Jeffrey C. Drazen

The influence of depth on mercury levels in pelagic fishes and their prey

Mercury distribution in the oceans is controlled by complex biogeochemical cycles, resulting in retention of trace amounts of this metal in plants and animals. Inter- and intra-specific variations in mercury levels of predatory pelagic fish have been previously linked to size, age, trophic position, physical and chemical environmental parameters, and location of capture; however, considerable variation remains unexplained. In this paper, we focus on differences in ecology, depth of occurrence, and total mercury levels in 9 species of commercially important pelagic fish (Thunnus obesus, T. albacares, Katsuwonus pelamis, Xiphias gladius, Lampris guttatus, Coryphaena hippurus, Taractichthys steindachneri, Tetrapturus audax, and Lepidocybium flavobrunneum) and in numerous representatives (fishes, squids, and crustaceans) of their lower trophic level prey sampled from the central North Pacific Ocean. Results indicate that total mercury levels of predatory pelagic fishes and their prey increase with median depth of occurrence in the water column and mimic concentrations of dissolved organic mercury in seawater. Stomach content analysis results from this study and others indicate a greater occurrence of higher-mercury containing deeper-water prey organisms in the diets of the deeper-ranging predators, X. gladius, T. obesus, and L. guttatus. While present in trace amounts, dissolved organic mercury increases with depth in the water column suggesting that the mesopelagic habitat is a major entry point for mercury into marine food webs. These data suggest that a major determinant of mercury levels in oceanic predators is their depth of forage.

Aggregations of egg-brooding deep-sea fish and cephalopods on the Gorda Escarpment: a reproductive hot spot.

Localized areas of intense biological activity, or hot spots, in the deep sea are infrequent but important features in an otherwise sparsely occupied habitat (1). Hydrothermal vents, methane cold seeps, and the tops of seamounts are well documented areas where dense communities persist for generations (2–5). Reproductive aggregations where conspecifics concentrate for the purposes of spawning or egg brooding could be thought of as transient hot spots. It is likely that they occur in populations with low densities to maximize mate location and increase reproductive success (6). However, only a few deep-sea reproductive aggregations have ever been documented (7–9), demonstrating the paucity of present-day information regarding reproductive behavior of deep-sea animals. In this paper we describe a unique multispecies reproductive aggregation located on the Gorda Escarpment, California. We document some of the highest fish and octopus densities ever reported in the deep sea, with most individuals of both species brooding eggs. We describe the nesting behavior of the blob sculpin, Psychrolutes phrictus, and the egg-brooding behavior of an octopus, Graneledone sp. observed during annual dives of a remotely operated vehicle (ROV) on the Gorda Escarpment. The animals are concentrated at the crest of the local topography and near cold seeps where they may benefit from enhanced current flow and local productivity. These findings provide new information on the reproductive behaviors of deep-sea animals. More importantly, they highlight how physical and bathymetric heterogeneity in the environment can result in reproductive hot spots, which may be a critical resource for reproductive success in some deep-sea species. Fifteen ROV dives were conducted on the Gorda Escarpment and Mendocino Ridge during three visits in August 2000, August 2001, and July 2002 (Fig. 1). The Gorda Escarpment is a submarine plateau offshore of northern California. The Mendocino Ridge extends westward from its northern edge at 40.35° N. The Escarpment’s northern side is characterized by steep topography, frequent rocky outcrops and talus fields, sediment slumps, and drainage channels (10). The depth of investigation ranged from 1300 to 3000 m. Reproductive aggregations of both blob sculpin and octopus were present at Site 1 (Fig. 1). The biomass of P. phrictus alone at this site was equivalent to the average total biomass of fishes on the continental slope. Likewise, the density of Graneledone sp. was considerably greater than previously published estimates (Fig. 2). Eighty-four individuals of P. phrictus and 64 nests (Fig. 3A) were observed. They were present at two sites, with the highest density occurring at Site 1 in both August 2000 and August 2001 (Fig. 1). The fish were found over the steepest topography and at a topographic break between the steep northern side of the ridge and the more gently sloping top (Fig. 4). P. phrictus and associated nests were absent in July 2002. Two hundred and thirty-two individuals of Graneledone sp. (Fig. 3B) were observed across all locations, with the highest densities observed at Site 1 during all three visits (Fig. 1). The octopus co-occurred with the blob sculpin, with 51% of the octopus observed within 5 m of sculpin adults or nests in 2001. Smaller aggregations of brooding blob sculpin and octopus were observed at Site 2. Site 1 (depth 1547–1603 m; dives T208, T349, T448) was Received 14 February 2003; accepted 12 May 2003. * To whom correspondence should be addressed. E-mail: jdrazen@mbari.org Reference: Biol. Bull. 205: 1–7. (August 2003)

Correlation of Trimethylamine Oxide and Habitat Depth within and among Species of Teleost Fish: An Analysis of Causation

Most shallow‐water teleosts have moderate levels of trimethylamine N‐oxide (TMAO; ∼50 mmol/kg wet mass), a common osmolyte in many other marine animals. Recently, muscle TMAO contents were found to increase linearly with depth in six families. In one hypothesis, this may be an adaptation to counteract the deleterious effects of pressure on protein function, which TMAO does in vitro. In another hypothesis, TMAO may be accumulated as a by‐product of acylglycerol (AG) production, increasing with depth because of elevated lipid metabolisms known to occur in some deep‐sea animals. Here we analyze muscle TMAO contents and total body AG (mainly triacyglycerol [TAG]) levels in 15 species of teleosts from a greater variety of depths than sampled previously, including eight individual species caught at two or more depths. Including data of previous studies (total of 17 species, nine families), there is an apparent sigmoidal increase in TMAO contents between 0‐ and 1.4‐km depths, from about 40 to 150 mmol/kg. From 1.4 to 4.8 km, the increase appears to be linear ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Metabolism and enzyme activities of hagfish from shallow and deep water of the Pacific Ocean.

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Establishing species–habitat associations for 4 eteline snappers with the use of a baited stereo-video camera system

\end{document} ), rising to 261 mmol/kg at 4.8 km. The trend also occurred within species: in most cases in which a species was caught at two or more depths, TMAO was higher in the deeper‐caught specimens (e.g., for Coryphaenoides armatus, TMAO was 173, 229, and 261 mmol/kg at 1.8, 4.1, and 4.8 km, respectively). TMAO contents not only were consistent within species at a given depth but also did not vary with season or over a wide range of body masses or TAG contents. Thus, no clear link between TMAO and lipid was found. However, TMAO contents might correlate with the rate (rather than content) of TAG production, which could not be quantified. Overall, the data strongly support the hypothesis that TMAO is adaptively regulated with depth in deep‐sea teleosts. Whether lipid metabolism is the source of that TMAO is a question that remains to be tested fully.

Fish Heart Rate Monitoring by Body-Contact Doppler Radar

In marine osmoconformers, cells use organic osmolytes to maintain osmotic balance with seawater. High levels of urea are utilized in chondrichthyans (sharks, rays, skates, and chimaeras) for this purpose. Because of urea’s perturbing nature, cells also accumulate counteracting methylamines, such as trimethylamine N-oxide (TMAO), at about a 2∶1 urea∶methylamine ratio, the most thermodynamically favorable mixture for protein stabilization, in shallow species. However, previous work on deep-sea teleosts (15 species) and chondrichthyans (three species) found an increase in muscle TMAO content and a decrease in urea content in chondrichthyans with depth. We hypothesized that TMAO counteracts protein destabilization resulting from hydrostatic pressure, as is demonstrated in vitro. Chondrichthyans are almost absent below 3,000 m, and we hypothesized that a limitation in urea excretion and/or TMAO retention might play a role. To test this, we measured the content of major organic osmolytes in white muscle of 13 chondrichthyan species caught with along-contour trawls at depths of 50–3,000 m; the deepest species caught was from 2,165 m. Urea and TMAO contents changed significantly with depth, with urea∶TMAO declining from 2.96 in the shallowest (50–90 m) groups to 0.67 in the deepest (1,911–2,165 m) groups. Urea content was 291–371 mmol/kg in the shallowest group and 170–189 mmol/kg in the deepest group, declining linearly with depth and showing no plateau. TMAO content was 85–168 mmol/kg in the shallowest group and 250–289 mmol/kg in the deepest groups. With data from a previous study for a skate at 2,850 m included, a second-order polynomial fit suggested a plateau at the greatest depths. When data for skates (Rajidae) were analyzed separately, a sigmoidal fit was suggested. Thus, the deepest chondrichthyans may be unable to accumulate sufficient TMAO to counteract pressure; however, deeper-living specimens are needed to fully test this hypothesis.

Dining in the Deep: The Feeding Ecology of Deep-Sea Fishes

Although hagfishes are ecologically important members of benthic communities there has been little data available on their metabolism. The oxygen consumption, enzyme activities, and muscle proximate composition of shallow living Eptatretus stoutii and deeper living E. deani were measured to investigate hagfish metabolism. Very low rates of oxygen consumption and both aerobic and anaerobic enzyme activities in the body musculature confirmed the low metabolism of hagfishes. However, significant variation in oxygen consumption existed. E. stoutii had significantly lower rates compared to those of the deeper living E. deani and two other shallow living species for which literature data was used. Both species could regulate their oxygen consumption to very low oxygen concentrations. Epatretus deani, which lives in an oxygen minimum zone, had a significantly lower critical oxygen tension (0.83 kPa) compared to E. stoutii (1.47 kPa). The deeper E. deani had greater lipid stores than E. stoutii which may reflect its deeper habitat and more sporadic food supply.

Evaluation of potential sustainability of deep-sea fisheries for grenadiers (Macrouridae)

This is a report of evidence of a close symbiotic relationship between the scyphomedusa, Stygiomedusa gigantea and the fish, Thalassobathia pelagica. Images from remotely operated vehicles (ROV) were obtained of the fish swimming on and around the large scyphomedusa. This is the first ever documented symbiosis between an Ophidiform fish and a medusa.