The size-frequency distribution of the commercial catch is often used as the basis of fisheries stock assessments (Paul and Morgan, 1987; Gulland and Rosenberg, 1992) because most dynamic processes of populations (growth, survival, 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 (Penaeus esculentus and P. semisulcatus) 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. However, 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 composition since 1985, when provision for this was made in operators' daily logbooks (between 30% and 45% of the tiger prawn catch reported in the logbooks contain size information). These books are therefore the most comprehensive source of information on the spatial and temporal size distribution of the commercial catch of the NPF. Present assessments of the fishery are based on deterministic growth and deterministic seasonal recruitment patterns (Wang and Die, 1996) and do not use size-structured data. If available, these data would help relax the 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 the assessed. This paper examines the accuracy of grading tiger prawns, by using data collected 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 grading is widespread in other fisheries around the world. Therefore our methods have potential application to other fisheries

Die, D. J.

Faddy, M. J.

ONeill, M. F.

Taylor, B. R.

University of Queensland eSpace

396
The size-frequency distribution of
the commercial catch is often used
as the basis of fisheries stock as-
sessments (Pauly and Morgan,
1987; Gulland 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 (Penaeus esculentus and P.
semisulcatus) 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
Accuracy of at-sea commercial size grading
of tiger prawns (Penaeus esculentus and
P. semisulcatus) 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. O’Neill): Southern Fisheries Centre
Queensland Department of Primary Industries
PO Box 76, Deception Bay, Queensland 4508, Australia
E-mail address (for M. F. O’Neill): oneillm@dpi.qld.gov.au)
Malcolm J. Faddy
University of Queensland
Department of Mathematics, Brisbane, Queensland 4072, Australia
Manuscript accepted 12 May 1998.
Fish. Bull. 97:396–401 (1999).
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
45% 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
397NOTE O’Neill et al.: Accuracy of at-sea commercial size grading of Penaeus spp.
these digital scales also makes it possible to pack in
more precise grades of prawns.
Prawns for these small packs are initially sorted
by eye by experienced crew, and many are verified
by individual weighing. Those prawns that fall out-
side a particular size grade are removed and the re-
mainder are graded according to corresponding
count-per-unit-of-weight tables.
Large packs Prawns for onboard grading into the
large packs are sorted by eye into groups of about
the same size (occasionally by counting the number
into a unit of weight [often a pound measured on
analogue scales] and grouping them accordingly).
Very large and very small prawns are removed and
regraded.
Quality control assessment in the factory—source of
data for analysis
A. Raptis and Sons process tiger prawns caught by
their own large fleet of trawlers working in the NPF,
as well as prawns purchased from other fishermen
operating in the same area. The company randomly
checks the quality of the packs entering its factory,
including the accuracy of the grading.
Packs for quality-control assessment were selected
at random for every vessel and from all consignments
entering the factory. All packs were  clearly marked
with the vessel’s name, the prawn species group, the
grade of the prawns, and the date caught.
The selected samples were thawed individually.
For the small packs, net weight was recorded, and
size grading was checked by counting all prawns from
each pack and averaging the count. Large and small
prawns that did not fit the grade category were se-
lected by eye, weighed, and graded individually, and
the percentage by weight and the true grade of these
prawns were recorded along with the percentage of
those correctly graded.
With the large packs, a variable 2.5–3 kilogram
sample of prawns was randomly taken from each
pack, counted, and checked as above. The percent-
age by weight and the true grade of incorrectly graded
prawns in the sample were recorded.
Categorical analysis
The results of all factory quality-control checks on
both the small and large packs between mid-1993
and the end of 1994 were examined. Over this time,
samples from 51 of the 127 boats that fished different
areas in the NPF had been taken. We split the data
into three time periods to find out whether the accu-
racy of grading early in the year differed from grading
that took place later in the year when smaller prawns
recruited to the fishery. Period one was from July to
December 1993, period two from January to June 1994,
and period three from July to December 1994.
Data from the large packs, where the grades used
were not the same as those for the small packs, were
omitted from the analysis because they could not be
compared directly. The commonly used size grades
for both pack sizes are shown in Table 1, as well as
the equivalent carapace length of the prawns.
The number of prawns (n) contained in the small
packs was recovered by converting the net weight of
the pack to pounds (weight in kilograms divided by
0.45359), and then by multiplying by the count-per-
pound derived from the quality-control inspection.
The number of misgraded prawns (r) in the small
packs was estimated by multiplying the number of
prawns in the sample by the percentage misgraded
in that pack. The numbers misgraded for each pe-
riod and each size grade were analyzed by fitting
binary regression models by means of iterative
weighted least squares. The number of misgraded
prawns was assumed to have a binomial distribution:
(r=0, 1...n)
with P(misgraded) = π and P(correctly graded) = 1 – π.
The probability π was modelled in terms of the log-
odds or logistic transformation (log(π/(1 – π)). The
computed deviance statistic, approximately distrib-
uted as chi-squared, was used in goodness-of-fit tests.
Table 1
Commercial size grades (*) used in grading and quality-
control assessments of tiger prawns for two different pack
sizes. Also shown are the ranges of carapace lengths (mm)
for each size grade.
Grade Carapace
(prawn count length Small Large Quality
(per lb) (mm) pack pack control
Under 6 >46 * * *
6 to 8 46–42 * * *
Under 10 >39 *
9 to 12 41–36 * * *
10 to 15 38–33 *
13 to 15 35–33 * * *
16 to 20 32–30 * * *
10 to 20 38–30 *
Over 20 <29 *
21 to 25 29–28 *
26 to 30 27–26 *
21 to 30 29–26 * *
Pr =



 − −
n
r
r n rπ π( ) ,1
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 = π (1)
and
(2)
where π = 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
(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):
(3)
(4)
where ρ = count per pound;
0.45359 kg = 1 lb; and
w = 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.
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. 1A). Overall, grading accuracy tended to in-
crease over the 18 months examined (Fig. 1A).
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. 1A). 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. 1A);
2 not the same for each size grade over the three
periods examined (Table 4; Fig. 1A).
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 size j (i, j=1 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 θij, then the
proportions, pi, of all prawns graded as size i at-sea
can be adjusted with the equation:
(5)
to give a corrected grade size distribution (j=1, 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
Variance E
n
=
− 
π π( ) ,1
=
× ×



0 45359 2 3 2 5. ln( . )
ρ
E
n
1 




E
n w
dw
1 0 45359
2
2 5
3  = ∫ .. ρ
pi ij
i
θ
=
∑
1
5
399NOTE O’Neill et al.: Accuracy of at-sea commercial size grading of Penaeus spp.
Figure 1
The proportion (+SE) of misgraded tiger prawns from each size grade, for (A) the small packs and
(B) the large packs. The proportion misgraded was split between prawns that were misgraded too
large (white) and those that were misgraded too small (gray).
changes in the proportion misgraded with period of
catch and size grade, with significant differences in
the proportion misgraded with period of catch, and
generally higher proportions of the smaller size
grades misgraded (Table 2; Fig. 1B).
There was a tendency for smaller-prawn grade
packs to contain larger prawns (Fig. 1B). The pro-
portion of misgraded prawns that should have been
in a larger-size grade, however, was
1 not constant over all size grades, within any one
period of catch (Table 3; Fig. 1B);
2 not the same for each size grade, over the three
periods examined (Table 4; Fig. 1B).
400 Fishery Bulletin 97(2), 1999
Table 2
Binomial model fits for each period of catch (1: July–December 1993, 2: January–June 1994, and 3: July–December 1994) and
pack-size combination. (a, b, c) denote groups of size grades with no significant differences in the proportion of misgraded prawns.
Size grade
Pack Time
size period Under 6 6 to 8 9 to 12 13 to 15 16 to 20 χ2 df P
Small a a a
1 b b 1.517 3 0.68
a a
2 b 0.476 1 0.52
c c
a
3 b b 3.244 2 0.20
c c
Large a
1 b 1
c
a a
2 b b b 2.695 3 0.44
a a
3 b 2.892 2 0.24
c c
1 Zero degrees of freedom.
Table 4
Chi-squared statistics for the constant model of misgraded
prawn size in each size grade for both small and large
packs. *P< 0.0001, **P= 0.13, ***P= 0.03.
Size grade Small pack Large pack
6 to 8 4.1494 ** 14.408 *
9 to 12 33.271 * 4.7639 ***
13 to 15 220.58 * 27.039 *
16 to 20 16.159 * 106.83 *
Table 3
Chi-squared statistics for the constant model of misgraded
prawn size in each period of catch (1: July–December 1993,
2: January–June 1994, and 3: July–December 1994), for
both small and large packs. * P< 0.0001
Period Small pack Large pack
1 45.088 * 17.963 *
2 45.916 * 55.701 *
3 32.915 * 83.060 *
Of the prawns misgraded, 92% were sized either
one grade larger or smaller. Of those misgraded in
the 16 to 20 grade, 11% were misgraded by as much
as two grades, whereas less than 2% were so
misgraded for the other size grades.
The misgrading proportions can again be used in
Equation 5 to obtain the at-sea grade-size distribu-
tion for these large packs. Shown in Table 5 are these
corrected distributions compared with at-sea
gradings; here the adjustments can be seen to be
more substantial than those for the small packs, par-
ticularly for the smaller grades 13 to 15 and 16 to 20
prawns per pound, owing to the tendency of the fish-
ermen to classify the prawns to smaller size grades.
Discussion
Our analysis indicates that small prawns are graded
less accurately than large ones. Given that the length
401NOTE O’Neill et al.: Accuracy of at-sea commercial size grading of Penaeus spp.
Table 5
Size-grade distributions, expressed as proportions, from grading at sea and adjusted according to Equation 5.
Period 1 Period 2 Period 3
(July–December 1993) (January–June 1994) (July–December 1994)
Size
grade at-sea adjusted at-sea adjusted at-sea adjusted
Small packs
Under 6 0.15 0.14 0.012 0.015 0.098 0.097
6 to 8 0.16 0.17 0.19 0.19 0.11 0.11
9 to 12 0.30 0.33 0.31 0.30 0.32 0.30
13 to 15 0.26 0.23 0.28 0.28 0.24 0.23
16 to 20 0.13 0.11 0.21 0.20 0.24 0.24
Over 20 0 0.015 0 0.014 0 0.018
Large variable packs
Under 6 — — 0.11 0.071 0.12 0.15
6 to 8 — — 0.059 0.11 0.17 0.15
9 to 12 0.079 0.12 0.12 0.16 0.22 0.28
13 to 15 0.055 0.43 0.10 0.22 0.14 0.21
16 to 20 0.87 0.42 0.61 0.38 0.36 0.20
Over 20 0 0.030 0 0.069 0 0.017
range corresponding to the small commercial grades
is narrow (Table 1), it is perhaps  not surprising that
small prawns tend to be misgraded more frequently.
Alternatively the grading of small prawns may be
less accurate because they are less valuable than
large prawns and therefore less time is spent on grad-
ing each individual.
In small packs, misgraded prawns were generally
graded into larger categories, whereas in large packs,
those misgraded were generally placed into smaller
categories.
Incorrectly graded prawns from both pack sizes,
however, tend to be incorrectly graded by only one
size category, so that all prawns were graded to
within three and six millimeters carapace length of
their corresponding size grade.
The high proportion of landings graded and the
accuracy of some of this grading suggest that size
information contained in the NPF logbooks could be
valuable for stock assessment. Most prawns sold in
small (3-kg) packs have been accurately graded by
fishermen, and these gradings could be used as a
reasonable measure of the length-frequency distri-
bution of the prawns. However, prawns sold in larger
(12–15 kg) packs are graded less accurately, espe-
cially for the smaller grade sizes, and it is recom-
mended that data from quality inspections be used
to correct fishermen’s grade-size distribution.
Although the work outlined in this paper was done
for the Australian northern prawn fishery, similar
analyses using similar methods could be carried out
for other fisheries if comparable data on size gradings
were available.
Acknowledgments
We thank A. Raptis and Sons and, in particular,
David Crighton and Paul Hand for providing the
quality-control data set and describing the quality-
control procedures. Ted Wassenberg, Chris Jackson,
and Carolyn Robins made constructive comments on
an earlier version of this manuscript.
Literature cited
Gulland, J. A., and A. A. Rosenberg.
1992. A review of length-based approaches to assessing fish
stocks. FAO Fisheries Tech. Paper 323, FAO, Rome,
100 p.
Jones, R., and N. P. van Zalinge.
1981. Estimates of mortality rate and population size for
shrimp in Kuwait waters. Kuwait Bull. Mar. Sci. 2:273–88.
Pauly, D., and G. R. Morgan.
1987. Length-based methods in fisheries research.
ICLARM Conference Proceedings 13, 468 p.
Simpson, A. C., and C. P. Kong.
1978. The prawn fisheries of Sabah, Malaysia. Ministry of
Agriculture, Malaysia. Fish. Bull. (Malaysia) 22, 25 p.
Wang, Y-G., and D. J. Die.
1996. Stock-recruitment relationships of the tiger prawns
(Penaeus esculentus and Penaeus semisulcatus) in the Aus-
tralian Northern Prawn fishery. Mar. Freshwater Res.
47:87–95.


Accuracy of at-sea commercial size grading of tiger prawns (Penaeus esculentus and P-semisulcatus) in the Australian northern prawn fishery

O'Neill, M.F.

Die, D.J.

Taylor, B.R.

Faddy, M.J.

Queensland DAF eResearch Archive

Accuracy of at-sea commercial size grading of tiger prawns (Penaeus esculentus and P. semisulcatus) in the Australian northern prawn fishery

http://era.daf.qld.gov.au/id/eprint/137/1/O%27NeillAccuracyTiger-SEC.pdf

Accuracy of at-sea commercial size grading of tiger prawns (Penaeus esculentus and P-semisulcatus) in the Australian northern prawn fishery

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