Conservation restocking of the imperilled European eel does not necessarily equal conservation

Abstract

To stop the decline of the European eel population, one of the measures taken is translocating eels for restocking, despite its conservational value being largely unknown. We aimed to contribute to this knowledge gap by (i) investigating the origin of eels caught in coastal waters of Estonia and Finland using otolith microchemistry and (ii) directly estimating restocked eel escapement from Narva River Basin District (NRBD), which is part of the primary Eel Management Unit in Estonia. In Estonia, 74% of the sampled eels (n = 140) were natural recruits and 26% were restocked. In Finland, 27% of the sampled eels (n = 235) were natural recruits and 73% were restocked. Only 1% of all the coastally collected eels were originally restocked to NRBD. These new data together with the reported commercial landings from the escapement route conflict with the current silver eel escapement estimation for NRBD and question the accuracy and value of such indirect calculations compiled for most Eel Management Units throughout the European Union. It is concluded that restocking eels to freshwaters may be futile as a conservation measure in some situations, and better escapement is likely achieved in restocking eels to coastal waters or undammed freshwater systems with a direct connection to the sea.

Habitat use and growth of yellow-stage European eel in coastal and freshwater ecosystems in Norway

“…The prevalent chemical marker used in eel otolith microchemistry studies is the strontium to calcium ratio (Sr:Ca) (e.g., Tsukamoto et al 1998;Arai et al 2019), which usually displays a positive relationship with salinities up to 20 PSU (Tabouret et al 2010). Complementary use of Sr:Ca and the barium to calcium ratio (Ba:Ca) (which is inversely related to salinity) could be valuable, but are less common (Tabouret et al 2010;Daverat et al 2011;Rohtla et al 2021), because Ba has a nutrient-like profile in the water column (Coffey et al 1997) and its application is, therefore, less straightforward in estuarine areas (Jessop et al 2012).…”

Variability in stream water chemistry and brown trout (Salmo trutta L.) parr otolith microchemistry on different spatial scales

“…seasonal) variation in water chemistry and study which elements have the highest discriminating power among different waterbodies, (ii) test the relationship between water chemistry and otolith microchemistry and (iii) explore whether and to what extent S. trutta parr from different sea trout spawning streams and hatcheries (sensu stricto) and regions (sensu lato) can be distinguished from one another. We predicted that due to the differences in the composition of the bedrock between the study regions (Löfvendahl et al, 1990) and based on previous studies (Rohtla et al, 2017(Rohtla et al, , 2021, these regions would also be separable based on the collected water samples, and as such, these differences would be reflected in S. trutta otoliths at least on a regional level. We further predicted that hatchery-origin S. trutta are distinguishable from wild S. trutta based on their otolith chemical composition.…”

Utvärdering av ålyngelutsättningar – Svenska väst- och sydkustområden

“…Den här studien visar att det har fångats få utsatta ålar jämfört med naturligt ankommande ål, så kallade naturliga rekryter. Den totala andelen omärkta, naturliga rekryter på 91 % visar dessutom på en högre andel naturliga rekryter i övervakningsfisket på väst-och sydkusten än vad som tidigare har observerats i studier utförda i länder runt om Östersjön (Limburg et al 2003;Shiao et al 2006;Sjöberg et al 2017;Rohtla et al 2021). Naturliga rekryter, och unga gulålar, tros först anlända till den svenska västkusten för att sedan ta sig vidare längs med kusten och upp i vattendrag och Östersjön (Andersson et al 1991) En förklaring till att inga utsatta ålar påträffas på majoriteten av elfiskelokalerna, trots flertalet utsättningar i västra och södra Sveriges vatten, kan vara att många av dessa utsättningar har skett i sjöar.…”

“…Dock kan det bero på slumpmässiga förhållanden då det inte har fångats så många ålar på denna lokal. I Finland har däremot en liknande situation med högre andel utsatta ål än naturligt rekryterade ål observerats i fångsten vilketRohtla et al (2021) anser beror på att fisket där gjordes i ett område där det även gjorts utsättningar. Detta eftersom tidigare studier har visat tendenser på att utsatta ål stannar kvar i det område de sätts ut(Shiao et al 2006).…”