Biological Environment of the West Coast Coastal Marine Area: an outline of Marine and Coastal Ecosystems and Habitats
3.6 The Deep Nearshore Domain
 Figure 3.6 Deep nearshore substrate types. Source: Stevenson 2004, RNZN (various dates), Mitchell 1987, Price 1983a&b, McDougal 1975 & 1982 |
The deep nearshore domain mostly comprises the waters and seabed of the continental shelf. However, some areas of continental slope (as well as the heads of underwater canyons) do lie within this domain (i.e., within the 12 nautical mile limit) in the southern part of the West Coast marine area. The extent of the continental shelf, the transition to the continental slope, and the position of the underwater canyons are shown in Figure 2.7 in Chapter 2. The distribution of sea floor substrates in the deep nearshore domain is mapped in Figure 3.6. Chapter 2 noted that for the purposes of this report, the deep nearshore zone extends from the 30 metre depth contour out to the 12 nautical mile limit. Both the 12 nautical mile limit and the 200 m depth contour are shown in Figure 3.6. 3.6.1 Biodiversity Associated with different Substrates The distribution of substrates in the deep nearshore domain is not extensively documented. Probert and Swanson (1986) surveyed sediment texture between north of the Karamea Bight and the Whataroa River and other researchers have documented specific features in the region. Substrates and the biological environment in the deep nearshore domain are most easily discussed in relation to the three topographical features noted in section 2.3.5 and illustrated in Figures 2.7 and 2.12 – the continental shelf, the continental slope and underwater canyons. Continental Shelf The shelf is mostly composed of mobile soft sediment beds of sand, silt and mud63. These mobile sediments provide habitat for a diverse range of marine worms and shellfish. The seabed-dwelling worms of the West Coast continental shelf are similar to those elsewhere around New Zealand and other places in the world64. However, Probert et al (2001) considered that certain environmental aspects of the West Coast (such as variable freshwater inflows, high sediment inputs, storm waves and episodic upwelling) may make it a favourable habitat for opportunistic species such as some marine worms and other types of invertebrates. Some areas of the shelf comprise substrates of coarser materials. Rocky reefs occur in some places, but they are difficult to survey so there are only a few studies that have researched these deep reefs (e.g. greater than 30 metres depth). Stevenson (2003) presents maps of ‘foul ground’ areas in the Kahurangi to Haast area that are regarded, for the purpose of the NIWA trawl surveys, as unsuitable for bottom trawling due to the potential for nets to get snagged by seabed rocks, logs and other debris. While deep reefs and foul ground are relatively uncommon in the northern parts of the West Coast marine and coastal area65, they are more common in the more dissected continental shelf landforms of South Westland66. There is some anecdotal information derived from fishers about the species that occur in some of these deep rocky reefs and foul ground sites (for example, there are reports of black coral in South Westland), but there is very little published material. |
Few specific sites on the continental shelf have been identified in the published literature as of particular significance. The main exceptions are:
- the ‘Harvester Prospect’, that was found in the 1980s and 1990s to contain significant concentrations of placer gold, but not in economic quantities;
- the Kahurangi Shoals, an area of coarse bed materials where the currents are modified by the seabed topography;
- an area of freshwater upwelling off Point Elizabeth, presumably originating from limestone structures related to the karst (limestone) areas of the Paparoa Syncline.
Fish assemblages associated with the West Coast continental shelf area feature species such as67 flatfish, stargazer, school shark, spiny dogfish, barracouta, red cod, gurnard and rig. Towards the edge of the shelf, deeper water species become more prevalent, including: rattails, tarakihi, hoki and ghost sharks. Fish (and invertebrate) assemblages are discussed in more detail at the end of this section.
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Tarakihi. Photo: P Ryan, DOC collection |
Continental Slope
The continental slope begins at the ‘shelf break’, at a depth
of about 200 metres. Here, the seabed drops off more steeply into deeper waters
towards the Challenger Plateau and to even greater depths of about 4000 m
in the Tasman Basin. The shoreline was positioned at about the shelf break
during periods of lower sea level in the Ice Ages. The continental slope extends
to within the coastal marine area only south of about Heretaniwha Point. Depths
of about 2000 metres are reached in some places within that area. The continental
slope is dissected in several places by steep-sided canyons.
Like the continental shelf, the upper continental slope is mostly composed
of soft sediments like sand, silt and mud. Studies of bottom-dwelling worms,
shellfish, etc, in northern parts of the West Coast show a change in invertebrate
assemblages with increasing depth across the continental shelf and slope68;
however, these surveys did not extend into South Westland, where deep waters
(greater than 200 metres) occur within the 12 nautical mile boundary that
marks the extent of the West Coast coastal marine area.
Trawl surveys in the 200–400 metre depth range between Okarito and Haast
River69 give an indication of the fish assemblages
of the upper parts of the continental slope. The most common species caught
in these surveys were: ling, stargazer, hoki, ghost shark, red cod, tarakihi,
spiny dogfish, arrow squid and barracouta. Compared to the continental shelf,
some species such as ling, hoki, ghost shark and stargazer become more common
in these greater depths, while others such as spiny dogfish and barracouta
become less abundant; gurnard and rig are virtually absent. However, the trawl
survey data (see elsewhere in this chapter) also shows that the catch rates
for each species can be quite variable from year to year.
Canyons
Submarine canyons in the West Coast coastal marine area occur from the Hokitika
Canyon southwards. They extend well beyond the 12 nautical mile limit, the
combined Hokitika-Cook Canyon system running for over 650 kilometres offshore70.
The South Westland area between the Hokitika Canyon and Awarua Point includes
the largest submarine canyon features on New Zealand’s western coast,
and one of the main concentrations of nearshore canyon ecosystems in New Zealand.
Canyons provide a great variety of seabed terrain, and their form tends to
alter the current systems, in particular by channelling the upwelling of the
Tasman Current onto the continental shelf. In physical terms, recent studies
indicate that the Hokitika and Cook Canyons act as major ‘sediment sinks’,
draining fine sediments off the continental shelf down into the deeper waters
of the Tasman Basin71. This is probably also the
case for the less studied canyons to the south.
The Hokitika Canyon is the largest of the West Coast canyons, encroaching
to within about 8 kilometres from the coastline72.
Other canyons to the south encroach much closer to the shore; these include
the major features of the Cook, Moeraki, Haast, Arawata, Jackson and Cascade
Canyons. Depths of up to 2000 m are thought to occur within 12 nautical miles
offshore in the Cascade Canyon, and depth ranges of about 400–1000 m
in the other canyons73. The Hokitika and Cook Canyons
have been recently mapped in great detail by a joint NIWA-French expedition,
which included geological and substrate surveys74.
Some trawl surveys have been located along the margins of submarine canyons,
and so the fish assemblages described later in this chapter could be expected
to apply to areas of similar substrate at the heads of the canyons to depths
of 400 metres. However, those surveys also indicated considerable areas of
foul ground, and these substrate differences could be expected to produce
corresponding differences in the biological communities that live there. There
is no information from the West Coast inshore trawl surveys for depths greater
than 400 metres (depths which occur in territorial waters within the canyons
of South Westland). It is to be expected that the fish assemblages change
with depth in these canyons. Some fish species, such as orange roughy, seal
shark and other deep-sea sharks, rattails, and dories are known to occur in
South Westland; furthermore, these species are known to become more common
in New Zealand waters at these greater depths75.
However, there is no specific published report on the fish assemblages of
waters deeper than 400 m in the West Coast coastal marine area.
The association between canyons and spawning fish such as hoki, ling and hake
is an indication of their biological richness and their suitability for the
propagation of some marine species and the dispersal of their offspring by
upwellings and other coastal current systems. In addition to these commercial
species, large aggregations of non-commercial fish (e.g. lanternfish) are
also know to occur in these canyon (and associated continental slope) areas
of the West Coast. These fish and their spawn in turn provide a large food
source for larger predators such as barracouta, fur seals and orca76.
3.6.2 Invertebrate and Fish Species in the Deep Nearshore Domain
New Zealand’s nearshore invertebrate and fish species have been documented
in a number of studies for a variety of commercial and noncommercial purposes.
However, detailed information on the distribution of species is limited.
Seabed Invertebrates
Seabed invertebrates have been assessed in a number of studies77.
The samples were mostly taken by the New Zealand Oceanographic Institute (NZOI)
in the 1980s along transects out from Karamea, Westport, Greymouth and Whataroa,
in depths of 30 to 1120 metres.
Shelf seabed invertebrate assemblages in the West Coast study area have been
assessed by several authors. Grange (1990) identified two communities within
that area, separated by depth, substrate type and location:
- a coastal and sandy bivalve community north of about Heretaniwha Point of mostly common New Zealand species; and
- a deeper and muddy bivalve community south of there dominated by a different group of species and with a higher species diversity.
- an inner shelf community on silty sand (32–51 m depth);
- a mid to outer shelf community on sandy mud (87–297 m);
- an outer shelf community on sand at the northern end of the study area (195–248 m); and
- an upper slope community on sandy mud (477–1120 m).
The first two of these types included sample sites from within the West
Coast coastal marine area. This pattern was largely confirmed by an assessment
of marine worm distributions by Probert et al (2001). In addition, this
latter study found that the community structure changes across the shelf
and upper slope, and to a lesser extent with latitude. The study concluded
that the distribution pattern of seabed worms appears to be mostly related
to water depth and seabed clay content.
Probert (1986) found that pelagic phytoplankton (sea-surface plant microbes)
are the main source of energy through the food chain for shelf invertebrates
and bottom-feeding fish. River inputs provide a lot of freshwater and sediment,
but relatively little in the way of nutrients.
Fish Assemblages
The relative annual abundance of a variety of nearshore fish species that
occupy the waters of the West Coast continental shelf and slope (in depths
ranging from 20 to 400 metres) has been determined through periodic trawl
surveys. Trawl surveys of the West Coast South Island and Tasman and Golden
Bays were undertaken on five occasions from 1994 to 200378. In all, 90 different
species of fish have been recorded from within territorial waters of the
West Coast region in these trawl surveys.
Stevenson and others assessed the distribution and abundance of fish in
three depth strata throughout Marlborough, Nelson and the West Coast. The
West Coast information was summarised and further analysed by Neale (2006c).
The results of these surveys for the whole survey area show that seven main
species (spiny dogfish, barracouta, red cod, giant stargazer, arrow squid,
tarakihi, and hoki) and one species group (rattails) comprise over 60% of
the catch (see Figure 3.7).
Figure 3.7
Common benthic fish species on the West Coast.
(Graph derived from 5 NIWA trawl survey reports. See Neale 2006c, Stevenson 2004, Stevenson 2002, Stevenson & Hanchet 2000, Stevenson 998, Drummond & Stevenson 996, Drummond & Stevenson 995a)
Barracouta
Photo: Mfish photo library
However, a wide variety of species were caught during the NIWA surveys,
with 21 species each comprising at least 1% of the catch. While the relative
abundances of these species might vary between the Nelson and West Coast
regions, available information indicates that this general pattern also
applies to the West Coast region on its own.
Hurst et al (2000) summarised the known areas of importance for New Zealand
coastal and continental shelf fish species, with particular reference to
spawning, pupping or egg-laying, and juvenile populations. Their report
selected 35 “commercially important” fish species that are found
regularly in research trawls in depths of less than 200 metres. O’Driscoll
et al (2003) undertook a similar exercise for 32 deepwater and pelagic fish
species that are found regularly in trawl and longline fisheries in the
200 to 1500 metres depth range. Those two reports found the South Island
West Coast included marine areas significant for the species listed in Table
3.1. Data from O’Driscoll et al (2003) indicates that deeper waters
off parts of the West Coast (especially north of about the Cook and Hokitika
Canyons) are areas where large aggregations of juveniles of a number of
commercially important fish species occur – such as hoki, hake, ling,
frostfish, orange roughy and lookdown dory.
Table 3.1
Abundances of fish species reproducing in West Coast marine areas.
Summarised from Hurst et al 2000 (inner & outer shelf species); O’Driscoll et al 2003 (deepwater & pelagic).
[NOTE: The ‘outer shelf’ and ‘deep’ classes in the table below are relevant to the West Coast Marine Protection Forum process only where these environments occur within the coastal marine area defined for this study].
The West Coast was considered to be of low or unknown importance for the other commercial fish species listed by Hurst et al (2000) and O’Driscoll et al (2003) and indicated in the table above. Also note that these two reports did not consider all fish species, including some that are known to be common on the West Coast (e.g. some flatfish, many reef fish species and all non-commercial fish).
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Giant stargazer Photo: S Wing, DOC collection |
Fish Spawning
The winter months are when the most fish spawning activity occurs off the
West Coast. The greatest amount of spawning by hoki, hake, ling and associated
species occurs along the continental shelf edge and associated canyons from
about the Heaphy Bluff to Hokitika. Similar areas off South Westland are
known spawning grounds for some deepwater species, such as orange roughy.
The West Coast of the South Island (during August-September) is one of four
known spawning areas in New Zealand for blue warehou. The South Island West
Coast has a warehou fishery during August/September and again later in summer.
The spawning seasons of hake, silver warehou, ling, gemfish, blue warehou,
giant stargazer, barracouta, jack mackerel, tarakihi and red cod overlap
with the hoki spawning season on the South Island West Coast79.
Some of the 90 fish species recorded from NIWA trawl surveys in the deep nearshore domain:
Top row: Black flounder (patikimohoao). Photo: W Farelly, DOC;
Spiny dogfish (koinga). Photo: S Wing, DOC collection
Bottom row: Gurnard (kumukumu). Photo: W Farelly, DOC collection;
Snapper (tamure). Photo: J Major, DOC collection.
Hurst & Bagley (1984) surveyed for spawning barracouta in early September
1984, from Cape Foulwind to Cook Canyon, and found that:
“....catch rates of barracouta and some of the other main species
(e.g. spiny dogfish, tarakihi, red cod, common warehou, stargazer) were
significantly greater in the southern half of the survey area, between Hokitika
Trench and Cook Canyon. Several species were spawning or about to spawn
in the period of the survey, namely barracouta, common warehou, silver warehou,
red cod, red gurnard and ling.”
3.6.3 Existing Protection in the Deep Nearshore Domain
These New Zealand territorial waters are governed by general controls under
a variety of legislation (e.g. the Fisheries, Resource Management, Wildlife,
Foreshore and Seabed and Maritime Transport, Acts). Other than these general
controls, there are no specific protection or management areas over the
continental shelf, continental slope or underwater canyons within the West
Coast coastal marine area.
3.6.4 Uses Associated with the Deep Nearshore Domain
The waters of the continental shelf, continental slope and underwater canyons
are presently used mostly for fishing (including trawling, net and line
fishing) and for shipping and navigation; most of these are commercial users
but there are also some recreational users. Mining or other extraction operations
on the seabed have not been undertaken to date, although there has been
some mineral exploration (e.g. for hydrocarbons and gold) and associated
geological studies80.
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Commercial fishing is a predominant use of the deep nearshore domain: stern trawler approaching Greymouth Photo: S Nimmo |
63 Probert and Swanson 1985
64 Probert et al 2001, p980
65 I McKenzie, pers comm 2006
66 e.g. Stevenson 2004
67 e.g. as indicated by the NIWA fishery trawl survey reports, such as Stevenson & Hanchet 2000
68 Probert et al 2001
69 Stratum #16 from Stevenson & Hanchet 2000
70 Cox2005
71 H Neil pers comm 2006
72 Probert and Swanson 1985
73 Carter 1981
74 H Neil pers comm 2006
75 e.g. see Anderson et al 1998
76 McDiarmid et al 2005, Livingston 2002
77 Probert et al 2001, Probert & Grove 1998, Probert & Anderson
1986, Probert 1986, Probert & Swanson 1985
78 Neale (2006c), Stevenson 2004, Stevenson 2002, Stevenson & Hanchet
2000, Stevenson 1998, Drummond & Stevenson 1996, Drummond & Stevenson
1995
79 Livingston 2002
80 e.g. Corner 1989, Kingett & Associates 1987, Price 1985, Price 1983a,
Price 1983b, Carter 1980, Alpine Geophysical Associates 1968