Distribution, Spawning and Migration of Pelagic Animals in Relation to Oil Exploration

Home , Burdwood Bank, Falkland Current, Patagonian Shelf, Southern blue whiting

AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 39–50 (2002) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/aqc.474

Critical aspects of the Falkland Islands pelagic ecosystem: distribution, spawning and migration of pelagic animals in relation to oil exploration

DAVID J. AGNEW* Renewable Resources Assessment Group, Imperial College, Royal School of Mines, Prince Consort Road, London SW7 2BP, UK

ABSTRACT 1. The oceanography and topography of the southern Patagonian shelf, with the strong Falkland current deriving from the Antarctic Circumpolar current moving northwards both west and east of the Falkland Islands, creates an area of very high zooplankton productivity immediately to the north of the islands. 2. Information on the distribution, spawning times and larval distribution of the most important fish and squid species is reviewed in this paper. High densities of macroplanktonic euphausid and hyperiid amphipods, especially in the summer, attract and sustain squid stocks (the pelagic Illex argentinus and bentho-pelagic Loligo gahi) and pelagic fish (Micromesistius australis and Sprattus fuegensis). 3. There is an important spawning ground for three fish species having pelagic eggs and larvae (Micromesistius australis, Salilota australis and Sprattus fuegensis) on the shelf break immediately to the south and southwest of the Islands. The shelf surrounding the islands, and west and south towards the Argentine coast, forms a nursery area for the larvae of these and a number of other fish and squid species. 4. Pollution emanating from the oil exploration tranches to the north of the islands or oil-based activities on the north shores of the Islands, although coincident with the area of high plankton productivity, would be unlikely to affect, in any major way the pelagic ecosystem around the Falkland Islands unless it became entrained in the area of slack water to the north of East Falkland. However, water flows from the Special Co-operation Area over critical spawning areas for a number of fished species (red cod, southern blue whiting and L. gahi) and has the potential to affect not only these but the Falkland shelf waters which act as a nursery area for many marine species. Copyright # 2002 John Wiley & Sons, Ltd.

KEY WORDS: Falkland Islands; macrozooplankton; ﬁsh larvae; spawning; oil exploration; pelagic ecosystem

INTRODUCTION

The distribution and biology of pelagic and demersal resources around the Falkland Islands is heavily inﬂuenced by the oceanography in the region. The dominant oceanographic feature is the Falkland current,

*Correspondence to: David J. Agnew, Renewable Resources Assessment Group, Imperial College, Royal School of Mines, Prince Consort Road, London, SW7 2BP, UK. E-mail: [email protected]

Copyright # 2002 John Wiley & Sons, Ltd. Received 22 May 2000 Accepted 18 March 2001 40 D.J. AGNEW a strong cold current that branches off the Antarctic circumpolar current around the southern tip of South America and travels northwards until about latitude 368S where it meets the warm Brazil current (Sanchez and Ciechomski, 1995). Both currents are then deflected eastwards into the Atlantic. The Falkland current contains a northward extension of the Polar Frontal Zone, whose western boundary (the Subantarctic Front) is entrained on the continental shelf edge, and occasionally encroaches on the shelf north of the Falklands (Peterson and Whitworth, 1989). The Falkland current itself splits into two around the Falkland Islands, the easterly current being the strongest (Glorioso and Flather, 1995) and joining up with the westerly current further north. The western current is often termed the Patagonian current (Haimovici et al., 1998). Just north of the Islands, in the ‘gap’ between the converging east and west branches of the Falkland current, there is an eddy system with rather little water movement (Glorioso and Flather, 1995). There is also an eddy system close inshore immediately south of Falkland Sound and inshore off the west coast of West Falkland (for instance around the Jason Islands). There is a counter-current of warm water flowing south close the Argentine coast to about 478S, although the origin of these waters (whether from the Brazil current or from the inshore edge of the Falkland current) is not clear (Sanchez and Ciechomski, 1995). There is strong upwelling of Antarctic surface water along the Falkland Islands shelf edges, as the Falkland current moves onto the shelf from relatively deep water. This results in high oxygen saturation and nutrient levels (Peterson and Whitworth, 1989). Primary production values observed around the Falkland Islands are amongst the highest in the SW Atlantic (Angelescu and Prenski, 1987; Sanchez and Ciechomski, 1995). The highest zooplankton densities are reported from the northern shelf edge in the summer (Ciechomski and Sanchez, 1983). The shelf and shelf break therefore become important feeding areas for many fish and squid species, and the circulation of currents around the Falklands is important for the dispersion of their larvae (Sanchez and Ciechomski, 1995). Of the commercial species taken around the Falkland Islands only the squid Illex argentinus and southern blue whiting (Micromesistius australis) may be described as pelagic, being caught in mid-water. However, a number of the other (demersal) commercial species feed on pelagic animals or use the pelagic ecosystem as an early feeding ground and as a distribution mechanism for their larvae. This paper will review available knowledge about the distribution and life-history stages of all commercial species which have a significant pelagic connection. It will also review non-commercial pelagic species (including some rare commercial species such as Martialia hyadesi), and the general distribution of macrozooplankton which are usually the food of the commercial species. However, this is a very large topic, and many of the species have been very extensively researched by a number of authors. Of necessity, therefore, this review will concentrate on only the general features of each species in an attempt to draw common conclusions of importance to the Falkland Islands environment.

COMMERCIAL SPECIES

The Falkland Islands are part of an eastward extension of the Patagonian shelf and, as explained above, form extensive feeding grounds for a number of commercial species. Few of these comprise local stocks specific to the Falkland Islands. Some, such as I. argentinus and Merluccius hubbsi, use the Falkland shelf area as summer (in the case of I. argentinus) or winter (in the case of common hake) feeding grounds at the southern end of their distribution and might be described as a warm-water group. For others, a cold-water group, the Islands are the northward extension of a more southerly distribution, and are used for feeding and/or spawning. In general this is the Angelescu and Prenski (1987) fish assemblage 4 (Micromesistius australis–southern blue whiting, Macruronus magellanicus–hake, Genypterus blacodes–kinorliol, Merluccius australis–southern hake, Salilota australis–Red cod and Dissostichus eleginoides–toothfish).

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Pelagic species M. australis (southern blue whiting) is a pelagic or mesopelagic fish associated with subantarctic waters occuring off Chile, New Zealand and in the southwest Atlantic. In the north of the southwest atlantic region (38–478S) it is confined to the shelf break and slope (Sanchez and Ciechomski, 1995) but extends onto the shelf in the southern part of its range, extending to Burdwood Bank and the Scotia Sea. It spawns in the productive upwelling waters to the southwest of the Islands (Figure 1), usually in September–October although southern blue whiting may exhibit spawning concentrations before this (in August; Macchi and Pajaro, 1999). Southern blue whiting is a batch spawner with determinate fecundity, meaning that mature eggs are released in batches but that the number of oocytes that will develop is fixed prior to the spawning season (Macchi and Pajaro, 1999). Eggs are thus distributed to the southwest and south of the islands and are often found in the same area as gravid females and larvae (Ehrlich et al., 1999). Sanchez and

Figure 1. Major zooplanktonic features around the Falkland Islands. Zooplankton densities (medium (shaded)=101–1000 mm3.m3, high (black) >1000 mm3.m3) are from Ciechomski and Sanchez, (1983). The general direction of currents is taken from Sanchez and Ciechomski (1995) and Glorioso and Flather (1995). The width of the arrows represents current speed, but in this diagram is indicative only. The Falklands current originates in the south, and moves north and also west both around the eastern edge of Burdwood Bank (bb) and in the channel between Burdwood Bank and Staten Island (si). The western current then moves onto the southern Patagonian shelf, and also curves back easterly through the Special Co-operation Area (sca) following the depth contours and splits, one branch rejoining the main Falklands current (fc) ﬂowing around East Falkland (ef) and the other ﬂowing west and north around West Falkland (wf) as the west Falkland current (wfc). There is an eddy system of low current speed north of the islands (lc). The northern oil tranche area (nt) is coincident with the area of high summer zooplankton density (sz). The areas of winter high zooplankton density (wz) and summer medium zooplankton density (msz) are also shown. The larval area for hoki described by Machinandiarena and Ehrlich (1999) is shown (h), together with the spring spawning area for southern blue whiting, sprat and red cod (sa). Other labels are: FICZ: Falkland Islands Interim Conservation Zone; FOCZ: Falkland Islands Outer Conservation Zone; ji: Jason Islands.

Figure 2. Position of main ﬁsheries for bentho- and meso-pelagic species: I. argentinus, L. gahi and southern blue whiting (SBW).

Ciechomski (1995) and Perrotta (1982) have found that the larvae are distributed to the south and northeast of the Islands, presumably distributed there by the anticlockwise passage of the Falkland current around the Islands and feeding in the area of high summer productivity and zooplankton reported over the northern shelf edge (Figure 2). They are also, however, found further south and west on the shelf (Ehrlich et al., 1999). Adult southern blue whiting are thought to spend the winter around Staten Island (see Figure 2) and on the southern Patagonian shelf, migrating to the southwest Falkland area to spawn on the shelf and shelf edge in the spring. They then disperse to feed over the shelf in general, to the north of the Falkland Islands, possibly returning to the southern Patagonian region via Burdwood Bank as well as simply over the shelf. Southern blue whiting is a zooplanktivore (Cousseau and Perrotta, 1998), with euphausids forming the bulk of the diet throughout the year, particularly Thysanoessa gregaria and Euphausia vallentini which are thought to supply the feeding requirements of juvenile and adult blue whiting around the Falkland Islands (Sabatini et al., 1999). Copepods and hyperiid amphipods (Themisto gaudichaudi) may be more seasonally important (Sabatini et al., 1999). Hoki (M. magellanicus) is widespread over the Argentine and Falkland shelves and is the subject of growing commercial interest. It follows the general distribution pattern for cold-water species of being restricted to the cold Falklands current along the shelf slope northwards from 45oS but being more widespread on the shelf south of this line (Cousseau and Perrotta, 1998). The species exhibits diurnal vertical migrations (Angelescu and Prenski, 1987) and should be considered to be largely pelagic. Spawning animals are found in the western FICZ (Falkland Islands Interim Conservation Zone) in October and

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November, but they are less common than spent animals at this time and generally confined to the very western areas. It is likely that most spawning takes place in midwater in Argentine coastal waters to the south of the Patagonian shelf in mid- to late-winter (Chesheva, 1995; Prenski et al., 1996). Larvae have been found only in the southwest of the Patagonian shelf and around Staten Island (Machinandiarena and Ehrlich, 1999) (Figure 1). Hoki is a bentho-pelagic predator, feeding primarily on zooplanktonic crustacea, secondarily on both demersal and pelagic fish (hake, red cod, sprat) and cephalopods (mostly L. gahi) (Prenski et al., 1996). Prenski et al. (1996), summarizing studies in Argentine waters, found 35% crustacea by weight of the diet in summer, the bulk of which was T. gaudichaudii, euphausids E. valentinii and E. lucens, and Munida gregaria. The latter was more abundant by weight than amphipods, but less common in the diet. The migration and distribution of I. argentinus has been well described by numerous authors. Three stocks are recognized, a Bonarensis-northpatagonic stock forming a fishery north of 438S, a summer spawning stock which forms a fishery on the Patagonian shelf between 42 and 448S from December to February, and the main south Patagonian stock. This latter stock is subject to a fishery around the Falkland Islands, and on the shelf in Argentine waters immediately to the west of the FICZ, mostly between March and June (Haimovici et al., 1998). Both earlier and later than the main fishery, squid from this stock are also caught to the north of the Falkland Conservation Zones in both Argentine and high seas waters on the shelf break (Basson et al., 1996). I. argentinus is presumed to hatch in northern shelf waters off Brazil and Uruguay (Arkhipkin, 1993), and larvae and juveniles have been found in the mixed zone between subtropical and subantarctic waters surrounding the warm meanders and eddies originating from the Brazil current (Brunetti and Ivanovic, 1992). Squid move south entering their adult feeding grounds to the north of the Falkland Islands in March (Basson et al., 1996; Rodhouse et al., 1995). In April–June squid leave their feeding grounds and migrate northwards to deeper water on the shelf slope, in the north flowing Falkland current, and thence north to their spawning grounds (Arkhipkin, 1993, 2000). The distribution of I. argentinus in the Falklands zone is consistent from year to year (Rodhouse et al., 1995) and takes the shape of an inverted V north of the Islands (Figure 2). The eastern arm follows the shelf slope and the waters of the eastern Falkland current, and the western arm is over the shelf in the western portion of that current, extending into Argentine waters. There may be considerable variation within the season, however, and although not generally common I. argentinus do sometimes appear in the eddy area north of the Islands. The species is bentho-pelagic in behaviour, undertaking diel feeding migrations (Haimovici et al., 1998). I. argentinus feeds primarily on pelagic crustacea (T. gaudichaudii, M. gregaria and euphausids and chaetognaths (Haimovici et al., 1998). Fish (mainly young hake, myctophids) and squid (including I. argentinus) become more important for larger animals.

Demersal or bentho-pelagic species with pelagic feeding or pelagic life stages L. gahi is a demersal cold-water squid, which forms the basis of a substantial fishery to the south and east of the Falkland Islands on the shelf but concentrated on the shelf break between 150 and 200 m (Figure 2). There appear to be two primary cohorts (stocks) (Agnew et al., 1998). The first is assumed to spawn in April/May, hatches between June and October and recruits to the fishery between October and January. The second cohort is spawned in October/November, hatches between October and January and recruits from April/May to July. Adult feeding concentrations (the subject of the fishery) are distributed mostly on the 200 m bathymetric contour to the south of the Islands and to the northeast (between 51 and 528S). Adults mate and return to shallow waters to deposit eggs on kelp stipes and holdfasts (FIG, 2000; see also Hatfield et al., 1990; George and Hatfield, 1995). Eggs are present all around the islands (Tingley et al., 1996), and concentrations of eggs have recently been found on both the north coast and south coast of East Falkland (FIG, 2000). After hatching, pelagic larvae settle as juveniles in the lower part of the water column, showing some diurnal migration (Hatfield and Rodhouse, 1994). Hatfield and Rodhouse (1994), in

Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 39–50 (2002) 44 D.J. AGNEW four surveys around the islands, found most juveniles on the southeast coast, in Falkland sound and along the north coast of East Falkland. Few juveniles were found to the west of the islands. Loligo gahi feeds on euphausids and other zooplankton (Guerra et al., 1991) and are, in some circumstances, also cannibalistic (Rasero and Portella, pers. comm.). Merluccius hubbsi, the common hake, is present around the Falkland Islands but its main fishery is in Argentine waters north of 488S (Bezzi et al., 1995). There appear to be a number of distinct spawning grounds in this northern area, the most southerly being identified at about 468S on the Argentine shelf (Bezzi et al., 1995). There is a clear seasonal component to the distribution of common hake around the Islands, with hake moving southwards over the shelf to concentrate to the north of and around the islands in winter. Throughout the Patagonian shelf, winter schools are relatively isolated (Angelescu and Prenski, 1987), but in spring and summer, the Falkland Island animals appear to move north (Bezzi et al., 1995). Perrotta and Sanchez (1992) found in a study of morphometrics that there were significant differences between hake collected around the Falkland Islands and those from further north (north of 488S). However, although the fish around the Islands and on the Patagonian shelf are not considered to belong to the main M. hubbsi stock (Prenski and Angelescu, 1993; Tingley et al., 1995) no spawning areas, or juveniles, have been found in Falkland Island waters or south of 468S on the Patagonian shelf. In the northern part of its range, M. hubbsi is a bentho-pelagic feeder, taking pelagic species such as macrozooplanktonic crustacea, anchovy, squid, myctophids and hake as well as more demersal fish species. Prenski and Angelescu (1993) estimate pelagic crustacea to form 41% of diet by weight, Sanchez and Garcia de la Rosa (1999) that crustacea occur in 48% of stomachs; primarily T. gaudichaudii and euphausids. Hake feed close to the bottom during the day, on epibenthic crustacea and fish, rising in the water column to feed on pelagic crustacea during the night. However, Sanchez and Garcia de la Rosa (1999) found that there was an increase in fish and cephalopods in the diet of hake in larger animals and south of 448S. Like southern blue whiting, the demersal red cod (S. australis) appears to spawn in the spring on the shelf edge southwest of the Falkland Islands (Figure 1). Although little is known about the distribution of its larvae, eggs are present in the plankton in late spring on the southern Patagonian shelf (Ciechomski et al., 1981; Ciechomski and Sanchez, 1983). Red cod is a demersal predator, with squid, fish and benthic crustacea making up its diet (Cousseau and Perrotta, 1998).

PELAGIC SPECIES WITH SOME COMMERCIAL INTEREST

Two small pelagic species are common on the Patagonian shelf, the anchovy Engraulis anchovita to the north and Sprattus fuegensis (referred to as Falklands herring, or sprat) to the south (Sanchez and Ciechomski, 1995). Adult sprat appear to be concentrated into two areas, near the southern Patagonian coast and around the Falkland Islands, and it has been suggested that these may represent two different populations (see Sanchez and Ciechomski, 1995). In common with some of the other species mentioned above, sprat spawns normally in September–October, though a study of maturity by Shirokova (1978) indicated that the species is a partial spawner with a 4-month reproductive season September–December. Sprat eggs have been found in September and October (Ciechomski and Sanchez, 1983) to the west and north of the Islands, and larvae over the whole of the southern Patagonian shelf and Burdwood Bank from late summer to early autumn (Sanchez and Ciechomski, 1995). A major nursery ground has also been found in the coastal area off Santa Cruz and Tierra del Fuego (Sanchez and Ciechomski, 1995; Ehrlich et al., 1999). Sprat, like southern blue whiting, are zooplanktivores, eating euphausids, copepods, mysids, chaetognaths, hyperiid amphipods and fish larvae (Cousseau and Perrotta, 1998). The species has only been targeted once by commercial vessels around the Falkland Islands since 1987 when the Falkland Island zones were set up. From July to October 1988 a number of Polish vessels took

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Figure 3. CPUE of sprat, S. fuegensis, by Polish vessels, 1988. Large circles are 4 t/h1, small are zero catches. about 8500 t of sprat1. Most catches were made in July and August, and show a distribution close inshore to the west and south of the Islands, but by September and October the sprat appear to have moved to the south of the islands (Figure 3). These vessels were fishing under general finfish licences. Large shoals of adults (growing up to 23 cm in length) often come very close inshore, and have frequently been reported as stranding or entering coves both on the Patagonian coast and around the Falklands (Sanchez and Ciechomski, 1995). Inshore shoals have been exploited around the Falkland Islands, with up to 250 kg being taken for local consumption fairly regularly in bays and creeks around Darwin and on the west coast of West Falkland from December to April (Brook Hardcastle, Greta Skene, pers. comm.). This is consistent with sprat moving close inshore to feed during the summer following spawning. Most other reports of sprat catches in Falkland Island waters are from Falkland Islands Fisheries Department observers on second season L. gahi vessels, which occasionally encounter shoals of sprat to the south of the Islands. The occurrence in observer samples seems to be very variable from year to year, but is usually associated with the second season (July–November) rather than the first season. Samples taken by observers in September 1998 were of maturing and spawning animals. Shoals of sprat are sometimes encountered to the south of the Islands on the Joint UK/Argentine acoustic surveys for southern blue whiting which take place in September–October. Martialia hyadesi is a nectonic pelagic ommastrephid squid inhabiting waters of the Polar Frontal Zone that is occasionally caught to the north of the Falkland Islands on the shelf break around 498S by the squid jig vessels targeting I. argentinus. Its occurrence there appears to be related to incursions of cold polar frontal waters of the Falklands current over the shelf, and it is usually caught in April or May. Recent years with significant M. hyadesi catch have been 1995 and 1997, almost 6000 t being reported from the former (FIFD, 1999). The species has a general circumpolar distribution associated with the Antarctic circumpolar zone and is an important component of the diet of grey-headed albatross to the north of South Georgia (Rodhouse et al., 1996). The potential for a fishery has also been suggested for South Georgia waters (Rodhouse, 1997; Gonzalez and Rodhouse, 1998). Studies of statoliths have suggested that while most M. hyadesi probably live about 1 year, a significant proportion of the population may live for longer than

1 Although this species is not recorded by name in FIFD fisheries statistics, being classified as ‘other’, Poland reported 8563 t of S. fuegensis from the Southwest Atlantic in 1988 in FAO fisheries statistics. A number of Polish vessels report transhipping sprats in Falkland Island waters in 1988, and these vessels report catching 8510 t of ‘others’ in that year. I have therefore assumed that these 8510 t were S. fuegensis. Although this species may have been caught prior to 1987, there are no FAO statistics relating to it for this earlier period.

1 year (Arkhipkin and Silvanovich, 1997). The species appears to hatch year-round (Arkhipkin and Silvanovich, 1997) and the Falkland shelf appears to be a nursery ground, with both larvae and juvenile squid having been found on the shelf (Rodhouse et al., 1992; Nolan et al., 1998). Another important pelagic squid species around the Falkland Islands is Moroteuthis ingens, being a major food item for hoki and hakes (Jackson et al., 1998). This species has never been taken commercially. There are also a number of myctophid species which spawn in the waters of the Falklands current, to the south of and surrounding the Falkland Islands and Burdwood Bank, and which are important in the diet of some commercial fish species (see Ehrlich et al., 1999; Cousseau and Perrotta, 1998). While there was a brief commercial fishery for the Polar Frontal Zone species Electrona carlsbergi in Antarctic waters in 1990, there has not yet been any fishery in Falkland waters.

MACROZOOPLANKTON

It has been previously noted that there are two areas of high zooplankton density around the Islands, one in the upwelling area of the western Falklands current (the Patagonian current) to the southwest of the Islands (Sabatini et al., 1999) and a stronger one to the northeast of the islands in the eastern Falklands current (Ciechomski and Sanchez, 1983). Tarling et al. (1995) identified specific macrozooplankton assemblages with different water masses in the SW Atlantic, that around the Falkland Islands being characterised by Sagitta gazellae, euphausids Euphausia lucens and E. vallentini and the hyperiid amphipod T. gaudichaudii. Two other species are also common, the euphausid Thysanoessa gregaria and the decapod M. gregaria (lobster krill) (Sabatini et al., 1999). A number of juvenile fish species (Sanchez and Ciechomski, 1995) and cephalopods (especially Semirossa patagonica, Gonatus antarcticus and Octopus sp.: Rodhouse et al., 1992) are also present in the zooplankton. T. gregaria and E. vallentini reach maturation at the end of winter and appear to reach peak density to the southwest of the Falkland Islands at this time (Sabatini et al., 1999). Total zooplankton around the Falkland Islands, however, does not reach a peak until January–February, and at that time is most dense north of the Islands along the shelf break (Ciechomski and Sanchez, 1983). A cruise by RV Atlantida in the summer of 1994 found E. lucens, E. vallentini and T. gaudichaudii to be common and distributed primarily in the inverted ‘V’ north of the islands that follows the confluences of the eastern and western branches of the Falklands current (Emma Jones, unpublished). Calanus tonsus was also very common. Conversely, M. gregaria, which forms dense shoals inshore, was largely absent from these more oceanic waters, being largely restricted to the eddy system within the angle of the V and close to shore. All species were also present over the shelf to the south and east of the Islands, once again with M. gregaria being restricted to inshore waters. M. gregaria also forms dense shoals in New Zealand inshore waters, where it has been considered as a potential target for a fishery (Zeldis, 1989). The distribution of these macrozooplankton in relation to oceanographic and topographic features surrounding the Falkland Islands is clear. The complex current patterns around the islands, together with the rising bathymetry and the extensive shelf area creates stable areas to the north, and to a certain extent to the southwest of the islands, where high salinity, high nutrient loaded water wells up onto the shelf creating areas of high phytoplankton productivity on the shelf break (Sanchez and Ciechomski, 1995). This in turn supports high productivity of macrozooplankton which creates good conditions for larval fish feeding especially to the southwest of the Islands and on the southern Patagonian shelf in late spring and early summer. Highest densities of macrozooplankton are seen in the summer (January–March), especially to the north of the Islands (Ciechomski and Sanchez, 1983), and these are exploited, for example, by the planktivorous I. argentinus and southern blue whiting. Seabirds also feed on macrozooplankton. Thompson (1993) found the diet of breeding Magellanic penguins (Spheniscus magellanicus) to include small fish (principally notothenids, southern blue whiting and

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Falkland herring), M. gregaria and small squid (predominantly Gonatus antarcticus). Diet varied seasonally and across sites. At New Island Gonatus antarcticus and fish (sprats and southern blue whiting) were most important, although M. gregaria formed 15% of the diet during chick rearing (December–February). At Volunteer Point cephalopods (G. antarcticus mostly, but with significant quantities of L. gahi) were most important, followed by M. gregaria and fish. The predominance of inshore species (M. gregaria and young fish) in the diet of penguins during their chick rearing periods when foraging is restricted is perhaps unsurprising.

DISCUSSION

It is clear from the above review that due to its particular current and topographic structure the Patagonian shelf around the Falkland Islands is a highly productive area in both summer and winter. The two most productive areas appear to be to the southwest and to the north of the Islands, over the shelf break (approximately the 200 m depth mark). Zooplankton density is highest in the summer, although there is an area of very high density to the north or northeast of the islands in both winter and summer. The persistent and predictable macrozooplankton densities attract zooplanktivorous fish and squid, in particular resident populations of sprat and L. gahi, and summer and autumn feeding populations of I. argentinus and southern blue whiting. These animals are distributed generally within the two branches of the Falklands current to the south, west and northeast of the islands. Bentho- pelagic fish species (hoki and common hake) also make use of the high abundance of pelagic crustacea around the islands. Three fish species (sprat, southern blue whiting and red cod) make use of the southwestern shelf break as a spawning ground, and the shelf is a nursery area for many species. The western branch of the Falkland current serves to entrain eggs and larvae from spring spawning on this site within the high summer productivity on the shelf around the Falkland Islands and further west on the Patagonian shelf. The oceanographic situation thus creates high productivity areas on the shelf and shelf edge around the Falkland Islands that support an ecosystem based on zooplanktonic crustacea and squid, which in turn supports a large number of fish, mammal and bird predators. The northern, main area of productivity coincides with the northern oil exploration area (Figure 1) and this might be considered to create a potential danger for the marine ecosystem. Any disruption of this area of productivity might have the effect of changing the carrying capacity of the northern shelf ecosystem, which in turn might change the productivity of especially the I. argentinus squid fishery but also affect the winter feeding area of common hake (M. hubbsi). However, it is difficult to conceive of how such disruption might be effected. The Falkland current travels around the islands and proceeds northwards, following the shelf edge. Any pollution events, whether catastrophic from carrier or holding tank ruptures, or chronic (from the drilling process itself) are likely to have an effect further to the north, and probably only a limited effect on the pelagic shelf ecosystem. It is possible that such northern effects might have an impact on the I. argentinus spawning and nursery grounds at the Falkland/Brazil current confluence, but as noted in the introduction this water mass gets deflected out into the Atlantic after travelling north. On the other hand, any oil related activities based on the north coasts of the Falkland Islands do have the potential to release chronic pollution that might impact the area of high productivity to the north. Such pollution may well become entrained in the eddy system to the immediate north of the islands, which although it does not contain euphausids does have significant populations of M. gregaria and young L. gahi which are preyed on by breeding penguins. The physical effects of seismic exploration activity on the behaviour of I. argentinus feeding concentrations were considered at the time of licensing the original tranches in the north. However, any such effects are likely to be short lived, and restricted to the exploration phase rather than to oil production.

The spawning areas to the southwest of the Islands are potentially more vulnerable, but to oil- exploration-based pollution emanating from the Special Co-operation Area rather than the northern oil tranches. Water from the Special Co-operation Area enters the western and eastern currents (Glorioso and Flather, 1995), both of which carry eggs and larvae from the spawning areas in the southwest of the Islands onto the shelf (Figure 1). Disruption to these spawning areas would be likely to have a significant effect on stocks of southern blue whiting and red cod which form fisheries both around the Falkland Islands and in the Argentine EEZ. Obviously, the most vulnerable time to such disturbance would be in the spring, from September to November, both during spawning and larval entrainment onto the shelf. This might arise from catastrophic accidents in the Joint Cooperation Area, or close to the Falkland Islands from oil-related activities, in the winter or early spring, when they would presumably be more likely anyway. Another species that is potentially vulnerable to oil-based pollution arising to the south of the Falkland Islands is L. gahi. As noted above, water from the Special Co-operation Area flows north both west and east of the Islands. The eastern current flows along the south and east coasts of East Falkland Island, directly over the main fishing areas for L. gahi (Figure 2). The species is also known to spawn along the south, west and east coasts of the Falkland Islands, all areas where eggs and larvae would be vulnerable to pollutants emanating in the Special Co-operation Area and flowing close inshore. Unlike the fish spawning to the southwest of the Islands, L. gahi spawn in two peaks, in the early winter and late spring, corresponding to the two cohorts that are fished in the early autumn and early spring respectively. The L. gahi fishery is particularly important to the Falkland Islands, so it is significant that this species and its fishery would be vulnerable to oil-based pollution for almost the whole year, rather than just the spring and early summer. Long-term chronic effects might also be seen if polluted water flows on to the shelf from the south over a long time period. As we have seen, the Falklands shelf forms the nursery area for a number of species of fish and squid. These include not only those mentioned above such as red cod, southern blue whiting and L. gahi but also species not subject to major fisheries such as M. hyadesi and S. fuegensis. Both the euphausids that form the diet of these juveniles, and the fish and squid themselves support large numbers of breeding birds on the Falkland Islands. Disruption of the pelagic ecosystem over the Falklands shelf through either chronic or catastrophic events arising from oil exploration in the Special Co-operation Area (or from oil- related activities based on the Falkland Islands themselves) could have potentially serious effects on both exploited and non-exploited species comprising the marine ecosystem around the Falkland Islands.

ACKNOWLEDGEMENTS

I am extremely grateful to Greta Skene who compiled the sprat ﬁsheries data for me, and to Emma Jones who allowed me to use her unpublished data on zooplankton distributions. My gratitude is also extended to all observers and scientists who have worked on Falkland Island ﬁsheries data both at the Fisheries Department and at Imperial College since 1987. John Barton (Director of Fisheries), Alexander Arkhipkin, David Middleton and Joost Pompert provided helpful comments on the manuscript and access to various Falkland Island Fisheries Department data. I am grateful to Simeon Hill, Tom Marlow and Lynne Purchase for comments on the paper and Jeremy Smith for providing access to some references.

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