James E. Craddock

New development in the MOCNESS, an apparatus for sampling zooplankton and micronekton

Four variants of the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) have been constructed to sample a broad size spectrum of oceanic animals from microzooplankton to micronekton. The systems differ in mouth opening dimensions (ranging from 1/4 to 20 m2), the number of nets carried (from 5 to 20), and the mesh size of the netting (from 64 μm to 3.0 mm). A new electronics package enables an operator to send commands down a single conductor, armored cable to open/close the nets and provides 12-bit resolution for the environmental (temperature, depth, conductivity) and net operation data (flow, net-frame angle, net-bar release), which are transmitted up the cable to the deck unit at 2-s intervals. A microcomputer system, interfaced to the deck unit, calculates salinity, volume filtered by a net, net trajectory velocity, and vertical velocity. The data are printed out and stored on disc, and profiles of temperature and salinity versus depth are plotted during the course of the tow. Analysis of the relationship between the geometry of the MOCNESS under tow and the past and present methods used to estimate the water filtered by a net revealed that significant bias is introduced when the ascent or descent angle of a net is disregarded. The bias is a function of the ratio of vertical velocity to net trajectory velocity and results in an underestimate of volume filtered while shooting a net and an overestimate while hauling.

Mesopelagic fishes and thermal fronts in the western Sargasso Sea

Analysis of 25 midwater trawl collections, disposed along the meridian 70°20′W from off Hispaniola to the Gulf Stream, showed a change in the mesopelagic fish fauna at about 27°N. The point of faunal change corresponded to a change in the temperature structure of the upper part of the water column, i.e., at a (the?) so-called “thermal front”, perhaps identical to the so-called “North Atlantic subtropical convergence”. Of 44 species occurring in four or more collections, 13 species were collected only to the north of the front and 1 species only to the south. By most criteria, north-of-the-front collections were larger than southern ones. This is in accord with the north-south difference in primary production noted by other workers which, in turn, seems atributable to the north-south difference in temperature structure. To the north the upper part of the water column is well stratified in summer only, while to the south it is well stratified at all seasons. Taken altogether, what is now known suggests that the thermal front logically divides the upper Sargasso Sea into northern and southern portions that differ in many ways.

Ceratoscopelus maderensis: Peculiar Sound-Scattering Layer Identified with This Myctophid Fish

A sound-scattering layer, composed of discrete hyperbolic echosequences and apparently restricted to the Slope Water region of the western North Atlantic, has been identified from the Deep Submergence Research Vehicle Alvin with schools of the meyctophid fish Ceratoscopelus maderensis. By diving into the layer and using Alvins echo-ranging sonar, we approached and visually identified the sound scatterers. The number of echo sequences observed with the surface echo-soutnder (1/23.76 x 105 cubic meters of water) checked roughly with the number of sonar targets observed from the submarine (1/7.45 x 105 cubic meters). The fish schools appeared to be 5 to 10 meters thick, 10 to 100 meters in diameter, and on centers 100 to 200 meters apart. Density within schools was estimated at 10 to 15 fish per cubic meter.

Biomass of the micronekton in Gulf Stream ring 82-B and environs: changes with time

Micronekton biomass was sampled in warm-core Gulf Stream ring 82-B (at ages 2, 4 and 6 months), the Slope Water, the Gulf Stream and the Sargasso Sea during three multi-ship cruises. There was no significant diel difference in the 0–1000 m integrated biomass for fishes or invertebrate micronekton in 24 paired day-night tows, and no evidence for diel vertical migration across the 1000 m level. Biomass showed a consistent upward shift at night; although it varied with hydrographic regime and sampling date, median biomass depth shift for invertebrates averaged 184 and 137 m for fishes. For total micronekton, median biomass depth ranged from 550 to 750 m by day and 300 to 600 m by night. Fish biomass was always centered deeper than 250 m, while the center of invertebrate biomass rose above that level in only one of 12 night tows. In April, ring-core biomass for fishes and invertebrates (2.2 and 11.9 cc m−2, respectively) was significantly lower than that for the high velocity region (3.6 and 19.8 cc m−2) or that for the Slope Water (4.7 and 20.8 cc m−2). By June, ring-core standing stocks had doubled while that for the high velocity region had been reduced to half of the April levels. No significant changes in Slope Water biomass occurred over the April–August period. Fluctuations in ring-core micronekton biomass were probably due to both in situ processes and advective exchange. On the average, invertebrate biomass was about 4 times that of fish biomass. Total micronekton was 20–30% of the macrozooplankton biomass in April and June, and 7–16% in August. Vertically, the invertebrate fraction overlapped the macrozooplankton more than did the fish fraction. Greater overlap of micronekton and macrozooplankton occurred at night.

Vertical distribution and species composition of midwater fishes in warm-core Gulf Stream meander/ring 82-H

Abstract The integrated abundance and vertical distribution of midwater fishes in the upper 1000 m of warm-core Gulf Stream ring 82-H show that the fauna of the ring was very similar to that of the northern Sargasso Sea. The data, indicate that warm-core rings have a large impact on the fauna of the Slope Water even though only a small fraction of the volume of rings is mixed into the Slope Water and the fishes (mostly non-migratory) are located in the “unreactive” parts of the ring.

Midwater fish data report for warm-core Gulf Stream rings cruises 1981-1982

Funding was provided by the National Scten.ce Foundation under Grant Numbers OCE 80-17270 and OCE 86-20402.

Food habits of Sowerby’s beaked whales (Mesoplodon bidens) taken in the pelagic drift gillnet fishery of the western North Atlantic

This paper is not subject to U.S. copyright. The definitive version was published in Fishery Bulletin 111 (2013): 381-389, doi:10.7755/FB.111.4.7.

Midwater fish data report : cold core rings time series cruises, Knorr 62, 65, and 71 and Endeavor 11

This work was done under Contract N00014-79-C-0071 NR083-004 and its predecessors with the Office of Naval Research.