Macewicz et at: Fecundity, egg deposition, and mortality of Lo/igo opalscens 



309 



We used the gravimetric method (Hunter et al., 1985, 

 1992) to estimate the standing stock of oocytes in 98 

 L. opalescens ovaries. The gravimetric method overes- 

 timated the total number of oocytes of Loligo pealeii, 

 but the difference between a count of all oocytes and a 

 weight-based estimate was slight (Maxwell and Hanlon, 

 2000). We did not compare our estimates with a count of 

 all oocytes in the ovary because we used a portion of the 

 ovary for our histological examinations, and each value 

 is the mean of the counts from two tissue samples (aver- 

 age coefficient of variation between samples was 0.12). 

 All oocytes in each tissue sample were macroscopically 

 classified (Fig. 3) as either unyolked, yolked, mature, or 

 atretic; they were then counted by class and all stages 

 were summed. "Atretic" was defined as oocytes in the 

 alpha stage of atresia (Hunter and Macewicz, 1985b), 

 recognizing, however, that poor preservation can create 

 oocytes of similar macroscopic appearance. The number of 

 ova in the oviduct was also counted directly (usually when 

 n was less than 300) or the mean number was estimated 

 from two tissue samples by using the gravimetric method. 

 To illustrate the form of the oocyte-size distribution in 

 the ovary, we measured (to 0.01 mm) the major axis of 

 all the oocytes in one tissue sample from each ovary of 

 six females by using a digitizer linked by a video camera 

 to a dissection microscope. In all other ovaries used for 

 fecundity estimation, we measured only the smallest and 

 largest oocyte in the sample. The length of the major 

 axis of the smallest oocyte (D) was used as an index of 

 the extent of ovarian maturity. D is a crude index of time 

 elapsed during the spawning period — as long as oocyte 

 maturation continues throughout the spawning period 

 and no new oocytes are produced — both of which appear 

 to be true for L. opalescens. 



To monitor body condition we cut a tissue sample disc 

 from the mantle using a number 11 cork borer (area of 

 251.65 mm 2 ) and removed the outer dermis and the in- 

 ner membrane. The mantle sample discs were frozen and 

 subsequently dried at 56°C to a constant weight. An index 

 of mantle condition (C) was calculated as the weight of the 

 dry mantle in milligrams divided by disc surface area and 

 is expressed as mg/mm-. 



We evaluated the extent that body reserves might be 

 used to support egg production by comparing dry weight 

 of the eggs and capsules to prespawning female body dry 

 weight. For these calculations we made the following mea- 

 surements: 1) the mean dry weight of one squid egg was 

 0.00177 g, including a fraction of the egg capsule because 

 the value is based on the dry weight of 34 egg capsules 

 (1-2 days old) containing 2 to 403 eggs each (total of 7341 

 eggs, capsules collected from La Jolla Canyon 6 July and 

 11 September 2000); 2) the relationship of dorsal mantle 

 length (L) and whole-body wet weight (W w ) for immature 

 and mature preovulatory females of W u . = 0.000051L 2 8086 , 

 where W w is in grams and L is in mm (Fig. 4); and 3) the 

 mean wet weight to dry weight conversion factor of 0.24 

 (2SE = 0.001), based on the wet and dry weights of mantle 

 tissue sampled from 214 mature females. The latter con- 

 version factor was constant regardless of mantle condition 

 index; apparently, in L. opalescens, starvation does not 



Figure 3 



Whole L. opalescens oocytes as viewed under a dis- 

 section microscope used for counting and classifying 

 oocytes. Bar = 1.0 mm. 



result in the replacement of muscle tissue with water as it 

 does in fishes (Woodhead, 1960). 



In addition to the specimens taken during the research 

 surveys, we also estimated the fecundity of 60 L. opal- 

 escens from the commercial catch sampled by California 

 Department of Fish & Game (CDF&G) during 1998 and 

 1999. Landed specimens were not analyzed histologically 

 because their ovarian tissues had begun to deteriorate 

 before preservation. The 60 females were selected by dor- 

 sal mantle length and mantle condition index to provide a 

 wide and uniform distribution of length and mantle condi- 

 tion. The number of oocytes in the ovaries was estimated 

 (as described above) and the number of ova in the oviducts 

 were predicted from oviduct weight (Fig. 5). CDF&G also 

 provided data on the dry mantle disc weights of 1275 ma- 

 ture females taken from the catch from December 1998 

 through December 1999 as random samples taken during 

 the Southern Californian Bight market squid fishery. 

 About 100,000 tons of market squid were landed during 

 this sampling period . 



Modeling egg deposition 



To identify egg deposition and mortality rates most consis- 

 tent with our current understanding of spawning biology, 

 we developed a model to estimate the proportion of the 

 potential fecundity deposited by a cohort in its lifetime. 

 The mean proportion of the potential fecundity deposited 

 is the proportion of eggs deposited weighted by the propor- 



