Didymo effects on river invertebrates: not as bad as feared?
Drift net used for gathering samples
of drifting invertebrates. The
'trash rack' upstream kept floating
clumps of didymo out of the net.
Photo: John Hayes, Cawthron Institute
One of the many downsides of a didymo infestation is its potential impact on invertebrate life – and, therefore, on the trout that rely on them for a food source.
New Zealand's rivers support a large recreational trout fishery that draws tourists from around the world. Fears that didymo might seriously reduce a river's capacity to support trout have been allayed to some extent by the results of a Cawthron Institute study, carried out for MAF Biosecurity New Zealand in Southland's Oreti and Mararoa rivers.
Preliminary research by the National Institute of Water and Atmospheric Research (NIWA) had shown that high didymo biomass was associated with a greater density of invertebrate life in affected rivers, although the proportion of smaller invertebrates was greater. This qualification is important for trout, because the smaller their individual prey, the greater the energy they have to invest in feeding on them. This energy demand could limit trout growth and affect the health of the fishery as a whole. Nevertheless, even though the research showed that there was a higher proportion of small invertebrates, the greater density all round meant that even the larger invertebrates that trout prefer were also more abundant at sites affected by didymo.
The Cawthron Institute research, carried out by Karen Shearer, Joe Hay and John Hayes, sought to quantify the effects of didymo infestation on the drift of invertebrates, and how this may affect trout growth potential.
The researchers carried out drift sampling by using submerged nets at two sites in the Oreti River and one site in the Mararoa River, first in autumn and then again in winter 2006.
The upstream site in the Oreti was intended as a didymo-free control. However, the alga had invaded there by the time the winter sampling was done, although its effect on invertebrates appeared minimal.
The researchers found that, in the didymo-affected rivers, the invertebrate drift density and biomass was actually highest at the sites with intermediate (in autumn) and highest (in winter) levels of didymo biomass. But the survey also confirmed that the invertebrate biomass contained a higher proportion of smaller species than is usually found in non-affected rivers, an effect that was more pronounced where volumes of didymo were higher.
While this effect was observed, the researchers said that, based on bioenergetics modelling, their results did not show that didymo affected invertebrate drift enough to negatively affect growth potential of drift-feeding brown trout.
This is encouraging news for anglers, but the Cawthron Institute researchers said that their results had to be interpreted with caution, because they were based on sampling in autumn and winter only, and limited to three sites.
In his presentation to the didymo science seminar, Joe Hay said trout energy requirements are much higher during the warmer months, and measurements taken throughout the year would be needed for a full picture of the effects of didymo on invertebrate life and trout growth potential.
He said there was no evidence yet that didymo was having an adverse effect on the abundance or size of trout. At this stage, negative effects for anglers were matters of aesthetics and inconvenience (fouled fishing lures and the need to clean equipment). Indeed, anecdotal accounts from Fish and Game New Zealand suggest that there had been excellent mayfly hatches (an important source of trout food) and good fishing in some didymo-infested rivers last summer.
Nonetheless, more research was needed to provide enough information for making management decisions. Joe Hay recommended additional drift sampling and trout growth modelling in summer. He further noted that Fish and Game New Zealand, in addition to its routine monitoring of trout abundance by drift diving, was planning to sample trout populations to assess size and age structure and growth, which ultimately would help to provide a better understanding of the impacts of didymo.
For a copy of the full report:
Return to:
Page last updated: 30 April 2008