North Pacific Research Board Research Articles

High-resolution biological net community production in the Pacific-influenced Arctic as constrained by O2/Ar and O2/N2 observations

Spatial and temporal patterns of primary productivity in the Arctic are expected to change with warming-associated changes in ice cover and stratification, yet productivity measurements are historically spatially and temporally limited. Over the last two decades, an approach that uses measurement of dissolved oxygen/argon ratios (O2/Ar) from a vessel's underway seawater system has emerged as an established method to assess net community production (NCP) rates with high spatial and/or temporal resolution. More recently, the measurement of oxygen/nitrogen ratios (O2/N2) with a gas tension device (GTD) and optode have been piloted in underway settings to provide comparable NCP estimates. The GTD/optode approach has several advantages: instrumentation is small, inexpensive, and suitable for autonomous deployments; however, dissimilarity in solubility between O2 and N2 makes this tracer pair less accurate than O2/Ar. We conducted a side-by-side ship-based comparison of a GTD/optode and Equilibrator Inlet Mass Spectrometer (EIMS) in the Pacific Arctic during one of the North Pacific Research Board Integrated Ecosystem Research Program cruises in 2019. NCP from O2/Ar and O2/N2 approaches were coherent throughout this cruise, with median mixed layer integrated NCP of 9.3 ± 2.8 and 7.9 ± 3.2 mmol O2 m−2 day−1, respectively. The range of NCP was large, from less than zero to >100 mmol O2 m−2 day−1, with some of the largest NCP estimates measured at well-established hotspots in the Pacific Arctic. While O2/Ar and O2/N2 largely tracked each other, deviations were observed, principally in the Bering Sea where wind-induced bubbles were a primary driver, while a combination of temperature and wind drove differences over the majority of the cruise. The GTD/optode can be used to enhance spatial and temporal coverage of NCP measurements, yet the uncertainty makes this approach better-suited to regions with higher overall rates of NCP, while regions near-equilibrium may result in unacceptably high uncertainty. Additionally, the GTD/optode is reliant on well-calibrated oxygen observations, a potential challenge if autonomously deployed.

Reliable lower bound for number of calling marine mammals in Chukchi Sea

A reliable lower bound is derived for the number of vocalizing bowhead whales and walruses in the Chukchi Sea from ten acoustic receivers deployed during NOAA's CHAOZ experiment in 2011. The lower bound is derived from extremely reliable 100% confidence intervals (CI) of the sound's locations. CI are derived from measurements of the Time Differences of Arrival (TDOA) of sound between receivers. Sequential Bound Estimation (SBE) is used to estimate the CI by assimilating the TDOA. SBE was successfully and independently evaluated by the U.S. Navy. It is a nonlinear non-Bayesian technique capable of explicitly accounting for all errors affecting uncertainty while being computationally efficient. To date, its CI always contain the true location of the source. The CI are affected by uncertainty of sound speed, TDOA, receiver location, and clock time. For two-dimensional models of location, i.e., longitude/latitude, it appears necessary to utilize isodiachrons instead of hyperbolas to obtain reliable CI because of the extreme non-homogeneity of sound speed needed to obtain correct location. [Supported by the North Pacific Research Board and Microsoft.]A reliable lower bound is derived for the number of vocalizing bowhead whales and walruses in the Chukchi Sea from ten acoustic receivers deployed during NOAA's CHAOZ experiment in 2011. The lower bound is derived from extremely reliable 100% confidence intervals (CI) of the sound's locations. CI are derived from measurements of the Time Differences of Arrival (TDOA) of sound between receivers. Sequential Bound Estimation (SBE) is used to estimate the CI by assimilating the TDOA. SBE was successfully and independently evaluated by the U.S. Navy. It is a nonlinear non-Bayesian technique capable of explicitly accounting for all errors affecting uncertainty while being computationally efficient. To date, its CI always contain the true location of the source. The CI are affected by uncertainty of sound speed, TDOA, receiver location, and clock time. For two-dimensional models of location, i.e., longitude/latitude, it appears necessary to utilize isodiachrons instead of hyperbolas to obtain reliable CI because...

Verification of historical smooth sheet bathymetry for the Gulf of Alaska – Integrated Ecosystem Research Program

We verified the accuracy of interpolated bathymetric surfaces created from historic (1924–2003) National Ocean Service smooth sheet bathymetric soundings and shorelines with single-beam echosounder measurements of seafloor depth obtained in the spring and summer of 2013. Independent comparisons were made at ten inshore locations in the central and eastern Gulf of Alaska as an effort to groundtruth the fish habitat layers provided for NPRB's (North Pacific Research Board) sponsored Gulf of Alaska – Integrated Ecosystem Research Program (GOA-IERP). Ordinary least squares linear regressions determined that the GOA-IERP soundings could successfully predict interpolated smooth sheet bathymetry (gridded or raster surface) at all sites (best R2 = 1.0), although the oldest smooth sheets from 1924 (R2 = ~ 0.90) and 1925 (R2 = ~0.80) had the poorest fits. Standardized residuals were geographically clustered at all sites, with larger residuals often observed in areas of rapid depth transition, but ≥ 93% of residuals at all sites were within two standard deviations. Residual analysis indicates that the standardized residuals increase with depth, slope, distance to nearest smooth sheet sounding, and distance to nearest smooth sheet navigation station. This indicates that errors in navigation were greater farther offshore, interpolations were worse in areas of sparse soundings, and the consequences were more significant in steeper and deeper areas. Overall, we conclude that the smooth sheet bathymetry was successfully ground truthed and useful for fish habitat descriptions.

Comparison of the physical attributes of the central and eastern Gulf of Alaska integrated ecosystem research program inshore study sites

In this analysis, five eastern Gulf of Alaska (eGOA) inshore study sites from the North Pacific Research Board's (NPRB) Gulf of Alaska Integrated Ecosystem Research Program (GOAIERP: http://www.nprb.org/gulf-of-alaska-project) were characterized using multiple Geographic Information System (GIS) physical measurements including water volume, surface area, and shoreline length. These eGOA sites include three small bays (Islas Bay, Torch Bay and Graves Harbor), a large semi-protected body of water (Salisbury Sound), and a large bay (Whale Bay). These measurements provide quantitative metrics of study sites that may aid in the interpretation of biological differences among the sites. The analysis is similar to a previously published study of five central Gulf of Alaska (cGOA) sites that also included three small bays, an unprotected body of water, and a large bay. In addition, this analysis included new measures of the surface area of lakes within each sites' watershed, and the amount of nearby shallow (< 200 m) continental shelf area. Multivariate analyses were used to examine similarities among the cGOA and eGOA sites and suggested that location and size were both important grouping factors, with the cGOA and eGOA small bays forming unique groups. The cGOA non-bay and large-bay sites were the most distinct locations. It appears that there is some link between geographic location and physical similarity of study sites chosen for the GOAIERP.

Deducing environmental mismatch from matched-mode processing ambiguity surface sidelobes

The application of non-linear time sampling to broadband acoustic signals propagating in shallow water has made modal filtering possible using a single hydrophone. As a result, incoherent matched-mode processing (MMP) techniques are now practical using only single-hydrophone data. When MMP ambiguity surfaces are constructed from pairs of modes and plotted as a function of range and frequency, both the mainlobe and sidelobes form striations that embed information about the type and amount of environmental mismatch present between the modeled and true environment. Thus the degree of symmetry that a two-mode frequency-averaged ambiguity surface displays around the mainlobe provides a metric for identifying environmental mismatch. Acoustic invariant theory, combined with simulations, demonstrate how mismatched waveguide replicas can be used to (1) estimate the true bottom interface sound speed, (2) determine whether a downward-refracting waterborne profile is present in data, and (3) estimate waterborne sound speed profile slopes. The theory also explains why certain mismatched environments can generate very high MMP correlations, even with broadband signals. In a shallow water environment with a downward-refracting sound speed profile, taking full advantage of this approach requires reformulation of non-linear time-sampling methods to extract refractive modes. [Work sponsored by the North Pacific Research Board.]

Inversion of normal mode functions and seabed properties using non-linear time sampling of low-frequency bowhead whale calls on a vertical array

Previous research has demonstrated that individual modal components of signals propagating in an acoustic waveguide can be isolated from a single hydrophone using non-linear time sampling (Bonnel et al., 2010) and that mode shape functions can be directly extracted from vertical array data in a laboratory setting (Bonnel et al., 2011). Here, we apply and extend the technique to experimental data collected in 2010 from a 15-element vertical array, covering only 63% of the 53 m water column off the continental slope of the Beaufort Sea, during the fall bowhead whale migration. We demonstrate how up to five distinct mode shapes can be extracted from frequency-modulated bowhead whale calls, along with the vertical array tilt and tilt direction, without resorting to assumptions about mode orthogonality across the array aperture. The extracted mode shapes can then be used to estimate whale call depth. Filtered modes (and mode cutoff information) can also be used to invert for whale call ranges and ocean bottom properties. [Work supported by the North Pacific Research Board and permitted by Shell Exploration and Production Company.]

Acoustic and foraging behavior of tagged sperm whales under natural and depredation foraging conditions in the Gulf of Alaska

Sperm whales have been depredating black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska for decades, but the behavior has now become pervasive enough that it may be affecting government estimates of the sustainable catch, motivating further studies of this behavior. Over a three-year period, 11 B-Probe bioacoustic tags have been attached to seven adult sperm whales off Southeast Alaska, permitting observations of the animals’ dive profiles and acoustic behavior during natural and depredation foraging conditions. Two rough categories of depredation were identified: “deep” and “shallow.” “Deep depredating” whales consistently surface within 500 m of a hauling fishing vessel, have maximum dive depths greater than 200m, and display significantly different acoustic behavior than naturally foraging whales, with shorter inter-click intervals, occasional bouts of high “creak” rates, and fewer dives without creaks. “Shallow depredating” whales conduct dives that are much shorter, shallower, and more acoustically active than both the natural and deep depredating behaviors, with median creak rates three times that of natural levels. Occurrence of slow clicks and the behavioral context in which these vocalizations are produced were also investigated. These results provide insight into the energetic benefits of depredation behavior to sperm whales. [Work conducted under the SEASWAP program, supported by the North Pacific Research Board and the National Geographic Society.]

Local and traditional knowledge regarding the Bering Sea ecosystem: Selected results from five indigenous communities

We documented local and traditional knowledge (LTK) about the Bering Sea ecosystem through interviews with Alaska Native elders, hunters, and fishers in the coastal communities of Akutan, St. Paul, Togiak, Emmonak, and Savoonga. Their observations describe a complex and changing ecosystem, with indications of divergent impacts of change in the south (many species in decline) and the north (a productive ecosystem). Observed changes in species abundance suggest that the marginal zone of maximum (March) sea-ice extent is experiencing the most rapid directional changes, including shifts in distribution of ice-associated species such as bearded seal (Erignathus barbatus). Causes of declines in other species such as northern fur seals (Callorhinus ursinus) and murres (Uria spp.) are harder to identify, and seabird abundance trends appear to vary greatly with location. Connections between the LTK findings and other research under the North Pacific Research Board and National Science Foundation's Bering Sea Project were modest due to mismatches in temporal and spatial scales of reference and the fact that LTK observations were not initially made with scientific relevance in mind. We found, however, the overall observations to be consistent with the emerging picture of high spatial variability in the Bering Sea ecosystem.

Using warping processing to range bowhead whale sounds from a single receiver

In certain shallow water environments, the acoustic propagation of low-frequency marine mammal calls can be well-modeled as a discrete set of normal modes. Each mode propagates with a different group velocity, and thus in principle the range of the call can be inferred by comparing relative arrival times of the modal arrivals. Traditionally, several time-synchronized hydrophones are required to spatially filter out individual modes in order to measure relative arrival times. In this presentation a nonlinear signal processing method classed “warping” is used to identify individual mode arrival times on a single receiver, even when the mode arrivals are overlapping in time. Warping processing is limited to frequency-modulated sources with monotonic increases or decreases of frequency with time. It is thus applicable to whale calls that consist of simple frequency-modulated upsweeps or downsweeps. Once the modes are separated, the source range can be estimated using conventional modal dispersion techniques. This method is applied on several bowhead whale vocalizations recorded near Kaktovik (Alaska) in 2012. Bowhead whale calls are ranged up to 35 km under median ambient noise conditions. These single-receiver range estimates are consistent with estimated ranges previously obtained via other methods [Work supported by North Pacific Research Board and Shell Exploration and Production Company.]

Using bioacoustic and satellite tags to study sperm whale depredation behavior, and to test new acoustic tracking methods

Sperm whales have been depredating black cod from demersal longlines in the Gulf of Alaska for decades, but the behavior has now become pervasive enough that it is starting to affect government estimates of the sustainable catch, motivating further studies of this behavior. In 2007 and 2009, 11 bioacoustic “BProbe” tags were attached to adult sperm whales off Southeast Alaska under both natural and depredation foraging conditions. Measurements of the animal's dive profiles, acoustic behavior, and angular velocities allowed two categories of depredation to be identified. The dive depths and durations of “deep depredating” whales are similar to those of natural dives, but acoustic parameters show significantly significant differences. By contrast, “shallow depredating” whales conduct dives that are much shorter, shallower, and are four times more acoustically active than during natural foraging dives. In 2010, both a satellite and bioacoustic tag were deployed on a sperm whale near a two-element vertical tracking array. The location and acoustic data from the tags were used to confirm the long-range tracking ability of the fixed array system to ranges of at least 5 kms. [Work conducted under the SEASWAP program, supported by the 2011 North Pacific Research Board graduate student research award]

Depth and range tracking of sperm whales in the Gulf of Alaska using a two-element vertical array, satellite and bioacoustic tags

A two-element vertical array was deployed between August, 15 and 17 2010, on the continental slope off Southeast Alaska, in 1200 m water depth. The instruments were attached to a longline fishing anchorline, deployed at 300 m depth, close to the sound-speed minimum of the deep water profile. The anchorline also served as a decoy, attracting seven depredating sperm whales to the area. Three animals were tagged with a satellite tag and one of them was tagged with both a satellite and bioacoustic “BProbe” tag. Both tags recorded dive depth information. Relative arrival times of surface- and bottom-reflected paths are used to estimate animal range and depth on a single hydrophone, and compared with tagging results. The two-element array is then used to estimate vertical arrival angles of the direct and surface-reflected paths to determine whether range and depth localization can occur without the use of bottom multipath. This data will be useful in determining whether long-range tracking of sperm whales is possible using a single compact instrument deployment. Potential applications include observing what ranges whales are willing to travel to depredate. [Work conducted under the SEASWAP program, supported by the National Oceanic and Atmospheric Administration and the North Pacific Research Board.]

Matched-field processing and modal filtered range estimates of bowhead whale calls detected in the Alaskan Beaufort Sea

In 2010, a 15-element autonomous vertical array was deployed roughly 35 km north of Kaktovik alongside a distributed array of directional autonomous seafloor acoustic recorders (DASARs). Matched-field processing and geoacoustic inversion techniques were used to extract the range, depth, bottom sound speed profile, density, and attenuation from one close-range whale call. The inversion localized the call at 1.2 km range and 44 m depth in 55 m deep water. The inverted propagation model was used to derive the group and phase velocities of the normal modes in the region surrounding the array. The vertical array spanned sufficient aperture in the water column to permit isolation of the first and second mode arrivals from any given call. A range- and frequency-dependent phase shift was applied to each modal arrival to remove geometric dispersion effects. The modeled range that time-aligned the modal arrivals was selected as the range estimate. The modal filtering technique is demonstrated on additional whale calls produced at 17.3 km and 35 km range from the vertical array, with the range estimates independently confirmed by triangulating bearings of call detections on surrounding DASARs. [Work supported by the North Pacific Research Board, Shell Exploration and Production Company, and Greeneridge Sciences Incorporated.]

Differences between natural and depredation behaviors in sperm whales (Physeter macrocephalus), using dive, acoustic, and orientation data from short‐term tags.

Sperm whales depredate black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska. Over a 3‐year period 11 bioacoustic tags have been attached to adult sperm whales off Southeast Alaska under both natural and depredation foraging conditions. Measurements of the animals’ dive profiles, acoustic behavior, and angular velocity under both behavioral modes were distilled and examined for statistically significant differences. Two rough categories of depredation are identified. The dive depths and durations of “deep depredating” whales are similar to those of natural dives, but acoustic parameters and roll rates show significantly significant differences. By contrast, “shallow depredating” whales conduct dives that are much shorter, shallower, and more acoustically active than natural foraging dives. Acoustic “creak” rates, which serve as proxies for prey capture attempt rates, are four times greater during shallow depredation than during natural foraging. During all behaviors, whales generating creak sounds had significantly greater roll rates when the creak was followed by a pause, suggesting that acoustic monitoring might distinguish prey capture successes from failed attempts. Fish that naturally “spin‐off” the line during a haul may explain facets of deep depredation. [Work supported by the North Pacific Research Board, the National Geographic Society, and OGP.]

Southeast Alaska Sperm Whale Avoidance Project: Collaboration among fishermen, acousticians, biologists and managers to reduce longline depredation in the Gulf of Alaska.

In the eastern Gulf of Alaska, depredation of demersal longline fishing gear set for sablefish (Anoplopoma fimbria) by sperm whales (Physeter macrocephalus) has occurred since at least the mid‐1970s. In 1995, with the implementation of Individual Fishing Quotas, the season expanded from a 2 week derby style fishery to 8 months. This change allowed more opportunity for whales to depredate longline gear and reports of depredation increased resulting in economic loss to the fleet. Beginning in 2003, the North Pacific Research Board funded a collaborative study among fishermen, scientists, and managers to collect quantitative data on longline depredation. The goal of the Southeast Sperm Whale Avoidance Project (SEASWAP) is to determine the mechanics of the depredation, characterize the whales involved, and to recommend changes in fishing behavior to reduce depredation. The longline fleet provided fishing data and allowed researchers to collect behavioral, genetic, and acoustic data in conjunction with their fishing operations. Through this effort, SEASWAP has estimated numbers and sexes of depredating whales, identified long‐range acoustic cues that attract animals to the gear, and systematically tested various depredation reduction strategies, including decoy deployments, active playbacks, and modifications to fishing gear.

How Does Climate Change Affect the Bering Sea Ecosystem?

The Bering Sea is one of the most productive marine ecosystems in the world, sustaining nearly half of U.S. annual commercial fish catches and providing food and cultural value to thousands of coastal and island residents. Fish and crab are abundant in the Bering Sea; whales, seals, and seabirds migrate there every year.In winter, the topography, latitude, atmosphere, and ocean circulation combine to produce a sea ice advance in the Bering Sea unmatched elsewhere in the Northern Hemisphere, and in spring the retreating ice; longer daylight hours; and nutrient‐rich, deep‐ocean waters forced up onto the broad continental shelf result in intense marine productivity (Figure 1). This seasonal ice cover is a major driver of Bering Sea ecology, making this ecosystem particularly sensitive to changes in climate. Predicted changes in ice cover in the coming decades have intensified concern about the future of this economically and culturally important region. In response, the North Pacific Research Board (NPRB) and the U.S. National Science Foundation (NSF) entered into a partnership in 2007 to support the Bering Sea Project, a comprehensive $52 million investigation to understand how climate change is affecting the Bering Sea ecosystem, ranging from lower trophic levels (e.g., plankton) to fish, seabirds, marine mammals, and, ultimately, humans. The project integrates two research programs, the NSF Bering Ecosystem Study (BEST) and the NPRB Bering Sea Integrated Ecosystem Research Program (BSIERP), with substantial in‐kind contributions from the U.S. National Oceanic and Atmospheric Administration (NOAA) and the U.S. Fish and Wildlife Service.

Automated detection of sperm whales (Physeter macrocephalus) creaks and association with depredation events.

Sperm whales have been depredating black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska for decades, but the behavior has now become pervasive enough that it is starting to affect government estimates of the sustainable catch, motivating further studies of this behavior. In July 2007 and June 2009, bioacoustic tags were attached to adult sperm whales off Southeast Alaska under both natural foraging conditions and situations wherein the animals were depredating sablefish from commercial longlining vessels. Tags remained attached for a cumulative hours, yielding more than 100 h of depth, orientation, and acoustic data. Results show that when depredating, sperm whales often dive at depths shallower than 50 m, compared to natural foraging depths of 300–400 m in the area. Dive durations, pitch, and roll rates are also distinctively different. Techniques such as spectrogram cross-correlation and cepstral analysis are used to automatically detect creak events, which have lower intensities and narrower frequency ranges than conventional clicks. Acoustic measurements such as creak production rate, duration, and intensity are then extracted. Correlations are derived between creak events, sudden changes in body orientation and depredation rate, with a goal of determining an acoustic measure of depredation activity and efficacy. [Work conducted under the SEA SWAP Program, supported by the North Pacific Research Board and the National Geographic Society.]

Introduction to a Special Section: Atka Mackerel Distribution, Life History, Ecology, and Management

This suite of papers on the life history, ecology, and management of Atka mackerel was originally presented at a research symposium (29–30 April 2008) in Seattle, Washington. The studies were done as a collaborative effort between the Alaska Fisheries Science Center, the University of Alaska–Fairbanks, the Alaska Sea Life Center, and the North Pacific Fisheries Foundation. This research was funded by the National Marine Fisheries Service, the North Pacific Research Board (project 522), and the North Pacific Fisheries Foundation. Atka mackerel Pleurogrammus monopterygius are widely distributed along the continental shelf extending from the Kuril Islands in Asia through the Gulf of Alaska. They are the most abundant groundfish in the Aleutian Islands region, with an estimated exploitable biomass of 388,500 metric tons in 2010 (Lowe et al. 2009), and are a key component of the Aleutian Islands ecosystem. Within Alaska waters they are concentrated in the Aleutian Islands chain, where they form dense aggregations in island passes. Atka mackerel are obligate spawners on hard rocky bottoms and exhibit a polygamous mating system, elaborate sexual dichromatism, and social behaviors that have been documented in other hexagrammids. Nesting colonies are widespread across the continental shelf and are associated with areas of strong currents and hard substrate as deep as 144 m (Lauth et al. 2007). In June, the reproductive phase begins with adult males establishing individual territories within nesting colonies. Adult females aggregate in dense patches close to the nesting colonies during the spawning season, presumably to feed (Cooper and McDermott, unpublished). In July spawning starts, and during a 12-week period ending in mid-October females spawn four to five batches of adhesive eggs in the territories of different males (McDermott et al. 2007). After spawning, females disperse and males guard the nests until the eggs hatch following an incubation period of 39–75 d in the eastern and central Aleutian Islands, with maximum hatching occurring in late November (Lauth et al. 2007). Atka mackerel exhibit a size cline across their range, with individuals having smaller sizes at age in the western Aleutian Islands area and larger sizes at age in the eastern areas. Historical morphological and meristic studies provide some indications of separate populations within Alaska. However, a genetic study of allozyme variation showed no evidence of discrete stocks (Lowe et al. 1998), a result that is reexamined in this special section using another class of nuclear markers. Atka mackerel’s highly localized aggregations and their overall large biomass make them an important prey species in the Aleutian Islands ecosystem. Atka mackerel have been identified as the main prey item for the endangered Steller sea lion Eumetopias jubatus and fish species such as Pacific cod Gadus macrocephalus and arrowtooth flounder Atheresthes stomias (I. Ortiz and E. Logerwell, National Marine Fisheries Service, unpublished). Adult Atka mackerel consume a variety of prey but principally feed on calanoid copepods, euphausiids, and myctophid fishes (Rand 2007). Egg cannibalism during the spawning period has been observed for both sexes. Heterocannibalism by both sexes and male filial cannibalism are common phenomena in species that have evolved to provide exclusive paternal care. Atka mackerel are the target of a large offshore commercial fishery in the Aleutian Islands. The spatial patterns of the fishery generally reflect the distribution and behavior of the species; the fishery is highly localized and focuses on the same locations each year, almost exclusively at depths shallower than 200 m. The fishery is currently managed by the North Pacific Fishery Management Council under a quota system whereby the overall total allowable catch (TAC) is divided into two equal seasonal allowances: an ASubject editor: Donald Noakes, Thompson Rivers University, British Columbia, Canada

Depth, orientation, and acoustics of sperm whales (Physeter macrocephalus) under natural and depredation foraging conditions in the Gulf of Alaska

In July 2007 bioacoustic tags were attached to adult sperm whales in the Gulf of Alaska under both natural foraging conditions, and situations wherein the animals were depredating sablefish from commercial longlining vessels. A small Rigid‐Hull Inflatable was used to approach and attach a suction‐cup acoustic recording tag on 13 occasions and stayed on animals for a total of 168 hours, yielding 80 h of depth, orientation, and acoustic data. These results, combined with passive acoustic tracking and underwater video‐camera data, indicate that sperm whales depredate at depths shallower than 50 m, compared to natural foraging depths of 300‐400 m in the area. During depredation the animals demonstrate changes in pitch and roll that are greater when compared with normal foraging behavior. Observations on the acoustic behavior of the animals are noted as well. The fact that these normally deep‐diving animals depredate so close to the surface has interesting implications for both depredation‐reduction strategies and biosonar research. [Work conducted under the SEASWAP program, supported by the North Pacific Research Board and the National Geographic Society.]

Direct underwater visual and acoustic observations of echolocation behavior of sperm whales around a longline

Sperm whales (Physeter macrocephalus) have learned to approach and depredate fishing vessels longlining for black cod in the Gulf of Alaska. In May 2006, off Sitka, Alaska, a video camera was placed inside an underwater housing with an external hydrophone mount, attached to a longline that had been partially hauled to the surface, and lowered between depths of 90 and 120 m, with black cod (Anoploploma fimbria) attached 2 to 4 m above the camera as bait. The camera was mounted facing towards the surface to exploit the ambient light and to eliminate the need for artificial lighting. An additional hydrophone was deployed at 17-m depth during the 40–60-m deployments, and a third autonomous acoustic recorder was usually mounted 1.3 m below the camera. During one encounter a whale investigated the line, producing characteristic creak sounds that were recorded on the three hydrophones, and which were subsequently time-aligned using vessel noise as a reference, in order to permit acoustic tracking. During the second encounter a whale interacted with two fish within 4 m of the camera, but only the camera hydrophone was available. Preliminary analysis of the echolocation behavior displayed by both animals is presented. [Work sponsored by the National Geographic Society and the North Pacific Research Board.]

Acoustic and optic characterization of forage fish distribution in the Bering Sea

Mesopelagic forage fish species are important components of the Bering Sea ecosystem, but information on species distribution and identity is limited. Using acoustics and optics [LIght Detection And Ranging (LIDAR)], this study compared distribution patterns detected by the two gears. Forage species distribution and quantity differed between shelf (6–100 m) and slope (6–100 m, 100–300 m, 300 m to bottom) regions. Acoustic results suggested that shallow and deep depth zones contained dispersed backscatter while the middle slope layer contained patchy schools associated with the shelf break. Variogram results for repeated LIDAR surveys of the shelf and slope regions indicated that backscatter distribution between 6 and 30 m was dynamic at the scale of days. This result was expected given the strong frontal nature of the area. When LIDAR results were compared with coincident acoustic transects on the shelf and slope, differences were found in gear detection of backscatter. Based on acoustic results, 25%–63% of forage fish in the shelf and slope regions were deeper than the LIDAR detection range. Although both LIDAR and acoustics are constrained to portions of the water column, the utility of remote sampling technologies is dependent on survey objectives. [Funding was provided by the North Pacific Research Board.]