Updated and expanded estimates of bottom area in IPHC regulatory

Updated and expanded estimates of bottom area in IPHC regulatory areas Steven R. Hare, Ray A. Webster, and Thomas M. Kong

Abstract IPHC estimates of bottom area by Regulatory Area are revised, updated, and expanded to facilitate assessment and apportionment needs.

Introduction In 2007, the halibut assessment switched from a set of closed-area models to a single coastwide model (Clark and Hare 2007a, 2007b). While this resulted in a more reliable estimate of coastwide biomass, it complicated the means for determining regulatory area biomass and catch allowances. The current method weights regulatory area survey CPUE by bottom area to produce relative estimates of abundance. Each regulatory area’s share of the relative abundance is then applied to the estimated coastwide abundance to produce area-specific abundance estimates. Robust estimates of bottom area are therefore of critical importance. This paper reports updated and expanded estimates of bottom area. New bottom area estimates are produced for the Bering Sea islands where survey stations have been fished since 2006. Also, a review of commercial landings shows that commercial fishing is taking place north of 61°N (Fig. 1), the previous northern limit used for bottom area calculations. Similarly, there has been increased commercial fishing in waters deeper than 300 fathoms (Fig. 2); previous bottom area calculations extended only to 300 fathoms. To produce relative abundance indices for each regulatory area, it is necessary to determine a suitable index of density to apply to each area. For all regulatory areas except 4CDE, the IPHC setline survey can be used. For the Bering Sea flats, the NMFS trawl survey is used, indexed by the special 2006 IPHC survey (Soderlund et al. 2007) conducted in the area. Because we use three-year averages of survey CPUE, the inclusion of 29 Bering Sea island survey stations in the annual survey since 2006 now allows the regions around the islands to be indexed separately from the rest of the shelf. The expanded 4D and 4E areas that are north of 61°N are mostly outside of the range of the annual Bering Sea NMFS trawl survey which is used to compute halibut density in the parts of 4D and 4E that are south of 61°N. It is likely that the regions of 4D and 4E north of 61°N areas have lower halibut densities than the region within the NMFS survey grid. The Alaska Department of Fish and Game conducts a Norton Sound trawl survey approximately every three years. Investigations are underway to determine the suitability of that survey to represent halibut density in northern 4D and 4E. Data from the NMFS trawl survey are available to 62°N so we anticipate that the region between 61°N and 62.5°N in Area 4D may be treated separately (i.e., different densities applied) from the 4D region north of 62.5; thus separate area estimates are provided for two northern regions of Area 4D. This report presents bottom area estimates for all the divisions described above, from zero fathoms out to depths of 300, 400, and 500 fathoms.

347 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

Methods The bottom area estimates are derived from a 30-arc second digital bathymetry database, termed the “Bathymetric Chart of the Ocean”, and available from http://www.gebco.net. This database represents a four-fold increase in the density of depth recordings over the digital database (ETOPO1) previously used to compute bottom area (Hare 2008). The software used to compute bottom between identified contour lines is the Generic Mapping Tools (GMT) routine grdvolume (Wessel 1998) provided in version 4.4 of the GMT set of software mapping routines (Wessel and Smith 1991). GMT software was also used to produce the bottom area maps illustrated in Figures 3a-i.

Results Bottom areas were computed for a range of depth intervals for each IPHC regulatory area (Table 1). These include 0-300 fathoms, 0-400 fathoms, 0-500 fathoms, and 20-275 fathoms which is the depth range presently used in the annual setline survey. In the Bering Sea, the 4D edge estimate is computed using an interval of 75-275, 75-300, 75-400, and 75-500 fathoms, respectively. The 4D edge setline survey occupies depths between 75 and 275 fathoms and this region represents a habitat and density distinct from the shallower waters of the Bering Sea shelf. As noted earlier, previous calculations of bottom area only considered the Bering Sea region south of 61°N. The bottom area estimates reported here differ by at most a few percent from those reported in Hare (2008). The area represented by the three islands in 4C and 4D is newly broken out in this report. The area represented by the islands encompasses all of the survey stations out to 5 nautical miles in each direction of the outside stations for each island (Fig. 3i). To compare with Hare (2008), the island area should be added to the 4CDE shelf area. However, if the region north of 61°N is included in the bottom area definition, then the bottom area estimates for 4D and 4E increase substantially - the Bering Sea shelf area represented by 4D and 4E increases by 50%. The corresponding share of coastwide bottom area represented by each of the other regulatory areas, therefore, decreases. Table 2 presents regulatory bottom area shares both including and excluding the region north of 61°N.

References Clark, W.G. and Hare, S.R. 2007a. Motivation and plan for a coastwide stock assessment. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2006: 83-96. Clark, W.G. and Hare, S.R. 2007b. Assessment of the Pacific halibut stock at the end of 2006. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2006: 97-128. Hare, S.R. 2008. New estimates of bottom area in IPHC regulatory areas. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2007: 297-307. Soderlund, E., Dykstra, C.L., Geernaert, T., Ranta, A., and Anderson, E. 2007. 2006 standardized stock assessment survey. Int. Pac. Halibut Comm. Report of Assessment and Research Activities 2006: 335-366. Wessel, P. 1998, An empirical method for optimal robust regional- residual separation of geophysical data, Math. Geol., 30(4), 391-408. Wessel, P., and Smith, W.H.F. 1991. Free software helps map and display data, EOS trans. AGU 72: 441 & 445-446. 348 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

Table 1. Revised estimates of bottom area (in square nautical miles) between specified depth contours for all IPHC regulatory areas. Area 4D edge estimates have 75 fathoms as the shallow contour (instead of 0 or 20 fathoms). Two sets of values are given for the shelf areas of Areas 4D and 4E, one for the region south of 61°N and the other for the region north of 61°N. A new set of island depths is also presented. These represent the regions surrounding the survey stations at St. Paul and St. George islands (Area 4C - islands) and St. Matthew island (Area 4D - island). end up eatin i for break Regulatory Area 2A 2B 2C 3A 3B 4A - GOA 4A - BS 4B 4C - islands 4C - shelf 4D - edge 4D - island 4D - shelf to 61°N

Depth range (in fathoms) 20-275 0-300 0-400 0-500 10725 13047 14132 15039 23770 28884 29601 30334 11915 14001 14580 14914 41998 48319 49178 49920 25581 28893 29584 30501 9858 10697 11769 12532 7131 7403 8120 8611 11865 15377 19711 23234 1951 2094 2094 2094 7079 7540 7542 7544 14318 14553 15313 15966 1693 1925 1925 1925 42509 45583 45583 45583

4D - shelf 61-62.5°N

21077

24093

24093

24093

4D - N of 62.5°N

13096

20393

20393

20393

4E - to 61°N

47853

76256

76256

76256

4E - N of 61°N

615

26400

26400

26400

Total - to 61°N

258244

314572 325388

334453

Total - inc. 61°N

293032

385458 396274

405339

349 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

Table 2. Regulatory area shares of bottom area. Column A does not include the 4DE region north of 61°N; this was the bottom area definition used in 2008 and earlier. Column B includes the region north of 61°N in computing regulatory area shares. Depth range (in fathoms) 20-275 Regulatory Area 2A 2B 2C 3A 3B 4A 4B

0-300

0-400

0-500

A

B

A

B

A

B

A

B

4.2%

3.7%

4.1%

3.4%

4.3%

3.6%

4.5%

3.7%

9.2%

8.1%

9.2%

7.5%

7.5%

9.1%

7.5%

4.6%

4.1%

4.5%

3.6%

4.5%

3.7%

4.5%

3.7%

16.3%

14.3%

15.4%

12.5%

15.1%

12.4%

14.9%

12.3%

9.9%

8.7%

9.2%

7.5%

9.1%

7.5%

9.1%

7.5%

6.6%

5.8%

5.8%

4.7%

6.1%

5.0%

6.3%

5.2%

4.6%

4.0%

4.9%

4.0%

6.1%

5.0%

6.9%

5.7%

4CDE - edge to 61°N

5.5%

4.9%

4.6%

3.8%

4.7%

3.9%

4.8%

3.9%

4CDE - islands to 61°N

1.4%

1.2%

1.3%

1.0%

1.2%

1.0%

1.2%

1.0%

37.7%

33.3%

41.1%

33.6%

39.8%

32.6%

38.7%

31.9%

4CDE - shelf to 61°N

9.1%

4D - 61-62.5°N

7.2%

6.3%

6.1%

5.9%

4D - N of 62.5°N

4.5%

5.3%

5.1%

5.0%

4E - N of 61°N

0.2%

6.8%

6.7%

6.5%

Total

100%

100%

100%

350 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

100%

100%

100%

100%

100%

Bering Sea commercial halibut catch 2004-2008 160°W

170°W

180° Legend Aggregated IPHC Stat Areas

65°N

65°N

Net weight (lbs) 72686

667 - 150000

341002 Nome

150001 - 600000 Savoonga

600001 - 1500000 56164

1500001 - 3000000 3000001 - 6000000

667

9047

St. Lawrence I

Not displayed for confidentiality reasons IPHC Reg Areas 10128

St. Matthew I

Mekoryuk

5205020 32560

60°N

Toksook Bay

507149 540196 Kipnuk

60°N

Nunivak I 901824 415096

37470

294360

11390 Togiak

Z

199666 848097 1826862

145917 162028

St. Paul I

EE

641248 993851 2491348 68351 Saint Paul

421447 390681 718580

21900

43479

5511

Sand Point King Cove

55°N 72763

55°N

2866292 Akutan Dutch Harbor

1778018 4369416

546886 287548 22707 52534

983377 1293323 552877 536157

Atka

Adak

180°

170°W

160°W

Figure 1. Summary of commercial catch by aggregated IPHC statistical area in the in the Bering Sea. The landings, in net pounds, are summed over the period 2004-2008. Catch amounts for areas for which there are fewer than three vessels contributing catch are not displayed.

351 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

2B

2C

0 75

175

300

425

0.00

0.00

0.04

0.05

0.08

0.10

0.12

0.20 0.15 0.10 0.05 0.00

Proportion of catch

0.15

2A

0 75

425

300

425

300

425

300

425

300

425

0.15 0.10 0.00 0 75

175

300

425

0 75

4C

175

4D

0.10 0.05 0.00

0.0

0.00

0.1

0.04

0.2

0.3

0.08

0.4

0.5

0.15

4B 0.12

175

0.05

0.20 0.10 0.00 175

0 75

4A

0.30

0.20 0.15 0.10 0.05 0 75

Proportion of catch

300

3B

0.00

Proportion of catch

3A

175

0 75

175

300

425

0 75

depth (fm)

175

300

depth (fm)

425

0 75

175

depth (fm)

Figure 2. Distribution of commercial catch by depth (in 25 fathom increments) by regulatory area for 2008.

352 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

127˚W

126˚W

125˚W

124˚W

123˚W

49˚N

122˚W 49˚N

2B 48˚N

48˚N

47˚N

47˚N

2A 46˚N

46˚N

45˚N

45˚N

44˚N

44˚N

Depth (fm) 43˚N

42˚N 127˚W

126˚W

0 -20 -75 -275 -300 -400 -500

43˚N

42˚N

125˚W

124˚W

123˚W

122˚W

Figure 3a. IPHC Area 2A, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations. 353 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

136˚W

134˚W

132˚W

130˚W

128˚W

126˚W

124˚W

55˚N

122˚W

55˚N

2C 54˚N

54˚N

53˚N

53˚N

52˚N

52˚N

2B

51˚N

51˚N

50˚N

50˚N

Depth (fm) 49˚N

48˚N 136˚W

0 -20 -75 -275 -300 -400 -500

134˚W

49˚N

48˚N

2A 132˚W

130˚W

128˚W

126˚W

124˚W

122˚W

Figure 3b. IPHC Area 2B, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations.

354 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

138˚W

136˚W

134˚W

132˚W

130˚W

59˚N

59˚N

58˚N

58˚N

3A 57˚N

57˚N

2C

56˚N

56˚N

Depth (fm) 55˚N

54˚N 138˚W

0 -20 -75 -275 -300 -400 -500 136˚W

55˚N

2B 54˚N 134˚W

132˚W

130˚W

Figure 3c. IPHC Area 2C, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations.

355 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

156˚W

154˚W

152˚W

150˚W

148˚W

146˚W

144˚W

142˚W

140˚W

138˚W

136˚W

61˚N

61˚N

60˚N

60˚N

59˚N

3A

58˚N

Depth (fm)

57˚N

3B

56˚N 156˚W

154˚W

152˚W

150˚W

148˚W

146˚W

59˚N

58˚N

0 -20 -75 -275 -300 -400 -500

144˚W

2C

57˚N

56˚N 142˚W

140˚W

138˚W

136˚W

Figure 3d. IPHC Area 3A, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations.

356 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

164˚W

162˚W

160˚W

158˚W

156˚W

154˚W

58˚N

152˚W 58˚N

4E 57˚N

d se o Cl

57˚N

ea ar

56˚N

3A

55˚N

56˚N

55˚N

Depth (fm)

3B

54˚N

4A 53˚N 164˚W

162˚W

160˚W

158˚W

156˚W

154˚W

0 -20 -75 -275 -300 -400 -500

54˚N

53˚N 152˚W

Figure 3e. IPHC Area 3B, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations.

357 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

173˚W 172˚W 171˚W 170˚W 169˚W 168˚W 167˚W 166˚W 165˚W 164˚W 163˚W 57˚N 57˚N

4C

C lo se d

ar ea

4D

56˚N

56˚N

4A -B S

55˚N

ed

55˚N

ge 3B 54˚N

54˚N

Depth (fm) 53˚N

4B

A

/AI

4A

O -G

52˚N

0 -20 -75 -275 -300 -400 -500

53˚N

52˚N

173˚W 172˚W 171˚W 170˚W 169˚W 168˚W 167˚W 166˚W 165˚W 164˚W 163˚W

Figure 3f. IPHC Area 4A, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations.

358 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

170˚E

172˚E

174˚E

176˚E

178˚E

180˚

178˚W

176˚W

174˚W

172˚W

56˚N

56˚N

Depth (fm)

Russia 55˚N

4B

54˚N 53˚N

0 -20 -75 -275 -300 -400 -500

55˚N 54˚N

4A 53˚N

52˚N

52˚N

51˚N

51˚N

50˚N

50˚N 170˚E

172˚E

174˚E

176˚E

178˚E

180˚

178˚W

176˚W

174˚W

172˚W

Figure 3g. IPHC Area 4B, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations.

359 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

180˚ 66˚N

176˚W

172˚W

168˚W

164˚W

160˚W 66˚N

65˚N

65˚N

Depth (fm) 64˚N 63˚N 62˚N

0 -20 -75 -275 -300 -400 -500

4DN

64˚N

4EN

63˚N 62˚N

61˚N

61˚N

4DS

60˚N

60˚N

4ES

59˚N

4D ed

58˚N

58˚N

ge

4C

57˚N

ed s o Cl

4A

56˚N

-B

ea ar

57˚N 56˚N 55˚N

S

55˚N

ed

176˚W

3B

ge

4B 180˚

59˚N

172˚W

168˚W

164˚W

160˚W

Figure 3h. IPHC Area 4CDE, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. Red dots indicate locations of IPHC setline survey stations. The dashed lines indicate where additional bottom area calculations were made for areas 4D (4DS and 4DN, north and south regions of 4D, respectively) and 4E (4EN and 4ES, north and south regions of 4E, respectively).

360 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

174˚W 61˚N

172˚W

170˚W

168˚W

166˚W 61˚N

4D

60˚N

60˚N

4E

59˚N

59˚N

Depth (fm) 58˚N

4C

0 -20 -75 -275 -300 -400 -500

58˚N

57˚N

57˚N

56˚N 174˚W

56˚N 172˚W

170˚W

168˚W

166˚W

Figure 3i. IPHC Area 4CDE islands with survey coverage, with depth contours drawn at 20, 75, 275, 300, 400 and 500 fathoms. Waters deeper than 500 fathoms are not shaded. the boxes surounding the three islands show the area included in the island bottom area calculations. Red dots indicate locations of IPHC setline survey stations.

361 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009

362 IPHC REPORT OF ASSESSMENT AND RESEARCH ACTIVITIES 2009