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International Council for the ICES CM 2002/Q:01 Exploration of the Sea

The effect of Falkland Current inflows on offshore ontogenetic migrations of the squid Loligo gahi on the southern shelf of the

Alexander I. Arkhipkin, Alexander M. Sirota, Ryszard Grzebielec, and David A.J. Middleton

A.I. Arkhipkin, R. Grziebelec and D.A.J. Middleton: Fisheries Department, Falkland Islands Government, P.O. Box 598, Stanley, Falkland Islands [tel: +500 27260, fax: +500 27265, e-mail: [email protected], [email protected]]. A.M.Sirota: Atlantic Research Institute of Marine Fisheries and Oceanography (AtlantNIRO), 5 Dmitry Donskoy street, Kaliningrad, 236000, Russia [tel: +7 0112 225525, fax: +7 0112 219997, e-mail: [email protected]]

ABSTRACT

After reaching the continental slope to the south of the Falkland Islands (South Atlantic Ocean), the northern branch of the Antarctic Circumpolar Current splits into two main northward flowing streams negotiating the Islands from the west (weaker Patagonian Current) and east (stronger Falkland Current). The Falkland Current meets the slope near causing a strong upwelling of the Subantarctic Superficial Water mass and flows up onto the shelf mixing with the Shelf Water mass and producing a Transient Zone (TZ) rich of nutrients. Every austral summer juveniles of the Patagonian longfin squid Loligo gahi migrate from their nearshore spawning and nursery grounds and aggregate near the bottom of their offshore feeding grounds which are closely associated with the inshore boundary of TZ. Three complex oceanographic and biological surveys were made in the period of the beginning of squid offshore migrations in February 2000- 2002. It was found that the shelf inflows of the Falkland Current varied from year to year both in strength and scale, causing significant variability in the location and resolution of the transient zone. Both the starting time of offshore migrations and the spatial distribution of feeding aggregations of L. gahi also varied in these three years. Possible correlations between the distribution of different oceanographic parameters, including the position of TZ, with the distribution of L. gahi were analysed, and their implication for fishery management were discussed.

Keywords: squid, Loligo gahi, Falkland Current, migrations INTRODUCTION The Falkland Islands are situated in the south-eastern part of the , on the highly productive boundary between southern temperate and subantarctic ecosystems (Boltovskoy, 2000). One of the main oceanographic features of the Falkland Islands waters is the cold-water oceanic Falkland Current originating from the Circumpolar Antarctic Current and flowing northwards. After reaching the continental slope to the south of the Islands, this current splits into two main northward flowing streams negotiating the Islands from the west (weaker Patagonian Current or western branch of the Falkland Current) and east (stronger eastern branch of the Falkland Current) (Bianchi et al., 1982). The Falkland Current meets the slope near Beauchene Island causing a strong upwelling of the Subantarctic Superficial Water mass and flows up onto the shelf mixing with the Shelf Water mass and producing several quasi- stationary eddies (Zyrjanov and Severov, 1979). Until now, very little is known about the interannual dynamics of these inflows and their potential impact on the shelf ecosystem of the Falkland Islands. The Patagonian longfin squid Loligo gahi is relatively small near-bottom squid (13–17 cm mantle length, ML) inhabiting the waters around the Falkland Islands. This squid is abundant in near-bottom water layers in the southern and eastern parts of the Falkland Shelf, supporting one of the main squid fisheries in the Southwest Atlantic (Csirke, 1987, Agnew et al., 2000). Like most loliginids (Hanlon and Messenger, 1996), L. gahi undertakes ontogenetic horizontal migrations. Juveniles move from their nursery grounds located in shallow inshore waters (<100 m depths) to offshore feeding grounds on the shelf edge (200–300 m) where they aggregated into dense feeding schools targeted by commercial trawlers (Hatfield and Rodhouse, 1994, Hatfield and des Clers, 1998). After the feeding period, mature adults return to shallow waters to spawn (Hatfield et al., 1990). Two cohorts of L. gahi having different spawning seasons are recognised; the first, autumn–spawning cohort and the second, spring–spawning cohort (Patterson, 1988). Squid of both cohorts have different preferences to oceanographic parameters during their ontogenetic migrations (Arkhipkin et al., 2001). As abundance and catches of both L. gahi cohorts varied from year to year (Agnew et al., 2000; Fisheries Department, 2001), the main task for the stock management is to find some possible environmental factors, influencing or relating to such variations. In February of 2000, 2001 and 2002 the Fisheries Department of the Falkland Islands Government undertook three oceanographic and trawl surveys of the southern part of the Falkland Shelf with an attempt to investigate the variability of main oceanographic features and their possible correspondence with time and extent of offshore feeding migrations of the first cohort of L. gahi. The results of this study made a background of the present report.

MATERIAL AND METHODS Oceanographic data Oceanographic data were collected during three complex oceanographic and biological surveys carried out on board R/V Dorada in the southern part of Falkland Islands shelf and slope during austral summer (Fig.1). In each survey, oceanographic stations were arranged into four transects running from inshore to offshore direction and crossing perpendicularly shelf and slope. On each transect, stations were made at standard depths and positions at 60 m, 100 m, 200 m, 300 m and 500 m. Occasionally, some stations were made between transects as well. A total of 24 stations were sampled in February 2000, 23 stations in February 2001 and 43 stations in February 2002. (Fig. 2). Duration of all oceanographic surveys was 7 days in 2000-2001 and 9 days in 2002. A CTDO (conductivity, temperature, depth and oxygen) sealogger SBE-25 (Sea- Bird Electronics Inc., Bellevue, USA) was deployed from the surface to 5–10 m above the bottom to get depth profiles of temperature (°C), salinity and dissolved oxygen (ml/l). Deployment speed was ca 1 m/sec and it was monitored continuously by a wire counter. Temperature was measured directly, whereas salinity and dissolved oxygen were calculated using Seasoft v. 4.326 software (Sea-Bird Electronics Inc.) from the following data (rpt): pressure (db), conductivity (S/m), oxygen current (µA) and oxygen temperature (°C). All sensors of the CTDO were calibrated once a year at Sea-Bird Electronics Inc. Vertical profiles of temperature, salinity and water density were interpolated along each transect using Surfer v. 7.02 software, and the data were smoothed by kriging. Temperature–salinity (T–S) diagrams on the isopycnal surface were made for each station of the surveys (Mamayev, 1987; Timofeev and Panov, 1962). To analyse the temporal and spatial variability of water mass distribution along each transect of the surveys, a complex method (Miller, 1950) was used based on the T–S analyses and spatial and temporal variability in vertical and horizontal gradients of temperature and salinity, respectively. Temperature and salinity parameters of water masses in the Southwest Atlantic were used to identify the main water masses and their derivatives, as well as the borders between them (Bianchi et al., 1982). Maps were constructed of near bottom and near surface temperature, salinity and density. Near bottom data was defined as the deepest point on each CTD cast, for data aggregated in 1m depth bins. Near surface values were obtained by averaging the up and down cast values for the depth bin at 1m depth. Data from all CTD casts falling in the region 57.5 to 62°W and 51.7 to -53.3°S in each of the three surveys were fitted by a natural bicubic spline surface evaluated on a 0.05° grid (Smith and Wessel, 1990). Grid nodes more than 50km from any CTD cast were masked. Bathymetry was mapped in a similar way using along track depth readings. A 0.02° grid was used, taking the median when more than one data point fell within a grid square, and the surface interpolated using splines with a tension value of 0.75. Grid nodes more than 10km from a depth reading were masked.

Biological data Three sets of data were used to study distribution, abundance and biological condition of L. gahi. Research biological data were collected during the three complex surveys of the southern shelf of the Falkland Islands on board the r/v Dorada. Trawls were made on the same transects as oceanographic stations at depths of 60-80, 100-120 m, 180-200 and 280-330 m. In 2000 and 2001, the trawls were made using a two-panel bottom trawl (vertical opening 4.5 m, horizontal opening 30 m) equipped with a small mesh liner (40 mm stretched mesh). In 2002, an Engel combi trawl (vertical opening 10 m, horizontal opening 40 m with #40 mm mesh in a mesh liner) was used to make trawl hauls on the bottom. Towing distance on the bottom ranged from 3.7 to 4.6 km with an average speed of 7.5 km/hr. From each trawl, a random sample of 200 individuals was taken from the L. gahi catch. If the total number of L. gahi caught in a given trawl was less than 200 specimens all squid were analysed. The CPUE of L. gahi in trawls observed by FIFD Scientific Observers was represented on maps as circles centered on the trawl start position and with a diameter proportional to the cube root of trawl CPUE (kg/hr). Daily CPUE from the entire fleet is available at a resolution of 0.5° longitude by 0.25° latitude. The midday position of a vessel was taken as an indication of its fishing position. Only grid squares with more than five catch reports during February were included. Mean daily CPUE was calculated for each grid square meeting this criteria and was weighted by the proportion of days that the grid square was fished.

RESULTS

Oceanography of the southern part of Falkland Shelf

Temperature Distribution of near-bottom temperatures in February is presented in Fig. 2. In 2000, warm temperatures over 10ºC were observed only in shallow waters east of , with 9.5ºC isotherm spreading southwest of . Three cold water inflows were observed; the strongest to the south of Cape Meredith in the western part of the survey (inflow A), rather weak inflow was traced to the west of Beauchene Isl. (inflow B) and intermediate to the east of Sea Lion Islands (C). In 2001, near-bottom temperatures in shallow waters were generally cooler than in 2000, with 9.5ºC isotherm being situated to the north of Sea Lion Isl. Two inflows of cold water were observed; the strongest in the western part of the survey (A) and another one to the east of Beauchene Isl. (C). Compared to the hydrographic situation in 2000, both inflows A and C were stronger and situated more westerly. Inflow B was not pronounced. February 2002 was the warmest among all three years, with temperatures being over 9.5ºC over the whole shallow area to the north of Sea Lion Island. Cold water inflow A was well-resolved and situated more easterly than in previous two years. Inflow B was not pronounced, and inflow C returned to the same position as in 2000. Interestingly, despite all variations in temperature in shallow waters and on the shelf in the western part of the survey, near-bottom temperatures around Beauchene Isl. were stable ranging from 5.5º to 6ºC in all three years.

Salinity Distributions of near-bottom salinity in February are presented in Fig. 3. In 2000, low near-bottom salinity (<33.8) was observed only inshore at depths <60 m mainly to the north of Sea Lion Isl. The 34.0 isohaline was located to the north of Beauchene Isl. All three inflows (A-C) were recognizable and located at the same locations as on temperature maps. However, the inflow B was less resolved than on temperature map. In 2001, freshened waters (<33.8) were distributed further offshore in the eastern part of the survey. However, it was a well-pronounced inflow of more saline waters (>33.9) into shallow depths to the east of Sea Lion Isl. The 34.0 isohaline was situated further offshore than in 2000, laying south of Beauchene Isl. In 2002, distribution of freshened waters (<33.8) was maximal in all three years. Two regions of their offshore penetration were observed; the first weaker near Cape Meredith in the western part of the survey and another occupied the whole northwestern part of the survey. Inflow A was traced also by salinity and located more easterly as in previous years. The 34.0 isohaline occupied the most southern position near Beauchene Island in all three years of studies.

Density Generally, distribution of density in all three surveys showed the same hydrographic structures as in distribution of temperatures and salinity (Fig. 4). Distribution of less-dense waters was maximum to the south of Sea Lion Islands in 2002.

Water masses Comparative analysis of distribution of near-bottom temperatures and salinity showed that distribution of water masses was different in years studied. Several water masses were revealed: Shelf waters, Sub-Antarctic Superficial Water mass and Transient Zone located between them (Arkhipkin et al., 2001). Shelf waters were the warmest in 2002, and coldest in 2001. Distribution of these waters on the shelf was the narrowest in 2000 and the widest in 2002. The temperature gradient on the periphery of shelf waters was the strongest in 2000 and it was situated closest to the shore near Sea Lion Islands. Upper boundary of the Transient Zone was located close inshore in 2000, and it was farthest offshore between Sea Lion and Beauchene Islands in 2002. The lower boundary was situated more inshore in 2000 and more offshore in 2001-2002.

L. gahi distribution on the southern part of Falkland Shelf

Research data In 2000, maximum catches were encountered on the shelf (100-120 m depth) between Sea Lion and Beauchene Islands (Fig. 5). In all other parts of the survey, abundance of L. gahi was low. In 2001, the highest catches were observed to the south-east of Sea Lion Island. In 2002, patterns of L. gahi distribution were different, with the highest catches observed more offshore than in previous years (around Beauchene Island). Small amounts of squid were captured in the shallow water area north of Sea Lion Islands.

Observer data Observer data set had much larger coverage than research data set (Fig. 3). In 2000, squid were caught mainly to the south of Sea Lion islands along 100-m isobath. Some smaller catches were taken to the north of Beauchene Island at 120-140 m depth. In 2001, squid were encountered in both regions, but were opposite to those of 2000 with higher catches being around Beauchene Isl. and lower catches to the south of Sea Lion Isl. In 2002, the bulk of L. gahi was caught to the east of Beauchene Isl. at 130-140 m depths.

Commercial fleet data The resolution of these data is much lower than those of two previous data sets, but it included commercial data of all vessels fishing for L. gahi to the south of the Falkland Islands (Fig. 6). In general, these data showed the same trends in L. gahi distribution as the observer data set. In 2000, the highest catches were continually observed in two eastern inshore grid squares of the Loligo box to the south of Sea Lion Isl. Some catches were also recorded in the western shallow water grid square which did not appear in the observer data set. Overall, catches were quite stable throughout the month with the mean CPUE of ~ 25 mt/day. In 2001, the highest catches were observed in the offshore grid square around Beauchene Isl. with lower catches in both shallow water grid squares. Catches were low in the first half of February and increased up to 25 mt/day only in the last week of the month. In 2002, high catches were observed only in offshore grid squares around and east of Beauchene, and only in the first half of the month (Fig. 6).

DISCUSSION

Oceanography Until now very little is known about mesoscale dynamics of water masses around the Falkland Islands. Zyrjanov and Severov (1979) pointed out that inshore waters flow in counterclockwise direction around the Islands and may create some quasi-stationary eddies. In the present study, distribution of water temperature and salinity indicated the presence of two quasi-stationary eddies in the region studied, one clockwise cyclonic eddy in the western part of the survey and another counterclockwise anti-cyclonic eddy in the eastern part of the survey. The western periphery of the largest western eddy consists of the strong inflow of the Falkland Current waters onto the shelf near Cape Meredith. Sometimes, like in 2002, it deviated further to the east and almost entered . Waters of the eastern periphery of this eddy flow southward and transport both shelf and Transient zone waters onto the shelf edge near Beauchene Island. Another eddy is located to the south-west of Sea Lion Islands and is less pronounced than the western eddy. The eastern part of this eddy originated from another inflow of the Falkland Current onto the eastern part of the Falkland Shelf, and the western periphery coincides with the eastern periphery of the clockwise eddy (Fig. 2). In some years (2000) these dynamic structures are not well resolved, and the whole region near Beauchene Island was occupied by Falkland Current waters with salinity > 34.0. In 2001 both eddies were well-resolved, resulting in stronger but more localized inflows of the Falkland Current waters onto the shelf and corresponding offshore outflow of Shelf/Transient Zone waters near Beauchene Island. In 2002, a shift of the western inflow (A) of the Falkland Current to the east (western periphery of clockwise eddy) resulted in appearance of strong outflow of Shelf waters near Beauchene Isl., as the position of the eastern counterclockwise eddy was the same as in previous year.

Distribution of L. gahi The timing of the three surveys coincided with the start of offshore feeding migrations of the first cohort (autumn–spawning cohort) of juvenile L. gahi in the southern part of the Falkland Shelf which usually happens in January-February (Patterson, 1988, Hatfield and Des Clers, 1998). Arkhipkin et al. (2001) revealed that squid of this cohort migrate first into the warmer waters of the inner boundary of Transient Zone and then move further down into cooler waters of the Zone, but never penetrating into the waters of the Falkland Current (which constitutes of SASW with salinity >34.0). The results of the present study revealed that spatial distribution of L. gahi corresponded well with hydrodynamic structure of the region studied. In 2000, when the offshore distribution of the Shelf waters was minimal, squid concentrated mainly in the region of warm-water penetration to the south of Sea Lion Islands, and the 34.0 isohaline was a marked boundary of their offshore penetration. In 2001, shelf waters (and correspondingly inshore boundary of the transient Zone) were spread further offshore of the Islands, achieving Beauchene, and squid distributed much wider than in 2000, concentrated both near Sea Lion and Beauchene Islands. Colder than in 2000 temperatures in the nursery grounds (to the north of Sea Lion Island) delayed offshore migrations. Therefore, in 2001 squid arrived to their feeding ground almost two weeks later than in 2000. Much warmer conditions in the nursery grounds in summer 2002 favoured earlier migrations of juveniles and small immature squid to their feeding grounds. Again, as in 2001, offshore flow of the Shelf waters were intense, and by February 2002 squid already attained their feeding grounds, concentrated in offshore shelf area near Beauchene along the 34.0 isohaline. Both temperatures in the nursery grounds and strength of inshore inflows of the Falkland Current affected both time and the extent of distribution of juvenile and immature ASC L. gahi on the Falkland Shelf. Warm temperatures in the nursery grounds induce faster growth rates and squid started to move to the offshore feeding grounds earlier than in years with cooler temperatures in the nursery grounds (2000 and 2002 versus 2001). Stronger inflows of the Falkland Current into the western part of the area induced stronger outflows of warmer and less saline shelf waters to the Beauchene area and corresponding farther offshore movement of squid in their feeding grounds. Thus, in case of implementing protective measures to restrict the fishing of small immature squid during their offshore migration period, a closure of geographical grid squares contained their nursery grounds and main migratory routes is not enough – oceanographic conditions which have happened just before the migratory period should be also taken into account.

REFERENCES

Arkhipkin, A., Grzebielec, R., Sirota, A.M., Remeslo, A.V., Polishchuk, I.A., Middleton, D.A.J. (2001). The influence of seasonal environmental changes on ontogenetic migrations of the squid Loligo gahi on the Falkland shelf. ICES CM 2001/K:1. Agnew D.J., Hill, S. and Beddington, J.R. (2000) Predicting the recruitment strength of an annual squid stock: Loligo gahi around the Falkland Islands. Can. J. Fish. Aquat. Sci. 57:2479–2487. Bianchi A., Massonneau M. and Olevera, R.M. (1982) Analisis estatistico de las caracteristicas T–S del sector austral de la Plataforma Continental . Acta Oceanog. Arg. 3:93–118. Falkland Islands Government (2001). Fisheries Department Fisheries Statistics, Vol. 5. Falkland Islands Goverment Fisheries Department, Stanley. Hanlon, R.T. and Messenger, J.B. (1996) Cephalopod behaviour. Cambridge University Press, Cambridge, 232 pp. Hatfield, E.M.C. and des Clers, S. (1998) Fisheries management and research for Loligo gahi in the Falkland Islands. CalCOFI Rep. 39:81–91. Hatfield, E.M.C. and Rodhouse, P.G. (1994) Migration as a sort of bias in the measurement of cephalopod growth. Antarctic Sci. 6:179–184. Hatfield, E.M.C., Rodhouse, P.G. and Porebski, J. (1990) Demography and distribution of the Patagonian squid (Loligo gahi d’Orbigny) during the austral winter. J. Cons. Perm. Int. Explor. Mer. 46:306–312. Mamayev, O.I. 1987. Thermohaline Analysis of World Ocean Waters. Leningrad, Hydrometeoizdat, p-296. (in Russian). Maslennikov, V.V. and Parfenovich, S.S. (1979) Some peculiarities of the water dynamics around the Falkland Islands. Trudy VNIRO 36:57–60. Miller, A.G. (1950) A study of mixing processes over the edge of the continental shelf. J. Mar. Res. 9:145–160. Patterson, K.R. (1988) Life history of Patagonian squid Loligo gahi and growth parameter estimates using least square fits to linear and von Bertalanffy models. Mar. Ecol. Progr. Ser. 47:65–74. Smith, W. H. F, and P. Wessel, 1990, Gridding with continuous curvature splines in tension, Geophysics, 55: 293-305. Timofeev V.T., Panov V.V. 1962. Indirect methods of water masses identification and analysis. Leningrad, Hydrometeoizdat, p-351. (in Russian). Zyrjanov, V.N. and Severov, D.N. (1979) Water circulation in the Falkland –Patagonian region and its seasonal variability. Okeanologiya 29:782–790.

Cape Meredith Sea Lion Isl.

Beauchene Isl.

Figure 1. Bottom topography of the southern shelf of the Falkland Islands.

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Fig. 2. Profiles of near-bottom temperature and positions of oceanographic stations in February 2000-2002. 2000

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Fig. 3. Profiles of near-bottom salinity and overlaid CPUEs in L. gahi fishery (observer data) in 2000-2002.

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Fig. 4. Profiles of near-bottom density and overlaid CPUEs in L. gahi fishery (observer data) in 2000-2002.

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Fig. 5. CPUEs of L. gahi during research surveys fishery (research data) in 2000-2002. 2000

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Fig. 6. CPUEs in L. gahi fishery (commercial data) in February 2000-2002.