High Fish Densities Research Articles

<b>Integration of Nile tilapia (<i>Oreochromis niloticus </i> L.) production <i>Origanum majorana</i> L. and <i>Ocimum basilicum </i> L. using aquaponics technology

This study aims to evaluate the effect of different initial densities of Nile tilapia fingerlings on water quality, fish growth, and production of marjoram ( Origanum majorana L.) and sweet basil ( Ocimum basilicum L.) in a recirculating aquaponic set up. The experiment was carried out on randomized block design and comprised four fish densities (0, 150, 250 and 500 fish m -3 ) and two crops, in which each treatment was continued for 45 days and replicated three times. Except for pH and total alkalinity, all water physicochemical parameters were increased significantly at high fish densities. In contrast, most of the biological parameters for fish were unaffected by culture density, although at the highest density crude protein percentage was significantly increased and ether extract percentage was significantly reduced. Plant productivities were highest when fish were cultured at density of 500 fish m -3 , a density that was found to be appropriate for maintaining water quality and promoting fish growth while preventing toxicity and mortality. The present study has demonstrated the technical viability of coupling Nile tilapia culture coup with aquaponic production of marjoram and sweet basil.

Evaluation of water treatment capacity, nutrient cycling, and biomass production in a marine aquaponic system

The need to expand marine fish production and improve the economic viability and sustainability of recirculating aquaculture systems led to the development of a zero-discharge, marine aquaponic system. In this study, water treatment capacity, nutrient cycling, and biomass production were evaluated in a prototype, commercial-scale marine aquaponic system that included a moving bed bioreactor (MBBR) for nitrification, a sand filter for solids removal and denitrification, and hydroponic plant beds. Red drum (Sciaenops ocellatus), two species of edible halophytes sea purslane (Sesuvium portulacastrum) and saltwort (Batis maritima), and organic solids, were successfully produced over a 9-month period. Extensive analysis of solids, organic matter, and nutrients (nitrogen and phosphorous) in water and plant biomass was used to develop detailed mass balances on the system. Simultaneous operation of the moving bed bioreactor (MBBR) and plant beds resulted in high ammonia removal rates, allowing the system to support a high fish biomass density (38.8 kg/m3). Passive denitrification was the main nitrate removal mechanism, contributing to approximately 59% of aqueous nitrogen removal. Conversion of a sand filter to a side-stream denitrification reactor resulted in removal of 17% of the daily aqueous nitrogen load and prevented nitrate accumulation in the system. In addition to fish and edible halophyte production, 34 kg of organic solids were harvested from the sand filter and provided to a commercial nursery that used the solids as a fertilizer. This study demonstrates that marine aquaponics is an effective way to simultaneously produce marine fish, edible halophytes, and fertilizer. Addition of biological MBBR and a denitrifying sand filter was shown to be beneficial in situations where there are space limitations for plant growth, unexpected plant losses, or to support high densities of fish.

How many fish? Comparison of two underwater visual sampling methods for monitoring fish communities.

BackgroundUnderwater visual surveys (UVSs) for monitoring fish communities are preferred over fishing surveys in certain habitats, such as rocky or coral reefs and seagrass beds and are the standard monitoring tool in many cases, especially in protected areas. However, despite their wide application there are potential biases, mainly due to imperfect detectability and the behavioral responses of fish to the observers.MethodsThe performance of two methods of UVSs were compared to test whether they give similar results in terms of fish population density, occupancy, species richness, and community composition. Distance sampling (line transects) and plot sampling (strip transects) were conducted at 31 rocky reef sites in the Aegean Sea (Greece) using SCUBA diving.ResultsLine transects generated significantly higher values of occupancy, species richness, and total fish density compared to strip transects. For most species, density estimates differed significantly between the two sampling methods. For secretive species and species avoiding the observers, the line transect method yielded higher estimates, as it accounted for imperfect detectability and utilized a larger survey area compared to the strip transect method. On the other hand, large-scale spatial patterns of species composition were similar for both methods.DiscussionOverall, both methods presented a number of advantages and limitations, which should be considered in survey design. Line transects appear to be more suitable for surveying secretive species, while strip transects should be preferred at high fish densities and for species of high mobility.

The effect of filtration method on the efficiency of environmental DNA capture and quantification via metabarcoding.

Environmental DNA (eDNA) is a promising tool for rapid and noninvasive biodiversity monitoring. eDNA density is low in environmental samples, and a capture method, such as filtration, is often required to concentrate eDNA for downstream analyses. In this study, six treatments, with differing filter types and pore sizes for eDNA capture, were compared for their efficiency and accuracy to assess fish community structure with known fish abundance and biomass via eDNA metabarcoding. Our results showed that different filters (with the exception of 20-μm large-pore filters) were broadly consistent in their DNA capture ability. The 0.45-μm filters performed the best in terms of total DNA yield, probability of species detection, repeatability within pond and consistency between ponds. However performance of 0.45-μm filters was only marginally better than for 0.8-μm filters, while filtration time was significantly longer. Given this trade-off, the 0.8-μm filter is the optimal pore size of membrane filter for turbid, eutrophic and high fish density ponds analysed here. The 0.45-μm Sterivex enclosed filters performed reasonably well and are suitable insituations where on-site filtration is required. Finally, prefilters are applied only if absolutely essential for reducing the filtration time or increasing the throughput volume of the capture filters. In summary, we found encouraging similarity in the results obtained from different filtration methods, but the optimal pore size of filter or filter type might strongly depend on the water type under study.

Fish thermal habitat current use and simulation of thermal habitat availability in lakes of the Argentine Patagonian Andes under climate change scenarios RCP 4.5 and RCP 8.5

Habitat use in relation to the thermal habitat availability and food source as a forcing factor on habitat selection and use of Percichthys trucha (Creole perch), Oncorhynchus mykiss (rainbow trout), Salmo trutta (brown trout) and Salvelinus fontinalis (brook trout) were determined as well as future potential thermal habitat availability for these species under climate change scenarios Representative Concentration Pathways 4.5 and 8.5. This study was conducted in three interconnected lakes of Northern Patagonia (Moreno Lake system). Data on fish abundance was obtained through gill netting and hydroacoustics, and thermal profiles and fish thermal habitat suitability index curves were used to identify current species-specific thermal habitat use. Surface air temperatures from the (NEX GDDP) database for RCP scenarios 4.5 and 8.5 were used to model monthly average temperatures of the water column up to the year 2099 for all three lakes, and to determine potential future habitat availability. In addition, data on fish diet were used to determine whether food could act as a forcing factor in current habitat selection. The four species examined do not use all the thermally suitable habitats currently available to them in the three lakes, and higher fish densities are not necessarily constrained to their “fundamental thermal niches” sensu Magnuson et al. (1979), as extensive use is made of less suitable habitats. This is apparently brought about by food availability acting as a major forcing factor in habitat selection and use. Uncertainties related to the multidimensionality inherent to habitat selection and climate change imply that fish resource management in Patagonia will not be feasible through traditional incremental policies and strategic adjustments based on short-term predictions, but will have to become highly opportunistic and adaptive.

Benthic grazing in a eutrophic river: cascading effects of zoobenthivorous fish mask direct effects of herbivorous fish.

Benthic grazing strongly controls periphyton biomass. The question therefore arises whether benthic grazing could be used as a tool to reduce excessive growth of periphyton in nutrient-enriched rivers. Although benthic invertebrate grazers reduce the growth of periphyton, this is highly context dependent. Here we assessed whether the only obligate herbivorous fish in European rivers, the common nase (Chondrostoma nasus L.), is able to reduce periphyton biomass in a eutrophic river. We conducted three consecutive in situ experiments at low, intermediate and high densities of nase in the river using standard tiles on the river bottom naturally covered with periphyton that were accessible to fish and tiles that excluded fish foraging with electric exclosures. The biomass of benthic invertebrate grazers was very low relative to nase. We hypothesised that nase would reduce periphyton biomass on accessible tiles and therefore expected higher periphyton biomass on the exclosure tiles, at least at intermediate and high densities of nase in the river. Contrary to our expectation, the impact of fish grazing was low even at high fish density, as judged by the significantly lower chlorophyll a concentration on exclosure tiles even though the ash-free dry mass on accessible and exclosure tiles did not differ. The lower chlorophyll a concentrations on exclosure tiles might be explained by a higher biomass of invertebrate grazers on the exclosure tiles, which would indicate that the effect of invertebrate grazers was stronger than that of herbivorous fish grazers. The high biomass of invertebrate grazers on exclosure tiles likely arose from the exclusion of zoobenthivorous fish, which occur in the river at high densities. The results of our small-scale experiments suggested that cascading top-down effects of zoobenthivorous fish have a higher impact on periphyton biomass than direct effects of herbivorous nase.

Indicator-Based Geostatistical Models For Mapping Fish Survey Data

Marine research survey data on fish stocks often show a small proportion of very high-density values, as for many environmental data. This makes the estimation of second-order statistics, such as the variance and the variogram, non-robust. The high fish density values are generated by fish aggregative behaviour, which may vary greatly at small scale in time and space. The high values are thus imprecisely known, both in their spatial occurrence and order of magnitude. To map such data, three indicator-based geostatistical methods were considered, the top-cut model, min–max autocorrelation factors (MAF) of indicators, and multiple indicator kriging. In the top-cut and MAF approaches, the variable is decomposed into components and the most continuous ones (those corresponding to the low and medium values) are used to guide the mapping. The methods are proposed as alternatives to ordinary kriging when the variogram is difficult to estimate. The methods are detailed and applied on a spatial data set of anchovy densities derived from a typical fish stock acoustic survey performed in the Bay of Biscay, which show a few high-density values distributed in small spatial patches and also as solitary events. The model performances are analyzed by cross-validating the data and comparing the kriged maps. Results are compared to ordinary kriging as a base case. The top-cut model had the best cross-validation performance. The indicator-based models allowed mapping high-value areas with small spatial extent, in contrast to ordinary kriging. Practical guidelines for implementing the indicator-based methods are provided.

Hydroacoustics to examine fish association with shallow offshore habitats in the Arabian Gulf

In order to implement spatial fisheries management in the Arabian Gulf, a better understanding of the distribution of fish in relation to benthic habitats is required. To facilitate this, hydroacoustic fish surveys were conducted over oyster bed/reef (“shallow”) and surrounding soft sediment (“deep”) habitats in the offshore central Gulf, within Qatari waters. Transects at ‘shallow’ sites had significantly higher mean fish density and biomass. Mean target strength of individual fish was also significantly higher at ‘shallow’ sites. Fish positions in the water column were examined and overall there was a closer association with the seabed at the ‘shallow’ sites. Larger fish were found significantly closer to the seabed than smaller fish across all sites, but more so at ‘shallow’ sites than at ‘deep’ sites. Acoustic return from the seabed was extracted to provide information on the habitat type both using ‘Sonar5′ and ‘Visual Habitat’ software. The different site categories (‘shallow’ vs ‘deep’) were significantly different for all the measures of acoustic habitat. Fish density was significantly related to ‘Visual Habitat’ data, more so than depth alone. Our results show that fish distribution in the offshore Gulf is associated with complex, shallow oyster bed/reef habitats, and this is particularly the case for larger demersal fish that are commercially exploited. The ability to characterise benthic habitats from acoustic fish survey data shows promise, with important time saving implications for the monitoring of marine environments and developing a spatial approach to fisheries management. This may include the identification of habitats with a relatively high density of larger fish for inclusion in candidate marine protected areas.

Transmission through the water of the bacterial diseases of the euryaline fish cultured in the Mediterranean. Elements of epidemiology and risk factors which concern their spread and transmission of the respective pathogens

Intensive fish culture practiced in the Mediterranean requires fish to live under stressful conditions and high fish densities, something that facilitates transmission and spread of bacterial pathogens within cultured fish populations. One of the most important means of transmission of bacterial diseases of the euryaline fish that are cultured in the Mediterranean is the seawater. The main pathogenic bacteria that infect the euryaline fish species that are cultured in the Mediterranean are: Listonella anguillarum, Photobacterìum damselae subsp piscicida Tenacibaculum marìtimum, Piscirikettsia salmonis, Pseudomonas anguilliseptica as well as various species of the genus Vibrìo, various species of the genera Streptococcus and Lactococcus, and species of the genus Mycobacterium. In the natural environment the bacteria, which are usually shed by infected fish, after surviving for some time in the water column and the bottom of the sea can, under appropriate conditions, infect other, cultured or wild susceptible fish. This review presents the published information that concerns the ability of the various bacterial pathogens of the euryaline fish to spread via water. Most of the published information concerning the bacterial diseases of the Mediterranean cultured euryaline fish deal with the characteristics, resistance and means of transmission of the respective bacteria, as determined in experimental studies. This information is very important for the design and application of prevention and control programs for the respective diseases, however, these programs must also consider epidemiological information concerning risk factors that may affect the onset and severity of the diseases. The investigation of risk factors, in analytical epidemiological studies, focuses on characteristics that relate to the pathogen, the host and the environment, which can concern and be measured at the farm level or at any other level of organization of the population. Especially useful in these cases is the determination of management practices which can be related to increased or decreased morbidity or mortality (total or due to specific diseases). In the current study, reference is made to analytical epidemiological studies in which risk factors that may affect the transmission of several fish pathogens, as well as the occurrence and severity of the respective diseases, have been investigated. Although these studies refer to environments outside the Mediterranean, however, they are particularly interesting because they concern infectious agents that are transmitted by water in conditions and culture environments similar to those of the Mediterranean seawater aquaculture. The approach of prevention of bacterial diseases through management of the populations and their environment is particularly important in the intensive culture of euryaline fish, since the methods that are used, today, for the reduction of the concentration of bacterial pathogens in the water have limited application and effectiveness in such systems. At the farm level, and especially in land-based facilities, three methods are used to decrease the concentration of pathogenic bacteria in the incoming sea water: ozonation, UV radiation and manipulation of the microbial flora of the water using non pathogenic microorganisms, which act antagonistically to the pathogenic bacteria. The first two methods are especially effective in flow-through systems, where water is not recirculated but not so much in closed systems where because the water is re-used fish are infected by the few bacteria that survive the treatment. The alteration of the microbial flora of the incoming water, through antagonism, can be applied in hatcheries during the initial stages of fish culture, but not at the grow out stage in the cages.

Hot eats and cool creeks: juvenile Pacific salmonids use mainstem prey while in thermal refuges

Thermal refuges form important habitat for cold-water fishes in the face of rising temperatures. As fish become concentrated in refuges, food resources may become depleted. In this study, we used invertebrate drift sampling and fish density surveys to quantify potential in-refuge food limitation, temperature-sensitive radio-tagging studies to quantify thermal habitat use, and isotopic analyses to determine diet sources for juvenile Pacific salmonids using thermal refuges in California’s Klamath River. Juvenile salmonids using refuges formed by tributary junctions with the mainstem river obtained the majority (range = 47%–97%) of their diet from mainstem prey sources. Mean steelhead (Oncorhynchus mykiss) body temperatures were significantly cooler (∼3.5 °C) than diet-inferred foraging temperatures. Thus, while fish seek cooler habitat for physiological benefits, they rely primarily on mainstem prey. Moreover, consistently high densities of fish in refuges (mean = 3.5 fish·m−2) could lead to density-dependent food limitation. Thus, mobile consumers like fish can exploit existing heterogeneity associated with cold-water refuges by gaining thermal benefits from a food-limited cold-water habitat while deriving the majority of their prey from the warm mainstem river.

Could artificial plant beds favour microcrustaceans during biomanipulation of eutrophic shallow lakes?

Introduction of artificial plants may facilitate the transition from a turbid to a clear-water state in shallow lakes, particularly when plant establishment is delayed. We investigated the usefulness of artificial plants as a restoration tool in an experimental setup mimicking open submerged plant beds with high plant density [80%, HPD] and low plant density [20%, LPD] in shallow Lake Vaeng, Denmark, having undergone biomanipulation in the form of extensive fish removal. Biological measures of the fish, and of both free-swimming (FSM) and plant-attached microcrustaceans (PAM) within the experimental beds and in the lake, were obtained from before, during and after biomanipulation. We found that microcrustacean measures (density, biomass and Cladocera:FSM) were significantly larger in the HPD beds, before and during fish removal, while the effect of plants was not significant after biomanipulation, with low fish biomass. On PAM, these effects were less pronounced and only significant after biomanipulation. Microcrustaceans were larger-bodied at HPD in all years, for both FSM and PAM. In conclusion, artificial plant beds acted as an effective microcrustacea refuge against fish, particularly for the FSM at HPD and in the years with high fish densities, providing further evidence that artificial plant beds could assist lake restoration efforts.

Synergistic negative effects of small-sized benthivorous fish and nitrogen loading on the growth of submerged macrophytes – Relevance for shallow lake restoration

Rapid recruitment of small fish after biomanipulation in warm lakes may delay the reestablishment of submerged macrophytes, not least at high nutrient concentrations. Success has recently been obtained in controlling phosphorus (P) loading to many lakes, but nitrogen (N) inputs often remain high. To determine the interactive effects of N loading and the abundance of small-sized fish on the growth of the submerged macrophyte Vallisneria natans, we conducted an outdoor mesocosm experiment with a factorial design on the north shore of Lake Taihu, China. The experiment involved two densities of small crucian carp – low (10gm−2) and high (40gm−2) – crossed with two levels of N loading – present-day external nutrient loading (P: 5μgL−1day−1, N: 130μgL−1day−1) and P: 5μgL−1day−1 with a three times higher N loading (N: 390μgL−1day−1). The results showed that nitrogen-fish interactions significantly hindered the growth of V. natans, particularly at the high N loading. At low N loading, high densities of fish decreased the relative growth rate, mean leaf length, leaf mass and root mass of V. natans by 16%, 5%, 8%, and 23%, respectively, compared with these measures at low fish densities. The effect of fish was even stronger when N loading was high, with decreases of 232%, 32%, 57%, and 47% for the respective plant growth measures. The stronger effect at high N loading was attributed to higher turbidity due to enhanced phytoplankton biomass and to increased consumption or damage of plants by the fish in response to the more nutrient-enriched plant tissue. Our results indicate that high abundance of small crucian carp in warm lakes may reduce the resilience of submerged macrophytes to external N loading, thereby lowering the chances of successful restoration by biomanipulation.

Combined effects of hydrologic alteration and cyprinid fish in mediating biogeochemical processes in a Mediterranean stream

Flow regimes are important drivers of both stream community and biogeochemical processes. However, the interplay between community and biogeochemical responses under different flow regimes in streams is less understood. In this study, we investigated the structural and functional responses of periphyton and macroinvertebrates to different densities of the Mediterranean barbel (Barbus meridionalis, Cyprinidae) in two stream reaches differing in flow regime. The study was conducted in Llémena Stream, a small calcareous Mediterranean stream with high nutrient levels. We selected a reach with permanent flow (permanent reach) and another subjected to flow regulation (regulated reach) with periods of flow intermittency. At each reach, we used in situ cages to generate 3 levels of fish density. Cages with 10 barbels were used to simulate high fish density (>7indm−2); cages with open sides were used as controls (i.e. exposed to actual fish densities of each stream reach) thus having low fish density; and those with no fish were used to simulate the disappearance of fish that occurs with stream drying. Differences in fish density did not cause significant changes in periphyton biomass and macroinvertebrate density. However, phosphate uptake by periphyton was enhanced in treatments lacking fish in the regulated reach with intermittent flow but not in the permanent reach, suggesting that hydrologic alteration hampers the ability of biotic communities to compensate for the absence of fish. This study indicates that fish density can mediate the effects of anthropogenic alterations such as flow intermittence derived from hydrologic regulation on stream benthic communities and associated biogeochemical processes, at least in eutrophic streams.

Bottom-Up Effects on Biomass Versus Top-Down Effects on Identity: A Multiple-Lake Fish Community Manipulation Experiment

The extent to which ecosystems are regulated by top-down relative to bottom-up control has been a dominant paradigm in ecology for many decades. For lakes, it has been shown that predation by fish is an important determinant of variation in zooplankton and phytoplankton community characteristics. Effects of fish are expected to not only be a function of total fish biomass, but also of functional composition of the fish community. Previous research on the importance of trophic cascades in lakes has largely focused on the role of zooplanktivorous and piscivorous fish. We conducted a large-scale multiple-lake fish community manipulation experiment to test for the effect of differences in fish functional community composition on the trophic structure of lakes. We examine the effect of top-down and bottom-up factors on phytoplankton and zooplankton biomass as well as on their community composition. We put our data in a broader perspective by comparing our results to data of a survey that also included ponds with low fish densities as well as ponds with very high densities of fish. Our results indicate that the overall food web structure under relative high fish densities is primarily structured by bottom-up factors, whereas community characteristics seem to be primarily regulated by top-down factors. Our results suggest a subtle interplay between bottom-up and top-down factors, in which bottom-up factors dominate in determining quantities while top-down effects are important in determining identities of the communities.

Alternative reproductive tactics and inverse size-assortment in a high-density fish spawning aggregation

BackgroundAt high densities, terrestrial and marine species often employ alternate reproductive tactics (ARTs) to maximize reproductive benefits. We describe ARTs in a high-density and unfished spawning aggregation of the squaretail grouper (Plectropomus areolatus) in Lakshadweep, India.ResultsAs previously reported for this species, territorial males engage in pair-courtship, which is associated with a pair-spawning tactic. Here, we document a previously unreported school-courtship tactic; where territorial males court multiple females in mid-water schools, which appears to culminate in a unique ‘school-spawning’ tactic. Courtship tactics were conditional on body size, local mate density and habitat, likely associated with changing trade-offs between potential mating opportunities and intra-sexual competition. Counter-intuitively, the aggregation showed a habitat-specific inverse size-assortment: large males courted small females on the reef slope while small males courted equal-sized or larger females on the shelf. These patterns remained stable across two years of observation at high, unfished densities.ConclusionsThese unique density-dependent behaviours may disappear from this aggregation as overall densities decline due to increasing commercial fishing pressure, with potentially large consequences for demographics and fitness.

Competitive and predatory interactions between invasive mosquitofish and native larval newts

Invasive fish have a high disruptive potential in aquatic ecosystems, in which amphibians may be highly impacted due to intense competition and/or predation on their eggs and larvae. Most studies have focused on the effect of large invasive fishes such as salmonids, whereas the effect of smaller fish on amphibians has been seldom investigated. We experimentally studied effects of the invasive Eastern mosquitofish (Gambusia holbrooki) on pygmy newts (Triturus pygmaeus), a species endemic to the Iberian Peninsula. We set up outdoor mesocosms in Donana National Park with native aquatic flora and invertebrate fauna, and containing larval newts at two experimental densities. Density of larval newts was also crossed with presence or absence of mosquitofish, either free-swimming or caged, in order to distinguish consumptive and non-consumptive effects. Increased density of coexisting larval newts did not reduce their survival, but reduced their growth and development. Newt survival and size at metamorphosis were dramatically reduced in the presence of free-swimming mosquitofish, whether at low or high fish densities. Caged mosquitofish, however, had no effect on larval newts. In laboratory trials, mosquitofish preyed more efficiently on insect larvae than did larval pygmy newts, highlighting the high competitive potential of mosquitofish. This was confirmed by the depletion of zooplankton that free fish caused in the experimental outdoor mesocosms. Our study suggests that invasive mosquitofish exert a high negative impact on coexisting newt populations. Such effects can be explained by a combination of direct predation, injuries caused by predation attempts, and intense competitive exploitation of common food resources.

Histopathological changes and zootechnical performance in juvenile zebrafish (Danio rerio) under chronic exposure to nitrate

Zebrafish is currently intensively reared in laboratories around the world due to its increasing importance as an experimental model in biomedical research. Recirculating aquaculture systems (RAS) have been generally used for zebrafish rearing, allowing the maintenance of high densities of fish with minimal water renewal. Biofilters play a pivotal role in RAS by converting the highly toxic ammonia excreted by fish into nitrate. Due to its much lower acute toxicity, nitrate accumulation in RAS is usually neglected, and fish can be exposed to relatively high levels of this compound for extended periods. This study evaluated the impact of chronic exposure to relevant levels of nitrate on the zootechnical performance and histology of selected organs (gills, integument, kidney, liver, and intestine) of juvenile zebrafish, with the aim to define safety levels of this nitrogenous compound for zebrafish rearing. For that, groups of 30-day-old zebrafish were exposed to <7 (control), 100, 200, and 400mgL−1 nitrate-N for 28days. No mortality was registered in fish exposed up to 200mgL−1 nitrate, and all individuals seemed externally healthy; however, in fish exposed to the highest nitrate concentration mortality reached 47% at the end of the trial, and many individuals showed lethargy, abnormal swimming, emaciation, lordosis, and/or superficial lesions. Although final growth was not significantly different among groups, growth parameters tend to decrease with increasing levels of nitrate, and a significant negative correlation was found between weight gain and nitrate levels, suggesting a dose-dependent negative effect of nitrate on growth. Except for the lowest nitrate concentration (100mgL−1 nitrate-N), the histological survey revealed significant changes induced by nitrate in all examined organs, and a dose-dependent effect of nitrate on the overall histopathological changes is suggested. In conclusion, this study shows that the chronic exposure of zebrafish juveniles to nitrate induces histopathological changes that would lead to a negative impact on the general health condition of fish. Fish growth tended to decrease and the overall histological damages tended to increase with increasing nitrate levels, particularly above 100mgL−1 (the lowest tested value). Thus, we recommend that this limit of 100mgL−1 nitrate-N should not be exceeded in RAS during rearing of juvenile zebrafish.

Consequences of High-Density Fish Culture as Monitored By Growth, Behavior and Immune Responses of Nile Tilapia (Oreochromis niloticus)

An experiment was performed in a twelve weeks period at three varied stocking densities: low density (LD, 100 fish m-3), medium density (MD, 200 fish m-3) and high density (HD, 400 fish m-3) to analyze the biological performances and ingestive behaviour, of Nile tilapia (Oreochromis niloticus) and Lately, fish were challenged with Aeromonas hydrophila to evaluate the cumulative mortality (%). Stocking density had no effect on survival rate, the final weight markedly reduced with increased density, ingestive behaviour was higher in fish reared in MD than fish reared in LD, however, fish at HD have a higher feed intake (%) and feed conversion ratio. Fish at LD showed marked higher levels of total protein, albumin and globulin compared with those reared at HD. plasma glucose and cortisol levels were high in HD contrast to LD and MD groups. LD fish had higher RBCs and WBCs parallel to MD and HD. similarly, lymphocytes, neutrophils, and monocytes (%) decreased with increasing density. In addition, Serum antioxidant and lysozyme enzymes were considerably (P

Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa spp vs. seagrass species in Mediterranean meadows

Mediterranean seagrass ecosystems are endangered by increased colonization of Caulerpa species, which may replace them, affecting key ecosystem processes. The fish Sarpa salpa (L.) is one of the main macroherbivores in the western Mediterranean seagrass meadows which is known to feed on a wide range of macroalgae such as Caulerpa species. In order to elucidate if this consumption could minimize the spread of invasive species, during summer-autumn 2012, we investigate the importance of S. salpa herbivory pressure on C. prolifera and C. cylindracea compared to Posidonia oceanica and Cymodocea nodosa in a mixed meadow. A combination of field experiments and dietary analyses were used to investigate consumption rates, dietary contributions, and feeding preferences for the different macrophytes, including the role of epiphytes and nutrient contents in mediating fish herbivory. In summer, C. nodosa was the most consumed macrophyte (12.75 ± 3.43 mg WW·d−1), probably influenced by higher fish densities, higher nutritional quality of leaves and epiphytes, and by differences in epiphyte composition. Feeding observations suggest that fish may have a variable diet, although with a consistent selection of mixed patches with C. nodosa and C. prolifera. Indeed, food choice experiments suggest that when seagrass leaves are not epiphytized, fish prefer feeding on C. prolifera. Gut content and stable isotopic analyses supported the dietary importance of epiphytes and C. prolifera but also suggested that C. cylindracea could occasionally be an important food item for S. salpa. Our results highlight the role of epiphytes in S. salpa feeding decisions but also suggest that C. nodosa and C. prolifera may have an important contribution to fish diet. The variability in S. salpa diet confirm the need to carry out multiple approach studies for a better understanding of its potential influence over different macrophytes species.

Effects of culture densities on feed demand, behavioural tests and on the hepatic and cerebral oxidative status in tilapia (Oreochromis sp.)

The present study examines whether fish density affects behavioural tests, feed demand, and different parameters indicative of the oxidative status of the liver and brain of Orechromis sp. to identify welfare indicators for fish culturing. The fish were maintained at two commercial densities (5.6 and 16.2kg/m3) and hand fed for 4 weeks (four replies/treatment). At the third week, two replies at each density were provided for self-feeders. At the fourth week, fish were subjected to behaviour tests and sampled for oxidative status studies. To examine the effect of chronic stress caused by the density of farming and animal response to acute stress on oxidative stress, six fish/treatment was sampled before behaviour testing and six fish/treatment after behaviour testing. To study behavioural tests 10 fish/treatment were submitted three behaviour tests: an open-field test followed by a neophobia test and an object presentation repeat test. The oxidative stress was determined by quantifying free malondialdehiyde (FrMDA), protein-bound malondialdehyde (PrMDA), protein hydroperoxides (PrOH), hydroperoxides (LOOH) and thiobarbituric acid reactive substances (TBARs). In addition, the total antioxidant capacity (TEAC) and expression and activity ofglucose-6-phosphate dehydrogenase were determined. No differences among treatments for oxidative status were found. Under acute handling stress, both lots, LD/HF and HD/HF, in both tissues increased the parameters indicative of oxidative damage in brain (p<0.05). The feeding behaviour shows a diurnal activity in low-density, while high-density didńt demand feed. The results suggested that farmed tilapia culture at a density of 16kg/m3shows slightly altered fish behaviour, and only significant differences were observed open field test in low density fish, between self-feeder and hand feeding groups. The object presentation repeat, showing higher exploration movement in high-density fish regarding low density and hand −feeding fish. Furthermore, the fish at high density did not use demand-feeders and showed a decrease in the expression and activity of G6PDH. These two markers can be used to assess the welfare status of tilapia and to study the welfare status in other farmed fish.