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. | |
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| 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. | |
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| 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. |
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| Subadult stage of the New Zealand mayfly Ameletopsis perscitus. Photo: Olly Ball/Steve Pohe collection. | |
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| Adult stage of the New Zealand mayfly Mauiulus luma. Photo: Olly Ball/Steve Pohe collection.
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| 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. | |
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| 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. |
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| 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
The importance of mayflies is indicated in the following “reel-life” update: Will Spry Fishing Report Dec 2016Published on Tuesday, December 20, 2016
The season kicked off to a great start, with some excellent numbers of fish still working back down the river systems to the lakes and lower stretches of the rivers. Post-spawning rainbows were mixed in with well-conditioned brown trout and were feeding voraciously on most offerings. However, a few ingredients were missing along the way, with a very noticeable absence of mayflies on certain rivers in the region. These vital links in the food chain did not show up in the usual numbers on the Tekapo River in particular, meaning that the traditional large hatches on this river were not happening. Likewise, the black-fronted terns which rely on the mayflies during their nesting period, were absent along with their food supply. In the mayflies’ place, caddis have become the mainstay and are becoming the mainstay of the trout’s diet. So what has caused the demise of the mayfly on the Tekapo River? Numbers were certainly reduced there at the end of April, when it is usually a great time to see mayflies re-appearing for a late season burst – however, there is almost now a complete dearth of them for the spring of 2016. This is a rapid decline in the population and one which needs some serious investigation, in order to restore some of the balance of life in this river. It has been said that the decline of mayflies and increase in caddis species is a sign of a depleting river system, and it would be a shame to think that this is happening on what was once an iconic Central South Island river. Perhaps it is time to have a good hard look at ourselves and start answering some of the obvious questions which may be arising. Enough of the negatives. On the positive side, higher rivers and good inflows make for a great start to summer fishing in the region, with plenty of water keeping the rivers of the district running well and in good health. With more settled conditions over the summer, we hope to see some excellent summer fishing and a good run of cicadas later in January and into February. The high country lakes are in great shape and some excellent fish are moving around them. High country rivers are dropping in height and cleaning up nicely for the summer. My picks for taking the family fishing are of course, the hydro canals, which although quite clear at the moment are still producing good numbers of excellent numbers of good-sized fish. However, venturing out onto the lakes is also an option, where a bit of trolling behind a boat can be a great way to spend an afternoon. For those seeking a bit of shade, lower river levels on the high country streams can leave you looking for fish in the shaded areas, where a nymph or lure run under a tree can pull well shaded fish out of the darkness and onto your lure or fly. The options are nearly endless and despite the huge numbers of anglers moving into the district over the Christmas break, it is not at all difficult to find a quiet corner to call your own and fool a few fish. Tight lines and Season’s Greetings. Will Spry |
