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| As winter sets in ice made landing here at Cape Adare impossible for the Our Far South team. |
Well
the Our Far South crew are on their way north now. Unfortunately the ice around Cape Hallett and
Cape Adare had moved in and made any landing impossible – the first signs of
the winter freeze are appearing down here – pancake ice and the coming of
night. We did get a good close look at
the Possession Islands where a number of our intrepid explorers took a sub-zero
dip. We leave with mixed feelings;
regret for leaving such a magical place and the anticipation of the welcome
call of home we are now heading for Campbell Island which we will reach in a
few days time. As we travel north the team is busy with seminars and wide
discussions about fishing, climate change and its effects on the Antarctic, and
tourism.
It’s
about time I talked about the lead actor in the fishing scene down here – the
Antarctic toothfish. A definite case of
beauty being in the eyes of the beholder, the one adjective that first comes to
mind is ‘big’. Imagine a blue cod the
length of a 12 year old human and of a similar weight, colour it grey, paint a
few black bands on its fins, give it several rows of stubby teeth and a wide
gaping mouth and you’ll start to get the idea.
The ‘Toothfish’ moniker applies more its close cousin the Patagonian
toothfish which has a serious row of wolf-like teeth at the front of its jaw –
not so it’s more southern relative.
Antarctic toothfish can grow to a very large
size. The largest I have seen was over 2
metres in length and weighed 155 kg. At
this size we generally find that their reproductive organs have atrophied -
they are long past the age of spawning.
Generally though, they live to about 35 years and by age 13 at least
half the males are spawning. Females
seem to mature a bit later so that by 16 years at least half of them are
reproductively active.
We know that spawning takes place during the
winter, when the Ross Sea Region is covered by ice, on the shallow hills and
ridges of the Pacific Antarctic Ridge to the north of the area. Some of the research I have been working on
indicates that before they spawn they feed up and gain condition in the more
southern 'slope' region (a steep area of seafloor where the shallower 'shelf' descends
into the deep sea). During this time we
think that they 'bulk up' and store energy as lipid fat - like fuel if you like
- to sustain them over their spawning period in the north, where food is more
scarce and toothfish are crowded on restricted grounds. As most spawning has finished by the start of
the season it is uncommon to find fully developed females. The few that I have seen have carried huge
egg masses, one fish I examined of about 60 kg in weight contained 26 kilograms
of eggs, a huge investment in reproduction. The eggs are large - about 3mm in
diameter or the size of tapioca, but still very numerous. What happens next is
still not fully understood. The most
likely hypothesis, and one we are now piecing together with direct observations,
is that the eggs and larval fish drift with the easterly flowing currents
associated with the Antarctic Convergence and possibly reach as far as the
Drake Passage between Antarctica and South America; in fact, some may continue further eastwards
into the Scotia Sea. Generally however, as the small toothfish develop they
move south close to the coast and ice shelves where there is a westerly flowing
counter-current, the Antarctic Coastal Current.
Over a number of years as they grow larger they continue a gradual
movement back westward. We can see this
trend when we measure the lengths (sizes) of these 'juvenile' fish from
different areas of the shelf along the Pacific sector of the continent. In fact this measurement is a major aim of
the pre-recruit survey that I have already covered in another blog. Eventually
this westward movement tends northward as they reach Ross Island and the
western side of the Ross Sea proper into deeper trench around Terra Nova Bay -
from where they move north again onto the 'slope', thus completing the cycle.
What do they eat? Literally anything… As part
of our research during the years I have worked down here we have examined the
stomach contents of many thousand Antarctic toothfish. They are non-selective and voracious feeders
– we have found many species of Antarctic fish in their stomachs - some of
which have been fresh enough to supply as museum specimens to Te Papa. They also eat, squid, prawns, skates, the
remains of penguins killed by leopard seals, we find rocks, in fact just about anything that they
encounter and find remotely interesting and that will fit between their stubby
toothed jaws ends up inside.
Interestingly there are records from krill
trawlers and whalers from the past era of commercial activity that indicates
that these large fish do not restrict their activities to the bottom but move
up and down in the water column. The
lack of a swim bladder which suits rapid vertical movements and the fact that they see using natural light
rather than having a yellow filter on the eye (as is the case for many species
adapted for the deeper sea) supports this.
The fishery catches them on the bottom in water depths from about 800 to
1700 metres deep.
The New Zealand fleet working in the Ross Sea
region have gained Marine Stewardship Certification. This is a reflection of
the effectiveness of the CCAMLR management system, an independent evaluation of
the sustainability of the stock, and a clear tick to the vessels’ responsible
operations. The effect of this is that another
body additional to CCAMLR is monitoring our advances in knowledge and
management of this precious resource.
Which leads on to the next blog – Fishing and
the Antarctic Environment.
