Can the models used in fisheries management

conserve fish?

C. Tara MarshallSchool of Biological Sciences

University of AberdeenEmail: c.t.marshall@abdn.ac.uk

“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