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The effect of didymo on fish life


Recently a study by NIWA was published that investigated what the effect of didymo infestations had on fish life. Until this study was done there was conflicting views as to whether didymo was detrimental to fish life and if it was, to what extent.

Didymo in NZ

Didymo was first discovered in New Zealand waters in 2004. Although some scientists have suggested that it may have lain largely dormant in our waters for long before that, it is probable that it was brought to NZ from a region of the world where it is indigenous.


Since it's discovery in the Waiua river in Southland, it has spread and is now found throughout many waters around the South Island yet has yet to be found in any North Island water. (see previous paper on Didymo for a discussion as to the reasons for this).

Despite the amount of research on didymo, until Jellyman and Harding published their paper, there was no hard evidence on the effect of didymo on fish populations.

Drift dives

Fish and Game undertake regular drift dives where they assess the number and size of the fish in a particular stretch of river. By doing this over a number of years they can look at cycles and trends in fish numbers.

Drift Dives

Fish and Game officers undertaking a drift-dive fish survey

What Fish and Game found in the Nelson region The annual report on the Nelson Marlborough fishery was published in May 2016. It found that in some of the rivers such as the Buller and Takaka which have dense didymo growths, fish numbers and the condition of the fish was down. This indicated that there appeared to be a direct correlation between didymo density and fish numbers (and fish size and condition).
Anecdotal reports

At nzfishing.com we receive reports on the fishing around the country. There seems to be a lot of confusion about the effects of didymo though there is consensus that in areas where didymo is found in heavy concentrations, fishing is avoided. This is borne out by angler surveys that show that angler numbers are down significantly on rivers such as the Buller and Takaka during times when didymo blooms are highest.

While these declines in angler activity of these infected waters may be due to aesthetic reasons (it is not a nice experience fishing rivers that have a thick coating of didymo), it is also assumed that the decline is partly due to lower fish numbers.

The Cawthron study

In 2006, Shearer, Hay and Hayes conducted a small research project on the Mararoa and Oreti rivers to see what impact the didymo blooms were having on trout and other aquatic populations.

From their studies they found there to be no noticeable impact and stated they found " no significant negative effect attributable to didymo on invertebrate drift density and biomass". (For anglers this read that they found that didymo did not seem to greatly impact on the food trout targeted and consequently trout numbers and size).

But Shearer et al were aware of the limitations of their study. They noted they had studied only a very small sample of waterways and the study was conducted in autumn and winter and had not looked at the spring / summer months when trout were most actively feeding (and didymo blooms were at a peak).

It was not until ten years later that Jellyman (NIWA) and Harding (University of Canterbury) conducted a more widespread investigation examining all fish, not just trout, and their findings cast a very different light on the problem that didymo causes.

Jellyman and Harding's study

Jellyman and Harding conducted a detailed study on 20 South Island rivers to see the impact of didymo on fish and other aquatic life. This large scale research project found the following:

  • Where there was heavy didymo infestation, the primary food sources of trout (mayflies, caddis and stoneflies) was greatly reduced
  • As a consequence both native fish and introduced fish species (most notably trout) were greatly reduced in numbers with the total fish biomass declining by up to 90%.
  • Juvenile trout were found at less than half of the study sites where didymo regularly blooms.

This very important study has shown that didymo is not just an unpleasant inconvenience to anglers but it is having a highly detrimental affect on fish numbers in didymo infected waters.

The economic impact of didymo

Fishing provides recreational activities for many New Zealanders and is an important part of the tourism industry with tens of thousands of tourists coming to New Zealand to fish annually.

In 2011 a MAF report on the economic impact of didymo to the economy found that between 2006 and 2011 didymo's financial impact was $127.8 million and from 2011 to 2020 would be between $210.6 and $845.8 million.


From the anecdotal evidence, Fish and Game surveys and now the hard evidence of the research by Jellyman and Harding, it is clear that didymo is having a highly detrimental effect on our freshwater fisheries. This is leading to other consequences such as the economic impact it is having, particularly on remote rural communities where fishing tourism is an important resource.

With the evidence that fish biomass can be reduced by up to 90%, we at nzfishing.com state it is time that more money was put into research into how the didymo problem can be addressed.

As it is unlikely, or even impossible, that we will be ever able to totally eradicate didymo, we need further research such as that carried out by Dr Kilroy, into ways we can control it's spread in our waters and how we can reduce the blooms that regularly occur in many waters.

Further reading

The above is a very short synopsis of research that should be of interest to all anglers and those who wish to see our outdoor heritage protected.

If you would like to read more see:







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