ALBERT

Notes: Abstract. Sections of the bottom of three earthen fish ponds in central Scotland were enclosed to prevent access by juvenile brown trout, Salmo trutta L., stocked at different densities from July to October. Monthly benthic samples were taken inside and outside the enclosures with a suction sampler. The benthos was dominated by Oligochaeta, whose density and biomass were significantly lower outside the enclosures; production was also lower outside but the P:B ratio was much higher. Total Chironomidae were more abundant outside, but not the larger predatory species which appeared to control the inside population. Species diversity of Oligochaeta and Chironomidae was greater outside while mean size was reduced, presumably as a result of size-selective predation by trout. Numbers of Mollusca and Asellidae were lower in the presence of fish, but Hirudinea and Sialidae were unaffected. Increasing fish density boosted chironomid numbers and biomass, indicating a predominant response to organic enrichment, but reduced the numbers of most other benthic groups presumably as a result of predation.

Notes: A polyculture experiment with the large carp rohu, Labeo rohita (Hamilton), catla, Catla catla (Hamilton) and either mrigal, Cirrhinus mrigala (Hamilton) or common carp, Cyprinus carpio (L.) (as cash crop fish), and the small indigenous fish punti, Puntius sophore (Hamilton) (as food for the small-scale farmer family) was carried out at the Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh. The main objective was to compare polycultures of large carp in which the bottom feeder is either the native mrigal or the exotic common carp. Secondary objectives were to assess the effects of adding the small indigenous species punti to polycultures of large carp, and to compare the effects of mrigal and common carp on punti production and reproduction. It was found that (i) common carp damaged embankments, had no effect on catla, improved rohu performance by 50% and total fish production by 20%; (ii) punti addition did not affect rohu, catla and total yield, improved mrigal performance by 50%, and decreased common carp performance by 20%; and (iii) punti was not affected either by common carp or by mrigal. However, its performance was not satisfactory, probably owing to frequent netting, which might have hindered growth and breeding. In spite of the embankment damage caused by common carp, this bottom feeder seems to be more promising than mrigal, because it leads to higher fish production. The addition of punti to the large carp polyculture is a viable proposition, as it does not reduce cash crop production, and might be a good food source for a small-scale farmer's family.

Notes: A polyculture experiment with the large carp rohu, catla and either mrigal or common carp (as cash crop fish), and the small indigenous fish punti (as food for the farmer's family) was carried out at Bangladesh Agricultural University, Mymensingh. The main objectives were to compare polycultures of large carp in which the bottom feeder is either the native mrigal or the exotic common carp, and to assess the effects of adding the small indigenous species punti to those polycultures. The results of fish–fish interactions and overall fish production have already been reported. The present paper presents the effects on the water quality, and discusses fish–environment interactions. The main conclusions are: time changes in the pond environment were stronger than fish composition effects. The main practice affecting water quality was liming, that incresed alkalinity, pH and water transparency and decreased ammonia. Rain affected photosynthesis and the match-mismatch of the two steps of nitrification. The more that bottom feeding fish species disrupt the mud bottom, the stronger their effects on pond environment. Common carp produce the strongest disruption of the mud bottom, followed by punti and then by mrigal. Mud disruption produced by common carp leads to a stronger liming effect, nutrient release into the water, and provides more particles that rain-floods wash out, facilitating the mismatch of the two steps of nitrification, and increased phosphorus adsorption into the mud bottom. Mud disruption by punti is only enough to improve the liming effect. Mud disruption by mrigal is the least, hence less particles are resuspended, nitrification is not affected during floods and relatively more phosphate remains in the water available for photosynthesis. The bottom feeder common carp can be seen not only as a target-cultured fish but also as a management tool. Farmers can get double benefit in introducing common carp in the ponds as it enhances the effectiveness of lime application and increases the availability of nutrients to phytoplankton. Through the manipulation of species in the polyculture alone, farmers can maintain the environment better and also reduce input costs.

Notes: Abstract. The fry of four species of planktivorous carp, silver carp, Hypophthalmichthys molitrix (Valenciennes), bighead carp, Aristichthys nobilis (Richardson), rohu, Labeo rohita (Hamilton), and catla, Catla catla (Hamilton), were grown in monoculture in ponds in Mymensingh, Bangladesh in order to determine electivity and the extent of dietary overlap. Other than an avoidance for Cyanobacteria, electivity was found to be low in most instances, although the low productivity of the ponds may have had some influence. Dietary overlap was greatest between catla and silver carp (0–52) and between catla and bighead carp (0–48).

Notes: The effects of four rates of application of fertilizer, with cow manure (3000 kg ha−1), urea (100 (kg ha−1) and triple super phosphate (TSP) (100 kg ha−1) (treatment F)), treatment F × 0.5 (treatment 0.5F), treatment F × 1.5 (treatment 1.5F) and treatment F × 2 (treatment 2F), on periphyton, plankton and water quality in tropical freshwater ponds were studied. The highest periphyton biomass in terms of dry matter (3.27 mg cm−2 substrate), ash-free dry matter (2.06 mg cm−2 substrate) and chlorophyll a (7.49 µg cm−2 substrate) developed in treatment 1.5F. The ash content of periphyton was lower in treatment 1.5F (38% of dry matter) than in other treatments (57–66% of dry matter). Total ammonia and chlorophyll a of water increased with fertilization rate. Treatment 1.5F (cow manure, urea and TSP at rates of 4500, 150 and 150 kg ha−1 respectively) appears to be the optimum, yielding high quantity and quality periphyton. By supplying a substrate area for periphyton equivalent to the pond surface, it was estimated that this level of fertilization could support a fish production of around 5000 kg ha−1 y−1, without recourse to supplementary food.

Notes: The project evaluated the effect of installing scrap bamboo (‘kanchi’) as a substrate for periphyton on growth and production of the indigenous major carp calbaush, Labeo calbasu (Hamilton). The impacts of fish grazing on the periphyton community were also assessed. Six ponds were used, three of which were provided with kanchi poles (700 per pond, spaced 30 cm apart). Ponds were limed and fertilized and stocked with L. calbasu fingerlings (mean total length = 5.16 cm; mean weight = 2.10 g) at a rate of 10 000 fingerlings ha–1 (75 fish per pond). There were no statistically significant differences in water quality between treatments, although differences in phytoplankton community composition were observed. Zooplankton numbers were the same in both treatments. While there was clear evidence that periphyton was being exploited by the fish, Chlorophycae being most affected, grazing was insufficient to cause significant reductions in total periphyton densities. Fish survival and specific growth rates (SGRs) were significantly higher in ponds with substrates, production in treatments with and without scrap bamboo substrate being 712.90 and 399.11 kg ha–1, respectively, over the 120-day period. However, production in both treatments was low in comparison with other studies, water temperatures (23.6–32.7 °C) being less than optimum for growth. It was concluded that kanchi and other locally available materials might be used to increase the production of some species of fish, although further evaluation of production economics is required.

Notes: This experiment was carried out in the framework of a project to develop a viable fish polyculture technology under Bangladeshi conditions that allows simultaneous fish production of small indigenous species for the farmers' family consumption and of large carp species as a cash crop. The objectives of this experiment were to assess the effects of adding punti and mola in different proportions on the large carp and on the environment, and to assess the effects of punti on mola and mola on punti. The polyculture included the large carp rohu, catla and common carp (as cash crop fish), and the small indigenous fish punti and mola (as food for the small-scale farmer family). The total large carp stocking density was 10 000 fish ha−1, at a species ratio of 1:1:1. The total small fish stocking densities were 0 in the control and 30 000 punti and mola ha−1 in the treatments, these at rates 2:1, 1:1 and 1:2. Stocking punti and mola at the density and all ratios tested were viable solutions to obtain simultaneously large carp cash crops and small fish to feed the farmer's family. Statistically marginal differences in large carp production were obtained in stocking small fish in the different proportions. These marginal differences could be explained by food competition between punti and common carp and between mola and rohu, which had different outcomes depending on the proportions of the small fish stocked. Stocking punti and mola at a 1:1 ratio would result in more small fish for the farmer's family, while the individual size of rohu, the most expensive large carp, would be somewhat smaller, but not necessarily small enough to decrease its selling price. Stocking one of the small fish in higher proportion than the other (2:1 or 1:2) would result in less mola for the family consumption, while harvesting of common carp would be somewhat lower and of smaller fish. Since common carp is the cheapest of the large carps, this small reduction would not necessarily affect the family income in an important way. With these results, farmers would now be able to reorganize their stocking practices with large carps and small fish and decide the appropriate small fish stocking ratios to meet their needs.

Notes: The effects of introduction of common carp, Cyprinus carpio (L.). in polyculture with major Indian carps and silver carp were studied in fertilized and fed earthen ponds in Mymensingh, Bangladesh. Bottom-dwelling common carp were found to exert an influence on the quality and quantity of planktonic food organisms. The presence of common carp significantly increased (P 〈 0.05) the growth of other carps including the bottom-feeder mrigal. Cirrhinus mrigala (Hamilton). A significantly higher fish yield (P 〈 0.05) was observed in the fish ponds containing common carp when the combined yields of all species in three treatments were compared.

Notes: The effects of periphyton, grown on bamboo substrates, on growth and production of two Indian major carps, rohu, Labeo rohita (Hamilton) and gonia, Labeo gonius (Linnaeus), were studied at the Bangladesh Agricultural University, Mymensingh. For each species, five ponds were provided with bamboo substrates and five ponds without substrate (control). Ponds were stocked at a rate of 10 000 ha−1 in both treatments. There was no discernible difference in water quality parameters between treatments. A large number of plankton (39 genera) showed periphytic nature and colonized the bamboo substrates. Rohu grew faster, resulting in a 77% higher net production (P 〈 0.05) in the ponds with bamboo substrates compared with the ponds without substrate. In contrast, the growth and production of gonia did not vary significantly (P 〉 0.05) between the substrate and control ponds. Rohu seems to be a more suitable candidate for periphyton-based aquaculture systems than gonia.

Notes: Abstract— A mathematical model is developed to predict the effect of weld toe undercut, misalignment and residual stresses on the fatigue behaviour of butt welded joints subjected to zero-to-tensile loading. Linear Elastic Fracture Mechanics (LEFM), Finite Element Analysis (FEA) and superposition approaches have been used for the modelling. It has been found that an undercut at the toes of welded joints is one of the most important weld geometry parameters. The reduction of fatigue strength of welded joints with a weld toe undercut is at least twice that of joints without an undercut in comparison with flush-ground welded plate. A misalignment of 5% of plate thickness and an undercut of 2% of plate thickness are fairly representative for the lower boundary of S-N curves of butt joints. The improvement of fatigue limits by means of surface treatments is shown to be effective for both undercut and misaligned joints. This approach is practical for a “Fitness-for-Purpose” assessment of welded joints subject to fatigue conditions.