ALBERT

Notizen: The sensitivity of any treatment comparisons in pond experiments is often limited by large variability among ponds. Standard techniques of increasing the number of replicate ponds to account for the large variability may be inappropriate as only a limited number of ponds may be available for any one experiment. This paper considers various ‘balanced incomplete block’ designs and compares their use with ‘completely randomized designs’ and ‘randomized complete block’ designs. With simulated data, it is shown that ‘balanced incomplete block’ designs can reduce the standard error of a treatment estimate by as much as 50%, and reduce confidence intervals by 25%, although increases of similar sizes may be experienced. The pattern of allocation of blocks to ponds by neighbour or by pond number shows no clear distinction in estimation improvement. Where missing ponds occur a large increase in imprecision may be experienced. These results are supported by data from non-uniformity experiments. Further work is needed to explore block structures for specific types of treatment that may influence the patterns of variability to different extents.

Notizen: A decline in the quality of the freshwater fish seed available to small-scale farmers in Asia has been identified as an important constraint on the development of aquaculture in the region. There is a need for simple and reliable methods of determining the quality of seed at the farm gate or hatchery. The present study was carried out, in the field, to develop and evaluate a stress challenge test for Nile tilapia (Oreochromis niloticus, Linn.) fry as the effects of stress on fish performance have been well documented and similar tests are used elsewhere in the sector. Formalin and iodized salt were used as reference toxicants. LC50 values were estimated using mortality figures resulting from exposure of marketable-sized fry to saline and formalin solutions. The two test concentrations, 24 ppt for saline and 500 p.p.m. for formalin, were used as reference toxicants in acute, static bioassays to challenge fry reared under different conditions during the post sex-reversal, nursery period. Using existing farm practice as controls the conditions were: 50% increased stocking density, formalin treatment, 40% reduction in feed rate and no supplementary feed. Fry length proved to be a very significant co-variable; adjusted means showed no significant difference between treatments. The importance of only marketing tilapia fry weighing over 0.2 g was emphasized. It was recommended that the challenge testing approach could be modified, but that it was vulnerable to unquantifiable and uncontrollable factors arising from management and the environment, which may have altered responses to the stressor.

Notizen: Two experiments (E1 and E2) to assess the performance of tilapia broodstock and tilapia sex-reversed fry in overwintering were conducted at the Research Institute for Aquaculture No.1 (RIA-1) in the cold seasons of 1995–96 and 1996–97. Nile tilapia Oreochromis niloticus (L.) broodstock of the Thai, GIFT, Egypt and Viet strains were overwintered in deep and shallow ponds, as well as in deep and shallow hapas suspended in a single deep pond for evaluation of the influence of overwintering systems on the survival and growth of fish. Large (〉 1 g) and small (〈 1 g) tilapia seed were overwintered in deep hapas-in-ponds for comparison of their performance. In 1995–96, the coldest pond water temperature was 10–11 °C, and survival of tilapia broodfish overwintered in deep and shallow hapas-in-ponds was 99.6–100%. This was significantly (P 〈 0.05) higher than fish stocked in deep and shallow ponds (74.4–90%). The survival rate of larger monosex tilapia fry was 54%, which was significantly (P 〈 0.05) higher than that of smaller fry (33.4%). In 1996–97, the lowest pond water temperature was 15.8 °C, and fry showed similarly high survival rates in all treatments (97–100%). There was no significant difference between fry in the two size classes. The results of this study clearly indicate that hapas-in-ponds are useful for reducing the risk and improving the survival of tilapia broodstock and fry in the cold season. Differences in the decline in ambient temperatures year on year mean that the need for special overwintering conditions varies. Hapas-in-ponds are a low-cost overwintering method that can be one of the appropriate strategies for tilapia seed production under the variable, cool temperature regimes in northern Vietnam.

Notizen: Direct use of pig wastes as inputs into fish culture systems may be unacceptable or an inferior use of valuable inputs. High value, but non-filter feeding fish, such as African catfish, Clarias gariepinus (Burchell), may be unable to recover nutrients efficiently through the pond food web and require complete diets in intensive culture. Live feeds such as the larval stage of the green blow fly. Lucilia sericata, can be used as intermediate organisms to utilize pig waste and subsequently be fed live as part of a complete ration for catfish raised in cages. The nutrient efficiency of the system is further enhanced by the stocking of phytophagous fish, the Nile tilapia, Oreochromis niloticus (L.), in the pond in which the catfish culture cages are suspended. A model derived from on-farm experimentation is presented that demonstrates system design and nutrient efficiencies. An extrapolated catfish production of 61 year1 using only fly larvae produced from a standing herd of approximately 1000 fattening pigs was demonstrated. The static water pond in which the catfish were cultured ensured that the environmental impact of both pig and catfish systems was minimal compared to conventional production systems.

Notizen: This study investigated the effects of nursing duration on the subsequent performance of rohu (R) Labeo rohita and mrigal (M) Cirrhina mrigala in polyculture with monosex male Nile tilapia (T) Oreochromis niloticus at four levels of pond fertilization. Nile tilapia, rohu and mrigal were stocked at a ratio of 4:1:1 in a 90-day trial based on 40 20-m2 pens fixed in four 400-m2 earthen ponds. Growth of carp fingerlings during prolonged nursing (5 or 12 months) was stunted compared with fish nursed over a conventional duration of 3 months (3) but showed superior growth subsequently. Mean daily weight gain of stunted rohu (12) ranged from 2.2 to 2.8 g per fish day−1 compared with 1.1–1.6 g per fish day−1 for younger fish (3). The comparable ranges for mrigal were 1.9–2.8 and 1.4–2.1 g per fish day−1. Growth of Nile tilapia was inversely related to duration of carp nursing at the four levels of fertilization. Nile tilapia showed more response to increasing levels of fertilizer input (Y=−1.421+1.716X, where Y is the daily weight gain of Nile tilapia and X is the fertilizer level, r2=0.98, P〈0.01, n=12). At a high level of fertilization (3.0 kg N:1.5 kg P ha−1 day−1), performance of stunted fingerlings (5 and 12) of both rohu and mrigal was similar (range 2.3–2.8 g per fish day−1, P〉0.05), but younger mrigal (M3) grew faster than rohu (2.1 g per fish day−1 and 1.6 g per fish day−1 respectively). Older rohu (12) appeared to perform particularly well, and Nile tilapia poorly at the lowest level of fertilization (1.5 N:0.75 kg P ha−1 day−1), suggesting the impact of age of seed on competition within polycultures. The net fish yield (NFY) of tilapia was not affected significantly (P〉0.05) by differential stocking age of carps; therefore, combined NFY of the three experimental fish species was not affected by the age of carp, as tilapia was the dominant species in polyculture. The highest combined NFY of all species in the most intensively fertilized pond (3.0 N:1.5 P kg ha−1 day−1) was calculated at 4.06±0.08 g·m−2 day−1, which was significantly higher (P〈0.001) than the yield (1.82±0.12 g·m−2 day−1) from the pond with the lowest fertilization. At the highest fertilizer level, tilapia, rohu and mrigal contributed 72%, 14% and 14%, respectively, to the NFY, whereas the ratio was 60%, 20% and 20% at the lowest fertilization level. The study indicated that yields from tilapia in polyculture with the two carp species in more eutrophic water can be optimized if advanced nursing of carps is practised. Moreover, higher inputs of inorganic fertilizer and advanced nursing of carp are economically attractive under Bangladeshi conditions. Advanced nursing of rohu also improves its performance in more extensive systems when tilapia densities are high.

Notizen: Traditional artisanal aquaculture systems are commonly assumed to be mainly for subsistence, to use predominantly on-farm inputs, and to have been developed by farmers themselves. Such systems with a long history in South East Asia exist mainly in northern Lao PDR, northern Vietnam and in West Java. In most other areas the traditional fish supply, wild fish, has declined only relatively recently, providing a stimulus for growth of aquaculture over the past few decades. An overview of artisanal aquaculture so defined in the South East Asian region is presented from a systems context considering social and economic aspects (micro- and macro-level perspectives), production technology (rice fields, ponds, cages), and environmental aspects (fitting into the local resource base without adverse environmental impact). Most artisanal aquaculture systems are integrated with crops and livestock but generally resource-poor farms constrain production. Rising expectations mean that productivity must be enhanced by off-farm inputs for aquaculture to contribute significantly to the farm household livelihood system. It is proposed that the term ‘small-scale’ be used rather than ‘artisanal’ because of increasing farmer interest in income rather than subsistence, because of increasing use of off-farm inputs, and because of the increasingly important role of science in the promotion of such systems.

Notizen: Substrate-spawning Tilapia zillii failed to spawn in crowded holding tanks but exhibited a marked tendency to spawn soon after transfer to individually partitioned aquaria. Confined conditions suppressed the levels of serum 17β-oestradiol (E2) and testosterone (T) in females, which remained low throughout the period of confinement. Levels of both steroids rose significantly following transfer of fish to individual aquaria and were maintained at consistently higher levels than those of fish which remained confined. Proportions of stage 3 (late perinucleolar) oocytes were significantly lower (P〈0.001) in individual fish 21–30 days after transfer, coincident with significantly higher (P〈0.01) proportions of stage 6 or 7 oocytes (late vitellogenic or maturing oocytes). However, no significant differences (P≥0.05) were detected between individual or confined groups of fish in the relative proportions of stage 2 (early perinucleolar), stage 4 (cortical alveolar) or stage 5 (early vitellogenic) oocytes. Atresia increased as the period of confinement increased. Following return of individual fish to confined conditions, blood steroids fell rapidly to levels seen in fish that had remained confined throughout. It is suggested that the reduced sex steroid levels detected during confinement were insufficient to allow developing oocytes to complete oocyte growth and maturation. The detection of significantly lower proportions of stage 3 and significantly higher proportions of stage 6/7 oocytes soon after transfer suggest that fish were preparing one batch of oocytes for spawning (stage 6/7 oocytes) whilst recruiting from a pool of previtellogenic oocytes (stage 3) for the ensuing spawning cycle.