Kramer: Growth and mortality rates of juvenile Paralichthys californicus 



203 



Rate of movements into bays 



I estimated the proportion of the population by age- 

 class emigrating each day from the open coast to the 

 bays by calculating the difference between the percent- 

 age of juvenile halibut lost daily from the total popula- 

 tion and from the open coast using age-specific instan- 

 taneous mortality rates (Tables 4, 5) in the following 

 equation: 



% emigrating/day = 



U\ — g-z(total population) _ Q — e~ z (°P en coast; )Y) x 100. 



The decline in abundance of juvenile halibut on the open 

 coast between days 30 and 70 was 182,100, and for the 

 total population was 145,500 (Tables 4, 5). During this 

 time, the daily emigration rate increased from 1.99% 

 for juveniles from age 30-43 days, to 3.67% from age 

 43-53 days, then declined slightly to 3.35% by 70 days. 



Discussion 



Extent of bay utilization 



Juvenile halibut appear to be dependent upon bays as 

 nursery areas, since nearly all halibut between 76 and 

 115 days of age occurred in the bays rather than the 

 open coast (Fig. 8). Transforming larvae and newly- 

 settled juvenile halibut < 70 days old occurred on the 

 open coast (97% of the transforming larvae were on 

 the open coast), but over 95% of the total population 

 of halibut >70 days were in the bays (Table 4). 



An alternative explanation for the decline in abun- 

 dance of juvenile halibut on the open coast is that they 

 move somewhere other than the bays, or suffer heavy 

 mortality. If halibut moved offshore, one would expect 

 a positive relationship between size of juvenile halibut 

 (31-70 mm SL, or 76-115 days) and bottom depth. This 

 is contrary to the observed size-structured distribution 

 pattern (Fig. 7). The decrease in abundance of juvenile 

 halibut on the open coast may have included higher in 

 situ mortality rates, but the corresponding increase in 

 abundance in the bays suggests that movement from 

 the coast to the bays probably accounts for about half 

 of the coastal decline. 



Advantage of bays as nursery areas 



Growth The potential advantages of using bays as 

 nursery areas are increased growth and decreased mor- 

 tality. Increased growth was not observed for juvenile 

 English sole Parophrys vetulus in Oregon estuaries: 

 they grow at about the same rate as juveniles on the 

 Oregon coast, but were more variable in size-at-age 



than those on the coast (Rosenberg 1982). Similarly, 

 growth rates of juvenile California halibut < 40 mm SL 

 on the coast and in the bays were not significantly 

 different. 



California halibut 70- 120 mm SL grew faster than 

 all other length-classes with rates approaching 1 mm/ 

 day (Fig. 6). These fast and variable growth rates 

 occurred during the period when juvenile halibut oc- 

 curred only in the bays (>115 days of age). Unfor- 

 tunately, comparisons could not be made between open 

 coast and bay habitats during this period of fast 

 growth, which coincides approximately with a change 

 in the food habits of halibut >55mm SL, from a diet 

 composed primarily of small crustaceans (copepods, 

 amphipods, mysids, and cumaceans) to one composed 

 of an increasing proportion of fish by weight (mostly 

 gobies) (Haaker 1975, Allen 1988, Drawbridge 1990). 

 Juvenile halibut feeding on gobies in the laboratory re- 

 main partially buried in the substrate, only striking at 

 gobies passing within a distance of three headlengths 

 (Haaker 1975). Gobies are abundant in bays (mean den- 

 sity of Ilypnus gilberti in Mission Bay, 8.1/m 2 ), but 

 not in shallow coastal waters <30m (Brothers 1975, 

 Allen 1985, Plummer et al. 1983). The diet of larger 

 juvenile halibut becomes increasingly piscivorous: 

 juvenile halibut > 150 mm SL on the open coast eat 

 primarily northern anchovies by weight (Plummer et al. 



1983, Allen 1982). 



Predation risk Predation risk may be higher for small 

 halibut on the open coast than in the bays. At least six 

 fish species on the open coast are known to eat flat- 

 fishes: these include California halibut, thornback ray 

 Platyrhinoidis triseriata, fantail sole Xystreurys lio- 

 lepis, bigmouth sole Hippoglossina stomata, speckled 

 sanddab Citharichthys stigmaeus, and California lizard- 

 fish Synodus lucioceps (Ford 1965, Allen 1982). Ford 

 (1965) found many small halibut (TL <10mm) in the 

 stomach contents of thornback rays, with a maximum 

 of 15 newly-settled halibut in the stomach of one ray 

 alone. The combined density of rays Platyrhinoidis 

 triseriata, Urolophus halleri, and Gymnura mar- 

 morata) on the shallow open coast (<10m) is about 

 100/hectare (Ford 1965). Speckled sanddab is the most 

 abundant flatfish in shallow open coast waters, with 

 a mean density of 950/hectare at Torrey Pines (Ford 

 1965, Allen 1982, Love et al. 1986, DeMartini and Allen 



1984, Kramer 1990). Although the diet of speckled 

 sanddab is composed primarily of mysids, they are 

 probably capable of eating newly-settled halibut, since 

 small unidentified flatfish juveniles have been found in 

 their stomachs (Ford 1965). 



In the bays, two potential predators include the round 

 stingray Urolophus halleri, and the staghorn sculpin 

 Leptocottus armatus (Allen 1985, Tasto 1975, Babel 



