AGE (doyj) 



Figure 3. — Age at length for field-captured prejuvenile 

 Sebastes diploproa as determined by analysis of daily growth 

 increments on otoliths. Lt = length at time t (days). 



lifetime of each fish, ranged from 0.154 to 0.225 

 mm SL/d, with the mean value indicated by the 

 slope of the line, 0.194 mm SL/d. These rates were 

 similar to predicted laboratory growth rates under 

 a variety of temperature and initial length condi- 

 tions at 16L:8D (Figure 2 A) but only to the small- 

 est fish between 10° and 18° C at 12L:12D (Figure 

 2B). Sebastes diploproa apparently grows more 

 slowly in the field than pelagic juvenile S. 

 melanostomus of similar size, the only other 

 species of Sebastes whose growth has been esti- 

 mated using daily growth increments (Moser and 

 Ahlstrom 1978). 



Discussion 



Fish growth generally reaches a maximum at 

 some optimum temperature and decreases at 

 temperatures above and below this level (Brown 

 1957; Brett 1979), approaching zero near lethal 

 temperatures. Upper lethal temperatures (critical 

 thermal maxima) for prejuvenile S. diploproa 

 range from 26° to 30° C, depending upon the tem- 

 perature of acclimation (Boehlert 1981), and pre- 

 juveniles tolerate surface temperatures up to 23° 

 C. The optimum temperature for growth of 

 juvenile S. diploproa depends upon both photo- 

 period and fish length. The growth models suggest 

 optimum temperatures for growth which increase 



with length at 12L:12D from 13.8° C at 30 mm SL to 

 15.7° C at 50 mm SL ( Figure 2B) and decrease with 

 length at 16L:8D from 16.0° C at 30 mm to 13.8° C 

 at 50 mm SL (Figure 2A). These temperatures 

 clearly exceed those experienced by later benthic 

 juveniles and adults. 



Longer photoperiod enhanced growth at nearly 

 all conditions offish length and temperature (Ta- 

 ble 1; Figures 1, 2). Increased growth with longer 

 photoperiod has been observed in green sunfish 

 (Gross et al. 1965), plaice and sole (Fonds 1979), 

 and Baltic salmon parr (Lundqvist 1980). Brown 

 (1946b), however, observed lower growth rate of 

 brown trout in long photoperiods. As the fish in the 

 present experiments were fed to satiation only 

 once daily, it is reasonable to assume that rations 

 under both photoperiods were similar. Surface pre- 

 juvenile S. diploproa acclimated to short photo- 

 periods have greater standard metabolic rates 

 than those acclimated to long photoperiods at the 

 same acclimation temperatures (Boehlert 1978). 

 Thus the enhanced growth in the longer photo- 

 period may be related to a greater scope for growth 

 (Elliot 1976) due to lower standard metabolic 

 rates. 



The dependence of growth on fish size in the 

 present study is interesting in relation to the life 

 history of this species. Other investigators have 

 observed both increases and decreases in the size 

 range of fishes at the beginning and end of growth 

 experiments. Brown (1946a) described the "size 

 hierarchy effect," which apparently results from 

 development of a peck order with larger fish dom- 

 inant (Stringer and Hoar 1955). No dominance or 

 peck order with respect to feeding was apparent in 

 the experiments with S. diploproa, and in four of 

 six experiments, growth rate decreased with in- 

 creasing size (Figure 1), as is predicted by the 

 growth models (Figure 2). Laboratory and field 

 growth rates were similar for fish at approxi- 

 mately the same lengths, but it is uncertain 

 whether the decreased growth apparent with in- 

 creasing length observed in the laboratory occurs 

 in the field. The largest specimen from the field 

 successfully aged using daily growth increments 

 on the otoliths was 42.7 mm SL, below the size at 

 which significantly decreased growth rates oc- 

 cured in the laboratory (Figure 1). If an asymptote 

 does exist in the field, it fits well with the largest 

 pelagic prejuvenile captured in the field (59 mm 

 SL; Boehlert 1977) and with the maximum size 

 after growth in the present study (60 mm SL, 

 Table 1). 



792 



