Proximate composition and energy content of mesopelagic fishes from the eastern Gulf of Mexico

The proximate composition of 33 species of mesopelagic fishes collected from the eastern Gulf of Mexico during May–June 1984, July–November 1985 and January 1986 was determined. Water level increased and ash-free dry weight (% dry weight) and protein level (% wet weight) decreased with increasing species' minimum depth of occurrence (that depth below which 90% of a species' population lives). Lipid level (% wet weight or % ash-free dry weight), did not change with species' minimum depth of occurrence. Skeletal ash level (% wet weight) generally decreased with minimum depth of occurrence, whereas carbohydrate level did not change with depth. The variable water level, low lipid level, and high protein level in eastern Gulf of Mexico fishes resulted in a low energy content. These results are generally characteristic of fishes from warm, stable regions of low productivity, such as the eastern North Pacific Gyre. The constant food supply provided by a stable environment may obviate the need for large lipid reserves, in contrast to colder waters at higher latitudes where food availability is seasonal. In addition, the large energy requirements for diel excursions into high-temperature surface waters by the many vertically migrating fishes of this region may influence lipid deposition. The relatively high protein level found in migrators compared to that in non-migrators or weak migrators indicates that locomotory demands influence the percentage of protein found in Gulf fishes. The lack of a positive correlation between protein level and the food availability of a zoogeographic region, suggested in previous studies, is supported here.     

Hydrocarbons in benthic algae from the eastern Gulf of Mexico

Thirty-six samples of benthic algae were collected from the continental shelf along the eastern Gulf of Mexico. The algae contain 82.8±143 ppm aliphatic hydrocarbons by dry weight and 11.8±22.7 ppm aromatic and polyolefinic hydrocarbons. The aliphatic constituents of red algae are composed almost entirely of n-C₁₇ (70 to 95% of aliphatic weight); green algae have varying amounts of n-alkanes in the range of n-C₁₅ to n-C₁₉, with homologous series of odd carbon number n-alkenes. Phytadienes occur as high as 740 ppm dry weight in the green algae and do not appear to be related to time or place of collection or to presence of any petroleum pollutants. The single brown algal species contains predominantly n-C₁₅. About 30% of the samples have hydrocarbons resembling degraded petroleum residues, but no petroleum pollution of recent origin was detected in any specimen.     

Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico

The proximate and elemental chemical compositions of 25 species of pelagic decapod and mysid crustaceans collected from the eastern Gulf of Mexico (27°N; 86°W, 1984 to 1989) was examined. Water level ranged from 63 to 95% and increased slightly with species' increased depth of occurrence. Protein levels were generally high (1.5 to 18.3% wet wt, WW; 27.6 to 62.4% ash-free dry wt, AFDW) and comprised the primary organic component in the majority of species. Protein, both as % WW and % AFDW, decreased with increased depth of occurrence. In contrast to protein, lipid levels were low (0.5 to 8.9% WW; 5.7 to 60.9% AFDW), and increased with increased depth of occurrence. Carbon and nitrogen best mirrored measured lipid and protein levels when considered as non-protein carbon and non-chitin nitrogen, respectively. C:N ratios increased with increased depth, consistent with changes in protein and lipid with depth. Because of their compositional attributes, resident Gulf of Mexico species have a low total wet weight energy content relative to species from more productive regions. Energy content showed no significant trend with depth. Vertical migration patterns were distinctly different between shallow-and deep-living gulf species and these differences were largely responsible for the relationships observed between composition and depth. In migrating species, the protein and nitrogen content were higher, the lipid and carbon contents and C:N ratio lower, than in non-migrating species. Three deep-living species of the genus Acanthephyra were found to be compositionally atypical, resembling shallow, migrating types rather than other deep-living, non-migratory species.     

Distribution and community structure of midwater fishes in relation to the subtropical convergence off eastern Tasmania,Australia

Midwater fishes were sampled at night to a depth of 400 m in oceanic waters east of Tasmania, Australia, in May/June of 1992, 1993 and 1994. We examined whether there were differences in the distribution and density of these fishes in relation to the subtropical convergence (STC) separating East Australia Current (EAC) water from subantarctic water (SAW) to the south. A total of 23999 fishes from 107 taxa and 43 families were identified. Four new records were identified from the study area. Myctophids and stomiatoids were the main fish taxa captured. We found no evidence of increased density of the combined fish catch in any particular area. However, differences between areas in the density of individual species were noted, although no species was confined to any one area. Ceratoscopelus warmingi, Lobianchia dofleini and Vinciguerria spp. were most abundant in the EAC, whereas Lampichthys procerus was most abundant in the SAW. Only Diaphus danae was found in significantly higher numbers in the STC. Multivariate analysis revealed that community structure in the EAC was significantly different from that of the SAW but not from the STC. Further, when separated by depth, the shallow EAC group was significantly different in all group comparisons bar that with the shallow STC. Our data indicated that the thin EAC layer above 200 m could be distinguished by its fauna from the SAW. However, we could not detect a separate community in the STC. We conclude, therefore, that the STC is not an area of increased micronekton abundance, nor does it contain a distinct community, during the Australian autumn/winter.     

Food availability as a selective factor on the chemical compositions of midwater fishes in the eastern North Pacific

In September and October 1980 we examined the relationships between food availability, depth, and chemical composition among 12 midwater fish species, from three adjacent areas of the eastern North Pacific: the eastern gyre, the California Current, and the transitional region between them. By comparing trends in chemical composition across a geographical productivity gradient, the influence of food availability could be examined both with and without depth as a dependent variable. In general, caloric density, lipids, and water content showed consistent trends along both vertical and horizontal gradients of food availability. Lipids and caloric contents were lowest among bathypelagic species and among fishes from the gyre. Water content was highest in the gyre and among deeper-living species. While protein content as a percentage of the total wet weight declined with depth, there was no clear trend among mesopelagic species between stations. Protein as percent of the total organic content showed no decrease with depth, but was significantly higher in the gyre among mesopelagic species. These findings suggest that compositional trends in water, lipids, and caloric content are correlated with food availability, regardless of any depth-related factors. In contrast, protein did not vary consistently with food availability and thus may be linked to other factors.     

Reproductive biology of three species of midwater fishes associated with the continental slope of eastern Tasmania,Australia

The reproductive biology of Lampanyctodes hectoris (Günther, 1876), Maurolicus muelleri (Gmelin, 1789) and Diaphus danae Tåning, 1932, from continental-slope waters of eastern Tasmania, was examined between April 1984 and June 1985. L. hectoris spawned in winter and M. muelleri spawned from late winter to early summer. Apart from one ripe male, no reproductive activity was detected in D. danae; this species may be an expatriate in these waters. Fecundity was positively correlated with standard length in L. hectoris, but not in M. muelleri. The ratio of females to males increased with length in all three species. The spermatozoa of L. hectoris and D. danae are atypical of vertebrates and have no tail.     

Feeding ecology of three species of midwater fishes associated with the continental slope of eastern Tasmania,Australia

The feeding ecology of Maurolicus muelleri, Lampanyctodes hectoris and Diaphus danae was examined from samples collected from continental slope waters of eastern Tasmania between April 1984 and April 1985. A total of 2 232 stomachs was analysed. M. muelleri, L. hectoris and D. danae fed primarily on euphausiids and secondarily on copepods, although larger D. danae (> 60 mm standard length) fed on other lanternfish (chiefly L. hectoris). The diets of M. muelleri and L. hectoris overlapped substantially. Diet overlaps between D. danae and the former species was low, however, due to the large biomass of fish present in D. danae. The prey taxa consumed changed with time of year and predator size. Stomach fullness (feeding intensity) varied seasonally in all three species, but only M. muelleri showed significant diel differences in fullness. The synchronization of the size structure of the predator populations and their feeding intensity, with seasonal variations in preferred prey, is proposed as a mechanism whereby each species maximizes its share of the available food resources.     

The bathypelagic Decapoda,Lophogastrida, and Mysida of the eastern Gulf of Mexico

The Earth’s greatest living space is found in the bathypelagic zone of the oceans (depths >1,000 m), yet little research has been dedicated to these ecosystems. The micronekton of the bathypelagic zone in the eastern Gulf of Mexico (EGOM) was investigated with the goal of comparing its community structure with that of the well-studied mesopelagic micronekton. Herein is described a portion of that community, specifically species belonging to the orders Lophogastrida, Mysida, and Decapoda. A total of 46 species were collected, most of which have broad zoogeographic distributions. Seventeen of the species had not previously been collected above 1,000 m in the same location despite over 20 years of sampling. Compared to the mesopelagic zone, the bathypelagic community showed increased contributions to abundance and estimated biomass from the Oplophoridae and Eucopiidae, with a simultaneous decrease in the importance of the Dendrobranchiata. In addition, the bathypelagic zone was distinguished by a relatively high percentage of individuals that brood their eggs (77% vs. 15% in the mesopelagic zone). The results are interpreted as evidence that the bathypelagic zone contains a distinct pelagic community, with a biology and ecology fundamentally different from that of the mesopelagic zone.     

Life histories of three species of lanternfishes (Pisces: Myctophidae) from the eastern Gulf of Mexico

Morphology and microstructure of the sagittal otoliths from three species of mesopelagic, tropical-subtropical myctophids [Benthosema suborbitale (Gilbert),Diaphus dumerilii (Bleeker)Lepidophanes guentheri (Goode and Bean)], collected from September 1984 to May 1986 in the eastern Gulf of Mexico (27°N, 86°W), were examined and described. Analysis of the microstructure revealed microincrements corresponding to the daily growth rings reported in many studies. Using marginal increment analysis, the deposition of microincrements was verified as occurring daily, the first validation of daily growth rings in the otoliths of mesopelagic fishes. In all three species, the clear central (larval growth) zone of the sagitta was sharply delimited by a dark check accompanied by a series of accessory primordia. A wide, dark, sharply defined postlarval zone (width 100 µm) radiated out from this boundary inB. suborbitale andL. guentheri. While a darkened region was also observed around the larval growth zone inD. dumerilii, it was diffuse and differed in structure from that in the other two species. Within the dark zone in the otoliths ofB. suborbitale andL. guentheri, two different microincremental structures were observed. The narrower of these was determined to be the increment deposited on a daily basis. The structure and formation of the dark region in these three species appear to be related to larval transformation and behaviors of different species of myctophid larvae.     

Chemical composition and buoyancy of midwater crustaceans as function of depth of occurrence off Southern California

Water, ash, C, H, N, lipid, carbohydrate, chitin and protein contents were determined for 28 species of midwater crustaceans. Variation of these components as a function of depth of occurrence, relative buouancy and respiratory rate of these species was examined. Vertical distribution data for 10 of the species based on discrete depth crawls is provided. The relative bouyancies of 16 of these species were measured. The caloric contents of 21 species were calculated. Lipid content (percent wet weight) and caloric density increased at intermediate depths, but apparently decreased at the greatest depths; it is suggested that this decrease is related to the relatively large size of these organisms and the energy limiting conditions in the deep-sea. Protein content (percent wet weight) decreases with increasing depth of occurrence, and is apparently related to a substantial fraction of the decrease in respiratory rate with increasing depth of occurrence in these species. Within the range of values examined, relative buoyancy appeas to be independent of metabolic rate. It is, therefore, suggested that the energy involved in maintaining an animal in the water column is generally insignificant compared to its overall activity. Furthermory, advantages of neutral buoyancy other than energy conservation may be more important selective factors for this characteristic.     

Flux capacities and acclimation costs in Trichodesmium from the Gulf of Mexico

Phytoplankton function and acclimation are driven by catalytic protein complexes that mediate key physiological transformations, including generation of photosynthetic ATP and reductant, and carbon and nitrogen fixation. Quantitation of capacities for these processes allows estimation of rates for key ecosystem processes, and identification of factors limiting primary productivity. We herein present molar quantitations of PSI, PSII, ATP synthase, RuBisCO and the Fe protein of nitrogenase of Trichodesmium collected from the Gulf of Mexico, in comparison to determinations for a range of cyanobacteria growing in culture. Using these measurements, estimates were generated for Trichodesmium capacities for carbon fixation of 1–3.4 g C g chl a ⁻¹ h⁻¹ and nitrogen fixation of 0.06–0.17 g N g chl a ⁻¹ h⁻¹, with diel variations in capacities. ATP synthase levels show that ATP synthesis capacity is sufficient to support these levels of carbon and nitrogen fixation, and that ATP synthase levels change over the day in accordance with the ATP demands of nitrogenase and RuBisCO activity. Levels of measured complexes indicate that Trichodesmium manifests n-type diel light acclimation through rapid changes in RuBisCO:PSII, supported by significant investment of cellular nitrogen. The plasticity in the levels and stoichiometry of these core complexes show that changes in the abundance of core protein complexes are an important component of acclimation and regulation of metabolic function by Trichodesmium populations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Survival and remigration probabilities for loggerhead turtles (Caretta caretta) nesting in the eastern Gulf of Mexico

Few long-term mark-recapture tagging datasets exist to estimate population parameters for loggerhead sea turtle (Caretta caretta) recovery units. Using a two-state open robust design model, we analyzed a 20-year (1990–2009) mark-recapture dataset from the Keewaydin Island loggerhead nesting assemblage off the southwest coast of Florida (USA) in the eastern Gulf of Mexico. For this analysis, 2,292 turtle encounters were evaluated, representing 841 individual nesting turtles. Survival was estimated at 0.73 (95 % CI 0.69–0.76). This estimate is comparable with survival estimates elsewhere in the Peninsular Florida subpopulation and is among the lowest estimates for the Northwest Atlantic loggerhead population. We documented no changes in remigration rates or clutch frequency over time. These are the first survival and remigration probabilities estimated for a loggerhead nesting assemblage in the eastern Gulf of Mexico.     

Triacylglycerols and wax esters in the lipids of deep midwater teleost fishes of the Southern California Bight

Total lipid contents and the proportions of triacylglycerols and wax esters were examined in 23 species of deep-sea fishes collected between 1977 and 1980 in the Santa Catalina, San Clemente and San Pedro Basins off Southern California, USA. Mean total lipid content ranged from 1 to 22% of wet weight. Triacylglycerols accounted for 1 to 91% of total lipids present and were more abundant than wax esters in the lipids of all but a single species. Wax ester contents ranged from 0 to 24% of total lipids but in 20 species were 10% of the lipids. These results do not substantiate the previously suggested idea that wax esters are important constituents of the lipids of deep-sea fishes. Moreover, the hypothesis that wax ester content increases with increasing depth of occurrence is not supported. To assess the importance of the lipids present in bouyancy regulation, the water content and swimbladder inflation of these species were also examined. Neither total lipids nor wax esters appear to play an important role in buoyancy regulation in the majority of these fishes. Eight species have gas-filled swimbladders. Of the species lacking inflated swimbladders, 7 have watery bodies (87 to 95% H₂O) and 6 with low lipid (8.3%) and water (85%) contents may need to swim continuously to maintain their position in the water column. Lipids, largely in the form of triacylglycerols, may be important in the buoyancy of three species with lipid contents>10% of wet weight. Two of these species also possess inflated swimbladders. The reasons for the production of large quantitites of was esters by certain deep-sea teleosts, especially gonostomatids and myctophids, remain unknown.     

Gulf of Mexico hydrocarbon seep communities

Sediment and water samples were collected by submersible in September 1986 at 16 locations on the carbonate cap overlying a conical diapir, which was formed by the upward migration of oil and gas through a subsurface fault on the continental slope off Louisiana, USA (27°47N; 91°30.4W). The biological community at the site was photographed quantitatively with still and video cameras. Rigorous spatial sampling indices were maintained so that variation in chemical parameters and in the abundance of photographed organisms could be estimated within the bounds of the study site. Concentrations of extractable organic material (EOM) ranged from 0.24 to 119.26 in the sediment samples, while methane concentrations in the water samples were from 0.037 to 66.474 M. The visible biological community was predominantly composed of the chemosynthetic tube worms (Vestimentifera) Lamellibrachia sp. and Escarpia sp., and an undescribed, methane-oxidizing mussel (Mytilidae: Bathymodiolus-like), as well as diverse non-chemosynthetic organisms. The ranked abundance of tube worms was significantly correlated (p<0.05) with the concentration of EOM in the sediment samples, while the abundance of mussels was significantly correlated (p<0.05) with the concentration of methane in the water samples. Tube worms and mussels both occurred in dense clusters; however, the clusters of mussels had a more restricted distribution within the study site than did clusters of tube worms. Both organisms were most abundant in the vicinity of the subsurface fault.     

Mercury bioaccumulation and trophic transfer in sympatric snapper species from the Gulf of Mexico.

Consumption of marine fish is a major route of toxic methyl mercury (MeHg) exposure to ocean apex predators and human populations. Here we explore the influence of trophic structure on total mercury (Hg) accumulation in red snapper (RS, Lutjanus campechanus) and gray snapper (GS, Lutjanus griseus) from the coastal Louisiana region of the Gulf of Mexico, west of the Mississippi River. The objectives of this investigation were to: (1) determine the effectiveness of the use of offshore recreational fishing charter boats and marinas as sources of fish samples and (2) compare species differences in Hg bioaccumulation, trophic position, and carbon sources. Our data show that length-normalized Hg concentrations (> or = 97% as MeHg in tissue of both species) were 230% greater in GS in comparison to RS collected from the same general area. Stable C and N isotope signatures (delta15N and delta13C) indicate that GS occupy a slightly higher trophic position (approximately 30% of one trophic position higher) on the Gulf food web in comparison to RS and that GS appear to incorporate higher trophic positioned prey, continually and at smaller sizes. Mercury was strongly correlated with combined delta15N and delta13C in pooled species data, arguing that most of the substantial difference in Hg bioaccumulation between RS and GS can be explained by modest differences in their trophic position and, to a lesser degree, carbon sources, which had low variation and high overlap among species. These observations demonstrate that even minor to moderate differences in trophic position and food habits in sympatric species can create relatively large differences in bioaccumulation regimes and underscores the importance of quantitative characterization of trophic structure in marine MeHg bioaccumulation studies.     

Eyes and extraocular photoreceptors in midwater cephalopods and fishes: Their roles in detecting downwelling light for counterillumination

The means of detecting downwelling light for counterillumination in several midwater animals has been examined. Eyes and extraocular photoreceptors (drosal photosensitive vesicles in the enoploteuthid squid Abraliopsis sp. B and pineal organs in the myctophid fish Myctophum spinosum) were alternately exposed to overhead light or covered by a small opaque shield above the animal and the bioluminescent response of the animal was monitored. Covering either the eyes or the extraocular photoreceptors resulted in a reduction in the intensity of counterillumination. Preliminary experiments examining the bioluminescent feedback mechanism for monitoring intensity of bioluminescence during counterillumination in the midwater squid Abralia trigonura indicated that the ventral photosensitive vesicles are responsible for bioluminescent feedback.     

Distribution and ecology of deep-water mollusks from the continental slope,southeastern Gulf of California,Mexico

Specimens of deep-water mollusks were collected with a bottom dredge in the SE Gulf of California during four cruises (TALUD project) in 2000–2001. A total of 24 species (16 Pelecypoda, 5 Gastropoda, and 3 Scaphopoda) were collected. Analyses of environmental data (depth, epibenthic temperature and oxygen content, sediment texture and organic matter content) associated with each record indicated that the community diversity of these deep-water mollusks was related to oxygen concentration and density was correlated with depth. A canonical correspondence analysis (CCA) indicated that the five environmental variables explained 53.8% of observed variance in the model. In a multiple regression analysis, density (ind l⁻¹) was best correlated with dissolved oxygen concentration (R = −0.25), followed by temperature (R = −0.20), organic matter content (R = −0.15), and depth (R = −0.12). As oxygen best explained variance in the CCA it was selected to perform a single correspondence analysis (CA) using the most abundant and frequent species (two Pelecypoda and three Scapophoda). The analysis shows that pelecypods occur in near anoxic values while scaphopods occur in intermediate oxygen concentrations.     

Contrasting C and N isotope ratios from sperm whale skin and squid between the Gulf of Mexico and Gulf of California: effect of habitat

To investigate feeding variation between populations of an apex oceanic predator, stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) have been compared in skin of female and immature male sperm whales (Physeter macrocephalus) from the northern Gulf of Mexico (GoM) and the Gulf of California (GC). Whale sexes were determined genetically. The δ¹³C and δ¹⁵N values from squid muscle were used from the GC, and from inshore and offshore sites in the GoM. We documented contrastingly lower δ¹³C and δ¹⁵N from whales and squid of the GoM compared with those from the GC. While this difference may be associated with variation in trophic position, geographic variation in biochemical cycling influenced significantly the contrasting isotope values between gulfs. Within the northern GoM, the highly distinct δ¹⁵N values of neritic squid versus mesopelagic squid provide further evidence of habitat specificity in δ¹⁵N.