Kramer: Growth and mortality rates of juvenile Paralichthys cahfornicus 



205 



between March and October, also suggesting increased 

 onshore transport of surface water (Schwartzlose 

 1963). 



Once nearshore, transforming larvae or settled 

 juveniles may search for bays by using longshore 

 transport (Boehlert and Mundy 1988). Net longshore 

 transport of shallow shelf waters in the Southern 

 California Bight is to the south (Winant and Bratkovich 

 1981). Longshore current speed measured in shallow 

 water (15 m) averages less than 5 cm/second; at this 

 speed, after 12 hours longshore movement could be as 

 great as 2km (Winant and Bratovich 1981). 



My data on the abundance of transforming larvae and 

 newly-settled juveniles provide an estimate of the time 

 required for halibut to locate and enter the bays from 

 the open coast. The time required can be considered 

 to be equivalent to the difference in the age of peak 

 abundance between the coast and the bays. Halibut 

 reached peak abundance in the bays at an age of about 

 70 days in 1988, whereas they were most abundant at 

 age 30 days (transformation) on the open coast (Fig. 

 8). Thus the time required to locate and enter the bays 

 was about 40 days in 1988 (70-30 = 40 days) (Fig. 8). 

 Over a 40-day period, halibut potentially could be 

 transported about 80 km alongshore (40 days x (2km 

 at 12 hours in the neuston)), which is greater than the 

 total distance between the northern sampling block at 

 San Onofre and Mission Bay (64 km) (Fig. 1). I mea- 

 sured the maximum distance between adjacent bays in 

 southern California at less than 60 km, thus larvae 

 using this transport mechanism would probably en- 

 counter a bay within 30 days of reaching the shallow- 

 water coastal environment. 



The potential cues used to find the entrances to bays 

 include temperature, currents, odor, turbidity, and bot- 

 tom substrate (Boehlert and Mundy 1988). A probable 

 cue in southern California is temperature: during 

 spring and summer, when larvae and juveniles are mov- 

 ing into the bays, the temperature is as much as 5°C 

 warmer in the bays than on the open coast (Kramer 

 1990). 



Once a bay entrance is located, the mechanism used 

 to migrate into the bay probably is tidal transport, 

 using incoming tidal currents to aid movement into the 

 bay, and remaining at the bottom to avoid transport 

 out of the bay (Weinstein et al. 1980, Boehlert and 

 Mundy 1988, Fujii et. al. 1989, Tsuruta 1978, Weihs 

 1978, Runsdorp et al. 1985). To use tidal stream trans- 

 port to move into bays, individuals must be able to 

 orient to currents, control vertical movements, and re- 

 main on the bottom during unfavorable currents. These 

 abilities probably develop by the time larvae reach 

 transformation (Boehlert and Mundy 1988, Weinstein 

 et al. 1980). Only a few tidal cycles may be required 

 for halibut to move from the entrance into the bay. 



Larval flounder {Paralichthys sp.) on the North Caro- 

 lina coast use tides to augment movement into 

 marshes, migrating to the surface during night flood 

 tides and remaining on the bottom during ebb tides and 

 during the day (Weinstein et al. 1980). 



In conclusion, California halibut settle either in bays 

 or on the open coast, but ultimately nearly all halibut 

 settling on the coast enter and use the bays as nursery 

 areas during their first year of life. The advantages of 

 bays as nursery areas may be a decrease in risk of mor- 

 tality of newly-settled halibut, and an increase in 

 growth of larger juveniles that feed upon the abundant 

 small fishes in the bays. 



Acknowledgments 



This paper represents part of a dissertation submitted 

 to University of California San Diego, Scripps Institu- 

 tion of Oceanography. The research was supported by 

 funds provided by the Habitat Program of the National 

 Marine Fisheries Service and by the Southwest Fish- 

 eries Science Center of the National Marine Fisheries 

 Service. I thank John Hunter, Mike Mullin, and Richard 

 Rosenblatt for their support and comments. Sandor 

 Kaupp reared halibut larvae in the laboratory and 

 generously provided samples. John Butler provided 

 assistance with the apparatus and programs used for 

 interpreting otoliths. Darlene Ramon assisted in pre- 

 paring and photographing otoliths. Mike Davis, Steve 

 Swailes, and many others were invaluable in the field. 

 I also thank Douglas Chapman, Richard Charter, 

 Nancy Lo, Geoffrey Moser, and William Watson for 

 their advice and assistance. 



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