Oocyte maturation in Hecate Strait 

 English sole [Pleuronectes vetulus) 



Jeff Fargo 



Department of Fisheries and Oceans, Pacific Biological Station 

 Biological Sciences Branch. Nanaimo, British Columbia V9R 5V6 



Albert V. Tyler 



School of Fisheries and Oceans 



University of Alaska, Fairbanks, Alaska 99775 



English sole, Pleuronectes vetulus, 

 is an important component of the 

 bottom trawl fishery in Hecate 

 Strait, British Columbia, Canada. 

 It is a small-mouthed flounder 

 that feeds on sedentary inverte- 

 brates associated with sandy sub- 

 strate and is most common at 

 depths of 80-150 m (Hart, 1973). 

 The species is characterized by 

 moderate growth (&=0.22), mortal- 

 ity (M=0.20) and longevity (20 

 years) (Fargo, 1993). It recruits to 

 the fishery at an age of four years, 

 which is roughly equivalent to the 

 age of sexual maturity (Ketchen, 

 1956; Tyler et al., 1987 1 ). Most of 

 the exploited population is under 

 12 years of age (30-45 cm in 

 length) (Fargo, 1993). Results 

 from tagging studies (Ketchen, 

 1956; Fargo et al., 1984) and 

 analysis of landing statistics and 

 age composition data (Fargo, 

 1993) indicate that a single stock 

 exists in Hecate Strait. 



Since 1955, abundance for this 

 stock has fluctuated, primarily be- 

 cause of changes in recruitment 

 (Fargo, 1993). Factors influencing 

 recruitment for this stock are 

 poorly understood. Ocean tem- 

 perature and circulation have 



1 Tyler, A. V., J. Fargo, R. P. Foucher, and 

 J. B. Lucas. 1987. Studies on the repro- 

 ductive biology of Pacific cod and En- 

 glish sole in Hecate Strait from the 

 cruise of the FR/V W.E. Ricker, Novem- 

 ber 25-29, 1986. Can MS. Rep. Fish. 

 Aquat. Sci. 1937, 43 p. 



been found to influence spawning 

 time and oocyte maturation for 

 the stock off the Oregon coast 

 (Kruse and Tyler, 1989). These 

 authors postulated that 1) the 

 rate of gonadal development for 

 English sole was inversely related 

 to summer bottom temperatures 

 in the same manner as is somatic 

 growth, and 2) spawning was de- 

 layed by rapid increases in bottom 

 temperature caused by upwelling. 

 In Hecate Strait, where Ekman 

 transport is weak, these tempera- 

 ture changes may be brought 

 about by the fall transition when 

 strong winds from the south cause 

 mixing of the warm surface wa- 

 ters to depths of 150 metres 

 (Dodimead, 1980 2 ). Relatively 

 little information exists on spawn- 

 ing time and egg development for 

 the Hecate Strait stock. We inves- 

 tigated oocyte growth and devel- 

 opment to examine the length of 

 the oocyte maturation period and 

 the time and duration of spawn- 

 ing for the English sole stock in 

 Hecate Strait. 



Materials and methods 



Samples of English sole ovaries 

 were obtained from research 



cruises and at ports-of-landing 

 from commercial vessels between 

 November 1987 and November 

 1990. The fish were caught with 

 bottom trawls at five locations 

 throughout Hecate Strait (PMFC 

 Areas 5C-D, Table 1, Fig. 1). 

 Length-stratified samples were 

 collected to ensure that ovaries 

 were obtained throughout the size 

 range of fish collected. For each 

 collection we attempted to sample 

 fifteen sexually mature fish from 

 each 5-cm length interval over a 

 range of 30-50 cm, though this 

 was not always possible. The 

 minimum size fish (30 cm) from 

 which an ovary was dissected cor- 

 responds to the length at first 

 maturity for this stock (Ketchen, 

 1956; Tyler et al., 1987 1 ). Total 

 length and the condition of matu- 

 rity for each fish sampled was re- 

 corded. The right ovary was then 

 removed and preserved in a buff- 

 ered formalin-saline solution 

 (Foucher et al., 1987 3 ). Sampling 

 methods have been described in 

 previous reports (Foucher et al., 

 1987 3 ; Tyler et al., 1987 1 ). A list 

 of ovary samples examined is 

 given by sample type and month 

 in Table 1. 



Preserved ovaries were pre- 

 pared for histological examination 

 by soaking in Davidson's fixative 

 for approximately 24 hours. Sub- 

 sequently, tissue sections were 

 dissected from the anterior por- 

 tion of the ovary (which contained 

 the greatest amount of eggs), em- 

 bedded in paraffin wax, sectioned 

 at 5 u, stained with haematoxylin 

 and counterstained with eosin 

 (Yasutake and Wales, 1983). 



Oocyte diameter was measured 

 with a light microscope calibrated 



2 Dodimead, A. J. 1980. A general review 

 of the oceanography of the Queen Char- 

 lotte Sound-Hecate Strait-Dixon En- 

 trance region. Can. MS. Rep. Fish. 

 Aquat. Sci. 1574, 248 p. 



3 Foucher, R. P., J. Fargo, and J.B. Lucas. 

 1987. Cruise of the FV Nucleus. Janu- 

 ary 5-17, 1987 to Hecate Strait to study 

 reproductive biology of Pacific cod and 

 English sole. Can. MS Rep. Fish. Aquat. 

 Sci. 1941, 25 p. 



Manuscript accepted 8 October 1993. 

 Fishery Bulletin 92:189-197 (1994) 



189 



