Library Item
Demography and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon along the open coast of southern California, USA
Description:
The demography, spatial distribution, and movement patterns of leopard sharks (Triakis semifasciata)
aggregating near the head of a submarine canyon
in La Jolla, California, USA, were investigated to resolve
the causal explanations for this and similar shark aggregations. All sharks sampled from the aggregation site
(n0140) were sexually mature and 97.1 % were female.
Aerial photographs taken during tethered balloon surveys revealed high densities of milling sharks of up
to 5470 sharks ha?1. Eight sharks were each tagged with
a continuous acoustic transmitter and manually tracked
without interruption for up to 48 h. Sharks exhibited
strong site-fidelity and were generally confined to a
divergence (shadow) zone of low wave energy, which
results from wave refraction over the steep bathymetric
contours of the submarine canyon. Within this divergence
zone, the movements of sharks were strongly
localized over the seismically active Rose Canyon
Fault. Tracked sharks spent most of their time in shallow
water (?2 m for 71.0 % and ?10 m for 95.9 % of time),
with some dispersing to deeper (max: 53.9m) and cooler
(min: 12.7 °C) water after sunset, subsequently returning
by sunrise. These findings suggest multiple functions of
this aggregation and that the mechanism controlling its
formation, maintenance, and dissolution is complex and
rooted in the sharks’ variable response to numerous
confounding environmental factors.
