HINCKLEY: REPRODUCTIVE BIOLOGY OF WALLEYE POLLOCK 



stage 9 (nuclear migration) oocytes were called 

 stage 8; stage 11 (maturation) and stage 12 (ovu- 

 lation) were called stage 11, owing to the diffi- 

 culty in consistently differentiating these stages. 

 Counts from all points on the grid were combined 

 into a total egg-stage frequency for each ovary. 

 Development of the maturing oocytes could be 

 followed by comparing the egg-stage frequencies 

 for each level of maturity. 



Slides were also examined for evidence of 

 ovarian rematuration. Rematuring ovaries were 

 defined as those containing identifiable postovu- 

 latory follicles (the remnant of the egg membrane 

 left after ovulation and release of the oocyte); 

 thick ovarian walls; and resorbing, unspawned, 

 fully yolked oocytes in ovaries that also contained 

 vitellogenic oocytes. 



RESULTS 



Spatial and Temporal Distribution 

 of Spawning 



Observer information showed that walleye pol- 

 lock spawning in the Bering Sea began in the 

 Aleutian Basin in January. As the year pro- 

 gressed, spawning was observed further inward 

 over the continental slope and shelf (Fig. 2A 

 through 2H). Spawning occurred between Janu- 

 ary and March in the basin, between March and 

 June over the southeastern Bering Sea slope and 

 shelf, and between June and August over the 

 northwest slope and shelf. Scattered spawning 

 was noted in the northwestern areas as late as 

 October. 



Spawning walleye pollock were caught at 

 depths ranging from 46 to 360 m, most commonly 

 between 100 and 250 m. Temperatures at these 

 depths ranged from 1.8° to 6.0°C (x = 2.34°C). 



The monthly distribution of the commercial 

 fishing fleet in the Bering Sea was examined to 

 assess whether observer reports from the fleet 

 represented the true distribution of spawning. If 

 significant portions of the Bering Sea were not 

 fished by the fleet, concentrations of spawning 

 walleye pollock could have been missed. In Janu- 

 ary and February, coverage of the continental 

 shelf was scattered and in May most fishing oc- 

 curred in the southeast portion of the Bering Sea. 

 Coverage of the Aleutian Basin was minimal 

 after March because harvestable concentrations 

 of spawning walleye pollock could not be found in 

 the area after this time (R. Nelson'^). Neverthe- 

 less, the fishing fleet distribution appears to have 



been sufficiently extensive to detect the majority 

 of spawning walleye pollock, and the reports of 

 spawning obtained from the fleet appear to rea- 

 sonably represent the true spawning distribution 

 for 1984. 



Length-Age Characteristics of 



Walleye Pollock from Spawning 



Concentrations 



The plots of mean length at age for walleye 

 pollock males (Fig. 3) and females (Fig. 4) suggest 

 that length at age was similar for both sexes in 

 the Aleutian Basin and over the northwest slope. 

 Lengths for the older ages were smaller in these 

 areas than in the other three. Although the data 

 were widely scattered, length at age was similar 

 in the southeast shelf, southeast slope, and north- 

 west shelf, and was larger in these areas than in 

 the basin and the northwest slope. 



Comparison of the length-at-age data from 5° by 

 1° cells with the growth curves generated by 

 Francis and Hollowed showed that most of the 

 cells assigned to the "northern" group were in the 

 northern slope and buffer areas and in the 

 Aleutian Basin (Fig. 5). Walleye pollock in these 

 areas were characterized by a smaller mean 

 length at age. Cells designated as "southern" 

 were mostly from the southeast and northwest 

 shelf areas and southeast buffer zone (Fig. 5) and 

 contained walleye pollock with a larger mean 

 length at age. The spawning concentrations of 

 walleye pollock, therefore, show the same geo- 

 graphic distribution of the two growth types as 

 seen by Lynde et al. (fn. 2) and Francis and Hol- 

 lowed (unpubl. data). 



Length-Fecundity Relationship 



In all areas of the Bering Sea, the length- 

 fecundity relationship for walleye pollock (Fig. 6) 

 was found to be curvilinear (F = aL^, Table 1), 

 similar to that observed for walleye pollock from 

 other regions. The overall test for coincidental 

 regression indicated (F = 5.51, P < 0.001) that 

 the length-fecundity relationships for the four 

 areas were not identical. 



Multiple range test results (Table 2) indicated 

 that the northwest and southeast shelf and slope 

 area regression slopes did not differ significantly. 



5R. Nelson, Observer Program, Northwest and Alaska Fish- 

 eries Center, National Marine Fisheries Service, NOAA, 7600 

 Sand Point Way N.E., Seattle, WA 98115, pers. commun. 1984. 



485 



