Small Sharks Research Articles

Anatomy and Evolution of Heterocercal Tail in Lamniform Sharks

Lamniformes is a small shark group consisting of 15 extant species with remarkably diverse lifestyles and a wide range in heterocercal tail morphology. The caudal fin morphology must be related to their lifestyle because the tail is a main locomotive structure in sharks, but such relationships have remained largely uninvestigated. Here, the morphology-lifestyle relationship in lamniforms is examined through phylogenetic mapping. This study suggests that, within Lamniformes, caudal fins with a more horizontally directed curvature of the vertebral column are plesiomorphic, whereas those with a large dorsally directed curvature of the vertebral column are apomorphic. It also shows that caudal fins with posteriorly directed hypochordal rays are plesiomorphic, and that those with ventrally directed hypochordal rays are apomorphic within Lamniformes. Four basic caudal fin types are recognized in lamniforms on the basis of these skeletal variables in which one corollary is that the evolution of external morphology of caudal fin does not necessarily correspond to the evolution of its skeletal anatomy. This study also demonstrates that specific lifestyles seen in different lamniforms are indeed correlative with different caudal fin types in which a less asymmetrical heterocercal tail is a derived feature in lamniforms that evolved for fast swimming to capture fast swimming prey.

Follicle Development in Immature Spiny Dogfish (Squalus acanthias): Histomorphometric Analysis

Squalus acanthias is a commercially exploited small coastal shark, highly sensitive to overexploitation due to a low reproductive potential and a low intrinsic rate of population increase. We collected specimens caught by commercial bottom trawls from April 2005 to January 2007 in the Adriatic Sea and investigated reproductive traits of the species in ovaries of 68 immature females (total body length range: 26.1-70.1 cm). Histomorphometric analyses included measurements of the growing follicle diameters, the width of the follicular layer and the width of the zona pellucida. We analysed 2700 ovarian follicles and grouped them into four categories: small previtellogenic, medium previtellogenic, large previtellogenic and small vitellogenic follicles. The growth of follicles was positively correlated with the body size of females (r=0.726; p<0.001). We documented significant changes in the thickness of the follicular layer and zona pellucida as follicles increased in size. Small vitellogenic follicles started to appear in small proportion (around 1% of follicles) in subadult females with the minimum size of 60 cm total body length. Yolk droplets first emerged in follicles larger than 600 μm in diameter while vitellogenesis was evident in follicles ranging 1.2-2.0 mm in diameter when both follicular layer and zona pellucida were the widest (41.1 μm and 49.1 μm, respectively).

Aerial Survey as a Tool to Estimate Abundance and Describe Distribution of a Carcharhinid Species, the Lemon Shark,Negaprion brevirostris

Aerial survey provides an important tool to assess the abundance of both terrestrial and marine vertebrates. To date, limited work has tested the effectiveness of this technique to estimate the abundance of smaller shark species. In Bimini, Bahamas, the lemon shark (Negaprion brevirostris) shows high site fidelity to a shallow sandy lagoon, providing an ideal test species to determine the effectiveness of localised aerial survey techniques for a Carcharhinid species in shallow subtropical waters. Between September 2007 and September 2008, visual surveys were conducted from light aircraft following defined transects ranging in length between 8.8 and 4.4 km. Count results were corrected for “availability”, “perception”, and “survey intensity” to provide unbiased abundance estimates. The abundance of lemon sharks was greatest in the central area of the lagoon during high tide, with a change in abundance distribution to the east and western regions of the lagoon with low tide. Mean abundance of sharks was estimated at 49 (±8.6) individuals, and monthly abundance was significantly positively correlated with mean water temperature. The successful implementation of the aerial survey technique highlighted the potential of further employment for shark abundance assessments in shallow coastal marine environments.

Palaeoecological implications of neoselachian shark teeth from the Bathonian (Middle Jurassic) ore-bearing clays at Gnaszyn, Kraków-Silesia Homocline, Poland

ABSTRACT Rees, J. 2012. Palaeoecological implications of neoselachian shark teeth from the Bathonian (Middle Jurassic) ore-bearing clays at Gnaszyn, Kraków-Silesia Homocline, Poland. Acta Geologica Polonica, 62 (3), 397-402. Warszawa. Systematic sampling through the Middle and Upper Bathonian strata at Gnaszyn has resulted in the discovery of 13 neoselachian teeth. Systematically, the teeth represent five taxa including Sphenodus sp., Protospinax sp. 1, Protospinax sp. 2, Palaeobrachaelurus sp. and another, indeterminate orectolobiform. The presence of two species of the flattened and bottom-dwelling Protospinax and two different orectolobiforms that are likely to have lived near the bottom, is a strong indication of oxygenated bottom conditions at the time of deposition. The dietary preferences of these taxa included a wide variety of benthic invertebrates. The synechodontiform Sphenodus may have been the first pelagic predatory neoselachian in the Jurassic, equipped with high and slender piercing teeth that formed a tearing-type dentition. The diet of Sphenodus probably included bony fish, smaller sharks and cephalopods.

CORRELATIONS OF CAPTURE, TRANSPORT, AND NUTRITION WITH SPINAL DEFORMITIES IN SANDTIGER SHARKS,CARCHARIAS TAURUS, IN PUBLIC AQUARIA

A number of captive sandtiger sharks (Carcharias taurus) in public aquaria have developed spinal deformities over the past decade, ranging in severity from mild curvature to spinal fracture and severe subluxation. To determine the frequency and etiologic basis of this disease, U.S. public aquaria participated in a two-stage epidemiologic study of resident sharks: 1) a history and husbandry survey and 2) hematology, clinical chemistry, and radiography conducted during health exams. Eighteen aquaria submitted data, samples, or both from 73 specimens, including 19 affected sharks (26%). Sharks caught off the Rhode Island coast or by pound net were smaller at capture and demonstrated a higher prevalence of deformity than did larger sharks caught from other areas via hook and line. Relative to healthy sharks, affected sharks were deficient in zinc, potassium, and vitamins C and E. Capture and transport results lead to two likely etiologic hypotheses: 1) that the pound-net capture process induces spinal trauma that becomes exacerbated over time in aquarium environments or 2) that small (and presumably young) sharks caught by pound net are exposed to disease-promoting conditions (including diet or habitat deficiencies) in aquaria during the critical growth phase of their life history. The last hypothesis is further supported by nutrient deficiencies among affected sharks documented in this study; potassium, zinc, and vitamin C play critical roles in proper cartilage-collagen development and maintenance. These correlative findings indicate that public aquaria give careful consideration to choice of collection methods and size at capture and supplement diets to provide nutrients required for proper development and maintenance of cartilaginous tissue.

Basking sharks in the northeast Atlantic: spatio-temporal trends from sightings in UK waters

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 459:121-134 (2012) - DOI: https://doi.org/10.3354/meps09737 Basking sharks in the northeast Atlantic: spatio-temporal trends from sightings in UK waters Matthew J. Witt1,2,*, Tom Hardy3, Louise Johnson4, Catherine M. McClellan2, Stephen K. Pikesley2, Sue Ranger2,5, Peter B. Richardson5, Jean-Luc Solandt5, Colin Speedie4, Ruth Williams3, Brendan J. Godley2 1Environment and Sustainability Institute, University of Exeter, Cornwall Campus, Penryn TR10 9EZ, UK 2Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Penryn TR 10 9EZ, UK 3Cornwall Wildlife Trust, Allet, Cornwall, UK 4Wave Action, 3 Beacon Cottages, Falmouth TR11 2LZ, UK 5Marine Conservation Society, Ross on Wye HR9 5NB, UK *Email: m.j.witt@exeter.ac.uk ABSTRACT: Basking sharks Cetorhinus maximus have undergone widespread historic exploitation in the northeast Atlantic and are of conservation concern. A greater knowledge of their spatial and temporal habitat use is required to better inform subsequent monitoring and management strategies. Techniques such as light-based geolocation have provided great insights into individual movements, but currently available data do not permit extrapolation to the population level. Public recording schemes may, however, help to fill shortfalls in data gathering, especially when analysed in conjunction with data from these other techniques. We analysed 11781 records (from 1988 to 2008) from 2 public recording databases operating in the UK. We describe 3 sightings hotspots: western Scotland, Isle of Man and southwest England, and highlight the marked seasonality of basking shark sightings, which were at their greatest during the northeast Atlantic summer (June to August). We further highlight a significant correlation between the duration of the sightings season in each year and the North Atlantic Oscillation, an atmosphere−ocean climate oscillation that has been linked to forcing of marine ecosystems. We augment patterns from public sightings records with effort-related data collected by boat-based transects at 2 regional sightings hotspots (western Scotland and southwest England). Analysis of reported body size data indicated that the annual proportion of small sharks (<4 m length) sighted by the public decreased, the proportion of medium-sized sharks sighted (4−6 m) increased, and the proportion of large sharks sighted (>6 m) remained constant. These patterns may be indicative of a population recovery following systematic harvesting in the 20th century. KEY WORDS: Basking shark · Cetorhinus maximus · Public sightings · Citizen-science · Marine vertebrates · Conservation Full text in pdf format Supplementary material PreviousNextCite this article as: Witt MJ, Hardy T, Johnson L, McClellan CM and others (2012) Basking sharks in the northeast Atlantic: spatio-temporal trends from sightings in UK waters. Mar Ecol Prog Ser 459:121-134. https://doi.org/10.3354/meps09737 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 459. Online publication date: July 12, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Estimating the Potential Impacts of Large Mesopredators on Benthic Resources: Integrative Assessment of Spotted Eagle Ray Foraging Ecology in Bermuda

Declines of large sharks and subsequent release of elasmobranch mesopredators (smaller sharks and rays) may pose problems for marine fisheries management as some mesopredators consume exploitable shellfish species. The spotted eagle ray (Aetobatus narinari) is the most abundant inshore elasmobranch in subtropical Bermuda, but its predatory role remains unexamined despite suspected abundance increases and its hypothesized specialization for mollusks. We utilized a combination of acoustic telemetry, benthic invertebrate sampling, gut content analysis and manipulative experiments to assess the impact of spotted eagle rays on Bermudian shellfish resources. Residency and distribution of adult spotted eagle rays was monitored over two consecutive summers in Harrington Sound (HS), an enclosed inshore lagoon that has historically supported multiple recreational and commercial shellfish species. Telemetered rays exhibited variable fidelity (depending on sex) to HS, though generally selected regions that supported relatively high densities of potential mollusk prey. Gut content analysis from rays collected in HS revealed a diet of mainly bivalves and a few gastropods, with calico clam (Macrocallista maculata) representing the most important prey item. Manipulative field and mesocosm experiments with calico clams suggested that rays selected prey patches based on density, though there was no evidence of rays depleting clam patches to extirpation. Overall, spotted eagle rays had modest impacts on local shellfish populations at current population levels, suggesting a reduced role in transmitting cascading effects from apex predator loss. However, due to the strong degree of coupling between rays and multiple protected mollusks in HS, ecosystem-based management that accounts for ray predation should be adopted.

Exploring the effects of spatial closures in a temperate marine ecosystem in Western Australia: A case study of the western rock lobster (Panulirus cygnus) fishery

An Ecospace model, was developed to investigate the potential impacts of spatial closures on key species and the food webs in the Jurien Bay Marine Park (JBMP), central Western Australia (823km2). The model comprised 80 functional groups (more than 200 species). The JBMP is a multiple use-marine park with several types of zoning and the effects of these on rock lobster biomass, mortalities and landings were explored using five Ecospace scenarios as baselines: (1) no-take Sanctuary Zones covering 4% of the total area; area of Sanctuary Zones changed to (2) 25%, (3) 50%; (4) 0%, i.e. No Sanctuary Zones; and (5) combining 25% of no-take areas with a 50% reduction in fishing mortality of the main targeted species. The results suggest that the introduction of the current management zones with 4% of the area in Sanctuary Zones produced a modest increase of ∼5% in the biomass of western rock lobster after 20 years, even with stable fishing effort. However, Western Rock Lobster biomass increased by ∼20% when the Sanctuary Zones covered 25% of the Park. Similar trends were observed for exploited fish species such as pink snapper, dhufish, and small sharks with their biomasses predicted to increase by up to 30% as the area of sanctuary zones increase from 4% to 50%. The largest predicted increases in biomass for the main target species were found when the protection from the sanctuaries (25%) was combined with a 50% reduction of fishing pressure. The catch of finfish species improved up to 70% (e.g. pink snapper) when a 25% of no-take area simulated was combined with a 50% reduction of fishing mortality during 20 years. To test the effects of habitat structure on the effectiveness of these fishing closures, a set of four scenarios were developed. We relocated the current 4% of sanctuaries, which are mainly associated with a reef-seagrass habitat, to the other three major habitats (reefs, seagrass and sand). The predicted fish biomass was higher when sanctuaries were placed on high structured habitats, such as reefs and seagrass-reefs (10–90%) than seagrass and sandy areas.

Systematics, palaeoecology and palaeobiogeography of the Neogene fossil sharks from the Azores (Northeast Atlantic)

The Azores Archipelago is a group of isolated islands located in the North Atlantic Ocean. One of these oceanic islands – Santa Maria – exhibits marine fossiliferous sediments of late Miocene/early Pliocene and also of Pleistocene age. Recent research provided new selachian fossil material, with three new records (Carcharias acutissima, Megaselachus megalodon, and Carcharhinus cf. leucas) increasing the number of fossil sharks reported from the Azores (Santa Maria Island) to seven species (Notorynchus primigenius, C. acutissima, Cosmopolitodus hastalis, Paratodus benedenii, Isurus oxyrinchus, M. megalodon, and C. cf. leucas). So far, no teeth of batoids or small sharks have been found despite the screen-washing of several sediment samples from Santa Maria. The Azorean Mio-Pliocene selachian fauna clearly differs from those described from sediments deposited on continental shelves, in which batoids and small benthic sharks (e.g., scyliorhinids) are usually well represented. During the late Miocene/early Pliocene, subtropical to warm-temperate seas were prevalent in the area of the Azores, as deduced from palaeontological, geological and isotopic data, all indicating a warmer climate than in the present.

A Streamlined DNA Tool for Global Identification of Heavily Exploited Coastal Shark Species (Genus Rhizoprionodon)

Obtaining accurate species-specific landings data is an essential step toward achieving sustainable shark fisheries. Globally distributed sharpnose sharks (genus Rhizoprionodon) exhibit life-history characteristics (rapid growth, early maturity, annual reproduction) that suggests that they could be fished in a sustainable manner assuming an investment in monitoring, assessment and careful management. However, obtaining species-specific landings data for sharpnose sharks is problematic because they are morphologically very similar to one another. Moreover, sharpnose sharks may also be confused with other small sharks (either small species or juveniles of large species) once they are processed (i.e., the head and fins are removed). Here we present a highly streamlined molecular genetics approach based on seven species-specific PCR primers in a multiplex format that can simultaneously discriminate body parts from the seven described sharpnose shark species commonly occurring in coastal fisheries worldwide. The species-specific primers are based on nucleotide sequence differences among species in the nuclear ribosomal internal transcribed spacer 2 locus (ITS2). This approach also distinguishes sharpnose sharks from a wide range of other sharks (52 species) and can therefore assist in the regulation of coastal shark fisheries around the world.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution.

Stable isotope profiles of large marine predators: viable indicators of trophic position, diet, and movement in sharks?

Understanding the role of predators is challenging but critical for ecosystem management. For community dynamics, predator-specific size-based variation in diet, trophic position, and habitat use are rarely accounted for. Using two applied tools (stable isotopes and stomach content data), we examined inter- and intra-species ontogenetic variability in diet (stomach contents), trophic position (TPSIA for δ15N and TPSCA for stomach contents), and habitat use (δ13C) of two large sharks, the scalloped hammerhead ( Sphyrna lewini ) and the dusky ( Carcharhinus obscurus ). Stomach contents identified size-based and gender-specific shifts in diet indicating resource partitioning for and between species. Calculated TP for the two sharks varied by method, either TPSIA or TPSCA and with species, size, and gender, but were complicated by differing baselines and broad functional prey groups, respectively. TP increased with size for S. lewini, but was low in large C. obscurus compared with small sharks. Size-based δ13C profiles indicated habitat partitioning by sex in S. lewini and a movement to shelf edge foraging in large C. obscurus. These results demonstrate that predators exert proportional size-based effects on multiple components of the marine system that are further complicated by species- and gender-specific strategies.

Trophic ecology of reef sharks determined using stable isotopes and telemetry

Establishing the ecological role of predators within an ecosystem is central to understanding community dynamics and is useful in designing effective management and conservation strategies. We analysed differences in the trophic ecology of four species of reef sharks (Carcharhinus melanopterus, Carcharhinus amblyrhynchos, Triaenodon obesus and Negaprion acutidens) at Ningaloo Reef, Western Australia, by analysing tissue stable isotopes (δ15N and δ13C). We also monitored animals using acoustic telemetry to determine long-term residency patterns in a bay at the southern end of the reef, Coral Bay. Overall, mean δ13C was similar among species, ranging between −10.9 and −11.8‰, suggesting a food-web dependency on coastal producers. Classification and regression tree analysis identified an effect of species on δ15N that separated C. amblyrhynchos and C. melanopterus from N. acutidens and T. obesus. For C. amblyrhynchos and C. melanopterus, animals were also divided by size classes, with smaller sharks having lower average δ15N than larger animals; this suggests that δ15N increases with size for these two species. Juvenile C. melanopterus, juvenile N. acutidens and adult T. obesus had trophic levels of 3.7, for juvenile C. amblyrhynchos and adult C. melanopterus it was 4, and adult C. amblyrhynchos had a value of 4.3. Trophic-level estimates for C. melanopterus and C. amblyrhynchos corroborate previous conclusions based on diet studies. We found no evidence for a difference in isotopic composition between resident and non-resident sharks. The lack of variation in isotopic composition was consistent with high mean residency of these species recorded using acoustic telemetry, which was 79% (±0.09 SE) of days monitored for T. obesus, followed by N. acutidens (57 ± 19.55%), C. amblyrhynchos (54 ± 13%) and C. melanopterus (33 ± 8.28%). High δ13C composition in reef sharks and long-term residency behaviour suggest that coastal marine reserves might provide effective conservation refuges for some species.

Isolation and characterization of microsatellite markers for the blacknose shark, Carcharhinus acronotus

The blacknose shark, Carcharhinus acronotus, is distributed in coastal waters of the western Atlantic Ocean from North Carolina to Brazil and is a component of both commercial and recreational fisheries throughout its range. In waters off of the United States, the species is managed as a component of the small coastal shark (SCS) complex. Concerns recently have arisen that blacknose sharks are overfished and experiencing overfishing. Here, we report polymerase-chain-reaction (PCR) primers for 37 polymorphic and 26 monomorphic microsatellites isolated from an enriched genomic library of C. acronotus DNA and characterized in 32 individuals. Polymorphic repeat motifs included 36 dinucleotides and one pentanucleotide. The characterized microsatellites will be useful in studies which involve conservation and management of blacknose shark resources.

Mouth-Size Estimation of a Primitive Lamniform Shark,Protolamna: Low Trophic Position in Lamniform Shark Origin

Abstract. The Lamniformes are a shark clade which occupies the highest trophic-level regimes in extant marine ecosystems, and includes the largest fish-eating shark, Carcharodon carcharias. However, the ancestor of this taxon is thought to be a small shark. In order to evaluate the trophic position of the lamniform shark ancestor, I first developed a new method to estimate the mouth size of fossil lamniform sharks from isolated teeth, and then estimated the mouth size of the oldest recorded lamniform shark genus, Protolamna. This method can predict the mouth size of a fossil lamniform shark if the original tooth position within a jaw or the most closely related extant species is unknown. The result suggests that the mouth size of Protolamna was smaller than most extant Lamniformes. This estimate indicates that they likely fed on small prey, considering that mouth size determines the upper limit of prey size. This finding indicates that extant lamniform sharks evolved from lower trophic-level ancestors.

Assessing the overlap between the diet of a coastal shark and the surrounding prey communities in a sub‐tropical embayment

An elasmobranch survey of sub-tropical Hervey Bay, Australia, captured the slit-eye shark Loxodon macrorhinus at only one of three sites sampled. The dietary composition of this small shark species was compared to the prey communities within Hervey Bay to test whether prey availability was driving this observation. Dietary analysis of prey groups revealed that teleosts dominated the diet, per cent index of relative importance, % I(RI) (79·5%) and per cent geometric index of importance, % G(II) (52·7%), with shrimp-like invertebrates and cephalopods identified as the most important invertebrate prey groups. Baited remote underwater video (BRUV) used to sample prey communities at each site, demonstrated a highly diverse and significantly different community composition among the sites. There was no significant overlap between the diet of L. macrorhinus and any of the prey communities detected by BRUVs according to one-way analysis of similarities and the simplified Morisita index. Habitat electivity analysis revealed affinity of L. macrorhinus for the site with the highest water clarity (Secchi disc depth), opposing that of three other shark species. Overall, the results suggest that the distribution of L. macrorhinus is not driven by prey availability but other factors such as water clarity, predator avoidance or a reduction in interspecies competition.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution. J. Exp. Zool. (Mol. Dev. Evol.) 318:79–90, 2012. © 2011 Wiley Periodicals, Inc.

Quantifying movement patterns for shark conservation at remote coral atolls in the Indian Ocean

Grey reef sharks (Carcharhinus amblyrhynchos) are apex predators found on many Indo-Pacific coral reefs, but little is known about their movement patterns and habitat requirements. We used acoustic telemetry to determine movements and habitat use of these sharks at the isolated Rowley Shoals atolls, 250 km off the coast of north-western Australia. We equipped 12 male and 14 female sharks ranging from 0.79 to 1.69 m in total length with transmitters that were detected by an array of 11 strategically placed receivers on two atoll reefs. Over 26,000 detections were recorded over the 325 days of receiver deployment. No sharks were observed to move between reefs. Receivers on the outer slopes of reefs provided nearly all (99%) of the detections. We found no differences in general attendance parameters due to size, sex or reef, except for maximum period of detection where larger sharks were detected over a longer period than smaller sharks. Male and female sharks were often detected at separate receivers at the outer slope habitat of one reef, suggesting sexual segregation, but this pattern did not occur at the second reef where males and females were detected at similar frequencies. We identified two patterns of daily behaviour: (1) sharks were present at the reef both day and night or (2) sharks spent more time in attendance during day than at night. Fast Fourier transforms identified 24-h cycles of attendance at the reef and a secondary peak of attendance at 12 h for most sharks, although no individuals shared the same attendance patterns. Our study provides baseline data that can be used to optimise the minimum area and habitat requirements for conservation of these apex predators.

Identification and evaluation of shark bycatch in Georgia’s commercial shrimp trawl fishery with implications for management

Abstract Many US states have recreational and commercial fisheries that occur in nursery areas occupied by subadult sharks and can potentially affect their survival. Georgia is one of few US states without a directed commercial shark fishery, but the state has a large, nearshore penaeid shrimp trawl fishery in which small sharks occur as bycatch. During our 1995–1998 investigation of bycatch in fishery-dependent sampling events, 34% of 127 trawls contained sharks. This bycatch totalled 217 individuals from six species, with Atlantic sharpnose shark, Rhizoprionodon terraenovae (Richardson), the most common and finetooth shark, Carcharhinus isodon (Muller & Henle) and spinner shark, Carcharhinus brevipinna (Muller & Henle), the least common. The highest catch rates for sharks occurred during June and July and coincided with the peak months of the pupping season for many species. Trawl tow speed and tow time did not significantly influence catch rates for shark species. Gear configurations [net type, turtle excluder device (TED), bycatch reduction device] affected catch rates for shark species. Results of this study indicate gear restrictions, a delayed season opening, or reduced bar spacing on TEDs may reduce shark bycatch in this fishery.

The Function of Bilateral Odor Arrival Time Differences in Olfactory Orientation of Sharks

The direction of an odor signal source can be estimated from bilateral differences in signal intensity and/or arrival time. The best-known examples of the use of arrival time differences are in acoustic orientation. For chemoreception, animals are believed to orient by comparing bilateral odor concentration differences, turning toward higher concentrations. However, time differences should not be ignored, because odor plumes show chaotic intermittency, with the concentration variance several orders of magnitude greater than the concentration mean. We presented a small shark species, Mustelus canis, with carefully timed and measured odor pulses directly into their nares. They turned toward the side stimulated first, even with delayed pulses of higher concentration. This is the first conclusive evidence that under seminatural conditions and without training, bilateral time differences trump odor concentration differences. This response would steer the shark into an odor patch each time and thereby enhance its contact with the plume, i.e., a stream of patches. Animals with more widely spaced nares would be able to resolve smaller angles of attack at higher swimming speeds, a feature that may have contributed to the evolution of hammerhead sharks. This constitutes a novel steering algorithm for tracking odor plumes.