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- The decline in recruitment of the European eel: new insights from a 40-year-long time-series in the Minho estuary (Portugal)Publication . Correia, Maria João; Costa, José Lino; Antunes, Carlos; De Leo, Giulio; Domingos, IsabelIn the past three decades the European eel Anguilla anguilla experienced up to 99% decline in recruitment in some parts of its distribution range. In addition to this long-term trend, glass eel recruitment exhibits significant inter-annual variability partially explained by large-scale oceanographic processes as well as local-scale meteorological and hydrological variables. A 40-year long time series of glass eel fishery catch in the Minho estuary, Portugal, was used to assess (a) whether local recruitment followed the general pattern of decline of A. anguilla observed elsewhere, and (b) whether environmental variables may explain inter-annual fluctuations in glass eel recruitment in the Minho estuary. The analysis shows that, in contrast to the majority of coastal systems in northern Europe and the Mediterranean, CPUE of glass eel in the Minho estuary did not exhibit a marked decline between 1974–2015 and never dropped below 20% of the 10-year mean (1974–1983), taken as baseline. The difference between the recruitment trend observed in the Minho river and that reported by WGEEL for wider European geographical scales highlights the need to calculate recruitment indices with a higher geographic resolution to better support the assessment of the status of the European eel population.
- Impact of freshwater inputs on the spatial structure of benthic macroinvertebrate communities in two landlocked coastal lagoonsPublication . Félix, Pedro; Correia, Maria; Chainho, Paula; Costa, José Lino; Chaves, Maria Luisa; Cruz, Teresa; Castro, João J.; Mirra, C.; Domingos, Isabel; Silva, A. C. F.; Cancela Da Fonseca, LuísLandlocked lagoons are naturally stressed environments. They are strongly influenced by freshwater input which not only varies naturally, but which is also impacted by anthropogenic activities. This study investigated the direct influence of freshwater discharges on the distribution patterns and abundance of benthic communities in two neighbouring landlocked coastal lagoons, assessing the whole system and the confluence area of each tributary. Sampling occurred in the wet and dry seasons of 2011 at two distances from freshwater discharge locations. Both lagoons were colonized by species from two different pools, freshwater and marine. Freshwater flow rates had a direct influence on the spatial structure of the benthic communities of brackish-water/freshwater interface areas, where also specific taxa can act as early indicators of freshwater input variations. The intensity of this influence is highly dependent on lagoon size, creating spatial heterogeneity or affecting the entire system. The benthic fauna at the confluence of the tributary that depends almost exclusively on groundwater showed the lowest variability, suggesting that the biogeochemical nature of the groundwater may be a central cause for setting specific ecotones. The results suggest that benthic communities of landlocked coastal lagoons can be highly impacted by flow reduction from freshwater aquifers under drought conditions or water abstraction activities.
- Glass eel (Anguilla anguilla) recruitment to the river Lis: Ingress dynamics in relation to oceanographic processes in the western Iberian margin and shelfPublication . Stratoudakis, Yorgos; Oliveira, Paulo B.; Teles-Machado, Ana; Oliveira, José Manuel; Correia, Maria João; Antunes, CarlosTo provide new information on the ingress dynamics of glass eels at the Portuguese coast, the study analyzes catch per unit effort (CPUE) data from river Lis, biometric data from recruits in basins of western Portugal and meteo‐oceanographic data from the Iberian margin and shelf. Biometric data of glass eels in western Portugal are congruent with the latitudinal size gradient of leptocephali in oceanic surveys and show clear seasonality, with longest and heaviest individuals arriving from October to December. Generalized additive models fitted to CPUEs from two experimental fishing periods (1996–1997 and 2013–2014) show that availability of glass eels to the sampling gear was inversely related to lunar phase and sea wave height. CPUE was lowest from May to September, increased towards the end of the year and peaked in February–March, when recruits were almost 40% lighter than autumn arrivals. Commercial CPUE during the 1989–1990 fishing season was significantly higher than experimental fishing data in 1996–1997. Higher variance and smaller sample size in 2013–2014 prevent conclusive interdecadal comparisons. Finally, CPUE was significantly higher during the prevalence of northward current flow off western Portugal and during strong cross‐shelf westerly winds. Findings support the suggestion that eel recruitment in Portugal occurs mainly by leptocephali travelling along the Azores Current, deflected northwards through the Iberian Poleward Current, with river ingress, and possibly oceanic metamorphosis, modulated by seasonal dynamics in coastal hydrology and shelf/upper margin oceanography.
- Tools for the management and conservation of the European eel (Anguilla anguilla): an application to Santo André lagoonPublication . Correia, Maria João; Costa, José Lino Vieira de Oliveira; Domingos, Isabel Maria Madaleno; De Leo, GiulioThe critical status of the European eel (Anguilla anguilla) population is an ongoing concern for fisheries and environmental managers. The European Eel Regulation approved in 2007 established the framework for the eel recovery, although incomplete knowledge about the stock hampers management that ensure the species’ sustainability. The present research investigated the dynamics of the European eel in a coastal lagoon, contributing to increase the knowledge about the species and to the assessment of the status of the stock in Portuguese inland waters. The information gathered on eel recruitment, and on the species exploitation and life history traits in a Portuguese brackish system, the Santo André lagoon, provided relevant data to develop stock assessment tools, and to analyse and understand the impact of management decisions on the local eel stock. Data on glass eel from the rivers Minho and Lis provide insights on recruitment trends in Portugal in recent decades and on the drivers of glass eel ingress into Portuguese coastal systems and were used as a proxy to the recruitment dynamics in Santo André lagoon. Results suggest that eel recruitment shows no evidence of a dramatic decline in Portugal, compared to trends reported for other European rivers. The dynamics of the eel population in continental waters was studied from a socio-economic and biological perspective in Santo André lagoon, where the eel yield is significant. The fishery statistics since the 1980’s and the field work conducted in 2011/12 and in 2015/16 have made possible to gather relevant information on the exploitation and biology and have provided the necessary data to characterize the local eel population. Data collected were used to calibrate a Bayesian state-space Integral Projection Model (SSIPM) that described the eel dynamics in the lagoon and estimated relevant eel biomass indicators. The eel population in the lagoon is male-skewed and characterized by a relatively young population (average age is 2.3 years), with only 13.7 % of individuals above 350 mm (13.7%). Eel growth in the lagoon is among the fastest reported for the species and explains the early onset of maturity in males, which metamorphose into silver eels, on average, at three years of age. The male dominance in the lagoon is most likely related to the high eel density estimates obtained with the SSIPM, ranging between 6 and 139 kg ha-1 in the years studied (2008-2017). The high productivity of the system supports one of the highest eel fishing yields in the species’ range, which represents a substantial share (50%) of local fishermen’s income. The increase in the catch per unit effort between 2006 and 2017 obtained from fisheries statistical data, was hypothesized to be a consequence of fishing effort reduction under the eel regulation, combined with an increase in natural recruitment. These results point out to the positive signs of the adaptive approach adopted in local fisheries management, although the eel regulation targets (silver eel biomass escapement) may be compromised. The closed nature of the lagoon translates into a late escapement of silver eels, which occurs only in the spring when the connection to the sea is artificially re-established, making them very vulnerable to capture by the fishery. Despite being an illegal activity, silver eel fishing occurs, showing that the management system must be improved. There are conditions to implement fisheries co-management in this socio-ecological system, and the estimated fishing yield and silver eel escapement under different management scenarios suggest that there are opportunities to improve the sustainability of the local European eel population. The SSIPM developed proved to be a useful tool to describe the European eel population in Santo André lagoon and can be used to support the evaluation of the Portuguese eel management plan, particularly in coastal brackish systems. The framework can be adapted to other eel habitats where eel abundance time-series are available, ideally incorporating information on sex ratio, growth, and length at silvering of the local population, since those life history traits are environmentally dependent.