Larval Fish Identification from Cruises at the Hancock Seamounts, TC-84-05 and TC-85-01

Ichthyoplankton sampling was conducted aboard the NOAA vessel Townsend Cromwell in 9-29 July 1984 and 4-10 February 1985. Collectors included George Boehlert, James Uchiyama, Robert Humphreys, Randolph Chang, Alan Everson, Victor Honda, Bert Kikkawa, Raymond Clarke, Thomas Kazama, and Michael Seki. Two locations were sampled intensively during these cruises; one location was over the seamount summit ("seamount" location); the other location was 20 km west, over a water depth of approximately 1,800-4,750 m ("reference" location).

Ichthyoplankton was sampled with an opening-closing Tucker trawl equipped with three nets and a double-release mechanism operated by messengers. The nets were 0.333 mm mesh (Nitex) with a 1.4 m2 mouth area. Ship speed was adjusted over tow speeds of about 0.9-1.1 m/second to maintain a wire angle at 45?. At this angle, the effective mouth area of the Tucker trawl is 1.0 m2 ; tow depths were estimated as a function of wire angle and meters of wire out. Four discrete depth strata (0-25, 25-50, 50-100, and 100-200 m) were sampled. Replicate tows at each depth comprised a sampling series. To sample at discrete depths without contamination by animals from shallower depths, the trawl was lowered with the first net open, and the second net was opened for the desired sampling time and then closed, and the trawl was retrieved with the third net open. To sample the two shallower strata, the trawl was lowered to 50 m, the second net was opened at that depth, and then the third net was opened at 25 m to sample the shallow stratum. For the two deeper strata, the second net was closed at the upper end of each stratum, and the trawl was retrieved with the third net open. Thus, a full set of duplicate samples for each depth stratum required six deployments of the trawl. The summer cruise had two series of night sampling (9-10 and 28-29 July) and one of day sampling (14-15 July); the winter cruise had one series of day and night sampling (4-10 February 1985). Each net was fished for 18 minutes and tows were conducted in a stepped oblique fashion, in an attempt to sample depths equally within each stratum. The volume of water filtered was estimated with calibrated General Oceanics flowmeters mounted in the center of each net. Plankton samples were preserved at sea in a 4% buffered formaldehyde seawater mixture. In the laboratory, plankton volume was determined from a known total volume minus the remaining water volume after the plankton were strained (Omori and Ikeda 1984); gelatinous plankton and fishes larger than approximately 50 mm were removed before the volume was determined. Whole samples were sorted for fish eggs, larvae, and squid para larvae under a dissecting microscope. Larval fishes were identified to the lowest possible taxonomic level by Bruce Mundy.

To compare ichthyoplankton species composition, distribution, and abundance in the Hancock Seamounts.

3,072 records

The data are available in Boehlert & Mundy (1992. Distribution of ichthyoplankton around Southeast Hancock Seamount, central North Pacific, in summer 1984 and winter 1985: data report. NOAA Tech. Memo. NMFS, NOAA-TM-NMFSSWFSC-176: 1?109).

SOURCE ID: RB006

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Data was collected in the field by PIFSC staff, and entered in table format into electronic spreadsheets.