Falkland Islands Fisheries Department Loligo gahi Stock

Transcription

Falkland Islands Fisheries Department
Loligo gahi Stock Assessment Survey, Second Season 2008
Vessel
Argos Vigo (ZDLU1)
Flag
Falkland Islands
Dates
30/06/2008-14/07/2008
Author
Ignacio Payá.
Scientific Crew
Ignacio Payá and Alastair Baylis
November 2008
_______________________________________________________________________________
SUMMARY
A research survey was conducted in the Loligo box on the Falkland Islands shelf on
board the F/V Argos Vigo between the 30th of June and 14th of July 2008. 41 daytime
and 11 night trawls were made at selected localities with a total Loligo catch of 123
tonnes. Only daytime trawls were used in biomass estimations because Loligo schools
disperse in the water column at night. The biomass was estimated at 13,790 tonnes
and standardised at 14,453 tonnes. This standardised biomass corresponds to 73%
and 63% of the standardised biomasses estimated in 2007 and 2006 July surveys,
respectively. Oceanographic information, collected by data storage tags attached to
the trawling net, showed that Loligo were concentrated in waters with bottom
temperatures of 5.22-5.44 ºC, salinities of 33.9-34.0 ‰, and TS (temperature/salinity)
of 6.2-6.4 ºC/‰, which correspond to the upper part of the Transient Zone. Loligo had
not completely entered the Loligo box because it was restricted by the extension of the
upper part of Transient Zone waters.
INTRODUCTION
The present survey is the ninth made since May 2004, when the first scientific survey
using commercial vessel was carried out (Roa-Ureta 2004, 2005a, 2005b; Payá and
Roa-Ureta 2006; Payá 2006, 2007a, 2007b and 2008). The first three surveys were
made long before the next fishing season and therefore the biomass at the beginning
of the next fishing season had to be estimated using projection models. To avoid the
uncertainty of these projections and possibility that the resource had not fully
recruited to the fishing area, since February 2006 the surveys have been conducted
just before the beginning of the fishing season. To compare the biomasses estimated
by using different fishing vessels, since February 2008 the biomasses have been
standardised by fishing powers. To understand the spatial distribution and the timing
of the Loligo arrivals to the fishing grounds, in the present survey the sampling and
analysis of oceanographic data was introduced.
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1
METHODOLOGY
All fishing activities were performed on F/V Argos Vigo; a Stanley registered stern
trawler with total length of 77.5 m and a beam of 13 m. The gross registered tonnage
is 2074 Mt with a net registered tonnage of 672 Mt and engine power of 3000 hp. A
total of 52 hauls were conducted at locations selected by the Chief Scientist yielding a
total catch of 123 tonnes. 11 trawls were made at night but they were not used for
biomass estimations due to Loligo dispersal at night. The survey covered the whole
shelf area of the Loligo box (depths ranging between 100 and 320 m). The survey
consisted of 15 transects, with several trawls on each transect depending on the width
of the shelf in the area (Fig. 1).
Between two and four trawls were conducted each day when the weather allowed. The
trawl was a standard Spanish bottom trawl with small mesh liner in the codend. The
door opening varied from 101 to 132 m with the mean of 121 m, and the horizontal
trawl opening ranged from 38 to 49 m with the mean of 45 m depending on the course
and trawl speed. The average duration of each trawl was two hours. Every fifteen
minutes during each tow the bridge officers and the chief scientist noted the position,
trawl speed, door opening and quantity and quality of marks observed on the
echosounder.
The net was hauled on board and lifted into place to allow the catch to flow into one
of two fish bins at stern of the trawl deck. The fish bins fed a conveyor system in the
factory. A random sample of 150 squid was taken from every trawl and as soon as
they were separated by sex and maturity they were measured for length frequency
analysis. Additionally, all by-catch species were collected from each trawl by crew
members working at the conveyor belt. After the contents of the trawl had been
processed, the bycatch was weighed and some species like Illex argentinus, rockcod,
icefish and skates were examined in greater detail.
The biomass was estimated using geo-statistical methods. The biomass was product
of the probability of presence, the mean density and the total area. The probability of
presence was estimated by Binomial logit-normal models, using geoRglm R package
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2
(Christensen and Ribeiro Jr, 2005). The mean density was estimated using the
likelihood-based methods of Diggle et al. (1998 and 2003) and the geoR R package
(Ribeiro Jr and Diggle, 2001). More mathematical and statistical details have been
described previously (Roa-Ureta 2005).
50° S
T14
T13
T12
51° S
Latitude
T11
T10
T9
52° S
T8
T7
T6
T0
53° S
54° S
63° W
T5
T1 T2 T3
62° W
61°W
60°W
T4
59°W
58°W
57° W
56° W
Longitude
Fig. 1. Adaptive sampling design of 41 daytime trawls (red dotted lines) and 11 night
trawls (black dotted lines) on the 15 transects (green lines) of the Loligo survey
in July 2008.
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3
In order to collect oceanographic information two data storage tags, DST centi and
DST CTD (Star-Oddi, [email protected]), were attached to the upper rope of
the net mouth. DST centi is a compact microprocessor-controlled temperature and
depth recorder with electronics housed in a waterproof housing. DST CTD is a
temperature and salinity recorder. Both recorders were attached at the same time to
the net in order to record simultaneously temperature, salinity and depth every 2
seconds. In every haul the recorders were removed from the net and put in the
communication box to retrieve the data and to program the recorders for the next haul.
The oceanographic information was analyzed using the likelihood-based geostatistic
methods of Diggle et al. (1998 and 2003) and the geoR R package (Ribeiro Jr and
Diggle, 2001). The same spatial resolution (5x5 km) used in the Loligo density
geostatistic analysis was used for the temperature and salinity data.
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4
RESULTS AND DISCUSSION
Loligo were distributed throughout the sampled area with the highest concentrations
in the southern area and very low concentrations in the central-northern areas. In the
southern area the highest concentrations were found in the western part (Fig. 2).
50° S
Latitude
51° S
52° S
CPUE (tonnes/h)
53° S
54° S
63° W
0 to
0.11
1 to
2 to
4 to
62° W
0.1
to 1
2
4
19
61° W
60° W
59° W
58° W
57° W
56° W
Longitude
Fig.2. CPUE (tonnes/h) of L. gahi observed during July 2008 Loligo Survey.
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5
In the central-north area, the highest Loligo concentrations were found in the farthest
north from 50º30’S to 50º40’S at a depth of 150-250 meters (Fig. 3)
50.5° S
50.7° S
50.9° S
51.1° S
Latitude
51.3° S
51.5° S
51.7° S
51.9° S
CPUE (tonnes/h)
52.1° S
0 to
0.11
1.01
2.01
4.01
52.3° S
0.1
to 1
to 2
to 4
to 18.4
52.5° S
58.25° W
57.75° W
57.25° W
56.75° W
56.25° W
Longitude
Fig. 3. CPUE (tonnes/h) of L. gahi observed during the July 2008 Loligo Survey in the
Central-North area.
In the southern area, the highest Loligo concentrations were found to the west of
Beauchene Island, close to the west Loligo box boundary, in the area called “El seco”,
which is the same place where the highest concentrations were found in July 2006 and
July 2007 surveys (Fig. 4). There were also some high concentrations to the east of
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6
Beauchene Island. This suggested that Loligo were still migrating from the western
areas to the Loligo box.
52.6° S
Latitude
52.8° S
53° S
CPUE (tonnes/h)
0 to
0.11
1.01
2.01
4.01
53.2° S
60.5° W
0.1
to 1
to 2
to 4
to 18.4
60° W
59.5° W
59° W
58.5° W
Longitude
Fig. 4. CPUE (tonnes/h) of L. gahi observed during the July 2008 Loligo Survey in the
southern area of Loligo box (blue line).
The observations made every 15 minutes on the SCANMAR sensor covered from 100
to 320 m of depth, Loligo were present along the whole range of depths, but were
most abundant at 180-239 and 260-319 m depths (Fig. 5). The number of observations
and proportion of presence by depth were similar to those found during the July 2007
survey, but much lower than in July 2006 at 220-279 m depth.
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7
Number of observations
80
40
300-319
280-299
260-279
240-259
220-239
200-219
180-199
160-179
140-159
120-139
1.2
0.6
240-259
260-279
280-299
300-319
240-259
260-279
280-299
300-319
180-199
180-199
220-239
160-179
160-179
220-239
140-159
140-159
200-219
120-139
120-139
200-219
100-119
0
100-119
Proportion of presence
100-119
0
CPUE (tonnes/Km2)
24
12
0
Depth (m)
2006
2007
2008
Fig. 5. Number of 15-minute observations (upper plot), proportion of observations with
Loligo (middle plot) and mean density of positive observations (down plot) by
depth and years for second season surveys.
Rockcod (marujito, Patagonotothen ramsayi) were abundant and were located mainly
to the north of 52º50’S (Fig. 6). There was an inverse relationship between the
proportions of Loligo and rockcod in total catches, however there were always some
amount of rockcod in trawls with high Loligo proportions (Fig. 7). This situation was
more similar to the survey carried out in May 2005 than to the one in July 2006, when
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8
Loligo and rockcod were more mixed. Therefore, during the current survey, there
could be some possible species interference in the acoustic mark identification.
50° S
Latitude
51° S
52° S
Proportion of RockCod in Catch
53° S
54° S
63° W
0 to 0.001
0.001 to 0.25
0.25 to 0.5
0.5 to 0.75
0.75 to 1
62° W
61° W
60° W
59° W
58° W
57° W
56° W
Longitude
Fig. 6. Proportion of Rockcod in total catch at the initial positions of each trawl.
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9
Rockcod Proportion
1
0.75
0.5
0.25
0
0
0.2
0.4
0.6
0.8
1
Loligo Proportion
Fig. 7.
Relation between proportion of Loligo and rockcod in total catch by each
trawl.
Loligo mantle length frequencies were unimodal with an average of 11.6 cm for
females and 12.4 for males (Fig. 8).
0.2
Proportion
0.16
0.12
Female
Male
0.08
0.04
0
5
7
9
11
13
15
17
19
Mantle Length (cm)
Fig. 8. Loligo mantle length frequency by sex found during the July 2008 survey.
The largest Loligo were found in the deepest trawls along the whole Loligo box (Fig.
9).
The female proportion in the catches was mainly 0.4-0.6, very low female
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10
proportion (0.0-0.2) were found from 51º30’ to 52º00’ S, low female proportions
(0.21-.04) were present in shallow trawls in the southern area and high female
proportion (>0.6) were scarce (Fig. 10).
50° S
Latitude
51° S
52° S
53° S
Average Mantle Length (cm)
5 to 8
8.1 to 10
10.1 to 12
12.1 to 14
14.1 to 24
54° S
63° W
62° W
61° W
60° W
59° W
58° W
57° W
56° W
Longitude
Fig. 9. Loligo average mantle length at the start positions of each trawl.
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11
50° S
Latitude
51° S
52° S
53° S
Female Proportion
0 to
0.21
0.41
0.61
0.81
54° S
63° W
0.2
to 0.4
to 0.6
to 0.8
to 1
62° W
61° W
60° W
59° W
58° W
57° W
56° W
Longitude
Fig. 10. Loligo female proportion found at the start positions of each trawl.
Female and male average mantle length in July 2008 was 1 cm and 2 cm shorter than
in July 2007, respectively (Fig. 11). As the scientific Loligo survey in 2005 was
carried out in May, the length frequency in commercial catches from the 3rd week of
July 2005 was used for comparisons.
In all these four years, the male length
distribution was wider than in females, with long tails to large sizes.
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12
0.2
Proportion
0.16
2005
0.12
2006
2007
0.08
2008
0.04
0
5
10
15
20
25
Mantle Length (cm)
0.16
Proportion
0.12
2005
2006
0.08
2007
2008
0.04
0
5
10
15
20
25
Mantle Length (cm)
Fig. 11. Mantle length frequency of female (upper plot) and male (down plot) found in
July surveys from 2006 to 2008 and in commercial catches of July 2005.
During the July 2008 survey only the spring-spawning cohort was found. The
maturity stages did not show any presence of the first cohort, most of individuals were
at both immature and maturing stages (Fig. 12). On the contrary, maturity stages
observed during the May 2005 survey showed the presence of two cohorts, which was
more evident in males.
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13
1.2
1
July 2008
0.8
July 2007
0.6
July 2006
0.4
May 2005
0.2
0
Nomat
Mat1
Mat2
Mat3
Mat4
Mat5
Mat6
0.6
0.4
July 2008
July 2007
July 2006
0.2
May 2005
0
Nomat
Mat1
Mat2
Mat3
Mat4
Mat5
Mat6
Fig. 12. Proportions of maturity stages found in females (upper plot) and males (down
plot) during July scientific surveys from 2006 to 2008 and May 2005.
The biomass available to the survey in July was estimated at 13,790 tonnes with a
coefficient of variation of 19% (Tables 1 and 2). The biomass, density and number
of individuals shown in tables 1 and 2 are the estimations made every year without
any standardization by fishing powers (see last session for standardized figures). The
biomass estimations by sex were more precise in males than in females (CV in Table
2). The 2008 biomass was composed by 53% of females; this percentage was greater
than in 2007 and similar to 2006. The whole 2008 biomass corresponds to 320
millions of individuals that had a mean body weight smaller than in 2007 but greater
than in 2006.
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14
Table 1. Main results of July surveys by year. Biomasses, density and number of
individuals are not standardized by fishing power.
Biomass (tonnes)
Female Proportion (in weight)
Mean Density (g/cm^2)
Number (million)
Area occupied by the stock
Fishing Grounds Area (km^2)
Mean Body Mass (g)
2006
22625
0.51
5.21
600
4341
7027
37.7
2007
19198
0.34
3.33
320
5765
7027
60.0
2008
13790
0.53
2.68
320
5145
7027
43.1
Table 2. Main results of 2008 July survey by sex. The total is not the
simple sum but the result of geostatistic analysis done with both sexes
combined.
Area occupied by the stock (km^2)
Biomass (Tonnes)
Standard Error Biomass (Tonnes)
CV of Biomass
Mean Body Mass (g)
Number (million)
SD(Number) (million)
CV of Number
Total
5145
13790
3227
23
43
320
75
23
Female
5145
7335
2322
32
43
170
54
32
Male
5145
6433
1564
24
43
149
36
24
The spatial statistics analysis for the presence/absence data, first component of
biomass estimation, showed that during July 2008 Loligo were present in 73% of
surveyed area (Table 3). This figure is lower than 2007 but greater than 2006 figures.
The spatial autocorrelation was fitted to the Whitle-Matern function and was greater
than in previous second seasons (Table 3 and Fig 13). The spatial correlation ended
at 100-kilometre distance in 2008, while in previous years ended at 50-kilometre
distance.
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15
Table 3.- Descriptive statistics and parameters of the spatial Loligo
presence/absence process in July surveys.
Presence/Absence of Stock
Area of locality
Total Number of Trials
Total Number of Successes
Spatial AC Function
Family
Link Function
Number of parameters Non Spatial Model
Log-likelihood Non Spatial Model
AIC Non Spatial Model
Number of parameters Spatial Model
Log-likelihood Spatial Model
AIC Spatial Model
Kappa (fixed)
Tau^sq (nugget) (fixed)
Sigma^sq (sill)
Phi (range) (km)
Beta
Spatial model
Kriging Mean p
Mean Interpolation SD of p
CV Mean Interpolation p
2006
2007
2008
128
175
141
25
25
25
329
416
403
229
336
308
Whittle-Matern Whitle-Matern Whitle-Matern
Binomial
Binomial
Binomial
Logit
Logit
Logit
2
2
2
-54.78
-117.6
-56.18
113.5655
239.2
116.4
3
3
3
3.282
4.69
1.33
-0.5646748
-3.38
3.33
1
1
1
0
0
0
1.98
1.79
1.15
11.01
10.98
9.433
0.4327
1.919
1.333
Isotropy
Isotropy
Isotropy
0.618
0.820
0.732
0.189
0.131
0.155
30.589
15.999
21.114
1.2
Presence/absence Correlation
Year
0.8
0.6
0.4
0.0
0.2
Correlation
1.0
2006
2007
2008
0
50
100
150
Kilometers
Fig. 13.
Spatial correlation for presence/absence information for July surveys.
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16
Loligo presence was concentrated in the western side of the southern area and in the
southern and northern extreme of the northern area. 2008 spatial pattern was more
4400
4400
similar to 2006 than to 2007 pattern (Fig 14).
2007
4350
4300
0.6
0.4
4250
Northing (km)
0.8
Presence
0.8
4200
4250
Presence
4200
Northing (km)
4300
4350
2008
0.6
0.4
0.2
4100
4100
4150
4150
0.2
400
450
500
550
600
650
400
Easting (km)
450
500
550
600
650
4400
Easting (km)
4250
Presence
0.8
4200
Northing (km)
4300
4350
2006
0.6
0.4
4100
4150
0.2
400
450
500
550
600
650
Easting (km)
Fig. 14. Estimations of Loligo presence during the July surveys from 2006 to 2008.
The spatial statistics analysis for the density, second component of biomass
estimation, showed that the July 2008 survey had similar number of positive
observations than in 2007 but greater than in 2006 (Table 4). In 2008 the mean
density was 80% and 50% of the density estimated in 2007 and 2006, respectively.
The female density (1.43 g/m2) was 14% greater than male density (1.25 g/m2).
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17
Table 4.- Descriptive statistics and parameters of the spatial Loligo density process in July surveys.
Number of Observations
Spatial AC Function
AIC Non Spatial Model
AIC Spatial Model
Lambda
Kappa
Tau^sq (nugget) (g^2/m^4)
Sigma^sq (sill) (g^2/m^4)
Phi (range) (km)
Beta (g/cm^2)
Spatial model
Kriging Mean (g/m^2)
Mean Interpolation SD (g/m^2)
Kriging Beta (g/m^2)
SD BT-Kriging Beta (g/m^2)
CV BT-Kriging Beta
2006
2007
Female
Male
Female
Male
Total
Female
229
229
336
336
336
308
Gaussian
Gaussian Whitle-Matern Whitle-Matern Whitle-Matern Whitle-Matern
1043
1181
815
1234
1532
1092
965.6
1038
544.4
1001
1298
933.1
-0.0593
-0.1039
-0.0604
-0.0375
-0.0443
-0.2349
inf
inf
2.1
4.445
3.554
1.154
0.7628
0.5581
0.4141
0.3813
0.3679
0.3405
0.8384
0.6482
1.564
1.248
1.161
0.6473
10.46
8.286
4.04
1.911
2.375
10.1
0.2837
0.2737
-0.725
-0.0398
0.4137
-0.0757
Isotropy
Isotropy
Isotropy
Isotropy
Isotropy
Isotropy
2.676
2.578
1.138
2.176
3.330
1.426
1.797
1.669
0.708
1.368
2.078
0.792
2.676
2.578
1.138
2.176
3.330
1.426
0.559
0.430
0.265
0.383
0.624
0.320
20.906
16.692
23.255
17.598
18.744
22.468
2008
Male
Total
308
308
Gaussian
Whitle-Matern
902.3
1428
804.7
1303
-0.2594
-0.2537
inf
2.126
0.419
0.2573
0.3877
0.3397
11.42
5.307
-0.1893
0.5357
Isotropy
Isotropy
1.250
2.680
0.696
1.481
1.250
2.680
0.164
0.448
13.099
16.705
_______________________________________________________________________________________________________________________________________
18
The 2008 and 2007 data were fitted to Whitle-Matern correlation model, but 2006
data did not converge to this model and therefore was fitted to a Gaussian model
(Table 4). In 2008 Loligo density was more concentrated than in the previous years
(Figure 15). The spatial correlation was present until a 10-kilometer distance, which
1.2
was half of distance observed in previous years.
Year
0.8
0.6
0.4
0.0
0.2
Correlation
1.0
2006
2007
2008
0
10
20
30
40
50
Kilometers
Fig. 15.
Spatial correlation for positive density information for July
surveys.
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19
The spatial density distributions by sex were similar, but with higher densities of
4300
4250
Density (g/m^2)
6
4200
Northing (km)
4350
4400
females in the western side of the southern area (Figs 16 and 17).
4
4100
4150
2
400
450
500
550
600
650
700
Easting (km)
4300
Density (g/m^2)
4250
Northing (km)
4350
4400
Fig. 16. Female density estimations for July 2008 survey.
6
4200
5
4
3
4150
2
4100
1
400
450
500
550
600
650
700
Easting (km)
Fig. 17. Male density estimations for July 2008 survey.
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20
To compare the spatial distribution of Loligo densities between years, it was necessary
to use different scales because the figures by year were too different (Fig 18). In 2008
Loligo were concentrated in the western side of the southern area, in the same place
than in previous years, but with lower densities. Loligo were also abundant in some
places in the extreme north of the study area, where in previous years were less
abundant. Unlike previous years the densities close to the Beauchene Islands were
4400
4400
low.
2007
4350
4300
Density (g/m^2)
4250
12
Northing (km)
4300
Density (g/m^2)
4250
Northing (km)
4350
2008
15
8
4200
4200
10
10
6
5
4100
4100
2
4150
4150
4
400
450
500
550
600
650
700
400
450
500
550
600
650
700
Easting (km)
4400
Easting (km)
4300
4250
Density (g/m^2)
40
30
4200
Northing (km)
4350
2006
4150
20
4100
10
400
450
500
550
600
650
700
Easting (km)
Fig. 18. Loligo density (tonnes/km2) in July surveys from 2006 to 2008. Please note the
different scales used in each plot in order to be able to compare the spatial distribution
rather than the density levels.
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21
Because different vessels participated in the Loligo surveys the biomass estimations
were standardized. As the trawl area is included in the biomass estimations but no the
trawl area per hour, the correction factor for standardizing the biomass was the ratio
of trawl speeds (Payá 2008). The reference vessel was the Capricorn vessel, which
was used in 2005 first season survey. The biomasses were increased by 3% in New
Polar and Sil and 5% in Argos Vigo surveys (Table 5). The 2008 biomass was 73%
and 64% of the biomasses estimated in 2007 and 2006 July surveys, respectively.
Table 5.
Biomass, correction coefficient and standardized biomass by year.
The correction factor (CF) is relative to the Capricorn Vessel.
Year
Date (dd/mm)
2006
29/6-13/7
2007
30/6-14/7
2008
30/6-14/7
Vessel
New Polar
Sil
Argos Vigo
Biomass (tonnes)
22000
19198
13790
CF
1.03
1.03
1.05
Standarized
Biomass (tonnes)
22592
19714
14453
The bottom temperature decreased with latitude and depth, the coldest waters were
found in the same area where Loligo was more abundant, that is the western extreme
of the southern area (Fig. 19). The salinity showed the opposite trend than the bottom
temperature, with the highest figures in the western extreme of the southern area (Fig.
20). The waters, where Loligo were concentrated, were characterised by bottom
temperatures of 5.22-5.44 ºC, salinities of 33.9-34 ‰ and TS (Temperature/Salinity)
of 6.2-6.4 ºC/‰, which corresponds to the Transient Zone water mass (Arkhipkin et
al. 2004)(Fig. 21). The 5.5 ºC isotherm has been postulated as a lower limit for
Loligo distribution, which is associated with the warmest possible water layers on the
feeding grounds in a given season (Arkhipkin et al. 2004). The survey data showed
that Loligo was located in the coldest waters available (Fig. 19), this apparent
contradiction could be explained by the fact that during the survey Loligo had not
fully immigrated into the Loligo box and therefore had not occupied the warmest
waters yet. On the other hand, this is the first time that oceanographic information and
Loligo abundance are simultaneously available throughout the whole Loligo box.
Arkhipkin et al. (2004) study was based on oceanographic data limited to one
oceanographic transect in the northern area at 51º45’S and Loligo abundance inferred
from commercial CPUE statistics.
__________________________________________________________________________________
22
50° S
Latitude
51° S
52° S
53° S
Bottom Temperature (C)
5.22
5.54
5.64
5.77
5.92
54° S
63° W
to
to
to
to
to
62° W
5.54
5.64
5.77
5.92
6.08
61° W
60° W
59° W
58° W
57°W
56° W
Longitude
Fig. 19.
Bottom temperature recorded at the net mouth. The points are the average
temperature every 15 minutes based on temperature records every 2 seconds.
In the case of salinity, the fact that during the survey Loligo distribution was
associated with salinity of 33.9-34 ‰, confirmed the Arkhipkin et al. (2005)
hypothesis that Loligo distribution is limited by the upper part of the Transient Zone
waters, which is indicated by salinity values of 33.9-33.95 ‰.
__________________________________________________________________________________
23
50° S
Latitude
51° S
52° S
53° S
Salinity
30.6
32.3
33.6
33.7
33.9
54° S
63°W
to
to
to
to
to
32.3
33.6
33.7
33.9
34
62° W
61° W
60°W
59° W
58° W
57° W
56° W
Longitude
Fig. 20. Bottom salinity recorded at the net mouth. The points are the average salinity
every 15 minutes based on salinity records every 2 seconds.
The TS spatial distribution showed a better relationship with Loligo densities than
temperature or salinity (Fig. 21). The TS values were higher in the extreme western
side of the southern area, exactly where the higher Loligo densities were found in the
southern area. In the extreme north, there were also high Loligo densities but the TS
values were lower than in the southern area. This could be explained by two reasons.
__________________________________________________________________________________
24
The first one is that the salinity recorder did not work properly in the extreme north,
therefore there were not salinity data available for this locality (Fig. 20), and
4350
4300
Temperature (C)
4250
12
Northing (km)
4300
Density (g/m^2)
4250
Northing (km)
4350
4400
4400
consequently the salinity was estimated using the kriging method (spatial correlation).
6
4200
4200
10
8
5.8
6
5.6
4150
4150
4
5.4
4100
4100
2
400
450
500
550
600
650
700
400
450
550
600
650
700
4400
4350
4400
4300
4350
TS
4250
Northing (km)
4300
Salinity
4250
Northing (km)
500
Easting (km)
Easting (km)
6.2
33
4150
4150
5.8
5.6
32
5.4
31
4100
5.2
4100
400
450
500
550
600
Easting (km)
Fig. 21.
6
4200
4200
34
650
700
400
450
500
550
600
650
Easting (km)
Loligo density (g/m2)(upper left plot), bottom temperature (upper right plot),
bottom salinity (down left plot) and bottom TS, Temperature/Salinity (down
right plot).
Loligo density in each locality (5*5 km) is the product of the
density and the presence proportion estimated by geostatistic methods.
Bottom temperature and salinity were estimated using spatial models with
Whitle-Matern correlation functions and anisotropic parameters. TS by locality
were calculated as the ratio between temperature and salinity estimated by
geostatistic methods.
The second reason is that the squid in extreme north were another group independent
of the southern area, with 2 cm smaller sizes than in the southern area (Fig. 9). In any
__________________________________________________________________________________
25
case, it is possible to conclude that Loligo had not completely entered to the Loligo
box because it was restricted by the extension of the upper part of Transient Zone
waters.
Monitoring the oceanographic conditions is very important to understand the timing
of Loligo arrivals to the fishing grounds and how this affects the biomass estimations
during the pre-recruitment survey and during the fishing season. Therefore, it is
recommended to continue using data storage recorders in future Loligo surveys and
ideally deploy them in the net of the fishery vessels during commercial operations.
CONCLUSIONS
1. Loligo biomass available to F/V Argos Vigo survey between 30th of June and 14th
of July 2008 was estimated at 13,790 tonnes and standardized at 14,453 tonnes.
This standardised biomass corresponds to 73% and 63% of the standardized
biomasses estimated in 2007 and 2006 July surveys, respectively.
2.
The biomass was composed by 53% of females. This figure was similar to the
percentages found in 2006 but higher than the ones of 2007.
3. Loligo were concentrated in the western side of the southern area, in the same
place than in previous years, but with lower densities. Loligo were also abundant
in some places in the extreme north of the study area, where in previous years was
less abundant.
4. Loligo were mainly concentrated over 180-239 and 260-319 m depth ranges.
5. Males (12.4 cm ML) were larger than females (11.6 cm ML). Female and male
mantle lengths were 1 and 2 cm shorter than in July 2007, respectively.
6. Male proportion was greater than female proportion in the areas of highest
concentration.
__________________________________________________________________________________
26
7. It was possible to collect important oceanography information using data storage
tags attached to the trawling net.
8. Loligo were concentrated in waters with bottom temperatures of 5.22-5.44 ºC,
salinities of 33.9-34 ‰, and TS of 6.2-6.4 ºC/‰, which corresponds to the upper
part of the Transient Zone water mass.
9. Loligo had not completely entered to the Loligo box because they were restricted
by the extension of the upper part of Transient Zone waters.
ACKNOWLEDGMENT
This study could not be done without the collaboration of the captain and crew of the
F/V Argos Vigo. The captain and bridge officers actively participated in the
identification and classification of Loligo acoustic marks. The crew helped attaching
and removing the data storage tags from the net in every haul and facilitate the
biological sampling in the factory. The survey was conducted in a very friendly
environment.
REFERENCES
Arkhipkin, A.I., R. Grzebielec, A.M. Sirota, A.V. Remeslo, I.A. Polishchuck &
D.A.J. Middleton. 2004. The influence of seasonal environmental
changes on ontogenetic migrations of the squid Loligo gahi on the
Falkland shelf. Fisheries Oceanography, 13:1-9.
Arkhipkin, A.I., D.A.J. Middleton. R., A.M. Sirota. & R. Grzebielec. 2005. The
effect of Falkland Current inflows on offshore ontogenic migrations of
the squid Loligo gahi on the southern shelf of the Falkland Islands.
Estuarine, Coastal and Shelf Science 60:11-22.
Christensen O.F. and Ribeiro Jr P.J. 2005. geoRglm - a package for generalised linear
spatial models.
Diggle PJ, Tawn JA, Moyeed RA. 1998. Model based geostatistics (with discussion).
Applied Statistics. 47:299-350.
__________________________________________________________________________________
27
Diggle PJ, Ribeiro PJ, Christensen OF. 2003. An introduction to model-based
geostatistics. In J. Moller (Ed.), Spatial Statistics and Computational
Methods, vol. 173, Lecture Notes in Statistics, Springer.
Payá, I and R., Roa-Ureta. 2006. Loligo gahi stock assessment survey, first season
2006. Technical Document, Falkland Islands Fisheries Department.
Payá, I. 2006. Loligo gahi stock assessment survey, second season 2006. Technical
Document, Falkland Islands Fisheries Department.
Payá, I. 2007a. Loligo gahi stock assessment survey, first season 2007. Technical
Payá, I. 2007b. Loligo gahi stock assessment survey, second season 2007. Technical
Payá, I. 2008. Loligo gahi stock assessment survey, first season 2008. Technical
Ribeiro Jr P.J., Diggle P.J. 2001. geoR: a package for geostatistical analysis. R-News
1:15-18.
Roa-Ureta, R. 2004. Loligo stock assessment survey and biomass projection, second
season 2004. Technical Document, Falkland Islands Fisheries
Department.
Roa-Ureta, R. 2005a. Loligo stock assessment survey and biomass projection, First
Department.
Roa-Ureta, R. 2005b. Loligo stock assessment survey and biomass projection, Second
Department.
__________________________________________________________________________________
28

Falkland Islands Fisheries Department Loligo gahi Stock

Transcription

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