Pelagic Sharks Research Articles

The biology and conservation status of the oceanic whitetip shark (Carcharhinus longimanus) and future directions for recovery

The oceanic whitetip shark (Carcharhinus longimanus) was once considered one of the most abundant and ubiquitous pelagic shark species in tropical seas globally. However, over the last several decades, the oceanic whitetip has experienced substantial population declines throughout its range due to fishing pressure and utilization in the international fin trade. In recent years, a significant amount of research has been undertaken on this species, revealing new information on life history, movements and behavior, and threats to the species. Additionally, a recent surge of protective measures has been implemented for the oceanic whitetip shark, both internationally and nationally. These include (but are not limited to) retention prohibition measures in every major tuna Regional Fishery Management Organization (RFMO), its listing in Appendix II of the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES), and its listing under the U.S. Endangered Species Act (ESA) as a threatened species. However, despite its global distribution and common occurrence in many commercial fisheries in tropical waters, little is still known regarding the oceanic whitetip shark’s biology and population status. Therefore, we summarize what is known on the biology and conservation of the oceanic whitetip shark, identify information gaps, and discuss future directions for recovery of this imperiled species.

A Time-Extended (24 h) Baited Remote Underwater Video (BRUV) for Monitoring Pelagic and Nocturnal Marine Species

Baited remote underwater video systems (BRUVs) are efficient devices for remotely characterising the assemblage and relative density of fishes and other marine organisms. However, technological constraints (e.g., battery life and limited storage memory) typically limit deployment times to <2 h making it very difficult to capture the presence of marine species, including rare and elusive ones. We developed an extended duration (24 h) BRUV to enable fisheries-independent surveying of a pelagic shark population in the western Mediterranean. Video data revealed seven visits (three nocturnal, four diurnal) by blue sharks (Prionace glauca) over a period of 1355 h. In the future, this cost-effective device could be fitted with a wireless video transmitter to provide real-time observations of marine animals for scientific or ecotourism purposes.

Are circle hooks effective management measures in the pelagic longline fishery for sharks in the Gulf of Gabès?

Abstract This study evaluated the circle hook use as a tool for shark management in the pelagic longline fishery in the Gulf of Gabès. The usual J‐hook No. 2 with 10° offset, which has been traditionally used by the fishery, was compared to the 18/0 non‐offset circle hook in an alternating fashion along the main line. In total, 22 experimental longline sets were deployed through the shark fishing seasons of 2016 and 2017 to examine the effects of hook types on the catch composition, the catch rates, the hooking location, and the status at haulback. The catch composition differed significantly among hook types. Moreover, an overall increase in catch rates for the main species, the sandbar shark Carcharhinus plumbeus, and the shortfin mako shark Isurus oxyrinchus, was observed when using circle hooks. There was no size‐selective effect of circle hooks for the common species. The circle hooks were not effective at reducing at‐haulback mortality of sandbar shark. Conversely, shortfin mako and smooth‐hound shark Mustelus mustelus showed significantly lower relative mortality at haulback with circle hooks than with J‐hooks. Furthermore, circle hooks were more frequently hooked externally than the J‐hooks for the three shark species. Results demonstrated that the use of 18/0 non‐offset circle hooks in the pelagic shark longline fishery can reduce mortality at haulback for some species without any benefit for the dominant species, the sandbar shark. Overall, it is difficult to promote the adoption of the use of circle hooks as a management measure in this specialized fishery. Management measures focusing on fishing effort controls, fishing closures in critical habitats, and size limits could have significant benefits for the conservation of shark species and may help to improve the sustainability of the shark fishery in the Gulf of Gabès.

New Example of Cosmopolitodus hastalis (Lamniformes, Lamnidae) from the Miocene of South Korea

The author reports the discovery of an isolated juvenile tooth of an extinct lamnid shark Carcharodon hastalis from the Duho Formation, Pohang City, South Korea. This tooth is approximately 11 mm in height, suggesting a juvenile affinity of an individual this tooth originated. In life, the shark is estimated to have been approximately less than 2 m in length. This is the second reported fossil record of Carcharodon hastalis in the Korean Peninsula and the first permineralized fossil remain as well. Although largely undescribed, fossil shark assemblage of the Duho Formation is similar to those of contemporaneous Japanese marine sediments, indicating epipelagic or pelagic sharks were already diversified throughout the East sea during the Middle Miocene.

Thresher sharks (Alopiidae) catch in the pelagic fisheries of Western Indonesia

Thresher sharks (family Alopiidae) are an evolutionarily distinct and globally endangered species group. Indonesia is the world’s largest shark fishing nation, and a global priority for reducing the capture of threatened shark species. Kutaradja port is one of Indonesia’s largest fishing ports, located in the capital of Aceh Province, Banda Aceh, Western Indonesia. Kutaradja is a large oceanic port serving a variety of fisheries, from small scale to industrial fleets. One of the main fisheries operating from Kutaradja port is an industrial purse seine fleet targeting valuable pelagic fish, particularly tuna. Pelagic sharks such as threshers are vulnerable to capture in these fisheries, due to their co-occurrence with stocks of valuable pelagic species. Here we describe the volume and composition of shark catch in Kutaradja’s purse seine fleet, based on daily landing monitoring conducted in January 2017 to May 2019. Our results show that the dominant species caught are thresher sharks (Alopiidae), which make up 51% of all shark landings. A total of 627 thresher sharks were landed during the monitoring period, consisting of 558 individuals of pelagic threshers shark (Alopias pelagicus) and 69 individuals of bigeye thresher shark (Alopias superciliosus). The catch is dominated by mature females, which could be particularly detrimental to the reproductive capacity of the population. Fishers in purse seine vessels often catch these threshers with hand-lines, which are used as a secondary gear while the main fishing gear is soaking. Based on these findings, we proposed recommendations for reducing thresher shark fishing mortality, including restricting the use of secondary fishing gears and releasing incidentally captured sharks.

Fisheries interactions and the challenges for target and nontargeted take on shark conservation in the Mexican Pacific.

Sharks have been of great cultural and socioeconomic importance in Mexico since the late 19th century, when the first fisheries were prosecuted in the Gulf of California to export fins to China. Mexican shark and ray fisheries are classified mainly by the size of the fishing vessel and include small- (7.5-10m), medium- (10-27m), and large-sized (>27m) fisheries. All are multispecies fisheries that use longline or gillnet gear, with their relative productivity varying over time. Off the Pacific coast, early shark small size vessel fisheries in the Gulf of California were driven by the need for vitamin A from livers, especially during World War II. As this fishery declined, new shark fishing opportunities arose because of government support and the development of the medium-sized fishery, which was capable of farther offshore excursions. Shark meat became an important part of the diets of poor and impoverished citizens during the 1950s and 1960s. The establishment of a Mexican Exclusive Economic Zone in 1976 pushed foreign vessels from Asia out of Mexican waters and led to the development of the large-sized vessel fishery to exploit pelagic sharks in offshore waters. By the early 1980s, Mexico shark fisheries were among the most productive in the world; however, a national economic crisis reduced effort and landings until the late 1980s, when several new fisheries emerged. Landings from Pacific states fluctuated between ~13,000 and 24,000t (dressed weight) during 1987-2012 but expanded steadily thereafter because of government support and offshore fleet expansion. Shark fisheries landings from the Mexican Pacific are currently at their highest recorded levels, exceeding 31,000t; however, a lack of species-specific landings and life history information has precluded population assessments of targeted stocks. In addition, though several recent management measures have been enacted to protect Mexican shark and ray fishery resources, the enforcement of these regulations is severely lacking. Therefore, the long-term sustainability of current fishing levels is unknown but should engender concern based on anecdotal evidence of serial depletion among historical elasmobranch targets in the Mexican Pacific.

Near-equal compressibility of liver oil and seawater minimises buoyancy changes in deep-sea sharks and chimaeras.

Whereas upper ocean pelagic sharks are negatively buoyant and must swim continuously to generate lift from their fins, deep-sea sharks float or swim slowly buoyed up by large volumes of low-density oils in their livers. Investigation of the pressure, volume, temperature (PVT) relationships for liver oils of 10 species of deep-sea Chondrichthyes shows that the density difference between oil and seawater, Δρ, remains almost constant with pressure down to full ocean depth (11 km, 1100 bar), theoretically providing buoyancy far beyond the maximum depth of occurrence (3700 m) of sharks. However, Δρ does change significantly with temperature and we show that the combined effects of pressure and temperature can decrease buoyancy of oil by up to 10% between the surface and 3500 m depth across interfaces between warm southern and cold polar waters in the Rockall Trough in the NE Atlantic. This increases drag more than 10-fold compared with neutral buoyancy during horizontal slow swimming (0.1 m s-1), but the effect becomes negligible at high speeds. Chondrichthyes generally experience positive buoyancy change during ascent and negative buoyancy change during descent, but contrary effects can occur at interfaces between waters of different densities. During normal vertical migrations buoyancy changes are small, increasing slow-speed drag no more than 2- to 3-fold. Equations and tables of density, pressure and temperature are provided for squalene and liver oils of Chimaeriformes (Harriotta raleighana, Chimaera monstrosa, Hydrolagus affinis), Squaliformes (Centrophorus squamosus, Deania calcea, Centroscymnus coelolepis, Centroscyllium fabricii, Etmopterus spinax) and Carcharhiniformes (Apristurus laurussonii, Galeus murinus).

Influência da Variabilidade Meteo-Oceanográfica no Oceano Atlântico Sudoeste sobre a Pesca Tubarão-Azul, Prionace glauca (Linnaeus, 1758)

O Brasil possui cerca de 8.500 km de linha de litoral e um número razoável de ilhas, totalizando uma área de aproximadamente 3,5 milhões de km² de Zona Econômica Exclusiva (ZEE), que se estendo desde o Cabo Orange (5 ºN) até o Chuí (34 ºS). A pesca com espinhel superfície é uma modalidade pesqueira mundialmente utilizada em oceano aberto. Atualmente os principais portos que abrigam a frota espinheleira na região Sudeste em Santos (SP) e Itaipava (ES) e na região Sul a localidade de Itajaí e Navegantes (SC). O tubarão-azul, Prionace glauca (Linnaeus, 1758), é uma das espécies de elasmobrânquios pelágicos, mais capturadas pela frota de espinhel de superfície. O tubarão-azul, é considerado o abundante dentre os grandes tubarões pelágicos presentes no Oceano Atlântico. É uma espécie oceânica, altamente migratória, apresenta distribuição global, entre as latitudes de 60°N e 50°S, ocorrendo em águas tropicais e temperadas, predominantemente naquelas com temperaturas entre 12°C - 20°C. A presente estudo visa avaliar a influência da variabilidade meteo-oceanográfica no oceano atlântico sudoeste sobre a população de tubarão-azul, Prionace glauca (Linnaeus, 1758), capturada pela frota industrial no período de 1994-2018 na modalidade de espinhel de superfície. Para a elaboração do estudo serão analisados ​​os dados de desembarques mensais da frota espinheleira da região do Atlântico Sudoeste entre os anos 1994 - 2018.

Local indicators for global species: Pelagic sharks in the tropical northeast Atlantic, Cabo Verde islands region

Abstract Pelagic sharks are an important bycatch in pelagic fisheries, especially for drifting longlines targeting swordfish. In the Cabo Verde Archipelago (tropical NE Atlantic), pelagic shark catches can reach a significant proportion of the total catches. Due to the increased concern on the status of pelagic shark species, this study was developed to enhance the current knowledge of those sharks in the Cabo Verde region in comparison to the adjacent areas, especially associated with European Union (EU) pelagic longline fishing activity. Stock status indicators for the two main species, blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus), were developed, based on fisheries data from logbooks and onboard scientific observers, including analysis of size frequency distributions and standardized catch-per-unit-of-effort (CPUE) indexes over time. The standardized CPUEs have been stable or increasing for both species in the past 10 years, indicating no signs of local depletion. In terms of sizes, the blue shark catch is composed mainly of adults, which can be a sign of a stable population. On the contrary, the catch of shortfin mako is composed mainly of juveniles, which in conjunction of a decrease of mean size might be a cause of concern, highlighting possible overfishing on the species in the region. Thirty satellite tags, 25 archival miniPATs and 5 SPOT GPS, were deployed in the Cabo Verde Exclusive Economic Zone (EEZ), showing that those species are highly mobile. The biomass and size distributions were modeled with spatial and seasonal models (GAMs) identifying locations where juveniles are predominantly concentrated and that should be prioritized for conservation. This work presents new information on the status of pelagic sharks in the Cabo Verde region in the context of those highly migratory species, and can now be used to promote more sustainable fisheries in the region.

Shark conservation hindered by lack of habitat protection

Many of the world’s shark populations are in decline, indicating the need for improved conservation and management. Well managed and appropriately located marine parks and marine protected areas (MPAs) have potential to enhance shark conservation by restricting fisheries and protecting suitable habitat for threatened shark populations. Here, we used shark occurrence records collected by commercial fisheries to determine suitable habitat for pelagic sharks within the Australian continental Exclusive Economic Zone (EEZ), and to quantify the amount of suitable habitat contained within existing MPAs. We developed generalised linear models using proportional occurrences of pelagic sharks for three families: Alopiidae (thresher), Carcharhinidae (requiem), and Lamnidae (mackerel) sharks. We also considered aggregated species from the Lamnidae and Carcharhinidae families (‘combined sharks’ in the models). Using a set of environmental predictors known to affect shark occurrence, including chlorophyll-a concentration, salinity, sea surface temperature, and turbidity, as well as geomorphological, geophysical, and sedimentary parameters, we found that models including sea surface temperature and turbidity were ranked highest in their ability to predict shark distributions. We used these results to predict geographic regions where habitat was most suitable for pelagic sharks within the Australian EEZ, and our results revealed that suitable habitat was limited in no-take zones within MPAs. For all shark groupings, suitable habitats were found mostly at locations exposed to fishing pressure, potentially increasing the vulnerability of the pelagic shark species considered. Our predictive models provide a foundation for future spatial planning and shark management, suggesting that strong fisheries management in addition to MPAs is necessary for pelagic shark conservation.

Effects of fishing practice changes on pelagic shark longline captures in the Gulf of Gabes, Tunisia

Abstract In order to increase their catches, longliners targeting sharks in the Gulf of Gabes, Tunisia, have increased the number of hooks per basket and have included new bait types instead of mackerel (Scomber scombrus). This paper reports the effects of these changes on catch composition, catch rates, size distribution, life‐stage captures, and mortality at haulback. Data from 48 and 96 longline sets, carried out during the shark fishing seasons of 2007 and 2008, with two hooks per basket, and 2016 and 2017, with five hooks per basket, respectively, were examined to assess the effect of gear change. Moreover, the effects of bait types were inspected based on 33 fishing sets using whole mackerel, 19 using salema (Sarpa salpa) halves, and 27 using pieces of stingray (Dasyatis spp.), sampled during 2016 and 2017. The species composition indicated that longliners expand the vertical distribution of their hooks to operate from the surface to the bottom. The catches with both longline designs were dominated by sandbar shark (Carcharhinus plumbeus). Despite gear changes, the catch rate of C. plumbeus decreased by 42.21%, suggesting population decline. The size composition indicated a shift towards larger specimens, mainly adult females, which could jeopardize the shark population. Moreover, the mortality of the main species at haulback seems to be affected by the design of the longline. The bait used, mainly pieces of stingray, significantly increased the catch rate of sandbar shark, which suggests a greater attractiveness of the new bait; however, fish size and mortality rates at haulback were unchanged in response to bait variation. The fishery operates in shark nursery grounds, which exposes these fish (principally C. plumbeus) to considerable exploitation pressure. The new fishing practices intensified the pressure and thus the risk of a rapid depletion of populations. To preserve the shark species in the area, the use of new gear and attractive bait should be banned.

Observation of a porbeagle shark Lamna nasus aggregation at a North Sea oil platform.

A minimum of 20 porbeagles Lamna nasus were observed circling the Alba oil platform (UK North Sea) over several days in July 2014. Although schools of fish often aggregate around oil platforms, less is known about their ability to aggregate pelagic sharks and L. nasus are rarely seen at the surface. This unusual observation provides new insights into L. nasus behaviour and habitat use and the potential role of offshore artificial structures on pelagic predators.

Negative allometric growth during ontogeny in the large pelagic filter-feeding basking shark

Many pelagic shark species change body and fin shape isometrically or by positive allometry during ontogeny. But some large apex predators such as the white shark Carcharodon carcharias or the tiger shark Galeocerdo cuvier show distinct negative allometry, especially in traits related to feeding (head) or propulsion (caudal fin). In particular, changes in propulsion are attributed to a shift in swimming mode. The more heterocercal caudal fin of younger individuals with its large caudal fin span seemingly aids in hunting small, agile prey. In contrast, the less heterocercal caudal fin with a larger fin area in larger individuals aids a long-distance slow swimming mode. We were interested if negative allometric effects can be observed in a planktivorous shark, the basking shark Cetorhinus maximus, a large species adapted to long-distance slow swimming. To address this question, we compared three size classes, specifically < 260 cm (juveniles), 299–490 cm (subadults), and from adults > 541 cm total length. Comparing literature data, we found negative allometric growth of the head and of the caudal fin, but a more rapid decrease of relative caudal fin size than of relative head length. Hereby, we provide the first evidence for early negative allometric growth of the caudal fin in a large pelagic filter-feeding shark. Our study further demonstrates that ecomorphological approaches may add valuable insight into the life history of animals that are challenging to study in their natural habitat, including large roving sharks such as the basking shark.

Organic contaminants as an ecological tool to explore niche partitioning: a case study using three pelagic shark species

Chemical contaminant profiles are linked to an animal’s niche, providing a potential tool by which to assess resource partitioning in pelagic species. As proof of concept, we examined contaminant signatures in three species of sharks (Isurus oxyrinchus, Prionace glauca, and Alopias vulpinus) known to overlap in both space and time. Since these sharks comprise a predatory guild within the Southern California Bight (SCB), we predicted that species may partition spatial and dietary resources to limit the extent of competitive exclusion. Indeed, species were distinguishable by both total contaminant loads and their contaminant fingerprint, as random forest analysis found that species could be correctly classified 96% of the time. Our results demonstrate the utility of chemical analyses for ecological studies, and how contaminant tracers can be used in combination with traditional methods to elucidate how species may undergo niche partitioning to reduce competition for overlapping resources within predatory guilds.

Mesoscale eddies release pelagic sharks from thermal constraints to foraging in the ocean twilight zone

Mesoscale eddies are critical components of the ocean's "internal weather" system. Mixing and stirring by eddies exerts significant control on biogeochemical fluxes in the open ocean, and eddies may trap distinctive plankton communities that remain coherent for months and can be transported hundreds to thousands of kilometers. Debate regarding how and why predators use fronts and eddies, for example as a migratory cue, enhanced forage opportunities, or preferred thermal habitat, has been ongoing since the 1950s. The influence of eddies on the behavior of large pelagic fishes, however, remains largely unexplored. Here, we reconstruct movements of a pelagic predator, the blue shark (Prionace glauca), in the Gulf Stream region using electronic tags, earth-observing satellites, and data-assimilating ocean forecasting models. Based on >2,000 tracking days and nearly 500,000 high-resolution time series measurements collected by 15 instrumented individuals, we show that blue sharks seek out the interiors of anticyclonic eddies where they dive deep while foraging. Our observations counter the existing paradigm that anticyclonic eddies are unproductive ocean "deserts" and suggest anomalously warm temperatures in these features connect surface-oriented predators to the most abundant fish community on the planet in the mesopelagic. These results also shed light on the ecosystem services provided by mesopelagic prey. Careful consideration will be needed before biomass extraction from the ocean twilight zone to avoid interrupting a key link between planktonic production and top predators. Moreover, robust associations between targeted fish species and oceanographic features increase the prospects for effective dynamic ocean management.

Global spatial risk assessment of sharks under the footprint of fisheries.

Effective ocean management and the conservation of highly migratory species depend on resolving the overlap between animal movements and distributions, and fishing effort. However, this information is lacking at a global scale. Here we show, using a big-data approach that combines satellite-tracked movements of pelagic sharks and global fishing fleets, that 24% of the mean monthly space used by sharks falls under the footprint of pelagic longline fisheries. Space-use hotspots of commercially valuable sharks and of internationally protected species had the highest overlap with longlines (up to 76% and 64%, respectively), and were also associated with significant increases in fishing effort. We conclude that pelagic sharks have limited spatial refuge from current levels of fishing effort in marine areas beyond national jurisdictions (the high seas). Our results demonstrate an urgent need for conservation and management measures at high-seas hotspots of shark space use, and highlight the potential of simultaneous satellite surveillance of megafauna and fishers as a tool for near-real-time, dynamic management.

Morphometric measurements of newborn blue shark Prionace glauca (Linnaeus, 1758) and characteristics of its potential parturition areas in coastal waters of Montenegro (Southeastern Adriatic)

The blue shark belongs to the group of the most widespread pelagic sharks in the world. It inha-bits the pelagic zone of almost all warm and temperate seas and oceans. As it is one of the most abundant shark species, it is frequently caught in different types of fishing gear operated in the pela-gic zone. This study provides the first data on the reproduction of this species in Montenegrin waters (in the Southeastern Adriatic Sea). Six newborn blue sharks were found during our field excursions. For three of them, detailed morphometric measurements are presented. Alongside the newborns, a gravid female was also recorded. The data was collected in the period from April to October 2017. According to the published literature and the data presented in this paper, species reproduction is likely to be seasonal. A review of the areas where the specimens were recorded was carried out in order to infer if the adult females choose them specifically as their parturition grounds

Post-release survivorship and movements of bigeye thresher sharks, Alopias superciliosus, following capture on deep-set buoy gear

The bigeye thresher shark (Alopias superciliosus) is a large pelagic predator routinely caught in a new deep-set fishery off California. Deep-set buoy gear (DSBG) was recently developed to selectively target swordfish (Xiphias gladius) at depth during the daytime and is currently being trialed by a limited fleet of vessels operating under an exempted fishing permit (EFP) issued through the National Marine Fishery Service (NMFS) and Pacific Fisheries Management Council (PFMC). Although marketable, bigeye thresher sharks (BETS) are often released or discarded to conserve limited hold space for more valuable catch. This study outfitted BETS with pop-up satellite archival transmitters (PSATs) programmed for 30-day deployments to evaluate acute post-release survival in the DSBG fishery and begin to understand depth and temperature distribution for this poorly known species. Fourteen BETS (43–93 kg) were tagged from 2016 to 2018 with PSATs (Wildlife Computers sPATs and MiniPATs) following fight times that ranged from 15 to 171 min. Out of the 14 deployments, twelve individuals survived the acute effects of capture, one BETS died shortly after release and one individual was preyed upon 6 h after release. Given the lack of information surrounding the predation induced mortality, two survivorship estimates are provided, one that includes the event as a mortality resultant of the capture event (86% survivorship) and one that excludes the event from the analysis (92% survivorship). All surviving BETS exhibited consistent diurnal dive patterns in which the sharks remained below the thermocline during the day and within the mixed layer at night. Mean daytime depths ranged from 250 to 439 m, while mean nighttime depths ranged from 20 to 106 m. Daytime temperatures at depth ranged from 6 to 10 °C with the lowest temperature value (6.0 °C) recorded on the deepest dive (548 m). For all BETS, maximum night temperatures averaged from 22.5 to 25.1 °C. Two tagged individuals remained proximal to the study area (<150 km) with others moving up to >1300 km from the tagging site (mean = 1009 ± 455 km). Mean daily rates of horizontal movement ranged from 3.4 to 41.6 km day−1. The most common movement trend entailed BETS moving in a southerly direction towards a series of offshore seamounts along southern Baja California, Mexico (22°-26 °N and 114°-126 °W), a purported offshore corridor for other large pelagic sharks of the eastern Pacific.

Opening the tap: Increased riverine connectivity strengthens marine food web pathways.

Reduction of ecosystem connectivity has long-lasting impacts on food webs. Anadromous fish, which migrate from marine to freshwater ecosystems to complete reproduction, have seen their historically larger ecosystem role undercut by widespread riverine habitat fragmentation and other impacts mainly derived from anthropogenic sources. The result has been extensive extirpations and increased susceptibility to a suite of environmental factors that currently impede recovery. Under this present-day context of reduced productivity and connectivity, aggressive management actions and enforcement of catch limits including bycatch caps and complete moratoria on harvest have followed. What remains less understood are the implications of changes to food webs that co-occurred. What benefits restoration could provide in terms of ecosystem functioning in relation to economic costs associated with dam removal and remediation is unknown and can limit the scope and value of restoration activities. Here we employ, historical landscape-based biomass estimates of anadromous alosine for the first time in an ecosystem modeling of the Northeast US large marine ecosystem (LME), to evaluate the value of improving connectivity by measuring the increase in energy flow and population productivity. We compared a restored alosine model to a contemporary model, analyzing the impacts of the potential increase of connectivity between riverine and oceanic systems. There was the potential for a moderate biomass increase of piscivorous species with high economic value, including Atlantic cod, and for a major increase for species of conservation concern such as pelagic sharks, seabirds and marine mammals. Our study highlights the benefits of increased connectivity between freshwater and ocean ecosystems. We demonstrate the significant role anadromous forage fish could play in improving specific fisheries and overall ecosystem functioning, mainly through the diversification of species capable of transferring primary production to upper trophic levels, adding to benefits associated with their restoration.

Development and testing of a Bayesian population model for the bigeye thresher shark, Alopias superciliosus, in an area subset of the western North Pacific

AbstractA Bayesian population modelling tool integrating separable virtual population analysis, per‐recruit models and age‐structured demographic analysis was developed for the bigeye thresher Alopias superciliosus (Lowe) population in an area subset of the western North Pacific. The mortality rates for years 1989–2016 were estimated, various biological reference points and associated risks of decline were also estimated, and alternative harvest strategies for the stock were evaluated. Estimates of the posterior mean of fishing mortality for bigeye thresher shark suggest fishing pressure has been high in recent years (2011–2016). The estimated population growth rate (λ) (without fishing) obtained from age‐structured demographic model was relatively low (λ = 1.01 per year; 95% confidence intervals of 1.00 and 1.03 per year). Risk analyses revealed that only low levels of fishing pressure (10% of the current fishing pressure) over a wide range of ages could maintain a relatively low risk of population decline for bigeye threshers. Sensitivity testing indicated that the model is robust to prior specification. The developed framework could be used as an assessment tool to evaluate the risk of decline for other widely distributed pelagic shark species where insufficient catch and effort data are available.