Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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    Coupled human and natural system dynamics as key to the sustainability of Lake Victoria’s ecosystem services
    Downing, A.S. ; Nes, E.H. van; Balirwa, J.S. ; Beuving, J. ; Bwathondi, P.O.J. ; Chapman, L.J. ; Cornelissen, I.J.M. ; Cowx, I.G. ; Goudswaard, P.C. ; Hecky, R.E. ; Janse, J.H. ; Janssen, A.B.G. ; Kaufman, L. ; Kishe-Machumu, M.A. ; Kolding, J. ; Ligtvoet, W. ; Mbabazi, D. ; Medard, M. ; Mkumbo, O.C. ; Mlaponi, E. ; Munyaho, A.T. ; Nagelkerke, L.A.J. ; Ogutu-Ohwayo, R. ; Ojwang, W.O. ; Peter, H.K. ; Schindler, D.E. ; Seehausen, O. ; Sharpe, D. ; Silsbe, G.M. ; Sitoki, L. ; Tumwebaze, R. ; Tweddle, D. ; Wolfshaar, K.E. van de; Dijk, J.W.M. van; Donk, E. van; Rijssel, J.C. van; Zwieten, P.A.M. van; Wanink, J. ; Witte, F. ; Mooij, W.M. - \ 2014
    Ecology and Society 19 (2014)4. - ISSN 1708-3087
    cyprinid rastrineobola-argentea - perch lates-niloticus - nile perch - east-africa - water hyacinth - mwanza gulf - oreochromis-niloticus - morphological-changes - introduced predator - biological-control
    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.
    Effect of C/N ratio levels and stocking density of Labeo victorianus on pond environmental quality using maize flour as a carbon source
    Magondu, E.W. ; Charo-Karisa, H. ; Verdegem, M.C.J. - \ 2013
    Aquaculture 410-411 (2013). - ISSN 0044-8486 - p. 157 - 163.
    oreochromis-niloticus - culture-systems - water-quality - nile tilapia - nitrogen - catfish - perch - fish - performance - aquaculture
    The main obstacle in developing intensive fish culture is waste management which is detrimental to the environment. To mitigate environmental impacts associated with effluent discharge, measures should be put in place to avoid deterioration of the pond environment. The addition of carbon materials into culture facilities while manipulating the levels of carbon and nitrogen ratios is one of the best strategies of controlling ammonia and nitrite in ponds. This study was carried out in 18 hapas suspended in six, 150 m2 earthen ponds to investigate the effects of C/N ratios (10 and 20) and stocking density (10, 15 and 25 fish m- 2) on water quality, sediment quality and growth of Labeo victorianus. All treatments were carried out in triplicate during a time period of 72 days. A locally formulated and prepared feed containing 30% crude protein with a C/N ratio of 10 was applied. Maize flour was used as the carbohydrate source for manipulating C/N ratio and applied to the water column separately from the feed. Increasing C/N ratio from 10 to 20 reduced (P <0.001) the total ammonia nitrogen (TAN), nitrite–nitrogen (NO2–N) and nitrate–nitrogen (NO3–N) in the water column and total nitrogen in the sediment (P <0.001). It also raised sediment pH, organic matter and total phosphorus (P <0.001). The lowest protein efficiency ratio (PER), specific growth rate (SGR) and the highest food conversion ratio for the feed were recorded with a C/N ratio of 10 (P <0.05). Based on highest growth, survival, production and net benefits, C/N ratio of 20 and a stocking density of 25 fish m- 2 are optimal. Therefore, carbohydrate addition in L. victorianus culture is a promising option for sustainable aquaculture.
    Dietary electrolyte balance affects the nutrient digestibility and maintenance energy expenditure of Nile tilapia
    Subramanian, S. ; Geurden, I. ; Orozco, Z.G.A. ; Kaushik, S.J. ; Verreth, J.A.J. ; Schrama, J.W. - \ 2013
    The British journal of nutrition 110 (2013). - ISSN 0007-1145 - p. 1948 - 1957.
    acid-base-balance - cation-anion difference - water rainbow-trout - juvenile african catfish - clarias-gariepinus burchell - fresh-water - alkaline tide - oreochromis-niloticus - gastrointestinal-tract - oncorhynchus-mykiss
    Acid–base disturbances caused by environmental factors and physiological events including feeding have been well documented in several fish species, but little is known about the impact of dietary electrolyte balance (dEB). In the present study, we investigated the effect of feeding diets differing in dEB ( - 100, 200, 500 or 800 mEq/kg diet) on the growth, nutrient digestibility and energy balance of Nile tilapia. After 5 weeks on the test diet, the growth of the fish was linearly affected by the dEB levels (P<0·001), with the lowest growth being observed in the fish fed the 800 dEB diet. The apparent digestibility coefficient (ADC) of fat was unaffected by dEB, whereas the ADC of DM and protein were curvilinearly related to the dEB levels, being lowest and highest in the 200 and 800 dEB diets, respectively. Stomach chyme pH at 3 h after feeding was linearly related to the dEB levels (P<0·05). At the same time, blood pH of the heart (P<0·05) and caudal vein (P<0·01) was curvilinearly related to the dEB levels, suggesting the influence of dEB on postprandial metabolic alkalosis. Consequently, maintenance energy expenditure (MEm) was curvilinearly related to the dEB levels (P<0·001), being 54 % higher in the 800 dEB group (88 kJ/kg0·8 per d) than in the 200 dEB group (57 kJ/kg0·8 per d). These results suggest that varying dEB levels in a diet have both positive and negative effects on fish. On the one hand, they improve nutrient digestibility; on the other hand, they challenge the acid–base homeostasis (pH) of fish, causing an increase in MEm, and thereby reduce the energy required for growth.
    Oxygen Consumption Constrains Food Intake in Fish Fed Diets Varying in Essential Amino Acid Composition
    Subramanian, S. ; Geurden, I. ; Figueiredo-Silva, A.C. ; Nusantoro, S. ; Kaushik, S.J. ; Verreth, J.A.J. ; Schrama, J.W. - \ 2013
    PLoS ONE 8 (2013)8. - ISSN 1932-6203 - 7 p.
    trout oncorhynchus-mykiss - european sea bass - rainbow-trout - dissolved-oxygen - nile tilapia - feed-intake - oreochromis-niloticus - salmo-gairdneri - self-selection - protein
    Compromisation of food intake when confronted with diets deficient in essential amino acids is a common response of fish and other animals, but the underlying physiological factors are poorly understood. We hypothesize that oxygen consumption of fish is a possible physiological factor constraining food intake. To verify, we assessed the food intake and oxygen consumption of rainbow trout fed to satiation with diets which differed in essential amino acid (methionine and lysine) compositions: a balanced vs. an imbalanced amino acid diet. Both diets were tested at two water oxygen levels: hypoxia vs. normoxia. Trout consumed 29% less food under hypoxia compared to normoxia (p0.05). This difference in food intake between diets under normoxia together with the identical oxygen consumption supports the hypothesis that food intake in fish can be constrained by a set-point value of oxygen consumption, as seen here on a six-week time scale.
    Responsible aquaculture in 2050: Valuing local conditions and human innovations will be key to success
    Diana, J.S. ; Egna, H.S. ; Chopin, T. ; Peterson, M.S. ; Cao, L. ; Pomeroy, R. ; Verdegem, M.C.J. ; Slack, W.T. ; Bondad-Reantaso, M.G. ; Cabello, F. - \ 2013
    Bioscience 63 (2013)4. - ISSN 0006-3568 - p. 255 - 262.
    life-cycle assessment - oreochromis-niloticus - marine aquaculture - animal health - systems - improvement - population - fish - technologies - biodiversity
    As aquaculture production expands, we must avoid mistakes made during increasing intensification of agriculture. Understanding environmental impacts and measures to mitigate them is important for designing responsible aquaculture production systems. There are four realistic goals that can make future aquaculture operations more sustainable and productive: (1) improvement of management practices to create more efficient and diverse systems at every production level; (2) emphasis on local decisionmaking, human capacity development, and collective action to generate productive aquaculture systems that fit into societal constraints and demands; (3) development of risk management efforts for all systems that reduce disease problems, eliminate antibiotic and drug abuse, and prevent exotic organism introduction into local waters; and (4) creation of systems to better identify more sustainably grown aquaculture products in the market and promote them to individual consumers. By 2050, seafood will be predominantly sourced through aquaculture, including not only finfish and invertebrates but also seaweeds.
    A high-resolution map of the Nile tilapia genome: a resource for studying cichlids and other percomorphs
    Guyon, R. ; Rakotomanga, M. ; Azzouzi, N. ; Coutanceau, J.P. ; Bonillo, C. ; Cotta, H. D'; Pepey, E. ; Soler, L. ; Rodier-Goud, M. ; Hont, A. D'; Conte, M.A. ; Bers, N.E.M. van; Penman, D.J. ; Hitte, C. ; Crooijmans, R.P.M.A. ; Kocher, T.D. ; Ozouf-Costaz, C. ; Baroiller, J.F. ; Galibert, F. - \ 2012
    BMC Genomics 13 (2012). - ISSN 1471-2164
    radiation hybrid map - genetic-linkage map - oreochromis-niloticus - sex determination - tetraodon-nigroviridis - zebrafish genome - fish - sequence - construction - evolution
    Background: The Nile tilapia (Oreochromis niloticus) is the second most farmed fish species worldwide. It is also an important model for studies of fish physiology, particularly because of its broad tolerance to an array of environments. It is a good model to study evolutionary mechanisms in vertebrates, because of its close relationship to haplochromine cichlids, which have undergone rapid speciation in East Africa. The existing genomic resources for Nile tilapia include a genetic map, BAC end sequences and ESTs, but comparative genome analysis and maps of quantitative trait loci (QTL) are still limited. Results: We have constructed a high-resolution radiation hybrid (RH) panel for the Nile tilapia and genotyped 1358 markers consisting of 850 genes, 82 markers corresponding to BAC end sequences, 154 microsatellites and 272 single nucleotide polymorphisms (SNPs). From these, 1296 markers could be associated in 81 RH groups, while 62 were not linked. The total size of the RH map is 34,084 cR(3500) and 937,310 kb. It covers 88% of the entire genome with an estimated inter-marker distance of 742 Kb. Mapping of microsatellites enabled integration to the genetic map. We have merged LG8 and LG24 into a single linkage group, and confirmed that LG16-LG21 are also merged. The orientation and association of RH groups to each chromosome and LG was confirmed by chromosomal in situ hybridizations (FISH) of 55 BACs. Fifty RH groups were localized on the 22 chromosomes while 31 remained small orphan groups. Synteny relationships were determined between Nile tilapia, stickleback, medaka and pufferfish. Conclusion: The RH map and associated FISH map provide a valuable gene-ordered resource for gene mapping and QTL studies. All genetic linkage groups with their corresponding RH groups now have a corresponding chromosome which can be identified in the karyotype. Placement of conserved segments indicated that multiple inter-chromosomal rearrangements have occurred between Nile tilapia and the other model fishes. These maps represent a valuable resource for organizing the forthcoming genome sequence of Nile tilapia, and provide a foundation for evolutionary studies of East African cichlid fishes.
    Constraints on Energy Intake in Fish: The Link between Diet Composition, Energy Metabolism, and Energy Intake in Rainbow Trout
    Subramanian, S. ; Schrama, J.W. ; Figueiredo-Silva, A.C. ; Kaushik, S.J. ; Verreth, J.A.J. ; Geurden, I. - \ 2012
    PLoS ONE 7 (2012)4. - ISSN 1932-6203
    european sea-bass - voluntary feed-intake - salmon salmo-salar - oncorhynchus-mykiss - food-intake - digestible energy - growing-pigs - utilization efficiency - oreochromis-niloticus - dicentrarchus-labrax
    The hypothesis was tested that fish fed to satiation with iso-energetic diets differing in macronutrient composition will have different digestible energy intakes (DEI) but similar total heat production. Four iso-energetic diets (2×2 factorial design) were formulated having a contrast in i) the ratio of protein to energy (P/E): high (HP/E) vs. low (LP/E) and ii) the type of non-protein energy (NPE) source: fat vs. carbohydrate which were iso-energetically exchanged. Triplicate groups (35 fish/tank) of rainbow trout were hand-fed each diet twice daily to satiation for 6 weeks under non-limiting water oxygen conditions. Feed intake (FI), DEI (kJ kg-0.8 d-1) and growth (g kg-0.8 d-1) of trout were affected by the interaction between P/E ratio and NPE source of the diet (P0.05). Our data suggest that the control of DEI in trout might be a function of heat production, which in turn might reflect a physiological limit related with oxidative metabolism.
    Did the loss of phytoplanktivorous fish contribute to algal blooms in the Mwanza Gulf of Lake Victoria?
    Witte, F. ; Silsbe, G.M. ; Hecky, R.E. ; Goudswaard, P.C. ; Guildford, S.J. ; Kishe-Machumu, M.A. ; Wanink, J.H. - \ 2012
    Hydrobiologia 679 (2012). - ISSN 0018-8158 - p. 283 - 296.
    sardine rastrineobola-argentea - perch lates-niloticus - introduced nile perch - east-africa - haplochromine cichlids - oreochromis-niloticus - nutrient enrichment - chlorophyll-a - waters - kenya
    Possible causes of the increased algal blooms in Lake Victoria in the 1980s have been disputed by several authors; some suggested a top-down effect by the introduced Nile perch, whereas others suggested a bottom-up effect due to eutrophication. In this article the potential impact is established of grazing by fish on phytoplankton densities, before the Nile perch upsurge and the concomitant algal blooms in the Mwanza Gulf. The biomass and trophic composition of fish in the sublittoral area of the Mwanza Gulf were calculated based on catch data from bottom trawls, and from gill nets covering the whole water column. Estimates of phytoplankton production in the same area were made from Secchi values and chlorophyll concentrations. The total phytoplankton intake by fish was estimated at 230 mg DW m 2 day 1 . The daily gross production ranged from 6,200 to 7,100 mg DW m 2 day 1 and the net production from 1,900 to 2,200 mg DW m 2 day 1 . Thus, losses of phytoplankton through grazing by fish were about 34% of daily gross and 1012% of daily net phytoplankton production. As a consequence it is unlikely that the phytoplankton blooms in the second half of the 1980s were due to a top-down effect caused by a strong decline in phytoplankton grazing by fish.
    Not All Bones are Created Equal - Using Zebrafish and Other Teleost Species in Osteogenesis Research
    Apschner, A. ; Schulte-Merker, S. ; Witten, P.E. - \ 2011
    Methods in Cell Biology 105 (2011). - ISSN 0091-679X - p. 239 - 255.
    salmon salmo-salar - upper pharyngeal jaws - astatotilapia-elegans teleostei - oryzias-latipes teleostei - atlantic salmon - acellular bone - chondroid bone - danio-rerio - oreochromis-niloticus - skeletal development
    Developmental osteogenesis and pathologies of mineralized tissues are areas of intense investigations in the mammalian field, but different from other areas of organ formation and developmental biology, zebrafish have been somewhat slow in joining the area of bone research. In recent years, however, genetic screens have provided a number of exciting mutants, and transgenic lines have been developed that permit visualization of osteoblasts and osteoclasts in vivo. We here review some of the recent literature and provide examples where insights from studies in zebrafish have complemented the information available from mammalian models or clinical studies. Furthermore, we provide a comparative overview about different forms of bone within the teleost lineage, and between teleosts and mammals
    Tailfin clipping, a painful procedure: Studies on Nile tilapia and common carp
    Roques, J.A.C. ; Abbink, W. ; Geurds, F. ; Vis, J.W. van de; Flik, G. - \ 2010
    Physiology and Behavior 101 (2010)4. - ISSN 0031-9384 - p. 533 - 540.
    branchial chloride cells - fresh-water - oreochromis-niloticus - oncorhynchus-mykiss - cyprinus-carpio - rainbow-trout - na+/k+-atpase - fish - nociception - perception
    The fish welfare debate is intensifying. Consequently, more research is carried out to further our knowledge on fish welfare in aquaculture. We define here a series of key parameters to substantiate an acute response to a supposedly painful stimulus: a standardized tailfin clip. Ultrastructural analysis of common carp (Cyprinus carpio) tailfin indicates the presence of A-d and C-type axons, which are typical for transmitting nociceptive signals in (higher) vertebrates. In Nile tilapia (Oreochromis niloticus), responses to a tailfin clip were studied and the unavoidable acute stress associated with the handling required for this procedure. A series of key parameters for further studies was defined. The responses seen in ‘classical’ stress parameters (e.g., changes in plasma cortisol, glucose and lactate levels) did not allow discrimination between the clipping procedure and the handling stress. However, three parameters indicated a differential, stronger response to the clip stimulus itself: first, swimming activity increased more and clipped fish spent more time in the light (in a tank where half the volume is covered by dark material); second, the gill's mucus cells released their content as observed 1 h after the clip, and this response is transient (no longer observed at 6 h post clipping). Third, branchial Na+/K+-ATPase activity assayed in vitro was not affected by the procedures, but a remarkable migration of Na+/K+-ATPase immunoreactive (chloride) cells into the lamellar epithelium was observed as of 6 h post clipping. We conclude that the differential response to clipping supports that this is a painful procedure that evokes a transient specific physiological status.
    Chronic stress impairs performance, energy metabolism and welfare indicators in European seabass (Dicentrarchus labrax): The combined effects of fish crowding and water quality deterioration
    Santos, G.A. ; Schrama, J.W. ; Mamauag, R.E.P. ; Rombout, J.H.W.M. ; Verreth, J.A.J. - \ 2010
    Aquaculture 299 (2010)1-4. - ISSN 0044-8486 - p. 73 - 80.
    high stocking density - sparus-aurata l. - rainbow-trout - physiological-responses - oreochromis-niloticus - clarias-gariepinus - plasma-cortisol - juvenile turbot - african catfish - common carp
    Rearing fish at high densities may act as a chronic stressor leading to a reduced performance. We hypothesise that this is due to an increase in energy requirements for maintenance and altered energy partitioning. To test this hypothesis, European seabass (mean initial weight 82.1 g) were stocked at four densities: 8.1 kg/m3 (L); 25.2 kg/m3 (ML); 50.5 kg/m3 (MH) and 75.4 kg/m3 (H) in 200 L tanks. To mimic practical conditions, water flow rate was not adjusted for density, leading to changes in water quality as a consequence of the increase in fish densities. Therefore, the chronic stress applied is a combination of crowding and water quality deterioration. The experiment (57 days) was carried out in metabolic chambers connected to a recirculating aquaculture system and fish were fed until apparent satiation. Swimming speed was recorded during the experiment. At the end of the experiment, blood was sampled to measure control values of plasma cortisol, glucose and lactate. Additionally, fish were subjected to an acute stress test (kept in a submerged net for 1 h) and their blood sampled to measure stress values for cortisol, glucose and lactate. Energy and nitrogen balances were quantified based on measurements of body composition and digestibility coefficients. Results showed that increased density levels reduced feed intake and growth. This feed intake reduction was partially compensated a decrease in maintenance requirements for energy at the highest density. Differences in the nitrogen and energy partitioning arose mainly from differences in feed intake. Feed conversion ratio increased in the treatments with the low (L) and the high (H) densities. Swimming speed was reduced with increasing densities. Regarding blood stress variables, control values of cortisol, glucose and lactate were not different among the treatments. However, after the acute stress test, fish at the highest density had significantly higher levels of plasma cortisol. The integration of performance, metabolic and stress parameters suggests that the highest density impairs feed intake and growth in seabass. Part of the reduction of feed intake was compensated by a decrease of maintenance requirements which were correlated with the behavioural changes.
    Submersion time, depth, substrate type and sampling method as variation sources of marine periphyton
    Richard, M. ; Trottier, C. ; Verdegem, M.C.J. ; Hussenot, J.M.E. - \ 2009
    Aquaculture 295 (2009)3-4. - ISSN 0044-8486 - p. 209 - 217.
    indian major carps - oreochromis-niloticus - artificial substrate - litopenaeus-vannamei - bacterial biofilm - stocking density - water-quality - polyculture - aquaculture - growth
    Periphyton is an additional food source in African and Asian brackish and freshwater fish ponds. The present study was a preliminary assessment of periphyton development on artificial substrates in temperate marine ponds. The effects of submersion time, substrate type, water depth, and total or partial sampling methods on the quantity and quality of periphyton collected, were evaluated. Four types of substrate (W: wooden poles, S: smooth fiber-glass strips, m: mosquito screen (1 mm-mesh) and M: garden netting (5 mm-mesh)) were deployed in a marine pond, and periphyton was collected 15 and 30 days later. The total amount of periphyton per substrate unit was collected as one sample or as 5 sub-samples. Results showed that (i) periphyton biomass in a marine pond increased between day 15 and day 30, (ii) more periphyton was collected on mosquito screen than on wooden poles, fiberglass strips and garden netting, (iii) periphyton biomass increased with submersion depth, (iv) sub-sampling leads to an underestimate compared to whole unit sampling, and (v) a correction of periphyton weight must be carried out considering the dissolved inorganic salts present in periphyton samples from marine and brackish ponds. Whole substrate unit sampling using a tube and stopper is recommended to avoid underestimation of periphyton development. Finally, the autotrophic fraction in the periphyton communities was very low compared to periphyton developed on biodegradable substrates in fertilized tropical ponds. Studies on fertilization and use of biodegraded substrates (i.e. long-time submerged wood) are recommended to further optimize periphyton development in temperate marine ponds.
    Effects of stocking density, periphyton substrate and supplemental feed on biological processes affecting water quality in earthen tilapia-prawn polyculture ponds
    Sharif Uddin, M. ; Milstein, A. ; Ekram Azim, M. ; Abdul Wahab, M. ; Verdegem, M.C.J. ; Verreth, J.A.J. - \ 2008
    Aquaculture Research 39 (2008)12. - ISSN 1355-557X - p. 1243 - 1257.
    macrobrachium-rosenbergii polyculture - oreochromis-niloticus - extensive aquaculture - fish production - nile tilapia - phytoplankton - fertilization - culture - system - carps
    The technical and economic potentials of tilapia, Oreochromis niloticus (L.), and giant river prawn, Macrobrachium rosenbergii (de Man), polyculture in periphyton-based systems are under investigation in an extensive research programme. This article is a combined analysis of data from four experiments exploring the effects of periphyton, fish, prawn and feed on water quality. Factor analysis and ancova models applied to the combined dataset allowed to identify the underlying ecological processes governing the system, and construct conceptual graphic models of the periphyton¿environment relationships observed. With the first factor, variability in water quality was due to autotrophic (photosynthesis and nutrient uptake) and heterotrophic (respiration and decomposition) processes affecting water quality in opposite directions. With the second factor, variability was related to decomposition on the bottom and nutrient release into the water column, algae production and sedimentation. The analysis of the relationships between both factors and the growth rates of tilapia and prawn in the different systems allowed a better understanding of the functioning of tilapia¿prawn ponds, and indicates that their joint culture is technically feasible and economically viable. Therefore, this technology is recommended for poverty alleviation and nutritional security in rural Bangladesh as well as in other countries of the region.
    Dietary shifts in Brycinus sadleri (Pisces : Characidae) from southern Lake Victoria
    Wanink, J.H. ; Joordens, J.C.A. - \ 2007
    Aquatic Ecosystem Health and Management 10 (2007)4. - ISSN 1463-4988 - p. 392 - 397.
    nile perch upsurge - rastrineobola-argentea - oreochromis-niloticus - east-africa - extinction - fishery - decline
    We studied the diet of Brycinus sadleri in Lake Victoria after extensive environmental changes during the 1980s. To check for diet expansion following these changes, as observed in some other fish species, we compared our results with data from the 1950s. Stomach contents were analysed in relation to fish size and time of the day to investigate ontogenetic and diel shifts in the diet. Comparison of diets before and after the environmental changes showed a shift to more generalist feeding. The main food sources in the 1950s were plants during daytime and surface insects at night. Hardly any plant material was eaten during the late 1980s, but chironomid larvae, chaoborid larvae and pupae, shrimps, Odonata nymphs and fish contributed to a broader daytime diet. These prey species were also commonly taken at night, though, like in the 1950s, surface insects accounted for the major part of the diet. We found dietary shifts during ontogeny. Juveniles fed exclusively on zooplankton. Chironomid larvae, relatively large food items in comparison with zooplankton, were the key prey of sub-adult fish. Their importance decreased in the diet of adults, which included even larger prey like shrimps, Odonata nymphs and fish. At night, surface insects replaced chironomid larvae in the diet of adult fish. Diet expansion may result from reduced competition after the decline in fish mass following the Nile perch boom. An alternative explanation, increasing the prey spectrum in response to deteriorated light conditions, is not likely to hold for B. sadleri. The ontogenetic shift to increasingly larger prey and the nocturnal shift to surface insects probably reflect optimal foraging behaviour and diel vertical migration patterns of the fish respectively. These dietary shifts emphasize the feeding flexibility of B. sadleri, which may have been essential in surviving the environmental changes in Lake Victoria.
    Digesta viscosity, nutrient digestibility and organ weights in African catfish (Clarias gariepinus) fed diets supplemented with different levels of a soluble non-starch polysaccharide
    Leenhouwers, J.I. ; Adjei-Boateng, D. ; Verreth, J.A.J. ; Schrama, J.W. - \ 2006
    Aquaculture Nutrition 12 (2006)2. - ISSN 1353-5773 - p. 111 - 116.
    sesbania-aculeata seeds - broiler-chickens - intestinal development - oreochromis-niloticus - growth - tilapia - fish - carboxymethylcellulose
    The objective of the present study was to investigate if dietary soluble non-starch polysaccharides (NSP) increase digesta viscosity, reduce nutrient digestibility and increase organ weights in African catfish. The fish (mean weight 80 g) were fed diets supplemented with the soluble NSP guar gum at three levels, 0 g kg1 (GG0), 40 g kg1 (GG4) and 80 g kg1 (GG8). Guar gum inclusion significantly increased digesta viscosity in the proximal (GG0: 1.7 centipoise or cP; GG4: 84.9 cP; GG8: 98.3 cP) and distal (GG0: 1.9 cP; GG4: 109.8 cP; GG8: 66.4 cP) intestine. Apparent digestibility coefficients (ADC) were significantly lower for the GG8 diet than the GG0 diet (dry matter: 52% versus 69%; protein: 77% versus 90%; ash: 41% versus 54%; energy: 60% versus 77%). The ADC of dry matter and energy were significantly lower for the GG4 diet than the GG0 diet. The relative growth rate of metabolic weight (14.5¿15.4 g kg0.8 day1) and feed conversion ratio (0.8) did not differ between diets. Fish fed the GG8 diet had a significantly higher somatic stomach index than GG0 fish (0.71% versus 0.65% body weight). The intestinal somatic index tended to increase with increasing guar gum supplementation (GG0: 1.08%; GG4: 1.23%; GG8: 1.59%). In conclusion, high digesta viscosities in the guar gum fed fish may explain the observed reduced nutrient digestibilities and increases in digestive organ weights
    Vulnerability to a small-scale commercial fishery of Lake Tana's (Ethiopia) endemic Labeobarbus compared with African catfish and Nile tilapia: An example of recruitment-overfishing?
    Graaf, M. de; Zwieten, P.A.M. van; Machiels, M.A.M. ; Lemma, E. ; Wudneh, T. ; Dejen, E. ; Sibbing, F.A. - \ 2006
    Fisheries Research 82 (2006)1-3. - ISSN 0165-7836 - p. 304 - 318.
    oreochromis-niloticus - species flock - reproductive segregation - perch upsurge - barbs barbus - cyprinidae - victoria - impact
    In 1986 a motorised, commercial gillnet fishery was introduced in Lake Tana, Ethiopia's largest lake (3050 km2) in addition to the artisanal, predominantly subsistence fishery conducted from reedboats. The three main species groups targeted by this fishery are a species flock of endemic, large Labeobarbus spp., African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus). The commercial gillnet fisheries was monitored during 1991-1993 and in 2001 (CPUE, effort allocation, catch composition, yield) and the development of the stocks of the three species groups was determined using an experimental trawl program during 1991-1993 and 1999-2001. In 1991-1993 the vast majority of fishing activities took place in the southern Bahar Dar Gulf (71%). Each species group contributed roughly one third to the total catch. In 2001, 41% of the effort was allocated to the north-eastern shores of Lake Tana. Both the CPUE and the contribution of O. niloticus to the total catch had doubled. However, the proportion of large specimen (>50 cm TL C. gariepinus; >20 cm FL O. niloticus) in the trawl surveys decreased significantly. A three-fold decline in abundance of the anadromous Labeobarbus species occurred. The 15 large labeobarb species are long-lived, ecologically specialised endemics. The seven riverine spawning Labeobarbus species form aggregations in the river mouths in August-September, during which period they are targeted by the commercial gillnet fishery. A sharp decrease in abundance by ca. 75% of the migratory riverine spawning Labeobarbus species in the sublittoral and pelagic zones of the lake, areas where no fishing takes place and the collapse of juvenile Labeobarbus (between 5 and 18 cm FL: by 90%) during the 1990s suggest recruitment-overfishing. To prevent the possible extinction of the unique Labeobarbus species flock all fishing effort should be severely restricted near the river mouths and on the upstream spawning areas during August-September (peak breeding period) to protect the vulnerable spawning aggregations
    Exploring the trophic structure in organically fertilized and feed-driven tilapia culture environments using multivariate analyses
    Muendo, P.N. ; Milstein, A. ; Dam, A. ; Gamal, N.E. ; Stoorvogel, J.J. ; Verdegem, M.C.J. - \ 2006
    Aquaculture Research 37 (2006)2. - ISSN 1355-557X - p. 151 - 163.
    oreochromis-niloticus - chemical fertilizers - aquaculture ponds - fish ponds - water - nitrogen - polyculture - phosphorus - manures - budgets
    Reports of similar yields in manure and feed-driven tilapia culture environments raise questions on food utilization in these environments. The possibility that similar production rates are because of utilization of different foods was investigated using exploratory techniques of multivariate analyses. Using factor analysis, trophic pathways through which food becomes available to fish were explored, and using anova models, water quality, sediment quality and tilapia growth and yields were compared. Conceptual graphic models of the main ecological processes occurring in feed-driven and organically fertilized environments are presented and discussed. In both environments, autotrophic and heterotrophic pathways are important processes that result in the availability of natural foods that are utilized by the fish. Extrapolated fish yield data indicate that with equal nutrient input and stocking density, organically fertilized environments could achieve production rates similar to those in feed-driven environments. The general assumption that supplemental or complete foods are well utilized by tilapia in outdoor stagnant ponds remains challenged, and further research on tilapia feeding behaviour and food selection in feed-and organic fertilizer-driven environments is needed.
    Adaptive bone formation in acellular vertebrae of sea bass (Dicentrarchus labrax L.)
    Kranenbarg, S. ; Cleynenbreugel, T. van; Schipper, H. ; Leeuwen, J.L. van - \ 2005
    Journal of Experimental Biology 208 (2005)9. - ISSN 0022-0949 - p. 3493 - 3502.
    teleost fish - oreochromis-niloticus - tissue - water - swimbladder - osteoblasts - osteoclasts - notochord - cichlidae - lordosis
    Mammalian bone is an active tissue in which osteoblasts and osteoclasts balance bone mass. This process of adaptive modelling and remodelling is probably regulated by strain-sensing osteocytes. Bone of advanced teleosts is acellular yet, despite the lack of osteocytes, it is capable of an adaptive response to physical stimuli. Strenuous exercise is known to induce lordosis. Lordosis is a ventrad curvature of the vertebral column, and the affected vertebrae show an increase in bone formation. The effects of lordosis on the strain distribution in sea bass (Dicentrarchus labrax L.) vertebrae are assessed using finite element modelling. The response of the local tissue is analyzed spatially and ontogenetically in terms of bone volume. Lordotic vertebrae show a significantly increased strain energy due to the increased load compared with normal vertebrae when loaded in compression. High strain regions are found in the vertebral centrum and parasagittal ridges. The increase in strain energy is attenuated by a change in architecture due to the increased bone formation. The increased bone formation is seen mainly at the articular surfaces of the vertebrae, although some extra bone is formed in the vertebral centrum. Regions in which the highest strains are found do not spatially correlate with regions in which the most extensive bone apposition occurs in lordotic vertebrae of sea bass. Mammalian-like strain-regulated bone modelling is probably not the guiding mechanism in adaptive bone modelling of acellular sea bass vertebrae. Chondroidal ossification is found at the articular surfaces where it mediates a rapid adaptive response, potentially attenuating high stresses on the dorsal zygapophyses.
    Periphyton-water quality relationships in fertilized fishponds with artificial substrates
    Azim, M.E. ; Milstein, A. ; Wahab, M.A. ; Verdegem, M.C.J. - \ 2003
    Aquaculture 228 (2003). - ISSN 0044-8486 - p. 169 - 187.
    oreochromis-niloticus - aquaculture - balance
    The potential of periphyton-based aquaculture in southern Asia is under investigation in an extensive research program. This paper is a further analysis of data from two experiments carried out in that framework, to explore periphyton-environment relationships in the absence of fish grazing pressure. Factor analysis and ANOVA models applied to data matrices of water quality, plankton and periphyton parameters allowed identifying the underlying ecological processes governing this novel periphyton-based pond system. The system was dominated by changes in time, and the main sources of variability were periphyton biomass increase through the culture period and its effect on water turbidity, processes affecting pond oxygenation and processes leading to surface-depth differences in water quality. All those processes act at the same time but with different intensities and time patterns, and affect the measured variables in different ways. Conceptual models of the periphyton-environment relationships observed are presented in graphical form. The knowledge generated provides background information required for fish culture management in periphyton-based ponds. (C) 2003 Elsevier Science B.V. All rights reserved.
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