Evaluating innovative technologies for multistage fish growth processes: Case study of an eel (Anguilla anguilla L.) farm

A new method for evaluating innovative technologies for multistage fish growth processes was developed. An earlier economic model was adapted to meet the real-world requirements of a commercial eel (Anguilla anguilla L.) farm. The economic benefits of different innovative technologies at the farm level were evaluated where the technology included multiple production alternatives at various intensities and several growth stages. Data were gathered into “bio-economic” tables that take into account the biological parameters of each production alternative and the costs related to its development and implementation. The adjusted data were then divided into groups of production alternatives and feasible combinations of alternatives for each of three growth stages were evaluated to determine the best technology for the overall growth process. A computer program enabled us to enumerate all potential innovative technologies, collect information on biological parameters and costs by growth stage, and compare additional profit expected after implementation of each innovation. Results reveal the most profitable innovative technologies by growth stage for the 5-year eel culture. Comparison of the 10 best technologies shows that combining the alternatives with the best biological parameters for each growth stage does not necessarily result in the maximum additional profit for the farm.     

Migration pattern of silver eel (Anguilla anguilla, L.) in an obstructed river system

Abstract –  The downstream migration of European silver eels in the River Frémur was examined to determine the potential effects of the numerous migration barriers that exist on waterways in western France. The Frémur has a 14 m high dam which creates a 3 × 10⁶ m³ water supply reservoir, 6 km from tidal limit. Based on 8-year records (1996–2004) of migrant silver eels captured in a Wolf trap located about 1 km below the dam, the influence of this dam on the migration was examined. These records indicate that 91% of silver eels were captured when the dam was overtopped (i.e. when the water reservoir was full). The timings of the overflow periods varied greatly between years mainly because they are function of the duration of the filling period which in turn is dependent on the level of summer water abstraction and annual hydrological conditions. Consequently, migration periods occurred at variable dates (between November and April) which is late in comparison with nonobstructed European river systems (generally between August and December). During overflow periods the migration of silver eels increased markedly during rain events (increasing river flow). This indicates that flow flushes, are essential so that settled silver eels can orientate themselves and pass over the dam. The hypothesis of a 'dam effect' that could stop temporarily or permanently some of silver eels in the reservoir is discussed. Finally, the influence of the migration delays on the condition of silver eels is considered.     

Errors in ocular index measurements in European eel Anguilla anguilla (L.)

Abstract –  Knowledge of maturation state of eels ( Anguilla anguilla , L.) based on ocular index (OI) serves management and research functions, such as estimating how much silver eel actually could escape from a particular basin or determining more precisely the developmental biology of a particular eels. OI implies the measurements of five variables (total length and eyes diameters) in a formula which can be sensitive to measurement error. To investigate this issue, we carried out a comparative study using 13 European eels harvested in Camargue (Rhône Delta, France) and three replicated measurements by three different observers. The estimated amplitude of OI imprecision at a 95% confidence level was 1.8 whatever eel and observer. Such error may lead to classification problems particularly when OI values are close to the threshold values used to discriminate between yellow and silver stages. This study not only raised the problem of the OI imprecision but also brought up the need to develop standardised protocol for identifying silver eels. To reach this goal, two different ways are proposed in conclusion.     

Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment

Abstract –  Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla , in the River Frémur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996.     

Differences in head shape of the European eel, Anguilla anguilla (L.)

Differences in eel, Anguilla anguilla (L.), head shape were the result of greater increases in head width, rather than changes in head length. The ratio of head width:total length (HW:TL) increased significantly ( t -test, P < 0.05) from the glass-yellow eel stage. Cultured yellow eels were exclusively narrow headed (mean HW:TL = 0.027, range = 0.023–0.032), while wild yellow eels displayed an array of head shapes (mean HW:TL = 0.033, range = 0.023–0.046). Therefore, broad heads (HW:TL 0.033) occurred only among wild yellow eels sampled and may have resulted from diet. Cultured yellow eels consumed only small pellet material. Of wild yellow eels stomachs containing food, 78% of broad-headed eels consumed large and/or hard-bodied organisms (e.g. beetles, fish, molluscs and Notonecta sp.), while 83% of narrow-headed eels consumed exclusively small/soft-bodied prey (e.g. amphipods and chironomids).     

A short note on the intensive culture of eel (Anguilla anguilla L.)

Preliminary experiments in intensive culture showed the possibility of raising healthy eel with obvious growth in water of moderate temperature, when fed a special diet.     

Spatial organisation of European eel (Anguilla anguilla L.) in a small catchment

Abstract –  Modelling-governing patterns of European eel ( Anguilla anguilla L.) distribution of four eel size classes (<150, 150–300, 300–450 and >450 mm) in the Frémur basin (northwest France) was done using artificial neural network (ANN) techniques and ecological profiles. Our results demonstrate the high predictive power of the ANN models. Some macro- and microscale factors, such as distance from the sea, depth and flow velocity, have the most significant influence on the models. Influence of distance from the sea appears to be very different from the spatial organisation usually described in river systems. In fact, the general tendencies of total eel densities according to the distance from the sea showed that densities increase weakly upstream. Another outcome was the variations in habitat preference according to the eel size, even if this species is spread over practically every type of microhabitat. Small eels were mainly found in shallow habitats with strong abundance of aquatic vegetation, whereas large eels tend to be found in intermediate to high depth with small to intermediate abundance of aquatic vegetation. Finally, we hypothesise that European eels change behaviour and microhabitat characteristic preference around a size of 300 mm.     

Morphological and histochemical study of intestine in wild and reared European eel (Anguilla anguilla L.)

Diet-related differences between the ratio of intestine length to body size and the enzymatic activity in the intestinal tract of wild and reared European eel (Anguilla anguilla L.) were studied. Compared with reared eel, wild eel showed significantly shorter relative intestine length. For the purpose of histochemical examination, different parts (anterior, middle and posterior) of intestine proper were used. Activities of non-specific esterase, alkaline and acid phosphatase, and aminopeptidase were examined in each segment. All enzymes were present in the intestines of both wild and reared European eel. Fish from both groups showed similar enzyme distribution within the enterocytes, but distribution and intensity of enzyme activity along the intestine vary depending on the group. Generally, reared European eel showed highest enzymatic activity and wider distribution of enzymes throughout all parts of the intestine. These results suggest that different diets could be one of the reasons for observed changes.     

Effects of dietary carbohydrate source on growth and body composition of the European eel (Anguilla anguilla L.)

Different carbohydrate sources—wheat meal, bread meal, soluble corn starch, native potato starch and sorghum meal—were included at 30% in isonitrogenous diets for European eels (Anguilla anguilla) weighing 4.6 g kept in tanks. Growth and body composition were determined and feed utilization was evaluated. Total gains/m² per day were as follows: on wheat, 3.20 g; bread meal, 2.77 g, soluble starch, 2.28 g; sorghum, 1.21 g; and potato, 0.78 g.Percent of protein in whole carcasses was similar for all diets, but percent of fat was higher with wheat and bread meal and lowest with potato starch. Food conversion, protein retention and energy retention were higher for eels fed wheat meal or bread meal than the other carbohydrates.     

Overwintering dormancy behaviour of the European eel (Anguilla anguilla L.) in a large lake

Overwintering dormancy behaviour was studied in female silver eels in Lake Mälaren in Sweden between 2008 and 2010. Depth choices and movements in relation to temperature were analysed from pressure and temperature records for 13 eels with implanted data storage tags, covering 17 overwintering periods and three intervening summer periods. Dormancy commenced in October–November as temperatures fell below 4–12 °C. Eels tended to remain motionless throughout the winter, with some short periods of activity signalled by small changes in depth distributions. During dormancy, the eel shows a clear avoidance of shallow areas <5 m in favour of the 10–25‐m‐depth interval. Activity tended to resume 4–6 months later in April–May as temperatures rose above 3–7 °C and ice cover broke, and eels spent more time at shallower depths of <5–10 m. The majority of the eels were assessed as being in the silver eel stage at the time of tagging. During the autumn months, the diving behaviour, with frequent and large vertical excursions and periods at the surface, was similar to that seen in migrating eels in the Baltic and Atlantic Ocean. In spring and summer, the behaviour differed, being dominated by more gradual depth variations, implying that the eels reverted from silver eel migration behaviour to yellow eel foraging behaviour. Body weight declined during dormancy, but other studies of starvation over comparable time periods showed significantly higher average specific weight losses, implying that the Mälaren silver eels must have fed between the end of dormancy and recapture.     

The yellow European eel (Anguilla anguilla L.) may adopt a sedentary lifestyle in inland freshwaters

Abstract – We analysed the movements of the growing yellow phase using a long-term mark–recapture programme on European eels in a small catchment (the Frémur, France). The results showed that of the yellow eels (>200 mm) recaptured, more than 90% were recaptured at the original marking site over a long period before the silvering metamorphosis and downstream migration. We conclude that yellow European eels >200 mm may adopt a sedentary lifestyle in freshwater area, especially in small catchment.     

Riverine migration of young eels Anguilla anguilla (L.)

Eels of length 6–50 cm, trapped in the course of up-stream migration in the River Shannon, were sampled during 10 seasons from 1973 to 1983. Migration normally began at the end of May at water temperatures of 13 or 14°C, but the extreme dates ranged from 17 May to 24 June. In five out of seven years, migration ended between 10 and 19 September; in the other two years it was on 29 July and 6 October. Positive correlations were observed between date of first migration and temperature between 15 and 21 May. No connection between temperature and date of end of migration was apparent.Length measurements of 5008 eels showed that large individuals (> 15 cm) migrated throughout the season, but that the majority of smaller eels (< 10 cm) had a shorter migration period from mid-June to mid-August.The ages of 158 specimens were determined. The maximum age was 10 +; the majority ranged from 1 + to 3 +. Elvers of 0+ were scarce or absent early in the season and were never plentiful. Numbers of eels caught ranged from 131000 to 417000 per year.It was concluded that a number of factors influenced migration of the eels, the time of year perhaps being the strongest, with water temperature in May being secondary. Migration of the greatest numbers occurred in the year following the greatest immigration of elvers to the river. This suggests that migration may be influenced by population pressure in the lower reaches of the river.     

The 'neck-cut' (spinal transection): not a humane way to slaughter eel, Anguilla anguilla (L.)

Abstract. Commercial slaughtering of eel sold fresh starts with the so-called 'neck-cut'. This tran5ectlon of the spinal column separates the spinal cord from the brain. In due course the slaughtering process is completed by gutting, skinning and, finally, by decapitation. There is a widespread belief that the advantage of the neck-cut is that it causes immediate death. However, inspection of eels with a neck-cut revealed no visible injury to the brain. Repeating the method as a laboratory experiment and subsequent observation of the experimental eels resulted in spinal regeneration, as reflected by behavioural recovery through four stages in about 6 weeks. It seems that after the neck-cut the brain is still intact, both structurally and functionally, and remains so as long as the slaughtering has not been completed. Thus a prolonged state of suffering cannot be ruled out. Consequently, slaughtering of eel should include instant destruction of the brain.     

The acute toxicity and sublethal effects of nitrite on growth and feed utilization of European eel, Anguilla anguilla (L.)

Eels cultured in recirculation systems are regularly confronted with high concentrations of nitrite, a well-known toxicant for fish. In this study, the acute toxicity of nitrite to European eel, Anguilla anguilla (L.), was assessed by determination of a 96-h LC₅₀. The 96-h LC₅₀ measured for eels was 143.7 ± 2.3 gm^-3 NO₂-N (mean SD), which is high compared with the LC₅₀ for other fish species. The sublethal effects of nitrite on growth and feed utilization were evaluated in a feeding trial lasting 77 days, divided into an acclimation period and two experimental periods. Eels of 24 g on average were divided over 20 aquaria, connected to five separate recirculation systems. In each system, the desired nitrite concentration level was maintained by water suppletion and continuous addition of NaNO₂. Fish were continuously exposed to levels of 0, 1, 5, 10 or 20 g m^-3 NO₂-N. Half of the experimental groups were fed ad libitum to study effects on feed intake, while the other half were fed a restricted ration to study effects on feed utilization. At the start and end of each experimental period, nitrite in the blood plasma, haemoglobin and methaemoglobin were measured. Fish weight and body composition were used to calculate specific growth rate and conversion efficiencies. In the range of concentrations studied, no significant effect of nitrite on maximum growth rate or feed utilization could be demonstrated. At the start of the experiment, low concentrations of nitrite were detected in the blood plasma, which suggests an ability of the eel to adapt to environmental nitrite. Nitrite, in the range normally encountered in intensive eel farms (max. 15 g m^-3 NO₂-N), can therefore be considered a factor of little significance.     

Age and growth of yellow eel, Anguilla anguilla (L.), determined by two different methods

Abstract– Ageing and calculation of growth rate of eels has always been controversial, and remarkably little validation has been reported. This article describes the growth of yellow eels from three lakes in the Burrishoole system, Ireland, as determined by back-calculation from otoliths prepared by burning and cracking. Eels captured by fyke net ranged in length from 25.0 cm to 98.5 cm and the ages ranged from 6 to 49 years. Back-calculated growth was slow, with mean annual increments of 2.08 cm, 1.40 cm and 1.97 cm for the three lakes. Yellow eels captured in the Burrishoole system in 1987 and 1988 were marked by Pan Jet inoculated alcian blue dye and released back into the three lakes. Recaptures made over the following three years yielded considerable growth data. Mean annual growth rates recorded were 2.30 cm, 1.07 cm and 0.97 cm. These data compare favorably with the calculated growth rates, thereby validating the otolith analysis for the eels in the Burrishoole system.     

Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area

Due to a recruitment decline of more than 90% in 30 years, the European eel (Anguilla anguilla L.) has been classified by IUCN as critically endangered. Although the species has been studied intensively to obtain knowledge to improve management, studies about the resident yellow stage are relatively scarce. In this study, 52 large female yellow eels were tagged with acoustic transmitters in a Belgian polder system and tracked by a network of 23 automatic listening stations. We studied both circadian and seasonal movement patterns and the effect of environmental variables on these patterns. Large female yellow eels were most active at night in late summer and early autumn. A generalised linear mixed model showed that their movement is only slightly influenced by environmental variables. Moreover, as yellow eels show high site fidelity (i.e., the majority was detected only in the habitat type of their catch‐release location), they do not encounter many human‐induced connectivity problems in polder systems, which makes these systems highly suitable as eel growth habitat. These results can contribute to an effective eel management regarding habitat protection and restoration.     

An improved grinding technique for examining fish otoliths for age and growth studies with special consideration of the eel, Anguilla anguilla L.

Abstract. A simple technique of grinding eel otoliths (sagittae) for age/growth studies is described. On thin, polished otolith layers the annual growth rings, as well as supernumerary zones formed in fresh water, could be easily distinguished. The described equipment and grinding procedure gave good results on permanent preparations, allowing repeated age-readings. The procedure appears to be applicable to other fish species as well.     

A feasible method for humane slaughter of eel (Anguilla anguilla L.): electrical stunning in fresh water prior to gutting

It was observed that farmed eels could be rendered unconscious and insensible instantaneously by passing an electrical current through fresh water. The general epileptiform insult on the EEG was characterized by a tonic/clonic and an exhaustion phase. After stunning, the ECG (electro‐cardiogram) revealed fibrillation. The electrical stunning parameters were on average 194 ± 4 V and 0.636 ± 0.040 A/dm² for 1.6 ± 0.4 s. Within a confidence level of 95%%, taking into account the number of animals with a reliable EEG (n = 29), at least 93% of all eels are effectively stunned in fresh water by an average current of 0.636 ± 0.040 A/dm². The behaviour of groups of five eels, which were able to move freely in the water was observed before and after stunning with 50 V and 0.17 A/dm². After 3‐s stunning, two eels were turned upside down. They changed to a normal position after 10 and 13 s respectively. Subsequently, all eels were very active in swimming behaviour and stopped swimming after 75 s. When stunned for a longer duration, all eels were turned upside down and stopped breathing for a limited period of time. In the last experiment the eels were stunned in fresh water (500 μS) with a voltage of 200 V for approximately 1 s, which was followed by 50 V for 5 min. As soon as the stunning started the water was de‐oxygenated by flushing nitrogen to kill the eels by suffocation during the period of unconsciousness and insensitivity. The oxygen saturation decreased from 74 ± 10 to 23 ± 11% at 22 °C. After stunning no brain activity and no responses to pain stimuli were observed on the EEG. The heart rate increased (P < 0.05) after stunning, which was followed by a significant decrease. Only 1 out of 18 eels returned partially from upside down to a normal position 2 h after stunning; however, the eel did not respond to pain stimuli in behaviour. The developed stunning procedure can be recommended for humane slaughter of 50‐kg batches of eels.     

Estimation of production and long‐term changes in the population of European eel Anguilla anguilla (L.) in an Irish lake

Monitoring of yellow‐phase and silver‐phase Anguilla anguilla during their continental life history is necessary for evaluation of stock recovery measures. Eel population data for an Irish lake (Lough Sheelin) were compiled for the period 1993–2014. Catch data from 2009 to 2014 provided minimum estimates of recent silver eel production ranging annually from 0.79 to 1.84 kg/ha. Long‐term changes in yellow eel abundance and silver eel size structure were assessed as part of a fishery monitoring programme. Yellow eel catch per unit effort (CPUE) in the lake (from longline surveys) was considerably higher in the 1990s (52.2–62.1 eels/100 hooks) than 2002–2008 (1.9–15.8 eels/100 hooks). Conversely, during 1993–2014, the mean size of silver eels migrating from the lake increased significantly (p < .001), from 659 mm to 838 mm. The results suggest that in the absence of direct yellow eel abundance data, interannual variation in silver eel size structure may be a useful monitoring tool for local eel stocks as part of Eel Management Plans (EMP's).