West Sutherland Elver Survey

West Sutherland Elver Survey 2008
densities are now favouring production of females and now predictions in 2006 based on
current stock (fig. 8) shows both yellow and silver eel fisheries are in serious continued
decline (Allen et al 2006).
Figure 8: Actual and predicted Silver eel output for Lough Neagh, N. Ireland (from Allen at al. 2006).
MONITORING ISSUES
Monitoring issues arise from: use of catch statistics without information on fishing effort to
estimate stock; collection of numbers of eels without lengths - missing any size structure
change; collection of total biomass without size structure (this is particularly relevant now
there is a better understanding of 2 phase dispersal of eels in river systems, as reduced
recruitment will cause eel abundance (not biomass) to decrease more in an upstream
direction whilst it may remain stable or decrease to a lesser extent down stream); and/or
sampling at bias positions that maintain numbers and size distributions long after changes in
the majority of the catchment, as the effects of reduced elver recruitment will not be easy
to identify down river as density remains constant with little pressure for any eel to move
up stream.
DETECTION LAG PERIOD
There appears to be a strong link between spawner (silver eel) escapement and recruitment,
with reduced recruitment adding to the existing problem of reduced stock levels. But,
because of eels long life cycle and relatively low mortality rates (20–70% during inland
stages according to sites and age groups), a delay of 10–20 years is observed between a
recruitment failure and the decline of a fishery (Moriarty and Dekker, 1997). In the case of
low mortality rates, there is even a biomass increase during 10 years following the
recruitment decrease (fig. 9). For these reasons scientists expect the recent (and ongoing)
21 West Sutherland Fisheries Trust