John A. Hildebrand’s research while affiliated with University of California System and other places

Species distribution models (SDMs) have been developed for multiple cetacean species within the California Current Ecosystem (CCE) from shipboard survey data collected by the Southwest Fisheries Science Center (SWFSC) in summer and fall, thus limiting the ability to inform management decisions in cool seasons when abundance and distribution patterns are substantially different. Winter and spring SDMs have been developed for a few species using California Cooperative Oceanic Fisheries Investigations (CalCOFI) shipboard survey data, but model predictions are limited to the waters off southern and central California. In this study, winter and spring density estimates for the entire CCE study area were made from SWFSC summer and fall model predictions (temporal extrapolation) and CalCOFI winter and spring model predictions (spatial extrapolation) for short-beaked common dolphin (Delphinus delphis delphis), Pacific white-sided dolphin (Lagenorhynchus obliquidens), Dall's porpoise (Phocoenoides dalli), and fin whale (Balaenoptera physalus). The performance of the models was compared based on available abundance estimates and documented distribution patterns in the cool seasons. Results reveal species-specific ecological factors to consider when extrapolating model predictions temporally or spatially, including whether a given study area includes a species “core habitat”, and whether static variables should be included when a species exhibits temporal distribution shifts.

Decadal variations of ocean soundscapes are intricately linked to large-scale climatic and economic fluctuations. This study draws on over 15 years of acoustic recordings at six sites within the Southern California Bight, investigating interannual, seasonal, and diel variations. By examining acoustic energy from fin and blue whales along with sounds from ships and wind, we identified changes in soundscape over time and space. This study reveals that sound levels associated with both biological and non-biological sound sources varied seasonally and correlated with large-scale climatic patterns and long-term oceanographic fluctuations. Baleen whale sound levels before, during, and after a marine heatwave were assessed; sound levels decreased in southern sites and increased in northern sites adjacent to the California Current, underscoring the potential for range shifts and habitat compression during warm years for these species. Ship-generated sound levels at high-traffic sites reflected economic events such as recessions, labor shortages and negotiations, and changes to port activities. Marine soundscapes offer an approach to assess the ocean's condition amid ongoing climatic and economic fluctuations.

Mysticete whales have bilateral bony ear complexes (tympanoperiotic complexes) that amplify low frequency vibrations in proximity to their vocalization ranges. Understanding the functional mechanics would enable precise predictions of mysticete hearing sensitivity, which is currently unknown. We conducted experiments on a juvenile and an adult gray whale skull from deceased animals to measure the vibrational dynamics between the tympanic bullae and the skull. Relative motions between bullae and skull indicate sound transfer to the inner ear. For the juvenile, assessments were performed on (1) a 3D-printed plastic skull-replica, (2) the original skull after much of the soft tissue had been removed by dissection, and (3) the denuded skull after hydrogen peroxide was used to erode the remaining soft tissues. We excited vibrations in the juvenile skull underwater by projecting sound in a test pool, ranging from 170–1000 Hz. Additionally, we measured in-air vibrations of the plastic, denuded, and adult skulls using a mechanical shaker to drive vibrations anteroposteriorly (rostrum-to-tail) from 150–1000 Hz. The results consistently showed amplification of vibrations at the tympanic bullae compared to the base of the skull, demonstrating a mechanism by which low-frequency sound is transferred from the environment, through the skull, to the inner ear.

Goose-beaked whales (Ziphius cavirostris) are a deep-diving toothed whale species and top predators in deep sea ecosystems. Much is yet to be learned about their social and foraging strategies due to their elusive behavior, but this information isincreasingly relevant given their demonstrated behavioral changes in association with anthropogenic sound. This study useddirection-of-arrival (DOA) localization to track the position of goose-beaked whales from echolocation clicks recorded onseafloor-mounted hydrophone arrays offshore Southern California. Overall, 2,738 tracks of diving goose-beaked whales wereprocessed from acoustic recordings collected at four long-term monitoring sites between 2018 and 2023. Results highlightdistinct spatial use patterns driven by bathymetric features at each site, with whales foraging closer to the seafloor at siteswith complex bathymetry and showing a preference for canyon slopes. Group sizes at depth ranged from 1 to 9 individualswith a mean of 2.34 and exhibited site-specific seasonal variability as well as a strong diel trend at one site. During manyof these encounters, individuals exhibited highly coordinated behaviors. This study demonstrates the value of long-termpassive acoustic tracking for studying elusive, deep-diving species and provides significant advancements in understandinggoose-beaked whale behavior at depth over long time scales.

Shortly after the Deepwater Horizon oil spill began in April 2010, a widely spaced passive acoustic monitoring array was deployed in the northeastern Gulf of Mexico to document the impacts of this unprecedentedly large and deep offshore oil spill on oceanic marine mammals. The array was subsequently maintained for over a decade. Here we document decadal density declines for seven of eight monitored species groups, including sperm whales (up to 31%), beaked whales (up to 83%), and small delphinids (up to 43%). Declines were observed both within and outside of the surface oil footprint. Though not conclusively linked to the oil spill, the broad spatial and temporal scale of these declines observed for disparate marine mammal species is consistent with Deepwater Horizon impacts. These declines have exceeded and outlasted post-spill damage assessment predictions, suggesting that the offshore ecosystem impacts of Deepwater Horizon may have been larger than previously thought.

Ship noise pollution significantly overlaps with critical habitats of endangered whales in the Santa Barbara Channel, prompting the need for effective noise reduction strategies. Various ship noise reduction approaches were assessed by simulating both source-centric (e.g., speed reduction or retrofit) and space-centric (e.g., routing changes) strategies to determine which would most effectively minimize noise within important marine habitats. Reducing the speeds of all ships achieved the highest noise reduction of the source-centric methods, although solely slowing cargo ships led to similar reductions. Implementing a single-route approach on the southern side of the Channel Islands achieved the greatest reduction of the space-centric strategies. For the multi-route approaches, some noise reduction was achieved by creating a buffer zone between the proposed shipping lanes and the critical habitat boundary. This simulation framework provides a mechanism for efficient exploration and assessment of noise reduction strategies across time and space. The framework can be updated to consider new approaches to changing ocean conditions.

Sperm whales Physeter macrocephalus are highly sexually dimorphic, with adult males having larger bodies, more powerful echolocation clicks, and slower echolocation clicking rates compared to females. This study introduces methods for estimating sperm whale population densities in the Gulf of Mexico (GoMex) by accounting for the population demographics using passive acoustic monitoring and reveals that ignoring the differences between demographic segments can introduce bias in density estimates. Weekly densities were estimated per 3 demographic segments: social groups consisting of adult females and their offspring, mid-size animals, and adult males. Analysis revealed that the GoMex sperm whale population is primarily composed of social groups, which account for 92 to 98% of the overall population. Mid-size animals and adult males made up a small proportion of the population and were only intermittently present. Our 7 yr GoMex density estimates, including the 2010 Deepwater Horizon (DWH) oil spill period and subsequent years, revealed demographic-specific trends. Declines found at 2 north-central GoMex sites, coupled with increases at a southeastern site, may indicate population movements and potential impacts from the 2010 DWH oil spill and elevated noise levels from anthropogenic activities in the north-central GoMex.

An accumulation of 26 859 days of acoustic recordings (spanning over 74 years at six sites) was processed and analyzed to investigate the interannual, seasonal, and diel patterns within six ocean soundscapes. Biological sound levels (18-25 Hz and 40–45 Hz) quantified acoustic energy from fin and blue whales, ambient sound levels (63-Hz and 800-Hz one-third-octave) quantified energy from ships and wind, and broadband spectra (15 Hz to 1 kHz) identified changes across time and space. Biological sound levels varied seasonally and correlated with large-scale climatic patterns and long-term ocean fluctuations. During marine heatwaves, baleen whale associated sound levels decreased in southern sites and increased in sites adjacent to the California Current. Ship sound levels at high-traffic sites reflected economic events such as the financial crisis, labor shortages and negotiations, and changes to port flows. Wind speeds and associated sound levels reflected on-shore/off-shore relationships and decreased during the morning hours. Understanding marine soundscapes aids in understanding the ocean’s ecological health amidst the ever-changing impact of climate change.

Sperm whales exhibit sexual dimorphism and sex-specific latitudinal segregation. Females and their young form social groups and are usually found in temperate and tropical latitudes, while males forage at higher latitudes. Historical whaling data and rare sightings of social groups in high latitude regions of the North Pacific, such as the Gulf of Alaska (GOA) and Bering Sea/Aleutian Islands (BSAI), suggest a more complex distribution than previously understood. Sperm whales are the most sighted and recorded cetacean in marine mammal surveys in these regions but capturing their demographic composition and habitat use has proven challenging. This study detects sperm whale presence using passive acoustic data from seven sites in the GOA and BSAI from 2010 to 2019. Differences in click characteristics between males and females (i.e., inter-click and inter-pulse interval) was used as a proxy for animal size/sex to derive time series of animal detections. Generalized additive models with generalized estimation equations demonstrate how spatiotemporal patterns differ between the sexes. Social groups were present at all recording sites with the largest relative proportion at two seamount sites in the GOA and an island site in the BSAI. We found that the seasonal patterns of presence varied for the sexes and between the sites. Male presence was highest in the summer and lowest in the winter, conversely, social group peak presence was in the winter for the BSAI and in the spring for the GOA region, with the lowest presence in the summer months. This study demonstrates that social groups are not restricted to lower latitudes and capture their present-day habitat use in the North Pacific. It highlights that sperm whale distribution is more complex than accounted for in management protocol and underscores the need for improved understanding of sperm whale demographic composition to better understand the impacts of increasing anthropogenic threats, particularly climate change.