FAHAY: OCCURRENCE OF SILVER HAKE EGGS AND LARVAE 



z 



< 

 > 



< 



Net 1 2 



Cru.se JUN 



1 2 



AUG 



1 2 



SEP 



1 2 



OCT 



1 2 



NOV- 

 DEC 



night tows 0= day tows 



Figure 11. — Comparisons of silver hake mean larval size, light 

 condition, and depth of capture. Net 1 sampled to a maximum 

 of 15 m; net 2 to a maximum of 33 m. 



sampling gear at about 17.0 to 20.0 mm NL. 

 This figure is somewhat smaller than that indi- 

 cated by Nichy (1969), whose smallest specimens 

 taken by otter trawl were about 50 mm. 



During 1966, all postlarvae larger than 21.0 

 mm NL (except one) were captured at night, and 

 most of those were taken in the deep net, which 

 sampled above, within, and below the thermo- 

 cline, if one was present. Postlarvae larger than 

 21.0 mm NL were taken in the shallow net only 

 when the thermocline was weak or nonexistent. 

 All this suggests that silver hake postlarvae seek 

 the bottom at about 17.0 to 20.0 mm NL and 

 migrate vertically at night, ascending at least to 

 the thermocline depth or, in the absence of a 

 thermocline, to levels nearer the surface. 



DISCUSSION 



The area encompassing the southern slope of 

 Georges Bank, Great South Channel, Nantucket 

 Shoals, and the shelf south of Martha's Vineyard 

 is evidently an important spawning center for 



silver hake. Unfortunately, neither the Dolphin 

 survey nor the Soviet surveys (Sauskan and 

 Serebryakov, 1968) sampled this area exten- 

 sively enough to determine all the possible drift 

 patterns of eggs and larvae. The 70°40'W merid- 

 ian approximates the eastern limit of the Dol- 

 phin survey and the western limit of the sampling 

 reported by Sauskan and Serebryakov. 



Within this wide area, eggs should be expected 

 to drift in several different directions, depending 

 on the location of spawning and on long-term pre- 

 vailing winds. One component of the westerly 

 current on the shelf south of Martha's Vineyard 

 and Nantucket Island originates on the southern 

 slope of Georges Bank where it forms the southern 

 part of a rotational eddy (Bigelow, 1927; Klimen- 

 kov and Pakhorukov, 1963; Bumpus and Chase, 

 1965; Bumpus and Lauzier, 1965; Harrison et al., 

 1967). Eggs spawned on Georges Bank may 

 1) drift wdth the eddy, develop, and recruit back to 

 Georges Bank, or 2) drift west and south of 

 Martha's Vineyard where we consistently found 

 concentrations. Walford (1938) described similar 

 patterns for haddock larvae spawned on Georges 

 Bank. A third possibility may result in the loss 

 of the brood. Colton (1959) reported that silver 

 hake larvae spawned on Georges Bank were killed 

 when a southerly drift carried them off the bank 

 and into warm slope water (the rate of warming 

 exceeding the larvae's rate of acclimation). Pre- 

 sumably, in addition to the perils of warming 

 waters, silver hake larvae carried off Georges 

 Bank into the slope water or Gulf Stream would 

 be carried to the east and, unable to find suitable 

 depths in which to begin the demersal stage, 

 would perish. 



Eggs spawned south of Martha's Vineyard drift 

 west but probably not far before hatching, for the 

 incubation period is only 48 h (Kuntz and Rad- 

 cliffe, 1917). Unfortunately, these authors did not 

 cite the temperature at which incubation or hatch- 

 ing occurred. If we assume (as did Sauskan and 

 Serebryakov, 1968) that Kuntz and Radcliffe 

 incubated their eggs at a maximum temperature 

 of 20°C, then the maximum incubation period in 

 degree-hours would be 960 (48 x 20 = 960). We 

 encountered the heaviest concentrations of 

 eggs on the Martha's Vineyard transect when sur- 

 face temperatures ranged from 13° to 22°C. Even 

 at the minimum temperature of 13°C, incubation 

 would occupy no more than 73.8 h (960/13 = 73.8). 

 Currents between Georges Bank and Delaware 



827 



