Combining Physiological Insights and Environmental Data to Investigate Immature Shortfin Mako Shark Habitat Use in the Southern California Bight Ecoregion
Written By: Emily Nazario (Fisheries Opportunity Fund Fellow 2023)
Shark conservation is as important as it is challenging
Large predatory fish such as tuna, billfish, and sharks are critical for maintaining healthy marine ecosystems. Declining predatory fish populations can have larger ecosystem-level effects, such as increased vulnerability to environmental disturbance and species invasions. One such example is the decline in shark populations, with some regions seeing up to a 90% decrease, which is primarily driven by overfishing, bycatch, and a lack of ecologically-based data-driven policy regulations aimed at their conservation. Population ranges spanning several national jurisdictions further complicate conservation efforts.
Targeted management of critical habitats (e.g., foraging and nursery grounds) that overlap with areas of high human activity could be one approach for effective shark management. Coastal nursery grounds are particularly vulnerable to fishing activity and coastal development, which can lead to species declines and disruptions in these crucial areas. Large declines in juvenile shark numbers can leave populations at unsustainable levels, and thus these areas have been identified as a critical component for shark conservation. However, such habitats may be lost or made unsuitable due to climate change, reinforcing the need to identify the indicators of immature shark habitat.
Tools for identifying habitat use now and in the future
Species distribution models have been used extensively to explore habitat use patterns by relating species presence data to environmental conditions. For sharks, habitat use has been associated with environmental variables such as sea surface temperatures and bathymetry. However, due to climate change, it is predicted that species distributions are likely to shift as populations begin encountering novel conditions. Thus, the foundations that these models rely on may no longer hold up, making it necessary to consider additional factors. The inclusion of physiological mechanisms provides insight into the biological constraints species face within a given habitat and are thus ideal for understanding how species distributions may be impacted by the predicted effects of climate change. This suite of modeling techniques, also referred to as hybrid species distribution models, is especially beneficial for species with specific thermal, metabolic, or habitat needs, such as large predatory fish.
North Pacific Shortfin Mako Sharks
Among shark species, Shortfin Mako Sharks (Isurus oxyrinchus) have some of the highest maximum and routine oxygen demands, resulting from their high energy foraging behaviors and impressive swimming speeds. Historically, these traits promoted widespread distributions, however, Mako Sharks in the North Pacific are projected to lose the most core habitat relative to other marine predators in the region as a result of climate change.
The environmental features within the Southern California Bight Ecoregion and surrounding waters are an important nursery area for the North Pacific Shortfin Mako Shark. However, due to recent environmental changes, the distributions of other juvenile shark species, such as White Sharks (Carcharodon carcharias) in this region have been observed to shift to higher latitudes. Previous work has been limited to investigating the vertical and horizontal movements of subadult Mako Sharks in the Southern California Bight. However, the environmental and physiological factors associated with juvenile Mako Shark habitat use are unknown.
New science: combining physiology and environmental data to investigate immature Shortfin Mako Shark habitat use in the Southern California Bight Ecoregion
To investigate the factors influencing immature Mako Shark habitat use, I am using location data (latitude, longitude) collected using bio-logging devices (see photo) deployed between 2004-2009 in the Southern California Bight Ecoregion. This region extends from Point Conception, California, USA, to Punta Abreojos, Baja California Sur, Mexico, and is suggested to better capture the distribution of juvenile Mako Sharks in this region relative to the traditional boundaries of the Southern California Bight.
I will use this data to develop physiologically informed habitat models to study the spatial use of immature mako sharks in the Southern California Bight Ecoregion. I will investigate which environmental features are most influential in describing predicted habitat use and how this changes with depth. Resting oxygen demands (the minimum bound of energy requirements) and temperature preferences will be incorporated to capture how their physiology may influence their habitat use. By assessing how oxygen demands shape habitat use throughout the water column, we can address how any changes in oxygen supply at depth may impact their distributions. In the Southern California Bight Ecoregion, this is especially important as climate change is projected to result in increased temperatures and shallower low oxygen conditions. Thus, the inclusion of immature Mako Shark physiological constraints may provide critical insights into how their habitat use may shift as a result of these changing environmental conditions.
Lastly, a recent addition to this work will include diet data for immature Mako Sharks in the Southern California Bight Ecoregion. By relating the prey that the sharks were most frequently targeting throughout the study period, I will be able to investigate how the shark diets and prey accessibility may have changed under annual variations in habitat suitabilities and environmental conditions (e.g., El Niño and La Niña years).
Early results indicate that under current conditions, the average sea surface temperatures where Mako sharks were present were between 15-23°C and when oxygen supply at 50 m depth (the median dive depth of sharks included in this study) was, on average, 1.8 – 2.8 times the oxygen needed to meet the sharks’s resting metabolic demands. Additionally, when comparing oxygen supply to resting oxygen demands, this ratio tended to be below average along coastal areas at lower latitudes, and habitat availability in these areas could be further constrained by future aerobic habitat loss. These results and model outputs will continue to be investigated and presented in March 2024 at the 8th Bio-Logging Symposium in Tokyo, Japan.
Working at the intersection of physiological demands and their influence on marine predator habitat use has been an exciting arena to develop my research questions. These fields have guided the development of each of my Ph.D. dissertation chapters, and watching these projects come together has been an incredible opportunity. For this study in particular, investigating resting immature Shortfin Mako Shark oxygen requirements and how they may or may not be shaping habitat use now and in the future has been an exciting research question I have had the opportunity to explore. I am eager to continue working on this topic and to connect my future results to the larger ecological context, such as changes in prey importance or the presence of El Niño or La Niña events. Physiologically informed habitat models can uncover the environmental needs for a species to persist and can indicate how species may be spatially impacted by climate change. These climate-ready tools will be increasingly critical for marine conservation.
Acknowledgments
This work would not be possible without the mentorship and support of my research team at UC Santa Cruz and NOAA’s Southwest Fisheries Science Center Ecological Research Division, Elliott Hazen, Nerea Lezama-Ochoa, Max Czapanskiy, Heidi Dewar, Antonella Preti, and Don Croll. The financial support for this work provided by the UCSC Fisheries Collaborative Program through the Fisheries Opportunity Fund and UCSC Ecology and Evolutionary Biology Departmental Funds has made this work possible and will continue to support me as I communicate this approach and my future findings to the broader scientific and conservation community.
