Daniele Bevacqua

Genetic patchiness in European eel adults evidenced by molecular genetics and population dynamics modelling.

Disentangling the demographic processes that determine the genetic structure of a given species is a fundamental question in conservation and management. In the present study, the population structure of the European eel was examined with a multidisciplinary approach combining the fields of molecular genetics and population dynamics modelling. First, we analyzed a total of 346 adult specimens of known age collected in three separate sample sites using a large panel of 22 EST-linked microsatellite loci. Second, we developed a European eel-specific model to unravel the demographic mechanisms that can produce the level of genetic differentiation estimated by molecular markers. This is the first study that reveals a pattern of genetic patchiness in maturing adults of the European eel. A highly significant genetic differentiation was observed among samples that did not follow an Isolation-by-Distance or Isolation-by-Time pattern. The observation of genetic patchiness in adults is likely to result from a limited parental contribution to each spawning event as suggested by our modelling approach. The value of genetic differentiation found is predicted by the model when reproduction occurs in a limited number of spawning events isolated from each other in time or space, with an average of 130-375 breeders in each spawning event. Unpredictability in spawning success may have important consequences for the life-history evolution of the European eel, including a bet-hedging strategy (distributing reproductive efforts over time) which could in turn guarantee successful reproduction of some adults.

No apparent genetic bottleneck in the demographically declining European eel using molecular genetics and forward-time simulations

The stock of the European eel is considered to be outside safe biological limits, following a dramatic demographic decline in recent decades (90–99% drop) that involves a large number of factors including overfishing, contaminants and environmental fluctuations. The aim of the present study is to estimate the effective population size of the European eel and the possible existence of a genetic bottleneck, which is expected during or after a severe demographic crash. Using a panel of 22 EST-derived microsatellite loci, we found no evidence for a genetic bottleneck in the European eel as our data showed moderate to high levels of genetic diversity, no loss of allele size range or rare alleles, and a stationary population with growth values not statistically different from zero, which is confirmed by finding comparable value of short-term and long-term effective population size. Our results suggest that the observed demographic decline in the European eel did not entail a genetic decline of the same magnitude. Forward-time simulations confirmed that large exploited marine fish populations can undergo genetic bottleneck episodes and experience a loss of genetic variability. Simulations indicated that the failure to pick up the signal of a genetic bottleneck in the European eel is not due to lack of power. Although anthropogenic factors lowered the continental stock biomass, the observation of a stable genetic effective population size suggests that the eel crash was not due to a reduction in spawning stock abundance. Alternatively, we propose that overfishing, pollution and/or parasites might have affected individual fitness and fecundity, leading to an impoverished spawning stock that may fail to produce enough good quality eggs. A reduced reproduction success due to poor quality of the spawners may be exacerbated by oceanic processes inducing changes in primary production in the Sargasso Sea and/or pathway of transport across the Atlantic Ocean leading to a higher larval mortality.

An integrated genetic-demographic model to unravel the origin of genetic structure in European eel (Anguilla anguilla L.)

The evolutionary enlightened management of species with complex life cycles often requires the development of mathematical models integrating demographic and genetic data. The genetic structure of the endangered European eel (Anguilla anguilla L.) has been thoroughly analyzed in several studies in the past years. However, the interpretation of the key demographic and biologic processes that determine the observed spatio‐temporal genetic structure has been very challenging owing to the complex life cycle of this catadromous species. Here, we present the first integrated demographic‐genetic model applied to the European eel that explicitly accounts for different levels of larval and adult mixing during oceanic migrations and allows us to explore alternative hypotheses on genetic differentiation. Our analyses show that (i) very low levels of mixing occurring during larval dispersal or adult migration are sufficient to erase entirely any genetic differences among sub‐populations; (ii) small‐scale temporal differentiation in recruitment can arise if the spawning stock is subdivided in distinct reproductive groups; and (iii) the geographic differentiation component might be overestimated if a limited number of temporal recruits are analyzed. Our study can inspire the scientific debate on the interpretation of genetic structure in other species characterized by complex life cycle and long‐range migrations.

Size selectivity of fyke nets for European eel Anguilla anguilla

Size selectivity of fyke nets for European eels Anguilla anguilla was investigated by reviewing the results of published experimental studies. A general size selectivity model was then derived that can be easily incorporated into demographic models to simulate population dynamics, assess and monitor abundance and length structure of eel stocks and forecast the consequences of different management options.

Density-dependent and inter-specific interactions affecting European eel settlement in freshwater habitats

Identifying the factors influencing the settlement of European eel (Anguilla anguilla) juveniles in continental habitats is crucial to designing effective management and conservation measures for this endangered species. A long-term data series (1993–2008) of European eel and European catfish (Silurus glanis) abundance in a freshwater canal of the Camargue water system (southern France), along with parallel data on water salinity and glass eel abundance in the adjacent Vaccarès lagoon, was analysed to identify the possible causes of decline in eel abundance observed in the canal during the last two decades. A model including glass eel recruitment and catfish abundance as covariates explained 78% of the observed variation in eel settlement success. Results suggest that (1) salinity does not play a significant role in determining the fraction of eels moving from the brackish lagoon to the canal; (2) density dependence affects settlement success, possibly through a reduction of juvenile survival in the adjacent lagoon; and (3) catfish abundance is negatively correlated with eel settlement. We discuss this latter point in terms of possible predation of catfish upon eels and/or inter-specific competition between the two species.

Allometric scaling of mortality rates with body mass in abalones

The existence of an allometric relationship between mortality rates and body mass has been theorized and extensively documented across taxa. Within species, however, the allometry between mortality rates and body mass has received substantially less attention and the consistency of such scaling patterns at the intra-specific level is controversial. We reviewed 73 experimental studies to examine the relationship between mortality rates and body size among seven species of abalone (Haliotis spp.), a marine herbivorous mollusk. Both in the field and in the laboratory, log-transformed mortality rates were negatively correlated with log-transformed individual body mass for all species considered, with allometric exponents remarkably similar among species. This regular pattern confirms previous findings that juvenile abalones suffer higher mortality rates than adult individuals. Field mortality rates were higher overall than those measured in the laboratory, and the relationship between mortality and body mass tended to be steeper in field than in laboratory conditions for all species considered. These results suggest that in the natural environment, additional mortality factors, especially linked to predation, could significantly contribute to mortality, particularly at small body sizes. On the other hand, the consistent allometry of mortality rates versus body mass in laboratory conditions suggests that other sources of mortality, beside predation, are size-dependent in abalone.

Genetic variability is unrelated to growth and parasite infestation in natural populations of the European eel (Anguilla anguilla)

Positive correlations between individual genetic heterozygosity and fitness‐related traits (HFCs) have been observed in organisms as diverse as plants, marine bivalves, fish or mammals. HFCs are not universal and the strength and stability of HFCs seem to be variable across species, populations and ages. We analysed the relationship between individual genetic variability and two different estimators of fitness in natural samples of European eel, growth rate (using back‐calculated length‐at‐age 1, 2 and 3) and parasite infestation by the swimbladder nematode Anguillicola crassus. Despite using a large data set of 22 expressed sequence tags‐derived microsatellite loci and a large sample size of 346 individuals, no heterozygote advantage was observed in terms of growth rate or parasite load. The lack of association was evidenced by (i) nonsignificant global HFCs, (ii) a Multivariate General Linear Model showing no effect of heterozygosity on fitness components, (iii) single‐locus analysis showing a lower number of significant tests than the expected false discovery rate, (iv) sign tests showing only a significant departure from expectations at one component, and, (v) a random distribution of significant single‐locus HFCs that was not consistent across fitness components or sampling sites. This contrasts with the positive association observed in farmed eels in a previous study using allozymes, which can be explained by the nature of the markers used, with the allozyme study including many loci involved in metabolic energy pathways, while the expressed sequence tags‐linked microsatellites might be located in genes or in the proximity of genes uncoupled with metabolism/growth.

A global viability assessment of the European eel

The global European eel (Anguilla anguilla) stock is critically endangered according to the IUCN, and the European Commission has urged the development of conservation plans aimed to ensure its viability. However, the complex life cycle of this panmictic species, which reproduces in the open ocean but spends most of its prereproductive life in continental waters (thus embracing a huge geographic range and a variety of habitat types), makes it difficult to assess the long-term effectiveness of conservation measures. The interplay between local and global stressors raises intriguing cross-scale conservation challenges that require a comprehensive modelling approach to be addressed. We developed a full life cycle model of the global European eel stock, encompassing both the oceanic and the continental phases of eels life, and explicitly allowing for spatial heterogeneity in vital rates, availability of suitable habitat and settlement potential via a metapopulation approach. We calibrated the model against a long-term time series of global European eel catches and used it to hindcast the dynamics of the stock in the past and project it over the 21st century under different management scenarios. Although our analysis relies on a number of inevitable simplifying assumptions and on data that may not embrace the whole range of variation in population dynamics at the small spatiotemporal scale, our hindcast is consistent with the general pattern of decline of the stock over recent decades. The results of our projections suggest that (i) habitat loss played a major role in the European eel decline; (ii) the viability of the global stock is at risk if appropriate protection measures are not implemented; (iii) the recovery of spawner escapement requires that fishing mortality is significantly reduced; and (iv) the recovery of recruitment might not be feasible if reproductive output is not enhanced.

Fishery-Induced Selection for Slow Somatic Growth in European Eel

Both theoretical and experimental studies have shown that fishing mortality can induce adaptive responses in body growth rates of fishes in the opposite direction of natural selection. We compared body growth rates in European eel (Anguilla anguilla) from three Mediterranean stocks subject to different fishing pressure. Results are consistent with the hypotheses that i) fast-growing individuals are more likely to survive until sexual maturity than slow-growing ones under natural conditions (no fishing) and ii) fishing can select for slow-growing individuals by removing fast-growing ones. Although the possibility of human-induced evolution seems remote for a panmictic species like such as the European eel, further research is desirable to assess the implications of the intensive exploitation on this critically endangered fish.

Nitrogen and water supplies affect peach tree–green peach aphid interactions: the key role played by vegetative growth

The availabilities of water and nitrogen resources are among the main abiotic factors modifying plant physiology and growth. Consequently, via bottom‐up processes, they also can have an impact on the performance of herbivorous insects. However, the extent to which the modification of plant growth is responsible for such an impact remains unclear. We conducted a factorial experiment quantifying Myzus persicae aphid abundance and the vegetative growth of Prunus persica peach tree shoots under contrasting levels of nitrogen and water supplies. We used a hierarchical analysis of multiple regression models to determine whether the consequences of the availability of nitrogen and water on aphid abundance could be the result of a modification of plant growth. Maximum aphid abundance was achieved under nitrogen and water comfort conditions. The best model explaining variance in aphid abundance took into account vegetative growth and water supply, as well as their interaction. The results of the present study suggest that a higher nitrogen supply increases aphid abundance by fostering plant growth. Additionally, the positive response of aphid abundance to vegetative growth is lower in the case of water restriction because, under such conditions, aphids cannot take full advantage of tree vigour. Such a result provides new insights into aphid control in agriculture, as well as on the possible effects of climate change.