Big catch,little sharks: Insight into Peruvian small‐scale longline fisheries

Shark take, driven by vast demand for meat and fins, is increasing. We set out to gain insights into the impact of small‐scale longline fisheries in Peru. Onboard observers were used to document catch from 145 longline fishing trips (1668 fishing days) originating from Ilo, southern Peru. Fishing effort is divided into two seasons: targeting dolphinfish (Coryphaena hippurus; December to February) and sharks (March to November). A total of 16,610 sharks were observed caught, with 11,166 identified to species level. Of these, 70.6% were blue sharks (Prionace glauca), 28.4% short‐fin mako sharks (Isurus oxyrinchus), and 1% were other species (including thresher (Alopias vulpinus), hammerhead (Sphyrna zygaena), porbeagle (Lamnus nasus), and other Carcharhinidae species (Carcharhinus brachyurus, Carcharhinus falciformis, Galeorhinus galeus). Mean ± SD catch per unit effort of 33.6 ± 10.9 sharks per 1000 hooks was calculated for the shark season and 1.9 ± 3.1 sharks per 1000 hooks were caught in the dolphinfish season. An average of 83.7% of sharks caught (74.7% blue sharks; 93.3% mako sharks) were deemed sexually immature and under the legal minimum landing size, which for species exhibiting k‐selected life history traits can result in susceptibility to over exploitation. As these growing fisheries operate along the entire Peruvian coast and may catch millions of sharks per annum, we conclude that their continued expansion, along with ineffective legislative approaches resulting in removal of immature individuals, has the potential to threaten the sustainability of the fishery, its target species, and ecosystem. There is a need for additional monitoring and research to inform novel management strategies for sharks while maintaining fisher livelihoods.     

Aspects of the reproductive biology of two pelagic sharks in the eastern Atlantic Ocean

This study used data provided by the Chinese Longline Fishery Scientific Observer Programme from the tropical eastern Atlantic Ocean to estimate the reproductive parameters of the blue shark (Prionace glauca) and crocodile shark (Pseudocarcharias kamoharai). Sizes ranged from 80 to 298 cm fork length (FL) for blue sharks and from 48 to 99 cm FL for crocodile sharks. Sexual segregation was observed during different months for both sharks. The sex ratio for blue sharks was 1.38 F:1 M, and 1 F:2.79 M for crocodile sharks. The size of adult blue sharks ranged from 144 to 280 cm for males and from 174 to 298 cm for females; and that of crocodile sharks from 63 to 97 cm for males and 78–99 cm for females. The size at 50% of maturity for blue sharks was estimated at 191.7 cm FL for females and 197.5 cm FL for males, and that of crocodile sharks was assessed at 84.9 cm FL for females and 78.5 cm FL for males. Most sexually matured females were pregnant; their means were 207.2 ± 16.4 cm FL for blue sharks and 89.4 ± 4.3 cm FL for crocodile sharks. Mature sizes for both species were significantly different among months. Embryonic sizes also varied widely among months for crocodile sharks, but a slight change was recorded for those of blue sharks. The observed mean size at birth and litter size were 34.5 cm FL and 37 ± 12 for the blue sharks, and that of the crocodile sharks, 39.5 cm FL and a dominant four embryos in the uterus. Due to the observed increasing catch trend of blue sharks and the slow reproductive cycle of crocodile sharks, this study presents the need of implementing conservation measures to ensure the sustainability of both species in their habitat.     

Discovery of a multispecies shark aggregation and parturition area in the Ba Estuary,Fiji Islands

Population declines in shark species have been reported on local and global scales, with overfishing, habitat destruction and climate change posing severe threats. The lack of species‐specific baseline data on ecology and distribution of many sharks, however, makes conservation measures challenging. Here, we present a fisheries‐independent shark survey from the Fiji Islands, where scientific knowledge on locally occurring elasmobranchs is largely still lacking despite the location's role as a shark hotspot in the Pacific. Juvenile shark abundance in the fishing grounds of the Ba Estuary (north‐western Viti Levu) was assessed with a gillnet‐ and longline‐based survey from December 2015 to April 2016. A total of 103 juvenile sharks identified as blacktip Carcharhinus limbatus (n = 57), scalloped hammerhead Sphyrna lewini (n = 35), and great hammerhead Sphyrna mokarran (n = 11) sharks were captured, tagged, and released. The condition of umbilical scars (68% open or semihealed), mean sizes of individuals (±SD) (C. limbatus: 66.5 ± 3.8 cm, S. lewini: 51.8 ± 4.8 cm, S. mokarran 77.4 ± 2.8 cm), and the presence of these species over recent years (based on fishermen interviews), suggest that the Ba Estuary area is a critical habitat for multiple species that are classified as “Near Threatened” or “Endangered.” Specifically, the area likely acts as a parturition ground over the studied period, and potentially as a subsequent nursery area. We identified subareas of high abundance and found that temperature, salinity and depth acted as small‐scale environmental drivers of shark abundance. The data suggests a tendency for species‐specific spatial use, both horizontally (i.e., between sampling areas) and vertically (i.e., across the water column). These results enhance the understanding of shark ecology in Fiji and provide a scientific basis for the implementation of local conservation strategies that contribute to the protection of these threatened species.     

Egg morphology of Swift Terns in South Africa

Morphology of Swift Tern Thalasseus bergii bergii eggs was examined on Robben Island, South Africa. A recently formed colony was found abandoned en masse, probably following human disturbance, and 146 freshly laid eggs were collected. The mean±SD nest density was 7±2.5 nests m?2 and 3% of nests contained two eggs. Eggs ranged in shape from oval to pyriform and displayed black markings (blotched, streaked, scrawled or speckled) overlaying the eggs’ colour. The mean length and width of a sample of 105 eggs was 62.2?mm (56.3–66.9?mm) and 42.3?mm (39.8–45.3?mm), respectively. All collected eggs were weighed and the mean mass was 57.9±3.72?g. Estimated volume of eggs was calculated to be 56.3±3.74 cm3. This is the first report of mass measurements obtained from freshly laid eggs Swift Tern eggs and provides insight on egg morphology, for which knowledge is limited for this species.     

Resource-use dynamics of co-occurring chondrichthyans from the First Coast,North Florida,USA

Recent studies on shark assemblages on the northeast Florida and southeast Georgia coast (hereafter referred to collectively as the “First Coast”) have demonstrated differences in species and age-class composition of catch from previously characterized estuaries and newly surveyed area beaches, demonstrating that these regions may provide a critical habitat to different segments (i.e., life stages) of local shark populations. In this study, carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N) from muscle tissue and blood plasma were used to examine trophic dynamics (and temporal variability thereof) of the three dominant co-occurring species found along First Coast beaches (the Atlantic Sharpnose shark Rhizoprionodon terraenovae, Blacknose shark Carcharhinus acronotus and Blacktip shark Carcharhinus limbatus) to determine if they exhibit overlap in resource use along with spatial and temporal habitat use. Although considered spatially segregated from the beach species, a dominant, age-class species found in First Coast estuaries (juvenile Sandbar sharks Carcharhinus plumbeus) was also included in this analysis for comparison. Temporal variability of resource-use characteristics was detected at the species level. Resource-use overlap among species varied by tissue type and was generally higher for blood plasma, suggesting greater resource sharing over more recent time periods. Over longer time periods Atlantic Sharpnose and Blacktip sharks exhibited resource-use expansion, whereas Blacknose sharks exhibited a narrowing in resource use, suggesting a more specialized foraging strategy compared to the other species. The resource-use breadth of Sandbar sharks also expanded between blood plasma and muscle tissue. Significant size relationships were detected in Blacktip and Sandbar sharks, indicating ontogenetic resource shifts for both species. A diversity of highly productive resource pools likely support shark populations along the First Coast such that resource-use differentiation is not required to facilitate species co-occurrence. This work may shed light on understanding patterns of species co-occurrence as well as aid in future conservation efforts.     

Analysis of variability in vertebral morphology and growth ring counts in two Carcharhinid sharks

Inter- and intra-regional variations in vertebrae morphology and growth increment counts (band counts) were analyzed for two carcharhinid shark species, Carcharhinus plumbeus (n = 10) and C. limbatus (n = 11). Five sequential vertebrae were removed from the cervical region, above the branchial chamber and posterior to the chondrocrainium, and thoracic region, below the first dorsal fin. Dorsal–ventral height, medial–lateral breadth, and caudal–cranial length were measured for each sampled vertebra. Results indicate no significant difference in vertebral morphology within a sampled region of the vertebral column. However, a significant difference in vertebral morphology was noted between regions for both shark species, with thoracic vertebrae consistently larger than cervical vertebrae. A sub-set of three vertebrae was taken from each sampled region of each shark for sectioning and counting of growth increments. Analyses of growth increment counts by two readers indicated no significant difference in band counts within and between sampled regions.     

The denticle surface of thresher shark tails: Three-dimensional structure and comparison to other pelagic species

Shark skin denticles (scales) are diverse in morphology both among species and across the body of single individuals, although the function of this diversity is poorly understood. The extremely elongate and highly flexible tail of thresher sharks provides an opportunity to characterize gradients in denticle surface characteristics along the length of the tail and assess correlations between denticle morphology and tail kinematics. We measured denticle morphology on the caudal fin of three mature and two embryo common thresher sharks (Alopias vulpinus), and we compared thresher tail denticles to those of eleven other shark species. Using surface profilometry, we quantified 3D-denticle patterning and texture along the tail of threshers (27 regions in adults, and 16 regions in embryos). We report that tails of thresher embryos have a membrane that covers the denticles and reduces surface roughness. In mature thresher tails, surfaces have an average roughness of 5.6 μm which is smoother than some other pelagic shark species, but similar in roughness to blacktip, porbeagle, and bonnethead shark tails. There is no gradient down the tail in roughness for the middle or trailing edge regions and hence no correlation with kinematic amplitude or inferred magnitude of flow separation along the tail during locomotion. Along the length of the tail there is a leading-to-trailing-edge gradient with larger leading edge denticles that lack ridges (average roughness = 9.6 μm), and smaller trailing edge denticles with 5 ridges (average roughness = 5.7 μm). Thresher shark tails have many missing denticles visible as gaps in the surface, and we present evidence that these denticles are being replaced by new denticles that emerge from the skin below.     

Eating catch of the day: the diet of porbeagle shark Lamna nasus (Bonnaterre 1788) based on stomach content analysis,and the interaction with trawl fisheries in the south-western Atlantic (52° S–56° S)

This work examined the diet of the porbeagle shark Lamna nasus in the south-west Atlantic Ocean (SWAO, Argentina, 52° S–56° S) by analysing the stomach content information obtained by scientific observers who sampled specimens captured as by-catch on-board commercial fishing vessels from 2010 to 2020. A total of 148 fishing sites were analysed, in which the estimated catch was composed mainly of hoki Macruronus magellanicus (56.00%) and southern blue whiting Micromesistius australis (33.13%). From 413 porbeagle sharks sampled (292 females and 121 males) ranging from 71 to 241 cm total length (L_T) (mean: 179.76 ± 26.74 cm), 310 (75.06%) contained food in the stomachs. The forage fish were mainly hoki M. magellanicus (23.53%) and southern blue whiting M. australis (19.05%), followed by the Patagonian sprat Sprattus fuegensis (4.48%) and nototheniids (1.4%). Cephalopods and crustaceans accounted for 10% of the diet. The estimated trophic level was 4.35. Generalized linear models revealed that the consumption of hoki M. magellanicus and southern blue whiting M. australis increased with the L_T of the porbeagle shark. Moreover, smaller porbeagle sharks preyed upon both small and large teleost fish, whereas larger porbeagle sharks predated exclusively upon large fish. The diet of porbeagle shark involved interactions with fisheries as it fed upon the fish species that constituted the main catch in the analysed fishing sites, as well as the main catches of the austral trawl fisheries. The ecological role of porbeagle shark observed in the SWAO exposed implications for fisheries management from a multispecies perspective.     

Size frequency,dispersal distances and variable growth rates of young sharks in a multi-species aggregation

During a mark–recapture survey from November 2014 until April 2017, 333 neonatal and juvenile blacktip reef sharks Carcharhinus melanopterus and 302 neonatal and juvenile sicklefin lemon sharks Negaprion acutidens were tagged and measured at the uninhabited and isolated St. Joseph Atoll (Republic of Seychelles). Both species demonstrated seasonal reproductive synchronicity and relatively large sizes at birth. Despite the extended times at liberty > 2.5 years, the majority of recaptures were found in close proximity to the initial tagging location (< 500 m). Annual growth rates of C. melanopterus (n = 24) and N. acutidens (n = 62) ranged from 6.6 to 31.7 cm year⁻¹ (mean ± SE; 16.2 ± 1.2 cm year⁻¹) and 0.2 to 32.2 cm year⁻¹ (11.8 ± 1 cm year⁻¹), respectively and are to date the most variable ever recorded in wild juvenile sharks. High abundances of both species coupled with long-term and repeated recaptures are indicative of a habitat where juveniles can reside for their first years of life. However, large variability in annual growth rates in both species may suggest high intra and interspecific competition induced by a possibly resource limited, isolated habitat.     

Aspects of maturation and reproduction in hexanchiform and squaliform sharks

Three species of hexanchiform sharks belonging to two families and 12 species of squaliform sharks belonging to three families were recorded in fish landing site surveys in eastern Indonesia. Of these, the Squalidae were the most abundant species landed, with Squalus hemipinnis, Squalus edmundsi and Squalus montalbani contributing 0·4, 0·4 and 0·5% to the total number of sharks recorded in a 5 year survey of Indonesian fish landing sites. In comparison, the hexanchid Hexanchus griseus contributed the largest percentage to the total shark biomass. For many species, the majority of the catch consisted of immature fishes, which had not yet been able to reproduce. The data presented in this article are the first biological data reported on most of these shark species and are thus vital for fisheries managers and conservation assessors.     

Growth trajectories of prenatal embryos of the deep-sea shark Chlamydoselachus anguineus (Chondrichthyes)

Chlamydoselachus anguineus, Garman 1884, commonly called the frilled shark, is a deep-sea shark species occurring up to depths of 1300 m. It is assumed to represent an ancient morphotype of sharks (e.g., terminal mouth opening, more than five gill slits) and thus is often considered to represent plesiomorphic traits for sharks. Therefore, its early ontogenetic developmental traits are important for understanding the evolution of its particular phenotype. Here, we established six stages for prenatal embryos and used linear measurements and geometric morphometrics to analyse changes in shape and size as well as their timing during different embryonic stages. Our results show a change in head shape and a relocation of the mouth opening at a late stage of development. We also detected a negative allometric growth of the head and especially the eye compared to the rest of the body and a sexual dimorphism in total body length, which differs from the known data for adults. A multivariate analysis of covariance shows a significant interaction of shape related to the logarithm of centroid size and developmental stage. Geometric morphometrics results indicate that the head shape changes as a covariate of body size while not accounting for differences between sexes. The growth pattern of stages 32 and 33 indicates a shift in head shape, thus highlighting the moment in development when the jaws start to elongate anteriorly to finally achieve the adult condition of terminal mouth opening rather than retaining the early embryonic subterminal position as is typical for sharks. Thus, the antero-terminal mouth opening of the frilled shark has to be considered a derived feature.     

Rhizobial Surface Biopolymers and their Interaction with Lectin Measured by Atomic Force Microscopy

Summary Atomic force microscopy (AFM) was used to measure the morphology and physicochemical properties of rhizobia and to probe cell-surface polymers with tips modified with soybean agglutinin (SBA). AFM measurements of the length, width, and height of Sinorhizobium fredii CCRC15769 were 1580±450, 870±70, and 270±50 nm, respectively (means±SD). Different AFM image modes revealed the morphology, adhesion, viscoelasticity, and surface roughness of rhizobia in air using the tapping operation. Force–distance relationships between SBA-terminated AFM probes and Bradyrhizobium japonicum USDA110 were recorded and the retraction curves showed an unbinding force of 106±48 pN with a loading rate of 1 nN/s in PBS containing 0.1 mM Mn²⁺ and 0.1 mM Ca²⁺ (pH 7.2). The technique of AFM provides information about the morphology and molecular interaction forces of rhizobia under physiological conditions.     

Re-evaluation of reproductive cycle and fecundity of finetooth sharks Carcharhinus isodon (Valenciennes 1839) from the Northwest Atlantic Ocean,with new observations on ovarian cycle and reproductive endocrinology of biennially reproducing sharks

To provide updated information on life history for improved fishery management, the reproductive cycle of the finetooth shark Carcharhinus isodon from Northwest Atlantic (NWA) populations was investigated by examining temporal changes in morphology and histology of reproductive organs. Changes in plasma concentrations of gonadal sex hormones in relation to reproductive stage were also examined. Increases in testis width, epididymis head width, plasma testosterone concentrations and occurrence of mature spermatozoa were observed in male sharks between December and April, suggesting a seasonal pattern in reproduction that culminates with copulatory activity in May. Increases in maximum follicular diameter, oviducal gland width, plasma 17β-estradiol concentrations and occurrence of vitellogenic follicles were observed in non-pregnant female sharks during the same time period along with the occurrence of newly pregnant females in May, demonstrating strong synchronicity between male and female reproductive cycles. Pregnant females bearing full-term embryos were also observed in May, indicating that parturition occurs between mid-May and early June and gestation requires 12 months. Only transient temporal changes in follicle size and oviducal gland width were observed in pregnant females, indicating that reproductive periodicity is biennial; nonetheless, a single female exhibiting signs of concurrent vitellogenesis and pregnancy was observed. Mean brood size ± S.D. was 3.9 ± 0.9 offspring/female. Fecundity was not significantly correlated with female size, in part due to an unexpectedly high rate of early embryo mortality, which occurred in 11% of pregnant females, and was more common in larger individuals. Changes in ovarian activity during mid-pregnancy were observed, suggesting possible roles for the ovary in regulating some aspects of early to mid-gestation. This study confirms that earlier characterizations of the reproductive cycle and fecundity in NWA finetooth sharks remain valid for use in fishery management. This study also highlights unusual features of finetooth shark pregnancy (e.g., early embryo death, mid-pregnancy changes in ovarian function) that may have broader relevance to understanding elasmobranch reproduction.     

Scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatus

We quantified placoid scale morphology and flexibility in the shortfin mako Isurus oxyrinchus and the blacktip shark Carcharhinus limbatus. The shortfin mako shark has shorter scales than the blacktip shark. The majority of the shortfin mako shark scales have three longitudinal riblets with narrow spacing and shallow grooves. In comparison, the blacktip shark scales have five to seven longitudinal riblets with wider spacing and deeper grooves. Manual manipulation of the scales at 16 regions on the body and fins revealed a range of scale flexibility, from regions of nonerectable scales such as on the leading edge of the fins to highly erectable scales along the flank of the shortfin mako shark body. The flank scales of the shortfin mako shark can be erected to a greater angle than the flank scales of the blacktip shark. The shortfin mako shark has a region of highly flexible scales on the lateral flank that can be erected to at least 50°. The scales of the two species are anchored in the stratum laxum of the dermis. The attachment fibers of the scales in both species appear to be almost exclusively collagen, with elastin fibers visible in the stratum laxum of both species. The most erectable scales of the shortfin mako shark have long crowns and relatively short bases that are wider than long. The combination of a long crown length to short base length facilitates pivoting of the scales. Erection of flank scales and resulting drag reduction is hypothesized to be passively driven by localized flow patterns over the skin. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Comparative feeding ecology of shortfin mako, blue and thresher sharks in the California Current

This study describes the feeding ecology of three pelagic shark species in the California Current: shortfin mako (Isurus oxyrinchus); blue (Prionace glauca); and thresher (Alopias vulpinus) sharks. Stomach contents of sharks collected from 2002 to 2008 were identified to the lowest taxonomic level and analyzed using univariate and multivariate methods. Of 330 mako sharks sampled (53 to 248?cm fork length [FL]), 238 stomachs contained 42 prey taxa, with jumbo squid (Dosidicus gigas) and Pacific saury (Cololabis saira) representing the most important prey based on the geometric index of importance (GII). In addition, 158 blue sharks were sampled (76 to 248?cm FL) and 114 stomachs contained 38 prey taxa, with jumbo and Gonatus spp. squids representing the most important prey. Lastly, 225 thresher sharks were sampled (108 to 228?cm FL) and 157 stomachs contained 18 prey taxa with northern anchovy (Engraulis mordax) and Pacific sardine (Sardinops sagax) identified as the most important prey. Overall, mako sharks had the most diverse diet based upon Simpson??s diversity index (1/D) (8.43?±?1.16), feeding on many species of teleosts and cephalopods, followed by blue sharks (6.20?±?2.11) which consumed a wide range of prey (primarily cephalopods), while thresher sharks were most specialized (2.62?±?0.34), feeding primarily on coastal pelagic teleosts. Dietary overlap was lowest between blue and thresher sharks (S?rensen similarity index?=?0.321 and Simplified Morisita Horn index?=?0.006), and seasonal variability in diet was greatest for blue sharks (Simplified Morisita Horn index?=?0.260, Analysis of Similarity (ANOSIM) p?<?0.001). In addition, size class, and subregion were significant factors that affected diet of each species differently (ANOSIM p?<?0.001). Despite similarities in life history characteristics and spatial and temporal overlap in habitat, diets of these three common shark species are distinct in the California Current.     

Elevated levels of trimethylamine oxide in deep-sea fish: evidence for synthesis and intertissue physiological importance

Tissue levels of trimethylamine oxide (TMAO) were compared for seven teleost and two elasmobranch species captured from three depth ranges: shallow (<150 m), moderate (500-700 m), and deep (1,000-1,500 m). Within the teleosts, the deep-caught species had significantly greater TMAO content than shallow- or moderate-caught species. In all teleosts, muscle had substantially more TMAO than all other tissues. Kidney or, in some cases, liver had elevated trimethylamine (TMA) content, 2.20-9.65 mmol/kg, along with appreciable trimethylamine oxidase (TMAoxi) activity, suggesting active TMAO synthesis. No correlation was found between TMAoxi activity and TMAO content. The elasmobranchs in this study, Squalus acanthias and Centroscyllium fabricii from shallow and deep water, respectively, were both squaliform sharks. The deep-caught species had significantly more TMAO in all tissues than the shallow species. Furthermore, urea was significantly less in the deep species in all tissues except liver, while the urea:TMAO ratio was significantly less in all tissues. As with teleosts, the TMAO content of muscle was substantially higher for both elasmobranchs than in all other tissues. TMAoxi was below levels of detection in both elasmobranch species, suggesting that TMAO is obtained solely from the diet. This study expands the trend of increased muscle TMAO in deep-sea fish to a variety of other tissues. The accumulation of TMAO in various tissues in deep-sea teleosts and the accumulation of TMAO and concurrent urea decrease in a deep-sea elasmobranch in comparison to a shallow water species strongly support the contention that TMAO is of physiological importance in deep-sea fish.     

Habitat use of the nurse shark, Ginglymostoma cirratum, off Recife, Northeast Brazil: a combined survey with longline and acoustic telemetry

The nurse shark, Ginglymostoma cirratum, is an abundant coastal species widely distributed on both sides of the tropical and subtropical Atlantic Ocean. Despite being an endangered species in Brazil, information on bio-ecological aspects of this species is still very scarce. This study investigates seasonal fluctuations in abundance, sex ratio and residency of nurse sharks in coastal waters off Recife, northeastern Brazil. Total length of the specimens caught ranged from 107 to 300 cm. Sex ratio for nurse sharks was 1.31♀:1♂ but it showed a strong variation throughout the year. About 8% of tagged sharks were recaptured after an average of 248 days at liberty, at distances between 0.04 and 6.23 km from the tagging site. A growth rate of 15.77?±?2.53 cm/yr was obtained from total length data collected from three recaptured specimens. Nurse sharks demonstrated some signs of affinity for temperatures between 25º and 30°C and salinities between 34 and 37. The combined analysis of longline and telemetry data indicated possible differences in habitat usage between sexes of nurse sharks off Recife.     

The efficacy of Scenedesmus morphology as a defense mechanism against grazing by selected species of rotifers and cladocerans

Phytoplankton often develop various defense mechanisms in response to zooplankton grazing, such as spines and colonies. While it is now known that increased spine length and cells in a colony of members of the genus Scenedesmus, when zooplankton grazing is intense, helps in reducing zooplankton filtering rates, the effect of these defense mechanisms at the population level has been observed in few studies. Here we present data on the growth rates of four zooplankton species, Brachionus calyciflorus, B. patulus, Ceriodaphnia dubia and Daphnia pulex at two food levels using two species of colony-forming Scenedesmus spp.: S. acutus (cell length = 18.2 ± 0.4 µm; width = 4.2 ± 0.1 µm; average colony length = 90 µm; width: 21 µm) and S. quadricauda (cell length: 21 ± 0.5 width 7.5 ± 0.3 µm; average colony length: 84 µm; width: 30 µm). Whereas S. acutus had no spines, S. quadricauda had spines of 6–10 µm. Population growth experiments of the test rotifers and cladocerans were conducted in 100 ml containers with 50 ml of the medium with test algae. Algae concentrations used were: 13 and 52 mg dw l^–1 of each of the two algal species offered in colonial forms. We used an initial inoculation zooplankter density of 1 ind. ml^–1 for either of the rotifer species and 0.2 ind. ml^–1 for either of the cladoceran species. In all, we had 64 test containers (4 test species of zooplankton × 2 test species of algae × 2 algal densities × 4 replicates). We found a significant effect of algal size on the growth rates of all the four tested species of zooplankton. The population growth rates of zooplankton ranged from ?0.58 to 0.66 and were significantly higher on diet of S. acutus than of S. quadricauda. Thus, our study confirms that the larger colony size and the formation of spines in S. quadricauda were effective defenses against grazing by both rotifers and smaller sized cladoceran Ceriodaphnia dubia but that larger-bodied Daphnia pulex could exploit both the algal populations equally.     

Assessment of multiple paternity in single litters from three species of carcharhinid sharks in Hawaii

We tested for presence or absence of multiple paternity in single litters from each of three congeneric shark species in Hawaii: the sandbar shark, Carcharhinus plumbeus, bignose shark, Carcharhinus altimus, and Galapagos shark, Carcharhinus galapagensis. Based on eight polymorphic microsatellite loci, we excluded paternity by a single sire in sandbar and bignose sharks, but could not exclude a single sire for the litter from the Galapagos shark. This study doubles the number of shark species tested for multiple paternity, and is the first demonstration of multiple paternity in sandbar and bignose sharks.     

Trophic ecology of juvenile bull sharks (Carcharhinus leucas) in the Coyote estuary,Costa Rica

Bull shark (Carcharhinus leucas) is a near-threatened elasmobranch species capable of moving between the fresh and salty waters of tropical and subtropical coastal areas, for which we still lack important ecological information. During their first years of life, bull sharks use estuarine systems as nursery areas, making them highly susceptible to environmental and anthropogenic pressures. We studied the trophic ecology of juveniles found in the Coyote estuary, a potential nursery area in Costa Rica, to understand the potential impact of further bull shark declines and gain knowledge that could aid in their conservation. We analysed the trophic ecology of juvenile bull sharks [81–103 cm total length (TL)] in the Coyote estuary, Costa Rica, using stable isotopes of δ¹⁵N and δ¹³C. Since one problem using this technique in juveniles is the confounding effect of the maternal signature, we sampled different tissues (muscle and plasma), verified the status of the shark's umbilical scar and identified the size at which the isotope signature is a result of the animal's current diet. The isotopic values of the muscle tissue reflected the maternal isotopic signature. In contrast, plasma values reflected the diet of juvenile bull sharks >95 cm TL and with a closed umbilical scar. Juvenile bull sharks fed primarily on teleost fishes of the order Anguilliformes and Siluriformes, and have a high trophic position (≥4.0) in the Coyote estuary. Our findings suggest that this estuary is an important feeding site for juvenile bull sharks of the Pacific of Costa Rica. Thus, the protection of essential habitats such as the Coyote estuary will benefit not only bull shark conservation, but also the conservation of an array of fish species that also use this habitat as a rookery, many of which are of commercial interest.