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Do Stocked Freshwater Eels Migrate?Evidence from the Baltic Suggests “Yes”

Karin E. LimburgSUNY College of Environmental Science & Forestry,

Syracuse, New York, USA

Håkan WickströmSwedish Board of Fisheries, Institute of Freshwater Research,

Drottningholm, Sweden

Henrik SvedängSwedish Board of Fisheries, Institute of Marine Research,

Lysekil, Sweden

Mikael Elfman and Per KristianssonDepartment of Nuclear Physics, Lund Technical University,

Lund, Sweden

Abstract.—In response to declining catches of eels in the brackish Baltic Sea, the Swedish gov-ernment stocks eels Anguilla anguilla (L.), both in lakes (mainly glass eels/elvers) and in the seaitself (mainly yellow eels). However, the degree to which these fish contribute to the spawningstock, if at all, was unknown. We collected silver eels at the exit of the Baltic Sea and analyzedindices of their maturity status. In addition, we used electron (WDS) and nuclear (microPIXE)microprobes to map out the strontium and calcium contents of their otoliths, as Sr:Ca correlateswith salinity. As a calibration, we analyzed otoliths from eels collected around the Swedishcoast and fresh water (0–25 psu) and derived a relationship between salinity and Sr:Ca. Ourresults show that, of 86 silver eels analyzed, 17 eels had Sr:Ca profiles consistent with havingbeen stocked into fresh water, six showed patterns consistent with stocking directly into theBaltic from marine waters, and 10 showed patterns indicative of natural catadromy. In all, 31.4%of silver eels showed histories of freshwater experience, including 24% of those found outsidethe Baltic. Silver eels caught exiting the Baltic had higher fat contents (21.1% of body weight)than those collected in the southern Baltic near Denmark (18.6%), but differences were notsignificant between wild and presumed stocked fish within geographic areas. The conclusionsof Tsukamoto et al. (1998), i.e. that freshwater eels are not supported by catadromous individu-als, do not appear to hold for the Baltic, although it is clear that noncatadromous fish composedthe majority of our silver eel samples.

Introduction

Baltic eel Anguilla anguilla (L.) catches have de-clined precipitously since the 1960s and asupplemental stocking program has been in ef-fect in Sweden since the late 1970s (Svedäng1996). The stocking program, at an annual costof nearly 10 million SEK (~ US$1 million; H.Wickström, personal communication), has con-sisted either of transplanting small eels (ca0.10–20 cm) collected as glass eels or elvers in the

Severn estuary in England, or transplanting yel-low eels (35–48 cm) caught off the Swedish westcoast (Wickström 1993). Today, stocking pro-vides an estimated 8–9% of all young eels inSweden. Whereas much has been learned aboutthe biology of stocked eels from experiments(e.g., Wickström et al. 1996), there has been nodirect evaluation of the contribution of stockedeels to Baltic catches, nor to their ultimate con-tribution to the spawning stock. However, pre-vious work raised two testable hypotheses.

American Fisheries Society Symposium 33:275–284, 2003© Copyright by the American Fisheries Society 2003

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In one study, experimentally stocked eels onGotland, that were tagged and allowed to mi-grate as silver eels, did not migrate through theoutlet of the Baltic at Öresund (Figure 1) as didnatural migrants, but rather ended up aroundthe Danish islands in the Belt Sea (Westin 1990).In a second study, Svedäng and Wickström(1997) found no correlation between maturitystage and muscle fat concentration, and sug-gested that silver eels (a non-feeding stage) withlow fat concentrations may temporarily haltmigration, revert to a feeding stage, and “bulkup” until fat reserves are sufficient to carry outsuccessful migration to the spawning area. Thus,by sampling silver eels at the exit point fromthe Baltic, measuring their fat reserves, and iden-tifying stocked versus natural fish, one shouldbe able to test whether: stocked eels can find

their way out of the Baltic (Westin 1990) andwhether there is a correlation between stocked/natural status and low fat concentrations(Svedäng and Wickström 1997).

In a third study, Tsukamoto et al. (1998) usedthe method of strontium:calcium ratios inotoliths (for a review of the method and otherotolith microchemistry, see Campana 1999) todetermine whether or not catadromous Euro-pean and Asian anguillids contribute to spawn-ing stocks. Briefly, Sr entrained in aragoniticotoliths typically reflects environmental concen-trations, and when normalized to otolith Ca itserves as a proxy for salinity because Sr is gen-erally higher in marine waters than in fresh.Because otoliths accrete over time, a temporalrecord of Sr:Ca is maintained and can be as-sessed with various microprobe techniques

Figure 1. Map of the study region. Key: S = Salnö, G = Gotland, Å = Lake Ången, D = area sampled inDenmark for silver eels, K = Kullen, F = Fladen, M = Marstrand, and S-K = Skagerrak.

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(Campana 1999). Otoliths from eels collected inthe North Sea and the East China Sea revealedno evidence of freshwater experience, leadingthe investigators to question whether catadro-mous eels really do contribute their genes tofuture generations. Thus a third hypothesis—that only noncatadromous eels recruit to thespawning stock—should also be testable.

In the present study, we used the Sr:Camethod to trace the environmental histories ofmigrating Baltic eels. We first calibrated themethod by measuring the Sr:Ca ratios in eelotoliths collected either in fresh water or along astable salinity gradient, and then used these toexamine and analyze otoliths of silver eels, col-lected either exiting the Baltic or down in itssouthern reaches. We found a substantial fractionof migrating silver eels whose Sr:Ca patterns areconsistent with having been stocked, and furtherfound fish showing catadromous patterns.

Methods

Study Area

Eels were collected at different places in the Bal-tic Sea, the Skagerrak-Kattegat area, and theSwedish west coast (Figure 1) either by trapping,trawling, or by purchase from fishermen. Eelsthat were experimentally stocked and monitoredin two lakes (Lake Ången on the Swedish main-land, and Lake Fardume on the island of Gotland)served as known freshwater endmembers. Yel-low eels collected from three areas of differentsalinity (Marstrand, approx. 25 psu, Gotland,approx. 7 psu, and Salnö, approximately 5 psu)were assayed to provide Sr:Ca ratios represen-tative of those salinities. Finally, silver eels werecollected at sites exiting the Baltic Sea (Kullen,Fladen, and in the Skagerrak) as well as in thesouthern Baltic in the vicinity of the Danish is-lands of Lolland and Falster. Eels were storedfrozen until ready for analysis.

Laboratory Procedures

Morphometric measures (total length, weight,eye diameter, jaw length, and pectoral finlengths) were made on defrosted fish. A sampleof tissue was taken just anterior of the anal ventand analyzed for lipid content as described inSvedäng and Wickström (1997). Eye diametersand total lengths were used to calculate an in-dex (I ) of maturation status (Pankhurst 1982):

I = (25�/8TL) � [(A + B)2L + (A + B)2

R ],

where TL is total length, and A and B are theheight and width of the left (L) and right (R)orbitals. Silver eels with values of I greaterthan 6.5 are classified as sexually maturing(Pankhurst 1982; Svedäng and Wickström 1997).

Sagittal otoliths were removed from the skull,cleaned, embedded in Spurr’s epoxy, and sec-tioned in the sagittal plane by grinding to thecore with a graded series of grinding papers, andfinally polished to 0.5 �m surface fineness. Fol-lowing carbon coating, otoliths were analyzedfor Sr and Ca by using either a wavelength-dispersive (WDS) electron microprobe at the De-partment of Geosciences, Uppsala University, ornuclear microscopy combined with proton in-duced X-ray emission analysis (�PIXE) at theDepartment of Nuclear Physics, Lund Techni-cal University. The latter method has the advan-tages of higher resolution and more rapid datacollection and was therefore the method ofchoice, given availability of machine time.Thirty-eight otoliths were analyzed using WDSand 100 with �PIXE.

Analyses using WDS involved visually locat-ing the core of the otolith with transmitted light,then laying out a transect of 30–50 points spaced20–40 �m apart traversing the otolith from thecore to the outer posterior edge (a so-called “life-history transect”). The parameters for operationwere: accelerating voltage, 20 kV; current, 20nA;electron beam diameter, 15 �m. Strontium wascounted for 40 s on the peak and 40 s on the back-ground (only on one side, to avoid the stronginterference from a second order Ca K-� peak).Calcium was counted until a precision of at least0.1% was reached (usually < 20 s), and back-ground was counted for 10 s on each side of thepeak. Strontianite (SrCO3) and calcite (CaCO3)were used as calibration standards. The detec-tion limits were 0.03 ± 0.004 weight percent forboth elements.

�PIXE analyses were made at the LundNuclear Microprobe facility, using a standard2.55 MeV proton beam. X-rays were detectedwith a Kevex Si(Li) detector of 5 mm2 active areaand a measured energy resolution of approxi-mately 155 eV at the 5.9 keV Mn K� peak. A thickabsorber (mylar + aluminum) was used duringthe analysis to suppress the Ca X-ray peaks; thispermitted increasing the current to enhance thesignals of Sr and other trace elements. The totalcharge was approximately 1 micro-Coulomb.

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278 Limburg et al.

The normal procedure for a scan was to rasteras much of the otolith as possible in a grid of128 × 128 pixels. Thus, we mapped out largeareas of the otolith rather than being confinedto a line transect, which facilitated interpreta-tion of the data. Following data collection, thedata sets were normalized to counts per charge.

Data Analysis

Strontium:calcium ratios were calculated,graphed, and examined for patterns. Yellow eelotoliths that had been analyzed with �PIXE wereused to calibrate Sr:Ca to salinity. Subsamples(two replicate 6 × 6 groups of pixels) were cho-sen from near the outer growing edge on differ-ent parts of each otolith and mean Sr:Ca ratioswere calculated. A nonlinear regression of Sr:Caon salinity yielded the following relationship:

Sr:CaPIXE = a � (1 – b e–K � Salinity), where

a = 3.467 ± 0.191 (standard error)

b = 0.905 ± 0.0254

K = 0.132 ± 0.0251

N = 29, R = 0.97.

Because of the mylar absorber used to suppressCa peaks in the �PIXE analyses, the Sr:Ca ratiosdo not reflect true mass ratios. Therefore, otolithsfrom six fish were analyzed with both methods,and regression analysis was used to relate elec-tron microprobe Sr:Ca measurements to �PIXE:

Sr:CaWDS = 0.477 + 1.531 � Sr:CaPIXE

(R2 = 0.84, p < 0.05).

Based on otolith Sr:Ca and the associated esti-mated salinity histories, silver eels were classi-fied into eight distinct groups. Five of thesegroups describe wild fish: entirely marine (M);marine moving into brackish water (MB); en-tirely brackish after the glass eel stage (B); “com-plex migration” (CM) when moving back andforth between waters of different salinities, butnever into fresh water; and catadromous (CAT)when the movements following the glass eelstage included residence in fresh water. The lastthree groups are called “stocked,” and showpatterns that are consistent with either havingbeen stocked as glass eels and remained virtu-ally until capture in fresh water (S1), captured

along the Swedish west coast as yellow eels andtransferred into the Baltic (S2), or having beenstocked as glass eels but migrating out to brack-ish or marine waters to feed and grow (S3). Ex-amples of these types are given in Figure 2.

Analysis of variance (ANOVA) and goodness-of-fit tests were conducted on silver eels to testthe hypotheses that differences exist betweenfish caught exiting the Baltic and those caughtin the southern Baltic (Denmark), and that dif-ferences exist between wild and stocked eelswith respect to lipid content. All statistical analy-ses were conducted with Statistica (Statsoft 1999).

Results

Collectively, the silver eels we analyzed dis-played a wide repertoire of habitat use patterns.Many eels appeared to move around among dif-ferent areas over the course of their lifetimes,among different salinity zones, up into fresh wa-ter, or both (Figure 2). Distributions of eel habi-tat use patterns differed between the eels caughtexiting or outside the Baltic, and those collectedin the Baltic around southeastern Denmark (Fig-ure 3; goodness of fit �2 = 32.06, df = 7, p <<0.001). Categories that differed the most in-cluded MB (fish moving from marine to brack-ish water), CM (movement back and forthamong zones of different salinity), catadromous,S1 (entirely freshwater, presumed to be stocked),and S3 (stocked but moved into saline waters),with all being more numerous in the Danishsamples except for CM and S3.

Overall, eels presumed to have been stockedcomposed 26.7% of the silver eels collected forthe study, not counting several fish that wereused in the calibration. Aggregating the threestocked categories, there was no statistical dif-ference between the ratios of wild and stockedeels between the geographic areas (Figure 4; �2

= 0.723, df = 1, p < 0.39). However, among thestocked categories, S1 fish were almost exclu-sively found in the Danish sample, and S3 fishwere three-fold higher exiting the Baltic than inthe Danish collection (Figure 3). Counts of eelsclassed as S2 were the same in both areas. Incontrast, the proportion of silver eels that hadspent time in fresh water, either stocked or wild,was marginally higher in the Danish sample(38% in Danish waters versus 24% outside; �2 =3.20, df = 1, p < 0.07). Eels with freshwater expe-rience composed 31.4% of all silver eels (exclud-ing individuals used in calibration).

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Do Stocked Freshwater Eels Migrate? Evidence from the Baltic Suggests “Yes” 279

Figure 2. Examples of the eight habitat use categories as discerned by strontium:calcium ratios. Imagesare of �PIXE scans of otolith chemistry converted to estimated salinity values. Note that the high values inthe otoliths’ centers are due to high levels of Sr associated with the leptocephalus stage. Key to categories:M = entire life spent in marine waters; MB = moved from marine to brackish waters; B = spent entire postglass-eel life in brackish waters; CM = complex migration, moving back and forth between marine and brackishwaters; CAT = catadromous; S1 = stocked into fresh waters and remaining there until maturation; S2 =stocked from marine waters into the Baltic Sea; S3 = stocked into fresh water and leaving to spend timefeeding and growing in brackish and/or marine waters.

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280 Limburg et al.

Trends were observed in the lipid contentsof silver eels. Lipids tended to be higher amongwild versus presumably stocked eels, andgreater in eels exiting the Baltic than those col-lected in Denmark (Figure 5). Although therewas no statistical difference between wild andstocked eels, eels exiting the Baltic were signifi-cantly fatter than Danish-caught eels (ANOVA,

Figure 3. Distributions of habitat use patterns bygeographic area. Key to categories: M = entire lifespent in marine waters; MB = moved from marineto brackish waters; B = spent entire postglass-eellife in brackish waters; CM = complex migration,moving back and forth between marine and brack-ish waters; CAT = catadromous; S1 = stocked intofresh waters and remaining there until maturation;S2 = stocked from marine waters into the BalticSea; S3 = stocked into fresh water and leaving tospend time feeding and growing in brackish and/ormarine waters.

Figure 4. Wild versus stocked silver eels by geo-graphic area.

Figure 5. Box plots of lipid contents of silver eels,by geographic area and stocking status.

Figure 6. Box plots of lipid contents of silver eelsclassified as stocked.

F1,77 = 4.14, p < 0.05). Differences also existedamong eels presumed to have different stock-ing histories (F2,19 = 3.39, p < 0.055). Of the nineeels that spent their entire lives in fresh water(S1), only one exceeded 20% lipid content,whereas ten out of thirteen S2 and S3 eels hadlipid levels of at least 20% (Figure 6).

Silver eels collected in the Danish Baltic is-lands varied differently from exiting eels withrespect to Pankhurst’s maturation index (Table1). In the Danish collection, catadromous eelshad the highest mean I of 7.86, while two eelsclassified as marine had the lowest (mean I =5.92). Exited catadromous eels, conversely, hadthe lowest I-values (mean 6.26), but were alsothe most fatty (mean per cent lipid = 27.5). I-values less than or equal to 6.5 are generally in-terpreted as sexually immature. I-values were

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significantly higher (mean 8.28 ± 0.24 s.e.) in theexited eels when compared to those collected inthe Danish islands (mean 6.91 ± 0.18 s.e.;ANOVA F1,81 = 21.7, p < 0.0001). Four of the nineS1 stocked eels had I-values greater than 6.5, butall had low fat values (11–15%). There was nocorrelation between I-values and lipid contentsfor Danish silver eels (Pearson r = 0.009), al-though there was a very slight positive trend inthe data. In contrast, there was a significant nega-tive correlation (Pearson r = –0.47, p < 0.001) be-tween these parameters in eels exiting the Baltic.

van Ginneken and van den Thillart (2000)report that the energy cost of swimming for eelsis 0.137 cal per gram wet weight and kilometer.Based on data in their paper, the caloric valueof eel fat is 10.68 kcal g–1. Knowing an eel’s wetweight and fat content, one can therefore com-pute a “migration potential,” assuming that 60%of the lipids are reserved for gonadal develop-ment (van Ginneken and van den Thillart 2000).Results of such calculations indicate that cat-adromous eels in this study had the highest mi-gration potential (mean = 7149 km) and stockedeels that spent their entire lives in fresh water(S1) had the lowest (mean = 4938 km; Table 2).

Discussion

The strontium-calcium “tag” provides a pow-erful means to examine salinity histories, givencaveats. Although otolith Sr:Ca ratios have notbeen related to salinity under controlled experi-mental conditions for European eels, the cali-bration using geographic locations along astable salinity gradient yielded a good fit to an

asymptotic curve, similar in shape to one de-rived for striped bass by Secor and Piccoli (1996)and also similar to results presented for A.japonica (Tzeng 1996). Because of the asymptoticshape of the curve, it is difficult to distinguishbetween strongly brackish and marine salinities,but resolution improves at lower salinities.Other confounding influences on otolith Sr:Caratios include the effects of growth rate (Sadovyand Severin 1992) and temperature (Campana1999). Aside from the slow-growing leptoceph-alus stage, during which time otolith Sr accu-mulates to very high levels (Figure 2), we couldnot distinguish the influence of growth rate per

Table 1. Mean (± s.e.) values of Pankhurst’s index (I) of maturation status and mean muscle tissue lipidcontents of silver eels collected either exiting the Baltic or in Danish Baltic waters. Habitat use categoriesare described in the Methods section.

Exited eels Danish eels

Habitat use Maturation Lipid content Maturation Lipid contentcategory index (I) (%) N index (I) (%) N

M 9.54 (0.05) 14.0 (4.0) 2 5.92 (0.58) 12.92 (4.23) 2MB 8.28 (0.53) 21.1 (1.6) 9 6.60 (0.40) 19.63 (0.78) 14B 8.74 (0.39) 22.4 (2.1) 6 6.71 (0.30) 18.38 (0.77) 4CM 8.01 (0.77) 22.0 (2.8) 8 6.80 (0.49) 15.89 (4.00) 4CAT 6.26 (1.21) 27.5 (1.5) 2 7.86 (0.55) 21.61 (2.59) 7S1 8.56 (—) 16.0 (—) 1 6.67 (0.32) 15.81 (1.73) 8S2 8.31 (0.69) 20.3 (1.4) 3 7.11 (0.92) 20.28 (3.17) 3S3 8.16 (0.16) 21.0 (2.6) 5 6.57 (1.06) 21.68 (0.68) 2

Table 2. Mean (± s.e.) migration potentials (km) forsilver eels collected exiting the Baltic and aroundthe Danish islands in the southern Baltic, by habi-tat use type. Assumes caloric value of 10.68 kcalg–1 eel fat and that 40% of the lipid reserves may beused in migration (i.e., that 60% of reserves is re-quired for spawning: van Ginneken and van denThillart 2000). Habitat use categories are describedin the Methods section.

Habitat use Migrationcategory potential (km) N

M 5126 (1046) 3MB 6292 (234) 22B 6430 (533) 9CM 6100 (745) 10CAT 7149 (1091) 9S1 4938 (476) 9S2 6334 (486) 6S3 6611 (552) 7

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282 Limburg et al.

se in our specimens. On the other hand, Sr:Caratios could be seen to increase during winter,when temperatures drop and growth rate slows.These showed up as bands of higher Sr:Ca andwere only visible outside of freshwater environ-ments. Thus, classification of eels depended onexamination of the entire “gestalt” pattern, andcomparisons to known-origin eels. As with otherstudies (e.g., Tzeng 1996; Tzeng et al. 1997;Tsukamoto et al. 1998), however, the discrimi-nation of freshwater and nonfreshwater resi-dency was not a problem.

Recently it has been suggested (Tsukamotoet al. 1998) that catadromous forms of freshwa-ter eels do not migrate to the spawning grounds,and therefore do not contribute to maintenanceof populations. The conclusion was based upona survey of 18 A. anguilla individuals from theNorth Sea, of which only seven were silver, and12 A. japonica individuals (all silver stage). It iswell known that eels of all ages and develop-mental stages are found in marine waters (Tesch1977), and this was confirmed by our study aswell. However, we found that over 30% of allthe silver eels (total N = 86) in our collectionshad spent at least some time in fresh water, in-cluding 24% of eels exiting the Baltic.

Aside from otolith Sr:Ca patterns, we haveno other independent means of telling whetheran eel was stocked or wild; therefore we can onlyclassify our eels as having habitat use patternsthat are consistent with having been stocked. Foreels displaying the S2 pattern, an alternativeexplanation is that the eels swam very quicklyfrom marine to brackish water. An alternativefor the S3 pattern could be that glass eels en-tered a river somewhere outside of the Baltic,spent several years in that system, and then mi-grated into the Baltic or marine waters. How-ever, there is no good alternative explanationfor the S1 pattern, because historically eels havereached the Baltic as bootlace eels (small andpigmented) rather than as glass eels (Svärdson1976). Due to time and labor constraints, eel ageswere not determined, but in other studies ofBaltic eels, age did not correlate with maturation(Svedäng et al. 1996; Svedäng and Wickström1997) or growth (Holmgren 1996).

Although all silver eels classified as stockeddid not vary in their proportions whether in-side the Baltic or out, most of the S1 eels werecaptured in the Danish Baltic islands. Whereashalf of these fish are classified as mature byPankhurst’s index of maturity, all but one of

them had lipid contents less than 20% of bodyweight (one of them had a lipid content of 27%,however). This means, if the assumption that40% of muscle lipid is available for migration isvalid (van Ginneken and van den Thillart 2000),that the S1 eels have shorter migration poten-tials and on the whole would not have the en-ergy reserves to make the greater than 6000 kmjourney to the spawning grounds in the SargassoSea. Interestingly, whereas no relationship ex-isted between Pankhurst’s index and lipid con-tent in the Danish eels, lipid content declinedwith increasing eye size in the exiting eels. Thismay be consistent with eels burning off energyreserves as they migrate into deeper waters, thislast necessitating enlargement of their eyes.

One of the ongoing debates about stockedeels in the Baltic is whether or not they can ori-ent their way out to spawn (Westin 1990; 1998;Tesch et al. 1991). In successive tagging studies,Westin found that stocked eels took months toyears longer to reach the southern Baltic thaneels presumed to be wild in origin, and alsofound that a high proportion of stocked indi-viduals did not orient properly to locate the clos-est exit from the Baltic at Öresund. Rather, manystocked eels were recaptured in the southernBaltic along the coasts of Poland, Germany, andDenmark (Westin 1998). It was also shown thatwild eels rendered anosmic by plugging up ei-ther both nostrils or only the left nostril hadpoorer navigation abilities. From this Westin(1998) suggested that stocked eels, lacking ol-factory imprints for their way across the Balticto freshwater sites, cannot use smell as a cue andinstead must use the less reliable cue of declin-ing water temperature.

Although we cannot tell how long the eels inthis study had migrated in the silver stage, thefact that most of the S1 eels were collected at theDanish site appears to corroborate Westin (1990,1998). However, the eels with highest fat con-tents (catadromous eels), and therefore highestmigration potentials, were also mostly found atthis site. Further, as eels are stocked in southernBaltic countries such as Poland (Moriarty et al.1990; Bartel and Kosior 1991), it is also possiblethat the eels we identified as S1 did not origi-nate in Sweden and were undertaking a coastalmigration from Poland, Germany, or Denmark.

The results of our study are at odds withthose reported by Tsukamoto et al. (1998) in thatwe did find eels with freshwater histories, andmany of these showed evidence of maturity and

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of being able to undertake the long migrationto the spawning grounds. Nevertheless, it isstriking that nearly 69% of the silver eels showedno evidence of freshwater residency. One pos-sible reason is suggested by Tsukamoto et al.(1998), i.e., that catadromy is but one of manyhabitat use strategies employed by eels, whichhave high plasticity in many of their life historycharacters (e.g., Panfili and Ximenes 1994;Holmgren 1996; Glova et al. 1998; Secor andRooker 2000). Another possibility is that damsand other artificial obstructions within the Bal-tic Sea drainage may be preventing young eelsfrom ascending many rivers. Such barriers havebeen implicated in reducing the numbers of eelsmigrating upstream in British rivers (White andKnights 1997). Although glass eels and elverscan climb vertical surfaces such as dam walls(Tesch 1977), bootlace eels, such as those enter-ing the Baltic, cannot. The hypothesis that damsserve as barriers to eel recruitment might betested by assessing the frequencies of catadro-mous silver eels in drainages differing with re-spect to barriers, distance from the spawninggrounds (affecting the life stage at which eelsarrive at rivers), or both.

Conclusions

A most striking result of this study was the con-firmation of the remarkable plasticity of habitatuse patterns among European eels. Eels thatmigrate in the silver stage show a life history(based on otolith microchemistry) of exploitingall forms of aquatic habitat ranging from ma-rine to fresh waters. Our results indicate thatBaltic drainage eels that spend all their lives infresh water are less likely to be able to make thespawning migration, unless they remain in thesystem for a number of years to feed, as sug-gested by Svedäng and Wickström (1997). How-ever, contrary to Tsukamoto et al. (1998), ourstudy strongly suggests that catadromous eelsindeed begin the spawning migration, and showstrong potential for reproductive success.

Acknowledgments

We thank Hans Harryson, Department of Geo-sciences, Uppsala University, for assistance withthe WDS microprobe analyses, and KlasMalmqvist, Department of Nuclear Physics,Lund University, for support. We are gratefulto our colleague Lars Westin for his discussions

and ideas; Lars also provided the eels from Den-mark and Gotland. Leif Johansson prepared theotoliths, and Liselott Wilhelmsson conductedthe lipid analyses. We also thank C. Moriarity,R. Kraus, and one anonymous reviewer forthoughtful and constructive comments. Finan-cial support for this study came from the Swed-ish Council for Forestry and AgriculturalResearch.

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