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Het natuurherstel langs onze rivieren is goed voor jonge riviervis maar begintna gemiddeld vijftien jaar zijn effectiviteit te verliezen. Dat blijkt uit onderzoekvan Wageningen Universiteit in samenwerking met Rijkswaterstaat naar deontwikkeling van soortenrijkdom en het aantal typische riviervissen. Om dezenatuurwaarden van herstelprojecten te behouden zouden nevengeulen regelmatig opnieuw moeten worden uitgebaggerd.

Grote aantallen gevlekte gladde haaien bezoeken in de zomer de Voordelta, terwijl de Oosterschelde en Westerschelde belangrijke geboortegronden zijn. In winter trekken veel vrouwtjes zuidwaarts richting Kanaal en Golf van Biskaje. Met tien hightech zenders wordt het trekgedrag komend jaar nog beter in kaart gebracht.

Voor een gezonde visgemeenschap in laaglandrivieren zijn uiterwaarden, ook wel vloedvlakten genoemd, erg belangrijk. Daarom is sinds de jaren ’90 van de vorige eeuw in Nederland veel geïnvesteerd in het herstel van de ecologische kwaliteit van uiterwaarden en daarmee die van het hele riviersysteem.

In deze literatuurstudie is geëvalueerd wat de effectiviteit en ecologische effecten zijn van het uitzetten van graskarpers (Ctenopharyngodon idella) als beheermaatregel om plantengroei te beperken. Daarnaast zijn de effecten van het uitzetten van graskarpers vergeleken met die van maaibeheer. Belangrijkste conclusie is dat het uitzetten van graskarpers als beheermaatregelen voor het verwijderen van watervegetatie vraagt om een grondige afweging.

Steeds meer exotische vissoorten vestigen zich in Nederland. Sommige invasieve exoten doen het zelfs zo goed dat ze inheemse vissen verdringen. Welke kenmerken verklaren precies dat deze exoten zo succesvol zijn en andere vissoorten niet?

Introduced predatory fishes have had consistently severe consequences for native fishes in stream environments around the world, although the drivers of these effects are often unclear. In the Swartkops River headwaters in South Africa, native Eastern Cape redfin Pseudobarbus afer were always absent from sites occupied by non-native black basses Micropterus salmoides and Micropterus dolomieu, but generally co-occurred with the native predators Anguilla marmorata and Anguilla mossambica. A natural experiment provided by flood-mediated recolonization of black-bass occupied sites by P. afer demonstrated depletion in black-bass invaded sites. Field behavioural observations of P. afer indicated that they foraged among benthic cover during the day, but suspended in open water at night. As the nocturnal A. marmorata and A. mossambica foraged actively within structural cover at night and M. dolomieu and M. salmoides are diurnal or crepuscular predators, P .afer is thus optimized to avoid predation by native anguillid predators and not the functionally unique predatory black basses. The integration of distributional, temporal population dynamics and behavioural data suggests that the severe effects of Micropterus spp. are probably a consequence of prey naïveté and behaviour evolved to evade native predators.

Elasmobranches are popular animals in public aquariums. Worldwide more than 700 million people visit zoos and aquariums annually, enabling elasmobranches to become an important ambassador for their natural habitats. We conducted a census within the European Association of Zoos and Aquaria to gain a better overview of which species are present within European collections. The census showed that 102 chondrichthyan species are found in European zoos and public aquaria, accounting for 8,6% of all known species. Of the captive population 47.1% of species have reproduced in aquariums. Benthic species are found most commonly in aquariums. Of the species reproducing, 87.8% fall in the body size range of 51 to 250 cm. Categorising the reproductive results by reproductive mode, the most successful groups are oviparous and aplacental viviparous with uterine villi or trophonemata. A regional collection plan has been defined by using the results of the census and the IUCN status. Currently 42 species are managed by a species coordinator, within the ex-situ European elasmobranch population to ensure a genetically healthy population, to increase reproductive output, and to conduct husbandry research. Long-term breeding efforts will help to reduce the demand on wild populations to supply the aquarium population. Species coordinators will become the contact for in-situ conservation initiatives and international conservation bodies like IUCN. This study discusses further the future challenges in the captive management of chondrichthyan populations.

De afgelopen decennia is er in toenemende mate aandacht gekomen voor natuurontwikkeling in rivieruiterwaarden. Een belangrijk onderdeel hiervan vormt het graven van hoogwatergeulen: korte nevenlopen die (bij hoogwater) parallel aan de hoofdstroom met de rivier meestromen en die gedurende de rest van het jaar ook water houden. Zulke geulen worden vaak gekenmerkt door een lagere stroomsnelheid dan de rivier zelf, gevarieerde oevers met ondiepe zandbanken en een rijke oever- en onderwatervegetatie. In dit artikel wordt de visfauna beschreven in een recent aangelegde hoogwatergeul in natuurgebied de Raaijweide bij Venlo.

To assess whether the species distinctions of Lake Tana’s Labeobarbus spp. are supported by genetic information, microsatellite markers were used. A total of 376 Labeobarbus spp., belonging to 24 populations of 11 species from three regions of the lake (north, south and east), were sampled. Eight microsatellite markers were analysed. In general, differences between conspecific populations were smaller than differences between populations of different species. For six species, conspecific populations from different regions in the lake were consistently more similar than populations of other species from the same region. For four species this was not the case, while for one species two populations were similar, but different from the third population. River-spawning species appeared to be more distinct than presumed lake spawners. On the species level, there was a significant correlation between genetic and morphological differentiation, especially in morphological aspects associated with ecological functioning. This suggests that genetic differentiation arose together with adaptive radiation, although the overall genetic differentiation among the Lake Tana Labeobarbus spp. is small.

In this study we present a fishery independent survey of the finfish populations at depths ranging from 5-30 m in the shallow reef habitats around the island of St. Eustatius.

Contains fulltext : 135031.pdf (Publisher’s version ) (Open Access) East Africa's Lake Victoria provides resources and services to millions of people on the lake's shores and abroad. In particular, the lake's fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake's social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks' vulnerability to exploitation but reduce society's vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales. 18 p.

In dit rapport worden de risico’s geanalyseerd van exotische vissoorten die zijn opgenomen in de Visserijwet en hun hybriden. De volgende soorten en één specifieke hybride zijn in de analyse meegenomen: beekridder (Salvelinus alpinus); roofblei (Leuciscus aspius); karper (Cyprinus carpio); Amerikaanse hondsvis (Umbra pygmaea); graskarper (Ctenopharyngodon idella); hybride ‘kruiskarper’ (Cyprinus carpio X Carassius spp.); snoekbaars (Sander lucioperca); giebel (Carassius gibelio) en kleine marene (Coregonus albula). Hoewel zeeforel (Salmo trutta trutta) was opgenomen in de initiële selectie van exotische soorten, is in consensus met experts besloten deze soort als inheems te beschouwen. Op basis van dit rapport worden de volgende maatregelen aanbevolen: - Het uitzetten van exoten, met name karper, giebel, snoekbaars, graskarper en vruchtbare hybriden (kruiskarper), moet gestopt of gereguleerd worden om verdere verspreiding en nieuwe introducties te voorkomen. - Het screenen van nationale en international vistransporten en maatregelen te nemen om de verspreiding van ziekten en andere meeliftende exoten te voorkomen.

Vanuit de Tweede Kamer is gesteld dat het streefbeeld voor uittrek van schieraal te hoog is. In het huidige Nederlandse streefbeeld is te weinig rekening is gehouden met factoren als veranderende voedselrijkdom van de Nederlandse wateren, het uitzetten van glas- en pootaal en de aanleg van migratiebeperkende kunstwerken zoals sluizen en gemalen. In dat kader is ingesteld de commissie ‘Herberekening streefbeeld paling’ o.l.v. Prof. Dr. Rabbinge. De resultaten worden in dit rapport gegeven

Het afgelopen decennium zijn de Nederlandse rivieren in hoog tempo gekoloniseerd door vier Ponto-Kaspische grondelsoorten, namelijk de marmergrondel (Proterorhinus semilunaris), zwartbekgrondel (Neogobius melanostomus), Kesslers grondel (Ponticola kessleri) en Pontische stroomgrondel (Neogobius fluviatilis). Deze uitheemse grondels zijn invasieve vissen met een bodemgebonden levenswijze en kunnen plaatselijk in hoge dichtheden voorkomen. Hoewel de verspreidingpatronen van invasieve grondels in Nederland tot op heden grofweg bekend zijn, bestaat geen duidelijk inzicht in de verspreidingprocessen en dichtheden van de grondels in verschillende watertypen, hun (ecologische) effecten en kansrijke mogelijkheden om ongewenste effecten te beperken.

Natural river-floodplain systems are characterized by their dynamic hydrology and diverse geomorphology resulting in a wide range of habitats that support high fish diversity and production. Various factors (e.g. hydrological dynamics, water quality, and biotic processes) have been proposed to explain fish distribution in large river floodplains, but it is still widely acknowledged that the mechanisms involved may vary in diverse floodplain systems and that they are not fully understood. To determine how flooding dynamics and floodplain geomorphology influence fish species distributions across the Volga-Akhtuba floodplain, Russian Federation, we examined the distributions of eight species with respect to variables reflecting floodplain hydrology and geomorphology. On the basis of fish catches in 40 floodplain water bodies at the end of summer in 2006–2008, we found that frequency of occurrence of most fish species remained stable along the time. The distribution of fish species was strongly influenced by the size and shape of water bodies as well as flood extent. Therefore, the long-term flood variability that drives the geomorphic heterogeneity of the floodplain creates suitable habitats across ranges of fish flow guilds (rheophilic, eurytopic, and limnophilic), resulting in high diversity of the floodplain ichthyofauna. We conclude that this diverse habitat availability is a highly significant factor influencing fish distribution in the Volga-Akhtuba floodplain.

Periodic flooding plays a key role in the ecology of floodplain rivers. Damming of such rivers can disturb flooding patterns and have a negative impact on commercial fish yield. The Volga River, the largest river in Europe, has a regulated flow regime after completion of a cascade of dams. Here, we study effects of damming on long-term discharge variability and flood pulse characteristics. In addition, we evaluate the effects of the altered flood pulse on floodplain ecosystem functioning and commercial fish yields. Our results indicate that both flood pulse and fish populations of the Volga–Akhtuba floodplain have varied considerably over the past decades. After damming, annual maximum peak discharges have decreased, minimum discharges increased, but average discharges remained similar to pre-damming conditions. Moreover, because of bed level incision of over 1.5 m, a higher discharge is needed to reach bankfull level and inundate the floodplains. Despite this significantly altered hydrological regime and subsequent morphological changes, current discharge management still provides significant spring flooding. However, commercial fish catches did decrease after damming, both in the main channel and in the floodplain lakes. All catches were dominated by species with a eurytopic flow preference, although catches from the main channel contained more rheophilic species, and floodplain catches contained more limnophilic and phytophilic species. The strong increase of opportunistic gibel carp (Carassius gibelio) around 1985 was apparent in the main channel and the floodplain lakes. Despite the hydrological changes, the decrease in overall catches, and the upsurge of gibel, we found a strong positive effect of flood magnitude in the previous year on commercial fish yield in the floodplain lakes. This suggests that under the current discharge management there still is an increased fish growth and/or survival during high floods and that functioning of the floodplain is at least partly intact.

Periodic flooding plays a key role in the ecology of floodplain rivers. Damming of such rivers can disturb flooding patterns and have a negative impact on commercial fish yield. The Volga River, the largest river in Europe, has a regulated flow regime after completion of a cascade of dams. Here, we study effects of damming on long-term discharge variability and flood pulse characteristics. In addition, we evaluate the effects of the altered flood pulse on floodplain ecosystem functioning and commercial fish yields. Our results indicate that both flood pulse and fish populations of the Volga–Akhtuba floodplain have varied considerably over the past decades. After damming, annual maximum peak discharges have decreased, minimum discharges increased, but average discharges remained similar to pre-damming conditions. Moreover, because of bed level incision of over 1.5 m, a higher discharge is needed to reach bankfull level and inundate the floodplains. Despite this significantly altered hydrological regime and subsequent morphological changes, current discharge management still provides significant spring flooding. However, commercial fish catches did decrease after damming, both in the main channel and in the floodplain lakes. All catches were dominated by species with a eurytopic flow preference, although catches from the main channel contained more rheophilic species, and floodplain catches contained more limnophilic and phytophilic species. The strong increase of opportunistic gibel carp (Carassius gibelio) around 1985 was apparent in the main channel and the floodplain lakes. Despite the hydrological changes, the decrease in overall catches, and the upsurge of gibel, we found a strong positive effect of flood magnitude in the previous year on commercial fish yield in the floodplain lakes. This suggests that under the current discharge management there still is an increased fish growth and/or survival during high floods and that functioning of the floodplain is at least partly intact.

Eind 2007 verscheen in Visionair een artikel over een uniek onderzoek naar de relatie tussen vloedvlaktes en vis in de Russchische Wolga. Inmiddels is de eerste fase succesvol afgesloten. De onderzoeksresultaten bieden een schat aan informatie over de biologie en het beheer van riviervissen.

1. Large river floodplains are considered key nursery habitats for many species of riverine fish. The lower Volga River floodplains (Russian Federation) are still relatively undisturbed, serving as a suitable model for studying the influence of flooding and temperature on fish recruitment in floodplain rivers. 2. We examined the interannual variability in recruitment success of young-of-the-year (YOY) fish in the lower Volga floodplain in relation to flood pulse characteristics and rising water temperatures in the spring. We sampled four areas with different flooding regimes, in three consecutive years (2006–2008). 3. Extensive areas with a long duration of flooding accommodated high densities of young fish. This suggests that extended inundation improves the recruitment success of river fish. In areas with extensive flooding, the biomass of YOY of most fish species was about three times higher in 2006 and 2007 than in 2008. We hypothesise that low spring temperatures in 2008 may have caused this reduced recruitment and that a flood synchronised with rising temperature enhances recruitment success. 4. Extensive flooding was particularly favourable for species characterised by large body size, delayed maturation, high fecundity and low parental investment, such as pike Esox lucius, roach Rutilus rutilus and ide Leuciscus idus. Gibel carp Carassius gibelio, a species tolerant of high temperature and hypoxia, did particularly well in small waterbodies in the driest parts of the floodplain. 5. Structural characteristics of floodplain waterbodies explained much of YOY fish density. These species–environment associations varied from year to year, but some species such as common bream Abramis brama, roach and gibel carp showed consistent relationships with structural habitat characteristics in all years, despite large interannual fluctuations in flood pulse and spring temperature

1. Floodplains are a key habitat for foraging, spawning and as a nursery for many riverine fish species. The lower Volga floodplains (Russian Federation) are still relatively undisturbed, while in Europe and North America, about 90% of floodplains have effectively been lost. 2. We examined relationships between the extent and timing of the spring flood, lateral spawning movements of fish species and timing and duration of spawning in the floodplain by sampling during spring 2006 and 2007. 3. Only the spawning of rheophilic species, that released their eggs in the floodplain, coincided with the flood. In contrast, the timing of spawning by eurytopic and limnophilic species was unrelated to flooding. 4. For most fish species, we found no indication that the majority of spawners in the floodplain originated from the main river channel, with the exception of sabrefish Pelecus cultratus. 5. We postulate that in the vast Volga-Akhtuba floodplain, fish spawning stocks mainly originate from permanent floodplain waterbodies, whereas hydrological conditions are dominated by the river. Both the river and waterbodies on the floodplain may serve as sources for recolonisation after local extinction because of extreme environmental conditions, such as freezing or desiccation.