NOTE Kerstetter et al.: Shark predation and scavenging on fishes equipped with satellite archival tags 



753 



the next eight days. The depth record following 

 this change in light level was marked by several 

 discrete diving events, and depths (see Fig. IF) 

 ranged between the surface and over 699 m. 

 Recorded temperatures for this period varied 

 between 18.9° and 29.5°C, although sea surface 

 temperatures in the area where gear was set and 

 hauled back varied from 20.9° to 26.0°C (Ker- 

 stetter, unpubl. data). On 12 August, the light 

 level returned to its maximum value and the tag 

 remained at the surface for approximately one 

 day until its scheduled release date (13 August) 

 when it began transmitting data. 



Opah 



The PAT2 satellite tag was expected to pop-up 

 6 months after deployment, but the first trans- 

 mission was received after only 34 days from a 

 location about 330 km (178 nmi) northwest of the 

 deployment site. All the archived binned light 

 level, temperature, and pressure (depth) data 

 from this period were recovered (see Table 1). 

 This tag model collected eight temperature and 

 depth samples during each 12-hour period, result- 

 ing in 16 values per day or 528 total values for 

 the deployment period. The two 12-hour blocks 

 were removed from all analyses to more accu- 

 rately represent the differences in data between 

 specimens: 1) the 12-hour block after tagging in 

 order to allow for the recovery of the animal, and 

 2) the 12-hour block during which the predation 

 event putatively occurred in order to clarify the 

 potentially distant depth and temperature char- 

 acteristics of the ingesting animal. 



The measured sea surface temperature during 

 the tagging of the opah was 25.9°C. The ranges 

 of dive depths, temperature, and light based on 

 minimum and maximum values over the 12-hour 

 day and night periods showed two distinct pat- 

 terns (Fig. 2). During the first period (23 days), 

 the dive depths ranged from about 32 to 456 

 m (Fig. 2A). Water temperatures encountered 

 by the tag during this period ranged from 8.0° 

 to 25.6°C (Fig. 2B) and the light index values 

 ranged from about 50 to 150 (Fig. 2C). During 

 the second period (11 days), the dive depths ranged from 

 to 524 m, temperature ranged from 26.2° to 30.6°C 

 (higher than the 24.2-24.8°C SST recorded by the tag 

 after it was released from the fish), and the light index 

 recorded persistently low values. 



Discussion 



WM1 



Our interpretation of these data is that the PSAT 

 on WM1 was ingested by an animal scavenging the 

 marlin carcass. The first PSAT readings for WM1, 



recorded about one hour after its release, indicated 

 that the marlin was already dead or moribund by 

 that time and was descending to the ocean floor. For 

 the next ten hours, the tag and carcass remained at 

 a constant depth of 145 m (the depth of the nearest 

 sounding at the site of release, according to NOAA 

 depth chart 13003 [1998], was approximately 160 m) 

 and at a temperature of 11°C. The light level steadily 

 decreased at approximately 4:30 pm, corresponding to 

 changes in ambient light from the setting of the sun. 

 At approximately 9:00 pm local time, there was a dra- 

 matic change in conditions when temperature rapidly 

 rose to near 26°C and depths began to vary between 

 the surface and 600 m. 



