Thomas Len

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Last Name
Thomas
First Name
Len
ORCID
0000-0002-7436-067X

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  • Article
    Comparing methods suitable for monitoring marine mammals in low visibility conditions during seismic surveys
    (Elsevier, 2017-11-07) Verfuss, Ursula K. ; Gillespie, Douglas ; Gordon, Jonathan ; Marques, Tiago A. ; Miller, Brianne ; Plunkett, Rachael ; Theriault, James A. ; Tollit, Dominic J. ; Zitterbart, Daniel ; Hubert, Philippe ; Thomas, Len
    Loud sound emitted during offshore industrial activities can impact marine mammals. Regulations typically prescribe marine mammal monitoring before and/or during these activities to implement mitigation measures that minimise potential acoustic impacts. Using seismic surveys under low visibility conditions as a case study, we review which monitoring methods are suitable and compare their relative strengths and weaknesses. Passive acoustic monitoring has been implemented as either a complementary or alternative method to visual monitoring in low visibility conditions. Other methods such as RADAR, active sonar and thermal infrared have also been tested, but are rarely recommended by regulatory bodies. The efficiency of the monitoring method(s) will depend on the animal behaviour and environmental conditions, however, using a combination of complementary systems generally improves the overall detection performance. We recommend that the performance of monitoring systems, over a range of conditions, is explored in a modelling framework for a variety of species.
  • Article
    Understanding the combined effects of multiple stressors: a new perspective on a longstanding challenge
    (Elsevier, 2022-01-29) Pirotta, Enrico ; Thomas, Len ; Costa, Daniel P. ; Hall, Ailsa J. ; Harris, Catriona M. ; Harwood, John ; Kraus, Scott D. ; Miller, Patrick J. O. ; Moore, Michael J. ; Photopoulou, Theoni ; Rolland, Rosalind M. ; Schwacke, Lori ; Simmons, Samantha E. ; Southall, Brandon L. ; Tyack, Peter L.
    Wildlife populations and their habitats are exposed to an expanding diversity and intensity of stressors caused by human activities, within the broader context of natural processes and increasing pressure from climate change. Estimating how these multiple stressors affect individuals, populations, and ecosystems is thus of growing importance. However, their combined effects often cannot be predicted reliably from the individual effects of each stressor, and we lack the mechanistic understanding and analytical tools to predict their joint outcomes. We review the science of multiple stressors and present a conceptual framework that captures and reconciles the variety of existing approaches for assessing combined effects. Specifically, we show that all approaches lie along a spectrum, reflecting increasing assumptions about the mechanisms that regulate the action of single stressors and their combined effects. An emphasis on mechanisms improves analytical precision and predictive power but could introduce bias if the underlying assumptions are incorrect. A purely empirical approach has less risk of bias but requires adequate data on the effects of the full range of anticipated combinations of stressor types and magnitudes. We illustrate how this spectrum can be formalised into specific analytical methods, using an example of North Atlantic right whales feeding on limited prey resources while simultaneously being affected by entanglement in fishing gear. In practice, case-specific management needs and data availability will guide the exploration of the stressor combinations of interest and the selection of a suitable trade-off between precision and bias. We argue that the primary goal for adaptive management should be to identify the most practical and effective ways to remove or reduce specific combinations of stressors, bringing the risk of adverse impacts on populations and ecosystems below acceptable thresholds.
  • Article
    Estimating cetacean population density using fixed passive acoustic sensors : an example with Blainville's beaked whales
    (Acoustical Society of America, 2009-04) Marques, Tiago A. ; Thomas, Len ; Ward, Jessica ; DiMarzio, Nancy A. ; Tyack, Peter L.
    Methods are developed for estimating the size/density of cetacean populations using data from a set of fixed passive acoustic sensors. The methods convert the number of detected acoustic cues into animal density by accounting for (i) the probability of detecting cues, (ii) the rate at which animals produce cues, and (iii) the proportion of false positive detections. Additional information is often required for estimation of these quantities, for example, from an acoustic tag applied to a sample of animals. Methods are illustrated with a case study: estimation of Blainville's beaked whale density over a 6 day period in spring 2005, using an 82 hydrophone wide-baseline array located in the Tongue of the Ocean, Bahamas. To estimate the required quantities, additional data are used from digital acoustic tags, attached to five whales over 21 deep dives, where cues recorded on some of the dives are associated with those received on the fixed hydrophones. Estimated density was 25.3 or 22.5 animals/1000 km2, depending on assumptions about false positive detections, with 95% confidence intervals 17.3–36.9 and 15.4–32.9. These methods are potentially applicable to a wide variety of marine and terrestrial species that are hard to survey using conventional visual methods.
  • Article
    Estimating the effects of stressors on the health, survival and reproduction of a critically endangered, long‐lived species
    (Wiley, 2023-02-06) Pirotta, Enrico ; Schick, Robert S. ; Hamilton, Philip K. ; Harris, Catriona M. ; Hewitt, Joshua ; Knowlton, Amy R. ; Kraus, Scott D. ; Meyer‐Gutbrod, Erin ; Moore, Michael J. ; Pettis, Heather M. ; Photopoulou, Theoni ; Rolland, Rosalind M. ; Tyack, Peter L. ; Thomas, Len
    Quantifying the cumulative effects of stressors on individuals and populations can inform the development of effective management and conservation strategies. We developed a Bayesian state–space model to assess the effects of multiple stressors on individual survival and reproduction. In the model, stressor effects on vital rates are mediated by changes in underlying health, allowing for the comparison of effect sizes while accounting for intrinsic factors that might affect an individual's vulnerability and resilience. We applied the model to a 50-year dataset of sightings, calving events and stressor exposure of critically endangered North Atlantic right whales Eubalaena glacialis. The viability of this population is threatened by a complex set of stressors, including vessel strikes, entanglement in fishing gear and fluctuating prey availability. We estimated that blunt and deep vessel strike injuries and severe entanglement injuries had the largest effect on the health of exposed individuals, reinforcing the urgent need for mitigation measures. Prey abundance had a smaller but protracted effect on health across individuals, and estimated long-term trends in survival and reproduction followed the trend of the prey index, highlighting that long-term ecosystem-based management strategies are also required. Our approach can be applied to quantify the effects of multiple stressors on any long-lived species where suitable indicators of health and long-term monitoring data are available.
  • Article
    Managing the effects of multiple stressors on wildlife populations in their ecosystems: developing a cumulative risk approach
    (The Royal Society, 2022-11-30) Tyack, Peter L. ; Thomas, Len ; Costa, Daniel P. ; Hall, Ailsa J. ; Harris, Catriona M. ; Harwood, John ; Kraus, Scott D. ; Miller, Patrick J. O. ; Moore, Michael ; Photopoulou, Theoni ; Pirotta, Enrico ; Rolland, Rosalind M. ; Schwacke, Lori H. ; Simmons, Samantha E. ; Southall, Brandon L.
    Assessing cumulative effects of human activities on ecosystems is required by many jurisdictions, but current science cannot meet regulatory demands. Regulations define them as effect(s) of one human action combined with other actions. Here we argue for an approach that evaluates the cumulative risk of multiple stressors for protected wildlife populations within their ecosystems. Monitoring effects of each stressor is necessary but not sufficient to estimate how multiple stressors interact to affect wildlife populations. Examining the mechanistic pathways, from cellular to ecological, by which stressors affect individuals can help prioritize stressors and interpret how they interact. Our approach uses health indicators to accumulate the effects of stressors on individuals and to estimate changes in vital rates, driving population status. We advocate using methods well-established in human health and integrating them into ecosystem-based management to protect the health of commercially and culturally important wildlife populations and to protect against risk of extinction for threatened species. Our approach will improve abilities to conserve and manage ecosystems but will also demand significant increases in research and monitoring effort. We advocate for increased investment proportional to the economic scale of human activities in the Anthropocene and their pervasive effects on ecology and biodiversity.
  • Article
    Decreasing body size is associated with reduced calving probability in critically endangered North Atlantic right whales
    (Royal Society of Chemistry, 2024-02-28) Pirotta, Enrico ; Tyack, Peter L. ; Durban, John W. ; Fearnbach, Holly ; Hamilton, Philip K. ; Harris, Catriona M. ; Knowlton, Amy R. ; Kraus, Scott D. ; Miller, Carolyn A. ; Moore, Michael J. ; Pettis, Heather M. ; Photopoulou, Theoni ; Rolland, Rosalind M. ; Schick, Robert S. ; Thomas, Len
    Body size is key to many life-history processes, including reproduction. Across species, climate change and other stressors have caused reductions in the body size to which animals can grow, called asymptotic size, with consequences for demography. A reduction in mean asymptotic length was documented for critically endangered North Atlantic right whales, in parallel with declines in health and vital rates resulting from human activities and environmental changes. Here, we tested whether smaller body size was associated with lower reproductive output, using a state-space model for individual health, survival and reproduction that quantifies the mechanistic links between these processes. Body size (as represented by the cube of length) was strongly associated with a female's calving probability at each reproductive opportunity. This relationship explained 62% of the variation in calving among reproductive females, along with their decreasing health (20%). The effects of decreasing mean body size on reproductive performance are another concerning indication of the worsening prospects for this species and many others affected by environmental change, requiring a focus of conservation and management interventions on improving conditions that affect reproduction as well as reducing mortality.
  • Article
    Assessing North Atlantic Right whale health: a review of threats, and development of tools critical for conservation of the species
    (Inter Research, 2021-02-25) Moore, Michael J. ; Rowles, Teresa K. ; Fauquier, Deborah A. ; Baker, Jason T. ; Biedron, Ingrid S. ; Durban, John W. ; Hamilton, Philip K. ; Henry, Allison G. ; Knowlton, Amy R. ; McLellan, William A. ; Miller, Carolyn A. ; Pace, Richard M., III ; Pettis, Heather M. ; Raverty, Stephen A. ; Rolland, Rosalind M. ; Schick, Robert S. ; Sharp, Sarah M. ; Smith, Cynthia R. ; Thomas, Len ; van der Hoop, Julie M. ; Ziccard, Michael H.
    Whaling decimated North Atlantic right whales (Eubalaena glacialis - NARW) since the 11th century and southern right whales (E. australis - SRW) since the 19th century. Today, NARWs are critically endangered and decreasing, whereas SRWs are recovering. We review NARW health assessment literature, NARW Consortium databases, and efforts and limitations to monitor individual and species health, survival, and fecundity. Photographs are used to track individual movement and external signs of health such as evidence of vessel and entanglement trauma. Post mortem examinations establish cause of death and determine organ pathology. Photogrammetry is used to assess growth rates and body condition. Samples of blow, skin, blubber, baleen and feces quantify hormones that provide information on stress, reproduction, and nutrition, identify microbiome changes, and assess evidence of infection. We also discuss models of the population consequences of multiple stressors, including the connection between human activities (e.g., entanglement) and health. Lethal and sublethal vessel and entanglement trauma have been identified as major threats to the species. There is a clear and immediate need for expanding trauma reduction measures. Beyond these major concerns, further study is needed to evaluate the impact of other stressors, such as pathogens, microbiome changes, and algal and industrial toxins, on NARW reproductive success and health. Current and new health assessment tools should be developed and used to monitor the effectiveness of management measures, and will help determine whether they are sufficient for a substantive species recovery.