Lively views on the merits of mangroves in intertidal environments were expressed at the freshwater seminar organised by Allegrow Environmental Consultants at Ōpōtiki Golf Club on Thursday.

Entitled, There’s Something in the Water, the event comprised presentations from a fish ladder expert; a regional council scientist and a Landcare water bug “enthusiast”.

The event followed a similar one held in the Edgecumbe War Memorial Hall the previous day.

During the seminar, an Ōpōtiki resident claimed he and his friends had cleared many hectares of mangrove swamp with the result that the cockles and other shellfish returned to the area and flourished once more.

The work allegedly had to be abandoned after he and his friends were taken to court by Forest and Bird. But the man declined to enlarge on his comments when asked to do so by the Ōpōtiki News.

Meanwhile, a pressing need to cull Canada geese and black swans – blamed for worsening faecal contamination of waterways – was also raised at the seminar.

One woman said Canada geese consumed pasture grass and farm crops and had been cut open to reveal “stomachs full of baby flounder”.

But again, those at the seminar calling for culling of Canada geese and black swans were not keen to go public.

The loudest applause followed the presentation from ATS Environmental scientist Kelly Hughes, an expert in so-called, “fish passage and waterway remediation solutions”.

In practice, this involves Mr Hughes and his colleagues spending a lot of time wriggling through culverts installing flexible plastic baffles to lower water velocity so that native fish can swim through.

The company’s expertise in this field is in demand nationwide and internationally.

Mr Hughes showed a video of whitebait clinging to the base of an overhanging culvert which drained into a stream. Baby eels had been carried on ocean currents all the way from the Tonga Trench on their quest to spawn in New Zealand rivers.

The culvert design stymied further progress upstream.

True, many native fish could briefly leave the water to wriggle and jump up steep rocky watercourses, but barriers such as swift flowing culverts and electricity turbines had them beat.

However, flexible plastic baffles could be screwed to the base of culverts in a pattern resembling the rungs of a ladder.

These allowed whitebait, eels and other native fish to make their way upstream.

The baffles were to some extent self-cleaning during flood events; a flexible design allowed them to let water and debris through when water levels rose, and flows speeded up.

Mr Hughes designed the baffles but did not patent them; he kept the design open-source, wanting them deployed as widely as possible.

He described the characteristics of the various species of whitebait, which are the immature (fry) of fish including inanga, banded kōkopu, giant kōkopu, kōaro, shortjaw kōkopu and common smelt, usually around 4-5 centimetres long.

Some species started life as eggs attached to grass along riverbanks. They hatched out when the river levels rose during higher-than-normal flows.

The larvae then floated downstream out to sea, carried by currents to the deep ocean off Tonga.

They developed into little fish we know as whitebait and returned on opposite currents, later being swept into New Zealand river mouths by tidal action. From there they swam upstream.

“Despite the myth that they return to the river in which they were hatched, whitebait do not have control over which river they enter from the sea,” Mr Hughes said.

Vastly more whitebait were predated at sea and within New Zealand rivers than ever got to spawn. After attaching to grass on riverbanks many eggs were eaten by mice and other creatures, or sometimes dried up and were lost if the rivers didn’t rise to float them away again within a few weeks.

But despite huge natural wastage, poorly designed industrial, agricultural or irrigation culverts had the ability to decimate native fish numbers, hence the need for mitigation.

“The good news is there’s huge demand for these measures from farmers, councils, electricity generators and the like.”

He said most Kiwis wanted fresh water eco-systems to succeed.

“But for some the ability of whitebait and eels to be able to spawn is also a matter of self-interest.”

Both eels and whitebait were exploited commercially, Mr Hughes explained, with those harvesting them grasping the need for these fish to be able to spawn. Landcare Trust freshwater ecologist Fran van Alphen described “creepy crawlies” inhabiting almost every waterway.

Some of these worms, snails and insects were prolific, in one case laying 8000 eggs a year, others were rarely seen.

In a healthy New Zealand stream, you’ll find macroinvertebrates like mayflies, stoneflies, caddisflies, and freshwater crayfish, which are sensitive to pollution.

The presence of mayflies, stoneflies, and caddisflies was a good indicator of stream health, as they were sensitive to pollution and habitat degradation.

Other macroinvertebrates found in healthy streams included freshwater crayfish, beetles, dragonflies, true flies, and some moths.

“The more invertebrates there are in a stream, and the greater the variety, the more likely the stream can support healthy populations of fish and other wildlife.

Bay of Plenty Regional Council senior groundwater scientist Paul Scholes gave an overview of water quality in the Ōpōtiki district, describing the testing for dissolved oxygen, nitrates, bacteria, and suspended solids. Testing was at least once a month, though once a week in the summer bathing season.

Results were published online showing our rivers varied in water quality throughout the year.

Some pollutants were from dairy runoff, but some occurred naturally.

Mostly, our rivers seemed to be in good shape, but the challenge was to establish base lines to better understand changes and trends.

The regional council used a range of testing methods, from satellite imagery to electrical testing equipment. Most of the testing equipment was available for the public to purchase.

“We hope a DNA ‘pen’ will be out soon which can provide an instant marker for E. coli.”

The next Fresh Water Ecology Field Day will be on April 10, location to be confirmed.  Email [email protected] or call 021 659 924.