The 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
