Davidson Seamount: 2010 Marine Mammal & Seabird Survey
- Kelly Newton
University of California, Santa Cruz
- Andrew DeVogelaere
Monterey Bay National Marine Sanctuary
End Date: July 26, 2010
The Davidson Seamount, located 129 km southwest of Monterey, California, was incorporated into the Monterey Bay National Marine Sanctuary on March 9, 2009 and is the first seamount within the National Marine Sanctuary system. The Sanctuary conducted a ship-based survey of the waters above and around the Davidson Seamount during July 2010. The three-day survey onboard the NOAA Ship McArthur II was the first dedicated at-sea survey of the Seamount to record marine mammal and seabird observations.
Overall, 8 transect lines were surveyed for a total of 605 km of on-effort observations. Seventeen species of seabirds and 6 marine mammal species were observed. Cook’s Petrel was the most abundant seabird observed (8.4 birds km-2), followed by Leach’s Storm-Petrel (5.6 birds km-2). Including off effort sightings, the greatest number on Cook’s Petrel ever recorded in California waters were observed. The seabird assemblage to the northwest of the seamount was distinctly different than that to the southeast with the northwest region characterized by more pelagic species such as Cook’s Petrels and Leach’s Storm-Petrel while the southeast region was characterized by more coastal species such as shearwaters, phalaropes, gulls, and alcids.
Of a total of 200 marine mammal sightings, fin whales (Balaenoptera physalus) were the most commonly encountered marine mammal (51% of sightings), comprising 94% of whales sighted. In addition, fishes and other ancillary sightings were recorded. This survey in combination with aerial surveys along the same transect lines will serve as a baseline for future studies of the Davidson Seamount.
Summary to DateClick on the Documents tab to access a PDF report, with figures and tables. Below is just some of the information from that report.
Overall, 200 sightings of 668 individual marine mammals were sighted over the 3 day survey period. Fin whales (Balaenoptera physalus) were the most commonly encountered marine mammals (102 sightings, or 51% of all marine mammal sightings), comprising 197 individuals (encounter rate of 0.33 per km). The majority of sightings were located above and to the west of the seamount. Fin whales comprised 94% of all whale sightings and were the only large whale identified to species. All other large whale sightings were identified as either 'unidentified rorqual' (7 sightings of 9 individuals) or 'unidentified ziphiidae' (1 sighting of 4 individuals). No sperm whales were observed.
Three species of small cetaceans were encountered. There were 21 sightings of 78 individual Dall's porpoise (Phocoenoides dalli). One large pod of dolphins was encountered (300 individuals) comprising 2 species: 65% northern right whale dolphins (Lissodelphis borealis) and 45% Pacific white-sided dolphins (Lagenorhynchus obliquidens).
Northern fur seals (Callorhinus ursinus) were the most common pinniped sighted (68% of all pinniped sightings). Observers identified 42 sightings comprising 48 individuals, the majority of which were on the southern two transects south of the seamount. Other pinniped sightings included California sea lions (Zalophus californianus; 6 sightings of 6 individuals) and northern elephant seals (Mirounga angustirostris; also 6 sightings of 6 individuals).
Overall there were 316 sightings of 1033 individual seabirds comprising 17 different species. Cook's Petrel (Pterodroma cookii) and Leach's Storm-Petrel (Oceanodroma leucorhoa) were the two most commonly encountered species (77% of seabird sightings and 82% of all seabirds observed). Cook's Petrels were encountered at a rate of 8.4 birds per square km (121 sightings of 507 individuals) and Leach's Storm-Petrels were encountered at a rate of 5.6 birds per square km (124 sightings of 338 individuals). The majority of the sightings of the two species were above and to the west of the seamount. Including off effort sightings, observers recorded the greatest number of Cook's Petrel ever observed in California waters (5,125 total birds; Rogers et al. 2011).
Other seabird species sighted included both Red and Red-necked Phalaropes (Phalaropus fulicarius and P. lobatus, respectively), Arctic Terns (Sterna paradisaea), and Scripp's Murrelet (Synthliboramphus scrippsi; formerly Xantus' Murrelet). There were several sightings of migrating shorebird species, 11 Short-billed Dowitcher (Limnodromus griseus), 15 Least Sandpipers (Calidris minutilla), and 6 Black-bellied Plovers (Pluvialis squatarola). One landbird species landed on the ship for a short period of time one morning, an Ash-throated Flycatcher (Myiarchus cinerascens).
Fishes, Marine Debris, and Other Sightings
In addition to the marine mammal and seabird sightings, observers sighted 13 ocean sunfish (Mola mola), 6 albacore tuna (Thunnus alalunga) schools, salmon shark (Lamna ditropis), mako shark (Isurus oxyrinchus), blue shark (Prionace glauca), numerous egg yolk jellies (Phacellophora camtschatica), and approximately 25 floating kelp patties. Observers recorded all marine debris sightings, which included three plastic bags, a large plastic drum, four buoys, a Kit Kat candy wrapper, one plastic water bottle, and a paper coffee cup.
Zooplankton Net Tows
Overall, we conducted 10 zooplankton net tows: four along the axis of the seamount, three to the west, and three to the east of the seamount. We identified 7 total species of krill in the 10 samples, with Euphausia pacifica being the most abundant and found at every station. The second most abundant species, Nyctiphanes difficilis, was also found at every station but in much lesser abundance. The greatest abundance of krill (Euphausia pacifica) occurred at the stations along the axis and to the west of the seamount. However, there are no significant differences between transects or locations.
We conducted CTD casts prior to each zooplankton net tow. We deployed each cast to 1000 m without incident. The depth of the 10° isotherm ranged between 52 m – 77 m and on average was located at 58.7 m ± 7.42 SD. See Appendix III for water column profiles from each CTD cast.
Humboldt Squid Sampling
Unfortunately, we did not catch a single Humboldt squid at any of the 10 stations.
Sea surface temperature along the survey transect lines (0700 – 1900 PDT) ranged between 13.38 – 15.18 °C and averaged 14.53 °C (± 0.378 SD).
- Size structure
- Trophic association
Study MethodsThe study area was focused on the water directly above the Davidson seamount, and to both the east and west of the seamount and encompassed a total area of approximately 4000 km2.
Transect lines were determined based upon the transect lines used during the April 19, 2010 aerial survey of the Davidson Seamount area. There are a total of 22 transects in the aerial survey grid. Each transect line is approximately 88km and includes 44.4 km of surface water perpendicular to each side of the axis of the seamount. To maximize our coverage of the area, we selected every 4th transect line starting at line 0. A half transect was added to the north of the northern most transect line at the same distance between the 2 lines. In addition to the 6 large and 1 half transect, a 30 km transect was conducted over the axis of the seamount.
Hydrographic stations (CTD casts & zooplankton net tows) were located on 4 transect lines, spaced equidistant along the axis of the seamount. Three stations were occupied along each transect except for transect 22, where only one station was occupied. Along each transect, one station was located along the axis of the seamount, one 7 km to the east, and one 7 km to the west of the axis. Only the station along the axis of the transect was occupied for transect 22 (Figure 2).
During daylight hours, approximately 0700 – 1900, a daily watch for marine mammals and seabirds was maintained on the flying bridge. Surveys were conducted at a ship speed of 10kt along the designated transect line. At the beginning of each day, search effort started on the eastern waypoint of the trackline, or at the breakoff point from the prior evening.
Marine Mammal Observations
Six marine mammal observers used line transect survey methods to collect cetacean and pinniped abundance data. Each observer worked in 2-hour rotations, manning each of the following three stations on the flying bridge for 40 minutes: a port side 7x 50 binocular station, a center-line “naked eye” position, and a starboard 7 x 50 binocular station. In addition, each observer occupied the data recorder position. An “independent observer” kept a separate watch of animals sighted during the cetacean survey operations, to be compared later with the observer team’s data. Big-eye (25 x 150) binoculars, mounted on both the port and starboard sides of the flying bridge were used to aid with marine mammal identification and group size estimation. Marine mammals were identified to the lowest possible taxonomic level and for each sighting, environmental conditions and sighting details (group size estimation) were recorded.
Two seabird observers conducted visual surveys of seabirds using handheld and 25 x 150 big-eye binoculars. Details of seabird methods are presented in Newton et al. (2009) and are summarized here. Seabirds were recorded during daylight hours on the side of the ship with the best viewing conditions. Seabirds were identified to species when possible, and recorded from the bow to 90 degrees and out to 300 m as estimated by the observer via a rangefinder as described by (1981). Seabirds following the ship were counted only once if they approached the ship and flew into the transect strip. Seabirds that flew into the strip as a result of being flushed were not counted. Group size and seabird behavior was recorded (sitting or flying) for each sighting. The data were not corrected for bird ‘flux’ (Spear et al. 1992) because flight direction was not recorded for flying seabird observations.
In addition to the “on effort” seabird sightings, all birds seen during the survey were recorded and counted by the seabird observers.
Night operations commenced at approximately 2000 and finished by 0300. Each night between 3 and 6 stations were occupied.
Zooplankton Net Tows
At each station, an oblique bongo net tow was deployed to a depth of 200 meters and for a duration of 45 minutes. The bongo has a 505 micron mesh on the starboard side and a 333-micron mesh on the port side. Samples were preserved in formalin and transported back to UC Santa Cruz for processing and archiving. For each sample krill were identified to species if possible, counted, and measured in mm. Overall krill counts are represented as the number of krill per 1000m3.
A SeaBird 9/11+ CTD was used each night to collect temperature and salinity profiles. At each station, a CTD cast to 1000m was conducted. Cast descent rates were 30m per minute for the first 100m of the cast and then 60m/min for the remainder of the cast. Raw CTD data was processed using standard processing techniques with Seasave V 7.20c software.
Humboldt Squid Sampling
At each station, during the CTD cast, one or more scientific personnel fished for Humboldt squid (Dosidicus gigas) using a pole with a squid jig.
Sea surface temperature was recorded by a SeaBird SBE38 recorded mounted near the bow of the ship. In addition, a SeaBird SBE45 thermosalinograph (TSG) recorded sea surface temperature and salinity approximately 125ft behind the SBE38. Both systems operated continuously while underway. Temperature data is reported from the SBE38 due to its close proximity to the sea water input. To determine prey (zooplankton, krill, and fish) abundance a SIMRAD EK60 scientific depth sounder was operated continuously at 38, 70, 120, and 200 kHz. Acoustic backscatter data were recorded to a depth of 200m. Only the SBE38 temperature data was processed for this report. All other shipboard data are archived at UC Santa Cruz and at the Monterey Bay National Marine Sanctuary for future analysis.
Figures and Images
Figure 1. Crew member preparing to deploy a CTD (Conductivity, Temperature, and Depth). Several probes are attached to a large metal rosette wheel and is lowered on a cable. The CTD is the primary tool for determining essential physical properties of sea water. It gives scientists a precise and comprehensive charting of the distribution and variation of water temperature, salinity, and water density.
Figure 2. Northern Right Whale Dolphin (Lissodelphis borealis) above Davidson Seamount. This is an easy to ID species due to the lack of a dorsal fin, the black top and white underside.
- Newton and DeVogelaere (2013)1.8 MB PDF