Survival and growth of European eels stocked as glass‐ and farm‐sourced eels in five lakes in the first years after stocking

Abstract

European eels Anguilla anguilla stocked as wild-sourced glass eels showed a better overall performance of growth and survival compared with farm-sourced eels after stocking in five isolated lakes within a 7-year study period. Eels stocked as farm eels lost their initial size advantage over eels stocked as glass eels within 3-5 years after stocking. Population sizes estimated for consecutive stocking batches indicated that 8-17% of eels stocked as farm eels survived 3-6 years after stocking compared with 5-45% of eels stocked as glass eels. This study coupled with results of previous studies suggests that stocking of farm eels may have no advantage in growth and survival compared with stocking of glass eels if stocking occurs at an optimal time in spring. In addition, the use of relatively expensive farm eels may provide no general advantage over stocking of glass eels. However, if glass eels are only available for stocking purposes very early in the year, lower survival rates than obtained in the present study can be assumed and stocking with relatively more expensive farm eels could possibly be a better option.

Short-term responses of glass eels transported from UK to small Belgian streams

“…The low recapture rate is a general problem on glass eel studies (Simon and Do¨rner, 2014) and the catch rate rises with increasing age and size (Naismith and Knights, 1990;Lambert et al, 1994;Simon et al, 2013). The number of stocked glass eels recaptured by electrofishing 1 year following stocking at elver stage was low in the three streams probably due to this issue of small amount of glass eel stocked, the size of the individuals (median length, 9 cm) and the low efficiency of electric fishing for this life stage.…”

“…Stocking is an old practice conducted in Germany since around 1900 (Walter, 1910;Lu¨bbert, 1923) and later in other European countries (Moriarty and McCarthy, 1982;Andersson et al, 1991;Wickstro¨m et al, 1996;Simon and Do¨rner, 2014). Tributaries, lakes and ponds with low or no natural immigration have been stocked using glass eels and elvers caught in river estuaries (Walter, 1910;Mu¨ller, 1975;Klein-Breteler et al, 1990;Simon et al, 2009) and farm eels (Wickstro¨m et al, 1996;Pedersen, 1998Pedersen, , 2000Pedersen, , 2009Simon et al, 2013).…”

“…Tributaries, lakes and ponds with low or no natural immigration have been stocked using glass eels and elvers caught in river estuaries (Walter, 1910;Mu¨ller, 1975;Klein-Breteler et al, 1990;Simon et al, 2009) and farm eels (Wickstro¨m et al, 1996;Pedersen, 1998Pedersen, , 2000Pedersen, , 2009Simon et al, 2013). The annual length increases and survival rates in the stocked eels vary strongly between the stocking environments, the stocking materials (farm and glass eels), the stages (smaller and larger eels) and times after stocking (Bisgaard and Pedersen, 1991;Pedersen, 1998;Lin et al, 2007;Simon et al, 2013;Simon and Do¨rner, 2014). In stocked (Pedersen, 2009;Simon et al, 2013) and non-stocked (Naismith and Knights, 1990;Lambert et al, 1994) eels, the recapture rates improved with increasing age and size.…”

Age and growth of European eels (Anguilla anguilla) in the Elbe River system in Germany

“…In addition to environmental variables, growth of eels is furthermore influenced by their early life history as they can either naturally immigrate into a certain water body or were stocked (Pedersen, 2000;Simon and Dörner, 2014;Couillard et al, 2014).…”

A question of scale: Replication and the effective evaluation of conservation interventions

“…However, making the comparison presumes that there are wild-hatched juveniles in the population, which may not be the case when natural recruitment is effectively zero (e.g., Ontario Wood Turtle Recovery Team 2010) or when re-establishing populations within a historical range (e.g., Amaral 2007). Furthermore, headstarted and wild-hatched individuals must be distinguishable in the field, e.g., through non-harmful marking such as passive integrated transponders (Gibbons and Andrews 2004), visible implant elastomer (Davy et al 2010;Antwis et al 2014;Simon and Dörner 2014;Kozłowski et al 2017), or other tools (Auger-Méthé and Whitehead 2007;Parker et al 2013;Schoen et al 2015). For example, short-term survival rates have been compared in wild and headstarted plains garter snakes (Thamnophis radix) of similar size (King and Stanford 2006), Mona Island iguanas (Cyclura cornuta stejnegeri; Pérez- Buitrago et al 2008), and European pond turtles (Emys orbicularis; Mitrus 2005).…”