Coastal wetlands are vital habitats for elasmobranchs, yet the spatial ecology of many species remains poorly understood, hindering effective conservation. The Pacific chupare ray, Styracura pacifica, listed as Vulnerable by the IUCN, is one such data-poor species with virtually unknown feeding habits and limited ecological data. This study aimed to address the spatial ecology of S. pacifica within the Térraba-Sierpe National Wetland (HNTS), Costa Rica, providing baseline data for management. We employed a participatory science framework, integrating Local Ecological Knowledge (LEK) from artisanal fishers with over 15 years of experience in the area, to identify key sampling sites in the main river channel and intertidal sand flat pools. Data were collected between 2022 and 2023 using floating longlines and modified cast nets. For each captured ray, sex and disc width (DW) were recorded, alongside environmental variables like temperature and precipitation. Due to non-normal catch per unit effort (CPUE) distributions, non-parametric analyses were applied. Relationships between CPUE, disc width (DW), and environmental variables were assessed using Spearman correlations, and differences between ecosystems, years, and groups were evaluated using a rank-based test. A total of 66 individuals were captured across both habitats. Captures were exclusively composed of immature individuals, with significantly smaller DWs recorded in intertidal pools (19.6–37 cm DW) compared to the main channel (31-55.7 cm DW), confirming strong size-based habitat segregation. CPUE was significantly and inversely correlated with temperature in the main channel and with rainfall in the intertidal pools, suggesting dynamic shifts in distribution linked to local environmental conditions. Our findings provide empirical evidence that the HNTS functions as a crucial nursery system for juvenile S. pacifica. The observed sensitivity to temperature and rainfall highlights the vulnerability of these life stages to climate-driven habitat instability. Methodologies that allow us to track individuals and their movements, adapted to small-sized rays, are recommended for a better understanding of habitat use.
Persistent inequities in marine science continue to shape who leads research, whose work is most visible, and which questions are prioritised. In elasmobranch science, a field often defined by intensive fieldwork and long-standing cultural norms, these dynamics have direct consequences for scientific capacity and conservation outcomes. Drawing on a decade of global publication data alongside existing scholarship, this Perspective evaluates how gender representation has changed within elasmobranch research and where structural barriers remain. Authorship patterns across more than twelve thousand peer reviewed publications indicate meaningful gains in participation by women, particularly at early career stages and in lead authorship. Progression into sustained senior authorship position remains uneven, reflecting institutional and cultural constraints rather than lack of expertise. These patterns align with broader evidence of leaky career pathways that continue to privilege narrow models of authority. Rather than framing equity as a matter of representation alone, we argue that inclusion is fundamental to the intellectual robustness of elasmobranch science. Diverse leadership expands the scope of inquiry, strengthens collaboration, and improves conservation relevance. As pressures on shark and ray populations intensify, retaining and empowering diverse scientific talent is not optional. It is essential. We call for coordinated structural reform across institutions, funding systems, publishing practices, and field operations to ensure that elasmobranch science reflects the diversity of expertise required to meet accelerating ocean challenges.
The pelagic stingray, Pteroplatytrygon violacea (Bonaparte, 1832), is the only fully pelagic member of the family Dasyatidae and is distributed across tropical and subtropical oceanic habitats worldwide. Despite its occurrence throughout the Indian Ocean and recent documentation in the eastern Mediterranean waters, the species has not previously been recorded from the adjacent Red Sea. Here we report the first confirmed record of P. violacea from the Red Sea (Gulf of Aqaba), based on a mature male (visually estimated ~45 cm disc width) incidentally caught by an artisanal fisher from epipelagic habitats within the territorial waters of Jordan (29.486°N, 34.965°E) on 30 May 2025. Identification was made from photographic and video evidence and is supported by a combination of morphological characters diagnostic for the species, including: uniformly dark purplish-brown to blackish coloration on both dorsal and ventral surfaces, a rhomboid disc wider than long with angular pectoral fin apices, a short and broadly rounded snout, and a single median row of small denticles on an otherwise smooth disc. Identification follows systematic exclusion of all dasyatid species currently recorded from the region based on this character combination. This record extends the known distribution of P. violacea into the Red Sea, raising important questions about the species’ occurrence patterns in the basin and its connectivity with the Indian Ocean or Mediterranean Sea.
The Benguela Current Large Marine Ecosystem (BCLME), renowned for its high productivity, sustains vital fisheries, including the ecologically and economically crucial Cape hake (Merluccius capensis and M. paradoxus). This system faces threats from anthropogenic heavy metal (HM) pollution, primarily from industrial and mining activities, which jeopardizes ecosystem integrity and seafood safety. This review synthesizes current knowledge on trophic interactions and heavy metal (HM) transfer in the BCLME, focusing on Cape hake (Merluccius capensis and M. paradoxus) and their crustacean prey. Using a PRISMA structured systematic literature review of published literature, the review evaluates patterns of HM bioaccumulation and trophic transfer across marine food webs. Evidence indicates that mercury (Hg) is the most extensively studied and bioaccumulative metal, with concentrations generally increasing with trophic position marine organisms. Spatial variability in HM concentrations is influenced by environmental drivers such as upwelling intensity, sediment interactions, and coastal inputs, which affect metal bioavailability. However, important gaps remain, including limited data on non-mercury metals, seasonal variability, and multi-trophic level analyses within the BCLME. In addition, the application of trophic ecology tools, such as stable isotope analysis (SIA) and Bayesian mixing models (MixSIAR), remains underutilized in assessing contaminant pathways. These limitations constrain a comprehensive understanding of HM dynamics in the region. The review highlights the implications of HM bioaccumulation for ecosystem functioning, fisheries sustainability, and seafood safety in Namibia. Evidence from the reviewed studies indicates that mercury concentrations in most marine organisms generally remain below established food safety thresholds, suggesting a low immediate risk to consumers. Strengthening region-specific monitoring and integrating ecological and human health assessments will be essential for improving management under increasing environmental and anthropogenic pressures.
With recovering seal populations, there is a need to rethink fisheries technology and application to overcome economic and ecological obstacles while overcoming socially challenging situations. We have evaluated catchability, seal-induced damage, and potential commercial viability, considering handling time, of four passive fishing gear types: pots, pontoon traps, fyke nets, and gillnets in a seal-dense area. Results indicated that while gillnets initially exhibited the highest catch rates, they also experienced the most substantial seal damage, with over 50% of the catch affected. In contrast, pots and pontoon traps showed no seal damage, and fyke nets had an almost negligible damage rate of 2%. When extrapolated to a full day of fishing and accounting for handling time, fyke nets could potentially yield the highest average daily catch, followed by gillnets, pontoon traps, and pots. We conclude that alternative gear types, particularly fyke nets, along with pots and pontoon traps, show potential as more seal-safe and commercially viable alternatives to traditional gillnets in regions with high seal interaction where future research should focus on optimizing the design and handling of these alternative gears, and evaluating their performance across diverse conditions and scales.
The Raja Ampat epaulette shark (Hemiscyllium freycineti) is an endemic species inhabiting shallow coastal habitats of the Raja Ampat Archipelago, Indonesia. Despite recent legal protection, information on its biology, population structure, and spatial ecology remains limited, constraining effective conservation. To address these gaps, a total of 64 nocturnal surveys (averaging 1.8–2.3 hours each) were conducted between February 2024 and April 2025 at six sites in the Dampier Strait region. Using photo-identification and passive integrated transponder tagging, 736 unique individuals were identified from 1,191 sightings, with most records from Arborek Island (n = 602). Of these, the population comprised 415 females and 321 males, resulting in a significantly female-biased sex ratio. Total length (TL) ranged from 19.4 to 75.0 cm (mean = 51.1 cm), and body mass ranged from 24 to 1,025 g (mean = 412.8 g). The length–weight relationships indicated negative allometric growth in both females (b = 2.91) and males (b = 2.81). Estimated length-at-maturity was 56.59 cm TL for females and 56.42 cm TL for males, with no significant difference between sexes. Relative density ranged from 333 to 2,462 individuals per km², representing the highest population density recorded globally for Hemiscyllium. A total of 256 individuals were resighted at least once, with resighting intervals extending up to 451 days. Immature individuals exhibited significantly higher growth rates than mature ones (7.8 vs 2.8 cm year-1), while no sex-specific difference in growth was detected. The best-supported and biologically meaningful growth model, seasonally oscillating ELEFAN with simulated annealing, estimated an asymptotic length (L∞) of 79.6 cm for all individuals, with a growth coefficient (K) of 0.747 year-1. Growth showed pronounced seasonal oscillations (C = 0.915). Sharks were most frequently observed in seagrass meadows. Habitat use analyses revealed ontogenetic partitioning. Coral reefs appear to function as a nursery habitat with 69% immature sharks, while seagrass and sand habitats supported higher proportions of adults. Individuals were highly resident, showed no inter-site movement, and occupied small spatial ranges, with maximum net displacements of 475 m. Collectively, these findings provide the first demographic and spatial baseline for H. freycineti in Raja Ampat.
Sturgeon are long-lived, late-maturing species and are typically maintained in captivity for many years prior to roe collection, raising distinct welfare considerations compared with most other farmed fish. European producers account for a substantial proportion of farmed caviar, yet little is known about welfare conditions in commercial sturgeon broodstock systems. This exploratory study examined European caviar producers’ perspectives on production practices and animal welfare. Semi-structured interviews were conducted with eight participants representing approximately 20% of European caviar production. Participants were recruited with support from the Federation of European Aquaculture Producers Sturgeon Commission. Interviews were transcribed and analysed using thematic analysis. Three main themes were identified: (1) history of caviar production and species used in farming, (2) living conditions for sturgeon and (3) market considerations and external pressures. Producers consistently described strong links between water quality, stress reduction and feed management and the quality of caviar, suggesting that economic incentives may align with certain welfare-supportive practices. Divergent views were expressed regarding traditional slaughter versus no-kill extraction, with perceived welfare and quality trade-offs in both systems. This study provides initial insight into European sturgeon farming practices and identifies priority areas for independent welfare assessment and future research.
IntroductionExposure to acute stress events, trauma, and intense anxiety can cause cognitive and memory deficits. This study investigated whether exposure to structured acoustic stimulation (classical music) modulates stress-induced cognitive impairments in adult zebrafish exposed to conspecific alarm substance (CAS), using the Y-maze test. MethodsFish were divided into groups, two of which were subjected to 15 days of Vivaldi’s classical music, with sound intensity between 65 and 75 dB SPL with predominantly measured frequencies between 330 and 506 Hz. One group was exposed to CAS before the acoustic stimulation (CAS + M), while another received the stimulation prior to CAS exposure (M + CAS). Additionally, two groups were exposed to CAS only, one on day 0 (CAS 0d) and the other on day 15 (CAS 15d), without any acoustic stimulation. A control group (CTL) was included without experimental manipulation. Spatial memory performance was assessed using the Y-maze test, and social behavior was evaluated through the shoaling test. Serum cortisol levels were measured as an index of physiological stress. ResultsExposure to CAS impaired exploratory behavior, evidenced by reduced time spent in the novel arm of the Y-maze. Consider this parameter as the main test outcome, post-stress acoustic stimulation restored exploratory performance to levels comparable to control fish, whereas prior exposure had no protective effect. Importantly, the CAS 15d group, which experienced the same temporal interval without acoustic stimulation, showed no recovery, supporting a specific effect of post-stress acoustic exposure on behavioral outcomes. CAS exposure significantly increased cortisol levels compared to controls; however, acoustic stimulation did not significantly attenuate this endocrine response. DiscussionTogether, these findings indicate that prolonged exposure to structured acoustic stimulation can modulate behavioral and cognitive responses following acute stress independently of cortisol normalization. The results support the use of zebrafish as an experimental model to investigate stress-induced cognitive dysfunction and highlight the relevance of acoustic environmental factors in post-stress behavioral modulation. Further studies are needed to elucidate the underlying neurobiological mechanisms and to determine the specificity of acoustic parameters involved.
Many giigoonyag (fishes) species worldwide display spawning site fidelity, making them especially susceptible to habitat disturbances in spawning areas. Identifying spawning sites and the degree of fidelity to these sites may be key to maintaining the relationship between humans and giigoonyag for future generations. Here, we use Indigenous knowledge to define Ogaa (Walleye Sander vitreus; singular) spawning periods, identify spawning sites, characterize habitat use, and quantify spawning site fidelity of 70 adult Ogaawag (Walleye; plural) during the 2019–2021 spawning seasons, in Mille Lacs Lake, Minnesota using an acoustic telemetry array. Ogaawag movements were tracked during the spring spawning period, and fish depth, number, and frequency of fish detected by each receiver were used to identify spawning sites and determine the relative importance of each spawning reef. Habitat characteristics such as substrate type, wind/wave action, and shoreline development were compared across reefs. We found that approximately 64% of Mille Lacs Ogaawag returned to the same site to spawn in 2020 as in 2019, 32% returned to the same site but also used other spawning areas, and only 4% used completely different spawning grounds across years. We identified areas with rocky and/or hard substrate, exposed to wind/wave action, and near undeveloped shorelines to be hotspots of Ogaawag activity during spawning. We also observed a rare example of skipped spawning by a female Ogaa. This study offers new insight into the rates of spawning site fidelity and the form that that fidelity takes in inland, fully-mixed waters, as well as provides evidence that Ogaa spawning hotspots may be correlated to areas of low or no onshore development. These findings may alter how humans act as stewards of this species, such as conserving spawning sites and adjacent habitats to improve recruitment and be a template for how Indigenous and Western societies can move toward a more inclusive life-history and ecosystem-based approach to sustaining their relationship with giigoonyag.
The inherent susceptibility of white sharks (Carcharodon carcharias), coupled with reported declines in abundance, has led to their global listing as “Vulnerable”, prompting substantial conservation efforts. However, white sharks' propensity to use coastal areas overlaps with human activities, causing safety concerns. In New South Wales (NSW), Australia, they are caught as part of the world's longest-running meshing bather protection program. After an unprecedented spate of shark bites in 2015, Shark-Management-Alert-in-Real-Time (SMART) drumline trials started in NSW as a novel catch-and-release program aimed at non-lethal bather protection. Using seven years of data, this study analyzed SMART drumline recapture events to describe white shark habitat use along the NSW coastline. In total, 890 individuals were tagged with a recapture rate of 24.5% (n = 218). Smaller individuals (fork length < 225 cm) and females were more commonly recaptured, suggesting size and sex-specific habitat preferences. Although recaptures occurred year-round and throughout the entire NSW coast, most occurred during the Austral winter and spring and along the northern NSW coast. There were few consecutive recaptures recorded in the same location, suggesting that white sharks move throughout relatively large ranges within coastal areas. Linear distances between recaptures were on average ~220 (SE ± 25) km, mostly representing northward travels. Despite some variation, most recaptures (~77%) were recorded within a year and ~45% within 90 days. These results are consistent with the described movement ecology for the species along NSW, corroborating the seasonal importance of NSW northern coast for juvenile white sharks, and the potential effect that the Eastern Australian Current has on their distribution. This study provides important insights into immature white sharks' ecology in inshore areas, highlighting the value of capture-mark-recapture data collected from SMART drumlines to forecast shark movement for enhanced bather safety and species management.
Sustainable fisheries require balancing ecological production with resource capture, but this balance is often obscured by changes in the fishing area. Further, expansion of fishing grounds is commonly proposed to mitigate overfishing by dispersing effort and promoting nearshore recovery, but its long-term consequences remain poorly understood. We evaluated artisanal coral reef fisheries along Kenya's fringing reef over a 23-year period, comparing ecological and fisheries production in non-expanded and expanded fishing grounds. Seventy percent of landing sites expanded by an average of 5% annually. Fishery catches were assessed against ecological production using fish censuses (1987–2024) and a biomass-production model (R2 = 0.78). Area changes significantly influenced per-area effort and catch estimates, requiring interpolation between initial and final measurements. Across all methods, capture consistently exceeded ecological production estimates, indicating unsustainable capture rates. Both area categories exhibited an effort–catch resource capture overshoot dynamic, with expanded areas showing delayed but more pronounced declines in per-area effort and yield after peak catches. Future yield projections varied with area estimation methods, expansion status, and catchability assumptions and projected that expansion would delay the time required to restore optimal yields. These findings highlight that fishing ground expansion is a less efficient use of fishing area, excludes several more efficient gears and livelihoods, slows recovery, and does not guarantee sustainability, or high long-term employment or yields. Detecting these patterns required regular evaluation of multiple fisheries and fisheries' independent variables, long-term assessments of effort, area, and ecological production. Moreover, simple time series and future projection methods can promote recommendations that hide the resource overshoot dynamic caused by a misalignment and time lags between production and capture rate estimates.
Some small-meshed fish-trawl codends require so-called “protective,” “strengthening,” or “lifting” bags for structural support, but these can occlude mesh openings and reduce size/species selectivity. This study compared two lifting-bag mesh sizes (94 and 216 mm stretched mesh opening; SMO) surrounding a 46-mm SMO codend in an Australian whiting-trawl fishery. During 37 deployments off Newcastle, New South Wales, compared to the 94-mm lifting bag, the 216-mm lifting bag significantly reduced the catches of juvenile eastern school whiting (Sillago flindersi <17 cm total length; TL) by ~35% and small unwanted velvet leatherjacket (Meuschenia scaber) by ~67%. Total bycatch declined by ~50%. However, ~6% of eastern school whiting ≥17 cm TL also escaped, along with some important byproduct species. Relative size–frequency analysis revealed no significant TL effects for eastern school whiting although there was evidence of fewer individuals being retained with the larger-meshed lifting bag. The data reiterate species-specific lifting-bag effects and the importance of evaluating both conservation benefits and economic impacts when subtly modifying trawl configurations.
We characterized the reproductive biology of the ecologically and commercially important small pelagic fish Trachurus novaezelandiae (yellowtail scad) in south-eastern Australian waters using a 25-year monitoring dataset. Sexual maturity occurred at 15.5 cm fork length (FL), with 95% of individuals mature by 23.2 cm FL. There were no significant differences in maturity ogives between sexes. Trachurus novaezelandiae exhibited an extended spawning season from July to March, peaking in October during the austral spring, consistent with sympatric small pelagic species in eastern Australian waters. Sex ratios in the commercial purse-seine fishery through 10 years were significantly skewed toward females (58%) and were consistent through years. Similarly to some other species within the Trachuridae, females predominated in all months except for those immediately following the spawning season, suggesting some differential sex-based availability to the fishery related to spawning. The fishery for T. novaezelandiae is currently classified as sustainable, with relatively low fishing mortality applied to the population. However, commercial harvests of small pelagic teleosts are rapidly increasing in Australian waters, and with developing markets it is likely that T. novaezelandiae will be fished more intensively in the future. The baseline information on reproductive biology in the present study may support sustainable exploitation through improved estimates of spawning stock biomass.
This study investigated the contribution of catecholamines to stress regulation in Atlantic salmon, with the goal of clarifying inconsistencies between the classical model of cortisol control in teleosts and recent observations that challenge it. According to the traditional theory, cortisol secretion is driven primarily by adrenocorticotropic hormone (ACTH) through activation of the hypothalamic–pituitary–interrenal (HPI) axis. However, several studies in salmonids have reported that elevations in cortisol can occur in the absence of, or prior to, measurable increases in ACTH. To examine whether catecholamines influence cortisol production we performed ex vivo incubations of head kidney tissue either with ACTH (10−6 M, 10−8 M, and 10−10 M), or catecholamine (adrenaline and noradrenaline, 10−6 M, 10−8 M for both) and monitored cortisol production up to 60 min post-incubation. The results confirmed that ACTH elicited a cortisol response, but not catecholamines. However, when head kidneys were incubated with combinations of ACTH (10−6 M) and catecholamines (adrenaline or noradrenaline, 10−7 M each) there was a massive increase in cortisol (by ~2.4-fold) production far exceeding that of ACTH alone. These findings suggest that catecholamines are unlikely to function as independent stimulators of cortisol synthesis but will enhance the responsiveness or sensitivity of interrenal cells to ACTH. Such a synergistic interaction could represent an adaptive mechanism enabling rapid cortisol elevation during acute stress, thereby helping to reconcile discrepancies between ACTH and cortisol profiles reported in vivo. Overall, this work provides new insight into the interplay between sympathetic activation and endocrine regulation in teleost fish.
Native to the Atlantic Ocean, anadromous sea lamprey (Petromyzon marinus) likely invaded the Laurentian Great Lakes in the mid 1800's-early 1900's following construction of the Erie Canal. Initially restricted to Lake Ontario, and some smaller nearby lakes, they entered Lake Erie via the Welland Canal in the early 1900s. Sea lamprey quickly became established in Lake Erie (1921), from which they invaded the three upper Great Lakes. Along with overharvest, predation (parasitism) by blood-feeding sea lamprey devastated commercial, sport and Indigenous fisheries including lake trout (Salvelinus namaycush) and whitefish and ciscoes (Coregonus sp.) populations. To deal with the crisis, the Great Lakes Fishery Commission (GLFC) was founded in 1955 with a mandate to eradicate sea lamprey. Sea lamprey were not eradicated, but a comprehensive sea lamprey control (SLC) program brought populations under control using barriers (dams) and traps to prevent spawning by adult lampreys, and chemical control using lampricides that selectively targeted larval sea lamprey in nursery streams draining into the lakes. In this synthesis the sea lamprey invasion is explored through the lens of “invasion theory” to characterize the likely vectors that introduced sea lamprey into the Great Lakes ecosystem, and to establish what eco-physiological features of sea lamprey led to their establishment and spread. The weight of evidence suggests that pre-existing adaptations and a robust physiology facilitated the sea lamprey's invasion of the Great Lakes. Key features likely included: (i) facultative anadromy, which allowed them to complete their entire life cycle in fresh water, (ii) a generalist diet enabling them to feed on a wide variety of fishes, (iii) the high fecundity of females that expedited their spread, (iv) a resilient thermal physiology, and (v) the availability of similar, suitable spawning and nursery habitat to that found in their native ranges. Many of these features may make sea lamprey relatively resilient to climate change, with changes in water temperature, water quality and hydrology having both negative and positive effects on the distribution of invasive populations in the Great Lakes, and imperiled populations native to the Western and Eastern Atlantic Ocean, and the Mediterranean Sea.
We compared tolerance of acute progressive hypoxia and warming in the three recognized populations of European seabass, Atlantic (AT), West Mediterranean (WM) and East Mediterranean (EM), that have evolved within a North-West to South-East thermal gradient, from AT to EM. We reared progeny of captive broodstock in common garden at two temperatures, 18 and 24°C, representing summer temperatures in Atlantic and East Mediterranean, respectively. At about 1 year of age, hypoxia tolerance was evaluated with static respirometry, as critical saturation for regulation of standard metabolic rate (Scrit) and regulation index (RI); while warming tolerance was evaluated by swimming respirometry, as critical thermal maximum for aerobic swimming (CTSmax). We expected AT fish systematically to be least tolerant and EM most, with WM intermediate. At 18°C, the tolerance traits were similar among populations, but they responded differently when reared at 24°C. In AT and WM, Scrit increased—tolerance declined—from 18 to 24°C, whereas in EM it did not change. In AT and WM, RI did not change from 18 to 24°C whereas in EM it increased—tolerance increased, and EM had higher RI than WM at 24°C. In AT and EM, CTSmax was similar at 18 and 24°C whereas in WM it increased—tolerance increased, and WM had higher CTSmax than AT and EM at 24°C. Therefore, the EM population is able to avoid negative effects of warmer water on hypoxia tolerance, compared to AT and WM, but this was not related to improved thermal tolerance at the warmer temperature, where WM performed better than AT and EM. Consequently, the seabass exhibits inter-populational variation in tolerance of the stressors but patterns are complex and not consistent between hypoxia and warming. We explored some potential patterns of inter-individual variation in tolerance. We found no evidence that individuals relatively tolerant of hypoxia (low Scrit, high RI) were also relatively tolerant of warming (high CTSmax). There was evidence of complex relationships with body mass, whereby tolerance of warming declined with increasing mass at 18°C but tolerance of hypoxia increased with mass at 24°C. Finally, the traits of tolerance were not dependent on individual standard metabolic rate at either temperature.
Oceanic whitetip sharks, Carcharhinus longimanus, are known to be common scavengers; however, observations of C. longimanus scavenging events are extremely rare due to their classification as an oceanic pelagic species, typically solitary in nature. On April 9, 2024, over 8.5 h, at least nine C. longimanus were observed scavenging from a heavily degraded carcass off the coast of Kailua-Kona, Hawai‘i, USA. Five tiger sharks (Galeocerdo cuvier) were also observed scavenging on the same carcass. Simultaneous feeding within and between species occurred; however, no agonistic or aggressive interactions were observed. Although a small snapshot, this stochastic event sheds new light on trophic relationships and social interactions among aquatic apex predators that do not normally overlap in space and time.
IntroductionDespite their size, relatively passive behavior, and commercial significance, knowledge of the behavioral ecology of whale sharks remains limited. The difficulty of tracking individual animals at sea encourages the use of retrospective biochemical approaches such as stable isotope analysis.MethodsWhale sharks at Mafia Island in Tanzania form a comparatively small and resident aggregation that has been monitored for several successive years, providing a rare opportunity to study biochemical changes in individuals over time. In this study, 53% of the identified individuals were sampled at the time of collection over a multi-year period. Stable isotope analysis was performed on these samples to investigate their feeding ecology.ResultsStable isotope data suggest that whale sharks at Mafia Island predominantly feed within the local food web at a trophic level consistent with current understanding of the species' general feeding ecology. These data also reveal a range of individual feeding strategies within the local aggregation, with some sex and size-related differences. However, the isotopic niche area at the population level was relatively constrained.DiscussionWe call for additional research and emphasize the importance of a multi-faceted approach incorporating diverse biochemical and tracking techniques to more accurately understand the long-term feeding ecology of this endangered and charismatic mega-planktivore.
River development affects fish connectivity, with intra-river issues exacerbated via sequential barriers. Remediation typically involves installing ‘fishways' to facilitate upstream movements. Here we evaluated species-specific upstream fish-passage efficiencies through three sequential vertical-slot fishways along the Nepean River in Australia via paired entry and exit trapping. Species-specific water velocity preferences associated with fishway entrance were informed by restricting head loss at the entry traps, but not at the exit traps. During 78 paired trap deployments 26,139 fish were caught, comprising 19 species; most of which successfully negotiated the fishways—albeit with considerable inter- and intra-specific variability among fishways. Catches of the most abundant species (38% of total), the amphidromous Gobiomorphus coxii (20–160 mm total length; TL), in the entry and exit traps were negatively and positively affected by water velocity, respectively at the second and third fishways, but not at the first. Catches of other species were also directly or indirectly affected by water velocity, with fewer catadromous Trachystoma petardi (145–460 mm fork length; FL) and Mugil cephalus (35–410 mm FL) recorded in entry than exit traps, implying (1) insufficient water velocity to permit entry and/or (2) confounding effects of the entry-trap design on capture. Conversely, two gudgeons [the potamodromous Philypnodon grandiceps (29–77 mm TL) and Hypseleotris galii (31–49 mm TL)] were caught in significantly greater abundances in the entry than exit traps implying some restriction to their passage and possibly due to deficits in fishway hydraulics and/or a lack of motivation to migrate in these species. The study highlights the value of location-specific monitoring for identifying key factors affecting fishway performance.
