Raw materials rich in EPA (C20:5n-3) and DHA (C22:6n-3) such as marine fish oils and meals are becoming increasingly scarce and valuable throughout the aquafeed industry, while human dietary intake of EPA and DHA is deficient in many parts of the world. Therefore, EPA and DHA sources have to be used judiciously and strategically in diets for especially freshwater fish, which, compared to their marine counterparts, tend to have a superior ability to biosynthesize these functionally important fatty acids (FAs) from their C18 precursors. Hence, this study tested the effect of a minimal dietary EPA + DHA level on growth performance, fillet and liver FA profile development, gene expression dynamics (elovl2, elovl5, and fads2), and nutrient excretion of African catfish (Clarias gariepinus) by replacing salmon oil with rapeseed oil in a marine-ingredient-free basal diet formulated with low EPA + DHA circular animal protein sources. Even though growth performance attained with a commercial African catfish diet utilizing the same salmon oil was not achieved with the two experimental diets, low EPA + DHA levels in the rapeseed oil experimental diet did not negatively affect growth. Expression of hepatic elovl2, elovl5, and fads2 was significantly elevated in the rapeseed oil-fed fish, while particularly EPA and DHA in the fillet, and EPA in the liver were consistently and significantly lower compared to the salmon oil-based diets. Concomitant examination of gene expression and FA profile development indicates the notable capacity of African catfish to de novo synthesize EPA and DHA from ALA (C18:3n-3), which suggests the potential of low EPA + DHA diets for this species. From a human nutritional point of view, however, this biosynthetic capacity was insufficient to compensate for the effect of the low EPA + DHA rapeseed oil diet on fillet FA profile quality, with EPA + DHA content and n-6:n-3 FA ratio being significantly negatively affected. Irrespective of oil source, the phosphorus-rich experimental diets resulted in a significantly improved ratio of excreted dissolved nitrogen to phosphorus compared to the industrial control from a plant nutrition perspective, confirming prior results on the ability to improve diets for use in aquaponics.

Fatty acids play an important role in the nutrition of fish and in assessing the quality of fish products for human consumption. The usual quantification of fatty acids using gas chromatography-mass spectrometry (GC-MS) requires the extraction and derivatization of fatty acids to fatty acid methyl esters (FAMEs). We compared the yields of fatty acids from liver samples of Oreochromis niloticus and the recovery of the internal standard C23:0 with five different methods for the production of FAMEs: (1) a method based on standard procedures according to EN ISO 12966-2:2017, (2) a modification of the first method with replacement of H 2 SO 4 by HCl for acid-catalyzed methylation, (3) a modification of the first method with use of HCl for acidic methylation and replacement of the solvent isooctane by MTBE, (4) chloroform extraction and subsequent acidic methylation, as well as (5) the rapid method EN ISO 12966-3:2016. The modified standard method according to EN ISO 12966-2:2017 with HCl for acid-catalyzed methylation and the extractant MTBE proved to be the most successful method with the highest fatty acid yields and recovery of the internal standard.

Fish can be infected with metacercariae (the final larval stage) of different species of potentially zoonotic digenetic trematodes (flukes). The fish-borne zoonotic trematodes thus compromise food safety and present a major threat for human health. Reducing the risk of human infections requires careful assessment and accurate taxonomic identification of these parasites. Here, we analysed metacercariae in muscle tissue of tench (Tinca tinca), sampled between March and September 2022 from three fish farms and three natural waterbodies in Germany. Whenever possible, we combined morphological and molecular data from the very same individual metacercariae using mitochondrial cytochrome oxidase c subunit 1 (cox1) and nuclear internal transcribed spacer 1 (ITS1) for species identification. Three morphotypes of metacercariae were found in the muscle of tench, corresponding to the trematode species Pseudamphistomum truncatum Rudolphi, 1819 (Opisthorchiidae), Hysteromorpha triloba Rudolphi, 1819 (Diplostomidae), and Paracoenogonimus ovatus Katsurada, 1914 (Cyathocotylidae). The high prevalence of metacercariae of P. truncatum and P. ovatus, both with zoonotic potential, poses a risk for human infections if undercooked or raw tench is consumed.

Aquaponic systems have potential to improve water and nutrient use efficiency compared to separately operated recirculating aquaculture systems (RAS) and hydroponic facilities. However, fish feeds are optimized for fish performance and low effluent nutrient discharge instead of integrated production of fish and plants, often leading to deficiencies of certain dissolved plant nutrients in RAS water. This study tested the effect of gradually replacing a blend of animal by-products (poultry by-product meal, catfish by-product meal, poultry blood meal) in a marine ingredient-free and entirely animal protein-based diet with a blend of common plant protein sources in four quasi-realistic isonitrogenous (42% crude protein) and isocaloric (20 MJ/kg) diets for African catfish on growth performance, body composition and nutrient release in dissolved and solid form into RAS water. Reducing the contribution of animal proteins to dietary protein from 100% over 75% and 50% to 25% resulted in the best biomass gain (%), feed conversion ratio (dry matter basis) and protein efficiency ratio for the 50% level, with differences, however, non-significant compared to the 100% and 75% level. At the 25% level, i.e. the highest level of plant protein inclusion, significant impairments regarding these growth indices as well as body ash, P and Ca were recorded compared to all other diets. No differences were observed for total dissolved 4 release per unit of feed between diets. Higher animal protein contribution lead to significantly higher release of dissolved P and Ca into RAS water, whereas increasing plant protein contribution caused significantly higher release of dissolved K, Mg and B. A mass balance of dietary nutrient fate revealed that N, P, K, S and Mg primarily ended up in fish biomass and in dissolved form in RAS water rather than in collected solids, while Ca, Fe, Mn, Zn and Cu predominantly ended up in solids and fish biomass rather than in dissolved form in RAS water. Modeling daily RAS water exchange rates needed to achieve the dissolved N concentration of a standard hydroponic nutrient solution on the basis of dissolved N release per unit of feed revealed that the entirely animal protein-based diet could reach 100%, 86% and 5% of the required dissolved N, P and K concentrations and that the diet with the highest plant protein inclusion could reach 100%, 14% and 15%, both at an exchange rate of 118 L/kg of feed/day.

Aquaponic (AQP) systems use aquaculture wastewater to supply the nutrients for plant growth. Recent fish meal (FM) production and price trends call for adopting of financially and environmentally sustainable protein sources for fish feed. However, different protein sources can alter the macro-and micro-nutrient effluxes from fish tanks and their nutritional value for plant growth. Early attempts to formulate the AQP diets have focused on enhancing nutrient efflux via dietary additives such as potassium diformate or potassium chloride. Here we present an alternative approach. We tested the effect of four aquafeeds differing in protein sources (complete substitution of FM with black soldier fly meal, BSFM; poultry blood meal, PBM; and hydrolyzed feather meal, HFM) on nutrient efflux in the water from Colossoma macropomum and Clarias gariepinus. The water from PBM and HFM-fed fish showed the highest efflux of TN (0.106 and 0.097 mg.L-1.g of feed, respectively), while the efflux of soluble reactive phosphorus and calcium was highest in FM-fed fish (0.012 and 0.028 mg.L-1.g of feed, respectively). BSFM-fed fish had the highest efflux of magnesium and potassium (0.008 and 0.023 mg.L-1.g of feed, respectively). Regarding macronutrients, the species-specific effects were observed only in sulphur, with higher efflux from C. macropomum. Iron efflux was not affected by diet, while higher efflux was observed in the trial with Clarias gariepinus. Effluxes of other micronutrients showed diet-and species-specific effects, except for zinc and manganese, with similar effluxes between species. In general, we found that it is possible to manipulate the nutrient efflux from RAS using alternative sustainable aquafeed components.

In order to further close nutrient cycles of aquaponic systems, it could be possible to integrate a third trophic level in the form of insect larvae production (i.e., black soldier fly larvae) to recycle internal waste streams into valuable nutrients. This would present opportunities to formulate sustainable circular aquafeeds that combine these internally available nutrients with complementary external raw materials. The ingredient composition of feeds for such circular multitrophic food production systems (CMFS) may affect fish performance as well as excretion of important dissolved plant nutrients such as N, P and K. Hence, fish meal from catfish processing (CM) as base ingredient was combined with variable levels of poultry by-product meal (PM) and black soldier fly larvae meal (BSFM) into three marine-ingredient-free experimental diets corresponding to hypothetical production scenarios of a CMFS that aims to integrate aquaponics with insect larvae production. These experimental diets and a commercial diet (COM) were compared using isonitrogenous and isolipidic formulations. They were fed to African catfish (Clarias gariepinus) in recirculating aquaculture systems (RAS) and evaluated concerning growth performance and nutrient excretion. All diets resulted in similar total inorganic nitrogen (TIN) excretion, whereas the increase of dietary PM inclusion from 0% (BSF diet) to 20% (MIX diet) and to 41% (PM diet) and concomitant reduction of BSFM inclusion led to increasingly higher soluble reactive phosphorus (SRP) excretion per unit of feed compared to the COM diet. While the PM diet enabled the best growth and feed conversion performance, the MIX and especially the BSF diet produced more similar performance to the COM diet, which generated the highest dissolved K excretion. The MIX and the PM diet resulted in the highest Ca and P, yet lower N content in the fish feces. Results indicate that combining CM with elevated levels of PM in the diet of African catfish could improve growth performance and reduce the need for P fertilization in aquaponics when compared to industrial diets optimized for low environmental impact. Findings are discussed regarding their implications for CMFS and aquaponic feed formulation.

In aquaponics and circular multitrophic food production systems, dietary protein source, as well as fish species choice, particularly in cases of different nutritional physiology, could be factors affecting excreted nutrient profiles. Accordingly, growth performance, dissolved nutrient accumulation and feces nutrient profiles were evaluated for African catfish (Clarias gariepinus) reared in recirculating aquaculture systems (RAS) and fed single protein source diets based on black soldier fly larvae meal (BSF), poultry by-product meal (PM), poultry blood meal (PBM) and fish meal (FM) and the results were compared to previous findings for Nile tilapia (Oreochromis niloticus). All diets resulted in significantly different growth performances of African catfish, with FM producing the best growth performance, followed by PM, BSF and PBM. PM resulted in the highest soluble reactive phosphorus concentrations (SRP) in the RAS water; whereas, BSF resulted in the highest K, Mg and Cu concentrations. The highest feces nutrient density was recorded for PBM; whereas, FM and PM yielded the lowest feces nutrient density. Comparing African catfish to Nile tilapia revealed that the former showed significantly better growth performance with FM and PM, however, significantly weaker performance with BSF. Although dissolved K accumulation was similar between species across diets, significant differences were recorded for total inorganic nitrogen and SRP production per unit of feed for individual diets. Despite similar feces nutrient profiles, African catfish produce significantly less feces dry matter per unit of feed for each diet compared to Nile tilapia. Findings are discussed regarding their implications for aquafeed development in the context of circular multitrophic food production systems.

With the general objective of optimizing internal nutrient recycling, circular multitrophic food production systems, e.g., combining fish, plant, and insect larvae production, rely on the quality and composition of sustainable nutritional inputs. Therefore, differences in dissolved and solid nutrient excretion patterns produced by Nile tilapia (Oreochromis niloticus) reared in recirculating aquaculture systems (RAS) with 5% daily water exchange and fed black soldier fly meal (BSFM), poultry by-product meal (PM), poultry blood meal (PBM) and fish meal (FM) as single protein sources were investigated to evaluate the potential for creating specific fish meal-free diets. Fish fed the FM and PM diet showed the significantly best (p < 0.05) and among each other similar (p > 0.05) growth performance (specific growth rate (SGR): 2.12 ± 0.04/2.05 ± 0.11; feed conversion ratio (FCR): 0.86 ± 0.03/0.92 ± 0.01), whereas the PBM diet caused significantly reduced performance (SGR: 1.30 ± 0.02; FCR: 1.79 ± 0.05) in comparison to the FM/PM diet as well as the BSF diet (SGR: 1.76 ± 0.07; FCR: 1.11 ± 0.05). The FM and PM diet resulted in a faster increase and significantly higher dissolved nitrogen and phosphorus levels, while the BSF diet caused faster accumulation and significantly elevated levels of dissolved potassium, magnesium, and copper. The PBM diet resulted in the feces with the significantly highest nutrient density (gross energy, crude protein, and amino acids) but overall much lower dissolved nutrient levels in the water. Results are discussed with regard to implications for developing circular multitrophic food production systems.

Inflammation is an essential process as a reaction towards infections or wounding. Exposure to hazardous environmental pollutants can lead to chronic inflammations, where the resolving phase is delayed or blocked. Very contradictory studies have been reported on the pro- and anti-inflammatory effects of humic substances (HSs) leading to significant disagreements between researchers. To a certain extent, this can be attributed to the chemical heterogeneity of this group of xenobiotics. Here we show for the first time that pro- and anti-inflammatory effects can occur by one HSs. We adapted an assay that uses green fluorescence-labeled zebrafish larvae and CuSO4 to indue an inflammation. In wild-type larvae, exposure to 50 µM CuSO4 for 2 h activated the production of reactive oxygen species, which can be monitored with a fluorescence dye (H2DCFDA) and a microplate reader. This allows not only the use of wild-type fish but also a temporal separation of copper exposure and inflammatory substance while retaining the high throughput. This modified assay was then used to evaluate the inflammatory properties of a fulvic acid (FA). We found, that the aromatic structure of the FA protects from inflammation at 5 and 50 mg C/L, while the persistent free radicals enhance the copper-induced inflammation at ≥ 300 mg C/L.

Artificial light at night (ALAN) can disrupt biological rhythms of fish and other vertebrates by changing the light information of the nocturnal environment. Disrupted biorhythms can impair the immune system of vertebrates as it has been shown for conditions with continuous illumination or long-day photoperiod in many vertebrates, including fish. Nonetheless, this has not been shown so far for typical ALAN scenarios with high light intensities during day and low light intensities at night. Therefore, in this study, proxies for the innate immune system and oxidative stress as well as body indices of Eurasian perch Perca fluviatilis were measured under a wide range of intensities of nocturnal illumination. The authors found no changes in parameters of the innate immune system and no significant changes in proxies for oxidative stress after 2-week exposures to nocturnal illuminance ranging from 0.01 lx to 1 lx in one experiment or from 1 lx to 100 lx in a second experiment. A decrease in the hepato-somatic index at the highest tested light intensity of 100 lx compared to the dark control was the only significant difference in all parameters among treatments. After 2 weeks of exposure, ALAN does not seem to seriously challenge the innate immune system and seems to cause less oxidative stress than expected. The results of this study contradict the findings from other studies applying continuous illumination or long-day photoperiod and highlight the importance of further research in this field. Because ALAN represents a sustained modulation of the environment that may have cumulative effects over time, long-term studies are required for a better understanding of how ALAN modulates the health of fish.

Aquaculture plays a pivotal role in covering dietary animal protein demands and restocking endangered fish populations. However, high mortality takes place at the earliest life stages: prior and immediately after hatching. Improving growth and health parameters by immunostimulants is widely used in older fish, but rarely studied in larvae. Fulvic acids (FAs) are natural substances found in soil and water. Using zebrafish as a model organism, we evaluated the effects of exposure to a FA at concentrations ranging from 1 to 500 mg C/L (mg dissolved organic carbon per liter) on embryonic development. Furthermore, the concentration of reactive oxygen species (ROS) inside the larvae as well as the molecular mechanisms involved in growth, immune response, and antioxidative protection were determined at 5, 50, and 500 mg C/L. 20 to 200 mg C/L accelerated the hatching, which was mediated by increased expression of ifg-1, gh, and he1-α. Furthermore, lyz and mpx were significantly increased at 5 and 50 mg C/L. A concentration of 500 mg C/L induced genes involved in the protection against ROS (nrf-2, keap-1, cat, sod-1), increased the concentration of ROS inside the larvae and caused tissue damage and mortality. Interestingly, 50 mg C/L activated ROS protection as well (nrf-2, sod-2), while no increase of ROS was found in the larvae. Our results show, that FA at low to medium concentrations can increase the health of larvae, but becomes detrimental at higher concentrations.

Aquaculture has become imperative to cover the demands for dietary animal protein. Simultaneously, it has to overcome prejudices from excessive use of antibiotics and environmental impacts. Natural supplements are traditionally applied orally. In this study, we demonstrated another pathway: the gills. Humic substances are immunostimulants and a natural part of every aquatic ecosystem, making them ideal to be used as bath stimulants. Five and 50 mg C/L of a fulvic acid-rich humic substance was added for 28 days to the water of juvenile rainbow trout ( Oncorhynchus mykiss ). This fulvic acid is characterized by a high content of phenolic moieties with persistent free radicals and a high electron exchange capacity. The high concentration of the fulvic acid significantly increased growth and reduced the food conversion ratio and the response to a handling-stressor. Phagocytosis and potential killing activity of head kidney leukocytes were increased, as well as the total oxyradical scavenging capacity (TOSC) and lysozyme activity in the gills. In conclusion, immunostimulation via gills is possible with our fulvic acid, and the high phenolic content improved overall health and stress resistance of fish.

Intraspecific diet specialization, usually driven by resource availability, competition and predation, is common in natural populations. However, the role of parasites on diet specialization of their hosts has rarely been studied. Eye flukes can impair vision ability of their hosts and have been associated with alterations of fish feeding behavior. Here it was assessed whether European perch ( Perca fluviatilis ) alter their diet composition as a consequence of infection with eye flukes. Young-of-the-year (YOY) perch from temperate Lake Müggelsee (Berlin, Germany) were sampled in two years, eye flukes counted and fish diet was evaluated using both stomach content and stable isotope analyses. Perch diet was dominated by zooplankton and benthic macroinvertebrates. Both methods indicated that with increasing eye fluke infection intensity fish had a more selective diet, feeding mainly on the benthic macroinvertebrate Dikerogammarus villosus , while less intensively infected fish appeared to be generalist feeders showing no preference for any particular prey type. Our results show that infection with eye flukes can indirectly affect interaction of the host with lower trophic levels by altering the diet composition and highlight the underestimated role of parasites in food web studies.

Encapsulation of the parasitic nematode Anguillicola crassus Kuwahara, Niimi & Hagaki is commonly observed in its native host, the Japanese eel (Anguilla japonica Temminck & Schlegel). Encapsulation has also been described in a novel host, the European eel (A. anguilla L.), and there is evidence that encapsulation frequency has increased since the introduction of A. crassus. We examined whether encapsulation of A. crassus provides an advantage to its novel host in Lake Müggelsee, NE Germany. We provide the first evidence that encapsulation was associated with reduced abundance of adult A. crassus. This pattern was consistent in samples taken 3 months apart. There was no influence of infection on the expression of the two metabolic genes studied, but the number of capsules was negatively correlated with the expression of two mhc II genes of the adaptive immune response, suggesting a reduced activation. Interestingly, eels that encapsulated A. crassus had higher abundances of two native parasites compared with non-encapsulating eels. We propose that the response of A. anguilla to infection by A. crassus may interfere with its reaction to other co-occurring parasites.

Visual performance and environmental conditions can influence both behavioral patterns and predator-prey interactions of fish. Eye parasites can impair their host’s sensory performance with important consequences for the detection of prey, predators, and conspecifics. We used European perch (Perca fluviatilis) experimentally infected with the eye fluke Tylodelphys clavata and evaluated their feeding behavior and competitive ability under competition with non-infected conspecifics, in groups of four individuals, for two different prey species (Asellus aquaticus and Daphnia magna). To test whether the effect of T. clavata infection differs at different light conditions, we performed the experiments at two light intensities (600 and 6 lx). Foraging efficiency of perch was significantly affected by infection but not by light intensity. The distance at which infected fish attacked both prey species was significantly shorter in comparison to non-infected conspecifics. Additionally, infected fish more often unsuccessfully attacked A. aquaticus. Although the outcome of competition depended on prey species, there was a general tendency that non-infected fish consumed more of the available prey under both light intensities. Even though individual prey preferences for either A. aquaticus or D. magna were observed, we could not detect that infected fish change their prey preference to compensate for a reduced competitive foraging ability. As infection of T. clavata impairs foraging efficiency and competitive ability, infected fish would need to spend more time foraging to attain similar food intake as non-infected conspecifics; this presumably increases predation risk and potentially enhances transmission success to the final host.

Invasive parasites have been implicated in the declines of several freshwater species. The swim bladder nematode Anguillicola crassus was introduced into Europe in the 1980s and is considered a threat to the European eel (Anguilla anguilla). Infection affects stress resistance and swimming behaviour. European eels produce an immune response against the parasite during the late stages of infection and after repeated infections. We used RNA-seq to examine the molecular response to infection during the poorly understood early stage and identify expression of genes and associated processes that are modified in two immune organs of European eels 3 days post infection with A. crassus. In the spleen, 67 genes were differentially expressed, 32 of which were annotated. Most of these were involved in immune processes and their regulation. Other differentially expressed genes in the spleen were important for heme metabolism and heme turn-over. In the head kidney, 257 genes (134 annotated) were differentially expressed. Several of these were associated with immune functions. Other differentially expressed genes in the head kidney were related to renal function, in particular osmoregulation and paracellular flow. We conclude that the early response of European eels to A. crassus is complex and involves various processes aside from the immune system. We identified molecular changes occurring early during the infection and identified candidate genes and processes which will facilitate future studies aimed at determining the factors affecting European eel viability in the face of this invasive parasite.

A present ecological issue causing secondary salinization in different countries is the discharge of effluents by the potash mining industry. In Germany, the River Werra is used as a sink for potash mining discharges containing high concentrations of ions, predominantly Cl-, K+, Na+, and Mg2+ resulting in a strong decline of the biodiversity and abundance of local species. However, hardly anything is known about the acute and chronic physiological effects of high concentrations and imbalances of ions being prevalent in potash mining effluents in fish. Therefore, the stress response and selected immune and growth parameters were investigated in standardized laboratory experiments. A native freshwater fish species, Rutilus rutilus, was exposed to concentrations of the high currently allowed (HT) and lowered future thresholds (LT) and three different ion solutions (containing high Mg2+ (Mg), high K+ (K) and high Mg2++K+ (Mg+K) concentrations) for four different exposure times (24 h, 7 d, 21 d, 8 wk). Tank water (additionally after 9 and 12 h) and plasma cortisol, glucose and protein, hematocrit and hemoglobin were determined after each exposure time. Furthermore, plasma lysozyme and head kidney leucocyte respiratory burst activity (only after 21 d) were evaluated as well as growth parameters. A transient stress response was induced in almost all groups. Tank water cortisol was elevated after 9 h in HT, LT and Mg+K and in HT after 12 h, whereas glucose concentrations increased after 24 h in all exposure groups except K. HT led to enhanced hematocrit and hemoglobin content after 24 h. Plasma protein, immune system and growth were not affected in any group. None of the ion solutions induced acute toxicity but most triggered typical acute stress reactions. Rather the sum of high ion concentrations than single ions challenged the fish. Even though the effects observed in adult roach were only transient and indicate acclimatization under laboratory conditions, adverse effects observed in the river are evident and further research on physiological endpoints including reproductive parameters and impacts on younger life stages seem to be needed to scientifically base protective thresholds.

Abstract Invasive parasites are involved in population declines of new host species worldwide. The high susceptibilities observed in many novel hosts have been attributed to the lack of protective immunity to the parasites which native hosts acquired during their shared evolution. We experimentally infected Japanese eels (Anguilla japonica) and European eels (Anguilla anguilla) with Anguillicola crassus, a nematode parasite that is native to the Japanese eel and invasive in the European eel. We inferred gene expression changes in head kidney tissue from both species, using RNA‐seq data to determine the responses at two time points during the early stages of infection (3 and 23 days postinfection). At both time points, the novel host modified the expression of a larger and functionally more diverse set of genes than the native host. Strikingly, the native host regulated immune gene expression only at the earlier time point and to a small extent while the novel host regulated these genes at both time points. A low number of differentially expressed immune genes, especially in the native host, suggest that a systemic immune response was of minor importance during the early stages of infection. Transcript abundance of genes involved in cell respiration was reduced in the novel host which may affect its ability to cope with harsh conditions and energetically demanding activities. The observed gene expression changes in response to a novel parasite that we observed in a fish follow a general pattern observed in amphibians and mammals, and suggest that the disruption of physiological processes, rather than the absence of an immediate immune response, is responsible for the higher susceptibility of the novel host.

In freshwater ecosystems, snails can significantly influence the competition between primary producers through grazing of periphyton. This activity can potentially be modified by trematodes, a large group of parasites which mostly use molluscs as the first intermediate host. Available studies, however, show contradictory effects of trematodes on snail periphyton grazing. Here, we used four different freshwater snail–trematode systems to test whether a general pattern can be detected for the impact of trematode infections on snail periphyton grazing. In our experimental systems, mass-specific periphyton grazing rates of infected snails were higher, lower, or similar to rates of non-infected conspecifics, suggesting that no general pattern exists. The variation across studied snail–trematode systems may result from differences on how the parasite uses the resources of the snail and thus affects their energy budget. Trematode infections can significantly alter the grazing rate of snails, where, depending on the system, the mass-specific grazing rate can double or halve. This underlines both, the high ecological relevance of trematodes and the need for comprehensive studies at the species level to allow an integration of these parasite–host interactions into aquatic food web concepts.

The Ram cichlid Mikrogeophagus ramirezi (Myers & Harry, 1948) was used to examine whether flash photography can be a stressor for fish in aquaria. The point in time of the highest cortisol concentration in whole-body homogenates was determined by the temporal course of the cortisol response following air exposure as stressor. Thus, the potential stress response to camera flashlight was examined 22 min after exposure to a single flash and after repeated flashes by applying 10 flashes per minute for 8 hr/day over 2 weeks. In both experiments the stress parameters cortisol and glucose were not increased due to exposure to the flash light. In contrast, after a single flash mean cortisol values tended to be lower and mean glucose values were significantly lower than in the control group, and after repeated flashes mean cortisol and glucose values were significantly lower than in the control group. Furthermore, treated fish showed less intraspecific aggressive interactions. These results can be explained by a possible dazzling or irritation of the fish by camera flashes, thus reducing the natural aggressive behaviour and, consequently, the concentration of stress hormones and mobilisation of glucose. In summary, the physiological stress parameters cortisol and glucose do not reveal that flash photography induces stress in M. ramirezi, and, on the contrary, might even reduce stress effects by lowering intraspecific aggressive behaviour of the fish.