Can the models used in fisheries management
conserve fish?
C. Tara MarshallSchool of Biological Sciences
University of AberdeenEmail: [email protected]
“Cod - It is amazingly prolific. Leewenhoek counted 9,384,000 eggs in a cod-fish of
middling size -- a number that will baffle all the efforts of man to exterminate” -- J. Smith Homans Sr. & Jr. (1858) - quoted in
"Cod" by Mark Kurlansky (1997)
If history repeats itself, and the unexpected always happens, how incapable must Man be of learning
from experience
George Bernard Shaw
Lecture outline
The collapse of the Newfoundland cod stock
The scientific models used to estimate stock
abundance
How these models failed Newfoundland cod
stock
How these models failed North Sea cod stock
Relevant highlights from the recent inquiry into
the Scottish Fishing Industry (Royal Society of
Edinburgh)
The collapse of the “Northern” cod stock
Commercial fishing began in the 16th century
Moratorium on fishing declared in 1992
Since closure there has been little rebuilding and excessive mortality
0
200000
400000
600000
800000
1000000
1958 1968 1978 1988 1998
Year
Bio
mass (
ton
nes)
Landings TAC
Landings closely tracked the TACs but the stock
collapsed anyhow(owing to “mismanagement” taxpayers of Canada were stuck paying the
compensation)
Example 1: Managing by TACs alone doesn’t work
After the moratorium 40,000 people in the five Maritime provinces became unemployed
Canadian government paid out over $4 billion in compensation
Compensation made continued attachment to the fishery profitable even in the absence of fish
Overcapacity remains a serious problem
The population decline observed in Newfoundland and Labrador has been attributed to the collapse of the cod fishery
Management advice in 2003: “without crisis management, the stock will not recover”
Northern cod was put on the “endangered” list on May 2 2003
11 years after closure of the fishery
May 9 2003 DFO offices were trashed by fishers demanding that the federal Fisheries Minister reverse his decision to close the cod fishery in the Gulf of St. Lawrence as a way to protect the dwindling stock
Did the models used in fisheries
management contribute to the
collapse of Northern cod?
To answer this we need to delve into the mechanics of age-structured population modelling
Recruitment
Growth
Biomass Fishing mortality (F)
Natural mortality ( M = 0.2)
If you remember only one thing from this
talk, it should be this model
We need a book-keeping method that keeps track of these basic elements (growth, recruitment, mortality and biomass) over time
N a,y - Numbers at age a in year y
e – (M + sa Fy) – survivorship after natural and fishing mortality
N a+1,y+1 - Numbers at age a+1 in year y+1
saFy – the selectivity for age a times the fully selected fishing mortality in year y
N a+1,y+1 = N a,y e – (M + sa Fy)
Starting point in the book-keepingconstant
unknown
In age-structured models we can go forward or backward through time to construct a matrix of numbers at age
Age
Years
Cohort analysis orVirtual population analysis
Must make assumptions to get the calculations
started
N0,1 N1,1 N2,1 N3,1 N4,1 N5,1
N0,2 N1,2 N2,2 N3,2 N4,2 N5,2
N0,3 N1,3 N2,3 N3,3 N4,3 N5,3
N0,4 N1,4 N2,4 N3,4 N4,4 N5,4
N0,5 N1,5 N2,5 N3,5 N4,5 N5,5
N0,6 N1,6 N2,6 N3,6 N4,6 N5,6
N0,7 N1,7 N2.7 N3,7 N4,7 N5,7
N0,8 N1,8 N2,8 N3,8 N4,8 N5,8
N0,9 N1,9 N2,9 N3,9 N4,9 N5,9
Terminal year
Age
Year
Read along the
diagonal to follow an individual
cohort through
time
There is insufficient information to
estimate the F in the most recent (or
terminal) year. Without this, we
cannot estimate abundance in the
terminal year which prevents us from
doing the backward reconstruction of
abundance at age!
N a+1,y+1 = N a,y e – (M + sa Fy)
constant
unknown
To get around this obstacle, “tuning” procedures have been developed which use auxiliary information to estimate F in the terminal year. This auxiliary information can include:
commercial landings of each age class (fisheries dependent information)
numbers of each age class caught by research vessel surveys (fisheries independent information)
Lessons for stock assessment from the northern cod collapseWalters, C. and Maguire, J.-J. 1996. Reviews in Fish Biology and Fisheries. 6: 125-137.
1. The terminal Fs were tuned so that the results would closely match the time trends in commercial catch rates expressed as catch per unit effort (CPUE)
2. CPUEs are an inherently biased estimate of stock abundance
Can you think of a reason why?
The fishery is extremely good at locating fish+
Fish school
High CPUEs can be maintained until the very end
This causes F to be
underestimated because
abundance is being overestimated
by using information from the
commercial catch (generated by
very non-random sampling!)
Spaw
nin
g s
tock
bio
mass
(th
ou.
t)
What have we learned from the collapse of northern cod?
Progressive revision
downwards with each updated
assessment
Retrospective bias – F in the terminal year is consistently underestimated leading to downward revisions of abundance at age as the cohorts converge towards final values
Did retrospective bias contribute to the
collapse of North Sea cod?
Year
Bio
mass (
ton
nes)
0
100000
200000
300000
400000
1962 1972 1982 1992 2002
Landings TAC
North Sea cod ICES sub-div IV, VIId, and IIIa
Example 2: Managing by TACs alone doesn’t work
Retrospective bias has been detected in the North Sea cod
stock“Since the mid 1990s, estimated
reductions in fishing mortality in the final
year of the assessment have been revised
to higher F when more years of data
became available.”ICES Adisory Council on Fisheries Management 2003
Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak ICES C.M. 2003
Retrospective bias in the North Sea cod stock assessment is now routinely examined
Retrospective plots for
North Sea cod
Spawning stock biomass - overestimated
Fishing mortality - underestimated
Fisheries scientists now keep a paper trail!
Difference between scientific advice and agreed can be quantified
Year Basis of ICES Advice Advice Agreed TAC Landings
1987 SSB recovery; TAC 100-125 175 167
1988 70% of F(86); TAC 148 160 142
1989 Halt SSB decline; TAC 124 124 110
1990 80% of F(88); TAC 113 105 99
1991 70% of effort (89) 100 87
1992 70% of effort (89) 100 98
1993 70% of effort (89) 101 94
1994 Significant reduction 102 87
1995 Significant reduction 120 112
1996 80% of F(94) 141 130 104
1997 80% of F(95) 135 115 100
1998 F(98) not exceed F(96) 153 140 114
1999 F=0.60 to rebuild SSB 125 132 80
2000 F less than 0.55 <79 81 62
2001 Lowest possible catch 0 48.6 42.3
2002 Lowest possible catch 0 49.3 44.2
2003 Closure 0 27.3
2004 Zero catch 0
Stock summary for North Sea (Subarea IV)
Difference is not consistently positive (only in recent years is agreed consistently > advice)
This would suggest that the scientific advice has failed to conserve the North Sea cod stock
North Sea cod
-40
-20
0
20
40
60
80
1987 1989 1991 1993 1995 1997 1999 2001 2003
Year
Agre
ed -
Advic
e (
thou.
t)
Chronology of the decline in North Sea cod
In 1997 scientists warned that North Sea cod stocks were about to collapse (Cook et al. 1997 Nature 385:521-522)
At that time the 1996 year-class was then estimated to be strong and management stated: “explicit measures to prevent targeting or wastage of this year-class should be implemented”
In 1997 and 1998 scientists advised a TAC of 135,000t and 153,000t, respectively, which were then estimated to correspond to Fs that were higher than target F of 0.6
Lessons for stock assessment from the northern cod collapse -> “retrospective problem” diagnosedWalters, C. and Maguire, J.-J. 1996. Reviews in Fish Biology and Fisheries. 6: 125-137.
In 1997 and 1998 the advised TACs for North Sea cod were based on achieving a target F of 0.65 -> F for
these years was shown retrospectively to be 0.85 and 1.06
Stock summary for North Sea (Subarea IV)
Year Advised F Catch to achieve F
Agreed TAC
Official Landings
Actual F
1988 0.68 148 160 142 0.989
1989 Halt SSB decline 124 124 110 1.010
1990 0.8 113 105 99 0.912
1991 70% of 89 effort 100 87 0.874
1992 70% of 89 effort 100 98 0.866
1993 70% of 89 effort 101 94 0.921
1994 Reduce effort 102 87 0.877
1995 Reduce effort 120 112 0.875
1996 0.7 141 130 104 0.797
1997 0.65 135 115 100 0.848
1998 <F(96) 153 140 114 1.061
1999 0.65 125 132 80 1.037
2000 <0.55 <79 81 62 1.210
2001 Lowest possible 0 48.6 42.3 0.747*
2002 Lowest possible 0 49.3 44.2 0.612*
2003 Closure 0 44.2
2004 Zero catch 0
* Considered uncertain
High F after 1997
warning of stock collapse!
To put those F values into perspective:
F Annaul percentage of stock caught
0.1 10
0.2 18
0.3 26
0.4 33
0.5 39
0.6 45
0.7 50
0.8 55
0.9 59
1.0 63
1.1 67
1.2 73
between 63 and 73% of stock was
being removed annually
The recent Royal Society of Edinburgh Inquiry into the Scottish Fishing industry concluded:
To download their report see: http://www.royalsoced.org.uk/
“Following the abundant 1996 year-class in the
North Sea, scientists recommended increases in
TACs. Had they recommended lower values, the
current crisis in North Sea cod could have been
averted. In retrospect, this would appear to be a
major misjudgement in management”
Furthermore, they noted:
To download their report see: http://www.royalsoced.org.uk/
“There is a fundamental problem in trying to
regulate F through TACs. If there is an error in
estimation of SSB, … then F could vary
considerably from the desired value. Management
of the fishery through TACs is then doomed to
failure; indeed, it appears to have consistently
failed over two decades”
Among their recommendations:
The EU Commission should manage demersal fish stocks so that F is much lower than over the past 15 years, aiming for a value of F<0.4 corresponding to removal of < 33% of the stock annually
Demersal stocks should be managed as a mixed fishery with a single overall limit on effort and no discarding
To download their report see: http://www.royalsoced.org.uk/
Can the models used in fisheries management
conserve fish?No – Northern cod; F too high and consistently underestimated
No – North Sea cod; F too high and consistently underestimated
Can the models used in fisheries management
conserve fish?
Such a low F means that errors in VPA and predicted catches are less critical
Royal Society of Edinburgh. 2004 Inquiry into the future of the Scottish Fishing industry
Yes – North Sea herring; use of VPA can be considered a success; F is currently around 0.25 and catches are stable
In an analysis of “paper trails” like the ones shown
here for North Sea cod (advised vs. agreed TAC) it was
concluded that “more than any other factor examined,
failure to comply with science advice greatly increases
the risk of unsustainability” for flatfish stocks
Management of flatfish fisheries – what factors matter?Rice, J and Cooper, J.A. 2003. J. Sea Res. 50: 227-243.
Can the models used in fisheries management
conserve fish?
Post mortems of cod crises
1983: The Kirby Commission report1987: The Alverson Commission 1989: Northern Cod Review Panel
2004: The Royal Society of Edinburgh2004: The UK Fisheries Project
Northern cod
North Sea cod