As a key parameter in the management of fish populations, individual growth rate (GR) variations were examined in the European eel (Anguilla anguilla; > 150 mm) using extensive mark-recapture surveys in the lotic habitats of two small rivers of western France: the Fr emur, supposed to be saturated, and at the same latitude, the Oir with densities fivefold lower than those of the Fr emur. In both systems, generalised linear models were used to test whether spatiotemporal factors such as dominant habitat type or local density affect GR variability. In the presumed unsaturated system, the Oir, GR variability is mainly explained by a set of habitat suitability drivers (density, dominant habitat type). In the Fr emur, GRs appear independent of differences in habitat density or productivity. Below saturation, an increase in density will decrease the GR through intraspecific competition. At saturation, intraspecific competition reaches such high levels that regardless of eel density and productivity, the resources available by individual are similar throughout the system. In these circumstances, the effect of density on growth was presumed undetectable. Despite these contrasted results, mean GRs observed in both catchments were closed (~20 mmÁyear À1 ). This is an unexpected result as GR is expected to be higher in unsaturated systems. This similarity could be explained by the difference between the two systems in terms of: (i) sex ratio (the Fr emur is dominated by male, whereas the female is dominant in the Oir), (ii) habitat type distribution or (iii) possible interspecific competition (important salmonid populations in the Oir).

Summary Invasive species may have strong negative impacts on ecosystems. Invasive crayfish are well known to cause ecological imbalances in freshwater systems and have become targets in eradication programmes. The red swamp crayfish (Procambarus clarkii) is vulnerable to predation by several fish species. The European eel (Anguilla anguilla) appears to be a very good candidate given its benthic feeding activity and ability to recognise prey by odour. Our study took place from 2009 to 2013 in a closed system in south‐eastern France in which ponds were sampled twice a year by passive capture methods to evaluate trends in populations of potential prey. Eel population dynamics were assessed by means of a multistate capture–recapture model, and diet assessment was performed using stable isotope analysis in October 2010 and October 2012. Our results show that the invasive crayfish was the most important prey in the eel diet, and every size‐class of crayfish was preyed upon by the predator. A high predator density, approaching carrying capacity for the European eel in this system, strongly affects P. clarkii populations, leading them to their collapse, but not eradication.

Wild fish populations are currently experiencing unprecedented pressures, which are projected to intensify in the coming decades. Developing a thorough understanding of the influences of both biotic and abiotic factors on fish populations is a salient issue in contemporary fish conservation and management. During the 50th Anniversary Symposium of The Fisheries Society of the British Isles at the University of Exeter, UK, in July 2017, scientists from diverse research backgrounds gathered to discuss key topics under the broad umbrella of 'Understanding Fish Populations'. Below, the output of one such discussion group is detailed, focusing on tools used to investigate natural fish populations. Five main groups of approaches were identified: tagging and telemetry; molecular tools; survey tools; statistical and modelling tools; tissue analyses. The appraisal covered current challenges and potential solutions for each of these topics. In addition, three key themes were identified as applicable across all tool-based applications. These included data management, public engagement, and fisheries policy and governance. The continued innovation of tools and capacity to integrate interdisciplinary approaches into the future assessment and management of fish populations is highlighted as an important focus for the next 50 years of fisheries research.

1. Estimating accurate age-specific survival probabilities and understanding the processes (density dependent or independent) that regulate this demographic parameter are fundamental to propose sustainable management options for the endangered European eel (Anguilla anguilla). 2. In the present study, we used extensive mark-recapture data sets (13 and 17 years) on eel >150 mm to estimate accurate natural survival probabilities of two eel sub-populations, from the Oir and Fr emur rivers, western France, and then we analysed survival probabilities in relation to density-independent (temperature) and density-dependent factors to identify those causing survival variability. 3. The Fr emur and Oir rivers are two small (<100 km 2 ) river systems in close proximity (65 km apart). The Fr emur River is a small river obstructed by dams with medium water quality, high eel recruitment and density (0.37 eel m À2 ), and a male dominant population; whereas, in the Oir River, low densities of eels were observed (0.04 eel m À2 ) and the sex ratio was skewed towards females. Furthermore, previous research suggested that the lotic habitats of the Fr emur River have reached carrying capacity, whereas habitats in the Oir River are below habitat saturation. 4. In the Fr emur River, there were significant spatial and temporal variations in the survival probabilities. However, survival probabilities observed in the Oir River were stable over time and space. The results highlight that the differences in the characteristics of the two systems and the two sub-populations prompt different responses to regulatory processes. 5. The contrasting pressures applied on these two sub-populations impact survival, which possibly lead to different life history strategies such as sex differentiation.