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Fishery Bulletin 88(4), 1990 



mussel growth in British waters. SFG analysis may 

 thus prove useful in interpreting relationships between 

 mussel growth and environmental factors in Califor- 

 nia waters. 



The present study, which continued work reported 

 in Page and Hubbard (1987), (1) compared mussel 

 growth rates in highly productive inshore coastal 

 waters north of Point Conception with rates measured 

 concurrently at offshore Platform Holly (Atlantic Rich- 

 field Company), (2) evaluated the relationship between 

 temporal and spatial variation in growth and measure- 

 ments of potential food availability, and (3) used pub- 

 lished physiological data and the "scope for growth" 

 concept (Bayne et al. 1976a) to provide an independent 

 assessment of the response of mussel growth to envi- 

 ronmental conditions in California waters. 



Materials and methods 



Study sites 



Avila Beach (35°10'N, 120°43'W) is located appro.x- 

 imately 84 km north of Point Conception, California 

 (Fig. 1), in a region characterized by episodic upwell- 

 ing and high primary productivity (Owen 1974, 1980; 

 Lasker et al. 1981). The study site was located at the 

 end of the Unocal pier which extended 0.8 km into 

 semiprotected Avila Bay. Water depth at this location 

 was about 12 m. Mytilus edulis were collected inter- 

 tidally on pier pilings, since subtidal mussels were 

 scarce due possibly to starfish predation (Landenberger 

 1967). 



Platform Holly is a 20-year-old oil and gas pi-oduc- 

 tion platform located in 60 m of water, about 3 km off- 

 shore of Goleta, California (34°25'N, 119°52'W; Fig. 

 1). Mussels colonize the support members of this plat- 

 form from the intertidal zone to depths greater than 

 18 m, and grow rapidly, achieving 50 mm shell-length 

 in 6-8 months (Page and Hubbard 1987). 



Physical and biological parameters 



We collected water samples at a depth of 2 m with a 

 Van Doren bottle every 7-10 days at each location from 

 October 1986 to June 1987. Temperature of samples 

 was measured by hand-held thermometer. Estimates 

 of potential food available to mussels were made from 

 the concentrations of seston, particulate organic mat- 

 ter (POM), chlorophyll a (an estimate of phytoplank- 

 ton biomass: Lorenzen 1970, Hunter and Laws 1981), 

 and particulate organic carbon (POC) in replicate 

 500-mL water samples. All water samples were pre- 

 filtered through a 300-;.(m nylon mesh. Seston concen- 

 tration was measured using standard methods (Wid- 



I Morro Bay 

 _Avila Beach 



Cajifomla 



Pacific Ocean 



Poinl 

 Conception 



Platform 

 Holly 



Figure 1 



Locatidns (if the Avila Beach and I'latfdfiii Holly 

 study sites. 



dows 1985b). Percent of POM (% POM) witliin the 

 seston was calculated as 100  [POM]/[seston]. Chloro- 

 phyll a concentration was determined using standard 

 fluorometric methods (Parsons et al. 1984) and a 

 Turner Designs Fluorometer. Particulate organic car- 

 bon concentration was determined using a Perkin- 

 Elmer CHN analyzer following the methods of Rod- 

 house et al. (1984). 



Growth rate of Mytilus edulis 



Twenty individually numbered mussels (~20 mm shell- 

 length), collected intertidally (~ - 15 cm), were enclosed 

 in a cylindrical vexar plastic cage (12 x 20 cm, 5-mm 

 mesh) monthly at each site from November 1986 to 

 June 1987. Using calipers, shell-lengths of caged mus- 

 sels were measured to the nearest 0.1 mm initially and 

 after 4 weeks. No growth data are available at Avila 

 from January and February 1987, due to lack of small 

 mussels. Cages were submerged at depth (-2m) either 

 by suspension on a weighted line (Holly) or by attach- 

 ment to a pole (Avila). Cages were cleaned as needed 

 to keep fouling to a minimum. 



Transplant experiments 



We conducted transplant experiments in Fall 1986 to 

 evaluate the potential influence of mussel stock on 

 growth rate. Forty M. rdulls were collected from Holly 

 on 27 September 1986 and transplanted (2 cages of 20 

 individuals of ~30 mm shell length) to Avila on 29 

 September 1986. Mussels of equal size and numlicr 

 were collected from Avila on 29 September 1986 and 

 transplanted to Holly on 4 October 1986. Mussels were 

 covered with a moist cloth in transport and maintained 

 in unfiltered seawater prior to placement in the field. 



Mention of trade names does not imply emlor.semeiit by the National 

 Marine Fisheries Service. N().\A. 



