How to infer population trends in sparse data: examples with opportunistic sighting records for great white sharks

New Evidence for the Contemporary Presence of Juvenile White Sharks (Carcharodon carcharias) in the Adriatic Sea

The presence of the white shark (Carcharodon carcharias) in the Mediterranean Sea is well documented, but mainly through historical and opportunistic records. Historically, the Adriatic Sea, particularly its eastern coastline, was considered a hotspot for white sharks, with relatively frequent reports of juvenile specimens suggesting a potential nursery area. However, since the second half of the 20th century, the abundance of white sharks in the Adriatic has experienced a dramatic decline, with the last confirmed sighting recorded in 2011. In this study, we report the recent capture of a young-of-the-year (YOY) white shark off the Croatian coast, previously misidentified as a porbeagle shark (Lamna nasus). In addition, we revisit historical records of white sharks in the Adriatic Sea to assess whether this region can be considered a nursery area. Our findings highlight significant gaps in the understanding of white shark spatial ecology and reproductive biology in the Mediterranean Sea. Furthermore, this study emphasizes the critical role citizen science and public engagement can play in documenting occurrences of these elusive and endangered predators, offering valuable insights for future conservation efforts.

Scientific progress made towards bridging the knowledge gap in the biology of Mediterranean marine fishes

The Mediterranean Sea is a renowned biodiversity hotspot influenced by multiple interacting ecological and human forces. A gap analysis on the biology of Mediterranean marine fishes was conducted in 2017, revealing the most studied species and biological characteristics, as well as identifying knowledge gaps and areas of potential future research. Here, we updated this gap analysis five years later by reviewing the literature containing information on the same eight biological characteristics, namely length-weight relationships, growth, maximum age, mortality, spawning, maturity, fecundity and diet, for the 722 fish species of the Mediterranean Sea. The results revealed a considerable knowledge gap as 37% of the species had no information for any of the studied characteristics, while 13% had information on only one characteristic. Out of all the biological characteristics, the smallest knowledge gap was found in the length-weight relationships (studied for 51% of the species, mainly in the eastern Mediterranean), while the least studied characteristic was mortality (studied for 10% of the species). The western and eastern Mediterranean Sea were leading forces in data collection exhibiting the narrowest gaps between current and desired knowledge. The most studied species across the entire region were the highly commercial European hake (Merluccius merluccius), red mullet (Mullus barbatus), European anchovy (Engraulis encrasicolus), European pilchard (Sardina pilchardus), common pandora (Pagellus erythrinus), and annular seabream (Diplodus annularis). The knowledge gap has shrunk by 6% during the last five years, with 40 new species having at least one study on their biology. Moreover, research has slightly shifted towards species that have been traditionally neglected, e.g., sharks, rays and chimaeras (chondrichthyans). It is recommended that research becomes less focused on commercial species and more targeted towards the identified gaps, vulnerable species (e.g., deep-sea species and chondrichthyans) and species that could potentially pose a threat (e.g., non-indigenous species) to the ecosystems of the everchanging Mediterranean Sea.

Fine-scale vertical habitat use of white sharks at an emerging aggregation site and implications for public safety

Abstract ContextOver the past decade, the coastal waters off Cape Cod, Massachusetts, have emerged as the only known aggregation site for the white shark (Carcharodon carcharias) in the western North Atlantic. During periods of seasonal residency, white sharks patrol the shoreline in search of pinniped prey, bringing them in close proximity to popular beaches where people recreate. AimTo examine whether white sharks off Cape Cod are more likely to occupy shallow depths (and consequently more likely to overlap with recreational water users) under certain conditions. MethodsWe deployed short-term, pop-up satellite archival transmitting (PSAT) tags and acoustic transmitters on 14 subadult and adult white sharks off the coast of Cape Cod during the summer and fall of 2017. PSAT tags provided fine-scale depth and temperature data, which were combined with high-resolution location data obtained from an acoustic telemetry array, to identify the depth and temperature preferences of white sharks when resident in the area. Key resultsSharks spent the majority (95%) of tracked time at depths of 0–31m and at temperatures from 8.9°C to 20.7°C. During resident periods along Cape Cod, individuals spent almost half (47%) of their time at depths of less than 4.5m, but made frequent excursions to mid-shelf depths, alternating between the surf zone and deeper offshore waters. Sharks were slightly more likely to occupy shallow depths at night during the new moon. The relationship between shark depth and lunar phase varied over the course of the day, suggesting the mechanism underlying lunar effects differs among diel periods. ConclusionsAlthough the overall risk posed to humans by white sharks is low, there is a high potential for overlap between white sharks and recreational water users off Cape Cod. The risk of interaction may be slightly higher during periods when local environmental conditions favour the species’ predatory stealth by influencing prey behaviour or detectability. ImplicationsThis study provides the first glimpse into the fine-scale vertical habitat use of white sharks off Cape Cod, which can be used to better understand the risk to recreational water users and to inform public safety practices.

The power to detect regional declines in common bird populations using continental monitoring data

Anthropogenic environmental change is driving the rapid loss of biodiversity. Large declines in the abundance of historically common species are now emerging as a major concern. Identifying declining populations through long-term biodiversity monitoring is vital for implementing timely conservation measures. It is, therefore, critical to evaluate the likelihood that persistent long-term population trends of a given size could be detected using existing monitoring data and methods. Here, we test the power to detect declines in Australia's common landbirds using long-term citizen science monitoring. We use spatially explicit simulations of occupancy dynamics and virtual sampling, designed to mimic bird monitoring in better-sampled regions of Australia, to assess likely power in these data to detect trends relevant for conservation. We predict the statistical power for 326 common species that meet minimum requirements for monitoring data across 10 regions of Australia, estimating the number of species for which we would have a high (≥80%) chance of detecting declines of different sizes. The power to detect declines of ≥30% per decade was predicted to be high for at least one-third of the common species in 7 of 10 regions, with a total of 103 (32% of 326) unique species sufficiently monitored in at least one region. These species spanned 12 taxonomic orders, four orders of magnitude in body mass, and a broad diversity of dietary guilds, suggesting the current species pool will likely serve as robust indicators for a broad range of environmental states and pressures. Power was strongly affected by species' detectability, and power to detect even large declines was negligible when species are detected on ≤50% of visits to an occupied site. Predicted power for many species fell just short of the 80% threshold in one or more regions, which suggests an increase in effort targeting these species could greatly enhance the species and regional representation of these data. Against the backdrop of unprecedented biodiversity losses, this study shows how critical evaluation of existing monitoring schemes is valuable both for assessing the contribution of citizen science schemes to biodiversity monitoring and for designing strategic monitoring to significantly improve the knowledge these schemes provide.

Trends in Stranding and By-Catch Rates of Gray and Harbor Seals along the Northeastern Coast of the United States: Evidence of Divergence in the Abundance of Two Sympatric Phocid Species?

Harbor seals and gray seals are sympatric phocid pinnipeds found in coastal waters of the temperate and sub-Arctic North Atlantic. In the Northwest Atlantic, both species were depleted through a combination of subsistence hunts and government supported bounties, and are now re-occupying substantial portions of their original ranges. While both species appear to have recovered during the past 2 decades, our understanding of their population dynamics in US waters is incomplete. Here we describe trends in stranding and bycatch rates of harbor and gray seals in the North East United States (NEUS) over the past 16 years through an exploratory curve-fitting exercise and structural break-point analysis. Variability in gray seal strandings in Southern New England and bycatch in the Northeast Sink Gillnet Fishery were best described by fitting positive exponential and linear models, and exhibited rates of increase as high as 22%. In contrast, neither linear nor exponential models fit the oscillation of harbor seal strandings and bycatch over the study period. However, a breakpoint Chow test revealed that harbor seal strandings in the Cape Cod, Massachusetts region and harbor seal bycatch in the Northeast Sink Gillnet Fishery increased in the 1990s and then started declining in the early to mid-2000s. Our analysis indicates that ongoing variation in natural and anthropogenic mortality rates of harbor and gray seals in the NEUS is not synchronous, and likely represents diverging trends in abundance of these species as they assume new roles in the marine ecosystems of the region.

Effects of recent environmental change on accuracy of inferences of extinction status

Correctly classifying a species as extinct or extant is of critical importance if current rates of biodiversity loss are to be accurately quantified. Observing an extinction event is rare, so in many cases extinction status is inferred using methods based on the analysis of records of historic sighting events. The accuracy of such methods is difficult to test. However, results of recent experiments with microcosm communities suggest that the rate at which a population declines to extinction, potentially driven by varying environmental conditions, may alter one's ability accurately to infer extinction status. We tested how the rate of population decline, driven by historic environmental change, alters the accuracy of 6 commonly applied sighting-based methods used to infer extinction. We used data from small-scale experimental communities and recorded wild population extirpations. We assessed how accuracy of the different methods was affected by rate of population decline, search effort, and number of sighting events recorded. Rate of population decline and historic population size of the species affected the accuracy of inferred extinction dates; however, faster declines produced more accurate inferred dates of extinction, but only when population sizes were higher. Optimal linear estimation (OLE) offered the most reliable and robust estimates, though no single method performed best in all situations, and it may be appropriate to use a different method if information regarding historic search efforts is available. OLE provided the most accurate estimates of extinction when the number of sighting events used was >10, and future use of this method should take this into account. Data from experimental populations provide added insight into testing techniques to discern wild extirpation events. Care should be taken designing such experiments so that they mirror closely the abundance dynamics of populations affected by real-world extirpation events.

Seasonal distribution and historic trends in abundance of white sharks, Carcharodon carcharias, in the western North Atlantic Ocean

Despite recent advances in field research on white sharks (Carcharodon carcharias) in several regions around the world, opportunistic capture and sighting records remain the primary source of information on this species in the northwest Atlantic Ocean (NWA). Previous studies using limited datasets have suggested a precipitous decline in the abundance of white sharks from this region, but considerable uncertainty in these studies warrants additional investigation. This study builds upon previously published data combined with recent unpublished records and presents a synthesis of 649 confirmed white shark records from the NWA compiled over a 210-year period (1800-2010), resulting in the largest white shark dataset yet compiled from this region. These comprehensive records were used to update our understanding of their seasonal distribution, relative abundance trends, habitat use, and fisheries interactions. All life stages were present in continental shelf waters year-round, but median latitude of white shark occurrence varied seasonally. White sharks primarily occurred between Massachusetts and New Jersey during summer and off Florida during winter, with broad distribution along the coast during spring and fall. The majority of fishing gear interactions occurred with rod and reel, longline, and gillnet gears. Historic abundance trends from multiple sources support a significant decline in white shark abundance in the 1970s and 1980s, but there have been apparent increases in abundance since the 1990s when a variety of conservation measures were implemented. Though the white shark's inherent vulnerability to exploitation warrants continued protections, our results suggest a more optimistic outlook for the recovery of this iconic predator in the Atlantic.

The last frontier: catch records of white sharks (Carcharodon carcharias) in the Northwest Pacific Ocean

White sharks are highly migratory apex predators, globally distributed in temperate, sub-tropical, and tropical waters. Knowledge of white shark biology and ecology has increased recently based on research at known aggregation sites in the Indian, Atlantic, and Northeast Pacific Oceans; however, few data are available for the Northwest Pacific Ocean. This study provides a meta-analysis of 240 observations of white sharks from the Northwest Pacific Ocean between 1951 and 2012. Records comprise reports of bycatch in commercial fisheries, media accounts, personal communications, and documentation of shark-human interactions from Russia (n = 8), Republic of Korea (22), Japan (129), China (32), Taiwan (45), Philippines (1) and Vietnam (3). Observations occurred in all months, excluding October-January in the north (Russia and Republic of Korea) and July-August in the south (China, Taiwan, Philippines, and Vietnam). Population trend analysis indicated that the relative abundance of white sharks in the region has remained relatively stable, but parameterization of a 75% increase in observer effort found evidence of a minor decline since 2002. Reliably measured sharks ranged from 126–602 cm total length (TL) and 16–2530 kg total weight. The largest shark in this study (602 cm TL) represents the largest measured shark on record worldwide. For all countries combined the sex ratio was non-significantly biased towards females (1∶1.1; n = 113). Of 60 females examined, 11 were confirmed pregnant ranging from the beginning stages of pregnancy (egg cases) to near term (140 cm TL embryos). On average, 6.0±2.2 embryos were found per litter (maximum of 10) and gestation period was estimated to be 20 months. These observations confirm that white sharks are present in the Northwest Pacific Ocean year-round. While acknowledging the difficulties of studying little known populations of a naturally low abundance species, these results highlight the need for dedicated research to inform regional conservation and management planning.

Use of Long-Term Opportunistic Surveys to Estimate Trends in Abundance of Hibernating Townsend's Big-Eared Bats

The advent of broad-scale threats to bats such as white-nose syndrome and climate change highlights the need for reliable baseline assessment of their populations. However, few long-term, rigorously designed assessments of bat populations exist, particularly in western North America. Consequently, results of informal monitoring efforts are often the only data available upon which to base population assessments. We evaluated whether an opportunistic collection of surveys recorded over a 22-y period could be used to assess population trend of Townsend's big-eared bats (Corynorhinus townsendii) at Lava Beds National Monument in northern California. We used records of counts of hibernating bats conducted during 1991–2012 to estimate the number of bats in 52 individual caves as well as cumulatively. Seventeen of 22 caves surveyed in four or more years had an increasing trend in the number of hibernating bats. We estimated the cumulative annual growth rate over the period to be 1.79%. Stable or increasing number of hibernating Townsend's big-eared bats may be a result of management actions taken to limit disturbance of bats during maternity and hibernation seasons. We found no evidence that annual counts depressed the number of hibernating bats, thereby broadening monitoring options and the ability to link population trends to extrinsic factors. Our results demonstrate that opportunistically collected, long-term data sets may be useful for establishing first approximations of population trends for bats.

Travelling light: white sharks (Carcharodon carcharias) rely on body lipid stores to power ocean-basin scale migration

Many species undertake long-distance annual migrations between foraging and reproductive areas. Such migrants depend on the efficient packaging, storage and utilization of energy to succeed. A diverse assemblage of organisms accomplishes this through the use of lipid reserves; yet, it remains unclear whether the migrations of elasmobranchs, which include the largest gill breathers on Earth, depend on such a mechanism. We examine depth records from pop-up satellite archival tags to discern changes in buoyancy as a proxy for energy storage in Eastern Pacific white sharks, and assess whether lipid depletion fuels long-distance (approx. 4000 km) migrations. We develop new algorithms to assess body condition, buoyancy and drift rate during drift dives and validate the techniques using a captive white shark. In the wild, we document a consistent increase in drift rate over the course of all migrations, indicating a decrease in buoyancy caused by the depletion of lipid reserves. These results comprise, to our knowledge, the first assessment of energy storage and budgeting in migrating sharks. The methods provide a basis for further insights into using electronic tags to reveal the energetic strategies of a wide range of elasmobranchs.

Long-term change in a meso-predator community in response to prolonged and heterogeneous human impact

Sharks and rays' abundance can decline considerably with fishing. Community changes, however, are more complex because of species interactions, and variable vulnerability and exposure to fishing. We evaluated long-term changes in the elasmobranch community of the Adriatic Sea, a heavily exploited Mediterranean basin where top-predators have been strongly depleted historically, and fishing developed unevenly between the western and eastern side. Combining and standardizing catch data from five trawl surveys from 1948–2005, we estimated abundance trends and explained community changes using life histories, fish-market and effort data, and historical information. We identified a highly depleted elasmobranch community. Since 1948, catch rates have declined by >94% and 11 species ceased to be detected. The exploitation history and spatial gradients in fishing pressure explained most patterns in abundance and diversity, including the absence of strong compensatory increases. Ecological corridors and large-scale protected areas emerged as potential management options for elasmobranch conservation.

Supporting Red List threat assessments with GeoCAT: geospatial conservation assessment tool

GeoCAT is an open source, browser based tool that performs rapid geospatial analysis to ease the process of Red Listing taxa. Developed to utilise spatially referenced primary occurrence data, the analysis focuses on two aspects of the geographic range of a taxon: the extent of occurrence (EOO) and the area of occupancy (AOO). These metrics form part of the IUCN Red List categories and criteria and have often proved challenging to obtain in an accurate, consistent and repeatable way. Within a familiar Google Maps environment, GeoCAT users can quickly and easily combine data from multiple sources such as GBIF, Flickr and Scratchpads as well as user generated occurrence data. Analysis is done with the click of a button and is visualised instantly, providing an indication of the Red List threat rating, subject to meeting the full requirements of the criteria. Outputs including the results, data and parameters used for analysis are stored in a GeoCAT file that can be easily reloaded or shared with collaborators. GeoCAT is a first step toward automating the data handling process of Red List assessing and provides a valuable hub from which further developments and enhancements can be spawned.