396 



Accuracy of at-sea commercial size grading 

 of tiger prawns iPenaeus esculentus and 

 P. semisuicatus) in the Australian 

 northern prawn fishery 



Michael F. O'Neill 

 David J. Die 



Brian R. Taylor 



CSIRO Marine Research Laboratories 



PO Box 120 Cleveland, Queensland 4163, Australia 



Present address (for M F ONeill); Southern Fisheries Centre 



Queensland Department of Primary Industries 



PQ Box 76, Deception Bay, Queensland 4 508, Australia 



E-mail address (for M F ONeill) oneillma dpi qld govau) 



Malcolm J. Faddy 



University of Queensland 



Department of Mathematics, Brisbane, Queensland 4072, Australia 



The size-frequency distribution of 

 the commercial catch is often used 

 as the basis of fisheries stock as- 

 sessments (Pauly and Morgan, 

 1987; GuUand and Rosenberg, 

 1992) because most dynamic pro- 

 cesses of populations (growth, sur- 

 vival, recruitment) are reflected in 

 changes in this distribution. The 

 data are generally collected, often 

 at great expense, by sampling the 

 catch at landing sites and markets, 

 or onboard fishing vessels. 



Size-frequency distributions of 

 prawns iPenaeus esculentus and P. 

 semisuicatus) can also be obtained 

 from fish processors, who grade 

 landings by size. These data are 

 easier and cheaper to obtain than 

 research samples, but unfortunately 

 they are also considered less accurate 

 and lack spatial information. How- 

 ever, they have been used in stock 

 assessment of prawns in Kuwait 

 (Jones and van Zalinge, 1981) and 

 Malaysia (Simpson and Kong, 1978). 



It is often difficult to relate size 

 data obtained from a processor to 

 time and place of capture of the 

 prawns, but this is not the case when 



the product is packed onboard, as in 

 Australia's northern prawn fishery 

 (NPF). 



Trawler operators in the NPF 

 have voluntarily recorded size com- 

 position since 1985, when provision 

 for this was made in operators' 

 daily logbooks (between 30% and 

 459c of the tiger prawn catch re- 

 ported in the logbooks contain size 

 information). These books are 

 therefore the most comprehensive 

 source of information on the spa- 

 tial and temporal size distribution 

 of the commercial catch of the NPF. 

 Present assessments of the fishery 

 are based on deterministic growth 

 and deterministic seasonal recruit- 

 ment patterns (Wang and Die, 1996) 

 and do not use size-structured data. 

 If available, these data would help 

 relax the assumption of determinis- 

 tic recruitment and improve current 

 stock assessments of the NPF. 



Before the size data recorded in 

 the logbooks can be used, however, 

 the accuracy of size grading at sea 

 needs to be assessed. This paper 

 examines the accuracy of grading 

 tiger prawns, by using data col- 



lected from a private firm, A. Raptis 

 and Sons, that operates a large 

 modern processing factory that 

 regularly assesses the onboard 

 grading of product purchased from 

 NPF trawler operators. 



Although the work presented 

 here relates specifically to the NPF, 

 the practice of onboard size grad- 

 ing is widespread in other fisher- 

 ies around the world. Therefore our 

 methods have potential application 

 to other fisheries. 



Methods 



At-sea commercial grading 

 procedures 



Prawns landed from the NPF are 

 size-graded at sea because both the 

 demand and price structure differ 

 for prawns of different sizes. Com- 

 mercial sizes are based on the num- 

 ber of prawns of the same size per 

 unit of weight (per pound), and the 

 sizes are then grouped in a range 

 to constitute a commercial grade. 

 For example, "9 to 12 grade" means 

 prawns in a range of sizes individu- 

 ally equivalent to between 9 and 12 

 per pound. 



The size grades ( especially for the 

 larger sizes) used for tiger prawns 

 are often more precise than those 

 used for other species, and the 

 grades selected by fishermen at sea 

 vary with operator, pack size, and 

 target market. For this project we 

 examined the data for the two pack 

 sizes that were most commonly 

 used during 1993 and 1994: small 

 packs (3 kg) and large, variable 

 weight ( 12-15 kg) packs. 



Small packs Since the early 1990s, 

 the use of accurate digital scales on 

 many vessels has improved the ac- 

 curacy of procedures for packing 

 prawns into 3-kg or smaller packs, 

 as well as into more conventional 

 larger packs. The sensitivity of 



Manuscript accepted 12 May 1998. 

 Fish. Bull. 97:396-401 (1999). 



