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The potential contribution of small coastal streams to the conservation of declining and threatened diadromous fishes, especially the European eel
Abstract
Human-generated pressures are continuing to have a detrimental effect on diadro-mous fishes, resulting in legislative initiatives to conserve and manage these species. Field studies to inform these initiatives focus almost exclusively on larger marine, estuarine, and inland waters, neglecting the role of small coastal streams and tributaries in population recovery. As an example of the potential contribution of very small coastal streams to diadromous fish recruitment, we report here on the densities and distribution of European eel Anguilla anguilla L. and flounder Platichthys flesus (L.) in one such stream, "La Warenne," in northeast France in October 1998 and 1999. Both species were found mainly in downstream sites in both years-flounder were captured in low numbers only, but mean CPUE values for eel were similar to the highest densities reported in the ICES WGEEL database for larger rivers of Great Britain, France and Spain in the Interreg Atlantic and North Sea areas. This small dataset illustrates that small coastal streams may represent an important, but overlooked , role in conservation strategies for European eel, and possibly flounder, indicating the need for further field studies and investigations of existing data sources to evaluate the contributions to diadromous fish recruitment of these poorly studied water courses. K E Y W O R D S dune stream, flounder, juvenile fish nursery habitat, Pas-de-Calais
... During their continental life cycle, three main habitat use strategies have been identified: marine and brackish resident, freshwater resident and inter-habitat shifter (Daverat et al., 2005;Shiao et al., 2006;Tabouret et al., 2010;Tzeng et al., 1997). The eels remain in coastal areas (Copp et al., 2021) and estuaries (Daverat et al., 2005;Daverat and Tomás, 2006;Tabouret et al., 2010), due to higher biological productivity than in freshwater, particularly at low latitudes (Gross, 1987;Tsukamoto and Arai, 2001). It has been shown that resident eels in marine and brackish habitats grow faster and mature earlier than those in freshwater (e.g. ...
... yellow and silver eels), in order to assess how local estuarine characteristics and habitat use strategies may influence the eel condition. In this study, we focused on six small and medium-sized French estuaries, which are less studied than large estuaries (e.g. the Severn estuary; Bird et al. (2008) and the Gironde estuary; ; Patey et al. (2018)), although small-sized catchments can represent a significant proportion of the overall global stock (Copp et al., 2021;Denis et al., 2022a). Specifically, we assessed the eel condition using an individual-scale multi-index approach based on four condition and health indices (i.e. ...
Brackish habitats are considered important for the facultatively catadromous European eel, but knowledge of eel habitat use strategies and the consequences on their condition, particularly in the estuaries areas, is limited and yet necessary for understanding some features such as growth and maturation in the different habitats that eel inhabit during the continental phase, that might also support assessment and management of local stocks, and contribute to the stock-wide assessment of this panmictic species. This study aimed to characterise and compare the condition of European eels according to their habitat use strategies and local estuarine characteristics. Eels were collected along the salinity gradient in six small and medium-sized estuaries located along the French coast in the eastern English Channel (i.e. the Slack, Wimereux, Liane, Canche, Authie and Somme estuaries). Four condition indices (i.e. Fulton condition factor K, lipid content, hepatosomatic index and health status) were measured on 119 individuals to explore variation with habitat characteristics at the small geographical scale and their habitat use strategies. Eel condition showed clear spatial differences between the six estuaries, with better condition in smaller estuaries. The spatial differences in eel condition appear to be related to variations in their diet composition, corresponding to different availability of macrozoobenthos prey among sites, in turn due to the local hydro-morpho-sedimentary characteristics. Environmental history and movements were reconstructed from the Sr:Ca and Ba:Ca ratios of otoliths from eel samples (N = 37) in both small- and medium-sized estuaries. The Sr:Ca and Ba:Ca ratios were used to distinguish the habitat use strategies and showed that both estuaries had a high proportion of resident eels (81%). Within each estuary, the Sr:Ca and Ba:Ca ratios were sufficiently contrasted to track movements of estuarine resident eels between three resident sectors (i.e. upper, middle and lower estuary). The relationship between eel condition and habitat use showed that inter-habitat shifter eels were in poorer condition than estuarine residents. Eel condition also varied between the three resident sectors, with decreases between eels from the lower to upper estuaries.
... In addition, investigations on eels focused almost exclusively on large estuary (e.g., in the Severn Estuary [30] and in the Gironde Estuary [10,25]), neglecting the role of small estuaries, which are the most numerous. Conservation and implementation of management measures for eel populations require an understanding of the importance of small estuaries for eels, particularly in terms of their carrying capacity and growth potential [36,37]. In the present study, we analyzed the European eel abundance and growth attributes in six small estuaries along the French coast of the eastern English Channel and explored whether local habitat characteristics influence them. ...
... In addition, CPUE values were in range with the highest densities previously reported in larger habitats. These results reinforce the idea that small estuaries are important habitats that contribute significantly to the eel population and, therefore, must be considered in both population status assessments, conservation and management strategies for the European eel [36]. ...
Abundance and growth of the European eel from six small northern French estuaries during their growth phase were examined to explore variations according to the local habitat characteristics. The length–weight relationships and growth models fitted to length-at-age back-calculated otolith growth increments were used to compare the growth. Higher abundances were observed in the smaller estuaries (2.4 to 10.5 ind. fyke nets 24 h−1). The eel length ranged from 215–924 mm with an age range of 4–21 years. There was no significant difference in fish eel lengths or age except in the Liane estuary where the individuals were larger. The length–weight relationships showed an isometric or positive allometric growth in most estuaries. The Gompertz growth models, which best fits the growth, showed no significant differences between estuaries except for female eels from the Liane and the Somme estuaries where the growth performance index was higher. The estimated annual growth rate varied from 2.7 to 115.0 mm·yr−1 for female and from 4.4 to 90.5 mm·yr−1 for male. The present study shows that eels in the six estuaries had CPUE and growth rates similar to those previously reported in larger habitats. These results reinforce the idea that small estuaries are important habitats that contribute significantly to the eel population and, therefore, play an essential role in conservation strategies for European eel.
... If the dispersal matrix included more catchments that were each a shorter distance away, it is possible that the resulting increase in the number and survival of dispersing spawners could allow surrounding catchments to be "rescued" from the Allee effect due to an annual influx of dispersing fish (Kanarek et al., 2015). Recent studies have suggested that small catchments could be more important for the resilience of diadromous species than previously thought (Melo-Merino et al., 2020;Copp et al., 2021), and so future applications of the HyDiaD model would benefit from the inclusion of more small catchments in future projections. ...
Diadromous species are particularly vulnerable to climate change because they utilize both marine and freshwater habitat to complete their life cycles. Dispersal plays an important role in restraining the distribution of plant and animal species, and is a key mechanism to allow diadromous species to adapt to changes in habitat suitability, but it is often not included in species distribution models that explore population trends under climate scenarios. The objective of this study was to develop a model to estimate potential shifts in diadromous populations in the Atlantic area of Europe under two climate change scenarios and multiple global climate models. To address the question of range-shift responses, a hybrid approach for diadromous species distribution (HyDiaD) was developed that incorporated two components: i) statistical static models of habitat suitability describing the influence of environmental factors on species occurrence, and ii) biological processes relevant for the distribution of the species, such as population demography and dispersal dynamics. Hybrid models were developed using a novel approach that incorporated both population and between-catchment dispersal dynamics specific to each species. Occupancy data for diadromous species in a subset of Atlantic Area catchments were first validated by regional experts, and boosted regression trees were applied to estimate habitat suitability within each catchment based on historical physical and climatic environmental predictors from the continental and marine domains. Habitat suitability was then used in a population dynamics model that incorporated between-catchment dispersal and local population growth. Results for different-sized catchments were compared using time series of spawner density and saturation rate, which estimated how much of the available habitat was being utilized. Many of the species-specific values used in HyDiaD were estimated through a survey of diadromous species experts, and group consensus was reached by calculating weighted averages. The HyDiaD model was applied to two shad species (Alosa alosa and A. fallax) to explore population trends projected annually from 1951 to 2100. Projected trends indicated that under XXIst century climate scenarios, habitat suitability is expected to increase for A. fallax, but decrease for A. alosa. Projected trends also indicated an increase in the rate of annual variability for A. alosa, particularly in the southern part of its range. Future studies can utilize the HyDiaD model to explore distribution trends for other diadromous species under climate change scenarios.
... A lack of reliable monitoring of smaller, more numerous, catchments often means that plans concentrate on larger catchments. This belies the likely collective and cumulative importance of numerous small catchments to eel stocks and overall conservation strategies (Copp, Daverat & Baši c, 2020). This study illustrates the importance of catchment-scale approaches to monitoring using different methods, especially PAS by electric fishing and even limited tracking of eels (see 4.1 above). ...
Effective conservation management of the Critically Endangered European eel ( Anguilla anguilla ) is hindered by incomplete understanding of distribution, abundance, and habitat requirements at the catchment scale.
All habitats available to eels within a small, highly regulated river catchment, representative of many used across the species' range, were sampled using several methods (including point abundance sample electric fishing and fyke nets) and supplemented by individual telemetry to investigate movements.
Eels were found throughout the catchment (59% of n = 131 sites) from the coastal marshes to the headwaters, although the probability of presence declined with distance from the estuary. The lack of a clear relationship with perceived barriers may illustrate a mismatch with the reality experienced by eels, as telemetry identified connectivity across obstacles between paludal habitat and estuary, and detected escapement of mature silver eels from both lotic and lacustrine habitat.
Different size/age classes occurred in different parts of the catchment, partly linked to different habitat associations, with coastal paludal habitat supporting >50% of the catchment population and especially smaller (possibly male dominated) yellow eels. Recently recruited elvers were most abundant in the lower reaches of lotic habitat. The largest (probably female) eels were concentrated in lacustrine sites, especially at the ‘end‐of‐the‐line’ in the headwaters.
Experiences here suggest that conservation management for eels in small catchments is best focused on: (i) improving connectivity and assisting migration of elvers across ‘problem’ barriers that cannot be removed or modified, (ii) river restoration and rewilding, especially measures that increase instream woody material to provide refuge habitat and, (iii) enhancement or creation (where necessary) of suitable lacustrine habitat to benefit large females in particular. Such action across numerous small river catchments may ultimately help support the recovery of eel stocks.
... It came close to extinction during the 20th century due to overharvesting and migration barriers on rivers. Currently, eels do not form sustainable populations in most countries in Europe, and their yields have been steadily decreasing in Europe in the 21st century [1]. Since eels are highly valued in commercial fisheries, the price of juveniles has been increasing rapidly on the black market [2,3]. ...
The European eel Anguilla anguilla is a critically endangered catadromous migratory fish species. To conserve eel populations, angling restrictions and stocking activities are often used. This paper aimed to analyze the effect of an increased minimum legal angling size, eel stocking, fishing effort, and important environmental and biological factors on eel yield. This study used data on eel stocking and yield collected by the Czech Fishing Union using angling logbooks. Data regarding 41 tons of harvested eels were collected on 176 fishing sites from 38,000 anglers over the years 2005-2018 in central Bohemia and Prague (the Czech Republic). Eel made up only 0.006% of the overall fish harvest by biomass. It was found that the increased minimum legal angling size led to decreased yield of eel and to a decreased percentage of eel in the overall fish harvested. It also led to larger harvested eels, while the number of fishing sites where anglers harvested eels stayed constant over time. The eel yield was strongly correlated to the angling effort but not to the eel stocking intensity or the environmental and biological factors. In conclusion, implementing the minimum legal angling size did achieve its goal, because it led to decreased eel yield.
... The robust evaluation of eel stock development requires data from the full range of aquatic habitats occupied by eels. As a facultative-catadromous species, yellow eels settle in coastal, brackish and inland aquatic habitats during their continental life phase (ICES, 2009), and the importance of such areas for the eel stocks has been highlighted by various studies ICES, 2009;Shiao, Ložys, Iizuka, & Zeng, 2006), although the contribution of small coastal streams to eel recruitment remains virtually unstudied (Copp, Daverat, & Bašić, 2021). ...
Within recent years, a slight but significant increase of European eel Anguilla anguilla (L.) recruitment has been documented, but it remains questionable whether or not the increased recruitment levels resulted in higher eel numbers at the regional scale. To detect the changes in yellow eel numbers, logbook data covering a 15‐year time series of catch per unit effort (CPUE) data from the German Baltic Sea were analysed. Monthly mean catch rates were calculated for two different size classes for two passive gears: fyke and stationary trap nets. Change‐point analysis was applied to discover changes in the catch data. After a period of decreasing or constant catch rates, the fyke net data indicated that yellow eel numbers increased slightly in recent years in the Baltic Sea. Besides increasing numbers of immigrating juvenile eels, other population dynamics or conservation efforts might have added to the observed positive stock trend.
The impoundment of the Aoos Springs, which was created in 1990 for the needs of hydroelectric production, is characterised by the lack of studies on its fish fauna across years. Seasonal sampling efforts were conducted with Nordic gillnets and an electrofishing device to describe the fish fauna after 30 years from the construction of the impoundment. Our results revealed the presence of establish species in the impoundment and highlighted their connectivity with the inflow streams. The fish fauna has been enriched throughout the years, starting from four observed species few years after the formation of the impoundment (during 1996-1997) to nine species 20 years later (2015-2019). This increase in the number of species across years was mainly attributed to human impacts through unintentional or intentional stocking conducted to support the local recreational fishery. In any case, the importance of each species' ecological niche and the complexity of freshwater ecosystems should be considered in future initiatives aimed at maintaining fish biodiversity.
The cover image is based on the Short Communication The potential contribution of small coastal streams to the conservation of declining and threatened diadromous fishes, especially the European eel by Gordon Copp et al., https://doi.org/10.1002/rra.3746.
Photography by Alan Walker.
Small, 1st and 2nd-order, headwater streams and ponds play essential roles in providing natural flood control, trapping sediments and contaminants, retaining nutrients, and maintaining biological diversity, which extend into downstream reaches, lakes and estuaries. However, the large geographic extent and high connectivity of these small water bodies with the surrounding terrestrial ecosystem makes them particularly vulnerable to growing land-use pressures and environmental change. The greatest pressure on the physical processes in these waters has been their extension and modification for agricultural and forestry drainage, resulting in highly modified discharge and temperature regimes that have implications for flood and drought control further downstream. The extensive length of the small stream network exposes rivers to a wide range of inputs, including nutrients, pesticides, heavy metals, sediment and emerging contaminants. Small water bodies have also been affected by invasions of non-native species, which along with the physical and chemical pressures, have affected most groups of organisms with consequent implications for the wider biodiversity within the catchment. Reducing the impacts and restoring the natural ecosystem function of these water bodies requires a three-tiered approach based on: restoration of channel hydromorphological dynamics; restoration and management of the riparian zone; and management of activities in the wider catchment that have both point-source and diffuse impacts. Such activities are expensive and so emphasis must be placed on integrated programmes that provide multiple benefits. Practical options need to be promoted through legislative regulation, financial incentives, markets for resource services and voluntary codes and actions.
European eel (Anguilla anguilla) is considered as critical endangered and even under the best circumstances it may take decades before the stock recovers.
Estimation of eel escapement biomass, Bescapement, is of critical importance to evaluate management schemes and to predict the recovery potential for the eel stock. Westerberg
and Wickström (2015. Stock assessment of eels in the Baltic: reconciling survey estimates to achieve quantitative analysis.
ICES Journal of Marine Science, doi: 10.1093/icesjms/fsv049) attempt to estimate potential Bescapement based on the assumptions that all elvers at the entrance of the Baltic also migrate into the Baltic Sea and that natural
mortality is low under the whole growth stage (close to 0.02 at the age of 10 years and older). As a consequence, Westerberg
and Wickström (2015. Stock assessment of eels in the Baltic: reconciling survey estimates to achieve quantitative analysis.
ICES Journal of Marine Science, doi: 10.1093/icesjms/fsv049) estimated the present potential Bescapement at ∼10–20 000 tonnes and fishing mortality close to 0.05–0.10, while it was also suggested that other sources of anthropogenic
mortality may reduce the actual escapement to unknown levels. Here we argue that these conclusions are entirely speculative
and contradicted by tagging experiment and fishery data, which instead indicate a much higher fishing mortality (mortality
induced by legal professional fishery) rates and a considerably smaller eel biomass.
Seven yellow eels (572–643 mm, 318–592 g) Anguilla anguilla (L.) were tagged with surgically implanted radio transmitters (activity circuit, 1.6–1.7 g) and tracked in the Awirs stream,
a small (width -1) tributary of the Belgian River Meuse. The eels were positioned daily from late April to mid-August, and their diel activity
was studied during twenty four 24-h cycles. During day-time, the eels were resting in rootwads or in crevices inside stone
walls or in crevices in between rocks. They became more active in the late afternoon but generally did not leave their residence
before sunset, except under overcast weather. Activity peaked during the first part of the night then progressively vanished,
and always ended before sunrise. The area exploited during night-time never extended over more than 40 m2, except when the eel changed its residence. The intensity and timing of nocturnal activity and the extent of the daily activity
area were dependent on water temperature (respectively P < 0.0001, P < 0.05 and P < 0.0005), with eels showing little or no
activity when the diurnal temperature did not exceed 13 °C. Eels showed higher agitation under full moon and maintained their
activity later in the night (P < 0.05). The eels showed restricted mobility, and occupied small stream areas (from 0.01 to
0.10 ha) in a non sequential mode, except for two fish which were displaced to the River Meuse by a spate in early June and
were never recovered. The length and frequency of net daily journeys were higher (P = 0.005) at water temperatures above 16
°C in late May and June, which also corresponded to the period of immigration of eels from the River Meuse. This study thus
shows that large yellow eels may adopt a highly sedentary lifestyle in a continental, fast flowing and densely populated environment,
even at periods of the year when these stages usually show upstream migrations.
Habitat use patterns of 3 species of temperate eels, Anguilla anguilla, A. japonica and A. rostrata, were investigated using otolith strontium:calcium ratio life history transects. Published and unpublished data from 6 sites (Canada, United States, Sweden, France, Taiwan and Japan) sam- pled across the geographical range of each eel species were compiled. Sr:Ca patterns indicated that the 3 species displayed similar patterns of habitat use. In all sites, patterns of habitat use consisted of either residency in one habitat (fresh, brackish, or marine) or movements between habitats. One movement pattern consisted of either a single change or 2 changes of habitat from fresh to brackish waters, or from brackish water to freshwater. Seasonal movements between fresh and brackish waters were observed for all 3 species. When only a single habitat switch event was detected, it occurred between 3 and 5 yr of age. Occurrence of eels with no freshwater experience was demon- strated, but such eels accounted for a smaller proportion of the overall sample than eels with some (even brief) freshwater experience. Contrary to the common convention that these are obligate catadromous species, we must now consider them as facultative catadromous, with far more flexibil- ity in habitat use. The most variable parameter among study sites was the relative proportion, rather than the diversity, of lifetime spent in the various habitat use patterns. Eels found at higher latitudes exhibited a greater probability of remaining in the lower reaches of watersheds in brackish water. Diversity of habitat use appears to be a common strategy of temperate eel species, and, as a life his- tory tactic, is under environmental control.
The Frémur is a 60 km2 catchment of Brittany where many hydraulic works (dams, weirs, gauging structures, etc.) have reduced the recruitment by elvers and glass eels
since at least 50 years. In 1992, an eel lift was built on the main dam (14 m high) and in 1996, two other dams (5 m high) located downstream were equipped with eel passes to
improve upstream migration. In 1995, before the construction of the passes, eel population parameters (density, biomass and size distribution) were assessed at the scale of the
whole watershed by electrofishing (removal method) in 33 stations. The average biomass was high (19 g/m2 ) despite the numerous physical obstructions. However, there was an
accumulation of eels downstream many hydraulic works and very low densities immediately upstream. Moreover, downstream the first obstruction, at 2 km from the sea,
the population is dominated by boot lace eels and elvers (< 100 mm), whereas this size class is absent in upstream reaches. These results suggest that the standing capacity of
the watershed is not reached and that the population could be enhanced in the next years by the recently installed eel passes.
The construction of the Arzal dam eel ladder in 1996 allowed enhancing fluvial recruitment from a negligible level, limited to yellow eels crossing sluice and dam overflow, to a level ranging from 0.2 to 2.4 million glass eels per year. The effect of such recruitment on the eel population (Anguilla anguilla) of the Vilaine watershed was analysed at 19 electro- fishing sampling stations. From 1998 to 2003, average densities of eel varied from 0.34 to 0.72 eels.m-2, with a maximum reached in 1999 of 0.82 eels.m-2. Fluvial recruitment failure, together with density dependent mortalities, explains the drop in yellow eel densities observed from 1998 in the downstream area (< 50 rkm) and from 2000 in the middle stream area (50- 100 rkm). The largest densities consisted of a large proportion eels classified as age 1 and the increased density in middle stream sectors was interpreted as the consequence of density-dependent migration at the periphery of a saturated area. The construction of 13 eel ladders on the Vilaine waterway in 1999 and 2000 was followed by increased densities in the upstream area (> 100 rkm) in 2001. The comparison to an electrofishing survey performed at 17 stations in 1981, ten years after dam construction, confirmed that ladder installation increased densities by a factor 6 and modified the population structure, with densities of eels age 0 and 1 multiplied 29 fold. This change corresponded to reduced escapement rates to the glass eel fishery (1% to 5%). Considering this result, a preliminary escapement target of 240 glass eel per km2 of watershed area or 1 500 glass eel per ha of water surface, is proposed and discussed for glass eel fisheries.
The European eel population is in decline. Urgent management actions are required. To monitor their effect, we developed a fast sampling method, applicable in shallow rivers and tributaries. We compared the Carle & Strub estimator from 35 double pass electric fishing samples with Point Abundance Sampling by Electrofishing or PASE (24 point samples on average by section) which we adapted for the eel. The correlation between both methods was highly significant (r2 = 94%, p < 0.001) for densities lower than 150 eels · 100 m-2. PASE, being a cheaper sampling method, thus constitutes an efficient and cost-effective method for evaluating management actions and further biological studies.
European flounder Platichthys flesuslife history patterns were investigated in 3
basins along a latitudinal gradient (Minho, N Portugal; Gironde, SW France; Seine, N France). We
used coupled Sr:Ca and Ba:Ca otolith signatures and microstructure to retrospectively determine
habitats occupied by flounder during their life, including early larval ontogeny. Flounder exhib-ited high life history plasticity among and even within basins, apparent by the diversity of habitats
used during larval ontogeny and throughout their lives, and by the age at which flounder
migrated to freshwater. Egg signatures probably had a strong maternal influence, and our inter-pretation suggests that flounder spawned and/or hatched predominantly in brackish waters in the
Minho, while in the Gironde and Seine, flounder spawned and/or hatched either in coastal, brack-ish or freshwater environments. The freshwater egg signature was most frequent in the Seine.
These interpretations contradict the current general assumption that flounder spawn exclusively
in coastal waters. During pre-metamorphosis and metamorphosis, flounder were predominantly in
brackish waters in the Minho, while in the Gironde and Seine, they were mainly in coastal and
freshwater environments, respectively. The diversity of flounder life histories (LH) (i.e. sequence
of habitat residence: freshwater, brackish or coastal) after metamorphosis was similar between the
Minho (LH = 13), Gironde (LH = 13) and Seine (LH = 14) basins. The age at which flounder
migrated to freshwater also varied among sites, at an earlier age in the Minho and Gironde
(< 0.5 yr old) than in the Seine, where flounder migrating from the coast into freshwater reached
maximum frequencies at age 1.3 yr old. Thus, catadromy in European flounder may be facultative,
and the factors influencing flounder high LH plasticity deserve thorough research.
For 2years, the flatfish community of Minho estuary (Northwest of Iberian Peninsula) was surveyed monthly with a beam trawl.
Flounder, Platichthys flesus, was the most representative species contributing the most (74%) to the total flatfish abundance and was the only species
occurring throughout the sampling period and along the entire salinity gradient. The majority of flounder sampled consisted
of 0- and 1-year group individuals. In both years, a peak in abundance was recorded in late spring–summer months (from May
to August) which coincided with seasonal settlement events. Higher densities of newly-settled flounder were obtained in the
most upstream site, located in the tidal freshwater wetland, indicating that this area is an important nursery area. Of the
environmental variables tested, temperature and salinity were the best predictors for 0-year group abundance explaining around
30% of the variability, whereas sediment was also important when considering all age groups together. This study supports
the generally prevalent idea that estuaries act as nurseries for the early life stages of flounder, with particular relevance
to the tidal freshwater estuarine wetlands.
The purpose of this present study was to use the growth rates, RNA–DNA ratio and morphometric (Fulton’s K) condition indices measured on 0-group juveniles European flounder, as a means of comparing the quality of estuarine nursery
habitats. The Seine estuary, heavily impacted by manmade modifications and one of the most contaminated in Europe, was compared
to two smaller estuaries considered as clean (low domestic, agricultural and industrial effluents). Indicators of human disturbance
(sediment chemical contaminants) as well as biotic data (macrobenthos abundances) were used to establish between-site differences
in environmental quality. The abundance of macrobenthos (potential preys for juvenile) was high and not significantly different
among the three estuaries confirming the important role played by estuaries as feeding grounds. Sediment metal concentrations
(Cd, Cr, Cu, Pb, Zn) were significantly higher in the Seine than in the Canche and Authie estuaries. Otolith growth was closely
related to somatic fish growth, and we used daily otolith increments from the previous 10days before capture as an indicator
of 0-group flounder recent growth (RG). RG did not vary significantly among flounders from the Canche and Authie estuaries
and individuals from the Seine had slower RG. RNA–DNA ratio and K values were also significantly lower in individuals from the Seine compared to those of the Authie or the Canche. Pollution
(e.g. metal contaminants) is the most likely cause of growth and condition differences among estuaries. The results of this
study suggest that habitat-specific fish growth and condition are useful proxies for habitat quality since they integrate
the effect of multiple environmental factors.
Following a multi-decadal decline of the European eel stock all across the continent, the EU adopted a protection and recovery
plan in 2007, known as the Eel Regulation. Implementation, however, has come to a standstill: in 2015, the agreed goals had
not been realised, the required protection had not been achieved, and from 2012 to 2015, no further reduction in mortality
has been accomplished—while the stock is at a historical minimum. To analyse this manifest impasse, this article characterises
the steering framework of the Eel Regulation as a governance problem. The Eel Problem is found to be extremely complex, due
to many knowledge uncertainties and countless societal forces having an influence. The Eel Regulation divides this complexity
along geographical lines, obliging national governments to implement national protection plans. This deliberate distribution
of control has improved communication between countrymen-stakeholders, and has stimulated protective action in most EU Member
States and elsewhere. In the absence of adequate international coordination and feedback on national plans, however, coherence
is lacking and the common goals are not met. Actions and achievements have been assessed at the national level, but these
assessments have not been evaluated internationally. Full geographical coverage has not been attained, nor is that plausible
in future. Meanwhile, ICES’ advice remained focused on whole-stock management, a conservative approach not matching the structure
of the Eel Problem or the approach of the Eel Regulation. Hence, essentially localised problems (non-reporting, insufficient
action) now lead to a hard fail, paralysing the whole European eel recovery plan. Here, I argue that immediate re-focusing
protective actions, assessments, evaluations and advice on mortality goals and indicators, for each management area individually,
will enable feedback on national protection plans, and in that way, will break the impasse.
Connectivity between coastal spawning grounds and estuarine nurseries is a critical step in the life cycle of many fish species. Larval immigration and transport-associated physical–biological processes are determinants of recruitment success to nursery areas. The recruitment of the European flounder, Platichthys flesus, to estuarine nurseries located at the southern edge of the species distribution range, has been usually investigated during its juvenile stages, while estuarine recruitment during the earlier planktonic life stage remains largely unstudied. The present study investigated the patterns of flounder larval recruitment and the influence of environmental factors on the immigration of the early life stages to the Lima estuary (NW Portugal), integrating data on fish larvae and post-settlement individuals (< 50 mm length), collected over 7 years. Late-stage larvae arrived at the estuary between February and July and peak abundances were observed in April. Post-settlement individuals (< 50 mm) occurred later between April and October, whereas newly-settled ones (< 20 mm) were found only in May and June. Variables associated with the spawning, survival and growth of larvae in the ocean (sea surface temperature, chlorophyll a and inland hydrological variables) were the major drivers of flounder occurrence in the estuarine nursery. Although the adjacent coastal area is characterized by a current system with strong seasonality and mesoscale variability, we did not identify any influence of variables related with physical processes (currents and upwelling) on the occurrence of early life stages in the estuary. A wider knowledge on the influence of the coastal circulation variability and its associated effects upon ocean-estuarine connectivity is required to improve our understanding of the population dynamics of marine spawning fish that use estuarine nurseries.
Most of the European diadromous fish species are endangered and listed in the habitats fauna and flora directive, the bern convention and the IUCN Red List. Current conservation plans do not address global warming issues and consider the 1900 distribution range as the reference without taking into account the potential re-distribution of these species under global warming. However, for other taxa, predictive models have been used successfully to relate conservation assessment with the impact of global warming. We investigated the distribution of the various diadromous species regularly encountered in Europe, North Africa and the Middle East at the beginning of the 20th century. For each species, we related the observed distribution to a set of environmental variables describing the prevailing climate in the basins, the physical nature of the basins and reflecting historical events known to have affected freshwater fish distribution. We successfully built twenty-one accurate species-specific models out of twenty-eight during both the calibration and the validation phases. Longitude, a factor we interpret as related to biogeographical history, was selected in all models and temperature was included in fifteen models, in six it was the most explanatory variable. These models allowed us to separate the roles of ecology and history in explaining distribution patterns at species level. Historical events such as glaciations constrained the continental distribution of all diadromous species and six of these were also highly temperature-dependent. Based on these results, we claim that these models can be used to predict the potential distribution under changing climatic conditions and provide two relevant examples (i.e. for Alosa alosa and Pe tromyzon marinus). Predictive models could be useful as a preliminary tool to prepare long-term conservation plans on European, national and regional scales.
We examined the status of diadromous (migratory between saltwater and freshwater) fishes within the North Atlantic basin, a region of pronounced declines in fisheries for many obligate marine species. Data on these 24 diadromous (22 anadromous, 2 catadromous) species are sparse, except for a few high-value forms. For 35 time series, relative abundances had dropped to less than 98% of historic levels in 13, and to less than 90% in an additional 11. Most reached their lowest levels near the end of the observation period. Many populations persist at sharply reduced levels, but all species had suffered population extirpations, and many species are now classified as threatened or endangered. Habitat loss (especially damming), overfishing, pollution, and, increasingly, climate change, nonnative species, and aquaculture contributed to declines in this group. For those diadromous fishes for which data exist, we show that populations have declined dramatically from original baselines. We also discuss the consequences of these changes in terms of lost ecosystem services.
Natural or anthropogenic induced variations in estuaries and the dynamics of marine fish populations potentially promote differences in connectivity between estuaries and marine areas, i.e. in their importance as nursery grounds. Within this context, an integrated assessment of the differential nursery function of the main estuaries along the Portuguese coast for commercial fish species common sole Solea solea, Senegalese sole Solea senegalensis, flounder Platichthys flesus and sea bass Dicentrarchus labrax was performed through several indicators based on available data. Contribution of individual estuaries to marine subpopulations was measured with potential metrics (juvenile density, habitat quantity, juvenile number and habitat quality within estuaries) and effective metrics (estuarine source of young adults in marine environment measured via otolith elemental fingerprints). The relationship between the two types of metrics was also assessed. Estuaries identified as important nursery and/or effective juvenile habitat (EJH) differed with species and no single estuary was best for all, highlighting species-specific regulation of nursery function. Multiple species assessment of nursery and EJH function differed among estuaries. Management and conservation of estuaries should focus on sites with higher contributions to adult subpopulations of multiple species. The importance of defining precise scientific and management objectives was emphasized by the different rankings of estuaries obtained with nursery or EJH criteria. Potential and effective contribution of estuaries were not significantly correlated, but in a quantitative analysis juvenile densities and number of juveniles seem related with effective contribution in some species. An agreement between potential and effective contributions of estuaries is concurrent with the acknowledged minor role of juvenile stage processes in regulation of recruitment to adult subpopulations.
Daverat F, Martin J, Fablet R, Pécheyran C. Colonisation tactics of three temperate catadromous species, eel Anguilla anguilla, mullet Liza ramada and flounder Plathychtys flesus, revealed by Bayesian multielemental otolith microchemistry approach. Ecology of Freshwater Fish 2011: 20: 42–51. © 2010 John Wiley & Sons A/S
Abstract – The colonisation of Gironde (SW France) river catchment by juvenile, eel, Anguilla anguilla, flounder Platychtys flesus and thinlipp mullet Liza ramada was investigated comparatively using Sr:Ca and Ba:Ca composition of otolith. The relation between Sr, Ba and habitat was investigated based on Sr and Ba water composition sampled each month along the estuary-river gradient. A total of 50 mullets, 30 eels and 47 flounders were collected in the Gironde river catchment. Analysis was performed with a Femtosecond LA-ICPMS along a trajectory from the core to the edge. Sr and Ba water concentrations discriminated three habitats within the Gironde system, the lower estuary, the upper estuary and the freshwater sites. A signal processing method based on Gaussian hidden Markov models was applied to the multielemental life-history data. The linear model used to allocate a Gironde habitat to coupled Sr, Ca values was parameterised with seasonal patterns and magnitude of Sr and Ba water values in the different habitats. The results showed that the three species used three different habitats and they had a large diversity of habitat use patterns with resident and nomadic tactics. Resident tactics were less frequent than nomadic tactics that suggested individual fish used two or more habitats. Mullet used a wider range of habitats in the lower part of the estuary than eel and flounder and switched habitats more frequently. Flounders tended to colonise initially freshwater, and then estuarine habitats later in life while mullets used the entire range of available catchment habitats throughout their life.
The performance of fyke netting was compared with that of electrofishing in an artificially stocked closed pond and in open waters with natural populations in the Thames catchment, England. Seine netting, trapping and fyke nets of different mesh size were also compared using wild populations. Studies of mark-recapture and catch-depletion techniques for estimating population sizes and structures were included. It was concluded that no single technique was ideal because of the difficulties inherent in adequately sampling eels in all aquatic environments. Recommendations are made and the implications of fyke net efficiency for commercial fishing are discussed.
Bio-indicators were measured on juvenile fish to assess the quality of eight coastal and estuarine nursery grounds in the Eastern English Channel and in the Bay of Biscay during 3 years. Growth (size and otolith daily increment width), body condition (morphometric index) and abundance of juvenile common soles were analysed together with xenobiotic concentrations (heavy metals and organic contaminants).Condition indices displayed important variations and did not allow relevant estimation of environmental quality. On the contrary, growth and density indicators showed good steadiness above years but varied among sites. In spite of difficulties of interpreting these indicators on such a meso-scale approach, analyses highlighted the estuaries of Seine and Gironde. In these nursery areas, the levels of contamination were especially high, and the combination of fish growth performances and density was significantly lower than in other sites. The combination of these variables appears to provide reliable indicators of habitat quality and anthropogenic pressure on nursery grounds, especially highlighting contaminated areas. Such indicators may thus contribute to improve assessment of environmental quality of essential fish habitats with the aim of a sustainable management of fisheries resources. A study at a different scale, from this meso-scale nursery approach with more precise analyses, on local habitats, will nevertheless be necessary to optimize the relevance of these indicators for the assessment of essential fish habitat quality.
It is shown that the catch-effort electrofifhing methods, which are those most often employed for estimates of riverine fish density and biomass, are often not precise. These multiple sampling procedures are time- and manpower-consuming, and can change the habitat and fish community structure. The results, collected in rivers of different size and character, show curvilinear relationships between the average specimen size and the percentage, number and biomass of fish caught by one electrofishing. Using such relationships minimizes the alteration of fish community structure by the sampling procedure. Conditions which must be fulfilled for proper use of the proposed relationships are discussed.
In this paper, we review the role of fish populations in generating ecosystem services based on documented ecological functions and human demands of fish. The ongoing overexploitation of global fish resources concerns our societies, not only in terms of decreasing fish populations important for consumption and recreational activities. Rather, a number of ecosystem services generated by fish populations are also at risk, with consequences for biodiversity, ecosystem functioning, and ultimately human welfare. Examples are provided from marine and freshwater ecosystems, in various parts of the world, and include all life-stages of fish. Ecosystem services are here defined as fundamental services for maintaining ecosystem functioning and resilience, or demand-derived services based on human values. To secure the generation of ecosystem services from fish populations, management approaches need to address the fact that fish are embedded in ecosystems and that substitutions for declining populations and habitat losses, such as fish stocking and nature reserves, rarely replace losses of all services.
EuroDiad 4.0 User's Guide. Report on Assessing and Enhancing Ecosystem Services Provided by Diadromous Fish in a Climate Change Context. Deliverable reference: WP 5-Action 1
- B Barber
- G Lassalle
- P Lambert
- E Quentin
- J King
- W Roche
- E Pereira
Barber, B., Lassalle, G., Lambert, P., Quentin, E., King, J., Roche, W., …
Pereira, E. 2020. EuroDiad 4.0 User's Guide. Report on Assessing and
Enhancing Ecosystem Services Provided by Diadromous Fish in a Climate Change Context. Deliverable reference: WP 5-Action 1.
Retrieved from https://diades.eu/wp-content/uploads/2020/07/Dia
dES-5.1-EuroDiad-User-Guide_Final_CorrectedJune2020.pdf
Regulation (EU) no 1237/2010 of the European Parliament and of the Council of December 15, 2010 amending council regulation (EC) no 2187/2005 as regards the prohibition of highgrading and restrictions on fishing for flounder and turbot in the Baltic Sea, the belts and the sound
- EC
EC. (2007). Council regulation (EC) no 1100/2007 of September 18, 2007
establishing measures for the recovery of the stock of European eel.
Official Journal of the European Union, 248, 17.
EC. (2010). Regulation (EU) no 1237/2010 of the European Parliament
and of the Council of December 15, 2010 amending council regulation
(EC) no 2187/2005 as regards the prohibition of highgrading and
restrictions on fishing for flounder and turbot in the Baltic Sea, the
belts and the sound. Official Journal of the European Union, 348, 34.
Un étrange poisson: L'anguille ses mœurs et son intérêt économique
- Hornyold G.
Hornyold, G. (1931). Un étrange poisson: L'anguille ses moeurs et son
intérêt économique. La Terre et la Vie, Revue d'Histoire Naturelle, 11,
657-674.
A review of the European flounderPlatichthys flesus-biology life history and trends in population
- D J Skerritt
Skerritt, D. J. (2010). A review of the European flounder Platichthys flesusbiology, life history and trends in population. Eastern Sea Fisheries
Joint Committee Report. Newcastle University. Retrieved from
https://www.eastern-ifca.gov.uk/wp-content/uploads/2016/03/Flou
nderLitReviewDS.pdf
Comment on stock assessment of eels in the Baltic by
- H Svedäng
- M Cardinale
Svedäng, H., & Cardinale, M. (2016). Comment on stock assessment of eels
in the Baltic by Westerberg and Wickström (2015): Do we need more
unknowns? ICES Journal of Marine Science, 73(6), 1610-1612.
- Baras E.
Council regulation (EC) no 1100/2007 of September 18, 2007 establishing measures for the recovery of the stock of European eel
- EC