Diving behavior and movements of juvenile hawksbill turtles Eretmochelys imbricata on a Caribbean coral reef

As historically abundant spongivores, hawksbill turtles Eretmochelys imbricata likely played a key ecological role on coral reefs. However, coral reefs are now experiencing global declines and many hawksbill populations are critically reduced. For endangered species, tracking movement has been recognized as fundamental to management. Since movements in marine vertebrates encompass three dimensions, evaluation of diving behavior and range is required to characterize marine turtle habitat. In this study, habitat use of hawksbill turtles on a Caribbean coral reef was elucidated by quantifying diel depth utilization and movements in relation to the boundaries of marine protected areas. Time depth recorders (TDRs) and ultrasonic tags were deployed on 21 Cayman Islands hawksbills, ranging in size from 26.4 to 58.4 cm straight carapace length. Study animals displayed pronounced diel patterns of diurnal activity and nocturnal resting, where diurnal dives were significantly shorter, deeper, and more active. Mean diurnal dive depth (±SD) was 8 ± 5 m, range 2–20 m, mean nocturnal dive depth was 5 ± 5 m, range 1–14 m, and maximum diurnal dive depth was 43 ± 27 m, range 7–91 m. Larger individuals performed significantly longer dives. Body mass was significantly correlated with mean dive depth for nocturnal but not diurnal dives. However, maximum diurnal dive depth was significantly correlated with body mass, suggesting partitioning of vertical habitat by size. Thus, variable dive capacity may reduce intraspecific competition and provide resistance to degradation in shallow habitats. Larger hawksbills may also represent important predators on deep reefs, creating a broad ecological footprint over a range of depths. Communicated by Biology Editor Dr Mark McCormick     

Summer and fall movement of cownose ray, <Emphasis Type="Italic">Rhinoptera bonasus</Emphasis>, along the east coast of United States observed with pop-up satellite tags

Cownose ray, Rhinoptera bonasus, is a common elasmobranch species along the southeast United States coast that recently has received negative attention. These rays can consume considerable amounts of commercial shellfish raising concerns regarding their control and need for effective management. However, limited information is known regarding their population abundance and migration patterns. We addressed the latter by reviewing 25 tagged cownose rays in Chesapeake Bay with pop-up satellite archival tags (PSATs) to study their movement patterns during summer and fall and identify wintering grounds. Eleven tags provided useful data on temperature, depth, light level and/or end locations. The migration tracks were deciphered through geolocation based on light levels, sea surface temperatures and depth constraints. PSAT end locations indicated southern wintering grounds in the coastal waters of central Florida. Female rays migrated out of Chesapeake Bay at the end of September to early October and continued their southerly migration to Florida. Male rays exited the bay in July and migrated northward based on their estimated movement tracks. The male rays appeared to have a second summer feeding ground off the coast of southern New England. In the fall, males migrated south from New England to the same wintering grounds as the females. No diel differences in habitat use were detected; however, males tended to occupy a wider depth and temperature range compared to females. Information on the movement patterns and habitat use for cownose rays will assist in determining more effective recreational and commercial management plans.     

Observations of mesoscale movements in the short-tailed stingray, Dasyatis brevicaudata from New Zealand using a novel PSAT tag attachment method

Short-tailed stingrays (Dasyatis brevicaudata) are the largest stingrays in the world. They are common in New Zealand waters and aggregate seasonally in large numbers at the Poor Knights Islands Marine Reserve (PKMR), where they are a major attraction. Pop-up satellite archival tags (PSAT) were attached to two female short-tailed stingrays at two New Zealand offshore islands (within and nearby the PKMR) to investigate patterns of movement. These are the first PSAT data published for a stingray species. The rays were tagged for 62 and 151 days using a novel method of tag attachment developed specifically for ray species. Analyses of geolocation, depth and behavioural data indicate neither ray moved large distances (≤ 25 km) from the tagging locations. However, they showed a seasonal shift to deeper waters, progressively increasing time spent at greater depths and decreasing time spent at shallow depths towards winter. In addition, one ray displayed strong diel vertical movements. The method developed for PSAT attachment to stingrays proved successful and can be applied for long-term (> 6 months) movement and behavioural studies.     

Validation of photo‐identification as a mark–recapture method in the spotted eagle ray Aetobatus narinari

The spotted eagle ray Aetobatus narinari is characterized by pigmentation patterns that are retained for up to 3·5 years. These pigmentations can be used to identify individuals through photo‐identification. Only one study has validated this technique, but no study has estimated the percentage of correct identification of the rays using this technique. In order to carry out demographic research, a reliable photographic identification technique is needed. To achieve this validation for A. narinari, a double‐mark system was established over 11 months and photographs of the dorsal surface of 191 rays were taken. Three body parts with distinctive natural patterns were analysed (dorsal surface of the cephalic region, dorsal surface of the pectoral fins and dorsal surface of the pelvic fins) in order to determine the body part that could be used to give the highest percentage of correct identification. The dorsal surface of the pectoral fins of A. narinari provides the most accurate photo‐identification to distinguish individuals (88·2%).     

Site affinity of whitespotted eagle rays Aetobatus narinari assessed using photographic identification

Photographic identification was used to track the movements of the whitespotted eagle ray Aetobatus narinari around South Caicos, Turks and Caicos Islands. A total of 165 individuals were identified, aided by the computer program I³S Spot. The sex ratio across all study sites in 2015 was not significantly different from 1:1 (χ² = 2·8, P > 0·05). 33·9% of all individual rays were resighted at least once and the maximum number of days between the first and last sighting was 1640 (median 165, interquartile range, IQR = 698). Sightings of individuals occurred at locations differing from the original sighting location 24·6% of the time (0·7–20 km away). The entire population around South Caicos has yet to be sampled and these rays exhibited site affinity during the study period; they are either resident to South Caicos or are using the area for parts of the year before making movements elsewhere and then returning. Given these results, A. narinari is suited to local‐scale management and conservation efforts.     

Marine depth use of sea trout Salmo trutta in fjord areas of central Norway

The vertical behaviour of 44 veteran sea trout Salmo trutta (275–580 mm) in different marine fjord habitats (estuary, pelagic, near shore with and without steep cliffs) was documented during May–February by acoustic telemetry. The swimming depth of S. trutta was influenced by habitat, time of day (day v. night), season, seawater temperature and the body length at the time of tagging. Mean swimming depth during May–September was 1·7 m (individual means ranged from 0·4 to 6·4 m). Hence, S. trutta were generally surface oriented, but performed dives down to 24 m. Mean swimming depth in May–September was deeper in the near‐shore habitats with or without steep cliffs (2·0 m and 2·5 m, respectively) than in the pelagic areas (1·2 m). May–September mean swimming depth in all habitats was slightly deeper during day (1·9 m) than at night (1·2 m), confirming that S. trutta conducted small‐scale diel vertical movements. During summer, S. trutta residing in near‐shore habitat progressively moved deeper over the period May (mean 1·1 m) to August (mean 4·0 m) and then reoccupied shallower areas (mean 2·3 m) during September. In winter (November and February), individuals residing in the innermost part of the fjords were found at similar average depths as they occupied during the summer (mean 1·3 m). The swimming depths of S. trutta coincide with the previously known surface orientation of salmon lice Lepeophtheirus salmonis. Combined with previous studies on horizontal use of S. trutta, this study illustrates how S. trutta utilize marine water bodies commonly influenced by anthropogenic factors such as aquaculture, harbours and marine constructions, marine renewable energy production or other human activity. This suggests that the marine behaviour of S. trutta and its susceptibility to coastal anthropogenic factors should be considered in marine planning processes.     

Pop‐up satellite archival tag effects on the diving behaviour,growth and survival of adult Atlantic salmon Salmo salar at sea

The effects of large, externally attached pop‐up satellite archival tags (PSATs) were compared with those of small implanted data storage tags (DSTs) on adult Atlantic salmon Salmo salar during their ocean migration in regards to depth utilization, diving depth, diving rate, diving speed and temperatures experienced. Additionally the return rate and growth of individuals tagged with PSATs was compared with those of small acoustic tags and DSTs. Overall, the depth distribution of individuals tagged with PSATs was similar to that of those tagged with DSTs, reflecting the pelagic nature of S. salar at sea. Individuals tagged with PSATs, however, dived less frequently and to shallower depths, and dived and surfaced at slower velocities. Sea surface temperatures experienced by individuals tagged with PSATs were similar to those experienced by those tagged with DSTs for the same time of year, suggesting that there were no large differences in the ocean migration. Return rates did not depend on whether individuals were tagged with PSATs or not, indicating that survival at sea was not impacted by PSATs in comparison to small internal tags. Individuals tagged with PSATs, however, had a smaller increase in body mass than those tagged with acoustic tags or DSTs. It was concluded that PSATs are suitable for use in researching large‐scale migratory behaviour of adult S. salar at sea, but that some effects on their behaviour from tagging must be expected. Effects of PSATs may be largest in the short term when S. salar are swimming in bursts at high speeds. Even though individuals tagged with PSATs performed deep and frequent dives, the results of this study suggest that untagged individuals would perform even deeper and more frequent dives than tagged individuals.     

Habitat and behaviour of yellowfin tuna Thunnus albacares in the Gulf of Mexico determined using pop-up satellite archival tags

This study presents the first data on movement, habitat use and behaviour for yellowfin tuna Thunnus albacares in the Atlantic Basin. Six individuals were tracked in the Gulf of Mexico using pop-up satellite archival tags. Records up to 80 days in length were obtained, providing information on depth and temperature preferences as well as horizontal movements. Thunnus albacares in the Gulf of Mexico showed a strong preference for the mixed layer and thermocline, consistent with findings for this species in other ocean basins. Fish showed a diel pattern in depth distribution, remaining in surface and mixed layer waters at night and diving to deeper waters during the day. The vertical extent of T. albacares habitat appeared to be temperature limited, with fish generally avoiding waters that were >6° C cooler than surface waters. The vertical and thermal habitat usage of T. albacares differs from that of bigeye Thunnus obesus and bluefin Thunnus thynnus , Thunnus orientalis and Thunnus maccoyii tunas. These results are consistent with the results of earlier studies conducted on T. albacares in other oceans.     

Kin structure and social organization in the spotted eagle ray,Aetobatus narinari,off coastal Sarasota,FL

Observations of elasmobranchs in groups suggest sociality in sharks and rays. However we currently lack a strong understanding of social structure and the role kinship has in structuring group organization in cartilaginous fishes. The spotted eagle ray, Aetobatus narinari (Euphrasen, 1790) frequents the shallow waters near Sarasota, FL, often in pairs or groups suggesting a social component to their behavior. In the present study, eight eagle ray-specific microsatellite markers were used to investigate relatedness in A. narinari groups, and used to determine if kin structure contributed to group organization. Using regression-based and maximum-likelihood approaches, relatedness was quantified and compared within and among groups of juveniles, and adults in mixed sex and same sex groups. Results showed a lack of kin-structured sociality in A. narinari, suggesting factors apart from relatedness shape social interactions among spotted eagle rays in the near-shore waters of Sarasota, FL. Our results add to the limited amount of published literature on elasmobranch kinship, which are important for understanding implications of anthropogenic disturbance on genetic variability for coastal populations.     

Intraspecific variation in vertical habitat use by tiger sharks (Galeocerdo cuvier) in the western North Atlantic

Tiger sharks (Galeocerdo cuvier) are a wide ranging, potentially keystone predator species that display a variety of horizontal movement patterns, making use of coastal and pelagic waters. Far less, however, is known about their vertical movements and use of the water column. We used pop‐up satellite archival tags with two data sampling rates (high rate and standard rate tags) to investigate the vertical habitat use and diving behavior of tiger sharks tagged on the Puerto Rico–Virgin Islands platform and off Bermuda between 2008 and 2009. Useable data were received from nine of 14 sharks tagged, tracked over a total of 529 days. Sharks spent the majority of their time making yo‐yo dives within the upper 50 m of the water column and considerable time within the upper 5 m of the water column. As a result, sharks typically occupied a narrow daily temperature range (~2°C). Dives to greater than 200 m were common, and all sharks made dives to at least 250 m, with one shark reaching a depth of 828 m. Despite some similarities among individuals, a great deal of intraspecific variability in vertical habit use was observed. Four distinct depth distributions that were not related to tagging location, horizontal movements, sex, or size were detected. In addition, similar depth distributions did not necessitate similar dive patterns among sharks. Recognition of intraspecific variability in habitat use of top predators can be crucial for effective management of these species and for understanding their influence on ecosystem dynamics.     

Meristic and morphometric variation in the lancet fish,Alepisaurus, with notes on the distribution of A. ferox and A. brevirostris

Abstract

Meristies and morphometries are reported for 81 specimens of Alepisaurus ferox and 31 of A. brevirostris, mostly from the Pacific Ocean. The holotype and topotype of A. ferox and two paratypes of A. brevirostris, all from the Atlantic Ocean, were also examined. For specimens longer than aJoout 500 mm SL, ferox is distinguishable from brevirostris by its relatively longer head and snout. In ferox the dorsal fin origin is a1bove or behind the hind margin of the operculum; in brevirostris it is in front. Paoific Ocean ferox have significantly fewer dorsal fin rays (mean 36.6) than Atlantic (40.5) and Indian Ocean (40.8) examples. In the Pacific, ferox can be distinguished from brevirostris by its fewer dorsal rays, but in the Atlantic and the Indian Ocean there is considerable overlap in dorsal ray counts. A. ferox is found world-wide, whereas brevirostris is not known from the North Pacific. The northernmost record of brevirostris in the Pacific is from 14°47′S.     

Vertical Movement Patterns and Ontogenetic Niche Expansion in the Tiger Shark,Galeocerdo cuvier

Sharks are top predators in many marine ecosystems and can impact community dynamics, yet many shark populations are undergoing severe declines primarily due to overfishing. Obtaining species-specific knowledge on shark spatial ecology is important to implement adequate management strategies for the effective conservation of these taxa. This is particularly relevant concerning highly-mobile species that use wide home ranges comprising coastal and oceanic habitats, such as tiger sharks, Galeocerdo cuvier. We deployed satellite tags in 20 juvenile tiger sharks off northeastern Brazil to assess the effect of intrinsic and extrinsic factors on depth and temperature usage. Sharks were tracked for a total of 1184 d and used waters up to 1112 m in depth. The minimum temperature recorded equaled 4°C. All sharks had a clear preference for surface (< 5 m) waters but variability in depth usage was observed as some sharks used mostly shallow (< 60 m) waters whereas others made frequent incursions into greater depths. A diel behavioral shift was detected, with sharks spending considerably more time in surface (< 10 m) waters during the night. Moreover, a clear ontogenetic expansion in the vertical range of tiger shark habitat was observed, with generalized linear models estimating a ~4-fold increase in maximum diving depth from 150- to 300-cm size-classes. The time spent in the upper 5 m of the water column did not vary ontogenetically but shark size was the most important factor explaining the utilization of deeper water layers. Young-of-the-year tiger sharks seem to associate with shallow, neritic habitats but they progressively move into deeper oceanic habitats as they grow larger. Such an early plasticity in habitat use could endow tiger sharks with access to previously unavailable prey, thus contributing to a wider ecological niche.     

Atlantic sturgeon Acipenser oxyrinchus Mitchill, 1815 seasonal marine depth and temperature occupancy and movement in the Bay of Fundy

Pop‐up satellite archival tags were used to collect fisheries‐independent data that characterized the seasonal habitat occupancy and movement of Atlantic sturgeon Acipenser oxyrinchus Mitchill, 1815 in the Bay of Fundy (BoF). Atlantic sturgeon from Canadian and United States stocks aggregate annually for feeding in Minas Basin, inner BoF (45.28N, 64.18W), during May to September but depart to other locations from October to April. Sixteen PSAT tags were applied to sturgeon ranging from 152 to 203 cm total length captured and released in Minas Basin during May to August. Ten of the tags were reported after 1 year at large and provided pop‐off locations. Seven were recovered with archived data, or provided transmitted data sets, which were analyzed for depth and temperature occupancy from June 2011 to August 2013. During June to August while in the Minas Basin the sturgeon spent >90% of time at depths <10 m and temperatures of 16–22°C. Departure from Minas Basin through the Minas Passage was in September and October, when depth occupancy varied from <10 to 120 m. From November to April sturgeon were in the outer BoF, where mean depth occupancies ranged from 40 to 100 m at mean temperatures of 3–14°C. Deepest mean depth occupancy of 60 to 90 m was recorded during December 2011 and 2012, and coldest mean temperature occupancy of 0–4°C in March 2011 and 2012. During April and May mean depth and temperature occupancy ranges shallowed from 40 to <10 m and increased from 4 to 15°C, respectively. Tag pop‐off locations indicated that sturgeon spent the winter season in the outer BoF but by June had either migrated back to the Minas Basin or off the mouth of the Saint John River, a known spawning location.     

Influence of estuarine and continental shelf water advection on the coastal movements of apron ray Discopyge tschudii in the Southwestern Atlantic

Evaluated was the seasonal and inter‐annual association between habitat variables and distribution pattern of the apron ray (Discopyge tschudii) and its relationship, with the main force on the environmental condition to understand the influence of estuarine and continental shelf processes in the coastal fishes of the Southwestern Atlantic Coastal System (34°–41°S). Two winter and five spring research surveys in the SACS were analyzed and the Perry and Smith methodology applied to determine the seasonal association of depth, temperature and salinity and the abundance of apron ray. The season with the highest inter‐annual spatial distribution variation was related to the main external force on environmental conditions (e.g. wind, atmospheric temperature, and Río de la Plata discharge and shelf water intrusion). Apron ray showed persistent habitat selection, with the water temperature (<16°C) and salinity (higher than 31.8) being environmental factors most influencing its spatial distribution. In the spring, the apron ray spatial distribution showed higher inter‐annual variation than in winter. The persistence of the spring habitat selection of D. tschudii suggests that its onshore‐offshore as well as north‐south movements are influenced by water mass movements forced by a combination of local conditions (wind) and regional‐scale weather patterns (e.g. strength of the Malvinas Current). This study provides evidence on the importance of considering environmental conditions on the spatial distribution of apron ray and improves the knowledge on interactions between estuarine and shelf water dynamics as determinants of the spatial distribution of a coastal fish species in the Southwest Atlantic Coastal System.     

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Direct underwater observation of micro‐habitat use by 1838 young Atlantic salmon Salmo salar [mean L_T 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (L_T 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro‐habitat variables snout water velocity and height above bottom, but with differences between size‐classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size‐classes. On a meso‐habitat scale, brown trout were more frequently observed in slow pool‐glide habitats, while young Atlantic salmon favoured the faster high‐gradient meso‐habitats. Small juveniles <7 cm of both species were observed most frequently in riffle‐chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species‐specific habitat overlap between streams may be complete. However, between‐species habitat partitioning remains similar.     

Diel vertical migration of adult Pacific cod Gadus macrocephalus in Alaska

The diel vertical migration (DVM) of Pacific cod Gadus macrocephalus was examined using depth and temperature data from 250 recaptured archival tags deployed on G. macrocephalus in the eastern Bering Sea and in the Gulf of Alaska near Kodiak Island. DVM of two types, deeper during daytime (type I) and deeper during night‐time (type II), occurred frequently (15–40% of all days) in G. macrocephalus released at all sites. Most individuals displayed both diel types, with each type of behaviour lasting up to 58 contiguous days, and day and night depth differences averaging c. 8 m. Despite high among‐individual variability, the occurrence of DVM varied significantly with the release site, season (i.e. day‐of‐year) and bottom depth, with the trend in seasonal occurrence nearly opposite for type I compared to type II DVM. No significance could be attributed to G. macrocephalus fork length, sex or ambient (tag) temperature. Trends in the magnitude of G. macrocephalus depth change were observed, with increased movement often occurring during night‐time, dawn and dusk, and at release sites where the bathymetry was more complex. Both type I and type II DVMs were attributed to foraging on prey species that also undergo DVM, and increased vertical movements of G. macrocephalus during crepuscular and night‐time periods were attributed to more active foraging during dim‐light conditions when G. macrocephalus can potentially exploit a sensory advantage over some of their prey.     

Interspecific habitat associations of juvenile salmonids in Lake Ontario tributaries: implications for Atlantic salmon restoration

Diel variation in habitat use of subyearling Chinook salmon (Oncorhynchus tshawytscha), subyearling coho salmon (O. kisutch), yearling steelhead (O. mykiss), and yearling Atlantic salmon (Salmo salar) was examined during the spring in two tributaries of Lake Ontario. A total of 1318 habitat observations were made on juvenile salmonids including 367 on steelhead, 351 on Chinook salmon, 333 on Atlantic salmon, and 261 on coho salmon. Steelhead exhibited the most diel variation in habitat use and Chinook the least. Juvenile salmonids were generally associated with more cover and larger substrate during the day in both streams. Interspecific differences in habitat use in both streams occurred with Atlantic salmon (fast velocities) and coho salmon (pools) using the least similar habitat. Chinook salmon and Atlantic salmon used similar habitat in both streams. These findings should help guide future management actions specific to habitat protection and restoration of Atlantic salmon in Lake Ontario tributaries.     

Spatial dynamics and expanded vertical niche of blue sharks in oceanographic fronts reveal habitat targets for conservation

Dramatic population declines among species of pelagic shark as a result of overfishing have been reported, with some species now at a fraction of their historical biomass. Advanced telemetry techniques enable tracking of spatial dynamics and behaviour, providing fundamental information on habitat preferences of threatened species to aid conservation. We tracked movements of the highest pelagic fisheries by-catch species, the blue shark Prionace glauca, in the North-east Atlantic using pop-off satellite-linked archival tags to determine the degree of space use linked to habitat and to examine vertical niche. Overall, blue sharks moved south-west of tagging sites (English Channel; southern Portugal), exhibiting pronounced site fidelity correlated with localized productive frontal areas, with estimated space-use patterns being significantly different from that of random walks. Tracked female sharks displayed behavioural variability in diel depth preferences, both within and between individuals. Diel depth use ranged from normal DVM (nDVM; dawn descent, dusk ascent), to reverse DVM (rDVM; dawn ascent, dusk descent), to behavioural patterns where no diel differences were apparent. Results showed that blue sharks occupy some of the most productive marine zones for extended periods and structure diel activity patterns across multiple spatio-temporal scales in response to particular habitat types. In so doing, sharks occupied an extraordinarily broad vertical depth range for their size (1.0-2.0 m fork length), from the surface into the bathypelagic realm (max. dive depth, 1160 m). The space-use patterns of blue sharks indicated they spend much of the time in areas where pelagic longlining activities are often highest, and in depth zones where these fisheries particularly target other species, which could account for the rapid declines recently reported for blue sharks in many parts of the world's oceans. Our results provide habitat targets for blue shark conservation that may also be relevant to other pelagic species.     

Vertical Movements and Patterns in Diving Behavior of Whale Sharks as Revealed by Pop-Up Satellite Tags in the Eastern Gulf of Mexico

The whale shark (Rhincodon typus) is a wide-ranging, filter-feeding species typically observed at or near the surface. This shark’s sub-surface habits and behaviors have only begun to be revealed in recent years through the use of archival and satellite tagging technology. We attached pop-up satellite archival transmitting tags to 35 whale sharks in the southeastern Gulf of Mexico off the Yucatan Peninsula from 2003–2012 and three tags to whale sharks in the northeastern Gulf off Florida in 2010, to examine these sharks’ long-term movement patterns and gain insight into the underlying factors influencing their vertical habitat selection. Archived data were received from 31 tags deployed on sharks of both sexes with total lengths of 5.5–9 m. Nine of these tags were physically recovered facilitating a detailed long-term view into the sharks’ vertical movements. Whale sharks feeding inshore on fish eggs off the northeast Yucatan Peninsula demonstrated reverse diel vertical migration, with extended periods of surface swimming beginning at sunrise followed by an abrupt change in the mid-afternoon to regular vertical oscillations, a pattern that continued overnight. When in oceanic waters, sharks spent about 95% of their time within epipelagic depths (<200 m) but regularly undertook very deep (“extreme”) dives (>500 m) that largely occurred during daytime or twilight hours (max. depth recorded 1,928 m), had V-shaped depth-time profiles, and comprised more rapid descents (0.68 m sec^-1) than ascents (0.50 m sec^-1). Nearly half of these extreme dives had descent profiles with brief but conspicuous changes in vertical direction at a mean depth of 475 m. We hypothesize these stutter steps represent foraging events within the deep scattering layer, however, the extreme dives may have additional functions. Overall, our results demonstrate complex and dynamic patterns of habitat utilization for R. typus that appear to be in response to changing biotic and abiotic conditions influencing the distribution and abundance of their prey.     

Movement and habitat use by the marbled lungfish Protopterus aethiopicus Heckel 1851 in Lake Baringo, Kenya

The marbled lungfish, Protopterus aethiopicus, a recent introduction into Lake Baringo, Kenya is now an important commercial species there. Because little is known about its behaviour, we used ultrasonic telemetry to investigate its movements and use of habitat as part of a broader biological study. Twelve marbled lungfish were implanted with ultrasonic tags and tracked for variable periods between September 2001 and 2002. Two individuals were tracked for most of the study period. Daily movement ranged from little or none to 5.2 km. Mean hourly rates of movement for three fish located twice a day (morning and late afternoon) over several days suggested that individuals were active throughout the diel period. Maximum lake depth was about 3 m and fish utilized all depths greater than 1 m. Six home ranges described for four lungfish varied in size from 5.8 to 19.8 km ² and were occupied for between 2 and 4.5 months. Use of habitat and the movement of marbled lungfish in Lake Baringo appeared to be influenced more by biotic than abiotic factors.