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The Secret Life of Marine Mammals

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The Secret Life of Marine Mammals FEATURE THE SECRET LIFE OF MARINE MAMMALS NOVEL TOOLS FOR STUDYING THEIR BEHAVIOR AND BIOLOGY AT SEA By Daniel P. Costa Given the interest by both the lay and sci- on the diving performance of marine mammals entific communities, it is surprising how little is (Kooyman 1989: Costa 1991a,b). Typically these known about the open ocean ecology of marine devices are attached to the animal (using a harness mammals. Their patchy distribution and low or glue; Fig. 1), and the data are retrieved when abundance leads to infrequent encounters at sea. the device is recovered after recapture of the ani- Most of our information on marine mammals has mal. The need to recapture the animal limits this been obtained from shipboard or aerial observa- system, but it has been used quite successfully on tions, which provide a very limited perspective studies of pinnipeds (sea lions, fur seals, and on their life at or near the surface, with little in- seals). An example of a time-depth record obtained sight into their behavior under the water where for an Australian sea lion is shown in Figure 2. they spend up to 90% of their time. Recent ad- A summary of the diving data obtained indicates vances in technology are providing an opportu- that the "true" or earless seals (Phocidae) are capa- nity to gain new insights into the underwater ble of deeper longer dives than sea lions and fur lives of marine mammals. Preliminary informa- seals (Otariidae: Fig. 3). These differences reflect tion indicates that marine mammal distribution optimization for exploitation of different habitats and resources (Costa, 1993). Sea lions and fur seals • . technologies and abundance is highly correlated with oceanic features like frontal systems, thermocline depth, feed on vertically moving prey that approach the now available on bathymetry, eddies, jets, and warm core rings. surface, whereas true seals feed on sedentary ben- thic or midwater prey. The considerable difference under development These features concentrate or aggregate prey, between mean dive depth/duration and maximum permitting effective predation. The technologies dive depth/duration suggests that seals and sea lions will enable an now available or under development will enable rarely reach their physiological limit with respect to an examination of how oceanic features and examination of how dive depth or duration. Other factors such as prey processes affect marine mammal biology. This type, depth, and availability determine the optimal oceanic features and article summarizes some of the novel technolo- diving pattern, and prey availability is directly re- gies that are currently available or under devel- processes affect lated to oceanic features and processes. opment. It is hoped that an increased awareness Coordination of data on diving pattern with in- marine mammal of the research potential of these approaches will formation on prey species indicates that prey size, stimulate interdisciplinary collaborations between biology. behavior, and energy content influence the forag- marine mammal biologists, fisheries biologists, ing pattern employed (Costa, 1991a,b). The most and physical and biological oceanographers. detailed study of foraging behavior carried out on Recoverable Data Loggers Antarctic fur seals (Croxall et al.. 1985), showed Recoverable multichannel digital data loggers that these seals made most (75%) of their dives at are available with variable sampling rates that night, when dives were consistently shallower record depth, light level, water temperature, swim (<30 m) than dives during the daytime (mostly velocity, and surface location. The first instruments 40-75 m). This pattern closely followed the verti- to be deployed measured dive depth as a function cal distribution of krill, which during daylight of time and have provided a wealth of information hours was below a depth of 50 m and at night was present in substantial quantities above 50 m. Even though >40% of the krill was below 75 m depth at all times of the day, fur seal dives seldom (3%) D.P. Costa, Life Sciences Department, Office of Naval Re- exceeded this depth. The authors concluded that search, 800 N. Quincy St.. Arlington VA 22217 and Biology krill are captured only from shallow depths, since Department, University of California, Santa Cruz, CA 95064, USA. this is when they are most efficiently obtained. 120 OCEANOGRAPHY*VoI.6, NO. 3"1993 Fig. 1: An Australian sea lion female with a Wildlife Computers time-depth recorder glued to the hair on her back. In a similar study it was found that female squid to move into shallow water before preying northern fur seals exhibit distinct diving patterns on them. specific to the type of prey consumed. During a The most impressive data obtained so far on foraging trip (typically lasting 7 days), an indi- pinniped diving behavior come from studies of vidual northern fur seal female exhibited one of northern and southern elephant seals (Le Boeuf the following dive patterns: those composed ex- and Laws, 1994). Northern elephant seals dive clusively of deep dives with a mean depth of 185 continuously, day and night, for the entire trip to m, those composed exclusively of shallow dives sea, which lasts between 2 and 8 months (Fig. Deep-diving with a mean depth of 50-60 m, and those with a 4). Two-month trips are associated with females mixture of both deep and shallow dives (Gentry who have just completed lactation and are re- northern fur seals did and Kooyman, 1986). Deep-diving northern fur turning to molt, whereas the 8-month foraging not exhibit diurnal seals did not exhibit diurnal fluctuations in div- trip follows the molting period and is when ges- ing depth, which implied that they were feeding tation occurs. While at sea, they spend 90% of fluctuations in diving on demersal or benthic species, whereas shallow their time underwater, with dives averaging 20 depth .... divers exhibited a striking diurnal fluctuation in minutes (maximum dives in excess of ~ hr) fol- diving pattern quite similar to that observed for lowed by surface intervals of less than 3 min- Antarctic fur seals eating krill. A subsequent utes. Their diving patterns follow a diurnal cycle study using a combination of time-depth recorder with the deepest dives occurring during the day and VHF telemetry determined that deep-diving and shallowest at night. Modal dive depths are fur seals feed on demersal fish such as pollock 300-600 m with maximum dives exceeding on the Bering Sea shelf, whereas shallow-diving 1,500 m! fur seals feed on vertically migrating squid over Recently, the track of elephant seals during deep water beyond the Bering Sea shelf (Goebel these foraging trips have been determined by mea- et al., 1991). Like krill, squid are available surements of light level and water temperature throughout the day, and like krill-consuming while they are at the surface. Location is calculated Antarctic fur seals, northern fur seals wait for by extrapolation of light levels to provide time of OCEANOGRAPHY'Vo1.6, NO. 3"1993 121 20:00 . _ . 24:00 northeastern Pacific, as far as 150°W (about due north of the Hawaiian Islands), in the range 8E 4 44-52°N (De Long et al., 1992) (Fig. 5). Although these results are impressive, they B0 L_ _ I ._ • O0 O0 04.00 have yet to be integrated into models that relate animal location and behavior with oceanographic phenomena such as frontal systems, eddies, up- welling zones, warn] core rings, and thermoclines. 04.00 O0 However, one recent study on southern elephant seals used a time-depth recorder that incorporated data on water temperature (Boyd and Arnbom, 08 O0 1991). An elephant seal was observed to descend rapidly to the discontinuity between cold surface water and warmer deep water. The animal spent a • . marine mammal substantial amount of its time (57%) at or near the 12 O0 thermocline indicating the likelihood that it was distribution, foraging there. Further, the structure of the water abundance, and mass indicated that the animal was foraging south of the Antarctic Polar Front. Ultimately marine behavior is related to 16 O0 mammal distribution, abundance, and behavior is the oceanographic related to the oceanographic factors that determine prey distribution. Our understanding of this rela- factors that determine 2O oo tionship is limited and is thus a fertile area for in- prey distribution. vestigation. Significant insights have been gained from data on time and depth and additional sensors are being oo'oq 04 o0 incorporated into the devices. For example, inves- tigators are looking for ways to incorporate pre- cise information on location derived from the Global Positioning System Satellite (GPS). Equip- ment is being designed and tested that would allow the data logger to be released from the ani- I I I mal upon command from a radio transmitter, or 08:00 . -2:00 permit stored data to be recovered via short-range radio or acoustic telemetry. Data loggers currently available can record at-sea location (within 40 P 12:00 16:00 km), dive frequency and profile, swim velocity, heart rate, and water temperature. Other sensors that can measure and record information on ambi- ............... ...... ' ..... t I I .__ | ent acoustic environment, ocean structure (salin- ity), and feeding behavior are being tested or are Fig. 2: The diving pattern (dive depth versus time) under development• over a complete 42-h foraging trip obtained.from an Australian sea lion female (from departure to return to shore). Time is divided into 4-h blocks Dive duration (min) and depth extends j~kom 0 to 80 meters. 10 100 ,o ......... 100 sunrise, sunset, and local apparent noon. Latitude 300 A is calculated from day length, and longitude from 500 .£ local apparent noon as referenced to Greenwich Jc 700 mean time (DeLong et al., 1992)• Previously, it had o been assumed that northern elephant seals existed 900 in offshore waters from California to British Co- 1100 00tariids lumbia.
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