series will be affected by changes in the system.
(^) Most spawner–recruit relationships are affected by the sheer variation from year to
year in the success of larvae in reaching recruitment, the year-class effect. We cite
only one example among a vast assortment. The Alaska pollock (Theragra
chalcogramma) stock in the Bering Sea is closely monitored by annual fishery
surveys that determine the number of fish reaching the age of one year. The variation
(Ianelli 2005; Fig. 17.12a) is very great and depends, in ways often difficult to
specify, on winter weather, predator activity, mesozooplankton production, variation
in currents during the larval drift, and more. In this fishery, as in many others, the
yields for multiple years can depend upon the availability and growth of a particularly
successful year class. The ridge of increasing mean size of pollock landed from 1992
to 1997 (Fig. 17.12b) was caused by the dominance of the yield by the strongly
successful 1989 year class. Huge effort has gone into understanding year-class
strength variation of different stocks, with varying but always modest success.
Fig. 17.12 (a) Recruitment variation of age 1 walleye pollock (Theragra
chalcogramma) in the eastern Bering Sea (EBS) stock as determined by fishery-
independent trawl survey. Bars shown for each year are upper and lower recruitment
confidence limits from the survey results. Notice the strong recruitment in 1989. (b)
Year-on-year size–frequency distributions for the summer catches of the EBS pollock
fishery. The ridge from 40 cm in 1992 to 55 cm in 1997 is the 1989 year class.
(^) ((a) from data in Ianelli 2005; (b) after Ianelli 2005.)