John H. Wormuth

Vertical distributions and diel migrations of Euthecosomata in the northwest Sargasso Sea

Vertical distributions and seasonal variations in abundance of nine abundant or frequent pteropod species or subspecies in the northwest Sargasso Sea are described. Factor analyses yielded two groups, diel migrators and non-migrators. In terms of water column abundances, tows taken in August and November are similar, as are tows in December and April. Most species show significant within-species agreement in depth distribution over the year but high variability in abundance. Regression analyses using environmental parameters as independent variables show significant correlations of species abundances with temperature.

A LARGE SWARM OF EUPHAUSIA SUPERBA: OVERVIEW OF PATCH STRUCTURE AND COMPOSITION

ABSTRACT Twenty-five MOCNESS (Multiple Open/Closing Net and Environmental Sensing System) tows were taken in and around a large swarm of Euphausia superba near Elephant Island. Krill from all 25 tows (201 samples) and the copepods from 9 tows (63 samples) have been analyzed. Significant agreement in rank order of abundance is found for both krill and nonkrill fractions. Most copepod species show significant covariation; there is less significant covariation among euphausiid categories. Larvae of E. superba and Thysanoessa spp. cooccur and show strong positive covariation. There are large day-night differences in catches of both adult and, to a lesser degree, larval euphausiids. Vertical boundaries between larval and adult E. superba are often sharp, and larvae may occur either above or below significant adult concentrations.

Distribution of antarctic zooplankton around Elephant Island during the austral summers of 1988, 1989, and 1990

Year to year variation and vertical distributions of epipelagic Zooplankton around Elephant Island and King George Island were examined with samples collected with bongo nets and a 1 m2 MOCNESS during the austral summers (Jan–Feb.) of 1988, 1989 and 1990. Copepods were the major components of epipelagic Zooplankton (in numbers) with dominance of Metridia gerlachei (1988 and 1989) and small calanoids and cyclopoids (1990). Euphausiids and salps were the next most abundant groups. The percent composition of euphausiids decreased from 1988 to 1990 while that of salps increased. The abundance of salps exceeded euphausiids and major taxa of copepods in 1990. Local patches of polychaetes and amphipods were also found. Statistically significant annual variations with increased numbers in 1990 were found by analyses of variance in total abundance, abundances of copepods, salps, chaetognaths and amphipods, but abundances of euphausiids, polychaetes and fishes showed no significant annual variations. When the study area was divided geographically, horizontal variability in abundance within each year showed no significance in total abundance, abundances of copepods, euphausiids, amphipods and fishes, but significance in salps, polychaetes and chaetognaths. Results of site clustering based on covariances of abundances of eleven major taxa were well matched, though not perfect, with the distribution of surface water temperatures which could be used as a tracer of source water masses suggesting that spatial variation was related to hydrodynamic conditions. Factor analyses showed that annual and spatial variations in abundance were mainly caused by only two taxa, Metridia gerlachei and salps (mostly Salpa thompsoni). These two taxa were responsible for about 60% and 30% of total variance, respectively, and were useful indicators of the interannual variation. That is, 1988 and 1989 were the years of M. gerlachei, and 1990 was the year of salps. From vertically stratified MOCNESS samples, it was shown that the major taxa in this study area were active vertical migrators. While most samples obtained by relatively shallow tows (uppermost 100 m depth) were composed of exclusively one or two taxa, those from relatively deep tows (down to 200 m) showed various patterns of vertical stratification suggesting that the patterns of vertical migration were species specific. Species specific ontogenetic vertical migration associated with elevated habitat temperatures also seemed to be responsible for the annual variation in zooplankton distribution in the upper water column.

The role of cold-core Gulf Stream rings in the temporal and spatial patterns of euthecosomatous pteropods

Abstract Diel vertical distributions and seasonal variations in abundance of three species of euthecosomatous pteropods characteristics of the Slope Water Region and two warm-water species are presented for two cold-core rings of the Gulf Stream. The cold-water species, Limacina retroversa, disappears from the cold-core rings within the first 5 months. One warm-water species, L. inflata, shows population increases of more than an order of magnitude within 5 months and then a decline to background Sargasso Sea levels. Daytime depths for this latter migratory species suggest a shallower migration in young to middle age cold-core rings.

Antarctic krill stock distribution and composition in the Elephant Island and King George Island areas, January–February, 1988

Information is provided on the distribution, size and maturity composition of Antarctic krill (Euphausia superba) stocks in the Elephant Island and King George Island areas, and at repeatedly sampled sites to the north of each island, during January–February, 1988. The overall distributional patterns of different sizes and maturity stages demonstrated a seasonal progression of those observed in the Antarctic Peninsula region during November–December, 1987 by Siegel (1989). The krill sampled at each island site represented different size-maturity groups and demonstrated different horizontal and vertical distributional characteristics. These distributional differences may be related to the demographic differences and/or hydrographie differences between the two sites.

Trace elemental patterns in Humboldt squid statoliths from three geographic regions

Statolith trace elemental concentrations can be used as natural tracers to better understand life history and stock structure of squid. A highly variable Humboldt squid population was targeted to determine if ontogenic patterns could be revealed and to compare variations among squid collected from three geographic areas. Statoliths from Humboldt squid collected off Westport, WA; Oceanside, CA; and the Galapagos Islands were sampled for ten trace elements using laser ablation inductively coupled plasma mass spectrometry. Similar patterns were found for some elements despite geographic collection site distances indicating a potential to explain ontogenic shifts experienced by this species. Other elements displayed high variation in spatial pattern, which suggests the potential to distinguish among the squid collected from the three geographic regions and better understand stock structure. Within the California and Washington squid, eight of the elements varied in relation to calcium across the statolith with only yttrium and zirconium remaining statistically invariant. In addition to these two elements, copper and zinc also did not vary significantly across the statoliths collected from Galapagos squid. All elements demonstrate potential to influence a multivariate elemental fingerprint, which may provide a useful measure of population discrimination needed for stock assessment.

Acoustic estimates of zooplankton and micronekton biomass using an ADCP

A calibrated 153‐kHz narrow‐band ADCP (Acoustic Doppler Current Profiler) has been used to collect acoustic backscatter intensity (Sv) data during several cruises in the Gulf of Mexico. Data have been gathered both while on station and while underway along transects through different hydrographic regimes, enabling an examination of both temporal and spatial trends in backscatter. In addition, zooplankton and micronekton stocks were intensively sampled with a 1 m2 MOCNESS (Multiple Opening‐Closing Net Environmental Sensing System). Empirical correlations between spatial and temporal variations in Sv and in standing stocks of zooplankton/micronekton will be presented. Sv measured with an ADCP may be useful as an index of zooplankton and micronekton biomass. The location and abundance of these food stocks are hypothesized to be important in determining the distribution and abundance of marine cetaceans in the Gulf of Mexico. In addition, Sv‐derived estimates of such stocks may allow inferences about seconda...

Sample variability of zooplankton in the nearshore off Louisiana with consideration of sampling design

Abstract Variability in zooplankton samples was examined to identify a proper sampling design for unbiased estimates of zooplankton abundances. Samples were selected in the nearshore about 16 km south of Louisiana during one night and 2 days in October 1985 using a 1 m2 multiple Opening/Closing net and Environmental Sensing System fitted with 0.333 mm mesh nets. Data obtained from 21 tows of three different tow lengths at mid depth (about 5 m, water depth 10 m) were analysed. There seemed to be different patterns of vertical migration and these vertical migrations were shown to explain about 75% of total sample variability in the study area. These were: usual vertical migration (Centropages velificatus, Chaetognatha, Eucalanus spp., Phialidium spp., Paracalanus spp. and Temora turbinata), weak vertical migration with elapsed phase (Doliolida A and Oikopleura spp.), and reversed vertical migration (Acartia tonsa). The relationship between mean abundances and tow distance was weak, but the variance of the abundance estimates showed an exponentially decreasing trend with an increase of tow distance when populations were at their maximum, probably due to vertical migration. From nonlinear regression analyses with the model (variance)= A + B ec(tow distance), the minimum tow distance that would provide a stabilized variance of abundance estimate was determined. It varied among taxa from 43 to 140 m with an average of 80 m. Vertically stratified sampling with a minimum tow distance of about 140 m is suggested as a proper sampling scheme for the unbiased estimation of abundances in a nearshore environment like the sampling site of this study.

Statolith extraction method improvements for use in microchemistry studies with laser ablation inductively coupled plasma mass spectrometry

Major hindrances for most scientific studies involve a lack of funding, sampling, or equipment. Without large, high resolution sample sets spanning significant spatial scales, long temporal scales, or both, it can be difficult to discern accurate results. This can be the case for some population studies and fisheries management efforts. One novel approach to squid population studies involves harvesting statoliths, but the power of the studies is limited by the scales at which samples are collected. Approaches that involve the characterization of trace elemental chemistry in these aragonite structures could provide a better understanding of the life cycles and behaviors of targeted fisheries species than the common practice of length-frequency studies. Often statoliths are not collected and the absence of consistently periodic or geographically distinct samples potentially limits large scale population studies. One possible reason statoliths are not always removed may be the lack of a thorough, clearly illustrated instructional method on dissection of the statoliths from the cranial cartilage. In an effort to encourage sampling of statoliths, previously published methods are improved upon in this work by including a description of the dissection of the statoliths using layman terms accompanied by detailed photographs. The methods were tested successfully with the Ommastrephid squid Dosidicus gigas and the statoliths obtained will be used to examine trace element variations determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).