by Rex Gibson – ecologist and scientist
The Federation of Freshwater Anglers Inc. (NZFFA) has long believed that one of the major factors in the dramatic demise of the trout populations across the Canterbury Plains has been increasing nitrate levels in those streams and rivers that remained after dairy-based irrigation had drained many of the available water sources for the lowland rivers.
On a recent Saturday two members of the Federation’s executive offered free bore-water nitrate testing. The Asburton Courier advertised the event. A table was set up at the Ashburton racecourse. The NZFFA had acquired, last year, a portable apparatus for nitrate testing (a NICO real-time testing unit) which was used to give almost instant results. The testing unit was funded by ”Pub Charities” and cost in the vicinity of $10,000. It is “state of the art” technology.
The NICO apparatus has been used by the NZFFA to monitor rivers and streams across Mid and North Canterbury. Various studies have shown that any Nitrogen levels over about 3.8 mg of nitrogen per litre (nitrate-N) are toxic to trout ova and fingerlings. They appear to also deter trout from spawning. Similar figures apply to salmon.
The disappearance of trout from the plains is related to a cocktail of possible causes: dewatering of streams, rising water temperatures as flow volumes decrease, the damage to their food chains from the glyphosate used on river beds by ECan and others, chemical and effluent runoff from intensive farming, and physical damage to spawning streams from regional council contractors and farmers. The Federation is keen to see how significant nitrate levels have been in this demise.
Much has been written elsewhere about the consequences of nitrate leaching from both cow urine and over-application of artificial nitrogenous fertilisers. Their perceived role in the local economy is often explained as a necessary evil. That however was not the reaction the NZFFA members encountered from Mid-Canterbury locals at the testing day. Most were anxious. The first sample was tested at 9.30 a.m. the queue was continuous until 12.20 p.m. Seventy three samples were processed from sixty one locations. Most were from farms or lifestyle blocks. The readings ranged from 13.2 down to 0.06 mg N/L. Every person was appreciative of the opportunity to know their nitrate level.
Interestingly the 9 a.m. news on Radio New Zealand that morning featured the item stating that Otago University is about to begin a nation-wide study of the nitrate and CRC link in New Zealand.
What do the figures mean? I asked Dr Mike Joy (Victoria University of Wellington). He stated “of the samples taken, 90% exceed increased significant risk of colo-rectal cancer (CRC) (from Danish study, see below), 80% exceed 15% increased risk of CRC, 54% exceed half current maximum allowable value (MAV) Ministry of Health (MOH) trigger level and 10% exceed MAV.
Clearly we have a freshwater crisis that is fast becoming a human health crisis. The costs to Cantabrian’s of the unrestrained intensification that led to this freshwater contamination as well as other impacts; like less water availability are becoming more obvious every day. This will be looked back on I’m sure as a very dark time in Canterbury’s History revealing the utter failure of Environment Canterbury”.
To the anglers it meant that 27 of the 67 water sources would still support trout i.e. they were below the toxicity level for trout (3.5 mg/L)). All of the rest, 40 in all, would not and were also above ECan’s precautionary maximum value for Christchurch water as approved after the proposal by the Waimakariri Water Zone Committee (3.8 mg/L).
The testing was done by NZFFA President and well-known Mid-Canterbury Veterinarian Dr Peter Trolove. The checking and recording was done by NZFFA’s ecologist, Rex Gibson, from Christchurch. The results were referred immediately to Dr Alastair Humphrey, Medical Officer of Health. These results were much higher than previously recorded, at least publicly. Dr Humphrey’s response was that he has the statutory power to require the regional council to conduct a detailed investigation where the results were higher than the MAV (Maximum acceptable value). We await the response from ECan.
Two people who came in for the testing separately volunteered that water races on their properties which formerly held trout, cockabullies, etc., were now devoid of any fish. One farmer explained that in years gone by he followed the diggers that cleared weeds from the water races and flicked the trout back into the water. Today there are no trout to flick back.
Is there a solution? Two families had Reverse Osmosis Filters on their drinking water. For one the level dropped from 12.80 to 9.97. For the other it dropped from 9.22 to 2.52. Clearly, more work needs to be done here. These filters can cost up to $2.000 each, depending on household size, and a further $500 a year in replacement filters. Filtration systems are an ambulance at the bottom of the cliff solution. Reducing the nitrogen levels at source may be the best alternative.
The health issue was highlighted nearly two years ago when a Danish longitudinal study, published in the International Journal of Cancer, firmly linked (correlated) drinking water nitrate levels with colorectal cancer (CRC), usually known in New Zealand as “bowel cancer”. They had followed up on 5,944 CRC CRC cases from the 2.7 million people in the study. At least six other recent international studies have reached similar conclusions. New Zealand has one of the highest CRC rates in the world; along with Denmark. The highest nitrate rates occur from Canterbury to Southland. This parallels the dairy conversions areas of recent decades.
Canaries in the Mine
The study’s “tipping point” for increased CRC risk was 3.87 mg N/L. Interestingly that is the same level where trout “turn up their toes”. It is a cliché, but in our waterways trout and cockabullies are thus “the canaries in the mine”.
One colleague has reminded me of the old Biblical conflation (Ecclesiastes and Isaiah) of “Eat drink and be merry for tomorrow we die”. Let us hope it is not our drinking water that is responsible.
Footnote: (1)Rex N. Gibson is a Spokesman for the NZ Federation of Freshwater Anglers. He is an ecologist and scientist and with a deep personal interest in rivers and public health.
(2)Journal Reference: Jörg Schullehner, Birgitte Hansen, Malene Thygesen, Carsten B. Pedersen, Torben Sigsgaard. Nitrate in drinking water and colorectal cancer risk: A nationwide population-based cohort study. International Journal of Cancer, 2018; DOI: 10.1002/ijc.31306