Last year I finished up a freshwater and marine biology Honours Degree and for my final thesis I decided to investigate an area of Lough Corrib, or rather, one of its feeder streams. There has been an ongoing and aggressive ‘predator control program’ in the lake where gill nets are laid out at great expense to remove spawn laden pike as they congregate to procreate in the shallows in springtime. Pike have been long referred to as a nuisance in Lough Corrib and one that has been blamed on the disappearance of huge numbers of trout, a conclusion based on estimates and guess work which is hardly a sound basis for any scientific reasoning. A 2012 Lough Corrib survey noted in its summary that despite the ongoing removal of pike, trout recruitment was low, particularly in the north east of the lake. Based on this and the thought that it is more than the pike responsible for poor trout numbers I decided to investigate the Cross River, a feeder to the Corrib that joins the lake in this location. I will attempt to condense days of fieldwork, hundreds of hours of lab work and scores of pages of writing into an article a few hundred words in length for your consideration.
The Cross River flows for approximately 15kms across fertile agricultural grazing land to enter the lake at Derries Bay. It flows through two small villages and features a waste water treatment facility with a second one planned. The river has also been extensively modified through dredging for drainage with obvious spoil heaps seen along most parts of the watercourse. Prior to starting the project I had assumed that water quality was the issue with scope for contamination from agricultural runoff and domestic inputs being the chief culprits. Methodology for the project would see the selection of five sampling sites along the river’s length which would be subject to water sample removal for chemical analysis and kick sampling efforts. The kick sampling would remove macroinvertebrates for laboratory identification, macroinvertebrates analysis being a key method for identifying water quality using various indices. Water samples were collected with four kick samples taken from each site.
Chemical analysis for phosphates and nitrates stumped me when they came back at negligible levels. I was sure that water quality was going to be an issue but the results showed otherwise. That said, sampling occurred when the river was at normal level but in the preceding weeks there had been a few massive rainfall events. It is plausible that any runoff may have been flushed from the system but seeing as that is purely speculative the only way to determine this would be through further sampling during drier spells. Excessive plant growth in the river and at its mouth would indeed suggest that there are nutrient influences affecting the water but were not detected during this sampling effort.
Next up was the macroinvertebrate analysis which required recovering all samples to the lab and identifying them with the aid of a stereoscope. When processing macroinvertebrates there are three main groups that will indicate water of better quality just by their presence – Plecoptera (stoneflies), Ephemeroptera (mayflies) and Tricoptera (sedges). Stoneflies will inhabit only the cleanest of water, many mayflies will tolerate very clean water and many sedges can be found in water of reasonably good quality. Samples that contain only Oligochaetes (worms), Coleoptera (beetle larvae) and large amounts of Asellidae (crustaceans, shrimps) are generally of a poorer quality. Please note that this is an oversimplification of the process for the sake of this article. The top two sites on the river yielded only worms, beetles and crustaceans and seemed to initially back up my water quality theory but further down the river sedges, mayflies and the odd stonefly started to appear. To have stonefly present would indicate very clean water so I had to rethink my strategy.
Taking my samples into account I started to realise that although there was representation there from all of the major groups and with juvenile salmonids seen at most sites, there were only a couple of species present from each group where in a healthy river system there would be a far greater range in species diversity. I started to look at the physical form of the river to see if I could figure out why there was such a small range of macroinvertebrate species in this body of water. Further research uncovered that reports made for the Water Framework Directive had listed the Cross River as being in danger of canalisation; previous dredging efforts had almost resulted in transforming what was a river with pool, riffle and glide habitats into a featureless, canal-like drain.
Many species of invertebrate like to occupy different sections of each habitat – some prefer the swifter flowing areas, some the slacker. With dredging removing a lot of the river habitats and leaving only slacker water with few riffles and glides left in large sections of the river it stands to reason that a huge portion of their aquatic inhabitants are also going to be missing. Indeed the two uppermost sites on the river were devoid of substrate, the river flowing over bare bedrock. Dredging is incredibly ecologically destructive and although that last major OPW drainage scheme occurred here decades ago it was clear to see that the river ecology had still not recovered sufficiently.
Dredging is not only ecologically destructive by removing the building blocks of a lot of aquatic life it also drastically alters the physical environment of the river. To dredge a river essentially what happens is heavy machinery is moved in to deepen, and in some cases straighten, the river. Spoil heaps along many sections of the Cross River give some clue as to why trout recruitment may be poor in the north eastern area of the Corrib; the very material that the trout need to spawn over is now piled high on the bank some 10 – 15 metres form the river’s edge. I know salmonids are an ‘athletic’ species but even they couldn’t manage to leap from the river to cut redds and spawn over dry land. Fish cannot spawn unless there is the right substrate there for them to spawn onto, on observation that one would wager could go a long way towards explaining the lower levels of trout recruitment in the north east of the Corrib.
Only in Ireland
And so it continues, Ireland, probably the only country in the world where prime spawning habitat can be removed and the subsequent dearth of trout is blamed on another species. If it wasn’t so tragic it would be funny. Almost €120,000 has been earmarked for the removal of healthy pike that could attract great tourism revenue into the area all year round, not just during trout season. The Corrib angling product is in decline where mixed fisheries like Lough Ree are providing better sport for both pike and trout anglers. The money put aside for the folly of predator removal could be far better spent restoring and improving the lake’s spawning feeders but alas predator control continues almost unhindered on Ireland’s ‘wild’ trout fisheries.
Surely ‘wild trout fisheries’ is a misnomer? The word wild would imply that there is no intervention by man. What with predator control and hatcheries on many of these waters it is seen by many that the ‘wild’ tag is nothing more than a cynical marketing ploy used to convince those that don’t tend to delve a little deeper. Some would call it deceit. These policies are having a severe impact on coarse and pike angling revenue that was created by visiting tourists who now prefer to spend their money in more enlightened angling destinations around Europe.
Images supplied by Off the Scale, Bill Brazier and Ecofact Environmental Consultants
This piece was featured in issue 14 of Off the Scale, Ireland’s number one free digital angling publication. Click the logo above to enjoy what the magazine has to offer.