Oceanic rafting by a coastal community

Oceanic rafting is thought to play a fundamental role in assembling the biological communities of isolated coastal ecosystems. Direct observations of this key ecological and evolutionary process are, however, critically lacking. The importance of macroalgal rafting as a dispersal mechanism has remained uncertain, largely owing to lack of knowledge about the capacity of fauna to survive long voyages at sea and successfully make landfall and establish. Here, we directly document the rafting of a diverse assemblage of intertidal organisms across several hundred kilometres of open ocean, from the subantarctic to mainland New Zealand. Multispecies analyses using phylogeographic and ecological data indicate that 10 epifaunal invertebrate species rafted on six large bull kelp specimens for several weeks from the subantarctic Auckland and/or Snares Islands to the Otago coast of New Zealand, a minimum distance of some 400–600 km. These genetic data are the first to demonstrate that passive rafting can enable simultaneous trans-oceanic transport and landfall of numerous coastal taxa.     

Residency and spatial use by reef sharks of an isolated seamount and its implications for conservation

Although marine protected areas (MPAs) are a common conservation strategy, these areas are often designed with little prior knowledge of the spatial behaviour of the species they are designed to protect. Currently, the Coral Sea area and its seamounts (north-east Australia) are under review to determine if MPAs are warranted. The protection of sharks at these seamounts should be an integral component of conservation plans. Therefore, knowledge on the spatial ecology of sharks at the Coral Sea seamounts is essential for the appropriate implementation of management and conservation plans. Acoustic telemetry was used to determine residency, site fidelity and spatial use of three shark species at Osprey Reef: whitetip reef sharks Triaenodon obesus, grey reef sharks Carcharhinus amblyrhynchos and silvertip sharks Carcharhinus albimarginatus. Most individuals showed year round residency at Osprey Reef, although five of the 49 individuals tagged moved to the neighbouring Shark Reef (~14 km away) and one grey reef shark completed a round trip of ~250 km to the Great Barrier Reef. Additionally, individuals of white tip and grey reef sharks showed strong site fidelity to the areas they were tagged, and there was low spatial overlap between groups of sharks tagged at different locations. Spatial use at Osprey Reef by adult sharks is generally restricted to the north-west corner. The high residency and limited spatial use of Osprey Reef suggests that reef sharks would be highly vulnerable to targeted fishing pressure and that MPAs incorporating no-take of sharks would be effective in protecting reef shark populations at Osprey and Shark Reef.     

Annual social behaviour of basking sharks associated with coastal front areas

Comparatively little is known about reproductive behaviour in wild sharks as it has proved extremely difficult to study, especially in large pelagic sharks. Here we describe annual courtship-like behaviour in the second-largest fish species, the basking shark (Cetorhinus maximus), from 25 separate episodes observed and tracked during a five-year study period (1995-1999) off south-west England. Social behaviours observed between paired, or three or four, sharks were consistent with courtship behaviours seen in other shark species, namely nose-to-tail following, close following, close flank approach, parallel and echelon swimming. Mature individuals between 5 and 8 m total body length (L(T)) exhibited these behaviours whereas smaller sharks (3-4 m L(T)) did not. Lead individuals were identified as female on a number of occasions and interactions were prolonged; the longest continuous observation of socializing was 1.8 h, although intermittent track data indicates bouts may last for up to 5-6 h. Locations of courtship-like behaviour events were not distributed randomly and were significantly associated with thermal fronts. Our results indicate that putative courtship behaviour occurs between May and July along oceanographic fronts, probably as a consequence of individuals aggregating to forage in rich prey patches before initiating courtship. Thus, locating the richest prey patches along fronts may be important for basking sharks to find mates as well as food in the pelagic ecosystem. As courtship-like behaviours occur annually off south-west England we speculate that this region may represent an annual breeding area for this protected species, but mating itself probably takes place at depth as it was not seen at the surface.     

Diel movements of scalloped hammerhead sharks, Sphyrna lewini Griffith and Smith, to and from a seamount in the Gulf of California

Occurrences of 18 scalloped hammerhead sharks tagged with coded ultrasonic transmitters were recorded at seamount El Bajo Espiritu Santo in the Gulf of California over a 10-day period by automated, data-logging monitors. These sharks remained grouped at the seamount during the day and moved separately into the surrounding pelagic environment at night. The temporal pattern of shark occurrences was related to the light-dark cycle: individuals departed prior to dusk and returned near dawn next morning. However, arrivals and departures of more than one shark within short daytime periods indicated group movements to and from the seamount. Sharks returned repeatedly to the tagging site and not to another site centred less than 240 m from the former site.     

Relative abundance and size of coastal sharks derived from commercial shark longline catch and effort data

In the north-west Atlantic Ocean, stock assessments conducted for some commercially harvested coastal sharks indicate declines from 64 to 80% with respect to virgin population levels. While the status of commercially important species is available, abundance trend information for other coastal shark species in the north-west Atlantic Ocean are unavailable. Using a generalized linear modelling (GLM) approach, a relative abundance index was derived from 1994 to 2009 using observer data collected in a commercial bottom longline fishery. Trends in abundance and average size were estimated for bull shark Carcharhinus leucas, spinner shark Carcharhinus brevipinna, tiger shark Galeocerdo cuvier and lemon shark Negaprion brevirostris. Increases in relative abundance for all shark species ranged from 14% for C. brevipinna, 12% for C. leucas, 6% for N. brevirostris and 3% for G. cuvier. There was no significant change in the size at capture over the time period considered for all species. While the status of shark populations should not be based exclusively on abundance trend information, but ultimately on stock assessment models, results from this study provide some cause for optimism on the status of these coastal shark species.     

鲨鱼

鲨鱼,古代称鲛、鲛鲨、沙鱼。是指软骨鱼纲板鳃鱼亚纲鲨鱼总目鱼类,体呈鱼雷形或纺锤形,鳃裂每侧6-7个,鳍1个或2个,尾鳍发达,歪形,臂鳍有时消失。海生,少数种类进入淡水,为一群游速快的中大型海洋鱼类。鲨鱼与人类关系密切,是重要食品,工业原料,药用材料,科研用品。     

Living on the edge: latitudinal variations in the reproductive biology of two coastal species of sharks

Differences in the reproductive biology of both the Australian weasel shark Hemigaleus australiensis and the Australian sharpnose shark Rhizoprionodon taylori were apparent between individuals from the southern‐most extent of their range in eastern Australia (Moreton Bay) and those from northern Australia. For H. australiensis from Moreton Bay the total length (L_T) at which 50% of individuals were mature (L_T50) was 759 mm for females and 756 mm for males, values that were respectively 17–26% larger than reported for the species in northern Australia. The relatively low percentage (63%) of pregnant mature females and presence of small, similar‐sized, embryos in utero in both May and November suggested a semi‐synchronous, annual reproductive cycle in Moreton Bay, whereas a synchronous, biannual reproductive cycle occurred in northern Australia. It is likely that H. australiensis has a resting phase between gestation cycles at the southern‐most extent of its range. For R. taylori from Moreton Bay the L_T50s were 588 and 579 mm for females and males, respectively, values 2–3% larger than for individuals from the mid‐Queensland coast and 31–35% larger than for individuals from northern Australia. The length at which 50% of the females were maternal (611 mm L_T) in Moreton Bay was greater than the L_T50, indicating that not all sharks mate immediately after maturing. Rhizoprionodon taylori in the south had an annual reproductive cycle incorporating a 7–8 month embryonic diapause, with pups probably born in February. A mean fecundity of 7·5 was almost double that reported from northern Australia. Regional variations in the reproductive characteristics of H. australiensis and R. taylori may influence their resilience to fishing and other anthropogenic pressures. The substantial differences reported here highlight the importance of region‐specific life‐history parameters to successful management and conservation.     

The breeding biology of lemon sharks at a tropical nursery lagoon

Surprisingly little is known about the reproductive behaviour and breeding biology of most shark species, especially in natural populations. Here, we characterize reproductive patterns and use of a natal nursery at Bimini, Bahamas by lemon sharks, Negaprion brevirostris. We systematically and exhaustively sampled young lemon sharks at Bimini annually from 1995 to 2000 and opportunistically sampled adults over the same period. Out of the 897 young sharks sampled, 119 could be assigned to five sampled mothers using microsatellite genotyping. Reproductive females showed strong philopatry to the nursery, returning to Bimini every two years to give birth. Each of these females may rely entirely on the Bimini nursery for recruitment. The protection of known nursery grounds should therefore figure prominently in conservation efforts for large coastal shark species. The reconstruction of paternal genotypes indicates that litters are sired by multiple males, and females mate with different males nearly every breeding cycle. The ubiquitous polyandry reported here raises the possibility that genetic incompatibility and post-copulatory paternity-biasing mechanisms may operate in viviparous sharks.     

Dynamics of Whale Shark Occurrence at Their Fringe Oceanic Habitat

Studies have shown that the whale shark (Rhincodon typus), a vulnerable large filter feeder, seasonally aggregates at highly productive coastal sites and that individuals can perform large, trans-boundary migrations to reach these locations. Yet, the whereabouts of the whale shark when absent from these sites and the potential oceanographic and biological drivers involved in shaping their present and future habitat use, including that located at the fringes of their suitable oceanic habitat, are largely unknown. We analysed a 16-year (1998–2013) observer dataset from the pole-and-line tuna fishery across the Azores (mid-North Atlantic) and used GAM models to investigate the spatial and temporal patterns of whale shark occurrence in relation to oceanographic features. Across this period, the whale shark became a regular summer visitor to the archipelago after a sharp increase in sighting frequency seen in 2008. We found that SST helps predicting their occurrence in the region associated to the position of the seasonal 22°C isotherm, showing that the Azores are at a thermal boundary for this species and providing an explanation for the post 2007 increase. Within the region, whale shark detections were also higher in areas of increased bathymetric slope and closer to the seamounts, coinciding with higher chl-a biomass, a behaviour most probably associated to increased feeding opportunities. They also showed a tendency to be clustered around the southernmost island of Santa Maria. This study shows that the region integrates the oceanic habitat of adult whale shark and suggests that an increase in its relative importance for the Atlantic population might be expected in face of climate change.     

An unprecedented aggregation of whale sharks, Rhincodon typus, in Mexican coastal waters of the Caribbean Sea

Whale sharks, Rhincodon typus, are often perceived as solitary behemoths that live and feed in the open ocean. To the contrary, evidence is accumulating that they are gregarious and form seasonal aggregations in some coastal waters. One such aggregation occurs annually north of Cabo Catoche, off Isla Holbox on the Yucatán Peninsula of Mexico. Here we report a second, much denser aggregation of whale sharks (dubbed "the Afuera") that occurs east of the tip of the Yucatán Peninsula in the Caribbean Sea. The 2009 Afuera event comprised the largest aggregation of whale sharks ever reported, with up to 420 whale sharks observed in a single aerial survey, all gathered in an elliptical patch of ocean approximately 18 km(2). Plankton studies indicated that the sharks were feeding on dense homogenous patches of fish eggs, which DNA barcoding analysis identified as belonging to little tunny, Euthynnus alletteratus. This contrasts with the annual Cabo Catoche aggregation nearby, where prey consists mostly of copepods and sergestid shrimp. Increased sightings at the Afuera coincide with decreased sightings at Cabo Catoche, and both groups have the same sex ratio, implying that the same animals are likely involved in both aggregations; tagging data support this idea. With two whale shark aggregation areas, high coastal productivity and a previously-unknown scombrid spawning ground, the northeastern Yucatán marine region is a critical habitat that deserves more concerted conservation efforts.     

Odontocete occurrence in relation to changes in oceanography at a remote equatorial Pacific seamount

Seamounts are considered hot spots of biodiversity and can aggregate pelagic predators and their prey. Passive acoustic monitoring was conducted over 3 mo in 2012 to document the occurrence of odontocetes near a seamount chain in the central equatorial Pacific in relation to oceanographic changes over time. Beaked whale echolocation signals were most frequently encountered. The main beaked whale signal was an unknown type, BW38, which resembled signals produced by Blainville's beaked whales. It had high occurrence during high sea surface temperature and low sea surface salinity. Cuvier's beaked whales were the second most detected. They had an opposite pattern and were encountered more often when sea surface temperature was low and net primary productivity was high. Risso's dolphins and short‐finned pilot whales had high acoustic densities, and echolocated predominantly at night. Risso's dolphins occurred more often during low sea surface height deviation. False killer whales were less frequently detected and mostly occurred during the day. Sperm whale detections were fewer than expected and associated with high chlorophyll a. Short duration Kogiidae encounters occurred on average every third day. These types of long‐term site studies are an informative tool to comparatively assess species composition, relative abundance, and relationship to oceanographic changes.     

The importance of unmanaged coastal wetlands to waterbirds at coastal Ghana

Ghana's 550 km coastline has about 100 wetlands out of which five large ones are managed as Ramsar sites, the rest being unmanaged. Recent pollution and misuse of unmanaged wetlands have necessitated a study into their roles in the support of waterbirds. Waterbird survey on four wetlands between September 2005 and April 2006 revealed that 51 waterbird species make use of the managed compared to 44 in the unmanaged wetlands. Average numbers of waterbirds were higher on the managed wetlands (10,510 ± 4862) than on the unmanaged wetlands (1348 ± 602) (P < 0.05) but mean population density of waterbirds (n/ha) was the same for both the managed (1860 ± 310) and unmanaged (1400 ± 220) (P > 0.05) wetlands. The population densities of waterbirds belonging to guilds 1, 5 and 7 were significantly higher on the managed wetlands (P < 0.05). In contrast, guilds 2 and 4 have significantly higher population densities on the unmanaged wetlands while the population density of guild 3 was the same (P > 0.05). There was an indication that during mid and late periods of the nonbreeding season, the unmanaged wetlands altogether may support a larger number of waterbirds belonging to guilds 2, 3 and 4 than the five managed wetlands.     

Occurrence and biological characteristics of broadnose sevengill sharks (Notorynchus cepedianus) in Pacific Northwest coastal estuaries

The broadnose sevengill shark (Notorynchus cepedianus) is a high-order marine predator distributed worldwide in shallow coastal waters of temperate seas. Recent reports have suggested it may be a prevalent component of Pacific Northwest coastal estuarine communities, although biological characteristics of the shark population remain undocumented despite growing interest in recreational harvest of the species. Longline sampling was conducted in Willapa Bay and Grays Harbor, Washington, USA seasonally during 2003–2006 to collect sevengill shark size, maturity, and sex ratio data, and establish some baseline catch rate information. Sevengill sharks were collected on 65% of longline sets and catches were composed of subadult and mature individuals (122–283 cm TL) of both sexes. Most male sevengill sharks were large sexually mature adults, based on external clasper calcification levels, whereas most comparably sized females were considered subadults, based on literature-based size-at–maturity estimates. Neonates and young sharks <120 cm were not collected, nor have they been reported in other historic estuary sampling efforts. Sex ratios were skewed toward males in Willapa Bay and suggest some degree of sexual segregation for the species, as has been shown for populations elsewhere. We suggest sevengill sharks are a largely ignored but potentially important predator in Pacific Northwest estuaries. This study therefore provides some of the first, basic information for guiding management decisions associated with a late-maturing, slow-growing shark species in these coastal habitats.     

The behaviour of sharks

Summary Despite the recent upsurge of interest in shark research, the current status of knowledge of the behavioural repertoire of most species is alarmingly incomplete. Clearly, from the steadily decreasing numbers of sharks caught by commercial and sport fishermen, sharks are highly vulnerable to human exploitation. Although education is making inroads, there is still steady opposition to the enforcement of catch limits and management strategies for most species. Accurate life history and behavioural information is required to enforce management policies. Wetherbee et al. (1990) cited a case in which commercial fishermen accused the spiny dogfish of stripping the commerical and recreational fisheries of their herring and salmon catch. A detailed study of the spiny dogfish diet disproved their claims.Sharks are clearly not mindless eating machines, as they have been labelled in the past. They are intelligent and have complex patterns of movement, space utilization, and social organization. Using a combination of remote and direct observational techniques, the scientific community is beginning to have a more complete understanding of these important apex predators in coral reef and oceanic ecosystems. More importantly, researchers who are interested in pursuing the fascinating field of shark behaviour still have a wide choice of direction.     

Feeding ecology and trophic segregation of two sympatric mesopredatory sharks in the heavily exploited coastal ecosystem of the Adriatic Sea

The ecological roles and trophic interactions of two commercially important mesopredatory shark species, Squalus acanthias and Mustelus punctulatus that co‐occur on the continental shelf of the north‐central Adriatic Sea were investigated. Both shark species are dietary specialists, with a significant dietary overlap recorded only during the spring season. They showed different patterns of feeding as they grew: S. acanthias extended its trophic niche with an increase in size, while M. punctulatus developed a more specialized diet. These two sharks partition food resources and reduce niche overlap by foraging at different trophic levels. Mustelus punctulatus is a crustacean feeder, specialized in foraging on scavenging malacostracans frequently found along trawl tracks or on discards in the Adriatic fishing zone. Conversely, S. acanthias prefers small pelagic fishes, which are commercially exploited and in decline. The different foraging strategies adopted by these two species suggest that they should be managed separately. Dietary specialization, direct competition with humans for prey and their higher intrinsic vulnerability make S. acanthias particularly susceptible to the effects of anthropogenic perturbations.     

Reproductive biology of lamnoid sharks

Synopsis The capture of recently inseminated or pregnant specimens ofCarcharias taurus, Isurus paucus, I. oxyrinchus, Alopias superciliosus andA. vulpinus has allowed new information to be obtained on the reproductive biology of these species. Oophagy and embryonic cannibalism (adelphophagy) have been documented inC. taurus, but only oophagy in other lamnoid species. The occurrence of up to nine embryos of similar size per uterus inIsurus and no indication of functional erect teeth in embryos leaves considerable doubt that embryophagy occurs in this genus. Considerable data has been collected onCarcharias taurus which allows a lamnoid reproductive model to be developed and tested, in spite of the obvious differences between the reproductive biology of this species and other lamnoids. Gonad structure, ovarian development, fertilization, early embryonic differentiation, embryonic nutrition and parturition, inC. taurus and other lamnoids differs significantly from other elasmobranchs.