Kyne & Simpfendorfer.. - Shark Specialist Group

SEGREGATION, MOVEMENT AND MIGRATION PATTERNS OF DEEPWATER
CHONDRICHTHYANS
Deepwater chondrichthyans are often segregated bathymetrically by size, sex or maturity
stage. Segregation has been demonstrated by changes in the catch composition with depth,
including for such species as the New Zealand lanternshark Etmopterus baxteri, the black
dogfish Centroscyllium fabricii, the great lanternshark E. princeps, the roughskin dogfish
Centroscymnus owstoni, the Portuguese dogfish C. coelolepis and the spotted ratfish
Hydrolagus colliei (Yano and Tanaka 1988, Wetherbee 1996, Jakobsdóttir 2001, Didier and
Rosenberger 2002). Several studies have noted a lack of gravid females, and it has been
suggested that these may make movements into, or occur in, deeper water (possibly
occupying nursery areas) or that they may be bathypelagic (and thus less susceptible to
capture in benthic fishing gear) (Yano and Tanaka 1988, Wetherbee 1996, Jakobsdóttir 2001).
Nursery areas have not been identified for deepwater shark species.
The short and long-term (including seasonal) movement and migration patterns of deepwater
chondrichthyans are poorly-known. The problems associated with tagging animals caught
from depth and ensuring their survival once returned to the water, along with logistical
constraints, has limited the tagging and tracking of deepwater species. As such tracking
studies of 'deepwater' species has largely been restricted to species which also regularly occur
in the epipelagic zone, at or close to the surface, such as the basking shark Cetorhinus
maximus (discussed briefly under that species' account in the main body of this Section),
sleeper sharks (Somniosus microcephalus and S. pacificus) and the bigeye thresher shark
(Alopias superciliosus). Preliminary tracking of deeper water species has been trialled on the
bluntnose sixgill shark Hexanchus griseus (Carey and Clark 1995), the needle dogfish
Centrophorus acus (Yano and Tanaka 1986) and the Portuguese dogfish Centroscymnus
coelolepis (Bagley et al. 1994).
A number of deepwater squaloid sharks, particularly small pelagic species in the family
Dalatiidae, are vertical migrators. These sharks undertake daily migrations from deep water
towards to the surface at night, returning to depth during the day. Daily vertical movements
may be in excess of 3000m (Compagno in prep. a) and these appear to be linked to the diel
migrations of their prey. Hulbert et al. (2006) showed that diel vertical migrations in S.
pacificus occurred only 25% of the time (i.e. 177 out of 726 days) and as such movement
patterns are more complex than daily repeated vertical movements. Sharks were seen to also
undertake 'systematic vertical oscillations' ('methodical ascents and descents with little pause
in transition') and 'irregular vertical movements' ('small-amplitude movements with random
frequency') with the most time spent at depths of 150-450m (Hulbert et al. 2006).
Diel vertical migration also occurs in the bignose shark Carcharhinus altimus. Anderson and
Stevens (1996) reviewed catch records of the poorly-known species determining that the
sharks occupy epibenthic waters at depths of 90–500m during the day, moving either into
shallower water or upwards into the epipelagic zone at night. Telemetry studies employing
acoustic tags (short-term) and pop-up satellite archival tags (long-term) of another vertical
migrator, A. superciliosus, has revealed consistent diel movement patterns with the species
occurring at depth during the day (generally 200–500m, but with dives to 732m) and moving
up in the water column to occupy depths of 10–130m by night (Nakano et al. 2003, Weng and
Block 2004).
Bagley et al. (1994) used acoustic transmitters imbedded in baits employed at 1517–1650m to
briefly track three C. coelolepis, the deepest tracked chondrichthyans. All sharks moved
outside the range of the recording equipment (500m) within 6 hours of bait deployment and
Bagley et al. (1994) suggested that this indicated no site fidelity. A single C. acus acoustically
tracked for nearly 21 hours in Japan generally swam parallel to the 500m depth contour,
mostly remaining close to the seafloor although making some vertical movements to between
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