Opinion: Fish can’t die twice - flawed water report

Dr Mike Joy 

By Dr Mike Joy

Our Freshwater Report 2017 from the Ministry for the Environment definitely shows an improvement in honesty on their water chapter in the recent state of the environment report. However, there are still some really obvious attempts to shift focus away from agricultural impacts.

Once again waterways in urban catchments are given the same prominence as pastoral, even though urban make up less than one per cent of river length and pastoral are 40 per cent.

First, I’ll go through their key findings in the media release to demonstrate some of the biases.

Nitrogen levels: the report says 55 per cent are getting worse and 28 per cent getting better, but that’s for all sites. If, as would be logical, you look at landcover classes separately (figure 11), then let’s look at pasture sites. ­Of the sites that showed a trend, 72 per cent got worse and 28 per cent got better.

Phosphorus is a non-issue because the algae can in most cases get all the phosphorus they need to cause problems from the sediment where levels are high, so don’t need it from water, which is where it is measured.

For E. coli the report inexplicably switches from a 20-year record (for other parameters) to 10 years and as would be expected after removing most of the data then most (52 per cent) of sites have no trend because of this lack of data. However, of the significant trends left then for pasture catchments, 60 per cent got worse and 40 per cent better (for urban one site got worse). This sounds a bit different to the “22 times higher in urban and 9.5 in urban” reported.

In the detail of the main report, a number of issues arise, apparently from a lack of freshwater ecological knowledge. In the section on nitrogen then figures are present using the banding from the National Policy Statement.

This approach is flawed for two reasons: 1) As far as I’m aware the limits are still subject to public submissions so could and should be changed and 2) Nitrate toxicity is a red-herring because we know that fish or invertebrates cannot die twice.

Much lower levels of nitrate (actually in the A band) are well known to have algal blooms that cause fish deaths through oxygen depletion, so the amount of nitrogen that is toxic to them is a non-issue and only occurs in laboratory experiments and not real-life.

The ANZECC guideline level is around 0.5 mg/l about one tenth of the toxic limit, thus using these toxic limits is disingenuous.

Next, the section on algal blooms starts with the statement that 83 per cent of the length of rivers is not expected to have prolonged algal blooms. This statement misses the point because just one day of a bloom is just as lethal to fish and other life as any longer length of time, once again they can’t die more than once.

Dr Mike Joy is a freshwater ecologist in the Institute of Agriculture and Enviroment.

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