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Annex B
Evolution of global discard estimates

As already noted, the current updated estimate of global discards is substantially lower than that given in FAO Fisheries Technical Paper No. 339 (the Alverson assessment).

Annex B aims to:

Following the publication of the Alverson assessment, FAO held a Technical Consultation at which regional experts provided revised estimates of discards for selected FAO statistical areas and suggested reasons why the assessment may have overestimated discards in certain fisheries and areas.

B.1 METHOD USED IN THE ALVERSON ASSESSMENT

The Alverson assessment estimated discards by region and in relation to target species, using FAO Fishstat for the global catch data. The 1 700 discard records included information both on numbers of fish discarded and on weights of fish discarded. The fisheries of the North Atlantic and Northwest Pacific provided over 70 percent of the records. Average discard rates associated with target species and each FAO statistical area were applied to nominal catch by species or species group, as provided in FAO Fishstat. As there is no a priori relationship between landings of target species and discards, and as the nominal catch of a species may often represent the retained catch of several different fisheries, each with a different fishing gear, target species and different level of discards, the extrapolation to area and global level may have resulted in some double counting (Murawski, 1996).

In 1998, the lead author of the 1994 assessment recognized its various shortcomings together with the substantial changes that were occurring in many fisheries, and an update on discarding practices and unobserved fishing mortality was published (Alverson, 1998). However, the global estimate was not recalculated.

B.2 TECHNICAL CONSULTATION ON REDUCTION OF WASTAGE IN FISHERIES

In 1996, the FAO Technical Consultation on Reduction of Wastage in Fisheries identified a number of difficulties arising with the methods used in the Alverson assessment, which were considered to contribute to an overestimate of global discards. As part of the contributions to the Technical Consultation, several authors prepared revised estimates of discards for selected FAO statistical areas, e.g. 4 million rather than 9.13 million tonnes for the Northwest Pacific.

The comments on the Alverson assessment in Box 8 are taken from the various papers in FAO Fisheries Report No. 547 (Clucas and James, 1997). It is stressed that the authors of the comments indicated that these were not intended to undermine the major contribution made by FAO Fisheries Technical Paper No. 339, but to contribute to a more accurate estimate of global discards.

B.2.1 Alternative approach to global discard estimation

The Technical Consultation suggested an enhanced approach[135] to include reference to the type of fishing method and provided a practical demonstration of the methodology. The current study has attempted to apply the methodology proposed by the Technical Consultation in the wider global context. The matrix of three spreadsheets (Table 30) was found unmanageable at the global level, partly because of the lack of information on many fisheries. Species-by-species information was also considered too detailed and unnecessary in the context of a global study, although clearly of considerable value at the country or fishery level.

BOX 8
Specific comments on the Alverson assessment1

Area 21 Duthie, 1997a,b

· Significant digits and error variance lacking (remains a problem in current study)
· Further consideration of factory vessels (remains a problem in current study)

Area 21 Kennelly, 1997

· Lack of detailed explanation as to how the estimates were made
· Lack of clarity regarding assumptions
· Impossible to judge validity of assumptions
· Use of target species, particularly in multispecies fisheries

Area 27 Smith, 1997

· 20-30 references for entire Northeast Atlantic and possible application of North Sea discard rates to all of Area 27
· Species-by-species approach requiring greater number of records for multispecies fisheries
· Species with low discard rates accorded high discard rates by default as no discard information exists
· Interpretation problems regarding Norway pout (110 000 tonnes discarded), sand eels (806 000 tonnes discarded), capelin (492 000 tonnes discarded) and blue whiting

Area 34 Balguerías, 1997

· Limited source material and some reference material on discards overlooked

Area 47 Japp, 1997

· Lacking specific information on Area 47

Area 61 Matsuoka, 1997

· Overestimate of discards because of double counting and an estimate of approximately 5 million tonnes lower provided

Area 71 Harris, 1997

· No allowance made for retained bycatch in shrimp fisheries. Major difference between estimate for shrimp fisheries and that made by Andrew and Pepperell (1992) - 1.38 million tonnes compared with 0.29-0.59 million tonnes. Discard ratios from temperate waters apparently applied in tropical waters. No allowance made for artisanal fisheries


1 From papers presented at the Technical Consultation on Reduction of Wastage in Fisheries, Tokyo, November 1996. FAO Fisheries Report No. 547 (Suppl.). (FAO, 1996b).

TABLE 30
Matrix for calculation of discards as proposed by the Technical Consultation

B.3 GLOBAL DISCARD ESTIMATES PRESENTED IN SOFIA

The conclusions reached in the Alverson assessment were presented in The State of Fisheries and Aquaculture 1996 (SOFIA) (FAO, 1996a). A revised estimate of 20 million tonnes was presented in SOFIA 1998 (FAO, 1998). This estimate has been largely ignored and is rarely cited in the literature, possibly because the revised estimate was not substantiated by FAO in any published documents.

Based on information provided at the Technical Consultation, the probable basis for the global discard estimate in SOFIA 1998 has been reconstructed (Table 32) and derives a similar quantum of discards. Although not directly comparable, but in order to demonstrate the evolution of the discard estimates, the results of the Alverson assessment, the Technical Consultation, the SOFIA estimate and this reassessment are presented in Table 31.

B.4 REVISION OF ALVERSON ESTIMATE USING TOKYO WORKSHOP INFORMATION

The sources of major changes in discard estimates (see also Box 8) indicated in the Tokyo workshop were as follows:

BOX 9
Discard estimates in SOFIA 1996 and SOFIA 1998

SOFIA1996
In 1994, FAO showed that the proportion of the world fish catch made up of bycatch might be much larger than previously considered and estimated that discarding amounted to an average of 27 million tonnes per year (or about 32 percent of the total reported annual production of marine capture fisheries).”

SOFIA1998
“A subsequent re-evaluation of these estimates, together with adjustments allowing for subsequent reductions in discarding, indicates that current levels are at the lower end of the range. The most recent FAO estimate of 20 million tonnes, if correct, is equivalent to 25 percent of the reported annual production from marine capture fisheries, which are those from which most of the discards derive.”

However, assuming that the 20 million tonne estimate in SOFIA 1998 was based on the type of calculation provided in Table 32, it is clear that it was not really a reestimate, but a modified version of the estimate provided in the Alverson assessment. As the papers presented in the Tokyo workshop did not cover many of the FAO areas (e.g. South America, Indian Ocean), the SOFIA 1998 figure was at best a partial reestimate. The adjustments made as a result of the Tokyo workshop reduced the discard estimate by approximately 45 percent for the six FAO areas considered.

Again, it is stressed that the different methods and approaches used in the two studies do not make the estimates directly comparable and considerable caution is required in drawing conclusions.

TABLE 31
Evolution of discard estimates (tonnes), 1994-2004

FAO area


FAO Fisheries
Technical Paper
No. 339 (Alverson
Table 5)

FAO Fisheries
Report No.
547/SOFIA 1998
(approx.)1

Current
study

Arctic Sea

18



0

Northwest Atlantic

21

685 949

699 689

92 926

Northeast Atlantic

27

3 671 346

2 891 080

1 408 931

West Central Atlantic

31

1 600 897

1 600 897

831 808

East Central Atlantic

34

594 232

185 956

309 718

Mediterranean/Black Sea

37

564 613

564 613

17 954

Southwest Atlantic

41

802 884

802 884

197 618

Southeast Atlantic

47

277 730

116 652

120 283

West Indian Ocean

51

1 471 274

1 471 274

205 428

East Indian Ocean

57

802 189

802 189

151 190

Northwest Pacific

61

9 131 752

4 000 000

1 355 822

Northeast Pacific

67

924 783

734 069

192 829

West Central Pacific

71

2 776 726

1 200 000

407 826

East Central Pacific

77

767 444

767 444

167 351

Southwest Pacific

81

293 394

293 394

35 475

Southeast Pacific

87

2 601 640

2 601 640

530 582

Multiple area

31, 77



27 335

Multiple area

67, 77



150 161

Multiple area

71, 77



2 138

Global shark fin




206 815

Tunas, bonitos, billfish


Atlantic and Mediterranean (ICCAT area) (21, 27, 31, 34, 41, 48)

159 466


Indian Ocean (IOTC area) (51, 57)

139 465


Pacific E. Central (IATTC area) (67, 77, 87)

56 508


Pacific SW and W. Central (SPC area) (71, 81)

162 068

Subtotal tuna

517 507

Antarctic

Atlantic, Antarctic

35 119

35 119


Indian Ocean, Antarctic

10 018

10 018


Pacific, Antarctic

109

109


Subtotal Antarctic CCAMLR (48, 58, 88)

2 079

Global estimate of discards

27 012 099

19 185 303

6 931 776

1 See Table 32 for derivation of estimate.

TABLE 32
Possible derivation of the estimate of discards referred to in SOFIA 1998
(in tonnes)

FAO Area

FAO TR339 (Alverson, Table 5)

FAO R547 Suppl. Tokyo discard estimate

FAO R547 Reference

Reduction

Comment

Northwest Pacific 61

9,131,752

4,000,000

Matsuoka, Zhou

-5,131,752


Northeast Atlantic 27

3,671,346

2,791,080

Smith

-780,266

R547 estimate excludes Norway Estimated total discards less than in TR339. There are major differences in the source of discards.



100,000

Norway (current study, not R547)



West Central Pacific 71

2,776,726

1,200,000

Harris, Chee, Zhou (combined)

-1,576,726

Southeast Asian countries (not directly equivalent to area 71 but difference of order of magnitude evident (Chee). Shrimp discards (1.34 million tonnes) overestimated by approx. 0.45 million tonnes (Harris). Zero discards in Chinese fisheries (Zhou and Ye).

Southeast Pacific 87

2,601,640

na




West Central Atlantic 31

1,600,897

na




West Indian Ocean 51

1,471,274

na




Northeast Pacific 67

924,783

734,069

Newton

-190,714

Change in the fisheries

Southwest Atlantic 41

802,884

na




East Indian Ocean 57

802,189

na




East Central Pacific 77

767,444

na




Northwest Atlantic 21

685,949

699,689

Duthie/Kennelly

13,740


East Central Atlantic 34

594,232

185,956

Balguerias


Shrimp and cephalopod fisheries only

Mediterranean/Black Sea 37

564,613

na




Southwest Pacific 81

293,394

na




Southeast Atlantic 47

277,730

116,652

Nolan/Yau

-161,078


Atlantic Antarctic 48

35,119

na




Indian Ocean Antarctic 58

10,018

na




Pacific Antarctic 88

109

na




Total TR 339 (Alverson)

27,012,099



-7,826,796


Revised estimate derived from R547 (Tokyo)



19,185,303

Approximates the value in SOFIA 1998



[135] Developed by Smith (1997) and Duthie (1997a,b). See Appendix C to the Technical Consultation (FAO Fisheries Report No. 547).

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