16 



Fishery Bulletin 103(1) 



from histological sections. Unlike gravimetric methods 

 (e.g., Hunter et al., 1992) where whole oocytes are used 

 to estimate fecundity, stereological methods do not rely 

 on oocyte diameter or other proxies for vitellogenesis. A 

 collection of shortspine thornyhead ovaries from Alas- 

 ka contained few specimens considered suitable for a 

 gravimetric fecundity method because too few of the 

 specimens contained all developing oocytes in stage 5 

 or beyond. However, enough samples were suitable for 

 the stereological method. 



This study provides a fecundity estimate based on 

 stereological and gravimetric techniques for shortspine 

 thornyhead off Alaska. Benefits and limitations of the 

 stereological method in this case are discussed. A gravi- 

 metric technique is also used to estimate fecundity for 

 longspine thornyhead and shortspine thornyhead from 

 samples off the West Coast of the United States. In 

 addition, we examine the hypothesis that thornyheads 

 are batch spawners, and that fecundity consequently 

 declines over the course of the spawning season (Wake- 

 field, 1990). 



Materials and methods 



Ovaries were collected from a large geographic area 

 in Alaska, including the Gulf of Alaska, the Aleutian 

 Islands, and the Bering Sea. National Marine Fisheries 

 Service (NMFS) observers aboard commercial fishing 

 vessels collected ovaries from April through June 2000. 

 Length and somatic weight (ovaries and stomach con- 

 tents removed) (±5 g) were recorded at sea. Ovaries were 

 excised and placed in 10% formalin solution buffered 

 with sodium bicarbonate. 



Ovaries from shortspine thornyhead and longspine 

 thornyhead were also collected during the 1999 NMFS 

 West Coast trawl survey. Samples were collected be- 

 tween Northern California and Washington (34°57'N lat. 

 121°33'W long, to 48°04' lat. 125°58'W long.). Length 

 and somatic weight (±2 g) were recorded at sea. 



Additional West Coast longspine and shortspine 

 thornyhead ovaries were collected from commercial 

 fishing vessels by the Oregon Department of Fish and 

 Wildlife in Astoria. Ovaries were collected off Oregon 

 and Washington from February through May 2000, 

 during December 2000, and during January 2001. Af- 

 ter shipment to the NMFS Alaska Fisheries Science 

 Center in Seattle, length, somatic weight (±2 g), and 

 ovary weight (±0.001 g) were recorded. Ovaries were 

 excised and placed in 10% formalin buffered with so- 

 dium bicarbonate. 



A cross section was removed from one ovarian lobe 

 (middle or middle posterior region) for histological pro- 

 cessing. When a whole cross section was too large to fit 

 on a microscope slide, a wedge was cut from the cross 

 section that included both the ovarian wall and the 

 center of the ovary. Samples were processed through 

 a dehydration series, embedded in paraffin, and sec- 

 tioned at 4 um. Slides were stained with hematoxylin 

 and eosin. 



Gravimetric fecundity estimation 



Histological ovary sections were examined at 100 x mag- 

 nification to select samples for the gravimetric method. 

 Oocytes were identified to one of eight developmental 

 stages as described by Pearson and Gunderson (2003). 

 To differentiate between oocytes to be spawned in the 

 current year and reserve oocytes for future years, only 

 ovaries with all maturing oocytes in stage-5 (late vitel- 

 logenesis) and beyond were used. By definition, yolk fills 

 more than 50% of the cytoplasm within stage-5 oocytes, 

 and the dark yolk made it easy to distinguish these 

 oocytes. Stage-4 oocytes would also be spawned in the 

 current year but overlapped significantly in size with 

 nonmature stage-3 oocytes, and early stage-4 oocytes 

 did not always have enough yolk (0-50%) to differentiate 

 them from stage-3 oocytes with the gravimetric method. 

 Specimens containing any stage-4 oocytes were omitted 

 as a result. Ovaries with stage-8 oocytes were also omit- 

 ted because the increased amount of gelatinous material 

 which surrounds the oocytes in Sebastolobus could not 

 be contained within the ovaries during subsampling. 



Ovaries were weighed (±0.001 g) after they had been 

 stored in formalin. Subsamples were cut from the ova- 

 ries and weighed (±0.001 g). For smaller ovaries, an 

 entire cross section was taken. For larger ovaries, a 

 pie-piece-shaped wedge was cut from the cross section 

 to ensure a representative sample of outer ovarian wall. 

 When cut correctly, a wedge starting at the center of 

 the cross section would have the same weight ratio of 

 ovarian wall to wedge subsample as the original cross 

 section. Subsamples usually contained approximately 

 1000 oocytes (mean=1133), but this number varied ac- 

 cording to stage of development and the amount of ge- 

 latinous material in the ovary (range: 108-3711). 



Gelatinous material could not be subsampled by cut- 

 ting at room temperature; therefore ovaries were briefly 

 frozen before subsampling. This procedure enabled the 

 gelatinous material to be cut, and also made it easier to 

 obtain a representative sample of the ovarian wall. Ini- 

 tially, parts of three ovaries were frozen, and no effects 

 of the freezing were detected with a light microscope. 

 Only samples for gravimetric fecundity estimates were 

 briefly frozen. 



No difference in oocyte density was found among 

 the different regions of the ovaries (see "Results" sec- 

 tion); however, gravimetric subsamples were still taken 

 randomly along the length of the ovaries to minimize 

 potential bias from any location. 



The oocytes in the subsamples were counted under a 

 stereomicroscope, and fecundity was estimated by 



W 

 Fec = —N, 



w 



where Fee = estimated fecundity; 

 W = total ovary weight; 

 w = subsample weight; and 

 n = number of oocytes in the subsample. 



