Falkland Islands Fisheries Department

Loligo gahi Stock Assessment Survey, Second Season 2006

Vessel New Polar (ZDLF2)

Flag Falkland Islands

Dates 29/06/2006-13/07/2006

Author Ignacio Payá.

Scientific Crew Ignacio Payá and Melissa Pritchard

1

SUMMARY

A research survey was conducted in the Loligo box of the Falkland Islands shelf on

board the F/V New Polar between the 29th of June and 13th of July 2006. 40 daylight

and 12 night trawl hauls were carried out in selected localities with a total catch of

240 tons. Only daylight trawls were used in biomass estimations because Loligo

availability at bottom decreases during the night. The estimated biomass at the time of

the survey was 22632 tons of only one cohort squid. The survey was carried out just

before the second fishing season therefore biomass projection was not required.

INTRODUCTION

The current survey is the fifth made since May 2004, when the first scientific survey

using commercial vessel was implemented (Anon. 2004, 2005a, 2005b and 2006).

The first three surveys were made long before the next fishing season and therefore

the biomass at the beginning of next fishing season had to be estimated using

projection models. To avoid the natural uncertainty of these projections and any

possibility that resource had not fully recruited to the fishing area, the last February

2006 and the current survey were made just before the beginning of the fishing season

The 2006 survey is the first survey made in July, previous second season surveys were

made in May 2004 and May 2005. Two practical implications arise from this new

date, first the number of daylight trawls was less than in previous surveys because of

few daylight hours available and second the sampling design had to be adapted to a

deeper Loligo distribution.

The mathematical and statistical methodology has been described previously (Anon.

2004, 2005) so it is omitted here.

METHODOLOGY

All fishing activities were performed on the F/V New Polar; a Stanley registered stern

trawler with total length of 74.5m and a beam of 13m. The gross registered tonnage is

1794 Mt with a net registered tonnage of 672 Mt and an engine power of 4000 hp. A

2

total of 52 hauls were conducted on locations selected by the scientist yielding a total

catch of 240 tons. 12 trawls were made during the dark hours but they were not used

for biomass estimations due to Loligo being less available to bottom nets during the

dark hours.

Between two and four trawls were conducted each day when the weather allowed. The

trawl was a standard Spanish made bottom trawl with small mesh liner in the codend.

The door opening varied from 93 to 178 m with a mean of 118 m, and the horizontal

trawl opening ranged from 37 to 71 m with a mean of 47 m depending on the course

and trawl speed. Trawl duration varied but was normally between two to three hours.

Every fifteen minutes during each tow the bridge officers and the scientist noted the

position, trawl speed, door opening and quantity and quality of the 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

analyses. Additionally, all by-catch species were collected from each trawl by

crewmembers 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 survey covered the whole shelf area of the Loligo box (depths ranging between

100 and 300 m). The survey consisted of 14 transects, with several trawls on each

transect depending on the width of the shelf in the area (Fig. 1).

3

T1

T2 T3 T4T5

T6

T7

T8

T9

T10

T11

T12

T13

T14

-63 -62 -61 -60 -59 -58 -57 -56

LONGITUDE

-54

-53

-52

-51

-50

LA

TIT

UD

E

Fig. 1. Adaptive sampling design of 40 daylight trawls (red dotted lines) and 12 dark

trawls (black dotted lines) throughout 14 transects (blue lines) of the Loligo

survey in July 2006.

4

RESULTS AND DISCUSSION

Loligo were distributed throughout the sampled area with high concentrations in the

southern area and moderate concentrations in the northern area. In the southern area

the highest concentrations were found in the western zone (Fig. 2).

-63 -62 -61 -60 -59 -58 -57 -56

Longitude

-54

-53

-52

-51

-50

La

titu

de

CPUE (ton/Km2)

0 to 0.0001 0.0001 to 4 4 to 8 8 to 16 16 to 32 32 to 175

Fig.2. CPUE (ton/km2) of L. gahi observed during July 2006 Loligo Survey.

5

In the southern area, the highest Loligo concentrations were found to the west of

Beauchene Island, close to the west Loligo box boundary, but there were also some

high concentrations to the east of Beauchene Island (Fig. 3). This suggests Loligo

were still migrating from the west areas to the Loligo box.

-60.5 -60 -59.5 -59 -58.5

Longitude

-53.2

-53

-52.8

-52.6

Latitu

de

CPUE (ton/Km2)

0 to 0.0001 0.0001 to 4 4 to 8 8 to 16 16 to 32 32 to 175

Fig. 3. CPUE (ton/km2) of L. gahi observed during the July 2006 Loligo Survey in

southern area of Loligo box (blue line).

In the observations made every 15 minutes on the SCANMAR sensor, Loligo were

present from 100 to 300 m of depth, but were most abundant between 220 and 280 m

of depth (Fig. 4).

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0

200

400

600

800

1000

1200

100-

120

120-

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Depth (m)

Mea

n C

PU

E (

ton

/km

^2)

0

0.2

0.4

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0.8

1

100-

120

120-

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Depth (m)

Pro

po

rtio

n o

f P

rese

nce

01020304050607080

100-

120

120-

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220-

240

240-

260

260-

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280-

300

Depth (m)

Nu

mb

er o

f o

bse

rvat

ion

s

Fig. 4. Number of 15-minute observations (upper plot), proportion of observations with

Loligo (middle plot) and mean density of positive observations (down plot) by

depth.

7

Rockcod (marujito, Patagonotothen ramsayi) was abundant and located in shallower

and northern waters than Loligo (Fig.5). There was an inverse relationship between

the proportions of Loligo and rockcod in total catches (Fig. 6). These situation

contrasts with the survey carried out in May 2005 when Loligo and rockcod were

more mixed. Therefore, during the current survey, possible species interference in the

acoustic mark identification was smaller.

-63 -62 -61 -60 -59 -58 -57 -56

Longitude

-54

-53

-52

-51

-50

La

titu

de

Proportion of RockCod in Catch

0 to 0.001 0.001 to 0.25 0.25 to 0.5 0.5 to 0.75 0.75 to 1

Fig. 5. Proportion of Rockcod in total catch at the initial positions of each trawl.

8

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2

Loligo Proportion

Roc

kCod

Pro

port

ion

Fig. 6. Relation between proportion of Loligo and rockcod in total catch by each

trawl.

Loligo mantle length frequencies were unimodals with an average of 11.2 cm for

females and 12.3 for males (Fig. 7).

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 5 10 15 20 25 30

Mantle Length (cm)

Pro

port

ion

Female

Male

Fig. 7. Loligo Mantle length frequency by sex found in July 2006 survey.

The largest Loligo were found in the high-density aggregations located in the southern

area (Fig. 8). As females had smaller size than males the proportion of females was

lower than of males in the high-density aggregations areas (Fig. 9).

9

-63 -62 -61 -60 -59 -58 -57 -56

Longitude

-54

-53

-52

-51

-50L

atitu

de

Mean Mantle Length (cm)

8 to 9 9 to 10 10 to 11 11 to 12 12 to 13

Fig. 8. Loligo mean mantle length at the initial positions of each trawl.

10

-63 -62 -61 -60 -59 -58 -57 -56

Longitude

-54

-53

-52

-51

-50

La

titu

de

Female Proportion

0 to 0.2 0.2 to 0.4 0.4 to 0.6 0.6 to 0.8 0.8 to 1

Fig. 9. Loligo female proportion found at the initial positions of each trawl.

There are not scientific Loligo surveys made in July before 2006, therefore the mantle

length frequency is comparable only with Loligo caught by the commercial fleet

during the 3rd week of July 2005 (Fig. 10). Mean mantle length in July 2006 was 2

cm smaller than in July 2005.

11

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

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5 10 15 20 25

Mantle Length (cm)

Pro

port

ion

2005

2006

0

0.02

0.04

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5 10 15 20 25

Mantle Length (cm)

Pro

port

ion

2005

2006

Fig. 10. Mantle length frequency of female (upper plot) and male (down plot) found in

July 2006 survey and July 2005 commercial catches.

During July 2006 survey only the second cohort was found. The maturity stages does

not show any evidence of the presence of the first cohort, all females were in

immature or in preparatory stages and males were in maturing or mature stages. This

situation contrasts with the maturity stages observed in May 2005 survey, where the

presence of the two cohorts was evident (Fig. 11)

12

0

0.1

0.2

0.3

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Nomat Mat1 Mat2 Mat3 Mat4 Mat5 Mat6

July 2006

May 2005

0

0.1

0.2

0.3

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0.5

0.6

0.7

0.8

Nomat Mat1 Mat2 Mat3 Mat4 Mat5 Mat6

July 2006

May 2005

Fig. 11. Maturity stages proportions found in females (upper plot) and males (down

plot) during July 2006 and May 2005 scientific Loligo surveys.

The geostatistical analysis of the presence/absence shows that Loligo were present

throughout the Loligo box with three main zones, two in the southern area and one in

the northern area (Table 1 and Fig. 12)

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Table 1.- Descriptive statistics and parameters of the spatial Loligo

presence/absence process in July 2006 survey.

Geostatistical Analysis of Presence/Absence TotalNumber of Localities (5 km x 5 km squares) 128Area of locality 25Total Number of Trials 329Total Number of Successes 229Minimum Distance (km) 5Maximum Distance(km) 332Minimum Count of Success 0Maximum Count of Success 18Minimum Count of Trials 1Maximum Count of Trials 20Spatial AC Function Whittle-MaternFamily BinomialLink Function LogitNumber of parameters Non Spatial Model 2Log-likelihood Non Spatial Model -55AIC Non Spatial Model 114Number of parameters Spatial Model 3Log-likelihood Spatial Model 3.3AIC Spatial Model -0.565Kappa (fixed) 1Tau^sq (nugget) (fixed) 0Sigma^sq (sill) 1.98Phi (range) (km) 11.01Beta 0.4327SD(Beta)BackTransformed Beta – Probability of observing the stock, p 0.607SD(p)Kriging Mean p 0.618Mean Interpolation SD of p 0.189CV Mean Interpolation p 30.6

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400 450 500 550 600 650

4100

4150

4200

4250

4300

4350

4400

Easting (km)

Nor

thin

g (k

m)

0.2

0.4

0.6

0.8

Presence

Fig. 12. Loligo presence proportion estimations for July 2006 survey.

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The average density estimated by geostatistical analysis were 5.21 (ton/km2) for total,

2.67 for females and 2.58 for males (Table 2). The highest densities were estimated

in the western-southern area (Fig.13).

Table 2.- Descriptive statistics and parameters of the spatial Loligo density

process in July 2006 survey.

Geostatistical Analysis of Density Total Female MaleFishing Grounds Area (km^2) (fgpol.bln) 7027 7027 7027Number of Observations 229 229 229Minimum Distance (km) 0.019 0.019 0.019Maximum Distance(km) 327 327 327Minimum Density (g/m^2) 0.28 0.05529 0.1691Mean Density (g/m^2) 10.4 4.213 6.19Maximum Density (g/m^2) 170.2 90.5 6.19Spatial AC Function Gaussian Gaussian GaussianAIC Non Spatial Model 1449 1043 1181AIC Spatial Model 1337 965.6 1038Lambda -0.079 -0.059 -0.104Kappa inf inf infTau^sq (nugget) (g^2/m^4) 0.562 0.763 0.558Sigma^sq (sill) (g^2/m^4) 0.549 0.838 0.648Phi (range) (km) 9.072 10.46 8.286Beta (g/cm^2) 0.991 0.284 0.274VAR(Beta) (g/m^2) 0.024 0.040 0.026Kriging Mean (g/m^2) 5.212 2.676 2.578Mean Interpolation SD (g/m^2) 3.412 1.797 1.669CV Kriging Mean % 65 67 65BackTransformed Beta Mean (g/m^2) 2.845 1.364 1.339SD BackTransformed Beta Mean (g/m^2) 0.4887 0.2851 0.2235CV BT Beta Mean % 17 21 17Krging Beta (g/m^2) 5.212 2.676 2.578SD BT-Kriging Beta (g/m^2) 0.895 0.559 0.430CV BT-Kriging Beta 17 21 17

16

400 450 500 550 600 650 700

4100

4150

4200

4250

4300

4350

4400

Easting (km)

Nor

thin

g (k

m)

10

20

30

40

Density (g/m^2)

Fig. 13. Loligo density (kg/m2) estimations for July 2006 survey.

Loligo biomass available to the trawling survey in July 2006 was estimated in 22625

tons and it was composed of 51% females (Table 3). This biomass corresponds to

600 million individuals with 55% of females. As there are no other July surveys to

compare the results to, the only comparable biomasses are those projected to July

based on the May survey biomasses (Table 4). July 2006 survey biomass was 22%

and 33% lower than the projected biomass for July 2005 and July 2004 respectively.

The western locations of highest squid concentrations and the gradually decrease of

densities and individual sizes to the east suggests that the resource is still migrating

from the west to Loligo box.

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Table 3. Loligo biomass and number of individuals in July 2006 survey. The

total biomass was estimated by geoestistical analysis so it is not the simple sum of

estimations by sex.

Total Female MaleArea occupied by the stock (km^2) 4341 4341 4341Biomass (Ton) 22625 11615 11190Standard Error Biomass (Ton) 5799 3777 3192CV Biomass % 26 33 29Mean Body Mass (g) 38 34 40Number (million) 600 341 278SD(Number) (million) 154 111 79Var(Number) (million^2) 23641 12282 6291

Table 4. Second season biomass by year. Initial biomass is the total stock

biomass, survey and projected biomass is the biomass available to trawling.

Only the second cohort composed the July 2006 survey biomass.

2004 2005 2006Depletion Model

Initial Biomass 37220 47201Survey Biomass

May (1rst Cohort) 4808 30000May (2nd Cohort) 19808 28000

May (total) 24616 58000July (Total, 2nd Cohort only) 22635

Projected Biomass from Survey15th July 34232 29000

Second Season

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CONCLUSIONS

1. Loligo biomass available to New Polar survey between 29th of June and 13th of

July 2006 was 22632 tons. This biomass was a 22% lower than the biomass that

was projected from the May 2005 survey to July 2005 .

2. Loligo were distributed throughout the sampled area with high concentrations in

the southern area and moderate concentrations in the northern area. In the southern

area the highest concentrations were found in the western zone.

3. Loligo were mainly concentrated in 220-280 m of depth.

4. Males (12.3 cm ML) were larger than females (11.2 cm ML).

5. Male proportion was greater than female in the highest concentration areas.

6. Loligo length was 2 cm smaller than in July 2005.

7. It is likely that during the survey Loligo was still migrating from west to Loligo

box.

REFERENCES

Anon. 2004. Loligo stock assessment survey and biomass projection, second season

2004. Technical Document, Falkland Islands Fisheries Department.

Anon. 2005a. Loligo stock assessment survey and biomass projection, First season 2005. Technical Document, Falkland Islands Fisheries Department.

Anon. 2005b. Loligo stock assessment survey and biomass projection, Second season

2005. Technical Document, Falkland Islands Fisheries Department. Anon. 2006. Loligo gahi stock assessment survey, first season 2006. Technical

Document, Falkland Islands Fisheries Department.