The stocking programs seem to be beneficial for the growth of stocked eels. The length-at-age between 5 - 8 years were significantly larger in the stocked eels than the naturally-recruited eels although there is no difference found in body length, weight, age and mean growth rate. Some possible reasons are discussed as follows:
Environment factor: Temperature
Yearly water temperature in the inland freshwater lake (mean: 8.8 °C) was significantly lower than that in the Curonian Lagoon (mean: 9.6 °C) (student’s t test, p<0.05), however did not differ (p>0.05) from that in the Baltic Sea (mean: 8.9°C) in 2004. Differences in water temperature between Curonian Lagoon and the Baltic Sea were significant (p<0.05). The physiological optimum for eel growth is found at 22 – 23 °C for yellow eels (Sadler, 1979) and glass eels (Elie & Daguzan 1976), however at temperatures < 10 °C eels become inactive (Tesch 2003) and as the result growth should stop. According to Elie & Daguzan (1976) from 10 °C and below, eel growth is inhibited, the individual activity and the alimentary taking are very low or non-existent; however, after temperature is elevated, from 11.5 °C, growth can start off again. In the Curonian Lagoon mean daily water temperatures > 10 °C were during 180 days (mean: 16.5 °C) in 2004, during 161 days in the coastal waters of the Baltic Sea (mean: 15.4 °C) and 157 days in the inland lake (mean: 16.2°C). The water temperature in periods >10 °C was significantly different (p < 0.05) in Curonian Lagoon than the Baltic Sea coast region, whereas that between the Baltic Sea and inland lake was not different (p > 0.05).
By the viewpoint of water temperature, the Curonian Lagoon seems to be the best habitat for growth, followed by the inland lake, and the last one is the Baltic coastal region, which is contrary to observed growth phenomenon. Hence, it is indicated that the water temperature played a relatively minor role in the growth of the eels in Lithuania.
Stocking population density
Population density is one of factors influencing growth of all individuals in a population.
Determination of growth rate of eels in a water body show that the slow growth of eels in natural waters results mainly from competition in a population that is too densely populated (Tesch 2003). Einsel (1961) found that eels stocked in the eel-free lake in Austria have reached longer body length in the 4th year compared to other natural populations, which might be due to less intra-species competition. But in Russian eels introduced in reservoirs with high density (200/ha) were smaller than in previous case in the 2nd year (Kokhnenko & Borovik 1958). The stocking rate is relatively lower: (only 1.7 ind.*ha-1 in 1995-2003) in the Curonian Lagoon, and this low population density may favor better growth condition for the eel growth.
However, the stocked eels only displayed relatively higher growth rate at 5 - 8 year, suggesting other effects may also be involved
Environment factor: Salinity
Effect of water salinity on fishes is not well revealed. Some authors have demonstrated the positive influence of external salinity on growth capacities in other fish species (e.g.
Tsintsadze 1991, Konstantinov & Martynova, 1993, Ložys 2004a). However, the effect of salinity on growth of freshwater species appears to vary among species. Specific growth rates for the channel catfish (Ictalurus punctatus) and goldfish (Carassius auratus) were highest in fresh water and decreased with increasing salinity, while within the same experiment three other species, i.e. rainbow trout (Oncorhynchus mykiss), striped bass (Morone saxatilis) and Gulf sturgeon (Acipenser oxyrinchus desotoi) grew faster in 3 and 9 psu than in lower salinities (Altinok & Grizzle, 2001). For eels, Tzeng et al. (2003) speculated that eels living in brackish waters may save more energy cost in osmo-regulation and resulting in better growth.
In addition, Edeline et al (2005) also found for glass eels living in SW (salt water), the length
started increasing earlier and they born higher growth rate than for those living in freshwater.
Some studies (Pic, 1978; Hales et al., 1990; Mazik et al., 1991) suggest that isosmotic environment could be beneficial for both freshwater and marine fishes and fish growth should be improved in an isosmotic environment (Foss et al., 2001). However, recent ones indicate that the osmotic cost is not as high (roughly 10%) as this (Bœuf, 2001). Data are available in terms of food intake and food conversion stimulation, depending on the environmental salinity (e.g. Lambert et al., 1994; Conides et al., 1997; Altinok & Grizzle, 2001). Many hormones (e.g. growth hormone GH, insulin-like growth factor IGF1, Prolactin PRL, cortisol, thyroid hormones) are known to be active in both osmoregulation and growth regulation (e.g.
Bern & Madsen, 1992; Bœuf, 1993; Sakamoto et al., 1993). It was found that GH cells were activated in eels acclimatized to brackish water (Olivereau & Ball, 1970). Pituitary gland explants from SW-adapted eels were shown to release less GH than those from FW-adapted specimens (Ball, 1981). However, the stocked eels living in the freshwater lagoon still grow fast, indicating that salinity does not overcome effect of other factors in this case. This is in contradiction with study done on another fish species Perca fluviatilis in freshwater Curonian Lagoon and brackish Baltic Sea coastal waters (Ložys 2004a). In this study low salt concentrations overcome other factors such as food and temperature and was crucial factor determining faster growth. Moreover, highly significant effect of brackish environment on perch growth was proven under experimental conditions. However, the fast growing eels in freshwater lagoon is in accordance with Tesch (2003), who refer to several studies when slower growth in coastal waters compare to freshwater was observed. (but the examples Tesch refered are old, about 1970~1980, and recently research ex Jessop 2004, Morrison and Secor 2002 has found that the eel in brackish water may have greater growth condition, thus here I may accept that the freshwater environment
Food availability and productivity of water sites
One of the parameters most well characterizing water body trophic status is the chlorophyll-a concentration in the phytoplankton, since it well reflects inflow of trophic elements into water body and is an indicator of primary production. According to data on chlorophyll-a concentrations in lake Dringis in 1988 provided by Kavaliauskienė (1996) and calculations of trophic index done as suggests Carlson (1977), trophic index of Dringis lake is 47, while trofic index of Baluošai lake in 2005 according to D. Kalytytė (pers. commun.) was calculated to be 39.8. According to Vaičiūtė (2004) trophic index of the Curonian Lagoon was on average 67 in 2004. The same index at the Baltic Sea according to the data on chlorophyll-a concentrchlorophyll-ations by Schrimpf et chlorophyll-al. (2005) during the period 1998-2004 trophic index wchlorophyll-as rather stable, with values ranging from 38.7 to 39.6. However, Lithuania coastal waters as well as other coastal water areas of the Baltic Sea where eels spend most their lifes are under some influence of trophic elements inflow from inland waters and is higher than average trophic index values for the whole Baltic Sea. E.g. according to the chlophyll-a data provided by Marine Research Center, trophic index in the coastal waters was on average 47.5 in 2004.
Hence, according to the Carlson (1977) clasification both lakes and Baltic Sea are mezotrophic water bodies, while Curonian Lagoon is hypertrophic.
Available information suggests that feeding conditions in the Curonian Lagoon may be better than those in the coastal waters. The abundance of fish juveniles in the lagoon is significantly higher (mean g per 100 m2 ± SD: 4025 ± 572, dominant species: Perca
fluviatilis, Rutilus rutilus, Gobio gobio, vs. 502 ±431, dominant species: Osmerus eperlanus,
Sprattus sprattus sprattus, Ammodytes tobianus, t = 11.8, p < 0.0001; according to Repečka et
al., 1996). Test fishing with 17-mm and 22-mm mesh size gill nets was conducted at both sites in 2000 and 2001. Significantly higher CPUE (catch per unit effort per 30 m of net per 8 h) was established in the Curonian Lagoon (CPUE: 5707 ± 2184 vs. 1123 ± 596, in g, t = 7.01,all their diet can consist of different species of benthic invertebrates (Tesch, 2003). A zoobenthos study in the lagoon and the Lithuanian coastal zone down to the depth 40 m (Olenin, 1997), demonstrated that invertebrate biomass (molluscs excluded) was greater in the lagoon (Curonian Lagoon: 11.8 g m-2; Baltic Sea: 5.5 g m -2). Hence, the available information suggests that faster growth of eels in the Curonian Lagoon could be the resulted by higher food availability.
Genetic structure factors
The stocked eels were caught mainly from UK and French coasts then transported to Lithuanian. In one hand, Wirth & Bernatchez (2001) has demonstrated genetic differentiation occurred in European eels in different regions. In the other hand, Dannewitz et al. (2005) recently argued this approach and suggested that genetic variation in temporal samples European eel within sites clearly exceeds the geographical component, i.e. isolation-by-distance and supported the panmixia hypothesis. The question of the European eel genetic structure is still under the discussion and needs more studies to clarify this phenomenon.
Consequently, there are no sufficient evidences to support the genetically driven growth difference in this study but the possibility can not be fully expelled.
Energy cost due to difference in migration distance
Comparing to naturally-recruited eels, the stocked eels colonized in the Curonian Lagoon without the extended migration from north Europe coast across the Baltic Sea. Bohlin et al (2001) has suggested energy cost of migration in trout could reduce fitness. According to evaluations done under experimental conditions, silver eel for migration use 10% of total energy allocations (fats) per 1000 km (Thillart et al. 2004). However, unlike migrating adults, eel juveniles or small yellow eels lack fat depots and other available stores of energy. Hence, during the migration, a regression in growth (Jellyman, 1977) and a reduction of caloric availability per eel (Tarr & Hill, 1978) occurs. Thus the longer migration of naturally-recruited eels have a negative effect on the growth due to the energy allocation to migration while stocked eels settling down at the elver stage have less energy spending in migration.
Moreover, the mean age of naturally-recruited eels first entering to freshwater is 5.2 year (range: 1 – 10 yrs, Shiao et al. 2006), coincidental to the range where the significant differences in length-at-age were observed between naturally-recruited and stocked eels. The difference in energy cost due to migration may be accumulated from the elvers, and reached its maximum in about the 5th years when the eels arrived at Lithuania. The differences in length might be later replenished by compensation growth, as indicated by Graynoth & Taylor (2000), or also by the better energy transformation efficiency from feeding of naturally-recruited eels due to longer distant migration compared with stocked eels (Bernatchez &
Dodson 1987).
Conclusion
The growth condition of European eels would be different among habitats, which eels caught in the inland freshwater lake grew slowly, and those caught in the lagoon and coast grew fast. Eels with different migratory histories were found no significant difference in growth, presumably due to connectivity, similarity and seasonal movement between the lagoon and the coast. Although in gross (??) the restocked were not different with naturally-recruited eels, but in the 5 - 8 year it was found the stocked eels were larger than the naturally-recruited eels. The difference during this period might result from different migration cost or different migration routes artificially induced due to stocking programs.
Acknowledgements
Temperature data for Curonian Lagoon and Baltic Sea were provided by Marine Research Center (Ministry of Environment); temperature records on Tauragnai Lake temperatures provided Lithuanian Hydrometeorological Service (Ministry of Environment). Financial support was provided by the Ministry of Environment, Republic of Lithuania and the Lithuanian Fisheries Produces’ Association and by the Lithuania-Latvia-Taiwan (Republic of China) Mutual Fund (Contract No. NSC 94-2313-B-002 -043) for the field studies and elemental analysis.
References
Acou A, Lefebvre F, Contournet P, Poizat G., Panfili J and Crivelli AJ (2003) Silvering of female eels (Anguilla anguilla) in two sub-populations of the Rhone delta. Bull Fr Peche Piscic 368: 55 – 68.
Altinok, I. & J. M. Grizzle, 2001. Effects of brackish water on growth, feed conversion and energy absorbtion efficiency by juvenile euryhaline and freshwater stenohaline fishes. J Fish Biol 59: 1142–1152.
Audenaert V, Huyse T, Goemans G, Belpaire C, Volckaert FAM (2003) Spatio -temporal dynamics of the parasitic nematodeAnguillicola crassus in Flanders, Belgium. Dis Aquat Org 56: 223-233.
Ball, J. N. (1981). Hypothalamic control of the pars distalis in fishes, amphibians and reptiles.
Gen Comp Endocrinol 44, 135-170.
Bern, H.A. & S.S. Madsen, 1992. A selective survey of the endocrine system of the rainbow trout Oncorhynchus mykiss with emphasis on the hormonal regulation of ion balance.
Aquaculture 100: 237–262.
Bœuf, G. & P. Payan, 2001. How should salinity influence fish growth? CompBiochem Physiol A 130: 411–423.
Bohlin T, Pettersson J and Degerman E (2001) Population density of migratory and resident brown trout (Salmo trutta) in relation to altitude: evidence for a migration cost. J Anim Ecol 70: 112 – 121.
Bonsdorff E. and Pearson TH (1999) Variation in the sublittoral macrozoobenthos of the Baltic Sea along environmental gradients: A functional-group approach. Aust J Ecol 24: 312-326.
Bernatchez L and Dodson JJ (1987) Why do so few anadromous population minimize the energetic cost of their upstream migrations. In Common Strategies of anadromous and catadromous fishes. Amer Fish Soc Sym 1. p 556.
Cairns DK, Shiao JC, Iizuka Y, Tzeng WN, Macpherson CD (2004) Movement patterns of American eels in an impounded watercourse, as indicated by otolith microchemistry.
North Am J Fish Manage 24: 452-458.
Carlson RE (1977) A trophic state index for lakes. Limnol Oceanogr 22 (2): 361-369.
Cone RS (1989) The need to reconsider the use of condition indices in science. Trans Am Fish Soc 118: 510-514.
Cone RS (1990) Properties of relative weight and other condition indices. Trans Am Fish Soc 119: 1048-1058.
Conides, A.J., A.R. Parpoura & G. Fotis,1997. Study on the effects of salinity on the fry of the euryhaline species gilthead sea bream Sparus aurata L. 1758. J Aquac Trop 12: 297–303.
Daverat F and Tomás J (2006) From tactics to population dynamics in the European eel (Anguilla anguilla): the case study of the Gironde watershed (Southwest France). Mar Ecol Prog Ser
Daverat F, Limburg KE, Thibault I, Shiao JC, Dodson JJ, Caron F, Tzeng WN, Iizuka Y, Wickström H (2006) Phenotypic plasticity of habitat use by three temperate eel species
Anguilla anguilla, A. japonica and A. rostrata. Marine Ecology Progress Series. 308:
231-241
Einsel W (1961) Das Wachstum des Aales in oesterreichischen Gewässern. Öst. FischZtg. 14, 136-138. in Tesch 2003.
Edeline E, Defour S, Elie P (2005). Role of glass eel salinity preference in the control of habitat selection and growth plasticity in Anguilla anguilla? Marine Ecology Progress Series 304: 191-199.
Edeline E, Elie P (2004). Is salinity choice related to growth in juvenile eel Anguilla anguilla?
Cybium 28: 77-82.
Elie P and Daguzan J 1976. Feeding and growth of elvers of anguilla Anguilla L. (eel-like teleosten fish) experimentally reared at various temperatures in the laboratory. Ann Nutr Aliment. 30(1): 95-114
Feunteun E (2002) Management and restoration of European eel population (Anguilla
anguilla): An impossible bargain. Ecological Engineering 18: 575–591.
Foss, A., T. H. Evensen, K. Imsland & V. Oiestad, 2001. Effects of reduced salinities on growth, food conversion efficiency and osmoregulatory status in the spotted wolffish. J Fish Biol 59: 416–426.
Francis RICC (1990) Back-calculation of fish length: a critical review. J Fish Biol. 36: 833-902.
Graynoth E and Taylor MJ (2000) Influence of different rations and water temperatures on the growth rate of shortfinned eels and longfinned eels J Fish Biol 57: 681 – 699.
Gross MR (1987). Evolution of diadromy in fishes. Amer Fish Soc Sym 1: 14-25.
Hales, L. S. Jr., C. C. Lay & G. S. Helfman, 1990. Use of low-salinity water and gel-coating to minimise handling mortality of spot, Leiostomus xanthurus (Perciformes: Sciaenidae).
Aquaculture 90: 17–27.
Holmlund CM and Hammer M (2004) Effects of Fish Stocking on Ecosystem Services: An Overview and Case Study Using the Stockholm Archipelago. Environ Manage 33: 799-820.
Jessop BM, Shiao JC, Iizuka Y and Tzeng WN (2002). Migratory behaviour and habitat use by American eels Anguilla rostrata as revealed by otolith microchemistry. Mar Ecol Prog Ser 233: 217-229.
Jessop BM, Shiao JC and Tzeng WN (2004) Variation in the annual growth, by sex and migration history, of silver American eels Anguilla rostrata. Mar Ecol Prog Ser 272:
231-244.
Kavaliauskienė J. (1996) Lietuvos ežerų dumbliai [Algae of Lithuanian lakes]. Geografijos institutas, Vilnius, 173 pp.
Kokhnenko SW and Borovik EA (1958) Results obtained after two years’ investigation on growth and development of eel fry in the water reservoirs of Belorussia. Bull Inst Biol Minsk 3: 269 – 272. in Tesch (2003) The eel, 3rd edition. Blackwell Science.
Konstantinov, A.S. & V.V. Martynova, 1993. Effect of salinity fluctuations on energetics of juvenile fish. J Ichthyol 33: 161–166.
Lambert, Y., J.D. Dutil, & J. Munro, 1994. Effect of intermediate and low salinity conditions on growth rate and food conversion of Atlantic cod Gadus morhua. Can J Fish Aqua Sci 51:
1569–1576.
Lee TW (1982) Ageing and growth of eel popultion Anguilla anguilla in the lagoons of Arcachon bay (France). J Oceanol Soc Korea 17: 83 – 94.
Limburg KE, Wickström H, Svedäng H, Elfman M and Kristainsson P (2003) Do stocked freshwatr eels migrate? Evidence from the Baltic suggests „Yes“. Amer Fish Soc Sym 33: 275-284.
Ložys, L., (2004 a). The growth of pikeperch (Sander lucioperca L.) and perch (Perca fluviatilis L.) under different water temperature and salinity conditions in the Curonian Lagoon and Lithuanian coastal waters of the Baltic Sea. Hydrobiologia 514: 105–113.
Ložys L (2004 b) Natūralių gamtinių žuvų populiacijų įvairovės vidaus vandens telkiniuose įvertinimas ir dirbtinio žuvų veisimo biologinis pagrindimas. Ataskaita Aplinkos Ministerijai (I dalis). Vilniaus universiteto Ekologijos institutas, Vilnius.
McKeown BA (1984) Fish migration 1st edition. Timber Press, Beaverton, USA.
Mazik, R. M., B. A. Simco & N. C. Parker, 1991. Influence of water hardness and salts on survival and physiological characteristics of striped bass during and after transport.
Trans Amer Fish Soc 120: 121 – 126.
Moriarty C and Hackett N (1976) An exceptional large eel Anguilla anguilla (L.) Ir Nat J 18 (10): 307-308.
Morrison WE and Secor DH (2003) Demographic attributes of yellow-phase American eels (Anguilla rostrata) in the Husdson River estuary. Can J Fish Aquat Sci 60: 1487 – 1501.
Olenin S (1996) Comparative community study of the south-eastern Baltic coastal zone and the Curonian Lagoon. Proceedings of the 13th Symposium of the Baltic Marine Biologists: 153–161.
Olivereau, M. & J. N. Ball, 1970. Pituitary influences on osmoregulation in teleosts. Memoirs of the Society for Endocrinology 18: 57–82.
Pic, P., 1978. A comparative study of the mechanism of Na+ and Cl- excretion by the gill of Mugil capito and Fundulus heteroditus: effects of stress. J Comp Physiol 123: 155–162.
Poole WR, Reynolds JD and Moriarty C (2004) Early post-larval growth and otolith patterns in the eel Anguilla anguilla. Fish Res 66: 107-114.
Sadler K (1979) Effect of temperature on the growth and survival of the European eel,
Anguilla anguilla L. J Fish Biol 15: 499–507.
Sakamoto, T., S. D. McCormick & T. Hirano, 1993. Osmoregulatory actions of growth hormone and its mode of action in salmonids: A review. Fish Physiol Biochem 11: 155–
164.
Svedäng H (1996) The development of the eel (Anguilla anguilla L.) stock in the Baltic Sea:
an analysis of catch and recruitment statistics: Polish-Swedish Symposium on Baltic Coastal Fisheries Resources and Management 255-267.
Shiao JC, Ložys L, Iizuka Y Tzeng WN (2006) Migratory patterns and contribution of stocking to the population of European eel in Lithuanian waters as indicated by otolith Sr:Ca ratios. J Fish Biol. In publish.
Shiao JC, Iizuka Y, Chang CW, Tzeng WN (2003) Disparities in habitat use and migratory behavior between tropical eel Anguilla marmorata and temperate eel A. japonica in four Taiwanese rivers. Marine Ecology Progress Series 261: 233-242.
Schrimpf W, Zibordi G, Mélin F, Djavidnia S (2005) Chlorophyll-a concentrations, temporal variations and regional differences from satellite remote sensing.
http://www.helcom.fi/environment2/ifs/ifs2005/Chlorophyll-a/en_GB/chlorophyll/
Tesch FW (2003) The eel, 3rd edition. Blackwell Science.
Thorpe JE (2004) Life history responses of fish to culture. J Fish Biol 65: 263-285.
Tsintsadze, Z.A., 1991. Adaptational capabilities of various size-age groups of rainbow trout in relation to gradual changes of salinity. J Ichthyol 31: 31–38.
Tsukamoto K, Nakai I, Tesch WV (1998) Do all freshwater eels migrate? Nature 396: 635–
636.
Tzeng WN, Severin KP, Wickström H (1997) Use of otolith microchemistry to investigate the environmental history of European eel Anguilla anguilla. Mar Ecol Prog Ser 149: 73–81.
Tzeng WN, Wang CH, Wickström H, Reizenstein M (2000) Occurrence of the semi-catadromous European eel Anguilla anguilla (L.) in Baltic Sea. Mar Biol 137: 93–98.
Tzeng WN, Shiao JC and Iizuka Y (2002) Use of otolith Sr:Ca ratios to study the riverine migratory behaviors of Japanese eel Anguilla japonica. Mar Ecol Prog Ser 245: 213–
221.
Tzeng WN, Iizuka Y, Shiao JC, Yamada Y and Oka HP (2003) Identification and growth rates comparison of divergent migratory contingents of Japanese eel (Anguilla
japonica).Aquaculture 216: 77-86.
Vaičiūtė D (2005). Potencialiai toksinių dumblių ir cianobakterijų vystymosi ypatumai šiaurinės Kuršių marių dalies planktone, 2004 m. vasarą [Development peculiarities of potentially toxic algae and cianobacteria in the plankton of the northern part of the Curonian Lagoon] . Graduation dissertation. Vilniaus Universitetas, Vilnius, 66 pp.
Vyšniauskas I (2003) Vandens druskingumas pietrytinėje Baltijoje [Water salinity in the Southeastern Baltic Sea]. In Stankevičius, A. (ed.), Baltijos jūros aplinkos būklė [State of the environment in the Baltic Sea]. Aplinkos ministerijos Jūrinių tyrimų centras, Kaunas: 35–38.
Vyšniauskas I and Lesys H (1998) Hidrologinio režimo ypatumai Lietuvos jūrinėje ekonominėje zonoje 1992-1996 metais [Peculiarities of the hydrological regime in the Lithuanian economic zone in 1992-1996]. In Tilickis, B. (ed.), Kuršių marių ir Baltijos jūros aplinkos būklė [State of the environment in the Curonian Lagoon and the Baltic Sea]. Jūrinių tyrimų centras, Klaipėda: 57-67.
Westin L (1998) The spawning migration of European silver eel (Anguilla anguilla L.) with particular reference to stocked eel in the Baltic. Fish Res 38:257–270.
Westin L (2003) Migration failure in stocked eels Anguilla anguilla. Mar Ecol Prog Ser 254:
307-311.
Wickström H, Westin L and Clevestam P (1996) The biological and economic yield from a longterm eel-stocking experiment. Ecol Freshw Fish 5: 140–147.
Wickström H, Westin L and Clevestam P (1996) The biological and economic yield from a longterm eel-stocking experiment. Ecol Freshw Fish 5: 140–147.