An oceanic manta ray outfitted with a satellite tag at the Revillagigedo Islands, Mexico. From Stewart et al 2016
Large marine vertebrates are some of the most ecologically, economically, and culturally important animals on the planet. They maintain ecosystem structure and function, support multi-billion-dollar fisheries and thriving tourism industries, and hold a substantial social value across cultures. Large marine vertebrates have also historically been the most heavily targeted commercial species, leading to precipitous population declines across a wide range of taxa. My work focuses on improving the science-based management and conservation of threatened marine megafauna. I combine field methods, emerging technologies, and advanced quantitative analyses to improve our understanding of population dynamics, spatial ecology, and human impacts on threatened species. My research interests are diverse, but my work generally falls into three broad categories:
Filling knowledge gaps for threatened marine species
In many cases, effective management is hindered by a lack of information on fundamental biological or ecological traits of impacted species. I use a diversity of approaches (satellite & acoustic tagging, animal-borne video cameras, stable isotope analysis, genetics, photogrammetry) to answer questions about where migratory species go, what habitats are important to their life history, what is driving their movements and behavior (e.g. foraging or reproduction), and how this information influences their vulnerability to human impacts. This work has contributed to national-level species protection and marine protected area design in four countries.
Deploying a Crittercam on an oceanic manta ray in the Revillagigedo Archipelago, Mexico. From Stewart et al. 2019
From a management and conservation perspective we are typically focused (with very good reason) on acute threats to species and populations, such as fisheries and other impacts that cause direct mortalities. Recently, I have become increasingly interested in sub-lethal impacts, or stressors that do not result directly in mortalities, but which may still affect individual fitness and ultimately population viability and trends. For example, in a recent study I led with collaborators from NOAA, Woods Hole Oceanographic Institution, and the New England Aquarium, we found that sub-lethal entanglements in fishing gear are stunting the growth of critically endangered North Atlantic right whales. Smaller body sizes may contribute to lower reproductive output, further imperiling this declining population. The drone-based aerial photogrammetry methods used in this study allow for extremely precise measurements of body size. As drone technology becomes more advanced, my collaborators and I are finding countless opportunities to put these powerful tools to work collecting information on size structure, body condition, individual behaviors, and fine-scale movements and habitat use of marine vertebrates.
North Atlantic right whales are stunted. From Stewart et al. 2021
Ongoing related projects: (1) Linking body size to fecundity in right whales; (2) Estimating anthropogenic injury risk to oceanic manta rays in Mexico and their influence on survival probability; (3) Movements and habitat use of oceanic manta rays in Pacific Mexico; (4) Diet and foraging behavior of manta rays; (5) Dynamics and vulnerability of super-aggregations of Munk’s devil ray; (6) Habitat use of Southern Resident killer whales in the Salish Sea.
Developing quantitative tools to improve inference and support management
A major focus of my work is using advanced quantitative tools to better understand population dynamics and prioritize effective management strategies. While I am the first to apply tried-and-tested, packaged analytical tools when the situation allows for it, I tend to gravitate to Bayesian approaches as they allow for a huge amount of flexibility when dealing with the messy data we often encounter in fisheries and complex ecological systems. There are two major advantages to Bayesian statistics. First, it is straightforward and relatively seamless to integrate many different data sources into a single analytical framework. This is great for both data poor and data rich scenarios. In data poor scenarios, we can squeeze every last drop of information out of the available data, plus keep parameter estimates in the realm of plausibility using priors. In data rich scenarios we can create integrated population models that draw on many informative data sources to estimate high-level population parameters with greatly increased precision. Second, Bayesian methods provide posterior distributions as their output, which can be interpreted as probability distributions. These are especially powerful in management contexts because they allow us to make statements like “There is a 32% probability of the population declining below 200 individuals in the next 25 years”. These posterior distributions can also be easily integrated into projection models, allowing us to quickly and easily evaluate, for example, how population dynamics might respond to different management interventions.
In a recent example, I worked with colleagues from NOAA, SR3, and Oregon State University to apply these tools to body condition data from endangered Southern Resident killer whales. I developed a custom Bayesian multi-state mark recapture model that allowed us to link fluctuations in body condition to changes in the abundance of Chinook salmon, the primary prey of these whales. In addition, we found that skinny whales have higher mortality probabilities, and identified thresholds of prey availability to help prevent body condition declines.
Estimating the effects of salmon abundance on body condition in endangered Southern Resident killer whales. From Stewart et al. 2021
Ongoing related projects: (1) Integrated population models to evaluate Arctic environmental drivers of gray whale population dynamics; (2) Estimating total catch, catch trends, and population impacts of small-scale fisheries on manta and devil rays in Sri Lanka; (3) Linking body condition to fecundity in killer whales; (4) Abundance and trends of oceanic manta ray populations in the eastern Pacific.
Working with stakeholders to translate science into conservation action
I work closely with stakeholders to ensure that the science we are doing supports the communities that rely on the resources we are studying. We learn invaluable lessons about ecology and resource management from stakeholders, and by incorporating their values, needs, and perspectives into our work, we maximize the real-world impact of our research. I am currently leading an international research collaboration that aims to reduce the bycatch mortality of threatened manta and devil rays—one of the most substantial sources of mortality for this highly vulnerable group of species. The collaboration brings together two non-profit conservation organizations, academic scientists, a regional fisheries management agency, a fishery sustainability project representing the fishing industry, fishing vessel captains and crews, and fishery observers. This unique coalition is facilitating logistically challenging field research, real-world testing and evaluation of improved handling and release practices, and bringing stakeholders directly into the research process to contribute their experience and suggestions for reducing fishery impacts.
Since 2014 I have been leading a research program focused on the population dynamics and habitat use of oceanic manta rays in Bahia de Banderas, Mexico. Injuries from ship strikes and fishing gear entanglements are ten times as common in this population than in a neighboring population of mantas at an offshore marine protected area. We are working directly with the local fishing cooperative to study the seasonal patterns of occurrence and environmental drivers of manta ray visitation and abundance along the heavily trafficked coastline. Through this partnership we have identified the locations and times of year when mantas are most likely to interact with local fisheries and boat traffic, while at the same time having a measurable impact on the perspectives of local community members on the value of marine conservation efforts. We also focus on capacity building, involving undergraduate students from the local college in our research activities, providing training workshops, and supporting their thesis research. Our student participants have gone on to complete graduate degrees, work as our project leaders, work at regional governmental management agencies, and teach at local colleges.
Left: an undergraduate student from Instituto Tecnologico de Bahia de Banderas collects data for his thesis with the help of Don Fiti, the head of the Yelapa fishing cooperative. Right: an oceanic manta ray with recent scars from a boat propeller.