SCRS/2004/096
Col.Vol.Sci.Pap.ICCAT, 58(4): 1271-1274 (2006)
CONVERSION OF FARMED BLUEFIN TUNA PRODUCT WEIGHT TO LIVE
WEIGHT OF THE FISH
Vjekoslav Tičina 1, Leon Grubišić1, Ivan Katavić2, Vlasta Franičević2,
Vanja Emrić Tičina, Mia Ožić
SUMMARY
This paper presents results on relationship between farmed bluefin tuna gilled and gutted (GG)
products weight and live weight of fish. In total, 165 fish from four different cages located at
two farming sites, were analysed immediately upon harvesting. Live weight of these fish ranged
from 19.4 to 81.7 kg. It was noticed that as a result of processing live fish in GG product,
farmed bluefin tuna loss in average 11.2% of its respective live round weight (RWT). Thus, use
of conversion factor of 1.112 to estimate RWT of farmed tuna from known GG product weight
can be suggested.
RÉSUMÉ
Le présent document fait état des résultats sur le rapport entre le poids des produits du thon
rouge d’élevage éviscéré et sans branchies et le poids vif du poisson. Au total, 165 poissons
provenant de quatre cages différentes situées dans deux fermes ont été analysés immédiatement
après leur mise à mort. Le poids vif de ces poissons oscillait entre 19,4 kg et 81,7 kg. Il a été
observé qu’à l’issue de la transformation du poisson vivant en produit éviscéré et sans
branchies, le thon rouge d’élevage perdait en moyenne 11,2% de son poids vif respectif. C’est
pourquoi l’on peut suggérer d’utiliser le coefficient de conversion de 1,112 pour estimer le poids
vif du thon rouge d’élevage à partir du poids du produit éviscéré et sans branchies.
RESUMEN
Este documento presenta los resultados del estudio de la relación entre peso de producto
eviscerado y sin agallas (GG) y peso en vivo de los peces. Se analizaron, justo en el momento
de su sacrificio, 165 peces en total, extraídos de cuatro jaulas diferentes situadas en dos
instalaciones de cría. El peso en vivo de estos peces osciló entre 19,4 y 81,7 kg. Se constató que
al transformar el pez vivo en producto GG, el atún de granja perdía como media un 11,2% de
su peso en vivo respectivo (RWT). Por tanto, se puede sugerir la utilización de un factor de
conversión 1,112 para estimar el peso en vivo del atún de granja partiendo del peso del
producto GG conocido.
KEY WORDS
Aquaculture products, fish conversion factors, bluefin tuna
1
2
Institute of Oceanography and Fisheries, Šet. I. Mešrovića 63, 21000 Split, Croatia (E-mail: [email protected])
Directorate of Fisheries, Ministry of Agriculture, Forestry and Water management of Republic of Croatia.
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1. Introduction
Since 1997, a new activities in aquaculture concerning the bluefin tuna (Thunnus thynnus) rearing in the floating
cages, took place along eastern part of the Adriatic Sea (Katavić et al. 2003). Recently, almost the whole purse
seine catch of bluefin tuna from the eastern Adriatic Sea has been transferred into floating cages for farming
and/or fattening purposes. After harvesting, majority of farmed tuna (over 90%) have been gilled and gutted, i.e.
processed and sold as GG product.
It should be recognised that tuna farming practice cause difficulties in terms of the catch statistics and
concurrence of catch data with trade data. Since tuna farming is recognised as a very important activity
(particularly in the Adriatic Sea), based on some specific knowledge about tuna farming activities, there is a need
to improve conversion factors currently used. Due to the fact that farmed tuna are fattened, its length-weight
relationship (Katavić et al. 2002) is significantly changed in comparison to length-weight relationship of wild
tuna (Tičina 1994) as it is shown on Figure 1. For this reason it was necessary to find out more accurate
conversion factors for estimating live fish round weight (RWT) from farmed product weight, than those
currently used for tuna products that originate from wild fish. It was one of the principal objectives of the study
on the bluefin tuna farming in the growth-out floating cages, conducted within framework of ICCAT Bluefin
Year Programme.
2. Material and methods
In total, 165 fish of different size (FL from 95.0 to 158.0 cm) were analysed in this research. Fish were harvested
from four different growth-out floating cages, located at two different farming sites in the Adriatic Sea. Before
harvesting, tunas were crowded into a small area of the cage using a net. Divers capture fish by hand, and then
swim with them to the harvesting pontoon where tunas are pulled out from the water. Once on board, the lateral
veins are cut immediately (behind the pectoral fins) for bleeding purposes. After that, the spinal chord was
destroyed, and fish were gilled and gutted. Fish were weighted individually using platform digital scale for quick
weight check, with accuracy of ±0.1 kg. Corresponding gills and guts of each fish were weighted separately on a
smaller scale with accuracy of ±0.05 kg. These weight measurements were done immediately, within a period of
approximately 1-2 minutes, after the fish was killed. Furthermore, based on direct observations, it was assessed
that weight loss due to partial bleeding was approximately 1% of corresponding GG product weight. Thus, live
round weight (RWT) of each fish was determined as a sum of GG product weight, weight of gills and gut, and
estimated blood weight loss.
3. Results and discussion
During this research on conversion factor of GG bluefin tuna product originated from farmed fish into respective
RWT, it was noticed that fish after processing (i.e. eviscerating gills and guts) loss different portion of its live
weight. Total GG product weight of all analysed fish in this research was 4.863 MT. Weights of GG products in
this research ranged between 17.5 and 73.0 kg, with an average weight of 29.48 kg (Table 1). The minimum
recorded weight loss was 7.6%. Removed gills and guts of this fish weighted 2.95 kg, while its GG product
weighted 41.0 kg. On the other side, the maximum recorded weight loss was 15.9%. In this case, removed gills
and guts of this fish weighted 4.40 kg, while its GG product weighted 24.5 kg only. Average weight loss, as a
consequence of fish processing (i.e. by eliminating gills and guts from the fish and partially by bleeding), was
11.2% (std=0.99%). Results are shown in Figure 2, and as it can be seen, it seems that there is no significant
dependence of fish size on amount of weight loss, if expressed as percentage of respective live fish RWT.
So, based on the given results, it could be suggested that, to estimate RWT of bluefin tuna from known GG
product weight that originate from farming facilities, conversion factor of 1.112 should be used. Consequently,
the following equation can be proposed:
RWT (kg) = 1.112 x GG product weight (kg).
Currently used ICCAT conversion factor for weight conversion from gilled and gutted weight (GWT) to RWT,
used in creation of ICCAT databases, is 1.16 (ICCAT). It suggests that average weight loss as a result of
processing bluefin tuna caught in the wild is 16%. This is considerably higher than our findings on weight loss
due to the processing farmed bluefin tuna.
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Thus, it can be concluded that conversion factor of 1.16, if applied to farmed bluefin tuna, overestimates weight
loss due to processing of farmed tuna for 4.8%, and consequently overestimate its original live RWT.
References
ICCAT. Conversion factors used in creation of ICCAT databases. http://www.iccat.es/Data/forms/convers.pdf.
Downloaded on 13.07.2004.
KATAVIĆ, I., V. Tičina and V. Franičević. 2002. A preliminary study of the growth rate of bluefin tuna from
Adriatic when reared in the floating cages. ICCAT Coll. Vol. Sci. Pap., 54(2): 472-476.
KATAVIĆ, I., V. Tičina, L. Grubišić, V. Franičević. 2003. Tuna farming as a new achievement in mariculture of
Croatia. In: ORAY, I.K. and KARAKULAK, F.S. (Eds.), 2003. «Workshop on Farming, Management
and Conservation of Bluefin Tuna», Turkish Marine Research Foundation, Istanbul – TURKEY,
Publication No. 13: 10-20.
TIČINA, V. 1994. Morphological characteristics, feeding and fishing on the bluefin tuna (Thunnus thynnus L.)
in the Adriatic Sea. (In Croat) M.Sc. Thesis, University of Zagreb; 84 pp.
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Table 1. Simple statistical description of farmed bluefin tuna GG product weights, weights of removed gills and
guts, live fish round weight and weight loss due to fish processing (N=165).
Weight (kg)
GG weight
Gills and guts
(kg)
weight (kg)
Average
29.48
3.41
Std.
11.01
1.28
Min.
17.50
1.70
l
Max.
73.00
8.00
l
* estimated
as 1% of GG weight
Blood weight
loss (kg)*
0.29
0.11
0.18
0.73
100
90
80
70
60
50
40
30
20
10
0
Live RWT
(kg)
33.18
12.35
19.38
81.73
Weight loss
(% of RWT)
11.2%
0.99%
7.6%
15.9%
3,2891
W = 0.000005 x FL
2,87
W = 0,00003 x FL
60
70
80
90
100
110
120
130
140
150
160
FL (cm)
In the cage (Katavić et al., 2002)
open sea (Tičina, 1994)
Figure 1. Comparison of allometric growth of bluefin tuna caught in the open sea and from the cages.
18%
Weight loss (% RWT)
16%
14%
Average value:
11,2%
12%
10%
8%
6%
15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
RWT (kg)
Figure 2. Relationship between weight loos due to the processing into GG product and corresponding live round
weight (RWT) of farmed bluefin tuna (N=165).
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