Can the models used in fisheries management conserve fish

1
Can the models used in fisheries management
conserve fish?
C. Tara Marshall School of Biological
Sciences University of Aberdeen Email
c.t.marshall_at_abdn.ac.uk
2
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
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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)

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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

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Example 1 Managing by TACs alone doesnt work
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• 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
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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 2 2003

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
10
Did the models used in fisheries management
contribute to the collapse of Northern cod?
11
To answer this we need to delve into the
mechanics of age-structured population modelling
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
N a1,y1 - Numbers at age a1 in year y1
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
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In age-structured models we can go forward or
backward through time to construct a matrix of
numbers at age
14
Age
Year
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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)

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Lessons for stock assessment from the northern
cod collapse Walters, C. and Maguire, J.-J. 1996.
Reviews in Fish Biology and Fisheries. 6 125-137.

• 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?
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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
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
21
Did retrospective bias contribute to the collapse
of North Sea cod?
22
North Sea cod ICES sub-div IV, VIId, and IIIa
400000
Landings
TAC
300000
Biomass (tonnes)
200000
100000
0
1962
1972
1982
1992
2002
Year
Example 2 Managing by TACs alone doesnt work
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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
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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
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Fisheries scientists now keep a paper trail!
Stock summary for North Sea (Subarea IV)
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• Difference is not consistently positive (only in
  recent years is agreed consistently gt advice)
• This would suggest that the scientific advice has
  failed to conserve the North Sea cod stock

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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 385521-522)
• 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

28
Lessons for stock assessment from the northern
cod collapse -gt retrospective problem
diagnosed Walters, 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 -gt
F for these years was shown retrospectively to be
0.85 and 1.06
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Stock summary for North Sea (Subarea IV)
High F after 1997 warning of stock collapse!
Considered uncertain
30
To put those F values into perspective
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 http//www.royalsoce
d.org.uk/
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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 http//www.royalsoce
d.org.uk/
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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 Flt0.4
  corresponding to removal of lt 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.royalsoce
d.org.uk/
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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
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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
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Can the models used in fisheries management
conserve fish?
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Post mortems of cod crises
1983 The Kirby Commission report 1987 The
Alverson Commission 1989 Northern Cod Review
Panel
Northern cod
North Sea cod
2004 The Royal Society of Edinburgh 2004 The UK
Fisheries Project