Tilapia Size Research Articles

Weight-dependent susceptibility of tilapia to tilapia lake virus infection.

The emergence of tilapia lake virus (TiLV) has had a severely negative impact on global tilapia aquaculture. TiLV infection has been reported at different life stages of tilapia, with more emphasis on fry and fingerlings; however, the virulence and pathology of TiLV at different tilapia size remains unexplored. In this study, tilapias from a single spawning were raised to 5 g, 25 g, and 65 g, and subsequently challenged by the intraperitoneal injection and cohabitation of a virulent strain of TiLV. The cumulative mortality, viral load, and histopathology of the fish were determined until 22 days post-infection (dpi). The cumulative mortality of the 5 g, 25 g, and 65 g fish was 85% (±1.67), 55% (±2.89), and 51.67% (±7.49), respectively. At 14 dpi, the mean TiLV load in the liver of the 5 g fish was significantly higher than in the 25 g and 65 g fish. All the weight groups showed severe pathological changes in the liver, spleen, and intestine after TiLV infection, but no particular difference was otherwise noted during the study with the exception of higher pathological scores in the liver of the small fish at 14 dpi. Overall, this study indicated that small fish are more susceptible to TiLV infection than large fish. Although multiple factors, including environmental factors, farm management practices, strains of virus could contribute to different susceptibility of fish to viral infection, the present study provides the evidence to support that fish weight affects the mortality and clinical outcome during TiLV infection. More intensive measures such as strict biosecurity and disease surveillance during the susceptible weight should therefore be emphasized to reduce the impact of this virus.

The immune efficacy of a Streptococcus agalactiae immersion vaccine for different sizes of young tilapia

In order to explore the immune efficacy of a Streptococcus agalactiae immersion vaccine for young tilapia, three sizes of tilapia were selected and immunized a single time with an S. agalactiae immersion vaccine. The results that showed the whole cell antigen was crushed into nanoparticles and positive signals of antigen uptake were found in different sizes of tilapia and were mainly distributed in the gill, oropharynx, kidney, liver, spleen, intestine, and fish skin. The relative peak expression values for IgM, IgT, CD8+, and C3 from 1 g tilapia were significantly higher than those in 0.1 g and 0.5 g groups (P < 0.05). The differential expression of IgT was most significant, and the peak expression of C3 was maintained the longest time. The antibody level peak values appeared in 0.5 g and 1 g groups at the second week post-vaccination; both were significantly higher than that in 0.1 g tilapia (P < 0.05), and the higher antibody level in 1 g tilapia lasted longer than in the other two sizes. During the three months post-vaccination, the peak RPS in three different sizes of tilapia were all higher than 60% and the effective protection period in 1 g tilapia was longer than those of the 0.1 g and 0.5 g groups (P < 0.05). These results suggested that the immersion vaccine employed in the present study was effective against S. agalactiae infection for young tilapia (0.1–1 g) through one-time bath. In addition, the RPS and effective protection period in 1 g tilapia was higher and longer than those of the 0.1 g and 0.5 g groups.

Effect of seaweed, Kappphycus alvarezii fermentation by various fermenters combinations as thickener on gel strength, attractiveness and palatability of gel diet in Tilapia, Oreohromis niloticus

Gel diet is one of the aquaculture diets that use seaweed, K. alvarezii, as a thickener made with cooking with water contents of between 50-70%. One effort to increase the nutritional content of seaweed is fermentation. However, fermentation affects the attractiveness, palatability, and gel strength of aquaculture artificial diet. This study aims to compare various fermenter combinations that are most effective in fermenting seaweed, K. alvarezii meal in terms of attractiveness, palatability, and gel strength. This research was carried out at the Laboratory of Nutrition and Feed Technology, Faculty of Marine Sciences and Fisheries, Hasanuddin University, Makassar, Indonesia. Experimental fishes used were tilapia size 7, 15, and 28 g. Various combinations of fermenters as treatment were seaweed meal without fermenter (treatment A), + 1.5g Bacillus sp. (treatment B), + 1.5g Bacillus sp. + 1.5 g of Rhizopus sp. (treatment C), + 1.5 g Bacillus sp. + 1.5g Saccharomyces sp. (treatment D), and + 1.5g Bacillus sp. + 1.5g Aspergillus sp. (treatment E). Fermentation is carried out on an an-aerobic basis. The parameters measured were attractiveness, palatability, and gel strength. The experimental design used was a completely randomized design with 5 treatments and 5 replications. Data were analyzed by ANOVA and W-Tukey test. The results of this study indicate that the attractiveness and gel strength of the experimental diet were the same in all treatments (p>0.05), while the highest palatability of the experimental diet was obtained in the experimental fish that consumed D diet, namely seaweed meal fermented with 1.5g Bacillus sp. + 1.5g Saccharomyces sp. Based on the results of this study, it was concluded that tilapia, which consumed the gel diet, contained seaweed fermented with 1.5g Bacillus sp. + 1.5g Saccharomyces sp. has the best palatability.

FALHAS DE MANEJO NA FASE DE ENGORDA DE TILÁPIAS E SUA RELAÇÃO COM A DISPARIDADE DE PESO FINAL

O presente estudo foi realizado na região Oeste do Paraná, considerada o polo agropecuário da produção de peixes do Estado, região que concentra 17 frigoríficos de abate e industrialização de pescados, tem como objetivo informar o produtor em relação à engorda do lote, bem como sobre as análises da qualidade da água do viveiro com o intuito de verificar as falhas de manejo que contribuem para a disparidade de tamanho final dos peixes. Durante cinco meses (out/18 a fev/19) foram monitorados seis viveiros em quatro pequenas propriedades localizadas no município de Toledo-PR. Para a biometria foram utilizadas amostragens com 50 indivíduos por viveiro, pesados e medidos individualmente. A disparidade de peso foi calculada utilizando-se o desvio padrão amostral de cada lote. A sonda multiparâmetros foi utilizada para mensurar: temperatura, pH, condutividade elétrica, sólidos totais dissolvidos, oxigênio dissolvido e turbidez. Os resultados demonstraram forte correlação entre turbidez, sólidos totais dissolvidos e condutividade elétrica com a disparidade de tamanho das tilápias. Foi observado superlotação dos viveiros. O desvio padrão de peso aumentou à medida que os peixes cresceram. Conclui-se que a qualidade da água afeta diretamente o desenvolvimento dos peixes e a falta de manejo adequado deste ambiente, contribui para com o aumento da disparidade de peso e tamanho das tilápias.

Defensible standardized ploidy assessments for Grass Carp (Ctenopharyngodon idella, Cyprinidae) intercepted from the commercial supply chain

Abstract Although methods are in place through the U.S. Fish and Wildlife (USFWS) program for ploidy testing of feral caught Grass Carp (Ctenopharyngodon idella) and black carp (Mylopharyngodon piceus), no guidelines exist for carp hauled across state lines. Using 1200 Grass Carp purchased by undercover Ohio law enforcement during 2015–2016, we developed a standardized protocol for discriminating ploidy by using two parameters, nuclear size and DNA content. Bead standards at 2 μm or 4 μm were used to establish nuclear size from Nile Tilapia (Oreochromis niloticus), known diploid (n = 20) and triploid (n = 20) Grass Carp blood, and cells derived from eyeballs of purchased field carp. The control for establishing DNA content was cryopreserved or fresh tilapia blood (2.40 pg). Time postmortem indicated nuclear size was similar over 4 days, but DNA quality from triploids was best at 24 h. Occasionally, only size or DNA content was measurable. Tilapia mean nuclear size (n = 501 samples) was 4.61 μm (SE 0.05) (R2 = 0.94). Known diploid and triploid blood nuclear sizes, compared with tilapia size, were 3.62 μm (SE 0.13) (R2 = 0.96) and 7.58 μm (SE 0.27) (R2 = 0.96), respectively. Mean field carp eye nuclear size was 5.83 μm (SE 0.13). Mean DNA content of cells from field carp eyes (n = 698 fish) was 3.51 pg (SE 0.06). No diploid Grass Carp were detected in the USFWS certified triploid Grass Carp transports. This standard protocol reliably discriminates ploidy and can be used for enforcement of regulations that differ among state jurisdictions.

Pemanfaatan bungkil kelapa fermentasi dalam pada ikan nila (Oreochromis niloticus)

The purpose of this study was to determine the concentration of the use of the best fermented coconut pulp in feed on the growth of tilapia. The research used 12 nets measuring 1x1 m placed in a concrete pond measuring 3.5 x 5 cm. The treatments were A (without fermented coconut pulp), B (10% fermented coconut pulp), C (20% fermented coconut pulp), and D (30% fermented coconut pulp). The experimental method used was a completely randomized design with 4 treatments and 3 repetitions. Data collected were absolute growth, daily growth, relative growth, and feed efficiency. The results of the variance analysis showed an influence in the growth of tilapia size 5-8 cm. The best growth and feed efficiency value occurred in treatment D Thus the use of 30% fermented coconut pulp in fish food could improve the growth of tilapia.Key words : fermented coconut pulp, tilapia, growth feed efficiency

Enhancing aquaculture development:mapping the tilapia aquaculture value chain in Ghana

Fish is important in the diet of millions of Ghanaians supplying protein and micronutrients. Ghanaians have developed taste preferences for the consumption of Oreochromis niloticus (Nile tilapia), thereby booming the tilapia aquaculture industry. This paper looks comprehensively at the value chain of the tilapia aquaculture sector in Ghana to identify particular areas of intervention and development opportunities in achieving food security and nutrition needs, safety, quality and trade benefits. Mixed methods (including stakeholders’ workshop, interviews and document analysis from official sources) were employed in collecting data between January and December 2014. The analysis indicates that farmed tilapia products are currently not exported but locally consumed. In 2012, approximately 28 000 t of tilapia were supplied to the local market. The bulk of the supply came from cage culture (85%) with the remaining coming from ponds and dugouts. About 98% of tilapia coming from aquaculture is supplied directly to the local markets, whilst 2% is family consumed (subsistence). The marketing channel involves farmers selling to wholesalers/retailers and to consumers/restaurants in nearby villages or secondary towns. The processing technology is generally low (mainly sun drying, small size smokers). Currently, due to the growing middle-class Ghanaians, high domestic demand for fish and high levels of undernutrition among many poor Ghanaians, there is a good and readily available market for all sizes of tilapia produced locally. However, farmed tilapia has the potential for export to the international markets if it is processed as smoked and salted. To achieve this, there is the need to develop a practical guide for aquaculture exporters by the relevant stakeholders, strengthen capacity building of public institutions to deal with the export of aquaculture products and train more aquaculture personnel, and collate proper documentation of aquaculture products.

Enhancing aquaculture development: mapping the tilapia aquaculture value chain in Ghana

AbstractFish is important in the diet of millions of Ghanaians supplying protein and micronutrients. Ghanaians have developed taste preferences for the consumption of Oreochromis niloticus (Nile tilapia), thereby booming the tilapia aquaculture industry. This paper looks comprehensively at the value chain of the tilapia aquaculture sector in Ghana to identify particular areas of intervention and development opportunities in achieving food security and nutrition needs, safety, quality and trade benefits. Mixed methods (including stakeholders’ workshop, interviews and document analysis from official sources) were employed in collecting data between January and December 2014. The analysis indicates that farmed tilapia products are currently not exported but locally consumed. In 2012, approximately 28 000 t of tilapia were supplied to the local market. The bulk of the supply came from cage culture (85%) with the remaining coming from ponds and dugouts. About 98% of tilapia coming from aquaculture is supplied directly to the local markets, whilst 2% is family consumed (subsistence). The marketing channel involves farmers selling to wholesalers/retailers and to consumers/restaurants in nearby villages or secondary towns. The processing technology is generally low (mainly sun drying, small size smokers). Currently, due to the growing middle‐class Ghanaians, high domestic demand for fish and high levels of undernutrition among many poor Ghanaians, there is a good and readily available market for all sizes of tilapia produced locally. However, farmed tilapia has the potential for export to the international markets if it is processed as smoked and salted. To achieve this, there is the need to develop a practical guide for aquaculture exporters by the relevant stakeholders, strengthen capacity building of public institutions to deal with the export of aquaculture products and train more aquaculture personnel, and collate proper documentation of aquaculture products.

Methionine and lysine requirements for maintenance and efficiency of utilization for growth of two sizes of tilapia (Oreochromis niloticus)

Different ration levels were used to determine the digestible methionine (DMet) and lysine (DLys) maintenance requirements and the utilization efficiencies for gain above maintenance for two different sizes of tilapia (20.7 and 165 g), by feeding a soybean meal–based diet. Protein gain and amino acid (AA) gain (e.g. methionine, Met; lysine, Lys; R2 = 0.98) were best-fit linear functions of DMet and DLys intake in both fish size classes. Slopes of these regression lines showed that the DMet utilization efficiencies for growth were 0.76 and 0.55 for juvenile and adult fish, respectively. The DMet maintenance requirements were 3.12 and 16.5 mg BW(kg)−0.7 day−1 for juvenile and adult fish, respectively. The DLys utilization efficiencies for gain were 0.72 and 0.52, whereas the DLys maintenance requirements were 16.9 and 68.8 mg BW (kg)−0.7 day−1, for juvenile and adult fish, respectively. These results suggested that there was an obvious difference in the maintenance requirements and utilization efficiencies for gain above maintenance for DMet and DLys in two different sizes of tilapia. The AA maintenance needs increased as fish increased in size, being greater in adult fish than in juvenile; however, the AA utilization efficiencies for gain above maintenance decreased with the increment of fish size.

Effects of addition of tilapia Oreochromis niloticus and substrates for periphyton developments on pond ecology and production in C/N-controlled freshwater prawn Macrobrachium rosenbergii farming systems

The present research investigated the effect of addition of tilapia and substrates for periphyton development on pond ecology, production and economic performances in C/N controlled freshwater prawn farming system. The absence and presence (0 and 0.5 individual m − 2 ) of tilapia were investigated in 40 m 2 ponds stocked with 3 prawn juveniles (individual weight 5 g) m − 2 with or without added substrates for periphyton development. A locally formulated and prepared feed containing 30% crude protein (C/N ratio10) was applied daily, initially at 10% of the prawn stocked biomass and was gradually reduced to 3% of prawn biomass. Tapioca starch was used as carbohydrate source for increasing the C/N ratio from 10 (as in feed) to 20 and was applied to the water column separately from the feed. Addition of periphyton substrates significantly reduced the inorganic N-species (NH 3–N, NO 2–N and NO 3–N) in the water column. It decreased the abundance of plankton in the overlying water and increased the abundance of benthic macroinvertebrates. The abundance of periphytic algae and periphyton biomass (dry matter, ash free dry matter and chlorophyll a) were significantly higher in tilapia free ponds compared to tilapia added ponds. Both substrates and tilapia had significant effects on feed conversion ratio (FCR) of freshwater prawn: substrates decreased FCR by 14% while tilapia addition increased it by 16%. The addition of substrates did not influence prawn and tilapia size at harvest but improved the survival of prawn from 54 to 77%. Substrates contributed 44% and 19% higher net yield of prawn and tilapia, respectively whereas tilapia addition decreased the net yield of prawn by 14%. The economic analysis showed that addition of tilapia and periphyton substrates jointly improved the benefit–cost ratio. Addition of tilapia and periphyton substrates in C/N controlled system benefited the freshwater prawn culture practices through (1) reducing toxic inorganic nitrogenous compounds in water (2) enhancing the utilization of natural foods (3) improving survival, production and economic benefit.

Effects of fish biomass and planktivore type on plankton communities

to filter (suction) feeding on both zooplankton and phytoplankton, we expected differences in the biomass-related effects of juveniles and adults on plankton communities. Our results show that rotifer abundances were higher in treatments stocked with juvenile tilapia whereas cladoceran abundances were higher in treatments stocked with adults. However, total zooplankton and copepod abundances, chlorophyll a concentrations and Secchi depth were not affected by tilapia size, suggesting that variation in the feeding mode of tilapias affects the structure of zooplankton communities but that this does not cascade down to affect phytoplankton biomass and water transparency. Competitive interactions among the fish constrained their potential to depress zooplankton abundances and increase algal biomass as fish-stocking biomass increased. Competition among size classes of tilapias was highly asymmetric and juveniles were better competitors than adults for plankton resources in our experimental ponds.

Effect of Nile tilapia, Oreochromis niloticus (L.), size on phytoplankton filtration rate

Four different-sized (390±3, 140±2, 40±2, 16±1 g) Nile tilapia, Oreochromis niloticus (L.), stocked at the same biomass in timed pulse feeding chambers were provided 27–29°C water dominated by Microcystis (82%) and Scenedesmus (18%) to determine the effect of fish size on filtration rates. The number of Microcystis and Scenedesmus units filtered from the water decreased significantly with increasing tilapia size. The shaping constants and maximum filtration rates for Ivlev's feeding model used to describe the relation between filtration rates and the suspended particulate organic carbon (POC) concentrations were significantly different among the four sizes. Filtration rates of 763, 671, 512 and 300 mg C kg−1 h−1, which correspond to 70%, 82%, 86% and 90% saturation levels, were achieved at POC levels of 30, 32, 32 and 33 mg C L−1 for 16, 40, 140 and 390 g Nile tilapia respectively. Smaller tilapia achieved these rates at lower POC concentrations than larger tilapia.

Ingestion and utilization of periphyton grown on artificial substrates by Nile tilapia, Oreochromis niloticus L.

The study was carried out to quantify the periphyton biomass developed on glass substrates over time, to investigate the effects of periphyton quantity and fish size on the ingestion rate by fish, and to determine the feed conversion ratio (FCR) of periphyton by tilapia Oreochromis niloticus. Periphyton was grown in two fertilised 1000-l tanks on glass slides and monitored as dry matter (g), ash-free dry matter (g) and chlorophyll a concentrations (mg) per unit surface area (m2) over a six week period. Ingestion rate was determined for two sizes of tilapia (7 and 24 g) which were provided with four different periphyton densities. Determination of FCR was made after feeding three individual fish ad libitum with periphyton for two weeks. Periphyton ash-free dry matter increased sharply during the first half of the trial with a peak being recorded at week 3 (75.5 g m-2). Productivity was 2.4 g ash-free dry matter m-2 d-1 during the first three weeks. Mean chlorophyll a concentration showed a cyclic pattern throughout the study with the lowest value being measured during the last week. Ingestion rates were 0.90 and 0.18 mg dry matter g fish body weight-1 h-1 for small and medium fish respectively. Ingestion rate among small fish increased significantly (P <0.05) with periphyton density, but not for medium size fish. Although periphyton ash content was high (55% dry matter), fish growth was sustained. Fish harvested 70% of total periphyton dry matter that was offered to them. The FCR for periphyton was 2.81 on a dry matter basis and 1.34 on an ash-free dry matter basis.

Effects of biomass of caged Nile tilapia ( Oreochromis niloticus) and aeration on the growth and yields in an integrated cage-cum-pond system

An experiment was conducted to determine the appropriate biomass of caged Nile tilapia ( Oreochromis niloticus) supporting maximum production of small tilapia in open water and maintaining acceptable water quality, and to investigate effects of aeration on the growth of both caged and open-pond tilapia and water quality in an integrated cage-cum-pond system. One, two, three or four cages were suspended in unaerated ponds as experimental treatments with three replicates each. An additional treatment with four cages in aerated ponds was also tested. Large tilapia (91±2.6–103±2.8 g) were stocked in 4-m 3 net cages at 50 fish/m 3, while tilapia fingerlings (13±0.2–16±0.8 g) were stocked at 2 fish/m 3 in open water of all ponds. Caged tilapia were fed with floating pelleted feed twice daily at 3%, 2.5%, and 2% body weight/day during the first, second and third month, respectively. Important water quality parameters, such as dissolved oxygen, unionized ammonia nitrogen and chlorophyll a, were analyzed biweekly. The biomass of caged tilapia had significant ( P<0.05) effects on the growth of both caged and open-pond tilapia. Survival of caged tilapia decreased with increasing biomass of caged tilapia per pond, while survival of open-pond tilapia in the treatment with one cage was significantly ( P<0.05) lower than that in treatments with more cages. Final mean weight of caged tilapia decreased significantly ( P<0.05) from 478±20.0 g in the treatment with one cage to 261±10.0 g in the treatment with four cages. However, the growth of open-pond tilapia (0.74±0.02–1.57±0.10 g/fish/day) increased significantly ( P<0.05) with the increased biomass of caged tilapia per pond, in response to the increased feed inputs to cages. Total extrapolated net yield, including both caged and open-pond tilapia, was highest (6.20±0.36 t/ha/crop) in the treatment with four cages; however, the treatments with one or two cages gave significantly ( P<0.05) better overall feed conversion ratio (0.82±0.02–0.98±0.02). Nighttime aeration for 5 h enhanced the growth of tilapia in the treatment with 4 cages/pond, and increased the carrying capacity in the integrated cage-cum-pond system. Net yield of caged tilapia in aerated ponds averaged 6.92±0.60 t/ha/crop, which was significantly ( P<0.05) higher than that (3.65±0.22 t/ha/crop) in unaerated ponds with four cages each. However, the growth of open-pond tilapia in aerated ponds was significantly ( P<0.05) lower than that in unaerated ponds. Further study should focus on determining the appropriate stocking density of open-pond tilapia to optimize the harvested tilapia size both from cages and open ponds in order to develop an integrated cage-cum-pond system for tilapia rotation culture.

A bioenergetics growth model for Nile tilapia ( Oreochromis niloticus) based on limiting nutrients and fish standing crop in fertilized ponds

A bioenergetics growth model for Nile tilapia in fertilized ponds, which linked Nile tilapia growth with limiting nutrients in pond water, was developed. The model incorporated six key variables affecting Nile tilapia growth in fertilized ponds: body size, water temperature, photoperiod, dissolved oxygen, unionized ammonia and food availability. In the model, food availability was estimated by a relative feeding level parameter, which was a function of potential net primary productivity based on limiting nutrients, and standing crop of Nile tilapia. The model was validated using growth data of Nile tilapia in 30 fertilized ponds, and successfully detected growth variations among ponds receiving the same nitrogen and phosphorus inputs. The model described 76% of the variance in growth in these ponds, and the relationship between predicted and observed growth rates had a slope of 0.93 and an intercept of 11.51, not significantly different from 1 and 0, respectively. The model indicated that the growth variations were caused by carbon limiting primary production during 55–99% of the culture period. Sensitivity analysis indicated that the parameters related to net energy from feeding were more sensitive than those related to fasting catabolism, and that the growth was most sensitive to photoperiod and then food availability when DO was above its critical limit (1.0 mg l −1), but was most sensitive to DO when it was below the critical limit. Tilapia growth was more sensitive to DO than UIA. Initial tilapia size was the least sensitive variable when UIA was above the critical limit in the model. Compared with previous models, this study provides a more reasonable and accurate way to estimate relative feeding level ( f) based on fish standing crop and potential net primary productivity derived by a limiting nutrient.

Diel Feeding Pattern and Ration of Two Sizes of Tilapia, Oreochromis spp. in Pond and Paddy Field

The diel feeding pattern, electivity and ration of tilapia, Oreochromis spp. (4.3-9.3 and 9.5-13.8 cm total length) were investigated to determine the ontogenetic shifts in diet and the importance of habitat on diet and feeding along with water quality characteristics from a nursery pond and a ricefield in Bangladesh. Feeding activity was continuous with diurnal variation in stomach content and per cent of fish feeding. Small tilapia were more active feeders than large ones. Both sizes exhibited a single feeding peak around afternoon-dusk in the pond but irregular feeding peaks in the ricefield. Stomach content did not differ qualitatively with size, ontogenetic shift in diet apparently was lacking, although resource exploitation was different in the two habitats. Periphytic detrital aggregate was the princi­pal diet in the paddy field while filamentous and colonial algae (Anabaena sp. and Melosira sp.) occurring as periphytic epipelon were the main food in the pond. Zooplankton was an insignificant dietary component in both habitats. Oreochrornis spp. was found to be an om­nivorous opportunistic-generalistic benthophagic browser or surface grazer. Its detritivory should not be confused with iliophagy. Feeding intensity and food consumption decreased with the increased fish size. Daily rations for small and large fish were estimated at 2.2 and 2.3% of bw in the pond and 0.91 and 0.45% of bw in the paddy field. Stomach content was computed to be completely evacuated in about 9-13 h in the pond and 1-4 h in the paddy field at water temperatures of 29.0-33.3°C. Total plankton density was higher in the pond than in the ricefield. Phytoplankton densities were higher in the pond while zoop­lankton densities were higher in the ricefield. Water quality properties were well within the acceptable ranges for aquaculture in both habitats. Lack of an ontogenetic dietary shifts suggest that caution should be taken in mixed-size rearing of tilapia.

刺し網のサイズ選択性の計算法とその水槽実験による検証

A method to assess size-selectivity of gillnets on the basis of variation of fish body forms was considered. Size-selectivity of a gillnet was defined as the probability both if a girth at anterior (around operculum) section of fish does not exceed a mesh perimetre and if the girth at the maximum body section exceeds it. In the present method, the probability distribution at each section against fish lengths were calculated according to normal distributions of the girth approximated with its means and standard deviations linearly correlated with lengths. Size-selectivity was derived from multiplication of the respective probability distributions. A series of tank experiments were conducted to obtain cartch ratios by tested gillnets for various sizes of tilapia, carp and rainbow trout. Selectivity curves of the tested gillnets for the species were calculated on measurements of body forms of the tested fish individuals, which were compared to the catch ratios. Regarding the wedged capture, the calculated selectivity curves were well supported by the catch ratios. It was concluded that the present method assesses e%-selectivity lengths of gillnets with errors less than 4% for species and size ranges caught by mainly the wedging function of gillnets.

Carbohydrate utilization versus body size in tilapia Oreochromis niloticus × O. aureus

1. 1. Two carbohydrate diets (glucose, starch) were each fed to two sizes of tilapia for 8 weeks. 2. 2. Large fish gained more body weight than small fish when fed the glucose diet, whereas similar weight gain, feed conversion ratio, body protein deposition and body energy deposition were measured in large and small fish when fed the starch diet. 3. 3. The body lipid content was higher in large fish than that in small fish irrespective of the carbohydrate sources, while small fish fed the starch diet had more body lipid than those fed the glucose diet. 4. 4. Plasma glucose level of tilapia fed the glucose diet peaked at 2 and 4 hr after the meal in small and large fish, respectively. When the starch diet was fed, no peak was observed in plasma glucose.