MORSE: CATCHABILITY. GROWTH, AND MORTALITY OF LARVAL FISHES 

 0.40 I 



0.00 



0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 

 INSTRNTRNEOUS GROWTH RRTE 



Figure 6 — Plot of instantaneous larval mortality rates versus instantaneous growth 

 rates for 26 larval fish taxa, 1977-84. Labeled points (ii = 5) were excluded fi-om the 

 fitted curve. 



ratios of mortality to growth coefficients for the 

 rest of the taxa appear to be reasonable, i.e., 

 approximately 0.7-0.8, and indicate that net 

 avoidance is not a serious problem. It is interest- 

 ing to note that the relationships of tempera- 

 ture- and length-dependent mortality (Fig. 5) 

 showed that Atlantic mackerel, haddock, blue- 

 fish, and Sebastes spp. exhibit high mortality 

 rates relative to the temperature, which lends 

 support to the conclusion that net avoidance is 

 the primary factor producing high mortality esti- 

 mates. Offshore hake also shows a high mortality 

 versus temperature, but does not exceed a ratio 

 of one in the mortality versus gi'owth rate plot. 

 It is unclear which factor, temperature, net 

 avoidance, mortality estimation, or a combina- 

 tion, is the dominant factor affecting the offshore 

 hake data. 



Contrary to this analysis, Houde (1989) found 

 from a literature review of larval growth and 

 mortality rates that mortality exceeded gi'owth 

 by about 40-100% over a temperature range of 

 5°-30°C. The relationship of gi'owth to mortality 

 during the larval stage has important implica- 

 tions about the life history dynamics during the 

 first year of life. For example, when the ex- 

 pected growth and mortality rates at 10°C from 

 Houde's study and this study are applied to 10'' 

 newly hatched larvae (weighing 0.05 mg each) 

 until metamorphosis (38.2 mg), quite different 



results are found. 



Zt 



La r-val 



stage 



(d) 



Metamorphosis 



Weight 

 No. (g) 



This study 

 Houde (1989) 



0.094 0.1266 59.09 

 0.1486 0.0904 80.11 



3,799 145.12 

 7 0.26 



The 3-4 orders of magnitude difference in num- 

 bers and weight at metamorphosis between the 

 two studies indicate that the gi'owth and mortal- 

 ity rates during the juvenile stage must be very 

 different for recruitment to be successful. If the 

 biomass of a year-class declines during the larval 

 stage, as indicated by Houde (1989), then the 

 ratio of growth to mortality must be high during 

 the juvenile stage. Since growth rate tends to 

 decline during the juvenile stage (Gushing 1975), 

 compared to the larval stage, then mortality of 

 the juvenile stage must be very low. 



The close interdependence of larval gi'owth 

 and mortality rates on temperature is clearly 

 demonstrated in this study. It is important to 

 realize that these results represent average con- 

 ditions for larvae during the eight years from 

 1977 to 1984. The average values for mortality, 

 gi'owth, and temperature form a baseline against 

 which areal, seasonal, or annual variations in 

 these important early life history parameters 

 can be compared. Thus hypotheses about larval 



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