CAREY Mild KOBISON: DAILY PATTERNS IN ACTIVITIES OF SWORDFISH 



isotherms at the end of the track in Figure 8 and 

 from the position of the Gulf Stream indicated on 

 the 9 November 1977 Experimental Ocean Frontal 

 Analysis Chart. Swordfish concentrate along the 

 edge of the continental shelf, but are widely 

 distributed over the ocean. The course that no. 7 

 followed may have been a normal one for sword- 

 fish in the Hatteras area, or no. 7 may have been 

 influenced to swim offshore by the trauma of being 

 caught on longline fishing gear. 



Vertical Movements and Light 



The tagged swordfish showed a clear diel pat- 

 tern of vertical movements, going deep during the 

 daylight hours and coming to the surface at night 

 (Figures 5, 8, 9). This is best illustrated by 

 swordfish no. 7 on 11 November (Figure 8). The 

 fish spent the night at about 20 m. About 1 h before 

 dawn, when light was just noticeable in the east, it 

 swam down rapidly, reaching a depth of 400 m by 

 sunrise, and worked gradually deeper until it 

 reached 617 m at midday. After noon the swordfish 

 gradually rose again, increasing its rate of ascent 

 sharply around sunset and reaching the surface 

 about 1 h later. 



There is an obvious relationship between the 

 vertical movements of swordfish and light. The 

 most rapid changes in depth were during a 2-h 

 period at dawn and dusk when surface illumina- 

 tion changes by six or seven orders of magnitude 

 (Brown 1952), and the greatest depth was reached 

 at noon when light at the surface was at a 

 maximum. The U-shaped pattern of the depth 

 curve would be expected for an animal following 

 an isolume ( Blaxter and Parrish 1965; Boden and 

 Kampal967). 



The swordfish also appeared to respond to moon- 

 light. There was no moon during experiment 2 

 (new moon, 21 April) and only a thin crescent for 

 swordfish no. 3. On these nights, which were 

 starlit and calm, the swordfish were usually at 

 depths <10 m and often right on the surface 

 (Figure 9). In the other experiments there was a 

 full moon shining through clouds and it was 

 windier. Nighttime depths for no. 5 and 6 were 10- 

 50 m (Figure 5) and for no. 7 about 25 m (Figure 8). 

 The fish were probably swimming at a greater 

 depth in response to moonlight, although the wind 

 might also have had an influence. 



Guitart Manday (1964) analyzed the depth at 

 which swordfish were taken in a Cuban fishery at 

 various phases of the moon. He concluded that 

 moonlight did not affect vertical migration, but 

 noted that the fishermen felt that the phase of the 

 moon was important and there is some suggestion 

 in his data that fish were taken deeper during the 

 full moon. Tesch (1978) used acoustic transmitters 

 to follow the silver eel stage o{ AnguiUa anguilla 

 and reported that one which swam at a depth of 

 100 m while the moon was up rose to 50 m when 

 the moon set. Our similar records for swordfish 

 suggest that they also respond to moonlight. 



If swordfish move vertically to maintain a 

 constant level of illumination, the light level they 

 follow should be somewhat greater than that on 

 the clear, starry nights when they came to the 

 surface and somewhat less than in moonlight 

 when they swam at 10-50 m depth. We may use the 

 values for irradiance of 3 x 10^ /xW/cm^ under a 

 clear night sky with full moon, 3 x 10"^ /uW/cm^ 

 foraclear sky withnomoon,and3 x lO^"* /u,W/cm^ 

 for & dark night from Clarke and Kelly ( 1964). At 

 noon, 11 November 1977, swordfish no. 7 was at lat. 

 35° N and at a depth of 600 m. Surface illumina- 

 tion, /o, for this time, recorded in Woods Hole 

 and corrected for latitude, was 5 x 10^ /xW/cm^ 

 (Payne ). Light, /, at depth, L, of the fish can be 

 calculated assuming an attenuation coefficient, k, 

 of 0.028 for the clear Gulf Stream-Sargasso Sea 

 water in this area, (Clarke and Backus 1964). 

 The relationship 



I = he 



-kL 



gives 2.5 X 10^^ ^iW/cm^ a value similar to that 

 on a clear, moonless night when the fish might be 

 expected to come to the surface. The fact that 

 swordfish may also come to the surface during the 

 day indicates that they are not locked to an 

 isolume, but under some conditions they do appear 

 to adjust their vertical position in a manner which 

 would maintain a constant, dim illumination 

 throughout the day. 



Vision is obviously important to swordfish. The 

 eyes of a 150 kg fish are as large as an orange and 

 almost touch in the midplane of the skull. The 

 amount of light we calculate for the isolume 

 followed by swordfish is many orders of magnitude 

 greater than the 3 x 10^" ^iW/cm^ suggested as a 



"U.S. Navy Oceanographic Office NSTL Station, Bay St. 

 Louis, MS 39522. 



"R. E. Payne, Woods Hole Oceanographic Institution, Woods 

 Hole, MA 02543, pers. commun. 1977. 



287 



