Dredging

Impact of dredging and drainage maintenance on lampreys

Drainage maintenance of river channels (often derogatively referred to as ‘cleaning’ of rivers  – an archaic euphemistic term used by some engineers), including dredging and channelisation, leads to the loss of silt habitat where juvenile lampreys live resulting in many age classes of lampreys being significantly impacted, or removed from the river. Riffle and side channel habitats may be destroyed or reduced leading to loss/reduction of spawning habitat and ammocoete rearing habitat. Inappropriate timing of channel maintenance has the potential to disrupt lamprey spawning, directly affect lamprey redds, cause suspended solids pollution of spawning areas, affect larvae dispersal etc.

River dredging and drainage maintenance works is banned from the winter months to protect salmon and trout; it should be banned in the spring and early summer to protect lampreys as theses works can have the same types of effects on lampreys as they can have on salmonids. Currently both drainage maintenance works and ‘fisheries development works’ take place in Ireland within the lamprey spawning season (all three species).

OPW River maintenance works should not be referred to as "cleaning" rivers
OPW River maintenance works should not be referred to as “cleaning” rivers

Removal of silt is a typical procedure in “channel cleaning” or maintenance, as practised by the Office of Public Works Drainage Division, River Drainage Boards and Local Authorities. Such silt often contains juvenile lamprey that become stranded on the bank slope or spoil heaps. It is clear that such a process can lead to substantial losses of juveniles and has the potential for greater conservation status effects if works are being undertaken within designated Natura 2000 sites, where lamprey species are listed as qualifying interests of the designation.

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In Ireland drainage maintenance works are supposed to follow the mitigation measures outlined in King et al (2008) and other measures outlined in the ‘Series of Ecological Assessments on Arterial Drainage Maintenance’ produced by the Environmental Section of the Office of Public Works. However, there is no evidence that even the basic mitigation measures described in King et al (2008) actually get implemented on any drainage maintenance site in Ireland. Indeed, during monitoring of OPW drainage maintenance works on the River Boyne in 2008, O’Connor and McDonnell (2008) found that there “was no evidence that any mitigation measures for protecting lampreys, crayfish or otters were being undertaken at the site, although all three species were considered to be present“. Even King et al (2008) notes that the ‘bottom line is the potential for maintenance to impact on lamprey, particularly juvenile lamprey, populations and habitat in almost any channel of OPW’s arterial drainage scheme networks‘.

Visual elevation of suspended solids levels downstream a working machine. OPW waorks take place within lamprey spawning season.
Visual elevation of suspended solids levels downstream of a working machine. OPW works take place during the lamprey spawning season, and have significant direct and indirect effects on lampreys. The machine driver is supposed to look out for spawning lampreys according to King et al (2008). This is clearly a ridiculous (and probably illegal) approach to protecting lampreys.
Upper River Boyne. Banks in this area have been destabilised as a result of these counter-productive OPW drainage maintenance works.
Upper River Boyne. Banks in this area have been destabilised as a result of these counter-productive OPW drainage maintenance works.

It is recommended that all dredging and drainage maintenance works be preceded by a lamprey survey (and other relevant ecological studies), and where lampreys are present detailed site-specific mitigation should be designed and implemented. Where targeted dredging is unavoidable a suitably qualified ecologist has to be on site to collect lampreys from extracted spoil and return these to undisturbed areas. It is not appropriate to leave this work to machine drivers and general operative staff, as is currently supposed to occur in Ireland. Instream works like drainage maintenance should never be undertaken during the lamprey spawning season on rivers where they occur.

This is key mitigation proposed for Natura 2000 rivers by OPW and Inland Fisheries Ireland and is both ridiculous and probably illegal

Although it is clear that dredging is required to maintain navigation schemes,  there is a growing awareness that dredging rivers may not be helpful in relation to increasing flood conveyance, and may even be counter-productive. Dredging can make river banks prone to erosion, and hence stimulate a further build-up of silt, exacerbating rather than improving problems with water conveyance capacity. Removing river bank vegetation actually decreases bank stability and increases erosion and siltation. For more on this subject see here.

Most ecologists cannot drive track machines, and likewise most machine operators and general operatives cannot do the job of an aquatic ecologist

It is noted that monitoring of spoil for lampreys (and crayfish) dredged from river during drainage maintenance works in Ireland is not regularly undertaken, although it is feasible to do this.  It is claimed by Irish state authorities (particularly the OPW) that machine operatives look out for lampreys, crayfish (and indeed even Freshwater Pearl Mussels – see OPW, 2007) and keep records and return specimens to the water. However, it is clear that none of this is ever done in the real world of digging out our SAC rivers and other watercourses under political pressure from the farming lobby. Most ecologists cannot drive track machines, and likewise most machine operators and general operatives cannot do the job of an aquatic ecologist.

No benefit for flood conveyance; Juvenile lamprey habitats being destroyed within River Nore SAC
No benefit for flood conveyance; Juvenile lamprey habitats being destroyed within River Nore SAC (Laois County Council)

One of the OPW’s mitigation measures, disgracefully recommended by the state agency charged with protecting lampreys in Ireland (King et al, 2008) is as follows:-

Sea lamprey spawn during the summer months and, as such, the proposed maintenance restrictions to start June would not be adequate to protect spawning activity of this species. Sea lamprey construct large redds at spawning sites, of similar dimensions to Atlantic salmon. Such structures would be evident in any channel and appropriate avoidance action could be undertaken under the supervision of the resident foreman“.

So let’s get this straight, what King et al (2008) is proposing is that OPW can work in Natura 2000 rivers, in sea lamprey spawning areas, during the sea lamprey spawning season, and the plan is for the guy digging (usually on contract with targets to meet) with the machine to contact the foreman if he just happens to see a pair of sea lampreys actively spawning or sees an area that looks like a redd. This is key mitigation proposed for Natura 2000 rivers by OPW and Inland Fisheries Ireland and is both ridiculous and probably illegal. Basic measures like checking spoil and putting lampreys back into the water is not done, based on our experience and questioning of operatives on-site. There is no strategic environmental assessment that can ever cover digging out even one bank of a Natura 2000 river. There needs to an ecological assessment each time, and proper site specific mitigation. There then needs to be full enforcement of the law where agreed mitigation is breached. None of this happens in Ireland, and we have to ask is anyone really protecting lampreys in Ireland?

  • PS: The OPW’s approach to Freshwater Pearl Mussels (OPW, 2007) is unbelievably worse, the following statement is an example“Thus, it is not possible to give a degree of certainty to the assessment of potential significant impact. The ideal ‘residual significance’ would be for all OPW activities to be beneficial  and for OPW to become champions for the species across Ireland” and “New opportunities have been identified for OPW staff to become effective protectors and champions for the pearl mussel across Ireland”. This would be funny if we were not dealing with an endangered species here.

And what about ‘illegal’ dredging works?

But things are even worse than this in Ireland. In 2012 we found that a 1km stretch of the River Arigna, Co Leitrim, had been dug out by local farmers. As well as damaging the river they had spread Japanese Knotweed everywhere. In good faith we reported this event immediately to IFI. However there have been no prosecutions here and IFI have ignored several enquiries from us regarding giving us an explanation of why no action was taken. We now suspect that IFI may have authorised this work, and that works like this are actually legal in Ireland as long as they occur outside of the salmonid close season.

Illegal river works
Dredging works on the River Arigna, Co Leitrim – why no prosecutions?

Flood schemes

As well as dredging rivers, there are a large number of flood schemes involving the construction of river walls being progressed in Ireland. In the Environmental Impact Statement (EIS) for the River Fergus (upper) Ennis flood scheme the OPW said that there would be no instream works. Likewise in Clonmel the EIS said that there would be no intream works. However, there were extensive instream works on both of those schemes and significant damage to lamprey spawning and nursery habitats. On the River Fergus, it is our opinion that there were integrity level impacts on the Lower Shannon SAC as a result of this scheme. For more on this see this link.

Flood defence works, due to the scale of works and the extent of channel length affected, can have adverse impacts on lampreys where in-stream works are involved including; destruction of spawning and ammocoete habitat, ammocoete fatalities, disturbance of spawning adults, elevated suspended solids levels, watercourse constriction (construction and operation phase) leading to an alteration in the flow regime within the channel and also the depositional rate within the stretch upstream and downstream. This all happened during the River Fergus scheme:-

See these links here:-

References

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