Some biometric parameters of Auxis thazard (Lacepède, 1800) (frigate tuna) -data from fishery dependent and fishery independent surveys conducted in Sri Lankan waters

Abstract

The neritic tuna catch in Sri Lankan waters is mainly composed of Auxis thazard (frigate tuna), Auxis rochei (bullet tuna), Euthynnus affinis (kawakawa) and Scomberomorus commerson (narrow-barred Spanish mackerel). Among them, frigate tuna is the dominant species presently contributing over 40% to the total neritic tuna production. Though several studies have been conducted to estimate some biometric parameters of the frigate tuna in Sri Lankan waters, all of the studies have focused on fishery dependent data. This paper attempts to estimate the biometric parameters of frigate tuna using both fishery independent data and fishery dependent data. Length-weight relationship (LWR) was calculated using the equation W = aL b and the Fulton's condition factor (K) was estimated from the relationship K=100W/L 3 (W = total weight; L = total length) to assess the condition of the selected fish. Fishery dependent data from 373 specimens was obtained from the samples collected from the coastal fisheries catches from October 2015 to September 2017. Fishery independent data of 254 specimens was obtained from the samples collected from R/V Dr. Fridtjof Nansen Ecosystem survey conducted in Sri Lankan waters from 24 June 2018 to 16 July 2018. The total length and the weight of the fishery dependent samples ranged from 21.50 cm-44.20 cm and 118.89 g-1430.90 g respectively while those parameters of fishery independent samples ranged from 14.00 cm-19.00 cm and 20.00 g-80.00 g respectively. The LWR for the commercial catch and the fishery independent catch were W = 0.003L 3.428 and W = 0.037L 2.540 respectively. The estimated K value for the commercial catch and fishery independent catch were 1.48 ± 0.15 and 1.03 ± 0.16 respectively. Considering the growth pattern of the two studies, commercial catch showed a positive allometric growth while fishery independent survey showed a negative allometric growth. Based on the results of the K, it can be concluded that the population consisting of larger fish from the commercial catches was at a better condition than the juvenile population studied during the fishery independent survey. According to the results of the IOTC-2019-WPNT09-12 fishery independent survey, two possible nursery grounds for Auxis thazard in the Sri Lankan waters were identified.

Full text

IOTC–2019–WPNT09–12
Some biometric parameters of Auxis thazard (Lacepède, 1800) (frigate tuna) – data from
fishery dependent and fishery independent surveys conducted in Sri Lankan waters.
K. R. Dalpathadu*, D.R. Herath & S.S.K. Haputhantri.
National Aquatic Resources Research and Development Agency (NARA) Crow Island,
Colombo 15, Sri Lanka.
Abstract.
The neritic tuna catch in Sri Lankan waters is mainly composed of Auxis thazard (frigate tuna),
Auxis rochei (bullet tuna), Euthynnus affinis (kawakawa) and Scomberomorus commerson
(narrow- barred Spanish mackerel). Among them, frigate tuna is the dominant species presently
contributing over 40% to the total neritic tuna production. Though several studies have been
conducted to estimate some biometric parameters of the frigate tuna in Sri Lankan waters, all
of the studies have focused on fishery dependent data. This paper attempts to estimate the
biometric parameters of frigate tuna using both fishery independent data and fishery dependent
data. Length–weight relationship (LWR) was calculated using the equation W = aLb and the
Fulton’s condition factor (K) was estimated from the relationship K=100W/L3 (W = total
weight; L = total length) to assess the condition of the selected fish. Fishery dependent data
from 373 specimens was obtained from the samples collected from the coastal fisheries catches
from October 2015 to September 2017. Fishery independent data of 254 specimens was
obtained from the samples collected from R/V Dr. Fridtjof Nansen Ecosystem survey
conducted in Sri Lankan waters from 24 June 2018 to 16 July 2018. The total length and the
weight of the fishery dependent samples ranged from 21.50 cm – 44.20 cm and 118.89 g –
1430.90 g respectively while those parameters of fishery independent samples ranged from
14.00 cm – 19.00 cm and 20.00 g – 80.00 g respectively. The LWR for the commercial catch
and the fishery independent catch were W = 0.003L3.428 and W = 0.037L2.540 respectively. The
estimated K value for the commercial catch and fishery independent catch were 1.48 ± 0.15
and 1.03 ± 0.16 respectively. Considering the growth pattern of the two studies, commercial
catch showed a positive allometric growth while fishery independent survey showed a negative
allometric growth. Based on the results of the K, it can be concluded that the population
consisting of larger fish from the commercial catches was at a better condition than the juvenile
population studied during the fishery independent survey. According to the results of the

IOTC–2019–WPNT09–12
fishery independent survey, two possible nursery grounds for Auxis thazard in the Sri Lankan
waters were identified.
Keywords: Frigate tuna, Length weight relationship, Fulton’s condition factor, fishery
dependent survey, fishery independent survey
Introduction.
Tuna fishery in Sri Lanka.
Sri Lanka is one of the oldest and most important tuna producing island nations in the Indian
Ocean (Maldeniya & Amarasooriya, 1998). The tuna fishery in Sri Lanka was developing
rapidly with the expansion of offshore and deep sea /high seas fishing as with more and more
tuna fishing boats making multi-day fishing trips in offshore waters (Jayasooriya & Bandara,
2013). Exploration and exploitation of tuna fishery resources around this island have shown
that the tuna resources of Sri Lanka are mainly comprised of Yellow fin tuna (Thunnus
albacares), big eye tuna (Thunnus obsesus), skipjack tuna (Katsuwonus pelamis), kawakawa
(Enthynnus affinis), frigate tuna (Auxis thazard) and bullet tuna (Auxis rochei) (Jayasooriya &
Bandara, 2013). Among tuna species, skipjack dominates in both coastal and off shore areas,
followed by yellow fin in the offshore area. The contribution of small tuna species (frigate tuna,
kawakawa and bullet tuna) is also substantial in coastal waters (Maldeniya & Amarasooriya,
1998).
Neritic tuna fishery in Sri Lanka.
Neritic tuna is a major component in the coastal large pelagic catch. The neritic tuna catch in
Sri Lankan waters is mainly composed of Auxis thazard (frigate tuna), Auxis rochei (bullet
tuna), Euthynnus affinis (kawakawa) and Scomberomorus commerson (narrow- barred Spanish
mackerel) (Bandaranayake, et al., 2015; Haputhantri, 2016). Among them, frigate tuna is the
dominant species presently contributes over 40% to the total neritic tuna production
(Haputhantri, 2016). Though there are several gear types and combinations in use for catching
neritic tunas, gillnet (GN), and ring net (RN) are the two major fishing gears that significantly
contribute to neritic tuna production (Perera et al, 2014).

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Even though Auxis thazard (frigate tuna) is the dominant species in the neritic tuna catch in Sri
Lankan waters, few studies have been conducted relating to the biological aspects of Auxis
thazard. Further all studies were based on fisheries dependent data. This paper discusses some
biological aspects related to this species using the data from the commercial catch of the coastal
waters as well as from the fishery independent survey.
Materials and Methods.
Fishery dependent data
Data from 373 specimens from the single day boat catch was obtained from the samples
collected from three provinces around the Sri Lankan coast. All the specimens had come from
the coastal waters. Sample collection was carried out from October 2015 to September 2017.
The samples were collected from Negombo and Beruwela in the Western Province, Chilaw
and Kalpitiya in the North western Province and Dodanduwa, Galle and Weligama in the
Southern Province of Sri Lanka (Fig. 1). The samples were obtained by visiting the landing
sites twice a month.
Figure 1: Sampling locations of A. thazard for fishery dependent data.

IOTC–2019–WPNT09–12
Fishery independent data
Data of 253 specimens was obtained during the ecosystem survey carried out by the RV Dr
Fridtjof Nansen Survey Programme in Sri Lankan waters from 24 June 2018 to 16 July, 2018.
Biological sampling of the fish was carried out using pelagic and bottom trawls. In shallow
water (<30 m) or at night when pelagic fish was close to the surface, the pelagic trawl with
floats or bottom trawl with floats were operated. At the end of the survey, a total number of
111 trawling stations (bottom and pelagic) had been completed among which 4 stations had
recorded Auxis thazard (Figure 2). Details of the trawling stations that found Auxis thazard in
the catch are tabulated in the Table 1.
Table 1: Details of the trawling stations.
Station No:
Gear type
Depth (m)
66
Bottom trawl
10 – 15
68
Bottom trawl
14 – 14
73
Bottom trawl
70 – 73
80
Pelagic trawl
46 – 46
Figure 2: Trawling stations of which Auxis thazard recorded.

IOTC–2019–WPNT09–12
In the fishery dependent samples, the weight of the fish was measured to the nearest 0.1 g
whereas the total length (TL) was measured to the nearest 0.1 cm. In the RV Dr Fridtjof Nansen
Survey, total weight was measured to the nearest 0.5 g whereas the total length (TL) was
measured to the nearest 1cm. Length–weight relationships (LWR) were calculated by the
equation W = aLb (Schneider, et al., 2000). The relationships between the length and weight of
the fish was estimated using least squares method applied to the log transformed data of males
and females given as: log W = log a + b log L, where ‘W’ is the body weight of the fish, ‘L’ is
the total length, ‘a’ is the intercept of the regression curve and ‘b’ is the regression coefficient.
The Fulton’s condition factor (K) was estimated according to Htun-Han (1978) from the
relationship K=100W/L3 to assess the fish condition in Sri Lankan waters.
Results.
Bio metric parameters.
Figure 3 illustrate the estimated LWR for of Auxis thazard in the Sri Lankan waters using
fishery independent and dependent data.
A
y = 3.4279x - 2.4842
R² = 0.9811
1.2
1.7
2.2
2.7
3.2
3.7
1.1 1.2 1.3 1.4 1.5 1.6 1.7
Log10(W(g))
Log10(TL(cm))
Auxis thazard (Fishery dependent)

IOTC–2019–WPNT09–12
B
Figure 3: A – LWR from the commercial catch; B – LWR from the fishery independent survey
Results of the LWR of Auxis thazard for samples collected via fishery independent survey and
fishery dependent survey is tabulated in the Table 2.
Table 2. Length- Weight Relationship (LWR) of Auxis thazard
Survey type
N
LWR
Parameters of
LWR
r2
Growth type
a
b
Fishery dependent
373
W =
0.003TL3.43
0.003
3.43
0.98
Positive
Allometric
Fishery
independent
253
W = 0.04TL2.54
0.04
2.97
0.56
Negative
Allometric
Table 3 shows a summary of the values for parameters obtained for Auxis thazard from fishery
independent and fishery dependent surveys: Length (cm), Weight (g) and Fulton’s Condition
Factor (K).
y = 2.54x - 1.4334
R² = 0.5561
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
1.12 1.14 1.16 1.18 1.2 1.22 1.24 1.26 1.28 1.3
Log10(W(g))
Log10(TL(cm))
Auxis thazard (Fishery independent)

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Table 3. Parameter values of Length (cm), Weight (g) and Fulton’s Condition Factor (K) of
Auxis thazard
Survey type
TL(cm)
Weight (g)
K
Min - Max
Mean ±
SD
Min - Max
Mean ± SD
Mean ±
SD
Fishery dependent
21.50 –
44.20
33.84 ±
5.36
118.89 –
1430.90
634.81 ±
324.72
1.48 ±
0.15
Fishery
independent
14.00 –
19.00
16.46 ±
1.09
20.00 – 80.00
46.20 ± 10.19
1.03 ±
0.16
Discussion and Conclusions.
The estimated mean TL of Auxis thazard from the samples obtained from RV Dr Fridtjof
Nansen Survey (fishery independent data) was 16.46 ± 1.09 cm. The size at first sexual maturity
of the Auxis thazard has been estimated in the other scientific studies as 29.5cm FL (Bahou, et
al., 2016; FishBase, 2019). It was statistically proven that the corresponding sample mean FL
of Auxis thazard from fishery independent survey data was significantly less than the size at
maturity. Therefore, it is evident that fishery independent data has been gathered from a
juvenile population of Auxis thazard in the Sri Lankan waters.
On the other hand, the corresponding mean FL value for the commercial catch from the single
day boats was 32.11 cm. Accordingly, it was statistically proven that the samples obtained
from commercial catches of Auxis thazard in the Sri Lankan waters are represented a matured
fish population. Thus, single day boat fishery operated in the coastal zone in the Sri Lankan
waters is sustainable in the means of not capturing immature juveniles of Auxis thazard.
According to Froese (2006), the exponent b should normally fall between 2.5 and 3.5. In this
study the exponent b for the two studied populations were within this range and therefore the
parameters can be used as the referred length range. The results further revealed that immature
population of Auxis thazard showed a negative allometric growth (b < 3) while mature
population showed a positive allometric growth (b>3). Positive allometric growth results of the
mature population of Auxis thazard is in agreement with those obtained previously in the Indian
Ocean (Tampubolon, et al., 2016; Herath, et al., 2019). Thus, it can be concluded that the

IOTC–2019–WPNT09–12
mature Auxis thazard gets relatively plumb as it grows. Considering the immature population
of Auxis thazard in the Sri Lankan waters, their negative allometric growth is hard to compare
with other similar studies in the region as all most all of those studies were concentrated on
mature populations. However it can be concluded that the Auxis thazard in the Sri Lankan
waters are favor increase in length than in mass as it grows till maturation. The LWR are not
stable for different populations thus may vary in relation to their environmental factors like
temperature, salinity, food (quality, quantity and size), habitat and gonad maturity, spawning
period, season, and sex etc. (Froese, 2006; Jayaprabha, et al., 2015). Therefore, the differences
in the LWR for two populations of the same species may be due to the environmental factors
and/or due to different growth stages.
According to Fulton (1902), a standard condition factor of 1.6 implies excellent condition, 1.4
- good and well-proportioned fish, 1.2 - fair condition, 1 - a long and thin fish in poor condition,
and 0.8 - extremely poor condition. Thus, according to the results, it can be concluded that the
mature population of Auxis thazard in the Sri Lankan waters was in a good condition. The
resulted K value for the immature population was ~ 1. Thus, it can be concluded that the
immature population of Auxis thazard in the Sri Lankan waters was in a poor condition at least
during the period of the survey. The sustainability of a given fishery is a function of the number
of sexually matured fish present in those water. If there is adverse condition for immature
population, the natural replenishment in the fishing grounds through their maturation and
spawning processes get disturbed which may lead to growth over fishing (Ganga, et al., 2014).
However, the value of K is influenced by age of fish, sex, season, stage of maturation, fullness
of gut, type of food consumed, amount of fat reserve and degree of muscular development
(Barnham & Baxter, 1998). Therefore it is imperative to monitor the condition of the immature
fish population of Auxis thazard in the Sri Lankan waters in order to ascertain the sustainability
of the resources.
According to the results of the RV Dr Fridtjof Nansen Survey, Auxis thazard had been only
recorded in four trawling stations at shallow depths. When considering the depth ranges of the
stations, it varies from 10 m (station 66) to 73m (station 73) thus all of them were located in
the coastal zone. According to the other studies of Uchida, 1963 and Siriraksophon, 2014,
though the juveniles were recorded in coastal as well as oceanic environments, most of the
nursery and spawning grounds of Auxis thazard had been identified in the shallow coastal areas.
Therefore, it is fair to conclude that there are two possible nursery grounds of Auxis thazard in

IOTC–2019–WPNT09–12
the Northern and Western coastal waters of Sri Lanka. This might be the first record of possible
nursery grounds reported for Auxis thazard in the Sri Lankan waters.
Acknowledgment.
This research was supported by EAF Nansen program of Food and Agriculture Organization
(FAO), the Norwegian Agency for Development Cooperation (NORAD) and the government
of Sri Lanka. We greatly appreciate the support given the National Aquatic Resources Research
and Development Agency (NARA) and at the Institute of Marine Research (IMR).
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