Suppliers of Glass Eels for Farming and Restocking
Developing a
Sustainable Fishery
8. FAO global catch landings, which cannot be directly linked to population due to stocking and harvest effort (though scientific evidence supports this decline)… The statement underlined conflicts with the assumption that reconstructed FAO landings are a reliable proxy for stocks. Furthermore, scientific evidence in the form of actual data for stocks post-1970 does not match the declines in landings (Knights & Bonhommeau, 2008).
9. The recent decline in recruitment will translate into a future decline in adult stock, at least for the coming two decades (ICES 2006). This statement is not well supported by evidence from recruitment-stock relationships (Knights & Bonhommeau, 2008).
10. Noting the longevity of this species, and the extremely depleted state, restoration of the stock is expected to take several generations (Astrom and Dekker 2007), from 60 to >200 years depending on the protection level. These conclusions are not based on actual stock-recruitment or recruitment-stock data. Stock-recruitment relationships are unclear and evidence shows that recruitment and stocks vary widely over much shorter time scales (Knights & Bonhommeau, 2008). This conflicts with the assumption that Temporary increases (10-15 years) in abundance following the implementation of protective measures thus do not guarantee ultimate recovery, if not severely protecting the stock (Dekker pers. comm.).
11. Although a reliable population decline in mature individuals is not known, This statement conflicts with the definitive claims made elsewhere for ‘stock’ declines – and evidence is now available re. trends in populations of yellow and silver eels (see above)
12. It is inferred that there has been a decline of over 80% in the past three generations (60 years) based on the massive decline in recruitment (>95% in 24 years) and also supported by the decline (in) catch landings of 76% between 1968 and 2005. See criticisms above of these inferences and generalisations. Stocks have declined by ~35% from a peak in the early 1960s in Baltic Sea sites but to have been relatively stable since the mid-1970s (Knights & Bonhommeau, 2008). The Baltic is towards the edge of the range of the European eel and factors such as a lack of major inflows of sea water from the North Sea in recent decades may be associated with low recruitment.
13. re. three generations (60 years) – this is again based on the assumption of a long average generation time of ~20 years, whereas in NW Europe, males mature and emigrate at an average of ~7 years and females at ~11 years - and at younger ages from warmer Mediterranean habitats (e.g. see Tesch, 2003).
Range Description:
14. Stocked in most inland waters. This is far too broad a generalization.
15. Large parts of the population remain at sea (northwestern Atlantic and Mediterranean). Very probably true, and if so this counteracts the emphases in the citation on factors affecting the freshwater component of stocks.
Population:
16. Glass eels: since the early 1980's, a steady and almost continent wide decline of 90% has been observed in the recruitment of glass [juveline] eels (Dekker 2003), and according to ICES and FAO (2006) recruitment reached an historical low in 2001 of 1 to 2% of the pre-1980 level, it has not improved since and is an indication that the reproduction is seriously impaired and that the stock is severely depleted. See previous comments on ‘% declines’ and ‘the stock is severely depleted’.
17. Yellow/silver eels: ICES and FAO (2006) states that even though there is no analytical assessment of the state of [continental] European eel stock, all available information indicates that the stock is at a historical minimum in most of the distribution area and continues to decline. The statement underlined is incorrect, data and analytical assessments were presented to the Working Group on Eel in 2007 and 2008 by Knights & Bonhommeau.
18. all available information indicates that the stock is at a historical minimum in most of the distribution area and continues to decline. Incorrect, stock time series have shown wide fluctuations over decadal time scales and were relatively low in the early-1970s and in the early 1900s.
19. From However, even though catch effort…to the last line in the ‘Population’ section – this appears to say that landings data are not very reliable but goes on to discuss changes in fishery yields. The relevance of these points in the ‘Population’ section is unclear.
20. This is supported by the possibly only long-term scientific data [from Lake Ijsselmeer in the Netherlands] where there has been a gradual decline since 1960 (Dekker 2004a). The IJsselmeer is an unusual fishery, being an enclosed and heavily-exploited water body off the southern North Sea where recruitment would be expected be relatively low and where access for glass eels is difficult.
21. All European catches have decreased, possibly because the eel fishery was developed over this period. In Norway the catches seem to be stable. (ICES 2002). The meaning and relevance of both of these statements in relation to ‘Population’ is obscure.
22. As the recruitment rate is so low and declining the population is continuing to decline as older eels dissapear from the stock. See previous criticism of such statements
Habitat:
23. Estuarine, brackish and marine habitats aren’t mentioned(see 14 above)
Habitat & Ecology:
24. Glass-eels are observed in autumn on Portuguese coasts, in winter in North Sea and in spring. But appear all year round in some Mediterranean sites, e.g. in S France
25. The generation length of the species ranges from 2 to > 50 years, with a typical mean of 20 (Dekker pers comm.). Females are twice the size and age of males). Inaccurate estimate of typical mean generation time (see above) and of female v. male size and age differences.
Threats:
26. The North Atlantic Oscillation (NAO) may have reduced larval survival and/or growth rate (Castonguay et al. 1994). The NAO itself cannot have any effects, it is only an index of atmospheric pressure difference in the N Atlantic region which can be related to climatic factors, such as temperature, winds and precipitation. There is increasing evidence that these factors can have major effects on recruitment (Knights, 2003; Freidland et al., 2007; Bonhommeau et al., 2008a,b; Kettle et al., 2008a; Knights & Bonhommeau, 2008).
27. However, Dekker (2004b) shows that the NAO index correlation is strong for growth rate but weak for glass eel numbers (in 2000 the NAO index returned to normal but recruitment still declined) and Climate change may have an impact upon the breeding grounds (Sargasso Sea). Relationships with the NAO have indeed weakened in recent years, but significant negative correlations between recruitment and temperature and primary productivity in the Sargasso Sea have been demonstrated by Bonhommeau et al., 2008a,b). Significant negative correlations have also been demonstrated between recruitment and longer-term increases in the Northern Hemisphere Temperature index by Knights and Bonhommeau (2008)