U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications


Observed patches of walleye pollock eggs and larvae in Shelikof Strait, Alaska: Their characteristics, formation and persistence

Phyllis J. Stabeno,1 James D. Schumacher,1 Kevin M. Bailey,2 Richard D. Brodeur,2 and Edward D. Cokelet1

1National Oceanic and Atmospheric Administration, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115

2National Oceanic and Atmospheric Administration, Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, WA 98115

Fisheries Oceanography, 5(Suppl. 1), 81-91 (1996)
Copyright ©1996 by Blackwell Science Ltd. Further electronic distribution is not allowed.

Sampling Methods

We use egg and larval abundance data from 38 ichthyoplankton cruises conducted mainly in the Shelikof sea valley and adjacent waters from May 1979 through May 1992 (Tables 1 and 2). The maximum sampling domain was from Shelikof Strait to the Shumagin Islands, and from the Alaska Peninsula to the shelf break. Generally, sampling ceased when no larvae were found. Ichthyoplankton were routinely sampled by double-oblique tows of 60-cm diameter bongo samplers equipped with 333 Ám or 505 Ám (in late May) mesh nets. Additional surveys of late larvae and/or early juveniles used 1 m2 Tucker trawl or 5 m2 Methot trawls. Tow depth varied according to the predominant life stage occurring at the time of sampling. During egg surveys, maximum tow depths were 200 m (before 1985) or within 10 m of the bottom (after 1985). The nominal maximum depth of each larval tow was 100 m. A pressure sensing device was used to determine the maximum sampling depth and to verify that tow trajectories were not biased. Sampled volumes were calculated from mechanical flow meters in the mouth of each net. All samples were preserved in a 5% buffered formalin/seawater solution for later analysis; pollock in the samples were sorted and enumerated. Eggs were staged according to the six age groups of Kendall and Kim (1989). A random subsample of up to 50 larvae were measured to the nearest 0.1 mm. An age/length relationship developed for larvae from the study area provided estimates of age (Bailey et al., 1996, see pp. 137-147 in this supplement).

 

Table 1. Observations and statistics of pollock eggs sampled for each cruise (the last two digits are year). The area of the grid, the number of tows, the mean number of eggs caught and the standard deviation are given. Lloyd's index was calculated following eqns (1-3). [Graphic image of Table 1]


Cruise Date Area
(km2)
No. of tows Mean
(no./10 m2)
Standard
deviation
Lloyd's
index

1MF81 12 Mar.-20 Mar. 10,600 25 35 49 4.4
2MF81 30 Mar.-8 Apr. 13,600 79 254 288 4.5
3MF81 26 Apr.-2 May 15,200 75 393 289 4.9
4MF81 20 May-24 May 13,600 66 98 76 3.9
2MF86 3 May-16 May 5000 106 248 247 4.5
2MF87 2 Apr.-17 Apr. 11,000 129 790 522 5.3
1MF88 1 Apr.-13 Apr. 12,100 104 730 152 5.3
1MF90 8 Apr.-12 Apr. 10,000 104 850 129 5.5
1MF91 31 Mar.-13 Apr. 8800 49 653 221 6.1
2MF91 15 Apr.-28 Apr. 8800 130 628 339 5.0

 

Table 2. Observations and statistics of pollock larvae sampled for each cruise (the last two digits are year). A 60-cm bongo net was used unless otherwise indicated, by † for Methot net and * for Tucker trawl. The "% in patch" column is the percentage of the total number of larvae that were caught at stations where the "no./Tow" > mean + 1 SD for the survey. In the Sutwik Island column "yes" indicates a patch existed near or northeast of Sutwik Island, and * indicates that the survey did not include the Sutwik Island region. [Graphic image of Table 2]


Cruise Date Area
(km2)
No. of tows Mean
(no./10 m2)
Standard
deviation
Lloyd's
index
% in
patch
Sutwik
Island

5TK79 17 May-20 May 85,000 30 225 530 15.0 88 *
2MF81 30 Mar-8 Apr. 16,000 73 9 28 15.9 62 *
2SH81 16 Apr.-24 Apr. 58,000 45 2615 10,100 16.1 92 no
3MF81 26 Apr.-2 May 23,000 79 14,338 21,600 9.3 65 *
3SH81 23 May-28 May 23,000 46 1750 2700 9.6 53 yes
4MF81 20 May-24 May 50,000 75 3118 4420 7.4 39 *
1DA82 7 Apr.-23 Apr. 64,000 45 35 67 14.4 62 no
2DA82 22 May-29 May 68,000 55 50 74 4.7 50 yes
1CH83 16 May-25 May 47,000 58 341 425 5.7 51 yes
1PO85 30 Mar-13 Apr. 22,000 73 141 674 16.2 80 no
2MF85 1 May-12 May 18,000 45 26 58 9.7 54 no
2PO85 16 May-8 Jun. 48,000 97 210 349 7.7 53 yes
1MF86 2 Apr.-13 Apr. 12,000 80 272 357 5.3 44 *
2MF86 1 May-19 May 28,000 103 935 371 5.5 43 no
2MF87 2 Apr.-17 Apr. 10,000 101 35 32 5.7 43 *
3MF87 18 May-29 May 10,000 42 383 396 5.8 26 yes
4MF87† 20 Jun.-11 Jul. 65,000 72 11 16 3.6 40 no
1MF88 1 Apr.-13 Apr. 14,000 105 6 11 11.2 72 *
2MF88 16 Apr.-16 May 15,000 55 477 319 6.4 38 *
4MF88A* 20 May-6 Jun. 17,000 66 255 201 4.4 30 yes
4MF88B* 20 May-6 Jun. 18,000 87 177 168 3.9 35 yes
1MF89A 5 Apr.-16 Apr. 12,000 28 210 218 5.5 45 *
1MF89B 5 Apr.-16 Apr. 12,000 72 83 119 6.6 53 *
2MF89 19 Apr.-5 May 18,000 83 1246 1420 7.4 44 yes
3MF89 9 May-25 May 23,000 112 767 845 6.8 42 yes
4MF89* 28 May-6 Jun. 32,000 95 539 448 5.1 34 yes
1MF90 8 Apr.-12 Apr. 14,000 107 463 643 5.4 52 *
2MF90 6 May-15 May 26,000 76 661 602 5.9 45 no
4MF90 27 May-5 Jun. 34,000 126 357 315 4.3 40 yes
1MF91 31 Mar-13 Apr. 5000 39 21 15 8.9 65 *
2MF91 15 Apr.-28 Apr. 28,000 138 733 727 6.2 45 *
3MF91 30 Apr.-15 May 27,000 102 230 322 5.2 40 no
4MF91 17 May-25 May 45,000 82 62 64 4.0 50 yes
5MF91† 22 Jul.-1 Aug. 27,000 42 3 2 9.0 49 *
1MF92 3 Apr.-11 Apr. 45,000 93 121 136 5.9 53 *
3MF92 30 Apr.-16 May 48,000 85 560 932 6.5 50 yes
4MF92 17 May-29 May 12,000 136 531 680 5.7 39 yes


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