Balmy Lyttelton Harbour waters ready for a dip

Swimmers searching for the perfect place for a dip off Canterbury’s coast may want to head to Lyttelton Harbour after scientists found its waters are considerably warmer than a year ago.

Data gathered by the Lyttleton Port Company show temperatures in December were about 5 degrees Celsius warmer than the same time in 2016.

A ‘‘marine heatwave’’ triggered by La Nin˜ a climate conditions in the tropical Pacific Ocean was observed across the Tasman Sea in November and December, leading to New Zealand coastal waters rising to between 2C and 4C above average, according to Niwa, the National Institute of Water and Atmospheric Research.

Climate scientists have warned that warming seas will have a ‘‘significant’’ impact on marine environments, potentially triggering a movement of tropical and sub-tropical fish species south towards New Zealand.

But while there are signs of waters off the North Island heating up, experts say there is little sign of temperatures increasing in the oceans off the South Island.

Fourteen monitoring buoys in Lyttelton Harbour and the wider coastal area recorded warming that mirrored a broader increase in sea surface temperatures around the country late last year.

Temperatures in December 2016 were about 17C, while a year later they were closer to 22C, Lyttelton Port Company said.

Water at shallower inshore sites are always warmer and fluctuate more than deeper waters exposed to oceanic currents and are more susceptible to changes in air temperature.

Dr Leonie Andersen, of Vision Environment, installed the buoys in September 2016 to monitor the harbour’s natural fluctuations before dredging to deepen the channel deepening begins later this year.

‘‘The water temperatures within the harbour itself strongly respond to air temperatures and it was a pretty hot December,’’ she said. ‘‘We can see the water temperature cycling during the day – heating during the day and cooling at night. Sometimes it can vary up to five degrees in 24 hours.’’

The marine heatwave began in September and is expected to continue for the rest of summer.

Climate scientist Dr Jim Salinger, from the University of Otago, suggested warming seas could trigger a movement further south of subtropical species of pelagic fish, those that inhabit the midwaters of the oceans.

‘‘It is going to cause a lot of change in the marine ecosystem, changes in seaweeds and probably towards more fish coming from warmer waters.

‘‘Already snapper have been caught during December 2017 in Doubtful Sound in Fiordland, which has never occurred before, and snapper have been spawning three months earlier than normal.

‘‘Many other impacts are likely, such as changes in shellfish and seaweed around the coasts.’’

Professor Dave Schiel, a marine science expert at the University of Canterbury, said the temperatures found in Lyttelton Harbour did not indicate a wider trend of warming and were not a major cause of concern. ‘‘There is a lot of evidence that water is warming in the North Island and will continue to do so, but there is not much evidence of offshore temperature increases in the south.’’

The buoys in Lyttelton Harbour also measure currents, wave height and water clarity, information that can help to surfers, swimmers and divers.

The data can be viewed in realtime via the Port of Lyttelton Company’s website.