How fast are eels

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In another experiment Van den Thillart et al. Two eels stopped swimming during the first 2 months, and the experiment was stopped after 3 months. Oxygen consumption data of the five eels was measured continuously during swimming over 95 days at 0. These first long distance swimming results with European silver eels show their impressive endurance.

The capacity of individual fish to swim can be tested in swim-tunnels as described and used by many authors Brett more or less in the same way as mammals can be tested on runways.

The fish are usually trained to prevent interference by handling stress, followed by stepwise increase of swimming speed. At each speed, the individual fish are usually left swimming for 1—2 h, as it takes 30—60 min to establish a new stable condition with respect to circulation and ventilation Jones and Randall During this start-up period, the animal reaches a certain rhythm that helps to optimise its swimming efficiency.

With each increase of speed, the drag increases to the 2-nd power, and therefore the oxygen consumption increases also to the 2-nd power with speed. There is not a complete metabolic switch, but the anaerobic processes are activated to supply the extra energy required for muscle contraction.

Initially, the lactic acid is buffered by the alkaline reaction of phospho-creatine hydrolysis van den Thillart and van Waarde Metabolic acidosis develops when most of the PCr is depleted; the low pH together with the high inorganic phosphate causes muscle fatigue, which ultimately results in collapse. The collapse point is a reproducible parameter, as is the oxygen consumption rate at the collapse point. Likely the maximal oxygen consumption rate determines the maximal sustained swimming speed, which is the speed where the animal does not fatigue when swimming for a few hours.

Somewhere between resting and maximal oxygen consumption, the optimal swimming speed U opt can be found. This is by definition the speed where the cost of transport is the lowest. At low speed because the standard metabolic rate is a large part of energy consumption, which is not used for swimming, and at high speeds it is high because of the drag which increases with 2nd power of the speed.

So, the lowest COT is found at an intermediate speed, which is then called the optimal swimming speed. Particularly for migrating animals the cost of transport is crucial as it determines the maximal distance that can be covered for the available energy, when swimming at the optimal speed.

A low cost of transport is certainly a prerequisite for high endurance. Therefore, maximal endurance should be expected at the optimal swimming speed. According to biomechanical criteria, the best endurance is the maximal speed where the oxygen consumption rate remains stable and the mode of swimming does not change Videler These two criteria are based on different assumptions, either lowest energy cost or stable swimming mode.

It is likely though that both will come together at the same speed, as such would be selected out in the population over several generations. To determine maximal endurance conditions for eels, they have to be swum for extended periods at different swimming speeds. Swimming fitness can be described by a number of parameters such as maximal swimming speed, optimal swimming speed, and minimal cost of transport. These were recently determined for European eel in a single day protocol, using the previously described swim-tunnels.

In this study Palstra et al. At each speed, the oxygen consumption was measured continuously for 90 min. The maximal aerobic speed was interpolated according to the method of Brett A group of 40 farmed eels were tested twice test 1 and test 2 with 2-h intervals, and in between at the same speeds with h intervals endurance.

The endurance test lasted 5 days, so each series with three tests took 7 days; speed test 2 was a way to test whether the results were repeatable and to see whether training effects would occur.

Results are summarised in Table 1 showing the relation between oxygen consumption and swimming speed for each of the 3 tests. The results show also that the two different tests were similar; a 12 h run gave the same result as the two 2 h speed tests. This implies that the 2-h speed test can be used for testing endurance as well.

This corresponds with our observations; once the eels are swimming they do not change their mode of swimming nor their oxygen consumption at that speed. When considering the COT at the different swimming speeds, we clearly see that the values remain almost the same, eels swim over 0. As the results were the same for the 2- and h speed tests in this series with farmed eels, three other eel groups were tested only in a 1 day speed test with 2-h intervals Fig.

Oxygen consumption and cost of transport COT. Although increase in level per speed increment shows a linear relation in the range 0. Comparison between means of the pooled group for the different tests shows that data of the first speed test and endurance tests are similar.

Source: Palstra et al. Swimming endurance tests. For all five groups, the fitness parameters were calculated Table 1 ; Fig. The average COT remained similar while fish were swimming at the different speeds. The optimal swimming speeds, were very similar for all 4 groups and were found 0. This would imply that female eels may reach the Sargasso Sea within 3.

During long-distance migration, all animals are likely to maximise the distance covered per given fuel unit, thus they will try to migrate at the lowest cost of transport. The migration distance of the different eel species varies: the European eel A.

These eel species migrate impressive distances, but European eels need to be the most efficient swimmers among eels. The long-term swimming experiments with 5 eels of about 0. Five out of seven eels were able to swim 3 months at 0. In the literature, limited data are available on swimming performance of eels or other anguilliform swimming teleosts Webb ; McCleave It is even suggested that the swimming movement of eel is less efficient than that of for example salmonids Videler ; Bone et al.

However, biomechanical efficiency of propulsion is different from overall swimming efficiency. The first relates to the transfer of kinetic energy from muscle to environment and the resulting displacement of the body. The overall swimming efficiency, however, is the total energy required by the animal to transport itself over a certain distance.

This determines the amount of fuel stores i. Based on a day swimming trial with European silver eel, we demonstrated earlier that the energy cost of transport of those eels was extremely low: 0. This is 2. In a more recent study, we exposed female yellow eels of about g to a 6-month swimming trial at a mean swimming speed of 0. The eels swam in this experiment a complete simulated migration run of 5, km. Oxygen consumption data demonstrate that the swimming eels have a twofold higher O 2 consumption than the resting eels, while over a period of almost 6 months the energy consumption remained almost constant.

As the length of the animal did not change, the drag must have remained the same, so the energy cost of transport for the whole animal did not change. From the oxygen consumption, the energy consumption can be calculated based on fat combustion data given by Brafield and Llewellyn Thus, we can estimate how much fat the eels would have used, assuming that fat is the only energy source, based on the oxicalorimetric data for fat combustion.

The values calculated for the two long-term swimming trials gave about the same oxygen consumption per km 28 vs. As for a trans-Atlantic journey of 5, km, this would then require about 60 g fat for an eel of 1 kg. The calculated values for the cost of transport COT correspond rather well for the two experiments. On the other hand, the COT values for eel are some four to fivefolds lower than those obtained for salmonids Schmidt-Nielsen Recent data on salmonid swimming including from our own experiments confirmed earlier results from Schmidt-Nielsen that salmonids swim indeed at much higher cost of transport.

Lee et al. These data are between four and seven times higher than those observed with the female eels in our swim trials, which clearly indicate again that eels are very efficient swimmers. Long-term swimming experiments with eels have been published for eels swimming at 0. During long-term swimming the eels lost weight, which can be due to diminished energy stores, but also due to water loss. Therefore, the total body composition was determined of the eels after swimming 5, km.

The body composition of the three eel groups in this experiment control, resting, and swimming remained the same in all three conditions, which is remarkable Table 2. This implies in the first place that the buoyancy of the animal did not change and that therefore no volume compensation is required by the swim-bladder during swimming or resting. Another implication is that the energy consumption calculated from oxygen consumption and weight loss cannot be based on fat alone, but needs to be compensated for protein oxidation as well.

Since body composition does not change, it is possible to calculate the energy consumption from weight loss as well. Therefore, we need to recalculate the energy content of the dry weight according to the composition given in Table 2. The recalculated data are presented in Table 3. The energy content of the dry weight was found to be Source: van Ginneken et al. Calculation of the calorimetric value of eel whole body based on body composition Combustion energies of fat, protein, and carbohydrates are from Brafield and Llewellyn From the oxygen consumption per hour and the swimming speed 1.

From this, the total oxygen consumed must have been When dividing by The observed wet weight loss over 5,km was Considering the errors occurring in the different measurements we can conclude that the respirometry data oxygen consumption corroborate well with the calorimetric data weight loss.

It is important to emphasise that these two techniques for measuring the energy consumption are fully independent from each other. Our new calculations based on The resultant COT for both methods is respectively, 0. A break down of the fuel usage for a simulated migration over 6, km provides the following results: fat Thus, the minimal fat requirement for migration is about 5. This value is lower than the first estimate of 6.

Since we recently found that optimal swimming speeds of European silver eels are about 0. Remarkably, oxygen consumption rates of the wild silver eels remained similar during the induced temperature profile Fig.

Table 4 presents an overview of the costs of migration for all experiments. Oxygen consumption profiles during simulated migration. The swimming speed was increased the first 4 days from 0. Starting at day 4, the water temperature was lowered with 0. Comparison of estimated lipid costs for migration COT of eel recalculated on basis of using The U opt for wild European female silver eels swimming in SW is 0. Other tracking studies revealed speeds of 0. Thus, U opt corresponds only to the fastest of these migration speeds.

Besides the already discussed temperature effects on swimming behaviour, other environmental conditions—like depth, currents and predators—experienced during oceanic migration may also affect the swimming speed. Hence, the actual ground speeds may differ significantly from the U opt.

The exact route of migration of European eels is still largely unknown reviewed by Tesch and Rohlf Trackings of considerable numbers of eels in the North Sea and on the east Atlantic shelf have shown that eels swim uninterrupted in a compass direction geographically north and west. With decreasing latitude, directional preference turns over farther northward and attains in a NW swimming direction.

The NW course must lead them to the continental slope where they start to swim in a SW direction. This kind of navigational ability could be based on magnetic sensing of the inclination or strength of the magnetic field.

Furthermore, by using currents like the North Equatorial Current, ground speeds in the field may be even higher than the optimal swimming speed. Eels use all depth zones, except for bottom layers, during all tidal phases reviewed by Tesch and Rohlf A diel vertical migration during deep-sea migration has been shown repeatedly during tracking experiments using pop-up tag methodology reviewed by Tesch and Rohlf and very recently by Tsukamoto ; recent research papers by Jellyman and Tsukamoto and Aarestrup et al.

Eels were ascending during dusk and descending during dawn indicating that they do this to escape predators. They migrated at depths between and m both in continental and deep-sea waters which probably persists as far as the spawning grounds.

They preferred a depth of about m which is in accordance with the depth range of newly hatched yolk sac larvae Kleckner and McCleave A point of discussion remains however, that none of these studies has tried to determine the effects of the pop-up tag on swimming behaviour. Since these tags are relatively large for eels van Ginneken and Maes , they may increase drag, decrease swimming speed and efficiency, influence route choice and diel vertical migration itself. Therefore, we believe it is of utmost importance that potential effects are investigated to validate the results of pop-up tag methodology.

Experiments with male eels swimming in FW at Diel vertical migration in the field — m may result in higher efficiency at higher depth and pressure during the day. It remains a major question whether silver eels are capable to swim at those extreme low temperatures.

Possibly, they hide in the dark during day time, and continue to swim at lower depths and higher temperatures at night. Obviously this aspect of their migration can only be solved by more advanced tracking experiments. The 22 Blazka-type calibrated swimming flumes at Leiden University have also been used for swimming trials elucidating aspects of swimming induced silvering and maturation.

In , a new 6,L swimming gutter was built to allow group-wise swimming of males and females, expected to lead to lower stress levels thus avoiding possible negative effects on maturation. Swimming induces an increase of eye diameter Table 5. This has been observed repeatedly in different fresh water FW swimming trials.

Continuous swimming at 0. The observed changes appeared even stronger after 6 weeks of swimming. Significant increases in EI were also apparent in FW swimming trials with migrating eels from the river Loire aged 10—28; otolith analysis by Palstra et al. Since younger farmed eels did not show swimming-induced enlargement of the eyes, age-dependent maturation sensitivity is suggested.

Arguments for age-dependent maturation also come from other observations: 1 older eels showed increased capacity to incorporate more lipid from the muscle into the oocytes Palstra et al. Repeated yearly silvering and subsequent regression Durif et al. Age might thus be a key factor for successful maturation. Eels may nowadays compromise in this perspective and leave to the Sargasso at a younger age, since a long life-time increases the impact of anthropogenic factors that negatively interfere with reproduction capacity PCBs: van Ginneken et al.

The asterisk marks an estimated age corresponding with findings for other Lake Balaton eels. It appears that the increase of the eye diameter occurs solely in freshwater, since no changes were detected in salt-water SW trials Table 5 , neither in males nor females.

Since the enlargement of the eyes is used for discriminating between the yellow and silver phase Pankhurst , it can thus be stated that FW-swimming induces silvering. Changes in the length Durif et al. However, in none of the swimming trials were such changes detected Palstra et al.

The ovaries of European silver eels show oocytes after transformation of the oogonia in the first developmental stages stage 1—2; Adachi et al. Further progression requires incorporation of lipids stage 3 and vitellogenin stage 4.

After 6 weeks of swimming, changes were much more pronounced than after 2 weeks of swimming: both GSI and oocyte diameter were significantly higher. In contrast to resting eels, the swimming eels had oocytes in the lipid droplet stage 3. These results indicate that a high level of lipid mobilisation induced by FW- swimming is required not only for fuel but also for a natural incorporation of lipid droplets in the oocytes.

Oocytes of eels that had swum contained more than large droplets. We can thus conclude that swimming activates lipid metabolism. This may represent a crucial step in oocyte maturation, since the amount of lipid droplets influences the subsequent developmental events before and after fertilisation Palstra et al.

However, we did not observe any yolk globuli in the oocytes of swimming eels. The oocytes also did not reach sizes that are characteristic for vitellogenesis stage 4. Adachi et al. Pituitary LH is increased in swimmers and KT tends to increase but individual variation is high.

Based on data from van Ginneken et al. Yolk deposition however remained absent. Based on data from Palstra et al. After swimming 5, km in freshwater, young farmed eels showed increased LH levels in the pituitary Fig. FW-swimming thus stimulates gonadotropin production by the pituitary but it is still unclear whether secretion is also stimulated. Just recently, Lokman et al.

E2 did not show these effects in vitro Lokman et al. A significant role for KT is thus clear, but a role for E2 cannot be excluded. Wild migratory silver eels swimming in SW even showed decreased levels of plasma Vtg after swimming 1, km in salt water while testosterone T and E2 were unaffected Palstra et al. Recently, we measured blood plasma E2 and estimated Vtg indirectly through plasma calcium Ca concentration Palstra et al.

Swimming, but also resting, increased E2 levels but only in first instance. Ca levels were found to be lower in SW-swimming eels. Results thus show that swimming does not stimulate vitellogenesis which corresponds with histological findings; the absence of yolk globuli in the oocytes of swimmers.

Recently, we have partially cloned 4 different genes from A. In swimming eels, the expression of the esr1 was lower than in resting eels for both timepoints Palstra et al. This reduction in expression probably resulted in the reduced expression of vtg1 and vtg2 in swimmers. From this, we can conclude that hepatic vitellogenesis is indeed reduced in swimming silver eels in salt water.

On the basis of evidence from different angles, it can be concluded that swimming inhibits the whole process of vitellogenesis, at least in the first instance. Firstly, esr1 -, vtg1 - and vtg2 -expression were reduced in the livers of swimming females Palstra et al. Secondly, plasma Vtg was repeatedly determined as not detectable and plasma Ca as not elevated in swimming females van Ginneken et al.

Thirdly, oocytes of swimming females from Lake Balaton Palstra et al. In a recent experiment, we have injected eels that swam or rested for 3 months with carp pituitary extract up to full maturation. The swimming-induced suppression of vitellogenesis resulted in delayed ovulation of 2—3 weeks. Female eels stay 7—30 years in the freshwater before migration, in contrast to just 4—9 years for males.

As a consequence, females reach a tenfold larger size than males at the onset of migration resp. As the energy requirements for males are far less than those for females, it is possible that the observed dopaminergic inhibition is sex-specific. We have tested this hypothesis by subjecting male and female eels to a GnRH-agonist GnRHa , specifically the commercial product Gonazon For Fish Intervet , as well as to stimulation by long-term swimming in seawater SW that is supposed to stimulate GnRH excretion by the hypothalamus Palstra et al.

Both treatments also caused a three- to five-fold increase in GSI Fig. One male swimmer even showed the formation of spermatocytes. Thus, in contrast to the response of the females, we observed sexual maturation in males upon GnRHa-injection, indicating that dopaminergic inhibition is not effective in males.

Males were also stimulated after 3 months SW-swimming, suggesting that swimming acts via a similar mechanism. Females can therefore only be stimulated by circumventing dopaminergic control Dufour et al. In females, regression occurred during the experimental period, an effect which was more pronounced in the resting group than those stimulated by SW-swimming and GnRHa.

Gonad development: 3 m rest, 3 m swimming or 3 m GnRHa vs. So while gonadotropin expression and vitellogenesis in females during SW-swimming are suppressed, full spermiation can be expected in males after longer swimming trials. Our latest results show that male eels that swam for 3 months in salt water and that were subsequently treated with human chorionic gonadotropin started their spermiation earlier, and they produced more sperm of higher density.

When results from the laboratory are extrapolated to the field situation, neglecting effects of other possible triggers, it can be assumed that migratory eels do not necessarily silver before their freshwater migration rather than, or especially, during it. Old swimming silver eels showed increase of eye size but it appears that this occurs only in freshwater trials.

As far as we know, it is unknown if eels from salt or brackish water generally have larger eyes than eels from freshwater leading to only the latter showing this swimming-induced change in eye size. During freshwater migration, lipid mobilisation occurs.

Extensive lipid incorporation in the oocytes was apparent during freshwater swimming trials. In the field, Cottril et al. Vitellogenesis seems to be inhibited during freshwater migration. Vtg and Ca levels are low in wild silver eels. Versonnen et al. In a recent study Palstra et al. Silver eels exhibit large variability in plasma Vtg levels as demonstrated by homologous radioimmuno- and immunoenzymatic assays Burzawa-Gerard and Dumas-Vidal ; Sbaihi et al.

Van Ginneken and Dufour unpublished data measured plasma Vtg in large female silver eels from the brackish Lake Grevelingen The Netherlands. This cycle means most Europeans will never see the earliest eel life stages and has led to much speculation over the years.

Where, people wondered, do eels come from? In his History of Animals, written in the 4th century BC, Aristotle claimed that eels were produced not through sexual reproduction but by spontaneous generation.

Lacking in milt the fish equivalent of semen or eggs, nor possessing the passages for either, he asserted that eels emerged from earthworms:. There can be no doubt that the case is so. For in some standing pools, after the water has been drained off and the mud has been dredged away, the eels appear again after a fall of rain.

In time of drought they do not appear even in stagnant ponds, for the simple reason that their existence and sustenance is derived from rainwater.

It is easy to laugh at this fanciful notion, armed with the wealth of knowledge accumulated in the centuries since the ancient Greeks. Schmidt never actually saw any adult eels there, though. Instead, he collected larvae from across the Atlantic, and noted that the smallest, youngest larvae were found in the Sargasso Sea.

To date, however, despite considerable research efforts, European eels have still not been observed reproducing in the wild, nor have any adult eels been captured in the Sargasso Sea.