Sea Lamprey migration blocked by weir

Sea lampreys Petromyzon marinus running at Annacotty weir on the Lower River Mulkear, Co Limerick, during early June 2018. Overnight rain caused river levels to rise and this stimulated upstream migration. However, lamprey migration is blocked by this ornamental weir.

As part of the Mulkear Life project, a ramp pass was installed on this weir at Annacotty for the purpose of allowing sea lampreys to pass this obstacle. However, I have never seen a lamprey use this so called “lamprey pass”, and no scientific information has ever been presented to show that it has any significant benefits.  This ramp is too steep and flows are too high: our native lampreys do not exhibit the “attach-twitch-attach” strategy that Pacific lampreys use to scale obstacles. Sea Lampreys cling to the bottom of this pass – but just get exhausted and can’t climb it. They are also exposed to predators doing this.

Some Sea Lampreys get past this weir using the denil pass in the centre of the weir – but this fish pass was blocked with trees when the above video was taken. Perhaps at very specific flows some Sea Lampreys scale the ramp – but I have never seen this and IFI never produced any scientific evidence to show that this occurs.

Claims that 95% of lampreys use this pass are just false. It is claimed that this monitoring was done by walking out on the weir at night – which is impossible! Yes the lampreys are attracted to the ramp – but they just cling on and get exhausted before dropping off.

Dead Sea lamprey at Annacotty Weir. Lampreys die of exhaustion and are exposed to predators at migration barriers.

The migration of River Lampreys is almost totally blocked – they definitely can’t use the ramp however a few may get through via leaks under the sluice gates, and during the very rare occasion that the weir floods out. But salmon also have problems, and critically endangered elvers have great difficulty.

The migration of River Lampreys is almost totally blocked by Annacotty weir – thousands of River Lampreys were blocked below this weir in March 2017.

There is now overwhelming evidence that Annacotty weir is a major fish migration barrier and should be removed, or lowered and a rock ramp (with no jump at the top) installed. Temporary solutions are also available. This is the Lower River Shannon SAC and, with water quality deteriorating, we need to urgently increase the range of anadromous lampreys in this river and the Lower Shannon catchment in general.

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