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:
briefly outline the method used in the Alverson assessment;
examine some of the reasons for the differences; and
provide an overview of the evolution of the discard estimates.
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 |
|
Area 21 Duthie, 1997a,b |
· Significant digits and error variance
lacking (remains a problem in current study) |
Area 21 Kennelly, 1997 |
· Lack of detailed explanation as to how
the estimates were made |
Area 27 Smith, 1997 |
· 20-30 references for entire Northeast
Atlantic and possible application of North Sea discard rates to all of
Area 27 |
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:
Area 27 - substantial change in source of discards although little change in total quantity (Smith);
industrial and artisanal fisheries in South and Southeast Asia (Chee, Harris);
China, which is reported to have no discards (Zhou and Ye); and
revision of Area 61 discard estimate (Matsuoka).
BOX 9 SOFIA1996 SOFIA1998 |
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 |
FAO Fisheries |
Current |
|
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). |