Sperm whales, Physeter macrocephalus, were tracked by means of a recording depth sounder in the waters off the Galapagos Islands. At depths of less than 300 m the whales generally dived nearly vertically at 60–100 m/min. At greater depths their descents were usually slower. Between February and April 1985, they dived to about 420 m, which is approximately the depth of the oxygen minimum. In 1987, a year of warmer water temperatures, they usually dived about 70 m shallower. There was no apparent diurnal variation in dive depths. None of the whales tracked dived to the ocean floor. Whales dived for about 40 min, followed by 10 min at the surface. Sperm whales usually started to make regular clicks when 150–300 m deep. Young calves appeared not to make prolonged deep dives. Our results are generally consistent with other direct information on the diving behaviour of relatively undisturbed sperm whales, but often conflict with results obtained using sonar for sperm whales being chased by whale catchers.

Killer whales or orcas, Orcinus orca, are known to prey occasionally upon members of larger whale species, including sperm whales, Physeter macrocephalus (Hoyt, 1981; Martinez and Klinghammer, 1970). Despite several references in the Russian literature, Berzin (1972) considered the reports to be too rare for killer whales to be "branded serious enemies of the sperm whale." Schevchenko (1975) stated that, in the Antarctic, 65.3% of sperm whale carcasses examined showed signs of killer whale tooth marks, although he did not say if or how he differentiated between marks of killer whale teeth and those made by other sperm whales. Yukhov et al. (1975) found remains of sperm whales in killer whale stomachs in the subtropical and temperate waters of the southern hemisphere, and mention a movie-film showing an attack by a group of killer whales on a school of female sperm whales and their calves. However no description was given. We report here observations made during an encounter between a group of sperm whales and a group of killer whales off the Galapagos Islands, and the subsequent flight of the sperm whales from the area of the attack. During the early part of 1985, we used the 10-m sloop Elendil to follow groups of sperm whales west of the island of Isabela (1*00'S, 91*00'W) in the Galapagos Islands, Ecuador. The methodology employed was based on that described by Whitehead and Gordon (in press). The groups of whales were tracked acoustically using a directional hydrophone at night, and acoustically and visually during the daytime. Sperm whales could be heard at about 8 km using our acoustic equipment. Whenever possible, photographs of dorsal fins and flukes were taken to identify individuals from distinctive marks and scars. At 5-min intervals we recorded the compositions, relative positions, speeds (estimated by comparison with nautical speedometer on boat), headings, and behavior of each visible subgroup. A subgroup constituted a set of whales swimming in a coordinated manner, each less than 100 m from its nearest neighbor within the subgroup. Except in exceptional circumstances, once each hour on the hour an acoustic recording was made through an omnidirectional hydrophone lasting 5 min. The steady click (at about 1 click per second) of the sperm whale is an indicator that the sperm whale is feeding at depth (Whitehead and Gordon, in press). In the following, "many clicks" indicates that we were hearing the output of roughly 10 or more sperm whales. Depth sounder traces of diving whales were obtained when possible. Positions were from a satellite navigator. The encounter between the sperm whales and the killer whales occurred between 0945 and 1230 h local time (GMT-6 h) on 18 April 1985. The observations were divided into five periods: 1. The day before the encounter; 2. The night before the encounter; 3. The morning just before the encounter; 4. The encounter; 5. After the encounter. Analysis of photographs showing individually identifying marks indicated the same group of animals to be present throughout the...

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Sperm whales Physeter macrocephalus were observed off the Galapagos Islands between late February and April 1985, a year of cool sea-surface temperatures (SST), and January to June 1987, an 'El Nino' year of warm SST Distributlon, abundance and diet of sperm whales were similar in both years. However, in 1987 they appeared to have a lower feeding success, as ind~cated by a reduced rate of observing faeces, and dived to shallower depths. Excretion rates were negatively correlated with sea-surface temperatures.The 'El Nino' phenomenon, an irregular event in which exceptionally warm surface waters appear in the central and eastern tropical Pacific, has considerable effects on most marine life in the area (Merlen 1984, Arntz 1986, Barber & Chavez 1986). El Nido is unfavourable to most pelagic species, but there is little information concerning its effects on mesopelagic species, largely because organisms in this depth range are hard to sample for biomass.The sperm whale Physeter macrocephalus, which feeds in this ecosystem, has indirectly provided considerable information on mesopelagic squid through studies of stomach contents and diving behaviour (Clarke 1980). Recently, research on living sperm whales has also given results on sperm whale feeding behaviour and the ecosystems in which they feed (Gordon 1987, Papastavrou 1987.In this note we relate indications of the feeding success of sperm whales around the Galapagos Islands to sea-surface temperature. We principally use data collected in 1985, a year of cool sea-surface temperatures (mean SST in early 1985 near Galapagos = 25.4 "C), and 1987, an 'El Nilio' year (mean SST in early 1987 near Galapagos = 27.5 "C). We also summarize results on differences and similarities in the distribution, abundance and behaviour of the whales between the 2 years. The rate of observing faeces was used as an indicator of the feeding success of the whales. For this measure we only used faeces observed as the whales fluked-up at the start of a feeding dive, and calculated the proportion of fluke-ups observed within 250 m of the research vessel which were accompanied by a defaecation. This meant that defaecations during breaches (leaps from the water) and other energetic acitivity were not included. Such defaecations may be more related to physiology than feeding success. A total of 3921 fluke-ups were observed within 250 m of the research vessel, and of these 125 were accompanied by defaecations. Sea-surface temperatures were measured every 3 h, but only 06:OO h readings are used in this paper, to avoid the effects of the warming of the surface layer by the sun. Rates of observing faeces and mean sea surface temperatures were calculated for each calendar month while in the Galapagos.The rate at which we observed faeces was significantly, = 21.96, p < 0.01) higher in 1985 (4.8 % of fluke-ups observed with faeces) than in 1987 (2.1 % of fluke-ups observed with faeces). The rate of seeing faeces during each month was negatively correlated with the mean 06:OO h SST during tha...

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