Ultrasonic tracking of Greenland sharks,<Emphasis Type="Italic"> Somniosus microcephalus</Emphasis>, under Arctic ice

Six Greenland sharks, Somniosus microcephalus (Bloch and Schneider, 1801), 190–355 cm fork length, were tracked under land-fast sea ice off northern Baffin Island (73.2°N; 85.3°W) between 16 and 28 May 1999, using ultrasonic telemetry. The sharks were tracked continuously for periods of 5.5–13.0 h, with the tracks of two individuals lasting 31.4 and 42.8 h, respectively, each with an interval when the track was lost. Several sharks dove after release and moved along the ocean bottom for the duration of the tracking period, while others varied their movements regarding course and depth. Two sharks made repeated visits to within 11 m of the ice–water interface from deeper water. The tracked sharks exhibited no apparent depth or temperature preferences, and pooled data indicated that sharks remained deep during the morning and gradually moved into shallower depths through the afternoon and night. Rates of descent (average=0.099 m s^–1) were significantly greater than rates of ascent (average=0.058 m s^–1) for all sharks, and the average rate of horizontal movement over ground was estimated as 0.215 m s^–1. Based on the movements of tracked sharks and information contained in the literature, S. microcephalus may prey on seals in areas covered by land-fast sea ice.Communicated by J.P. Grassle, New Brunswick     

Quantitative analysis of aggregation behavior in juvenile blacktip sharks

Aggregation patterns of a population of juvenile blacktip sharks (Carcharhinus limbatus) within a summer nursery area were examined over a period of 3 years using an array of acoustic monitors. Individually coded acoustic tags were surgically implanted into 33–40 neonate sharks each year from 2000 to 2002 and their presence monitored continuously by the acoustic array. Data from the acoustic monitors was processed to estimate the center of activity location of each tagged shark every 30 min. Nearest neighbor analysis of shark locations revealed that sharks aggregated within the nursery in all years of the study. Sharks were aggregated most commonly during the early and late summer months (June, October and November) and became less common through the middle of the study period each year (July–September). Temporal periodicity within the data revealed a strong diel pattern with sharks aggregating during the day and dispersing at night, except in June when aggregations often persisted through the night. Aggregations were generally restricted to the mid and northern sections of the study site. Reasons for aggregations may include predator avoidance or improved feeding efficiency.     

Satellite tagging of blue sharks (Prionace glauca) and other pelagic sharks off eastern Australia: depth behaviour, temperature experience and movements

Satellite telemetry was used to study the movements and behaviour of ten blue sharks and one individual each of shortfin mako, thresher and bigeye thresher off eastern Australia. The tracks showed latitudinal movements of up to 1,900 km, but none of the sharks travelled away from the eastern Australian region. Tracking periods did not exceed 177 days. All species showed oscillatory dive behaviour between the surface layers to as deep as 560–1,000 m. Blue sharks spent 35–58% of their time in <50 m depths and 10–16% of their time in >300 m. Of these four species, the bigeye thresher spent the least time in the surface layers and the most time at >300 m depth. All four species showed clear diel behaviour generally occupying shallower depths at night than during the day. Blue sharks were mainly in 17.5–20.0°C water, while the thresher sharks showed a more bimodal temperature distribution.     

Movement and environmental preferences of Greenland sharks (<Emphasis Type="Italic">Somniosus microcephalus</Emphasis>) electronically tagged in the St. Lawrence Estuary,Canada

Three Greenland sharks (Somniosus microcephalus) were tagged with electronic tags, in Baie St. Pancrace, St. Lawrence Estuary, Quebec, Canada. One shark was tagged on 23 July 2004, with an acoustic telemetry tag. Two sharks were each tagged with a pop-up satellite archival tag (PSAT) on 27 August 2004. Two of the sharks remained in or close to the bay, one for 47 days and the other for at least 66 days. The third shark left the bay immediately after tagging on 27 August 2004. This shark entered the main channel of the St. Lawrence Estuary, and had moved 114.9 km upstream by 1 November 2004 when the tag reported to ARGOS satellites. The tags provided a total of 179 days of data on the movement and environmental preferences of Greenland sharks in the St. Lawrence Estuary. Sharks that reported depth and ambient water temperature data from the bay showed significant diel differences in depth preferences and corresponding ambient temperatures. The sharks remained near the bottom of the water column during the day and displayed increased vertical movements at night. The shark that resided in the main channel did not show this pattern, but generally remained at depths between 325 and 352 m. Sharks in the bay experienced water temperatures that ranged from −1.1 to 8.6°C at depths from 0 to 67 m. In the main channel the shark experienced temperatures that ranged from 1.0 to 5.4°C at depths from 132 to 352 m. This is the first report of numerous Greenland sharks inhabiting shallow near shore bays during summer and autumn.     

Movement patterns of young Caribbean reef sharks, Carcharhinus perezi, at Fernando de Noronha Archipelago, Brazil: the potential of marine protected areas for conservation of a nursery ground

The movement patterns and long-term site-fidelity of primarily juvenile Caribbean reef sharks, Carcharhinus perezi, were investigated using tag-recapture and automated telemetry at an insular nursery area, the Fernando de Noronha Archipelago, Brazil. Of the 143 externally tagged juvenile sharks (<110 cm), 22 (15.3%) were recaptured between 0 and 5 km from the site of tagging after 5–800 days at liberty, suggesting some site-fidelity in young individuals of this species. Site-fidelity and movement patterns of ten juvenile sharks ranging from 78 to 110 cm total length (TL) and one opportunistically captured adult female (224 cm TL) were also investigated for periods of up to 2 years with an array of automated telemetry receivers. Tagging and telemetry data from both inside and outside a marine protected area (MPA) show that shark abundance and activity is greatest along the part of the archipelago’s coastline least disturbed by human activity. Telemetry tracking also showed that juvenile reef sharks demonstrated a high degree of site-fidelity and occupied specific locations along the coast throughout the year, with some evidence of an increase in activity space with ontogeny. Sharks appeared to range more widely at night and there were no seasonal variations in habitat use. Our results suggest that MPAs may be a useful conservation tool to protect young C. perezi and potentially other reef-dwelling carcharhinid sharks during their early life history.     

Distribution, seasonal abundance and bycatch of basking sharks (Cetorhinus maximus) in New Zealand, with observations on their winter habitat

Basking sharks occur throughout New Zealand, but are most common in cool temperate latitudes of 39-51°S. Inshore records from miscellaneous sources peaked in spring-summer, with few winter records. Two records were of sharks observed in a large brackish water lake. About 203 basking sharks were observed caught by commercial trawlers between 1986 and 1999. Multiple captures were common, including 14 in one tow. Most trawl-caught sharks were taken near or outside the 250 m depth contour, and 91% came from three small regions - East Coast (EC) and West Coast (WC) of South Island and Snares-Auckland Islands (SA). The highest catch (93 sharks) and catch rate (58 sharks per 1,000 tows) were from EC, where sharks were caught only in spring-summer. In SA, sharks were caught mainly in summer, and in WC all were caught in winter. The modal seabed depths for shark tows were 300-400 m at EC, 700-800 m at WC and 150-250 m at SA. Sharks were therefore caught in the deepest water in winter at WC. It was impossible to determine the actual depths of capture, but circumstantial evidence indicates that most sharks were caught on or near the bottom. The capture of some sharks in midwater in winter argues against hibernation, because hibernating sharks are unlikely to hover in midwater. Males dominated catches in all regions, particularly in WC and SA. In WC and SA, most sharks (94%) were 7-8 m long, whereas in EC most sharks (73%) were <7 m. Based on their lengths, many of the WC and SA males could have been mature, but most EC males were probably immature. Few of the females would have been mature. This study provides support for the hypothesis that basking sharks over-winter in deep water on the continental slope.     

A multiple instrument approach to quantifying the movement patterns and habitat use of tiger (Galeocerdo cuvier) and Galapagos sharks (Carcharhinus galapagensis) at French Frigate Shoals, Hawaii

We equipped individual tiger (Galeocerdo cuvier Péron and Lesueur, 1822) and Galapagos (Carcharhinus galapagensis Snodgrass and Heller, 1905) sharks with both acoustic and satellite transmitters to quantify their long-term movements in the Papahanaumokuakea Marine National Monument (Northwestern Hawaiian Islands). Tiger sharks exhibited two broad patterns of behavior. Some individuals were detected at French Frigate Shoals (FFS) year round, whereas others visited FFS atoll in summer to forage on fledging albatross, then swam thousands of kilometers along the Hawaiian chain, or out into open ocean to the North Pacific transition zone chlorophyll front, before returning to FFS in subsequent years. These patterns suggest tiger sharks may use cognitive maps to navigate between distant foraging areas. Different patterns of spatial behavior may arise because cognitive maps are built up through individual exploration, and each tiger shark learns a unique combination of foraging sites. Galapagos shark detections were all associated with FFS, suggesting these sharks may be more resident around oceanic islands. Both Galapagos and tiger sharks primarily used the mixed layer (<100 m depth) and made occasional deeper dives through the thermocline down to 680 m. Results show reef-associated sharks utilize a wide variety of habitats ranging from shallow atoll lagoons to deep reefs and open ocean and may provide important trophic links between these habitats.     

Resident mysids: community structure,abundance and small-scale distributions in a coral reef lagoon

Seasonal and diel variations in community structure and abundance of coral-reef lagoon mysids were examined at Davies Reef in the central region of the Great Barrier Reef (GBR) between June 1980 and May 1981. Twenty-five mysid species belonging to three subfamilies of the family Mysidae were captured during the study, with six new records for the GBR. The epibenthic mysid community differed from that in the overlying water, was faunistically uniform, but formed characteristic seasonal and diel groupings. The dominant epibenthic species were Erythrops sp., Anisomysis pelewensis, Doxomysis littoralis, A. laticauda, Prionomysis stenolepis, A. lamellicauda, and A. australis, five of which formed schools. Total mysid abundances ranged between 110 and 790 m^-3 with peak abundance in October. Schooling species occurred at local densities of up to 500 000 m^-3. Mysids were absent from shallow and midwater depths during the day, but were distributed throughout all depths at night with peak abundances in mid-water and deep layers. The dominant species in the water column at night were Pseudanchialina inermis, A. laticauda and Gastrosaccus indicus, in descending order of abundance. Lagoonal mysids contribute little to the food of sessile reef planktivores, as all but three species remain concentrated near or on the lagoon floor both day and night. The contribution of resident lagoon mysids to reef trophodynamics is probably through remineralization of lagoon detritus. Given the vast reef areas comprised of sandy lagoons, the large populations and relatively large size of lagoon mysids, this trophodynamic role may be of considerable importance.A.I.M.S. Contribution No. 477     

Vertical distribution and migration of fishes of the lower mesopelagic zone off Oregon

An exceptionally large midwater trawl (50 m² mouth area) with 5 opening and closing codends was towed horizontally in the lower mesopelagic zone at depths of 500, 650, 800 and 1000 m off Oregon (USA) from 1–6 September, 1978. In comparison to more conventional trawls, ours collected more fish, including rare species and large individuals of common species. Comparison of collections made by day and by night revealed that 12 of the 15 most common species probably migrated vertically. Bathylagus milleri evidently migrates from 650 m during the day to 500 m at night. Cyclothone acclinidens and C. atraria were more abundant by night than by day at 800 m, possibly due to an upward migration from deeper depths at night. C. pseudopallida, C. signata, Chauliodus macouni, Tactostoma macropus and Stenobrachius leucopsarus were more abundant by day than by night at 500 m, suggesting that they migrated out of this depth horizon at night. Lampanyctus regalis, and large individuals of B. pacificus were more abundant by night than by day at 500 m, possibly because they migrated upward from near 650 m. Many species exhibited trends of increasing or decreasing size with depth, and several species showed changes in migratory behavior with size. For example, only small (<240 mm) T. macropus migrated vertically, whereas only large (>110 mm) B. pacificus appeared to migrate. Depths of maximum abundance of congeneric species were usually separated. B. milleri and B. pacificus had similar distributions by day, but the former was shallower at night. S. leucopsarus tended to live shallower than S. nannochir both day and night. Congeners always occurring at the same depth were Cyclothone pseudopallida and C. signata (both most abundant at 500 m) and C. acclinidens and C. atraria (both most abundant at 800 m).     

Trace metal content in deep-water sharks from the eastern Mediterranean Sea

Trace metals were analyzed in the muscle and other organs from several species of deep-water sharks (particularly Centrophorus granulosus and Galeus melastomus) from 1280 to 1500 m depth in the eastern Mediterranean between 1985 and 1991. As has been shown with other pelagic fish, there was a significant correlation between specimen size and the mercury concentration in the muscle as well as in the liver and kidneys. For a given size, the level of mercury in sharks from the eastern Mediterranean was considerably higher compared with the same species caught off the west coast of Italy. There was no significant correlation between the concentrations of Cd, Cu, Zn and Fe in the muscle (or other organs) and body size. The overall trace metal content was considerably higher in the specimens from the eastern Mediterranean compared with the same species from the N. E. Atlantic. It is suggested that these higher-than-expected trace metal levels might be due to a relatively high trace metal content in the waters of the region or to the unusual physiology of these fish, possibly related to the extreme oligotrophic conditions in the area.     

Highly directional swimming by scalloped hammerhead sharks,Sphyrna lewini,and subsurface irradiance,temperature, bathymetry,and geomagnetic field

The homing behavior of scalloped hammerhead sharks (Sphyrna lewini) to and fro between Espiritu Santo Seamount and Las Animas Island and the surrounding pelagic environment was studied to reveal their mechanisms of navigation in the oceanic environment. Four sharks were tagged with ultrasonic transmitters and tracked at the former location and one shark at the latter site during July, August, or September between 1981 and 1988. Hammerhead swimming movements were highly oriented: the mean coefficient of concentration (r) for sets of ten consecutive swimming directions recorded during eight homing movements by three hammerhead sharks ranged from 0.885 to 0.996. Drift within a current could not explain this directionality, since highly variable directions were recorded from a transmitter floating at the sea surface after becoming detached from a shark. Forward swimming momentum was an unlikely explanation, since highly directional swimming was maintained for a period of 32 min with only a gradual change in course. To maintain directionality over this period, an environmental property should be necessary for guidance. The hammerheads swam at night, with repeated vertical excursions ranging from 100 to 450 m deep, out of view of either the sea surface or the sea floor. The sharks' vertical diving movements were compared to distributions of spectral irradiance (relative to elasmobranch scotopic and photopic visual sensitivities), temperature, and current-flow directions in the water column. No relationships were evident between these properties and the sharks' oriented swimming movements. Movements of scalloped hammerhead sharks to and from a seamount were compared to topographic features in bathymetry and geomagnetic field leading away from the seamount. Sharks swam repeatedly over fixed geographic paths, and these paths occurred less often along submarine ridges and valleys than maxima and minima in the geomagnetic field. No significant difference existed between the degree of association of points from the sharks' tracks and points from track simulations and 20° changes in the slope of the depth record. On the other hand, significantly more points from the sharks' tracks were associated with slope changes in the magnetic intensity record than points from track simulations. A magnetic intensity gradient of 0.037 nanoteslas/m (nT/m) existed at 175 m depth, where a shark swam directionally, and this gradient was three times steeper than that measured at the sea surface and exceeded that recorded at a depth of 200 m. The hammerheads are hypothesized to find the seamount using geomagnetic topotaxis. The shark could be attracted to and move back and forth along ridges and valleys, features in the relief of magnetic field intensities occurring over a geographical area.     

Observations on the short-term movements and behaviour of whale sharks (Rhincodon typus) at Ningaloo Reef, Western Australia

The short-term movements and behaviour of whale sharks (Rhincodon typus Smith, 1828) during March 1994 and April 1997 are reported from data collected by acoustic tracking and archival tags at Ningaloo Reef on the north west coast of Western Australia. Sharks were tracked for up to 26 h and generally swam slowly at ≃0.7 m s⁻¹ parallel to the reef edge; occasionally they swam in a wide arc adjacent to passes in the reef. All tracked sharks made regular dives through the water column, mostly from the surface to near the bottom. These dives did not appear to be related to hydrographic features, and the sharks were probably searching the water column for food. Most sharks were accompanied by other fishes, usually the golden trevally Gnathanodon speciosus. Received: 19 January 1999 / Accepted: 22 June 1999     

Anguilliform larvae collected off North Carolina

The distinctive larval stage of eels (leptocephalus) facilitates dispersal through prolonged life in the open ocean. Leptocephali are abundant and diverse off North Carolina, yet data on distributions and biology are lacking. The water column (from surface to 1,293 m) was sampled in or near the Gulf Stream off Cape Hatteras, Cape Lookout, and Cape Fear, North Carolina during summer through fall of 1999–2005, and leptocephali were collected by neuston net, plankton net, Tucker trawl, and dip net. Additional samples were collected nearly monthly from a transect across southern Onslow Bay, North Carolina (from surface to 91 m) from April 2000 to December 2001 by bongo and neuston nets, Methot frame trawl, and Tucker trawl. Overall, 584 tows were completed, and 224 of these yielded larval eels. The 1,295 eel leptocephali collected (combining all methods and areas) represented at least 63 species (nine families). Thirteen species were not known previously from the area. Dominant families for all areas were Congridae (44% of individuals, 11 species), Ophichthidae (30% of individuals, 27 species), and Muraenidae (22% of individuals, ten species). Nine taxa accounted for 70% of the overall leptocephalus catches (in order of decreasing abundance): Paraconger caudilimbatus (Poey), Gymnothorax ocellatus Agassiz complex, Ariosoma balearicum (Delaroche), Ophichthus gomesii (Castelnau), Callechelys muraena Jordan and Evermann, Letharchus aliculatus McCosker, Rhynchoconger flavus (Goode and Bean), Ophichthus cruentifer (Goode and Bean), Rhynchoconger gracilior (Ginsburg). The top three species represented 52% of the total eel larvae collected. Most leptocephali were collected at night (79%) and at depths > 45 m. Eighty percent of the eels collected in discrete depth Tucker trawls at night ranged from mean depths of 59–353 m. A substantial number (38% of discrete depth sample total) of larval eels were also collected at the surface (neuston net) at night. Daytime leptocephalus distributions were less clear partly due to low catches and lower Tucker trawl sampling effort. While net avoidance may account for some of the low daytime catches, an alternative explanation is that many species of larval eels occur during the day at depths > 350 m. Larvae of 21 taxa of typically shallow water eels were collected at depths > 350 m, but additional discrete depth diel sampling is needed to resolve leptocephalus vertical distributions. The North Carolina adult eel fauna (estuary to at least 2,000 m) consists of 51 species, 41% of which were represented in these collections. Many species of leptocephali collected are not yet known to have juveniles or adults established in the South Atlantic Bight or north of Cape Hatteras. Despite Gulf Stream transport and a prolonged larval stage, many of these eel leptocephali may not contribute to their respective populations.     

Temporal changes in feeding habits and daily rations of<Emphasis Type="Italic"> Hoplostethus mediterraneus</Emphasis> in the bathyal Ionian Sea (eastern Mediterranean)

Data on the diet, feeding habits and daily rations of Hoplostethus mediterraneus Cuvier, 1829 in the bathyal eastern Ionian Sea (Mediterranean Sea) are presented. A total of 430 specimens collected by bottom trawls at depths ranging from 473 to 603 m during four 24-h day–night sampling cycles covering the four annual seasons was examined. H. mediterraneus diet consisted of pelagic and vagile epibenthic prey, mainly crustaceans, and was dominated by benthopelagic natantian decapods (83.35% IRI, index of relative importance). Seasonal changes in diet were apparent and related to seasonal fluctuations in suprabenthic and zooplanktonic prey in the environment. Diel patterns in stomach fullness and trends in diel feeding cycles are discussed in relation to the vertical migratory movements of available prey (i.e. suprabenthos and zooplankton). Daily-ration estimates were determined by evacuation-rate models and ranged from 0.143% to 0.397% WW/WW. Overall, daily-ration estimates were within the range of the daily consumption of other deep-sea fish. Deduced from diet contents, we found a constant gross energy intake (305–316 kcal g^–1) during all seasons. As a possible response to the reproductive peak of mature females observed in summer, H. mediterraneus increases its food consumption, which, in turn, is coupled with an increase in food availability.Communicated by S.A. Poulet, Roscoff     

Archival tagging of subadult and adult common thresher sharks (Alopias vulpinus) off the coast of southern California

The common thresher shark (Alopias vulpinus) is a secondary target species of the California drift gillnet fishery (CA-DGN) and supports a growing recreational fishery in California waters. This study used archival tags to examine the movement patterns and habitat preferences of common threshers of the size range captured in the CA-DGN (>120 cm fork length). Depth and temperature-logging archival tags were deployed on 57 subadult and adult common threshers in the Southern California Bight. Tags from five individuals (8.8%) were recovered, and 154 days of data were successfully obtained from four of these. By night, shark movements were primarily limited to waters above the thermocline, which ranged in depth from 15 to 20 m. Sharks were significantly deeper by day, and daytime vertical distribution consisted of two distinct modes: a ‘shallow mode’ (wherein sharks occupied only the upper 20 m of the water column) and a ‘deep mode’ (characterized by frequent vertical excursions below the thermocline). This modal switch is interpreted as relating to regional differences in abundance of surface-oriented prey and prey in deeper water. Maximum dive depth was 320 m, greatest dive duration was 712 min, minimum temperature experienced during a dive was 9.1°C, and dive descent rate was significantly greater than ascent rate. Sharks inhabited waters corresponding to a sea surface temperature range of 16 to 21°C. The nocturnal depth distribution of common threshers has implications for management of drift gillnet deployment depths in the CA-DGN.     

Using time-depth-light recorders to measure light levels experienced by a diving marine mammal

Acoustic telemetry was used to track vertical and horizontal movement patterns and to monitor the stomach temperatures of seven juvenile shortfin mako sharks (Isurus oxyrinchus Rafinesque) in the Southern California Bight from July to November 2002. Makos (80–145 cm fork length, FL) were attracted to the tracking vessel, where they were fed a mackerel containing an acoustic transmitter that reported temperature and pressure. Tracks ranged from 6.8–45.4 h. Collectively, the mako sharks spent 80% of the track record at 0–12 m, 15% at 12–24 m, and 5% at depths >24 m. The average horizontal swimming speed was 2.3 km h^–1 or 0.55 FLs s^–1, and the greatest distance traveled was 145 km in 45.4 h. For the six tracks >21 h, there was a positive correlation between body size and maximum depth. Makos used more of the water column during daylight hours. Mean stomach temperature was 3.8±1.5°C above ambient, and body size was positively correlated with both maximum and average stomach temperature. Stomach content analyses of four makos captured at the end of tracking verified the occurrence of feeding events as indicated by changes in stomach temperature.Electronic Supplementary Material Supplementary material is available in the online version of this article at Communicated by J.P. Grassle, New Brunswick     

Genetic structure of leopard shark (<Emphasis Type="Italic">Triakis semifasciata</Emphasis>) populations in California waters

The leopard shark (Triakis semifasciata) is an important predator in coastal marine ecosystems of California, targeted by recreational and commercial fishermen and of specific interest in fisheries management. From October 2003 to August 2006, 169 leopard sharks were collected from the coast of California (between 40.750°N and 32.678°N) and analyzed for mitochondrial and nuclear genetic structure. Analyses of mtDNA control region sequences revealed relatively low levels of genetic variation (five haplotypes, average pairwise divergence π = 0.0067). In contrast, leopard sharks were highly polymorphic for inter simple sequence repeats (ISSRs), which characterize a broad range of the nuclear genome. The null hypothesis of panmixia in California waters was rejected for both genetic markers, and ISSRs displayed a statistically significant pattern of isolation by distance (IBD) across the species range (P = 0.002). A variety of analyses showed that divergence is most pronounced in the northernmost population of Humboldt Bay. Natal philopatry in T. semifasciata was tested using Siegel-Tukey tests on data partitioned by breeding site status, and sex-specific philopatry was tested by comparing IBD plots between sexes. Although there was some evidence for natal philopatry in leopard sharks (P = 0.038), and population divergence may be related to the proximity of breeding sites (P = 0.064), we found no support for sex-specific philopatry. In addition to identifying a novel set of highly variable genetic markers for use in shark population studies, these results may be used to better inform management decisions for leopard sharks in California.     

The hunting strategy of white sharks (Carcharodon carcharias) near a seal colony

The degree to which white sharks, Carcharodon carcharias, are social while hunting is unclear. Our aim was to describe the behavior and interactions among white sharks hunting seals near a seal colony. We attached ultrasonic beacons to five adult white sharks, 4.5–5.2?m long, and recorded their movements and behavior toward each other over a 15-day period in October 1997 at Año Nuevo Island, California. This site is home to colonies of four species of seals and sea lions. Two additional sharks, females 5.5 and 4.7?m in length, were later tracked intensively during periods of 12 and 3?days during October 1998 and November 1999, respectively. We recorded stomach temperature (indicative of feeding on warm-bodied seals) and swimming depths from the 5.5-m female, swimming speed and depth from the 4.7-m female. We monitored the movements and behavior of these sharks using an array of sonobuoys moored near the island; the receptive field measured 1?km². Our principal findings were: (1) the sharks spent a mean time of 39.5% of each day patrolling within the receptive field; (2) no shark ever moved far out of it; (3) the sharks spent an equal amount of time and activity in the receptive field at all times of the day, daytime, twilight, and nighttime; (4) movements with respect to the island rookery were most often back and forth parallel to the shoreline, (5) tracks of three sharks, tagged at the same time and place, overlapped more often than those of the other two sharks; and (6) some sharks patrolled certain areas in the field preferentially, but there was no conclusive evidence that they defended these areas as territories. Feeding appeared to be infrequent: only two likely feeding bouts occurred during a cumulative 78-day/shark period that individuals were monitored at Año Nuevo Island. The behavior and movements of the sharks were consistent with a hunting strategy, in which individuals search for prey independently but, at the same time, remain close enough to each other to “sense” and exploit a kill by any one of them by joining in on the kill to feed.     

Occurrence of inflated swimbladders in five species of lanternfishes (family Myctophidae) from waters off southern California

The swimbladders of three species of myctophid fishes (Symbolophorus californiensis, Tarletonbeania crenularis and Diaphus theta, collected in the Southern California Bight between 1963 and 1978), previously observed to have both inflated and noninflated swimbladders as adults, were examined to determine if the swimbladders either vary in their inflation on a diel basis or cease to be inflated and possibly become nonfunctional in some individuals. The swimbladders of two lanternfishes reported to lack inflated swimbladders as adults (Lampanyctus regalis and L. ritteri) were included in the study for comparison. Only the swimbladder of L. regalis did not increase in size with increasing fish size. In the remaining four species, both the lengths and volumes of noninflated swimbladders were positively correlated with fish standard length (SL). Although the swimbladder continued to increase in size, inflated swimbladders were not found in L. ritteri>2 mm SL. Inflated and noninflated swimbladders occurred in overlapping size ranges of S. californiensis, T. crenularis, and D. theta. Only 11% of the D. theta swimbladders were inflated, and the occurrence of inflated swimbladders dit not differ significantly with day vs night capture or fish size. Although the frequency of occurrence of inflated swimbladders decreased significantly in larger S. californiensis, both S. californiensis and T. crenularis caught at the surface at night with a neuston net had significantly higher proportions of inflated swimbladders than did those collected below the surface with either daytime or night-time trawls. The swimbladder does not appear to become nonfunctional in either species. Rather than maintaining either constant volumes or constant masses of gas in their swimbladders during their vertical migrations, both species may inflate their swimbladders only while in the surface waters at night, and have noninflated swimbladders while at other depths of their vertical ranges.     

Stereophotography for the field biologist: measurement of lengths and three-dimensional positions of free-swimming sharks

A stereophotographic technique for determining size and relative position of free-swimming sharks is described and illustrated for schooling scalloped hammerhead sharks (Sphyrna lewini). The method yields total length and nearest-neighbor, interindividual distance; and each of these dimensions is expressed as a function of the shark's distance into a school on the vertical and horizontal planes. Stereopairs of photographs were taken by an aligned, beam-mounted pair of cameras (Nikonos III). The scale to determine the length of a shark from the paired photographic images was obtained from the horizontal displacement between the images. Displacement was correlated with optical axis separation from photographs of a scaled staff at known distances from the camera. Image dimensions on the photographs were measured by projecting a scale onto the stage through a camera lucida. The precision of repeated measurements of a 50 cm section of a scaled staff at increasing distances from the cameras of 2, 4, and 8 m was ±5.0%. Lengths of the sharks ranged from 109 to 371 cm, with a median of 178 cm, for 3 offshore sites in the Gulf of California during July and August 1979. At one site, El Bajo Gorda, lengths increased with both distance from the camera and distance into the group; in contrast, the interindividual distances (head-to-head) did not vary with distance into the group and possessed a median of 232 cm.