Håkan Wickström

Panmixia in the European eel: a matter of time...

The European eel (Anguilla anguilla L.) has been a prime example of the panmixia paradigm because of its extraordinary adaptation to the North Atlantic gyral system, semelparous spawning in the Sargasso Sea and long trans-oceanic migration. Recently, this view was challenged by the suggestion of a genetic structure characterized by an isolation-by-distance (IBD) pattern. This is only likely if spawning subpopulations are spatially and/or temporally separated, followed by non-random larval dispersal. A limitation of previous genetic work on eels is the lack of replication over time to test for temporal stability of genetic structure. Here, we hypothesize that temporal genetic variation plays a significant role in explaining the spatial structure reported earlier for this species. We tested this by increasing the texture of geographical sampling and by including temporal replicates. Overall genetic differentiation among samples was low, highly significant and comparable with earlier studies (FST=0.0014; p<0.01). On the other hand, and in sharp contrast with current understandings, hierarchical analyses revealed no significant inter-location genetic heterogeneity and hence no IBD. Instead, genetic variation among temporal samples within sites clearly exceeded the geographical component. Our results provide support for the panmixia hypothesis and emphasize the importance of temporal replication when assessing population structure of marine fish species.

Too short to spawn? Implications of small body size and swimming distance on successful migration and maturation of the European eel Anguilla anguilla

Individual net fat reserves after migration and reproductive investments were calculated for migrating female silver eels Anguilla anguilla (n = 387) collected in the outlet region of the Baltic Sea during the autumn run. It is estimated that 20·4% of the A. anguilla had completely exhausted all initial fat reserves and that 45·0% of A. anguilla were within 90% of complete energy depletion after migration and reproduction. This study concludes that a combination of body size and distance (6900 km) to the spawning area in the Sargasso Sea explains the results. An increase in the costs of migration due to heavy infection with Anguillicoloides crassus was also evaluated in an additional scenario with results showing that 26·4% of the A. anguilla had completely depleted all fat reserves. It is hypothesized that a large proportion of female silver A. anguilla from the Baltic Sea catchment area will have inadequate or suboptimal reserves for successful migration and reproduction.

Effects of the swimbladder parasite Anguillicola crassus on the migration of European silver eels Anguilla anguilla in the Baltic Sea

In a mark-recapture study in 2006, migrating European Anguilla anguilla silver eels were caught, tagged and released in the Baltic Sea and recaptures in commercial pound nets examined for possible effects on migration of infection with the swimbladder parasite Anguillicola crassus. The overall recapture rate was 36%. The prevalence of infection was lowest at the northernmost sampling site. There were no significant differences between infected and uninfected A. anguilla in condition indices, body fat content and estimated migration speeds. Parasite infection intensity levels were significantly negatively correlated with times and distances covered between release and recapture, but did not correlate with migration speed. It appears that more heavily infected A. anguilla were relatively more vulnerable to recapture in pound nets. It is hypothesized that parasite-induced damage to the swimbladder inhibited vertical migrations and infected A. anguilla tended to migrate in shallower coastal waters, relatively close to the shore.

Elemental composition of otoliths as a discriminator of life stage and growth habitat of the European eel, Anguilla anguilla

The hypothesis that elemental composition of otoliths of the eel (Anguilla spp.) changes with life stage and growth habitat was tested in the present study. The minor elements Cl, Na, K, Mg, Ca, Sr and P in otoliths of European eels (Anguilla anguilla) were examined by using an Electron Probe Microanalyser (EPMA) equipped with wavelength dispersive spectrometers (Cameca SX-50). Yellow-stage eels were collected from coastal waters and lakes of Sweden in 1987, 1988, 1991, and 1994, with ages ranging from 5 to 18 years old. Strontium maps and profiles of Sr : Ca ratio, as well as the elver check in otoliths, were used to classify life history stages of the eels as leptocephalus, and freshwater- and seawater-resident yellow eels. Canonical score plots of the otolith elemental compositions of the freshwater-resident yellow eel were completely separated from those of leptocephalus and seawater-resident yellow eel, but the latter two partially overlapped. Strontium is the primary component in determining the discrimination, but the nutrient-related (S and P), and the physiologically controlled elements (Na and Cl), may also play an important role in the discrimination. These results indicate that multiple-elemental information can provide additional insight into the migratory environmental history of diadromous fishes.

A decade’s worth of otolith PIXE analyses

Over the past 10 years, several thousand otoliths have been analyzed with PIXE (using 2.55 MeV protons) at LIBAF (Lund Ionbeam Analysis Facility, formerly LNMP Lund Nuclear Micro Probe). Over 40 elements have been identified in otoliths, many at levels suitable for PIXE analysis. Readily detectable elements in otoliths starting with Ca are: Ca (the matrix), Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Sr, Y, Zr, Mo, Cd, Sn (difficult), I, Ba (sometimes difficult), Pb (difficult). The detector system, used over this time period, is more sensitive than many other X-ray detector systems, since it consists of eight HPGE detector elements (100 mm2 each), in an annular formation around the beam entrance. Using a thick absorber allows us to use quite high beam current, typically 12 nA, but sometimes up to 20 nA. This permits us to have low detection limits within short analysis times. Additionally, light stable isotope research is widespread in the sciences including ecology. Stable isotopes of N provide information about trophic level (“who eats who”), providing the opportunity to map out the switching of diets from one food type to another. Oxygen isotopes are useful as “environmental thermometers”. Currently, most of such analyses require destruction of the otolith, and nitrogen isotope analysis may require dissolving entire otoliths, thus losing all temporal information. We present new techniques using new types of detectors, double side silicon strip detector (DSSSD). The detectors, electronics and the laboratory setup are described in detail; for our analysis, a MeV proton and a deuterium microbeam at LIBAF is used. The analysis is performed immediately after the PIXE analysis, without moving the sample.