Environmental Engineering Reference
In-Depth Information
in 2004 and 2005. Up to age 4
, roach were smaller than suggested by the
Hickley Growth Standard (Hickley & Dexter
1979
), which may indicate stunting
through increased competition for food and spawning habitat. Of particular
interest is that a consistent rate of growth (denoted by the slope of the line) of
these younger fish was sustained, possibly due to the ample food supply, such
that age 5
þ
fish were larger than the standard would suggest. It is important to
note that the sample size of scaled fish in 2006 was smaller (n
þ
¼
17) than
previous years (in 2003 n
53), which may affect the overall shape of the line.
Nevertheless, the current trends in fish growth are likely to continue until the
community stabilises and ultimately, if it is assumed that all individuals
exhibit similar growth characteristics albeit with some degree of variability,
the growth curve superimposes the Hickley Growth Standard (Hickley & Dexter
1979
). A similar situation occurred within the comparatively clean waters
of Salford Quays in 1999 following years of water quality improvements and
10 years after the introduction of the first fish. It is likely that long-term
improvements in WwTW infrastructure through the AMP process would lead
to a similar trend in the MSC, albeit over an extended time frame, although the
oxygenation scheme has undoubtedly accelerated the rate of improvement in
condition. As a measure of the achievement to date, also given in
Fig. 14.12
,is
the curve derived from roach within the River Thames during the 1960s
(Williams
1967
) which shows markedly slower growth rates.
The sewage-derived effluents continuing to enter the MSC contain endocrine
disrupting chemicals largely originating from anthropogenic oestrogen (the
natural steroids oestrone and 17
b
-oestradiol) and synthetic steroids (ethinyles-
tradiol), which can cause sex reversal of fish. This is an increasing problem in
many inland waters running through heavily populated areas and leads to the
feminisation of the fish population. Conlan et al.(
2006
) provides a comprehen-
sive review of the subject and includes results from a study investigating
intersex in roach and perch in the MSC and Salford Quays in 2000 (Nash et al.
2003
). Results showed that the sex ratio in roach was biased towards females in
the MSC and Salford Quays, though to a far lesser extent in the latter (Conlan
et al.
2006
). In addition, the incidence of intersex revealed that this condition
was twice as common in males from the MSC as for those from Salford Quays.
However, the discovery of intersex individuals in Salford Quays requires fur-
ther investigation given that these waters have received no sewage derived
inputs since isolation 20 years ago. It was suspected that potentially oestro-
genic compounds may be present in the sediments originating prior to the
isolation. The high level of intersex in the MSC is comparable with studies of
roach in other polluted waterways (e.g., Jobling et al.
1998
). It was postulated
that the level of intersex and the highly skewed sex ratio may be due to oestro-
genic effects of chemicals in the MSC (Conlan et al.
2006
). The study was the first
to identify intersex in male perch; in the MSC 100% of males had ovotestes.
¼
