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Habitat Simulation – RHYHABSIM – The Work of Fiction?


For the past three decades River Hydraulic Habitat Simulation (RHYHABSIM) has been misinforming commissioners at New Zealand water allocation hearings resulting in over abstraction of water from our valued rivers and streams.

The worth of habitat simulation has been the subject of much debate and even legal challenges internationally as hydrologists seduced by the simplicity of the assumptions on which the models are based have clashed with biologists who are concerned that the models cannot replace comprehensive biological studies of the interactions between aquatic inhabitants and their environment.

Whether you side with those that see habitat simulation as nonsense or those who come to its defense, both groups agree that habitat modelling alone is not a substitute for a comprehensive Instream study – IFIM of which habitat modelling plays a small part.

In New Zealand we have got it wrong and drifted from completing the detailed river specific studies that make up IFIM. We have been seduced by a simplistic Weighted Useable Area (WUA) “habitat” model because of the quantitative “answers” it gives. Where “habitat” or space in a river is not limiting, habitat modelling is meaningless.

[“There is extensive ecological literature on habitat selection modeling which indicates that a simple selection of flow recommendations from a static set of WUA versus flow curves is not a credible approach (for example Marthur et al. 1985, 1994; Osborne et al.1988; Gan and McMahon 1990; Elliot 1994; Castleberry et al.1996; Ghanen et al.1996; Williams 1996; Lamouroux et al.1998). It has long been recognized that WUA versus flow relations alone lack biological meaning. Instead, the flow-habitat relations as a component of IFIM are intended as input for evaluation of habitat analyses (Stalnaker et 1979; Stalnaker et al.1995; Bovee etal. 1998; NRC 2005a, 2005c.)There is no justifiable way to base flow-management recommendations on habitat relations alone. This limitation of WUA has been recognized for more than 30 years, but WUA continues to be inappropriately used by some as a shortcut approach for selecting flat-line “minimum flows”….

[Hydrology, Ecology, and Fishes of the Klamath River Basin. NRC of the National Academies, 2008]]

Bovee et al. (the originators of IFIM) stated: “IFIM … is widely misconstrued, misinterpreted, and in some cases misused”.

A Critique of IFIM-instream habitat simulation in the New Zealand context.Science for Conservation 231 Hudson et. Al. 2003, is a detailed 68 page review of IFIM – instream habitat simulation written with the objective of encouraging a critical re-evaluation of IFIM and improving the way in which IFIM is used in New Zealand.

Having witnessed the misuse of RHYHABSIM, the New Zealand habitat model, at two consent hearings I say enough is enough. For the sake of what is left of our rivers it is time for an independent review of the model to determine whether it should be dropped &/or its limitations exposed.

RHYHABSIM – The model

Put simply proponents of RHYHABSIM believe populations of aquatic species of interest can be predicted from simple hydrological measurements.

Habitability suitability curves (HSCs) have been developed from observations of the presence or absence of species of interest at different flows, depth, and substrate averaged over multiple (57) New Zealand rivers and streams.

The physical space in the river is then measured by dividing the stream into cells of variable width, length, and mean water column velocity measured between transects. Estimates of the point suitability of each cell for the specific species of fish or insect of interest are made by multiplying the habitat suitability weightings at that point.

The cells are summed to produce the Weighted Useable Area (WUA) of the “habitat” which is then plotted against simulated flows.

The resultant curves show a “break point” where reducing flows begin to sharply reduce the amount of modelled “habitat” or space in the river which has been estimated to suit the fish or invertebrate.

This modelled “break point” is presented to hearing commissioners as answers as to how a proposed water abstraction will affect the river.

Using this method Ian Jowett, John Hayes, and Maurice Duncan (2008) have determined that;

Benthic invertebrates have an optimal flow requirement of 15 m3

Large trout require flows of 10 m3

Trout fingerlings have optimal flow requirements of 2m3.

Ian Jowett goes so far as to claim this RHYHABSIM enables him to “improve” rivers by reducing their flow to the optimum predicted by this model.

No wonder the model finds favor with corporate water users and these experts are hired to feature regularly at consent hearings!

Despite vast increases in computing power allowing more sophisticated data management and elaborate graphics, habitat simulation involves a form of modelling that has never been validated nor verified.

Along with a large body of biologists and fishermen I believe these experts are wrong.

I believe this model would have been exposed and discarded but for poor study design, unethical science, baseless/dishonest assumptions, and the self-interest of those that earn their living through selling their “expertise” to corporate water raiders.

[HSCs must be river specific. HSCs can vary strongly with environmental and biological variables, such as temperature, turbidity, habitat structure, and fish density. As a consequence mixing habitat-suitability data from a variety of sites and conditions may produce criteria that represent no site. NRC Klamath Study, 2008. The point was also stressed by Dr Richard Laven.]

Ethics in Research

Ethical research is important it requires;

· Honesty Do not fabricate, falsify, or misrepresent data. Do not deceive colleagues granting agencies, or the public.

· Objectivity Strive to avoid bias in experimental design, data interpretation, expert testimony and other aspects of research where objectivity is expected. Avoid or minimize bias or self-deception

· Integrity Act with sincerity: strive for consistency in thought and action

· Carefulness Avoid careless error and negligence. Critically examine your work and the work of your peers

· Respect Intellectual property

· Responsible publication Publish to advance research and not just your career

· Social responsibility Promote social good or mitigate social harms through research, public education, and advocacy.

· Competence

[David B. Resnik, J.D., Ph.D. “What is ethics in Research & Why it is Important” NIEHS National Institute of Environmental Health Sciences.]

In a recent article in the Dominion Post, Dr Mike Joy speaks of “Agency capture” of the Ministry for the Environment which he accuses of concentrating on making the policies of the government of the day look good. The focus of Mike Joy’s concerns are the tricks used to cover for the problems of irrigation and water pollution in particular the inappropriate NPS FW Management Standards

Dr Joy introduces the term agnotology which is defined as “culturally induced ignorance or doubt, particularly the publication of inaccurate or misleading scientific data to spread confusion and deceit, usually to sell a product or win favor.

This definition could readily be applied to the way expert witnesses (deceitfully) present RHYHABSIM evidence at consent hearings despite the expert(s) having sworn to abide by the Code of Conduct for Expert Witnesses in the Environment Court.

Perhaps this explains why RHYHABSIM is listed on the MfE website as the “gold standard” for water allocation?

Crown Research Institutes, (CRIs), and not for profit institutes such as NIWA and The Crawthron Institute are also vulnerable to the pressures of having to pay their way. What client would accept studies that found against their needs? Unfortunately their clients are government and corporations wishing to mine our water resources.

Two Case Studies – Trustpower’s Arnold River HEPS application (2007) and Trustpower’s 2012 application to amend the Rakaia NWCO (1988)

1. The Arnold River

In 2007 I was asked to become a West Coast Fish & Game councilor in order to respond to Trustpowers application to take 80% of the mean Arnold River flow, (60 m3), for a hydroelectric power scheme. In their Assessment of Environmental Effects, (AEE), Trustpower’s experts argued that by reducing the river’s mean flow to 12 m3 the river would become more suitable for large brown trout. [RHYHABSIM modelling was used to predict a flow reduction of 4/5 would result in a five-fold increase in large trout thus removing any need for mitigation to local anglers].

My challenge was to learn about habitat simulation and Instream Flow Incremental Methodology, (IFIM).

While intuitively I found the suggestion outrageous, I methodically set about assembling the published papers describing the development of RHYHABSIM, (the New Zealand version of Physical Habitat Simulation, PHABSIM). PHABSIM was developed in North America by the US Fish and Wildlife Service to predict the effects of altered flows on populations of fish.

First I compared the assumptions made to develop the model against my knowledge and experience of trout biology and behavior. Little made sense – the habitat or space in a stream or river was determined by measuring flows remote from where I would expect fish to be. No allowance was made for drift, migration, or feed. No allowance was made for key water quality parameters such as temperature, dissolved oxygen, shade, or turbidity. No allowances were made for geological factors –water hardness/mineralization, channel structure etc. The model simply relied on a set of hydrological measurements multiplied by some simple biological observations. The model was described by one critic as “Multiplying hydrological apples with biological oranges”.

Secondly I was troubled to find a number of overseas peer reviewed studies that specifically criticized the New Zealand developer of RHYHABSIM in which he was accused of “data dredging”, (selecting data to fit his theory), and his alleged misuse of statistical methods.

I decided it was time to seek some expert help so I contacted Dr Richard Laven, a research veterinarian at Massey University and sent an email to Dr John G. Williams a consultant fish biologist from Davis Ca., who had previous experience in the New Zealand environment court. Both men kindly put themselves out to bring me up to speed.

I also phoned Dr Henry Hudson, author of the DOC publication above to get a feel for his views.

I sent Dr Laven the papers which supported the use of habitat modeling to predict the abundance of large brown trout. While he accepted the basic approach he felt the “clean” graphs were probably a spurious representation of the biological data, and was emphatic that HSCs should be developed specifically for the Arnold River – it was not appropriate to use averaged HSCs.

John Williams’ response was blunt and pointed. He and his colleagues had spent a lot of time critically examining the claims made for RHYHABSIM and found them wanting. He emailed a large amount of material critical of the model.

From my reading I learned that even the basic hydrological measurements of flow, depth, substrate, and transect selection all have potential for error that can affect the results.

My submission achieved little other than to elicit the comment from one of the hearing commissioners that the habitat model evidence was accepted simply because it was the best we have.

2. The Rakaia River – The Lake Coleridge Project Ecan Hearing 2012

While my Arnold river submission was made out of a sense of duty, the Rakaia is a river is my river.

I have fished the Rakaia for nearly 60 years and have owned a bach at the Rakaia Huts since 1994. I had an undemanding job as the supervising meat vet at a nearby Deer Slaughter Premises from 1994 – 1999 so I spent nearly every afternoon &/or evening on the river.

I have worked for farmer Vet Clubs on both banks of the river and my father generously took me along on his salmon fishing expeditions from the age of 7. I know this river well.

Having completed my “apprenticeship” on the Arnold, I downloaded the latest mass of evidence produced by Trustpower’s expert witnesses and set to work.

Ian Jowett was the lead expert witness around whose evidence the others framed their reports.

Jowett used RHYHABSIM modeling to support his expert opinion that the altered Rakaia flows applied for by Trustpower would have “minor or less than minor effects”. He produced a table of the predicted gain or loss of habitat that would result for the various fish, aquatic insects, and birds that inhabit the middle braids with an apparent precision of 0.1%, without giving confidence limits, or explaining that if space in the reaches studied were not limiting, then his (modeled) results would be meaningless. [In fact the “experts” only studied reaches in the inhospitable middle braids where few fish are present so the results did not have biological meaning.]

The Rakaia catchment is complex with a nursery fishery in the headwaters, relatively inhospitable middle braided reaches, and a prolific fishery within 3 km of the coast. It is dynamic with variable flows and variable turbidity. The fish and other aquatic life have changing habitat requirements as they evolve from egg to adult. Who knows what bottlenecks may occur as the flow patterns are modified by water abstraction. Temperature, a master modifier of the ecology of the river, was not addressed by the habitat model. Important catchment wide connections were not considered. Temporal effects from changing flows were missed.

The Muldoon government of the late 1970s had looked at a large scale project using water from the Rakaia to irrigate much of Central Canterbury. Muldoon’s various ministries were set to work completing the necessary studies, but ultimately the economics of the scheme simply did not stack up.

Both Trustpower’s experts and I assembled the published references of the studies completed at this time, (1981-1986), the centerpiece of which was a study by Glova & Duncan (1985) where the flows of the Rakaia were artificially manipulated by shutting off and then releasing water from both the Highbank and Lake Coleridge power stations. This was a way of getting flow data from the complex multi-braided reaches studied.

Jowett described how he used this flow data along with the 2010 transect measurements of Ross Dungey and referenced flow data from the gorge as inputs for his in vitro study. Jowett did not visit the river in order to conduct his study.

[Thirty year old flow data, married to 2010 transect measurements, and flows measured 15 – 20 km upstream. No allowance could be made for gains/losses to groundwater which can be up to 40m3. There was no discussion or explanation of the potential errors this methodology might cause.]

When I tried to follow his method (Appendix D), it appeared Jowett had inaccurately transcribed flow data from one reach.

When this was pointed out to the hearing commissioners, Jowett was unfazed, in his rebuttal of evidence he simply stated that the original authors had made a mistake and he had access to the original flow data. I subsequently emailed Maurice Duncan the original hydrologist to get his version. He replied that as NIWA was working for Trustpower on this one he felt he was unable to comment!

Naturally I used my Arnold River evidence to shed doubt on RHYHABSIM. Jowett’s response was to use his rebuttal of evidence to state that in his “expert opinion” Peter Trolove did not see the fish he reported spawning in up-welling water in a deep cut hole in the North Rakaia River.

Subsequently one of the hearing commissioners, Mr Michael Bowden, died allowing Jowett’s rebuttal to be challenged. Not surprisingly this part of the rebuttal was “lost”.

The format of the hearing and the constitutionally repugnant Environment Canterbury Temporary Commissioners and Improved Water Management Act (2010) ensured Trustpower’s application would succeed.

The subsequent collapse of the Rakaia’s once internationally and nationally recognized outstanding fishery supports my view that Trustpower’s troupe of expert witnesses together with the controversial habitat model were incorrect when they gave their collective “expert” opinion that the reduced Rakaia flows would have “minor or less than minor effects”.

I felt Jowett’s actions at the hearing fell short of the standards proscribed in the Code of Conduct for Expert Witnesses and saw his modeling of habitat for black fronted terns as farcical – how could modelled habitat or space in the river’s braids calculated using flow, depth, substrate, and wetted surface area have any connection to a bird that hawks emerging insects, and near-surface fish from the drift lines of a river? I have even seen these birds hawk cicadas from thermals generated from the heated stones of dry Taramakau braids!

Ninety percent of the Rakaia fishery exists within 3 kilometers of the coast. This can simply be explained by feed, temperature, and turbidity. The largest fishery by mass involves Stokell’s smelt. When they are present in the lower braids they are concentrated by the swift currents where sea trout gorge themselves. When they are not present these trout drop back to the sea or rest in pockets of cold upwelling ground water. The same applies where high water temperature or sediment levels make conditions unappealing to the trout.

In the photograph I have grouped a large sea trout, a salmon, and an aged resident trout from the braids caught on the same outing. The sea trout was caught just above the tidal region on the edge of hard swift water where the smelt were in greatest numbers. The salmon was caught in a “resting hole” 5 – 6km upstream. Adult salmon do not feed in the river. They simply use the braids to access spawning creeks at the headwaters. I was successful as I had fished slow and deep in pocket water where a fish would be likely to take a breather. The river trout was caught in the same hole after I switched to my lure rod to sound the subtle currents and structures of the hole. If I had relied on knowledge of the “habitat” or space in the river without considering the biological factors I would have had no “luck”. Feed &/or cover determined the location of these fish. [The river fish (6lb) had experienced much slower growth having had to feed opportunistically on the slim pickings in the (inhospitable) braids. His size suggested a cannibal fish.]

The Case for the Defense – The Six Rivers Validation

One of the rare peer reviewed published papers in the world purporting to validate habitat simulation involves six New Zealand rivers:

[Flow regime requirements and the biological effectiveness of habitat-based minimum flow assessments for six rivers

IAN. G. JOWETT., J.F. BIGGS Intl J. River Basin Management Vol. 4. No3 (2006), pp. 179-189]

Much of the evidence is based on inaccurate assumptions regarding the presence or absence of fish populations present in the when the rivers’ flows were in their un-modified state.

The authors still appear to see habitat simulation as IFIM studies

This paper reads like those written by post-modernist academics more commonly found in the arts faculties of North American institutions – “I believe it therefore it is”.

In this study the authors looked at rivers that had had their flows partially or completely abstracted and subsequently the flows were partially restored either by downstream tributaries or managed flows.

“The assessment is based on information and data collated from a range of unpublished and published studies that were usually carried out on each river for other purposes, but allow evaluation of the success of the flow regime recommendations. All data and study methodologies were scrutinized and found to be adequate for the purposes of the present assessment”

The authors appear to have completely lost their objectivity. The paper is full of inaccurate assumptions and bias. This is article is not proof that habitat simulation has meaning, this article is an example of researchers’ escalating commitment to a failed theory.

· Tekapo river The whole flow of this river was diverted at the lake outlet leaving the downstream tributaries to provide the residual flows. Down river streams such as Irishman Creek, Mary Burn and Gray’s Stream provided water free of glacial sediment. The lower river connects with Lake Benmore a productive and popular fishery. It is my contention that improved water quality and the lake connection explain the current fishery better rather than the “optimized” flows caused by the total abstraction of the main river.

· Waiau River All of the 450m3 natural flow of the Waiau was diverted through the Manapouri Power Station in 1977. A remedial flow regime was implemented of 12 m3 winter, and 16 m3 summer. The authors claim these flows have restored the river without any detrimental effects on the fish community. How would they know without evidence? (My father took me to fish the outlet of Manapouri in the 1960s. Many trout were visible in the clear deep flow. The banks were accessible with tracks and access points. The challenge was one of presentation in the massive river. An old Tourist Dept. booklet kept from that visit referred to the excellent fishing at the outlet).

· Monowai River Monowai flows are regulated at the outlet of the lake. Previous hydropower needs saw the flows vary from near zero to full flow. A modelled flow assessment saw a minimum flow of 6 m3 negotiated. As a result benthic invertebrates doubled from a low base to achieve densities that are about 1/3 of the median counted in other NZ rivers. The taxon richness showed a similar improvement. So what. What was the state of the un-modified river? [The productivity and water quality at the outlet of a Fiordland National Park lake should have been excellent.]

· Moawhango River The natural flow of this river was diverted to the Tongariro River for hydro-power generation. As part of the re-licensing process a “habitat”/flow assessment saw 6% of the natural flow returned. Not unsurprisingly even with a flow of 0.6 m3 the invertebrate community improved. So what again. The authors then introduce the concept of “flushing flows” to help revive a dewatered river. There is evidence of improvement however flushing flows are not a feature of RHYHABSIM. Again there is no base-line data of the un-modified river.

· Ohau River Again the whole mean flow of 80 m3 was diverted. Later a reinstated flow of 10 m3 was determined. In this case the “excellent trout habitat” as predicted by the model has failed to impress either trout or anglers. The authors attempt to explain this by suggesting “historically the fishery may have been poor, as there is no mention of angling use in the Ohau River in an early angling diary scheme. The lack of trout in the river may be related to poor food production because of glacial silt deposits on the substrate and lack of flow variation, and/or problems with recruitment and fish passage between the lake and river or simply a preference for the lake environments.

This is the only case where predictions of trout abundance and maintenance of habitat were not successful”.

More poppycock! My father took us on a camping holiday to the Ohau River in the 1960s prior to the creation of Lake Benmore at the insistence of a local civil engineer, Doug Hood, a neighbor and friend who had the contract to clear the willows that lined the river prior to inundation by the lake. Doug was impressed by the numbers of large trout he saw while removing the trees. The problem here was simply one of angler access and opportunity. In the 1960s few people had private cars. We had to make our own track to the river.

· Waipara I know little of the Wiapara except that while it is prone to drying up in the middle reaches it has slow pond like pools relatively independent of flow where it passes through the hills downstream and upstream of SHWY 1. Again no baseline data was available.

Remove the incorrect assumptions regarding the un-modified productivity of these six rivers and this paper proves little about the effectiveness of habitat modeling.

Some other failures of RHYHASIM

The Ashburton River

Jowett was at a loss to explain why a 12 m3 flow could not sustain the fishery in the Ashburton River despite modelling the river several times. A biologist would have identified that a flow of around 20 – 40 m3 is required to keep the river open to the sea thus enabling the smelt on which the fishery is based to enter the river to breed. This connection was not captured by the model. [The Maori name for the Ashburton River is Hakatere or “shining waters” a reference to the once abundant smelt.]

The Onekaka River

This spring I was fortunate to meet a veterinary locum whose partner has a property both sides of the Onekaka River. She related a sad tale of her river that had been devastated by a private electric power scheme.

DOC commissioned NIWA to conduct a study to determine a “sustainable” minimum flow for the river. An assessment was carried out by Richardson and Jowett (1995). [NIWA Science and Technology Series No. 21]

This paper is recommended reading as it provides a clear and informative discussion of habitat modeling. Due to the accessibility of this river site specific HSCs were developed by electric fishing the various stream structures. Although this study comes closer to a true IFIM assessment than most, the primary focus remained on the hydrology of the river.

Sadly the modeled flows did not achieve their predicted outcomes and a case was brought to the environment court in the hope of gaining remedial flows to restore the river’s ecology. The court did not elect to alter the flow but ordered ongoing monitoring of the affected native fish. The decline in population size and diversity has continued. The 2018 fish count found the lowest fish densities to date.

This is a useful case study where the predictions of habitat simulation has and continues to be monitored.

[Stark Environmental Report No.2018-03 199]

Some hope at last?

The CAWTHRON Coastal and Freshwater news 17 June 2016 “Trout and fish need more water than we think research finds….” could be the game changer that will save our freshwater fisheries.

Thankfully CAWTHRON fresh water scientist Dr John Hayes has not lost his objectivity despite being involved with habitat modeling in New Zealand since its inception. Dr Hayes has long recognized RHYHABSIM has not provided adequate explanations for many modified rivers and streams. Rather than resort to evasive devices such as “expert opinion” when habitat modeling fails to deliver, he has always been prepared to revisit the river to look for reasons for the failure.

Dr Hayes’ admission that for 40 years we have got it wrong reflects well on his scientific objectivity, honesty and integrity. His eureka moment recognizing the importance of insect drift, and the productivity of larger flows is welcome news to all anglers who have intuitively known we have been short-changed in the water management game.

Perhaps we will see other eureka moments regarding water quality, connectedness, temporal effects, and temperature?


For too long RHYHABSIM has been most valued for its misinformation rather than its insights. It is not and has never been capable of providing answers.

When we think about science and scientific theory, we must always remember the underlying premise: as additional information is gathered, the theory may be modified or even rejected.

New Zealand is limited by a small pool of expertise that we rely on for informing the public of matters of importance to us. We need to understand in order to participate in our democratic processes.

“Agency capture” by government and corporates is a deliberate strategy to diminish our participation.

The RMA, MfE, NIWA, etc. are all susceptible to such influences of money, power, and politics.

We need to develop independent agencies, with open dialogue and peer reviewed “quality assurance” of our environmental science if we are to have any chance of protecting our natural resources while achieving sustainable development. We need to be able to look beyond our shores for expertise when it is lacking. It is ridiculous to expect the adversarial legal processes we currently rely on for water allocation to deliver the outcomes we require.

We need experts capable of honestly explaining what science can and cannot achieve.

We need well informed, independently funded advocates for our valued aquatic resources.


The New Zealand version of habitat simulation, RHYHABSIM, has been misunderstood and misused for far too long. We have seen many valued New Zealand fisheries diminished or destroyed by its (mis)use.

North American authorities have found it wanting. New Zealand fisheries biologists and ecologists have repeated their concerns.

John Hayes has realized its limitations.

It is time the Ministry for the Environment to re-evaluate this form of modeling and update its recommendations regarding defensible science for water allocation.

Peter Trolove