229 research outputs found
Afscheid Professor Johan Verreth van Wageningen Universiteit
No full textOp 22 maart 2018 nam Professor Johan Verreth afscheid van de leerstoelgroep Aquacultuur en Visserij van Wageningen Universiteit. Dr Verreth heeft daar 17 jaar geleden de positie van hoogleraar ingenomen en bereikte eerder dit jaar de pensioengerechtigde leeftijd. Voorafgaand aan Johans afscheidsrede in de aula van Wageningen Universiteit (titel: ‘Fish on the menu’) was een mini-symposium waarin visteelt en visserij centraal stonden
Kweekvis steeds vegetarischer. Zoektocht naar vismeelarm of zelfs visvrij voer. Interview met Johan Verreth
No full textKweekvis steeds vegetarischer. Zoektocht naar vismeelarm of zelfs visvrij voer. Interview met Johan Verreth
No full textProtein and energy nutrition of marine gadoids, Atlantic cod (Gadus morhua L.) and haddock (Melanogrammus aeglefinus L.)
No full text Primary goals of this thesis were to: 1) examine the in vivo digestion of macronutrients from conventional or alternative feed ingredients used in practical diets of juvenile gadoids (Atlantic cod and haddock), 2) document growth potential of fish at the juvenile grower phase given varying levels of dietary protein and energy and 3) assess the potential of in vitro pH-Stat methods for rapid screening protein quality of feed ingredients, specifically for gadoids. All primary research questions were linked to and built upon one another with the goal of gaining a better understanding of protein and energy utilization of juvenile grower phase gadoids. Studies showed that cod and haddock have a high capacity to utilize a wide range of dietary feed ingredients, such as fish meals, zooplankton meal, soybean products (meal, concentrate and isolate) and wheat gluten meal. New dietary formulations for gadoids may also utilize pulse meals, corn gluten meal, canola protein concentrate and crab meal. Digestibility data in this thesis is currently the only research that examined both in vivo and in vitro macronutrient digestibility of a large number and wide range of individual ingredients, specifically for gadoids. This is essential to gain new knowledge on protein and energy utilization as well as for least-cost ration formulations and effective substitution of ingredients into new formulations. Data has demonstrated a dietary digestible protein/digestible energy (DP/DE)ratio of 30 g DP/MJ DE is required for gadoids during the juvenile phase (in vitro closed-system pH-Stat assay for rapid screening protein quality of test ingredients that is ‘species-specific’ to gadoids. It is demonstrated that in vitro results generally reflected results obtained through conventional in vivo protein digestibility methods. Studies resulted in the first generation of a ‘gadoid-specific’ proteolytic enzyme extraction method and in vitro closed-system pH-Stat assay which may be useful to investigate protein digestion, absorption and metabolism of gadoids and further development of their feeds. </p
Individual variation in growth of African catfish Clarias gariepinus: a search for explanatory factors
No full textAmong farmed animals, fish exhibit the largest individual variation in growth, yet most of the studies reporting data on growth do not take individual variation into account. Usually a mean value is considered and although the variation around the mean is also mentioned, it is generally viewed as a sort of "statistical noise". The importance of individual variation in growth should not be underestimated since it has important consequences for water quality, aggression, stress levels, farm management, selection programmes, etc. Among the factors responsible for growth variation, social hierarchy is often considered as the most important. Social hierarchies may induce behavioural inhibition and stress on subordinate fish, affecting their feed intake, feed efficiency and as a consequence reducing their growth. However, for most fish species there is no unambiguous proof that individual differences in feed intake, feed efficiency and growth result from social hierarchies. Some studies suggest that inherent (genetic) factors may also cause the variation in growth.The general aim of this study was to understand the underlying factors responsible for the individual variation in growth of African catfish Clarias gariepinus . The following factors were investigated: 1) if individual variation in growth is mainly a consequence of social hierarchies, 2) the contribution of individual differences in feed intake and feed efficiency to the individual differences in growth and 3) the contribution of feeding behaviour and stress response in explaining individual differences in feed efficiency.Chapters 2 and 3 investigated the importance of social hierarchy as an explanatory factor for the individual variation in growth of African catfish. The growth performance, behaviour (feeding behaviour, aggression levels) and stress response between groups of low-, medium- and heavy- weight fish were compared. Chapter 2 showed that low-weight fish do not exhibit increased growth rates in the absence of heavier fish. Apparently, the growth differences were not induced by social hierarchies where the larger fish suppress the growth of smaller fish. Instead, this study suggests that feeding behaviour is a crucial factor. Heavier fish exhibit feeding behaviours that may give advantage when feed is limited, such as being more active swimmers, spending more time at the feeding areas and eating their meal faster than low-weight fish. These differences in behaviour may result in growth variation, as found in this study.Chapter 3 showed that the aggression and stress levels did not increase in heterogeneous (weight) groups as compared with homogeneous (weight) groups. Furthermore, low-weight fish did not exhibit a higher number of skin lesions and higher stress levels when reared in heterogeneous groups as compared with low-weight fish reared in homogeneous groups. These results suggested that low-weight fish were not behaving as subordinates and heavy fish as dominants.To further investigate the importance of inherent differences in growth variation, a set of experiments were designed using individually housed fish. Housing fish individually enabled the study of individual differences in the absence of social interactions and to measure individual feed intake accurately. This raised the question whether the results obtained from housing fish individually could be representative of a group housing situation. Chapter 4 compared the growth performance, feeding behaviour and stress response of isolated and non-isolated fish. This study suggested that in African catfish feed intake is stimulated by the presence of conspecifics resulting in higher feed intake and growth rates. However, isolation per se seems not to act as a stressor in the short term or to affect the stress response, probably because periods of isolation are part of the African catfish lifestyle. In addition, Chapter 5 compared the growth of fish housed individually and afterwards in a group. The average growth of individually housed fish was lower than fish in group housing. However, slow and fast growing fish under individual housing remained slow and fast growing fish, respectively, under group housing. This suggests that the different growth rates observed when fish are housed individually are a characteristic of the individual and not simply a consequence of isolation.Chapters 5 to 7 used individually housed fish to supply experimental data on inherent factors responsible for individual variation in growth. Chapter 5 quantified individual differences in performance traits and feeding behaviour and focused on the repeatability of such individual differences when fish were fed ad libitum. Fish exhibited pronounced individual variation in growth (CV = 52.8 %), in feed intake (34.3 %) and in total feeding time (>100 %). The repeatability estimates were 0.55 for growth, 0.70 for feed intake, 0.49 for feed efficiency and 0.81 for total feeding time. These high repeatability estimates suggested that individual differences in growth, feed intake/efficiency and feeding behaviour are consistent over time and therefore probably inherent. Individual differences in growth were explained mainly by individual differences in feed intake (~85 %). Individual differences in feeding behaviour were shown to be related to feed efficiency, measured as residual feed intake (i.e., the difference between actual feed intake and that predicted from mean observed requirements for growth and maintenance). With increasing total feeding time, the maintenance requirements also increased suggesting that slow eaters have higher maintenance costs. Chapter 6 tested whether individual differences in feeding behaviour explained the differences in growth rate by affecting feed efficiency, using restrictively fed fish. This study showed that despite the low variation in initial body weight (6.5 %) and in cumulative feed consumption (7.5 %) over the experimental period, catfish exhibited high variation in final body weight (18.1 %), specific growth rate (17.2 %) and feed conversion ratio (27.9 %), suggesting that individual variation in growth/feed efficiency is important in determining growth. This individual variation may be related to individual differences in protein/fat deposition since faster growing fish deposited more protein and less fat than slower growing fish. Pronounced individual differences in feeding behaviour (reaction towards feed and time spent eating) were also observed and correlated to individual differences in growth/feed efficiency. Fast eaters were the fast growers.Chapter 7 presented two experiments to investigate individual differences in basal and post-stress levels of glucose, lactate and cortisol and their relation to individual differences in feed efficiency. There was a pronounced individual variation in both basal and post-stress levels of plasma glucose, lactate and cortisol. Basal levels of glucose, lactate and cortisol did not contribute significantly to explain differences in feed efficiency. However, glucose levels obtained after a stress test (netting) could explain differences in feed efficiency by 1.3 % in experiment 1 and 5.9 % in experiment 2. In experiment 2, the cortisol levels obtained after the stress test also explained part of the differences in feed efficiency (8.7 %). Apparently, high stress responders are less efficient fish. The stress response probably adds to differences in maintenance costs, thereby affecting the feed efficiency.The findings of this thesis are discussed and the main conclusions are presented in Chapter 8 . The importance of social hierarchy in explaining individual growth variation should be considered species-dependent. In addition, social hierarchy should not be accepted a priori as the major cause of individual growth variation without previous investigation. The results of this thesis suggested that in African catfish the individual variation in growth is not the result of marked dominance-subordinance relationships. Instead, genetic-based differences in feed intake, feed efficiency, feeding behaviour and stress response seem to play a role in explaining growth variation in African catfish. In practical terms, one may question the use of grading in this species as grading is done under the assumption that it disrupts an existing social hierarchy. Furthermore, the results of this thesis called for the development of selection programmes in African catfish. Selecting for feed efficiency (residual feed intake) is a promising direction to pursue. The most efficient fish (low residual feed intake) were shown to be fast eaters and low stress responders which may be advantageous under aquaculture conditions.It was also concluded that individual differences in feed intake and feed efficiency (residual feed intake) contributed ~85 and ~15 %, respectively, to the individual differences in the growth of African catfish. Individual differences in feeding behaviour (total feeding time) and stress response (plasma glucose and cortisol after an acute stress) contributed to explain variation in residual feed intake (maintenance requirements) up to 8.7 %.Despite the results obtained in this thesis, our understanding of the causes of growth variation in African catfish is far from being complete. The challenge is to find the mechanisms responsible for this variation and how they are related to the individual differences in behaviour and stress response found in this thesi
Optimisation of the pond rearing of Nile tilapia (Oreochromis niloticus L.). The impact of stunting processes and recruitment control
No full textStunting is often considered as a major bottleneck for the pond rearing ofNiletilapia ( Oreochromis niloticus niloticus L.) and was a major topic in this study. Neonteny: the earlier breeding of tilapia in ponds takes place, but is not a bottleneck as with appropriate feeding levels the somatic growth is maintained. The study indicated that aggravation of the living conditions is not a major cause of the observed earlier breeding. Real stunting takes place at low feeding levels but can be avoided by higher feeding levels or by elimination of recruitment through polyculture with either the African catfish ( Clarias gariepinus Burchell) and the African snakehead murrel ( Parachanna obscura Günther). Recruitment ofNiletilapia is completely controlled at stocking densities of 8300 large catfish ha -1 or725 large snakehead ha -1. The difference in predation efficiency between the two species is related to their feeding strategies: omnivorous vs piscivorous. Mass production of fingerlings of the African catfish was carried out in ponds protected against frogs by aluminium roof plates. The results showed that the developed system is labour orientated, technical reliable and economic feasibly whenthefingerlings can be sold for US$ 0.07 a piece. An Individual Based Simulation model for the pond rearing of theNiletilapia for mixed or mono sex culture, and for poly culture with the African Catfish or African Snakehead is presented. The model visualized major underlying ecological processes in tilapia farming and indicated that growth and its relation to feed quality, recruitment and prey-predator relations are major topics in tilapia farming modelling. The model can serve as a predictive and decision-making support tool after some slight adaptations discussed
Dietary carbohydrate and faecal waste in the Nile Tilapia (Oreochromis niloticus L.)
No full textPublic concerns about environmental pollution are putting increasing pressure on fish farms to treat their wastewater before release. The wastewater produced by the fish can also deteriorate water quality within the system. Aquaculture waste can be divided into solid waste and dissolved waste.<span class=GramE>Faeces is</span>main sources of solid waste. An increased consistency of faeces may improve its removal efficiency, thereby improving water quality. The removal efficiency of faeces by a collector (settling tank and/or Choubert) was proposed as an indicator for faeces consistency. Manipulation of the diet composition changed faeces consistency in tilapia. Soluble non-starch polysaccharide increased digesta viscosity and reduced faeces removal efficiency. Insoluble non-starch polysaccharide did not change faeces removal. Replacement of native starch by gelatinized starch improved faeces removal efficiency. Fermentation and viscosity of digesta are the main parameters affecting faeces consistency. An increased faeces removal, induced by dietary composition, lowered the organic matter load into the system, thereby increasing nitrification and dissolved oxygen. A detailed investigation on fermentability and viscosity of different ingredients can provide insight in characterization of faeces, thereby giving suggestion for diet formulation geared to maximize solid removal
Dietary carbohydrates and denitrification in recirculating aquaculture systems
No full text Due to overfishing of global fish stocks and increasing fish meal prices, plant ingredients are being increasingly used as an alternative source of protein in fish feeds. However, the inclusion of unpurified plant ingredients will also increase the content of fibers in feeds. Fibers are nearly indigestible and will therefore increase solid waste production in aquaculture. This solid waste can be used to as a carbon source for denitrification to control nitrate levels in recirculating aquaculture systems (RAS), thereby reducing both solid and dissolved waste production. Additionally, fibers can change the recovery characteristics and lower the degradability of fecal waste. Therefore, this study investigates how changes in the dietary carbohydrate composition can affect waste production, system performance and denitrification in RAS. Furthermore, ultrasound treatment (to decrease particle size in fecal waste) and enzymatic conditioning (to increase fiber degradability) were tested as possible means to increase the bioavailability of carbon in fecal waste for denitrification. Comparing a high fiber (HNSP) and low fiber (LNSP) diet in RAS stocked with rainbow trout confirmed that the fibers in the HNSP diet increase fecal waste production. Although the HNSP diet produced more fecal waste than the LNSP diet, both diets produced the same amount of biodegradable fecal carbon. Since feces removal was higher in RAS using the HNSP diet, the load of degradable organic matter on the biofilters was lower with the HNSP diet than with the LNSP diet. Furthermore, fecal waste produced with the HNSP diet contained larger particles than feces of the LNSP diet, which could also improve the recovery of fecal waste with microscreens. Feces produced with the HNSP diet were also less degradable than feces produced with the LNSP diet. By using fecal waste as an internal carbon source for denitrification, solid and dissolved waste emissions from RAS could be reduced by ~50% for the HNSP diet. However, only approximately half of the supplied cellulose and hemicellulose were degraded in the denitrification reactors, whereas lignin was not degraded at all. Thus, the overall degradability of organic carbon in fecal waste was limited by fibers as hemicellulose, cellulose and lignin. Ultrasound and enzymatic conditioning did not sufficiently increase the degradability of fecal waste. Nonetheless, fibers originating from unpurified plant ingredients may also have beneficial effects on RAS performance by increasing fecal recovery. A more selective choice of feed ingredients could be used to increase the recovery and degradability of fecal waste in RAS.</p
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