1. Can the models used in fisheries management conserve fish?
C. Tara Marshall
School of Biological Sciences
University of Aberdeen

2. “Cod - It is amazingly prolific
“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)

3. If history repeats itself, and the unexpected always happens, how incapable must Man be of learning from experience
                   
George Bernard Shaw

4. 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)

5. 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

6. 200000
400000
600000
800000
1958
1968
1978
1988
1998
Year
Biomass (tonnes)
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

7. 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

8. The population decline observed in Newfoundland and Labrador has been attributed to the collapse of the cod fishery

9. 11 years after closure of the fishery
Management advice in 2003: “without crisis management, the stock will not recover”
Northern cod was put on the “endangered” list on May
                                                                                      
May 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

10. Did the models used in fisheries management contribute to the
collapse of Northern cod?

11. If you remember only one thing from this talk, it should be this model
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

12. We need a book-keeping method that keeps track of these basic elements (growth, recruitment, mortality and biomass) over time
constant
N a+1,y+1 = N a,y e – (M + sa Fy)
Starting point in the book-keeping
unknown
N a+1,y Numbers at age a+1 in year y+1
N a,y Numbers at age a in year y
e – (M + sa Fy) – survivorship after natural and fishing mortality
saFy – the selectivity for age a times the fully selected
fishing mortality in year y

13. Must make assumptions to get the calculations started
In age-structured models we can go forward or backward through time to construct a matrix of numbers at age
Age
Years
Cohort analysis or
Virtual population analysis
Must make assumptions to get the calculations started

14. Read along the diagonal to follow an individual cohort through time
Age
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
Year
Read along the diagonal to follow an individual cohort through time
Terminal year

15. N a+1,y+1 = N a,y e – (M + sa Fy)
constant
unknown
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!

16. 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)

17. Can you think of a reason why?
Lessons for stock assessment from the northern cod collapse
Walters, C. and Maguire, J.-J Reviews in Fish Biology and Fisheries. 6:
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)
CPUEs are an inherently biased estimate of stock abundance
Can you think of a reason why?

18. The fishery is extremely good at locating fish+
Fish school
High CPUEs can be maintained until the very end

19. This causes F to be underestimated because abundance is being overestimated by using information from the commercial catch (generated by very non-random sampling!)

20. Progressive revision downwards with each updated assessment
What have we learned from the collapse of northern cod?
Spawning stock biomass (thou. t)
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

21. Did retrospective bias contribute to the collapse of North Sea cod?

22. Example 2: Managing by TACs alone doesn’t work
North Sea cod ICES sub-div IV, VIId, and IIIa
400000
Landings
TAC
300000
Biomass (tonnes)
200000
100000
1962
1972
1982
1992
2002
Year
Example 2: Managing by TACs alone doesn’t work

23. 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

24. Retrospective plots for North Sea cod
Retrospective bias in the North Sea cod stock assessment is now routinely examined
Spawning stock biomass - overestimated
Retrospective plots for North Sea cod
Fishing mortality - underestimated
Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak
ICES C.M. 2003

25. Difference between scientific advice and agreed can be quantified
Fisheries scientists now keep a paper trail!
Stock summary for North Sea (Subarea IV)
Year
Basis of ICES Advice
Advice
Agreed TAC
Landings
1987
SSB recovery; TAC
175
167
1988
70% of F(86); TAC
148
160
142
1989
Halt SSB decline; TAC
124
110
1990
80% of F(88); TAC
113
105 99
1991
70% of effort (89)
100 87
1992 98
1993
101 94
1994
Significant reduction
102
1995
120
112
1996
80% of F(94)
141
130
104
1997
80% of F(95)
135
115
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
48.6
42.3
2002
49.3
44.2
2003
Closure
27.3
2004
Zero catch
Difference between scientific advice and agreed can be quantified

26. North Sea cod
-40
-20 20 40 60 80
1987
1989
1991
1993
1995
1997
1999
2001
2003
Year
Agreed - Advice (thou. t)
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

27. Chronology of the decline in North Sea cod
In 1997 scientists warned that North Sea cod stocks were about to collapse (Cook et al Nature 385: )
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

28. Lessons for stock assessment from the northern cod collapse -> “retrospective problem” diagnosed
Walters, C. and Maguire, J.-J Reviews in Fish Biology and Fisheries. 6:
In 1997 and 1998 the advised TACs for North Sea cod were based on achieving a target F of > F for these years was shown retrospectively to be 0.85 and 1.06

29. High F after 1997 warning of stock collapse!
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
110
1.010
1990
0.8
113
105 99
0.912
1991
70% of 89 effort
100 87
0.874
1992 98
0.866
1993
101 94
0.921
1994
Reduce effort
102
0.877
1995
120
112
0.875
1996
0.7
141
130
104
0.797
1997
0.65
135
115
0.848
1998
<F(96)
153
140
114
1.061
1999
125
132 80
1.037
2000
<0.55
<79 81 62
1.210
2001
Lowest possible
48.6
42.3
0.747*
2002
49.3
44.2
0.612*
2003
Closure
2004
Zero catch
High F after 1997 warning of stock collapse!
* Considered uncertain

30. Annaul percentage of stock caught
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

31. The recent Royal Society of Edinburgh Inquiry into the Scottish Fishing industry concluded:
“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”
To download their report see:

32. Furthermore, they noted:
“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”
To download their report see:

33. 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:

34. 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

35. Can the models used in fisheries management conserve fish?
Yes – North Sea herring; use of VPA can be considered a success; F is currently around 0.25 and catches are stable
Such a low F means that errors in VPA and predicted catches are less critical
Royal Society of Edinburgh Inquiry into the future of the Scottish Fishing industry

36. Can the models used in fisheries management conserve fish?
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 J. Sea Res. 50:

37. Post mortems of cod crises
Northern cod
1983: The Kirby Commission report
1987: The Alverson Commission
1989: Northern Cod Review Panel
                                                                
North Sea cod
2004: The Royal Society of Edinburgh
2004: The UK Fisheries Project