Split Ponds Effectively Overwinter Golden Shiners

Baitfish producers have expressed interest in adopting the split‐pond production system. However, confining fish to 20% of the pond area in split‐pond systems effectively quintuples fish density within the culture unit as compared with densities in open ponds. Winter conditions are known to be relatively more stressful on smaller fish, and high densities within split‐pond culture units could increase losses. A 139‐d study was conducted during the winter to compare the production of golden shiners, Notemigonus crysoleucas, in traditional earthen ponds and split ponds at two densities. Golden shiners were stocked at 646 kg/ha or 1292 kg/ha (ca. 370,500 or 741,000 fish/ha, respectively) into 12, 0.04‐ha, netted earthen ponds (six split ponds and six traditional). Feeding rate, nightly aeration hours, and daily circulation hours were reduced when water temperature decreased. At harvest, net yields were significantly lower in the split ponds as compared with traditional ponds at each density (53 and 113 kg/ha less in the low‐ and high‐density split‐pond treatments, respectively). Estimated survival was high (>87%) and did not differ among treatments. Results showed that, although net yield was reduced, small baitfish could be successfully overwintered in split‐pond culture units in preparation for the spring crappie market.     

Studies of fish traps as stock assessment devices on a shallow reef in south-western Puerto Rico

Traditional Antillean arrowhead fish traps were deployed around and on a shallow, 8-ha coral reef of south-western Puerto Rico in two 1987 studies: Study I, 2–18 March, a comparison of 30 trap catches and 30 visual transect counts; Study II, 8 April–18 June, a tag and recapture study of 95 trap hauls. Study I considered model taxa and conditions for calculating effective fishing area (EFA) of the traps on the basis of catch per haul and fish density from transects. Of the 185 fish of 24 species caught, EFA could only reasonably be calculated for three taxa: redband parrotfish, Sparisoma aurofrenatum, 25 m² and 90 m²; stoplight parrotfish, Sparisoma viride, 24 m²; ocean surgeon, Acanthurus bahianus, 93 m². Other taxa were considered unsuitable because of near-zero density or catche, e.g. white grunt, Haemulon plumieri, was often caught but seldom seen. Ability to assess EFA depended on trap location; future surveys of reef fishes will necessarily have to utilize stratified sampling based on habitat characteristics. Study II considered fish movements within a five-trap, cross-shaped array deployed on a back reef area. Of 702 captures from 28 species, redband parrotfish, white grunt and ocean surgeon dominated. Of 272 fish tagged, 65 were recaptured once and 24 were recaptured twice or more. In the short term, traps seemed to sample a discrete population. Except for one individual, all tagged fish were recaptured at the same trap or at a nearby trap of the array, i.e. no more than 25 or 35 m away; ocean surgeon were always caught at the trap where tagged. Using the mark-recapture data, population estimates were calculated for dominant species. These were not used to estimate EFA because the area sampled by the traps could not be reliably estimated. However, properly designed marking studies could be used to obtain this information. It was concluded that using EFA in trap-based assessments is a promising technique for residents like redband parrotfish or ocean surgeon. Mark and recapture techniques may also return useful density estimates for cryptic or diurnally migrating residents such as white grunt.     

Effects of stocking density of Labeo rohita on survival, growth and production in cages

Labeo rohita (139.92 ± 0.76 mm/24.33 ± 0.45 g) was reared for 92 days in floating square cages (10 m² area, 1.5 m height) in a pond (2 ha) at six stocking densities (5, 7.5, 10, 15, 20 and 25 fish m^?2) each with 3 replicates. Fish were fed daily once in the morning with rice polish and groundnut oil cake (1:1) in dough form at 3 % of the total body weight. Survival ranged from 96 to 100 % in different stocking densities. Final average body weight, average body weight gain, mean daily body weight gain and SGR decreased (P < 0.05) with increasing stocking density. Conversely, final biomass, biomass gain and FCR increased (P < 0.05) with increasing stocking density. The highest growth rate of fish could be achieved up to 60 days at 5 fish m^?2 and 92 days at other densities. The reduced growth rate at 10–25 fish m^?2 for 60 days of culture indicated that stress is related to size and density of the fish, suggesting that utmost care is required to reduce the stress at high densities. Maximum production and profit was observed at the highest stocking density. Non-lethal levels of water and soil qualities at different sites (cage premises, and 20 and 200 m away from cage area) suggested that cage aquaculture could be done safely covering 0.9 % of pond area. Production of advanced fingerlings in cages was found a viable alternative to their culture in pond.     

Comparison of density estimates derived from strip transect and distance sampling for underwater visual censuses: a case study of Chaetodontidae and Pomacanthidae

Despite its wide use in terrestrial ecology, distance sampling is as yet rarely used in underwater visual censuses. The present study attempts to compare density estimators based on distance sampling and on strip transects. Three stations with increasing densities of Chaetodontidae and Pomacanthidae were sampled twice by two divers of unequal experience, using two different transect types. A total of 96 transects and 2 970 records of Chaetodontidae and Pomacanthidae were analysed. Nine estimators based on distance sampling were calculated and only the best fit (DT estimator) was kept for comparison with other estimators. These were either based on the average distance of the fish to the transect (AD estimator), or a 3-m- or 5-m-wide strip transect estimator (FW3 and FW5, respectively). There were no significant differences between the means found by DT, AD and FW3. Lower density estimates were given by FW5 in all cases. FW3 and FW5 did not detect several significant differences between stations which were otherwise detected by DT or AD. The number of transects needed to detect a significant difference between stations was four to ten times higher with FW3 or FW5 than with DT or AD. Diver experience was found to be a significant factor in density estimates. However, this factor was less important than the choice of the density estimator. Transect type or the day of sampling had no consequence for the estimates. The distance distributions of fish were divided into three different patterns which may be explained by a combination of detectability function and a behavioural component.     

An experiment in rearing larval Siganus vermiculatus (Valenciennes) and some observations on its spawning cycle

Siganus vermiculatus spawned in captivity on three occasions, and limited numbers of newly hatched larvae were stocked in an 80-m³ outdoor concrete tank and a 2000-m² fish pond in Fiji. Survival through the larval stage was 9% in the tank, in spite of high temperatures and relatively low food densities. No larvae survived in the pond.Data collected on the appearance of fry, spawning, and the gonadal state of adults suggest that S. vermiculatus in Fiji spawns monthly on or around the first lunar quarter during its spawning season.Possible ways to solve problems encountered in this experiment are discussed.     

Growout production of camouflage grouper, Epinephelus polyphekadion (Bleeker), in a tank culture system

Growout production of the camouflage grouper, Epinephelus polyphekadion (Bleeker), in a 10-m³-capacity fibreglass tank culture system was evaluated, using hatchery-produced fingerlings (56-59 g initial weight) at stocking densities of five, 15 and 45 fish m^?3. During the first 9 months of a 12-month growout period, the fish were fed twice a day with a moist pellet feed containing 40.9% protein. From month 10 onwards until harvest, the fish were fed moist pellets in the morning and trash fish in the evening at a 1:1 ratio. The final weight of fish at harvest was up to 900 g, with mean weights of 544.6 ± 170.72 g at five fish m^?3, 540.2 ± 150.82 g at 15 fish m-^?3 and 513.3 ± 134.52 g at 45 fish m^?3. The results showed no significant differences (P > 0.05) in growth rate and fish size between the different stocking densities tested. The average daily growth rate ranged from 0.62 to 3.38 g fish^?1 day^?1, with mean weights of 1.49 ± 0.74 g fish^?1 day^?1 at five fish m^?3 through 0.53 to 2.38 g fish^?1 day^?1, 1.32 ± 0.57 g fish^?1 day^?1 at 15 fish m^?3 to 0.48-3.32 g fish^?1 day^?1 and 1.31 g fish^?1 day^?1 at 45 fish m^?3 stocking density. Although up to 100% survival was observed at the lowest stocking density, the survival rate significantly decreased (P < 0.05) with increasing stocking density. The food conversion ratio (FCR) significantly decreased (P <0.05) with increasing stocking densities, showing efficient feed utilization with increasing stocking densities of E. polyphekadion. The FCR averaged 2.1 at a stocking density of 45 fish m^?3. The yield in terms of kg fish produced m^?3 of water used in the culture system significantly increased (P < 0.001) from five to 45 fish m^?3. The yield averaged 17.3 ±0.53 kg m^?3 at a stocking density of 45 fish m^?3. The present results show that the present tank culture system could sustain more biomass in terms of increasing fish stocking densities. The growth performance of E. polyphekadion observed during this investigation has been reviewed with other grouper species.     

Preliminary Evidence of Sturgeon Density and Other Stressors on Manifestation of White Sturgeon Iridovirus Disease

Two studies were conducted using 5- to 6-month-old juvenile (mean weight: 3-4 g) white sturgeon, Acipenser transmontanus, to examine the effects of density and other stressors on manifestation of disease caused by the white sturgeon iridovirus (WSIV). In the first study, replicate groups of Snake River white sturgeon were stocked at three densities (953, 1,907, and 3,178 fish/m³) in 0.31-m³ fiberglass aquaria with spring water flow through of 30.0 L/minute. No significant (P > 0.05) differences were observed among the three groups during the initial 6-week period. However, the high density group cumulative mortality was significantly (P < 0.05) greater than the mortality detected in the other groups at 59 days. Pathognomonic signs of WSIV were detected in moribund fish from each group by histological evaluation; however, no virus was isolated. These results suggested that maintaining low sturgeon densities in fish younger than I year may be a prudent strategy for minimizing mortality caused by WSIV. In the second study, triplicate or replicate groups of Kootenai River white sturgeon that had been transported for approximately 14 hours at 2-6°C, prior to acclimation at lS°C, were stocked at high and low densities previously tested. No signs of WSIV or abnormal mortality were observed during the initial 2-week period. However, after 36 days the mortality increased to 10-18% and 14-18% in the low and high density groups, respectively, and signs of WSIV disease were detected in moribund fish examined from each treatment by histological evaluation. The presence of the virus was further confirmed by electron microscopy of gill tissue. Although fish density did not appear to affect the occurrence of disease or cumulative mortality, the results suggested that a stressor (e.g., handling, transport, and temperature) in subyearling sturgeon may enhance clinical manifestation of WSIV.     

Premiers resultats de l'elevage en cages de Tilapia nilotica (L.) dans le Lac Kossou,Cote d'IvoirePreliminary results of cage rearing Tilapia nilotica (L.) in Lake Kossou,Ivory Coast

The experimental rearing of T. nilotica in 1-m³ cages floating in Lake Kossou, Ivory Coast, was performed between March 1974 and April 1975. The fish were fed water-stable pellets (24.7% protein), the daily ration generally varying between 6 and 3.4% of the ichthyomass present. Fingerlings (9–55 g each) were stocked at densities of 200–400 fish/m³. They reached commercial size (more than 200 g) within 4–6 months, depending on the biomass present in the cage (stocking density and average individual weight of fish), as well as on limological conditions. The average annual mortality was 5.9%. Annual production varied from 36 to 64 kg/m³. The feed conversion rate averaged 2.8. A reduction of the daily ration for fish larger than 40 g reduced this value.Fingerlings should average at least 20–30 g in weight at initial stocking of the cage. Two crops a year can then be easily realized. If only male fish are reared, three harvests per year appear to be possible, increasing the annual cage production of fish close to 200 kg/m³ with proper management.The possible development of T. nilotica cage culture is actually hampered due to the lack of adequate fish feed available commercially, and to the lack of sufficient quantities of calibrated fingerlings throughout the year.     

Stressing fish in Recirculating Aquaculture Systems (RAS): does stress induced in one group of fish affect the feeding motivation of other fish sharing the same RAS?

As a consequence of water re‐use and high stocking densities, Recirculating Aquaculture Systems (RAS) may lead to an accumulation of substances released by the fish into the water, e.g. cortisol and alarm pheromones. This study investigated the effect of stressing fish on the feeding motivation of other fish not subjected to stress but sharing the same water of stressed fish. Two identical RAS were used (operated at 30 L kg feed^?1 day^?1) and contained grouped (stressed fish) and individually (receiving water from stressed fish) housed Nile tilapia. A stress test was applied in grouped housed fish on days 17 and 55. Feeding behaviour (intake and latency) was recorded in the individually housed fish 3 days before, during and 3 days after the stress test. The results showed no effect on feeding behaviour in fish receiving the water from stressed fish. These results could be a consequence of insufficient cortisol/alarm cues' release by the stressed fish into the water or inactivity of such substances, either due to a trapping effect of humic acids or due to degradation in the nitrification and denitrification processes. Future research is needed to clarify how these processes may affect the water concentration of cortisol and alarm pheromones and should be extended by measuring other behavioural and physiological traits.     

Variation in larval fish communities: implications for management and sampling designs in reservoir systems

Understanding spatial and temporal patterns in larval fish distributions is important for investigating factors related to recruitment variability and for developing sampling methodologies. Variation in species diversity (Shannon index) and density (number 100 m^?3) of larval fishes from nearshore and pelagic habitats in Glen Elder Reservoir, KS, USA, was assessed during 1999 and 2000. Species diversity and densities of individual taxa were significantly higher (P < 0.05) in nearshore habitats than pelagic habitats during both years. Larval fish densities were generally higher in the year 1999 with higher spring water levels. Gizzard shad Dorosoma cepedianum (Lesueur) was the most abundant taxon in nearshore and pelagic habitats during both years. In addition, larval gizzard shad was sampled for the longest duration in the upper water column and overlapped with all taxa collected. High densities of gizzard shad in both habitats suggested that the potential for competitive interactions was high, especially given the lack of aquatic vegetation and habitat heterogeneity in littoral habitats. Most of the variation in larval fish diversity and density was because of temporal variation. If sampling effort must be limited and the purpose of sampling is to monitor larval diversity or density, attention should focus on sampling the same location over long time periods rather than across a variety of sampling locations.     

Muscle activity as a key indicator of welfare in farmed European sea bass (Dicentrarchus labrax L. 1758)

Groups of adult sea bass were reared at either low (10 kg m^?3) or high (50 kg m^?3) stocking densities respectively for 84 and 116 days. To monitor the red muscle activity, about 20 fish from both densities were surgically implanted with EMG (Electromyograms) radio transmitters, after EMG calibration during exhaustive swimming exercise (U_crit test). Blood samples and morphometric measurements were also taken. EMG showed that the muscle activity of fish reared at 50 kg m^?3 was on average twofold higher than fish kept at lower density. Cortisol was significantly more elevated at higher density and haemoglobin, haematocrit and RBCC (red blood cells count) showed the same trend, while lysozyme decreased. Patterns for glucose and lactate were less clear. The results showed that the contemporary use of functional (EMG) and physiological (haematological and biochemical) profiles could give a more comprehensive view of the fish status validating the diagnosis of fish stress induced by culture practices.     

A conditional simulation of acoustic survey data: advantages and potential pitfalls

Standard geostatistical techniques provide effective methods for estimating the global abundance and precision of a variable of interest, for mapping its spatial distribution and for describing its spatial structure. In the case of acoustic survey data, however, obtaining a measure of precision of the global abundance estimate is confounded by the combination of variances from the interpolation of both the acoustic data and the concomitant fish length data. Even if the global estimation variance could be calculated, the distribution of the estimation error is not known and so confidence intervals cannot be determined. Furthermore, kriged distribution maps, in minimising the estimation variance, tend to smooth out local details of the attribute’s spatial variation: small values can be overestimated and larger ones underestimated, such that the kriged map is smoother than reality. This can lead to serious shortcomings when trying to detect patterns of extreme attribute values, such as the high densities encountered in some fish schools. Stochastic geostatistical simulations, conditional on sampled locations, provide a solution to many of these problems. They can deliver a measure of uncertainty for local (density) estimates, a confidence interval estimation for the global mean density, and finally, reproduce global statistics, such as the sample histogram and variogram. In so doing, they also provide maps of the attribute, which are spatially realistic because the variogram is reproduced; these are generated as a number of equiprobable realisations. In the present paper, we apply these techniques to acoustic data from an acoustic survey of North Sea herring. Sequential gaussian simulations are used to generate realisations for fish length and values of the nautical area scattering coefficient. These are then combined to produce realisations of herring density. The combined set of multiple realisations is then used to provide confidence intervals for the global abundance estimate: 95% of the herring abundance estimates are between 5677 and 6271 millions of individuals. Although the method presented in this paper contributes to the assessment of total uncertainty for acoustic surveys, the approach may have suffered from bias due to the use of off-the-shelf data transformation algorithms on fisheries acoustic data, which are often very positively skewed. We discuss this limitation and propose corrections for future work.     

Can fish sampling protocol (CEN, 2005) be amended for Mediterranean lakes?

The CEN (2005) methodology for fish monitoring of mesotrophic to eutrophic Mediterranean lakes was evaluated. The contribution of gillnet (benthic and pelagic) and electrofishing catches on species richness was examined. Additionally, the contribution of benthic gillnet sampling effort on species richness and precision of catch‐per‐unit‐effort (CPUE) estimates, considering both number of specimens (ΝPUE) and biomass (BPUE), was assessed. Subsequently, the impact of reduction in sampling effort on the parameters mentioned above was estimated and the effort needed to acquire the desired precision level considering different NPUE values was modelled. The use of different sampling gears was found to contribute to species richness assessment, while sampling effort could be reduced in the deepest zones of the lakes. The resultant model can reliably guide assessment of the sampling effort required in the Mediterranean and central European lakes with high fish densities.     

Effect of stocking density on the growth performance and yield of Nile tilapia [Oreochromis niloticus (L., 1758)] in a cage culture system in Lake Kuriftu,Ethiopia

This research was conducted to investigate the effect of stocking density on the growth performance and yield of Oreochromis niloticus in cage culture in Lake Kuriftu. The treatments had stocking densities of 50 (50F), 100 (100F), 150 (150F), and 200 (200F) fish per m^?3. All treatments were in duplicate. Juveniles with an average weight of 45. 76±0.25 g were stocked in the treatments. The fish were fed a composite mixture of mill sweeping, cotton seed, and Bora food complex at 2% of their body weight twice per day using feeding trays for 150 days in powdered form. The growth performance of O. niloticus was density dependent. The final mean weight of O. niloticus ranged 147.76±0.28–219.71±1.42 g and the mean daily weight gain was 0.69±0.01–1.15±0.02 g day^?1. Fish held in cages with lower density were heavier than the ones held at higher densities, and showed higher weight gain and daily weight gain. The most effective stocking density, in terms of growth parameters, was 50 fish m^?3. The gross yield (4.5–20.55 kg cage^?1) showed a significant difference with increasing stocking density (P<0.05). Moreover, the apparent food conversion ratio (2.48–7.22) was significantly affected by stocking density (P<0.05). However, survival rate was not affected by stocking density (P>0.05). It can be concluded that the most effective stocking densities were at 50 fish m^?3 cage for larger size fish demand in a short period and 200 fish m^?3 for higher gross production with supplementary feed.     

The effect of large-scale spatial variation of pelagic fish on hydroacoustic estimates of their population density in Windermere (northwest England)

From July 1989 to December 1992, an echo sounder provided monthly estimates, usually for both day and night, of pelagic fish densities in the north and south basins of Windermere, the largest natural lake in England. Sampling was along contiguous transects, 3 in the north basin and 5 in the south basin. It was impossible to separate records for Arctic charr (Salvelinus alpinus) from those for brown trout (Salmo trutta), but previous sampling by gill-nets and anglers showed that charr formed about 90% of this mixed population in the north basin and 60–75% in the south basin. In each basin, estimates of population density for the combined transects did not agree with estimates obtained by treating the transects as a contiguous sample of 9 sampling units; only the latter, of course, provided a measure of the precision of each estimate. Analyses of the contiguous samples showed that the variance was significantly greater than the arithmetic mean in most samples, indicating that the fish were distributed nonrandomly in a patchy or clumped pattern. The relationship between the large-scale spatial variance and mean for these samples was well described by a power function; the parameter estimates did not vary significantly between basins, day and night samples or years. As the power in this equation did not differ greatly from 2 (value for pooled data was 1.70 ± 0.11, n= 136), the variance was stabilized by a log transformation of the data, and the geometric mean, rather than the arithmetic mean, provided the best estimate of population density when some measure of precision was required. These conclusions may be applicable to other echo-sounding estimates of population abundance, and similar comparisons should be made for pelagic fish in other lakes.     

Effects of a research trawl on a hard-bottom assemblage of sponges and corals

The effects of a research trawl on several sponge and coral species was assessed in a shallow-water, hard-bottom area located southeast of Savannah, Georgia. The study entailed a census of the numerically dominant species in replicate 25-m² quadrats located along five transects established across a trawling alley. The density of undamaged sponges and corals was assessed in trawled and non-trawled (control) portions of each transect immediately before, immediately after, and 12 months after a 40/54 roller-rigged trawl was dragged through the alley once. Some damage to individuals of all target species was observed immediately after trawling, but only the density of barrel sponges (Cliona spp.) was significantly reduced. The extent of damage to the other sponges (Ircinia campana, Haliclona oculata), octocorals (Leptogorgia virgulata, Lophogorgia hebes, Titanideum frauenfeldii) and hard corals (Oculina varicosa) varied depending on the species, but changes in density were not statistically significant. Twelve months after trawling, the abundance of specimens counted in the trawled quadrats had increased to pre-trawl densities or greater, and damage to the sponges and corals could no longer be detected due to healing and growth. Trawl damage observed in this study was less severe than the damage reported for a similar habitat in a previous study. Differences between the two studies are attributed to (1) differences in the rollerrig design of the trawls used, and (2) differences in the number of times the same bottom was trawled.     

Comparative study of growth performance of three strains of Nile tilapia, Oreochromis niloticus, L. at two stocking densities

This study was conducted to investigate the effect of stocking density (125 or 200 fish m^?3) on the growth performance of three strains of the Nile tilapia, Oreochromis niloticus: the non‐improved strain (NS), the genetically improved farmed tilapia (GIFT) and the Freshwater Aquaculture Center selected tilapia known as the FaST selected line (SL). Each strain and density combination was triplicated in 0.42 m³ fibreglass tanks within a re‐circulating water system. Water temperature was maintained at 29.0±1.0°C. Large Nile tilapia having a mean body weight of 100–110 g were stocked in each tank and hand‐fed four times daily with commercial tilapia pellets (35% protein) for 104 days. Results showed that at the two stocking densities, the GIFT and SL strains showed a significantly higher (P<0.05) mean weight (MWT), daily growth rate (DGR), specific growth rate (SGR), feed conversion ratio (FCR) and gross yield (GY) than the NS. In all three strains, growth performance was negatively affected by stocking density. The lower density (125 fish m^?3) treatments had significantly higher MWT, DGR and SGR than the higher density one (200 fish m^?3). However, higher FCR and GY were observed at the higher density. Survival rates were high in all treatments and were not affected by strain or density. In general, the SL strain had better growth parameters than the GIFT strain. The findings of this study demonstrated the superior growth performance of the improved strains at both densities compared with the NS. The higher density (200 fish m^?3) could be more profitable for the tilapia farms in Kuwait than the lower density of (125 fish m^?3) in terms of reduced land cost and facilities, demand on the limited low‐salinity underground water and manpower.     

Effect of Stock Density and Ploidy in Jundia,Rhamdia quelen,Larvae Performance

The performance (growth and survival) of diploid and triploid jundia, Rhamdia quelen, was evaluated at six different stocking densities (10, 60, 110, 160, 210, 260 larvae/liter) during 31 days after rearing in an intensive larviculture system. Triploid fish exhibited a significantly higher survival rate than diploids at all stocking densities (27.1 ± 4.3% vs. 12.1 ± 3.3%; P < 0.01). Survival was not affected by stocking density (P > 0.05). Length gain was not affected by either ploidy or stocking density. Diploid fish gained more weight than triploids (P < 0.05), though this difference could result from lower fish densities in diploid treatments resulting from the higher mortality rate of diploid fish. This hypothesis is strengthened by the higher biomass present in triploid treatments (P < 0.01).     

Water Quality and Growth of Rohu,Labeo rohita,in a Biofloc System

We evaluated the technical feasibility of reducing water dependency of rohu, Labeo rohita, culture with biofloc under light-limited indoor culture. Biofloc and control treatments were conducted in 700-L indoor tanks at three different stocking densities (STD): 1.3, 2.6, and 3.9 Nos. fish m^?2 of surface area of tank for a period of 90 days. In biofloc treatment, fish were fed 20% crude protein feed and extra organic carbon in the form of wheat flour, whereas in case of control treatment they were fed 30% crude protein feed only. Fish survival was 100% in both the treatments. Lower stocking density produced larger fish, but growth was similar within stocking densities among control and biofloc treatments. The nutritional quality of biofloc was found to be quite suitable for rohu. Frequency of water exchange was significantly less in biofloc treatments as compared to the control ones.