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Advancements in Alaska Harbor Seal Census Methodology

(Quarterly Report for Oct-Nov-Dec 1999)

by Jack Cesarone and David Withrow

picture of Dave Withrow holding a juvenile harbor seal (31216 bytes)
Alaska Harbor Seal Task Leader and coauthor Dave Withrow holding a juvenile harbor seal.

The Pacific harbor seal, Phoca vitulina, is a common pinniped species found off the west coast of North America, throughout most of coastal Alaska, and from Baja California to Cape Newenham in the Bering Sea. Fifty years ago, the Pacific harbor seal was so abundant in Alaska (and perceived a conflict with commercial fisheries) that the state issued a bounty for the animal that continued until the early 1970s. Since that time, the numbers of harbor seals in areas of Alaska have declined dramatically—up to 90% at some haul-out sites.

Accurate range-wide abundance figures for the 1970s are not available to assess the impact this decline has had on the population as a whole, its availability to native subsistence hunters, or the degree of the species’ interactions with commercial fisheries.

State and Federal biologists have been collecting harbor seal count data sporadically since the 1940s. However, until the past decade most of these counts have been  incidental to other ongoing studies such as pelagic fur seal research and counts for Steller sea lions, sea otters, and other marine mammals conducted in the 1970s under NOAA’s Outer Continental Shelf Environmental Assessment Program (OCSEAP). The Alaska Department of Fish and Game (ADF&G)  also began to record numbers of harbor seals in the 1970s in selected areas in Alaska in order to establish sites for long-term trend analysis. Data from the ADF&G studies have shown that  at the  trend site Tugidak Island, south of Kodiak Island, counts of harbor seals declined 85% between 1976 and 1988.

With  the reauthorization of the Marine Mammal Protection Act in 1988, an increased effort began on Federal and State levels to establish reliable population estimates for Alaska pinnipeds. In 1991, the Alaska Harbor Seal Task of the National Marine Mammal Laboratory (NMML)  initiated a survey project to generate a minimum population estimate for Alaska harbor seals.  Over the past 10 years, the Harbor Seal Task has conducted comprehensive surveys specifically designed to assess harbor seal abundance in Alaska. The surveys represent the first state-wide attempt targeting harbor seals throughout their Alaskan range. The Task has developed aerial census procedures and continues to update and refine its survey methods using state-of-the-art imaging, mapping, and computer technologies.  NMML biologists have also developed new capture techniques for tagging studies in order to generate correction factors to improve the accuracy of Alaska harbor seal abundance estimates.

The size of the harbor seal’s geographic range in Alaska in conjunction with budgetary constraints prohibited its coverage in a single survey season. The NMML divided the range into four sections covering the Gulf of Alaska, the Aleutian Islands chain, Bristol Bay and the north side of the Alaska Peninsula, and Southeast Alaska.  At the onset of the 1998-99 seasons, Southeast Alaska was further divided into northern and southern sections.  One section per survey season is covered, and the entire Alaska range is now completed every 5 years.  The Task recently completed its third survey season within the 5-year survey cycle: the northern half of Southeast Alaska—1997;  the southern half of Southeast Alaska—1998; the Aleutian Islands—1999; the north side of the Alaska Peninsula and Bristol Bay—2000; and the Gulf of Alaska, including Prince William Sound—2001. Within the Pacific harbor seal’s Alaska  range, the National Marine Fisheries Service  recognizes three distinct management units or stocks:  1) the Southeast Alaska stock occurring from Cape Suckling (144EW) south to the Alaska/British Columbia border, 2) the Gulf of Alaska stock from Cape Suckling to Unimak Pass and the Aleutian Island chain and, 3) the Bering Sea stock including all waters north of Unimak Pass.

Survey zones and survey years for the Alaska harbor seal census
Survey zones and survey years for the
Alaska harbor seal population census.

Aerial Surveys

Aerial surveys have long been used in wildlife biology to establish minimum population estimates for certain animal species. Airplanes cover large areas quickly and efficiently.  Within the field of marine mammal research, aerial surveys are especially appropriate for assessing pinniped populations because the animals frequently and predictably haul out on dry land or ice. In the case of the Alaska harbor seal, aerial surveys are the standard methodology by which the population is assessed.

NMML aerial surveys are done in cooperation with ADF&G surveys and are scheduled to coincide with the Alaska harbor seal’s annual molt in August, the longest time the animals spend hauled out on land or ice.  During the second or third week in August, a tidal cycle is selected when the tides are low during daylight hours and the cycle of near minus tides lasts from 8-10 days.  Surveys are flown within 2 hours on either side of low tide when we expect the greatest number of seals to be hauled out.

Most surveys are flown in small, high-wing, single engine, amphibious aircraft. Amphibious planes are an important safety factor since many of the surveys are conducted in remote areas where landing facilities are few, fuel is scarce, and land masses are separated by large expanses of open water. Certain survey areas require the use of more specialized aircraft. For the area west of Adak Island, we charter only twin-engine, amphibious aircraft with turbine engines because they are extremely reliable and use jet fuel, the only aviation fuel available in this region.

Surveys are generally flown at altitudes between 500and 800 ft (150-200 meters) with a standard survey crew of one pilot and one observer.  Observers use their naked eyes to spot haulouts and use binoculars to make visual population estimates and to note the type of substrate and potential sources of disturbance to the animals, including the aircraft itself.  Examples might be natural, such as a bear on the beach, or human caused, such as fishing boats nearby.   Disturbance is a major factor that can, if severe enough, nullify a count. Each haulout is photographed using high speed color slide film and a 70-210 mm zoom lens.  An entire survey section may require seven or more aircraft surveying simultaneously to ensure total coverage.

A typical NMML aerial survey generates 4,000-5,000, 35-mm, color slides. The slides are processed at the NMML facilities in Seattle and projected onto a white acrylic board where each image is enlarged to facilitate identifying individual animals for counting. Each slide contains images of between 1 and 500 seals. Technicians use erasable marking pens to cross out each image when counted to prevent double counting and to track their way across each slide. Slides are counted at least twice and often by more than one technician.  We are also experimenting with computers to assist and automate the counting process. Computer software allows digital enhancement of poor images and stores and retrieves each counter’s marks, as well as archives the entire slide inventory.

During the early years of the Alaska harbor seal survey, NMML observers kept track of their survey positions by referencing the aircrafts’ loran navigation equipment. Due to the large number of sites and relatively imprecise location data, seal numbers were usually recorded along a strip of coastline; for example, “328 seals between SW cape and NE point.”  Beginning in 1994, NMML observers began using hand-held global positioning systems (GPS). The GPS records individual haul-out sites as way points in the unit’s memory that can be downloaded into a computer database. This has greatly increased the precision of haul-out data by using a latitude/longitude format, eliminated transcription errors, and allowed the data to be imported directly into newer navigation and presentation software programs.  Data stored in the new format can also be incorporated in GIS (global information system) programs and used to analyze certain aspects of harbor seal life history, such as population trends, migration, and residence times.

aerial photo of harbor seal haul-out site (41167 bytes)
The NMML's Alaska harbor seal assessment surveys generate up to 5,000 images of harbor seal haul-out sites documented by aerial observers.

Correction Factor

Aerial surveys only account for animals present during the actual fly over. However, a substantial number of a site’s population are not present during the aerial survey passes.  A goal of the Alaska Harbor Seal Task is to determine a correction factor to account for the proportion of animals not present when aerial surveys take place.  Counts from aerial assessment surveys are then multiplied by the correction factor to provide a better estimate of the true number of harbor seals in an area.  Depending on local geography, a haul-out site may be composed of rocky ledges, sand bars, gravel beaches, or floating ice.  These factors are important because haul-out behavior is specific to haul-out substrate.  For example, seals that use sand bars don’t haul out during higher tides (since their beach is underwater). Seals that haul out on ice are not affected by tide, but prefer to haul out at midday.  Covariates or variables, such as tide, time of day, weather (wind speed and direction), all affect seal haul-out behavior, and these effects are different for each type of substrate used.

Development of a correction factor for population assessment of a particular survey area  necessitates the capture, tagging, and release of seals within that area. The basic strategy of the Harbor Seal Task is to capture and tag about 35 seals annually. A select number of seals utilizing a given haul-out substrate are tagged with small radio-frequency transmitters which broadcast signals detectable when a seal is on land or ice, but undetectable when submerged. Signals generated from the electronic tags are received by the aerial survey aircraft outfitted with a VHF receiver and special external antennas mounted to the wing struts of the aircraft. Signals are also recorded by remote data collection computers stationed on shore nearby.  The remote data collection computers record each animal’s presence or absence at 15-minute intervals for approximately 2 months and are retrieved at the end of the field season. Data provided from the tagged seals along with data gathered during the aerial censuses provide the proportion of tagged seals hauled out during each aerial survey, along with the total number of all seals hauled out.  This method evaluates the combined effect of all the above mentioned covariates. The mean of all the daily proportions is calculated and the reciprocal of this value is the correction factor. This corrected or adjusted value more accurately reflects the number of seals in an area.

Capture and Tagging

photo of transmitter being attached to harbor seal flipper  (32662 bytes)
Transmitters are attached to approximately 35 Alaska harbor seals
annually.  Data generated from the tagged seals are used to develop
a correction factor for more accurate population estimates.

The Alaska Harbor Seals Task’s capture methodologies have been adapted from earlier work by the ADF&G and the Oregon and Washington departments of  Fish and Wildlife.  Capture nets are similar to salmon gill nets; each panel measures 30 m long and 3-7 m deep, with 30-cm, stretched-mesh openings.  A buoyant float line attaches along one long edge of the net, a weighted lead line along the other.  Depending on site conditions, NMML biologists lace any number of these panels together to create a net of any length; 30- to 100-m lengths have proved the most useful.

Capture methodology is a function of haul-out substrate.  Rocky haulouts are generally found on shorelines composed of moderate and large boulders and exposed bedrock.  Such low-lying areas are frequently sheltered by sloping cliffs or large rock formations at lands edge.  When possible we use these formations as screens to hide our slow approach by boat until the last possible moment when we accelerate and set the net.  Sand and pebble haulouts are broad, flat areas which lack sheltering land masses.  When these are islands or spits we approach at high speed from an area opposite the seals, keeping the land between us and the seals for as long as possible.  This gives us a flying start to get the net in the water before all seals leave the beach.

Capture operations are planned to coincide with the lowest point of the tide cycle.  Using a Boston Whaler, we encircle a haulout with a capture net, much like a purse seine capturing tuna.  Rather than use two boats to set the net, we use a diver to jump into the water with one end of the net and swim it ashore while the capture boat rapidly lays net in front of the haulout. Most seals head in the direction of the diver, swim into the net, and are entangled.

We generally average three to four seals per set, although empty sets are not unusual especially during ice captures.  Entangled animals are recovered into a capture boat,  the net cut away, and the seals transferred into restraining hoop nets.   Each animal is sexed, weighed, and measured for length and girth; age, stage of molt , and general health are noted; tissue samples are taken for DNA analysis, whiskers for stable isotope work, blood and in some cases, blubber samples are collected for condition studies. Processing and release takes place as close to the point of capture as possible. Animals are usually returned to the water within 2 hours of capture.  All animals are physically restrained.  No drugs are used. Aerial surveys are flown and remote data recording stations set out immediately following capture operations.

During the 1994 season the NMML focused on harbor seals among rocky haulouts in a remote area southwest of Ketchikan. Sixteen female and 20 male seals were captured, tagged, and released. The following two survey seasons (1995-96) were devoted to sandy haulouts outside of the town of Cordova in Prince William Sound.  We took 25 and 34 animals respectively during this time; a total of 34 females and 25 males. Beginning in 1997 our attentions focused on harbor seals on glacial ice.

NOAA vessel John N Cobb in front of glacier (38407 bytes)
The NOAA vessel John N. Cobb served as base of operations during the 1997-99 seasons in Southeast Alaska.

Ice-Associated Capture

While it had long been recognized that a substantial number of seals inhabit glacial fjords, the difficulties associated with operations in these remote and often hostile locations prevented any reliable population assessments.  NMML aerial surveys over Southeast Alaska and the Gulf of Alaska supplied information concerning the number of seals visible on the ice but had no successful capture methodology for ice-associated seals to generate a correction factor.

In spring 1997 the Task, in collaboration with Dennis McAllister of the ADF&G, began work in the Tracy and Endicott Arms glacial fjords in Southeast Alaska, supported by the NOAA vessel John N. Cobb. The Tracy and Endicott Arms are typical  glacial fjords: deep, steep-walled canyons created by advancing and retreating glaciers.  Each is about 15 km  long  and 10-300 m deep. Ice conditions vary throughout the year. During midwinter a sheet of ice often extends from the face of the glacier to near the mouth of the fjord; by June a boat can usually navigate the length of the fjord, dodging occasional icebergs.

Unlike previous tagging projects which allowed us to modify capture techniques to fit our specific environmental conditions, information concerning seal capture in glacial fjords was virtually nonexistent. From the start we adopted a trial and error methodology, observing the animals’ reactions to such variables as types of water craft, engine noise levels, speed of approach, and smell. We used painted wood decoys to setup artificial haulouts near our capture nets; experimented with net deployment at low speed in ice-choked water;  placed divers in the water next to our nets to bring curious seals closer.  We experimented with a net-firing gun used to capture caribou from helicopters.  At various points we disguised ourselves with white Tyvek suits (over our dry-suits) and covered our boat  with a white tarp as camouflage.

With only rudimentary knowledge for approaching ice-hauled seals, we moved our operations to the Kenai fjords National Park and continued to refine our capture techniques. Here we adopted a stealth approach using a small Zodiac powered by a quiet electric motor. The two-person crew spent many hours experimenting with net design and deployment  The remote nature of Alaska’s glacial fjords posed logistical problems to our capture operations. Pervasive cold and massive ice flows as the glaciers calved added stadium-sized pieces of ice to the already crowded surface ice. This rearranged our nets from a straight line to a jumbled mess in a matter of minutes, necessitating frequent retrievals and resets. Frustrated by bad weather, lack of experience, and poorly designed equipment we were still successful in capturing four seals during the 1997 field season.

scientists in zodiak approach seals hauled out on glacial ice
A small Zodiac and quiet motor allowed the closest approach to seals hauled out on glacial ice.  The majority of seals were captured when approximately 50% of the water's surface was covered by floating ice.  This provided a degree of cover for both biologists and seals

Leading up to the 1998 field season, the Task worked to modify its capture equipment as dictated by the lessons of the previous year. We constructed new nets incorporating larger mesh openings (30cm).  This allowed the animals room to push their head and neck through the mesh before actually contacting the net with their shoulders and increased the likelihood of their tangling.  We also increased the amount of net from the previous year and had several hundred feet at our disposal. Each of our boats, two Boston Whalers and a small Zodiac, were equipped with gear necessary to deploy and recover the net.  In this way we could work together or separately as conditions dictated.  For instance, one boat could be recovering tangled net while another made a new set.  Or all three boats could combine their resources to surround a small group of animals.

The Alaska Harbor Seal Task’s 1998 season in the Kenai fjords National Park proved highly successful.  Combining the stealth approach techniques worked out the previous year with our newly designed equipment we captured 19 seals around the Aialik and Pederson Glaciers.  We noted while working around theAialik Glacier that the majority of captures happened when the surface ice cover averaged about 50%.  This seemed to offer the greatest degree of compromise between cover for both seals and biologists and may have increased the animals willingness to allow our approach. Because Pederson Glacier is offset from the main fjord and is accessible only through a tidal channel, we captured several animals by stretching our nets across the narrower portions of the channel where all seals had to pass in order to enter or exit the glacial area.

The success of the 1998 season allowed us for the first time to tag a large enough number of ice-associated seals, providing us with an estimated corection with adequate precision (cv<0.2).  Data derived from aerial reconnaissance flights gave the average number of tagged seals hauled out  each day as 52%, providing  a correction factor of 1.92.  Using data generated from the 1998 field season efforts on ice-associated seals,  researchers can now begin to examine in detail the dynamics of a largely ignored, but substantial, population of Alaska harbor seals.

photo of diver in water surrounded by glacial ice (45458 bytes)
Members of the Alaska Seal Task tried various methods to improve their capture techniques for seals hauled out on glacial ice, including assessing the seals' reaction to divers in the water.

During the 1999 field season, the NMML collaborated with Peter Olesiuk of the Canadian Department of Fisheries and Oceans (DFO) in British Columbia to examine the diving behavior of harbor seals occupying glacial fjords.  Nine seals were captured in April 1999  in the Tracy Arm glacial fjord using the methodology described above.  These seals were equipped with time-depth recording devices (TDR) that monitored and stored the animals’ swimming and diving profiles over a period of several months.  The instruments were glued to the hair on the seal’s back at capture. The hairs was shed during the annual molt and the TDR units shed along with the hair.  The free-floating units contained VHF transmitting devices to allow for location and retrieval.  Seven of the nine TDRs were recovered by the end of September 1999. Analysis of the TDR data continues during winter 1999-2000 by NMML and DFO biologists.  Correction factors will be developed along with diving and potentially feeding profiles for each seal.

This project represents the first successful attempt to examine the diving and feeding behavior of fjord-dwelling harbor seals. Additionally, tissue samples taken from these animals will be used in a genetic analysis examining the population stock structure of seals utilizing this unique habitat. The NMML’s pioneering work in ice-captures has opened new opportunities for research in previously unstudied habitats.

We gratefully acknowledge the assistance of the NOAA vessel John N. Cobb, the rangers of the Kenai Fjords National Park, and the ADF&G for their invaluable contributions during these projects.