THRESHER ET AL.: LARVAE OF GADOID. MACRURONUS NOVAEZELANDIAE 

 16 



12 . 



>, 



o 

 C 





-_n 



12 . 



4 . 



West Coast 

 n = 96 



-.3 



-.1 .1 



aa 



■O-^ 



ri- 



SE and East Coast 

 n = 106 



-.3 -.1 .1 .3 .5 



Seasonally Adjusted Growth Residual 



Figure 14.— The distribution of seasonally adjusted residuals from the semilog regres- 

 sion of In total length against age for Macruronus novaezelandiae larvae ^25 d postfirst- 

 feeding tor larvae collected off the west coast (transects 5 and 6) and the southeast and 

 east coasts (transects 8 and 9). Residuals were adjusted for seasonal variations in mean 

 rates of larval growth by fitting a polynomial to the seasonal patterns and extracting 

 new, detrended residuals. 



productivity between the two coasts were less pro- 

 nounced; measured values for two sites off the east 

 coast were 1.51 and 2.89 mg C-m-^h'^ versus 

 values for the west coast that ranged from 1.04 to 

 2.24. We suspect, therefore, that growth rates of 

 these older larvae are driven by local differences in 

 the abundance of copepods and other larger zoo- 

 plankters that constitute their primary diet. Why 

 such regional differences in productivity did not 

 result in a parallel difference between coasts in 

 growth rates of younger larvae is not known. It may 

 be that the effects of food availability on growth 

 rates of first-feeding larvae are overridden by those 

 of water temperatures. 



Regardless of how location affected the growth 

 rates of young and older larvae, summarized in 

 Figure 11, the net effect remains that conditions 

 favorable for early larval growth were not spatial- 



ly coincident with those favoring growth by older 

 larvae. Specifically, growth rates of larvae aged 

 <10-15 d postfirst-feeding were highest closest to 

 the spawning area of M. novaezelandiae, whereas 

 growth rates of larvae older than 25 d postfirst- 

 feeding increased the farther away from the spawn- 

 ing area the larvae were caught. Why M. novae- 

 zelandiae aggregate to spawn off the west coast of 

 Tasmania in the winter, rather than at any other 

 site or time, cannot be known. Winter spawnings 

 are not the norm in gadoids (Breder and Rosen 1966; 

 Hislop 1984) nor, with the possible exception of a 

 weak gyre off the coast, is there any conspicuous 

 oceanographic feature or condition, such as a highly 

 localized plankton bloom, yet documented that 

 would uniquely characterize the site as a particularly 

 good one for spawning. Nonetheless, the enhanced 

 growth rates of early stage larvae at the site argue 



45 



