TRM Daily Report does not usually get too ‘bio-technical’ in terms of fly fishing – we leave that to the experts. But we thought this was so interesting as Stephen is – to our knowledge – the only boffin researching mayflies in NZ. As nymphs, duns and mayflies are a major part of trouts diet we thought we should share it with you. Thanks Stephen.
Distribution and body size patterns of mayflies in New Zealand
|Hi all, I’m Stephen Pohe. I stayed with Ross and Pip recently. I’m a PhD student with the School of Biological Sciences at the University of Canterbury in Christchurch, and am conducting research on New Zealand mayflies. I’m interested in where different species of mayflies live within New Zealand, and also how big they grow in different regions. I’ll explain why shortly.|
|Me (left) and the light trapping equipment I use to collect adult specimens (right).|
|I’ve been travelling the country each summer for the past three years and have now visited much of New Zealand. The blue dots on the map show all the places I’ve surveyed to date.Mayflies are an ancient lineage of insects, dating back to before some of the well-known dinosaurs. Their ancestors may have been the first winged insects to evolve. Today mayflies occur on all the contents of the planet except Antarctica. In New Zealand 52 species are described, all of which are endemic (only found in New Zealand), but there are more New Zealand species that haven’t been described yet. New Zealand’s actual mayfly biodiversity is probably about 75 species.|
|The immature stages, called nymphs, are aquatic and live in cool, clean rivers for about a year. In other countries some species also live in lakes, but New Zealand doesn’t strictly have lake-dwelling mayflies.|
|Mayfly nymphs Coloburiscus humeralis (left) and Acanthophlebia cruentata (right). Photos: Stephen Moore, Landcare Research.
The adult mayfly (imago) is terrestrial, and lives only a few days. There is also a transitional subadult (subimago) stage, known by anglers as a dun. The adults, subadults and nymphs are an important source of food for many other animals, forming an essential link in the food-chain between basal resources like algae, fungi and bacteria, and hungry predators like eels, native galaxiids and introduced trout, as well as many forest and riverine birds.
|Subadult stage of the New Zealand mayfly Ameletopsis perscitus. Photo: Olly Ball/Steve Pohe collection.|
| Adult stage of the New Zealand mayfly Mauiulus luma. Photo: Olly Ball/Steve Pohe collection.
|However, freshwater habitats are increasingly being degraded, which cause declines and the eventual demise of many mayfly populations. In addition, in recent decades there has been a pronounced warming trend in global surface temperature, which is predicted to continue. Freshwater species have been identified as particularly vulnerable to this warming climatic situation, and in New Zealand cold-adapted aquatic species may be at risk.|
|Graph of mean air temperature with latitude across New Zealand showing a clear decline in temperature, from north to south. Data source: http://cliflo.niwa.co.nz/
For mayflies, increases in water temperature can affect their reproductive success by influencing metabolism, growth and adult body size. Smaller individuals (of the same species) produce fewer offspring, and fewer offspring mean a lesser chance of a mayfly’s offspring surviving to reproduce. My research suggests that New Zealand mayflies are smaller in size, the further north they are (see below), which is also a clear temperature gradient (see above). Thus, it might be inferred that as average temperatures increase, all mayfly populations will reduce in size, which may in turn reduce population numbers. This may ultimately have implications on other species; both for those like algae that mayflies consume (inducing habitat changes), and for those that rely on them as a food-source.
|Changes in adult body size (wing length) across the country for males and females of the mayfly Coloburiscus humeralis.|
| Stephen Pohe, December 2016
University of Canterbury