Dr. PAUL M. WEBB

MARINE BIOLOGY

 

Dr. Paul Webb
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Conference Abstracts (*denotes undergraduate author)

 

Nicotri, H.* and P. M. Webb. 2006. Impacts of seals on fish populations in Narragansett Bay, Rhode Island, USA. Rhode Island Natural History Survey Annual Meeting, Warwick, RI, Mar. 3, 2006

 

Norris, A.* and P. M. Webb. 2006. Nocturnal behavior of the harbor seals (Phoca vitulina) from Prudence Island, Rhode Island. Rhode Island Natural History Survey Annual Meeting, Warwick, RI, Mar. 3, 2006

 

Nicotri, H.* and P. M. Webb. 2005. Impacts of seals on fish populations in Narragansett Bay, Rhode Island, USA. 16th Biennial Conference on the Biology of Marine Mammals. San Diego, CA, Dec. 12-16, 2005

 

Weaver, K. M.*, S. K. O'Shea, and P. M. Webb. 2004. Distribution and localization of essential and heavy metals in relation to polyaromatic Hydrocarbons (PAHs) in Stranded Seal Tissue. 227th American Chemical Society National Meeting, Anaheim, CA, Mar. 28-Apr. 1, 2004

 

Keach, S. E.*, S. K. O’Shea, and P. M. Webb. 2003. Analysis of contaminant levels in various tissues of local seals. 225th American Chemical Society National Meeting, New Orleans, LA, Mar. 23-27, 2003

 

McKamey, K.* and P. Webb. 2002. Haul-out and scanning behavior of the harbor seal (Phoca vitulina concolor) from Rome Point, Rhode Island. Rhode Island Natural History Survey Annual Meeting. Warwick, RI. Mar 1., 2002.

 

Crocker, D.E., P.M. Webb, and D.S. Houser. 2001. Reproductive energetics of adult male northern elephant seals. Fourteenth Biennial Conference on the Biology of Marine Mammals. Vancouver, B.C. Nov. 28-Dec. 3, 2001.

 

Webb, P.M., D.E. Crocker, D.S. Houser, D.P. Costa and B.J. Le Boeuf. 1999. Effects of body composition on the buoyancy and diving behavior of northern elephant seals. Thirteenth Biennial Conference on the Biology of Marine Mammals. Wailea, Maui, HI. Nov. 28-Dec. 3, 1999.

 

Crocker, D.E., D.P. Costa, B.J. Le Boeuf, P.M. Webb and D.S. Houser. 1999. Impact of the 1997-1998 El Niño on the foraging behavior of northern elephant seals. Thirteenth Biennial Conference on the Biology of Marine Mammals. Wailea, Maui, HI. Nov. 28-Dec. 3, 1999.

 

Le Boeuf, B.J., D.P. Costa, S.B. Blackwell, D.E. Crocker, P.M. Webb, J. Gedamke and J. Grayson. 1999. Surface breathing rate and heart rate in free-ranging elephant seals. Thirteenth Biennial Conference on the Biology of Marine Mammals. Wailea, Maui, HI. Nov. 28-Dec. 3, 1999.

 

Crocker, D.E., D.P. Costa, B.J. Le Boeuf, P.M. Webb and D.S. Houser. 1999. Impacts of the 1997-1998 El Niño on the foraging behavior of northern elephant seals, a large marine predator. The Oceanography Society 1999 Scientific Meeting, Reno, NV April 27-30, 1999.

 

Webb, P.M., S.B. Blackwell, D.E. Crocker, D.P. Costa and B.J. Le Boeuf. 1998. Effects of buoyancy on the diving behavior of northern elephant seals. World Marine Mammal Conference, Monaco, Jan. 20-24, 1998.

 

Costa, D.P., D.E. Crocker, J. Gedamke, P.M. Webb, D.S. Houser, S.B. Blackwell, D.M. Waples, S. Hayes, J. Ganong and B.J. Le Boeuf. 1998. Response of elephant seals to ATOC sound transmissions. World Marine Mammal Conference, Monaco, Jan 20-24, 1998.

 

Le Boeuf, B.J., D.P. Costa, D. Crocker, S. Blackwell, P. Webb and D. Houser. 1998. Foraging and migratory behaviour of northern elephant seals. World Marine Mammal Conference, Monaco, Jan 20-24, 1998.

 

Costa, D., D. Crocker, J. Gedamke, P. Webb, B. Le Boeuf, D. Waples, S. Hayes and J. Ganong. 1997. Responses of elephant seals to Acoustic Thermometry of Ocean Climate transmissions. 134th Acoustical Society of America Meeting. San Diego, CA. Dec. 1-5, 1997

 

Crocker, D.E., S.B. Blackwell, P.M. Webb, B.J. Le Boeuf, and D.P. Costa. 1997. Instrumentation for studying the diving, foraging and migratory behavior of northern elephant seals. 127th Annual Meeting of the American Fisheries Society. Monterey, CA. Aug. 24-28, 1997

 

Costa, D.P., D.E. Crocker, D. Croll, D. Goley, D. Houser, B.J. Le Boeuf, D. Waples, P. Webb, and J. Calambokidis. 1996. Effects of the California ATOC experiment on marine mammals. Journal of the Acoustical Society of America 100(4):2581

 

Costa, D.P., Crocker, D.E., Le Boeuf, B.J., and Webb, P.M. 1996. Foraging behavior of northern elephant seals using time depth recorders coupled with ARGOS satellite locations. Fifth European Conference on Wildlife Telemetry. Strasbourg, France. Aug. 25-30, 1996

 

Crocker, D.E., Costa, D.P., Le Boeuf, B.J., Webb, P.M., and Blackwell, S.B. 1996. Swim speed and foraging strategies of the northern elephant seal. Fifth European Conference on Wildlife Telemetry. Strasbourg, France. Aug. 25-30, 1996

 

Costa, D.P., D.E. Crocker, B.J. Le Boeuf and P. Webb. 1996. Foraging behavior and habitat utilization of northern elephant seals. Society for Integrative and Comparative Biology, Annual Meeting. Dec. 26-30, 1996

 

Webb, P.M., Crocker, D.E., Costa, D.P., and Le Boeuf, B.J. 1995. Glomerular filtration rate in lactating northern elephant seals. Eleventh Biennial Conference on the Biology of Marine Mammals. Orlando, FL. Dec. 14-18, 1995

 

Blackwell, S.B., Le Boeuf, B.J., Crocker, D.E., and Webb, P.M. 1995. Hold your breath and stay fat: An estimate of the water savings from terrestrial apnea in northern elephant seals. Eleventh Biennial Conference on the Biology of Marine Mammals. Orlando, FL. Dec. 14-18, 1995

 

Houser, D.S., Crocker, D.E., Webb, P.M., and Costa, D.P. 1995. Dynamics of glomerular filtration in northern elephant seal pups (Mirounga angustirostris) during suckling and fasting periods. Eleventh Biennial Conference on the Biology of Marine Mammals. Orlando, FL. Dec. 14-18, 1995

 

Webb, P.M., Andrews, R.D., Le Boeuf, B.J., and Costa, D.P. 1993. Heart rate and oxygen consumption in northern elephant seals diving in the laboratory. Tenth Biennial Conference on the Biology of Marine Mammals. Galveston, TX. Nov. 11-15, 1993

 


IMPACTS OF SEALS ON FISH POPULATIONS IN NARRAGANSETT BAY, RHODE ISLAND, USA

Nicotri, H. and P. M. Webb

 

Seals have been protected in the U.S. since the MMPA took effect in 1972. Since that time, seal populations in New England waters have increased 5-fold. At the same time that seal populations have been expanding, the commercial landings for several fish species in Narragansett Bay, RI, have declined significantly. Winter flounder landings in particular, have declined from over 8 million lbs/year in the early 1980’s, to about 1.3 million lbs/year during the late 1990’s, and suggested among the possible reasons for this decline is an increase in predation by an expanding seal population. We used bioenergetic models to estimate the potential impact on this fishery by seals. With a maximum population of approximately 1000 seals, each consuming 6% of its body mass per day, we calculated that seals would take 88,155-244,875 kg of prey per season (193,941-538,725 lbs/season). This take represents 0.15-0.4% of the total commercial annual landings for all fish species, suggesting minimal impacts on overall fish stocks. If seals were eating winter flounder exclusively, seal predation would be equivalent to 14-41% of the commercial and recreational take, however an exclusively flounder diet is highly unlikely, as in other nearby areas flounder makes up only 7-15% of a seal’s diet. We conclude that seals would play only a minor role in the observed decline in fish stocks.

 


DISTRIBUTION AND LOCALIZATION OF ESSENTIAL AND HEAVY METALS IN RELATION TO POLYAROMATIC HYDROCARBONS (PAHs) IN STRANDED SEAL TISSUE

Weaver, K. M., S. K. O'Shea, and P. M. Webb

This study not only demonstrates that seals can be used as bioindicators of the contaminants in the environment, but also creates a model of varying distribution and localization of contaminants in different tissues. which is thus far incomplete in literature. Marine mammals are the top of the trophic levels, and many of the contaminants have bioaccumulated in their food sources. As a result. these contaminants build up in the tissues of the, marine mammal. Such contaminants include the heavy metals cadmium, lead, and tin and organics like polyaromatic hydrocarbons (PAHs). Tissues and blood were analyzed for contaminants from several seals. A survey of other essential, nutrient metals, iron, copper, zinc. cobalt, Manganese, chromium, and vanadium, was conducted to find basal levels. Tissue and blood samples were analyzed for metals using acid digestion and flame atomic absorption and for PAHs using high pressure liquid chromatography. in line fluorescence, and UV/Vis detection.


HAUL-OUT AND SCANNING BEHAVIOR OF THE HARBOR SEAL (PHOCA VITULINA CONCOLOR) FROM ROME POINT, RHODE ISLAND

McKamey, K., and P. Webb

 

A haul-out site of harbor seals (Phoca vitulina concolor) was observed from Rome Point, Rhode Island. The stage of tide and number of seals hauled-out was analyzed. A statistically negative correlation between the number of harbor seals hauled-out and the time before or after low tide (P<0.05) was discovered, meaning that more seals hauled-out at low tide. Scanning behavior was also analyzed. As the group of harbor seals becomes larger, an individual seal reduces its scanning time without lessening its predator surveillance. The median time scanning for an individual harbor seal in a small group was 96% (s.d.=3.94), in a medium group was 62% (S.d.=24.32), and in a large group was 1% (s.d.=0.84). these results were also statistically significant (P<0.05).
 


REPRODUCTIVE ENERGETICS OF ADULT MALE NORTHERN ELEPHANT SEALS

Crocker, D.E., P.M. Webb and D.S. Houser

 

The energetics of breeding was investigated in 21 adult male northern elephant seals associated with three adjacent harems at Año Nuevo, CA. Mass and body composition were measured in January and again in March. Body composition was determined from water isotope dilution. Water flux was estimated from isotopic turnover. Mean measurement duration was 49 + 8 days. Movements, dominance interactions, dominance rank, and copulations were monitored over the measurement interval. The estimated number of females inseminated (ENFI) by each male was determined based on the proportion of observed copulations with marked females. Estimates of metabolism from metabolic water production were within a mean of 4% of those derived from changes in mass and body composition. On average adult males expended energy at a rate of 195 + 49 MJ/day which was equivalent to 3.2 times the standard metabolic rate predicted from Kleiber's equation. Males derived a mean of 7% of their metabolism from protein catabolism, with the remainder provided by lipids. Mean body composition late in breeding varied among ranks (F = 6.7, p < 0.01) with alpha males having lower proportions of body fat (16.3%) than others. Energy expenditure was significantly greater in alpha males (F = 11.5, p < 0.01). When controlled for metabolic body size, energy expenditure was also significantly different among ranks (F = 3.9, p < 0.05) being greater in alpha males than in peripheral males. The rate of energy expenditure was significantly related to the ENFI (r2 = 0.43, p < 0.05). Topography appeared to have an important impact on the energy expenditure of alpha males. The number of movements between harems was significantly related to energy expenditure in peripheral males (r2 = 0.74, p < 0.05). These data emphasize the importance of body reserves in the breeding strategies of male phocids.
 


EFFECTS OF BODY COMPOSITION ON THE BUOYANCY AND DIVING BEHAVIOR OF NORTHERN ELEPHANT SEALS

Webb, P.M., D.E. Crocker, D.S. Houser, D.P. Costa and B.J. Le Boeuf.

 

The deposition of blubber plays an important role in energy storage and thermoregulation in marine mammals, but the accumulation of large amounts of fat may also alter the buoyancy of a swimming animal. Northern elephant seals experience seasonal changes in body composition that should affect their buoyancy, and may also impact their diving behavior. This study examined the effects of buoyancy on the diving behavior of northern elephant seals, both through experimental buoyancy manipulation, and through natural changes in body composition. Pregnant adult female elephant seals departed the rookery for their post-molt migration with a body composition of 26.4 ± 2.8 % blubber, and returned 7-8 months later, having increased their blubber stores to 31.8 ± 1.2 % of body mass. The pregnant seals showed consistent changes in descent rates over the course of the migration that appear to be related to fluctuations in body composition, and therefore, buoyancy. Descent rates initially increased as the females deposited lean mass, then declined rapidly as blubber mass increased, perhaps in association with implantation of the blastocyst. Late in the migration, descent rates stabilized as the fetus developed, offsetting the buoyancy effects of continued blubber deposition. The changes in descent rate were not observed in a non-pregnant female, suggesting that they may be related to gestation. In both the natural and the experimental setting, descent rates appeared to be affected by buoyancy changes, but ascent rates remained relatively constant, suggesting that ascent rates are independent of buoyancy. These data indicate that changes in body composition play a role in the buoyancy and diving behavior of northern elephant seals, and that these buoyancy differences might be used to track the timing of implantation and gestational progress of seals at sea.

 


IMPACT OF THE 1997-98 EL NINO ON THE FORAGING BEHAVIOR OF NORTHERN ELEPHANT SEALS

Crocker, D.E., D.P. Costa, B.J. Le Boeuf, P.M. Webb and D.S. Houser.

 

The foraging behavior of northern elephant seals, Mirounga angustirostris, was examined during the 1997-1998 ENSO using ARGOS satellite tracking and time-depth recorders. Given their deep diving and spatial distribution, the response of northern elephant seals to a severe El Niño should provide important clues to both the scale and the impact of rapid ecosystem change on a large marine predator. Instruments were deployed on nine females in February and March 1998 at the Año Nuevo rookery in central California. Trip duration and rates of mass gain were compared to data from females carrying instruments in 1983-1996 and to data on diving behavior collected during 1990-1996. Tracks of females in 1998 covered much of the northeast Pacific Ocean and were typical of those observed in non-ENSO years. Females exhibited significantly fewer dives with bottom times than in previous years. The temporal pattern of dive shapes was significantly different from that seen in previous years. Rates of mass gain at sea for nine adult females was 0.29 + 0.36 kg/day in 1998, the lowest ever measured. One female lost mass at the rate of 0.44 kg/day. Whereas declines in the mass gain rate of females were pronounced in severe ENSO years such as 1998 and 1983, they were not evident in a moderate El Niño year such as 1986-87. The long-term data strongly suggest that females increase foraging trip duration to compensate for decreases in the rate of mass gain. Elephant seals had a harder time finding prey during the recent El Niño event and attempted to compensate by increasing foraging effort.  Moreover our data confirm that the impact of the 1997-98 ENSO was not limited to the nearshore coastal margin, but extended far out into the North Pacific Ocean in the mesopelagic zone.
 


SURFACE BREATHING RATE AND HEART RATE IN FREE-RANGING ELEPHANT SEALS

Le Boeuf, B.J., D.P. Costa, S.B. Blackwell, D.E. Crocker, P.M. Webb, J. Gedamke and J. Grayson.

 

Our aim was to measure respiratory rate (RR) and heart rate (HR) at the surface between dives in free-ranging northern elephant seals, Mirounga angustirostris. The need for diving mammals to replenish oxygen stores, adjust acid-base balance, and process excess metabolites at the surface constrains all activities. We recorded RR and HR acoustically in six translocated juveniles, 1.8 to 2.4 years old, and three migrating adult males from the Año Nuevo, California, rookery. To the dorsum of each seal we attached a diving instrument to record the diving pattern, a digital audio tape recorder with an external hydrophone to record sounds of respiration and HR at the surface, and a VHF transmitter to facilitate recovery. On average, adult males breathed 32.7 + 5.4 times at the rate of 15.3 + 1.8 breaths per min during the surface intervals averaging 2.2 + 0.4 min. Mean HR at the surface was 84 + 3 beats per min. The RR of juveniles was 26% faster than adult males, averaging 19.2 + 2.2 breaths per min for a mean total of 41.2 + 5.0 breaths during surface intervals lasting 2.6 + 0.31 min. Mean HR at the surface was 106 + 3 beats per min. RR and HR did not change significantly over the course of surface intervals. Surface variables were not clearly associated with measures of immediately previous or subsequent diving performance such as diving duration, diving depth or swim speed. There was no change in surface RR or HR as adult males moved from rapid horizontal transit to slow transit and apparent foraging. These results are consistent with previous reports that surface interval duration in this species is virtually invariable, even following extraordinarily long or deep dives. Elephant seals are like marathon athletes that manage their energy supplies aerobically to serve the long run.
 


IMPACT OF THE 1997-98 EL NINO ON THE FORAGING BEHAVIOR OF NORTHERN ELEPHANT SEALS, A LARGE MARINE PREDATOR

Crocker, D.E.,  D.P. Costa, B.J. Le Boeuf, P.M. Webb and D.S. Houser

 

The foraging behavior of northern elephant seals, Mirounga angustirostris, was examined during the 1997-1998 ENSO using a combination of ARGOS satellite tracking and time-depth recorders. Given their deep diving and spatial distribution, we expected ENSO events to have lesser impacts on elephant seals than other more coastal pinnipeds. Their response to severe El Niño's should provide important clues to both the scale and the impact of rapid ecosystem change on a large marine predator. Instruments were deployed on nine females in February and March 1998. Trip duration and rates of mass gain were compared to data from females carrying instruments in 1983-1996 and to data on diving behavior collected during 1990-1996. Rates of mass gain in 1998 were the lowest ever measured suggesting that foraging trip duration increased to compensate for decreases in mass gain. Tracks of females in 1998 covered much of the northeast Pacific Ocean and were typical of those observed in non-ENSO years. Elephant seals had a harder time finding prey during the recent El Niño event and attempted to compensate by increasing foraging effort.  As elephant seals forage in deep water far offshore, these data confirm that the impact of the 1997-98 ENSO was not limited to the nearshore coastal margin, but extended far out into the North Pacific Ocean. 


RESPONSES OF ELEPHANT SEALS TO ACOUSTIC THERMOMETRY OF OCEAN CLIMATE TRANSMISSIONS

Costa, D., D. Crocker, J. Gedamke, P. Webb, B. Le Boeuf, D. Waples, S. Hayes and J. Ganong

 

The hypothesis that northern elephant seals would respond to acoustic transmissions from the ATOC sound source was tested. Elephant seals were chosen because they have the best low-frequency hearing of any pinniped, are abundant, naturally migrate past the Pioneer Seamount, and are deep divers. ARGOS satellite tags provided information on animal location while at sea, while archival tags provided information on swim speed, time, depth, ambient acoustic environment, and ambient sound pressure levels. Instruments were deployed on 14 naturally migrating adult male elephant seals that were expected to swim near the source site, and upon 29 juvenile animals that were translocated and released 1 h prior to transmission. Measured mean intensity of ATOC exposure ranged from 120 to 135 dB for 60--90 Hz (n=6) compared to ambient levels of 100--107 dB (60--90 Hz). Animals did not alter return track, diving pattern, or swim speed, did not go to the surface, and often continued to dive closer to the sound source if on the descending segment of a dive. [Work funded by ONR and ARPA.]

 


EFFECTS OF THE CALIFORNIA ATOC EXPERIMENT ON MARINE MAMMALS

Costa, D., D. Crocker, D. Croll, D. Goley, D. Houser, B. Le Boeuf, D. Waples, P. Webb, and J. Calambokidis.

 

The effect of the California ATOC experiment on marine mammals has been studied using aerial surveys to observe changes in marine mammal abundance. Dramatic changes in behavior have not been observed while the source was operating. The following species have been sighted in the area humpback, sperm, gray, fin, pilot, and killer whales, Baird's and Cuvier's beaked whales, elephant seals, California sea lions, northern fur seals, Risso's, Pacific white-sided and northern right whale dolphins and Dall's porpoise. Satellite and archival tags were used to record the behavior of elephant seals as they passed by the ATOC sound source. Tracks of male elephant seals passing by the Pioneer Sea mount were closer on their return migration when the source was on than on their outbound path when the source was off. Acoustic data loggers were deployed on juvenile elephant seals that were released by boat near the ATOC sound source. These units collected data on diving and swimming behavior concurrently with recordings of the ATOC source sound pressure level at the animal.

 


FORAGING BEHAVIOR OF NORTHERN ELEPHANT SEALS USING TIME DEPTH RECORDERS COUPLED WITH ARGOS SATELLITE LOCATIONS

Costa, D.P., Crocker, D.E., Le Boeuf, B.J., and Webb, P.M.

 

Our aim was to determine the diving behavior, travel routes, destination and foraging strategy of northern elephant seals, Mirounga angustirostris, during biannual foraging trips from Año Nuevo, California, lasting 2.5 to 8 months. We attached archival depth recorders and ARGOS satellite linked transmitters to 14 adult males and 18 adult females during the years 1995 and 1996. The data confirm earlier studies, based on geolocation using light levels, that there is habitat separation by sex during foraging migrations. Males traveled rapidly (about 100 km/day) and directly to foraging areas along the North American continental margin from Puget Sound, north to the Queen Charlotte Islands, the northern reaches of the Gulf of Alaska, and as far west as the eastern Aleutian Islands. Once males reached their destination, they spent up to two months diving in localized areas, exhibiting a dive pattern suggesting foraging on benthic prey. Individual males returned to the same foraging location in the subsequent migration taking the same migratory path. In contrast, females meandered across a wide area of the northeastern Pacific Ocean from an eastern boundary of 122o W to a western boundary of 175o E and from a southern boundary of 37o N to a northern boundary of 58o N. Episodes of concentrated diving were not associated with specific bathymetric features and the diel pattern of diving suggested foraging on prey in the deep scattering layer. Like males, individual females took similar routes in both migrations but they were more variable. We conclude that males migrate to feed in specific locations associated with strong bathymetric features such as the continental shelf break; females forage on prey in the water column that occur in patches associated with oceanic currents or large eddies. This work was supported by research grants from the Office of Naval Research and the ATOC Program Scripps Institution of Oceanography ARPA.

 


SWIM SPEED AND FORAGING STRATEGIES OF THE NORTHERN ELEPHANT SEAL

Crocker, D.E., Costa, D.P., Le Boeuf, B.J., Webb, P.M., and Blackwell, S.B.

 

Our objective was to elucidate the foraging strategies of male and female northern elephant seals, Mirounga angustirostris, during their biannual foraging migrations in the North Pacific. We used swim speed measurements and location data from ARGOS linked satellite tags to: 1) Shed light on the functions of the four major dive types found in elephant seal diving records; 2) Test the predictions that males utilize a 'sit-and-wait' foraging strategy or a 'moving-on-bottom' foraging strategy in pursuing benthic prey near continental margins; and 3) Describe sex differences in the swim speed and temporal patterning of foraging in male and female elephant seals. Instruments were attached to the back and head of 6 adult female and 5 adult male elephant seals before they departed the Año Nuevo rookery in central California in 1995 and 1996 following breeding or molting haul-outs. Instruments were recovered when the animals returned to Año Nuevo 78-112 days later. Males exhibited significantly higher swim speeds than females during the initial 3-4 weeks of the migration, moved continually when not at the surface and made fewer dives with bottom times during this period. Subsequently, males exhibited both active swimming dives that appeared to follow bottom topography as well as dives with extended periods without movement at the deepest part of the dive. Males appear to frequently utilize a sit-and-wait foraging strategy while foraging on the continental margin. In contrast, females always moved continually while diving, exhibiting long series of mesopelagic dives with a bottom time characterized by rapid changes in swim speed. These dives occurred every day after the first few days at sea and appeared to follow diurnal migrations of the deep scattering layer. These data are consistent with satellite location data which demonstrate a rapid transit phase for males followed by extensive localized foraging on the continental slope and slower, more consistent rates of pelagic movement in females. Both males and females exhibited periodic 'drift dives', with a dramatic reduction in swim speed during descent. These data provide further support for the putative functions of the four major dive types and sex differences in the foraging strategies of male and female elephant seals.

 


FORAGING BEHAVIOR AND HABITAT UTILIZATION OF NORTHERN ELEPHANT SEALS

Costa, D.P., D.E. Crocker, B.J. Le Boeuf and P. Webb

 

Our aim was to determine the diving behavior, travel routes, destination and foraging strategy of northern elephant seals, Mirounga angustirostris, during biannual foraging trips from Año Nuevo, California, lasting 2.5 to 8 months. We attached archival depth recorders and ARGOS satellite linked transmitters to 14 adult males and 18 adult females during the years 1995 and 1996. The data confirm earlier studies, based on geolocation using light levels, that there is habitat separation by sex during foraging migrations. Males traveled rapidly (about 100 km/day) and directly to foraging areas along the North American continental margin from Puget Sound, north to the Queen Charlotte Islands, the northern reaches of the Gulf of Alaska and as far west as the eastern Aleutian Islands. Once males reached their destinations, they spent up to two months diving in localized areas, exhibiting a dive pattern suggesting foraging on benthic prey. Individual males returned to the same foraging location in the subsequent migration taking the same migratory path. In contrast, females meandered across a wide area of the northeastern Pacific Ocean from an eastern boundary of 122o W to a western boundary of 175o E and from a southern boundary of 37o N to a northern boundary of 58o N. Episodes of concentrated diving were not associated with specific bathymetric features and the diel pattern of diving suggested foraging on prey in the deep scattering layer. Like males, individual females took similar routes in both migrations but they were more variable. We conclude that males migrate to feed in specific locations associated with strong bathymetric features such as the continental shelf break; females forage on prey in the water column that occur in patches associated with oceanic currents or large eddies.


GLOMERULAR FILTRATION RATE IN LACTATING NORTHERN ELEPHANT SEALS

Webb, P.M., Crocker, D.E., Costa, D.P., and Le Boeuf, B.J.

 

Previous investigations of lactation energetics in female northern elephant seals, Mirounga angustirostris, suggest potential physiological constraints on reproductive effort in this species. Fasting in non-lactating elephant seals is characterized by effective protein sparing and reduced glomerular filtration rate (GFR). In lactating females, urea turnover increases across lactation without a corresponding change in serum urea concentrations. This may be indicative of changes in GFR. GFR was measured in five adult females during mid- and late lactation from the clearance of a single injection of [3-H] inulin. Mean absolute GFR increased dramatically across lactation, from 167.8 to 420.3 ml/min. Mass-specific GFR increased 115 + 41% between measurements, from 0.49 + 0.09 ml/min-kg for mid-lactation females to 1.03 + 0.12 ml/min-kg for late-lactation females, a highly significant difference (t = 8.07, p < 0.01). Comparisons with predicted mammalian GFR based on allometric equations showed that GFR was 50 + 7.9% of the predicted value in mid-lactation females, and 97.3 + 12.4% of the predicted GFR in late-lactation. That this important adaptation for fasting water conservation is modified over lactation supports the idea of physiological constraints on reproductive effort in this species.

 


HOLD YOUR BREATH AND STAY FAT: AN ESTIMATE OF THE WATER SAVINGS FROM TERRESTRIAL APNEA IN NORTHERN ELEPHANT SEALS

Blackwell, S.B., Le Boeuf, B.J., Crocker, D.E., and Webb, P.M.

 

We tested the hypothesis that northern elephant seal weanlings reduce evaporative water loss by breathing irregularly during the 2.5 months on land before going to sea. We estimated time per day in apnea for 17 free-ranging weanlings from gross changes in heart rate measured by attached data loggers; we recorded resting breathing rate from observations of 15 known-age weanlings sleeping on the beach; and we measured average water loss per breath from 8 weanlings placed in a small metabolic dome from which all expired air was collected and its water content determined, while simultaneously recording breathing activity. On average, weanlings spent 6.2 out of 24 hrs in terrestrial apnea. Resting breathing rate decreased by over 50% between weaning and the end of the post-weaning fast, when it reaches about 7 bpm. Water loss per breath averaged about 50 mg. This study shows that in the absence of irregular breathing on land, the seals would have had to increase their metabolic rate by at least 20% to stay in water balance, potentially reducing the duration of their stay on land by the same percentage. We conclude that terrestrial apneas are an efficient mechanism for water and energy economy in fasting elephant seals.

 


DYNAMICS OF GLOMERULAR FILTRATION IN NORTHERN ELEPHANT SEAL PUPS (MIROUNGA ANGUSTIROSTRIS) DURING SUCKLING AND FASTING PERIODS

Houser, D.S., Crocker, D.E., Webb, P.M., and Costa, D.P.

 

In water stressed mammals regulation of glomerular filtration is often associated with the simultaneous maintenance of homeostatic water balance and excretion of metabolic wastes. To assess the dynamics of glomerular filtration in developing northern elephant seal pups standard [3-H]inulin clearance techniques were employed during suckling and fasting periods. Glomerular filtration rates (GFR) and extracellular volumes (ECV) were determined in five pups (3M, 2F) at 9 and 20-23 days post-parturition and 2 and 8 weeks into the post-weaning fast. Neither absolute GFR nor ECV significantly changed throughout development but mass specific GFR dropped from a neonate mean of 1.40 + 0.77 ml/min/kg to 0.71 + 0.08 ml/min/kg at the end of lactation and rose slightly by late in the fast. Absolute GFR was not significantly related to mass or ECV. Mass specific and ECV specific GFR were significantly related to mass (r2 = 0.33, p = 0.008; r2 = 0.37, p = 0.004) and a correlation between the means of the dependent variables was high (r = 0.97). Fluctuations in mass specific GFR inversely followed known patterns of adipose deposition and depletion and, with respect to suckling, may be related to exponential decreases in the water composition of maternal milk through lactation. These data suggest that GFR is regulated as a function of differential body stores and resultant levels of interstitial fluids and cannot be explained by linear models alone. More importantly depression of glomerular filtration rates occurs before weaning and entrance into the fasting stage, possibly pre-adapting kidney function to the fasting condition.

 


HEART RATE AND OXYGEN CONSUMPTION IN ELEPHANT SEALS DIVING IN THE LABORATORY

Webb, P.M., Andrews, R.D., Le Boeuf, B.J., and Costa, D.P.

 

Our aim was to investigate the relationship between heart rate and oxygen consumption (VO2), to determine whether heart rate could be used as an indicator of metabolic rate. Six juvenile elephant seals, Mirounga angustirostris, were allowed to dive freely in a metabolic chamber at Long Marine Lab, Santa Cruz, California, while simultaneous measurements of heart rate and VO2 were made. The seals spent an average of 74% of the time under water, with a mean dive duration of 6.43 + 0.6 minutes. Mean VO2 during diving was 3.32 + 0.4 ml/min/kg, a decrease of approximately 26% from resting values. Heart rate during dives was on average 36% lower than resting values, and was independent of dive duration, percent time submerged, or VO2. Heart rate while breathing increased slightly with increases in the preceding dive duration, but was not correlated with VO2. Mean heart rate, measured during a dive and the following breathing interval, increased with increasing VO2, and was inversely correlated with percent time submerged. An inverse relationship was also observed between VO2 and percent time submerged. These results are consistent with other studies supporting the hypothesis that diving is not energetically costly, however, preliminary analysis leads to the conclusion that heart rate is not an accurate method for determining metabolic rate in this species.