Macewicz et al.: Fecundity, egg deposition, and mortality of Lo/igo opa/scens 



319 



Table 6 



Estimates of number of days of egg deposition, the mean number of eggs deposited, mean standing stocks of oocytes and ova 

 remaining in female L. opalescens, mean number of eggs deposited at the end of the first night (all means are expressed as a 

 fraction of the potential fecundity), for various combinations of possible egg deposition (u) and total adult mortality (z) rates. 

 Model provided estimate nearest observed data when z = 0.45, v = 0.25, and t max = 8 days. 



Daily 

 total 



mortality 

 Cz) 



Daily 



egg 



deposition 



rate 



(v) 



Fraction of 

 potential 

 fecundity 

 deposited 



Qsp 



(Equation li 



Fraction of 

 potential 

 fecundity 



remaining 

 in females 



a-Qsp) 



0.2 



0.2 



0.2 



0.45 



0.45 



0.45 



0.8 



0.8 



0.8 



Observed 



0.25 

 0.45 

 0.65 

 0.25 

 0.45 

 0.65 

 0.25 

 0.45 

 0.65 



0.458 

 0.617 

 0.706 

 0.341 

 0.486 

 0.580 

 0.237 

 0.359 

 0.447 

 0.326 (SE 0.075) 



0.542 

 0.383 

 0.294 

 0.659 

 0.514 

 0.420 

 0.763 

 0.641 

 0.553 

 0.674 



when the data were weighted by the distribution of mantle 

 conditions in the catch. The mean fraction of the poten- 

 tial fecundity deposited (Q SP ) by L. opalescens was 0.326 

 (1-2599/3859). That much of the fecundity had escaped 

 (eggs were deposited) before the market squid were taken 

 by the fishery does not seem unreasonable because 22- 

 36% of E P may be deposited during the first day of spawn- 

 ing. The mean Q SP is an important index because it mea- 

 sures egg escapement as a fraction of potential fecundity 

 over its lifetime (Eq. 1). It is used in subsequent sections 

 to identify a daily total mortality rate and egg escapement 

 rate for the average female in the population that best 

 characterizes the sampled L. opalescens population. 



Preferred mortality and egg deposition rates We used 

 Equation 1 to evaluate which combination of a range of 

 plausible values for the rates of daily total mortality (z of 

 0.2, 0.45, and 0.8) and daily egg deposition (v of 0.25, 0.45, 

 and 0.6 5) provides an estimate closest to observed Q sp 

 (E SP IE P ) (Table 6). The combination of an adult daily total 

 mortality (z) rate of 0.45, a daily egg deposition (v) rate of 

 0.25, and using a £ max of 8 days gave an estimate that was 

 most consistent with the observed value for Q SP of 0.326 

 (Table 6, Fig. 10). This combination of rates also gave an 

 egg depletion of 78% of the potential fecundity in 6 days 

 which was consistent with our best guess for maximum 

 longevity and maximum fecundity. On the other hand, the 

 model (using 1-e-"' and t=l) predicts that about 22% of the 

 potential is deposited by the end of the first 24 hours (day 1) 

 which is less than our preferred estimate (36%) based on 

 the reduction in standing stock of oocytes but is closer to the 

 one based on the standing stock of ova ( 27%). A possible bio- 



logical explanation for the difference might be that some of 

 the ova produced during the first day of deposition might 

 remain in the oviduct and then be deposited on the second 

 day. Regardless of the uncertainties regarding the fit for 

 the initial day of egg deposition, a daily total mortality rate 

 of 0.45 and daily egg deposition rate of 0.25 are most con- 

 sistent with the field data known at the present time. This 

 means that the average spawning period is very short; the 

 average female lives only 1.67 days after spawning begins 

 (ln(0.659)/-0.25; Eq. 2). It is interesting that 1.67 days for 

 the average animal is not a radical departure from Fields's 

 (1965) original conclusion of a single night of spawning. 



Egg escapement from the fishery In L. opalescens, where 

 the fishery targets spawning adults that die after spawn- 

 ing, it is important to know the effect of fishing mortality 

 on the egg escapement rate with respect to the lifetime 

 fecundity deposited, R L , t (Eq. 12). However, not all terms 

 in Equation 12 are observable and it may be practical 

 to manage the fishery by monitoring the fraction of the 

 potential fecundity that is deposited on the bottom (Q SP = 

 1-[E YD /E P ]). Nevertheless, we examined the potential 

 effects of fishing mortality if) on the egg escapement rate, 

 B f(mii , when natural mortality (m) is 0.1, 0.25, or 0.4, and 

 egg deposition (v) is 0.25, 0.45, or 0.65 (Fig. 11). Because 

 our preferred rates from the previous section are v = 0.25 

 and 2 = 0.45, then m is <0.45 with fishing because z = m 

 + f. If we use v = 0.25 and set daily natural mortality rate 

 high (»i=0.4), then /"is 0.05 andi? f Jum is 93%. Doubling the 

 fishing mortality (to 0.1) produces an absolute difference of 

 6% in egg escapement (Fig. 11C). Thus at a high m of 0.4, 

 escapement is relatively insensitive to changes in daily 



