MID-PACIFIC OCEANOGRAPHY Part IV, Transequatorial Waters, January-March, 1952 SPECIAL SCIENTIFIC REPORT-FISHERIES No. 135 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 ;;uides for administrative or legislative action. It is issued in limited quantities for the official use of Federal, State or cooperating Agencies and in processed form for economy and to avoid delay in publication. 5T801 United States Department of the Interior, Douglas McKay, Secretary Fish and Wildlife Service, John L. Farley, Director // si ■ r-=l : -d \ m : _D : a 1 ° i □ i m o MID-PACIFIC OCEANOGRAPHY, PART IV TRANSEQUATORIAL WATERS JANUARY-MARCH 1952 By E . D. Stroup Physical Science Aid Pacific Oceanic Fishery Investigations Honolulu, T. H. Special Scientific Report: Fisheries No. 135 WASHINGTON: AUGUST 1954 CONTENTS Page Introduction : 1 Vessel, equipment, and procedures 1 Some results of Hugh M. Smith cruise 14 2 Computed currents = . 3 Temperature . . 3 Sigma-t . . 4 Salinity 4 Phosphate phosphorous 4 Acknowledgements o 5 Literature cited •• 6 List of figures 7 Notes on tabulated data 19 Introduction This report is concerned with cruise 14 of the M/V Hugh M. Smith, the fifth in a series of equatorial oceanographic cruises by the Pacific Oceanic Fishery Investigations of the U. S. Fish and Wildlife Service. _' The cruise was carried out in January and February 1952 in conjunction with longline tuna fishing in the same area on cruise 11 of the M/V John R. Manning; the results of the fishing have been described by Murphy and Shomura (195 3). The purpose of these simultaneous operations was to help relate the occurrence of tunas to the oceanography of the upper layers in the equatorial Pacific between 155 W. and 180 longitude. Measurements of surface and subsurface temperature, salinity, and inorganic phosphate as well as plankton collections were made during Smith cruise 14. The purpose of this report is to present the tabulated station data and various vertical sections, with minimum analysis of results. There are, in addition, brief descriptions of one of the smaller features of the area. The results of the plankton collections will appear in later reports. Vessel, Equipment, and Procedures The shipboard equipment was essentially as described by Cromwell (1951), except that the hydrographic winch and work platform were raised to a safer and more workable loca- tion on the boat deck, about 6 meters above the sea surface (see frontispiece). The main-deck laboratories were also redesigned for greater convenience. The methods of collection and processing of the raw data have been described in detail (Cromwell 1951, 1954). The treatment of the observed data from Smith cruise 14 leading to their ultimate presentation as interpolated data and vertical sections differs somewhat from the previous cruises. Smith cruises 5 and 8 (Cromwell 1954) and 11 (Austin 1954) were presented using modifications of the methods proposed by Montgomery in his evaluation of a Smith cruise 2 section (Montgomery 1954). The procedures used in the analysis of Smith cruise 14 are, in effect, an extension of Montgomery's method, embodying, in addition to the methods he em- ployed, several further details suggested in his paper. An outline of the method of analysis will be given here; reference to the reports above (especially Montgomery 1954) should be made for detailed discussion and theoretical consideration. The basic representation of each oceanographic station was made on a graph embody- ing a temperature scale vs. scales of depth, salinity, and phosphate phosphorous, with lines of constant sigma-t corresponding to the temperature and salinity arguments also included on the graph (fig. 1). Temperature-depth curves for each station were first drawn, passing through the values obtained from the reversing thermometers but utilizing the shape of the station bathythermograph (BT) trace to define detail in the upper 270 meters. Temperature-salinity and temperature-phosphate curves (station "characteristic curves") were then drawn. Values from adjacent stations, and in many cases from stations two or three removed on the same longitude, assisted in construction of the curves between observed points; this was readily accomplished by superposing the station graphs on a light table during the drawing. The neces- sity for this procedure is evident in an examination of the vertical sections (see below). The depths sampled by the Nansen bottles (entered as dots on the sections) necessarily often miss some of the major features of the distributions, especially in the region of the thermocline, but by utilizing adjacent-station values the continuity of these features is not destroyed. The varia- tions in these characteristic curves with latitude were made as regular as possible, and, so far as the observed data permitted, maxima and minima were located on the same respective values of sigma-t from station to station. 1/ Certain results of the previous cruises in this series appear in reports by Cromwell (1951, ~ 1953, 1954), Austin (1954), and Montgomery ( 1954). Depths of sigma-t, salinity, and phosphate phosphorous at the desired isopleth values were taken from each station graph by referring to the common temperature scale and the temperature-depth curve, and each of these properties was plotted on a meridional section. Temperature sections to the same scales of latitude and depth were drawn for each station line of the cruise from the complete BT data (these were used in the analysis but are omitted here in favor of temperature sections with greater vertical exaggeration). The plots of sigma-t were then placed over the temperature sections on a light table and the isopleths were drawn, allowing the grosser detail of the temperature sections to influence the sigma-t sections between stations. The salinity and phosphate phosphorous plots were next drawn over the sigma-t sections in a similar fashion, with their isopleths made as nearly as possible parallel to the isopleths of sigma-t. In view of the extensive adjusting of the characteristic curves to obtain continuity between stations, the interpolated values of sigma-t, salinity, and phosphate phos- phorous plotted on the sections were followed exactly during the drawing. Values of temperature and salinity at standard depths were taken from the station graphs for use in the computation of geostrophic currents. The values of sigma-t tabulated at standard depths were computed from these temperature and salinity values, rather than read directly from the station graphs. The most evident new procedure, that of allowing the BT temperature sections to influence construction of the sigma-t (and thus salinity and phosphate) sections, has been instituted to achieve consistency between the distributions of temperature and density, and internal consistency in the method of analysis. It is certainly not argued that all the detail thus introduced into the density distribution is meaningful from every point of view, but a more accurate presentation of the observed field of density is obtained than by merely drawing smooth curves through the station points. As a practical consideration, it is a great deal easier to in- clude all the larger detail than to set up any arbitrary criterion for smoothing which would then have to be invoked continuously during the drawing of a section. The method of mutual adjustment of the station characteristic curves is based on the fact that the ocean is continuous; with the present sampling methods this assistance between stations probably gives a closer representation of actuality than methods treating each station individually. The location of the maxima and minima of these curves on continuous values of sigma-t from station to station, and the subsequent drawing of the isopleths of the salinity and phosphate sections parallel to the isopleths of sigma-t, are based on the consideration that flow in the ocean normally takes place along surfaces of constant potential density, which are closely approximated by surfaces of constant sigma-t, and that mixing of by far the greatest magnitude takes place along these same surfaces. Some Results of Hugh M. Smith Cruise 14 The station pattern for Smith cruise 14 appears in figure 2. The stations along 155 W. longitude consisted of casts to at least 1, 000 meters; to permit greater definition of detail in the region of interest the other two lines of stations were mostly limited to the upper 700 meters. Figures 3 to 7, describing the section at 155° W. longitude are in the sequence: dynamic topography, geostrophic currents, temperature, sigma-t, salinity. The second series of figures (8 to 11), describing the section at 180° longitude and the third (12 to 15), describing the section at 169 W. longitude are in the sequence: temperature, sigma-t, salinity, phosphate. he section at 169 W. longitude are in the sequence: tempera rigure 16 presents temperature sections on 155° W. , 169 W. o Figure 16 presents temperature sections on 155° W. , 169 W. and 180 longitudes, taken on the fishing station lines of John R. Manning cruise 11. The numbers and positions of the stations are indicated on each section. For each variable except temperature, the location of every observation carried in the tabulated data is indicated by a spot on the vertical section in question. When there was serious doubt regarding the accuracy of an observation, it was not used in constructing the vertical section, and a foot- note to this effect was entered in the tabulated data. Observations unquestionably in error were dropped from the tabulations. To make the relations between the different fields more readily apparent, selected isopleths of sigma-t have been plotted as dashed lines directly on the vertical sections of salinity and phosphate and on the section of computed currents. The two shallower isopleths roughly define the upper and lower boundaries of the strong density gradient in the thermocline. Comparison of the temperature sections from Smith cruise 14 and Manning cruise 1 1 indicates that changes in the minor features of the various distributions may be quite rapid; this has been verified by the results of a subsequent cruise (Smith cruise 15, Austin MS. ). For this reason no attempt has been made to plot any form of horizontal distribution over the area covered by Smith cruise 14. It should be noted that the winds, atypically, were predominantly north of east over the entire area during the period of the cruise (wind data are listed for each station in the tabulated data). Computed currents--As indicated above, only the stations along 155 W. longitude were of sufficient depth to permit computation of geostrophic currents relative to a 1, 000-meter level of no motion (except for three stations on 180° longitude, see below). The smoothed dynamic topographies of selected isobaric surfaces with respect to the 1 , 000-decibar surface are shown in figure 3; the computed dynamic heights at each station are carried as dots. Owing to equipment malfunctions the observations at 0°54'N. latitude and 3°00'S. latitude were too shallow to permit computation relative to this surface. From these smoothed topographies the currents shown in figure 4 were computed. The computation of currents becomes highly un- certain, and is not attempted, within 3 of the Equator. In the surface layer, the Equatorial Countercurrent is found from the northernmost station at 8° N. latitude to the southern boundary at 4° N. latitude; the Smith temperature section on this longitude (see below) indicates the northern boundary at approximately 9 30'N. latitude. The westerly South Equatorial Current is found south of 4° N. latitude. The three southernmost stations of the 180 longitude line reached 1,000 meters, and computation of dynamic heights of isobaric surfaces relative to this level have been carried out; these are carried in the tabulated data but not in a figure, and no currents have been computed. Examination of the data indicates that the currents in the surface layer were variable and perhaps partly easterly. Temperature— Temperature sections from the BT data appear in figures 5, 8, 12, and 16. As mentioned above, in order to reveal detail these sections were constructed with greater vertical exaggeration than those of the other variables. The BT casts from which the Smith sections were drawn were taken every 10 miles between stations, and every 30 miles along 155° W. longitude north of 8 N. latitude. Two casts were taken at each station; the one nearest in time to the hydrographic cast was plotted in each case. The Manning sections are drawn from casts made on the fishing stations only. The Manning fished one station per day; the date of the first station on each line is given in the figure caption. On each section the downward slope of the thermocline away from the Equator to north and south (as distinguished from the pronounced arching of the shallower isotherms at the Equator) is evidence of the distribution of mass associated with the South Equatorial Current, and the northward rise starting a few degrees north of the Equator is associated with the easterly flow of the Equatorial Countercurrent. Only the Smith section along 155 W. longitude extends beyond this into the North Equatorial Current, indicated by a second northward deepening. The southward rise of the isotherms at the southern end of the 180 longitude line is reflected in the weakening of the westerly flow indicated here by the computed dynamic heights of isobaric surfaces. These gross "topographic" features, associated primarily with zonal currents, are reflected in the distributions of each of the variables. In the surface layer, lower temperatures at the Equator are evidence of the addition of water from somewhat greater depths. As mentioned above, a comparison of the Smith and Manning sections indicates the possibility of quite considerable changes in detail in a relatively short time, although the basic features remain distinct. Surface temperatures are indicated at the top of each temperature section. These are often influenced by shallow diurnal effects, as at 3°15'N. latitude, 180° longitude (fig. 8); no attempt was made to include these temporary variations in the vertical temperature sections. Several of the details of the temperature field, such as the reduction of the vertical temperature gradient at the Equator and the presence of stable inversions in the thermocline near the Equator, are described in reports of this series (Cromwell 1954, Austin M3. ),, Both of these features are seen on each of the Smith cruise 14 temperature sections. A detail not so far described is the persistent, relatively "sharp" bottom of the main thermocline at about 2 - 3° N. latitude. Figure 17 presents BT traces taken near these latitudes on various longitudes during several cruises. The traces best illustrating the phenomenon were chosen from a meridional series in each case, demonstrating a remarkable consistency in position; it has moreover never been completely absent. Figure 18 gives the detailed temperature structure on the oblique section marked on figure 2. The most evident "sharp" bottom of the thermocline is seen at 2°06'N. latitude, 175°04'W. longitude; this particular trace is second from the left in figure 17. Sigma-t--The distribution of sigma-t is shown in figures 6, 9, and 13. These figures are closely associated with, and share most of the details of, the temperature sections (see procedures). For an exception to this rather general rule, the surface layer decrease in density at the southern end of the 180° longitude line is largely associated with a decrease in salinity. The expected slight east to west thickening of the light surface layer may be seen in a comparison of the sections. There is a marked meridional change in the character of the water just beneath the main thermocline in approximately the 2°-3° N. latitude region discussed above; this feature is reflected in the sharp southerly increase in depth of the sigma-t isopleths 26.6 to 27.0 in this area. The effect of this distribution of density is evident in the plot of dynamic heights of -iso- baric surfaces, but at these latitudes the interpretation is extremely uncertain. Barnes et al. (1948, p. 872) describe what seems to be a similar case in the Marshall Islands area, with regard to a dip in the 10 C. isotherm. Salinity — The vertical sections of the field of salinity are shown in figures 7, 10, and 14. The major features of the distribution are as described previously for the central Pacific (Cromwell 1951, 1953, 1954; Austin 1954; Montgomery 1954; Mao and Yoshida 1953). The similarity from section to section of Smith cruise 14 is self-evident. The detailed vertical distribution of salinity in the region of 2 -3 N. latitude is of interest and is not readily evident from the sections. Figure 1, illustrating the method of re- presenting the oceanographic stations is taken from Smith cruise 14, station 27 at 2°07'N. latitude, 179°57'W. longitude and shows this distribution. The sharp bottom of the thermocline seems to be a boundary between overlying water associated with a salinity minimum extending from the north, in the thermocline, and deeper water with a temperature-salinity relation identical with that of water to the south. The salinity decreases downward through the thermo- cline, increases sharply across the boundary, then decreases gradually again below; the relative maximum and minimum values lie close together vertically. With the isopleth values used, this feature is most evident on the 180° longitude section. The increase in salinity across the bound- ary is often accompanied by a stable temperature inversion. Farther to the north the vertical transition below the salinity minimum is more gradual; toward the Equator the salinity minimum disappears into the very salty water extending into the area from the south. Phosphate phosphorous — The vertical sections of inorganic phosphate for longitudes 180° and 169° W. appear in figures 11 and 15. On the 155° W. longitude line the colorimeter broke down after the first five stations; data for these are tabulated but no figure was drawn. A relatively higher concentration of phosphate about the Equator in the surface layer, seen on the two sections included, is evidence of enrichment from below. Acknowledgements Many more persons than can be mentioned here share the credit for the successful completion of Smith cruise 14, and for the publication of this report. The field party personnel during the cruise comprised: Garth I. Murphy (field party chief), E. D. Stroup, H. Yuen, T. Hida, and T. Roseberry. Special thanks must go to Captain Ralph Johnson and the crew of the Hugh M. Smith for the utmost cooperation with the scientific field party. Ashore, Mary Lynne Godfrey was largely responsible for the processing of the data. Tamotsu Nakata drafted the figures. T. S. Austin, T. Cromwell, and Dr. R. B. Montgomery gave invaluable assistance and encouragement in the many stages of preparation of the data, and during the writing of the report. LITERATURE CITED AUSTIN, T. S. 1954. Mid-Pacific oceanography III, transequatorial waters, August-October 1951. U. S. Fish and Wildlife Serv. , Spec. Sci. Rep.: Fish. No. 131. (In press). MS. Mid-Pacific oceanography V, transequatorial waters, May-June 1952, August 1952. BARNES, C. A., D. R. BUMPUS and J. LYMAN 1948. Ocean circulation in the Marshall islands area. Trans. Amer. Geophys. Union, Vol. 29, No. 6, pp. 871-876. CROMWELL, T. 1951. Mid-Pacific oceanography. January-March 1950. U. S. Fish and Wildlife Serv. , Spec. Sci. Rep. : Fish. No. 54, 9 pp. 1953. Circulation in a meridional plane in the central equatorial Pacific, Jour, of Mar. Res., Vol. 12, No. 2, pp. 196-213. 1954. Mid-Pacific oceanography II, transequatorial waters, June-August 1950, January- March 1951. U. S. Fish and Wildlife Serv., Spec. Sci. Rep.: Fish. No. 131. (In press). MAO, HAN-LEE and K. YOSHIDA 1953. Physical oceanography in the Marshall islands area. Univ. of Calif. , Scripps Inst, of Ocean. , Ref. 53-27, April 15, 1953. MONTGOMERY, R. B. 1954. Analysis of a Hugh M. Smith oceanographic section from Honolulu southward across the Equator. Accepted for publication in the Jour, of Mar. Res., Vol. 13, No. 1. MURPHY, G. I. and R. S. SHOMURA 1953. Longline fishing for deep-swimming tunas in the central Pacific, January-June 1952. U. S. Fish and Wildlife Serv. , Spec. Sci. Rep.: Fish. No. 108, 32 pp. Figures Frontispiece: Hydrographic platform and winch, Hugh M. Smith cruise 14. Photo by G. I. Murphy. 1. Example of station data representation. 2. Station positions. 3. Dynamic topography, 155 W. 4. Geostrophic currents, 155° W. 5. Vertical section of temperature, 155° W. 6. Vertical section of sigma-t, 155° W. 7. Vertical section of salinity, 155 W. 8. Vertical section of temperature, 180 . 9. Vertical section of sigma-t, ISO . 10. Vertical section of salinity, 180°. 11. Vertical section of phosphate, 180 . 12. Vertical section of temperature, 169 W. 13. Vertical section of sigma-t, 169 W. 14. Vertical section of salinity, 169° W. 15. Vertical section of phosphate, 169 W. 16. Vertical sections of temperature, John R. Manning cruise 11. 17. BT traces showing "sharp" thermocline bottom. 18. BT section showing "sharp" thermocline bottom. TEMPERATURE IN *C Fig. 1 — (Revised from Cromwell 1954). Example of graph used to present tem- perature, salinity, and phosphate data for each station. Points under inverted V represent reversing thermometer temperatures; plain points are taken from the station BT trace (both plotted against depth). Points in a square represent salinity, and those in a circle phosphate, both plotted against temperature. Smith cruise 14, station 27, 2°07'N. latitude, 179°57'W. longitude, February 19, 1952, 1336 GCT. 180° 175° 170° 165° 160° 155° Fig. 2 — Hydrographic station positions. Hugh M. Smith cruise 14, January-March 1952. Location of temperature section in figure 18 indicated by dashed line. o 2 0 1.8 16 1.4 1.2 1.0 8 6 .4 2 0 J I L J I L J L J L J L 0° S- LATITUDE -N Fig. 3 — Smoothed anomaly of topographies of isobaric surfaces rel- ative to the 1,000-decibar surface. Hugh M. Smith cruise 14, stations 1-16, 155° W. longitude, 8° N. to~7° S. latitude, Jan- uary-February 1952. Points represent computed values. 16 15 14 13 12 II 10 9 8 7 6 5 4 3 2 I 100 - 80 80 80 80 80 2'07'N 2"06'N 2*43'N 7 2-50'N 2'H'N 1 2"47'N 2°I0'N / 2*33'N II 1 3*I7'N I79«57'W I75°04'W 171'58'w 169'07'W ise'os'w 155'12'W I5l*<»5'w I39°58'W 1 130'28'W _ 200 Q- 19-52 D-24-52 SD-9-50 01-11-52 2D-3I-50 D-3-52 D-3-52 1-31-52^ _---' HD-ll-52y K UJ UJ 400 / *° j 60 u. J** TO ^ — t" ^ ^ /* r^X^-^^ X 1- 600 /"TO r- -£ p K r — 60 ™( 60 f 6 0. •"60 j J UJ a 800 innn Fig. 17 — BT traces illustrating the "sharp" thermocline bottom, 2°-3° N. latitude. Temper- atures in degrees F. , depths in feet. Position and date of each trace indicated. 17 ** \ ,0* M \^ *%v \ \ V \ y- | Vy \A ^~~ /' N / *s^ / ^ *J r* ~v S^ ^"^ / / I ^^ ^ 1 „'' / / s \J*^ 1 / / / /\ ""* /. y •— tS / / a. / / v / ^ / / / \ / / ^ '\ ,A6 Fig. 18— Isometric block diagram presenting BT traces, 3°25'N. latitude, 175°57'W. longitude to 0°46'N. latitude, 174°05'W. longitude, Hugh. M. Smith cruise 14. Depth in feet, temperature in degrees F. 18 NOTES ON THE TABULATED DATA In every case, any variation from the standard 13-bottle cast has been explained in a footnote. Where more than one cast was made on a station, they are divided in the observed data by a horizontal line. The later casts are the deeper ones except on station 34, where a single bottle was recast. Where the corrected paired protected thermometer readings differed by more than 0.05 C. , the depth and salinity are repeated, and both temperature values are carried. Sigma-t values calculated using each temperature are tabulated. Where the duplicate determinations of inorganic phosphate differ in extinction value by more than 0.01 (approximately 0.05 ug at/l), the depth is repeated, and both phosphate values are carried. Weather is recorded in the ww (present weather) code given in the U. S. Weather Bureau Circular M, eighth edition. Manual of Marine Meteorological Observations. Cloud coverage is given in tenths of sky. Wind velocity was measured with an anemometer 30 meters above the sea surface. The direction (given to the nearest 10 ) is that from which the wind was blowing, measured clockwise through 360 from north. No phosphate determinations were made from station 6 to station 16, and no data interpolated at standard depths for dynamic computations after station 19 (see text). 19 STATION 1 M/V Hugh M. Smith: Cruise 14, 7°57,N, 154°57'W. January 27, 1952. Messenger time: 2103 GCT. Weather: 03, cloud cover- age 7. Wind: 070°, 24 kt. Sea: 5-8 ft. Wire angle: 12° OBSERVED DEPTH T S g at/1) 05 27.45 35.49 22.96 0.92 20 27.48 35.50 22.96 0.95 39 27.39 35.48 22.97 0.96 76 27.16 35.48 23.05 0.99 146 22.26 35.25 24.35 1.21 217 15.87 35.13 25.90 1.56 289 11.70 34.85 26.55 2.30 355 10.88 34.79 26.65 2.62 426 9.60 34.72 26.79 2.88 503 8.52 34.67 26.95 3.05 576 7.39 34.61 27.08 3.17 576 7.43 34.61 27.07 715 5.88 34.54 27.23 3.16 866 4.82 34.53 27.35 3.24 l/ Underway to reduce wire angle. 41 STATION 26 M/V Hugh M. Smith: Cruise 14, lo03'N, 179°58'W, February 19, Messenger time: 0346 GCT. Weather: 03, cloud cover- 1952. age 8. Wind: 090^ 13 kt. Sea: 1-3 ft. Wire angle: 49 OBSERVED DEPTH T S 0"t 02 PO4-P (m) (°C) (°/oo) (g/1) (ml/1) (M« at/1) 04 28.03 35.49 22.77 0.91 15 27.58 35.47 22.91 0.93 ?6 27.50 35.47 22.93 0.94 49 27.50 35.48 22.94 0.94 88 27.48 35.46 22.93 0.91 121 27.36 35.42 22.94 0.93 148 26.63 35.27 23.06 0.96 172 19.37 34.88 24.85 1.24 200 16.22 35.08 25.78 1.51 233 12.52 34.69 26.27 1.75 266 11.46 34.30 26.55 2.18 333 10.66 34.76 26.67 2.50 430 9.36 34.69 26.84 2.71 STATION 27 M/V Hugh M. Smith: Cruise 14, 2°07'N, 179057'W, February 19, 1952. Messenger time: 1336 GCT. Weather: 02, cloud coverage not recorded. Wind: 070°, 14 kt. Sea: 3-5 ft. Wire angle: U 25° OBSERVED DEPTH T S *t O2 PO4-P (m) (°C) (°/oo) (g/D (ml/1) ("g at/1) 04 27.35 35.34 22.72 0.82 18 27.86 35.35 22.72 0.85 36 27.74 35.34 22.76 0.87 74 27.68 35.36 22.79 0.88 144 26.78 35.22 22.98 1.03 214 11.72 34.63 26.37 2.09 283 10.96 34.79 26.64 2.45 348 10.30 34.75 26.73 2.58 418 9.72 34.73 26.81 2.74 493 8.90 34.66 26.89 2.83 564 8.14 34.61 26.97 2.84 564 3.19 34.61 26.97 703 6.14 34.53 27.19 3.26 855 4.92 34.53 27.33 3.28 1/ Underway to reduce wire angle. 42 STATION 28 M/V Hugh M. Smith: Cruise 14, 3°06'N, 179°57'W, February 19. 1952. Messenger time: 2237 GCT. Weather: 02, cloud cover- age 8. Wind: 100°, 7 kt. Sea: 1-3 ft. Wire angle: 32° OBSERVED DEPTH T S