The influence of diel period on fish assemblage in the unstructured littoral of reservoirs

Abstract Diel changes in littoral fish assemblage were studied in four reservoirs in the Czech Republic (Central Europe). The sampling was performed by beach seining in an unstructured littoral zone. Perch, Perca fluviatilis L., roach, Rutilus rutilus (L.), bream, Abramis brama (L.), carp, Cyprinus carpio L., ruffe, Gymnocephalus cernua (L.), pikeperch, Sander lucioperca (L.), eel, Anguilla anguilla (L.) and A. brama × R. rutilus hybrids exhibited higher densities at night. Only bleak, Alburnus alburnus (L.) exhibited higher densities during the day. The number of species was higher in night hauls, and mean body size of roach and bleak was also higher at night. Diel changes in fish densities resulted in the change in species composition between day and night. The study has implication for the sampling design to assess littoral fish assemblages using beach seine netting and recommends night sampling for a representative assessment or sampling during both diel periods for a robust assessment.     

Size and age are important factors for marine sanctuaries: evidence from a decade of systematic sampling in a subtropical marine park

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Larger biomass of targeted reef fish in no‐take marine reserves on the Great Barrier Reef,Australia

• 1. An expansion of no‐take marine reserve zones of Australia's 348 000 km² Great Barrier Reef (GBR) Marine Park from 4.6% to 33.4% of the park area is proposed in 2004. However, limited evidence currently exists that no‐take marine reserves on the GBR have increased abundance of reef fish targeted by fisheries. This study provides such evidence for inshore reefs of the GBR.
• 2. Underwater visual surveys were used to estimate the effect of no‐take reserves on abundance of species targeted by hook‐and‐line fisheries around the Palm, Whitsunday and Keppel Islands, spanning 600 km of the length of the GBR. The reserves had been zoned ‘no fishing’ for 14 yr.
• 3. Densities of Plectropomus spp. and Lutjanus carponotatus, both targeted by fisheries, were much higher in protected zones than fished zones in two of the three island groups. Plectropomus spp. were 3.6 and 2.3 times more abundant in protected than fished zones of the Palm and Whitsunday island groups. L. carponotatus were 2.3 and 2.2 times more abundant in protected zones than fished zones of the Whitsunday and Keppel island groups.
• 4. The biomasses of Plectropomus spp. and L. carponotatus were significantly greater (3.9 and 2.6 times respectively) in the protected zones than fished zones at all three island groups.
• 5. Legal minimum sizes of Plectropomus spp. and L. carponotatus are ?38 cm and 25 cm TL respectively. There were significantly higher densities and biomasses of Plectropomus spp. >35 cm TL (density: 3.8 times; biomass: 5.1 times) and L. carponotatus >25 cm TL (density: 4.2 times; biomass: 5.3 times) in protected zones than fished zones at all three island groups.
• 6. No significant difference in abundance between protected and fished zones was found for two species not captured by fisheries (Siganus doliatus and Chaetodon aureofasciatus), and there were no significant differences in benthic characteristics between protected and fished zones.
• 7. Results suggest that no‐take marine reserves have increased stock biomass of targeted fish species on inshore GBR reefs.

Copyright © 2004 John Wiley & Sons, Ltd.     

Is European catfish a threat to eels in southern France?

• 1. European eel (Anguilla anguilla) is recognized as a critically endangered species in the northern hemisphere. Threats such as overfishing or pollution are well known as risks for eel populations. However, much less is known about the impact of introduced fish on European eels. In particular, introduced large‐bodied predators could become new predators to eels.
• 2. The potential impact of European catfish (Silurus glanis L) on an eel population in the Camargue, southern France was studied using a combination of stable isotope and gut content analyses.
• 3. Only large‐bodied catfish (>500 mm) can consume numerous fish prey. However, catfish mostly consumed crayfish (Procambarus clarkii), these prey items being found in 79% of the guts of the largest individuals. Eel was absent from the dissected catfish guts. A mixing model based on Bayesian inference revealed that catfish diet included only 5% (0–8.5%) of marine sources (both eel and mullet).
• 4. While local economic interests prompted ecological studies to assess whether catfish exerted a new and strong predatory pressure on eel, this study found that European catfish behaved as an opportunistic omnivore, and as such was not a direct threat as a predator on eel populations in the Camargue. Copyright © 2011 John Wiley & Sons, Ltd.

     

The home range and behaviour of yellow‐stage European eel Anguilla anguilla in an estuarine environment

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Coastal river connectivity and the distribution of ascending juvenile European eel (Anguilla anguilla L.): Implications for conservation strategies regarding fish‐passage solutions

1. Many diadromous fish populations are declining and at risk of collapse. Lack of river connectivity is a major contributor to these declines, as free migration routes between marine and freshwater habitats are crucial for life‐history completion. For the conservation and ultimately recovery of such species, it is imperative that remedial measures aimed at increasing connectivity are effective.
2. This study investigated the distribution patterns of ascending juvenile European eel (Anguilla anguilla L.) in rivers in south‐western Sweden, with a focus on the effects of barriers and measures that aim to reduce the impact of barriers, i.e. fish‐passage solutions (FPSs).
3. Eel occurrence data were spatially and temporally integrated with the national databases of dams and FPSs in a Geographic Information System (GIS) environment to evaluate their effect on ascending eel distribution. The types of barriers assessed were: (i) dams with nature‐like fishways; (ii) dams with eel ramps; (iii) dams with technical fishways; and (iv) dams without FPSs.
4. Dams fitted with eel ramps or technical fishways, as well as dams without FPSs, produced a significant negative effect on the probability of eel occurrence upstream. This negative effect was not found for dams fitted with nature‐like fishways, indicating that these solutions may function better than the other FPS types in this study. The probability of eel occurrence decreased with distance from the sea and increased with area sampled, number of electrofishing runs, water temperature, and with the size of the bottom substrate.
5. We suggest that future conservation strategies for improving the natural immigration of juvenile eels should include optimizing FPS function (e.g. placement and design), the continued maintenance of FPSs, the construction of nature‐like fishways, and preferably the removal of dams, which will also benefit the downstream migration of maturing eels as well as restoring other ecosystem services.

     

Conservation implications of establishment success of the Critically Endangered Twee River redfin ‘Pseudobarbus’ erubescens (Skelton, ) in an artificial impoundment in South Africa

1. This study reports the first known record of breeding of the Critically Endangered Twee River redfin ‘Pseudobarbus’ erubescens in an artificial impoundment. This followed an introduction of 48 individuals into a 10 ha impoundment within the species' native range more than a decade ago.
2. Sampling the impoundment using three fyke nets set overnight yielded 2838 P. erubescens, which included both juveniles and adults capable of spawning. Fork length measurements of a subsample of 250 individuals ranged from 29 to 125 mm with length cohorts indicating multiple spawning events.
3. This demonstrates that this species can successfully reproduce in lentic environments and suggests that artificial impoundments could be stocked to provide refugia for P. erubescens and other highly threatened small cyprinids while conservation strategies are developed to mitigate against habitat loss resulting from alien fish invasions, increased human use of water, and from climate change in rivers.

     

The use of fyke nets as a quantitative capture technique for freshwater eels (Anguilla spp.) in rivers

Abstract  Fyke netting is a convenient and effective technique for capturing freshwater eels, and catch-per-unit-effort is usually assumed to be an index of eel abundance. The present study investigated the potential of depletion fishing using baited fyke nets to obtain population estimates of longfin eels Anguilla dieffenbachii Gray, in a river in the South Island of New Zealand. The probability of capture (capture efficiency) of a single night's fishing increased with increasing size of eel, and ranged from 0.2 to 0.9 for eels <400 mm, to 0.7–0.9 for eels ≥500 mm. The capture efficiency of baited vs unbaited nets was assessed in a small stream that was electric fished after netting trials were completed. Baited nets proved to be an effective method of assessing abundance of longfin eels (>400 mm) but not shortfins ( A. australis Richardson); capture efficiency of the population of longfins (the sum of fyke caught and electric fished eels) was 0.4 for a single night's fyke netting, increasing to 0.8 over four nights. Comparable efficiencies for shortfins were <0.1 and 0.3 respectively. Unbaited nets were markedly less efficient for both species.     

Gambusia holbrooki,the ‘tadpolefish’: The impact of its predatory behaviour on four protected species of European amphibians

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Hydroacoustic assessment of mono‐ and polymorphic Coregonus density and biomass in subarctic lakes

Density and biomass estimates of pelagic fish are essential to understand food web interactions and ecosystem functioning. We conducted surveys of six subarctic lakes for assessing both mono‐ and polymorphic whitefish Coregonus lavaretus (L.) populations. Monomorphic whitefish lakes were inhabited by a habitat and diet generalist, large sparsely rakered (LSR) morph, whereas polymorphic whitefish lakes had a littoral benthivorous LSR morph, a pelagic planktivorous densely rakered (DR) morph and in two cases a benthivorous small sparsely rakered (SSR) morph inhabiting the profundal zone. In addition, an introduced specialist zooplanktivore, vendace Coregonus albula (L.), inhabited one of the monomorphic lakes. Hydroacoustics was found to be an appropriate method for estimating coregonid densities and biomasses in large and deep polymorphic lakes occupied by the planktivorous DR morph or vendace, but only during dark nights in autumn. The suitability of hydroacoustic assessment for benthivorous LSR and SSR morphs was low, especially in polymorphic whitefish lakes due to their preference for near‐bottom habitat or shallow areas not sampled with hydroacoustics. The pelagic density of DR morph varied from 330 to 1780 fish·ha^?1 and biomass from 1.4 to 13.3 kg·ha^?1 in polymorphic whitefish lakes, whereas corresponding estimates for LSR morph were 10–320 fish·ha^?1 and 0.5–8.4 kg·ha^?1 in monomorphic whitefish lakes. In general, polymorphism tended to increase the density and biomass of whitefish in the pelagic area compared with monomorphic systems.