20 



Fishery Bulletin 105(1) 



170°E 



160°W 



55°N- 



50°N- 



45°N- 



Legend 



▲ Samples 1994 



• Samples 1993 



55°N 



■50°N 



180° 



170=W 



leo-w 



Figure 1 



Sampling locations for Atka mackerel (Pleurogrammus monopterygius) collected 

 for estimates of annual fecundity and batch fecundity in 1993-94 in the Aleu- 

 tian Islands, Alaska. 



oocytes into which no new oocytes are recruited once 

 spawning begins; 2) females used to estimate potential 

 fecundity have not spawned; and 3) atretic losses are 

 negligible or can be estimated. Recent studies have 

 shown that many fish species reabsorb a substantial 

 number of their oocytes before spawning or at the end 

 of the spawning season — a process known as atresia 

 (Hunter and Macewicz, 1985; Kjesbu et al., 1991; Ma et 

 al., 1998) Atretic losses in Icelandic cod (Gadus morhua) 

 and Atlantic herring (Clupea harengus) occur mainly 

 during the earlier phase of vitellogenic development, 

 decrease as oocytes mature, and are negligible near the 

 time of spawning (Hardardottir et al., 2001, Kurita et 

 al., 2003). Scombroid fishes (mackerels and tunas) have 

 been described as having extensive atresia at the end of 

 their spawning season (Hunter and Macewicz, 2001). 



Previous studies (Zolotov, 1993) have described Atka 

 mackerel as determinate spawners. However, no study 

 to date has examined the oocyte-size distribution or 

 shown that fecundity decreases after spawning. We ex- 

 amined the oocyte-size distribution for mature oocytes 

 over time. Additionally, all ovaries were examined his- 

 tologically to identify advanced oocyte stages, presence 

 of postovulatory follicles, and oocyte atresia. We esti- 

 mated potential annual fecundity and batch fecundity. 

 The number of batches to be spawned was calculated 

 by dividing total fecundity by batch fecundity. Poten- 

 tial fecundity was studied by comparing prespawning 

 individuals with those that had spawned at least one 

 batch of eggs in order to determine if advanced oocytes 



are added throughout the spawning season. Atresia of 

 mature oocytes was estimated from ovaries of post- 

 spawning females to calculate realized fecundity as a 

 measure of oocytes actually spawned per year. 



Materials and methods 



Data collection 



All Atka mackerel examined in this study were either 

 collected by the National Marine Fisheries Service 

 (NMFS) fisheries observers aboard commercial fishing 

 vessels during the commercial Atka mackerel fishery 

 in 1993 or by Alaska Fisheries Science Center (AFSC) 

 scientists during the 1994 NMFS bottom trawl survey 

 of the Aleutian Islands (Table 1, Fig. 1). 



During both years, Atka mackerel were subsampled 

 from individual trawl tows. Collections were stratified 

 by fish size; no more than five fish per size group were 

 collected in each NMFS statistical area during a sam- 

 pling year. The fork length of each fish was measured 

 to the nearest centimeter, and each fish was weighed 

 to the nearest 0.1 kg. In most cases, the stomach was 

 emptied before weighing the fish. The ovaries were ex- 

 cised and placed in labeled cloth bags in 10% formalin 

 solution buffered with sodium acetate (20 g per liter 

 solution). In order to determine maturity stage, and 

 presence of atresia, all ovaries were processed histologi- 

 cally. Four ovarian sections (4-f<m thick) were stained 



