Increasing fish production from wetlands at Lake Victoria, Uganda using organically manured seasonal wetland fish ponds

The processes driving primary productivity and its impacts on fish production were investigated in field trials in eight seasonal earthen wetland ponds ‘Fingerponds’ (192 m²) in Uganda between 2003 and 2005. The ponds were stocked by the seasonal flood with predominantly Oreochromis spp. at densities ranging from 0.1 to 0.5 fish m⁻². Chicken manure (521, 833 or 1,563 kg ha⁻¹) was applied fortnightly. Results showed that primary productivity was enhanced with maximum average net primary productivity (±Standard Error) of 11.7 (±2.5) g O₂ m⁻² day⁻¹ at the Gaba site and 8.3 (±1.5) g O₂ m⁻² day⁻¹ at the Walukuba site. Net fish yields were higher in manured ponds with up to 2,670 kg ha⁻¹ yield for a 310 day growth period compared to less than 700 kg ha⁻¹ in unmanured ponds. Fish production was limited mainly by high recruitment, falling water levels, light limitation from high suspended solids and turbidity, and low zooplankton biomass. It was concluded that Fingerponds have a high potential for sustainable fish production and can contribute to the alleviation of protein shortages amongst the riparian communities around Lake Victoria. Production can be enhanced further with improved stock management.     

Growth and body composition of rohu, Labeo rohita (Hamilton), fed compound diet: winter feeding and rearing to marketable size

A 122‐day growth trial was conducted to observe the impact of polyhouse in winter on growth, conversion efficiencies and body composition of fingerling Labeo rohita (7.9 ± 1.1 cm; 4.60 ± 0.18 g). Fish were stocked (1.5 m^?2) in polyhouse and outdoor concrete tanks (250 m²) in duplicate and fed a compound diet [35% crude protein (CP)] to apparent satiation twice daily, at 09.00 and 17.00 hours. A natural photoperiod was maintained during the trials. At the end of the trials, polyhouse‐reared fish produced significantly (P < 0.01) higher values for weight increment, specific growth rate (SGR %), protein efficiency rate (PER), protein productive value (PPV) and better feed conversion ratio (FCR). Polyhouse‐reared fish showed higher (P < 0.01) CP and fat, and lower (P < 0.01) moisture and ash contents in the muscle. In the second part of the study, which also lasted 122 days, polyhouse‐reared young L. rohita (21.7 ± 1.3 cm; 127.8 ± 0.69 g) were raised to marketable size in outdoor earthen ponds (500 m²) at a stocking density of 0.37 m^?2 to examine the effects of feeding traditional feed mixture and compound diet (30% CP) on growth and body composition of fish. Labeo rohita fed the compound diet showed higher (P < 0.01) values for weight increment, SGR (%), PER and PPV, and better (P < 0.01) FCR than those fed traditional feed. Proximate composition of fish muscle showed higher (P < 0.05) CP and fat, and lower (P < 0.05) moisture and ash contents in fish fed the compound diet. The results suggest that polyhouse may be used successfully to culture L. rohita during winter, and that thereafter the fish may be reared in earthen ponds using the compound diet to obtain better production.     

Optimal stocking density for beluga,Huso huso,and ship sturgeon,Acipenser nudiventris during the grow‐out phase

The influence of three different initial stocking densities (SD) in flowthrough systems was evaluated on growth performance and feed utilizaition in beluga, Huso huso, and ship sturgeon, Acipenser nudiventris, juveniles in three different grow‐out phases for 228 days. In each grow‐out phase, fish were randomly distributed in 18 concrete square tanks (2.0 × 1.0 × 1.2 m³; 2 m³ of volume) according to the experimental SD. At the first phase, both fish species were subjected to three SD including 1.5, 3 and 6 kg/m² for 60 days. In the second phase, fish were submitted to three SD including 3, 6 and 9 kg/m², whereas in the third phase, beluga were stocked at 6, 9 and 12 kg/m² and ship sturgeon were stocked at 6, 8 and 10 kg/m² and each phase lasted for 12 weeks. In both species, the growth performance and feed intake (FI) significantly decreased with increasing SD at the end of the first phase (p < 0.05). In the second phase, growth performance and FI in beluga decreased with increasing SD, whereas in ship sturgeon neither growth nor FI were affected by SD. Interestingly, the growth performance and feed utilization in both species were not decreased with increasing SD in the third phase indicating adaptability of these species to high stocking density during grow‐out period. According to the result of this study, the appropriate initial SD for beluga within the range of 100–500, 500–2,000 and 1,500–3,000 g were at 1.5, 6 and 12 kg/m², respectively in an open flow‐throw system. Regarding to ship sturgeon, initial SD at 1.5 and 10 kg/m²recommended for fish within the range of 100–300 and 300–1,200 g, respectively in an open flow‐throw system.     

Growth and contribution of stocked channel catfish,Ictalurus punctatus (Rafinesque, 1818): the importance of measuring post‐stocking performance

In this study it was sought to quantify post‐stocking growth, survival, and contribution of advanced size (178 mm total length [TL]) channel catfish Ictalurus punctatus fingerlings, something rarely done. Channel catfish populations were evaluated before (May 2010) and after (May to August 2011 and 2012) stocking. Relative abundance, stocking contribution, and growth were different (P < 0.05) in the two study impoundments (lakes Lone Chimney and Greenleaf, Oklahoma). For fish stocked in Lake Lone Chimney, stocking contribution was lower (3–35%), and average length and weight of stocked fish by age‐2 reached 230 mm TL and 85 g, whereas the stocking contribution (84–98%) and growth in length (340 mm TL) and weight (280 g) were higher by age‐2 in Lake Greenleaf. Given these unambiguous differences of post‐stocking performance, benchmark metrics that represent population‐level information such as relative abundance and average length and weight of the sample masked these significant differences, highlighting the importance of marking hatchery‐fish and then following them through time to determine the effectiveness of stocking. These results suggest that stock enhancement programmes would benefit from studies that quantify post‐stocking performance of hatchery fish.     

Mass culture ofTilapia zillii [Cichlidae] in pond ecosystems

The culture ofTilapia zillii (Gervais) in 18 and 34 cm deep, 5.5×7.6 m earthen ponds at 3 initial stocking densities, 6, 9 and 12 pairs, revealed an optimum 9 pair initial density for the maximum production of fry after 104 days. However, biomass increase was significantly greater at the 6 pair stocking density and in deeper ponds. Pond depth had no significant effect on size and weight gain of parental fish. Nest and brooding tube construction was variously influenced by pond depth and stocking density. Partial shading of ponds, while not affecting the maximum water temperature, did apparently reduce fish biomass production, the average length and weight of fry, and the average depth of nests. An estimated 1 million fish could be produced in 104 days from 190 similar ponds with a water surface equivalent to 0.8 hectares. This would be equivalent to ca. 725 kg fish biomass production and would require ca. 1,900 kg of catfish pellets or their equivalent. Possible ways to increase rearing efficiency are discussed.     

Evolving water quality in a common carp and blue tilapia high production pond

Intensive polyculture of blue tilapia, Oreochromis aureus, and common carp, Cyprinus carpio was conducted to determine their growth capacity and the accompanying water quality transitions in a 376 m² (207 m³) fish pond without aeration or water exchange. A total of 485 fingerlings (290 tilapia and 195 carp) averaging 72 g each (total 35 kg) was stocked and fed commercial floating pellets (36% protein-6%-N) at the approximate daily rate of 3% of the fish biomass six days each week. The total net fish growth was 159.4 kg (4.2 t ha^–1) with an average growth of 199.5 g per fish through the 96 days feeding period — 6 June to 7 September 1984. The s conversion ratio for the commercial feed was 1.3.Physiochemical patterns in the pond water were directly related to the rise and fall of turbidity through the course of the production trial. During one period of low turbidity, a biological inversion was evident with pH and dissolved oxygen measured at higher levels near the pond bottom and increasingly lower toward the surface. It is postulated that settled phytoplankton formed a photosynthetically-active algal mat on the pond bottom. The implications for potential fish kills can be significant.     

Preliminary investigation of an integrated aquaculture–wetland ecosystem using tertiary-treated municipal wastewater in Los Angeles County, California

The objective of this study was to determine the feasibility of using a designed integrated aquaculture–wetland ecosystem (AWE) for experimental food production and inorganic nitrogen removal from tertiary-treated wastewater. The AWE connected polyculture aquaculture ponds with in-pond aquatic plant systems (water hyacinths, Eichhornia crassipes, and Chinese water spinach, Ipomea aquatica), a solar energy aeration system, and an artificial wetland. Ponds were stocked with hybrid tilapia (Oreochromis mossambicus×O. urolepis hornorum), common carp (Cyprinus carpio), mosquitofish (Gambusia affinis), and red swamp crayfish (Procambarus clarkii), and were flushed weekly with new wastewater at 20%. Fish were fed a 32% protein floating ration at 1% fish body weight per day, and wheat bran was added at 1 mg l⁻¹ when water conductivities exceeded 900 μmhos cm⁻¹. Plants were allowed to grow until they reached approximately 50% of the pond surface area, then maintained at this area by manual harvesting. Pond water quality (temperature, conductivity, pH, oxygen) was monitored twice daily, and weekly water samples were taken for analyses of inorganic nitrogen (ammonia and nitrate-N) in the ponds, wetland, and wetland discharge waters (n=30). Tilapia harvest from three ponds was 1134.5 kg. Fish standing crop biomass increased from 0.16 to 0.21 at stocking to 1.50–2.00 kg m⁻³ at harvest. Tilapia grew from an average stocking weight of 21 to 362–404 g at harvest but had poor survival (48–64%) due to heavy bird predation. Total food conversion ratios ranged 0.9–1.2. Approximately 70% of the tilapia were marketed live at $2.20 kg⁻¹. An estimated standing crop of 1.4 tons wet weight of Ipomea aquatica grew luxuriantly in one 200-m² polyculture pond which could be harvested sustainably at 20 kg week⁻¹. Water hyacinths removed approximately 90% of the ammonia and nitrate-N in wastewater, and the wetland removed an additional 7% (total removal was 97% of wastewater input concentrations). Overflow water exiting the wetland had less than 0.4 mg ammonia–nitrogen l⁻¹ and no detectable nitrate–nitrogen. The experimental AWE accomplished aquatic food production and almost complete removal of inorganic nitrogen from wastewater, functioning as a `quartenary' wastewater treatment/food production ecosystem. However, more rigorous experimentation is required to optimize fish- and plant-carrying capacities, nutrient cycles, and testing for bioaccumulation of metals in order for the AWE to be socially and economically relevant. The concept of using tertiary-treated wastewater for aquatic food production may be attractive in the peri-urban areas of many meagcities like Los Angeles, both for fish markets and to stem the growing discharges of wastewaters that are causing coastal pollution.     

Fatal,virus-associated peripheral neuropathy and retinopathy in farmed Penaeus monodon in eastern Australia. II. Outbreak descriptions

Outbreaks of 'peripheral neuropathy and retinopathy' (PNR) occurring during 2 consecutive growout periods (typically October-April) are described for an intensive Penaeus monodon farm in eastern Australia. In the 1998/99 growout period, outbreaks graded minor to severe occurred in 22 of 25 ponds, 12 to 25 wk post-stocking. In the severely affected index pond, harvested 8 wk after outbreak recognition in mid-January, estimated survival for the period late December to harvest was 50%. Minor to moderate losses could be attributed to PNR in the other ponds. Mean survival over the same period for the 14 ponds harvested within 5 wk of outbreak recognition was 93% (83 to 100%); for the 7 ponds harvested 5 to 8 wk after outbreak recognition was 79% (67 to 92%) and for the 3 unaffected ponds was 90% (86 to 95%). Analysis indicated a significantly lower risk (Fisher's exact p = 0.016) of an outbreak in the 2 ponds stocked only with postlarvae from one hatchery (D) versus the 18 ponds stocked only with postlarvae from 3 other hatcheries (A, B and C). In the 1999/2000 growout period, minor to severe PNR outbreaks occurred in all 26 ponds, each stocked with postlarvae from the same hatchery (E), 19 to 21 wk post-stocking. Stocking date in 1999/2000 appeared to influence PNR outbreak severity; for ponds stocked on 2 of the 7 stocking dates versus those stocked on remaining dates, the crude relative risks (CRR) of a severe outbreak, or either a moderate or severe outbreak, were 11.25 (1.55 < CRR < 81.40) and 2.63 (1.30 < CRR < 5.31), respectively. Although inconclusive, study findings are consistent with the hypothesis that 'gill-associated virus' (GAV), the putative causal pathogen identified in a separate pathological study, entered ponds via postlarvae, and that prevalence and/or severity of infection within postlarval batches influenced outbreak severity. The generally high survival in ponds harvested soon after outbreak recognition, together with PNR prevalence of approximately 50% in prawns collected from 4 ponds 7 wk before those ponds were recognised as affected, also suggest that GAV is highly infectious and that PNR has a relatively long incubation period and/or clinical course.     

Effect of stocking strategy on distribution and recapture rate of common carp Cyprinus carpio L., in a large and shallow temperate lake: implications for recreational put‐and‐take fisheries management

It is hypothesized that the stocking procedure influences survival, growth and distribution of introduced fishes; however, there is still limited information on the effect of various stocking strategies on recaptures in natural freshwaters. The present study aim was to investigate how the rate and distribution of anglers' catches of common carp (Cyprinus carpio) vary with the stocking season (spring, summer and autumn), lake area, method (shore and offshore releases), and fish size (≤500 and >500 g) in the large and shallow Lake Balaton, Hungary. In 2010, 4500 two‐summer‐old individually‐tagged common carp were stocked to test 36 release set‐ups (three seasons × three lake areas × two methods × two size groups). Anglers reported the date, location and fish size (standard length and weight) on 787 recaptures within 2 years after the release. Recapture rate was highest in summer and lowest in autumn stockings, but was not affected by the stocking area, method or fish size. Regarding space, the widest dispersals were in recaptures in autumn and in the centre of the lake, but fish movement was not influenced by the stocking method or fish size. In conclusion, in summer the stocking quotas should be evenly distributed along the entire shoreline; early spring stockings may be optimized for transport costs and concentrated by each lake basin. Late autumn stockings should be avoided, and the capacity of effective wintering ponds should be developed. This study also provides a good framework for testing fisheries management alternatives in other intensively fished habitats.     

As integrated culture system for outdoor production of microalgae and cyanobacteria

Cultivation of microalgae in closed photobioreactors suffers from high temperatures, whereas in open raceway ponds the optimal temperature is seldom reached. With an integrated device coupling a raceway pond with a suitably sized and positioned closed system (alveolar panel), the heat accumulated in the panel was efficiently transferred to the pond and a near-optimum temperature regimen for Arthrospira platensis was maintained, with no need for any additional cooling device. The productivity obtained in the integrated system was higher than the sum of the productivities (g reactor⁻¹ day⁻¹) of the pond and panel systems operating separately. This revised version was published online in September 2006 with corrections to the Cover Date.     

Effects of fish density on planktonic communities and water quality in a manipulated forest pond

We examined the effects of fish on lower trophic levels in a small pond in eastern Finland. The pond was divided into four sections with plastic curtains and stocked with crucian carp (Carassius carassius); two sections had low (4.4–5.5 g m^–3) and two high (10.4–13.7 g m^–3) densities of fish. In the summer of 1987, the pond was sampled weekly for phyto- and zooplankton until the fish were removed by rotenone in September after a three month experiment. Fish density as well as the extent of macrophyte cover had a considerable impact on planktonic communities and water quality. Mean zooplankton biomass was significantly lower and phytoplankton biomass higher, at high fish density. Water transparency was correlated negatively with chlorophyll-a at low fish density but turbidity appeared to reduce transparency at high fish density. The composition and dynamics of the plankton also differed at different fish densities. The mechanisms behind these effects, and the influences of habitat and fish behaviour on the results, are discussed.     

Pathways of ornamental and aquarium fish introductions into urban ponds of Epping Forest (London,England): the human vector*

To examine the role of humans in the non‐native fish introductions, we measured the frequency of occurrence and density of non‐native fishes in ponds (Epping Forest, Essex, England) that had been restored (drained of water and voided of fish or treated with rotenone) on a known date and into which no piscivorous or non‐native fishes had subsequently been stocked intentionally. For each pond, the period of time since pond restoration, pond area, distance to nearest residential housing, distance to nearest footpath, distance to nearest water body or stream, and the proportion of pond vegetated were measured. The occurrence of both non‐native and unexpected native fish species was non‐random, and the number of ornamental varieties was found to increase as pond distance from the nearest road decreased. Variety richness of each of three categories of fish (non‐native, goldfish Carassius auratus and native) was significantly correlated with at least two of the following variables: distance from nearest road, nearest footpath and nearest pond. The rate of non‐native fish introductions (adjusted variety richness per year) could also be estimated from pond distance to the nearest road, being about 3.5 ornamental varieties introduced per year in ponds adjacent to roads, but the rate appears to be much greater in ponds that had recently (<1.5 years) undergone restoration. Implications for conservation and management, as well as the potential role of societal issues such as recreational activities, cultural and religious practices, are discussed.     

Recovery from a fish kill in a semi‐arid Australian river: Can stocking augment natural recruitment processes?

Localized catastrophic events can dramatically affect fish populations. Management interventions, such as stocking, are often undertaken to re‐establish populations that have experienced such events. Evaluations of the effectiveness of these interventions are required to inform future management actions. Multiple hypoxic blackwater events in 2010–2011 substantially reduced fish communities in the Edward‐Wakool river system in the southern Murray‐Darling Basin, New South Wales, Australia. These events led to extensive fish kills across large sections of the entire system following a period of prolonged drought. To expedite recovery efforts, 119 661 golden perch Macquaria ambigua and 59 088 Murray cod Maccullochella peelii fingerlings were stocked at five locations over 3 years. All fish stocked were chemically marked with calcein to enable retrospective evaluation of wild or hatchery origin. Targeted collections were undertaken 3 years post‐stocking to investigate the relative contribution of stocking efforts and recovery via natural recruitment in the system. Of the golden perch retained for annual ageing (n = 93) only nine were of an age that could have coincided with stocking activities. Of those, six were stocked. The dominant year‐class of golden perch were spawned in 2009; before the stocking programme began and prior to blackwater events. All Murray cod retained (n = 136) were of an age that coincided with stocking activities, although only eight were stocked. Among the Murray cod captured, the dominant year‐class was spawned in 2011, after the blackwater events occurred. The results from this study provide first evidence that natural spawning and recruitment, and possibly immigration, were the main drivers of golden perch and Murray cod recovery following catastrophic fish kills. Interpreted in the context of other recent examples, the collective results indicate limited benefit of stocking to existing connected populations already naturally recruiting in riverine systems.     

Stocking density positively influences the yield and farm profitability in cage aquaculture of sutchi catfish, Pangasius sutchi

The sutchi catfish (Pangasius sutchi Fowler 1937) was cultured at 10 different densities in cages suspended in a river‐fed channel during the summer of 2000. Catfish fingerlings (mean length 9.14–9.74 cm; mean weight 5.92–6.7 g) were stocked at densities of 60, 70, 80, 90, 100, 110, 120, 130, 140, and 150 fish m^?3, which were equivalent to 397, 427, 488, 551, 604, 656, 710, 871, 981, and 899 g m^?3 respectively. At the end of 150 days, the growth and yield parameters were studied and a simple economic analysis was performed to calculate profitability. Gross yields were 15.6 ± 0.27, 17.1 ± 0.31, 19.5 ± 0.30, 21.9 ± 0.29, 26.8 ± 0.22, 28.6 ± 0.40, 30.0 ± 0.37, 31.1 ± 0.45, 32.7 ± 0.31, and 34.5 ± 0.44 kg m^?3; net yields were 15.2 ± 0.22, 16.7 ± 0.28, 19.0 ± 0.29, 21.3 ± 0.21, 26.2 ± 0.19, 27.9 ± 0.33, 29.3 ± 0.33, 30.3±0.37, 31.8 ± 0.29 and 33.5 ± 0.36 kg m^?3, respectively, with stocking densities of 60, 70, 80, 90, 100, 110, 120, 130, 140, and 150 fish m^?3. Mean weights of fish at harvest were inversely related to stocking density. Both gross and net yields were significantly different and were directly influenced by stocking density, but the survival rates and feed conversion were unaffected. Higher stocking density resulted in higher yield per unit of production cost and lower cost per unit of yield, with a net revenue being higher with increasing stocking density. It was concluded that the density of 150 fish m^?3 produced the best results among the densities tested in this experiment.     

Comparison between serum values of pond and intensive raceway cultured channel catfish Ictalurus punctatus (Rafinesque)

Blood serum components in channel catfish ( Ictalurus punctatus ) were compared between raceway (with low dissolved oxygen) and pond populations. Normal limits were defined for the pond populations, for each of 18 variables, as ± two standard deviations. Carbon dioxide, blood urea nitrogen, total protein, uric acid, creatinine, sodium, and total bilirubin were all lower in fish raised in raceways. Carbon dioxide and sodium had the greatest reductions; all reductions were statistically significant.
Alkaline phosphatase, inorganic phosphorus, calcium, cholesterol, and glucose values were higher in raceway fish, but only alkaline phosphatase and glucose were significantly higher.     

The influence of white seabream (Diplodus sargus) production on macrobenthic colonization patterns

The present work evaluates the influence of fish production on macrobenthic colonization over large areas (approximately 700 m²), where the colonizing populations are not nearby the disturbed area. Sampling was undertaken within newly created aquaculture earthen ponds under two contrasting conditions: white seabream (Diplodus sargus) production and no production (control). Macrobenthic and geochemical samples were collected 7, 23, 54, 93 and 180 days after filling the earthen ponds with seawater pumped from a water reservoir for the first time. The water reservoir was also sampled, and is used as a reference for the colonizing populations. Macrobenthic colonization rate in the ponds was low, probably due to the isolation of the disturbed habitat, to the large size of the defaunated area, and possibly to geochemical constraints. Initial colonization was by insect larvae (mainly chironomids), the bivalves Cerastoderma spp., the polychaetes Pseudopolydora paucibranchiata and Hydrodoides elegans, and nemerteneans. The number of species was similar in control and production ponds, even though under production higher total abundance values were observed. Although well represented in the water reservoir, the amphipod Microdeutopus gryllotalpa was only observed within the new ponds after 6 months. Preliminary results suggest that macrobenthic colonization patterns were influenced by fish production, as assemblages were significantly different among ponds. Higher food availability due to fish production may explain the results obtained, but ecological reasons, such as predation, may also contribute for shaping the macrobenthic communities.     

Characterizing the trophic niches of stocked and resident cyprinid fishes: consistency in partitioning over time,space and body sizes

Hatchery‐reared fish are commonly stocked into freshwaters to enhance recreational angling. As these fishes are often of high trophic position and attain relatively large sizes, they potentially interact with functionally similar resident fishes and modify food‐web structure. Hatchery‐reared barbel Barbus barbus are frequently stocked to enhance riverine cyprinid fish communities in Europe; these fish can survive for over 20 years and exceed 8 kg. Here, their trophic consequences for resident fish communities were tested using cohabitation studies, mainly involving chub Squalius cephalus, a similarly large‐bodied, omnivorous and long‐lived species. These studies were completed over three spatial scales: pond mesocosms, two streams and three lowland rivers, and used stable isotope analysis. Experiments in mesocosms over 100 days revealed rapid formation of dietary specializations and discrete trophic niches in juvenile B. barbus and S. cephalus. This niche partitioning between the species was also apparent in the streams over 2 years. In the lowland rivers, where fish were mature individuals within established populations, this pattern was also generally apparent in fishes of much larger body sizes. Thus, the stocking of these hatchery‐reared fish only incurred minor consequences for the trophic ecology of resident fish, with strong patterns of trophic niche partitioning and diet specialization. Application of these results to decision‐making frameworks should enable managers to make objective decisions on whether cyprinid fish should be stocked into lowland rivers according to ecological risk.     

Swimming, grazing and social behaviour of rohu Labeo rohita (Hamilton) and common carp Cyprinus carpio (L.) in tanks under fed and non-fed conditions

Fish behaviour was monitored in 1-m² tanks each stocked with three 67-g rohu (Labeo rohita). In addition, 80-g common carp (Cyprinus carpio) were stocked at 0, 1 or 2 fish per tank. All tanks were fertilized prior to stocking to stimulate natural food production. In addition, half of the tanks were fed a supplemental diet. Results confirm the general view that rohu mainly lives and feeds in the water column, while common carp is a bottom feeder. In the presence of common carp, rohu increased the time spend grazing on tank wall and bottom, and decreasing grazing time in the water column. Supplemental feeding had a similar, be it less pronounced effect. When both present, rohu and common carp spend 47–52% of their time together. Rohu spends more time close to the bottom in the presence of common carp than when no common carp is present, presumably to profit from increased zooplankton production, triggered by the resuspension of nutrients by burrowing common carp. This was reflected in a higher growth rate of rohu in the presence of carp. These effects were stronger with one common carp per tank than with two common carp. In this study, the results from behavioural observations in tanks nicely complemented results from a pond study analyzing growth, production and food availability. Behavioural observations in tank yielded useful additional information helpful to clarify species interactions and feeding ecology in polyculture ponds.     

Cohort‐specific estimates of first‐year survival are positively associated with size at stocking for lake sturgeon Acipenser fulvescens (Rafinesque 1817) stocked in Black Lake,Michigan, USA

The authors conducted a gillnet survey in 2013 in Black Lake, Michigan, USA to evaluate the lake sturgeon (Acipenser fulvescens) stocking programme that began in 2001. Objectives were to (i) estimate year‐class specific abundance of juvenile lake sturgeon in Black Lake; and (ii) determine year‐class specific survival of stocked year classes and determine whether year‐class‐specific first‐year survival was related to average size at the time of stocking. Deployed were 15 and 20 cm stretch mesh gillnets at 72 randomly selected sites in Black Lake over a 3‐week survey using a Schnabel multiple‐mark, multiple‐recapture estimator to determine overall abundance of stocked fish. Ages for captured fish were determined from fin ray cross sections and the presence of coded wire tags, and apportioned the overall abundance estimate of juveniles to year class using an age‐length key. Overall survival estimates were calculated by dividing the year‐class specific abundance estimates by the number of fish stocked that year. Also evaluated was the relationship between first‐year survival and average total length (TL) at time of stocking using logistic regression. Overall survival from stocking to 2013 ranged from 0.03 to 0.53. First‐year survival was positively associated with average TL at stocking, and ranged from 0.05 for fish stocked at 9 cm TL to 0.84 for fish stocked at 22 cm TL. Estimation of future cohort‐specific abundance based on size‐based expected survival allows managers to establish annual stocking targets that should lead to the achievement of long‐term population goals for adult abundance.     

The application of the Zero Emissions Research and Initiatives (ZERI) concept in an integrated industry-polyculture-farm system in Namibia: The case of Tunweni Sorghum Brewery

One major area dealt with by the concept of Zero Emissions Research and Initiatives (ZERI) is the utilisation of huge volumes of nutrient-rich waters from household toilets, kitchen sinks and municipal and organo-industries for integrated purposes. The arrangements at the Tunweni Sorghum Brewery, Tsumeb, Namibia are such that wastewater is utilised in the polyculture of locally available riverine fish species in large (3 000m²; 120m long × 25m wide), deep (3m) earthen ponds. Before reaching the fishponds the wastewater passes through a series of bio-system processes such as mushroom and earthworm beds, a pig sty, anaerobic and aerobic digesters and algae ponds. A pH of about 8.0 is desirable for maximum fish production in the large (9 000m³) fishponds. The old Chinese technique is followed whereby wastewater is utilised for growing fish without adding any feed supplement; various plankton and invertebrates produced naturally serve as feeds for various kinds of fish feeding at different trophic levels. A variety of locally-available riverine fish species, of different trophic levels, are useful for sustainable stocking in the ponds where no feed-supplement is necessary. Various plankton and invertebrates are produced naturally as feeds for the various kinds of fish feeding at different trophic levels, obviating the purchase of costly artificial feeds which can make fish culture uneconomical. Fish wastes are naturally mineralised into nutrients in the ponds and the nutrient-rich water supports phytoplankton, zooplankton and invertebrates, all of which comprise a varied food chain supporting the fish growth.