ANALYSIS - Understanding and preventing fish escapes was a big topic at AQUA 2012 in Prague, Czech Republic, from 1-5 September 2012, writes Lucy Towers, TheFishSite Editor.
Tim Dempster from the EU funded 7th Framework project 'Prevent Escapes' discussed how the project aims to create a baseline for escape events and to understand how escapes can be reduced, a benefit both to farmers and the enviroment,
Looking at the history of escapes, Mr Dempster noted that before 2006 the number of escapes were rising. The year 2006 then saw a sharp decline in escapes. However, from 2007 onwards the number escaping has gradually started to rise again.
An analysis of techical standards has shown the main causes of structural failiure which, from 2008 to 2010, was the main cause of escapes in Norway.
The main causes of failiure were identified as:
- Collapse of mooring line
- Collapse of floating collar
- Hole in net
Of these three causes, the first two have both been worked upon, with the number of escapes decling. Hole in the net however still remains a huge problem.
As well as documenting escapes, the project has also evaluated the the cost of escapes.
Mr Dempster said that 242 escape events were recorded across six countries in three years, with the number of escaped fish being nine million.
David Jackson from the Marine Institute in Ireland also discussed his work in the 'Prevent Escapes' project.
Mr Jackson looked at the causes of holes in netting which, as Mr Dempster discussed, is a major contributor to escape events.
Mr Jackson said that biting, predators, an unknown cause and chaffing snagging are the top four causes of holes in nets.
Mr Jackson carried out research and surveys across Greece, Spain, Malta, UK, Ireland and Norway to understand how nets have failed and how bad weather has caused equiptment failure, allowing for escape events.
It was seen that nets are often repaired inadequately leading to future failure or have become damaged beyond repair.
In order to maintain the strength of nets, Mr Jackson stated how when nets are brought up to be maintained, any damaged can be noted on a board with a tag. Each time a new failure or damage is seen it is marked on the board. When the board is covered it can clearly be seen that all areas have been damaged at some point and therefore the net is no longer usable.
In order to address the problem of net design, Mr Dempster discussed alternate net designs. One idea involved tripling the distance between the outer and inner nets and for the bottom ring to have the same diameter as the outer ring.
Mr Dempster also spoke of how to to minimise the damage escaped fish can cause through the use of DNA markers, which can allow fish to be tracked post escape, and the use of predatory 'Wall of Mouths'.
For more information on the prevent escape project, please see www.preventescapes.eu.net