Håkan Westerberg

Swedish University of Agricultural Sciences | SLU · Department of Aquatic Resources

The European eel (Anguilla anguilla) is critically endangered (according to the most recent IUCN assessment) and has suffered a 95% decline in recruitment since the 1980s, attributed in part to factors occurring during the marine phases of its life-cycle. As an adult, the European eel undertakes the longest spawning migration of all anguillid eels,...

Coexistence between seals and fisheries is desirable and possible. Some seal-safe gear and scaring methods work, but much remains to be done. Gear development requires strong and persistent public support. Awaiting technical mitigation methods economic compensation is possible. The efficiency of protective hunting is doubtful. Opinion differs on wh...

The Sargasso Sea was identified as the spawning area of the European eel (Anguilla anguilla) 100 years ago, and numerous subsequent surveys have verified that eel larvae just a week old are regularly recorded there. However, no adult eels or eel eggs have ever been found, leaving room for alternative hypotheses on the reproduction biology of this e...

Recent developments in tracking technology resulted in the mapping of various marine spawning migration routes of the European eel ( Anguilla anguilla ). However, migration routes in the North Sea have rarely been studied, despite many large European rivers and hence potential eel growing habitat discharge into the North Sea. In this study, we pres...

Anguillid eel populations have declined dramatically over the last 50 years in many regions of the world, and numerous species are now under threat. A critical life-history phase is migration from freshwater to distant oceans, culminating in a single life-time spawning event. For many anguillids, especially those in the southern hemisphere, mystery...

Nineteen female silver European eels (Anguilla anguilla L.) were tagged with satellite tags and released in the Gulf of Lion in the Mediterranean during the migration seasons 2013 and 2015. Sixteen tags transmitted data: five in the Atlantic Ocean, and eleven in the Mediterranean. Of those, 50% of migrating eels were consumed by marine mammals in e...

Anguillid eel populations have declined dramatically over the last 50 years in many regions of the world, and numerous species are now under threat. A critical life-history phase is migration from freshwater to distant oceans, culminating in a single life-time spawning event. For many anguillids, especially those in the southern hemisphere, mystery...

Anguillid eels are found globally in fresh, transitional and saline waters and have played an important role in human life for centuries. The population status of several species is now of significant concern. The threats to populations include direct exploitation at different life stages, blockages to migratory routes by dams and other structures,...

The avoidance behaviour of cod and herring to dredging-induced turbidity and the effects of sediment plumes on the buoyancy and mortality of cod egg and larvae have been studied as part of the EIA of the Öresund Link project. The avoidance threshold to suspended sediments of glacial clay or limestone origin was studied in a 15*6 m saltwater flume a...

Sunfishes or Molidae are a rarely encountered family within the teleost order Tetraodontiformes and most details of their reproductive biology including times and places of spawning and their larval ecology are rather unclear. Spawning of two species of Molidae was suggested in the Sargasso Sea before, yet comprehensive data on larval distribution...

Leptocephali of the shallow-water congrid eel Ariosoma balearicum are abundant during February–April in the Sargasso Sea, and larval and adult meristic data indicates this species includes several regional subpopulations/cryptic species. Four multiple-transect larval surveys (2011, 2014, 2015, 2017) were used to examine the geographic size distribu...

It has been known for about a century that European eels have a unique life history that includes offshore spawning in the Sargasso Sea about 5000–7000 km away from their juvenile and adult habitats in Europe and northern Africa. Recently hatched eel larvae were historically collected during Danish, German and American surveys in specific areas in...

The head morphology and body shapes of anguilliform larvae (leptocephali) vary widely by having fewer, much longer teeth in the small larvae and more numerous, but relatively smaller teeth in larger developmental stages. The feeding ecology of leptocephali has been difficult to understand because they do not appear to feed on typical zooplankton li...

The abundance and distribution of leptocephalus larvae of the European eel (Anguilla anguilla, Anguillidae) were examined using ten historic and recent Sargasso Sea expeditions that were selected on the basis of having the largest number of sampling stations and highest catches. The surveys cover the period 1920–2014. Station data were recalculated...

The distribution of the leptocephalus larvae of the European and American eel collected during recent Sargasso Sea surveys, was used to model the larval drift. The drift trajectories of individual larvae were back-calculated to the estimated time of spawning, using current data from two global oceanographic assimilation models. The results of both...

Infection with the swim bladder nematode Anguillicola crassus has been hypothesised to threaten the spawning migration success of the endangered European eel (Anguilla anguilla). To examine this assumption, we compared the swimming behaviour of one Anguillicola crassus infested eel in the North Sea and three parasite-free eels in the Baltic using d...

European eels (Anguilla anguilla) migrate between the southwestern Sargasso Sea and the European and Mediterranean coasts. In a recent paper in Current Biology, Naisbett-Jones et al. [1] claim to “provide the first evidence that they [eels] derive positional information from the Earth's magnetic field” and that this information guides their migrati...

The spawning migration of the European eel (Anguilla anguilla L.) to the Sargasso Sea is one of the greatest animal migrations. However, the duration and route of the migration remain uncertain. Using fishery data from 20 rivers across Europe, we show that most eels begin their oceanic migration between August and December. We used electronic taggi...

Scientific Reports 6 : Article number: 21817; 10.1038/srep21817 published online: 24 February 2016 ; updated: 20 May 2016 . In Figure 2 of this Article, the upper left graph is a duplication of the upper right graph.

The migration route and the spawning site of the European eel Anguilla anguilla are still uncertain. It has been suggested that the Mediterranean eel stock does not contribute to spawning because there is no evidence of eels leaving the Mediterranean Sea. To test this hypothesis, we equipped eight female silver eels from the south of France with po...

The data from two scientific surveys, where eel is part of the catch, have been studied to assess trends in recruitment and escapement to and from the Baltic. Abundance of juvenile eel was monitored in the Sound, between Sweden and Denmark, from 1992 to 2002. The total natural recruitment of new eels was estimated under the assumption that the numb...

Despite some recent progress, there are still large gaps of knowledge about migration routes and behaviour of European eels, Anguilla anguilla, during their long-distance oceanic migration. To achieve a better understanding of the migration behaviour, 28 large female silver eels were equipped with pop-up satellite transmitters and released at three...

Two scientific surveys where eel is a by-catch - the Baltic International Trawl Survey and herring larvae monitoring in Kattegat – allows estimates of time series of silver eel abundance in the Baltic from 1992 and glass eel abundance in the Sound from 1981. With the assumption that the integrated number of juvenile eels and silver eels represent a...

Overwintering dormancy behaviour was studied in female silver eels in Lake Mälaren in Sweden between 2008 and 2010. Depth choices and movements in relation to temperature were analysed from pressure and temperature records for 13 eels with implanted data storage tags, covering 17 overwintering periods and three intervening summer periods. Dormancy...

One objection to the stocking of translocated eels as a management measure for the European eel Anguilla anguilla L. is that these eels may lack the ability to find their way back to the spawning area in the Sargasso Sea because the translocation will confuse their imprinted navigation. We undertook a series of tagging experiments using satellite t...

One hypothesis to explain the large decline in the recruitment of the European eel (Anguilla anguilla) from the 1980s onwards is that a change in ocean circulation has influenced the drift of eel larvae, resulting in lower recruitment. To test this hypothesis, a simple Lagrangian model was constructed to simulate a passive drift from the spawning a...

Background Four methods for attaching pop-up satellite transmitters to European eel were tested in the laboratory by recording long-term tag retention, growth and survival; short-term behavioral responses; and physical damage from attachments. Results All eels survived until they lost their tag, or until end of the six-month study. Specific growth...

This chapter presents the results of case studies on human-wildlife conflicts conducted in Sweden and Finland. In both cases the conflict is between the conservation of the grey seal (Halichoerus grypus) and small-scale coastal fishing. The characteristics of the conflict between grey seal protection and fishery can be shown by way of a systematic...

Data on the diet of an aquatic top predator are essential to assess the impact of predation on the fisheries resources. Quantitative data are sparse however. Field data is usually collected by analysis of undigested parts in the food remains of scats, pellets, or in stomach and intestine of dead animals. Advanced techniques as fatty acid or stable-...

Effects of implanting data-storage tags in European eel, Anguilla anguilla, and the suitability of different suture materials (braided permanent silk, permanent monofilament, absorbable and absorbable antibacterial) were examined. The tags consisted of an electronic unit and three floats on a wire, making them flexible and able to follow the swimmi...

A list of 138 positions with records of Lophelia pertusa is compiled from all published and unpublished investigations in the area including material from the BIOFAR research programme. In addition, Solenosmilia variabilis, another species of branching coral new to the area is reported. The Lophelia records are from areas that are dominated by the...

The formation of an odour trail can be broken down into a chain of events: first, the emission of the attractant from the bait, second, a turbulent mixing into the immediate surroundings and third, a downstream advection and dispersion of the attractant by the current flowing past the bait. In this work, a simple analysis of the physical processes...

The behavior of sham-operated and anosmic Atlantic salmon, Salmo salar, was studied in a fjord system with close reference to the fine-scale hydrographic features. Control fish made small-amplitude vertical movements, with sudden large-amplitude excursions. The anosmic fish made large continuous searches up and down in the water column, descended b...

Abstract Coded acoustic tags and an array with moored receivers were used to study the effect of a sub-sea AC power cable on migrating European eel, Anguilla anguilla (L.) in the Baltic Sea. Sixty eels were tagged and the migration speed was measured in a strait with a 130 kV AC power cable. Observed swimming speed over the ground was corrected for...

With the large scale developments of offshore windpower the number of underwater electric cables is increasing with various technologies applied. A wind farm is associated with different types of cables used for intraturbine, array-to-transformer, and transformer-to-shore transmissions. As the electric currents in submarine cables induce electromag...

Westerberg, H., Lagenfelt, I., and Svedäng, H. 2007. Silver eel migration behaviour in the Baltic. – ICES Journal of Marine Science, 64: 1457–1462 Female silver eels (Anguilla anguilla L.) were tagged with data storage tags and released in the Baltic Sea at the same time at a single site on the east coast of Sweden. Data on temperature, light, and...

The Baltic seal populations have been threatened by extinction due to a combination of hunting and the effects of environmental pollution by organohalogens. By the 1970s, the number of gray and ringed seals had declined to very low numbers, and a complete ban on hunting was introduced. Since then the gray seal population has recovered rapidly and t...

With the large scale developments of offshore windpower the number of underwater electric cables is increasing with various technologies applied. A wind farm is associated with different types of cables used for intraturbine, array-to-transformer, and transformer-to-shore transmissions. As the electric currents in submarine cables induce electromag...

Acoustic harassment devices (AHDs) were deployed at salmon-trap nets in the Baltic Sea to reduce gear and catch damage by grey seals (Halichoerus grypus). The AHDs emitted pulses of 250–500-ms duration, worked at a frequency of 15 kHz, and a source level of 179 dB re 1 μPa rms at 1 m. AHDs were deployed during three consecutive fishing seasons. Cat...

The Swedish eel (Anguilla anguilla The Swedish eel (Anguilla anguilla The Swedish eel () fishery has suffered an increase both in damage to fishing gear and in catch losses caused by harbour seals (Phoca vitulina). Eel fyke nets, which are the principal type of fishing gear used, tend to incidentally catch species like cod (Gadus morhua), flounder...

The current knowledge on detection of, and reaction to, sound by fish is reviewed, with special emphasis on underwater noise from offshore wind farms. The detection distance to wind farms for 3 species of fish representing various hearing capabilities varies between 0.4 and 25 km at wind speeds of 8 to 13 m s -1. The detection distance depends on t...

A new design for a salmon trap aimed at minimizing damage to catch and gear caused by grey seals was tested. The traditional trap design used in the northern Baltic permits an efficient hunting strategy by seals, whereby chased fish entangle themselves in the side panels and can then easily be taken, with associated damage to the net. The side pane...

In the herring (Clupea harengus), the swim bladder is connected to both the alimentary canal and the anal opening. The anterior duct is used for filling the swim bladder with air. Gas release from the anal opening is often observed when the fish is scared or during ascent and descent. Here, the sounds produced by such a gas release are studied. The...

The behaviour of ultrasonically tagged whitefish Coregonus lavaretus was studied in a 15000m2 net enclosure. Avoidance manoeuvres of fish were recorded during short-time introductions of different obstructions at randomly chosen places in the enclosure. In all 28 individual fish were tested in 92 trials with different leader structures. There was a...

The processes by which eels arrive to the Baltic Sea are discussed in the light of an almost total lack of O+ eels in the Kattegatt and the Sound during 1996. The comparatively high elver abundance at the Skagerrak coast the same year indicates that this was due to a failure of the glass eels to penetrate into the Kattegatt, which can be attributed...

C.M. / International Council for the Exploration of the Sea ; 1994/Q:4, Ref. C Pages: 12 p. The spawning of cod in the deep parts of Kattegat and the sound takes place in late winter and spring. The buoyancy of cod eggs is intermediate between that of the Baltic surface water and the Kattegat deep water, which means that there is an accumulation of...

A telemetric system for automatic positioning of individual fish in the vertical or horizontal plane is described. The position of a transmitter in a fish is computed from the difference in arrival time of the ultrasonic signal to a fixed array of hydrophones. Sources of error in positioning are discussed. In a preliminary study, the resolution of...

Summary Mineralogical and magnetic properties of the otolithic mass in the sacculus of dogfish, Squalus acanthias, were investigated. The endogenous statoconia were found to be either rhombohedral crystals of calcite or spherical particles of other calcium carbonate polymorphs. From 20 to 60% of the total otolithic mass consisted of exogenous mater...

Meeresforschung / Reports on Marine Research, ISSN 0341-6836, vol. 33, nr. 2-3, 183-196 Some 16 silver eels (Anguilla anguilla ) tagged with pressure sensing ultrasonic transmitters were released for tracking studies in the Central Baltic Sea. Three eels were made anosmic. The initial swimming direction of the intact eels was close to SSW with litt...

Fishes representing the main groups of teleosts have been investigated for magnetic material by susceptibility measurements. All the investigated species contain magnetic material. Bone samples from the skull and the vertebral column contain magnetic particles which yield a saturation magnetization in the range 10(-4) emu g-1 to 10(-3) emu g-1 in a...

The magnetic susceptibility of bone samples of eel was measured. They contain magnetic precipitates with an even distribution in the skull and backbone, similar for yellow and silver eels.

1.1. Samples of the European eel (Anguilla anguilla L.) were investigated for magnetic material by measurements of magnetic susceptibility, χ.2.2. Bone samples from the skull, vertebral column and pectoral girdle contain magnetic precipitates with a saturation magnetization of the order of 10−3 emu g−1.3.3. In samples from the premaxilloethmovomeri...

Some characteristic properties of vertical fine structure and microstructure in natural waters are surveyed. Ubiquitous thermal microstructure, by its anisotrophy due to shear in the fine-structure gradient layers, is a property that could be used by fishes to orient relative to the local current shear. The geostrophic shear causes a bias in the pr...

Tracking times ranged up to 40 hr (mean 16h); tracking distance was up to 60 km (mean 19 km). About 60% of trackings which lasted for >5 hr showed erratic movements, statistically not significantly different from random walk. For this group, net progress was slow (typically 10 km/day) and the resulting direction could be both towards and away from...

The swimming depth of 8 adult salmon was monitored continuously during tracking in the coastal zone at the time of spawning migration. A behaviour common to all individuals were occasional rapid excursions down to the main thermocline, or, if the initial swimming depth was in the stratification, upwards to the mixed layer. Salmon tended to follow f...

Background Many attempts have been made to use sound to reduce the conflict between mammals and human activities in the marine environment. AHD-s (Acoustic Harassment Devices) produce a sound intense enough to cause discomfort and avoidance and are used extensively in aquaculture. Some trials have also been made in fisheries operations (Mate and Ha...

The instantaneous point-source, sub-surface dye diffusion experiments carried out during the IFYGL are used to study the small-scale vertical mixing in relation to the vertical temperature, and current structure. The experiments covered time scales from 5 to 80 hours as well as a wide range of environmental conditions. The dye tracing was done by t...

Following is a continuation of the list of titles and authors of the papers presented: Assessment of a Waste Outfall. By B. S. Kohli and M. D. Palmer. Experimental Study of Diffusion Characteristics in the Thermocline and Hypolimnion Regions of Lake Ontario. By G. Kullenberg, C. R. Murthy and H. Westerberg. Water Transparency in Southern Lake Michi...

Laxforskningsinstitutet, information ; 4 Pages: 16 p.

Information från Sötvattenslaboratoriet ; 1979:7 Pages: 12 p.

Miljökonsekvensbeskrivning för öresundsförbindelsen / Underlagsrapport ; 46 = Environmental impact assessment for the fixed link across the Öresund / Sub-report ; 46 Pages: 16 leaves

Fiskeriverket rapport, ISSN 1404-4110 ; 2000:3 Pages: 38 pp. The seal damages on catch and gears has increased dramatically during the 1990ies. Static gears - traps, gillnets and fykes - are the most exposed. In the Baltic the grey-seal causes the majority of the damages and ringed seals probably but a minor part of the damages to gillnets in the B...