Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology
Cost of transport and optimal swimming speed in farmed and wild European silver eels (Anguilla anguilla)
Introduction
The 5500-km migration to the spawning grounds in the Sargasso (Schmidt, 1923) is crucial for the reproduction of European eel. Most certainly the effective genitors contributing to the future generation must therefore be characterized by an excellent swimming performance. Female eels leave the continent in October–November and spawning is believed to occur primarily in the following March and April (McCleave, 2003). Migration in the field is thus generally assumed to last for 6 months. Since distance and duration are known, a minimal swimming speed of about 0.4 m s− 1 can be calculated (or 0.5 BL s− 1 for average female eels of 80 cm) which is generally assumed to be the cruising speed.
Recently, we subjected large female eels to long term swimming trials at this speed in order to estimate the energy costs of transport (van Ginneken and van den Thillart, 2000, van den Thillart et al., 2004, van Ginneken et al., 2005a). The results showed that eels swam four to six times more efficiently than non eel-like fish, utilizing ca. 60 g fat/kg during sustained swimming. Other experimental data available on swimming performance of anguillids are limited and were obtained from small yellow eels (< 15 cm) (Langdon and Collins, 2000) or intermediate sized yellow eels (Schmidt-Nielsen, 1972, Webb, 1975, van Ginneken et al., 2002). However, the onset of migration is preceded by a change of ‘continental’ yellow eels into ‘oceanic’ silver eels. This so called ‘silvering’ process is a metamorphic event; a physiological and morphological preparation for their journey to the Sargasso Sea (Tesch, 2003, Lokman et al., 2003, Durif et al., 2005) or rather a pubertal event; an expression of sexual maturation (Pankhurst, 1982, Aroua et al., 2005). Drastic changes occur during silvering; most apparent is the enlargement of the eyes which is widely used to discriminate between the yellow and silver phase (Pankhurst, 1982). The silver phase is characterized by the impressive swimming performance.
As silver eels, they cease feeding and rely primarily on their fat stores for swimming and reproduction. Because of this, the COT should be minimised and the swimming speed optimised. Like with salmons that cease feeding, energy management is the key to successful migration and maturation. Besides the costs for swimming of about 40% of the energy stores, an additional 28% is required for deposition in the oocytes of European silver eels (Palstra et al., 2006). With the total costs of 68% of the energy stores eel's lifestyle is most likely a semelparous one. Starting migration with the maximum fuel reserve can be viewed as an optimum strategy (Lucas and Baras, 2001) and the same accounts for optimal swimming. For salmonids, it is suggested that intraspecific migration energetics and abilities are correlated to migration distance (Bernatchez and Dodson, 1985, Lee et al., 2003a). When comparing the migration distances between salmonids (up to 1500 km) and European silver eels (5500 km), the need for optimal swimming in eels is very clear. It may well be that Uopt is higher than the generally assumed cruising speeds and it is likely that silver eels would cruise at Uopt.
In this study, we have investigated Uopt of silver eels and the minimal COT. A variety of swimming tests were performed to compare critical swimming speeds (Ucrit), Uopt and COT between farmed eels and wild eels, both in freshwater as in seawater.
Section snippets
Experimental eels
Eels were collected at three locations:
- 1.
Farmed eels were obtained from a commercial freshwater eel farm (Royaal BV, Helmond, The Netherlands) in October 2001 and in September 2004. They originated as glass eels from France. The October 2001 batch was immediately transferred to SW (32 ppt), acclimated during a 2-week period and used in SW experiments in November and December 2001. The September 2004 group was used in FW experiments within a week after arrival.
- 2.
Wild migratory eels in the FW River
Biometric comparisons between experimental groups of eels
All biometric data are presented in Table 1. Farmed eels had eye indices ≥ 9.6 and were therefore considered as silver according to Pankhurst (1982). The 2001 batch was split on the basis of their mass into a group of small (BW = 409–648 g, n = 20) and a group of large eels (BW = 659–1191 g, n = 22). The characteristics of the FW group (n = 20) were comparable with those of the large eels of the SW group.
The wild eels from the SW Lake Grevelingen (n = 19) were longer (P ≤ 0.01), and heavier (P ≤ 0.05) than the
Discussion
This study has been the first to test the swimming endurance of large female silver eels on a large scale. Earlier swimming experiments were performed only on small eels (< 60 cm) and often in low numbers. Our set-up of 22 Blazka swim tunnels allows large scale investigations. Recent long term simulated migration trials of large female eels swimming at a fixed speed of 0.5 BL s− 1 (on average 0.4 m s− 1) revealed that large silver eels have a very low COT (van Ginneken and van den Thillart, 2000,
Acknowledgements
This research was subsidized by the EU (EELREP no Q5RS-2001-01836). The authors wish to express their thanks to R. van der Linden and R. Heijmans for their technical support, P. Niemantsverdriet, S. van Schie and L. Wagenaar for animal care, M. Brittijn for the help with the illustrations and Royaal BV (Helmond, The Netherlands), Bout (Bruinisse, The Netherlands) and Dr. C. Durif for providing experimental eels. Prof. Dr. S. Dufour, Prof. Dr. F. Volckaert, Prof. Dr. M. Richardson and four
Glossary
- AS
- air saturation
- BL
- body length
- BW
- body weight
- COT
- cost of transport
- COTmin
- minimum cost of transport at optimal swim speed
- EDh
- eye diameter horizontal
- EDv
- eye diameter vertical
- EI
- eye index
- FW
- freshwater
- Hb
- hemoglobin
- Hct
- hematocrit
- K
- condition factor
- M˙O2
- oxygen consumption
- SW
- sea water
- U
- swimming speed
- Ucrit
- critical aerobic swimming speed
- Uopt
- optimum swimming speed
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