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Estimating the Abundance of Marine Mammal Populations
Motivated by the need to estimate the abundance of marine mammal populations to inform conservation assessments, especially relating to fishery bycatch, this paper provides background on abundance estimation and reviews the various methods available for pinnipeds, cetaceans and sirenians. We first give an “entry-level” introduction to abundance estimation, including fundamental concepts and the importance of recognizing sources of bias and obtaining a measure of precision. Each of the primary methods available to estimate abundance of marine mammals is then described, including data collection and analysis, common challenges in implementation, and the assumptions made, violation of which can lead to bias. The main method for estimating pinniped abundance is extrapolation of counts of animals (pups or all-ages) on land or ice to the whole population. Cetacean and sirenian abundance is primarily estimated from transect surveys conducted from ships, small boats or aircraft. If individuals of a species can be recognized from natural markings, mark-recapture analysis of photo-identification data can be used to estimate the number of animals using the study area. Throughout, we cite example studies that illustrate the methods described. To estimate the abundance of a marine mammal population, key issues include: defining the population to be estimated, considering candidate methods based on strengths and weaknesses in relation to a range of logistical and practical issues, being aware of the resources required to collect and analyze the data, and understanding the assumptions made. We conclude with a discussion of some practical issues, given the various challenges that arise during implementation.
Journal Article
Density-dependent effects on reproductive output in a capital breeding carnivore, the northern elephant seal (Mirounga angustirostris)
All organisms face resource limitations that will ultimately restrict population growth, but the controlling mechanisms vary across ecosystems, taxa, and reproductive strategies. Using four decades of data, we examine how variation in the environment and population density affect reproductive outcomes in a capital-breeding carnivore, the northern elephant seal (Mirounga angustirostris). This species provides a unique opportunity to examine the relative importance of resource acquisition and density-dependence on breeding success. Capital breeders accrue resources over large temporal and spatial scales for use during an abbreviated reproductive period. This strategy may have evolved, in part, to confer resilience to short-term environmental variability. We observed density-dependent effects on weaning mass, and maternal age (experience) was more important than oceanographic conditions or maternal mass in determining offspring weaning mass. Together these findings show that the mechanisms controlling reproductive output are conserved across terrestrial and marine systems and vary with population dynamics, an important consideration when assessing the effect of extrinsic changes, such as climate change, on a population.
Journal Article
The history and effects of seal–fishery conflicts in Denmark
Growing marine mammal populations have led to renewed conflicts with fisheries and discussions of culling as a management measure. In order to evaluate the effects of such measures, lessons from previous culling efforts and historic data on marine mammal abundance and distribution in response to different hunting and management regimes are pertinent. Here, we combined multiple data sources, including bounty data from the Danish seal culling programme of 1889 to 1927, zooarchaeological records, historical written accounts, 20th century hunting statistics on seals and recent population survey data, in order to assess the prehistoric and historic occurrence of seals in Denmark, and to evaluate the effects of hunting and culling on seal populations, as well as its efficacy as a mitigation measure in seal–fisheries conflicts. We found that past conflicts were driven primarily by developments of passive fishing gear technology in the late 19th century, and that—contrary to several modern interpretations—the primary motivation for culling was damage to catch and gear, not resource competition. Furthermore, we demonstrate that it took decades of heavy-handed culling to minimize the historic seal–fisheries conflicts. Moreover, the culling programme should be regarded in a broader context, where preceding hunting had already decimated grey seal stocks, and subsequent hunting led to an all-time low of a few thousand harbour seals in the early 1970s. We recommend that 21st century seal–fisheries conflicts, debates and associated management decisions should be seen in a historical context, and that there should be an aim towards the development of sustainable fisheries and ecotourism, rather than culling.
Journal Article
Avoidance of wind farms by harbour seals is limited to pile driving activities
1. As part of global efforts to reduce dependence on carbon-based energy sources there has been a rapid increase in the installation of renewable energy devices. The installation and operation of these devices can result in conflicts with wildlife. In the marine environment, mammals may avoid wind farms that are under construction or operating. Such avoidance may lead to more time spent travelling or displacement from key habitats. A paucity of data on at-sea movements of marine mammals around wind farms limits our understanding of the nature of their potential impacts. 2. Here, we present the results of a telemetry study on harbour seals Phoca vitulina in The Wash, south-east England, an area where wind farms are being constructed using impact pile driving. We investigated whether seals avoid wind farms during operation, construction in its entirety, or during piling activity. The study was carried out using historical telemetry data collected prior to any wind farm development and telemetry data collected in 2012 during the construction of one wind farm and the operation of another. 3. Within an operational wind farm, there was a close-to-significant increase in seal usage compared to prior to wind farm development. However, the wind farm was at the edge of a large area of increased usage, so the presence of the wind farm was unlikely to be the cause. 4. There was no significant displacement during construction as a whole. However, during piling, seal usage (abundance) was significantly reduced up to 25 km from the piling activity; within 25 km of the centre of the wind farm, there was a 19 to 83% (95% confidence intervals) decrease in usage compared to during breaks in piling, equating to a mean estimated displacement of 440 individuals. This amounts to significant displacement starting from predicted received levels of between 166 and 178 dB re 1 μPa(p-p). Displacement was limited to piling activity; within 2 h of cessation of pile driving, seals were distributed as per the non-piling scenario. 5. Synthesis and applications. Our spatial and temporal quantification of avoidance of wind farms by harbour seals is critical to reduce uncertainty and increase robustness in environmental impact assessments of future developments. Specifically, the results will allow policymakers to produce industry guidance on the likelihood of displacement of seals in response to pile driving; the relationship between sound levels and avoidance rates; and the duration of any avoidance, thus allowing far more accurate environmental assessments to be carried out during the consenting process. Further, our results can be used to inform mitigation strategies in terms of both the sound levels likely to cause displacement and what temporal patterns of piling would minimize the magnitude of the energetic impacts of displacement.
Journal Article
The deeper the rounder: body shape variation in lice parasitizing diving hosts
Seal lice, unique among insects, show remarkable adaptability to the extreme conditions of the deep sea. Evolving with their seal and sea lion hosts, they have managed to tolerate hypoxia, high salinity, low temperature, and elevated hydrostatic pressure. Given the diving capabilities of their mammalian hosts, which can reach depths of hundreds to thousands of meters, our study examines the morphological variation among closely related seal lice species infesting hosts with different maximum diving depths. In particular, our research reveals a significant morphological difference between lice associated with regular and deep-diving hosts, where lice from deep-diving hosts tend to be rounder. This could be an adaptation to withstand the high hydrostatic pressures found in the deep ocean. The rounded shape optimizes the louse’s ability to withstand external pressure by redistributing it over a larger ventral/dorsal plane. This in turn minimizes the internal energy required to support body deformations, thereby increasing the louse’s resilience in the deep sea environment.
Journal Article
A species-specific qPCR assay provides novel insight into range expansion of the Mediterranean monk seal (Monachus monachus) by means of eDNA analysis
The monk seal is the most endangered pinniped worldwide and the only one found in the Mediterranean, where its distribution and abundance have suffered a drastic decline in the last few decades. The limited understanding of the population demographics and conservation status of this species are due to both its rarity and evasiveness, with records biased towards occasional, mostly coastal, encounters. Current molecular techniques allow us to detect DNA traces released into the environment (eDNA) by any organism. We have developed three species-specific qPCR-assays targeting the 12S/16S rRNA mitogenome regions of the monk seal. The assays have been tested extensively on a comprehensive and diverse set of samples (n = 73), including positive controls from a breeding population in Madeira collected during their peak abundance, and two opportunistic collections of Mediterranean eDNA samples (offshore/coastal) from on-going projects. Monk seal DNA was detected in 47.2% of the samples collected from a ferry platform in the Tyrrhenian (2018–2019) and in 66.7% of those obtained in the Pelagie archipelago in the Strait of Sicily (2020). These findings anticipated subsequent visual observations in the proximity of these sampling areas by up to 2 years. At the Tyrrhenian site, monk seal detection increased between 2018 and 2019. The demonstrated approach provides a non-invasive and highly sensitive tool for defining the monk seal actual distribution and home range -enabling monitoring also in nocturnal hours-, for assessing its recovery rate and pinpointing coastal/offshore sites where prioritizing conservation, research, citizen-science, and educational opportunities.
Journal Article
Tracking interannual variation of a large‐scale ocean front influences foraging in a mesopelagic predator
Macroscale ocean fronts aggregate significant biomass and provide critical foraging habitat for large marine predators. These frontal systems shift in response to ocean climate variation, including basin‐scale oscillations, and the degree to which marine predators track these movements affects their foraging and reproductive success. Using two decades of adult female northern elephant seal ( Mirounga angustirostris ) movement data, we assessed their use of the Subarctic Frontal Zone (SAFZ) in the northeast Pacific Ocean, the SAFZ's influence on their diving behavior, and its importance to their foraging success. We found that elephant seals followed the interannual movement of the SAFZ and their diel diving behavior became more extreme as they moved closer to the SAFZ, likely reflecting a different vertical distribution of prey in the region. During their short foraging trip, elephant seals that spent time north of the SAFZ, where day and night diving depths were more similar, had greater foraging success. During the long, gestational foraging trip, their geographic distribution relative to the SAFZ did not influence foraging success, but larger animals were more successful, likely due to more efficient diving. Understanding the relationship between dynamic foraging habitat and the response capacity of predator species is critical for assessing the resilience of species and ecosystems as ocean climates become less predictable.
Journal Article
Evaluating the effectiveness of a large multi-use MPA in protecting Key Biodiversity Areas for marine predators
Aim Marine protected areas can serve to regulate harvesting and conserve biodiversity. Within large multi‐use MPAs, it is often unclear to what degree critical sites of biodiversity are afforded protection against commercial activities. Addressing this issue is a prerequisite if we are to appropriately assess sites against conservation targets. We evaluated whether the management regime of a large MPA conserved sites (Key Biodiversity Areas, KBAs) supporting the global persistence of top marine predators. Location Southwest Atlantic Ocean. Method We collated population and tracking data (1,418 tracks) from 14 marine predator species (Procellariiformes, Sphenisciformes, Pinnipedia) that breed at South Georgia and the South Sandwich Islands, and identified hotspots for their conservation under the recently developed KBA framework. We then evaluated the spatiotemporal overlap of these sites and the different management regimes of krill, demersal longline and pelagic trawl fisheries operating within a large MPA, which was created with the intention to protect marine predator species. Results We identified 12 new global marine KBAs that are important for this community of top predators, both within and beyond the focal MPA. Only three species consistently used marine areas at a time when a potentially higher‐risk fishery was allowed to operate in that area, while other interactions between fisheries and our target species were mostly precluded by MPA management plans. Main conclusions We show that current fishery management measures within the MPA contribute to protecting top predators considered in this study and that resource harvesting within the MPA does not pose a major threat—under current climate conditions. Unregulated fisheries beyond the MPA, however, pose a likely threat to identified KBAs. Our approach demonstrates the utility of the KBA guidelines and multispecies tracking data to assess the contributing role of well‐designed MPAs in achieving local and internationally agreed conservation targets.
Journal Article
Ice-algal carbon supports harp and ringed seal diets in the European Arctic
Sea-ice declines in the European Arctic have led to substantial changes in marine food webs. To better understand the biological implications of these changes, we quantified the contributions of ice-associated and pelagic carbon sources to the diets of Arctic harp and ringed seals using compound-specific stable isotope ratios of fatty acids in specific primary producer biomarkers derived from sea-ice algae and phytoplankton. Comparison of fatty acid patterns between these 2 seal species indicated clear dietary separation, while the compound-specific stable isotope ratios of the same fatty acids showed partial overlap. These findings suggest that harp and ringed seals target different prey sources, yet their prey rely on ice and pelagic primary production in similar ways. From Bayesian stable isotope mixing models, we estimated that relative contributions of sympagic and pelagic carbon in seal blubber was an average of 69% and 31% for harp seals, and 72% and 28% for ringed seals, respectively. The similarity in the Bayesian estimations also indicates overlapping carbon sourcing by these 2 species. Our findings demonstrate that the seasonal ice-associated carbon pathway contributes substantially to the diets of both harp and ringed seals.
Journal Article