Jaquet And Gendron 2002

Marine Biology (2002) 141: 591–601 DOI 10.1007/s00227-002-0839-0

N. Jaquet Æ D. Gendron

Distribution and relative abundance of sperm whales in relation to key environmental features, squid landings and the distribution of other cetacean species in the Gulf of California, Mexico

Received: 26 June 2001 / Accepted: 5 February 2002 / Published online: 20 April 2002
Springer-Verlag 2002

Abstract Sperm whales (Physeter macrocephalus) feed predominantly on meso- and bathypelagic cephalopods for which effective sampling methods have not been developed. The Gulf of California is one of the very few areas where sperm whales might feed on a commercially fished species of squid (jumbo squid, Dosidicus gigas), presenting a unique opportunity to investigate the impacts of variations in jumbo squid abundance on sperm whale distribution. This study examines sperm whale distribution and relative abundance in relation to the distribution of D. gigas, other cetacean species and key environmental features over spatial scales ranging from a few kilometers to a several hundreds of kilometers. Data were collected during two field seasons in spring– summer 1998 and 1999 using non-invasive techniques. Landing statistics show that the jumbo squid fishery collapsed in 1998 and started recovering in early 1999. Despite this collapse in 1998, sperm whales remained abundant during both years, but there were strong differences in their aggregative behavior. In 1998, sperm whales were roughly evenly distributed, while in 1999, there were three super-aggregations (55·75 km across), which were stable for over a month. During both 1998 and 1999, sperm whales were uniformly distributed with respect to mean depth, slope and sea surface temperature over spatial scales of 10, 19, and 37 km segments and over areas of 70·90 km. There was no close association between sperm whale distribution and the distribution of jumbo squid landings in Communicated by G.F. Humphrey, Sydney N. Jaquet (&) Texas A&M University, Marine Biology Department, Galveston, TX 77551, USA E-mail: [email protected] Tel.: +1-409-7414329 Fax: +1-409-7405002 D. Gendron Centro Interdisciplinario en Ciencias Marinas, Instituto Polite´cnico Nacional, A. P.l 592, CP 23000 La Paz, BCS, Mexico

1998. In 1999, about two-thirds of the individuals were found in areas of possibly high jumbo squid biomass. There was a significant correlation between the occurrence of sperm whales and that of bottlenose dolphins (Tursiops truncatus), despite the fact that they usually inhabit different water depths. This is the first study which was able to relate sperm whale distribution and relative abundance to the abundance of their main prey items. It suggests that sperm whales change their distribution in response to a decline in jumbo squid but that they do not leave the Gulf of California. However, this study encompassed only 2 years and further investigations are needed to gain an understanding of what may trigger large-scale movements.

Introduction During the 1970s and the 1980s, numerous studies attempted to relate sperm whale (Physeter macrocephalus) distribution to oceanographic features (Gulland 1974; Ramirez and Urquizo 1985; Kenney and Winn 1987). Despite past findings that suggest relationships between sperm whale distribution and areas of upwelling, steep temperature gradients and high underwater relief, the confusion regarding spatial and temporal scales makes these conclusions equivocal (Jaquet 1996a). As a consequence, recent studies have used a wider range of spatial and temporal scales to try to elucidate the relationship between sperm whale distribution and oceanographic factors (Jaquet and Whitehead 1996; Jaquet et al. 1996) or have used spatial scales relevant to the local oceanographic processes (Griffin 1999; Biggs et al. 2000). These recent studies have suggested that, while some biological and oceanographic factors (e.g. underwater relief, oceanographic fronts, biological productivity) may help us to understand sperm whale distribution, a large amount of variability remains unexplained. As phytoplankton biomass and sperm whales represent the opposite ends of a trophic spectrum, large

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temporal and spatial lags are expected to occur between a peak in chlorophyll concentration and the occurrence of sperm whales, blurring any kind of direct correlation (Jaquet 1996a). To better describe sperm whale habitat, it would be more meaningful to relate the distribution of sperm whales to that of their main prey items, rather than to primary productivity or high underwater relief. However, sperm whale diet consists primarily of meso- and bathypelagic cephalopods (Kawakami 1980), and methods of effectively sampling these deep-living squid have not yet been developed (Clarke 1987). It has therefore been impossible to directly relate sperm whale distribution to the distribution of their prey. Fig. 1. Map of the study area

The Gulf of California, Mexico, is an ideal region in which to study sperm whale distribution in relation to both key environmental features and squid distribution, over manageable spatial scales (ranging from a few kilometers to several hundreds of kilometers). It is a very productive marginal sea, with some of the highest surface nutrient concentrations of any ocean of the world (Santamarı´ a-del-Angel et al. 1994). Due to its monsoon climate and to its underwater topography (Fig. 1), there are tremendous changes in productivity both seasonally and spatially (Thunell 1998). The winter months are characterized by a high biological productivity while the summer months are characterized by a very low productivity (Santamarı´ a-del-Angel et al. 1994). Furthermore, it is one

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of the very few areas where sperm whales probably feed on a commercially fished species of squid (jumbo squid, Dosidicus gigas). Despite the absence of stomach content studies within the Gulf of California, in all other sites within the distributional range of the jumbo squid, D. gigas represents a major part of the total biomass of squid consumed (up to 99%, Clarke et al. 1988; Clarke and Paliza 2001). Therefore, it is not unreasonable to assume that D. gigas is also an important part of sperm whale diet in the Gulf of California. A large-scale fishery for D. gigas was established in 1979 in response to high jumbo squid biomass in the Gulf of California (Ehrhardt et al. 1983). However, from 1982 to 1993, very few jumbo squid were landed and the fishery collapsed (Neva´rez-Martı´ nez et al. 2000). After 1994, squid abundance increased again, stimulating an increase in fishing effort with up to 140,000 tons of squid fished during 1996. The fishery collapsed a second time in early 1998, but the second recovery was already observed during March 1999 (Morales-Bojo´rquez et al. 2001b; E. Morales-Bojo´rquez personal communication). This intensive fishery for jumbo squid in the Gulf of California is providing us with a unique opportunity to relate commercial catch data for squid to the distribution of sperm whales. There is currently very little information on sperm whales in the Gulf of California. Prior to 1990, there were only few records of sperm whale sightings within the Gulf (Vidal et al. 1993). However, this paucity of records may have been due to the fact that very few studies were conducted on cetaceans in the Gulf of California. In the last 10 years, several surveys for cetaceans have been conducted, and there have been many opportunistic reports of large concentrations of sperm whales, suggesting that sperm whales are abundant there year-round (Mangels and Gerrodette 1994; Gendron 2000). However, as there have been no dedicated studies on sperm whales in this area, very little is known on their distribution, abundance, residency, seasonal movements, and on the relationship between sperm whales and squid abundance/distribution. Furthermore, at least 20 other species of cetaceans are known to inhabit these waters (Mangels and Gerrodette 1994), but very little is known about their spatial utilization of the Gulf and whether their distributions overlap with the distribution of sperm whales. The goals of this study were to assess the distribution of sperm whales relative to the landings of D. gigas and to oceanographic features (underwater relief, sea surface temperature and depth) over a range of spatial scales (10, 19 and 37 km segments and areas 70·90 km), and to assess sperm whale association with other species of odontocetes within the Gulf of California.

mid-July 1998 and mid-May to early July 1999). The study area comprised the deep waters of the Gulf (>200 m deep) from 2420¢N to 2830¢N (Fig. 1). The search pattern for sperm whales was not systematic; greater search effort was given in areas where sperm whales were likely to be abundant (i.e. off Santa Rosalia, between Tortuga and San Pedro Martir, and within depressions characterized by a steep underwater relief, Fig. 1). However during each field-season, most of the study area was surveyed. Sea surface temperature (SST) was measured every hour using a mercury thermometer (accuracy =0.2C), and search effort and vessel track recorded automatically by linking a Garmin 12XL GPS to a Hewlett Packard 95LX palmtop computer. Cetacean sightings were recorded during daylight hours by the observer on watch (sitting about 3 m above sea level), using the naked eye and looking in front as well as on both sides of the boat. Acoustic encounters with delphinids (whistles) were also recorded. Sperm whales were located by listening every hour through an omnidirectional hydrophone for their characteristic clicks (Backus and Schevill 1966). Once located, sperm whales were followed both visually and acoustically (using a directional hydrophone) for periods ranging from a few hours to 4 days. Vessel track, dive locations, date, and fluke-up time were recorded using custom-written software on the HP palmtop. Fluke photographs were taken at the start of the dive to allow individual identification, only photographs with a quality (Q) ‡3 were taken into consideration (following Arnbom 1987). No systematic measurements of depth or intensity of the scattering layers were undertaken as our depth sounder (Furuno GP1810F, dual frequencies 50 kHz and 200 kHz) was not powerful enough to detect the very weak scattering layers encountered in the Gulf of California at this time of year. Furthermore, due to very low productivity and thus very high transparency, it was impossible to weight the Secchi Disk with enough lead that it would sink vertically and still be retrievable by hand. Therefore no systematic measurements of chlorophyll concentration were undertaken during this study.

Materials and methods

The mean number of hours spent searching for whales (i.e. searching time) gives a rough indication of the abundance of whale aggregations (Whitehead and Kahn 1992). Both the density of sperm whales and the number of encounters measured sperm whale density. However, once heard, sperm whales were not always followed, and even when followed some encounters only lasted a

Field methods Data were collected from a 12.8 m ocean-going sloop during 2 field seasons in the Gulf of California, Mexico (early June to

Data analyses The relative abundance of sperm whales was measured using three different variables. Biases inherent in each are discussed below. As sperm whales click almost continuously and are usually detected acoustically, we had as many chances to find sperm whales during the night as we had during daytime. Therefore, round-the-clock data with ‘‘only acoustic’’ as well as ‘‘acoustic and visual’’ encounters were used to calculate the three following variables:

•

•

•

Searching time = number of hours between the time we left an encounter with sperm whales and the time we found a new encounter. Encounters were considered independent when no whales were heard nor seen during two consecutive hours when sailing in a straight line at about 10 km/h. Number of encounters = number of times we encountered a ‘‘new’’ group or aggregation of whales (either visually or acoustically). This variable was used whenever transects of similar lengths were compared. However, when areas with different amount of search effort were compared, we standardized ‘‘Number of encounters’’ by using a very similar variable, ‘‘Encounter rate’’, which was the number of encounters per 100 km of search effort. Density of sperm whales = number of hourly listening sessions during which we heard or saw whales divided by the total number of hourly listening sessions.

594 couple of hours. Therefore, the density of sperm whales is positively biased towards areas where whales were followed and the number of encounters is positively biased towards areas where whales were not followed. To investigate the spatial and temporal variation in sperm whale relative density, the study area was divided into nine areas of roughly 70·90 km (6,300 km2, Fig. 2). The density of whales (calculated using both density of sperm whales and number of encounters and corrected for uneven effort) and the mean SST were calculated for each area and for each year. Sperm whale density was then related to what is known about jumbo squid distribution. To examine the association between sperm whale distribution and underwater topography, depth and SST, the track of the research vessel was divided into 9.2 km segments (5 nautical miles). For each segment, number of encounters, slope (calculated in a radius of half the segment length centered in the middle of the segment and expressed as meters per km), mean depth and SST were calculated. Mean depth and slope were calculated using the chart Carta Batimetrica, Golfo de California, CB-002, 1982 edition. To increase the spatial scale, two segments were combined to make 18.5- and 37-km segments, respectively. For each segment at each spatial scale, the four variables listed above were calculated. Log-likelihood ratios were then used to determine whether sperm whales were distributed non-uniformly with respect to mean depth, slope and SST at each spatial scale. As sperm whales groups were often followed for several hours, segments with sperm whales were highly auto-correlated (a segment had a higher chance of having sperm whales if the previous segment already had sperm whales). Therefore, only one segment per sperm whale encounter was used in the analyses. To avoid having to make a decision about which segment should be kept, and to keep all possible information, mean SST, depth and slope were calculated for all segments on which a particular encounter was followed (D. Fletcher, Otago

Fig. 2. Division in areas of roughly 70·90 km. In each area, the proportion of hourly listening during which sperm whales (Physeter macrocephalus) were either seen or heard is shown by the proportional size of the circle. The number below the circle represent the total number of hourly listening sessions

University, personal communication). The search effort was ‘‘normalized’’ by creating class sizes of more or less equal probability of encounter (following the methodology used by Baumgartner 1997) and not of equal distances as is conventional in the v2 test. The number of classes was chosen in such a way that the expected number of sightings for each class was larger than 3 (Zar 1996). The expected distribution was then compared with the observed distribution. Sightings with other species of cetaceans were considered independent when the sightings were separated by at least 30 min without observing an animal. Associations between frequently sighted delphinids and sperm whales were investigated using a G-test, with the Ho hypothesis that dolphins were uniformly distributed with regards to sperm whales. As the species of delphinids could only be identified during daytime hours, data collected at night were not included in these analyses. To avoid the problem of auto-correlation, only one segment per encounter (or sighting) was used as explained above.

Results A total of 55 days were spent at sea with a search effort covering 7,123 km. We listened 1,258 times for the characteristic clicks of sperm whale and in 491 of these (39%) whales were either seen or heard (Fig. 3A, B). We encountered sperm whales 44 times (21 encounters in 1998 and 23 in 1999). Out of these 44 encounters, 15 of them were only acoustic and we heard and saw the whales in 29 encounters. Two encounters were with single mature males. During the other 42 encounters, several sperm whales were always seen or heard. In total, 467 h were spent in contact with whales: 223 h in visual contact, and 244 h in acoustic contact. From these encounters, we took 414 good quality identity photographs (Q‡3) and identified 159 different individuals, 63 in 1998 and 96 in 1999. Only six individuals were identified during both years. The mean searching time between leaving an encounter and finding a new one (either visually or acoustically) was relatively low during both years (mean=16.4 h, SD=16.3) suggesting a high abundance of sperm whales within the Gulf. The searching time was slightly lower in 1998 mean=14.7 h, SD=15.7) than in 1999 (mean=18.0 h, SD=17.0); however the difference was not statistically significant (v2=0.148, df=1, 0.5
Fig. 3A, B. Track of the research vessel, the black crosses represent hourly listenings during which no whales were seen or heard and the light grey crosses represent hourly listenings while in contact with sperm whales. A 1998; B 1999

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were about 55–75 km across and two of them were stable for periods of at least a month. During both years, no whales were seen or heard in area 9 (Fig. 2). Table 1 shows the number of different sperm whales individually identified in each area during each year, 1998 and 1999, and in the two combined. In 1998, the 63 individuals identified were found in areas 1–6 and, except for a single whale in area 3, they were more or less evenly distributed between areas. In 1999, almost half of the individuals were identified in area 1 and the other half in the Depression de Guaymas and the Depression de Farallon, despite 25% of the effort expended in the other areas. The results obtained during hourly listens were similar and confirmed that whales were more aggregated during 1999 than during 1998. When both years are pooled, the higher relative abundance of sperm whales was in area 1 followed closely by area 3 (Table 1). SST was consistently about 2C higher in 1998 in comparison to 1999 (Fig. 4). In 1998, sperm whales were abundant in areas 1–6 which covered all values of SST, and almost absent from areas 7–9 (Table 1, Fig. 4). In 1999, sperm whales were mainly abundant in areas 1, 3, 7 and 8 which were characterized by high, medium and low SST respectively (Table 1, Fig. 4). Table 1. Total amount of effort and sperm whale density for each of the nine areas: for the 1998 field season; for the 1999 field season; and for the two seasons combined

The relative abundance of sperm whales was unchanged between 1998 and 1999, despite a difference in SST of about 2C, suggesting that SST has no effect on sperm whale ecology in the Gulf of California over the spatial scale (70·90 km) and temperature range examined there. Similarly, there was no relationship between sperm whale distribution and steepness of the underwater topography and/or maximum depth of the area (Fig. 2). Sperm whales were very abundant in area 1 which was the shallowest area characterized by the flattest underwater topography; and they were totally absent in area 9, which was the deepest area with the steepest underwater topography (Fig. 2). Clearly, over this spatial scale (70·90 km), factors other than depth and steep underwater relief are influencing the distribution of sperm whales in the Gulf of California during the summer season. Over a smaller spatial scale (9.2-km segments), the distribution of sperm whale encounters with respect to mean depth, slope and SST was not significantly different from an expected uniform distribution (Fig. 5). As the larger spatial scales (18.5- and 37-km segments) gave very similar results to the smaller one (9.2-km segments), and as no log-likelihood ratio tests were significant at

Total amount of effort (in km)

Encounter rate

1998 1 2 3 4 5 6 7 8 9 Total

259 404 493 187 487 400 409 156 261 3,056

1.163 0.743 0.814 1.610 0.821 0.499 0.490 0 0 0.688

57 68 82 37 76 79 59 20 32 510

0.807 0.426 0.390 0.514 0.276 0.278 0.068 0 0 0.339

15 14 1 14 11 8 0 0 0 63

1999 1 2 3 4 5 6 7 8 9 Total

1,363 211 683 126 217 263 796 204 204 4,067

0.734 0 0.586 0 0 0 0.881 0.983 0 0.566

285 27 137 17 29 48 118 64 25 750

0.488 0 0.628 0 0 0.417 0.263 0.609 0 0.420

40 0 33 0 0 2 12 15 0 102

1998 and 1 2 3 4 5 6 7 8 9 Total

1999 1,622 615 1,176 313 704 665 1,204 359 465 7,123

0.803 0.488 0.682 0.959 0.569 0.302 0.748 0.557 0 0.618

342 95 219 54 105 127 177 84 57 1260

0.541 0.305 0.539 0.352 0.200 0.331 0.198 0.464 0 0.387

51 14 33 14 11 10 12 14 0 159

Area

Total number of hourly listenings

Proportion of listenings during which whales were seen or heard

Number of different whales identified

597 Fig. 4. Average sea surface temperature for each of the nine areas for 1998 (grey) and 1999 (black). The error bars indicate 95% confidence intervals

the 5% level for any spatial scales, only the results for the 9.2-km spatial scale are given here (Fig. 5). Although sperm whales were encountered in all water depths from 600 to 2,500 m, a high percentage of the sightings (33.6%) occurred at depths shallower than 1,000 m. However, as 28.7% of the effort was spent in water depths <1,000 m, this result suggests that sperm whales were found as often in waters deeper than 1,000 m as in waters shallower than 1,000 m. The sightings of other species of cetaceans are summarized in Tables 2, 3. Bottlenose dolphins (Tursiops truncatus) were frequently sighted during the study (96.7 h were spent in visual contact with bottlenose dolphins). Short-finned pilot whales (Globicephala macrorhynchus), Risso’s dolphins (Grampus griseus) and common dolphins (Delphinus sp.) were also seen relatively frequently. Other cetacean species including longsnouted spinner dolphins (Stenella longirostris), dwarf sperm whales (Kogia simas), fin whales (Balaenoptera physalus), humpback whales (Megaptera novaeangliae) and minke whales (B. acutorostrata) were rarely sighted. There were also 116 occasions when delphinids were heard on the hydrophone but not sighted (these occurred mainly at night) and 39 encounters with unidentified dolphins (either at night or when positive identification was not possible). Bottlenose dolphins were sighted at all values of SST, depth and slope (Table 2), suggesting that their distribution was not tightly associated with these features. They were also sighted in all areas except for areas 9 and area 4 (Table 3). Pilot whales were sighted at relatively lower SST, and in areas with less underwater relief than bottlenose dolphins and sperm whales (Table 2); however, the number of sightings does not allow statistical analyses to be performed. The other species of cetaceans were well distributed

within all areas (Table 3). Area 9, which had no sightings of sperm whales, bottlenose dolphins or Risso’s dolphins, had a large proportion of the baleen whale, dwarf sperm whale and common dolphin sightings. Only bottlenose dolphins had a sufficient number of sightings to allow association analyses with sperm whales to be undertaken. From the 34 9.2-km segments on which different sightings of bottlenose dolphins were present, there were 22 segments where sperm whales were also present and 12 segments where bottlenose dolphins were found by themselves. The result of the G-test for association between the two species (with the Yates correction for continuity, Zar 1996) was highly significant (G=50.504, df=1, P0.001).

Discussion The mean searching time between sperm whale encounters in the Gulf of California (16.4 h) was slightly smaller than the mean searching time obtained by Whitehead and Kahn (1992) around the Gala´pagos Islands (23.0 h) and the Seychelles Islands (25.4 h), and considerably smaller than off mainland Ecuador (47.2 h). Our results suggest very high abundance of sperm whale aggregations in the Gulf of California. The small difference in mean searching time between 1998 and 1999 (14.7 h vs 18 h) suggests that the abundance of sperm whales was roughly similar between both years, despite the collapse of the squid fishery in 1998. There were large differences in distribution between 1998 and 1999, sperm whales being more aggregated in 1999 than in 1998. As sperm whales aggregate more in areas and at times of greater food abundance (Whitehead and Kahn 1992), our results suggest that there was

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Fig. 5. Encounter rate (= number of encounters per 100 km of survey effort) versus A depth class, B slope class, and C SST class. The results of the G-test for the 9.2-km scale are given for each of these variables

less food available to sperm whales in 1998 than in 1999 and thus these results are consistent with the decline in squid landings. It seems therefore that sperm whales did not respond to a shortage of food by moving away from the study area, but responded by changing their aggregative behavior. The size of the areas covered by sperm whale super-aggregations (55–75 km across) in 1999 was roughly similar to the extent of the super-aggregations found during an extensive survey around the South Pacific (Jaquet 1996b; Whitehead and Weilgart 2000). Furthermore, our present results confirm that the temporal scales of these super-aggregations are at least a month, scales which could be only hypothesized during these previous studies (Jaquet 1996b; Whitehead and Weilgart 2000). In the Gulf of California, D. gigas catches were highly clumped, and there was large variability in fishing success over scales of 30–55 km (Neva´rez-Martı´ nez et al. 2000). The sizes of these patches are roughly similar to the sizes of sperm whale super-aggregations seen by San Pedro Martir and in the Guaymas Depression, confirming that the size of superaggregations is likely to be related to the size of prey patches. This hypothesis was previously postulated by Jaquet (1996b) and Whitehead and Weilgart (2000). Within the Gulf of California, sperm whales were regularly found over shallower depths than previously reported for other ocean basins. It has been generally accepted that groups of female and immature sperm whales were mostly found in waters deeper than 1,000 m, and that only males venture into shallower areas (Whitehead et al. 1992; Scott and Sadov 1997; H. Whitehead, unpublished data). For example, from extensive work around the Gala´pagos Islands, H. Whitehead (unpublished data) found that groups of sperm whales spent only 1.6% of their time in water shallower than 1,000 m, despite extensive shallow areas close by. Similarly, in the Gulf of Mexico, Davis et al. (2000) found that ‘‘sperm whales were more likely to be seen in areas with a mean depth of 1,581 m’’. As D. gigas occupies an intermediate position between the shelf/ slope squid and the true oceanic squid (Nesis 1983), and as during May–July most of the squid are fished off Santa Rosalia with the highest abundance found around San Pedro Martir (Neva´rez-Martı´ nez et al. 2000), it is not surprising to find concentrations of sperm whales in these areas, despite a mean water depth <1,000 m. It seems, therefore, that the 1,000 m contour does not represent a barrier to the distribution of groups of female and immature sperm whales. The depth over which groups are found is likely to vary tremendously between regions depending on the distribution of their food resources. Due to the intense solar heating associated with weak southerly winds, productivity is always very low throughout the Gulf of California between June and October (<0.1 mg Chl/m3, Santamarı´ a-del-Angel et al. 1994). The very high water transparency (Secchi Disk always visible at depths >35 m) and the very low density of zooplankton and mesopelagic fish (no scattering

599 Table 2. Cetacean sightings for 1998 and 1999: summary of cetacean sightings versus SST and bathymetry features Cetaceans

Number of Mean sightings group size (SD)

Bottlenose dolphin Common dolphin Risso’s dolphin

34 5 5

Spinner dolphin Pilot whales Dwarf sperm whales Fin whale Humpback whale Minke whale Sperm whale Controls (9.2-km segments)

1 10 2 3 1 1 44 783

Mean sighting Mean SSTC, duration in minutes (SD; range) (SD; range)

Mean depth in m, (SD; range)

Mean slope m/km, (SD; range)

30.2 (27.2) 126.1 (175.4; 1–764) 27.9 (1.1; 25–30.3 1,177 (447; 450–2,150) 58 (107.4) 23.6 (20.2; 3–60) 27.9 (1.0; 26.2–28.8) 960 (585; 350–1,700) 7 (2) 46.6 (65.3; 13–177) 29.0 (1.0; 27.8–30.5) 1,690 (167; 1450–1,850) 170 50 28.5 1,850 17 (9) 13.5 (16.2; 1–51) 26.2 (1.0; 24.5–27.5) 1,045 (439; 500–1,850) 3 (1.4) 11 (1.4; 10–12) 26.2 (1.0; 26–26.4) 600 (212; 450–750) 1.6 (1.2) 1 1 – –

3.7 (2.9; 2–7) 17 2 449 (286; 30–1316) –

28.4 (24.8; 0–145.8) 42.2 (11.7; 21.6–48.6) 33.5 (27.0; 0–81) 21.6 24.8 (14.4; 0–48.6) 67.0 (33.6; 43.2–90.7)

26.6 (0.8; 26–27.5) 683 (305; 350–950) 41.0 (34.1; 16.2–79.9) 28.5 50 4.3 27 650 16.2 27.3 (1.7; 22.8–30.3) 1,388 (452; 650–2,400) 33.5 (25.3; 0–86.4) 26.9 (1.6; 22–31.2) 1,313 (461; 50–2,500) 30.8 (26.8; 0–167.4)

Table 3. Cetacean sightings for 1998 and 1999: sighting rate (number of sightings per 100 km of effort) per area for toothed whales and baleen whales Area

Total amount of effort (in km)

Sperm whale

Bottlenose dolphin

Common dolphin

Risso’s dolphin

Spinner dolphin

Pilot whale

Dwarf sperm whale

Baleen whales

1 2 3 4 5 6 7 8 9 Total

1,622 615 1,174 313 704 665 1,204 359 465 7,121

0.803 0.488 0.682 0.959 0.569 0.302 0.748 0.557 0 0.618

1.111 0.651 0.852 0 0.426 0.602 0.166 1.393 0 0.646

0.062 0 0.626 0 0.142 0 0.083 0 0.215 0.070

0 0.163 0.170 0.320 0 0 0 0.278 0 0.070

0 0 0 0 0 0 0 0.278 0 0.014

0.309 0.163 0.085 0 0 0.151 0 0.278 0.215 0.141

0 0 0 0 0 0 0.083 0 0.215 0.028

0.123 0 0 0 0 0 0.083 0 0.431 0.070

layer detected on our Furuno depth sounder) confirmed that overall productivity was also very low during late May to late July of both 1998 and 1999. Therefore, in 1998 and 1999, sperm whales were abundant during months when chlorophyll concentration was very low and they were uniformly distributed with regard to mean depth, slope and SST over every spatial scale investigated. At first glance, these results seem to contradict the general belief that sperm whales are found in areas of high underwater relief (Clarke 1956) and areas of high primary and secondary productivity (Gulland 1974). These results also seem to contrast with the recent work of Biggs et al. (2000) and Davis et al. (2000), which showed that, in the Gulf of Mexico, sperm whales were significantly more abundant in the ‘‘cyclonic oases’’ than in the ‘‘depauperate anticyclonic ocean deserts’’. However, these results are consistent with the work of Jaquet and Whitehead (1996) and Jaquet et al. (1996) who found correlations between sperm whale distribution and underwater relief and primary and secondary productivity only at spatial scales larger than 200 km. Over a large spatial scale (study area, 500 km·110 km), the underwater relief in the Gulf of California is very steep (with depth gradient of up to 200 m/km and with depth of 1,000 m less than 7 km from shore). Furthermore, the intense upwelling occurring yearly between November

and May supports a very high biological productivity in the Gulf of California (surface NO3 up to 15.8 lM, Gaxiola-Castro et al. 1999; and chlorophyll concentration of >10 mg/m3, Santamarı´ a-del-Angel et al. 1994) during these months, and thus when a large temporal scale is taken into consideration (>1 year), the overall productivity of the Gulf is high. Therefore, our results confirm that sperm whales are found in areas of high productivity and high underwater relief, but that these correlations tend to occur at large spatial and temporal scales. Temporal and spatial lags between peaks in productivity and peaks in squid abundance would be the most likely explanation for relationships to occur only at large scales. In such a case, one would expect substantial correlations between sperm whale distribution and their main prey item to occur at small scales. However, our results do not suggest a close association between sperm whale distribution and relative abundance and D. gigas landings during our study period. The relative abundance of sperm whale was roughly similar during both years, despite the paucity of D. gigas in 1998, and only their aggregative behavior suggested lower food abundance during 1998. Furthermore, during 1998, most squid landings occurred south of Loreto, on the Baja side of the Gulf (Morales-Bojo´rquez et al. 2001b), while

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very few sperm whales were found south of Loreto (Fig. 3A) as almost all sperm whale encounters were roughly evenly distributed north of Loreto. Unfortunately no exact data are available for the distribution and abundance of jumbo squid during 1999. However, as 1999 was a ‘‘normal year’’ (with regard to SST and El Nin˜o), it is likely that squid distribution during 1999 matched roughly their distribution in 1995– 1997. Research cruises during May–July of these years show an area of very high abundance between 28N and 2830¢N (Morales-Bojo´rquez et al. 1997, 2001b; Neva´rezMartı´ nez et al. 2000), corresponding roughly to our area 1. Jumbo squid were also very abundant in the shallow water east and south east of San Pedro Martir (an area we did not survey due to depth <200 m), and in the Depression de Guaymas. On the other hand, jumbo squid were scarce south of 2630¢N (Neva´rez-Martı´ nez et al. 2000; Morales-Bojo´rquez et al. 2001a), and thus very few squid were ever fished in the Depression de Farallon area. It seems therefore that only two (out of three) of the superaggregations of sperm whales found in 1999 were located in areas of possibly high jumbo squid abundance. The absence of a close match between sperm whale distribution and squid landing seems puzzling at first glance. However, 1998 was characterized by very low squid abundance throughout the Gulf; and in 1999, the landings were still about three to four times smaller than in 1996–1997 (Morales-Bojo´rquez et al. 2001b; E. Morales-Bojo´rquez, personal communication). It is therefore possible that in 1998 sperm whales changed their diet to other species of squid in response to D. gigas depletion (as it has been suggested for sperm whales off Kaikoura, New Zealand, Jaquet et al. 2000). Similarly, in 1999, the abundance of jumbo squid may not have been sufficient to feed the entire sperm whale population, and some of them may have foraged on different food resources in the Depression de Farallon. Alternatively, as sperm whales are known to feed on large size squid which may not be available to fisheries (Clarke 1987), it is conceivable that large D. gigas were inhabiting the central part of the Gulf near the Depression de Farallon. However, Ehrhardt et al. (1983) showed that during May to August, large squid were usually found close to the Baja Coast while smaller ones were found further offshore, rendering this last hypothesis unlikely. This study supports the opportunistic nature of sperm whale foraging behavior, and suggests that despite fluctuations in their food resources, sperm whales did not migrate out of the Gulf during 1998 when D. gigas was scarce. However, this study encompass only 2 years, and 1998 was preceded by 3 years of very high jumbo squid abundance (up to 140,000 tons were fished in a single year, Morales-Bojo´rquez et al. 2001b). It is possible that there is some inertia in sperm whale response to low squid biomass and that if the abundance of D. gigas stays consistently low, either due to overfishing and/or to environmental conditions, sperm whales may eventually leave the Gulf of California.

Smith and Whitehead (1999) found that the dolphin community around the Gala´pagos Islands was very similar to the one encountered in the Gulf of California and that bottlenose dolphins were also the most commonly sighted dolphins, followed by common dolphins, Risso’s dolphins and pilot whales. Although bottlenose dolphins and sperm whales usually inhabit waters of different depths (Scott and Chivers 1990; Davis et al. 1998), our study shows a surprisingly close and new association between these two species. It is most unlikely that this association lead to competition for food resources as they have very different diets. However, large swarms of sea birds as well as schools of tunas and dolphin fish (Coryphaena hippurus) were often observed in the vicinity of bottlenose dolphins. Because, off Baja California, dolphin fish feed primarily on D. gigas as well as on an important D. gigas prey item (red crabs, Pleuroncodes planipes, Aguilar-Palomino et al. 1998), it is probable that the areas where both bottlenose dolphins and sperm whales were encountered were also areas of high prey concentration for both species. Acknowledgements This study was funded by the Center for Field Research (Earthwatch, USA), Wildlife Conservation Society (USA), CONABIO (Mexico), and the ‘‘Socie´te´ Acade´mique Vaudoise’’ (Switzerland). CICIMAR-IPN, Mexico, and the University of Otago (New Zealand) loaned us equipment. We would like to extend special thanks to the skipper and owner of the research boat, John Botke, who lent us the sailing vessel and greatly helped with data collection, and to K. Babiak, L. Bertholet, J. Bourton, F. Dupont, K. France, D. Hauenstein, K. Hoelrich, G.King, D. McCutchen, F. Poma, C. Roelli, I. Ruiz-Castro, J. Smith, P. Sperisen, S. Thomas, R. Winjum and T. Worthen who helped us collecting the data at sea: Enrique Morales-Bojo´rquez provided us with the recent information on jumbo squid landing. This research was conducted under the scientific permit No. 040598–213–03 (1998) and 140598-213-03 (1999) from the Secretaria de Medio Ambiente Recursos Naturales y Pesca. The manuscript was much improved by comments from David Fletcher, Keith Jensen, Sarah Mesnick, Hal Whitehead and one anonymous reviewer.

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