Table 1 Comparison of model estimates of landed catch and recorded landings tonnes
1991 Species
1992 1993
TAC model
logbook model
logbook TAC
model TAC
logbook 68
Blue eye trevalla 125
125 57
125 125
94 243
1533 2215
3000 Flathead
1516 2064
3000 1635
1298 Gemfish
a
800 800
576 500
500 586
300 300
354 3196
3664 5000
2249 3184
3232 5000
5000 Blue grenadier
400 166
240 John dory
240 99
240 240
103 782
800 800
613 780
Ling 780
728 780
780 224
246 700
Mirror dory 302
196 800
286 284
1673 1095
1550 1550
789 1500
1500 Morwong
709 250
212 300
192 175
Ocean perch 300
221 192
300 300
334 500
500 173
500 500
Royal red prawns 184
966 1598
620 614
892 600
592 Redfish
785 22 650
22 650 Orange roughy
18 250 22 431
18 197 16 687
13 000 13 000
10 481 500
319 160
152 195
500 Silver trevally
198 182
2119 2694
Warehou 2211
2741 3000
1587 2159
3000 Whiting
1642 2100
1320 777
1471 2000
1091 1331
a
Both eastern and western gemfish combined.
substantial overestimate of the catch. Restricting effort ensures that, in the model, a variety of species
are caught in each month and reduces the potential for catches to be substantially overestimated.
Given this, additional constraints were added to the model to restrict the maximum and minimum
number of days that can be fished for any one group. These constraints are given by
DAYS
k, m, g
] n
k
for V
g, k
12 DAYS
k, m, g
5 0.3d
k
13 for k = inshore sector and Vg, k 0.
DAYS
k, m, g
= 0 for Vg, k = 0
14 where n
k
is the minimum number of days fished by a boat in sector k on any one group in any month.
Eq. 13 applies to boats in the inshore sector only, and is imposed to prevent concentration of effort
on any one species groups.
5. Model validation
The process of model verification and validation is not straightforward. Oreskes et al. 1994 claim
that verification and validation of numerical mod- els of natural systems is impossible. They argue that
the existence of uncertain parameters in a model ensure that it can never be verified as a true
representation of the system. While the ability of a model to replicate known events is often used to
validate models, this does not prove that the model accurately represents the system Oreskes et al.,
1994. Nevertheless, the converse would hold: if a model cannot reasonably replicate known out-
comes, then it definitely does not represent the system.
To test the ability of the model to replicate actual behavior, the model was used to estimate the
landings of each quota species over the period 1991 – 93, and these were compared with actual
landings. The model estimates were derived given the actual prices received each month in each year,
fuel and other variable costs in each year, and restrictions imposed by the TACs in each year.
Prices for each species in each year were obtained from the Sydney and Melbourne markets. Fuel
costs were estimated on a cost per day basis, and derived from logbook and survey data ABARE,
1993b. Crew costs and marketing costs were esti- mated as a percentage of the total revenue in each
year and were derived from surveys of the fishery over the time period examined ABARE, 1993b.
Table 2 Comparison of model and survey estimates of key economic variables m
1990–91 survey
a
1992 model 1991–92 survey
a
1993 model 1992–93 survey
a
Variable 1991 model
Revenue Inshore trawlers
26.4 25.3
31.3 23.6
28.8 23.3
50.5 60.4
45.2 62.3
40.5 Offshore trawlers
66.0 5.2
2.9 4.0
3.0 3.1
Danish seiners 3.3
Trip costs 13.4
17.2 14.2
Inshore trawlers 16.2
15.9 13.1
24.5 32.5
18.5 36.1
33.7 Offshore trawlers
19.2 1.9
Danish seiners 2.9
1.7 2.2
1.8 1.9
Gross margins 11.9
14.1 9.4
10.5 12.6
Inshore trawlers 10.2
26.0 27.9
26.7 Offshore trawlers
28.6 29.8
21.3 2.3
1.2 1.8
1.1 1.3
Danish seiners 1.4
a
Source: ABARE, 1993b.
The estimates of landings derived from the model were compared with recorded landings for each
year Table 1. ITQs were introduced on a broad basis in 1992, but TACs for orange roughy and
gemfish both eastern and western combined were in place in 1991. It was found that limits on
landings were also necessary on ling and royal red prawns to prevent overestimation of landings of
these species in the 1991 simulation.
2
For the 1992 and 1993 simulations, both model estimated land-
ings and recorded landings were less than the TAC for most species. In the majority of these cases, the
estimated landings were closer to the recorded landings than the TAC. The model does appear,
however, to consistently overestimate landings of a number of high value but low quantity species.
The significance of any divergence between the level of landings estimated using the model and
recorded landings is difficult to determine. The newness of the ITQ system, problems in allocation
and the lack of an established quota trading market may have resulted in difficulties in quota leasing
Pascoe, 1993. This in turn may have resulted in less catch than might otherwise have been taken,
even though quota may still have been available. Where quota could not be obtained by operators,
catch of some species may have been discarded, resulting in recorded landings being lower than the
true catch. An implicit assumption in the model is that quota can be transferred between boats within
each sector of the trawl fleet and between sectors with no impediments or transactions costs.
The estimates of revenue, costs, and gross margin derived from the model were also compared with
estimates derived from an economic survey of the fishery ABARE, 1993b Table 2. In most cases,
the model estimates of revenues and gross margins were higher than the survey derived estimates, but
were generally of similar orders of magnitude. As one estimate is based on a calendar year January –
December and the other on a financial year July – June, there is no expectation that the esti-
mates should be identical. The relatively higher revenues estimated for the offshore sector using the
model is largely a result of using market prices for orange roughy. Most orange roughy is sold directly
to processors and incurs little or no handling charges once it leaves the boat. As processor prices
were generally not available at the level of detail required in the model, it was assumed that the
market price would be similar to the processor price
2
The overestimation of ling and royal red prawns was thought to be because of the catch rates of these species being
too high in the model. These are relatively minor species in the fishery and may not have been recorded correctly in the
logbook data used in the analysis. Rather than adjust the parameters, constraints were placed on their landing. There-
fore, the validation process also involved an element of cali- bration, a common process in model development Oreskes et
al., 1994.
once the marketing charges had been deducted. As the revenue estimate is before marketing
charges are deducted, the total revenue is substan- tially higher than the revenue received from pro-
cessors. This higher revenue is largely offset by the higher costs, most of which is as a result of
the higher marketing charges assumed in the model. As a result, the gross margin estimated
using the model is similar to the survey estimate for the offshore sector.
6. Simulations and results
