Biological Environment of the West Coast Coastal Marine Area: an outline of Marine and Coastal Ecosystems and Habitats
3.5 The Shallow Subtidal (Open Coast) Domain
The shallow subtidal (open coast) domain extends out to depths of about 30 metres where wave action and light penetration begins to have a lesser influence at the seabed. The changes in physical environment that occur on the open coast over distances of just several metres can produce sharp changes and complex zonation patterns; these are mostly related to depth and substrate type, but also to the degree of the coast’s exposure to wave energy. As a consequence, biodiversity patterns can occur in distinct bands: such as a low-tide ‘fringe’ of bull kelp below intertidal mussel beds, and clam beds on subtidal sand next to expanses of filter-feeding invertebrates on rocky reefs. Some open coast species, such as reef star, giant triplefin, Fiordland crested penguin, Hector’s dolphin and juvenile inanga, are more common in the West Coast shallow subtidal domain than in most other parts of New Zealand.
Soft substrates such as clean sand (<10% mud) and silty sand cover most of the inner continental shelf seabed of the shallow subtidal depth zone but gravel beds and areas of hard substrates like submerged rocky reefs are also common. These sea floor sediment types are mapped in Figure 3.4. Logs and driftwood also occurs on the beaches, seabed and sea surface. Other than studies of some organisms (e.g. burrowing worms and other invertebrates) that live in or on the logs, and the biota of wharf piles and ship hulls48, no information has been published on natural wood habitats in New Zealand marine waters49. Shallow Subtidal Rocky and Boulder Reefs Where mountain ranges and hill country reach down to the coastline, the rock formations extend out into the sea to form intertidal and shallow rocky reefs. Patchy reefs are also found in other places, such as at the ends of bouldery moraine bluffs (see Figure 3.4). Subtidal rocky reef habitats in the West Coast region have been studied for:
|
 Figure 3.4 Shallow subtidal (open coast) substrate types. Source: Neale 2006f, RNZN (various dates), Mitchell 1987, Price 1983a & b, McDougal 1975 & 1982 |
 |
 |
| Filter-feeding musseld and robust starfish are a feature of northern subtidal rocky reefs such as these at aCape Foulwind Photo: N Shears |
Subtidal 'seaweed turfs' are a feature of the southern West Coast, such as here at Jackson Head Photo: N Shears |
Shears (in prep) surveyed shallow subtidal reef communities around the New Zealand coast, including 24 sites within eight general areas in the West Coast region, from Little Wanganui Head to Gorge Islands50. That report indicates that West Coast reef communities have clear ecological differences from other parts of the New Zealand coast. The marine biodiversity of the reefs of the northern West Coast were found to be different from those in the southern West Coast51, mostly because of variability in the physical environment from north to south (see Chapter 2).
In the northern reefs from Kahurangi to Greymouth, the rocks typically extend
to depths of only about 10 to 20 metres before ‘bottoming out’ to a sand/silt bed, and are usually dominated by filterfeeding invertebrates
(such as mussels and sea squirts) and robust seaweeds. Characteristic species
in these shallow reefs include bull kelp, blue and greenlip mussels, reef
stars, and giant triplefins.
Reefs in the central West Coast are less extensive and less well studied,
but they are mostly dominated by species that prefer bouldery habitats and
can withstand the heavy scouring effects of waves and sand.
In the southern reefs from about Bruce Bay to Awarua Point, a wider diversity
of species live among the rocks, including extensive subtidal seaweed ‘turfs’52
and a larger variety of reef fish53. These southern
reefs tend to reach to greater depths (sometimes well beyond safe diving depths)
and so support a wider diversity of species.
The subtidal reefs around rock stacks and islands tend to be less subjected
to the effects of wave action, sand scour and turbidity than those closer
to the coast, and the reefs often extend to greater depths. This feature can
result in a different (and often greater) variety of marine plants and animals,
such as is found at the Three Steeples off Cape Foulwind54
or at Open Bay Islands (Taumaka me Popotai)55. Rock
stacks and islands are also discussed in the intertidal section of this chapter.
The substrates of shallow rocky reefs include a variety of types, including
boulder ramps, bedrock slopes and vertical faces. Elsewhere in New Zealand,
these different substrates typically support different marine species assemblages,
but no attempt has yet been made to assess such variability with West Coast
reef type.
Each row from top left: Thornfish; Scarlet wrasse (püwaiwhakarua); Blue cod (räwaru);
Blue-eyed triplefin.
Photos: P Ryan, DOC collection.
Several reports56 examine the fish fauna of
shallow rocky reef habitats on the West Coast (including parts of Fiordland).
An analysis of the data from Roberts et al 2005 (Neale 2006b) shows that surveys
have recorded 78 species of coastal reef fish on the West Coast; nine of these
are common and widespread on the West Coast. The Buller/Westland area is a
stronghold for about six species that prefer shallow turbid pools and reefs
(such as the giant triplefin, orange clingfish and olive rockfish), while
South Westland is more suited to about 22 species that prefer holes, overhangs,
and open turf/seaweed reefs (such as the rockling, common roughy and several
species of triplefin).
Neale & Nelson (1996) identified about 175 species of seaweed from the
West Coast, growing in a variety of forms and habitats. That number continues
to increase with additional surveys and knowledge. For instance, fourteen
new records of crustose coralline algae species have recently been identified
for the West Coast region by Harvey et al (2005). The most obvious seaweeds
are the large brown types, such as the bull kelp Durvillaea that grows in
the low tide zone on most rocky coasts. Bull kelp is a habitat-forming species
which has a major effect on the structure of coastal reef communities. It
attaches firmly to low tide rocks in exposed sites, and its blades of up to
several metres length swirl in the surf, shading the rock surfaces below.
Two species are present on the West Coast, the honeycomb-fronded Durvillaea
antarctica and the stoutly-stemmed D. willana.
 |
Bull kelp, Durvillaea antarctica is a feature of many West Coast intertidal rocky shores Photo: P Ryan, DOC collection |
 Figure 3.5 Mussel beds are especially common in the northern parts of the West Coast. Source: Neale unpublished DOC data. |
Other brown kelps, such as zigzag weed and oak leaf weed, grow in low densities at some locations; other kelps, such as paddle kelp, flapjack and Lessonia, are present at only one or two locations. Seaweed ‘turfs’ are common on intertidal rocky shores in the northern West Coast; these turfs are mainly made up of species of Champia, Gigartina, Lophurella, Haliptilon and Scytothamnus. In contrast, the seaweed turfs in South Westland occur more commonly in the subtidal zone, and consist of a different variety of species of Plocamium, Anotrichium, Glossophora and Euptilota. An area of particular significance is Taumaka me Popotai (Open Bay Islands), which supports the West Coast region’s greatest recorded diversity of seaweeds.
|
The West Coast rocky reefs include a range of distinctive biota that are uncommon or absent in other parts of New Zealand. Together, these features produce ecological patterns that make this habitat on the West Coast significantly different to other parts of New Zealand. These biodiversity features include:
- a genetically distinctive wild population of greenlip mussels throughout the West Coast – Fiordland area57;
- large populations of the giant triplefin (a coastal reef fish that is rare elsewhere in New Zealand), especially in parts of the region north of about Okarito;
- a diversity of crustose coralline algae (the pink ‘crusts’ commonly seen on shore and sub-tidal rocks), which Harvey et al (2005) regard as a diverse and ecologically significant group of plants. A total of 15 species (out of 20 identified from the central New Zealand region) were recorded from four localities sampled on the West Coast, including ten species from the Steeples off Cape Foulwind;
- New Zealand fur seal colonies that support an estimated 20% of New Zealand’s breeding population of this species58, as well as large winter haulout colonies; and
- Unusual colour forms that have been recorded only from the West Coast, of two fish: a scorpionfish (at Taumaka in 1996) and a spotty (at Hokitika in 1867 and Jackson Bay/Okahu in 1995).
 |
 |
| Giant triplefin Drawing by F E Clarke |
An unusually yellow scorpionfish. Photo: P Ryan, DOC collection |
A number of conspicuous or readily-recognised marine species that are common
in other parts of New Zealand are known on the West Coast from only one
or a small number of localised sites. These include: seahorses (Jackson
Bay/Okahu), paddle kelp (Open Bay Islands and Jackson Bay/Okahu), Lessonia
weed (Open Bay Islands), and agar weed, Pterocladia lucida (Wekakura Point).
While they might occur elsewhere in the region, these species are unlikely
to be widespread due in part to their general aversion to exposed sites
with high wave energy.
Shallow Subtidal Gravel Beds
The occurrence of gravel beds are indicated by seabed surveys at locations
such as the Kahurangi Shoals and the ‘Harvester Prospect’, as
well as adjacent to cobble and gravel beaches along the coast. However,
the biology of such areas on the West Coast has not been specifically studied.
Shallow Subtidal ‘Soft’ (sand and silt) Beds
Sand and silt seabeds occur along most of the West Coast, often interrupted
by rocky reefs. These are mapped in Figure 3.4. Their physical composition
depends not only on the types of sediment that is supplied to the coast
by the nearby rivers, but also on the degree of wave exposure and the sediment
transport dynamics of the areas.
Soft sediment seabeds have mostly been studied in relation to their physical
dynamics and sedimentation patterns59. The first
thorough regional assessment of the physical character of the West Coast’s
shallow seabeds is nearing completion as part of NIWA’s “NZ
Coast” project. It is intended that this resource will be made publicly
available via the NIWA website in the near future60.
 |
 |
| Several species of clams and other animals such as crabs, worms and snails live buried in the subtidal sediments. Sea sediment core samples (left) can be sieved to reveal their inhabitants (right). Photos: R Davidson |
|
Soft sediment seabeds provide a habitat for a variety of worms, shellfish,
fish and marine mammals. Shellfish beds such as clam beds occur as close
inshore as the surf zone, and they include different species that are largely
determined by water depth. Shallow water species include the triangle shell,
tuatua and the venus shell, while further offshore there are heart urchins
and crabs.61 Several shallow sites have been found
to support clam beds of probably quite high densities; they include Kongahu
beach, the Cape Foulwind sand beaches from North Beach to Nine Mile Beach,
and the Jackson Bay/Okahu – Neils Beach area (Neale 2007; Davidson
et al, 2003).
Fish and other larger marine species occurring in shallow zone are generally
similar to those a little further offshore on the continental shelf. The
more common species close inshore in these shallower coastal waters, include:
rig, spiky dogfish, sevengill sharks, red cod, flatfish, kahawai, gurnard,
flounders, yellow-eyed mullet, Hector’s dolphins, terns, spotted shags
and gulls.62
 |
 |
| A subtidal seabed snail, Austrofuses glans. Photo: R Davidson |
Bottlenose dolphins Photo: D Neale, DOC |
As well as the seabed communities, the water column of these shallow subtidal
ecosystems is a habitat for demersal (near the seafloor) fish species and
marine mammals. Hector’s dolphins reside almost entirely in these
areas out to about 4–6 nautical miles offshore. New Zealand fur seals
and seabirds such as penguins, gulls and terns are also locally common,
and southern right whales are an infrequent but significant migrant making
use of this shallow seabed habitat type.
3.5.2 Existing Protection in the Shallow Subtidal (Open Coast) Domain
The West Coast Regional Coastal Plan recognises several types of protection
and management areas that include some shallow subtidal areas such as: Coastal
Protection Areas, Culturally Sensitive Areas, Coastal Recreation Areas,
Coastal Hazard Areas, Marine Mammal and Bird Sites and Outstanding Natural
Features and Landscapes. Coastal erosion can affect the position of the
foreshore in a variety of ways, and can sometimes cause Crown conservation
land to become part of the shallow subtidal (open coast) domain.
Other than these, and the general controls that apply within all territorial
waters, there are no other specific protection or management areas over
the shallow subtidal (open coast) domain within the West Coast coastal marine
area.
 |
 |
| 7-gill shark Photo: S Wing, DOC collection |
SSCUBA diving occurs mostly in shallow subtidal rocky area. Photo: N Shears |
3.5.3 Uses Associated with the Shallow Subtidal (Open Coast) Domain
The shallow subtidal (open coast) domain is used for fishing and shellfish
gathering, including rock lobster, blue cod, kina, paua and mussels. Other
recreational activities include boating and boat anchorage, diving, tourism,
education and scientific (e.g. biological and geological) studies.
48 e.g. Morton 2004
49 Arnold 2003
50 Karamea x3; Cape Foulwind x4; Moeraki x3; Open Bay Is x3; Jackson Bay
x2; Jackson Head x3; Cascades x3; Barn Bay Coast x3.
51 e.g. Shears in prep, Roberts et al 2005, Neale & Nelson 1998
52 Neale & Nelson 1998
53 Roberts et al 2005
54 Shears in prep, Roberts et al 2005, Harvey et al 2005
55 Shears in prep, Neale & Nelson 1998, Parsons & Fenwick 1984
56 Roberts et al 2005, Francis 1996, Neale 2006b
57 Star et al 2003
58 H. Best pers comm 2001
59 e.g. Benn & Neale 1992
60 T Hume pers comm 2006
61 e.g. Davidson et al 2003, Neale 2007
62 e.g. Neale 2007, Neale 2006c, Stevenson 2004, Rayment et al 2003, Anderson
et al 1998