CURRENTS IN SOUTHEASTERN BERING SEA AND POSSIBLE EFFECTS UPON KING CRAB LARVAE SPECIAL SCIENTIFIC REPORT- FISHERIES No. 293 Marine Biological Laboratory JUL i 7 i^i^y WOODS HOLE, MASS. UNITED STATES DEPARTMENT OF THE INTERIOR FISH AND WILDLIFE SERVICE EXPLANATORY NOTE The series embodies results of Investigations, usually of restricted scope, intended to aid or direct management or utilization practices and as guides for administrative or legislative action. It is issued in limited quantities for official use of Federal, State or cooperating agencies and in processed form for economy and to avoid delay in publication . SPECIAL NOTE The International North Pacific Fisheries Commission, established in 1953 by the International Convention for the High Seas Fisheries of the North Pacific Ocean, coordinates the research of the member nations: Japan, Canada, and the United States. The resulting investigations provide data to the Commission for use in carrying out its duties in connec- tion with fishery conservation problems in the North Pacific Ocean. Publication of this scientific report has been ap- proved by the United States Section of the Commission. United States Department of the Interior, Fred A. Seaton, Secretary Fish and Wildlife Service, Arnie J. Suomela, Commissioner CURRENTS IN SOUTHEASTERN BERING SEA AND POSSIBLE EFFECTS UPON KING CRAB LARVAE by James F. Hebard Fishery Research Biologist Bureau of Commercial Fisheries Contribution No. 9 to research conducted with the approval of the United States Section of the International North Pacific Fisheries Commission. United States Fish and Wildlife Service Special Scientific Report — Fisheries No, 293 Washington, D.C. April 1959 The Library of Congress catalogue card for this publication is as follows: Hebard, James F Currents in southeastern Bering Sea and possible effects upon kinp crab larvae. Washino^on, U. S. Dept. of the Interior, Fish and Wildlife Service, 1959. iv, 11 p. maps, diagrs., tables. 27 cm. (U. S. Fish and Wildlife Service. Special scientific report — fisheries, no. 293) "Contribution no. 9 to research conducted with the approval of the United States Section of the International North Pacific Fisheries Commission." Bibliography : p. 11. 1. Ocean currents — Bering Sea. 2. Paralithodes camtschatica. 3. Larvae — Crustacea. i. Title. (Series) [SH11.A335 no. 293] Int 59-62 U. S. Dept. of the Interior. Library for Library of Congress The Fish and Wildlife Service series, Special Scientific Report — Fisheries, is catalogued as follows: U. S. Fish and Wildlife Service. Special scientific report : fisheries, no. 1- [Washington, 1949- no. illus., maps, diagrs. 27 cm. Supersedes in part the Service's Special scientific report. 1. Fisheries — Research. SH11.A335 639.2072 59-60217 Library of Congress 11 TABLE OF coNTEr^rrs Page Introduction 1 Methods 2 Tidal currents 2 Average currents 7 Discussion of average currents 9 Effects of currents on the king crab larvae 10 Summary 11 Literature cited 11 1X1 ^ * ■" -— '^' ' " " "" " " _ — . . — _ — » I •« —1 - 1 . • 1 ^ 40 ! 53" ^ ^ fr " N \ 58° Figure 1. — Southeastern Bering Sea (Within dashed line) showing current stations and their depths along with the depth contours for the king crab fishery area. Depths given in meters. IV CURRENTS IN SOUTHEASTERN BERING SEA AND POSSIBLE EFFECTS UPON KING CRAB LARVAE by James F. Hebaxd U. S. Fish and Wildlife Service Seattle, Washington ABSTRACT During an investigation of the tidal and average currents in southeastern Bering Sea, carried out in June 1957, the currents were observed at four anchor stations by means of an Ekman current meter. Dextrally rotating tidal currents occurred on the two offshore sta- tions, and greatly compressed rotary tidal currents varying in direc- tion of rotation occurred on the two inshore stations. The flow of the average current indicated a counterclockwise circulation. These currents may affect the movement of king crabs in the planktonic stage. INTRODUCTION The movements and physical characteris- tics of the water in the king crab fishery of southeastern Bering Sea (figure 1), were investigated in June 1957 to determine the tidal and average currents in an effort to assess the possible effects they may have on the king crab (Paralithodes camtschatica Tilesius). This investigation was under- taken as part of the program of the King Crab Investigations, United States Fish and Wildlife Service. The results contribute primarily to the knowledge of the currents observed in southeastern Bering Sea and also to the determination of the effects of the currents on the king crab in the plank- tonic stage. Future investigations will attempt to relate the water movement to other stages in the life cycle of the crab. The area under investigation is trian- gular in shape, with an apex in the Bristol Bay region. The land adds freshwater run- off through many streams, among the largest being the Ugashik, Egegik, Naknek, Kvichak, and Nushagak Rivers, all of which empty into the Bristol Bay region. The bottom topography is one of gradually increasing depth from Bristol Bay toward the southwest (figure 1). The positions of the four current stations are shown in figure 1. The maxi- mum depths at the sampling stations A, B, C, and D are 66, 40, 60, and 71 meters, respectively. Previous current studies covering part of the same area (surrounding and between stations C and D) were made by scientific personnel aboard the Japanese king crab raothership, Tokei Maru, during the crabbing seasons of 1955 and 1956 (International North Pacific Fisheries Commission, 1957). The 1955 data show that the tidal current flowed west-southwest during the ebb-tide hours and east-northeast during the flood- tide hours. In 1956, this general pattern sometimes became obscure. The recorded tidal current velocities did not exceed 1.5 knots in either year. Past studies contributing to the knowl- edge of water circulation in and around southeastern Bering Sea include those by the U. S. Coast Guard (1936), Thompson and Van Cleve (1936), and the several mapping, tide, and current surveys made by the U. S. Coast and Geodetic Survey for the Tide and Current Tables for the Pacific Coast of North America. Dr. Richard H. Fleming and Dr. Maurice Rattray of the Department of Oceanography, University of Washington, generously helped plan the study. Thomas S. Austin, James W. McGary, and Gunter R. Seckel of the U. S. Fish and Wildlife Service, Pacific Oceanic Fishery Investigations, also reviewed the manuscript and made many useful suggestions. METHODS From an anchored vessel, the currents were observed at three meters, the top and bottom of the thermocline (where present), and near the bottom, every hour for 38 hours at each of four positions (ainchor stations). An Ekman current meter (Ekman, 1932), re- cording both current velocity and direction, wjis used for the observations. No correc- tion for the roll of the ship was attempted because during the observations, except at station B, the roll was usually nonexistent. Before the beginning of the observation period at each station, a bathythermograph (BT) trace was taken to determine the ther- mocline depth, thereby locating the sampling depths between the surface and the bottom observed depth. (Because of isothermal con- ditions on station B, only three depths were observed.) The mid-depths thus found were then used throughout the sampling period, regardless of possible later changes in the thermocline depth. In the analyses of the data for the currents involved, portions of the methods described in both the Manual of Current Observation (U. S. Coast and Geodetic Sur- vey, 1950) and the Admirality Manual of Tides (Doodson and Warburg, 1941) were used. Tables from the Manual of Current Observa- tion were used to resolve the observed hourly velocities for each station into their respective north and eaist component velocities which include both the tidal and nontidal currents. (Negative north or east values for the components indicate a current flowing south or west, respectively.) After the component velocities were obtained, the method for obtaining mean sea level by use of multipliers was used (Doodson and War- burg, 1941, p. Ill, table 13.6) to extract the average current from the hourly compo- nents. This method has the advantage of reducing casual error which would probably be found in the hourly observations. By then substracting the north and east compo- nents of the average current from the corresponding components of the observed current for each hourly observation, the north and east components of the tidal currents were obtained. TIDAL CURRENTS Observations of tidal currents occur- ring in southeastern Bering Sea show that both rotary euid reversing movements occur, the rotary on the more offshore stations (A and B), the usual locus for this type of tidal current, and the reversing on the inshore stations (C and D). The rotation period for all stations was semi-diurnal, accompanied by cyclic variations of the current velocity, also semi-diurnal, thus showing the relationship between the cur- rents and the tidal cycle. Figures 2 through 5 are hodographs of the tidal cur- rents, these showing the hourly observed velocity cuid direction, the change in cur- rent direction between observations and the characteristic rotation occurring on each station. In the northern hemisphere, the normal rotation of a rotary current is dextral unless some other interfering force modifies the rotation. On station A the rotation was normal at all observation depths, but on B the direction of rotation varied with depth. Dextral deviation occurred at the surface, while at 20 meters and 40 meters the pattern was confused, both dextral and sinistral rotation occurring at each depth. This pattern is shown in figure 3. To determine the principal flood and ebb directions of the rotary currents, the average maximum flood and ebb currents were used, these showing, with slight individual station variation, that the flood and ebb were generally in a northeast-southwest direction at all observation depths. The directions of the flood or ebb and their variations in average velocity and direction with increased depth are shown in table 1 (page 7). On stations C and D, the tidal currents were essentially reversing but with slight rotation, the amount increasing with depth to the bottom on C but remaining approxi- mately the same at all depths on D. Because of the slight rotation, apparently no slack water occurred as is associated with the completely reversing type of tidal current movement . y ■r. / j^^ N / - " " ~~y^-L \ / ^ ^ / r \ \ ^ / J' J \ / / 9 1 X 1^ / y /I \ \ ' - ' /~ ~ I \ \ ; ' " ^ / V~~~" --^ \ \ 1 i / / ^ \ \ / Q ^. T- -_— JV*^""^ \ \ / \ '4^ / "/ I ^ % \ 1 i ; Vjr/T§/^'^ \ 1 \ \ 1 , 1 ' ' pX/^^/ ^i" 1 IJ2 1 1.6 '^ /lK // J-^ 1 / i 1 \ ^ \%^^. \./L---^' / / / \ \ \ \ ^\ W / / / \ \ V 7^^^.,\^ / / / • \ \. \^^ cf^ ^"^ ^ / 1 \ \^^"^=i^ - ^ / 1 ^o ^ ^ — " ^ ^ / / \ / / STATION A (Surfoce) \ y / •^ y / / / / ^ ^ X x" N \ \ / / / \ / . - - - - - N \ ' ^ - '^ ' ^ " - ^ \ \ \ \ \ \ \ 1 / / ^-•''-"■*^ \ \ / / r \ \ \ \ t/ 1 ; / FA \ 1 \ ' 1 1 y/' X^r <^ ^ J l!2 1 ,.^ \ V 1 1 / / / 1 / / / / / \ \, //^ — i~ ~ — — ' " / / / / / \ \ ry / / 1 \ / / \ / STATION A (Botlom) ^ ^ y / , ^ / Figure 2. — Station A hodograph of surface and bottom tidal currents. r ^ ' -"-^ / /' --^ \ / N \ \ ■*• / / / ^ ," ,^ \ i \ \ / / > \ , \ / / ^'-^'^°\ / \ \ \ / / ,' . .^"^ \^ / \ ^ \ \ / / /X. C^^'^'^A^ \ \ 1 \ \ 1 1 1 ^>^^r^ ""^ V /x Ol8 'i2 l{.6 1 \ * / \ \ ^^^3 1 / / / 1 \ \ \ /\ II ^-"^l^^^^y f 1 / , / / \ \ f /^C^ ^C>-._^^'^ / / / / \ \ \\V < ^ ^ / / / \ \ ~ — - -^ / / \ \ / / N z' / \ y STATION B\(Surfoce) ' / •■- / / \ / ^ •^ \ / X \ \ / / N \ \ / / / ^^/^-"^ ■^ \ \ / / y^^ ^ \ \ / / i/^ (^ \ \ \ / / L \V P \_ J / / / \X ^ \ \ / 1 / s-^-^!y>x i ^^^— :r^i-^ ^\ \ \ 1 1 \ A/^^ ^^r^^^\^^^ / ols ,.t 1,6 \ \ yC> r Nl/y °<* ole Ij2 1.6 1 1 I (Iff 1 ( / I \ \ \ \ \ / / / / / / / i 1 1 \ \ \ \ / 1 1 / \ \ / / \ \ / / V y / STATION C (Surfoca) -^ ^ / / ^ ■^ -^ r\ y / / ^ X .- — ^ / ^ iLcy -^x / / / / / / / / y^,0 JDr\ / /^ 7 \ \ \ \ \ / \ \ \ / / 1 f \ \ \ \ \ I t \ 1 ( 1 1 1 'tn^yW oj. 1 1? 1 ,.|6 1 1 1 1 \ / / \ \ \ / / / / / / \ \ M ' / / \ \ V Jif -^ / STATION C (Bottom) ^OC^ ^ / / / / Figure 4. — Station C hodograph of, surface and bottom tidal currents. / ^ ^ "^ \ \ / . V. \ y" S . / / \ \ / / _ - - N \ / / / /''' ^fe^\ \ \ \ \ / / ;'' /^^K^ ^\ \ \ \ / 1 \ 1 1 ' ' Ik "0» / ^* °* ■f 1 1 \" \ \^^ ^ ! ' / 1 / \ \ ^ f^^^^^t/ y ' / \ \ V / T^SaCl / \ \ \ \^ '''-/Vy,.'-'' / / / 1 / \ ^ ■- --TOi^ -' / / /' y / / STATION D (Surfoce) ^-. ^ '' / / ■ — ~, "'"^ y / ' — *" / '' / " "". \ \ / '' N \ / / \ \ / / .^'^ '~ ^- \ \ / 1 / ^ \ \ \ / \ / If ^ -** — Q>^ ^/\. "^ \ \ / / / ''Vf^\ '' \ \ 1 ' '/t-^'^y^';'' "'' \z ,'6 \ 1 1 1 / / \ \ \ \ '\^;^*^ / 1 / / / / / \ \ \ ^ ^ ^ -- / \ ^ / / \ X ^ STATION 0 (Bottom) ^ ^ / ----'"' / Figure 5. — Station D hodograph of surface and bottom tidal currents. The sequence of observations on C and D indicated the rotation to be both clock- wise and counterclockwise. Above the ther- mocline on both stations the rotation was counterclockwise while below the thermocline the rotation vacillated between clockwise and counterclockwise. The directions of principal flood and ebb were approximately northeast and south- west, respectively. The variations of each with respect to one another and individually with respect to velocity and direction with greater depth are presented in table 1. The rotation period of the tidal cur- rents is significant in that it was semi- diurnal at all observed stations and depths, thereby showing relevance to the semi- diurnal tidal cycle. From the hodographs one can see that the time required for one complete revolution was approximately 13 hours . The velocities of the currents were less regular. Probably because of shallow depths and a smaller cross-sectional area at the inner stations (B and C) , the greater velocities occurred on these two stations. The maximum, minimum, and average velocities of the tidal currents (tidal phase consid- ered) and the variation of these with depth for the above stations as well as stations A and D are shown in figures 6 to 9. AVERAGE CURRENTS In the analysis of the original obser- vation data, the average currents were separated from the tidal currents by the method previously described. The results of the analysis indicated a counterclockwise circulation through southeastern Bering Sea (figure 10). On the inshore stations (C and D), the average current tended to flow into southeastern Bering Sea, the flow being more definite on the inner station (C), at average velocities of 0.09 (C) and 0.05 (D) knots with directions of 072"! and OOa^T, respectively. On station D, the average current varied, with depth, between north and east. Because very light winds pre- vailed during the observations, the proba- bility of the currents being caused by the winds is somewhat decreased. The velocities and flow direction of the average current on stations C and D and their variation with depth are illustrated both in table 2 aind in figure 10 (page 9). Tabl 5 1.— Mean tidal currents for each depth and for whole water cclumn. Obs. Ebb Flood Station Depth(M) Vel.(kt) Dlr. °T Vel.(kt) Dir. °T A 3 0.59 213 0.62 OltO 11 0.65 211+ 0.72 065 25 0.68 211* 0.70 01*7 65 0.67 220 0.51 062 Mean 0.65 215 0.65 054 B 3 0.68 238 0.61* 056 20 0.70 235 O.7I* 080 1*0 0.60 237 0.68 065 Mean 0.66 237 0.69 067 C 3 0.U3 196 0.55 026 13 0.53 207 0.56 031 18 0.50 211 0.63 032 60 0.54 219 0.65 037 Mean 0.50 208 0.60 032 D 3 0.42 186 0.38 027 15 0.1*6 217 0.36 023 1*6 0.1+1 215 0.1*2 026 70 0.1+0 209 0.35 037 Mean 0.1+2 207 0.38 028 Because of strong winds during the observations on station B, the offshore stations (A and B) had to be considered separately. On station A, the average cur- rents flowed essentially southwest at an average velocity of 0.05 knot and an aver- age direction of 223''T. The variations in the current velocity and direction with respect to depth for stations A and B are also shown in table 2. On Station B, where the wind tended to set up the average current, the wind current was effective throughout the water column. The average current flow was generally be- tween east and southeast: the flow at the surface, 153°T; at 20 meters, 146''T; and at the bottom (40 meters), 112°T. The whole column considered, the mean flow direction of the average current was 137°T at a velo- city of 0.11 knot. To determine whether this station (B) was consistent with the counterclockwise circulation in southeastern Bering Sea, an attempt was made to remove the wind effects in order to determine the average current less the wind current. The wind current velocity was considered to be 2 percent of the wind velocity and flowing 42° to the right of the wind (Rossby and Montgomery, 1935). Subtracting the vectors of the wind current from the observed average current results in an average current (only the STATION A eeB VELOCITIES (ktt) ■ a 14 r I 10 O.t 0 • 0 4 03 FLD VELOCITIES (kn) STATION B EBB VELOCITIES (ku) FLD VELOCITIES (hti) — 1 — ■ I ■ ' 4 It 10 o.t o» 04 0j2 I o,a 0-4 o« 0 1 lo it -It -It -10 -14 ■21 -SI -s« -40 -4« -4t -51 -B» -to -■4 -f - velocity cf tbe flood and ebb tidiX currests observe, earn Jeptb oa stiticsi A. Tbe lines on either side of the a iodicate tbe observed K^xi^Da xnd ainism tidal velocities. '. — Hem velocity of tbe flood and ebb tidal cuzrents observed at eacb depth oa station B. The lines on eitber side cf tbe aeans indicate the observed saxiaua sad BiniBux tidal velocities. STATION C E89 VELOCfTlES (his) IS 14 12 loot o.e a4 02 '■'■ I ■''■''■'■' ■ FLO VELOCITIES tkis) 01 04 oeos 10 It 14 STATION D EBB VELOCITIES (kts) 14 12 I 0 0 1 ac 0 4 02 '■'■'■■-''■■'■ FLO VELOCITIES (kit) Figure S, — Hran velocity of the flood and ebb tidal currents r:bserweii at each depth on station C. Tbe liiMS oa either side of tbe seans indicate the observed aaximxm and HinissiB tidal velocities. Figure 9. — Mean velocity of the flood and ebb tijal currents cbserved at each depth on station D. The lines on either side of the ^ans indicate the observed aajtisua and »*"■—"* tidal velocities. i 58' 57- 56° 55' LESS THAN 0-05 KNOT FROM 0 05 TO 0.10 KNOT -- GREATER THAN 0 10 KNOT