Boating New Zealand — February 2018

74 Boating New Zealand

They seem to be getting bolder as well, perhaps spurred on by

competition from their peers. Bronze whalers often hunt in

packs; once sharks home in on your boat you’ll lose every fish

you hook, so there’s nothing for it but to move somewhere else.

Other sharks that become common during summer include

threshers, which enter northern waters around October,

hammerhead, mako and blue sharks. Like bronze whalers,

hammerheads breed in sheltered bays; hammerhead pups are

very common in the Hauraki Gulf in early summer.

Mako and blue sharks are oceanic species more usually

encountered in the blue water some way offshore, but they also

come inshore at times. I have caught both species fishing from

the rocks.

Over the last 20 years or so, blue and mako shark became very

rare in our waters, but they seem to be making something of a

comeback, probably as a result of less drift-netting and reduced

surface longlining in the Pacific Ocean north and west of New

Zealand, though both species are still targeted for their fins.

Recognised as game fish by the International Game Fish

Association (along with thresher, hammerhead, blue and various

whaler sharks), mako sharks are good fighters on the line,

especially large specimens. Like all sharks they’re difficult and

even dangerous to handle boat-side, so take care.

It is possible to release sharks relatively unharmed, but it

takes courage and skill. Too often sharks are roughly handled

and probably die from their injuries after release. And since

sharks have little value as table fish (mako and thresher flesh

is good to eat, but full of mercury), it is better to avoid hooking

them if you can.

If you accidentally hook a shark and successfully bring it to

the boat, unhook it in the water if it is safe to do so – otherwise

cut the trace close to the hook. Where possible, avoid bringing

sharks into the boat.

While ocean temperatures around New Zealand are shaping

up to be exceptionally warm this summer, what is exceptional

now may soon be normal.

Scientists expect the average water temperature around

New Zealand to increase by 2.5 degrees Celcius by the end of

the century, as reported in a recent paper, published in the New

Zealand Journal of Marine and Freshwater Research. That

means summer water temperatures in parts of the North Island

will reach 25 degrees-plus.

The waters around Tasmania are warming rapidly and

scientists think what’s happening there is a good indication of

what we can expect here. Rapid warming affects how oceans mix

and impacts on the nutrients available for plankton growth.

Warm surface water is less dense than cold nutrient-rich

water from deeper regions and the warmer the water, the greater

the density difference, which forms a barrier to mixing. Less

mixing means the nutrient supply that reaches the surface is

restricted, which limits phytoplankton growth and affects all the

other organisms down the food chain, including fishes, birds and

sea mammals.

This has already happened in the water off Tasmania and

the southeast corner of Australia, as the East Australian current

pushes warmer water further south, causing huge changes to local

ecosystems. Coastal ecosystems around Tasmania are changing and

subtropical species are increasing in number and abundance.

As with all change, there will be winners and losers: in New

Zealand waters we could see more invasive species and possibly

new diseases that might affect local ecosystems. The Chatham

Rise east of New Zealand, which provides about 60 per cent

of the country’s commercial fish catch, will suffer significant

nutrient decline, as will our sub-Antarctic waters, which

currently support important commercial fisheries.

In addition, more acidic oceans (acidity is predicted to be

130 per cent higher by 2100) will have detrimental affects on

shellfish, which may become smaller because of the effort and

resources they must expend to maintain their shells.

However, tuna could become more plentiful in our waters

because they prefer warmer waters and other fisheries may

also benefit.

New Zealand is on the border between warmer sub-tropical

water, where things are moving from, and cooler temperate

water, where things are moving to, so it may not be as hard hit as

some of the islands in tropical regions where continual warming

will reduce the numbers of fish and plankton. BNZ

Rapid warming

affects how oceans

mix and impacts on

the nutrients available

for plankton growth.

LEFT Species that

prefer cooler water,

such as blue cod, may

be forced deeper or

further south.