Foraging Energetics and Diving Behavior of Lactating New Zealand Sea Lions, Phocarctos Hookeri

The diving patterns of freely diving, air-breathing vertebrates result from an interaction between the animal’s behavior and its physiological capability (Castellini et al., 1985; Kooyman, 1989; Ponganis et al., 1993). The resulting diving behavior, while being constrained by physiological and anatomical adaptations, generally reflects the distribution, abundance, depth and energy content of the prey (Costa, 1991a,b, 1993a).

Remote recorders have resulted in a wealth of information on the diving behavior of freely living marine mammals and seabirds (Kooyman, 1989; Costa, 1993b; Boyd and Croxall, 1996). These data reflect a wide range of foraging strategies among this group and have led to a discussion of the primary determinants of these strategies (Kooyman et al., 1980, 1983; Kooyman, 1989; Costa, 1993b; Boyd and Croxall, 1996). The diving capability of a marine mammal is determined by its total metabolic stores and the rate at which they are used (Kooyman, 1989). These metabolic stores can be separated into aerobic and anaerobic components. The aerobic component of diving metabolism is thought to be the major determinant of diving ability (Kooyman et al., 1980, 1983; Kooyman, 1989; Ponganis et al., 1993, 1997). The aerobic dive limit (ADL) is experimentally defined as the diving duration beyond which blood lactate levels increase above resting levels.

Lactating New Zealand sea lions, Phocarctos hookeri, have been reported to be the deepest- and longest-diving otariid (Gales and Mattlin, 1997). They typically dive for 3.9 min (median 4.3 min) and routinely attain depths of 123 m, with a maximum depth of 474 m. Some females routinely dive to depths of 187 m, with mean dive durations of 4.7 min. This diving pattern is characterized by almost continuous dives, most of which are to the benthos. Unexpectedly, an unusually high proportion (44 %) of dives exceed the theoretical ADL. Aerobic diving is the most efficient way to dive because it provides the greatest cumulative submerged time and the shortest inter-dive interval (Kooyman, 1989; Houston and Carbone, 1992; Ydenberg and Clark, 1989).

Gales and Mattlin (1997) hypothesized that the New Zealand sea lion is physiologically better equipped for prolonged, continuous diving than other otariids and/or that the availability of prey is such that the diving behavior reflects the sea lions’ physiological limit in a marginal foraging environment. It was subsequently found that New Zealand sea lions have the largest blood volume, and therefore oxygen-storage capacity, of any otariid, but it was still insufficient to account for the long-duration dives typical of this species (Costa et al., 1998). Measurements of swimming velocity in diving New Zealand sea lions showed that deep and shallow divers had similar surface intervals and that deep divers had higher descent rates than shallow divers. Further, these profiles indicated that deep divers employed burst-and-glide locomotion and that these animals were gliding over a substantial portion of the descent phase of the dive (Crocker et al., 2000). The goal of the present study was to determine whether New Zealand sea lions are physiologically better equipped for prolonged, continuous diving than are other otariids. Here, we test the hypothesis that the diving ability of New Zealand sea lions is in part due to a reduction in oxygen utilization during diving that would be reflected by a lower at-sea metabolic rate.

Fieldwork was conducted at Sandy Bay, Enderby Island, Auckland Islands Group, New Zealand (50°30′S, 166°17′E) during January and February 1997. Enderby Island is the site of the second largest of the Phocarctos hookeri (Gray) rookeries (approximately 400–500 pups per year) (Gales and Fletcher, 1999) and is the best site for capture and eventual recapture of lactating females. Measurements of foraging energetics and diving behavior were completed on females during their summer breeding season when their pups were between 1 and 2 months old. Metabolic rate (rate of CO₂ production) and water turnover were measured on 12 females during periods when they were fasting onshore suckling their pups and foraging at sea using the ¹⁸O doubly-labeled water method (Lifson and McClintock, 1966; Nagy, 1980; Nagy and Costa, 1980; Costa, 1987; Speakman, 1997).

The selection of experimental animals was based on the females appearing healthy, being seen to be suckling a pup and representing a range of body masses. Pup sexes were represented equally in the experimental group. Captures were made using a specially designed hoop net (Furhman Diversified, TX, USA) constructed from soft, strong, knotless mesh with a multi-layered head end to reduce the vision of the animal (but still allow free breathing) and a hole at the apex of the net through which the sea lion’s nose protruded. Once in the net, the net handle was removed, the sea lion was physically restrained by two handlers, and a rubber mask was placed over its nose. The anesthetic gas Isoflurane was delivered with oxygen to the mask via a field portable vaporizer (Acoma MK III, Japan), and the sea lion was usually anesthetized within 2 min (Gales and Mattlin, 1998). The sea lion was then placed on a restraint frame and weighed using a 250 kg capacity digital scale (Dyna-Link MSI-7200, Measurement Systems International, Seattle, WA, USA) suspended from an aluminum tripod to within ±0.2 kg. Standard body length and girth were measured to the nearest centimeter. Once restrained, blood (20 ml) was drawn from the lateral gluteal vein for measurement of isotope background specific activity, followed by an intraperitoneal injection of 140 ml of sterile saline that consisted of 10 % H₂¹⁸O and 4.92 MBq of tritiated water (HTO). Females were then released and recaptured 3–4 h later, when a 20 ml blood sample was collected to measure equilibrated isotope concentrations. In most cases, females were recaptured twice over the following 10 day period, as soon as practical after returning from a foraging trip, and a 20 ml blood sample was taken and body mass recorded. The presence or absence of an individual at the colony was determined from a VHF radio transmitter glued to the pelage of the sea lion using standard telemetry techniques.

Simultaneously with the isotopic measurements, time/depth recorders (TDRs) were deployed on the females to assess diving behavior. Two types of TDR were used; MK3e (N=3) and MK6 (N=9) (Wildlife Computers, Redmond, WA, USA). The MK3e TDRs are housed in a 149 mm long, 26.5 mm diameter titanium tube and weigh 136 g. They are able to record depths of up to 1000 m. The MK6 TDRs are encased in an ovoid plastic housing 69 mm×57 mm×35 mm, weigh 80 g and can measure depth to either 250 m or 500 m. Prior to deployment, each piece of equipment was glued to a piece of neoprene material cut to the same size as the unit’s base. This wet-suit base was then glued to the dorsal pelage of the sea lion using a two-part epoxy glue (Araldite 2017, Ciba-Geigy, Cambridge, UK). Because of the shape of the MK IIIe TDRs, they could not be glued but were anchored to the neoprene with three pre-sewn plastic cable ties and two stainless-steel hose clamps. The TDRs were set to sample every 5 s. The dive records were first normalized using Wildlife computers Zero Offset Correction program, and the subsequent dive record was analyzed using Wildlife Computers Dive Analysis software. A dive had to exceed 2 m in depth to qualify as a dive, and only dives greater than 6 m were used in the analysis.

Total body water (TBW) was determined by the initial dilution of H₂¹⁸O (Nagy and Costa, 1980). Final TBW was calculated as the final mass of the animal multiplied by the initial TBW/mass ratio. The specific activity of tritium was determined by scintillation spectrometry of triplicate samples of 300 μl of pure water (distilled from the serum samples) in 10 ml of Ecolight cocktail (ICN Costa Mesa, CA, USA) (Ortiz et al., 1978). The specific activity of H₂¹⁸O was determined by mass ratio spectrometry (Metabolic Solutions, Nashua, NH, USA) of pure water distilled from serum samples. CO₂ production was calculated using equation 3 of Nagy (1980), which assumes an exponentially changing body water pool. This equation does not include corrections for isotopic fractionation effects; no equations are available for correcting simultaneously fractionation effects and changing water volumes. For comparison, CO₂ production was also calculated using the two-pool model of Speakman (1997). Water influx was calculated using equations 5 and 6 of Nagy and Costa (1980) assuming an exponentially changing body water pool. Given that the diet of New Zealand sea lions is not well known, we used a conversion factor from California sea lions, Zalophus californianus, of 23.6 kJ l⁻¹ CO₂ for converting CO₂ production to energy consumption. This assumes that all the fat and protein contained in the diet are oxidized and that the diet is composed of 80.3 % water, 1.3 % fat and 16.7 % protein and that the energy content is 3.75 kJ g⁻¹ (Costa et al., 1990).

Metabolic rate measurements were completed on all 12 animals that had variable periods at sea and onshore. Mass changes, body water, metabolic rate and water influx data for these animals are presented in Table 1. Values were on average 13.6 % lower when FMR was calculated using the two-pool equation of Speakman (1997) than using the equation of Nagy (1980). Time spent at sea and onshore, FMRs calculated for animals while at sea and dive data are provided in Table 2.

Ten complete and one partial diving record were obtained from the 12 sea lions, yielding a total sample of 11 341 dives. One unit stopped collecting data after 2 days at sea (E-55), and another unit detached from the animal while it was at sea (E-39). One animal (E-40) reached the maximum resolution (236 m) of the attached TDR on 5 % of its dives, and data on dive depth for this animal were therefore deleted from the analysis. The mean maximum dive depth was 353±164 m, with a mean diving depth of 124±36 m. The deepest single dive was 550 m and the longest was 11.50 min, both by the largest animal in our study E-59. The mean maximum dive duration was 8.3±1.7 min, with an average duration of 3.4±0.6 min. The dive data for each animal are provided in Table 2, along with concurrently collected at-sea metabolic rates.

A significant limitation of using FMR measurements derived from doubly-labeled water is that they integrate the time/energy budget over the entire trip to sea. Therefore, the only comparable behavioral data are integrated metrics such as mean dive depth, mean duration, etc. For this reason, the analysis of diving behavior was limited to mean indices of behavior over the entire trip to sea. A more complete analysis of the diving behavior of this species can be found in Gales and Mattlin (1997) and Crocker et al. (2000). Individual sea lions exhibited different diving patterns in terms of mean dive depth, duration and indices of diving effort (Table 2). Dive duration increased significantly with dive depth (r²=0.46, P=0.032) (Fig. 1). While there was no significant correlation between dive depth and time spent at the bottom of a dive (r²=0.03, P=0.65) (Fig. 1), the mean surface interval increased significantly as animals made deeper dives (r²=0.66, P=0.005) (Fig. 2A). The mean surface interval was more dependent on mean dive depth than on either dive duration (r²=0.04, P=0.56) or bottom time (r²=0.20, P=0.19). The deeper the dive, the more time spent on the surface with significantly less time at sea spent diving (r²=0.45, P=0.006) (Fig. 2B). Surprisingly, body mass had little effect on diving behavior: there was no significant relationship between body mass and mean dive depth (r²=0.20, P=0.44), duration (r²=0.02, P=0.66), bottom time (r²=0.01, P=0.78) or mean surface interval (r²=0.28, P=0.10).

Given that deep divers spent more time at the surface with less time diving, it is not surprising that mean dive rate decreased with increasing dive depth (r²=0.88 P<0.001). However, there was no significant relationship between dive rate and metabolic effort in terms of at-sea FMR (r²=0.09, P=0.38). An index of dive effort that takes into account both the rate and absolute depth of the dive can be calculated from the rate of vertical distance covered in the water column as a function of time (Arnould et al., 1996). However, this metric is only applicable to divers that make spiked dives, which are dives that have no bottom time and no horizontal swimming. As New Zealand sea lions spend a considerable time at the bottom of a dive, we estimated the horizontal distance covered at the bottom of the dive by multiplying the bottom time by the mean swimming speed. New Zealand sea lions have a mean swimming speed of 1.64 m s⁻¹ at the bottom of a dive (Crocker et al., 2000). Using this index, there was no significant relationship between dive effort and dive depth (r²=0.20, P=0.19) (Fig. 3A).

Given the differences in diving behavior between deep- and shallow-diving sea lions, we might expect differences in their rate of energy expenditure. The relationship between mass-specific FMR and mean dive depth for all sea lions was not significant (r²=0.26, P=0.078; Fig. 4A), but it was highly significant (r²=0.71, P=0.003) when the data from one of the smaller females (E-61) with the highest FMR was removed from the analysis. This point fell outside the 99 % confidence interval for the regression and can be considered an outlier. In contrast, there were no correlations between FMR and dive effort (r²=0.22, P=0.17), dive rate (r²=0.09, P=0.38), bottom time (r²=0.14, P=0.30), dive duration (r²=0.1, P=0.35) or mean surface interval (r²=0.24, P=0.13). However, there was a significant correlation between FMR and the proportion of time spent at sea diving (r²=0.38, P=0.042) (Fig. 4B).

The most striking observation relevant to the diving physiology of New Zealand sea lions is the profound role of dive depth on dive pattern and foraging energetics. Our data indicate that the mean surface interval increased as the mean dive depth increased (Fig. 2A), while the overall amount of time spent diving declined (Fig. 2B). There are a number of possible explanations for this observation. One is that as sea lions dive deeper they are unable to maintain aerobic metabolism and must incorporate some component of anaerobic metabolism during the dive, thus increasing mean surface time. When Weddell seals Leptonychotes weddelli exceed the aerobic threshold, the post-dive surface interval increases disproportionately faster than dive duration (Kooyman et al., 1980, 1983). This greater surface interval is required to clear the lactic acid accumulated during the dive.

A disproportionate increase in mean surface interval relative to mean dive duration in New Zealand sea lions is consistent with a requirement to spend more time on the surface to clear lactic acid. One of the disadvantages of anaerobic metabolism during diving is that, while a diver may increase the duration of a single dive, the total accumulated time spent under water is reduced because the animal must spend proportionately more time at the surface clearing lactic acid. New Zealand sea lions follow this prediction: the ratio of mean surface interval to mean dive duration increased with increasing dive depth (r²=0.47, P=0.029) (Fig. 5).

It is also possible that deep divers spend more time on the surface for reasons unrelated to their diving metabolism. If deep-diving New Zealand sea lions eat large prey, they might need to return to the surface to consume them, whereas shallow divers could feed on small prey, consuming them entirely under water. Differences in the foraging behavior of diving birds and mammals have been observed for prey that are located at different depths in the water column (Costa, 1991a).

An important component of the dive response is a reduction in metabolic rate, which allows more efficient utilization of oxygen reserves (Scholander et al., 1942). Our initial hypothesis was that New Zealand sea lions would have a reduced diving metabolic rate and that this would result in a correspondingly lower at-sea FMR. However, the at-sea FMR of New Zealand sea lions was 5.8 times the predicted basal rate of a terrestrial animal of equal size, well within the 4.8–7 times BMR previously reported for Australian sea lions Neophoca cinerea, California sea lions, Northern fur seals and Antarctic fur seals (Fig. 6) (Costa, 1993a). This result leads us to reject the hypothesis that New Zealand sea lions have a greater diving capacity because of reductions in their diving metabolic rate. Given the variability in FMR observed for this species, one might predict that longer-diving sea lions would exhibit a reduced at-sea FMR. The data do not support this hypothesis: there was no correlation between FMR and dive duration (r²=0.1, P=0.35) or bottom time (r²=0.14, P=0.30). This does not preclude variations in metabolic rate that may have occurred within individual dives, but it does preclude a consistent reduction in metabolic rate as a means of increasing mean dive duration.

The time spent at the bottom of a dive was independent of depth in New Zealand sea lions. This was achieved by an increase in the mean duration of dives (Fig. 1) coupled with an increased rate of descent in deeper dives. Measurements of swimming speed using swim velocity/depth recorders on New Zealand sea lions confirmed that deep divers descend faster than shallow divers (Crocker et al., 2000). How, then, is it possible for deep-diving sea lions to descend faster during a dive and yet maintain a lower metabolic rate? New Zealand sea lions employ a burst-and-glide form of locomotion (Crocker et al., 2000). At the start of the dive, they use their pectoral flippers and accelerate rapidly. After an initial series of strokes, they then glide and increase speed as they descend. An example of this diving pattern can be seen in Fig. 7. This pattern was confirmed by attaching a video camera to the back of freely ranging sea lions. The video images clearly showed an initial period of powered swimming followed by free gliding until the animal reached the bottom (N. J. Gales and B. Buhlier, unpublished data). A lower metabolic rate would be consistent with deep-diving sea lions spending a greater proportion of their dive gliding, whereas shallow divers may spend more time actively swimming. A biomechanical analysis suggests that gliding during descent, along with the associated changes in lung volume, allows bottlenose dolphins Tursiops truncatus to conserve energy during deep dives (Skrovan et al., 1999). Similarly, Williams et al. (2000) found that four species of marine mammal diving deeper than 80 m increased their dive duration by prolonging the gliding phase during the descent. They also found that Weddell seals reduced their rate of oxygen consumption by between 9.2 and 56.9 % by increasing the gliding phase during the descent.

An alternative hypothesis for the lower FMR of deeper-diving sea lions is that deep divers are not working as hard, as indicated by the lower proportion of time at sea spent diving together with the greater time spent at the surface (Fig. 2). However, our data do not support this hypothesis. Even though deep divers make fewer dives per unit time (Fig. 3B), their diving effort is similar to that of shallow divers (Fig. 3A). This is indicated by a lack of correlation between FMR and dive rate (r²=0.09, P=0.38), dive effort (r²=0.22, P=0.17) or time at the surface (r²=0.24, P=0.13). In contrast, there was (with the one outlier animal removed) a highly significant correlation between FMR and mean dive depth (Fig. 4A, r²=0.71, P=0.003). These observations lend further support to the hypothesis that deeper-diving individuals have a decreased FMR as a result of using an efficient form of locomotion that relies on an initial burst followed by gliding to depth.

It is important to note that at-sea FMR is an average rate that integrates the variable costs of surface swimming, diving and resting, and the actual diving metabolic rate could therefore be higher or lower. The FMR, O₂ stores, calculated (cADL), mean dive duration and the ratio of observed dive duration to the cADL are presented in Table 3. These data support the hypothesis that individual animals exhibit different physiological responses while diving. For example, sea lions E-59, E-55, E-52 and E-41 carried out dives that were very close to the cADL, while other individuals, E-40, E-43 and E-61, routinely exceeded their cADL in one animal (E-40) by 2.6-fold. Further inspection of the data in Table 3 reveals that as dive duration and bottom time increase so does the ratio of observed dive duration to cADL (r²=0.62, P=0.004 for dive duration; r²=0.54, P=0.016 for bottom time) (Fig. 8), whereas there is no relationship between this ratio and mean dive depth (r²=0.07, P=0.47). This is surprising because it does not support the hypothesis that deep diving is achieved by incorporating anaerobic metabolism. Further, it implies that differences in the diving pattern of shallow and deep divers were due to differences in the type of prey consumed, which may vary with respect to size, energy content and/or how or where the prey was processed (handling time).

The efficacy of calculating ADL is this way is supported by measurements of aerobic thresholds in freely diving trained California sea lions (Ponganis et al., 1997). These authors found that increased formation of lactic acid corresponded with dives lasting greater than 2.3 min. The predicted ADL of these animals was based on estimates of body oxygen stores coupled with FMRs derived from doubly-labeled water measurements on wild adult female California sea lions (Costa et al., 1990). ADLs calculated in this way yielded predicted ADLs of between 1.8 and 2.0 min, whereas ADLs calculated from metabolic rates estimated from flume studies yielded much longer ADLs of 3–3.5 min. Although ADLs calculated from doubly-labeled water measurements underestimated the actual ADL by 13–22 %, they were far closer than ADLs calculated using flume studies, which overestimated the actual ADL by 43–57 %. If the same correction applies to New Zealand sea lions, anaerobic metabolism would only be expected in those animals that had cADLs greater than 1.3 times the predicted value. If this were the case, many of our animals routinely dived aerobically (E-41, E-52, E-55 and E-59), while others routinely dived outside their ADL and must have included an anaerobic component in their diving metabolism (E-40, E-42, E-43, E-53, E-56, E-58 and E-61). Furthermore, the animals that spent the greatest proportion of their time on the surface inactive were those that exceeded the cADL by the greatest amount (Table 3).

Comparable data on diving pattern and energetics are only available from the Antarctic fur seal Arctocephalus gazella (Arnould et al., 1996; Costa et al., 2000). In contrast to New Zealand sea lions, Antarctic fur seals make shallow short dives, routinely diving to mean depths of only 19 m, and remain submerged for 1.13 min (Table 4). Given such shallow dives, it is not surprising that at-sea FMR did not vary with dive duration or depth in fur seals. Unlike New Zealand sea lions, FMR in Antarctic fur seals increased with increasing trip duration and declined as the proportion of time spent diving and diving effort increased. Arnould et al. (1996) suggested that, for fur seals, a reduced diving effort and lower relative time spent diving indicated that the animals spent more time swimming between dense prey patches and thus spent less time per patch, but caught more prey for their effort (Boyd, 1996; Boyd et al., 1995). The energy cost of swimming is greater than that of diving, so individuals that spend more time transiting between patches expend more energy (Arnould et al., 1996). In contrast, the at-sea FMR of New Zealand sea lions increased as the proportion of time spent diving increased, reflecting the consequences of the deep diving pattern of this animal. New Zealand sea lions spent 2–4 times more time at sea diving with a diving effort that was 4–7 times greater than that observed for Antarctic fur seals (Table 4). Antarctic fur seals spend more time surface-swimming, so this should be a greater factor in determining their at-sea FMR.

These differences may also reflect the differences between animals that have highly patchy but dense prey resources (Antarctic fur seal) versus benthic foragers with a more evenly distributed (less patchy) prey resource (New Zealand sea lion). Nonetheless, comparisons of mass-adjusted FMRs between New Zealand sea lions and Antarctic fur seals indicate that, overall, they have surprisingly similar rates of energy expenditure while at sea, even though their swimming and diving patterns are so strikingly different. These similarities imply that otariids operate within a narrow range of FMRs. Differences in how fur seals and sea lions budget their energy expenditure are important; fur seals invest in travel between high-value patches that are near the surface, while sea lions invest in diving effort because their resources are spread out on the benthos.

The deep and long-duration diving ability of New Zealand sea lions is not due to a decreased diving metabolic rate, but rather to reliance in part on increased oxygen stores. Individual sea lions that performed deeper dives had lower FMRs that probably were due to reliance on burst-and-glide locomotion that reduced the cost of locomotion. Differences in the foraging pattern that correlate with the depth of the dive may be related to differences in diet. Lastly, our data indicate that, although New Zealand sea lions have increased their O₂ storage capacity, they have not or cannot significantly reduce their at-sea metabolic rates and are therefore likely to be operating near their physiological maximum.

This work was supported by National Science Foundation Grant OPP-9500072, Office of Naval Research grant N00014-94-1-1013, the National Geographic Society and the New Zealand Department of Conservation. The manuscript was prepared while D.P.C was the Zoologist in residence at the University of Tasmania. Simon Childerhouse, Wally Hockly and Cath Gales provided invaluable assistance in the field. We thank Mary Zavanelli for help with laboratory analysis. We thank two anonymous reviewers for their valuable input to an earlier draft of the manuscript.