562 



Fishery Bulletin 105(4) 



56°N - 



srN 



-^ Temperature data sets 

 Q^ Trawl locations 



V 



Bering 

 Sea 



Amchltka Island 

 187 egg masses 

 87-92 m 

 5.41-5.95°C 



Tanaga Island 

 1 8 egg masses 

 80-105 m 

 5.31-5.42°C 



j^JS^ 



Seguam Island 

 61 egg masses 

 109-149 m 

 4.84-5.69°C 



179°W 



174AA/ 



1 69''W 



164°W 



159°W 



Figure 1 



Map showing collection sites for water temperature data sets (asterisks; also see Table 1) 

 and trawl locations where Atka mackerel [Pleurogrammus monopterygius) egg masses were 

 collected (open circles). The number of egg masses collected and the range of water depth and 

 temperature are listed beneath each trawling location. 



Incubation model 



An incubation model was used for extrapolating spawn- 

 ing and hatching dates for egg mass samples collected 

 from three different Atka mackerel nesting sites. 

 During the period 7-23 October 2004, egg masses 

 were collected from trawl hauls aboard the commercial 

 stern-trawler FV Seafisher and samples were preserved 

 in 10% phosphate-buffered formalin. The October col- 

 lection period was chosen because histological studies 

 have indicated it is the month when spawning ends in 

 Alaska (McDermott and Lowe, 1997). A total of 266 

 egg masses were collected: 187 from the Amchitka 

 Island area, 18 from near Tanaga Island, and 61 from 

 the waters surrounding Seguam Island (Fig. 1). In the 

 laboratory, egg mass samples were placed underneath 

 a stereo dissecting microscope and aged to the nearest 

 day by using the the descriptions of embryonic devel- 

 opment from the incubation experiment described by 

 Lauth and Blood (2007). 



The calculation of spawning and hatching dates was 

 based on the assumption that the effect of temperature 

 on incubation rate for any given development stage was 



nearly a constant linear relationship for the entire in- 

 cubation period. The constant relationship is generally 

 true for temperatures normally encountered by eggs 

 in their habitat (Ahlstrom, 1943; Riley, 1974; Hempel, 

 1979). Spawning and hatching dates were estimated by 

 using the following relationship: 



d, 



A ^6° C 



where d, 

 D, 



= 6°C 



-"e-c 



age of the egg mass (days) at temperature t; 

 total incubation time (days) at temperature 

 t\ 



estimated age of the collected egg mass 

 (days) using the descriptions of embry- 

 onic development from Lauth and Blood 

 (2007); 



total number of days for first egg to first 

 hatching: 



Spawning date = Egg mass collection date - d^, and 

 Hatching date = ^Egg mass collection date - d^) + Z?,. 



