FISHERY BULLETIN: VOL. 72, NO. 3 



Table 9. — Contribution of deep net to total catch of larvae on stations where 

 both nets were used, compared to weighted mean surface temperature. 



Cruise 



IVIonth 



Percent 

 caught in 

 deep net 



Weighted mean 



surface 

 temperature (°C) 





 34 

 62 

 82 

 78 

 52 



14.4 

 14.5 

 20.6 

 19.9 

 15,5 

 11.7 



Total number 

 caught in 

 both nets 



1 



408 



2,940 



3,836 



3,164 



398 



Table 10. — Diel differences 



in captures of silver hake larvae, cruises D-66-7 (June) through 

 D-66- 14 (November-December). 



z 80- 



6 20- 



ooooooo ooot 



>oo oooooo 



I I I I I 1 I I 



NOTOCHORD LENGTH i 1 < 



Figure 10. — Percentage of silver hake larvae collected in non- 

 day tows per 1-mm size groups. 



in night tows, the most generally accepted being 

 that larvae avoid the approaching sampler during 

 daylight in response to visual warning. The differ- 

 ence cannot be attributed to vibration of the tow- 

 ing cable or inefficient filtration by the sampler 

 because these factors are equal during all light 

 regimes. Undersampling of larger larvae of other 

 species during daylight has been well documented 

 (Silliman, 1943; Bridger, 1956; Ahlstrom, 1959; 

 Colton, 1965). These authors, however, noted diel 

 differences resulting from tows made at 1 knot. 

 Miller, Colton, and Marak (1963) towed a high- 

 speed plankton sampler at 7 knots and found no 

 significant differences in the day and night 

 catches of haddock larvae and pelagic juveniles. 

 Ryland (1963) concluded that a towing speed of 

 5 knots (257.4 cm/s) was sufficient to prevent net 

 avoidance by plaice larvae up to 20.0 mm whose 



maximum "darting velocity" he found to be 20 

 cm/s. If, by towing at 5 knots, we were able to 

 overcome net avoidance by larger larvae, then the 

 presence of larger larvae in night tows only must 

 reflect some form of diel activity or vertical 

 migration. Kelly and Barker (1961) found a sig- 

 nificant difference in depth distribution with 

 growth of young redfish, the larger juveniles 

 occurring in deeper layers. A similar difference 

 plus a diel change in depth distribution is ob- 

 served with silver hake when the light regime, 

 capture depth (net 1 vs. net 2), and mean larval 

 length are combined (Figure 11). The largest 

 larvae were captured in the deep net during the 

 night, the smallest larvae in the shallow net 

 during the day. In both nets, night tows con- 

 tained larger larvae than day tows, and in both 

 light regimes the deep net contained larger 

 larvae than the shallow net. Evidently, with 

 growth, silver hake larvae seek deeper water, 

 perhaps in response to increasing negative photo- 

 tropism, perhaps simply approximating the adult 

 habitat. 



During the summer of 1970, we made two 

 cruises to investigate the size at which silver hake 

 larvae first occur on or near bottom. On 12 stations 

 northeast and southwest of Hudson Canyon, we 

 made reciprocal tows with Gulf V samplers and 

 an otter trawl (39-foot headrope) fitted with a V4- 

 inch mesh cover bag and separate cod end. Length 

 frequencies of the Gulf V catches, compared with 

 those of the otter trawl (Figure 12), indicate that 

 silver hake first become available to bottom 



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