Invasive Species Management

Lagarosiphon Project on Lough Corrib

In 2005 Curley Leaved Pondweed (Lagarosiphon major) was found to be growing wild in Lough Corrib, the species of water weed that is not native to Ireland. The uncontrolled and rapid growth of this plant can cause significant problems in water columns and channels as the plant form a dense cover on the surface of the lake or pond reducing light penetration and impacting on native weeds and the entire aquatic ecosystem. The growth of this weed was also preventing the free movement of boats throughout the lake. Lararosiphon major is listed by the EU as an Invasive Alien Species of Union concern since 2014.

Since Lararosiphon major was first discovered, many techniques were investigated on Lough Corrib to attempt to remove or control the weed. Following years of research, trials, and monitoring, two core techniques emerged as the most effective at managing the plant-specific to Corrib. These are:

• Jute Matting and
• Mechanical Cutting

Jute Matting

In Lough Corrib areas where Curly leaved waterweed is found to be emerging from the lakebed are treated by a weed control team. The Summer and Autumn months are ideal as the weed collapses to the lake floor and temperatures are optimal for diving. The team includes commercial divers and a jute deployment crew. The jute matting is deployed over infected areas of weed and secured to the lake bed by the dive team. The jute matting allows the native Chara species to re-establish through the woven mesh while preventing the Lararosiphon major weed nodes from emerging. The organic jute material will biodegrade in water naturally in approximately 2 years. This very effective weed control technique was a global first and has since been employed in other countries where the curly leaved waterweed has proven problematic. A video illustrating the Jute Matting process is included below.

Mechanical Cutting

Mechanical cutting of Lagarosiphon major is carried out on Lough Corrib between November and May using special harvesting methods and equipment. The weed grows more towards the surface in Winter months making it more suited to Mechanical Cutting and harvesting. The method involves the trailing of dual V-blades that are pulled along the lake floor by a boat. The blades do not cut but drag the base of the delicate stems and roots of the weed. When the root of the weed is detached from the lake floor it floats to the surface in bulk. The mass of weed is collected from the lake surface and disposed on the nearest dry shore point where it naturally decomposes quickly in a confined area due to its consistency of 90% water. Containment nets are deployed in exposed bays to prevent weed fragments that come loose during cutting operations from spreading onto other locations.

Equipment used in mechanical cutting include 2 X OSMA weed cutting boats, one to pull the V-blades and the other to lift the cut weed into a HasCraft 700 harvesting boat. The harvesting boat disposes of the weed on the dry shoreline, manages the containment net and monitors fragmentation. 

Review and innovation of methods, equipment and practices are regularly trialled in conjunction with research. Including a Berky weed harvester (pictured below), drone imagery to identify sites and track machine to improve harvesting efficiency at the shoreline.

The management operation of curly leaved waterweed from Lough Corrib has been ongoing since 2008 in conjunction with the Office of Public Works and Galway County Council. The aim of this operation is to reduce the amount of weed in the lake and to prevent it spreading to other parts of the lake. This work is essential to ensure there is a healthy environment for the lakes native plant, animal and fish species. The operation requires a lot of resources to manage including staff and equipment.