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individuals were always observed in this zone of high impact. How then can the presence
in 1992 of small
N. lapillus
populations near the civil and military harbors of Brest (Huet et
al. 1996) be explained? Was there a progressive migration from external areas to the heav-
ily affected zone, or did in some way resistance develop to the devastating TBT effects? A
similar event was encountered at Dumpton Gap (UK) by Gibbs (1993) and in Galicia (Spain)
by Barreiro et al. (1999). It was called Dumpton syndrome and proven to be a genetically
based imposex resistance (Gibbs 2005). The possible presence of this phenomenon must be
appreciated when determining imposex intensity measures (resistant individuals are not
suitable bioindicators and must be omitted from the calculation of imposex indices).
9.1.4.2 Case Study: Sullom Voe, Shetland
The oil terminal based at Sullom Voe in Shetland has received large volumes of liquid
petroleum gas and crude oil tanker traffic since 1978. Sullom Voe Association, through the
Shetland Oil Terminal Environmental Advisory Group (SOTEAG), has funded long-term
monitoring of the state of the environment surrounding the terminal, including both sur-
veys of rocky shore community structure and (after the discovery of imposex in dogwhelks
in the area in the early 1980s) effects of TBT (Gubbins 2009). This now represents one of the
longest time series of TBT effects recording with data from 1987 to 2011, and clearly demon-
strates the recovery (post-TBT inputs) both of the imposex condition in discrete dogwhelk
populations as well as the recruitment of this species back to areas where populations had
previously expired due to female sterility caused by TBT exposure (Gubbins et al. 2010).
TBT inputs to Sullom Voe have been declining since the 1980s through reduced tanker
shipping traffic (Figure 9.4), changes to free association antifouling paints with reduced
TBT leaching rates, and IMO restrictions on use on large vessels in 2003 and 2008, which
put a stop on the use of TBT-based paints.
Monitoring of dogwhelk populations has taken place since 1987 at a wide range of sites
(20) both close to the oil terminal jetties where tankers and TBT sources are based and fur-
ther afield in the open waters of Yell Sound, removed from the sources of TBT (Figure 9.5).
Thus, it has been possible to compare the temporal trend of imposex intensity at both
heavily impacted and reference sites in the area. Typically, imposex is highest at sites
within the confined waters of Sullom Voe and close to the terminal. Sites further away in
Yell Sound show much lower levels of imposex, typical for background levels of contami-
nation, and sites on the “boundary” between Sullom Voe and Yell Sound show intermedi-
ate levels of imposex (Figure 9.5). Large pelagic fishing vessels are likely responsible for
the high level of imposex shown on the West shore of Yell Sound in the figure (this has
since recovered to much lower values) (Gubbins 2009).
Historically, imposex has resulted in the sterility of female dogwhelks throughout the
voe, but only resulted in the complete expiry of populations at sites very close to the termi-
nal itself. Metrics of imposex (VDSI, RPSI) have declined substantially at monitoring sites
within the voe. RPSI values have declined substantially since the early 1990s and are now
close to zero (Figure 9.6a). VDSI has declined very slowly over the same period, but until
2007 was still >4 at most sites within the voe with many sterile females present in popula-
tions. After 2007 there was a very substantial decline in VDSI across all voe populations
(Figure 9.6b), and in 2009 for the first time no sterile females were observed anywhere in
the study area, a situation that persists in 2011 (Figure 9.6c).
Concurrently with these observations of declining TBT inputs and effects on dogwhelk
reproductive capacity, rocky shore community monitoring since 1981 in the same area has
observed both a recovery of adult and juvenile dogwhelk populations to heavily affected
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