572 



Fishery Bulletin 105(4) 



srN 



11°N 



3rN 





North 

 Pacific 

 Ocean 



N 



172"30'W 172°2rO-W 



500 km 



1 I I I I I 



^ Geographic distribution of Atka macl<erel 

 ^ Egg mass collection site 



176°N 



174°N 



164°N 



154°N 



144°N 



Figure 1 



Map showing the geographic distribution of Atka mackerel (Pleurogrammus monopterygius; 

 light gray shading) in the North Pacific Ocean and adjoining seas. Also shown is the location 

 (*) near Seguam Island where a newly fertilized Atka mackerel egg mass was obtained for 

 an incubation experiment. 



preserved embryo and the observations from the en- 

 tire progression were used to construct a chronologi- 

 cal series describing embryonic development at 6.0°C. 



Results 



The newly fertilized eggs were collected from a nest 

 containing several separate egg masses. The eggs were 

 light green and were relatively soft and loosely clumped, 

 in contrast to their character in the following days when 

 they became more firm, rubbery, and tightly attached to 

 one another. As the eggs aged, their color turned reddish 

 brown. The original egg mass had approximately 1500 

 eggs that ranged in diameter from 2.5 to 2.8 mm. The 

 average daily water temperature during the incubation 

 ranged from 5.42° to 6.57°C and the mean and standard 

 deviation were 6.2° and ±0.3°C, respectively. Incubated 

 eggs began hatching on day 74 and preserved length of 

 yolksac larvae was 9.4-10.9 mm SL. 



Embryonic development 



The description of embryonic development was divided 

 into two sections: the first section is a description of the 



development of morphological features, and the second 

 is a description of the development of pigmentation. For 

 brevity, each paragraph in the pigmentation section 

 describes one or more pigment type from the time they 

 first appear until they are fully developed. Table 1 pro- 

 vides descriptive summaries for the progression of both 

 morphological features and pigmentation. 



Morphological features 



Preserved eggs from the day of collection (fertilization) 

 had a single blastomere sitting on a yolk containing a 

 group of >50 oil globules varying in size (0.08-0.20 mm). 

 After 72 hours of meroblastic cleavage, a blastodermal 

 cap formed on top of the yolk. Over the next four days, 

 the blastodermal cap flattened and increased in diameter 

 as the edge began folding under (gastrulation) to form a 

 germ ring that started moving downward over the yolk 

 (epiboly; Table 1). The embryonic shield formed as the 

 germ ring advanced over the yolk. The blastopore closed 

 on day 15 when the germ ring completely enveloped 

 the yolk. During days 17-21, the optic capsule and eye 

 lenses formed, the head and anterior body became thick 

 and more defined, and the notochord, myomeres, and 

 Kupffer's vesicle became visible (Table 1). 



