by Dr J Floor Anthoni (2004-2005)
The map shows global ocean temperatures for the Pacific Ocean in false colours from purple to red. Absolute values for the sea temperature are thus hard to guess but a clear picture emerges. As expected, the sea warms gradually from the South Pole to the Equator in horizontal bands. But these bands are not strictly horizontal, particularly the green one of the subtropics. It curves all the way up to the equator along the coast of South America, due to a strong ocean current moving up this coast. As a result, the Galapagos Islands on the Equator are as cool as Sydney and Melbourne or even Auckland. This tongue of cool water moves just south of the equator as it gradually warms up.
Niue is located on the cool edge of the warm tropics but its waters are much cooler than those of Indonesia for instance.
that the Sea Surface Temperature (SST) map was produced by infrared sensing
satellites that cannot pierce the water's surface (infrared light is absorbed
immediately). Thus the SST is indeed the temperature of a very shallow
surface layer, ignoring any cooler water direct underneath. One would expect
waves to mix the water column, which they do, but in the Niue area, average
wave height is rather low, as seen by this satellite map of average wave
heights. The bulge of warm water near Niue may thus not at all represent
the actual situation.
This map was produced from the electrical noise in the reflected radar signal from various satellites. Scientists soon discovered that the strength of this noise related to wave height, which allowed them to plot the wave height map seen here. Note the power engine in the South Indian ocean and south of NZ. This wind and its waves drive the Antarctic circumpolar (going round the whole pole) current, and probably also the South Pacific gyre that affects Niue.
No records exist of the productivity of Niue's seas but the chlorophyll map below gives some important clues. This map shows the amount of green plant matter (chlorophyll) which is responsible for the productivity in plants. The map combines both land and sea. As one can see, the South Pacific is sandwiched between two deserts, Chile and Peru on the East and Australia on the West. Niue has been marked by a red dot. The productivity of the sea does not follow the temperature bands as above but depends very much on a few invisible factors.
|Look at the sea around New Zealand, for instance. NZ is surrounded by a very productive sea because of invisible currents south and north of it. One current runs southward along Australia's east coast, mopping nutrients into the New Zealand area. On its downward course, it encounters a cool sea which forms a cold front, diverting its course to southern New Zealand. Likewise, subantarctic currents experience this as a warm front as they too are diverted towards southern NZ where turbulence causes upwellings of nutrients. (See ocean/currents and oceans/specialNZ)|
Such turbulent upwellings occur also along South America's coast as
the coastal Peru Current cork-screws along the Peruvian coast. This current
becomes diverted by Coriolis forces (due to the spinning of Earth) and
follows a straight path on the equator (See Oceanography/currents/defection).
Here the Coriolis forces focus its beam horizontally into a narrow jet
that sends invisible eddies downward, causing nutrient upwellings along
its entire course as it also gradually warms up. Scientists are studying
the interaction of this
cool tongue and warm pool, as it produces
some unexpected pelagic productivity.
The centre of both North and South Pacific are pools of ultra clear water with very low productivity, while also located in the world's desert bands. Evaporation is high, resulting in a higher than normal salt content. Remarkably, this pool of infertile water travels in a NW direction, past Niue. This is probably because of the trade winds blowing in the same direction. Note that chlorophyll-sensing satellites can look down only about 10m into the sea, so they are unable to view what is happening further down.
For instance, it is quite possible that the ultra clear blue water overlays a basement of cool nutrient-rich water in which phyto plankton (plant plankton) can bloom, because the clear water admits so much light deep down. The productivity of this blue tongue could therefore be higher than thought, although always much lower than green water at the surface.
Note that some anomalies in green upwellings have been discovered, for instance a thin line running from NZ to Niue over the Kermadec Trench and Ridge and a cloud around French Polynesia and Hawaii. These are caused by the slow, deep tide waves 'stumbling' over sea bottom irregularities, resulting in surface currents and local upwellings. The map below shows how scientists calculated the dissipation (loss of energy) of the tide wave due to the shape of the sea bottom. It is based on comparing a mathematical model of the tide wave with actual tide heights measured from satellites. In places where this tide wave 'stumbles' (shown in red), turbulence occurs with possible surface mixing and nutrient upwelling. Compare it with the chlorophyll map above. For Niue this model does not suggest remarkable upwellings.
Scientists claim to have recorded from Niue, 189 species of coral in 43 genera and 243 species of fish (Spalding et al., 2001). This despite the fact that the seas around Niue are sparsely populated. Nearby Tonga, further West has 229 species of fish. Tonga has suffered a major coral bleaching event in 2000, whereas Niue has not. The world map below shows coral diversity, which is roughly related to the water's temperature. Niue is somewhere at the bottom rung with 50-100 species, a lot less than the 189 mentioned before. Our own observations do not justify the figure of 189 either.
Nature appears to thrive best under high temperatures, predictable moisture and accompanying evapo-transpiration, zones with varying degrees of stress and interconnectivity between them. Niue scores low on all these necessary conditions for maximum biodiversity, even when compared with cooler New Zealand.
Hurricanes (tropical cyclones) also leave their everlasting mark on the seascape. Ironically, they cause most damage to the side of the island which is also most sheltered. So Niue has no places of shelter, a necessary condition for high biodiversity. As soon as soft organisms have colonised rock space, the next hurricane will wipe them out again. Only in very deep water can soft and old organisms survive. We have not been able to investigate these deep reefs.
New Zealand's Kermadec Islands live in a similar predicament, although surrounded by a richer sea, sufficient areas of shelter and flats that function as small continental shelves. We observed signs of stress even though there are no unnatural threats such as those caused by people (See marine reserves/Kermadecs). In Niue we encountered the same signs of stress but more pronounced:
We also observed that the predators like small groupers were few and far between, pointing to intensive fishing in places where we dived. All fish, dolphins and turtles were rather shy, indicating the presence of spearfishers. If Niue wishes to become more attractive to visiting divers, places must be set aside where fishing (and gathering) is prohibited at all times!
The limiting factors around Niue are rather severe, reason to investigate what kinds of survival strategy would enable organisms to live and reproduce in the seas around Niue. An article has been written about this (diving with the survivors) but it pays to discuss these strategies here as well.
Hurricanes or tropical cyclones as they are called in our area of the world, do not strike frequently (Niue 1959, 1960, 1970, 1989, 1990, 2004) and therefore it is hard to believe that they have such a pronounced effect on the underwater environment. Not only do they affect the visual aspect of the seascape, but we observed that they also affect the structure of the cemented coral rocks that built Niue, and that they have been doing this for as long as Niue exists (1-3 million years). However, no scientific proof is yet available.
The map shows where hurricanes strike most frequently and the paths they take. New Zealand used to be struck frequently in the past, but for some time now, has not experienced any (Cyclone Bola 1983, Ola 1993). Cyclones are born over warm water and they dissipate over land and over cold water while raining down their amassed moisture.
hurricanes travel in one direction for some time, they send large waves
ahead that have a low frequency and high speed. Such waves reach deep and
their destructive power exceeds normal storm waves by far. Thus hurricanes
leave unerasable marks by creating barren areas. One would have thought
that the barrens will recover, but this is only partly so. Immediately
after denudation, the surface becomes covered in fine algae that prevent
other animals, like young corals from settling there. Then the grazing
armies of snails, sea urchins and fish take over, removing both algae and
whoever tries to settle there. So the barren areas are a stable habitat
form, comparable to the grasslands of the Serengeti. Only now and then
a large storm is needed to keep them barren.
The red band on the map shows where Cyclone Heta caused its damage, but other cyclones may arrive more from the NW, causing more damage to Avatele in the south and less to Uluvehi in the north.
Underneath the fine algae grows another algal form, a coralline alga
known as pink paint. It grows flat leaves on the rock although
these are not true leaves like green plants have. These coralline leaves
are almost as hard as a coral skeleton, and over time they form reefs almost
indistinguishable from those built by corals. In fact, most coral rocks
contain a high ratio of coralline algae. The coralline algae grow over
anything and everything, and in doing so encapsulate corals and coral debris.
When the temperature becomes too low for corals (as during the ice ages),
the coralline algae will still grow but perhaps insufficiently to build
reefs. We have observed that the coral rocks of the Alofi Terrace on the
hurricane side, are of this type: jumbled and erratic (see also Niue's
f220433: the sea has broken through the jumbled rock, providing a glimpse of a cross-section through the Alofi Terrace at Alofi on the hurricane side. Notice the platforms, walls, cavities and some brown soil formation. It even shows some dripstone and vertical percolation channels.
|Fishing around Niue
No good records exist of what fish is caught around Niue. Niuean fishermen seldom go out further than 2-3km from shore, and that brings them over a very deep ocean, towards 5000m deep. Most if not all of their catches consist of pelagic fish like wahoo (Acanthocybium solandri), dolphinfish mahimahi (Coryphaena hippurus), barracuda, yellowfin tuna (Thunnus albacares), albacore tuna (Thunnus alalunga), skipjack tuna (Katsuwonus pelamis), marlin, shark and a few others. But what do these fish feed on and how much is there of both predators and prey?
|From the chapters above, it is clear that Niue is not able to catch the fishes found around continents with continental shelves. Drawing a net through the water may well cost more energy than the catch can pay for. So, longlining for predatory fish is Niue's remaining option and judging from the experience of local fishermen, this may well be possible on a large scale further out over the entire EEZ of Niue. As mentioned before, scientists are puzzled over the unexpected density of large predators in these infertile seas. But some good information is available. Niue's EEZ counts ten sea mounts, and these stir the water as the tide wave passes by, resulting in some upwelling. However, most of these sea mounts are too deep to account for this effect and some seamount fisheries have collapsed elsewhere (see more detailed map ).|
The density of fish over seamounts that somehow attracts them, depends on :
|So, Niue is not well placed for catching fish that originated from
its own waters but many uncertainties remain. But what about strays, stragglers
and migratory fish arriving or passing through from other waters? Very
little is known of the migratory behaviour of fish schools and their predators.
Marine organisms usually wander up-stream because that is where smells
come from. Would Niue and Beveridge Reef provide a wake or trail for them
to swim towards? Obviously, more knowledge would be welcome.
What about the fish on outer reef slopes? There are ten seamounts with raised sea bottoms, but most unfortunately do not reach above 1000m. Note that in this respect the satellite bathymetry is uncertain as to the depths of the tops of the sea mounts, and could well be out by 500-1000m, depending on the track of the satellite. Traditional measurements are needed to confirm the depths of these sea mounts, but even so, the fishery will be very small indeed compared to a continental shelf fishery like New Zealand's.
The productivity table on right gives us estimates of what to expect. Note that the productivity of the open ocean is comparable to that of a desert, and the most productive seas over upwellings compare to temperate forests, now replaced by the most productive arable zones of the world.
Etherington: Environment and plant ecology, 1976.
The quantities are green productivity, which on land leads to crops directly and to livestock one step away (trophic level) with biomass losses to wastes and energy for heating, movement and internal transport (breathing, blood circulation). As a rule of thumb, each next trophic level contains less biomass, usually 6-10 times less. In the sea where life begins with microscopic plants (phytoplankton), it takes 3-5 trophic levels before fish are large enough to be eaten by humans (table fish). Thus biomass and productivity at this level is about 100 times less. The continental shelf for instance is estimated to produce 2-6t/km2 of edible fish. The open ocean accounts for less again, with a high degree of uncertainty.
Estimates for the kind of sustainable harvests that can be expected from Niue's EEZ are:
But Niue does not have a safe harbour, so fishing boats must be small enough to be lifted out of the water. For such boats to stay far out for prolonged periods, storing ice and catch while travelling fast and economically, and also light enough to be lifted out of the water, requires novel ship designs.
The fish expected between 500-900m depth are: alfonsino (Beryx splendens),
bluenose grouper (Hyperoglyphe antarctica), pelagic armourhead (Pseudopentaceros
richardsoni), barracuta (Rexea antefurcata) and small sharks
spp, Centrophorus spp, Etmopterus spp) .
Deeper down the roundnose grenadier (Coryphaenoides rupestris) can be caught to depths of 1200-2000m, but the deeper fish are all very slow growing and do not easily support a sustainable fishery. Besides, their flesh tends to be watery.
 Floor Anthoni (2004): Deductions from oceanographic
knowledge and facts, however not proved by scientific method.
 Satellite bathymetry survey of the EEZ of Niue. Seafloor Imaging Inc. Govt of Niue, MAFF.
Read about our most recent discoveries about symbiotic decomposition and mixotrophy which explain why the productivity of the deep blue sea around Niue can be much higher than originally estimated. Visit the Dark Decay Assay chapter.