GUNDERSON: POPULATION BIOLOGY OF SEBASTES AIMTUS 

 Females 



Female gonads are difficult to weigh during the 

 embryo release period, since they are easily rup- 

 tured then. Furthermore, eggs and embryos can be 

 extruded with slight pressure on the body cavity 

 during this period, and it is possible that sig- 

 nificant quantities of these sex products are lost 

 when fish are compacted in the cod end of a trawl. 

 For these reasons, no data on gonad weight of 

 mature females were taken during March. 



Between-season comparisons for "maturing" 

 females (Figure 18) show that their relative gonad 

 weights were virtually the same during August- 

 November as they were during the embryo release 

 period in March. This is conclusive evidence that 

 "maturing" fish are not sexually active, and they 

 were grouped with immature fish in all later 

 analysis. 



Differentiation of "maturing" and mature fish 

 was less difficult for females than for males. It was 

 most difficult during July-November, when most 

 adult fish were in maturity Class 3 (Table 10), and 

 had gonads that were similar to "maturing" 

 gonads in color. There was also some overlap in the 

 relative gonad weights of mature and "maturing" 

 individuals of the same length during this period 

 (Figure 18). 



During the embryo release period, or when 

 females were in the resting state, adult gonads 

 were readily differentiated from the small, yel- 

 lowish gonads of "maturing" fish. Consequently, 

 only maturity data collected during February- 

 June were used to examine the size and age at first 

 maturity for females. 



Length and Age at Maturity 



Data on maturity of Pacific ocean perch have 

 been gathered since 1968, during the course of 

 routine biological sampling of commercial 

 catches. In addition, some maturity data were 

 available from research cruises off the northern 

 Washington coast. The data for 1968-72 combined 

 were examined by stock to determine size and age 

 at maturity. 



In most instances, age, length, and maturity 

 data were available for individual fish, and the 

 proportion of mature fish in each cell of an age- 

 length matrix could be calculated. This type of 

 analysis was carried out for both males and 

 females from Queen Charlotte Sound (Tables 11, 



12) and for females off Washington and southwest 

 Vancouver Island (Table 13). 



Only 213 age-length-maturity observations 

 were available for WVI males, too few to allow 

 direct analysis of maturation by age-groups. 

 Examination of the relation between length and 

 maturity was possible, however, as 551 length- 

 maturity observations were available. 



Length-maturity relation 



Tables 11 through 13 show the proportion of 

 mature fish in each cell of an age-length matrix. 

 The region in which 50-80% of the fish were ma- 

 ture is delineated by the isopleths drawn in these 

 tables and can be interpreted as a maturity re- 

 sponse surface. For all three sets of data, the 

 50-80% region occupied a narrow range of size 

 groups (3-5 cm) and a relatively wide range of 

 age-groups (5-6 yr). Hence it seems that matura- 

 tion of both male and female Pacific ocean perch 

 depends more on the size of a fish than its age. 



Raw data on length versus proportion mature 

 were plotted for each area and sex (Figure 19) and 

 seemed to conform to a logistic equation (Finney 

 1971) of the form: 



1 



1 + exp 



-(^H 



where / = length in centimeters 



Pi = proportion mature at length / 

 l 50 = length where: P t = 0.5 = maturation 

 length 

 cr = constant. 



The length-maturity curves are quite steep in 

 the vicinity of P/ = 0.2-0.8 and maturation can be 

 regarded as knife-edged, taking place at l Q5Q . Be- 

 cause the above equation is symmetrical about 

 Z 50 , the area under the curve and to the left of /0.50 

 is equal to the area above the curve and to the 

 right of it. Hence, the errors introduced by assum- 

 ing knife-edged maturation at / 50 tend to 

 balance. 



By algebraic manipulation, the above equation 

 can be linearized to: 



- (k - 



/ 



0.50 

 cr 



l_ 



cr 



The equation was then in the formy = a + (31 and 



387 



