Marine Pollution Bulletin 64 (2012) 2815–2819
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Marine Pollution Bulletin
journal homepage: www.elsevier.com/locate/marpolbul
Entanglement of grey seals Halichoerus grypus at a haul out site in Cornwall, UK
Rebecca Allen a,⇑, Dan Jarvis c, Sue Sayer b, Cheryl Mills d
a
Cornwall College Newquay, Trenance Gardens, Newquay, Cornwall TR7 2LZ, UK
Cornwall Seal Group, Copperleaf Cottage, Phillack Hill, Phillack, Hayle, Cornwall TR27 5AD, UK
c
British Divers Marine Life Rescue, Lime House, Regency Close, Uckfield, East Sussex TN22 1DS, UK
d
Centre for Ecology and Conservation, School of Biosciences, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, UK
b
a r t i c l e
Keywords:
Halichoerus
Grey
Seal
Net
Entanglement
Debris
i n f o
a b s t r a c t
Entanglement in marine debris has been internationally recognised as a potential threat to marine species. Sightings records and a photo identification catalogue from a haul out site in southwest England
were used to establish entanglement records for grey seals Halichoerus grypus. Between 2004 and 2008
the annual mean entanglement rates varied from 3.6% to 5%. The maximum recapture period for entangled seals compared to paired control seals was significantly less (p = 0.045) suggesting an increased mortality rate for affected seals. Of the 58 entangled cases in the catalogue, 64% had injuries that were
deemed serious. Of the 15 cases where the entangling debris was visible, 14 were entangled in fisheries
materials. The entanglement reported at this site could indicate a high rate of mortality and should be
monitored carefully. On a more immediate level, entanglement represents a welfare issue for the affected
animals.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Entanglement in marine debris has been internationally recognised as a potential threat to marine species since the 1984 International Workshop on the Fate and Impact of Marine Debris
(Shomura and Yoshida, 1985). In a review of the impacts of entanglement Laist (1997) reported that 135 species of marine wildlife
had been recorded as entangled in marine debris. This included
19 of the 34 pinniped species.
Entanglement has welfare implications for individual pinnipeds;
injuries sustained by debris, or increased drag caused by trailing
material result in increased foraging times to meet raised metabolic
demands, and at the same time movement is often impaired making
foraging more difficult (Feldkamp, 1985; Feldkamp et al., 1989;
Bengston in Laist, 1997). In many cases the entangling item causes
a constriction around the neck or body of the affected animal and
it is assumed that these combined effects will eventually be fatal
(Derraik, 2002). As an indicator of mortality, entanglement rates
are likely to be an underestimate; for every entangled animal recorded on land an unknown proportion are likely to die at sea, entrapped in debris too large to swim against or entangled beneath
the surface, and therefore drowning (Laist, 1997). For some species
entanglement mortality has been significant enough to be
⇑ Corresponding author. Tel.: +44 (0) 1637 857957; fax: +44 (0) 1637 857969.
E-mail addresses: [email protected] (R. Allen), [email protected]
(D. Jarvis), [email protected] (S. Sayer), [email protected] (C.
Mills).
0025-326X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.marpolbul.2012.09.005
implicated in population level effects (Fowler, 1987). For the endangered Hawaiian Monachus schauinslandi and critically endangered
Mediterranean M. monachus monk seals it has been identified as a
major factor threatening the survival of these species (Henderson,
2001; Boland and Donohue, 2003; Karamanlidis et al., 2008).
Where identified, the majority of the entangling material is fisheries related, although debris from other sources are also commonly reported (Arnould and Croxall, 1995; Hanni and Pyle,
2000; Boren et al., 2006). Seals may become entangled due to interactions with either operational or discarded fishing gear although
it is difficult to ascertain which is responsible for the majority of
cases. As outlined by the United Nations (1995) incidental mortality caused by either live or discarded fishing gear can be regarded
as fisheries bycatch.
The British Isles are home to a substantial proportion (approximately 45%) of the world population of grey seals Halichoerus grypus which recent estimates by the Sea Mammal Research Unit
(2008) put at 180,000 (confidence intervals: 96,200–346,000) individuals living and breeding around the coasts. Data suggests that
grey seals found around Wales and SW England form a genetically
distinct subpopulation of approximately 5000 animals (Sea Mammal Research Unit, 2006).
Grey seals at a mainland haul out site in Cornwall have become
the focus of a long term study by the Cornwall Seal Group (CSG)
since 2000. Numbers of seals counted at the site averaged 25 between 2000 and 2008, but there was great seasonal variation and
sightings of over 100 animals were common during the spring
moulting period (Sayer et al., 2009; Leeney, 2010).
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R. Allen et al. / Marine Pollution Bulletin 64 (2012) 2815–2819
From the initial stages of the study it became apparent that a
number of individual seals were suffering from constriction injuries and from 2003 entangled seals have been recorded as part of
the sightings protocol.
2. Methods
2.1. Entanglement rates
Entanglement rates were calculated as the percentage of the
hauled out seals observed per sighting, then averaged for the year.
graphs of entangled individuals were studied to identify the entangling material where possible. In addition the nature of injuries
sustained by the entangled seals was examined and categorised
as: ‘‘constriction’’ – flesh was obviously drawn in by the impacting
material, ‘‘open wound’’ – pink flesh and/or blood was visible,
‘‘constriction and wound’’ – combination of previous categories,
or ‘‘evident’’ where entangling material or an encircling mark
was observed but no wound or constriction was visible (Fig. 1).
Presence of trailing material was also recorded.
2.3. Maximum recapture period of entangled versus non-entangled
seals
2.2. Entangling material and the nature of entanglement injuries
The group has also established a photo identification catalogue
of seals since 2001 alongside the sightings data. From this, photo-
‘‘Maximum recapture period’’ (time in days from initial to last
photo capture) was used as a proxy to examine whether net entanglement had an effect on survivorship. A sample of net entangled
Fig. 1. Categories of net entanglement injuries: (a) ‘constriction’, (b) ‘wound’, (c) ‘evident’ and (d) ‘constriction and wound’, (e) ‘trailing material evident’.
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Table 2
Nature of injuries caused by entangling material.
Type of Injury
Total
Constriction
Wound
Constriction and wound
Evident
25
4
8
21
Fig. 2. Mean entanglement rates as a percentage of total haul out. (Error bars
indicate 95% confidence intervals.)
seals from the photo identification catalogue was matched with a
control sample of seals of the same sex and age (i.e. similar size
and appearance). Controls were selected by finding a suitable
match which had also been photo identified within one calendar
month of the entangled individual. If more than one suitable match
was found then a random selection was made from the candidates
available. Matched pairs were discarded unless both individuals
had been reliably recaptured at least once and more than 6 months
after the initial photo identification. Data was analysed with the
paired t-test. To identify potential sightings bias for either group,
sightings frequency was calculated (total sightings/maximum recapture period), normalised by the Johnson transformation and
subjected to a paired t test.
3. Results
3.1. Entanglement rates
Mean entanglement rates ranged from 5% in 2004 to 3.6% in
2008 (Fig. 2).
3.2. Entangling material and the nature of entanglement injuries
At the end of 2008, 58 entangled individuals were recorded in
the photo identification catalogue. Of these 42 (72%) had either
constrictions or encircling wounds for which the entangling material was not visible, either because it was embedded in the animal’s
flesh, too fine to be resolved in the photograph or possibly no longer present. Identifiable material included monofilament line,
monofilament net, multifilament net, and rope (Table 1). Nine seals
(15%) had trailing material in evidence.
Of the 58 seals, 37 (64%) had injuries which were either causing
a constriction, had formed an open wound, or both (Table 2).
Table 1
Entangling materials observed on affected individuals.
a
b
Type of material
Total
Trailing material in evidence
Unidentifiablea
Monafilament net
Multifilament net
Monafilament line
Monafilament and rope
Rope
Rubber collar and tasslesb
42
6
6
1
1
1
1
0
1
5
1
1
0
1
Entangling material was not visible.
Source of black rubber neck collar was not determined.
Fig. 3. Mean maximum recapture period for net entangled versus control seals
(Error bars represent standard error.).
3.3. Maximum recapture period
The mean maximum recapture period for net entangled seals
(n = 20, x ± SE = 1210 ± 160 days) was significantly shorter
(p = 0.045) than that of the control group (n = 20, x ± SE = 1529 ±
156 days) (Fig. 3).
There was no significance difference in sightings frequency between the two groups.
4. Discussion
The level of entanglement for grey seals recorded at this Cornish
site is to the authors’ knowledge, the second highest level recorded
for a pinniped species and the highest recorded for a phocid seal
globally. A number of otariid species have been reported with similar levels for example: Californian sea lions Zalophus californianus
3.9–7% (Harcourt in Page et al., 2004), New Zealand fur seals Arctocephalus forsteri, 0.6–2.8% (Boren et al., 2006) and Australian fur
seals Arctocephalus pusillus 1.3–1.9% (Pemberton et al., 1992). However, many pinniped entanglement rates are at least one order of
magnitude lower (Fowler, 1987; Laist, 1997; Boland and Donohue,
2003; Hofmeyr et al., 2006; Campagna et al., 2007). It is notable
that even entanglement rates of 0.4% were considered indicative
of a level of mortality serious enough to have population level effects on northern fur seal Callorhinus ursinus colonies (Fowler,
1987). There is currently insufficient data to determine how the
levels seen in this study reflect those of grey seals throughout
the rest of their distribution. Although preliminary data from another important Celtic sea site on the Isles of Scilly indicates
slightly lower rates of entanglement 0.5–1.5% (Sayer et al., 2012).
Of the 58 cases of entangled seals from the photo identification
catalogue, 64% had injuries where the entangling material was
causing a visible constriction to the head or body, an open wound
or both. It would seem inevitable that injuries of this nature will
have a serious impact on the animals’ health and will eventually
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cause mortality through either starvation, strangulation, blood loss
or infection (Fowler, 1987; Derraik, 2002). In addition 9 cases (15%)
had trailing material evident. Seals thus affected would be expected to have an increased energetic demand and reduced foraging efficiency due to the additional drag compromising their diving
and swimming ability. Feldkamp (1985) discovered that energy
requirements increased fourfold for an average female Californian
sea lion carrying just 400 g of netting material.
Proving the effect of entanglement on survivorship is problematic because of the difficulties inherent in studying any marine
mammal, i.e. most of their life is spent unseen at sea, and most will
therefore die at sea undetected (Laist, 1997).
The maximum recapture period does indicate that survivorship
is decreased even over the relatively short seven year study period,
with control individuals being reliably observed over significantly
longer periods of time. The sightings frequency for individuals
showed no significant difference between entangled and nonentangled seals and goes some way to countering the argument
that there may be bias in the likelihood of re-sighting individuals
from either group.
Although these findings indicate the life expectancy for entangled seals is likely to be less than that of an unaffected individual;
it is also apparent that even seriously impacted grey seals are capable of surviving for many years and in the case of one very badly
constricted female, breeding successfully. This contrasts with the
findings in some other entanglement studies where entangled
individuals were rarely re-sighted in subsequent years (Page
et al., 2004).
For 14 of the 15 cases where the entangling material was visible
(or identified during rescue), the source was found to be fisheries
related, i.e. monofilament line or net, or multifilament net. In the
remaining 42 cases the entangling material could not been seen,
however in 12 cases where similarly affected seals have been successfully captured and disentangled, the offending material was
found to be monofilament net (D. Jarvis, pers. comm.). From this
it seems reasonable to assume that the vast majority of the unidentified material causing constriction was likely to be monofilament,
which would mean that nearly all of entangling materials could be
sourced to fisheries. This result may be a reflection of the intensity
of fishing effort around the southwest coast of Britain (Witt and
Godley, 2007; Irwin and Padia, 2008). Fisheries related material
has been the main source of entangling material in other studies,
being responsible for between 45% and 72% of entangling material
for various pinniped species (Fowler, 1987; Pemberton et al., 1992;
Arnould and Croxall, 1995; Laist, 1997; Hanni and Pyle, 2000; Hofmeyr and Bester, 2002; Page et al., 2004; Hofmeyr et al., 2006).
Whether seals become entangled due to interaction with live or
discarded fishing gear is open to speculation although both routes
are possible. Whilst interaction with fisheries have been recorded
in some of the earliest scientific papers on the Cornish population
(Steven, 1934) the true extent of this interaction and any resultant
bycatch remains unknown. Due to their natural curiosity pinnipeds
may interact with discarded fishing gear and other marine debris
(Fowler, 1987). On more than one occasion juvenile grey seals have
been filmed by a CSG member interacting with fragments of multifilament and monofilament net at the study site (S. Sayer, pers.
comm.). Discarded fishing gear has been implicated as the major
source for entangling material for some other pinniped species;
Hofmeyr et al. (2006) argue that discarded fisheries material is largely responsible for entangling Antarctic fur seals Arctocephalus
gazella at the island of Boutetøya in the Southern Ocean, as no fishing industries are thought to operate in the immediate area. There
are certainly opportunities for Cornish seals to interact with and
possibly entangle in discarded fishing gear and survive; large
quantities of fishing related litter (largely in the form of net fragments) can be found around the coast. Recent marine litter surveys
recorded on average 744 items of fishing related beach litter items
per kilometre on southwest beaches (Marine Conservation Society,
2008).
There is insufficient data to begin to determine the level of mortality due to entanglement. From 2005 to 2008, 284 grey seal carcasses were recorded by Cornwall Wildlife Trust Marine Strandings
Network volunteers around the Cornish coast (Loveridge and Loveridge 2006, 2007, 2008, 2009). Of 72 dead seals recorded in 2008,
four had obvious net entanglement injuries or rope indentations,
however it is often impossible to determine the cause of death or
even identify individuals as they are often in a state of decomposition and necropsies are rarely carried out (J. Loveridge, pers.
comm.). Of 58 juvenile and adult grey seals post-mortemed from
the coasts of England, Wales and the Isle of Man between 1989
and 1997, seven (12%) were recorded as drowned in fishing net
(Baker et al., 1998) with a further eight having signs of net entanglement. Drowning in fishing net was the largest cause of death
(30%) in the 20 juveniles autopsied. A Norwegian study tagged
3571 grey seal pups between 1975 and 1998, of the 259 tags recovered 204 (89%) were from animals drowned in fishing gear, mostly
bottom set nets (Bjørge et al., 2002). The study also reflected that
juveniles were most vulnerable with seals under eight months
accounting for 60% of the fishery related mortality.
The population of grey seals around the southwest coast of Britain needs careful monitoring as the level of entanglement seen at
this Cornish site implies a significant level of mortality. Although
the British population is considered to be stable, the numbers are
largely based upon pup production data collected around Scotland,
with southwest estimates based on data from 1994 (Sea Mammal
Research Unit, 2008).
Population effects aside, it is important to recognise the welfare
implications for individual seals, with so many animals suffering
from a presumably debilitating and painful condition for months
or even years.
5. Disclosure statement
The authors know of no actual or potential conflict of interest
that could inappropriately influence their work.
Acknowledgements
The authors would like to thank the following individuals and
organisations for their advice and support: Dr. M. Witt (University
of Exeter), Dr. P. McGregor (Cornwall College), Mr. and Mrs. Hick
(Polly Joke Car Park), National Trust, British Divers Marine Life Rescue, Cornwall Wildlife Trust Marine Strandings Network.
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