Table Of ContentARCHER-‐‑THOMSON (2016). FIELD STUDIES (http://fsj.field-‐‑studies-‐‑council.org/)
THE SEA EMPRESS INCIDENT AND THE LIMPETS OF FRENCHMAN’S STEPS,
TWENTY YEARS ON.
JOHN ARCHER-‐‑THOMSON
Honey Hook Cottage, Lower Freystrop, Haverfordwest, Pembrokeshire, SA62 4ET. UK
Students and staff from FSC Dale Fort Field Centre have studied limpet populations on Frenchman’s Steps
shore for over thirty years. Variations in population density and age structure have been measured and in
particular the effects of the Sea Empress oil spill have been investigated. The oil pollution reduced the
population density significantly and affected smaller (younger) limpets in particular, especially those on
the lower half of the shore. Since then further changes in limpet population density have occurred. These
are interpreted in the light of long-‐‑term oscillations in populations of barnacles and dogwhelks, which are
related to the use (and subsequent banning) of Tri-‐‑Butyl-‐‑Tin anti-‐‑fouling paint. Long-‐‑term monitoring,
using data collected by student groups, can be useful in highlighting variations in population density over
time as well as being a worthwhile educational experience in its own right for the students and staff
involved in the data collection.
FIGURE 1. Frenchman’s Steps , Pembrokeshire (Grid Ref. SM822053).
INTRODUCTION
Limpets of the Genus Patella are very important organisms on European rocky shores. Their differential
grazing activity on a wide range of microorganisms (biofilm) and seaweeds, including Fucus spp., and encrusting red
seaweeds, affects the shore community so significantly that limpets have been called a “keystone species” in that their
effect on community composition is greater than would be predicted from their abundance or biomass alone (Jenkins et
al., 1999, Little et al., 2009). Factors that affect limpet abundance are therefore likely to affect the shore as a whole, which
makes any long-‐‑term monitoring programme of limpets potentially interesting.
Meaningful statements about the effect of an event on a rocky shore population depend on knowledge of
what the population’s status was before the event. It is also imperative to know about “natural” variations in the
population before deductions can be made concerning “unnatural” ones. However, very few suitable long-‐‑term data
sets exist, for any ecosystem, to allow such deductions to be made with any confidence.
When the Sea Empress oil spill occurred (see Figure 2) in February 1996, long-‐‑term data sets on the numbers
and size-‐‑range of the common limpet Patella vulgata L, collected by student groups from FSC Dale Fort, did exist.
Details of the methodology are given below. Comparisons were made between pre-‐‑ and immediately post-‐‑pollution
results and the conclusions were presented in Field Studies (Archer-‐‑Thomson, 1999). Useful background information on
the effects of oil spills on rocky shores, the Sea Empress oil spill, chemical details of the oil and the biology of limpets
can be found in this paper, but see Branch (1981) for a comprehensive overview of limpet biology and Crump et al.,
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(1998) for an account of the oil spill on West Angle Bay, Pembrokeshire. In summary, the oil spill reduced the numbers
of limpets in total (Figure 3) with especially noticeable declines in smaller size classes on the lower part of the shore, as
a result the modal class for the population rose from a “normal” value in the 10-‐‑14.99 mm size class, in all pre-‐‑pollution
data sets, to the 15-‐‑19.9 mm size class in April 1996 (Figure 4). In April 1997 numbers of limpets on the study shore had
returned to what might be considered “normal” again (Figure 5) but the modal class was still shifted to the right
(Figure 6). By April 1998 numbers were towards the high side of the “normal” range (Figure 7) and the modal class had
returned to “normal” as well (Figure 8). Seemingly, the population had made a full recovery, in terms of numbers and
size distribution, within two years: a surprisingly quick revival given the magnitude of the event.
Student data sets vary in quality. However, if students are told that they will be contributing to long-‐‑term
monitoring, conscientious data collection is far more likely and has occurred in this instance. Two sets of observations,
a fortnight apart, by different A-‐‑level groups, in April 1996, gave similar and statistically significant results (Archer-‐‑
Thomson, 1999) (Figure 3).
Having realised the value of the limpet data, monitoring continued in April of each year, to gain further
insight into what might represent “normal” fluctuations of this population. To help with the consistency of recording,
final-‐‑year MSc students from the University of Leuven, who visit FSC Dale Fort annually, were used to collect the data.
In more recent years FSC Dale Fort teaching staff have collected the data. There is now an unbroken (except 2005) set of
data for the Frenchman’s Steps population up to April 2016.
FIGURE 2. Frenchman’s Steps and surrounding coastline. On 15th February 1996 the Sea Empress oil tanker grounded near St Ann’s
Head, 2 days later high winds drove her onto the rocks. Over 70,000 tonnes of oil was spilt. (Map from Archer-‐‑Thomson, 1999)
MATERIALS AND METHODS
The site chosen was Frenchman’s Steps, Grid Ref. SM822053 (Figure 2). This is a sheltered rocky shore, with a
north-‐‑north-‐‑easterly aspect (Figure 1), Ballantine’s Exposure Grade 4 (Ballantine, 1961). Data were collected by groups
of students attending field courses at FSC Dale Fort Field Centre and latterly by FSC Dale Fort teaching staff. All data
collection was directly supervised by the author. An interrupted belt transect, sampled at 0.75 m height intervals, was
established from a fixed starting height 2.25 m Above Chart Datum (ACD). This is where the bedrock starts; below this
the substrate is mobile and unsuitable habitat for limpets. Data collection continued up the shore until the upper
distributional limit of limpets was reached. 50 cm by 50 cm quadrats were placed at 0.5 m (horizontal) intervals along a
tape measure laid out at each height. In each quadrat the longest diameter of every limpet was measured and recorded
in its appropriate 5 mm size class. To prevent measuring the same limpet more than once, each shell was lightly
marked with chalk.
The number of quadrats used each year varied with the numbers of students involved in the data collection so
the results were standardised to give totals for ten quadrats at each height. No changes to the methodology have been
made over the years to facilitate comparisons with past data (Archer-‐‑Thomson, 1999).
Limpet population data are likely to show a degree of seasonality, especially as recruitment to the shore
occurs in the autumn, so it was decided to standardise data collection to April of each year where possible; however,
some data sets are from March and May.
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700
1985
600
1986
500 1989
ets 400 1996
p
m 1996
of li 300
r
e 200
b
m
u
n 100
al
ot
T 0
2.25 3 3.75 4.5 5.25 6 6.75
Vertical height ACD / metres
FIGURE 3. Limpet numbers at Frenchman’s Steps at the given heights ACD for two April 1996 data sets and three pre-‐‑pollution
examples (1985-‐‑1989).
A. April 1985
700
s 600
et
p
m
of li 500
er 400
b
m
u
n 300
al
Tot 200
100
0
Size class / mm
B. 29 April 1996
700
s 600
et
p
m
of li 500
er 400
b
m
nu 300
al
Tot 200
100
0
Size class / mm
FIGURE 4. Limpets at Frenchman’s Steps: size frequency data for a typical pre-‐‑pollution data set (A; April 1985)
versus an April 1996 example (B). Modal class size is in red.
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700
1985
600 1986
1989
s 500 1996
et 1996
p
m 400 1997
of li 300
r
e
b
m 200
u
n
al 100
ot
T
0
2.25 3 3.75 4.5 5.25 6 6.75
Vertical height ACD / metres
FIGURE 5. Limpet numbers at Frenchman’s Steps at the given heights ACD for two April 1996 data sets and three pre-‐‑pollution
examples (1985-‐‑1989), and a one-‐‑year post-‐‑pollution example (1997).
A. 29 April 1996
700
s. 600
et
p
m 500
r of li 400
e
b
m 300
u
n
otal 200
T
100
0
Size class / mm
B. 30 April 1997
700
600
s.
et 500
p
m
of li 400
r
be 300
m
u
n 200
al
ot
T 100
0
Size class / mm
FIGURE 6. Limpets at Frenchman’s Steps: size frequency data for a April 1996 data set (A)
versus (B) an April 1997 (one-‐‑year post pollution). Modal class size is in red.
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700
s 1985
et 600
p 1986
m
of li 500 1989
er 400 1996
b
m
1996
nu 300
al 1997
ot 200
T 1998
100
0
2.25 3 3.75 4.5 5.25 6 6.75
Vertical height ACD / metres
FIGURE 7. Limpet numbers at Frenchman’s Steps at the given heights ACD for two April 1996 data sets and three pre-‐‑pollution
examples (1985-‐‑1989), and two post-‐‑pollution examples (1997-‐‑one-‐‑year post pollution, 1998 – two-‐‑year post pollution).
A. 29 April 1996
700
s 600
et
p
m500
of li
r 400
e
b
m300
u
n
al 200
ot
T
100
0
Size class / mm
B. 30 April 1998
700
s 600
et
p
m
500
of li
r 400
e
b
m
u 300
n
otal 200
T
100
0
Size class / mm
FIGURE 8. Limpets at Frenchman’s Steps: size frequency data for an April 1996 data set (A) versus (B) an April 1998 (two-‐‑year post
pollution). Modal class size is in red.
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RESULTS AND ANALYSIS
Table 1 shows a data set for 30 April 1998. Table A contains class data from seven quadrats (student groups),
whereas Table B gives the standardised results, as if ten quadrats had been used.
Table 2 gives (standardised) results for the number of limpets found in ten 50 x 50 cm quadrats at various
heights up the shore. The three data sets before 1996 were chosen because they were gathered in April and deemed
typical for the shore. The two sets of data for 1996, (1) and (2), were recorded by two different A-‐‑level groups, on the 3
and 29 of April respectively. Analysis of these results is presented in Archer-‐‑Thomson (1999).
Table 3 similarly provides (standardised) data for size frequencies.
TABLE 1. Original (A) and standardised (B) data for limpet numbers and size range from Frenchman’s Steps on 30 April 1998.
(A) data from seven quadrats (student groups). (B) the converted (standardised) data for the equivalent of ten 50 x 50 cm quadrats.
The shaded cells show:
the height at which most limpets were found (3.75m ACD).
the mode for the size class data (10.00-‐‑14.99mm).
the shift in the modal class at each height to progressively larger limpet sizes as height up the shore increases.
(See Archer-‐‑Thomson (1999) for an explanation).
A: Data from seven quadrats (student groups)
Vertical height above chart datum / m
Size class / mm 2.25 3 3.75 4.5 5.25 6 6.75 TOTALS
< 4.99 7 36 61 29 4 0 0 137
5.0−9.99 26 118 116 52 17 6 0 335
10.0−14.99 24 100 136 103 43 4 0 410
15.0−19.99 12 90 104 91 39 9 0 345
20.0−24.99 12 69 50 89 53 12 0 285
25.0−29.99 2 25 17 21 32 22 0 119
30.0−34.99 9 8 5 6 17 10 0 55
35.0−39.99 3 1 0 0 5 7 0 16
40.0−44.99 5 2 7
45.0−49.99 1 1
50.0−54.99
55.0−59.99
60.0−64.99
65.0−69.99
TOTALS 101 449 489 391 210 70 0 1710
B: Converted (standardized) data for the equivalent of ten 50 x 50 cm quadrats
Vertical height above chart datum / m
Size class / mm 2.25 3 3.75 4.5 5.25 6 6.75 TOTALS
< 4.99 10 51 87 41 6 0 195
5.0−9.99 37 169 166 74 24 9 479
10.0−14.99 34 143 194 147 61 6 585
15.0−19.99 17 129 149 130 56 13 494
20.0−24.99 17 99 71 127 76 17 407
25.0−29.99 3 36 24 30 46 31 170
30.0−34.99 13 11 7 9 24 14 78
35.0−39.99 4 1 7 10 22
40.0−44.99 7 3 10
45.0−49.99 1 1
50.0−54.99
55.0−59.99
60.0−64.99
65.0−69.99
TOTALS 143 642 698 558 300 100 2441
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TABLE 2. The number of limpets found in ten 50cm x 50 cm quadrats at each 75 cm vertical height interval up the shore at
Frenchman’s Steps, in April . ( *Data collected in March. No limpets found above 6.75 m ACD).
April Vertical height above chart datum / m
data 2.25 3 3.75 4.5 5.25 6 6.75 Totals
1985 191 504 693 556 333 49 0 2327
1986 152 407 480 338 228 55 0 1660
1989 487 600 607 556 241 94 0 2585
1996 (1) 72 177 335 313 248 75 5 1225
1996 (2) 26 174 320 346 264 36 0 1166
1997 64 330 432 519 235 145 0 1725
1998 143 642 698 558 300 100 0 2441
1999 333 567 657 529 354 118 0 2558
2000 320 472 558 553 351 152 0 2406
2001 119 383 510 630 293 121 0 2056
2002 174 423 723 560 389 103 0 2372
2003 98 584 711 552 238 78 0 2261
2004 129 396 479 540 269 70 2 1885
2005 No data collected for this year
2006 228 364 421 490 293 104 0 1900
2007 190 410 423 328 228 15 0 1594
2008 208 422 392 330 266 146 30 1794
2009 220 222 424 498 290 18 0 1672
2010 159 775 818 862 493 268 0 3375
2011 163 811 1261 1126 553 203 0 4116
2012 168 716 856 1122 432 156 0 3450
2013 257 703 777 645 442 225 0 3048
2014 347 748 715 548 337 168 0 2863
2015* 396 806 728 560 318 114 0 2922
2016 374 846 648 616 388 94 0 2966
Table 4 presents the statistical analysis of the abundance data sets from Table 2. The total number of limpets at
each height, for any given year, is compared with those from every other year. It should be noted that this type of data
is difficult to analyse statistically. Biological data of this nature are unlikely to meet the requirements of parametric
statistics (e.g. assuming normally distributed data, with equal variances) so non-‐‑parametric tests were used. Because
the variation in numbers, with height up the shore was large (as a consequence of the environmental gradient on rocky
shores from fully marine conditions at the bottom to near terrestrial ones at the top of the shore) the analysis had to be
based on a matched-‐‑pair system (2.25 m ACD versus 2.25 m ACD for the years in question etc.). Although multivariate
techniques could have been used, these would have been inaccessible to A-‐‑level students. The Wilcoxon Matched Pairs
test was deemed the most suitable test. It is noted, however, that given the small sample size (seven heights
maximum), this test would only record a statistically significant difference if the quadrat totals in all the heights from
one year were greater (or lesser) than those from another. Inherent variation at site one (where the substrate varies
from year to year) makes this constraint significant. Even so, the analysis did largely show significant differences
where expected. An excellent description of the limitations associated with this statistics test is given in Fowler et al.
(1998).
Table 4 has been colour coded to aid interpretation. Normally, the Null Hypothesis is rejected, coded S, or
accepted, coded NS, at the 5% significance level or better. In Table 4 the results have been coded (S) if the Null
Hypothesis could be rejected at the 10% significance level. The decision to indicate rejection at this level is because of
the inherent variability in the data (increased by substrate variation at site one, for example) and because the Wilcoxon
test ideally needs a minimum sample size of six non-‐‑zero differences. Many of the data sets give six non-‐‑zero
differences, some give seven, but operation here is at the lower end of the ideal sample size spectrum. Thus, (S)
indicates a tendency in the data and is therefore still a useful guide.
A look at Table 4 shows a cluster of significant differences (S) for the 1996 (oil pollution) data sets as expected
but also another cluster of significant difference (Ss) for the 2010 data and later, which highlights how different the
2010, and later, population densities are.
When working at the 5% significance level, there is a risk of rejecting a Null Hypothesis (H0), when it is true,
five times out of a hundred or one time in twenty. This is referred to as a type 1 error. Table 4 contains a great many
Wilcoxon Matched Pairs test results and thus the risk of a type 1 error is increased. A Friedman test analyses the data
set in table two as a whole, whilst still operating on matched pairs (It is like a matched pairs Analysis of Variance
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TABLE 3. The number of limpets in each 5 mm size class on the transect up the shore at Frenchman’s Steps.
Data standardised for ten 50 x 50 cm quadrats at each height. Cells shaded:
show the modal class. (* Data collected in March. No limpets found bigger than 69.99 mm).
Size class in mm
Adaptrai l 99 9.99 14.99 19.99 24.99 29.99 34.99 39.99 44.99 49.99 54.99 59.99 64.99 69.99 Totals
< 4. 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑ 00-‐‑
5. 0. 5. 0. 5. 0. 5. 0. 5. 0. 5. 0. 5.
1 1 2 2 3 3 4 4 5 5 6 6
1985 96 476 658 494 327 171 72 18 13 1 1 2327
1986 55 239 438 410 266 145 67 27 10 3 1660
1989 324 586 590 469 240 200 108 41 14 14 2585
1996 (1) 33 157 285 327 221 94 72 19 11 5 1225
1996 (2) 8 91 248 376 286 115 29 8 3 1 1166
1997 19 172 360 472 359 187 112 34 6 3 1 1725
1998 195 479 585 494 407 170 78 22 10 1 2441
1999 57 543 702 489 375 169 125 73 13 9 1 1 1 2558
2000 109 366 668 589 316 197 84 39 16 12 5 2 1 2 2406
2001 41 373 639 424 292 171 57 46 10 1 1 1 2056
2002 262 452 617 476 324 144 57 24 12 3 1 2372
2003 47 492 679 554 214 186 55 25 3 4 2 2261
2004 48 292 559 474 232 145 64 44 16 2 7 2 1885
2005 No data collected this year
2006 54 396 639 435 230 94 35 6 6 2 3 1900
2007 75 337 470 305 210 76 59 30 21 3 8 1594
2008 76 413 622 338 199 77 48 16 4 1 1794
2009 50 264 424 454 234 134 56 28 20 2 6 1672
2010 75 547 1072 775 457 216 120 71 33 9 3375
2011 253 1120 1204 841 335 213 79 43 21 8 1 4116
2012 174 608 1236 740 382 166 76 44 16 4 4 3450
2013 142 617 865 722 348 183 95 40 23 13 3048
2014 55 558 862 742 387 177 70 5 7 2 2863
2015* 26 588 846 820 370 164 42 44 14 6 2 2922
2016 10 280 1090 884 508 128 46 6 8 4 2 2966
would be to a series of matched pair t tests, suitable if the data were normally distributed, or if sample sizes were
larger). The result of the Friedman test on the data as a whole (see FRIEDMAN tab in the Excel spreadsheet of RAW
data in the appendix*) allows the rejection of the H0 (that there is no significant difference between the medians of the
data sets) at P< 0.001, a very highly significant result. The chance of getting a result this significant by chance is less than
one in a thousand. Although the Friedman analysis reduces the chance of a type 1 error significantly, it only indicates
the data sets as a whole are different, and does not show which data sets (years) are significantly different from which
others. Thus, the many Wilcoxon tests are necessary despite the increased risk of type 1 errors.
(* For interested readers RAW limpet survey data may be found in an additional file supplied with this on-‐‑line paper.)
DISCUSSION
Explanations for the patterns in the data, from 1985 to 1998, have been presented previously (Archer-‐‑Thomson, 1999)
and summarised below.
The total number of limpets at each height [Table 2].
Limpet densities vary with height up the shore. Numbers are low at the base of the study area, 2.25 m ACD,
but increase with height to a maximum value in the middle shore (at approximately 3.75 m ACD), decreasing above
this again to zero by about 6.75 m ACD. Portions of the lowest site are covered with pebbles, the amount and extent
varying from year to year, which explains some of the variation at the bottom of the shore.
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TABLE 4. Wilcoxon Matched Pairs Test results for limpet numbers over the given years (April data). Data standardised for ten 50 x 50 cm
quadrats at each height. The shaded cells show:
NS Non-‐‑significant result, Accept H0 (of no significant difference between the median number of limpets at the 5% significance level).
S Reject H0 at 5% significance level.
(S) Reject H0 at 10% significance level (see text for explanation as you would normally Accept the H0 here).
1) 2)
Year 5 6 9 6( 6( 7 8 9 0 1 2 3 4 6 7 8 9 0 1 2 3 4 5 6
8 8 8 9 9 9 9 9 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1
9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
1985 NS NS (S) S NS NS NS NS NS NS NS NS NS S NS (S) (S) (S) (S) S NS NS NS
1986 S NS NS NS (S) S S NS S NS NS NS NS NS NS S S S S S S S
1989 (S) (S) NS NS NS NS NS NS NS NS NS S NS (S) NS NS NS NS NS NS NS
1996 (1) NS NS S S S S S NS (S) S NS S NS S S S S S S S
1996 (2) (S) S S S S S (S) S S NS (S) (S) S S S S S S S
1997 NS NS S (S) NS NS NS NS NS NS NS S S S S S (S) (S)
1998 NS NS NS NS NS S NS (S) NS NS S S S S S S NS
1999 NS NS NS NS (S) S S (S) S NS NS NS (S) NS NS NS
2000 NS NS NS S S S (S) S NS NS NS (S) NS NS NS
2001 NS NS NS NS NS NS NS S S S S NS NS NS
2002 NS S NS NS NS (S) (S) (S) (S) S NS NS NS
2003 NS NS NS NS NS S S S S (S) S (S)
2004 NS NS NS NS S S S S S S S
2006 NS NS NS (S) (S) NS S S S (S)
2007 NS NS (S) (S) (S) S S S S
2008 NS NS NS NS S (S) NS NS
2009 (S) (S) (S) S S S S
2010 NS NS NS NS NS NS
2011 NS NS NS NS NS
2012 NS NS NS NS
2013 NS NS NS
2014 NS NS
2015 NS
2016
There is a decrease in numbers towards the top of the shore, because conditions become increasingly harsh for
a marine snail. Temperature and salinity variation and dehydration create stresses that all get worse as emersion
(periods spent out of water) times increase. Limpets graze selectively on green algae (both macro and microscopic),
lichens and young fucoids. They often feed at night when the tide is out (Branch, 1981), or at any time when the sea is
calm and the tide is in. Feeding time and food supply are not necessarily reduced in the upper shore but a limpet might
feed less if already stressed by other environmental (abiotic) factors. In short, abiotic factors probably set the upper
distributional limits (Branch, 1981).
Numbers decrease towards the bottom of the shore because of inter-‐‑specific competition from macro algae in
more sheltered regions and from barnacles in more exposed ones. Exposure to wave action will vary considerably on
shores in different locations. A headland site receives far more wave energy than an embayed one, but there are subtle
variations along small horizontal distances as the slope and topography vary and this can contribute over the study
area width of approximately ten metres. Thus, lower distributional limits are probably set by biotic (living) factors.
It follows that optimal conditions, between the two extremes mentioned above, exist for limpets in the middle
shore. Over the thirty years in which data have been gathered, the trends described above have held true with no
significant deviation.
The total number of limpets on the shore over time [Table 2].
Looking at the population as a whole numbers were significantly reduced by the Sea Empress oil spill,
recovering to normal again within a year or two. From 1998 onwards, the population fluctuated within what had
become expected (“normal”?) limits up until 2010. In 2010 and 2011 limpet numbers exceeded anything hitherto
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experienced (Figure 9). Up until 2010 it was assumed that “normal” fluctuations in the study limpet population had
been established. Data from 2010 and 2011 threw this assumption into doubt but the question now became one of
whether the record population densities were “natural” or if there was another explanation that might be regarded as
“unnatural”?
4500
4000
3500
3000
s
et
mp 2500
of li 2000
r
e
b
m
1500
u
n
otal 1000
T
500
0
5 6 9 6 6 7 8 9 0 1 2 3 4 6 7 8 9 0 1 2 3 4 5 6
8 8 8 9 9 9 9 9 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1
9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
FIGURE 9. Total number of limpets on Frenchman’s Steps sample site, 1985-‐‑2016.
FIGURE 10. (Left) Dogwhelk Nucella lapillus plus egg capsules from Frenchman’s Steps. (Right). Typical dogwhelk Nucella lapillus
density in recent years (photograph taken September 2009).
In an attempt to explain the record high limpet densities of 2010 and 2011, it is necessary to include anecdotal
evidence and field observations from the author’s own 30-‐‑year-‐‑plus experience of the shores around FSC Dale Fort. In
1982 there were virtually no dogwhelks (Nucella lapillus) on the shores between the Field Centre and Dale village
(Figure 2). Local shores might have yielded two or three animals in an area of 1500 m2. Dogwhelks (Figure 10) are fairly
common rocky shore animals in the UK, but at the time their numbers were severely depleted by Tri-‐‑Butyl-‐‑Tin (TBT)
anti-‐‑fouling paint, which was widely used to prevent fouling organisms (barnacles, seaweeds, etc.) from growing on
the hulls of pleasure boats and larger commercial vessels.
In the 1980s unusual changes in the sexual characteristics of dogwhelks were noticed in estuaries and other
areas where small boats, painted with TBT anti-‐‑fouling paint, were concentrated (Little et al., 2009). Studies showed
10
© Field Studies Council (02/09/2016)
Description:particular the effects of the Sea Empress oil spill have been investigated. The oil pollution reduced the population density significantly and affected
