SGS Product & Process Certification - Marine Stewardship Council

Figure 3.Indices of the abundance of juvenile bass year-classes in the Thames Estuary (IVc),
1994-2008 (source CEFAS).
Fishery-independent estimates of relative abundance of juvenile bass year classes are also
available from a trawl survey (actually using a 3-m beam trawl to sample pre-recruit flatfish)
conducted in the Westscheldt Estuary (IVc) by IMARES each year, Figure 4 (source Henk
Heessen, WGNEW). These also show an increasing trend in year class strength since the
early 1990s, though the pattern is not the same as that in the Thames Estuary survey, probably
because different age groups are used in calculating the indices (0-groups especially can suffer
high mortality in cold winters).
N per hour
120
100
80
60
40
20
Figure 4. Indices of the abundance of juvenile bass year classes in the Westscheldt Estuary
from the IMARES DFS (source Henk Heessen, WGNEW).
3.2 Stock status
Sea bass Westerscheldt
0
1970 1975 1980 1985 1990 1995 2000 2005 2010
The results of the fleet-based separable model for the sea bass stock unit (Divisions IVb,c) that
includes the southern North Sea indicate levels of F around 0.2–0.4 between 1995 and 2004
(Pawson et al., 2007b). The gear selectivity patterns derived from the model confirm the results
of Pawson et al. (2005).The abundance of adult bass (SSB) in the North Sea appears to have
remained stable between 1985 and 2004, whilst there has been an increased level of recruitment
from 1989 onwards.
The patterns in F and recruitment derived from this model are consistent with the evidence
provided by the CEFAS logbook scheme and the fishery-independent pre-recruit survey (which
has not been used in the model), but there are inevitable problems with the scaling of F and
estimating biomass. In particular, the absence of data for the recreational rod-and-line fishery,
a substantial contributor to mortality on the younger sea bass, would affect estimates of
recruitment to the commercial fishery. However, these are consistent between areas over the
period 1984–2002, and that for VIId closely matches the fishery-independent abundance
indices for pre-recruit bass provided by the Solent trawl survey that covers the main nursery
areas for sea bass in Division VIId. Thus, whilst the model reflects reasonably the dynamics of
the bass stock (around the UK) both in stock numbers, SSB and in F, it is not possible to
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