Showing papers by "Johan A.J. Verreth published in 2005"
TL;DR: Analyzing nutrient conversions, which are taking place in integrated intensive aquaculture systems, finds that fish culture alone retains 20–50% feed nitrogen and 15–65% feed phosphorous and the combination of fish culture with phototrophic conversion increases nutrient retention of feed N and feed P by up to 53%.
259 citations
TL;DR: It is found that eels swim 4–6 times more efficiently than non-eel-like fish, removing a central objection to Schmidt's theory by showing that their energy reserves are, in principle, sufficient for the migration.
Abstract: One of the mysteries of the animal kingdom is the long-distance migration (5000-6000 km) of the European eel Anguilla anguilla L. from the coasts of Europe to its spawning grounds in the Sargasso Sea. The only evidence for the location of the spawning site of the European eel in the Sargasso Sea is the discovery by Johannes Schmidt at the beginning of the previous century of the smallest eel larvae (leptocephali) near the Sargasso Sea. For years it has been questioned whether the fasting eels have sufficient energy reserves to cover this enormous distance. We have tested Schmidt's theory by placing eels in swim tunnels in the laboratory and allowing them to make a simulated migration of 5500 km. We find that eels swim 4-6 times more efficiently than non-eel-like fish. Our findings are an important advance in this field because they remove a central objection to Schmidt's theory by showing that their energy reserves are, in principle, sufficient for the migration. Conclusive proof of the Sargasso Sea theory is likely to come from satellite tracking technology.
208 citations
TL;DR: Dietary soluble NSPs increase organic matter load in the culture system through a reduction in faeces recovery and nutrient digestibility, whereas insoluble N SPs improve the removal efficiency of particles by increasing faece recovery.
Abstract: The physico-chemical properties of nutrients influence the physical characteristics of faeces and thus may affect waste removal efficiency. The aim of this study is to assess the effect of type of non-starch polysaccharide (NSP) on digesta viscosity, faeces recovery and nutrient digestibility in Nile tilapia. Insoluble (cellulose) and soluble (guar gum) NSPs were included separately and combined at a level of 8%, thereby formulating four experimental diets. The diets were assigned to 16 tanks with 35 fish each, with four replicates for each diet. Cellulose inclusion did not influence digesta viscosity, growth and digestibility of protein and starch and tended to increase faeces recovery (P=0.06). Guar gum inclusion increased digesta viscosity and reduced the growth and digestibility of protein, fat and starch (P<0.01). Faeces recovery was reduced by 42% in diets containing guar gum. There were interaction effects (P<0.05) between cellulose and guar gum for the growth and feed conversion ratio, indicating that cellulose alleviated the negative impact of guar gum. In conclusion, dietary soluble NSPs increase organic matter load in the culture system through a reduction in faeces recovery and nutrient digestibility, whereas insoluble NSPs improve the removal efficiency of particles by increasing faeces recovery.
139 citations
TL;DR: The results showed that light plays an important role in the African catfish behaviour and its wellbeing, and data suggests that the fish were more stressed and aggressive, compared to those under a reduced number of light hours.
Abstract: Short periods of light or no light (18D : 06L and 24D : 00L) resulted in an increased growth compared to extended periods of light (06D : 18L and 12D : 12L) in African catfish Clarias gariepinus. Fish under longer periods of light (12D : 12L and 18D : 06L) showed higher swimming activity, more aggression (injuries on the body) and higher lactate, free fatty acids and cortisol levels compared to those who were reared at shorter periods of light (24D : 00L and 18D : 06L). Feeding activity during light and dark periods in this experiment showed that C. gariepinus had both night and day feeding activities, with a preference to diurnal feeding in the 12D : 12L photoperiod. The results showed that light plays an important role in the African catfish behaviour and its wellbeing. As the hours of light increased during the 24 h cycle, data suggests that the fish were more stressed and aggressive, compared to those under a reduced number of light hours.
79 citations
TL;DR: It is indicated that individual differences in growth, feed intake/efficiency and feeding behaviour are consistent over time and therefore probably inherent and may have implications on the use of feeding behaviour as a predictor of feed efficiency in juveniles of African catfish.
Abstract: Individual variation in growth, feed intake and feeding behaviour has been previously recognized in several fish species. However, there is a lack of information regarding the consistency of such individual differences, which is important to understand the probability of a certain individual trait to be inherent, i.e. genetically linked. The goal of this study is to quantify the consistency of individual differences in growth, feed intake/efficiency and feeding behaviour in African catfish Clarias gariepinus (Burchell 1822). Forty-eight juvenile fish (58.9±0.4 g) were housed individually for 47 days and weighed every second week. The consistency of growth, feed efficiency (residual feed intake (RFI)) and feeding behaviour (total feeding time (TFT)) was determined using repeatability estimates. Fish exhibited pronounced individual variation in growth (CV=52.8%), feed intake (34.3%) and in the TFT (>100%). The repeatability estimates were 0.55 for growth, 0.70 for feed intake, 0.49 for RFI and 0.81 for TFT. Individual differences in growth were mainly explained by individual differences in feed intake (80%). Individual differences in feeding behaviour contributed to explain differences in feed intake by affecting the RFI. With increasing TFT, the maintenance levels also increased suggesting that slow eaters were less efficient in feed/energy utilization. The results of this study indicate that individual differences in growth, feed intake/efficiency and feeding behaviour are consistent over time and therefore probably inherent. Moreover, this study may have implications on the use of feeding behaviour as a predictor of feed efficiency in juveniles of African catfish.
56 citations
TL;DR: Overall, this study indicates that size distribution does not affect growth performance and low-weight fish do not exhibit increased growth rates in the absence of heavier fish, and the use of grading in juveniles of African catfish should be re-considered.
46 citations
TL;DR: The magnitude of individual responses, independently of group effects (fish housed individually), in growth efficiency and feeding behaviour of African catfish Clarias gariepinus is investigated.
Abstract: Understanding the major causes of growth variation is crucial for the success of fish farming since its reduction contributes to maximize production efficiency, reduce food waste and improve water quality. The growth variation observed in aquaculture has been associated with the establishment of social hierarchies. However, some studies suggest that this variation may not be mainly a consequence of social hierarchies but mainly a result of inherent (genetic) differences. This study investigates the magnitude of individual responses, independently of group effects (fish housed individually), in growth efficiency and feeding behaviour of African catfish Clarias gariepinus (Burchell 1822). Despite the low variation in initial body weight (6.5%) and 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 efficiency is important in determining growth rate. This individual variation may be related with 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 with individual differences in growth efficiency. Fast eaters were the fast growers. We suggest that the growth variation observed in African catfish may be inherent and that the use of grading to increase uniformity should be further investigated
42 citations
TL;DR: In this article, the authors used the household life cycle of five phases to analyze the impact of integrated farming systems in Vietnam and found that farms with at least four flows of biomass between components earned more profits.
Abstract: Although specialization is the global trend in agriculture, integrated farming systems have emerged in Vietnam. An important motive was the desire to improve the livelihoods and the diet of the nuclear families; this was evident in the analysis using the household life cycle of five phases. Off-farm diversification was especially important for a new household. At the onset of expansion, the new mothers replaced off-farm with homebound activities. During expansion the farmers increased virtual farm size by keeping more livestock; during accumulation they invested in land or education, and during consolidation old couples adjusted farm activities to their labor capacity. Livestock, including fish, was essential for livelihood. The distribution of goats instead of cattle by credit or by âpassing-on-the-giftâ was far more effective for poverty alleviation. Technological innovations on the cultivation of rice and fruits, and the breeding of fish were essential for change. The improved food security and reduced cash income from rice after the 1986 reforms pushed farmers to take risks. The farm area and number of component farm activities providing cash determined the level of cash income from agriculture. Farms with at least four flows of biomass between components earned more, demonstrating that real integration improved profits. A minimum area of land in, or close to, the homestead, and know-how are required for an effective integration of components.
40 citations
01 Jan 2005
28 citations
TL;DR: Major underlying processes in tilapia farming and aspects for further improvement of the model are visualized, which would provide the basis of a model that can serve as a predictive and decision-making support tool.
Abstract: A simulation model for the production of the Nile tilapia (Oreochromis niloticus niloticus L.) for mixed- or mono-sex culture and for poly-culture with the African catfish (Clarias gariepinus Burchell 1822) or African snakehead (Parachanna obscura Gunther 1861) is presented. The model is based on an exponential decay model used in population dynamics and follows an individual-based approach. The model consists of a tilapia sub-model representing population dynamics, growth and recruitment and a predator sub-model representing the predation process and population dynamics and growth of the used predator. The model was calibrated with data on mixed- or mono-sex culture of Nile tilapia and for poly-culture with the African catfish or African snakehead obtained in Congo Brazzaville and validated with similar data from the Philippines, Thailand and the Ivory Coast. The model visualized major underlying processes in tilapia farming and aspects for further improvement of the model; growth is one of the most sensitive input parameters and should be quantitatively related to feeding level and feed quality; length at first maturity and quantification of the recruitment of Nile tilapia has a relatively large influence and recruitment should be related to the length of the females; prey?predator relations are too coarse and should be more fine-tuned with the relation between prey size and predator size. Incorporation of these features would provide the basis of a model that can serve as a predictive and decision-making support tool
27 citations
TL;DR: The differences in ADC and faeces recovery measured by two faecal collection methods depend on diet quality, and they are higher when using settling tanks compared with Choubert collectors.
Abstract: The need for unbiased digestibility estimation has led to the development of a number of faeces collection methods. However, there is still a large variation in apparent digestibility coefficient (ADC) of diet/ingredients between these methods. This study investigated the impact of dietary quality on ADC and faeces recovery measurements obtained by two faecal collection methods (Choubert and settling tank). The fish were fed five diets: a control diet as high-quality diet, two levels of dried duckweed (20% and 40%) and two levels of fresh duckweed (20% and 40%), which were added to the control diet, as low-quality diets. Apparent digestibility coefficient estimates were highly correlated (r>0.95) between both faecal collection methods. For all diets the ADC estimates were higher when using settling tanks compared with Choubert collectors. For the control diet differences in ADC between two collectors were small, but by inclusion of duckweed these differences increased. The recovery percentage of faeces was not correlated between both faecal collection methods (r=0.22, P=0.41). The estimated recovery percentage of faeces by settling tanks was about three times higher than by Choubert. In conclusion, the differences in ADC and faeces recovery measured by two faecal collection methods depend on diet quality
TL;DR: It is concluded that L. plantarum 44a maintained high viability after being dried and stored in feed even after exposure to gastric and intestinal fluids in vitro, and after 1 year of storage.
Abstract: A good probiotic strain should be able to survive the conditions of handling and storage to be delivered in high concentration to the host. That is especially important when stressful conditions are prevalent in the carrier, for instance in low water content foods like animal feed. The aim of this research was to study the survival of the probiotic candidate Lactobacillus plantarum 44a after spraying and drying in feed, and during storage and exposure to gastrointestinal tract fluids in vitro. In addition, the viability of the strain during exposure to distilled water and 2% NaCl was studied. Feed was sprayed with a suspension of ?2 × 1010 CFU of L. plantarum 44a in 10, 15, 20, 25 and 30% v/w of the feed and dried to constant weight (6% moisture) in a convective oven at 25°C. L. plantarum 44a survived 14.67, 36, 51.86, 78.9 and 105.3% respectively in relation to the original % v/w of the feed. After 3 weeks of storage at 25°C, survival was similarly low in all the treatments. L. plantarum 44a stored in feed containing 13% moisture, vacuum-packaged and stored in refrigeration, maintained high viability (?100%) after 1 year of storage. Survival was not affected after feed-containing lactobacilli was exposed to gastrointestinal fluids in a simulation model. Viability of L. plantarum 44a as a cell suspension in PBS added directly to distilled water or distilled water with 2% NaCl was maintained up to 48 h; after 72 h, viability started to decline. It is concluded that L. plantarum 44a maintained high viability after being dried and stored in feed even after exposure to gastric and intestinal fluids in vitro.
TL;DR: A dynamic simulation model of organic matter deposition/decomposition in fish ponds simulated well the concentrations of organic carbon and nitrogen in the sediments but it may be developed further, especially by considering the effects of resuspension
Abstract: In aquaculture, ponds with high loads of organic inputs, organic matter accumulates at the bottom over time. Uneaten feed, senescent phytoplankton and faeces are the principal sources of accumulated material, but quantifications are scarce. The sedimented organic matter develops into a flocculent layer in which different processes transform the material into inorganic forms. A better understanding of factors influencing organic matter accumulation/decomposition in the sediment is needed to better understand and manage the dynamics of nitrogen in fish ponds. In this study, the rate of mineralization of organic nitrogen and the nitrogen flux between the sediment and the water column were measured. Organic matter accumulation in fish ponds was quantified, and the data were used to construct, calibrate and validate a dynamic simulation model of organic matter deposition/decomposition in fish ponds. The accumulating material consisted of dead phytoplankton, fish faeces and uneaten feed. Through model calibration, the proportion of these materials in the total accumulated organic matter was determined. In the model, gross photosynthetic rate was estimated from an empirical relationship with feed input. After calibration, the model was validated using independent data. The model simulated well the concentrations of organic carbon and nitrogen in the sediments but it may be developed further, especially by considering the effects of resuspension
TL;DR: A suplementacao de carnitina dietetica e importante para peixes criados em sistemas intensivos e em fase inicial of crescimento, uma situacao tipica que o consumo de gordura e elevado podera evitar que o animal vivencie uma reducao brusca das reservas energeticas.
Abstract: Lipids, together with proteins, are traditionally considered as primary fuels during aerobic swimming. The effects of dietary fat and carnitine supplements and exercise on the energy metabolism of juvenile fish were investigated. One hundred African catfish (Clarias gariepinus) were fed four isonitrogenous diets containing a fat level of 100 or 190 g kg-1 diet and one of the two levels of carnitine (15 and 1000 mg kg-1). Fish grew from 61 to 162 g in 10 wk. Thereafter, 6 fish per group swam vigorously for 3 h and the results were compared with unexercised groups. Fish receiving 1,000 mg carnitine accumulated 2- to 3-fold more carnitine than fish receiving 15 mg carnitine. Plasma acyl-carnitine level was affected by an interaction between dietary treatment and exercise (P < 0.05). Adenosine triphosphate and phosphocreatine concentrations were higher in the white muscle (WM) of exercised fish fed the high-carnitine supplements, compared with the low-carnitine fed fish (P < 0.05). Adenilate energy charge indexes were higher and ammonia concentrations were lower in WM of fish fed high-carnitine and high-fat diets. Dietary carnitine supplements may be needed in growing fish when dietary lipid level is high. In that case extra dietary carnitine can maintain the body energy reserves at adequate level when fish is exposed to a short-term, exhaustive exercise, a physiologic stress common both in nature and in intensive aquaculture systems.
TL;DR: Nitrogen accumulated in the pond during periods of excessive feeding and was utilized by the fish during periodsof low feeding, meaning that this cycling should be further studied and may be an important pond management technique.
Abstract: This study quantified the accumulation of nitrogen (N) in the water column, sediments, fish and seepage water during a production cycle of Colossoma macropomum. By combining estimates of the deposition rates of uneaten feed, faeces and dead phytoplankton with measurements of N accumulation in the sediment, the rate of decomposition of organic matter in the sediment was estimated. The first-order rate constant for organic matter decomposition was 0.237±0.019 day1. Total N recovery during the first weeks of the experiment was about 65%. Later, the N recovery was close to 100%. The cumulative recovery at the end of the experiment was almost 100%, meaning that the N budget in the system studied can be fully explained without consideration of N volatilization, due to either denitrification or ammonia volatilization. In the beginning of the growth cycle, the major flux of N was sedimentation. Intensive microbial degradation process occurred about 34 weeks later, leading to a release of inorganic N and an approach towards a steady state as to the accumulation of organic N. Feed was irregularly applied during the experiment but fish growth was constant, showing that the fish utilized detrital or planktonic feed during periods of low feeding. Nitrogen accumulated in the pond during periods of excessive feeding and was utilized by the fish during periods of low feeding. This cycling should be further studied and may be an important pond management technique
01 Jan 2005
01 Jan 2005
TL;DR: In this paper, the authors analyzed the effect of the integration of farm components on income in two zones of the Vietnamese Mekong Delta, with data from 144 interviews, and found that the number of components providing cash income to the household contributed significantly to the level of well-being and to the cash income that farmers derived from agriculture.
Abstract: We analysed the effect of the integration of farm components on income in two zones of the Vietnamese Mekong Delta, with data from 144 interviews. Next to the farm area, the number of farm components providing cash income to the household contributed significantly to the level of well-being and to the cash income that farmers derived from agriculture. The integration of components in the hills was hampered by the distance to the fields and a small homestead, and in the delta by a small total farm area. The total number of components and the components providing cash determined two clusters of gross income from agriculture.