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

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Distribution, Spawning and Migration of Pelagic Animals in Relation to Oil Exploration 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). Copyright # 2002 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 12: 39–50 (2002) CRITICAL ASPECTS OF PELAGIC ECOSYSTEM 41 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.mÀ3, high (black) >1000 mm3.mÀ3) 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
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