources – macro zooplankton such as Chaoborus.

Trophic fingerprint of fish communities in subtropical floodplain lakes

AbstractStable nitrogen isotope ratios (δ15N) and gut content analysis are commonly used to detect trophic relationships in aquatic systems. Nonetheless, the use of δ15N in determining the suitability of quantitative and qualitative dietary data as representations of trophic relationships in lake ecosystems remains unverified. Quantitative and qualitative dietary data on 46 fish species were obtained from field surveys in the floodplain lakes of the Yangtze Plain to calculate trophic position and benthivory. Dietary measures using the stable isotope approach were validated. The dietary approach yields comparable results in detecting trophic relationships in lake ecosystems. Our estimation of the benthivory of the 46 fish species ranged from 1.5% to 100%, with an average of 60.3%. These figures support the assumption that benthic energy pathways account for more than half of total fish consumption. Our feeding guild data confirm that more than 50% (including 25 species) of the studied fish species in the floodplain lakes of the Yangtze Plain are omnivorous, suggesting that omnivory is prevalent among subtropical floodplain lakes.

Temporal variation of energy sources in a floodplain lake fish community

We compared the stable isotope ratios of δ 13C and δ 15N of four basal food sources and eight fish species collected in spring and summer from the largest freshwater lake in China, Lake Poyang. The aim was to study the temporal variations in the fishes’ energy sources. The δ 13C and δ 15N ratios of all eight species of fish were higher in summer. Employing the IsoSource mixing model, we identified that the terrestrial plant Carex spp. was the most important primary producer supporting fish biomass in spring when the water level was low; meanwhile, the submerged macrophyte Potamogeton malaianus made consistent contributions to fish. However, in summer, when the water level was high, the submerged macrophyte Ceratophyllum demersum was the most important primary production source for fish. Our study indicated that feeding links in floodplain lakes are complex and modified by hydrologic seasonality.

Smallmouth Bass in the Pacific Northwest: A Threat to Native Species; a Benefit for Anglers

As a popular sportfish, smallmouth bass (Micropterus dolomieu) generates considerable angling opportunities with benefits to local economies even outside of their native range. Smallmouth bass was first introduced to the Pacific Northwest region of North America as a sportfish over 80 years ago, and this species is now widely distributed. More recently, smallmouth bass have become a large component of the fish community in many streams, rivers, and lakes. Smallmouth bass thrive in the Pacific Northwest largely due to the habitat created by human modifications of the landscape. While a desired sportfish, smallmouth bass may also negatively affect native fishes. Of greatest concern is predation on threatened and endangered Pacific salmon; however, the current level of knowledge is inadequate to make informed management decisions for smallmouth bass. Management options for smallmouth bass are complicated further because fisheries agencies are simultaneously charged with enhancing fishing opportunities and controlling predators of threatened and endangered salmon. To advance conservation science, there is a need to determine the utility of different management approaches, and testing options in key areas of overlap between smallmouth bass and salmon is suggested.

Relationships between species feeding traits and environmental conditions in fish communities: a three-matrix approach

Understanding the relationships between species biological traits and the environment is crucial to predicting the effect of habitat perturbations on fish communities. It is also an essential step in the assessment of the functional diversity. Using two complementary three-matrix approaches (fourth-corner and RLQ analyses), we tested the hypothesis that feeding-oriented traits determine the spatial distributions of littoral fish species by assessing the relationship between fish spatial distributions, fish species traits, and habitat characteristics in two Laurentian Shield lakes. Significant associations between the feeding-oriented traits and the environmental characteristics suggested that fish communities in small lakes (displaying low species richness) can be spatially structured. Three groups of traits, mainly categorized by the species spatial and temporal feeding activity, were identified. The water column may be divided in two sections, each of them corresponding to a group of traits related to the vertical distribution of the prey coupled with the position of the mouth. Lake areas of low structural complexity were inhabited by functional assemblages dominated by surface feeders while structurally more complex areas were occupied by mid-water and benthic feeders. A third group referring to the time of feeding activity was observed. Our work could serve as a guideline study to evaluate species traits x environment associations at multiple spatial scales. Our results indicate that three-matrix statistical approaches are powerful tools that can be used to study such relationships. These recent statistical approaches open up new research directions such as the study of spatially based biological functions in lakes. They also provide new analytical tools for determining, for example, the potential size of freshwater protected areas.

Native fish diversity alters the effects of an invasive species on food webs

Aquatic communities have been altered by invasive species, with impacts on native biodiversity and ecosystem function. At the same time, native biodiversity may mitigate the effects of an invader. Common carp (Cyprinus carpio) is a ubiquitous, invasive fish species that strongly influences community and ecosystem processes. We used common carp to test whether the potential effects of an invasive species are altered across a range of species diversity in native communities. In mesocosms, treatments of zero, one, three, and six native fish species were used to represent the nested subset patterns observed in fish communities of lakes in Illinois, USA. The effect of the invader was tested across fish richness treatments by adding common carp to the native community and substituting native biomass with common carp. Native species and intraspecific effects reduced invader growth. The invader reduced native fish growth; however, the negative effect was minimized with increasing native richness. The zooplankton grazer community was modified by a top-down effect from the invader that increased the amount of phytoplankton. Neither the invader nor richness treatments influenced total phosphorus or community metabolism. Overall, the invader reduced resources for native species; and the effect scaled with how the invader was incorporated into the community. Higher native diversity mitigated the impact of the invader, confirming the need to consider biodiversity when predicting the impacts of invasive species.

Variations of fish composition and diversity related to environmental variables in shallow lakes in the Yangtze River basin

Variations in fish communities of shallow lakes in the Yangtze basins were investigated from September 2007 to September 2009. Six lakes were chosen for comparative study of species composition and diversity in relation to environmental variations. Lake heterogeneity was described with environmental physico-chemical variables, using principal component analysis. Sixteen families, composed of 75 species of fish were found in the studied lakes, Cyprinidae being the dominant group. Fish species were divided by habitat preference and trophic guild: benthopelagic and herbivorous fish were the most common guilds in all lakes. Species diversity and richness were significantly higher in spring, while the evenness, expressed by equitability of Simpson's index, was not significantly different among seasons. Species richness and diversity were significantly higher in vegetated lakes (e. g. Liangzihu Lake) than in non-vegetated lakes (e. g. Biandantang Lake), with the largest area (Liangzihu Lake) harbouring the largest species richness and the greatest diversity. The relationship between environmental variables and fish assemblage were analysed using canonical correspondence analysis (CCA). The dominant gradients describing species composition and abundance among the sampling sites were: total phosphorus, total nitrogen, chlorophyll a, transparency and water depth. Our study led to the following conclusions: 1) the water quality was better - i.e. high transparency, low total phosphorus (TP) and total nitrogen (TN) and chlorophyll a- in vegetated lakes than in unvegetated lakes; 2) vegetated lakes had higher fish diversity than unvegetated lakes; 3) fish relative abundance (CPUE: number of fish per fishing pass) was significantly related to water chemical parameters. Consequently, the details of the findings are useful and relevant for developing suitable conservation strategies to sustain the integrity of fish communities in these lakes.

Initial and long-term consequences of attempts to improve fish-food resources in Lithuanian waters by introducing alien peracaridan species: a retrospective overview

Summary In this overview, we consider the rationale and practice of invertebrate introductions into Lithuanian waters, the current distribution of introduced species, test the theoretical expectations of introduction of peracaridan species into lakes, and summarize the large-scale consequences of these translocations. After some early attempts to transfer locally available species within the country, peracaridan species from the Ponto-Caspian region were introduced into Lithuanian waters during the 1960s (Pontogammarus robustoides, Obesogammarus crassus, Chaetogammarus warpachowskyi, Paramysis lacustris, Limnomysis benedeni and Hemimysis anomala). After their establishment at the site of first introduction in a newly constructed water reservoir on the Nemunas River, further deliberate introductions into Lithuanian lakes and water reservoirs, as well as into more distant areas, were undertaken. These introduced species soon contributed to fish diet and a subsequent increase in the production of commercially important fish was envisaged. However, our collation of available data does not support the enhancement of fish production in Lithuanian lakes. Although perch (selected as a model fish species) assimilated the introduced species into diet and sometimes in large quantities, there was no subsequent influence on somatic growth rates when compared with perch from lakes devoid of alien peracaridans. Comparisons of littoral fish communities of different lakes, and commercial catches in the lake with the most numerous populations of introduced species also indicate no significant effect on fish production. Negative consequences of introduction are more than obvious in contrast. To date, most of the introduced Ponto-Caspian amphipod and mysid species occur in different sites of the Baltic Sea basin outside Lithuania, and their further dispersal may be expected. These species, especially amphipods, have proved to be highly ecologically aggressive, inducing change in resident macroinvertebrate communities and causing increase of biological contamination (i.e. detract from naturalness). Thus, the introduction of alien fodder species was ill advised from an economic, and even adverse from an environmental perspective.

An overview of fish species introductions to the English Lake District, UK, an area of outstanding conservation and fisheries importance

Fisheries management has a considerable capacity to precipitate legal and illegal introductions of fish species to new geographical areas. Consequently, there are many examples of such introductions and their subsequent short-term effects on native fish. However, longer-term impacts which may be expected to be more substantial have been less frequently studied in detail. For this overview, long-term datasets up to a number of decades in duration have been assembled from published and unpublished sources on the native lake fish communities of the English Lake District in north-west England, UK. These communities are dominated by simple salmonid-percid assemblages and contain England's only vendace (Coregonus albula), whitefish (Coregonus lavaretus) and Arctic charr (Salvelinus alpinus) populations, all of which are protected under nature conservation legislation. In addition, they have historically supported commercial or semi-commercial fisheries for perch (Perca fluviatilis), whitefish and Arctic charr, the latter of which still persist on two major lakes. In recent years, fisheries activities have resulted in the region experiencing a number of illegal fish introductions including the cyprinids roach (Rutilus rutilus) and dace (Leuciscus leuciscus) and the percid ruffe (Gymnocephalus cernuus). Although these introductions have benefited some local recreational fisheries, several instances of cultural eutrophication and a more widespread increase in lake temperature are likely to benefit these introduced species and disadvantage native salmonids and percids. The resulting changes in fish community compositions and the performances of local fisheries are described and discussed.

Invasive fish species in the largest lakes of Scotland, Northern Ireland, Wales and England: the collective UK experience

An invasive species is defined as an alien (or introduced or non-native) species whose establishment and spread threaten ecosystems, habitats or species with harm. Such threats to UK lake fish communities have long been appreciated and this review assembles case histories, including new data, from the largest lakes of Scotland, Northern Ireland, Wales and England to examine the hypothesis that at least some of these introductions have become invasive. Loch Lomond in Scotland has experienced six introductions [chub (Leuciscus cephalus), common bream (Abramis brama), crucian carp (Carassius carassius), dace (Leuciscus leuciscus), gudgeon (Gobio gobio) and ruffe (Gymnocephalus cernuus)], of which the most significant has been that of the percid ruffe, which has been implicated in a recent decline of the native coregonid whitefish (Coregonus lavaretus). In Northern Ireland, the introduction of the cyprinid roach (Rutilus rutilus) to Lough Neagh has apparently had a negative impact on some overwintering waterfowl, although the native coregonid pollan (Coregonus autumnalis) remains abundant. Llyn Tegid in Wales has received three introductions [rudd (Scardinius erythrophthalmus), ruffe and silver bream (Blicca bjoerkna)], although no impacts on the native whitefish or other fish populations have been observed. In England, individuals of at least 12 native and non-native fish species have been brought to Windermere for the purpose of live-baiting, although only those of the cyprinids roach and common bream have established abundant populations. At the same time, the native salmonid Arctic charr (Salvelinus alpinus) has declined markedly while the native esocid pike (Esox lucius) has shown changes in abundance, distribution and individual condition, although these developments have not been shown to be causally linked. None of these introductions were sanctioned by appropriate fisheries or other regulatory bodies and almost all of them probably arose from the release or escape of live-bait used by pike anglers. Of the 10 species introductions documented here, four (common bream, gudgeon, roach and ruffe) have established abundant populations and two of these (roach and ruffe) have apparently caused or currently threaten harm, supporting the hypothesis that at least some of these introductions have become invasive.

Comparative susceptibility of representative Great Lakes fish species to the North American viral hemorrhagic septicemia virus Sublineage IVb

The present study compared the susceptibility of representative Laurentian Great Lakes fish species to the emerging viral hemorrhagic septicemia virus (VHSV) Genotype IVb. The median lethal dose of infection by intraperitoneal injection (IP-LD50) was obtained from fish that were experimentally infected with the MI03 index strain of VHSV-IVb. Fish were injected at doses ranging from 7 x 10(7) to 7 x 10(-2) plaque-forming units (pfu) and maintained at 12 +/- 1 degrees C. The infection trials identified species of high, medium, and low susceptibility based on the IP-LD50 values. Pathogenicity of VHSV-IVb was highest in largemouth bass Micropterus salmoides, which resulted in an IP-LD50 of 1.5 x 10(2) pfu, while also demonstrating the clinical diathesis of VHSV-infected fish. The virus was moderately pathogenic in yellow perch Perca flavescens (IP-LD50 of 2.5 x 10(5) pfu), but also showed the classical signs of VHSV infection. Salmonids were the least susceptible to VHSV-IVb with IP-LD50 values of no less than 1.4 x 106 pfu; however, in fish that succumbed to infection, characteristic VHSV lesions were observed. Histopathologic alterations were most profound in gill, skin, muscle, gonads, and liver of largemouth bass and yellow perch, while in salmonids, hemorrhages in the swimbladder and/or degenerative changes in the liver were the most common lesions noticed. VHSV was isolated from infected fish, and its identity was confirmed by the reverse transcriptase polymerase chain reaction. These results highlight the variations among fish species susceptibility to this emerging strain of VHSV and offer insights into the potential impact of VHSV-IVb on the Laurentian Great Lakes fish community.

Factors influencing zooplankton size structure at contrasting temperatures in coastal shallow lakes: Implications for effects of climate change

We assessed the importance of temperature, salinity, and predation for the size structure of zooplankton and provided insight into the future ecological structure and function of shallow lakes in a warmer climate. Artificial plants were introduced in eight comparable coastal shallow brackish lakes located at two contrasting temperatures: cold‐temperate and Mediterranean climate region. Zooplankton, fish, and macroinvertebrates were sampled within the plants and at open‐water habitats. The fish communities of these brackish lakes were characterized by small‐sized individuals, highly associated with submerged plants. Overall, higher densities of small planktivorous fish were recorded in the Mediterranean compared to the cold‐temperate region, likely reflecting temperature‐related differences as have been observed in freshwater lakes. Our results suggest that fish predation is the major control of zooplankton size structure in brackish lakes, since fish density was related to a decrease in mean body size and density of zooplankton and this was reflected in a unimodal shaped biomass‐size spectrum with dominance of small sizes and low size diversity. Salinity might play a more indirect role by shaping zooplankton communities toward more salt‐tolerant species. In a global‐warming perspective, these results suggest that changes in the trophic structure of shallow lakes in temperate regions might be expected as a result of the warmer temperatures and the potentially associated increases in salinity. The decrease in the density of large‐bodied zooplankton might reduce the grazing on phytoplankton and thus the chances of maintaining the clear water state in these ecosystems.

Effects of stocked trout on native fish communities in boreal foothills lakes

Nasmith LE, Tonn WM, Paszkowski CA, Scrimgeour GJ. Effects of stocked trout on native fish communities in boreal foothills lakes. Ecology of Freshwater Fish 2010: 19: 279–289. © 2010 John Wiley & Sons A/SAbstract – Ecological effects of stocking nonnative trout into lakes are receiving increased attention, especially in alpine environments. We assessed effects of stocked trout on native forage fishes in the boreal foothills of Alberta (Canada) by comparing fish density, population size structure and spatial and temporal activities in stocked and unstocked lakes over 3 years (2005–2007). The numerically dominant dace (primarily Phoxinus spp.) were larger in stocked lakes, consistent with size‐limited predation. Dace were also more crepuscular and concentrated on the lake‐bottom in stocked lakes, compared to more daytime activity in the water column in unstocked lakes. There were, however, no demonstrable effects of trout on the abundance of forage fish. The lack of major population‐level impacts of stocked trout suggests that current stocking practices, characteristics of boreal foothill lakes (e.g. thermal structure, abundant invertebrates, dense macrophytes) and/or behavioural adjustments of forage fish contribute to healthy native fish populations in our stocked lakes.

An environmental assessment of a small shallow lake (Little Black Lake, MI) threatened by urbanization

A limnological survey was conducted of Little Black Lake, MI, and its tributaries during summer 2007. This small, shallow lake is located in a rapidly developing area of west Michigan. As such, our analytical approach and recommendations can serve as a model for other similar systems threatened by urbanization. Soluble reactive phosphorus and nitrate concentrations in both the inflows to (during baseflow) and Little Black Lake itself were low (≤0.007 and ≤270mg/L, respectively). Nutrient concentrations increased during stormflow conditions, although the magnitude of the increase depended on the nutrient and sampling location. Macrophyte growth was extensive throughout most parts of the lake, with Chara and Potamogeton spp. present in most sites; based on the coefficient of conservatism, plant composition was indicative of good water quality conditions. Chlorophyll a concentration averaged 1.7μg/L in Little Black Lake, with cryptophytes and cyanobacteria being the most dominant members (by biovolume) of the phytoplankton community. The fish community in Little Black Lake was dominated by bluegill (Lepomis macrochirus) and pumpkinseed (L. gibbosus), with no invasive species observed. Overall, abiotic and biotic conditions indicate that Little Black Lake is in good ecological health despite increasing pressures of urbanization in its watershed. To maintain this status, it is recommended that the local municipalities develop a comprehensive watershed management plan and implement best management practices to limit nonpoint source pollutant loading to Little Black Lake.

Larger zooplankton in Danish lakes after cold winters: are winter fish kills of importance?

Winter fish kills can be intense under ice in shallow lakes, and have cascading effects on the food web and ultimately on lake water clarity. In maritime Western Europe, winters are usually mild, but occasional colder periods may also have strong effects on lake fish communities. Global warming may have disproportionate effects by delaying freezing and shortening the period of ice coverage. We studied differences in zooplankton (cladocerans, copepods, and rotifers): phytoplankton biomass, zooplankton community structure, and individual body size among 37 Danish lakes of various depths, chemical characteristics, and trophy, by comparing four winters of different severity (mean winter temperatures ranging from −1.19°C in 1996 to +2.9°C in 1995). We found that crustacean mean body sizes were significantly larger in the summer following a severely cold winter. The zooplankton communities in the summer after a cold winter had a significantly larger proportion of larger-bodied species and taxa. Phytoplankton biomass, expressed as chlorophyll-a (chl-a), was lower and zooplankton herbivory (chl-a:TP index), higher, in the summer after the severely cold winter of 1995/1996. All these effects were stronger in shallow lakes than in deep lakes. Changes in zooplankton during summer 1996, compared with other years, were likely caused by fish kills under ice during the preceding severe winter of 1995–1996. Fish kills due to under ice oxygen depletion would be expected to occur earlier and be more complete in the shorter water columns of shallow lakes. With climate change, severe winters are predicted to become less frequent and the winters to be milder and shorter. In general, this is likely to lead to higher winter survival of fish, lower zooplankton grazing of phytoplankton the following summer and more turbid waters, particularly in shallow eutrophic lakes.

Impacts of climate warming on lake fish community structure and potential effects on ecosystem function

Fish play a key role in the trophic dynamics of lakes, not least in shallow systems. With climate warming, complex changes in fish community structure may be expected owing to the direct and indirect effects of temperature, and indirect effects of eutrophication, water-level changes and salinisation on fish metabolism, biotic interactions and geographical distribution. We review published and new data supporting the hypotheses that, with a warming climate, there will be changes in: fish community structure (e.g. higher or lower richness depending on local conditions); life history traits (e.g. smaller body size, shorter life span, earlier and less synchronised reproduction); feeding mode (i.e. increased omnivory and herbivory); behaviour (i.e. stronger association with littoral areas and a greater proportion of benthivores); and winter survival. All these changes imply higher predation on zooplankton and macroinvertebrates with increasing temperatures, suggesting that the changes in the fish communities partly resemble, and may intensify, the effects triggered by eutrophication. Modulating factors identified in cold and temperate systems, such as the presence of submerged plants and winter ice cover, seem to be weaker or non-existent in warm(ing) lakes. Consequently, in the future lower nutrient thresholds may be needed to obtain clear-water conditions and good ecological status in the future in currently cold or temperate lakes. Although examples are still scarce and more research is needed, we foresee biomanipulation to be a less successful restoration tool in warm(ing) lakes without a strong reduction of the nutrient load.

Habitat use and diet of sympatric Arctic charr (Salvelinus alpinus) and whitefish (Coregonus lavaretus) in five lakes in southern Norway: not only interspecific population dominance?

Stable coexistence of Arctic charr and whitefish does occur in a number of native lake fish communities in Scandinavia. Even so, whitefish introductions into Arctic charr lakes have resulted in serious decline and possibly local extinction of Arctic charr. In this article, we analyze the habitat use and diet of the two species in five Norwegian lakes differing in basin shape and environmental conditions. In two of the lakes, both species are native, and appear to live in a relatively stable coexistence. Here, whitefish mainly occupy the littoral and upper pelagic zone, while Arctic charr live in the deeper habitats. Diets are generally quite different in terms of the zooplankton species eaten. In the three other lakes, either whitefish or both species have been introduced. In the shallowest lake, habitat segregation is similar to that seen in the pristine lakes, although Arctic charr appears to be on the brink of extinction. In the remaining two lakes, however, Arctic charr dominates, and occurs in higher numbers than whitefish in all the habitats. Our observations indicate that coexistence of the two species in oligotrophic and relatively pristine lakes requires an extensive profundal zone to serve as a refugium for Arctic charr. If the littoral zone is rendered inaccessible or unprofitable for whitefish due to dominance of a third competitor or predator, or as a result of lake regulation, then Arctic charr may be the dominant species.

Lake of flies, or lake of fish? A trophic model of Lake Malawi

Ecosystem-focused models have, for the first time, become available for the combined demersal and pelagic components of a large tropical lake ecosystem, Lake Malawi. These provide the opportunity to explore continuing controversies over the production efficiencies and ecological functioning of large tropical lakes. In Lake Malawi these models can provide important insight to the effect of fishing on fish composition, and the potential competition that the lakefly Chaoborus edulis may have with fisheries production. A mass-balanced trophic model developed for the demersal fish community of the southern and western areas of Lake Malawi was integrated with an existing trophic model developed for the open-water pelagic. Input parameters for the demersal model were obtained from a survey of fish distributions, fish food consumption studies, and from additional published quantitative and qualitative information on the various biotic components of the community. The model was constructed using the Ecopath approach and software. The graphically presented demersal food web spanned four trophic levels and was based primarily on consumption of detritus, zooplankton and sedimented diatoms. Zooplankton was imported into the system at trophic levels three and four through fish predation on carnivorous and herbivorous copepods and Chaoborus larvae. It is proposed that the primary consumption of copepods was by fish migrating into the pelagic zone. Chaoborus larvae in the demersal were probably consumed near the lakebed as they conducted a daily migration from the pelagic to seek refuge in the sediments. This evidence for strong benthic–pelagic coupling provided the opportunity for linking the demersal model to the existing model for the pelagic community so producing the first model for the complete ecosystem. Energy fluxes through the resulting combined model demonstrated that the primary import of biomass to the demersal system was detritus of pelagic origin (72.1%) and pelagic zooplankton (10.6%). Only 15.8% of the biomass consumed within the demersal system was of demersal origin. Lakefly production is efficiently utilised by the lake fish community, and any attempt to improve fishery production through introduction of a non-native plantivorous fish species would have a negative impact on the stability and productivity of the lake ecosystem.