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A more circular food system is increasingly proposed to address the challenge of feeding a growing world population while limiting environmental impacts and resource use. A circular food system prioritises resources for direct food supply to avoid feed-food competition. The role of animals is to upcycle resources unsuitable or undesired for human consumption, so called low-opportunity-cost feeds (LCF) into animal-source food. This thesis evaluates the potential of various animals in upcycling LCF in a circular food system by applying an optimisation model that allocates available LCF to that combination of animals that maximise the supply of human digestible protein (HDP) to a EU-28 case study. We first explored the potential of common livestock species in the EU (e.g. pigs, laying hens, broilers, dairy cattle and beef cattle) under various productivity levels. Optimal use of LCF required livestock systems that had a high conversion efficiency (laying hens, dairy cattle), were best able to valorise specific LCF (dairy cattle for grass; pigs for food waste) and could valorise low quality LCF due to their low productivity. When, in addition, considering fish – currently the only natural source of the essential eicosapentaenoic (EPA) and docosahexaenoic (DHA) ω-3 fatty acids – while demanding EPA/DHA requirements are met, fish provide nutritious food via both capture fisheries and fish farming. Even if capture fisheries rebuilds stocks and prioritises edible fish for human consumption, it can only fulfil 40% of EPA/DHA requirements. The farmed fatty fish needed to meet these requirements depend on fisheries by-products to meet their EPA/DHA requirements and livestock slaughter by-products to meet their high fat and protein requirements. A circular food system thus requires a combination of co-dependent animal production systems, tailored to the available LCF and the desired nutrient output. As the availability of food leftovers as LCF is currently restricted by legislation and other barriers, we explored the potential of food leftovers currently not used as LCF. Potential to increase animal protein intake was highest for, currently banned, household waste (+12%) and livestock by-products (+18%) that are allowed in fish feed but currently not used and appear essential to meet human requirements of EPA/DHA ω-3 fatty acids in a circular food system. Improved use and legalisation of inevitable food leftovers can improve the resource use efficiency of both current and future circular food systems. When allowing all LCF in a circular food system, livestock and fish provide an HDP intake up to 39 g per capita per day, less than current animal protein supply but meeting 65% of total protein requirements. A circular food system, thus, requires a reduction in ASF consumption, and a change in the type of ASF we consume, where fish and milk become more prominent. While the used food systems approach illustrates the potential of animal production in a circular food system, it provides little direction to farmers in achieving sustainability objectives. Currently, they base their sustainability strategies on supply chain life cycle assessments (LCA) that does not account for feed-food competition. In a case study of a novel egg production system, such LCA underestimated the environmental benefit of feeding only LCF with 57% for global warming potential and 96% for land use. The proposed food-based allocation, better captures the complexity of the food system, a first step towards metrics that stimulate circularity. Besides improved understanding of our food system, such novel metrics and changed consumption patterns, the paradigm shift needed to move towards a circular food system requires that policy makers value social and economic aspects within the ecological boundaries of our planet. This way, farm animals can contribute to the resource use efficiency of the entire food system.

Global demand for food and bio-based products grows at a fast pace, and have led to the vision that natural resource use and emissions associated with modern systems can and should be substantially reduced by shifting towards a circular food system. To move towards a circular and climate-neutral society, implies minimizing the input of finite resources, encourage the use of regenerative ones, prevent leakages of natural resources (e.g. carbon), and stimulate the reuse and recycling of resources in a way that adds highest value. Carbon sequestration in shells locks away carbon which is unique for animal protein production, and it is therefore often compared to primary producers such as seaweed (marine) or forest (terrestrial) production. Just taking account for shell material, and not tissue, shows that carbon capture is somewhat in the lower range but still in the same order of magnitude compared forestry. Translating to theoretical monetary values (CO2 equivalents) indicates that carbon sequestration in shells only resembles a fraction of annual market revenues. As shells are now considered to be a waste product, the development of biobased products might add another (commercial) value to shellfish production, which at the same time may substitute fossil sources, e.g. in cement applications, adding to climate ambitions.

Summary This study determined prey consumption in common sole as a function of prey size (0–0.5, 1–1.5, 2–2.5 and 4–5 g), sediment thickness (20 cm and 2 cm) and fish size (50 g, 125 g or 300 g). Prey consumption (in numbers of prey eaten per fish per day) was reduced with increasing prey size and sediment thickness, and was increased with increasing fish size (p

Concerns about the welfare of production animals have extended from farm animals to fish, but an overview of the impact of especially capture fisheries on fish welfare is lacking. This review provides a synthesis of 85 articles, which demonstrates that research interest in fish welfare in capture fisheries has increased over time and that research has focused more on trawls and hooks than on purse seines, gillnets, traps and seines. We found that various gear characteristics, fish characteristics and context variables affect external injuries and mortality. Although the influence of gear characteristics on injuries and mortality can by nature not be compared across gear types, synthesis of the articles reviewed shows that fish characteristics and context variables influence injuries and mortality across gear types. In terms of fish characteristics, decreasing fish length and certain fish species were associated with higher mortality. In terms of context variables, greater capture depth and a longer fishing duration were associated with more injuries and higher mortality, whereas a large change in water temperature, a longer duration of air exposure and a high density in the net were associated with higher mortality. These relations provide options to reduce injuries and mortality from commercial capture fisheries. Implementation of such options, however, would require analysis of potential trade-offs between welfare benefits, and ecological and economic consequences.

This study compares the effect of food type (formulated diet vs. natural food) and fish size on protein and energy utilization efficiencies for growth in common sole, Solea solea (L.). Replicate groups of common sole (mean initial body weight ± SD was 45.7 g ± 2.1 and 111.2 g ± 4.2) received the diets at five (natural feed) or four (formulated diet) feeding levels. The protein utilization efficiency for growth (kgCP) was higher (P > 0.001) in common sole fed ragworms than in common sole fed the formulated diet (respectively, 0.40 and 0.31). Likewise, the energy utilization efficiency for growth (kgGE) was higher (P = 0.001) in common sole fed ragworms than in common sole fed the formulated diet (respectively, 0.57 and 0.33). The protein maintenance requirement was not different between food types (P = 0.64) or fish size (P = 0.41) being on average 0.82 g kg−0.8 day−1. The energy maintenance requirement was not different between food type (P = 0.390) but differed between fish size (P = 0.036). The gross energy maintenance requirement of small common sole was 35 kJ g−0.8 day−1. The gross energy maintenance requirement of large common sole was 25 kJ g−0.8 day−1. In conclusion, the low growth of common sole fed formulated diets was related to reduced feed utilization.

Today, fish farming faces an increasing demand in fish products, but also various environmental challenges. Genetic improvement in growth rate and feed conversion ratio is known to be an efficient way to increase production and increase efficiency in fish farming. The environmental consequences of genetic improvement in growth rate and feed conversion ratio, however, are unknown. In this study, we investigated the environmental consequences of genetic improvement in growth rate and feed conversion ratio in an African catfish farm, using Recirculating Aquaculture System (RAS). In RAS, total fish production of the farm is limited by rearing density or by the capacity to treat dissolved nitrogen. To evaluate the environmental consequences of genetic improvement in growth rate and feed conversion ratio, we combined life cycle assessment and bioeconomic modelling of genetic response to selection. We explored different impact categories, such as climate change, eutrophication, acidification and energy use, and we expressed impacts per ton of fish produced. Results show that the environmental impact of genetic improvement in growth rate and feed conversion ratio varies among impact categories and depends on the factor limiting production at farm level (i.e. rearing density or nitrogen treatment capacity). Genetic improvement of feed conversion ratio reduces environmental impacts in each scenario tested, while improving growth rate reduces environmental impacts only when rearing density limits farm production. Environmental responses to genetic selection were generally positive and show similar trends as previously determined economic responses to genetic improvement in growth rate and feed conversion ratio in RAS. These results suggest that genetic improvement of growth rate and feed conversion ratio for species kept in RAS will benefit both the environmental impacts and the economics of the production system.

Sexual maturation from puberty to adulthood in European eels (Anguilla anguilla) occurs during and/or after the ~. 6000 km reproductive migration from their freshwater habitats to the spawning grounds in the Sargasso sea. This is the first study to simulate an anorexic, mixed-sex, group-wise freshwater migration (2. weeks; 689 km) and subsequent seawater migration (9. weeks; 3,103. km) under mimicked photothermal conditions, using farmed silver eels. Silver eels swam under an 8 hour light:16. hour dark regime in freshwater at 11.5. °C, and subsequently in complete darkness in seawater at daily fluctuating temperatures between 11.7. °C and 10.1. °C mimicking the vertical migrations in the Atlantic Ocean. The aim of these two consecutive experiments was to determine the effects of a simulated reproductive migration on the progression of sexual maturation. The freshwater migration significantly increased plasma testosterone levels in both migrating males and females, but did not enhance sexual maturation further as no significant increases in gonad weight, gonadosomatic index (GSI) nor eye index (EI) were observed. The subsequent seawater migration significantly increased gonad weight and GSI of the migrant males and, particularly, of the females (1.40. ±. 0.06 vs. 1.00. ±. 0.10%) vs. control groups, suggesting advancement of maturation. Also EI was significantly higher in migrant males (14.0. ±. 0.6) as compared to their controls (12.3. ±. 0.4). Plasma levels of the gonadotropins FSH and LH remained near the detection limits of the assays and levels were not elevated in migrating eels. These results show that simulation of migration under mimicked photothermal conditions has significant stimulating effects on early maturation which are presumably under steroid control. This brings farmed silver eels to a similar state of maturity as their wild conspecifics that are ready to embark on their oceanic migration. Simulated migration may therefore be used to condition farmed eels for the use as broodstock eels for further hormonal stimulation with gonadotropins in maturation protocols.

Successive exposure to aquaculture-related stressors may compromise the allostatic capacity of African catfish and lead to allostatic overload and poor welfare. Therefore, we tested the effect of (i) feeding during the light or dark phase, (ii) density (51 fish per 140 L versus 51 fish per 43 L) and (iii) altered available resting space/shelter on stress handling were studied in African catfish (Clarias gariepinus). Available resting space/shelter was manipulated by providing PVC-tubes in the tanks. Growth, baseline stress load (cortisol and osmolality), metabolism (glucose, lactate and non-esterified fatty acids) and aggression (measured by skin scar incidence) were assessed. Upon completion of the experimental period, we determined the adaptive capacity of the fish by the response to air exposure. We show that night-feeding enhances growth and lowers feed conversion ratio compared to day-feeding. No effects of density were found for baseline stress hormones, basal metabolism or aggression. Low density seems to stimulate aggressive behaviour after air exposure. Unexpectedly, providing PVC-tubes under high-density conditions increased aggression, raised baseline cortisol levels and decreased basal metabolism as well as growth; also the endocrine response to air exposure was stronger. We argue that these effects relate to a reduction in available space, i.e. a further increase in density, or to an increased competition for shelter. Overall, our data suggest that night-feeding optimizes growth and that care should be taken in using objects as shelter/enrichment for African catfish.

Concern exists about the potential effects of pile-driving sounds on fish, but evidence is limited, especially for fish larvae. A device was developed to expose larvae to accurately reproduced pile-driving sounds. Controlled exposure experiments were carried out to examine the lethal effects in common sole larvae. No significant effects were observed at zero-to-peak pressure levels up to 210 dB re 1 μPa 2 and cumulative sound exposure levels up to 206 dB re 1 μPa 2 ·s, which is well above the US interim criteria for nonauditory tissue damage in fish. Experiments are presently being carried out for European sea bass and herring larvae.

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BACKGROUND: Oyster refinement using land-based pond systems is a new activity in the Dutch oyster sector. It increases the oyster's tissue weight and changes its sensorial properties. However, the response of Dutch consumers towards refined oysters is unknown. The research aim was to gain insight into the importance of oyster quality parameters, drivers for oyster consumption, and acceptance of refined oysters by Dutch consumers, taking into account the information given to them about the product and process. RESULTS: Taste, texture, and odor are the most important oyster quality characteristics for Dutch consumers. The outcome of questionnaires showed that willingness to buy and pay is influenced by factors such as the oysters' country of origin, cultivation area, and flavor profile. Refinement did not affect willingness to buy and pay. Furthermore Dutch consumers seem to have a preference for the flavor profile of refined oysters. Consumer evaluation showed that refined Pacific cupped oysters were perceived as sweeter compared with non-refined oysters. When information on the cultivation process was disclosed, overall appreciation of refined oysters by consumers increased. CONCLUSION: New insights in the importance of oyster quality characteristics for Dutch consumers are generated that can be used in the development of refined Pacific cupped oysters.

Background Male European seabass, already predominant (~ 70%) in cultured stocks, show a high incidence (20–30%) of precocious sexual maturation under current aquaculture practices, leading to important economic losses for the industry. In view of the known modulation of reproductive development by swimming exercise in other teleost species, we aimed at investigating the effects of sustained swimming on reproductive development in seabass males during the first year of life in order to determine if swimming could potentially reduce precocious sexual maturation. Methods Pre-pubertal seabass (3.91 ± 0.22 g of body weight (BW)) were subjected to a 10 week swimming regime at their optimal swimming speed (Uopt) in an oval-shaped Brett-type flume or kept at rest during this period. Using Blazka-type swim tunnels, Uopt was determined three times during the course of the experiment: 0.66 m s− 1 at 19 ± 1 g BW, 10.2 ± 0.2 cm of standard length (SL) (week 1); 0.69 m s− 1 at 38 ± 3 g BW, 12.7 ± 0.3 cm SL (week 5), and also 0.69 m s− 1 at 77 ± 7 g BW, 15.7 ± 0.5 cm SL (week 9). Every 2 weeks, size and gonadal weight were monitored in the exercised (N = 15) and non-exercised fish (N = 15). After 10 weeks, exercised and non-exercised males were sampled to determine plasma 11-ketotestosterone levels, testicular mRNA expression levels of genes involved in steroidogenesis and gametogenesis by qPCR, as well as the relative abundance of germ cells representing the different spermatogenic stages by histological examination. Results Our results indicate that sustained swimming exercise at Uopt delays testicular development in male European seabass as evidenced by decreased gonado-somatic index, slower progression of testicular development and by reduced mRNA expression levels of follicle stimulating hormone receptor (fshR), 3-beta-hydroxysteroid dehydrogenase (3βhsd), 11-beta hydroxysteroid dehydrogenase (11βhsd), estrogen receptor-beta (erβ2), anti-mullerian hormone (amh), structural maintenance of chromosomes protein 1B (smc1β), inhibin beta A (inhba) and gonado-somal derived factor 1 (gsdf1) in exercised males as compared with the non-exercised males. Conclusions Swimming exercise may represent a natural and non-invasive tool to reduce the incidence of sexually precocious males in seabass aquaculture.

We examined the influence of prey density and fish size on prey consumption in common sole (Solea solea L.) foraging on buried ragworm Alitta virens (Sars) (formerly known as Nereis virens (Sars)). The tested prey densities of 0.8, 2.2, 4.3 and 6.5 individuals dm−2 were exposed to common soles of either 100 g or 300 g. At each prey density common sole foraged for 48 h. At both common sole classes studied, a positive correlation between prey consumption and prey density was observed (P P P = 0.018). Small common sole reached satiety prey consumption rates at nearly every prey density while large common sole did not reach satiation rates even at highest prey densities. The data suggest that in nature, polychaetes such as A. virens may contribute to the diet of small common sole even when they are only moderately abundant. In contrast, polychaetes may not be an ideal prey for larger common sole as indicated by the absence of satiety regardless of prey density.

Common sole fed with commercial pellets develop anaemia and are restricted in their growth performance. The anaemia can be the result of a difference in feed intake, a nutritional deficiency, an inflammatory response to infection or combinations of these aspects. In this study, it was investigated whether feeding ragworm would alleviate the anaemia and stimulate growth. Sole were fed one of three diets: a commercial feed, a commercial feed treated with ragworm extract or chopped ragworm. By comparing groups, three hypotheses were tested: (1) feeding ragworm alleviates the anaemia and positively affectS the sole’s metabolic performance reflected in feed intake, feed efficiency and growth; (2) anaemia is alleviated by a higher feed intake when feeding ragworm and (3) anaemia is caused by an inflammatory response to infection. The sole fed with a commercial diet suffered from anaemia. Feeding sole with ragworm alleviated the anaemia as the average haematocrit level nearly doubled in these fish as compared to fish fed pellets. Investigation of the expression of genes in the liver indicated that the anaemia in sole fed pellets is a nutritional anaemia and not an anaemia due to an inflammatory response. Sole fed ragworm showed improved growth which may be a consequence of the higher haematocrit levels in these fish increasing their oxygen carrying capacity. Addition of ragworm extract to the pellets levelled the feed intake between pellets and ragworm, but did not improve the anaemic state of sole and had only a limited effect on growth

The effect of a high and low non-starch polysaccharide diet (HNSP and LNSP diet) and ultrasound treatment on particle size distribution and carbon bioavailability in fecal waste of rainbow trout (Oncorynchus mykiss) was studied. Feces were collected from four flow-through fish tanks, two tanks fed the HNSP diet and two the LNSP diet. The collected feces were sonicated (disintegrated) in duplicate with high-intensity (0.6 W/ml), low-frequency (f = 20 Hz) ultrasound at five different energy levels (0.6 W/ml for 0, 0.25, 1, 4, and 16 min). The particle size distribution of the treated feces samples was measured by wet sieving (1000, 500, 200, 100, 63, 36, 1.2 μm screen size) and total suspended solids (TSS) measurement. Carbon bioavailability in sonicated fecal waste samples was determined with oxygen uptake rate (OUR) tests. The results showed that: (1) feces from the HNSP diet contained significant more particulate material and bigger particles; (2) carbon bioavailability was almost three times higher in untreated LNSP feces when compared with HNSP feces; (3) almost 50% of HNSP feces could have been recovered on a microscreen of 36 μm after wet sieving, whereas it was only 10% for LNSP feces; (4) the production of small particles (1.2–36 μm), which could pass a drum filter screen and potentially accumulate in RAS, was approximately 50 g/kg feed, showing no significant differences between diets; (5) sonication increased fecal dry matter below 36 μm (p = 0.015), but it had no significant effect on the median particle size; (6) sonication increased carbon bioavailability with 7–10% for the HNSP feces (p = 0.037); (7) fecal particles withstood up to 16 min sonication at an intensity of 0.6 W/ml and a frequency of 20 Hz corresponding to specific energy input of 20,000 kJ/kg DM without major changes in particle size distribution. The results of this study indicate that the applied ultrasound treatment of fecal waste is not an effective method to increase short-term carbon bioavailability.

The nitrite threshold concentration in rearing water of African catfish (Clarias gariepinus) was assessed. African catfish with an initial mean (SD) weight of 219.7 (57.8) g were exposed to an increasing range of water nitrite from 6 (Control) to 928 µM nitrite for 28 days. Mean (SD) plasma nitrite concentrations increased from 5.0 (3.6) to 32.5 (12.6) µM at 928 µM ambient nitrite. The increase in nitrite was accompanied by gradual increase in plasma nitrate from 41.6 (28.4) µM to 420.2 (106.4) µM. Haematocrit, haemoglobin, methemoglobin, plasma concentrations of cortisol, glucose, lactate, osmolality, gill morphology and branchial Na+/K+-ATPase activity were not affected. Feed intake, final weight, SGR, FCR and mortality were not affected. We advise not to exceed a water nitrite concentration of 43 µM (0.6 mg L-1 NO2--N) to prevent the risk of reduced growth and feed intake in African catfish aquaculture.

Vis is gezond en veel consumenten beoordelen vis ook als zodanig. Toch is gezond als een kwaliteitsaspect van vis vaak geen doorslaggevende reden om voor vis te kiezen. Het voedingsadvies om vaker vis te eten krijgt onder veel Europese consumenten onvoldoende navolging. Het product vis heeft met gezond weliswaar een sterke troef in handen die gekoesterd moet worden, deze kwaliteit alleen schiet tekort om consumenten (meer) vis te laten eten. Productvernieuwing is een mogelijk hulpmiddel hierbij. Ter aanvulling op gezond, als 'klassieke' kernkwaliteit van vis, wordt binnen het Europese onderzoeksproject 'Diversity' gezocht naar innovatieve ideeën met aantrekkingskracht op hedendaagse consumenten, waarbij meer specifiek oog is voor vijf nieuwe kweekvissoorten.

In 2010 vatte drie ondernemers het plan op om bij de Rotterdamse Fruithaven een loods en het erom heen gelegen rangeerterrein een landbouwkundige bestemming te geven. Het duurde tot 2012 voordat voldoende kapitaal en vergunningen waren geregeld en met de aanleg van Uit Je Eigen Stad begonnen kon worden. Inmiddels is er veel bereikt (waaronder een werkend aquaponics systeem met een inhoud van 125 m3) en worden groente, tilapia, Afrikaanse meerval, paddenstoelen en kippen in het eigen restaurant en in Rotterdam en omgeving verkocht. Maar de medewerkers van Uit Je Eigen Stad rusten niet en gaan door met het testen en proberen van verschillende gewassen, productiewijzen, producten, markten en evenementen. We bezochten het bedrijf en spraken met Ivo Haenen.

Scientifically, this study aimed to validate the potential of ragworm to alleviate anaemia in common sole and to identify the dietary requirements of common sole to alleviate this anaemia. At the same time it was aimed to explain part of the difference in growth between sole fed a commercial pellet and sole fed ragworm. Practically, this study aimed to develop a diet that achieves similar growth rates in sole as when fed ragworm. Sole fed commercial pellets developed nutritional anaemia. Feeding ragworm or mussel alleviates this nutritional anaemia. It is suggested that the ability of mussel or ragworm (meal) to alleviate anaemia and improve growth in sole can be explained by heme iron and high B12 levels. Yet, iron absorption in sole is high and independent of iron source. Still, heme increases the absorption of copper. The high absorption of iron and copper in sole fed heme does not affect the haematocrit (Hct) and haemoglobin (Hb) levels, which indicates the anaemia in sole is not an iron or a copper deficiency anaemia. The Hct and Hb levels in sole are affected by dietary B12. Yet, the applied levels are unable to alleviate the anaemia in sole induced by feeding commercial pellets. More options to alleviate the nutritional anaemia in common sole are discussed. Nutrients as vitamin C, B1, B2, B5 and a possible role of dietary EPO are discussed. It is suggested that the slow growth of pellet-fed sole might be due to the low Hct and Hb levels, which hampers the uptake of oxygen, and thus also the overall metabolic capacity, including the scope for growth. Discussed is a 7°C difference in the “optimal” temperature between sole fed ragworm and the 2nd generation pellet and that the “worm effect” is dependent on temperature. However, the growth rate of sole fed the 3rd generation pellet at 18.4°C was comparable to the growth rate of sole fed ragworm, which could not have been the consequence of increasing Hct and Hb levels as these were comparable to levels found in sole fed commercial pellets. Yet, B12 levels in blood plasma of sole are up to 200 times those of other (fish) species. Hence, we inferred on the possibility of a specific metabolic function of B12 in respiration in sole. In addition, the economic and practical impact of the improved growth rate in sole culture is discussed. Finally, several suggestions for future research are given.

Objectives of this study were to determine the combined market share of breeding companies in aquaculture production in Europe, to describe the main characteristics of breeding companies and their programs, and to provide per species estimates on cumulative genetic gain in growth performance. Surveys were conducted among breeding companies of five major species cultured in Europe: Atlantic salmon, rainbow trout, European seabass, gilthead seabream, and turbot. The market share was estimated as the combined egg or juvenile production of breeding companies relative to the total egg or juvenile production in Europe for each species in 2012. Cumulative genetic gain was estimated from the number of selected generations in current breeding programs, combined with genetic trends, reported selection responses in literature, and phenotypic differences. The combined market share of breeding companies ranged from 43–56% for seabass to 100% for turbot. The total volume of fish production in Europe that originated from selective breeding was 1653–1706 thousand tonnes, corresponding to 80–83% of the total aquaculture production. Over species, there were 37 breeding programs of which the majority performed family selection. Growth performance was universally selected upon. Cumulative genetic gain in growth performance varied from + 65% for turbot to + 900% for trout in terms of harvest weight, and from + 25% for turbot to + 200% for trout in terms of thermal growth coefficient. It is concluded that selective breeding has a major impact on European aquaculture and will contribute to future growth of the sector. Statement of relevance This manuscript helps to understand the impact of selective breeding on European aquaculture. It shows that the adoption of selective breeding is much higher in Europe than globally. It demonstrates that selective breeding has led to major improvements. Forecasts of industry trends are made and the importance of selective breeding to future growth of aquaculture production is illustrated.

Mussel bottom culture is an extensive type of aquaculture; it depends on natural resources for feed, seed and space. It consists of the translocation of seed from natural beds to designed culture areas, where mussel farmers try to improve production efficiency. Production efficiency is measured by the relative biomass production (RBP) expressed as units of biomass harvested from one unit of biomass seeded, it increases with mussel growth and decreases with mussel mortality. Mussel bottom culture makes use of nature and also depends on nature. Cultured mussels are subject to similar environmental factors that influence growth and mortality on natural mussel beds, with additional effects of anthropogenic factors. In this thesis we focus on dynamics of mussel beds and the impact and effectivity of culture activities on mussel production yield. The major objectives are stated as: (1) to better understand the population dynamics of subtidal mussel populations, (2) to analyze what factors determine production efficiency in mussel bottom culture and how this can be improved. On natural mussel beds mussels organise in patterns that enhance food delivery and resilience of the bed. On culture plots mussels are seeded in concentric seeding patterns. Seeding techniques concentrate mussels locally within the culture plot area, resulting in high local mussel densities; this increases competition and limits the spatial re-organisation of mussels in the bed. Consequently, seeding on culture plots is followed by a large size and density dependent seeding loss that ranges from about 40% for seed from fishery to 69% for smaller SMC seed. This loss was the major factor in determining the maximum RBP. Losses in the grow-out stage were substantially lower, a subsequent density dependent loss was found for smaller mussels (30 mm). Shore crab predation is an important factor contributing to the higher losses at seeding. The effect of shore crab predation on mussel biomass production is higher than expected from previous studies. In an experiment on an intertidal culture plot in the Oosterschelde (NL), we observed that shore crab predation peaks directly after seeding and accounted for 33% of the total losses within five weeks after seeding. Spatial patterns in the survival rates of natural mussel beds in the Wadden Sea show better seed survival in areas with intermediate salinity (mean annual salinity 17.5-22.5 mg l-1). This suggests that mussel survival is negatively related to sea star distribution, which is largely controlled by salinity. Natural beds that escape predation are found at lower salinities and mussels on these beds showed low growth rates, also because of a lower food quality in these areas. Mussel culture strongly affects the population dynamics of the subtidal mussel population, through relaying of mussels from natural mussel beds to culture plots. Culture plots are located in more saline regions of the Wadden Sea (mean annual salinity 25.8 mg l-1), compared to natural mussel beds. This activity increased mussel growth and survival because food quality on culture plots is high and predation is prevented. As a result, average biomass production is higher on culture plots than on natural mussel beds and this difference increases over time. A more efficient seed use on the available area, that can be obtained by reducing seeding losses will increase RBP, maximum biomass production and increases maximum profit. Our results suggest that this can be achieved by seeding homogeneously in low densities.

Diet composition, ingredient and nutrients, are important to consider for maintaining intestinal functions. Studies on both positive (using feed additives) and negative effects (using high inclusion of plant ingredients) of fish feeds are numerous, however, between studies results are often highly variable, both in type of response and in significance. The central hypothesis of this study was that adverse environmental conditions may aggravate negative effects of plant ingredients on the intestinal functions to the extent that mild effects become severe and perceptible. To do so, dietary factors and environmental conditions were evaluated and the interaction between diet composition and environmental conditions were studied in Nile tilapia. In Chapter 2, six common raw materials including hydrolysed feather meal (HFM), soybean meal (SBM), rice bran (RB), rapeseed meal (RM), sunflower meal (SFM) and dried distiller grains with solubles (DDGS) were chosen to determine the effect of nutrient digestibility, nitrogen/energy balance and changes in intestinal morphology. The study demonstrated that feed ingredients do have an impact on the alteration in intestinal parameters, but also on the nutrient digestibility and the nitrogen/energy balance. Although being well digested, soybean meal caused the most obvious alteration in the intestinal morphology. These alterations were not related to the nutrient digestibility nor to nitrogen/energy balance parameters. Soybean meal, causing the most alterations in the intestinal morphology, was further used in all subsequent chapters of this thesis. Chapter 3 and 4 described the interaction between diet composition and environmental conditions on the intestinal functions. This was studied with two different environmental conditions, dissolved oxygen (Chapter 3) and salinity (Chapter 4). These two chapters evaluated whether suboptimal environmental conditions (low dissolved oxygen or elevated salinity in water) may interact with a soybean meal based diet in nutrient digestion and intestinal morphology of tilapia. The study demonstrated that environmental stressors can aggravate/reveal the negative intestinal morphology changes induced by a soybean meal based diet. However, these effects of adverse environmental conditions on the intestinal functions were not homogenously dispersed over the whole intestinal length. The effect of salinity on the intestinal morphology occurred predominantly in the distal intestine, whereas the effect of low oxygen concentration was more visible at the proximal intestine. Alterations in the intestinal morphology, as found in this study, have wider effects on the performance of the affected fish. In Chapter 3, the protein digestibility decreased under hypoxic conditions at week 8, which parallels with the time related alteration in intestinal morphology. Chapter 4 showed that when fish were raised at 15 ‰ salinity, nutrient digestibility increased; however, this positive effect decreased when the intestinal morphology changed. The study also found that the combined effect of a soybean meal based diet and hypoxia was stronger compared to the combination with elevated salinity. Therefore, the combination with hypoxia was further used in the next study of this thesis. In Chapter 5, the combination of hypoxic condition and a soybean meal based diet was chosen to test the hypothesis that only under stressful conditions, the effects of feed additives can be noticed. The impact of two dietary organic acids, formic acid and butyric acid, on nutrient digestibility and intestinal morphology was determined under optimal (normoxia) and suboptimal conditions (hypoxia). The results showed that although organic acids did not significantly improve growth performance and nutrient digestibility under normoxic condition, they did so under hypoxic conditions. Fish fed organic acid supplemented diets all showed improvements in the morphology of intestine under normoxic conditions, and these effects were more enhanced under hypoxic conditions. This indicates that environmental conditions can alter the effect of organic acid on nutrient digestibility and intestinal morphology in tilapia. Finally, Chapter 6 provided a synthesis of the main findings, and a reflection on the methodologies used in Chapters 2-5 as well as a discussion on the relevance of this study to aquaculture. It is concluded that although being well digested, soybean meal caused the most obvious alteration in intestinal morphology. The adverse environmental conditions aggravated negative effects of soybean meal based diets on the intestinal functions to the extent that mild effects become severe and visible. The negative effect on intestinal morphology of soybean meal in the diet is enhanced at low oxygen level and at elevated salinity. The effect of salinity on the intestinal morphology occurs predominantly in the distal intestine, whereas the effect of low oxygen concentration is more visible at the proximal intestine. The thesis indicated that the impact of organic acids on intestinal functions is dependent on environmental conditions, being more pronounced under challenging conditions (e.g. hypoxia). Therefore, studies on both positive (using feed additives) and negative effects (using high inclusion of plant ingredients) of dietary factors should be done under suboptimal conditions.

The aim of this thesis was to develop a sustainable low cost breeding program for Nile tilapia that addresses both genetic and economic aspects of smallholder fish farmers in Kenya. First, Analytical Hierarchy Process Technique was used to define a breeding goal based on farmer’s preferences for traits. Farmers’ preferences for traits differed significantly depending on income and market orientation. Low and medium income farmers preferred harvest weight (HW) while high income farmers preferred growth (GR) and survival (S) traits. Grouping farmers according to market objective (fingerling production or fattening) showed that fingerling producers preferred GR and S while fattening farmers preferred HW and S. Consensus preference values were obtained using weighted goal programming and these values were used to derive desired gains for a breeding goal that takes into account farmers’ diverse backgrounds and preferences for traits. Secondly, the existence of genetic variation for traits of interest was investigated. Substantial additive genetic effects for HW, GR and shape traits were present that can be exploited through selection under low input production system. Heritability estimates for HW, GR and shape were 0.21 ± 0.03, 0.26 ± 0.04 and 0.12 ± 0.03 for mixed sex (nucleus) respectively. The calculation of economic values for breeding goal traits revealed that economic values for GR differed depending on the definition of the breeding goal and that selection for feed efficiency is the key factor to economic profitability of Nile tilapia breeding programs. A significant genotype by environment re-ranking was found for GR between the mixed sex nucleus and monosex production environments. Genotype by environment interaction (G x E) led to lower genetic gain for GR in production environment. Incorporating sib information from monosex production environment into the selection index resulted in a more accurate estimation of breeding values which increased genetic gain in growth. Using desired gain approach, weights for desired gains in harvest weight, growth rate and survival were derived that maximized genetic gains for these breeding goal traits. It is concluded that these results can be used to develop a sustainable centralized breeding program. However, a reliable well planned and organized decentralized strategy for dissemination of genetically improved fry of Nile tilapia to farmers is paramount.