566 



Fishery Bulletin 105(4) 



Discussion 



The mating and brooding cycle (i.e., reproductive cycle) 

 of Atka mackerel lasts from June to January. The 

 mating phase begins with males aggregating in nest- 

 ing colonies where they establish individual nesting 

 territories. Time-lapse camera footage and archival tag 

 data show that male Atka mackerel begin aggregating 

 and establishing nesting territories in early June, 1.5 

 months before spawning commences. The reason for 

 the lengthy nest-establishment period is unknown, but 

 fishes that provide male parental care of eggs typically 

 put considerable effort into choosing and preparing an 



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Figure 5 



Ogive plots of incubation model results for Atka mackerel (Pleuro- 

 grammus monopterygius) egg masses collected from (A) Amchitka 

 Island, (B) Tanaga Island, and (C) Seguam Island. The y-axis is 

 the cumulative frequency of egg mass spawning and hatching dates 

 by week for low (dashed line) and high (solid line) average daily 

 water temperatures for each area (see Fig. 4). 



optimal nesting territory because it can be an impor- 

 tant determinant for maximizing reproductive success 

 (e.g., Sargent, 1982; Sikkel, 1988; Bisazza et al., 1989; 

 Ostlund-Nilsson, 2000). 



The second phase of mating consists of courtship and 

 spawning, during which time a territorial male can 

 mate with multiple females. Spawning begins mid-July 

 and ends in mid-October. A July to October spawning 

 period has been corroborated by a histological study of 

 978 Atka mackerel ovaries from collections made across 

 the entire North American geographical range (McDer- 

 mott and Lowe, 1997), and from observations of captive 

 spawning Atka mackerel used in the incubation experi- 

 ments from the present study. Because such 

 a high proportion of egg masses were found 

 in later stages of development at Amchitka, 

 it is possible that some hatching occurred 

 before the egg mass collections. This would 

 have the effect of shifting the onset of the 

 spawning and hatching period at Amchitka 

 to an earlier time. Because egg masses col- 

 lected from Tanaga and Seguam Islands 

 were much younger at the time they were 

 collected, it is not likely that some earlier 

 hatching took place, especially given the low 

 water temperatures characteristic of each 

 nesting site. 



Detailed analysis of spawning times by 

 area, with the use of aged egg masses, indi- 

 cated a later onset of and peak in spawning 

 as spawning activity moved from west to 

 east. Female Atka mackerel spawn an aver- 

 age of 4.6 separate batches of eggs during 

 the 12-week spawning period (McDermott 

 et al., 2007), but uncertainty about precise 

 ambient temperatures precludes conclusions 

 regarding the periodicity of separate spawn- 

 ing events and whether these events were 

 continuous or episodic. 



During the brooding phase, spawning has 

 ended and males guard the incubating egg 

 masses within their territory until all the 

 eggs hatch. The duration of the male brood- 

 ing phase is highly dependent on water tem- 

 perature. The average water temperature of 

 106 nesting sites covering a broad temporal, 

 bathymetric, and geographic range in the 

 Aleutian archipelago was 5.4°C (Lauth et 

 al., in press). If this water temperature rep- 

 resented a true mean for the entire range 

 of Atka mackerel, the average male brood- 

 ing period would be 11.5 weeks. Using a 

 video drop camera on 30 November 2005, 

 the principal author verified that males 

 were still aggregated and exhibiting nest- 

 ing behavior at a nesting site near Dutch 

 Harbor, AK. Considering that egg masses 

 are spawned until late October and that in 

 situ water temperatures as low as 3.99°C 

 are observed at nesting sites, it is plausible 



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