CHARACTERIZATION OF BEAKED WHALE (ZIPHIIDAE) AND SPERM WHALE (PHYSETER MACROCEPHALUS) SUMMER HABITAT...

MARINE MAMMAL SCIENCE, 17(4):703-717 (October 2001) 0 2001 by the Society for Marine Mammalogy

CHARACTERIZATION OF BEAKED WHALE (ZIPHIIDAE) AND SPERM WHALE

(PHYSETER MACROCEPHALUS) SUMMER

WATERS OFF THE NORTHEAST U.S. HABITAT IN SHELF-EDGE AND DEEPER

G. T. WARING T. HAMAZAKI~

Northeast Fisheries Science Center, National Marine Fisheries Service, NOAA,

166 Water Street, Woods Hole, Massachusetts 02543, U.S.A. E-mail: [email protected]

DANIEL SHEEHAN~ Northeast Regional Office,

National Marine Fisheries Service, NOAA One Blackburn Drive,

Gloucester, Massachusetts 01930, U.S.A.

GRAYSON WOOD Northeast Fisheries Science Center,

National Marine Fisheries Service, NOAA, Narragansett, Rhode Island 02882 U.S.A.

SHEEKELA BAKER Graduate School of Oceanography,

University of Rhode Island, Kingston, Rhode Island 02882, U.S.A.

ABSTRACT

Sperm whales (Physeter macrocephalzls) and beaked whales (Mesoplodon spp. and Ziphius cavirostris) are deep-diving cetaceans that frequent shelf-edge and Gulf Stream waters off the northeast U.S. coast. Sighting data collected during seven summer (1990, 1991, 1993, and 1995-1998) shipboard surveys were analyzed using a geographic information system to determine habitat use based on bathymetric and oceanographic features. Although sighting rates

Current address: Department of Fishery & Wildlife Science, New Mexico Stare University,

* Current address: MIT/Academic Computing Practice, Bld. N42, Cambridge, Massachusetts Las Cruces, New Mexico 88003, U.S.A.

02139, U.S.A.

703

7 04 MARINE MAMMAL SCIENCE. VOL. 17. NO. 4. 2001

were lower for beaked whales, both taxa occupied similar habitats. Beaked whales were concentrated at the colder shelf edge, whereas sperm whales were associated with warmer off-shelf water. Mean sighting rates for both taxa were higher in canyon features, but only beaked whale sighting rates were significantly different between canyon and non-canyon habitat (Wilcoxon signed rank test P = 0.007). Within the shared habitat, the two taxa were separated at fine-scale based on oceanographic features. Keywords: sperm whale, Pbysetw macrocephalus, beaked whales (Mesoplodon spp. and Ziphias ruvirostris), habitat use, satellite imagery, GIs, northeast U.S. shelf.

Sperm whales (Physeter macrocephaltls) and beaked whales (family Ziphiidaei) appear very similar in both their diet and patterns of habitat use. Studies conducted in various locations worldwide have demonstrated that both taxa inhabit primarily deeper and off-shelf waters, notably in areas associated with specific bathymetric and/or oceanographic features (Best 1979, Mead 1989, Rice 1989, Whitehead et al. 1992, Waring et al. 1993, Reilly and Fiedler 1994, Childer- house et al. 1995, Trove 1995, Jaquet 1996, Lockyer 1997, Jefferson and Schiro 1997, Tynan 1997, Ballance and Pitman 1998, Davis et al. 1998, Griffin 1999, Hooker and Baird 1999, Smith and Whitehead 2000). In the North Atlantic both sperm and beaked whales have been recorded along the continental shelf break (200-2,000-m isobaths) and near warm-core rings and Gulf Stream eddies (CETAP 1982, Kenney and Winn 1987, Waring et al. 1993, Trove 1995, Scott and Sadove 1997, Griffin 1999). In this region frontal boundaries are generated by numerous submarine canyons (Uchupi 1968), often resulting in the inter- action of three distinct water masses (i.e., shelf, slope, and Gulf Stream water; Brooks 1996), as well as ephemeral Gulf Stream features (i.e., warm-core rings and eddies; Richardson 1983). Frontal boundaries tend to increase productivity due to upwelling and consequent entrainment of nutrient-rich shelf-edge waters (Wroblewski and Cheney 1984, Olson and Backus 1985, Brooks 1996).

Both taxa consume mainly cephalopods (Clarke 1986, Gaskin 1985, Rice 1989, Mead 1989, Ostrom et al. 1993, Smith and Whitehead 2000), although, fish are also consumed. For example, off Iceland fish were an important com- ponent of the sperm whale diet (Roe 1968). Similarly, bentho-pelagic fish were the primary items in the stomach contents of six Sowerby’s beaked whales (Mesoplodon bidens), and one Cuvier’s beaked whale (Ziphius cavirostris) by- caught in the summer 1989-1996 U.S. Atlantic pelagic drift gillnet fishery along the continental slope off Georges Bank; species found included mycto- phids (Lobianchia gemellarii), morids (Gadella imberbis, and Laemonemu barbu- tula), grenadiers (Macrouridae), and a few squid beaks.*

j Five species of beaked whales reside off the northeast U S . coast. These include Cuvier’s beaked whales, Ziphius ravirostvis; True’s beaked whale, Mesoplodon mirus; Gervais’ beaked whale, M. europaeus; Blainville’s beaked whale, M. densirostrir; and Sowerby’s beaked whale, M. bidem. These speces are difficult to identify at sea; so all of the available classification included here is to the family or genus level(s) only.

* Personal communications from Jim Craddock, Woods Hole Oceanographic Institution, Bi- ology Department, Woods Hole, MA 02543, September 2000, and Damon P. Gannon, Duke University Marine Laboratory, Nicholas School of the Environment, 135 Duke Marine Lab Road, Beaufort, NC 28516, January 1998.

WARING E T A L . : HABITAT USE 705

List of marine mammal summer sighting surveys conducted by the North- Table 1. east Fisheries Science Center in waters off the northeast U.S. coast, 1990-1998.

MonthIYear Geographic region August 1990 June-July 1991

June-July 1993

July-September 1995

Gulf Stream north wall Cape Hatteras, NC to Georges Bank, shelf edge

Georges Bank to Scotian Shelf, shelf edge (200-

Virginia to southern New England (200 m to

(200-2,000 m)

2,000 m)

Gulf Stream north wall); Georges Bank (200- 2,000 m)

edge (200-2,000 m) only July-August 1996

August 1997 New England seamounts July-September 1998

Southern New England to Georges Bank, shelf

Virginia to western portion of Scotian Shelf

These broad similarities raise the question of whether competitive interac- tions occur between sperm and beaked whales, or whether they are somehow separated either spatially or by niche specialization. Interspecific competition is difficult to identify reliably (see Clapham and Brownell 1996), but any such assessment requires better data on smaller-scale differences in distribution and habitat use than have historically been available for sperm and beaked whales.

Such data are now available for the waters of the northeastern United States. The first comprehensive assessment of the cetacean community in this region was conducted by the Cetacean and Turtle Assessment Program (CETAP 1982). Since then, the National Marine Fisheries Service, Northeast Fisheries Science Center has conducted surveys along the shelf edge in this same region. Consequently, we are now able to use these large data sets to undertake the first detailed study of comparative habitat use by sperm and beaked whales in this region. Here, we present the results of this investigation, based on analyses of bathymetric and oceanographic features with which each taxon was associated.

Specifically, we address three questions: (1) do sperm and beaked whales share the same habitat, (2) are sperm and beaked whale sighting rates higher within or outside submarine canyons, and (3) can sperm and beaked whale habitats be reliably characterized (and separated) by particular oceanographic features, notably depth, sea-surface temperature, frontal boundaries, or bottom slope?

METHODS

Cetacean Surveys

Since 1990, seven shipboard line-transect surveys were conducted princi- pally in shelf-edge and deeper waters off the northeast U.S. coast (Table 1). Generally, transects followed a saw-toothed pattern (Fig. 1) designed to de-

706 MARINE MAMMAL SCIENCE, VOL. 17, NO. 4, 2001

Figzlre I. Transect lines completed during 1990-1998 summer shipboard marine mammal surveys in shelf-edge and deeper waters off northeast U.S. coast.

termine cetacean summer distribution and minimum abundances in high-use habitats between the 200-m and 2,000-m isobaths and Gulf Stream features (k, north-wall and warm-core rings).

Data were collected while the ship traveled at approximately 17-19 km/h (9-10 kn) in Beaufort sea states of 5 and less. Cetacean sightings were collected daily between 0600 and 1800, weather and light conditions permitting.

The sighting team(s) followed standard line-transect procedures (Buckland et al. 1993, Palka 1995). Each sighting team consisted of four observers that rotated through three observation positions (port, center, and starboard). On all surveys observers scanned the ocean from 90" port to 90" starboard, where 0" is on the track line. Over the 9-yr (1990-1998) survey period, data col- lecting and recording evolved from using the naked eye and paper logs (1990- 1991) to high-power (25 X 150) binoculars and computers (1993-1998) (Gar- rett-Logan and Smith 1997; unpublished data5). Observers recorded the time,

5 Unpublished data from Northeast Fisheries Science Center, Protected Species Branch, 166 Water St., Woods Hole, MA 02543.

WARING ETAL.: HABITAT USE 707

bearing, radial distance, species, group size, behavior, sighting cue, and swim- ming orientation of all species encountered. Position, time, vessel heading, wind speed and direction data were recorded at regular intervals by bridge personnel or via a computer link to the ship’s instruments.

Analytical Methods

Full resolution (1.2 km X 1.2 km) satellite derived Non-Linear Sea Surface Temperature (SST) images from NOAA Coastwatch and the University of Rhode Island (URI) were analyzed for the corresponding observation periods of the cetacean surveys. A set of five-day composite images were produced by selecting the warmest value for each pixel location in a five-day image se- quence, thus eliminating times of cloud cover. The single-image data used in the composites are from NOAA/NESDIS operational SST fields (Li et al. 1998) for 1996 through 1998, and historical Pathfinder SST fields (Smith et al. 1996) for 1990 through 1995.

Monthly probabilities of a front were derived from the SST data, obtained from the NOAA National Geophysical Data Center. Cloud cover was removed using an automated cloud detection algorithm developed at URI (Ullman and Cornillon 1999). Fronts were discerned using a multi-image edge detection algorithm (Cayula and Cornillon 1995). The monthly probability of a front occurring at each 4.8 X 4.8-krn region was calculated as: 100 X (number of images with a front at the region per month)/(total number of images per month). We selected a 4.8 X 4.8-km grid, because we could not detect clear spatial patterns ( i e . , high front occurrence along edge of Gulf Stream, warm- core rings or cold-core ring) at the 1.2 X 1.2-krn resolution.

Ocean bottom topography of the study region was obtained from the NOAA, National Geophysical Data Center (resolution: 5-min grids). The orig- inal data were collected by the U.S. Naval Oceanographic Office. The data were entered into an Arc/Info geographic information system6 (GIS) (ESRI 1992), and slope was calculated using the SLOPE command of the Arc/Info@ Grid module (ESRI 1992). Slope was defined as the maximum rate of change in depth (Oo-9O0).

Using GIS, we constructed a polygon coverage for 31 named submarine canyons off the US . northeast coast. First, each canyon was delineated by the 200-2,OO-m isobaths, in 100-rn increments. The head of each canyon was defined using the 200-m isobath. Then, we outlined the edge of each canyon by tracing the isobath to the inflection points where the 200-m isobath shifts from vertical to horizontal orientation. Straight lines were drawn down from the left and right inflection points, perpendicular to the deeper isobaths, until the cursor intersected the 2,000-m isobath, which we defined as the canyon mouth. Polygons were closed by a line connecting the two 2,000-m intersec- tion points. Although many canyons extend beyond 4,000 m (Emery and Uchupi 1972), our cut-off depth was consistent with survey line end points

Reference to trade names does not constitute endorsement by NMFS.


along the shelf edge. Further, we examined 1 and 10-min grid structures and deemed them to be unsuitable because of insufficient data, or variability in canyon topography and dimensions. For example, grids incorporated irregular sections of on-shelf and non-canyon slope water habitat.

Data Analyses

The study region was divided into 4-km2 grids. This grid size was chosen because it approximates the maximum distance between the ship and observed whales. For each grid cell containing track line, oceanographic and topographic data within five days of the date of the track line were assigned to the center of the grid cell. The grid cells were classified as either 1 (track line present) or 0 (track line absent).

Logistic regression was employed to identify beaked and sperm whale habitats. In the model, the probability of the presence of each species was calculated as:

e Y p = - 1 + e y

where y = a. + alxl + azxz + . . . aixi, xi is an environmental variable, and ai is the regression coefficient. Only 1990-1996 data were used to construct the model. Earlier and later sightings data were used to test the predicted model.

Significant environmental variables were selected using a stepwise selection procedure (Freund and Littell 199 1). The best-fitting regression models were used to predict habitat distribution maps. In projecting the predicted habitat distribution, the five-day (30 July-03 August 1995) SST composite image was used because it showed typical summer conditions off the northeast US. coast. For illustration, August sighting locations of each species for three time periods were plotted: the time period of the model (1990-1996), historical (<1986), and more recent (1997-1998). The historical data were from a copy of the CETAP data archived at the Northeast Fisheries Science Center, Woods Hole, MA.

To compare mean sighting rates of sperm whales and beaked whales within and outside of canyons, survey track lines between the 200 and 2,000-m isobaths were consolidated into canyon or non-canyon subsets by survey trip and calendar month. For each track line, the sighting rate (nl l ) was calculated as the number of animals observed (n) divided by the total length of the track line ( I ) . For each subset, differences between sighting rates in canyon and non- canyon were calculated. Wilcoxon signed rank test was used to examine whether the mean of the differences was greater than zero. Using monthly, rather than daily, subsets was deemed suitable because (1) daily survey effort was relatively low (< 100 km), and (2) interannual monthly survey regions were generally similar. To examine whether sightings of sperm whales and beaked whales were independent from each other we conducted a Chi-square analysis


Table 2. Mean (? 1 SE) oceanographic and topographic conditions associated with beaked whale and sperm whale sightings during the 1990-1998 summer surveys.

Variable Beaked whale Sperm whale Z-value P-value Temperature ("C) 22.3 ? 0.5 23.9 t 0.3 -2.716 0.007 Front (%) 6.2 5 0.5 5.9 2 0.3 0.45 1 0.652 Depth (m) 1774 ? 96 1942 2 58 1.267 0.205 Slope (") 2.5 5 0.2 2.0 2 0.1 1.918 0.055 Sample size 97 254

(Zar 1974). We tested whether the frequency of sightings of both taxa at 4- km2 grids was different from expected values.

RESULTS

Beaked and sperm whales generally were seen along the continental shelf break (200-2000 m) from Virginia to Georges Bank, and along the north wall of the Gulf Stream (Fig. 2A, B). Additionally, a few sightings were made along the New England seamounts, southeast of Georges Bank. Beaked whales were found in significantly cooler waters (Wilcoxon signed rank test, P = 0.007), than sperm whales, but there were no significant differences in other variables (Table 2).

Sighting probabilities (number of 4-km2 grids with anirnals/total number of grids with effort) were 0.016 and 0.043 for beaked whales and sperm whales, respectively. Despite these low sighting probabilities, the multiple logistic regression model was significant and exhibited a good fit (Table 3). We were not able to directly validate the predictive model using 1997-1998 survey data, because the 1997-1998 monthly front probability data were not available for this analysis.

Predicted habitat distribution maps (Fig. 3) created by logistic regression models indicated that the habitat regions for both taxa were located along the outer shelf edge. Predicted beaked whale habitat (Fig. 3A) was geographically limited to the southern New England to Georges Bank shelf-edge region. Predicted sperm whale habitat (Fig. 3B) was more widespread, including shelf- edge and oceanic waters. Seamounts were also predicted to be sperm whale habitat, which subsequently was supported by the 1997 survey data. We also created prediction maps for different dates between June and September; however, we did not discern any notable habitat shifts based on visual inspection of the maps.

On a finer scale, mean sighting rate (number of sightingdkm) was generally higher within canyon as compared to non-canyon shelf-edge regions for both taxa; 4.60 and 0.15 for beaked whales and 0.16 and 0.07 for sperm whales, respectively. There was a significant difference in mean sighting rates for beaked whales (Wilcoxon signed rank test df = 11, P = 0.007), but not for sperm whales (df = 12, P = 0.431). Further, the frequency of co-occurring


Figure 2. Sighting locations (open circles) of beaked whales (above) and sperm whales (opposite) during 1990-1 998 summer surveys.

sightings of beaked whales and sperm whales were independent from each other (A2 = 0.321, df = 1, P = 0.57).

D I s c u s s I o N

Based on the logistic regression model, habitat use by both taxa in this region can be predicted using topographic features. This is probably possible because shelf edges, submarine canyons, sea mounts, and other bathymetric formations are known to influence oceanographic processes that concentrate prey (Hui 1985, Fiedler and Bernard 1987, Kenney and Winn 1987, Podesta et al. 1993, Stefanescu et al. 1994, Childerhouse et al. 1995, Vetter 1995, Baumgartner 1997, Tynan 1997). The model predicted sperm whale habitat on the continental shelf proper, which is supported by August 1995 and 1998 aerial surveys (unpublished data).’

’ Personal communication from Debra Palka, Northeast Fisheries Science Center, 166 Water St., Woods Hole, MA 02543, October 2000.


Figure 2. Continued.

Table 3 . Coefficients and probabilities of variables in the multiple logistic regression model for beaked whales and sperm whales.

Beaked whales Sperm whales

Variable Coefficient P Coefficient P

Intercept -6.8427 0.0001 -6.4833 0.0001 Temperature 0.1063 0.0001 Depth -0.00325 0.0001 0.00233 0.0001 Depth X Depth -6.77 x 10-7 o.oooi -6.61 x 10-7 o.oooi Front -0.055 0.0132 -0.1081 0.0021 Slope 0.723 0.0001 Depth X Slope 0.0004 14 0.0001 Depth X Front - 0.00004 0.0355 C 0.755 0.784 Goodness of fit 0.6610 0.2725

Sighting probability: proportion of grids with sighting for effort grids. C: Area under regions of concordance curve, number of agreements between observed

and expected. Values greater than 0.7 are considered good fit (Moses and Finn 1997). Goodness of fit: (P < 0.05: poor fit; P > 0.05: good fit).


Figure 3. Predicted habitat maps of beaked whales (A) and sperm whales (B). Shaded areas depict probability of whale presence (0 to 1). Predicted maps were based on logistic regression model constructed from the August 1990-1996 sighting survey data. Locations of recent (1997-1998) and historical (<1986) August sightings were also plotted.

Off the northeast U.S. coast we believe that numerous foraging sites are established by both permanent (ie., shelf- slope interface, Gulf Stream north wall) and ephemeral (i e., warm-core rings, seasonal frontal boundaries) oceanographic features. Therefore, beaked whales and sperm whales may not always be associated with specific canyons as suggested by other studies (Kenney and Winn 1987, Jaquet 1996, Davis et ul. 1998). High use of submarine canyons has been documented for sperm whales and beaked whales in the Gully region off the Nova Scotian shelf (Whitehead et ul. 1992, Gowans and Whitehead 1995), and sperm whales in the Mississippi Canyon in the northcentral Gulf of Mexico (Davis et al. 1998). In a previous analysis of cetacean data (ie., CETAP) collected in this region, Kenney and Winn (1987) determined that cetacean biomass was significantly lower in canyon as compared to non-canyon defined sampling blocks. Using a finer scale canyon delineation, and larger


Figure 3 . Continued.

within-season sample sizes, as compared to Kenney and Winn (1987), we did detect a significant difference in beaked whale sighting rates between canyon and intercanyon habitats. However, the prediction map did not indicate that either taxa were concentrated in submarine canyons (Fig. 3). Differences between beaked whale and sperm whale association with canyon features in this region may be because of the steep gradient along the extensive shelf break and ephemeral hydrographic events. In contrast to the Nova Scotian shelf where the Gully is the dominant topographic feature (Whitehead et al. 1992, Gowans and Whitehead 1995), the US. Atlantic shelf break is delineated by numerous canyons (Uchupi 1968). Also, warm-core rings are typical hydrographic features off the northeast U.S. coast (Halliwell and Mooers 1979). Sperm whales are highly mobile and are known to concentrate along hydrographic features (Whitehead et al. 1992, Waring et al. 1993, Jaquet 1996, Griffin 1999). Therefore, their association with canyons is likely to be tran- sitory and influenced by the presence of warm-core rings.

Absence of significant association of sightings between beaked whale and sperm whale, and a significant difference in water temperature at sighting

7 14 MARINE MAMMAL SCIENCE, VOL. 17, NO. 4, 2001

locations suggests that the two taxa use different habitats at a 4-km grid scale. Based on average oceanographic values, beaked whales occupied cooler waters than sperm whales.

The small scale habitat separation may be related to differences in diet, although scant data are available on the trophic ecology of the two species within the study areas. Whitehead et al. (1992) and Gowans and Whitehead (1995) have suggested that habitat partitioning for similar prey may explain sperm whale and northern bottlenose whale (Hyperoodon ampullatus) distribution in the Gully, a major submarine canyon region on the Scotian Shelf. Based on comparisons of 815 N values for several cetacean species off Newfoundland, Ostrom et al. (1993) suggested that the diet of Sowerby’s beaked whale consisted of small squid, whereas sperm whales consumed larger squid (higher S15 N value). For instance, larger sperm whales make deeper dives (Lockyer 1997) to feed on larger cephalopods (Clarke 1980, 1996), which are known to occupy deeper waters (Lu and Clarke 1975). Thus, we suggest the differences in prey selection could be important factors contributing to the habitat partitioning observed in this study.

To better elucidate habitat preference and ecology of beaked whales and sperm whales off the northeast U.S. coast, finer scale surveys, with a comprehensive data collection program (i. e., multibeam hydro-acoustics, high-speed mid-water trawling, vertical water column profiles, and zooplankton sampling) would be required. Excluding some exploratory fishing for shelf-edge and slope-water resources and large pelagic fisheries, fin fish and cephalopod populations have remained unexploited in this region. This provides an oppor- tunity to study cetacean foraging ecology in a relatively undisturbed environment.

ACKNOWLEDGMENTS

We thank the many scientific observers who participated in the seven shipboard surveys, We also thank the officers and crew of the NOAA RIVs Chapman and Delawave 11; RIV Abel-J and RIV Pelican. The manuscript was improved by reviews and com- ments by Drs. Phil Clapham, Richard Merrick, Debra Palka, and Fred Serchuk, and two anonymous reviewers. Partial funding for the surveys was provided by the Office of Protected Resources, National Marine Fisheries Service, Silver Spring, Maryland. This research was conducted under Scientific Research Permits 520 and 917.

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Received: 20 March 2000 Accepted: 19 February 2001

CHARACTERIZATION OF BEAKED WHALE (ZIPHIIDAE) AND SPERM WHALE (PHYSETER MACROCEPHALUS) SUMMER HABITAT...

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