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The European sturgeon (Acipenser sturio)
In Western Europe, the last remaining wild population of the European sturgeon (Acipenser sturio) is listed in several international conventions and directives as critically endangered (Rochard 2011) (Table 2). Thus, the species is strictly protected and a subject of the European recovery strategy (Rosenthal et al., 2007) along with national action plans in France (MEDDTL 2011) and Germany (Gessner et al., 2010b). In the eighteenth century the species had almost pan-European distribution and spawning was occurring in 196 basins (Lassalle et al., 2011) (Figure 1). By 1850 the species was present in 26 basins and by 1950 only in 18 (Lassalle et al., 2011). Currently, there is only one functional population in the Gironde basin in France and its two last reproductions in the wild were reported in 1988 and 1994 (Lochet et al., 2004). The European sturgeon is a long-lived and slow-growing species that can reach 3.50m in length (Magnin 1962; Lepage and Rochard 1995). The wild population effective size is estimated to be around 20–750 individuals as consequence of habitat loss and overexploitation that reduced the population to 10% (IUCN 2015). The age at first maturation is estimated to be around 10–12 years for males and 14–15 years for females and spawning interval is reported between two and three to four years for males and females, respectively (Magnin 1962; Rochard and Jatteau 1991). In virtue of these extreme life history traits common in sturgeons: large species, late age at the first maturation, long spawning intervals, low density and endangered status (Jager et al., 2008), studies and manipulation under control and in situ conditions are not straightforward. But, this species is a perfect model to study behavior and ecology under the reintroduction programs context.
Monitoring the European sturgeon
The European sturgeon restoration program uses several monitoring approaches to assess the effectuation of the restoration plan: incidental capture declaration, standardized scientific monitoring in the estuary and specific studies (MEDDTL 2011; Acolas et al., 2016). For incidental capture, the program encourages the fishermen and the general population to declare sturgeon catches through awareness campaigns carried out by the fisheries association and a close partnership between managers and scientists to collect and analyze data (Rochard et al., 1997; Acolas et al., 2011a; Acolas et al., 2016; MEDDTL 2011). These data provide insights on fish distribution and habitat utilization which are consistent with the species life cycle (Acolas et al., 2016). To monitor the estuarine population since the recent stocking, trawling campaigns have been used since 2009 (Acolas et al., 2011b) on the same line as the historical protocol that was carried out for the wild population (Rochard et al., 2001). The protocol is standardized, and the sampling occurs once every two months throughout the year in the mesohaline area of the estuary. Each sturgeon captured is measured, weighed and fin samples are taken for age estimation (Rochard and Jatteau 1991; Jatteau et al., 2011), parental assignation (Roques et al., 2016) and diet is analyzed through non-invasive methods (Brosse et al., 2002; Vega 2016). Besides, each fish is tagged externally (Hallprint tags), to ease recognition by fishermen, and internally (PIT tags) (Acolas et al., 2016). This specific monitoring allows evaluating health state of the estuarine population, demographic parameters, life history traits and to estimate stocking efficiency. The estuarine population is mainly composed of 2−3 years old fish (age range 1−7 years, mean total length 71.4cm) and the preliminary annual survival estimation at the age of two years old range between 5.5−8.9% for 2007 to 2009 cohorts (mainly released at the age of 3 months) and between 0.8−3.3% for 2011−2013 cohorts (released as larvae and 3 months old fish) (Acolas 2017, pers. comm). In the estuary the population density and spatial distribution has shown an increasing trend since 2014. Specific studies on European sturgeon aim at better understanding the species life history, the effect of environmental factors on their ecology and to evaluate the ex-situ practices. Recent studies include research in hypoxia and pollutant’s tolerance in sturgeon of young stages (Delage 2015) , migration patterns and habitat utilization of 1 year-old stocked fish (Acolas et al., 2012; Acolas et al., submitted), diet specificity and estuarine carrying capacity (Vega 2016), species repositioning within the climatic change scenario (Lassalle et al., 2010), phylogeography of the species (Chassaing et al., 2016) and experimental releases of older juveniles in the watershed where the species has disappeared were tested (Brevé et al., 2014). The ex-situ practices are assessed through the genetic characterization of the broodstock to optimize mate crossings (Roques et al., 2015) and to develop genetic markers for parental assignment of the recaptured individuals (Roques et al., 2016) as well as by analyzing the individual capacity of the juveniles produced (Acolas and Gesset 2014).
The stocking program
Since the 1920s, the European sturgeon was exploited for flesh and caviar while its habitats were receding due to water dam constructions and gravel extraction in the Dordogne and Garonne Rivers (Rochard et al., 1990; Williot et al., 1997). By the 1970s, the species has disappeared from all the West Europe except France (Williot et al., 1997; Williot et al., 2009b) losing its commercial importance (Williot et al., 1997). Protection and conservation concerns lead to a total fishing, transport and trade ban in 1982 in France (Williot et al., 2009b). At the same time natural life history research on wild sturgeon and on artificial reproduction for stocking purposes started. The creation of a French captive stock started in the 1993, and the first successful artificial reproduction and larvae stocking was accomplished in 1995 (Williot et al., 2009b). Since then, the captive stock has been formed mainly by wild origin individuals from 1970s−1994 cohorts and hatchery produced fish of 1995 (Williot et al., 2009b). This broodstock has been supporting the wild sturgeon population by frequent larvae and juvenile stockings since 2007 (Acolas et al., 2016) (Figure 3). Up to date (January 2017), more than 1.5 million fish originated from assisted reproduction have been released into historical spawning grounds (Jego et al., 2002) in the Dordogne and Garonne rivers (Acolas et al., 2016). Most of the fish released are 7 dph larvae and 3-month old juveniles. Stocking of 1−2 years old fish and older is also done in lesser proportion to spread the mortality risk between different age classes (in total 3430 individuals between 2008 and 2015). Currently, the captive stock is composed of 6 females, 15 males and about 300 juveniles; the spawners number has drastically decreased in the last 4 years (39 females and 33 males in 2012 (Acolas 2013)). In addition to the French captive stock, another is held in Germany (Gessner et al., 2010a; Kirschbaum et al., 2011), which was built on hatchery reared individuals produced in France. The German stock has 6 broodstock fish of 1995 and about 808 juveniles (Gessner 2017, pers. comm). With the aim of increasing the broodstock, individuals obtained from the different genetic crossings that are produced every year are kept and incorporated into the French and German captive stocks (Acolas 2013; Jatteau 2014).
The species is anadromous. Reproduction occurs between May and June in rivers where it has been suggested that females mate with numerous males. Adults, return to the sea by the end of July (Magnin 1962; Castelnaud et al., 1991; Acolas et al., 2011a). The species early life stages are poorly documented but juveniles migrate to the estuary within the first year of life for further growth (Rochard et al., 2001; Acolas et al., 2011a). The estuarine phase is well documented (Taverny et al., 2002; Brosse 2003); after the second year of life, fish move seasonally between the estuary and the sea until they reach 3-7 years old to and then they become permanent sea residents (Castelnaud et al., 1991; Acolas et al., 2011a). Thus, most of the growth and the beginning of maturation take place at sea before they return to the rivers to spawn (Acolas et al., 2011a) (Figure 2).
Table of contents :
1.1. Biodiversity and conservation
1.2. Stocking programs
1.3. Rearing practices
1.3.2. Artificial selection, natural selection in captivity and relaxation of natural selection
1.3.3. The alternative approaches: pre-release training and environmental enrichment
1.4. Post-release monitoring
1.5. The European sturgeon (Acipenser sturio)
1.5.1. Study model context
1.5.2. Life cycle
1.5.3. The stocking program
1.5.4. Monitoring the European sturgeon
1.6. Thesis outline
2. European sturgeon (Acipenser sturio L.) young of the year performance in different rearing environments –study within a stocking program
3. Tracking juvenile sturgeon in the wild: minuature tag effects assessment in a laboratory study on Siberian sturgeon (Acipenser baerii)
4. Effects of rearing practices on the post-release behavior of European sturgeon (Acipenser sturio) young of the year
5. Discussion and perspectives
5.1. Summary results
5.3. Recommendations and perspectives