eel fisheries-recruitment-future. decline of eel populations??? outside safe biological limits? how...
TRANSCRIPT
Eel Fisheries-Recruitment-Future
Decline of Eel Populations???
• Outside safe Biological limits?
• How are we influencing change?
• Can we manage the population change?
• Will reducing fishing produce a predicable change in the eel population?
• No!
Glass Eel Recruitment Levels
• 1876 High
• 1930 Low
• 1950 High
• Company Low 1976 Company High 1979
• 1984 50% Stepped Reduction
• Then Progressive Slow Decline stabilising last 3 years
Glass Eel Recruitment Levels
• ICES - 1% of 1980 levels
• Industry - 15% - 20% of 1980 levels
• Back Calculation of recruitment 1980s based on average of last three years.
– United Kingdom 500 -1000 tonnes
– France 15000-20000 tonnes
Quantifiable Causes for Decline
• Industrial Revolution 1800. – Agriculture, Industry
– Loss of migratory Pathways
– Loss of Habitat
• Fishing???
• Water Mill Circa 1900
Tide Assisted Migration
• Ratchet Mechanism • Glass Eels – 1Y migration Spring
– Passive
• Small Eels– 2Y Migration Summer
– Active
Loss of Migratory pathways
• Soft barriers
– Migration possible some of the time
– Dissipate tidal energy
Tidal Effect
• Effect of soft barriers on tidal flows
• Impact on assisted glass eel migration
Loss of Migratory pathways
• Hard barriers– Major civil engineering projects. Weirs, dams,
locks, flood defences
– Minor works. Valves, sluices. Pumped systems, and poor design.
Loss of Migratory pathways Hard barriers
• Weir- • Tidal Flap-Submerged
Loss of Migratory pathways Hard barriers
• Tidal Flap- Raised • Drain-Raised
Water Management v. Migration
• Pumped Barrier • Nearly Every outlet Guarded
• Poor Design• No management of
migration• Process of change
unrecognised.
Loss of Migratory pathways Passes
• Ladders and Passes – for Glass eels
Ineffective
– for Small eels partially effective
– Poorly managed and maintained
• Glass Eel/Eel Pass
Loss of Nursery Habitat
• Eutrophic - Still Water• In area for assisted
tidal migration• Very little remains on
River Severn• Not accessible• Mortality in River
• Nursery Habitat
Habitat Change
• Spring • Summer
Loss of Habitat
• Wetlands Remaining– Year 0000 100%
– Year 1900 40%
– Year 2005 15%
• Artificially Maintained Wetland Habitat– poor access.
• Summer
Habitat Change
• Spring • Summer
Future of Eel Fisheries
• Recognise Environmental Change– Migration, Habitat, Engineering
• Not a Fishery Management Problem
• Choice between Economic Growth and the Environment.
• Problem pan European scale.
• Recovery of stock to previous historic levels not possible.