Re: ‘Black swans stinking up Tauranga Harbour' (The Weekend Sun, June 3).

The ‘Danger: toxic shellfish' signs are popping up everywhere due in no small part to the burgeoning numbers of black swan.

Eastern Fish and Game's Matthew McDougalls' response to my comments in The Weekend article was predictable but no amount of bureaucratic ducking and weaving, or convenient massaging of statistics can conceal the evidence of massive pollution and ecological damage black swans are causing in Tauranga Harbour.

Fish and Game, and by default our own city council, are derelict in their duty of care to not recognise the damage these birds are causing.

Mr McDougalls is so out of touch he appears confused about the birds status and origins. He claims they are native, when at best they could be classified as endemic.

They are more likely to be descended from escapees from the imported 100 from Melbourne, where they are native around 1864. However this argument is of no help to our harbour; they are here and in numbers sufficient to destroy our harbour completely as they have done elsewhere. Perhaps the most concerning comment from Mr McDougall is that he considers the environmental impact of the birds waste is miniscule.

If a single black swan invaded Mr McDougalls swimming pool it would dump 418grams of excrement in one day. This will contain 11 million faecal coliforms, 8.6 million ecoli, and 27,000 enteococci.

My question is would he encourage his kids to have a swim? Or pretend it did'nt happen? Or

shoot the swan and drain the pool?

The western extremities of Tauranga Harbour from Plummers Point to the Waihi gap is in a perilous state due to swan activity.

Surviving kaimoana is toxic and so is the water. Omokoroa's eastern beaches were visited last summer among high concentrations of black swan by blue green algae, a biological nightmare which has the potential to destroy all marine life and aquatic dependents where it occurs.

Without controls this threat extends to human health and safety. The threats are obvious and the remedies simple, we have to urgently reduce black swan numbers and then maintain strict control.

The authorities charged with this responsibility need to act now.

K Molloy, Omokoroa.

Fish and Game response:

During recent years there have been some outlandish claims in relation to black swan on Tauranga Harbour.

Just recently in SunLive, people have continued to make uninformed claims including adult swans being able to eat their own body weight every 24 hours, that swans are putting three tonnes of excrement into Tauranga Harbour every day, and that the harbour is home to tens of thousands of the birds.

And despite scientific evidence to the contrary, some people continue to claim swans aren't native birds.

Because not one of these claims is true, Fish & Game wants to set the record straight by presenting the facts.

First, black swan are a native species and harbours such as Tauranga are part of their natural habitat. There is solid evidence supporting this, with fossil black swan bones found throughout NZ, including in pre-European Maori middens.

The swan living here is the same species as that living in Australia. They arrived here well before being introduced by the Acclimatisation Society in 1863, and self-introduced again in 1897, along with a number of other Australian species that turned up about the same time.

Although movement from Australia seems remarkable, Fish & Game has had ducks banded in New Zealand turn up in Adelaide in Australia - despite this being against the prevailing wind - and New Caledonia.

As for the 'tens of thousands” of swan which are claimed to live on Tauranga Harbour, this too is incorrect.

Fish & Game has been conducting aerial counts of the harbour's black swan population since 1991. These counts are carried out three times a year - in January when the population is at its peak, in April and then again in August. The highest number recorded in January was in 2014, when 6458 were counted, far short of the 'tens of thousands” claim. The summer average is even less – 4600 during the last 10 years. By April, that population had decreased by about 30 per cent, and 90 per cent by August.

Most of the black swan move away from the harbour to breed - in the 21 years Fish & Game has been conducting aerial counts, cygnets have never been seen on the main part of the harbour.

The National Institute of Water and Atmospheric Research - known as NIWA - has looked at the impact of swan nutrients on waterways. NIWA's scientists found that in fact, black swan poop about 52 grams dry weight per day.

Using NIWA's figures and the highest swan count ever recorded, the total amount of swan poop going into the harbour is around 336kg a day in January, or about 21kg a day during the winter months.

Let us put this into perspective. Of the 52g of faecal dry weight produced per swan per day, just 0.07 of a gram is dissolved inorganic nitrogen and another 0.07 g is dissolved reactive phosphorus.

Based on the 10-year summer average of 4600 swan, this means these birds are putting a total of 118kg of nitrogen or phosphorus a year into Tauranga Harbour. The actual amount is much less, because as already noted, swan numbers plummet by 90 per cent in August.

Is that good or bad? As a comparison, the Waiohewa, a geothermal stream flowing into Lake Rotorua, discharges 40kg to 50kg of nitrogen a day into the lake, while the Hamurana Spring puts almost 41kg of phosphorus into Lake Rotorua each day.

The NIWA report notes inputs from black swan to a water body need to be put into the context of nutrient inputs from other sources, and consideration needs to be given to whether the birds are importing nutrients to the water body or merely recycling them. The evidence for large water bodies such as Taupo and Ellesmere is that the contribution by swans is very small. Most swans appear to feed within the harbour itself, and this being so are likely, at the very least, to be recycling nutrients if not removing a proportion of these during the course of their own digestive processes.

Faecal coliforms are bacteria which live in the gut and are excreted. Counts of these bacteria (or more specifically Escherichia coli) are used to assess the impact of faecal contamination of waterways. Using the Lake Rotorua example once more, the daily faecal coliform count in the Ngongotaha Stream as measured at the SH5 bridge is markedly higher than that which swans are likely to be inputting to Tauranga Harbour.

In fact, even at the very highest swan population ever counted on Tauranga Harbour, these birds would only contribute about six percent of what flows from non-swan sources down the Ngongotaha and into Lake Rotorua every day.

One question the swan's opponents have yet to answer is why focus simply on this species?

What about other native birds? Seagulls are believed to have one of the highest faecal coliform outputs, so they, along with godwits, oystercatchers and even introduced birds like sparrows are likely to collectively exceed the swan's efforts. Should we cull these species too?

The reality is the biggest impact on the harbour is surrounding land use. If residents really want to make a difference to the harbour they should be focusing their attention here rather than picking on the native wildlife.

It is exasperating that Fish & Game's evidence is based on decades of scientific research, gathered at great effort and expense, yet the black swan's opponents don't rely on science. It would appear some prefer to pluck their claims out of thin air and these masquerade as facts. This is not at all helpful and shifts attention away from where it should be directed.

Hopefully, the facts presented here will provide a more rational and informed basis for future discussion on the place of one of NZ's magnificent native birds in the modern environment.

A fully referenced version of this article has been posted on Fish & Game's website:

http://eastern.fishandgame.org.nz/newsitem/fish-game-corrects-black-swan-misinformation for those who wish to follow up on the science behind this article.

Fish & Game.