Chia, F.S. 1969. Histology of the pyloric caeca and its changes during tional Journal of Invertebrate Reproduction and Development, brooding and starvation in a starfish, Leptasterias hexactis. Biology 7(4), 249-257. Bulletin, 136(2), 185-192. McClintock, LB., and J.S. Pearse. 1987. Biochemical composition of Dearborn, J.H., and F.J. Fell. 1974. Ecology of echinoderms from the antarctic echinoderms. Comparative Biochemistry and Physiology, Antarctic Peninsula. Antarctic Journal of the U.S. 9(5), 304-306. 86B(4), 683-687. Emlet, R.B., L.R. McEdward, and R.R. Strathmann. 1987. Echinoderm Pearse, J.S., J.B. McClintock, and I. Bosch. 1991. Reproduction of larval ecology viewed from the egg. Echinoderm Studies, 2,55-136. Antarctic benthic marine invertebrates: Tempos, modes, and tim- Hendler, G., and D.R. Franz. 1982. The biology of the brooding seast- ing. American Zoology, 31(l), 65-80. ar, Leptasterias tenera, in Block Island Sound. Biology Bulletin, Slattery, M., and I. Bosch. In press. Spawning behavior of a brooding 162(3),273-289. antarctic asteroid, Neosmilaster georgianus. International Journal Lawrence, J.M., and A. Guile. 1982. Organic composition of tropical, of Invertebrate Reproduction and Development. polar, and temperate-water echinoderms. Comparative Biochem- Turner, R.L., and J.H. Dearborn. 1979. Organic and inorganic compo- istry and Physiology, 72B(2), 283-287. sition of post-metamorphic growth stages of Ophionotus hexactis Lawrence, J.M., J.B. McClintock, and A. Guffle. 1984. Organic level and (E.A. Smith) (Echinodermata: Ophiuroidea) during intraovarian caloric content of eggs of brooding asteroids and an echinoid incubation. Journal of Experimental Marine Biology and Ecology, (Echinodermata) from Kerguelen (South Indian Ocean). Interna- 36(l),41-51. Epipelagic communities in the northwestern Weddell Sea: Results from acoustic, trawl, and trapping surveys R.S. KAUFMANN, K.L. SMITH, JR., R.J. BALDWIN, and R.C. GLArrs, Marine Biology Research Division, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0202 B.H. ROBISON and K.R. REISENBICHLER, Monterey BayAquarium Research Institute, Pacific Grove, California 93950 ntil recently, little was known about the community instruments to monitor the abundance, vertical distribution, Uinhabiting the underside of seasonal sea ice in the and size distribution of animals in the upper 100 m of the Antarctic. Increasing understanding of this unique environ- water column beneath seasonal pack ice, two upward-look- ment has given rise to a number of questions about the ing, split-beam acoustic arrays were deployed at each of two under-ice community and its interactions with other portions locations (four deployments total) in the northwestern Wed- of the antarctic fauna, especially those of the underlying dell Sea during early October 1992: in ice-covered water at water column and the above-ice community, including 61 032.54S 41 054.28W and 61°30.62S 41°39.44W and in open seabirds and marine mammals. In particular, the discovery of water at 600 14.96S 49047.74W and 60 0 13.13S 49050.62W (fig- mesopelagic species in the guts of surface-feeding seabirds ure 1). These instruments were moored on the bottom and foraging in areas of open water (for example, leads and positioned approximately 100 m beneath the sea surface in polynyas) amidst heavy pack ice (Ainley et al. 1986; Ainley, areas with bottom depths of 1,050-1,100 m. Each instrument Fraser, and Daly 1988) has presented the startling possibility operated at a frequency of 72 kilohertz and was programmed of trophic coupling between two apparently disjunct commu- to ensonify an 8,600-cubic-meter conical section of the water nities. Although many of the mesopelagic species found in column [100 m vertically upward from the transducer with bird guts are known to migrate vertically on a daily basis (Tor- 20-centimeter (cm) vertical resolution and a beam angle of 501 res et al. 1985; Torres and Somero 1988), most have not been every 5 seconds for 1 minute, with intervals of 6 minutes caught at depths of less than 100 meters (m) in open water between 12-ping groups over a deployment duration of 2 (Lancraft, Torres, and Hopkins 1989); whereas the seabird days. In the ice-covered area, two lines of baited minnow species sampled are not known to forage deeper than 5 m traps (for the collection of scavenging animals) were deployed (Ainley et al. 1986). concurrently at depths of 0, 10, 50, 100, and 200 m through There are numerous difficulties in examining the ecology holes drilled in the ice. A similar trap array was deployed from of under-ice fauna, including the logistic intractability of the a floating buoy at the open-water site. In both the ice-covered environment and the distribution and behavior of the animals and open-water areas, the upper 250 m of the water column themselves. The animals, most commonly krill (Euphausia was also sampled during the day and at night using an open- superba), tend to be found in close association with the ing-closing Tucker trawl (two nets with a 10-square-meter under-ice surface and are known to actively avoid sampling mouth opening). The Tucker trawl was successfully employed gear (OBrien 1987; Marschall 1988). As part of a short-term 11 times in the pack ice and three times in open water. feasibility study to evaluate the possibility of using free-vehi- Acoustic targets were more abundant at the open-water cle (independent of a ship or any surface mooring) acoustic location than at the ice-covered site, 10.0 and 7.6 targets per ANTARCTIC JOURNAL - REVIEW 1993 138 500 Figure 1. Study site in the northwestern Weddell Sea (bounded by solid lines). Areas examined in previ- ous studies (AMERIEZ and EPOS) are enclosed by dashed lines. Locations of vertically profiling acoustic array moorings are labeled (stations 115S and 120S beneath pack ice and stations 140N and 141N in open water). hour of sampling, respectively. A portion of the open-water acoustic record spanning approximately 1.75 h contained an unusually large surface target, most probably an iceberg. Acoustic targets were nearly twice as abundant beneath this feature as in the surrounding water column (21.4 vs. 10.0 tar- gets per hour of sampling). The abundance of acoustic targets exhibited temporal variability at both sites, with more targets identified at night than during the day (figure 2). In addition, a greater percentage of targets was recorded in the upper 50 m of the ensonified field in the ice-covered area than at the open-water site. Target strengths for individual targets were used to gen- I erate estimates of animal body length, using the relationships TS = 10 log(-JL) I, and Ice Cover (Q-1 =0.02 1(ETL en Water 2) kAJ 0 400 800 1200 16W 2000 244.10 Daily Time where TS is the target strength in decibels (dB), o is the Figure 2. Hourly total numbers of targets (animals) detected acousti- cally with two upward-looking, vertically profiling acoustic arrays acoustic cross-section, X is the wavelength of the acoustic sig- during 2-day deployments in ice-covered and open-water areas of nal (in this case 2.00 cm), and ETL is the estimated target the northwestern Weddell Sea. length in meters (Love 1977). ANTARCTIC JOURNAL - REVIEW 1993 139 Targets detected in the open-water area were significant- The greater abundances of larger organisms, particularly fish- ly larger than those under the ice (mean estimated length 13.3 es, in the night trawls correlate well with the observation of vs. 9.2 cm; Mann-Whitney U-test, Pcz0.001), though the largest nightly peaks in acoustic target abundance (see above). Size target detected (target strength = -40.1 dB; estimated length = distributions for net-collected zooplankton were comparable 28.7 cm) was observed beneath the ice (figure 3A). Targets to those measured acoustically; the largest captured fish mea- beneath the "iceberg" were not significantly larger than those sured 25.4 cm (Notolepis coatsi) compared to 28.7 cm for the in the ice-covered area (mean estimated length 11.6 vs. 9.2 largest acoustic target recorded. cm; Mann-Whitney U-test, 0.05<P<0.10) or in open water. All baited traps from the open-water station were empty The greater size of acoustic targets observed in open water, upon recovery, but each of the surface traps from the baited relative to those recorded under the ice, is consistent with trap arrays within the pack ice contained hundreds of rela- reports of smaller prey being taken by seabirds foraging in or tively large (up to 3 cm in length) scavenging amphipods near pack ice, compared to those feeding in open water (Mn- Orchamene rossi. This observation suggests that these leyetal. 1988). amphipods may be present in large numbers just beneath the A temporal pattern in target size was apparent at the ice- ice surface, as has been noted for krill (see, for example, covered site; targets larger than 10 cm were observed only at OBrien 1987; Marschall 1988; Stretch et al. 1988). We believe night (figure 3B). In contrast, no such temporal variation in this to be the first record of scavenging amphipods occupying target size was recorded at the open-water station (figure 3B). this microhabitat. Temporal patterns in species distributions were also evident The epipelagic community in this area of the northwest- in the Tucker trawl samples. Fishes, hill (Euphausia superba), ern Weddell Sea appears to be affected by the presence of sea- and salps were substantially more abundant in night trawls sonal pack ice. These effects are reflected in the presence of than in those taken during the day.