398 



Fishery Bulletin 97(2), 1999 



It was not possible to recover the total number of 

 prawns in the large packs because the variable 

 sample weight (between 2.5 and 3 kilograms) was 

 not recorded in the Raptis database. Therefore esti- 

 mates of the number of misgraded prawns were de- 

 rived only for the samples and not for the whole pack. 

 Because these samples were randomly chosen, it was 

 possible to assume that the assessment of grading 

 accuracy was representative of the grading accuracy 

 for the whole pack. 



For the large pack samples, the proportion mis- 

 graded had a mean (Eq. 1) and variance (Eq. 2) over 

 the different samples 



Mean = n 



and 



Variance = E 



;rfl-;r) 



(1) 



(2) 



where n = the misgrading probability; and 

 n = the sample size. 



Because the weight range of the samples was small, 

 it was possible to estimate the expected reciprocal 

 sample size 



n 



(Eq. 4) by integrating over the sample weight range 

 (assumed for mathematical convience to be uniformly 

 distributed between 2.5 kg and 3 kg) (Eq. 3): 



E 



n j J2.5 



3 0.45359 



2dw 



piv 



f0.45359x2xln(3/2.5)'| 



(3) 



(4) 



Results 



Small packs 



Of the 21,443 tiger prawns in 293 small packs that 

 were assessed, an estimated 1937 (9%) prawns in 

 229 packs were misgraded. There were significant 

 changes in the proportion misgraded with both pe- 

 riod of catch and size grade, with higher proportions 

 of misgraded prawns in the small size grades (Table 

 2; Fig. lA). Overall, grading accuracy tended to in- 

 crease over the 18 months examined (Fig. lA). 



The size of the misgraded prawns over the differ- 

 ent grades did not show a consistent pattern, but 

 generally larger prawn grade packs tended to con- 

 tain smaller prawns (Fig. lA). The proportion of 

 misgraded prawns that should have been in smaller 

 grades, however, was 



1 not constant over all size grades within each pe- 

 riod of catch (Table 3; Fig. lA); 



2 not the same for each size grade over the three 

 periods examined (Table 4; Fig. lA). 



Of the misgraded prawns, 99^^ were size-graded 

 either one grade larger or one grade smaller. Only 

 grades 9 to 12 and 16 to 20 contained prawns 

 misgraded by as much as two size grades, with no 

 more than 2'/( so misgraded. Because there was no 

 larger grade, prawns misgraded in the under 6 size, 

 were graded as 6 to 8. 



If the proportion of prawns graded size i by fisher- 

 men at-sea that were actually sizejii, j=l for under 

 6, 2 for 6 to 8, 3 for 9 to 12, 4 for 13 to 15, 5 for 16 to 

 20, and 6 for over 20 prawns per pound) obtained 

 from the sample data are denoted by 9^ , then the 

 proportions, p^, of all prawns graded as size ; at-sea 

 can be adjusted with the equation: 



f^PA, 



(5) 



where p = count per pound; 

 0.45359 kg = 1 lb; and 



iv = sample weight. 



The relationship between Equations 1 and 2 here is 

 the same as that for the binomial distribution; there- 

 fore the data were analyzed by fitting binomial re- 

 gression models. 



The size of misgraded prawns was examined to 

 determine whether misgrading was a result of in- 

 cluding small prawns in large grades or vice versa. 

 The number of size grades in which misgrading oc- 

 curred was also assessed. 



to give a corrected grade size distribution (j=l, 2, ..., 

 6). Shown in Table 5 are the corrected distributions 

 compared with at-sea grading for the small packs. 

 The adjustments can be seen to be quite modest, and 

 the at-sea gradings provide a reliable assessment of 

 the size distribution. 



Large packs 



Samples containing an estimated 8210 tiger prawns 

 from 124 large packs were assessed. Of these 

 samples, an estimated 2914 (35%) prawns from 107 

 packs were misgraded. Again, there were significant 



