ICES Journal of Marine Science: Journal du Conseil

Castonguay, Martin; Durif, Caroline M. F.

doi: 10.1093/icesjms/fsv256pmid: N/A

AbstractThis symposium issue of the ICES Journal of Marine Science contains 16 contributions from the second International Eels Symposium held during the American Fisheries Society (AFS) Annual Meeting (August 2014, Québec City, Canada). This symposium followed the first International Eels Symposium held in 2003, which emphasized the international scope of the Anguilla conservation problem. This second symposium reviewed a decade of research on biology and life history, genetics and genomics, reproduction, oceanic biology, early life history, population dynamics, assessment and management, eel passage at hydroelectric facilities, stocking, and threats, focusing primarily on Atlantic anguillids. The symposium finished with a panel discussion that emphasized the need to develop inter-jurisdictional governance approaches for panmictic eels as a way to promote recovery of the various species.

Dekker, Willem; Beaulaton, Laurent

doi: 10.1093/icesjms/fsv132pmid: N/A

AbstractFew fish stocks are as influenced by (intentional and inadvertent) human impacts as the European eel, all across the continent. The dynamics of this stock, however, are poorly understood—neither the causes of the historically low abundance, nor minimal protection levels, are beyond discussion. Rather than analysing contemporary processes, this article turns back in time—two centuries or more—unravelling historical abundances and distribution patterns; reviewing historical actions and objectives; and discussing technical developments and scientific advice—picturing the slippery slope the eel stock has come down from. The first claim, that the continental stock was in decline, dates from the early 1800s; stock-enhancement actions were initiated shortly after. Diffuse objectives, technical innovations, eternal optimism, and—above all—no quantification impede the exact evaluation of historical reports. After 1950, when quantification improved, a slow but consistent decline was observed, but it is only two decades after the crash in glass eel recruitment (in 1980), that protection plans addressed the bad status of the stock. A slippery slope, full of pitfalls—yet, we now observe several years of increasing recruitment.

Hein, Jennifer L.; de Buron, Isaure; Roumillat, William A.; Post, William C.; Hazel, Allan P.; Arnott, Stephen A.

doi: 10.1093/icesjms/fsv097pmid: N/A

AbstractLittle is known about the infection status of glass eel and elver stages of the American eel Anguilla rostrata by the invasive swimbladder parasite Anguillicoloides crassus. This study examined infection by adult and larval A. crassus in glass eels (n = 274) and elvers (n = 199) collected during March–December 2013 from an eel ladder at a dammed creek near Charleston, SC, USA. Among all the eels examined [total lengths (TLs), 34–156 mm], the prevalence (±SE), mean abundance, and mean intensity of A. crassus worms was 29.4 ± 2.1%, 0.88 ± 0.12, and 2.98 ± 0.34, respectively. Infection by A. crassus was not detected in the earliest glass eel development stages (pigment stages 1–3), but it was detected in more advanced stages (pigment stages 4–7) and fully pigmented elvers. From March to July, parasite prevalence increased significantly with eel TL, and all eels 125 mm or longer (n = 13) were infected. From August–December, when fewer eels were caught, parasite prevalence was generally lower and less dependent on the eel TL. Our study demonstrates the potential risk of spreading A. crassus to new areas by transporting live glass eels and elvers. This is of particular relevance because our study site was located in the Cooper River drainage, one of the few locations in the USA that permits a glass eel harvest.

Tremblay, V.; Cossette, C.; Dutil, J-D.; Verreault, G.; Dumont, P.

doi: 10.1093/icesjms/fsv106pmid: N/A

AbstractThe American eel (Anguilla rostrata) population has experienced a marked population decline. Habitat loss resulting from dam construction to improve the control and use of freshwater discharge is one of the factors involved. There are some 5600 dams in rivers draining to the St. Lawrence River in Quebec (Canada). Their passability to eels migrating upstream and downstream has been assessed using the Québec Dam Database. Eighteen percent of the dams are used for supplying water and 13% for hydroelectricity, but >50% are used for recreational purposes. Although the majority of the dams are <3 m in height and are made of concrete or earthfill, dams present a great variety of physical characteristics. Passability ranks were assigned to each category of dam based on three assessment criteria: the height of the dam, the materials used in its construction, and its use. Passability to upstream migrants was also assessed from photographs for a subset of dams. The two methods (statistical analysis and the use of photographs) may yield different results, but the two methods were consistent to identify the impassable dams. This analysis shows overall that the problem of passability is more significant for upstream passage than it is for downstream passage.

Welsh, Stuart A.; Aldinger, Joni L.; Braham, Melissa A.; Zimmerman, Jennifer L.

doi: 10.1093/icesjms/fsv052pmid: N/A

AbstractMonitoring of dam passage can be useful for management and conservation assessments of American eel, particularly if passage counts can be examined over multiple years. During a 7-year study (2007–2013) of upstream migration of American eels within the lower Shenandoah River (Potomac River drainage), we counted and measured American eels at the Millville Dam eel pass, where annual study periods were determined by the timing of the eel pass installation during spring or summer and removal during fall. Daily American eel counts were analysed with negative binomial regression models, with and without a year (YR) effect, and with the following time-varying environmental covariates: river discharge of the Shenandoah River at Millville (RDM) and of the Potomac River at Point of Rocks, lunar illumination (LI), water temperature, and cloud cover. A total of 17 161 yellow-phase American eels used the pass during the seven annual periods, and length measurements were obtained from 9213 individuals (mean = 294 mm TL, s.e. = 0.49, range 183–594 mm). Data on passage counts of American eels supported an additive-effects model (YR + LI + RDM) where parameter estimates were positive for river discharge (β = 7.3, s.e. = 0.01) and negative for LI (β = −1.9, s.e. = 0.34). Interestingly, RDM and LI acted synergistically and singularly as correlates of upstream migration of American eels, but the highest daily counts and multiple-day passage events were associated with increased RDM. Annual installation of the eel pass during late spring or summer prevented an early spring assessment, a period with higher RDM relative to those values obtained during sampling periods. Because increases in river discharge are climatically controlled events, upstream migration events of American eels within the Potomac River drainage are likely linked to the influence of climate variability on flow regime.

Miller, Michael J.; Feunteun, Eric; Tsukamoto, Katsumi

doi: 10.1093/icesjms/fsv063pmid: N/A

AbstractThe three northern hemisphere anguillid eel species experienced recruitment declines at similar times beginning in the 1970s and 1980s, but the exact causes of the declines have remained unclear. Attention focused on two categories of possible causes that included (i) anthropogenic impacts on eel growth habitats, such as dam construction, degradation and pollution of habitats, introduction of parasites, overfishing and (ii) changes in ocean-atmospheric conditions affecting their marine life-history stages. The cumulative effects of reaching peaks in dam construction, levels of pollution, and eutrophication just before the eel declines likely had reduced eel production in many areas, and contamination by metallic and organic compounds and parasites may have reduced reproductive success. Shifts in ocean-atmospheric conditions also occurred just before the eel declines that could have reduced feeding success of larvae or disrupted larval transport. If oceanic regime shifts reduced production of the marine snow food of eel larvae, it may have affected larval survival and recruitment success, especially if there is a critical time-window for successful larval first feeding when marine snow particles need to be abundant. A reduction of these particles could result in density-dependent early mortality of the larvae of each spawning event, and competition for marine snow particles with sympatrically spawning mesopelagic eel larvae would amplify this effect. Nutrient reductions causing shifts in the relative abundance of phytoplankton contributing to marine snow production and of ubiquitous cyanobacteria may mediate levels of larval survival in areas with high spawning activity. Reductions of eels reaching the spawning area from species range margins that spawn outside of peak spawning periods could have reduced recruitment further. It appears likely that a variety of impacts, oceanic and anthropogenic occurred simultaneously causing sudden declines of these eel populations.

Béguer-Pon, Mélanie; Shan, Shiliang; Thompson, Keith R.; Castonguay, Martin; Sheng, Jinyu; Dodson, Julian J.

doi: 10.1093/icesjms/fsv169pmid: N/A

AbstractBoth the American eel (Anguilla rostrata) and European eel (Anguilla anguilla) undertake long-distance migrations from continental waters to their spawning sites in the Sargasso Sea. Their migration routes and orientation mechanisms remain a mystery. A biophysical particle tracking model was used in this study to simulate their oceanic migration from two release areas: off the Scotian Shelf (Canada) and off the Irish continental shelf. Two plausible swimming-directed behaviours were considered for simulating two different migratory paths: true navigation to specific spawning sites and innate compass orientation towards the vast spawning area. Several combinations of swimming speeds and depths were tested to assess the effect of ocean circulation on resulting migratory pathways of virtual eels (v-eels), environmental conditions experienced along their oceanic migration, and energy consumption. Simulations show that the spawning area can be reached in time by constantly swimming and following a readjusted heading (true navigation) or a constant heading (compass orientation) even at the lowest swimming speed tested (0.2 m s−1) for most v-eels. True navigation might not be necessary to reach the spawning area. The ocean currents affect mainly the migration of American v-eels, particularly for swimming speeds lower than 0.8 m s−1. The ocean circulation increases the variability in the oceanic migration and generally reduces the efficiency of the v-eels, although positive effects can be possible for certain individuals. The depth range of diel vertical migration (DVM) significantly affects the total energy expenditure due to the water temperature experienced at the various depths. Model results also suggest that energy would not be a limiting factor as v-eels constantly swimming at 0.8 BL s−1 spent <25 and 42% of energy available for migration for American and European v-eels, respectively.

Westerberg, Håkan; Wickström, Håkan

doi: 10.1093/icesjms/fsv049pmid: N/A

AbstractThe 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 number of juvenile eels observed in the surveys represented an accumulation of eels migrating into the Baltic. To assess the total recruitment, the stocking by imported glass eels, or transplanted juveniles from outside the Baltic, was added. Fish larvae monitoring at a nuclear power plant in the Kattegat gave an index of glass eel abundance which was well correlated with glass eel monitoring in the Sound. This was used to extrapolate the recruitment time-series to the full period from 1981 to the present. The main conclusions from the analysis were: (i) the recruitment to the Baltic region, including the contribution from stocking, has declined by 95% between 1981 and 2012. (ii) The stocking of glass eel and bootlace eel has been of the same order of magnitude as natural recruitment, averaging 25–30% of the total supply of glass eel to the Baltic, with a decreasing trend with time. (iii) The total landings in the Baltic countries have ranged from 5 to 15% of the estimated survival without anthropogenic mortality. This proportion has been stable and independent of a large decrease in potential escapement.

Bernotas, P.; Vetemaa, M.; Saks, L.; Eschbaum, R.; Verliin, A.; Järvalt, A.

doi: 10.1093/icesjms/fsv245pmid: N/A

AbstractEuropean eel stocks are in a decline all over Europe. In Estonia, we analysed three different dataseries (commercial landings 1969–2014, scientific eel monitoring 1998–2014, and recreational catches 2005–2012). In the commercial fishery, the biggest drop in landings occurred in the 1980s, with a small recovery in the end of the 1990s. We also interviewed fishers to evaluate the quality of the official catch figures. In the monitoring dataseries, gathered by the use of special eel fykenets, the steepest cpue decline occurred in the last 5 years. Finally, recreational eel fishery has almost stopped: in 2012, catches were only 3% of that in 2005. In conclusion, both professional and recreational eel catches have declined in all the coastal areas of Estonia. The number of professional and recreational fishers specialized to eel fishing has decreased and the number of licences for eel-specific fishing gear has been reduced.

Brämick, Uwe; Fladung, Erik; Simon, Janek

doi: 10.1093/icesjms/fsv113pmid: N/A

AbstractUnder the European Eel Regulation EG 1100/2007, Member States exhibiting natural habitats for the European eel (Anguilla anguilla) on their territory are obliged to prepare Eel Management Plans (EMP) containing appropriate measures to safeguard the escapement of a river system specific silver eel target biomass. Stocking is one management option to reach this target. We used various methodical approaches to study population parameters in a large lowland river under the application of a multi-annual intense stocking programme. The approaches were used to further enhance modelling of stock dynamics and silver eel escapement, in particular. Parameterizing the German Eel Model III (GEM III) with values and functions obtained for recruitment, growth, and mortality resulted in an annual escapement estimate of roughly 32 000–64 000 silver eels from 2010 to 2012. Escapement estimates based on a mark-recapture study conducted in parallel revealed somewhat lower values (11 000–25 000) for the same years. In view of the small number of natural recruits, such values are only contingent if stocking had a profound effect on silver eel production. Results from modelling annual silver eel escapement values indicate that escapement targets set in the EMP for this tributary cannot be reached without stocking. This constellation is likely to apply to other Eel Management Units with low current natural immigration values as well, and might be considered a key dilemma in eel management in such catchments due to the current confusion whether translocation of recruits yields a net benefit to the panmictic stock of the European eel.