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The Prestige oil spill
On 13 November 2002, the hull of the Prestige oil tanker split and the ship began to spill heavy
bunker oil off the coast of Galicia, NW Iberian Peninsula. During the next 6 days, the ship
followed an erratic course back and forth along the Galician coast, spreading around 19,000
tonnes of oil, and finally sank 130 miles west of Illas Cíes (42º12’N, 12º03’W). During the
following months, 40,000 tonnes of oil were additionally released. Wind and currents spread
the oil across a vast coastal area, as far as the Isle of Wight, on the south coast of England, and
the Canary Islands, close to West Africa, making this one of the largest oil spills in Europe. The
most heavily oiled area (thereafter ‘oiled’ area) was located between Illas Sisargas and Illas
Cíes, according to the oil presence on the coast and in aquatic organisms during the black tides
[1-3]. In this area, more than 50,000 tonnes of oiled products were collected from beaches
during the cleaning scheme [4]. Colonies outside this area were less affected by oil and they
were considered as “unoiled” (see [5]; figure S1). In the unoiled area, only small amount of oil
traces were found in the marine organisms [1-3], and only 4,400 tonnes of oiled products were
collected from the beaches in this vast area ([4], figure S1).
Figure S1. Map of the study colonies of European shag (Phalacrocorax aristotelis) in the oiled (closed circles) and
unoiled (open circles) areas. Colony codes inside circles as given in table S1.
Reproductive success
This study was carried out in the north-west of the Iberian Peninsula (figure S1) from 1994 to
2012. Reproductive success was defined as the number of chicks surviving to fully-grown per
nest, >35 days of age [6]. Nest sites in breeding areas of each study colony were monitored
three to five times during the breeding season (March to June) each year. In all cases, nests
were monitored at least once during the incubation period and twice during the chick growth
period. Hatching success was not recorded due to the breeding asynchrony in the study
colonies. The number of monitored nests varied among colonies and years mainly due to
annual fluctuations in breeding numbers. Prespill reproductive success was monitored during
seven years in an oiled colony (Illas Cíes), and during one to five years in three colonies in the
unoiled area. In the breeding season immediately after the Prestige oil spill (2003),
reproductive success was recorded in almost the whole study area (17 colonies, 11 colonies
in the unoiled area and 6 colonies in the oiled area). Thereafter, reproductive success was
monitored in four main colonies in the oiled area and six colonies in the control area (table
S1). The interaction between area and period was also significant (F1,39.6 = 21.15, p < 0.001)
when analysis was restricted to colonies with temporal replicated data for both periods (i.e.
A Forcada, Castrios-Represas, As Pantorgas and Illas Cíes).
Table S1. Sample size (number of monitored nests) from all the colonies along the study period (1994-2012).
Location of colonies (see code column) is shown in the figure S1.
Climate
To analyse climatic variability, we used the average monthly values during the shag breeding
period (February to July) of two climatic proxies: the North Atlantic Oscillation index (NAO,
from NOAA; ftp://ftp.cpc.ncep.noaa.gov/cwlinks/) and the Sea Surface Temperature (SST,
from NASA; http://neo.sci.gsfc.nasa.gov/Search.html) (see [6]). Since SST varies
geographically, we estimated two different SST values in the oiled and unoiled areas, using
the SST values in the central coastal point of these areas.
Statistical analyses
Annual reproductive success was analysed by a Linear Mixed Model (LMM), including area
(oiled and unoiled, figure S1), period (before and after the Prestige oil spill), NAO and SST as
fixed effects. Year, nested within period, and colony, nested within area, were included as
random effects. In two colonies, the presence of invasive American mink (Neovison vison) was
detected in certain years (2009-2012 at Illas Cíes and 2012 at Castríos-Represas). In order to
ensure that the results were not affected by the presence of this exotic predator, we included
this factor in the model, and we also re-ran the same model while excluding reproductive
events with predator presence.
The effect of environmental impact on a BACI analysis was tested by the interaction
between period (before-after factor) and area (control-impact factor). The statistical
significance of the difference in reproductive success between pair comparisons (e.g. before-
after, control-impact) was estimated by means of Monte Carlo randomization analysis (10,000
simulations without replacement). Randomization tests are robust in the analysis of
unbalanced and small samples [7]. We also tested any possible effect of recovery by restricting
our analysis of postspill data, including the number of years after the Prestige oil spill in the
model. In all models, Satterthwaite’s approximation of degrees of freedom was used and the
statistical significance of random effects was assessed using the restricted Likelihood Ratio
Test, LRT [8]. All models were simplified by deleting non-significant terms (backward deletion).
Data are expressed as mean ± SE.
References
1. http:// www.ccmm-prestige.cesga.es/
2. http://www.eea.europa.eu/data-and-maps/figures/impact-of-prestige-accident-on-
the-coastal-environment
3. http://otvm.uvigo.es
4. Santos, R. 2003 Listas para el verano. Ministerio de Agricultura, Alimentación y Medio
Ambiente; Ambienta 23, 6-14.
5. Velando A, Álvarez D, Mouriño J, Arcos F, Barros Á. 2005 Population trends and
reproductive success of the European shag Phalacrocorax aristotelis on the Iberian
Peninsula following the Prestige oil spill. J. Ornithol. 146, 116-120.
6. Barros Á, Álvarez D, Velando A. 2013 Climate influences fledgling sex ratio and sex-
specific dispersal in a seabird. PlosOne 8, e71358.
7. Manly BJF. 1997 Randomization, Bootstrap and Monte Carlo Methods in Biology, 2nd
edition. London: Chapman and Hall.
8. Littell RC, Milliken GA, Stroup WW, Wolfinger RD, Schabenberger O. 2006 SAS for
mixed models, 2nd edn. Cary NC: SAS Institute Inc. 633 p.
