Demersal Sharks Research Articles

Australian sea lions Neophoca cinerea at colonies in South Australia: distribution and abundance, 2004 to 2008

The Australian sea lion Neophoca cinerea is an endemic species listed as vulnerable under the Commonwealth 'Environment Protection and Biodiversity Conservation Act' and as Endangered by IUCN. Assessing its abundance is difficult because of its supra-annual (17 to 18 mo) breeding cycle, and pupping seasons that are extended (about 6 mo but varying between colonies) and asynchronous in their timing between colonies. Based mainly on surveys at most sites in South Australia (SA) between 2004 and 2008, and information from the literature, estimates of abundance are provided for 39 breeding colonies and 9 haulout sites where pups are recorded occasionally. From this study it is estimated that in SA, 3119 Australian sea lion pups are born per breeding cycle, an increase on former estimates by at least 16% resulting from recognition of new breeding colonies, targeting surveys to coincide with maximum pup numbers and using mark-recapture procedures at some colonies. With the addition of 503 pups in Western Australia, the overall estimate of pup abun- dance for the species is 3622. This leads to an estimate of 14 780 animals using the multiplier 4.08. Trend data for the Seal Bay colony on Kangaroo Island indicated that pup numbers decreased at 0.54% yr -1 in the 22 yr (16 pupping seasons) from 1985 to 2007. A cause of the decrease is believed to be bycatch in the demersal shark gillnet fishery, which overlaps with sea lion foraging areas nearby. Area closures declared during 2010 within several km of all Australian sea lion breeding sites in SA should reduce the incidence of bycatch mortality.

Spatio-temporal patterns in demersal elasmobranchs from trawl surveys in the eastern English Channel (1988–2008)

Elasmobranchs have life-history characteristics that make them more vulnerable to fishing than many teleost fish. As commercial landings data are usually not sufficient for the monitoring and assessment of elasmobranchs, fisheries-independent data, geostatistics and Geographic Information Systems were used to investigate the spatio-temporal patterns of sharks (6 species), skates (9 species) and stingray in the eastern English Channel. Temporal trends in relative abundance and distributions of total-length frequencies were investigated using data collected over 21 yr from an annual survey using a Grande Ouverture Verticale (GOV) trawl. Temporal trends in relative abundance were examined qualitatively in relation to the reported conservation status for the various species. Most individuals captured for 7 of the species were immature. The GOV trawl was more efficient at sampling demersal sharks than batoids (skates and rays), especially small-sized batoids. Geostatistics were effectively used to explore, identify and quantify the spatial structure of the distributions of 13 species. The more abundant species displayed significantly larger patch diameters. Three shark species (Scyliorhinus canicula, S. stellaris and Galeorhinus galeus) were found to be spatially segregated by sex. The shark species tended to occur in the deeper waters of the area, while skates were found to be more coastal in their distribution. Results have contributed to a greater understanding of the spatio-temporal patterns in elasmobranch species in the eastern English Channel and are of relevance to future marine spatial planning and the development of long-term management plans for elasmobranchs in the area and elsewhere.

Body morphometrics, swimming diversity and niche in demersal sharks: a comparative case study from the Mediterranean Sea

Seven demersal shark species belonging to six families, obtained from the southern Mediterranean Sea, were selected as representative of different putative swimming modes, inhabited depths and trophic levels. Caudal fin morphometrics (caudal fin span, area and aspect ratio), hepatosomatic index and muscular-skeletal amount per unit length were estimated and compared within and among species by means of both parametric and non-parametric multivariate and univariate statistical procedures. Intra-specific variation was related to size and, for one species only, also to sex. Two inter-specific patterns of variation were observed: the first pattern was interpreted as a swimming ability gradient between slow benthic and more pelagic species, as a function of caudal morphometry and body characteristics; the second pattern is related to the depth the species inhabit and/or their trophic level. These two hypotheses were tested with the current data and literature information, and discussed in the context of shark swimming modes, habit and environment. Based on observed differences in caudal fin morphometrics, liver and muscular masses, this paper is the first to provide a quantitative and comparative dataset for demersal sharks as a group, and attempts to describe sample variability as a function of expected swimming ability and occupied niche (depth inhabited and/or trophic level).

Mercury in 16 demersal sharks from southeast Australia: Biotic and abiotic sources of variation and consumer health implications

Total mercury (THg) and monomethylmercury (MMHg) concentrations were determined in the tissues of demersal shark (Order Squaliformes and the Families: Scyliorhinidae, Hexanchidae) and chimaera species (Families: Chimaeridae and Rhinochimaeridae) from continental shelf and slope waters off southeast Australia, including embryos, juveniles and adults. The distribution of THg in various tissues (muscle, liver, kidney and skin), examined in ten species, shows higher levels in the muscle tissue (1.49 ± 0.47 mg kg −1, ww), which accounted for between 59% and 82% of the total body burden of mercury and in the kidney (0.93 ± 0.14 mg kg −1, ww) and liver (0.61 ± 0.25 mg kg −1, ww) with lower levels observed in the skin (0.12 ± 0.06 mg kg −1, ww). Additional THg determinations were performed in the muscle tissue of five other species allowing geographical and inter-specific comparisons. Speciation analysis demonstrated that more than 90% mercury was bound in muscle tissue as MMHg with higher percentages (>95%) observed in sharks species occupying deeper environments. Species differences were observed. Highest THg levels in the muscle tissue (up to 6.64 mg kg −1 wet weight, ww) were recorded in Proscymnodon plunketi and Centrophorus zeehaani (mean values; 4.47 ± 1.20 and 3.52 ± 0.07 mg kg −1, ww, respectively). Consistent with the ongoing paradigm on mercury bioaccumulation, we systematically observed THg concentrations increasing with animal size from the embryos to the larger sharks. Embryos of Etmopterus baxteri and Centroselachus crepidater had average levels 0.28 and 0.06 mg kg −1 (ww), while adult specimens reached 3.3 and 2.3 mg kg −1 (ww), respectively. THg concentrations in Australian sharks were compared with the same genus collected in other world regions. Levels were closer to data reported for East Atlantic than for the epicontinental Mediterranean margins. At a smaller geographical scale, the habitat effect on mercury concentration in sharks seems less clear. Squalid sharks occupying shelf waters showed higher mean mercury levels relative to their size (body weight, bw) than mid-slope species (0.4–6.7 mg kg −1 bw and 0.3–2.2 mg kg −1 bw, respectively). However, local regional differences (East and South Tasmania vs. Victoria) in Hg levels were not detected for the majority of taxa examined. All species, with the exception of Figaro boardmani showed values greater than 0.5 mg kg −1 (ww) and all but four were above many international regulatory thresholds (1.0 mg kg −1, ww).

Temporal variation in the artisanal elasmobranch fishery of Sonora, Mexico

Baseline, species-specific information is largely unavailable for artisanal elasmobranch fisheries, but is essential for the monitoring of exploited populations and the development of effective management plans. Seasonal surveys were conducted during 1998–1999 in Sonora, Mexico to determine the extent and activities of artisanal elasmobranch fisheries operating in the eastern Gulf of California. Nineteen fishing sites were documented, the majority of which (84.2%) targeted elasmobranchs during some part of the year. Most small demersal sharks and rays were landed in bottom set gillnet fisheries that also targeted demersal teleosts, whereas large sharks were usually taken in directed surface longline or, to a lesser extent, drift gillnet fisheries. Rays numerically dominated sampled landings in Sonora (63.4%, n = 100,136), and catch rates exceeded those of sharks during spring and summer months. The shovelnose guitarfish, Rhinobatos productus, was the primary fishery target during these seasons. During autumn, small sharks, especially mustelids ( Mustelus spp.) were numerically dominant, but rays (e.g., Dasyatis dipterura) were also caught in large numbers. Winter landings in Sonora were principally composed of mustelid sharks, which represented the greatest seasonal catch rates of all elasmobranch taxa during this study. Large sharks were of comparably minor importance, with a limited summer fishery operating in the southern part of the state. Variation in catch composition was evident in association with differential interannual environmental conditions (El Niño and La Niña) and seasonal temperature fluctuations. Size composition of landings varied greatly by species, but relatively small size classes of sharks and rays were abundant and large, often gravid females of several ray species (e.g., R. productus and Narcine entemedor) supported spring and summer fisheries in nearshore waters. Populations of many large shark species (e.g., Carcharhinus leucas, Carcharhinus limbatus, Carcharhinus obscurus and Galeocerdo cuvier) have likely been overfished, prompting a shift in effort towards coastal populations of smaller elasmobranchs.

Habitat-Mediated Use of Space by Juvenile and Mating Adult Port Jackson Sharks, Heterodontus portusjacksoni, in Eastern Australia1

Studies of spatial ecology of demersal sharks are critical to understanding the significance of habitat variation; however, limited information exists. Spatial ecology of adult Heterodontus portusjacksoni was studied at three locations on the central and southern coast of New South Wales, Australia, from January 2002 to December 2005. Juveniles within a nursery area were studied from December 2002 to December 2005. Tag-recapture, day and night underwater visual census, and acoustic tagging were used. Adults returned annually to the same coastal breeding reefs for up to four consecutive years. Individual juveniles resided within a sea-grass nursery area for at least 2 yr and were not uniformly distributed throughout the nursery. Adult females often sheltered in aggregations in gutters as a male avoidance strategy, and both sexes utilized the sand/reef interface in the absence of gutters. Juveniles aggregated infrequently due to absence of habitat features that mediated aggregation. Acoustic tracks of adults revealed periods of inactivity up to 27 hr. Juveniles spent significant amounts of time inactive, punctuated with short bouts of swimming. Juveniles utilized moderate activity spaces (3,510–583,990 m2) centered over a core area of the sea-grass bed but also ranged over much larger areas of the bay. Use of space by H. portusjacksoni is strongly influenced by habitat characteristics throughout its life history.

Food-web dynamics in the South Catalan Sea ecosystem (NW Mediterranean) for 1978–2003

An ecosystem model representing the continental shelf and upper slope of the South Catalan Sea (NW Mediterranean) is calibrated and fitted to the available time series data from 1978 to 2003. We use a process-oriented model to explore the extent to which changes in marine resources and the ecosystem were driven by trophic interactions, environmental factors and fishing activities. Fishing effort and fishing mortality are used to drive the model, while observed (absolute and relative) biomasses and catches are compared with the predicted results. A reduction in the sum of the squared deviations of the observed and predicted values of the biomass is used as a metric for calibrating and assessing the fit of the model. A posteriori trophodynamic indicators are used to explore the ecosystem's structural and functional changes from 1978 to 2003, and a generalized least squares regression is used to assess the significance of the predicted trends. In general, a high proportion of the variability in the time series data is explained by the main trophic interactions (37–53%), fishing activities (14%), and indirectly by considering the environment (6–16%), as driving factors. The model's predictions match satisfactorily with the yearly data on the biomass for anglerfish, adult hake, demersal sharks, anchovy and mackerel, which show a statistically significant decrease over time, while the biomass of flatfish and seabirds are observed to increase. Catch data show a significant decrease in anglerfish, demersal sharks, anchovy and sardine, while there is an increase in red mullet, flatfish, juvenile hake and horse mackerel. These changes in biomass are predicted to have direct and indirect impacts on the ecosystem mediated by the trophic web, such as the proliferation of non-commercial species with lower trophic levels (e.g., benthic invertebrates) or higher turnover rates (e.g., cephalopods and benthopelagic fish). This is consistent with anecdotal information from the Mediterranean and is likely caused by trophic cascades due to the removal of demersal and pelagic higher trophic level organisms (predator release), and a decrease in small pelagic fish (competitor release). Trophodynamic indicators suggest a degradation pattern over time: both the mean trophic level of the community (mTL co, excluding primary producers and detritus) and a modified version of Kempton's index of biodiversity decrease with time, while the total flow to detritus and the loss of production due to fishing increase from 1978 to 2003. Additionally, the demersal/pelagic ratio increases due to an overall decrease in the abundance of small pelagic fish in the ecosystem.

Acoustic assessment of trophic dominance in a marine ecosystem

High-frequency acoustic surveys over the past 15 years show that the trophic structure in Prince William Sound (PWS), Alaska, functions as a wasp-waist ecosystem. Three dominant biomasses in PWS are: (1) Pacific herring (2) walleye pollock and (3) the large-bodied copepods (Neocalanus spps.) that dominate the spring zooplankton assemblage. The acoustic surveys and associated ecosystem observations suggest that the relative dominance of herring and pollock affects the composition of the apex predators. The near-shore and near-surface distribution of herring provides access by surface-oriented marine mammal and seabird predators, such as sea lions, seals, murres and cormorants, to a crucial winter-period food source. In contrast, the deep, off-shore distribution of the pollock favors large benthic predators, such as demersal sharks, halibut, and flounder. The acoustic surveys also demonstrated that the abundance of large-bodied copepods in PWS is critical to ecosystem productivity, including survival of juvenile pink salmon, and may affect the relative dominance of herring and pollock. An additional outcome of the long-term database on Pacific herring was the discovery of linkages between the 1989 Exxon Valdez oil spill and a subsequent collapse of the herring along with associated predators.

Morphological characteristics of five bycatch sharks caught by southern Chilean demersal longline fisheries

The by-catch of sharks in artisanal demersal pink cusk-eel (Genypterus blacodes) and yellownose skate (Dipturus chilensis) fisheries is frequent within their fishing effort. Nevertheless, there is no registry of landings, which could help to control this problem. This is particularly evident for endemic species, which includes most coastal and deep water Chilean sharks. The main systematic characteristic of these Chondrichthyan species is the external morphology of the neurocraneum. The form and arrangement of the teeth and dermal denticles allow specific differences to be identified. The objective of this paper is to contribute to the biology and systematic knowledge of demersal shark species, teeth and dermal denticle morphology and neurocraneum morphometrics of two species of Scyliorhinids, the redspotted catshark (Schroederichthys chilensis) and the dusky catshark (Halaelurus canescens), as well as three Squaliforms, the granular dogfish (Centroscyllium granulatum), the birdbeak dogfish (Deania calcea) and the spiny dogfish (Squalus acanthias).

Standardized diet compositions and trophic levels of skates (Chondrichthyes: Rajiformes: Rajoidei)

Skates by virtue of their abundance and widespread occurrence appear to play an influential role in the food webs of demersal marine communities. However, few quantitative dietary studies have been conducted on this elasmobranch group. Therefore, to better understand the ecological role of skates, standardized diet compositions and trophic level (TL) values were calculated from quantitative studies, and compared within and among skate and shark taxa. Prey items were grouped into 11 general categories to facilitate standardized diet composition and TL calculations. Trophic level values were calculated for 60 skate species with TL estimates ranging from 3.48 to 4.22 (mean TL = 3.80 ± 0.02 SE). Standardized diet composition results revealed that decapods and fishes were the main prey taxa of most skate species followed by amphipods and polychaetes. Correspondingly, cluster analysis of diet composition data revealed four major trophic guilds, each dominated by one of these prey groups. Fish and decapod guilds were dominant comprising 39 of 48 species analyzed. Analysis of skate families revealed that the Arhynchobatidae and Rajidae had similar TL values of 3.86 and 3.79 (t-test, P = 0.27), respectively. The Anacanthobatidae were represented by a single species, Cruriraja parcomaculata, with a TL of 3.53. Statistical comparison of TL values calculated for five genera (Bathyraja, Leucoraja, Raja, Rajella, Rhinoraja) revealed a significant difference between Bathyraja and Rajella (t-test, P = 0.03). A positive correlation was observed between TL and total length (L T) with larger skates (e.g. >100 cm L T) tending to have a higher calculated TL value (>3.9). Skates were found to occupy TLs similar to those of several co-occurring demersal shark families including the Scyliorhinidae, Squatinidae, and Triakidae. Results from this study support recent assertions that skates utilize similar resources to those of other upper trophic-level marine predators, e.g. seabirds, marine mammals, and sharks. These preliminary findings will hopefully encourage future research into the trophic relationships and ecological impact of these interesting and important demersal predators.

Inter- and intraspecific variation in the distribution and number of pit organs (free neuromasts) of sharks and rays.

The distribution of pit organs (free neuromasts) has previously been documented for several species of pelagic sharks, but is relatively poorly known for rays and bottom-dwelling (demersal) sharks. In the present study, the complete distribution of pit organs was mapped in the demersal sharks Heterodontus portusjacksoni, Orectolobus maculatus, Hemiscyllium ocellatum, Chiloscyllium punctatum, and Asymbolus analis, and the rays Rhinobatos typus, Aptychotrema rostrata, Trygonorrhina sp. A, Raja sp. A, and Myliobatis australis. All of these species had pit organs scattered over the dorsolateral surface. The sharks also had "mandibular" pit organs (and "umbilical" pit organs in C. punctatum and A. analis) on the ventral surface, while pit organs were sparse or absent on the ventral surface of rays. All of the species examined here, except for M. australis, also had a "spiracular" group of pit organs adjacent to the eye and/or spiracle. Spiracular pit organs were also recorded for the sawshark Pristiophorus sp. A and the skate Pavoraja nitida, although the remainder of pit organs were not mapped in these species. The distribution and number of pit organs varied both within and among species. Pit organ distribution was asymmetrical in each individual examined, but no particular trend towards left or right "handedness" was observed in any species. Although rays have been thought to have fewer pit organs than sharks in general, this was not the case in the present study. All of the species examined here had few pit organs compared to the pelagic sharks previously documented, but it is not clear whether this is due to ecological or phylogenetic causes.

Arsenocholine- and dimethylated arsenic- containing lipids in starspotted shark Mustelus manazo

We have investigated lipid-soluble arsenic compounds present in the tissues of a demersal shark, the starspotted shark Mustelus manazo. Arsenic compounds were extracted with chloroform–methanol from several tissues taken from each of five individuals, subjected to mild alkaline hydrolysis and fractionated into alkali-labile and alkali-stable fractions. Ordinary muscle, kidney and brain contained alkali-labile arsenic compounds; liver, stomach, heart and gall bladder contained alkali-stable compounds; and intestine, skin, dark muscle, spleen and bone contained both types of arsenic compounds. After further hydrolysis, the hydrolysates from ordinary muscle and liver were chromatographed with HPLC–ICP–MS. Arsenocholine was detected in the hydrolysates from the muscle, suggesting that arsenolecithins were present in the tissues. However, dimethyl-­arsinic acid was detected in the hydrolysates from the liver, suggesting the presence of di-­methylated arsenolipid in it. Copyright © 1999 John Wiley & Sons, Ltd.

Identification of arsenobetaine as a major arsenic compound in muscle of two demersal sharks, shortnose dogfish Squalus brevirostris and starspotted shark Mustelus manazo

1. 1. The major water-soluble arsenic compound was isolated from the muscle of shortnose dogfish Squalus brevirostris and of starspotted shark Mustelus manazo, both of which are demersal sharks. 2. 2. The isolated compound was identified to be arsenobetaine by its chromatographic and spectrometric analyses.

Identification of arsenobetaine as a major water-soluble arsenic compound in the liver of two demersal sharks, shortnose dogfish squalus brevirostris and starspotted shark mustelus manazo

1. Each major water-soluble arsenic compound was isolated from liver tissues of shortnose dogfish Squalus brevirostris and starspotted shark Mustelus manazo. 2. Both isolated compounds were identified to be arsenobetaine by comparison of IR and FAB-mass spectra with those of synthetic arsenobetaine.

Ommatokoita sp. (Copepoda: Lernaeopodidae) parasitic on a demersal shark, Etmopterus princeps, from the northwest Atlantic Ocean

Ommatokoita sp. is described from a single adult female specimen recovered from the skin of the demersal shark Etmopterus princeps collected in the northwest Atlantic Ocean. The specimen differs from the other species of the genus Ommatokoita, O. elongata, in the shape of the trunk, the structure of the first maxillae and maxilliped, and the size of the parasite.

Body Form and Locomotion in Sharks

A revised interpretation of the mode of action of the heterocercal tail in sharks shows that the upturned tail axis tends to produce a thrust directed downwards behind the centre of balance of the fish and thus gives a moment turning the head upwards. This is countered in two ways—by the rotation of the tail along its longitudinal axis during each lateral beat, and through the action of the ventral hypochordal lobe. The shape of the tail and the mode of action of the tail in all sharks so far considered reflects a balance between these three factors, in all of them the net effect being the production of a forward thrust from the tail that passes directly through the centre of balance of the fiish. There is normally therefore no tendency for the fish to turn around the centre of balance in a sagittal plane but there is a net sinking effect that is countered by the planning effect of the pectoral fins and the ventral surface of the head. A study of 56 species of sharks shows that the tail is constructed according to a remarkably consistent common plan, the extremes being the high angled rather symmetrical tail of pelagic sharks such as hums, Lamna and Rhincodon and the straight tails of benthic sharks such as Ginglymostoma in which a ventral hypochordal lobe is absent. When the general body shape of sharks, including the position of insertion of the median and paired fins and the pattern of growth of fin surface areas is considered, the uniformity of the shark body plan and locomolor function is further emphasised. Four patterns of body form in sharks are recognised: 1) The fast swimming pelagic sharks and the whale sharks have a tail with a high aspect ratio, a conical head, a lateral fluke on the caudal peduncle. 2) The generalised sharks typified by the Carcharhinidae, have lower heterocercal angles, a flattened ventral surface on the head and lack the caudal fluke. 3) The demersal sharks typified by the catsharks (Scyliorhinidae) have a very low, almost straight tail. The ventral hypochordal lobe is absent and the first dorsal fin is posterior in position. 4) The squalomorph sharks are distinct in the absence of the anal fin, presence of a marked epicaudal lobe in the tail and often an elevated insertion of the pectorals. The anal and second dorsal fins are always the smallest fins and the pectorals grow at the fastest rate. In general there is an inverse relationship between size and rale of growth of all fins and the ventral surface of the head. In hammerheads the growth data confirms that the head has a significant planing action in swimming. The pectoral, second dorsal and anal fins show an extreme constancy of position of insertion in all sharks studied. The locomotor mechanism of sharks is adapted for an efficient cruising swimming but at the same time, the potential instability in the sagittal plan allows for the production of turning moments that are used in attack and feeding.