Cascading effects of introduced Nile perch (Lates niloticus) on the foraging ecology of Nile tilapia (Oreochromis niloticus)

Abstract –  Nile tilapia ( Oreochromis niloticus L.) is the dominant of the introduced tilapiines in many East African lakes and has flourished in the presence of introduced Nile perch ( Lates niloticus L.). We explored the hypothesis that O. niloticus exhibits increased omnivory in response to a decline in abundance of haplochromine cichlids. First, we quantified variation in habitat use and diet of O. niloticus in Lake Nabugabo, Uganda. Second, we compared the diet of O. niloticus in lakes with (Nabugabo, Victoria) and without (Mburo, Wamala, Nyamusingiri, Kyasanduka) introduced Nile perch. In Lake Nabugabo, a higher level of phytoplanktivory was observed in small juveniles than in larger fish and in wetland ecotone areas where haplochromines were most abundant. An omnivorous diet dominated by detritus and invertebrates was recorded for O. niloticus in lakes Nabugabo and Victoria, while a predominantly herbivorous diet was characteristic of O. niloticus in lakes without Nile perch. Availability of a broad food base in lakes where inshore insectivores have been reduced may explain the increased omnivory recorded in lakes Nabugabo and Victoria.     

Implications of hypoxia tolerance for wetland refugia use in Lake Nabugabo,Uganda

Aquatic hypoxia can affect predator‐prey interactions by altering the success rate of the predator and/or the vulnerability of prey. For example, in the Lake Victoria basin of East Africa, native prey exploit hypoxic wetlands as refugia from predation by introduced Nile perch (Lates niloticus). Here, it is predicted that species exploitation of wetlands depends on their hypoxia tolerance relative to the heterogeneity of wetland hypoxia. In this study, we compared the hypoxia tolerance of four fish taxa that differ in their use of hypoxic wetlands in Lake Nabugabo, Uganda: the cichlid Pseudocrenilabrus multicolor victoriae that inhabits the dense swamp interior; and three taxa that inhabit wetland ecotones including Nile tilapia (Oreochromis niloticus), L. niloticus and juvenile endemic haplochromine cichlids. We characterised hypoxia tolerance by exposing fish to progressive hypoxia and quantifying variation in aquatic surface respiration (ASR). The effect of body size on tolerance was explored in L. niloticus by quantifying ASR behaviour across a range of size classes. ASR behaviour was also compared between O. niloticus groups from wetland versus open‐water habitats to detect habitat‐associated intraspecific variation. The most tolerant taxon was the swamp specialist P. multicolor, indicated by its low ASR thresholds and small percentage of fish using ASR during the final sample interval. The other three taxa did not differ in ASR behaviour, and no differences were detected between O. niloticus groups. Body size effects were present for L. niloticus suggesting a lower tolerance to hypoxia in larger‐bodied individuals, thus limiting their ability to penetrate wetlands.     

Fatty Acid Composition of Muscle,Liver, and Adipose Tissue of Freshwater Fish from Lake Victoria,Uganda

Fish oils may differ in fatty acid (FA) composition depending on diet. Oils extracted from muscle, liver, and adipose tissue of Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus), silver fish (Rastrineobola argentea), lungfish (Protopterus aethiopicus), Victoria squeaker (Synodontis victoriae), and two catfishes (Clarias gariepinus and Bagrus docmac) from Lake Victoria, a tropical freshwater lake, were evaluated for FA composition. Oil contents of muscles, livers, and adipose tissues were in the range of 3.16 to 13.8%, 3.62 to 53.4%, and 28.8 to 42.4%, respectively. Omega-3 polyunsaturated FA, particularly alpha-linolenic (ALA), eicosapentaenoic (EPA), docosapentaenoic (DPA), and docosahexaenoic (DHA) acids, were found to be in substantial amounts in oils from all seven fish species. Ratios of polyunsaturated FA to saturated FA (0.79 to 1.18) were in the range considered adequate for normal health. Overall, the results show that the fish species studied are a rich source of omega-3 polyunsaturated FA.     

The introduced species fishery of Lake Naivasha,Kenya: ecological impact vs socio‐economic benefits

Lake Naivasha is a shallow, freshwater lake in the eastern Rift Valley of Kenya. Its fish community now comprises only introduced species. Oreochromis spirulus niger was stocked in 1925 as a forage fish for the largemouth bass, Micropterus salmoides, introduced in 1929 for sport fishing. Further introductions of tilapiines followed for commercial exploitation. A gillnet fishery opened in 1959, and the annual species catch composition to 2000 was dominated by Oreochromis leucostictus. Following their accidental introduction, carp Cyprinus carpio appeared in catches in 2002; by 2010, it comprised >99% of landings by weight. Carp now provides a sustainable fishery in a lake heavily impacted by anthropogenic stressors, including water abstraction and nutrient enrichment. Oreochromis niloticus was reintroduced in 2011 to reinvigorate tilapia stocks following the collapse of its stocks in the 1990s and early 2000s, and the African catfish Clarias gariepinus is now captured in small but increasing numbers. The current status of the fishery, especially the predominance of carp, presents major management challenges; these are addressed by stakeholder engagement and co‐management. The introductions have artificially created a commercial fishery that provides substantial societal benefits in a semi‐arid region of a developing country with high poverty levels.     

鄱阳湖刀鲚的渔汛特征及渔获物分析

通过探究鄱阳湖刀鲚(Coilia ectenes)的渔汛特征和渔获物现状,为有效地保护其种群资源提供科学依据。2013-2014年在鄱阳湖设置3个调查点,对刀鲚进行逐月采样调查,共取样986尾、41.54 kg,随机抽样调查不同捕捞网具的整船次鄱阳湖刀鲚渔获物数据,测量体长、体重等生物学参数。采用Pearson相关分析刀鲚单船日渔获重量WB与水文因子关系。结果表明,鄱阳湖刀鲚体长范围为17.9~38.1 cm,其中21.2~23.6 cm是优势体长组,占总个体数的37.02%;体重范围为14.3~198.2 g,其中25~40 g是优势体重组,占总个体数的57.20%。鄱阳湖刀鲚体长L(cm)和体重W(g)幂函数方程为:W=0.0019L3.1378(R2=0.8997;n=986;P0.01)。湖口水域刀鲚出现时间主要集中在每年的5-6月,高峰期在6月,7月很少见;单船渔获数量NB为(3.2±4.1)尾/d,单船渔获重量WB为(0.12±0.11)kg/d。湖口水域刀鲚单船渔获重量WB与该水域水位、流量均呈显著正相关(P0.05),WB与水位的相关性更为显著。与历史资料相比,鄱阳湖刀鲚渔获规格以小个体居多;渔汛稍有推迟,持续时间大幅度缩短;种群规模急剧缩小,表明刀鲚资源正在严重衰退。     

Factors influencing the abundance of three common nearshore fishes in south‐west Lake Michigan

Variation in the distribution and abundance of nearshore fishes is critical to understand food web processes and fishery management issues in Lake Michigan. This study characterised patterns in abundance of three common nearshore species, yellow perch Perca flavescens (Mitchell), round goby Neogobius melanostomus (Pallas) and alewife Alosa pseudoharengus (Wilson), in relation to spatio‐temporal, abiotic and biotic factors using gillnet sampling conducted across 5 years at multiple locations representing different substrates. Significant variations were observed in alewife and round goby catches between locations. A negative relationship between round goby and age‐0 yellow perch catch was observed, indicating potential competition between the two species. This study demonstrates that variability in nearshore fish communities can be driven by factors including substrate and interspecific interactions. Given the prominent role these species play in Lake Michigan's food web and thus their importance to fishery production, a thorough understanding of these factors is warranted.     

Fish fauna in a Protected Greek lake: biodiversity, introduced fish species over a 80-year period and their impacts on the ecosystem

Abstract –  Lake Pamvotis is a tertiary natural shallow lake located in the NW of Greece. For biogeographical reasons, it historically contained only four fish species: two endemics ( Phoxinellus epiroticus , Squalius pamvoticus ), one native to the West Greece ( Barbus albanicus ) and the ubiquitous Anguilla anguilla . These species were almost exclusively present in the lake before 1920, forming remarkable commercially exploited populations. From the 1930s through the 1990s, 20 species were introduced into the lake for purposes of eutrophication control or to enhance the fishery. These introductions, some of them accidental have led to the apparent decline, even loss, of the native species. During the last three decades the fish fauna of the lake has shifted from the native, clear-water species to a predominance of introduced species, mainly those adapted for turbid eutrophic water ( Rutilus panosi , Cyprinus carpio and Carassius gibelio , and several Asian cyprinids). The current fish assemblage in the lake is dominated by introduced species particularly the Mosquitofish ( Gambusia affinis ) and the Lourogobios ( Economidichthys pygmaeus ) in the littoral zone, the benthopelagic allogynogenetic Prussian carp ( C. gibelio) , and the opportunistic Trichonis roach ( R. panosi ), in the pelagic zone.     

Prevalence of nematode parasite,Contracaecum, in Nile tilapia,African catfish and Barbus species in Lake Hawassa,Ethiopia

A cross‐sectional study was conducted from November 2017 to March 2018 at Lake Hawassa to determine the prevalence of larval Contracaecum infestation in Nile tilapia, (Oreochromis niloticus), African Catfish (Clarias gariepinus) and Barbus species (Barbus intermedius). Fifty‐two point six per cent (52.6%) previous prevalence and 5% precision were used to estimate the sample size. Accordingly, a total of 383 randomly sampled fish species comprising of 163 (42.6%) C. gariepinus, 159 (41.5%) O. niloticus and 61(15.9%) B. intermedius were examined. The length and weight of each sampled fish were recorded. All the sampled fish were examined for the evidence of Contracaecum parasites in Hawassa University Veterinary Parasitology and Pathology Laboratory. Chi‐square values and comparison of proportions were used to analyse the data. The overall prevalence of Contracaecum parasites of fish population was 31.6%. The distribution of parasite was significantly affected by fish species (p = .000), sexes (p = .018), length (p = .003) and weight classes of fish (p = .026). As a hygienic problem and gutting activity conducted at the sides of the Lake Hawassa and distribution of discarded fish wastes for surrounding piscivorous birds by butchers and other people, the life cycle of Contracaecum parasite was perpetuated. Thus, the parasite is of zoonotic significance. Therefore, awareness creation activities for societies and control of fish parasites should be conducted in the study area.     

Habitat use and activity patterns of roach (Rutilus rutilus (L.)), rudd (Scardinius erythrophthalmus (L.)), perch (Perca fluviatilis L.) and pike (Esox lucius L.) in the laboratory: the role of predation threat and structural complexity

Abstract– The role of predation risk and structural complexity in determining the habitat use and activity patterns of roach, rudd and perch was assessed using a series of video-recorded laboratory trials. The time spent in open and structured habitats, vertical swimming heights and activity levels of each species were observed in the presence and absence of a potential, pike, predator. Habitat complexity varied between treatments with artificial stem densities of 200, 400 and 600 stems'm⁻². Predator free trials showed that roach and rudd spent significantly less time in structured habitats than perch. Increasing stem density had no significant effect on the habitat choice of perch but did affect the distribution of roach and rudd. Stem density influenced the vertical swimming height of rudd or perch but not roach, although the effects of habitat complexity on swimming activity were more complicated. Pike were themselves influenced by increases in stem density, only selecting structured habitats when stem densities were less than 600 stems'm⁻². In the presence of a predator, both roach and rudd increased the amount of time spent in structured areas. These observed differences were independent of stem density. Perch, however, decreased the amount of time spent in structural habitats at all stem densities. Predation risk also prompted significant changes in both vertical swimming height and activity levels of potential prey fish. A pike predation success hierarchy of rudd-roach-perch was also observed during the study. The reasons for this pattern are discussed.     

Smalltooth sawfish (Pristis pectinata Latham, 1794) in the Casamance River,Senegal: A historical perspective

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The demise of Darwin's fishes: evidence of fishing down and illegal shark finning in the Galápagos Islands

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Understanding fishing and conservation of the Pacific seahorse (Hippocampus ingens) in Sinaloa,Mexico, through local ecological knowledge

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Integration of mapping and socio‐economic status of cage culture: Towards balancing lake‐use and culture fisheries in Lake Victoria,Kenya

Mapping of lacustrine aquaculture and socio‐economic assessment of cage farmers operations can be employed as decision support tools in an integrated fashion for fisheries management. We simultaneously mapped and reported the location of cages and characterized socio‐economic and indicators of cage farming in Lake Victoria, Kenya. Structured questionnaires and interviews from cage farmers generated socio‐economic data and management information. Vital water quality parameters were analyzed in selected sampling sites. Cage culture was found to be a male‐dominated activity with the majority of owners aged <45 years (n = 23; 59%). Siaya County had the highest cage establishments (n = 20) and number (n = 1,343). Proximity to Dominion Farm, pioneers of cage culture, as well as decreased presence of water hyacinth (Eichhornia crassipes) coverage may have contributed to high cage numbers in Siaya County. The only species cultured was Oreochromis niloticus. Most cage establishments (n = 30; 76%) were located within 200 m from the shoreline. Total operational costs for a cycle (8 months) amounted to USD 465,250 worth a total production value of USD 8,827,000, farmers often realizing a mark‐up of >100%, indicative of its robust viability within “The Blue Economy” concept. With the increasing number of cages in the lake, there is the need for policy and regulations to guide its investment, both to protect local economies through improved business practices and to ensure sustainability for the lake ecosystem due to the likelihood of exacerbation in water quality deterioration in cage culture sites.     

Controls of local young-of-the-year fish species richness in flood plain water bodies: potential effects of habitat heterogeneity, productivity and colonisation–extinction events

Abstract –  This study reports on the relationships between young-of-the-year (YOY) fish species richness and landscape and local factors in 20 backwaters of the Seine River flood plain. Using multiple linear regression, we tested the importance of three types of variable for explaining total species richness (TSR), phytophil species richness (PSR) and lithophil species richness (LSR): (1) colonisation–extinction processes, (2) habitat heterogeneity and (3) productivity. At the local scale, no variables describing habitat heterogeneity correlated with TSR, PSR or LSR. Productivity was correlated only with TSR according to a polynomial model. In contrast, variables relating to the colonisation–extinction process correlated with TSR, PSR and LSR: TSR varied mainly with backwater size, which is a surrogate of extinction, whereas PSR and LSR correlated with the relative longitudinal position of backwaters and the size of their connection to the main channel. In conclusion, it seems that the total fish richness varies as a function of local factors, whereas the components of fish diversity are rather influenced by landscape factors.     

The status and distribution of a newly identified endemic galaxiid in the eastern Cape Fold Ecoregion,of South Africa

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西藏巴松错（湖）夏季浮游植物优势种生态位及其种间联结性

为了解青藏高原高寒湖泊不同空间维度浮游植物群落优势种的生态位及种间联结性,本文选取青藏高原东南部最大的高寒冰山堰塞湖之一的巴松错为研究地点,于2020年7月对巴松错浮游植物进行了定性和定量采集,采集了水平和垂直维度浮游植物的水样275个,结合生态位宽度、生态位重叠值、生态响应速率、总体联结性、x2 检验、联结系数( AC) 、共同出现百分比( PC) 以及点相关系数(φ) 等方法,分析优势种的生态位及种间联结性。结果表明：1）共鉴定浮游植物优势种5属8种,均属于硅藻门;2）生态位宽度值差异较大,可分为三大生态位类群,且水平-垂直二维生态位宽度的变化幅度受水平维度生态位宽度影响比较明显;3）在水平维度上,生态位重叠度达到有意义的种对比例较高,具有明显的同质性;在垂直维度和水平-垂直维度上,生态位重叠度达到有意义的种对比例较低,具有明显的异质性;4）巴松错夏季浮游植物群落优势种生态响应速率在水平维度上形成正反馈,在垂直和水平-垂直维度维度上形成负反馈,表明巴松错夏季浮游植物群落存在群落多稳态,垂直和水平-垂直维度上群落较水平维度更稳定;5）巴松错夏季浮游植物群落总体处于演替的初期阶段且存在正向演替的趋势,物种间联结性不强,种间关系较为松散。本研究结果为进一步研究高寒湖泊优势种生态位和群落多稳态提供了基础资料。     

Fish distribution and diet in relation to the invasive macrophyte Lagarosiphon major in the littoral zone of Lake Dunstan, New Zealand

Abstract –  Invasive macrophytes are usually associated with negative impacts on habitat quality and a threat to native biodiversity. However, they might provide the same beneficial functions of native macrophytes, i.e., the provision of food and shelter for fish, in the absence of native macrophytes. To assess the value of the invasive macrophyte Lagarosiphon major as a fish habitat, we investigated the spatio–temporal variation in the distribution of a small littoral fish species (common bully) in the littoral of Lake Dunstan, a New Zealand hydro lake. Large- and fine-scale common bully distribution could partly be explained by the occurrence of dense L. major stands. Additionally, variability in catch per unit effort was partly explained by season and recruitment. Diet analysis indicated that common bullies in the Lagarosiphon-dominated littoral fed on invertebrates (Mollusca, Trichoptera, Chironomidae) found on exotic L. major , therefore suggesting its role as a food provider in the system. These results indicated that invasive macrophytes can provide important ecosystem functions in disturbed habitats that are otherwise devoid of native macrophytes. Any macrophyte management strategy should therefore carefully consider the costs and benefits associated with macrophyte control.     

Influences of local habitat and stream spatial position on fish assemblages in a dammed watershed,the Qingyi Stream,China

Abstract – Identifying the underlying mechanisms that explain the spatial variation in stream fish assemblages is crucial for the protection of species diversity. The influences of local habitat and stream spatial position on fish assemblages were examined from first‐order through third‐order streams within a dammed watershed, the Qingyi Stream, China. Based on linear regression models, the most important environmental variables influencing fish species richness were water temperature and wetted width, but stream spatial position variables were less important. Using canonical correspondence analysis, five environmental variables were identified to significantly influence fish assemblages, including three habitats (elevation, substrate and water depth) and two spatial variables (C‐link and Link). Our results suggest that, in a heavily dammed watershed, by blocking the migration routes of fishes, dams weaken the influence of stream spatial position on fish species richness. However, fish species compositions are significantly influenced by both local habitat environment and stream spatial position, which is perhaps owing to the distribution of fish species according to ecological requirements not related to spatial processes.