FISHERY BULLETIN: VOL. 79. NO. 2 



minimum layer is not well developed. They would 

 probably encounter less anoxic conditions than 

 the offshore fish which were well into the Pacific. 

 Because of this, an attempt to compare abundance 

 of swordfish in these two areas from the numbers 

 seen on the surface could be grossly misleading. 



On the continental shelf off the northeastern 

 United States and Canada where swordfish can be 

 seen "finning" on the surface during the warm 

 months, the water is well oxygenated from surface 

 to bottom. However, temperatures on the bottom 

 can be quite cold, and the swordfish which are 

 feeding deep may be coming to the surface to warm 

 their muscles or as an aid in digestion. Basking 

 behavior by swordfish may be part of a recovery 

 from a variety of stresses experienced at depth. 



Buoyancy 



Swordfish swimming on the surface seem to 

 have neutral or sufficient positive buoyancy to 

 raise the dorsal and caudal fins out of the water. 

 Swordfish taken on longline frequently float, 

 swim bladders distended, when hauled to the 

 surface, and would have been at neutral buoyancy 

 at a pressure of a few atmospheres. The swordfish 

 are clearly able to inflate their bladder to a 

 volume which will give them neutral density 

 at some near-surface depth. The capillary retia 

 mirabilia of the gas gland are short, about 1 mm 

 long, and similar to those of surface dwellers such 

 as the flying fish (Marshall 1960, see footnote 10). 

 Thus the structure of the gas gland does not seem 

 suitable for rapid pumping of large volumes of 

 oxygen under pressures up to 60 atm at 600 m. Our 

 depth records (Figures 5, 8) show many examples 

 of rapid vertical movements with the fish some- 

 times moving from 100 m to the surface in < 5 min. 

 Such changes in depth could cause a 10-fold 

 expansion in the volume of a free bubble. It seems 

 unlikely that the swordfish could pump gas into 

 and out of its swim bladder rapidly enough to 

 maintain constant volume during these excur- 

 sions. The necessity for doing this could be avoided 

 if the bladder were allowed to compress with 

 hydrostatic pressure as the depth increased. This 

 would increase the density of the fish, but even 

 with the bladder partially collapsed at depth, 

 the high lipid content and porous fatty bone of 

 the swordfish would lower its density and the 



flattened bill and fixed pectoral fins would give it 

 hydrodynamic lift while swimming. 



When at rest at depth, excess density would 

 prevent the swordfish from hovering easily and 

 it might find resting on the bottom to be a 

 convenient position. While on the bottom the fixed 

 pectoral fins would form an effective tripod with 

 the tail (R. H. Backus ). Frequent records of 

 swordfish caught in bottom trawls indicate that 

 resting on the bottom may be common in this 

 species (Bigelow and Schroeder 1953; Eschmeyer 

 1963). Martin Bowen,'^ a National Marine Fish- 

 eries Service (NMFS) observer on foreign squid 

 trawling vessels working between Cape Hatteras 

 and Cape Cod, Mass., reported 28 swordfish taken 

 in bottom trawls during 72 d at sea in 1977. 

 Observers in research submarines have seen 

 swordfish lying on the bottom (Zarudski and 

 Haedrich 1974), and in our records it appears that 

 swordfish no. 3 was on the bottom for several 

 hours. 



Temperature 



Water temperatures encountered by swordfish 

 in the Baja California area are illustrated in 

 Figure 5. A 10° C (18° F) gradient was present 

 between the surface and the depth of the deepest 

 dive, 300 m. The gradient between surface and the 

 usual daytime depth was 5°-7° C. The fish made 

 frequent excursions through the thermocline, 

 passing such gradients in a few minutes. While 

 these are significant temperature changes, they 

 did not seem to affect the activities of the fish, 

 which in this area appear to be more influenced by 

 the presence of anoxic water. 



Our record for swordfish no. 7 in the North 

 Atlantic shows the impressive ability of this 

 species to penetrate marked thermal boundaries 

 (Figure 8). The greatest temperature change 

 occurred on the morning of 10 November when 

 this fish moved from 27° C water on the surface 

 to 8° C water at 420 m, a 19° C excursion in 

 2.5 h. This is a large change for any heterothermal 

 organism to undergo and remain active. It was not 

 just a brief excursion, for it remained in the cold 

 water all day. The thermal history of the fish 

 before this dive was complex, but the preceding 



'"N. B. Marshall, Park Lane, Saffron Walden, Essex, Engl, 

 pers. commiin. 1975. 



" R. H. Backus, Woods Hole Oceanographic Institution. Woods 

 Hole, MA 02543, pers. commun. 1972. 



M. Bowen, Northeast Fisheries Center, National Marine 

 Fisheries Service, NOAA, Woods Hole, MA 02543, pers. 

 commun. 1978. 



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