The impacts of climate change and anthropogenic disturbance are pervasive throughout ecosystems. Yet particularly in aquatic and marine systems, animal responses to the resulting environmental changes can be difficult to determine, which inhibits appropriate management strategies. Conservation efforts are often based on research that examines the effect of stressors or human-induced impacts on a single species and/or scale. However, this approach can be too narrowly focused. Species do not exist in isolation, and there are many potential drivers of their survivorship and behavior that extend beyond the individual or population. Particularly for mobile species, such as many fishes, population dynamics can be strongly influenced by species interactions, individual movements within a habitat, or changes in habitat suitability. As a result, the impacts of environmental change can be highly complex or even counter-intuitive, particularly in locations where these changes are occurring rapidly.
Among these, California hosts a variety of aquatic and marine habitats that have been dramatically modified by human activity over the last century. These ecosystems now face further alteration due to a changing climate. Consequently, there is growing concern about how species will respond to additional stressors, such as increasing water temperatures. Our ability to forecast these effects is becoming increasingly important, because while many endemic fishes are already heavily managed, some populations are continuing to decline. The potential consequences of these declines can be dire, affecting ecological communities as well as human populations that rely on fish for sustenance. However, the mechanisms underlying patterns of fish presence and survival are multifaceted and remain undetermined in many cases, preventing a reevaluation of conservation efforts. To address this, my dissertation demonstrates the value of a more holistic perspective on organismal response to environmental change. Focusing on species that are threatened or data deficient in California waters, I use select case studies to examine fish responses to environmental changes at multiple scales, including fish physiology, behavior, and habitat use.
Chapter 1 is based in the Sacramento-San Joaquin River Delta of California’s Central Valley. Here, increased mortality of outmigrating juvenile salmon has been correlated to warming water temperature. It has been assumed that reduced survivorship is due to salmon thermal physiology. Yet emerging laboratory studies suggest that these salmon populations are relatively thermally robust. Instead, I test the hypothesis that decreases in salmon survivorship are due to increased activity of non-native predators. My results suggest that major predators of salmon in the Delta are more thermally adapted to warmer temperatures, increasing the possibility of their threat to juvenile salmon in light of global warming. This indicates that an understanding of fundamental thermal physiology can be useful in predicting predator-prey dynamics; however, I also show that such an approach is highly dependent on the physiological metric employed, and it is more effective for some species than others. Regardless, given that the Delta is a highly modified ecosystem, managers could hypothetically improve juvenile salmon survivorship by adjusting water flow, and therefore temperature, along primary outmigration routes based on predator thermal preferences.
Chapter 2 scales up to consider fish behavioral response to current flows in the San Francisco Bay Estuary. In estuarine habitats, seasonal and daily hydrological variance is affected by human activity, such as dredging and coastal wetland development, and rising sea levels due to global warming. However, assessing fish responses to hydrological patterns is logistically challenging, limiting our ability to predict how future changes will impact the behavior of local populations. In this chapter, I explore the use of hydrodynamic models for examining the influence of tidal flow in the San Francisco Bay Estuary on broadnose sevengill sharks (Notorynchus cepedianus). Combining acoustic tracking data with these models, I find evidence that sharks swim in and out of the bay with most of the ebb and flood tides, presumably to move in the most energetically efficient manner. However, this movement was not universal, nor was it dependent on the strength of the tide. This suggests that there may be other variables shaping sevengill shark behavior in this habitat, such as prey availability, which offsets the energetic cost of moving against the tide. Taken together, my results demonstrate that tidal movement is not the sole driver of sevengill shark behavior, but this species will likely be susceptible to potential hydrological changes in the future.
Chapter 3 examines environmental impacts at the largest scale, using a spatial modeling approach to determine population-level trends and habitat use in a large, highly migratory species: the basking shark. Historically targeted globally for their liver oil, fins, and meat, basking sharks were classified as globally endangered in 2019. While they seem to be responding well to protective measures in some parts of their range, there appears to be no sign of recovery in the Eastern North Pacific population. However, evaluating the status of this population is hindered by multiple data gaps. It has not been determined whether variation in shark sightings over time is due to changes in population size or the effect of environmental change on basking shark distribution. To fill these gaps, I compile recent and historical data to describe variation in basking shark sightings and school size over time in the California Current Ecosystem (CCE). I also build species distribution models using general additive mixed models to determine the environmental factors that may affect their distribution and sightings probability. My results suggest that the number and probability of sightings declined in the mid-1980s, as did the size of schools reported. Simultaneously, there was a shift in sighting seasonality, from fall and spring to summer starting in the 2000s. The species distribution models also revealed that sightings probability increased with low sea surface temperature and high chlorophyll a concentration, and was correlated with larger-scale climatic oscillations. Based on these analyses and previous studies, I conclude that the decline in sightings after the 1980s most likely emerged because of a decrease in the population following a century of culling and overfishing. This was perhaps exacerbated by an Allee effect based on the simultaneous decrease in group size. Furthermore, there is a high probability that the basking sharks that remain in the CCE will be affected by a combination of changing sea surface temperatures and shifting prey fields in the future, though the degree to which this will shift their spatial or temporal distribution is unknown. To better inform management of the CCE basking shark population, there should be more coordinated documentation of fisheries mortalities and sightings to inform population estimates and track potential changes in habitat use. I also advocate for improved monitoring of shark fin markets to ensure existing conservation regulations are being followed.
In conclusion, climate change and increasing anthropogenic activity are rapidly altering California’s aquatic and marine ecosystems, rendering conservation more difficult. My dissertation contributes to a large body of evidence highlighting numerous threats, from warming temperatures to habitat alteration, that are affecting species directly and ecosystem interactions more broadly. Together, these chapters show that species responses to potential threats will vary depending on environmental variables, animal behavior, and community interactions. It is critical to consider each of these levels of influence when predicting how populations will fare with environmental change. In turn, this broader understanding will improve dynamic management strategies for threatened species facing an uncertain future.