Gadus morhua off the West Coast of Scotland Joanne Clarke 1 David M Bailey 1 Peter J Wright 2 Gilbert Van Ryckevorsel WWFCanada 1 University of Glasgow 2 Marine Scotland Science ID: 505283
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Slide1
Measuring the effects of a seasonal fishing closure on the abundance of Atlantic Cod (
Gadus morhua) off the West Coast of Scotland
Joanne Clarke1*, David M Bailey1, Peter J Wright2
© Gilbert Van Ryckevorsel / WWF-Canada
1University of Glasgow; 2Marine Scotland Science
* j.clarke.1@research.gla.ac.uk
Catch control measures for commercial species are often at a spatial scale greater than that of local population dynamics, making individual spawning components vulnerable to extirpation
Spawning
aggregations often occur at times and places that are predictable making them more
vulnerable to high levels of exploitation at this timeAggregative behaviour causes localised increases in catchability compared to other times of the year, resulting in high fishing mortalityTherefore the introduction of an area closure during an aggregation period can reduce fishing mortality directly, whilst permitting sustainable exploitation outside of the spawning periodCod off the West Coast of Scotland are managed as a single stock within ICES Area VIa3 sub-populations of cod have been identified within this area (Wright et al., 2006)Spawning stock biomass of cod off the West Coast of Scotland crashed during the late 90sA seasonal area closure was introduced to the mouth of the Clyde in 2001 to protect spawning cod
Introduction
1
2
3Slide2
Objectives
Examine monthly Landings per Unit Effort (
LPUE)Compare annual trends in Catch per Unit Effort (CPUE)3. Compare the
length frequency distributions
Methods
Commercial landings per unit effort were investigated for each area prior to the area closure
Clyde
Trawls used were from within 60km distance of the known spawning aggregations of each of the 3 sub-populations
The seasonal area closure was introduced into the mouth of the Clyde in 2001 prohibiting certain types of fishing during the spawning period
MinchSouth West
A Before/After, Control/Impact analysis was conducted to identify if there had been a change in trend of mature cod in the Clyde sub-population after the area closure was introduced compared to the other 2 sub-populations
Aims
To identify if cod aggregate in survey areas and determine if there has been an effect of the area closure on the sub-population of cod in the Clyde.
Study Area
BACI
–
B
efore
= 1986-2000
A
fter
= 2001-2010
Control = Minch and SW
Impact = Clyde
CPUE was grouped into 5 cm length bins to compare the length frequency distribution between “Before” and “After” for each sub-population.
Catch
per Unit Effort (CPUE) of mature cod
caught by
the international bottom trawl survey (IBTS)
during quarter 1 were
used for the analysis
from 1986 to 2010. Slide3
Results
BACI-Catch per Unit Effort (CPUE)
Before
After
Commercial Data
Sum of Landings
Landings per Unit Effort
Left Figure
: Sum of total
landings in Kilograms for each location for each month during time period 1986-2000 for all vessel types. Right Figure: Mean (+/- SE) Corrected Landings Per Unit Effort (CLPUE) for vessels using Beam Trawl; Gill Nets Pelagic; Light Trawl; Trawl Single Demersal; Pair Trawl Demersal; Seine net and Nephrops trawl for each month for the time period 1986-2000 Conclusion: Landings and catch rate (LPUE) peak for each area during the spawning period indicating aggregating codThe log of the fitted values ±1.96*standard error taken from the model of best fit for Catch per unit Effort (CPUE) for each sub-population “Before” (left) and “After” (right) the area closure. Conclusion: Model of best fit does not include 3-way interaction indicating no effect of the area closure on changes in population trends in the Clyde sub-population
Evidence
of aggregations
from elevated landings
per unit effort (
LPUE
)
No
evidence of a local recovery on the Clyde
sub-population from trends in Catch per Unit Effort (CPUE)
No evidence of changes in length frequency distributions after the area closure in the Clyde sub-population, but no larger size class of cod being caught
Why no recovery of cod in the Clyde?too little too late?
Length
Frequency
Distribution using
Kolmogorov-Smirnoff (K-S) test
Clyde
K-S
test,
D=0.25
, p =
0.16
Before
After
Conclusion:
No significant difference in length frequency distributions between “Before” and “After” area closure for each of the sub-populations, but reduction in larger size classes being caught in the “After” period
Conclusions