Transcript Document

A decade in the doldrums:
depressed size-structure in the
Celtic Sea fish community
IRL
UK
UK
FR
Effects of fishing on community
size-structure
• Fishing is selective and directly removes
large individuals/species
• Removal of large piscivores leads to
predation release which indirectly
enhances small fish biomass
Shift from large to small fish dominated community
Percentage Frequency
5
4
1998-2004
1984-1997
3
2
1
0
0
10
20 30
40
50
60 70
80
Length (cm)
90 100 110 120 131 145
Size-based indicators
Direct and indirect effects of fishing can be
integrated by metrics like the Large Fish
Indicator (LFI):
Proportion by weight of ‘large’ fish =
LFI  B Xcm BTotal
Where ‘large’ is derived from the data
• LFI responds to F (lag). Sustained F in the Celtic Sea until 2002
• Peak LFI in 1990 - low levels throughout the 2000s
WCGFS
0.7
IGFS
1.4
0.6
1.2
0.5
1
0.4
0.8
0.3
0.6
0.2
0.4
0.1
0.2
0
1985
1990
1995
2000
Year
2005
0
2010
F
LFI
F
Fishing (+ discarding) of ‘recruits’ prevents
recovery (Monkfish, Hake, Cod, Haddock)
43000
R
R
R
M+G
38000
0.35
0.30
33000
R
M+G
0.25
23000
0.20
18000
0.15
LFI
Biomass
28000
13000
0.10
8000
G+F
G+F
G+F
0.05
3000
-2000
2004
2005
2006
Small Fish
2007
Large Fish
2008
LFI
2009
0.00
Models suggest a multi-decadal recovery
period, asymptote depends on F history
• Perturbed system with LFI well below 1990
• No recovery during 2000s due to sustained F
• Some good recruitments but low survival
• Recovery depends on F history
• Decadal process - full recovery not guaranteed