Як уникнути підйому рівня води?

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

Habitat-related birdsong divergence: a multi-level study on the influence of territory density and ambient noise in European blackbirds

Song plays an important role in avian communication and acoustic variation is important at both the individual and population level. Habitat-related variation between populations in particular can reflect adaptations to the environment accumulated over generations, but this may not always be the case. In this study, we test whether variation between individuals matches local conditions with respect to noise level and territory density to examine whether short-term flexibility could contribute to song divergence at the population level. We conducted a case study on an urban and forest population of the European blackbird and show divergence at the population level (i.e. across habitats) in blackbird song, anthropogenic noise level and territory density. Unlike in several other species, we found a lack of any correlation at the individual level (i.e. across individuals) between song features and ambient noise. This suggests species-specific causal explanations for noise-dependent song differentiation which are likely associated with variation in song-copying behaviour or feedback constraints related to variable singing styles. On the other hand, we found that at the level of individual territories, temporal features, but not spectral ones, are correlated to territory density and seasonality. This suggests that short-term individual variation can indeed contribute to habitat-dependent divergence at the population level. As this may undermine the potential role for song as a population marker, we conclude that more investigations on individual song flexibility are required for a better understanding of the impact of population-level song divergence on hybridisation and speciation

An overview of the pulse logbook data collected in 2017-2019

Knowledge on how fishers exploit their fisheries resources is important for understanding how fishing affects the population dynamics of the exploited species and how the fishery may affect the ecosystem. The introduction of a new gear may affect the way fishers deploy their gear in space and time. We compared the behaviour of pulse trawl vessels to the behaviour of traditional beam trawl vessels. Because the pulse logbook monitoring is still ongoing, the results are preliminary and will be updated when the complete data set will become available. The logbook data set analysed comprised catch and effort information per tow collected between 1 January 2017 and 31 December 2019. The results were compared with an analysis of logbook data of traditional beam trawl vessels collected between 2000 - 2005. This report repeated the analyses of the previous report (data from 01/01/2017-30/08/2018) and found very similar results regarding the differences between fishing patterns of the pulse trawl and the traditional beam trawl. In total, after quality filtering, data of 141,000 hauls from 75 pulse vessels were investigated. The previous study showed that pulse trawl (PT) and traditional beam trawl (BT) vessels had similar fishing patterns with alternating periods of searching, or sampling, for fishing grounds and exploitation of fishing grounds. The main differences with the results of the previous report is that a longer time series and therefore more data has been used to analyse the logbooks. Because of this similarity, the results of these analyses will not be reported in this provisional report but will be discussed in the final report. The cpue (catch per unit effort) of sole follows a clear seasonal pattern and seems to decrease slightly through time. Also the number of logbook received by fishermen clearly decreased as a result of the ban on pulse fisheries by the EU. The logbook data provide detailed information on what happens on the local fishing grounds which is fundamental to assess the impact of the pulse trawl fishery and beam trawl fishery on the fisheries resources and on the benthic ecosystem. The study of the total pulse fleet provides a unique data set to study not only the dynamics of the whole fleet, including the interactions among pulse vessels, but also provides a solid basis to study competitive interactions with other fisheries. The next step is to study the weekly fishery patterns of the fleet as a whole and to study the interaction between vessels

Inspannings- en vangstregistraties van trekvissen door beroepsvissers aan de buitenzijde van de Haringvlietsluizen in 2021

De Zeeuwse en Zuid-Hollandse delta vormden van oudsher een open estuarium met vrije toegang voor vissen tot het Rijn-Maas stroomgebied. Door de deltawerken is de toegankelijkheid van het gebied sterk ingeperkt. Dit heeft enorme consequenties gehad voor trekvispopulaties. Recent zijn grootschalige initiatieven gestart voor verbetering van doortrekmogelijkheden, waaronder het Kierbesluit (de ‘Kier’), dat in 2019 in werking is getreden. Rijkswaterstaat heeft de nodige monitoring en onderzoeken ingezet rondom de Kier in een traject van ‘lerend implementeren’. Binnen dit traject spelen veel beheersvragen in relatie tot herstel van vismigratie. Enkele van de hieraan verbonden beheersvragen centreren zich rondom het effect van de visserij op trekvissen, waarbij het ministerie van LNV heeft aangegeven hier in samenwerking met Rijkswaterstaat meer kennis over te willen vergaren. In het huidige onderzoek is getracht om de bijvangst van trekvissen door beroepsvissers zelf te laten registreren middels logboeken waarbij regelmatig een medewerker van WMR (opstapper) mee ging aan boord. Het doel is om voorafgaand én na instelling van de voorgenomen visserijvrije zone nabij de Haringvlietsluizen de bijvangst gedetailleerd in kaart te brengen. De nu voorliggende rapportage betreft de bijvangsten van trekvissen geregistreerd door beroepsvissers aan de buitenzijde van de Haringvlietsluizen in 2021. In totaal waren er 15 vissers actief in het Goereese Gat in 2021. Vier fuikenvissers, één staandwantvisser, één zegenvisser, zes kleine sleepnetvissers, vier grote sleepnetvissers. Drie van de vissers hebben hun inspanning en vangsten geregistreerd (fuikenvisser, staandwantvisser, zegenvisser). In deze drie visserijen zijn in totaal 88 trekvissen, verdeeld over negen soorten, bijgevangen in de periode juni 2021-januari 2022. De meeste trekvissen (55, voornamelijk rivierprikken en salmoniden) werden in hokfuiken gevangen, gevolgd door staandwant (23 finten) en zegen (6 finten, 3 zeeforellen) en in de schietfuiken werd één rivierprik bijgevangen. De hokfuiken vingen ook de grootste diversiteit aan soorten met zes van de negen soorten exclusief in hokfuiken. Bijzondere vangsten waren Siberische steur, regenboogforel en twee bultrugzalmen, allemaal gevangen met hokfuiken. De trekvissen elft, Atlantische steur en Europese steur zijn niet waargenomen. Opstappers zijn met vijf verschillende visserijen mee geweest om de vangsten te controleren. Tijdens deze opstapreizen zijn trekvissen gevangen. Bij drie van de 10 opstapreizen met garnalenvissers (kleine sleepnetvissers) is trekvis gevangen (1 juveniele Noordzeehouting, 2 juveniele Noordzeehoutingen, 4 zeeforellen resp.). Tijdens de opstapreis met de zegen zijn er twee zeeforellen bijgevangen. Tijdens de opstapreis met de fuikenvisser op aal/snoekbaars is één fint bijgevangen. Tijdens de opstapreis met de staandwantvisser is geen trekvis bijgevangen. Tijdens de opstapreis met de fuikenvisser op wolhandkrab is 1 juveniele Noordzeehouting bijgevangen. De meeste trekvissen die geregistreerd zijn in de vangsten, zijn volwassen dieren die mogelijk klaar waren om de rivieren op te trekken richting de paaigronden. Voor fint, zeeforel en mogelijk ook Noordzeehouting geldt dat deze dieren geregeld langs de Nederlandse kust foerageren, inclusief het Goereese Gat en dus niet noodzakelijkerwijs aan het intrekken waren. Deze tussenrapportage geeft een beeld van de bijvangst van trekvissen door beroepsvissers aan de buitenzijde van de Haringvlietsluizen. Dit beeld is niet volledig en representatief vanwege een aantal zaken. 1) Deze resultaten zijn gebaseerd op registraties van iets meer dan een half jaar waardoor er geen compleet beeld is ontstaan van alle seizoenen waarin trekvissen migreren en waardoor ook niet alle visserijen konden worden meegenomen. 2) Er zijn inspannings- en vangstregistraties van maar drie (fuiken, staandwant en zegen) van de zes visserijen. 3) De visserijen vinden op verschillende locaties plaats binnen het Goereese Gat. Aangezien niet alle vissers hun vangsten registreren ontbreekt er nog informatie over het verschil in de bijvangst van trekvis per vangstlocatie. 4) Het jaar 2021 lijkt een uitzonderlijk jaar te zijn met een hoge afvoer gedurende de zomermaanden waardoor er langer door gevist kon worden met fuiken en wellicht heeft deze hoge afvoer ook een grote(re) aantrekkingskracht gehad op trekvissen. Wageningen Marine Research rapport C027/22 | 5 van 24 Het verdient aanbeveling om de inspannings- en vangstregistraties uit te breiden zodat er een completer beeld van de inspanning en de bijvangst ontstaat. Daarnaast kan er meer nadruk worden gelegd op de behandeling van de bijgevangen trekvis, zoals snelle herkenning en terugzetten van trekvis voordat de rest van de vangst wordt verwerkt, wat de overleving van trekvis na vangst kan verbeteren

An update of the pulse logbook data collected in 2017-2020

Knowledge on how fishers exploit their fisheries resources is important for understanding how fishing affects the population dynamics of the exploited species and how the fishery may affect the ecosystem. The introduction of a new gear may affect the way fishers deploy their gear in space and time. From 01/01/2017, pulse trawlers started to register detailed catch data in logbook on board of the pulse trawler. Based on these data, we compared the behavior of pulse trawl vessels to the behavior of traditional beam trawl vessels. The logbook data set analysed comprised catch and effort information per tow collected between 1 January 2017 and 30 September 2020. The results were compared with an analysis of logbook data of traditional beam trawl vessels collected between 2000 - 2005. This report repeated the analyses of the previous report (data from 01/01/2017-31/12/2019) and found very similar results regarding the differences between fishing patterns of the pulse trawl and the traditional beam trawl. In total, after quality filtering, data of 150,000 hauls from 75 pulse vessels were investigated. The previous study showed that pulse trawl (PT) and traditional beam trawl (BT) vessels had similar fishing patterns with alternating periods of searching, or sampling, for fishing grounds and exploitation of fishing grounds. The main differences with the results of the previous report is that a longer time series and therefore more data has been used to analyse the logbooks. Because of this similarity, the results of these analyses will not be reported in this provisional report but will be discussed in the final report. The catch per unit effort (cpue) of sole, the most important target species of the pulse trawl fishery, follows a clear seasonal pattern. Also the number of logbooks received by fishermen clearly decreased as a result of the ban on pulse fisheries by the EU. Because the pulse logbook monitoring is still ongoing, the results are preliminary and will be updated when the complete data set will become available (end of 2021). The logbook data provide detailed information on what happens on the local fishing grounds which is fundamental to assess the impact of the pulse trawl fishery and beam trawl fishery on the fisheries resources and on the benthic ecosystem. The study of the total pulse fleet provides a unique data set to study not only the dynamics of the whole fleet, including the interactions among pulse vessels, but also provides a solid basis to study competitive interactions with other fisheries. The next step is to study the weekly fishery patterns of the fleet as a whole and to study the interaction between vessels

Supplementary video from The onset of ecological diversification 50 years after colonization of a crater lake by haplochromine cichlid fish

Adaptive radiation research typically relies on the study of evolution in retrospective, leaving the predictive value of the concept hard to evaluate. Several radiations, including the cichlid fish in the East African Great Lakes, have been studied extensively, yet no study has investigated the onset of the intraspecific processes of niche expansion and differentiation shortly after colonization of an adaptive zone by cichlids. Haplochromine cichlids of one of the two lineages that seeded the Lake Victoria radiation recently arrived in Lake Chala, a lake perfectly suited for within-lake cichlid speciation. Here, we infer the colonization and demographic history, quantify phenotypic, ecological and genomic diversity and diversification, and investigate the selection regime to ask if the population shows signs of diversification resembling the onset of adaptive radiation. We find that since their arrival in the lake, haplochromines have colonized a wide range of depth habitats associated with ecological and morphological expansion and the beginning of phenotypic differentiation and potentially nascent speciation, consistent with the very early onset of an adaptive radiation process. Moreover, we demonstrate evidence of rugged phenotypic fitness surfaces, indicating that current ecological selection may contribute to the phenotypic diversification

Spieringstand IJsselmeer en Markermeer 2021 : pilotproject maandelijkse monitoring - update 2020

Het Ministerie van Landbouw, Natuur en Voedselkwaliteit heeft Wageningen Marine Research gevraagd om een korte rapportage van de spieringstand in het IJsselmeer en het Markermeer op te stellen op basis van de jaarlijkse bestandsopnamen binnen het WOT-programma

Strong species structure but weak geographical structure in demersal Lake Victoria cichlids

Studying phenotypic and genetic differentiation between very young species can be very informative with regard to learning about processes of speciation. Identifying and characterizing genetic species structure and distinguishing it from spatial genetic structure within a species is a prerequisite for this and is often not given sufficient attention. Young radiations of cichlid fish are classical speciation study systems. However, it is only during the past decade that population genomics based on next-generation sequencing has begun to provide the power to resolve species and distinguish speciation from spatial population structure for the youngest of these radiations. The Lake Victoria haplochromine cichlids constitute the youngest large cichlid fish radiation, probably <20,000 years old. Earlier work showed that communities of rocky reef cichlids are composed of many reciprocally monophyletic species despite their very recent origins. Here, we build on this work by studying assemblages of offshore demersal cichlids, adding analyses of within-species spatial structure to the sympatricspecies structure. We sampled seven multispecies communities along a 6-km-long transect from one side of the Mwanza Gulf to the other side. We investigated whether phenotypically diagnosed putative species are reciprocally monophyletic and whether such monophyly is stable across species geographic ranges. We show that all species are genetically strongly differentiated in sympatry, that they are reciprocally monophyletic, and that monophyly is stable across distribution ranges. We found significant differentiation between geographically distinct populations in two species, but no or weak isolation by distance. We further found subtle but significant morphological differences between all species and a linear relationship between genomic and morphological distance which suggests that differences in morphology begin to accumulate after speciation has already affected genome-wide restrictions of gene flow

Efficiency changes in bottom trawling for flatfish species as a result of the replacement of mechanical stimulation by electric stimulation

Although fishing with electricity is illegal in the European Union, a number of temporary licences allowed converting beam trawlers to pulse trawling. To analyse how the adaption of pulse trawling changed this fishery, we studied fishing speeds and landings per unit effort as proxies for catch efficiencies for the main target species. Compared to conventional tickler chain beam trawls, pulse trawls were towed at lower speeds (small vessels -10%, large vessels -23%). Large vessels that switched from conventional beam trawls to pulse trawls at the end of 2009 gradually increased catch efficiency for sole over the period of almost 1 year. While pulse trawling was found to have higher catch rates (kg/h) for sole (small vessels +74%, large vessels +17%), lower catch rates were observed for plaice (small vessels -31%, large vessels -32%). Vessels that switched later achieved immediate gains in catch efficiency for sole. The change in catch efficiency is likely due to the difference in cramp response between the species

The origin and future of an endangered crater lake endemic; phylogeography and ecology of Oreochromis hunteri and its invasive relatives

Cichlids of the genus Oreochromis (“Tilapias”) are intensively used in aquaculture around the world. In many cases, when “Tilapia” were introduced for economic reasons to catchments that were home to other, often endemic, Oreochromis species, the loss of native species followed. Oreochromis hunteri is an endemic species of Crater Lake Chala on the slopes of Mount Kilimanjaro and is part of a small species flock in the upper Pangani drainage system of Tanzania. We identified three native and three invasive Oreochromis species in the region. Reconstructing their phylogeography, we found that O. hunteri is closely related to, but distinct from the other members of the upper Pangani flock. However, we found a second, genetically and phenotypically distinct Oreochromis species in Lake Chala whose origin we cannot fully resolve. Our ecological and ecomorphological investigations revealed that the endemic O. hunteri is currently rare in the lake, outnumbered by each of three invasive cichlid species. It is mitochondrially, phenotypically and trophically distinct from all others. The occurrence of the formerly abundant O. hunteri in such small numbers, its narrow habitat restriction and its limited morphological variability suggest recent population decline and loss of niche breadth in this critically endangered endemic cichlid species.</p