Classification of Ovarian Stages of Walleye Pollock (Theragra chalcogramma)

Walleye pollock, Theragra chalcogramma, is the most commercially important species in the eastern Bering Sea. Estimates of maturity are critical for setting an appropriate harvest rate for pollock spawning biomass, and correct classification of ovaries into a maturity condition is necessary for accurate estimation of maturity. Data on length, weight, and ovary weight and condition were collected from 4,996 pollock in 2002 and 5,201 in 2003 aboard pollock trawlers across the eastern Bering Sea. In 2003, 173 pollock ovaries were collected and prepared for histological analysis. Maturity condition was assessed by macroscopic inspection of gonads, a gonado-somatic index (GSI), and histological methods. Macroscopic inspection was based on alterations in ovary size and appearance, whereas histological methods evaluated changes in oocyte stages. The GSI was calculated as the proportion of ovary to body weight. The GSI was a good indicator of pollock that had spawned or were about to spawn. Histological analysis confirmed the overall general appropriateness of macroscopic staging for mature versus immature fish, but it also indicated relatively high misclassification rates for particular maturity stages when using macroscopic staging methods alone. Among ovaries macroscopically classified as developing, 16% were at immature oocyte stages and 84% were at primary yolk to more advanced oocyte stages. This indicates that pollock classified macroscopically as “developing” may mature in either the current or subsequent spawning season. Macroscopic inspection and 2 Stahl and Kruse—Ovarian Stages of Walleye Pollock GSI in combination may be useful to determine maturity condition for some maturity stages; however, histological examination of ovaries is the most accurate method for all stages.