The restoration of shallow eutrophic lakes,and the role of northern pike,aquatic vegetation and nutrient concentration

The feasibility of biomanipulation is related particularly to reducing the production and recruitment of planktivorous fish stocks. For assessing the level of predation needed to suppress planktivorous fish stocks, the relation between fish P and B, on one hand, and nutrient concentration on the other were analyzed. The carrying capacity of shallow lakes in terms of biomass is related to the total phosphorus concentration and nature of the lake bottom substrate. The production of planktivorous fish was 60–80% of the maximum carrying capacity. It is argued that aquatic vegetation and northern pike are effective tools to maintain water quality, but these are limited by the maximum nutrient concentration aquatic vegetation can sustain.     

Top-down or Bottom-up Effects by Fish: Issues of Concern in Biomanipulation of Lakes

A large-scale biomanipulation trial was carried out on Lake Vesijärvi in Finland during 1989–1993. Following the mass removal of coarse fish the biomass of cyanobacteria collapsed from 1.4 g/m^?3 to below 0.4 g/m^?3, while total phosphorus concentration declined from 45 μ g/L to 30 μ g/L. No relevant changes in zooplankton communities were observed. The results suggest that the success of food web manipulation as a tool for lake restoration is not necessarily dependent on the grazing rate of zooplankton. The effects of reduced fish-mediated internal loading and recycling of nutrients are in many cases stronger than those of reduced planktivory. Alternative stable states of water quality may also exist in lakes not covered by macrophytes, owing to the changes in the behavior of fish stocks. Year-to-year variation in the littoral zone may cause large oscillations in lake ecosystems—for example, through the recruitment of fish. In addition, the nutrients translocated by fish from the littoral zone may affect the nutrient dynamics of the pelagial plankton community. In terms of phytoplankton species composition and the ratio of phosphorus to chlorophyll a, the water quality in Lake Vesijärvi has improved in a stepwise fashion within the last 10 years. This is probably due to the fact that the five-year mass removal of fish in Enonselkä fulfilled the requirement of sustained management of fish stocks in order to maintain nonequilibrial conditions between alternate stable states. The prediction of the water quality development is obscured, however, by spatial and temporal within-lake variation, which sets high requirements for sampling programs.     

A Game of Cat and Fish: How to Restore the Balance in Sustainable Fisheries Management

This article argues that continued reliance on input/output controls and restrictions in fisheries management may be insufficient to protect global fish stocks. Instead, a transition beyond supply-side measures to those aimed at reducing demand for fish stocks may be necessary. The article offers a proposal for five types of demand-side or market-based measures: elimination of fishing subsidies, bolstering of import restrictions, ceasing trade in endangered and threatened fish stocks, strengthening civil and criminal penalties against illegal fishers, and pursuit of punitive trade sanctions against flag states flouting international fishery guidelines to help prevent and deter global overfishing.     

Fish stocking creates an ecological trap for an avian predator via effects on prey availability

In anthropogenic landscapes animals may be lured into low‐quality habitats where they survive or reproduce poorly (‘ecological traps’). I investigated breeding habitat selection in relation to intra‐seasonal changes in food availability and reproductive output in red‐necked grebes Podiceps grisegena, a size‐limited predator, of common carp Cyprinus carpio ponds. Carp farms constitute highly heterogeneous habitat mosaics due to separate stocking of different age/size fish. Pond features significant for grebe settling decisions, i.e. hydroperiod and emergent vegetation cover, had no obvious effects on prey abundance for chicks and on fledging success. Breeding grebes avoided ponds containing fish too large for them to ingest but exhibited little preference between ponds with medium‐sized one‐year‐old carp that could be exploited by pre‐laying birds, and ponds designated for young‐of‐the‐year carp, where only invertebrates and amphibians were available as prey in early spring. Red‐necked grebes settling on ponds with medium‐sized fish failed to predict future shifts in interactions with carp stocks; carp exceeded the prey‐size threshold of chicks and adversely affected their non‐fish prey levels. The resulting food shortage led to severe egg‐to‐fledging mortality rates compared to fishless ponds or those containing young‐of‐the‐year fish. This study shows that waterbirds vulnerable to competition from fish can risk maladaptive habitat selection due to unrecognised spatial and temporal variation in food resources caused by fish stocking practices. Ecological traps created by perturbations to trophic interactions may be common but difficult to detect because altered dynamics of trophic resources can affect wildlife indirectly. As with other types of ecological traps, manipulation of habitat features identified as attractive cues for settling animals, but not related to critical food resources, may help to reduce perceptual pitfalls. For example, wetland management to mitigate trap effects driven by commercially stocked fish should preserve abundant emergent vegetation in habitats with weak fish impact and extend their hydroperiod.     

Condition of Barbus sclateri from semi-arid aquatic systems: effects of habitat quality disturbances

This study investigated the relationships between fish condition and environmental variables in Barbus sclateri from semi-arid freshwater ecosystems in the south-eastern Iberian Peninsula. Two main habitats were studied: semi-arid streams characterized by strong seasonal fluctuations in flow level (droughts and floods) and reservoirs (artificial ecosystems characterised by waters of high conductivity). The mass–length relationships were used to test differences in fish condition between nine stream populations and five reservoir populations of B. sclateri from the Segura River basin. The relationships between seven ecosystem variables (conductivity, oxygen concentration, water temperature, pH, seasonal water flow, submerged vegetation and sub-basin location) and fish condition were analysed. The ecological variables that accounted for most of the variation in fish condition were seasonal water fluctuation and water conductivity. The condition of B. sclateri populations may be a good indicator of fish habitat quality in Mediterranean semi-arid freshwater ecosystems and should be considered when such populations are subjected to sports fishing regulations, recovery plans or any other management programme.     

Movement response of Orange-Vaal largemouth yellowfish (<Emphasis Type="Italic">Labeobarbus kimberleyensis</Emphasis>) to water quality and habitat features in the Vaal River,South Africa

Many threatened fish species that utilize riverine habitats are faced with habitat degradation and subsequent deterioration in their ecological surroundings. Habitat degradation is a consequence of water quality parameters associated with anthropogenic activities including mining, industrial, agricultural and urban activities. We examined how the movement behaviour of radio-tracked Orange-Vaal largemouth yellowfish (Labeobarbus kimberleyensis) responded to a suite of water quality chemical parameters and habitat features in the Vaal River, South Africa. We found that the probability of their movement increased with a decrease in water clarity, presence of emergent and overhanging marginal vegetation and fast flowing rapids. High mobility in conditions of low water clarity was probably related to low prey capture success of this piscivorous fish. High movement of largemouth yellowfish in emergent and overhanging marginal vegetation areas and rapid habitat biotopes were attributed to accessibility of prey within these important cover features. When water quality parameters were considered, the probability of largemouth yellowfish movement increased with increasing levels of dissolved chloride (Cl) and silicon (Si), whereas movement decreased with high phosphate (PO₄) concentrations and increased with alkalinity levels in the river. High nutrient levels associated with eutrophication caused reductions in the movement of largemouth yellowfish. The association of increased movement of largemouth yellowfish with increasing Cl and Si is indicative of degraded habitat condition in the Vaal River system. Our study showed the importance of monitoring fish behavioural movement patterns to multiple environmental parameters, as these fish are important ecological indicators when appropriate conservation and management plans of freshwater ecosystems are required.     

Architecture of collapse: regime shift and recovery in an hierarchically structured marine ecosystem

By the late 20th century, a series of events or ‘natural experiments’, for example the depletion of apex predators, extreme eutrophication and blooms of invasive species, had suggested that the Black Sea could be considered as a large ecosystem ‘laboratory’. The events resulted in regime shifts cascading through all trophic levels, disturbing ecosystem functioning and damaging the water environment. Causal pathways by which the external (hydroclimate, overfishing) and internal (food web interactions) drivers provoke regime shifts are investigated. Statistical data analyses supported by an interpretative framework based on hierarchical ecosystem theory revealed mechanisms of hierarchical incorporation of environmental factors into the ecosystem. Evidence links Atlantic teleconnections to Black Sea hydroclimate, which together with fishing shapes variability in fish stocks. The hydroclimatic signal is conveyed through the food web via changes in productivity at all levels, to planktivorous fish. Fluctuating fish abundance is believed to induce a lagged change in competitor jelly plankton that cascades down to phytoplankton and influences water quality. Deprived of the stabilising role of apex predators, the Black Sea's hierarchical ecosystem organisation is susceptible to both environmental and anthropogenic stresses, and increased fishing makes fish stock collapses highly probable. When declining stocks are confronted with burgeoning fishing effort associated with the inability of fishery managers and decision‐makers to adapt rapidly to changes in fish abundance, there is overfishing and stock collapse. Management procedures are ineffective at handling complex phenomena such as ecosystem regime shifts because of the shortage of suitable explanatory models. The proposed concepts and models reported here relate the hydroclimate, overfishing and invasive species to shifts in ecosystem functioning and water quality, unravelling issues such as the causality of ecosystem interactions and mechanisms and offering potential for finding ways to reverse regime shifts. We advocate a management approach aiming at restoring ecosystem hierarchy that might mitigate the costly consequences of regime shifts.     

Assessment of exploited fish species in the Lake Edward System,East Africa

The unknown status of inland fish stocks hinders their sustainable management. Therefore, increasing stock status information is important for sustainable inland fisheries. Fisheries reference points were estimated for five exploited fish species (11 stocks) in the Lake Edward system, East Africa, which is one of the most productive inland water systems. The aim was to ascertain the status of the fisheries and establish reference points for effective management. The reference points were based on four linked stock assessment approaches for data-limited fisheries. Estimates showed poor stock status with the stocks defined as either collapsed, recruitment impaired or overfished. However, higher catches could be obtained under sustainable management. Estimates of maximum sustainable yield (MSY) and supporting biomass (B_msy) are provided for 10 of the stocks as targets for rebuilding plans. The immediate target of management should be rebuilding biomass to B_msy. Applicable measures include shifting length at first capture to the length that maximizes catch without endangering size structure and biomass, and livelihood diversification out of fisheries.     

A model study on the role of wetland zones in lake eutrophication and restoration

Shallow lakes respond in different ways to changes in nutrient loading (nitrogen, phosphorus). These lakes may be in two different states: turbid, dominated by phytoplankton, and clear, dominated by submerged macrophytes. Both states are self-stabilizing; a shift from turbid to clear occurs at much lower nutrient loading than a shift in the opposite direction. These critical loading levels vary among lakes and are dependent on morphological, biological, and lake management factors. This paper focuses on the role of wetland zones. Several processes are important: transport and settling of suspended solids, denitrification, nutrient uptake by marsh vegetation (increasing nutrient retention), and improvement of habitat conditions for predatory fish. A conceptual model of a lake with surrounding reed marsh was made, including these relations. The lake-part of this model consists of an existing lake model named PCLake. The relative area of lake and marsh can be varied. Model calculations revealed that nutrient concentrations are lowered by the presence of a marsh area, and that the critical loading level for a shift to clear water is increased. This happens only if the mixing rate of the lake and marsh water is adequate. In general, the relative marsh area should be quite large in order to have a substantial effect. Export of nutrients can be enhanced by harvesting of reed vegetation. Optimal predatory fish stock contributes to water quality improvement, but only if combined with favourable loading and physical conditions. Within limits, the presence of a wetland zone around lakes may thus increase the ability of lakes to cope with nutrients and enhance restoration. Validation of the conclusions in real lakes is recommended, a task hampered by the fact that, in the Netherlands, many wetland zones have disappeared in the past.     

Biodiversity and fish farming intensification in French fishpond systems

Fishponds are artificial ecosystems in which biodiversity may be strongly affected by fish farming management. We studied biodiversity variation along a gradient of fish farming intensification within a 180-pond sample of a French region to establish whether biodiversity primarily depended on extensive management with the alternative hypothesis that some habitat characteristics could compensate for the effect of intensification. We compared the relationships of three biodiversity indicators (breeding birds, protected plants, macrophytes) to an index of intensification and to habitat characteristics likely to influence each indicator. In all selected indicators, species richness did not vary according to the composite intensification gradient. Protected plant richness was best explained by periodic drainage, which could thus benefit biodiversity in intensified fishponds provided that shallow littoral areas are still present. Bird richness was linked to macrophyte cover and probably to reed bed areas. Macrophyte richness and coverage were negatively influenced by low water transparency and high fish biomass which seemed likely to affect bird habitat above 350–400 kg/ha. Aquatic vegetation, which may reflect interactive effects of environmental factors and fish farming management, may then contribute to assessment of the ecological status of fishponds under the E.U. Water Framework Directive.