546 



Fishery Bulletin 100(3) 



Figure 1 



South Australian coastal waters. King George whiting iSillaginodes punctata) 

 are found predominantly in the two Gulfs, and along the West Coast. 



hood ratio (Ivimura, 1980; Rice, 1995). We sought to reduce 

 the original 8-parameter model and employed likelihood 

 ratio tests to evaluate the relative fit of the simpler mod- 

 els tested. We specify and apply a set of selection criteria 

 below, seeking one common model form for six data sets 

 to describe the length-at-age distributions. A 7-parameter 

 generalized seasonal von Bertalanffy model, with t^ fixed 

 at zero, was chosen. 



In addition, we applied an allometric relationship (a pow- 

 er curve) to fit weight versus length for this population. 



That the samples are not uniform by length in commer- 

 cial catch is common to most fisheries. The absence of fish 

 below LML or below the lengths selected by the fishing gear, 

 inevitable when sampling is from the landed catch, creates 

 an overestimation bias of mean length-at-age, because fast- 

 er growing fish reach the legal stock sooner and are thus 

 over-represented in those samples. This sampling bias was 

 taken into consideration by the use of a truncated model 

 likelihood assuming zero probability of capture below legal 

 size. In practice, the clear advantage of samples from the 

 landed (commercial or recreational) harvest is a substan- 

 tially lower cost than the alternative of fishery-independent 

 samples gathered, usually by researchers, at sea. 



The specific length-dependent bias in sampling changed 

 during the time of the study. The LML was raised from 280 



mm to 300 mm. Also, some of the samples were gathered 

 by researchers who landed all, notably smaller fish. By 

 applying one of three different likelihoods to any given 

 sample, having different truncation lengths of 280 mm, 

 300 mm, or no truncation, all fish samples were combined 

 in a single length-at-age estimation for each of three re- 

 gional subpopulations, and by sex. 



Materials and methods 



A total of 11,164 King George whiting were sampled 

 between 1995 and 1998 from across the main fishery 

 area of South Australia. Most samples were obtained by 

 subsampling the commercial catch at local fish processing 

 plants or by purchasing fish from commercial processors. 

 Some samples were provided as frozen fish frames (fillets 

 removed) by recreational fishermen, and the remainder 

 were caught on scientific cruises. Each fish was measured 

 for total (TL) and standard lengths (SL) to the nearest 

 mm, and weighed to 0.1 g. Gonads were removed, sexed, 

 and weighed. The sagittae, the largest pair of otoliths, 

 were removed from each fish for age determination. 



Samples were subdivided into six data sets: three spa- 

 tial regions and by sex (Table 1). Movement studies based 



