Behavioural polymorphism in one of the world's largest populations ...
Behavioural polymorphism in one of the world's largest populations ...
Behavioural polymorphism in one of the world's largest populations ...
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208<br />
Mar Ecol Prog Ser 418: 201–212, 2010<br />
dencies from Cape Verde, West Africa (Hawkes et<br />
al. 2006) and Japan (Hatase et al. 2002, 2007). In both<br />
cases smaller turtles used oceanic habitats with <strong>the</strong><br />
more usual neritic habitats only used by larger <strong>in</strong>dividuals.<br />
In contrast, most <strong>of</strong> <strong>the</strong> turtles <strong>in</strong> <strong>the</strong> present study<br />
extensively used pelagic habitats <strong>in</strong> addition to coastal<br />
neritic <strong>one</strong>s dur<strong>in</strong>g <strong>the</strong> post-nest<strong>in</strong>g phase, with <strong>the</strong><br />
<strong>largest</strong> <strong>in</strong>dividuals mostly utilis<strong>in</strong>g <strong>the</strong> oceanic realm.<br />
We suggest that a polymodal forag<strong>in</strong>g strategy for<br />
<strong>in</strong>dividuals is widespread <strong>in</strong> this population; this was<br />
recently suggested at population level for loggerheads<br />
breed<strong>in</strong>g <strong>in</strong> sou<strong>the</strong>astern USA (Reich et al. 2010).<br />
Additionally it should be noted that sizes <strong>of</strong> all turtles<br />
<strong>in</strong> <strong>the</strong> present study were equivalent to <strong>the</strong> larger<br />
neritic dwell<strong>in</strong>g <strong>in</strong>dividuals from Cape Verde (Hawkes<br />
et al. 2006). We suggest that oceanic forag<strong>in</strong>g may be<br />
equally if not more beneficial <strong>in</strong> energetic terms as<br />
neritic forag<strong>in</strong>g <strong>in</strong> <strong>the</strong> Arabian Sea region. The lack <strong>of</strong><br />
strong size differentiation between neritic and<br />
oceanic-dwell<strong>in</strong>g <strong>in</strong>dividuals has also been shown<br />
<strong>in</strong> green turtles (Hatase et al. 2006, Sem<strong>in</strong><strong>of</strong>f et al.<br />
2008).<br />
The spatial footpr<strong>in</strong>t <strong>of</strong> <strong>the</strong> turtles tagged <strong>in</strong> <strong>the</strong><br />
present study ranged from nor<strong>the</strong>ast Oman to <strong>the</strong><br />
entrance to <strong>the</strong> Gulf <strong>of</strong> Aden, with maximum migratory<br />
distances from <strong>the</strong> nest<strong>in</strong>g region rang<strong>in</strong>g from<br />
approximately 400 to 1400 km. Turtles tagged and<br />
tracked at Masirah Island later <strong>in</strong> <strong>the</strong> same season as<br />
part <strong>of</strong> a separate study also utilised <strong>the</strong> core forag<strong>in</strong>g<br />
area, above Socotra Island, but half <strong>the</strong> <strong>in</strong>dividuals<br />
ei<strong>the</strong>r progressed fur<strong>the</strong>r southwest <strong>in</strong>to <strong>the</strong> Gulf <strong>of</strong><br />
Aden or north and west <strong>in</strong>to <strong>the</strong> Arabian Gulf (Fig. 1,<br />
B. E. Wi<strong>the</strong>r<strong>in</strong>gton & E. Possardt pers. comm.). Limpus<br />
(1985) found that loggerheads from different forag<strong>in</strong>g<br />
areas <strong>in</strong> Australia beg<strong>in</strong> migrations to a s<strong>in</strong>gle nest<strong>in</strong>g<br />
area at different times. Therefore <strong>the</strong>re may be<br />
<strong>in</strong>traseasonal temporal sub-structur<strong>in</strong>g <strong>of</strong> <strong>the</strong> population,<br />
lead<strong>in</strong>g <strong>the</strong> cohorts to experience diverse threats,<br />
separated both spatially and temporally.<br />
Fur<strong>the</strong>rmore, long-distance recoveries <strong>of</strong> loggerhead<br />
turtles tagged on Masirah Island have been<br />
reported by Baldw<strong>in</strong> et al. (2003) (see Fig. 1). They<br />
<strong>in</strong>dicate that turtles travelled south and west to <strong>the</strong><br />
Gulf <strong>of</strong> Aden, north and northwest to <strong>the</strong> Gulf <strong>of</strong> Oman<br />
and <strong>the</strong> Arabian Gulf and additionally to <strong>the</strong> nor<strong>the</strong>ast<br />
to Pakistan or <strong>the</strong> Gulf <strong>of</strong> Kutch. Our present study and<br />
<strong>the</strong> o<strong>the</strong>r telemetry results broadly concur with this<br />
distribution. However, both satellite-track<strong>in</strong>g studies<br />
lack turtles that migrated across <strong>the</strong> Arabian Sea to <strong>the</strong><br />
Indian subcont<strong>in</strong>ent. The significance <strong>of</strong> this omission<br />
needs be determ<strong>in</strong>ed. It may be that dispersal to this<br />
region is rare or it is only demonstrated by mid-season<br />
turtles or those from a different year-cohort not yet<br />
tracked.<br />
Some sem<strong>in</strong>al sea-turtle satellite-track<strong>in</strong>g studies<br />
(e.g. Hays et al. 1991) noted <strong>the</strong> utility <strong>of</strong> satellite track<strong>in</strong>g<br />
for deriv<strong>in</strong>g clutch-frequency estimations. However,<br />
as <strong>the</strong> technique became more widespread, a<br />
shift <strong>in</strong> emphasis on exam<strong>in</strong><strong>in</strong>g post-nest<strong>in</strong>g migrations<br />
occurred (e.g. Luschi et al. 1998, Godley et al.<br />
2002, Hays et al. 2002b). Many studies <strong>the</strong>n focused on<br />
end-<strong>of</strong>-season deployment to avoid dangers to <strong>the</strong><br />
transmitter <strong>in</strong> <strong>the</strong> <strong>in</strong>ternest<strong>in</strong>g environment (e.g. clean<strong>in</strong>g<br />
behaviour <strong>in</strong> Sch<strong>of</strong>ield et al. 2006). Their f<strong>in</strong>d<strong>in</strong>gs<br />
may present a biased representation <strong>of</strong> <strong>the</strong> true spread<br />
<strong>of</strong> post-nest<strong>in</strong>g movements for <strong>the</strong> population as a<br />
whole, as suggested here. This bias may be exacerbated<br />
with results obta<strong>in</strong>ed from a s<strong>in</strong>gle nest<strong>in</strong>g<br />
season, strongly suggest<strong>in</strong>g that to obta<strong>in</strong> a full representation<br />
<strong>of</strong> a population, both <strong>in</strong>tra-seasonal temporal<br />
spread and multi-annual telemetry need to be undertaken,<br />
as demonstrated by Whit<strong>in</strong>g et al. (2007) and<br />
<strong>in</strong>dicated by Shill<strong>in</strong>ger et al. (2010). This flags <strong>the</strong><br />
necessity for exist<strong>in</strong>g studies <strong>of</strong> mar<strong>in</strong>e turtles and<br />
o<strong>the</strong>r megafauna to be revisited to determ<strong>in</strong>e if<br />
suitable spread <strong>of</strong> sampl<strong>in</strong>g has been achieved to<br />
substantiate <strong>the</strong> <strong>in</strong>ferences made.<br />
The results <strong>of</strong> <strong>the</strong> present study also identify plasticity<br />
<strong>in</strong> behaviour dur<strong>in</strong>g <strong>the</strong> breed<strong>in</strong>g season. No l<strong>in</strong>k<br />
between body size and behaviour was established and<br />
some <strong>in</strong>dividuals carried out coastal sedentary and<br />
wider-rang<strong>in</strong>g movements <strong>in</strong>volv<strong>in</strong>g oceanic loop<strong>in</strong>g.<br />
This is <strong>the</strong> third population <strong>of</strong> loggerheads, <strong>in</strong> addition<br />
to those <strong>of</strong> Japan (Sakamoto et al. 1990) and <strong>the</strong><br />
Cayman Islands (Blumenthal et al. 2006), <strong>in</strong> which<br />
<strong>in</strong>dividuals demonstrate oceanic habitat utilisation<br />
dur<strong>in</strong>g <strong>in</strong>ternest<strong>in</strong>g periods, contrast<strong>in</strong>g with o<strong>the</strong>r<br />
<strong>populations</strong> (e.g. Greece: Zb<strong>in</strong>den et al. 2007; Cyprus:<br />
Fuller et al. 2008; USA: St<strong>one</strong>burner 1982, Plotk<strong>in</strong> &<br />
Spotila 2002; Australia: Tucker et al. 1996) and fur<strong>the</strong>r<br />
highlights <strong>the</strong> highly plastic behaviour <strong>of</strong> this species.<br />
We would note, however, that <strong>in</strong> a recent study <strong>of</strong> 12<br />
loggerhead turtles <strong>in</strong> <strong>the</strong> USA (Hawkes et al. 2007),<br />
<strong>one</strong> female spent at least some time <strong>in</strong> oceanic habitats<br />
before return<strong>in</strong>g to neritic habitats. Thus, as our<br />
knowledge base expands it is likely that we will<br />
discover that multiple behavioural strategies are employed<br />
<strong>in</strong> o<strong>the</strong>r <strong>populations</strong> previously thought to<br />
conform to <strong>the</strong> typical model.<br />
Female loggerhead turtles typically have 2.5 to<br />
3 yr remigration <strong>in</strong>tervals between breed<strong>in</strong>g years<br />
(Schroeder et al. 2003); however, evidence is accumulat<strong>in</strong>g<br />
for <strong>the</strong> common occurrence <strong>of</strong> s<strong>in</strong>gle-year remigrations<br />
from flipper-tag returns (Broderick et al. 2001,<br />
Schroeder et al. 2003) and us<strong>in</strong>g satellite track<strong>in</strong>g<br />
(Cape Verde: B. J. Godley et al. unpubl.; Cayman<br />
Islands: Blumenthal et al. 2006). We confirm, as previously<br />
reported (Ross 1979), that s<strong>in</strong>gle-year remigrations<br />
occur with<strong>in</strong> <strong>the</strong> Masirah population at an un-