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The Working Group on Electrical Trawling creates a platform for supra-national joint research projects on electro-trawling and scientific publications. The group also reviews all relevant studies on marine electrofishing and discusses the ongoing and upcoming research projects in the light of knowledge gaps. Research areas covered by the group included fishing tactics and dynamics, organisms and ecological impacts and selectivity of electro trawling. A study into the exploitation of local fishing grounds revealed that pulse trawlers and conventional tickler chain beam trawlers had similar tactics spending 10% of their tows searching for a fishing ground and spending 90% of their tows exploiting a fishing ground. In-situ field campaigns revealed a lower impact of pulse trawls on biogeochemical parameters compared to traditional beam trawl methods. Laboratory experiments found that while alternating or pulsed bipolar currents readily penetrated the sediment, biogeochemical effects appeared to be inhibited from occurring. The combined results concluded that the environmental impact of electricity from pulse trawls is relatively minor compared to the mechanical disturbances created from the same gears. Behavioral response thresholds for pulsed electric fields were determined in laboratory experiments for electroreceptive as well as non-electroreceptive fish species. Comparison of these thresholds to simulations of electric fields around commercial fishing gears suggest that electrical pulses are unlikely to substantially affect the investigated fish species outside the trawl track. A field study into direct mortality among fish and benthic organisms in the wake of pulse trawlers refuted claims that pulse trawling causes mass mortality among non-target species. A study into the selectivity of shrimp pulse trawling vs. traditional trawling concluded that that shrimp fishing using pulse gear does not result in higher amounts of undesired bycatches of small shrimp, fish and benthos when compared to the traditional shrimp beam trawl fisheries. The outline of a PhD project that started in 2021 into organism and ecological impacts of electrofishing for razor clams in Scottish shallow coastal habitats was presented and preliminary results were shared.

Electric stimulation was used in the North Sea beam trawl fishery for common sole to reduce its environmental impact. Because electrical stimulation may cause internal injuries in fish, a laboratory experiment was conducted to study the effect of pulse exposure on lesser sandeel (Ammodytes tobianus) and greater sandeel (Hyperoplus lanceolatus), important mid-trophic species in the North Sea ecosystem. We exposed 244 sandeels between two electrodes to a pulsed bipolar current for 2 s in an experimental cage with 5 cm sediment; 221 control fish were handled similarly but not exposed. The occurrence of spinal injuries and internal haemorrhages were scored using X-radiography and dissection. None of the sandeels exposed to a field strength of up to 600 V m–1 showed spinal injury or haemorrhage. Equal numbers of minor spinal abnormalities were found in exposed and control fish. In the absence of spinal injuries, we estimated by bootstrapping the field strength below which spinal injuries are unlikely to occur, i.e. the lower limit threshold, and the corresponding limit dose–response relationship between field strength and injury probability. We conclude that it is unlikely that pulse trawl fishery will have an ecologically significant adverse effect on the population abundance of sandeels, because of the low probabilities of exposure and injury.

Aims Geosmin is associated with off-flavour problems in recirculating aquaculture systems (RAS) and represents an economic problem for the aquaculture industry. This study aims at investigating factors influencing the composition of the bacterial microbiota, in particular the presence of geosmin producers and the environmental and farming factors favouring geosmin accumulation. Methods and Results Several water quality parameters were correlated to the composition of the microbiota with special emphasis on the presence of geosmin producers within 26 different RAS from four European countries. Three novel groups of geosmin-producing bacteria were quantified to identify potential correlations with geosmin concentration. Conclusions The microbiome differed significantly between systems. However, phosphate levels, calcium levels and redox potential correlated to geosmin concentration in the water and the presence of the Actinomycetales geosmin-producers but not with the presence of other groups of geosmin-producing bacteria. Oxygen levels and conductivity were found to negatively correlate with geosmin concentration. A large proportion of the detected geosmin producers represented novel taxonomic groups not previously linked with this activity. Significance and Impact of the Study These results improve our understanding of the diversity of microbiota in RAS and the water quality parameters favouring the populations of geosmin-producing bacteria and the production of geosmin.

Beam trawls are widely used in European waters to target flatfish, in particular sole (Solea solea) and produce a substantial amount of discards. Tickler chains used to chase flatfish from the seabed have considerable adverse impacts on the ecosystem. Since 2010, innovative electrified beam trawls – pulse trawls – have been used in the North Sea to study whether the ecological impact can be reduced. Pulse trawls use pulsed bipolar current with a peak amplitude of 60 V, frequency of 30 Hz, and pulse width of 340 µm. The electrical stimulus elicits a muscle cramp response, immobilising the fish in front of the trawl. Exposure duration is ~1.5 s and the electric field strength quickly decays with increasing distance from the electrodes to values below fish response thresholds within the trawl width. We describe the innovative pulse trawl and present results of an impact assessment. Pulse trawls are 30% more efficient to catch sole but catch less undersized fish and benthos (57 and 80 % respectively). Lower towing speed reduces the trawling footprint by 23% and CO2 emissions by 30%. Abandoning tickler chains reduces mechanical disturbance of the seabed, penetration depth, and benthic impact by 20-60%. Pulse exposures on six invertebrate species showed varying acute responses but no lasting signs on locomotor activity and no increased mortality. Bio-geochemical processes were not affected by pulse exposure. The induced muscle cramp may result in spinal injuries and haemorrhages in fish. Extensive sampling of commercial pulse trawl trips (>15,103 fish, 14 species) showed an elevated injury probability in Atlantic cod, but not in other fish species, compared to samples from tickler-chain beam trawls. The population effect of pulse exposure on cod is negligible due to limited overlap between the fishery and population distribution. Although our study showed that pulse trawls can be safely used to exploit the sole quota with a substantial reduction in ecosystem and environmental impacts, societal concerns about the use of electricity fueled by a campaign of an environmental NGO against pulse fishing resulted in a European Union decision to maintain the prohibition of pulse trawls.

Fish farmers utilize the reversibility of the bioconcentration process to depurate geosmin and other off- flavour causing chemicals from their fish by placing them in clean water just before harvest. To better understand and improve this process, we investigated effects of temperature and water exchange of depuration tanks on geosmin elimination from Atlantic salmon (Salmo salar). Fish loaded with geosmin were depurated for 144 h during which they were subjected to combinations of water exchange (stagnant water or a water exchange rate of ~1200 L/kg fish/d) and temperature (~11.5 or ~14.5 °C) treatments. Model predictions indicated enhanced depuration by water exchange, elevated temperature and interactive effects of these two factors, plus geosmin accumulation in the depuration tank water. The latter was predicted but not observed in the experiment. Furthermore the elevated temperature did not enhance geosmin elimination from Atlantic salmon, possibly because in the tanks with water exchange, temperature treatments did not cause differences in oxygen levels and thus gill ventilation rates. The water exchange significantly increased geosmin elimination from Atlantic salmon, indicating that removal of excreted geosmin from the direct environment of this fish is needed to obtain maximal geosmin elimination from the fish.

The effect of feeding during off-flavour depuration on the elimination of geosmin from muscle tissue (fillet) and ovaries as a model for caviar was assessed in Nile tilapia (Oreochromis niloticus) (mean ± SD weight of 185 ± 15.0 g). The experiment had a 2 × 4 factorial design with feeding level (starved or fed) and depuration time (24, 48, 72 and 96 h) as factors with duplicates for each of the 8 treatment combinations. Fish were normally loaded with geosmin prior to the experiment. During off-flavour depuration geosmin levels in fillet and ovary declined over time in both fed and starved tilapia. In fed tilapia geosmin declined faster from the ovaries compared to starved fish (p = 0.018). The same trend of a faster decline was observed for the muscle tissue (fillets) of fed tilapia, though only numerically (p = 0.11). Because faster geosmin elimination paralleled with high blood lipids, we do not rule out that blood lipids are involved in geosmin transport via the circulatory system and that low blood lipid levels are limiting geosmin elimination in starved fish. No difference in geosmin elimination rate was detected between ovary and muscle tissue in Nile tilapia. Off-flavour depuration time is strongly reduced when farmers adopt a practice of feeding Nile tilapia during off-flavour depuration.

Off-flavour described as ‘earthy-musty’ is commonly reported in fish raised in land-based aquaculture systems. Bioconcentration in fish of lipophilic geosmin produced by microbiota is regarded as the most important cause for this off-flavour. Despite that off-flavour in fish is an extensively studied and well documented problem, satisfactory solutions are lacking. The most promising solution on a short notice is the optimization of geosmin removal from fish prior to harvest, i.e., the off-flavour depuration process. This depuration process however is not always effective nor reliable. Hence, further optimization is needed to prevent market entrance of off-flavoured fish. The general objective of this thesis was therefore to improve off-flavour depuration processes, with a focus on geosmin excretion. The experimental work conducted in this thesis leads to the following conclusions regarding the bioconcentration of geosmin in fish:Rainbow trout bioconcentrates waterborne geosmin, but in vivo bioconcentration is less than the general fish bioconcentration model for moderately lipophilic compounds predicts based on theoretical rate constants.Geosmin distribution within the body of rainbow trout and Nile tilapia is not exclusively governed by the lipid content of tissues/organs.Geosmin depuration from European eel is not affected by the water renewal rate of depuration tanks.Geosmin depuration from Atlantic salmon is enhanced by increased water renewal rate of depuration tanks.Geosmin elimination from European eel does not follow the generally accepted passive diffusion mechanism for excretion of lipophilic chemicals. We assume that geosmin biotransformation by the eel is probable.Exercise (swimming) enhances geosmin excretion by European eel and reduces the time required to depurate off-flavours from fish.Fed Nile tilapia eliminate geosmin faster from their ovaries compared to starved fish.The rate of geosmin elimination from muscle tissue and ovary is similar in Nile tilapia.This thesis shows that there is more to geosmin bioconcentration in fish than passive partitioning over water and lipid compartments. The physiology underlying the bioconcentration process remains to be fully elucidated. Yet this thesis presents various measures for the optimization of off-flavour depuration processes that can be readily adopted by the aquaculture industry.

Dutch demersal fisheries in the North Sea is a mixed fishery that mainly targets Dover sole (Solea solea) with plaice (Pleuronectes platessa), turbot (Scophthalmus maximus), brill (Scophthalmus rhombus) and other species as valuable bycatches. The fleet currently uses two gear types: pulse beam trawls and conventional tickler chain beam trawls. Pulse beam trawlers operate with a temporary exemption from the EU prohibition to use electric stimulation in fishing gears, of which the last exemptions will expire in June 2021.To assess the consequences of transitions between pulse and tickler chain beam trawling for discards mortality, knowledge on the discards survival probabilities as well as the amount of discards is required for both gear types. The objective of the current study was to estimate discards survival probabilities for undersized plaice, sole, turbot, brill and thornback ray discarded by tickler chain beam trawl fisheries using fish condition as a proxy for survival probability. To this end the condition and reflex impairment of undersized fish in the catches of tickler chain beam trawlers were assessed and compared to similar data collected from pulse trawl fisheries. For spotted ray we assessed fish condition in tickler chain beam trawling but could not estimate its discards survival probability because a relation between survival probability and fish condition is lacking for this species. In this study direct mortality imposed by the tickler chain beam trawling ranged between 10 and 32% in flatfish species and was between 2-4 times higher than in pulse beam trawling. Direct mortality in ray species was lowest among the investigated species (2-8%) and did not differ between the two gear types. Differences in direct mortality were reflected in the condition scores. Direct mortality of sole was higher in tickler chain beam trawling (17%) than in pulse beam trawling (8%). Brill, turbot and plaice discarded by pulse beam trawling are in better condition than when discarded by tickler chain beam trawl fisheries. For sole no effect of gear type on fish condition could be detected. We consider the lower fish condition scores of brill, plaice and turbot from tickler chain beam trawling a direct reflection of the higher mechanical impact of this gear on the fish. For thornback ray and spotted ray no effect of gear type on fish condition could be detected. The predicted survival of plaice, brill and turbot discards indicate that discards survival could indeed be lower in tickler chain beam trawl fisheries compared to pulse beam trawl fisheries. For sole and thornback ray discards we found no evidence for such difference between gear types. The discards survival probabilities for tickler chain beam trawling as presented in this study should be considered as predictions based on the currently best available information instead of definite values. Actual measurements of discards survival at sea are needed to confirm and quantify survival probabilities in tickler chain beam trawling.

This study established that geosmin depuration from European eel is not affected by the water renewal rate of depuration tanks. A general fish bioaccumulation model extended with terms that account for effects of tank water renewal rate and system losses of chemicals, predicted strong effects of the water renewal rate of depuration tanks on geosmin depuration from European eel. Model predictions were validated in a depuration experiment with geosmin-loaded European eel (n = 95) with a mean (SD) individual weight of 134.4 (5.0) g and a mean (SD) lipid content of 33.7 (2.8) % (w/w). Fish were depurated for 24, 48 or 72 h at three different tank water renewal rates (0.3, 3.3 and 33 day-1). Treatments were installed by three different mean (SD) water flow rates (13.8 (1.3), 143.5 (9.2) and 1511 (80) L kg fish-1 day-1) over 30-L tanks. Eels eliminated geosmin from their bodies, but unlike the model predicted, this was independent of the water renewal rate of the depuration tanks. Although being eliminated from the fish, geosmin hardly appeared and certainly did not accumulate in the water of the depuration tanks as the model predicted. This observation may be explained by geosmin being eliminated from eel as metabolite rather than the parent compound. Geosmin elimination from eel seems not to occur according to the generally accepted passive diffusion mechanism for excretion of lipophilic chemicals, and geosmin biotransformation by the eel seems indicated. Clearly geosmin depuration from European eel cannot be enhanced by increasing water renewal rates of depuration tanks.

Pulse trawl fisheries involve the use of electrical pulses to immobilize (cramp) target species and make them available for capture. A major concern related to pulse trawl fisheries is the passing pulse trawl causing direct, mass mortalities among benthic organisms, resulting in a ‘graveyard’ in the wake of a pulse trawler. This has never been investigated in situ and the current pilot study therefore aimed to develop a method for in situ assessments and to perform a first assessment. A pulse trawler equipped with double rigs made pulse trawl tracks specific for the current experiment. Within 15 to 30 minutes after passage of the pulse trawler, one of its trawl tracks was sampled with a shrimp trawler by a 10 minute tow with a small mesh shrimp beam trawl while the other, similar shrimp trawl was deployed outside the pulse trawl track to obtain control samples. The experiment consisted of two pulse trawl track treatments: a complete pulse trawl and a pulse trawl with its netting and ground rope removed to minimize its mechanical impact. This allowed for isolating electrical from mechanical impacts. In total two paired samples of treatments and controls were obtained per pulse trawl track treatment. The condition of three fish species and three species of invertebrates was assessed. Fish species included plaice (Pleuronectus platessa), dab (Limanda limanda) and solenette (Buglossidium luteum). Invertebrate species included flying crab (Liocarcinus holsatus), hermit crabs (Paguroidea spp.) and brittle stars (Ophiuroidea spp.). Direct survival ranged from 91-100% among treatments for the fish and 88-100% for the invertebrates. No significant differences in direct survival were detected between the two treatments and their respective controls for any of the species. Underwater video observations confirmed deployment of the sampling trawl inside the pulse trawl tracks, although part of swept area was outside the pulse trawl tracks. However, also when correcting the observed direct mortality for this, no differences between treatments and controls were detected. We conclude that with the right equipment, skilled skippers and a calm sea, it is possible to collect biota samples from a track recently trawled by a pulse trawler. However, it is very difficult to sample exclusively from a pulse trawl track; it is inevitable that part of the area swept by a sampling tow lies outside the pulse trawl track aimed for. Despite these limitations, any direct mass mortality caused by a passing pulse trawler would have been recorded in the current study, not only for the systematically observed species but also for other species in the samples. Our study thus did not find any evidence of direct mortality among plaice, dab, solenette, flying crab and brittle stars.

Discards survival probability in conventional commercial 80 mm pulse fisheries was assessed for undersized plaice (Pleuronectus platessa n=558), sole (Solea solea n=274), turbot (Scophthalmus maximus n=111), brill (Scophthalmus rhombus n=90), thornback ray (Raya clavata n=99) and spotted ray (Raya montagui n=23). In total nine sea trips were performed on three commercial pulse-trawlers with three trips per trawler. Sea trips were spread out over the year to account for potential seasonal variation in discards survival. All test-fish were randomly collected from the end of the sorting belt at both the start and end of the catch-sorting process from multiple hauls per sea trip. Reflex impairment and damages were assessed for all test-fish and summarized in a vitality index score indicating fish condition. Test-fish were housed in four custom-built monitoring units installed on board. Each unit contained 16 24L tanks. Numbers of fish housed per tank were five (plaice, sole), three (turbot, brill) and one (rays) fish per tank. Tank water was continuously renewed with sea water at a rate of at least one tank volume per hour to maintain proper water quality. Survival was monitored and dead fish were removed upon detection. Upon arrival in the vessel’s home port, monitoring units were taken from board and transported to and placed in a climate controlled room to continue survival monitoring for two more weeks. The total monitoring period ranged from 15 to 18 days among test-fish depending on the day of collection at sea. In the climate controlled room, tank bottoms were covered with coarse sand and fish were fed natural food. Control-fish, fish of the same species and in good condition collected in advance at sea, were deployed during all sea trips (30-35 control plaice, 10 control sole, 3 control turbot and brill, 2 control thornback ray, 2 control spotted ray per sea trip). Control-fish were treated equally as test-fish to separate fisheries related mortality from mortality caused by the experimental procedures. Discards survival probabilities were estimated from counts of surviving fish at the end of the monitoring period.Within all species, discards survival probabilities varied among sea trips. Discards survival probability estimates and their 95% confidence intervals (95%CI) based on all sea trips combined were 14% (95%CI 11-18%) for plaice, 19% (95%CI 13-28%) for sole, 30% (95%CI 20-43%) for turbot, 13% (95%CI 7-23%) for brill and 53% (95%CI 40-65%) for thornback ray. For spotted ray discards survival probabilities of 21% and 67% were observed during two sea trips. Given the agreement with a previous and comparable study and the large number of observations collected year-round, we consider the current results for plaice and for sole to represent the actual overall survival probability of undersized plaice and sole discards in the 80 mm pulse-trawl fisheries. The discards survival probability estimates for turbot, brill and thornback ray are based on limited numbers of observations per species. These estimates should therefore be considered and treated as a first indication of the actual discards survival probability for these species in the 80 mm pulse-trawl fisheries. Collection of more data should narrow down the current 95% confidence intervals and give more precise estimates for the survival probability. We expect however that these more precise estimates lie within the current 95% confidence intervals for the survival probability estimates.In all species tested, discards survival was strongly affected by fish condition, with large differences in survival probability between fish in best and worst condition. The proportion of fish in the best condition is small and their high survival probability has little effect on the survival probability of the entire population in the catches. We therefore recommend that measures aimed at increasing discards survival focus on improving the condition of discarded fish. Since catch-processing time seems to have no effect on fish condition nor discards survival, we recommend that measures aimed at improving the condition of discarded fish focus on the fish capture process rather than catch processing.

Measures to increase discard survival in the 80 mm pulse-trawl fisheries were assessed under commercial fishing conditions using plaice (Pleuronectes platessa) as model species. Measures tested were a water filled hopper (8 sea trips), short hauls (90 instead of 120 min, 4 sea trips) and a knotless cod-end (1 sea trip) with undersized plaice. In total nine sea trips were performed with three different commercial pulse-trawlers. Sea trips were spread over the year to account for potential seasonal variation in discards survival. Additional trials with a knotless cod-end (one sea trip) and water filled hopper (2 sea trips) were performed with undersized sole. Effects were assessed by comparing survival to conventional conditions (dry hopper, conventional haul duration, conventional cod-end) for which data were collected from the same or subsequent hauls.All test-fish were randomly collected from the end of the sorting belt at both the start and end of the catch-sorting process from multiple hauls per sea trip. Reflex impairment and damages were assessed for all test-fish and summarized in a vitality index score. Test-fish were housed on-board in custom-built monitoring units containing 16 (24L) tanks with five fish each tank. Tank water was continuously renewed with sea water at a rate of at least two tank volumes per hour to maintain proper water quality. Survival was monitored and dead fish were removed upon detection. Upon arrival in the vessel’s home port, monitoring units were road transported to the laboratory to continue survival monitoring for two more weeks. Total monitoring period ranged from 15 to 18 days among test-fish depending on the day of collection at sea. In the laboratory tank bottoms were covered with coarse sand and fish were fed natural food. In total 558 plaice from conventional fisheries (ca. 60 per sea trip) were collected, 478 plaice for the water filled hopper treatment (ca. 60 per sea trip), 200 plaice from short hauls (ca. 40 and 60 each in two sea trips) and 60 plaice from the knotless cod-end.Control-fish of the same species, in good condition and not exposed to the fishing gear were deployed during all sea trips (circa 30 control plaice and circa 15 control sole per sea trip). Control-fish were handled and tagged as test-fish to separate fisheries related mortality from mortality caused by the experimental procedures.Discards survival probabilities and their 95% confidence intervals (CI) were estimated from counts of surviving fish at the end of the monitoring period. For all sea trips combined, no significant effect of a water filled hopper on plaice discards survival probability could be detected with 16% (95%CI 12-19%) for the conventional dry hopper and 20% (95%CI 15-25%) for the water filled hopper. Within the individual sea trips, a significantly higher survival probability for plaice discards from the water filled hopper was found for three sea trips. In three other sea trips a lower survival of plaice discards was detected for the water filled hopper, although the difference with the dry hopper was not significant. Given this observation, it cannot be entirely excluded that the water filled hopper can also have a negative effect on discards survival. For sole discards the effect of a water filled hopper was tested during two sea trips only, yielding a higher survival probability of sole discards for the water filled hopper (14%, 95%CI 10-21%) compared to the dry hopper (5%, 95%CI 2-10%).Deployment of a water filled hopper results in a shift towards a better condition of the discarded fish. Despite this effect, the total proportion of fish in good condition within catches remained small. We therefore recommend to prioritize measures aimed at improving fish condition in the trawl to increase discards survival chances.For all sea trips combined, no effect of short (90 instead of 120 min) hauls on discards survival probability could be detected: survival probabilities for plaice discards were equal at 11% (95% CI 8-15%) for both short and conventional hauls. No effect of a knotless cod-end on plaice and sole discards survival probability could be detected.In conclusion, deployment of a water filled hopper does not result in higher survival probability for plaice discards than a conventional dry hopper in year-round pulse-trawl fisheries. However, it is clear6 of 39 | Wageningen Marine Research report C038/18that for individual trips the deployment of a water filled hopper can result in an increase of survival chances of discarded plaice, but as it seems only under certain specific, yet to be established, conditions. In addition, it cannot be excluded at this point that under certain conditions a water filled hopper may have a negative effect on discards survival. For sole a positive effect of the water filled hopper on discards survival was detected. However, since sole was tested during two sea trips only, the current findings may not be representative for year-round fisheries and the positive effect may be specific for the conditions that prevailed during the two trips. Survival probability of plaice and sole discards cannot be increased by reducing haul duration from 120 to 90 min or using a knotless cod-end.

The nitrite threshold concentration in rearing water of African catfish (Clarias gariepinus) was assessed. African catfish with an initial mean (SD) weight of 219.7 (57.8) g were exposed to an increasing range of water nitrite from 6 (Control) to 928 µM nitrite for 28 days. Mean (SD) plasma nitrite concentrations increased from 5.0 (3.6) to 32.5 (12.6) µM at 928 µM ambient nitrite. The increase in nitrite was accompanied by gradual increase in plasma nitrate from 41.6 (28.4) µM to 420.2 (106.4) µM. Haematocrit, haemoglobin, methemoglobin, plasma concentrations of cortisol, glucose, lactate, osmolality, gill morphology and branchial Na+/K+-ATPase activity were not affected. Feed intake, final weight, SGR, FCR and mortality were not affected. We advise not to exceed a water nitrite concentration of 43 µM (0.6 mg L-1 NO2--N) to prevent the risk of reduced growth and feed intake in African catfish aquaculture.

Het Ministerie van Economische Zaken wil komen tot wetenschappelijk onderbouwd duurzaam beheer van snoekbaars, baars, blankvoorn en brasem in het IJsselmeer en Markermeer. Voor alle vier bestanden is de beleidsdoelstelling voor visseizoen 2017/2018 geformuleerd in het document “Toekomstbeeld visstand IJsselmeer/Markermeer – synthesedocument’. Hierin wordt in ieder geval gestreefd naar ‘een evenwichtiger lengte-opbouw van de bestanden met meer grotere exemplaren en een groter aantal jaarklassen’, als ook ‘een toename van de (paai)bestanden’. Voor het behalen van deze beleidsdoelstellingen zijn inspanningsadviezen gevraagd over de staandwantvisserij en de zegenvisserij, gecombineerd voor het IJsselmeer en Markermeer.

The presence of geosmin in fish products causes an earthy or musty off-flavour. To avoid economic damage resulting from market entrance of off-flavoured fish products, fish farmers utilize the reversibility of geosmin uptake to depurate the off-flavours from their fish crops by holding them in clean water just before harvest. To improve this process, effects of exercise and temperature on the excretion of geosmin by European eel ( Anguilla anguilla ) were assessed. Fish loaded with geosmin were depurated for 23 h during which they were subjected to combinations of exercise (spontaneous swimming activity at 0.05 m s − 1 or forced swimming at optimal swimming speed of 0.55 m s − 1 ) and temperature (15 °C or 25 °C) treatments. Oxygen consumption was measured during depuration. Whole body geosmin concentrations were measured in samples collected at t = 0 and t = 23 h to assess geosmin excretion. Geosmin excretion by European eel was clearly enhanced by exercise, but temperature had no statistically significant effect. Exercise increased oxygen consumption, which in turn showed a positive linear relation with geosmin excretion. These findings support the idea that the physiological responses aimed at increasing oxygen uptake also affect the branchial exchange of lipophilic xenobiotic chemicals between the fish and its surroundings. Exercise can be used to reduce the time required to depurate off-flavours from fish. These findings are relevant for aquaculture industries confronted with off-flavoured fish crops. Statement of relevance Fish raised in land-based aquaculture systems may bio-concentrate geosmin, a lipophilic organic compound produced by microbiota. The presence of geosmin in fish products causes an earthy or musty off-flavour which human consumers do not appreciate and this poses one of the most significant economic problems for land-based aquaculture. To avoid economic damage resulting from market entrance of off-flavoured fish, fish farmers utilize the reversibility of geosmin bio-concentration to depurate the off-flavours from their fish crops. To this end fish are kept in water free of geosmin just before harvest. This procedure, however, is not always sufficiently effective and it is costly as it requires time, effort and use of specific facilities. Therefore better understanding of factors enhancing the geosmin excretion is needed to be able to improve the off-flavour depuration process. This study established that forced exercise during off-flavour depuration increases the rate at which European eel eliminates geosmin from its body. Forcing fish to swim during off-flavour depuration shortens the required depuration time. This finding is relevant for the aquaculture industries confronted with off-flavoured fish crops.

The threshold concentration for NH 3 in rearing water of African catfish ( Clarias gariepinus ) was assessed. African catfish with an initial mean (SD) weight of 141.0 (24) g were exposed to five different T amm [sum of NH 3 and NH 4 + ] concentrations: 0.37 (Control), 1.06, 2.12, 5.16 and 19.7 mM, which concurs with NH 3 concentrations of 4 (Control), 14, 38, 176 and 1084 μM. Plasma concentrations of NH 4 + , cortisol, glucose and lactate, plasma osmolality, gill morphology, branchial Na + /K + -ATPase activity, feed intake and specific growth rate were monitored. No effect of water NH 3 on plasma NH 4 + concentrations was detected. Feed intake and specific growth rate were severely affected at exposure to water NH 3 concentrations above 90 μM (calculated EC 10 values: 89 and 122 μM). No major disturbances in physiological blood parameters were observed at these NH 3 concentrations, but gill morphology (a remarkably sensitive stress indicator) deteriorated significantly. Based on the lower limit of the 95% confidence interval for EC 10 , we advise for African catfish not to exceed a water NH 3 concentration of 24 μM (0.34 mg NH 3 -N/L). This finding is relevant for design and management of African catfish production systems.

We wanted to optimize the procedure for the selenium enrichment of farmed African catfish, using garlic as dietary selenium source. In the first experiment we established the relation between the length of the selenium enrichment period and the resulting total selenium level in the fillet of the fish. It was found that at a dietary level of 11.7 mg kg-1 Se, a total selenium level in the fillet of 0.7 mg kg-1 was reached in a relatively short enrichment period of 10 days before harvest. In the second experiment we studied the effect of depuration on the selenium level in the fillet and the sensory properties of selenium-enriched African catfish. It was found that total selenium levels in the fillet were not affected during a 7-day depuration period, while garlic odours and flavours in the raw and cooked fillets were significantly reduced after 2 days of depuration. We concluded that selenium enrichment of farmed African catfish can be obtained by selenium-enriched finishing diets, while garlic odours and flavours resulting from dietary garlic can be effectively reduced in the fillet during a short depuration period without negatively affecting fillet levels of total selenium.

The effect of flow rate on growth was investigated in juvenile turbot. Fish with a mean (SD) initial weight of 102 (10.4) g were reared at 6 different flow rates, equaling 1, 2, 3, 4, 6 or 8 tank volumes/h in 196 L tanks during 29 days at 18 ± 0.29 °C, a salinity of 18.0 ± 0.77‰ and a pH ranging from 6.02 to 7.21. Total ammonia nitrogen, dissolved oxygen, pH, temperature and free carbon dioxide levels were monitored during the experimental period. Results show that increasing the flow rate promotes the growth of turbot up to a flow rate of 4.7 tank volumes/h. Accumulation of ammonia and carbon dioxide and oxygen levels in the tanks were affected by the treatments. It is concluded that increased flow rate resulted in higher SGR of juvenile turbot, possibly explained by differences in carbon dioxide levels. © 2009 Elsevier B.V. All rights reserved.

The anti-carcinogenic properties of selenium against certain types of cancer when present in organic forms justify the increasing interest in development of selenium fortified food. In this particular study, African catfish (Clarias gariepinus) were fed with a Se-enriched diet in order to enhance the selenium concentration in the fish fillet up to 0.85 ± 0.06 mg Se kg−1 (0.26 ± 0.02 mg Se kg−1 in control). Selenium distribution in proteins separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was studied for better understanding of the metabolism of this element in the organism. Soluble proteins (Tris buffer) were investigated by conventional application of a glass homogenizer in comparison to application of ultrasound probe sonication, and the latter was convincing due to higher protein extraction efficiency and the ease and speed of application. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was successfully applied for screening of selenium in proteins in fillets of African catfish separated by one-dimensional SDS-PAGE and after electroblotting onto nitrocellulose membranes, with a total Se amount below 0.4 ng loaded into a well. Selenium was detected in more than 11 protein spots. In order to improve the protein separation of one-dimensional SDS-PAGE, pre-fractionation of the Tris buffer soluble protein extract by size exclusion was carried out. Tryptic digestion of the Se-containing bands was performed for protein identification by nano-HPLC coupled to electrospray-MS/MS. A database search revealed several proteins which are located in muscle tissue. However, the very low Se concentration circumvents the detection of selenium containing amino acids in the tryptic peptides by electrospray-MS/MS.

We wanted to create functional seafood with high concentrations of organic selenium (seleno-methyl-selenocysteine and ¿-glutamyl-seleno-methyl-selenocysteine) with anti-carcinogenic properties for human consumers. Garlic containing high concentrations of these organic selenium compounds was used as a selenium source in five experimental feeds for African catfish (1.9, 2.8, 3.9, 5.1 and 8.5 mg kg¿1 Se); a sixth experimental feed was formulated without garlic (1.9 mg kg¿1 Se). The experimental feeds were fed to African catfish [initial mean (SD) weight 100.7 (2.7) g] for 43 days with three replicates per treatment. Whole fish fillets were sampled for total selenium analysis (start and end) and selenium speciation (end). We found a positive linear relationship between dietary and fillet concentrations for total selenium and selenomethionine. The dietary total selenium concentration of 8.5 mg kg¿1 resulted in a total selenium concentration of 0.9 mg kg¿1 in the fillet (wet tissue). The majority of the selenium compounds recovered in an extract made from the fillet consisted of selenomethionine, considered to be important from a nutritional point of view. Seleno-methyl-selenocysteine, one of the organic selenium species to which superior anti-carcinogenic properties are attributed, was detected in the fillet but could not be quantified.

The effects of dissolved oxygen concentration combined with sublethal doses of un-ionised ammonia nitrogen (UIA-N) on growth and physiology were investigated in juvenile turbot. Fish with a mean ± SD initial weight of 19.1 ± 4.1 g, were reared in replicate at normoxic (80–85% O 2 saturation) and hyperoxic (110–115% O 2 saturation) conditions, combined with either control or two sublethal concentrations of un-ionised ammonia (0, 0.17 and 0.31 mg UIA-N l − 1 ) added for 64 days at 18 °C and pH 8.04 in a flow-through system. The subsequent effect of fixed O 2 and UIA concentrations on growth and blood physiology was measured every 22 days. The results show that UIA-N concentrations only had a negative effect on growth under normoxic conditions, suggesting that hyperoxic conditions may increase tolerance to un-ionised ammonia in juvenile turbot. Only minor effects on the acid-base balance were found. A generally higher blood CO 2 and P CO 2 , along with an accumulation of HCO 3 − resulting in an increased blood pH was found in the hyperoxic groups towards the end of the experiment. We conclude that, under normoxic conditions, a chronic exposure of ammonia above 0.17 mg UIA-N l − 1 will have a negative effect on growth and that hyper-oxygenation can be a possible solution to counteract this harmful effect.

The effect of thermal history (16 and 20°C) on growth of juvenile turbot, Scophthalmus maximus (initial mean weight 72.6 g, n = 157) was studied. Fish were divided into four groups, two groups remaining at constant temperature (C16, C20), while fish in the other groups were transferred from either 16 to 20°C (F16-20) or from 20 to 16°C (F20-16). Between 35 and 42 fish in each tank were individually tagged at the start of the experiment. The final mean weights were significantly higher in the F20-16 group (230 g) than in the C20 (213 g), F16-20 (211 g) and C16 (205 g) groups. The overall growth rate was highest in the F20-16 group (1.17% day¿1) but comparable in the three other groups (1.00¿1.04% day¿1). Our findings indicate that, even at near-optimal temperature for a given size, the temperature history of the fish may influence future growth. Based on these indications, we conclude that as turbot grow larger, the temperature should be reduced to take advantage of the change in optimal temperature for growth with increasing fish size rather than rearing at constant temperatures.

The ammonia (NH 3 ) and nitrate (NO 3 − ) threshold concentrations in rearing water of juvenile pikeperch ( Sander lucioperca ) were assessed. Pikeperch with an initial mean (SD) weight of 17.7 (4.2) g were exposed to 0.9 (control), 3.6, 5.2, 7.1, 11.2 and 18.9 μM NH 3 in the water for 42 days. Plasma NH 4 + concentrations stayed at control levels (~ 650 μM) up to 11.2 μM NH 3 in the water. At the highest water NH 3 concentration tested, plasma NH 4 + had more than doubled to 1400 μM. Based on the specific growth rate, the EC 10 value for NH 3 was 5.7 μM. When pikeperch (initial mean (SD) weight of 27.0 (4.9) g) were exposed to 0.1 (control), 1.5, 2.3, 3.7, 6.1, 10.2, 15.8 and 25.6 mM NO 3 − for 42 days, mean (SD) plasma NO 3 − concentrations increased linearly from 88 (47) to 5993 (899) μM at the highest ambient NO 3 − level. Feed intake, specific growth rate and feed conversion ratio were not affected. Neither NH 3 nor NO 3 − exposure significantly affected haematocrit, plasma concentrations of cortisol, glucose, lactate, osmolality, gill morphology or branchial Na + /K + -ATPase activity in pikeperch. For juvenile pikeperch we advise not to exceed a water NH 3 concentration of 3.4 μM (0.05 mg NH 3 –N/L), the lower limit of the 95% confidence interval of the EC 10 value for SGR, to ensure proper physiology and growth. For NO 3 − we advise not to exceed 25 mM (350 mg NO 3 − –N/L). This criterion is based on the highest NO 3 − concentration tested (25.6 mM). As no negative effects were detected at the highest concentration tested, the actual NO 3 − threshold probably exceeds 25.6 mM.

Dover sole (Solea solea) is an obligate ectotherm with a natural thermal habitat ranging from approximately 5 to 27°C. Thermal optima for growth lie in the range of 20 to 25°C. More precise information on thermal optima for growth is needed for cost-effective Dover sole aquaculture. The main objective of this study was to determine the optimal growth temperature of juvenile Dover sole (Solea solea) and in addition to test the hypothesis that the final preferendum equals the optimal growth temperature. Temperature preference was measured in a circular preference chamber for Dover sole acclimated to 18, 22 and 28°C. Optimal growth temperature was measured by rearing Dover sole at 19, 22, 25 and 28°C. The optimal growth temperature resulting from this growth experiment was 22.7°C for Dover sole with a size between 30 to 50 g. The temperature preferred by juvenile Dover sole increases with acclimation temperature and exceeds the optimal temperature for growth. A final preferendum could not be detected. Although a confounding effect of behavioural fever on temperature preference could not be entirely excluded, thermal preference and thermal optima for physiological processes seem to be unrelated in Dover sole.

Flatfish culture in re-use or recirculation aquaculture systems (RAS) is a relatively young production sector and is hardly practiced on commercial scale (Blancheton, 2000; Labatut & Olivares, 2004...

Speciation of Se in fish is needed to elucidate the metabolism of this element in living organisms in the marine environment. In this paper, selenium concentration and its species distribution in several organs and tissues (liver, gills, kidney, muscle and gastrointestinal tract) of African catfish fed with a selenium-enriched garlic based diet was studied. The intention of this paper is focused on both the investigation of selenium distribution in the soluble protein fraction and the detection of selenoaminoacids. Thus, two different procedures have been developed. In the first procedure, screening of selenium in proteins in the Tris-buffer soluble fraction of different tissues was carried out by size exclusion chromatography-inductively coupled plasma-mass spectrometry (SEC-ICP-MS) and by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) after sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) separation and electroblotting onto membranes. For the amino acid analysis, several sample treatments for Se-species extraction, based on enzymatic hydrolysis, were compared. The best results were obtained for incubation at controlled temperature. Application of several sample treatments in conjunction with different chromatographic techniques (reverse phase, anion exchange and ion exchange/size exclusion) was crucial to unambiguous Se-species identification. In Se-enriched African catfish a noticeable increase in the content of selenium in different organs was observed, except for the liver, where the Se level remained unaltered. The kidney was the Se-target organ in animals fed with enriched Se food. Selenomethionine (SeMet) was the main Se species identified in fillet extracts, whereas the presence of selenocysteine (SeCys) was detected in the liver and both SeMet and SeCys were present in the kidney.

Juvenile turbot ( Scophthalmus maximus ) were exposed periodically and chronically to different levels of un-ionised ammonia (UIA-N) and the subsequent effect on growth and food conversion efficiency was studied. Fish with a mean (SD) initial weight 19.3 (3.9) g, were exposed to five treatments consisting of a control group, two groups (ChronicLow and ChronicHigh, UIA-N levels of 0.13 and 0.25 mg l − 1 , respectively) chronically exposed to ammonia and two groups (LowPulse and HighPulse) exposed to the same levels as above for a short period daily. The fish were held for 64 days at 18 °C, a pH of 8.04 and a salinity of 33.5‰. At the end of the experiment, fish from the Control group had a significantly higher mean weight (95.5 g) compared to all other groups. Fish from the ChronicHigh group displayed the lowest mean weight (74.6 g), whereas fish in the ChronicLow, LowPulse and HighPulse groups displayed mean weights of: 79.3 g, 82.8 g and 81.9 g, respectively. Blood ion concentrations were not affected significantly in any of the treatments, whereas minor reductions in blood pH, partial pressure of CO 2 and total CO 2 content were found in ammonia exposed groups up until day 44 of the experiment. Feed conversion efficiency, daily feeding rate and total feed consumption did not vary significantly between treatments. The study demonstrate that short daily ammonia peaks may result in negative effects on growth, equivalent to that found under chronic ammonia exposure in juvenile turbot.

Publisher Summary The primary role of a diet is to provide sufficient nutrients to meet the nutritional requirements of an individual. There is an increasing amount of scientific evidence to support the hypothesis that some foods and food components have beneficial health effects beyond the provision of basic nutrients. Scientific and technological developments in the field of food have led to a marked shift in how consumers deal with food and health. There is a growing awareness among consumers that the dietary source and form of food might affect their overall health. The role of food as an agent for improving health has initiated the development of new classes of tailor-made food – functional foods. Functional foods must remain food and must demonstrate their effects from amounts that can normally be consumed in the diet. A functional food can be produced from a food by natural enhancement of the health beneficial components through growing conditions. Addition of the component to the food in order to provide the benefits is another option. The removal of component(s) to reduce any reverse health effect can be regarded as a process for functional foods. Chemical modification of one or several components in the food or changing the bioavailability of its healthy components to improve absorption is another tool to develop functional foods. Natural enhancement of health-beneficial components via the feed during farming is an ideal opportunity to develop new functional foods. This natural enhancement might be better accepted by consumers than addition of the health-beneficial component to the final product.

The aim of this study was to investigate the possible benefit of ‘‘temperature-steps’’ (T-steps) rearing for juvenile turbot (initial weight 15.1 g) under realistic production scale and to determine whether initial growth advantage is maintained throughout the rearing period to market size. One group (called T-step 22-19-16) of juvenile turbot was reared at three different temperatures, that is, 22 C (from 17 to 60 g) followed by 19 C (from 60 to 100 g) and 16 C (.100 g); another group (called T-step 19-16) at two temperatures, that is, 19 C (from 17 to 100 g) and lowered to 16 C (.100 g); and the third group (called C16) at one constant temperature, that is, 16 C. Relative growth was significantly higher in the two T-step groups, with the T-step 19-16 showing the highest overall growth. Feed conversion efficiency was highest in the 19-16 group. Only minor effects of the experimental rearing on blood physiology were found, with one notable exception of inverse relationship between plasma glucose and growth. Overall, these findings indicate that a short interval of rearing fish at high temperature during the early juvenile phase may have a long-term effect on biomass increment in turbot. This is an important finding for the turbot industry.

Industrial processing and heat treatment of fish muscle generally lead to losses of water-soluble components, some of which may have beneficial health effects. The aim of this work was to determine the retention of taurine, selenium and n-3 polyunsaturated fatty acids when preparing African catfish by three traditional household techniques: boiling in pouches, deep-frying and baking. Cooking did not significantly reduce the content of selenium, having retention between 91and 104%. Deep-frying resulted in a taurine loss of 40%, which was significantly higher than in baking where losses were 25% The fatty acid profiles were similar for baked and boiled fillets, but were significantly different from deep-fried fillets, due to absorption of vegetable frying oil. Baking was the best preparation technique with regard to retention of 20:5n-3 (eicosapentaenoic acid; EPA) and 22:6n-3 (docosahexaenoic acid, DHA), retaining above 80% for both fatty acids, whereas boiling and deep-frying were able to retain only approximately 54 and 65% of each, respectively.

Dover sole were reared at 6 different stocking densities between 0.56 and 12.6 kg/m2 with duplicate tanks for each treatment. The experiment lasted for 55 days. Water quality effects on growth were minimised by making the flow rate per tank proportional to the feeding load. Individual initial and final weights were determined for all fishes to calculate the specific growth rate and the coefficient of variation within each culture tank. The specific growth rate of Dover sole significantly decreased with increasing stocking density. Mortality increased significantly with increasing stocking density. The difference between initial and final CV (?CV) increased with stocking density (P

A number of scientific studies have investigated aspects of soles ( Solea solea and S. senegalensis) ecology, population genetics and biology in their natural environment, and the species have been extensively studied in captivity during the last decade. Studies on the genetic population structure of sole indicate that several distinct breeding populations exist within its distributional range in European waters. Recent studies suggest a phylogenetic relatedness of S. solea and S. senegalensis, being found as closest sister lineages in most reconstructions. However, studies on molecular genetics and morphological traits give diagnostic differences that consistently lead to their taxonomic separation at the specific rank. Studies show that sole spawn readily in captivity, and the buoyant, fertilized eggs are easily collected. Stocking density during maturation should be 1-1.5 kg/m(2), and temperature should be kept above 16degreesC ( S. senegalensis) or between 8 and 12degreesC ( S. solea). In nature, the onset of spawning is related to a rise in temperature occurring during spring ( March-June). Salinity should be kept constant around 33-35& and the fish reared under simulated natural photoperiod ( LDN). In other cultured flatfish species, a change in the photoperiod is the key environmental signal used to manipulate and control maturation, but at present time there are no published work that verifies or contradicts this for either S. senegalensis or S. solea. Studies indicate that a mixture of inert and live food may increase the weaning success of sole fry, and this can be further enhanced by using attractants in the dry feed. Future experiments are needed to determine the ideal time to commence weaning and determine the minimum duration of this period. Studies on alternative feeding strategies are also required. The effect of temperature and photoperiod on juvenile growth has not been studied systematically in neither of the two species and the relative importance of a direct photoperiod effect on growth in sole therefore remains to be defined.