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

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Presentation on theme: "Can the models used in fisheries management conserve fish? C. Tara Marshall School of Biological Sciences University of Aberdeen"— Presentation transcript:

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. 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 16 th century Moratorium on fishing declared in 1992 Since closure there has been little rebuilding and excessive mortality

6 Year Biomass (tonnes) LandingsTAC 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 doesnt 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 Management advice in 2003: without crisis management, the stock will not recover Northern cod was put on the endangered list on May years after closure of the fishery 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 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 N a,y - Numbers at age a in year y e – (M + s a F y ) – survivorship after natural and fishing mortality N a+1,y+1 - Numbers at age a+1 in year y+1 s a F y – the selectivity for age a times the fully selected fishing mortality in year y N a+1,y+1 = N a,y e – (M + s a F y ) Starting point in the book-keeping constant unknown

13 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 N 0,1 N 1,1 N 2,1 N 3,1 N 4,1 N 5,1 N 0,2 N 1,2 N 2,2 N 3,2 N 4,2 N 5,2 N 0,3 N 1,3 N 2,3 N 3,3 N 4,3 N 5,3 N 0,4 N 1,4 N 2,4 N 3,4 N 4,4 N 5,4 N 0,5 N 1,5 N 2,5 N 3,5 N 4,5 N 5,5 N 0,6 N 1,6 N 2,6 N 3,6 N 4,6 N 5,6 N 0,7 N 1,7 N 2.7 N 3,7 N 4,7 N 5,7 N 0,8 N 1,8 N 2,8 N 3,8 N 4,8 N 5,8 N 0,9 N 1,9 N 2,9 N 3,9 N 4,9 N 5,9 Terminal year Age Year Read along the diagonal to follow an individual cohort through time

15 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 + s a F y ) constant unknown

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 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) 2.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 Spawning stock biomass (thou. 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

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

22 Year Biomass (tonnes) LandingsTAC North Sea cod ICES sub-div IV, VIId, and IIIa Example 2: Managing by TACs alone doesnt 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 Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak ICES C.M 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

25 Fisheries scientists now keep a paper trail! Difference between scientific advice and agreed can be quantified YearBasis of ICES AdviceAdviceAgreed TACLandings 1987SSB recovery; TAC % of F(86); TAC Halt SSB decline; TAC % of F(88); TAC % of effort (89) % of effort (89) % of effort (89) Significant reduction Significant reduction % of F(94) % of F(95) F(98) not exceed F(96) F=0.60 to rebuild SSB F less than 0.55< Lowest possible catch Lowest possible catch Closure Zero catch0 Stock summary for North Sea (Subarea IV)

26 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 Year Agreed - Advice (thou. t)

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 Stock summary for North Sea (Subarea IV) YearAdvised FCatch to achieve F Agreed TAC Official Landings Actual F Halt SSB decline % of 89 effort % of 89 effort % of 89 effort Reduce effort Reduce effort

30 To put those F values into perspective: FAnnaul percentage of stock caught between 63 and 73% of stock was being removed annually

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

32 Furthermore, they noted: To download their report see: 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

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? 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 Yes – North Sea herring; use of VPA can be considered a success; F is currently around 0.25 and catches are stable

36 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: Can the models used in fisheries management conserve fish?

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


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