Are trout to blame for declining water quality?

UPDATED: Scientists are questioning statements by a prominent dairy industry representative, suggesting trout are a ‘disastrous species’ — no better than ‘freshwater stoats’ — and that farmers have been unfairly blamed for their impacts on declining water quality.

These allegations were made in a speech last week by outgoing Federated Farmers Dairy chairperson Lachlan McKenzie, urging members to use good science and their own judgement to distinguish fact from opinion.

An excerpt:

“We can easily and graphically see the effect of stoats, possums and rats on our terrestrial environment… but what if we have an equally disastrous species in our waterways devouring native invertebrates, increasing algal growth and forcing our native fish to the brink?

“It’s time to test if trout is the benign tourism-friendly icon it is held up as. …

“Farming affects water quality, yes, but we seem to have been attributed with 100 percent of the blame.”

The following experts in freshwater, aquatic ecosystems and water quality contacted the Science Media Centre with their reactions to statements made in the speech.

To follow up with these or other local experts, contact the Science Media Centre.

Russell Death, Associate Professor – Centre for Freshwater Ecosystem Management and Modelling, Massey University comments:

“Research at Otago University has shown that brown trout (Salmo trutta) can affect New Zealand freshwater ecosystems. Lachlan McKenzie in a speech to Federated Farmers believes this research provides evidence that trout, and not agricultural land use intensification, are to blame for many of the algal proliferations found in New Zealand waterways.

“With any research investigation there is always a danger that those reading it will extrapolate more from the study than intended by the scientists presenting the research. For example, an experiment in which a new drug reduces tumours in mice signals that a cure for cancer has been found. Lachlan McKenzie in support of his assertion that trout, and not nutrient run-off from agriculture, cause algal blooms, quotes a study conducted in two Otago streams that found trout not the native Galaxias cause trophic cascades (increased algal growth from removal of invertebrate grazers).

“Nowhere in his paper does Huryn state, nor did he even consider, whether trout versus added nutrients from agriculture are more important in promoting algal growth.

“Mr McKenzie ignores or is unaware of another study by the same research team that did examine the relative role of nutrient enrichment and trout predation on algal growth and found that added nutrients caused increased periphyton in both trout and Galaxias streams. However, even this study examined only a limited level of nutrient addition and only in six Otago streams. It is not possible to conclude from a single study conducted in one region that nutrients or trout are leading to increased algal growth in rivers throughout New Zealand.

“As scientists we are always accused of having too many caveats around our research, but similarly we must be equally careful that the findings from any particular study are not used to make conclusions about wider issues for which they were not intended.”

Professor Colin Townsend, Dept of Zoology, University of Otago comments:

“In his speech, Lachlan McKenzie advises us not to accept someone’s opinion as gospel but to interrogate it and check it out. ‘If robust it will stand scrutiny’, he says, ‘if not, there is cause for concern’.

“As a researcher with 40 years experience and someone intimately involved in both trout research and the effects of agriculture on stream ecology I wish to comment on a few relevant points.

“Our research has shown that by reducing grazing by stream insects, trout can lead to a modest increase in algae on the streambed. These extra algae have the effect of sucking up some nitrogen from stream water and so the trout actually make a small contribution to cleaning up the mess caused by nutrient runoff from farms. In any case, the small changes to nutrient fluxes in streams associated with trout are swamped by the much larger amounts of nutrients entering as diffuse pollution from the land.

“It’s worth noting too that our research in Otago shows that soil erosion, and the resulting smothering of the streambed by fine sediment can be even more harmful to stream health than nutrient enrichment. Unless Lachlan McKenzie has witnessed trout emerging from streams and churning up the land with their big fat hooves, he will find it difficult to shift responsibility from cows to trout.

“Farming is important to New Zealand but so is the state of our environment. Thankfully, many farmers are already doing their best to be good stewards. What is needed now is more discussion, education and collaboration between all sectors with an interest in land and water management, not an untutored and distorted analysis of the evidence.”


Professor Angus McIntosh, Chair in Freshwater Ecology, University of Canterbury comments:

“I have studied the influence of trout on native biodiversity for most of my career, and as a researcher, I’m also involved in developing ways to mitigate the effects of land-use intensification on waterways.

“Yes, it is true that trout have negatively influenced native biodiversity and they do alter nutrient cycling. However, to compare their effects to stoats, and to imply they are somehow worse than, or equivalent to, the effects of land use intensification on water ways is a misrepresentation of the science.

“Firstly, they are clearly not ‘eating the basis of the food chain’ since we have highly productive trout fisheries in clean water streams. Native fish populations have been affected, but the resilience displayed by stream invertebrates in supporting predation by both native and introduced fishes is remarkable. Secondly, the effects of nutrient enrichment on algal accumulation and nutrient cycling are much more powerful than those of trout.

“Experiments conducted in New Zealand and elsewhere clearly establish that elevated nutrient concentrations quickly overwhelm any effect of trout on algae, which is actually small by comparison. Trout have not been responsible for what could be described as ‘algal blooms’ in New Zealand or elsewhere. Moreover, the effects of nutrient enrichment on stream invertebrate communities are also likely to be much stronger than those of trout.

“With respect to nutrient limitation in New Zealand waterways, the scientific message to farmers and others involved in nutrient inputs to streams should be very clear.  Both nitrogen (N) and phosphorus (P) are involved as limiting nutrients in aquatic systems.  Just because a waterway is regarded as P-limited doesn’t mean N isn’t important. In fact many of the situations of P-limitation are probably associated with situations of excessive nitrogen loading.

“The health of water ways in places like lowland Canterbury is very poor at present. The primary causes in agricultural areas are high sediment levels, low flow and high nutrient levels. In urban areas, storm water contaminated with heavy metals and sediment (especially since the earthquakes) are to blame. Discussion regarding urban and rural waterways should be revolving around plans for rehabilitation and management, and needs to based firmly on the best science possible.”


Prof Jenny Webster-Brown, Director – Waterways Centre for Freshwater Management comments:

“In a recent speech by Lachlan McKenzie at the Federated Farmers AGM in Rotorua, he has questioned the truth of some commonly espoused statements regarding our waterways.  There is an element of truth in Lachlan McKenzie’s comments, but the statements referred to need to be considered in light of all of the evidence – one fact alone cannot be used to discredit them.  That would indeed be the “Bad Science” that  is referred to in the speech.

“Regarding the assertion that trout have a significant adverse impact on freshwater ecology – Trout may be an introduced species, but they are also the most sensitive species to most contaminants, and the first to show the effects of water quality decline.   Because they are widespread globally, there is also a great deal of toxicity data (data that tell us when a particular species will be affected by increasing contaminant concentrations) available for trout; certainly when compared to the data available for species which are endemic to New Zealand.  Trout are therefore a very useful indicator of water quality, and protecting them ensures an additional level of protection for other species from the effects of poor water quality.

“Regarding the statement that nitrogen loss from farms is not the main factor promoting algae growth in rivers – The majority of NZ rivers are indeed phosphorus-limited. None-the-less,  algae and plants still require both nitrogen and phosphorous to grow, and controls on the leaching of BOTH nutrients from land surfaces is required if algal growth is to be controlled long term.  The tendency of phosphorus to bind strongly to soil limits the amount of phosphorous getting into our waterways, but the capacity of soil to take up phosphorous is finite (i.e., can be exceeded).   The main sources of phosphorous are natural rock weathering and the application of fertilisers.  Soaps and detergents are most unlikely to be a major source of phosphorous in our rural rivers … or even in urban waterways.  Such wastewaters from houses, commercial and industrial activities are not (unless sewerage infrastructure has been compromised) discharged untreated into streams and rivers,  and not into Lake Rotorua, as implied in the speech.

“Finally, the statement regarding the relationship between river water quality and algal growth – Algal cover is only one measure of water quality, and certainly not a robust indicator of water quality as it can respond to many other environmental changes. To say that river water quality is not declining because algal cover is not increasing, is like saying that water is safe to drink because it has  low arsenic (even though it contains disease-causing organisms or high aluminium, for example).”


Dr Jon O’Brien, Post-doctoral Fellow in Freshwater Ecology comments:

“Nitrogen and phosphorous are important elements in regulating algal growth in lakes, rivers, and marine ecosystems.

“In streams and rivers, excess nitrogen and phosphorous can contribute to excessive growth of periphyton (algae growing attached to hard surfaces).

“In lakes, excess nitrogen and phosphorous causes a process called eutrophication, which is characterized by algal blooms, loss of oxygen at the lake bottom, and growth of toxic cyanobacteria.

“In marine ecosystems, excess nitrogen can lead to ‘dead zones’ where prolific growth and collapses of algae blooms strip the oxygen from the water (famous examples include Gulf of Mexico, Chesapeake Bay and parts of the Baltic Sea).

“The ratio of nitrogen to phosphorous in the water will determine which is more important in driving algal blooms (sometimes nitrogen, ‘N’, is more important, other times phosphorus, ‘P’). The more important element, in any given circumstance, is called “limiting”.

The effects of excess nitrogen and phosphorous may be less evident in streams and rivers (algal growth in influenced by many things other than N and P: including light, floods, and grazing by invertebrates). However, streams and rivers transport nitrogen and phosphorous to downstream ecosystems (i.e. lakes), which are more sensitive.

“With regard to Mr Mckenzie’s comments :

“In a review of the literature, Abell and colleagues showed algal growth in New Zealand lakes is more likely to be limited by nitrogen than by phosphorous. Not P as stated.

“Rivers in many areas of New Zealand appear to be phosphorous limited because they are already receiving excess nitrogen. The ratio between N and P has already shifted in these streams.

“River periphyton (algae growing attached to hard surfaces) can build substantial biomasses despite P-limitation. Periphyton does this by recycling phosphorous within the algal mat. Such internal recycling can account for a large proportion of the biological phosphorous demand (e.g., see Steinman and colleagues, 1995).

“An example of this is Didymo, which is renowned for achieving amazing biomasses while living in very nutrient poor waters. Using a specialized mechanism recently demonstrated by Sundreshwar and colleges, Didymo can scrub phosphorous from the water column and recycle it internally within the mass of algae, effectively escaping P-limitation.  Didymo is an extreme example and the mechanism that it uses in rare (but not unique). Many other types of algal communities are able to use P recycling and partly escape P-limitation.

“Our own experience in rural Canterbury streams suggests that the main drivers of photosynthesis are light and nitrogen, not phosphorous. This is despite that apparent phosphorous limitation from the ratio of nitrogen to phosphorous in the water. We are currently researching the mechanism behind this pattern in the data.”

Abell, J.M., Ozkundakci, D. & Hamilton, D.P. (2010). Nitrogen and phosphorus limitation of phytoplankton growth in New Zealand lakes: Implications for eutrophication control. Ecosystems, 13(7), 966-977.

Steinman A.D., Mulholland P.J. & Beauchamp J.J. (1995) Effects of biomass, light, and grazing on phosphorus cycling in stream periphyton communities. Journal of the North American Benthological Society, 14, 371-381.

Sundareshwar et al. 2011. Didymosphenia geminata: Algal blooms in oligotrophic streams and rivers. Geophysical Research Letters  38: L10405, doi:10.1029/2010GL046599

Prof David Hamilton, Chair in Lake Ecology, University of Waikato and current President of the NZ Freshwater Sciences Society provided the following abstracts of relevant research he has been involved in:

Relationships between land use and nitrogen and phosphorus in New Zealand lakes

Jonathan M. Abell, Deniz Ozkundakci, David P. Hamilton and Steven D. Miller
Marine and Freshwater Research, 2011, 62, 162-175

We analysed total nitrogen (TN) and total phosphorus (TP) data for 101 New Zealand lakes and related these to land use and edaphic sources of phosphorus (P). We then analysed a sub-sample of lakes in agricultural catchments to investigate how lake and catchment variables influence the relationship between land use and in-lake nutrients. Following correction for the effect of co-variation amongst predictor variables, high producing grassland (intensive pasture) was the best predictor of TN and TP, accounting for 38.6% and 41.0% of variation, respectively. Exotic forestry and urban area accounted for a further 18.8% and 3.6% of variation in TP and TN, respectively. Soil P (representing naturally-occurring edaphic P) was negatively correlated with TP, owing to the confounding effect of pastoral land use. Lake and catchment morphology (zmax and lake : catchment area) and catchment connectivity (lake order) mediated the relationship between intensive pasture and in-lake nutrients. Mitigating eutrophication in New Zealand lakes requires action to reduce nutrient export from intensive pasture and quantifying P export from plantation forestry requires further consideration.

Nitrogen and phosphorus limitation of phytoplankton growth in New Zealand lakes: Implications for eutrophication control
Jonathan M. Abell, Deniz Ozkundakci, and David P. Hamilton
Ecosystems, 2010, 13(7): 966-977

We examine macronutrient limitation in New Zealand (NZ) lakes where, contrary to the phosphorus (P) only control paradigm, nitrogen (N) control is widely adopted to alleviate eutrophication. A review of published results of nutrient enrichment experiments showed that N more frequently limited lake productivity than P; however, stoichiometric analysis of a sample of 121 NZ lakes indicates that the majority (52.9%) of lakes have a mean ratio of total nitrogen (TN) to total phosphorus (TP) (by mass) indicative of potential P-limitation (>15:1), whereas only 14.0% of lakes have mean TN:TP indicative of potential N-limitation (<7:1). Comparison of TN, TP, and chlorophyll a data between 121 NZ lakes and 689 lakes in 15 European Union (EU) countries suggests that at the national scale, N has a greater role in determining lake productivity in NZ than in the EU. TN:TP is significantly lower in NZ lakes across all trophic states, a difference that is driven primarily by significantly lower in-lake TN concentrations at low trophic states and significantly higher TP concentrations at higher trophic states. The form of the TN:TP relationship differs between NZ and the EU countries, suggesting that lake nutrient source  and/or loss mechanisms differ between the two regions. Dual control of N and P should be the status quo for lacustrine eutrophication control in New Zealand and more effort is needed to reduce P inputs.