Falkland Island Fisheries Department. Loligo gahi Stock Assessment Survey, First Season Argos Vigo (ZDLU1) Falkland Islands

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1 Falkland Island Fisheries Department Vessel Argos Vigo (ZDLU1) Flag Falkland Islands Dates 9/02/ /02/2007 Author Ignacio Payá Scientific Crew Ignacio Payá, Bahadir Onsoy

2 SUMMARY A research survey was conducted in the Loligo box of the Falkland Islands shelf on board F/V Argos Vigo between the 9 th and 23 rd of February hauls were made in selected localities with a total catch of 100 tons of Loligo. The estimated biomass at the time of the survey was 2684 tons and it was composed by 71% females. This biomass corresponds to 26% and 7% of the biomass estimated in February 2005 and February 2006 surveys, respectively. It is likely that immigration of Loligo to fishing grounds was delayed and therefore the biomass estimated represents only a part of the total stock which was already in the fishing ground. INTRODUCTION The current survey is the sixth made since May 2004, when the first scientific survey onboard a commercial trawler was conducted (Anon. 2004, 2005a, 2005b; Payá and Roa 2006 and Payá 2006). 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 natural uncertainty of these projections and any possibility that resource had not fully recruited to the fishing area, since February 2006 the surveys were made just before the beginning of the fishing season The mathematical and statistical procedures were described previously (Anon. 2004, 2005). METHODOLOGY All fishing activities were performed on the F/V Argos Vigo, a Stanley registered stern trawler with total length of 77.5m, a beam of 13m and draught of 4.8 m. The gross registered tonnage is 2074 mt with a net registered tonnage 672 mt. A total of 65 hauls were made on locations selected by scientists yielding a total catch of 100 tons. 1

3 Between four and five trawls were conducted each day when the weather allowed. The trawl was a standard Spanish made bottom trawl with a small mesh liner in the codend. The door opening varied from 85 to 134 m with a mean of 117 m, and the horizontal trawl opening ranged from 40 to 65 m with a mean of 56 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 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 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 survey covered the whole shelf area of the Loligo box (depths ranging between 80 and 250 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). 2

4 -50 T14 T13-51 T12 T11 T10 Latitude -52 T8 T9 T7 T6-53 T1 T2 T3 T4 T Longitude Fig. 1. Adaptive sampling design of 65 trawls (red dotted lines) throughout 14 transects (black lines) of the Loligo survey in February

5 RESULTS AND DISCUSSION Loligo was concentrated in the south area of the Loligo box, and it was almost absent in the middle and north areas (Fig. 2). -51 Latitude CPUE (Kg/hour) 0 to 5 6 to to to to Longitude Fig.2. Loligo CPUE (Kg/hour) observed during February 2007 survey. 4

6 Most of the trawls made in the middle and the northern area had zero or very small Loligo catches. In the southern area, Loligo was concentrated to the north of Beauchene Island, along the first corridor formed by hard bottoms (Fig. 3). Loligo were not present near the border of the Loligo box, close to Sea Leon Island. This situation, and the presence of Loligo found in shallow water at same time during Skate survey conducted by FIFD, suggests that the Loligo immigration to fishing grounds had just started at the time of the survey. Latitude -53 CPUE (Kg/hour) 0 to 5 6 to to to to Longitude Fig. 3. Loligo CPUE (Kg/hour) observed during February 2007 Survey in the southern area of Loligo box (blue line). Rockcod (marujito, Patagonotothen ramsayi) was abundant and located in deeper waters than Loligo. It was also observed in the middle and northern areas of the Loligo box (Fig.4). As in 2006 February survey, there was an inverse relationship between the proportions of Loligo and rockcod in total catches (Fig. 5). Therefore, the possible species interference in the acoustic mark identification was small. 5

7 Latitude Proportion of RockCod in Catch 0 to to to to to Longitude Fig. 4. Proportion of Rockcod in total catch at the initial positions of each trawl. 6

8 Rockcod Proportion Loligo Proportion Fig. 5. Relation between proportion of Loligo and rockcod in total catch by each trawl. The Loligo mean mantle length was 10.8 cm for both sexes combined and 10.7 cm for females and 11 cm for males (Fig. 6). Squids were larger than in the two previous February surveys. FEM ALE FREQUENCY MANTLE LENGTH (cm ) M ALE 0.2 FREQUENCY MANTLE LENGTH (cm ) Fig. 6. Loligo mantle length frequency by sex and year found in February surveys. 7

9 Biomass available to the survey in February 2007 was estimated to be 2684 tons with a coefficient of variation (CV) of 125% (Table 1). This biomass corresponds to 26% and 7% of the biomass estimated in February 2006 and February 2005 surveys, respectively. The biomass was composed by 71% of females, which suggests that the male immigration to the fishing grounds was delayed. The whole biomass corresponds to 78 millions of individuals that had a greater mean weight than in the previous February surveys. Table 1. Main results of February surveys by year Biomass (Ton) Female Proportion Mean Density (g/cm^2) Number (million) Area occupied by the stock Fishing Grounds Area (km^2) Mean Body Mass (g) The spatial statistics analysis for the density process showed that the February 2007 survey had smaller number of positive observations, smaller density and greater coefficient of variation (CV) than previous February surveys (Table 2). Anisotropy condition was found in the data, that is the semi-variance of density at different distances (known as variogram) changed by spatial direction. The anisotropy angle was estimated in 86 degrees for female and 85 degrees for male. These angles are probably related with the East-West orientation of depth contours in the southern area and Loligo distribution by depth and also with the East-West orientation of trawling tracks that generates more observations in this axis than in the North-South axis. The density estimations by sex showed that Loligo were concentrated in just one small part of the southern area (Figures 7 and 8). The anisotropy was shown in the density estimations with an important East-West component. The spatial distributions of densities were similar for each sex but density values were greater in females than in males (different scales in the figures 7 and 8). 8

10 Table 2.- Descriptive statistics and parameters of the spatial Loligo density process in February surveys Female Male Female Male Female Male Number of Observations Spatial AC Function Gaussian Gaussian Whitle-Matern Whitle-Matern Whitle-Matern Whitle-Matern AIC Non Spatial Model AIC Spatial Model Lambda E Kappa Inf Inf Tau^sq (nugget) (g^2/m^4) Sigma^sq (sill) (g^2/m^4) Phi (range) (km) Beta (g/cm^2) Spatial model Isotropy Isotropy Isotropy Isotropy Anisotropy Anisotropy anisotropy angle (degrees) anisotropy ratio 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 Northing (km) Density (g/m^2) Easting (km) Fig. 7. Female density estimations for February 2007 survey. 9

11 Northing (km) Density (g/m^2) Easting (km) Fig. 8. Male density estimations for February 2007 survey. The spatial statistics analysis for the presence/absence showed that during the February 2007 Loligo were found in only 29% of surveyed area. This is a strong reduction in comparison with 49% and 63% found in 2006 and 2005 February surveys, respectively (Table 3). Loligo was present mainly in a small area in the southern area, and in less extent in two areas in central and northern zones (Figure 9). The presence proportion estimations had an important East-West component based on the same anisotropy parameters estimated for density data. 10

12 Table 3.- Descriptive statistics and parameters of the spatial Loligo presence/absence process in February surveys Number of Localities (5 x 5 km squares) Area of locality Total Number of Trials Total Number of Successes Spatial AC Function Gaussian Whitle-Matern Whitle-Matern Family Binomial Binomial Binomial Link Function Logit Logit Logit 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) Inf 1 1 Tau^sq (nugget) (fixed) Sigma^sq (sill) Phi (range) (km) Beta Spatial model Isotropy Isotropy Anisotropy anisotropy angle (degrees) (Fixed) 86 anisotropy ratio (Fixed) Kriging Mean p Mean Interpolation SD of p CV Mean Interpolation p Northing (km) Presence Easting (km) Fig. 9. Loligo presence proportion estimations for February 2007 survey. 11

13 A comparison between the correlation models fitted for each February surveys showed that Loligo in 2007 was more concentrated than in 2006 but less than in 2005, both in terms of positive densities and presence/absence information (Figures 10 and 11). The spatial correlation for the presence proportion was still observed at 50- kilometre distance in 2007, at 140-kilometre distance in 2006 and at 20-kilometre distance in The spatial correlations for the density had similar values, except for 2006 that was more close to 2007 correlations. Correlation Year Kilometers Fig. 10. Spatial correlation for presence/absence information for February surveys. Correlation Year Kilometers Fig. 11. Spatial correlation for positive density information for February surveys. 12

14 The spatial correlation differences by year are obvious in the density plots that show a small concentrated area of approximately 40 kilometres of length in February 2007; a big and concentrated area of approximately 50 kilometres of diameter in February 2006 and three main areas of 10 kilometres of diameter in February 2005 (Figures 12 to 14). Northing (km) Easting (km) Fig. 12. Loligo density (ton/km 2 ) in February 2007 survey. Northing (km) Easting (km) Fig. 13. Loligo density (ton/km 2 ) in February 2006 survey. 13

15 Northing (km) Easting (km) Fig. 14. Loligo density (ton/km 2 ) in February 2005 survey. The low biomass of Loligo encountered during the pre-recruit survey gave rise to concern, albeit the evidence from the Dorada survey showed that there were significant concentrations of Loligo in shallow waters. A meeting was held on 23 February 2007 with the Loligo producers group to discuss the results. A number of options were considered including delaying the season by 10 days, or starting as normal and possibly suspending the season if the situation did not improve. The outcome was to proceed with the fishery starting on 25 February 2007 and to assess the results. CONCLUSIONS 1. - Loligo biomass during the Argos Vigo survey between 9 th and 23 rd of February of 2007 was estimated at 2684 tons. This biomass corresponds to 26% and 7% of the biomass estimated in February 2005 and February 2006 surveys, respectively. 2. The biomass was composed mainly (71%) by females. 14

16 3. - Loligo was highly concentrated in the southern zone of the Loligo box in a very small area to the north of Beauchene Island. The spatial correlations were present until 50 kilometres of distance Modal mantle length was 10.8 cm that was larger than in the two previous February surveys It is likely that at the time of the survey the Loligo migrations to the fishing grounds were delayed, and therefore the biomass was underestimated. REFERENCES Anon Loligo stock assessment survey and biomass projection, second season Technical Document, Falkland Islands Fisheries Department. Anon. 2005a. Loligo stock assessment survey and biomass projection, First season Technical Document, Falkland Islands Fisheries Department. Anon. 2005b. Loligo stock assessment survey and biomass projection, Second season Technical Document, Falkland Islands Fisheries Department. Payá, I. and R. Roa Loligo gahi Stock Assessment Survey, First Season Technical Document, Falkland Islands Fisheries Department. Payá, I Loligo gahi Stock Assessment Survey, Second Season Technical Document, Falkland Islands Fisheries Department. 15