The habitat
The deep-sea is the largest habitat on earth.
The area over 4000m in depth covers 53% of the
sea's surface, which in turn covers 71% of the
world's surface! The continental slopes alone
occupy 8.8% of the world's surface, compared to
7.5% for the continental shelf and shallow seas. It
is a predominately dark and cold environment with
much lower productivity than shallower ones.
The "deep-sea" water mass can be sub-divided
into four depth zones: mesopelagic (150-1000m);
bathypelagic (1000-3000m); abyssopelagic
(3000-6000m); and hadal zone, below 6000m depth, in
the deep ocean trenches. From a demersal, or
seafloor perspective, the deep-sea region consists
of the continental slopes (starting at the shelf
break and corresponding to the mesopelagic and
bathypelagic zones) the continental rise which
extends down to the abyssal plane at around 6000m,
and the trenches. The seamounts stand out of the
abyssal plain
No light penetrates beyond 1000m and even at
depths of 150m light levels are reduced to 1% of
those at the surface, insufficient to support
photosynthesis. Thus, organic material must be
convected into the deep-waters, which occurs in
various ways. Dead phytoplankton and nekton sinks,
and though much is consumed as it settles,
sufficient amounts enter the deepwater to sustain
much of the biomass there. Many species undergo
extensive diel vertical migration, feeding in the
surface waters and moving down during the day,
reducing predation. In this way, surface production
is cascaded through progressively deeper layers. Of
relatively minor productive importance is organic
material from large carcasses sinking to the
seafloor, e.g. dead whales, and sulpha-based
organic production associated with deep-sea
seafloor hot-water vents. Nevertheless, the
concentration of organic material decreases
exponentially with depth.
In contrast to former views, it is now known
that seasonal effects in surface layers are
transferred into even deeper ocean regions so that,
despite the physical uniformity of the deep oceans,
an annual production signal exists resulting in
seasonal migrations and reproductive cycles in
deep-sea fauna.
The Fishes
Deepwater fishes comprise three major groups:
pelagic fish living largely in midwater, with no
dependence on the bottom; demersal fish, living
close to and depending on the bottom; and
benthopelagic fish, living close to the bottom but
undertaking short migrations in the watermass (e.g.
for feeding). In general, the deep-sea demersal
fishes come from phylogenetically much older groups
than the pelagic species (the first existing
demersal species were present around 80 million
years ago). While most of the demersal deep-sea
families are found worldwide, the existence of
isolated deepwater basins bounded by the continents
and mid-oceanic ridges has resulted in regional
differences believed to be a consequence of
continental drift and subsequent ocean formation.
Much remains unknown about deepwater fishes and new
discoveries continue, such as the megamouth shark
(a 4.5m and 750kg shark) and the six-gilled ray,
which both represent new families. Since the
demersal species are distributed according to
depth, those inhabiting the continental slope and
rise are spread along ribbon-like depth regions
along the perimeters of the oceans. Where deepwater
pelagic species and demersal species co-occur, they
usually prey on each other.
Life History Characteristics and
Productivity
Just as for epipelagic fishes, deepwater species
must successfully spawn, grow and return to the
area of the adult habitat. The extreme conditions
of the deep-sea are reflected in the variety of
reproductive strategies that exist. Low population
sizes notwithstanding, hermaphroditism, extreme
sexual dimorphism and unbalanced sex ratios occur.
Sebastes spp., certain ophidioids, as well
as deepwater sharks can be live bearers and the
pseudotriakid, Pseudotrakis microdon, is
oviphagous. Despite the fewer number of species in
the deep-seas, those that occur display a variety
of reproductive methods ranging from strongly
K-selected species, which may be semelparous
(e.g. Coryphaenoides armatus, a
widely occurring macrourid) through ovoviviparous
and oviparous species, to those that are strongly
r-selected. And, in the perpetual darkness
of the abyss, many species depend on photophores
and sound production for intra-species recognition
required for successful reproduction.
Many deepwater species grow slowly, so slowly in
fact that determination of their actual age remains
difficult and contentious. For some species,
particularly orange roughy (Hoplostethus
atlanticus), no convincing case has yet emerged
for any particular ageing technique based on
interpretation of otolith microstructure. Depending
on the assumptions made, this species may have
longevity ranging from 21 to more than one hundred
years. Because of these biological characteristics,
most deep-sea species are very fragile with reduced
resilience to intensive fishing.
Deep-sea Fisheries
Until most recently, the great depth of the
deep-sea has made it difficult to exploit and the
existence of relatively more abundant resources in
shallower seas have meant that little incentive
existed to fish in such difficult-to-exploit
regions. Few deepwater fisheries are of long
standing and those that are - the Portuguese
(Madeira) line fishery for black scabbardfish
(Aphanopus carbo), the Pacific Island
fisheries for snake mackerels (Gempylidae) and
cutlass fish (Trichiuridae) or the west African
fisheries for deep-sea sharks (for extraction of
scalene) - were initially artisanal.
With the reduction of opportunities for
development of inshore fisheries and the
improvement of gear technology and navigation
instruments, deep-sea fishing has expanded in the
1990s. A well-known example of recently developed
deepwater fisheries is that of the orange roughy, a
species that inhabits the slope waters and those of
seamounts (as well as the seafloor), particularly
around New Zealand and Southeast Australia where
this commercial fishery initially began. The
fishery later spread to the Walvis Ridge in the
Southeast Atlantic (Namibia) and the Southwest
Indian Ocean. A small fishery even exists in the
Bay of Biscay. This long-living fish reaches about
40cm and 2kg in size though the maximum size varies
with region. Specially--aimed trawling techniques
were developed after initial massive catches from
spawning aggregations were taken in a matter of
minutes resulting in split codends. Orange roughy
is particularly sensitive to approaching objects
(perhaps an adaptation to avoid predation) so that
acoustic assessment using towed bodies containing
the transducer have proved futile in some areas.
Maximum sustainable levels of exploitation of
orange roughy may be as low as 5-10% of unfished
biomass, corresponding to natural mortalities (M)
of about 0.04 per year. Accumulating evidence about
stock declines indicates that none of these
fisheries are being exploited sustainably and
ongoing yields will likely be around 5% of those
initially obtained.
A Trichiurid fishery, which exploits
Aphanopus carbo in the Atlantic, is a rare
example of a deepwater fishery that, because it has
traditionally used hook and line gear, has proved
sustainable over a period of about 150 years.
Adults of this species are benthopelagic living in
the deep range 400-1600m. The species ranges from
Greenland to the Canary Islands and on both sides
of the mid-Atlantic ridge. Unusual for a deepwater
species, A. carbo grows rapidly and has
longevity of around 8 years. However, as with
orange roughy, the usual ominous signs are now
evident for this fishery. Catch rose from 1100t in
1980 to 3000t in 1992, gear efficiency has improved
through the introduction of monofilament lines and
in a large increase in the number of hooks per line
set, now at 4000-5000 per line.
The Macroudidae are another group whose members
are widespread and, in particular locations,
abundant. They are typical pelagic 'cruisers' and
inhabit the mid-to-upper region of the continental
slope. In the North Atlantic, fisheries exist for
Macrourus berglax and Coryphaenoides
rupestris using bottom trawls initially fishing
in depths of 600-800m, and more recently extending
down to 1500m depth. However, experience in these
fisheries off Newfoundland shows the
all-too-familiar pattern of total allowable catches
tracking declining trends in reported landings of
this group. Coryphaenoides rupestris have a
potential longevity of 70 years, although in the NE
Atlantic fish ages are usually in the 20-30-year
range. Thus, as for other deepwater species,
Macrourids exhibit the characteristics of many
deepwater fisheries that render them susceptible to
overfishing.
The Pleuronectidae are a highly-evolved group
that are not usually associated with deepwater
fisheries, but important fisheries for members of
this group occur in both the North Atlantic and
North Pacific Oceans. In the Atlantic, the best
known has been that for Greenland Halibut
(Reinhardtius hippoglosoides) on the
continental slope depths. This fish had an average
size of around 1kg up until the mid-1980s, but has
since declined to around 200g in the early
1990s.
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