The fecundity of the walleye pollock, Theragra chalcogramma (Pallas), spawning in Canadian waters

Abstract

The ovaries of 113 walleye pollock from a resident stock in the Strait of Georgia, British Columbia were examined for determination of fecundity. Oocytes were sized and counted in 20 μm intervals of diameter. Without exception, ovaries contained a pronounced bimodal distribution of oocyte diameters with peaks at 100 and 400–600 μm. Oocytes ≥ 180 μm diameter were undergoing trophoplasmic growth leading to hydration. ‘Apparent’ fecundity is defined as the estimated number of yolked oocytes ≥ 180 μm diameter, regardless of potential resorbtion. Previous workers have not shown that significant resorbtion takes place in the post‐spawned ovary. Total oocyte complement (≥40μm diameter) was best expressed by a linear model where Ft= 33004 f.l.– 869627, where f.l. = fork length in cm and r= 0.86. Estimates of Ft, ranged from 117700 to 1394 100 oocytes ≥40μm. Age was weakly related to fecundity, reflecting large individual differences in annual growth after age 4 years. Apparent fecundity best suited a linear model where Fa= 23522 f.l.– 599713 and r= 0.91. Estimates of Fa fell within the range 58 379–1 151 527. Relative fecundity (eggs g−1) decreased over most of the length range encountered in the sample. The average‐sized female in Georgia Strait is twice as fecund as her counterparts in the north‐western Pacific Ocean, containing some 390 000 to 420 000 oocytes 7ge;180 μm diameter compared to about 200 000 oocytes in a north‐western female of comparable length.