Technical Report No. 273 1971

UBlIAII, "",'., .,w,~.~.·_, ....... FISHERIES RESEARCH BOARD OF CANADA TECHNICAL REPORT NO. 273 1971 FISHERIES RESEARCH BOARD OF CANADA Technical Reports FRS Technical Reports are research documents that are of sufficient importance to be preserved, but which for some reason are not appropriate for scientific pUblication. No restriction is placed on sUbject matter and the series should reflect the broad research interests of FRB. These Reports can be cited in pUblications, but care should be taken to indicate their manuscript status. Some of the material in these Reports will eventually appear in scientific pUblication. Inquiries concerning any particular Report should be directed to the issuing FRB establishment which is indicated on the title page. FISHERIES RESEARCH BOARD OF CANADA Technical Report No. 273 Oceanographic Features of and Biological Observations at Bowie Seamount, 14-15 August, 1969. by R. H. Herlinveaux Pacific Environment Institute 4160 Marine Drive West Vancouver. B.C. September, 1971 TABLE OF CONTENTS Page Introduction . Conclusions ·.·····.··· .. Acknowledgements . References . Lis t of Figures . Figures 1- 20 •••.•......••..•.....•.....•.......•. 8-27 Tables 1-3 •.•••....•..•...........•.•• ········•·· 29-33 Appendix 1 - Bathythermographs ....•.....•........ 35 Introduction During August 14-15, 1969, an oceanographic program was carried out on and over Bowie Seamount as part of a co-operative study with the Defence Research Establishment Pacific to deter mine the feasibility of using this and other seamounts (eg. Union or Cobb)--Fig. l--as oceanic observation platforms. Data from hydrographic and biological sampling in this program are presen ted in this report. (A preliminary bathymetric study (Fig. 2) had been carried out earlier (March 25-29, 1969); the results have been reported by 5crimger and Bird (1969). Physical Oceanography (a) Water Properties: Data on salinity, temperature, and nutrients--silicate (5i03), nitrate (N03), and phosphate (P04)--were gathered by hydrographic casts employing Nansen bottles, at stations along two transects over the seamount (Fig. 2). Temperatures were obtained from deep-sea reversing thermometers attached to the bottles; bathythermographs were also employed. Salinity determin ations were carried out by means of an inductive-type laboratory salinometer (Brown and Hamon, 1961). The nutrient samples were frozen, and later analysed at the Nanaimo Laboratory, by means of an auto-analyser (Stephens, 1970). The results are presented in Table 1 and Appendix 1. The vertical salinity and temperature structures of the waters overlying, and in the vicinity of the seamount are shown in Fig. 3. The vertical sections of temperature, salinity, density, and nutrients (P04, 5i03 , N03) to a depth of 250 m along the transects are presented in F1g. 4 and 5. The structures and distributions of properties reported here are consistent with those reported in earlier research (Bennett, 1959; Dodimead et al., 1963). The vertical ~emperature stratification between- depths of 25 and 50 m defines the position and magnitude of the seasonal thermocline at the times of sampling (Fig. 4A). It basically denotes the limiting depth of surface-induced heating and mixing processes. Below the seasonal thermocline, a temperature-minimal stratum, with values ranging from 5.3 to 5.5 C., was present at about 109 m depth. The stratum is considered to represent the remnants of water cooled during the previous winter. (Dodimead, et al., 1963). A temperature-maximum stratum with values ranging trom5.8 to 6.0, at about 150 m depth, is evident on most of the bathythermograms (Appendix 1) as well as in Fig. 3. The maximum - 2 - is most pronounced at Station 4; in this instance it is located on the southeast side of the seamount (Fig. 4A). The temperature structure over the seamount itself has a different character; the minlmum- and maximum-temperature strata were found not to be con tinuous. The lowest minimum temperature occurred south of the seamount (Station 4 and 5); the highest maximum temperature on the southeast side (Station 4). Below 150 m the temperature decrea ses slowly with depth. In the vertical distribution of salinity (Fig. 3) two haloclines generally appear to be present. The shallower, smaller, halocline, which is coincident in depth with the seasonal thermo cline, is a summer feature attributed to the addition of fresh water which is served in the 'surface layer' above the thermocline (Dodimead et al., 1963). The top of the permanent halocline occurs at a depthof"""'ibout 100 m and is coincident with the temperature minimum. The surface layer varies in depth between 17 and 30 m.; it was deepest at Station 4, and shallowest at Station 2 (Fig. 3). The sections involving the nutrients provide an indica tion of the gradients associated with these properties in the area. From the surface to about 30 m depth, nutrient concentra tions are low, probably as a result of utilization associated with primary production in this area. A relatively large increase in the amount of all nutrients measured occurs between 30 and SO m coincident WIth the thermocline and secondary halocline. Below SO m the nutrient concentrations increase more gradually with depth. A slgnlflcant feature 15 the relatively marked downward slope of the Isopelths across the seamount, from Station 2 to Station 4, in the upper 100 'ro. This slope occurs at least over a distance of 2.5 miles and may be a result to some degree of upwelling associated with flow in the vicinity of and across the seamount. (b) Current Observations: The research ship was anchored by the bow about 60 m. over the seamount at a position (~) shown in Fig. 2. Three velocity profiles were obtained using a Hytech lISavonius-rotor" type current meter (Fig. 6). In addition, continuous observations of the currents at 20 m depth were made for approximately 11 hours; these data are presented in Fig. 7 and listed in Table 2, and were obtained from the Current meter recorder paper which had been di vided into 36 equal intervals. Also, the ships head was recor- ded in order to determine the direction of surface drift (Table 3) . The data indicate that the surface movement during the sampling period was easterly (roughly diagonally across the sea mount). Consistency is evident between values from ships head ing and from meter during the same time, ego Fig. 6B and 6C. At 20 m (Fig. 7) the current rotates anti-clockwise from NE through north-to-south over an II-hour period. The flow varied from 0.25 to 0.50 knots. Using the predicted times for flood and ebb tide at Tofino (Fig. 7C). it would appear that the flood sets generally northerly at about 0.5 knots, and the ebb generally southerly at about 0.4 knots at Bowie Seamount. Satellite Navigation The ship was anchored as previously mentioned in about 60 m with about 160 m of anchor chain out. The satellite naviga tion system (Magnavox Dual Channel 702 GA) was Tun continuously. The 10 positions computed during the period at anchor are dis played in Fi~. 8; the black dot encompasses three positions. The figure thus lndicates the scatter in computed positions that can be expected by one of these systems from ,a slowly-rolling ship at anchor. Biological Oceanography While the ship was anchored on the seamount, the ship's Simrad 11 kHz echo sounder was operated to observe echo character istics. At the same time the ship's crew fished with hand jigs. Examples of the echo sounder traces which indicate a shallow scattering layer are presented in Figure 9. A diurnal vertical migration of such a layer is usually noted in most areas of the ocean; however, it is not evident over Bowie Seamount (60 m in depth) . Several hundred pounds of fish were caught by the crew and frozen. They were later identified, and their stomach con tents analysed, by staff of the Fisheries Research Board, Biological Station, Nanaimo. The fish which were caught consisted primarily of three species: Sebastodes entomelas (widow rockfish), Sebastodes ruberrimus (red snapper), and Sebastodes prioriger (red stripped rockfish). Figure 10 shows the age- and length-frequency distri bution of Sebastodes entomelas, which suggests that these fish are a complete population, because there is a complete range of ages and sizes which brackets the known average length (400 mm) at maturity (Westrheim (private communication)). Further the age and length frequency distribution shown in Figure 10 are well with in the known size distribution of S. entomelas population for the coast of British Columbia . (Westrheim et al .• 1968). The analysis of the stomach contents of the rockfisn-snow they are cannabalistic feeding predominately on juveniles. - 4 - After sampling the fish in the scattering layer the Navy divers carried by the ship, swam down to a peak of the seamount at about 90 to 120 feet depth to collect samples of rocks and bottom organisms, to photograph the fish life in the scattering layer and the bottom forms, to observe and describe the topography and note swell characteristics near the bottom. Their findings are presented. (a) Rock samples, floral and faunal forms: Several large rock samples collected on the seamount were covered with floral and faunal forms. Dr. D. B. Quayle. Biological Station. Nanaimo. B.C. took samples of the forms for analysis and reporting. The rock samples were sent to Mr. R. H. Herzer. University of British Columbia. who has been working on the geology of Bowie Seamount and his results are being prepared for publication. Dr. R. F. Scagel. University of British Columbia is preparing a brief note on the floral forms found on the sea mount; the interest in these samples is due to the fact that the forms at 100 feet are usually those found in the intertidal zone. The fact that these forms occur at this depth could be due to one or more factors·-such as secular changes in sea level.

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