Markle et al.: Metamorphosis of Microstomus pacificus 



297 



0.48, range 75-188 m). Compared with Stages 4 and 

 5, the greater average depth and variance of benthic 

 Stage-3 larvae indicate a much broader depth distribu- 

 tion (Fig. 14). 



Stage-3 larvae occupy a transitional "landing" zone 

 quantitatively distinct from, but overlapping, the late- 

 larval and juvenile nurserygrounds. Although Stage-3 

 larvae caught in bottom trawls quickly take to the bot- 

 tom when placed in aquaria (pers. observ.), their night- 

 time capture in midwater trawls and da3rtime capture 

 in bottom trawls suggest they may be engybenthic 

 (nearbottom) or benthopelagic, rather than exclusive- 

 ly benthic. 



Behavior associated with metamorphosis presumably 

 includes some short-term (hours to days) switching be- 

 tween midwater and bottom habitats. In one individual 

 in our data set, the behavior continued into Stage 4. 

 A 53.0 mm Stage-4 specimen was caught 19 April 1963 

 off the mouth of the Columbia River at 0411 hours in 

 a midwater trawl fished to 73 m over a bottom depth 

 of about 125 m. Its gut loop was well developed and con- 

 tained sand grains. Additional evidence is provided by 

 midwater Cobb trawl samples collected by W. Lenarz 

 and colleagues (NMFS Southwest Fish. Sci. Cent., 

 Tiburon, CA 94920) between Monterey and San Fran- 

 cisco, California, from 28 March to 2 April 1990. In 

 eight nighttime (2235-0447 hour) samples, fished at 

 0-1 10 m (most 0-30 m) over bottom depths of 33-1462 

 m, they collected 14 Stage-3 larvae (40. 4-51. 2 mm SL) 

 and 16 Stage-4 larvae (42.4-53.4 mm SL). Stage-3 

 larvae were collected over bottom depths of 73-1462 m, 

 and Stage-4 larvae were collected over bottom depths 

 of 33-91 m. Thus, settling Stage-3 larvae were found 

 in a "landing" zone at 55-377 m and in a wedge of the 

 water column above and seaward of that zone. 



Discussion 



Time-line 



Dover sole spawn in deep water in winter, December 

 to February, according to Hagerman (1952), and 

 November to April according to the circumstantial 

 evidence of Harry (1959). Yoklavich and Pikitch (1989) 

 provide evidence that smaller Dover sole have an 

 earlier, shorter spawning season than larger fish, and 

 that Dover sole now mature at significantly smaller 

 sizes than reported by Hagerman (1952) or Harry 

 (1959). These observations suggest the possibility that 

 size-selective exploitation might have shifted the 

 spawning season to earlier dates. 



However, other observations suggest that peak 

 hatching of Dover sole off Oregon is later, not earlier, 

 than indicated by Hagerman (1952) or Harry (1959). 

 Results of sampling the commercial Dover sole catch 



off southern Oregon (43°N) from March 1990 to 

 September 1991 indicate running ripe females were 

 caught from February through July with a peak in 

 April (Mike Hosie, Oreg. Dep. Fish Wildl., Charleston, 

 OR 97420, pers. commun.). Spent females increased 

 from less than 10% of all females in April to 100% by 

 early August. However, these observations may be 

 biased towards later-spawning fish because the com- 

 mercial catch is culled of small fish (Yoklavich and 

 Pikitch 1989). Experiments performed in 1972 by 

 S. WOliams at Newport, Oregon, showed that hatching 

 took 18 days at 12.5°C, 27 days at 10.0°C, and 38 days 

 at 7.5°C (Mike Hosie, pers. commun.). In agreement 

 with these observations, small larvae (<10mmSL) in 

 this study were collected from February to July, with 

 most caught in April and May (Fig. 13, Pearcy et al. 

 1977a). In ten NMFS ichthyoplankton cruises con- 

 ducted at 40-48°N from 1980 to 1987, high densities 

 of Dover sole eggs were found in each of six cruises 

 conducted in March, April, or May; none or trace 

 amounts were found in four cruises conducted in 

 August, November, or January (Urena 1989; M. Doyle, 

 NMFS Alaska Fish. Sci. Cent., Seattle, WA 98115, 

 pers. commun.). Finally, "spawning" adults off Alaska 

 have been collected primarily in May and June 

 (Hirschberger and Smith 1983), and eggs are collected 

 in June (Kendall and Dunn 1985). Thus, the weight of 

 evidence seems to indicate that most Dover sole off 

 Oregon hatch from February (Fig. 13) to August 

 (Urena 1989), with a peak in April and May (see also 

 the time-line in Hayman and Tyler 1980). 



Settlement is restricted to the period from January 

 to March or April (Table 5), whereas metamorphosis 

 requires a protracted period of up to one year, occurs 

 at sizes >40mmSL (Fig. 3), includes little growth in 

 body length, and may include loss of weight. Cessation 

 in growth of body length before and during metamor- 

 phosis has been documented in other flounders (Fuku- 

 hara 1986, 1988). If the modal size of Stage-1 larvae 

 is ~25 mm in March, then the average duration of the 

 planktonic period of Dover sole is about 21 months 

 (Fig. 15). However, the timing of settlement has a size 

 component; larger larvae tend to settle before smaller 

 larvae. It seems reasonable that larger larvae are those 

 that grow faster, but it is also possible that they are 

 slow growers or have otherwise delayed metamor- 

 phosis and, therefore, are more than 2 years old (see 

 Discussion below). 



Distribution and relative abundance of metamorphic 

 planktonic stages provide additional insight. Larger 

 planktonic specimens were generally rare in midwater 

 trawl collections (Fig. 13). However, Stage-2 larvae, 

 with developmental scores of 7 and 8, and Stage-3 

 larvae were the most abundant of all metamorphic 

 stages found in midwater (Table 4), even though they 



