Acoustic Characterization of Gelatinous-Plankton Aggregations: Four Case Studies from the Argentine Continental Shelf

ICES Journal of Marine Science, 60: 650–657. 2003 doi:10.1016/S1054–3139(03)00051-1 Acoustic characterization of gelatinous-plankton aggregations: four case studies from the Argentine continental shelf

Gustavo Alvarez Colombo, Hermes Mianzan, and Adrian Madirolas

Alvarez Colombo, G., Mianzan, H., and Madirolas, A. 2003. Acoustic characterization Downloaded from https://academic.oup.com/icesjms/article/60/3/650/660506 by guest on 29 September 2021 of gelatinous-plankton aggregations: four case studies from the Argentine continental shelf. – ICES Journal of Marine Science, 60: 650–657.

During routine acoustic surveys for the assessment of fish abundance in the Argentine Sea, large-scale, plankton-like scattering layers covering thousands of square nautical miles are commonly observed. Net sampling revealed that many of these scattering layers comprised gelatinous zooplankters aggregated in dense concentrations in the main. Even though echoes from gelatinous zooplankton are expected to be weak, because of the low reflectivity of their bodies, dense aggregations are capable of producing sound-scattering levels high enough to mask even the overlapping echoes from swimbladdered fish. The objective of this study is to relate the aggregations of four gelatinous species identified by means of nets to the presence of sound-scattering layers. Selected sections of echo recordings from aggregations of Lychnorhiza lucerna (Scyphozoa), Iasis zonaria (Salpidae), Mnemiopsis leidyi (Ctenophora), and Aequorea sp. (Hydrozoa) were obtained at 38 or 120 kHz, at different locations along the Argentine shelf. Some features of the spatial distributions of the aggregations are described. The feasibility of remote detection for different gelatinous groups is of great importance considering the impact that blooms of these organisms could have on some particularly sensitive ecosystems (e.g. fish spawning and nursery grounds). The characterization of specific aggregation and behavioural patterns will allow the mapping of jellyfish distributions by the analysis of previous cruise databases. This methodology will provide a baseline for the study of spatial and temporal changes and trends in their abundance. Ó 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved. Keywords: acoustic characterization, gelatinous-zooplankton aggregations, Rıó de la Plata estuary, southwestern Atlantic Ocean, spatial-distribution patterns. G. Alvarez Colombo, and A. Madirolas: National Institute of Fisheries Research and Development (INIDEP) CC 175, (7600) Mar del Plata, Argentina. H. Mianzan: National Council for Scientific and Technical Research (CONICETINIDEP) CC 175, (7600) Mar del Plata, Argentina. Correspondence to G. A. Colombo; tel: þ54 223 486 2586; fax: þ54 223 486 1830; e-mail: [email protected].

Introduction tent of their tissues with a very low-density contrast at the water–body interface. More recently, various authors have Blooms and aggregations seem to be the rule in the life suggested that different species of gelatinous plankton (the history of many gelatinous plankters (Nival and Gorsky, salp Iasis zonaria, the hydromedusae Aequorea victoria, 2001). Periodic blooms of gelatinous zooplankton, leading and the scyphomedusae Aurelia aurita and Chrysaora to high biomass concentrations occupying enormous areas hysoscella) are capable of producing significant levels of of the oceans, have been observed. These often have a large sound scattering even at low sound frequencies (38–50 kHz) impact on human activities such as fisheries and other (Toyokawa et al., 1997; Brierley et al., 2001; Mianzan et al., industries that, in turn, affect the ecosystem structure (see 2001a). Laboratory and in situ analyses of target strength Mills, 1995, 2001; Mianzan and Cornelius, 1999). (TS) have been made for several jellyfish species, some Regardless of the ecological importance of such aggre- of them showing TS values higher than 50 dB (Mutlu, gations, accurate estimates of their abundance and distribu- 1996; Brierley et al., 2001). Modelling of their TS has tion are scarce because of the methodological constraints contributed to discrimination between gas-bearing, siphon- characteristic of net sampling. Several alternative methods ophora species, and non-gas-bearing (fluid-like) species have been suggested, including acoustic surveys. Tradition- (Stanton et al., 1996b). Model formulation for umbrella- ally, these organisms were disregarded as conspicuous shaped jellyfish has recently been developed (Monger sources of sound scattering because of the high water con- et al., 1998). However, data are not completely reliable

1054–3139/03/000650þ08 $30.00 Ó 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved. Acoustic characterization of gelatinous-plankton aggregations 651 concerning the contribution of jellyfish to the observed high-speed sampler (Nellen and Hempel, 1969) (0.03-m2 echograms during field studies, because of the presence of mouth opening, 300-lm mesh net), and a 60-cm diameter other non-gelatinous organisms that may contribute to the bongo net (0.28-m2 mouth opening) fitted with 300-lm sound scattering. mesh net (Smith and Richardson, 1977). Filtered volumes During routine acoustic cruises for the assessment of fish by the larger nets were calculated as the product of the abundance in the Argentine Sea, large-scale, plankton-like travelled distance and the net-mouth area, considering scattering layers occupying thousands of square nautical a filtering efficiency of 100%. Digital flowmeters (Hydro- miles are commonly observed with echosounders. Net Bios and General Oceanics) were employed to calculate the sampling revealed that many of these concentrations are filtered volumes in the smaller nets. Trawling depth during composed mainly of gelatinous zooplankters of large in- all the net operations was monitored in real time by means of Downloaded from https://academic.oup.com/icesjms/article/60/3/650/660506 by guest on 29 September 2021 dividual size, which aggregate in dense concentrations. As Scanmar pressure sensors. an example, dense aggregations of the salp I. zonaria are After the net operations, the gelatinous organisms were capable of producing scattering layers at 38 kHz, at levels immediately sorted from the samples and analysed onboard. high enough to obscure the echo recordings of anchovy Taxonomical identification, number, total weight and schools (Engraulis anchoita) in the Rıó de la Plata surface- individual length, weight and volume, or just one of these salinity front (Mianzan et al., 2001a). last two parameters, were obtained. The non-gelatinous The objective of this study was to correlate the ag- organisms were fixed in a 5% formalin solution in seawater gregations of four gelatinous species identified by means for subsequent laboratory analysis. Identification of the of different plankton-net samples with the presence of gelatinous taxa was based on the studies of Mayer (1912), sound-scattering layers (SSLs). The criteria employed in the Kramp (1961), Esnal and Daponte (1999), Mianzan (1999), selection of the echograms were: first, a clear dominance of and Mianzan and Cornelius (1999). a particular gelatinous species, and second, the coverage Acoustic data were collected using a calibrated SIMRAD with plankton nets of most of the local planktonic-size EK500 with a 38 kHz split-beam transducer, except for range. As a result of this selection, echo recordings from the data relating to L. lucerna, where a SIMRAD EY500 aggregations of Lychnorhiza lucerna (Scyphozoa), I. echosounder and a 120 kHz split-beam transducer were zonaria (Salpidae), Mnemiopsis leidyi (Ctenophora), and employed. Echograms were recorded with a HP9000 Series Aequorea sp. (Hydrozoa) are presented. Features of the graphical station, or with a laptop computer (EY500), and spatial distributions of the aggregations are also described post-processed with the SIMRAD BI500 and SonarData and discussed. Echoview software. A threshold of 80 dB (Sv) was applied in most cases (Figure 2A, B, and D), while for M. leidyi the applied threshold was 70 dB (Figure 2C). Materials and methods From the cruise data, segments of the acoustic transects (Figure 1) were selected to illustrate the patterns observed. Acoustic and net-sampling data were obtained onboard three The four selected echogram sections (Figure 2) correspond research vessels (‘‘CapitańCańepa’’, ‘‘Doctor Eduardo to locations where net sampling revealed a clear dominance Holmberg’’, and ‘‘Capitań Oca Balda’’) of the National of a particular jellyfish species in the total-species com- Institute of Fisheries Research and Development (INIDEP), position. The presence of other species, such as small cope- over a large part of the northern and central Argentine pods and other crustaceans, at low densities in the net continental shelf (35–47S) (Figure 1). Data were collected samples was disregarded as a potential source of sound along acoustic transects during four routine fish-abundance scattering at the working frequencies. surveys, where jellyfish were particularly abundant in most Particular information on each cruise and the equipment of the surveyed area. In order to obtain an unbiased sampling employed are presented within the results for each of the of the whole range of the planktonic organisms present at gelatinous species considered here. each location, different net samplers were employed for ground-truthing of the observed echo traces. A modified Isaacs–Kidd midwater trawl (IKMT) (10-m2 mouth open- Results ing, mesh size reducing from 50 mm to 1500 lm) and a small bottom trawl (piloto) (2.4-m2 mouth opening, 6-m total Lychnorhiza lucerna (Scyphozoa: Lychnorhizidae) length, 6-m headrope and groundrope, 25-mm wing-mesh Acoustic data and net samples were obtained along a size, 10 mm in the codend, 10-m bridles, and 80-cm vertical transect (35109S, 56109W) during a survey focused on the opening) were employed to sample the larger organisms, study of the Rıó de la Plata salt-wedge regime during including most of the jellyfish species. The smaller zoo- February 2000 (Figure 1A). Net sampling consisted of plankton fraction was sampled by means of three differ- hauls with the small bottom trawl and the Multinet sampler. ent nets: a Hydro-Bios Multi-Plankton Sampler Type B Echo recordings were obtained at 120 kHz. The scatterers (Multinet) (0.25-m2 mouth opening, 300 lm mesh size), a aggregated close to the bottom in the main, where medusae Nackthai high-speed sampler, a modification of the Gulf V dominated the net catches (Figure 2A). 652 G. Alvarez Colombo et al. Downloaded from https://academic.oup.com/icesjms/article/60/3/650/660506 by guest on 29 September 2021

Figure 1. Map of the Argentine Sea showing the study areas of the four gelatinous species analysed. Acoustic tracks are shown by solid lines while the black rectangles correspond to the location of the echograms selected in Figure 2, where A ¼ Lychnorhiza lucerna; B ¼ Iasis zonaria;C¼ Mnemiopsis leidyi;D¼ Aequorea sp.

The section of the echogram selected (Figure 1A) shows the 50 and 200 m isobaths off the Rıó de la Plata estuary the bottom aggregation of L. lucerna (Figure 2A). A catch of (35–38S) in June 1999 (Figure 1B). Plankton sampling 137 kg of L. lucerna (3–31 cm bell diameter, mode ¼ 11 cm) included the use of the IKMT and Nackthai nets towed was obtained, representing a density of 14 individuals per obliquely to sample the whole water column. 100 m3. Some juveniles of demersal fish species (mainly During this survey, high quantities of the salp I. zonaria Paralonchurus brasiliensis and Micropogonias furnieri) were collected with both samplers. Up to 46 kg of salps ranging from 2 to 6 cm (density ¼ 5 per 100 m3) and two were collected in one single IKMT tow (Figure 2B). No Chrysaora lactea were also found in the sample (Table 1). SSLs were found at the stations where no salps or only a few individuals were captured. High density aggregations of this salp were observed with Iasis zonaria (Thaliacea: Salpidae) the echosounder over the whole study area. The vertical Sampling was carried out during a cruise directed at the distribution of salps, as observed in the echo recordings, assessment of common hake pre-recruits (Merluccius varied along the ship’s track occupying most of the water hubbsi) at its northern spawning ground, located between column and occasionally forming patches of higher Acoustic characterization of gelatinous-plankton aggregations 653 Downloaded from https://academic.oup.com/icesjms/article/60/3/650/660506 by guest on 29 September 2021

Figure 2. Echogram sections selected from the corresponding cruise data and images obtained of the four jellyﬁsh species catches: A ¼ Lychnorhiza lucerna;B¼ Iasis zonaria;C¼ Mnemiopsis leidyi;D¼ Aequorea sp. showing a characteristic migration cycle. Depth scales (m) are on the left of the echograms and Sv colour scales (dB) on the right. densities at variable depths reaching Sv values, averaged obtained with the IKMT and Nackthai nets showed that the by nautical mile, of 62.5 dB. salps represented up to 99% of total-catch weights. A total The selected section of the acoustic track shows a salp of 10 kg of salps (maximum length ¼ 7 cm), equivalent to distribution as described earlier (Figure 2B). Net samples approximately 3300 individuals with an average length of 654 G. Alvarez Colombo et al.

Table 1. The density of jellyﬁsh and bycatch species obtained with the diﬀerent samplers employed at the selected locations.

No. of stations Jellyﬁsh density Bycatch density Corresponding analysed (stations at selected Main bycatch at selected ﬁgure-associated at selected echograms echogram species at echogram species Net employed in brackets) (ind.per 100 m3) selected echogram (ind. per 100 m3)

Small bottom 4 [1] 14 Small ﬁsh (2–6 cm) 5 Figure 1A trawl (piloto) Lychnorhiza lucerna Hydro-Bios 2 [2] 1 Copepods, chaetognaths Scarce Multinet (1–2 cm), amphipods (<0.5 cm) Downloaded from https://academic.oup.com/icesjms/article/60/3/650/660506 by guest on 29 September 2021 IKMT 20 [1] 14 Fish-squid (3–10 cm) <1 Figure 1B Iasis zonaria Nackthai 20 [1] 300 Copepods, amphipods, mysiids Scarce (<1 cm)

Figure 1C Nackthai 1 673 Anchovy larvae (1 cm) 145 Mnemiopsis leidyi

Figure 1D IKMT 20 [2] 0.4 Amphipods and euphausiids <1 Aequorea sp. ﬁsh (5–6 cm)

4 cm, were captured with the IKMT along with eight fishes umbrella diameter), representing a numeric density of 2 (3–8 cm) and three squids (2–3 cm). A density of 300 salps Aequorea per 100 m3. Bongo catches showed the presence (length 2–7 cm) per 100 m3 was obtained from the Nackthai of crustaceans such as copepods, euphausiids, and amphi- sample. Sparse organisms of a smaller fraction (1 cm) pods, but no relation was observed between their presence consisting mainly of small salps and crustaceans (copepods, and the observed SSLs. No medusae or only a few amphipods, and mysiids) were also found in the Nackthai individuals were captured in the stations where no SSLs sample (Table 1). were observed. During daylight hours this species aggre- gates near the bottom, forming dense SSLs (average Sv Mnemiopsis leidyi (Tentaculata: Bolinopsidae) values of up to 62.1 dB) usually masking the presence of adult bottom fish. Large catches of this species are frequent Data related to this species were obtained in December in the area during hake bottom-trawl surveys. 1998 during a survey of the El Rincoń area focused on the Diel vertical-migration cycles were observed for this assessment of coastal-fish abundance. species, during which the organisms ascended at dusk to A dense scattering layer (maximum S averaged by v cover most of the water column and returned to their deeper nautical mile ¼62 dB) was observed at night along distribution after dawn. The selected echograms presented a transect parallel to the shoreline off Buenos Aires Prov- in Figure 2D were condensed to show the vertical-migration ince (39S, 60W) (Figure 1c). Echograms showed the cycle of the SSL, as described earlier. Only Aequorea sp. scatterers to be restricted to the lower half of the water specimens and two juvenile hake (5.5–6 cm) were obtained column (Figure 2C). The catch of the Nackthai sampler in the IKMT samples at that location (Table 1). towed at the depth of the SSL consisted of 98 M. leidyi with The average TS for Aequorea sp. was estimated from the a length range of 1–8 cm, representing a density of 673 in situ echo recordings obtained during the night from the ctenophores per 100 m3, along with 20 anchovy larvae of most dispersed organisms in the aggregation described 1–2 cm total length (Table 1). earlier. An average of 64.53 dB was determined for the upper 50-m layer ðn ¼ 4374Þ. In order to validate that Aequorea sp. (Hydromedusae: Aequoreidae) estimate, 50 individual target traces were isolated from the Acoustic and net samples were obtained during a cruise echograms and their individual TS distributions analysed, directed at the assessment of common hake pre-recruits in obtaining the similar TS average of 64.15 dB. their southern spawning area off Isla Escondida and Golfo San Jorge (44–47S) in June 2000. Acoustic and net samplings during the cruise were conducted between the 50 Discussion and 100-m depth contours (Figure 1D). Conspicuous SSLs were associated with a high incidence The term ‘‘gelatinous zooplankton’’ is used for a diverse of Aequorea sp. in the IKMT samples. Net sampling was group of marine animals having a body principally com- carried out after dusk, when the scatterers ascended posed of ‘‘jelly’’ tissue with a similar density to that of the dispersing through the water column. Catches of this surrounding medium. Although little sound reflectivity is species obtained with the IKMT reached 76 kg (8–20 cm therefore expected, according to our results jellyfish in fact Acoustic characterization of gelatinous-plankton aggregations 655 contribute significantly to the sound scattering even at fairly 46.6 to 68.5 dB. These results are consistent with the low frequencies. Several factors may account for this strong sound scattering observed in our study. Brierley et al. observation. The large size of individuals reached by several (2001), in particular, found an average TS of 66.3 dB for species, the hardness of their gelatinous tissue and the fact a mean umbrella diameter of 7.4 cm for A. aequorea, which that they often occur in enormous population densities closely agrees with the average of approximately 64 dB might combine to produce high levels of sound scattering at found in the TS distribution of our in situ observations of several locations of the Argentine shelf waters as observed the larger Aequorea sp. (umbrella diameters ranging over in the selected echograms. 8–20 cm). According to the classification given by Stanton et al. Few data are available regarding the scattering from salps

(1996a), jellyfish are in the category of ‘‘fluid-like scat- and ctenophores. The TS of the ctenophores Bolinopsis sp. Downloaded from https://academic.oup.com/icesjms/article/60/3/650/660506 by guest on 29 September 2021 terers’’. Given the absence of an elastic shell, the acoustic- (4.5 cm length) (Wiebe et al., 1990) and Pleurobrachia scattering strength of these ‘‘weak scatterers’’ would be bachei (maximum diameter ¼ 1 cm) (Monger et al., 1998) mainly a function of their size, shape, and material properties were 80 dB (420 kHz) and 75.9 dB (200–1000 kHz), (Stanton et al., 1996b). respectively. The TS of Salpa aspera (2.6-cm length) was The TS of several gelatinous species has been obtained approximately 90 dB at 120–200 kHz (Stanton et al., by various authors employing different techniques (Table 1994). David et al. (2001) estimated the TS of salps by 2). The size and shape of jellyfish have been shown by applying mathematical models to calculate the contribution Monger et al. (1998) to affect their acoustic response of the gelatinous bodies and their nucleus separately. They strongly. The simple act of swimming may produce vari- obtained a TS range of 105.8 to 63.2 dB at 38 kHz for ations of 10 dB in TS values as a result of the rhythmic a body-length range of 1.4–10 cm. We believe that the contraction of the umbrella (Mutlu, 1996; Monger et al., strong sound-scattering levels registered from our salp (I. 1998). High TS values have recently been verified for large zonaria) were in part related to its large size (2–7 cm) and medusae species at quite low frequencies (18–120 kHz). also to the very rigid nature of the tunic of this species Laboratory measurements of TS of A. aurita (9.5–15.5 cm compared with Salpa species (Esnal and Daponte, 1999; diameter) (Mutlu, 1996) and field estimates of C. hysoscella Mianzan et al., 2001a). In the case of the ctenophore M. (26.8-cm diameter) and Aequorea aequorea (7.4-cm di- leidyi (maximum length ¼ 8 cm in our samples), net samples ameter) (Brierley et al., 2001) have shown a TS range of probably underestimate the larger specimens because of the

Table 2. The TS of several gelatinous species from in situ and laboratory measurements and theoretical calculations.

Species Size (cm) Frequency (kHz) TS (dB) Source

Scyphozoa Chrysaora hysoscella 26.8 18 51.5 Brierley et al. (2001) 38 46.6 120 50.1 Aurelia aurita 9.5 120 60.24 Mutlu (1996) 9.5 200 64.27 11.5 200 62.48 15.5 120 57.10 15.5 200 56.47 Aurelia autrans 838 54 Nakken, in Mutlu (1996) 838 51.7 16 120 54.2 16 120 50.1 Hydromedusae Aequorea aequorea 7.4 18 68.1 Brierley et al. (2001) 38 66.3 120 68.5 Aequorea victoria 2.1–5.8 200–1000 72.6 (average) Monger et al. (1998) Thaliacea Salpa aspera 2.6 120–200 approximately 90 Stanton et al. (1994) Salps (body length) 1.4–10 38 105.8 to 63.2 David et al. (2001) Salps (nucleus radius) 0.1–0.75 38 110.9 to 63.8 David et al. (2001) Ctenophora Bolinopsis sp. 4.5 420 80 Wiebe et al. (1990) Pleurobrachia bachei 0.6–1 200–1000 75.9 Monger et al. (1998) 656 G. Alvarez Colombo et al. small mouth opening of the sampler employed on that aggregations formed at this edge or just outside it (Mianzan occasion. Specimens as large as 15 cm are commonly caught et al., 2001a). with larger nets in the area. Acoustic methods allowed us to establish some behav- Other factors have been addressed to explain the origin ioural patterns of Aequorea sp. This species performed of sound scattering in gelatinous species, and should be a marked diel vertical migration during which the organ- considered as contributing to our results. Demer and Hewitt isms ascended at dusk, spreading through the water column (1994) assumed that most of the scattering produced by and returning to the near-bottom layer at dawn. No evidence Salpa thompsoni results from their spherical stomach, has been registered yet for migration cycles of M. leidyi. where diatoms, copepods, and other organisms are actively Their aggregations were found mainly in the deeper half of concentrated. Monger et al. (1998) suggested that copepod the water column, even though visual observations of their Downloaded from https://academic.oup.com/icesjms/article/60/3/650/660506 by guest on 29 September 2021 carcasses in the guts of the ctenophore P. bachei could have presence near the surface, out of the detection range of accounted for the TS values measured in their study. David the sounder, were common in the area. Consistent with this et al. (2001) calculated similar contributions to sound observation, Costello and Mianzan (2003) reported that scattering from the salps’ nucleus and from their gelatinous M. leidyi appeared to be concentrated either near the bottom body. Also, many jellyfish species are known to be as- or at the surface, with relatively few individuals inter- sociated with small fishes that swim close to their bodies mediate between the two depths. (Mansueti, 1963; Brodeur, 1998). Direct observations made Assessments of the distribution of these organisms in the during the surveys indicated that small fish were often seen vast areas they inhabit are possible through the analysis of in close contact and swimming around the oral arms of data from routine surveys of fish abundance involving the L. lucerna. use of 38 and 120 kHz transducers. Our results indicate that Associated fish could introduce a source of error, causing certain sizes and densities of jellyfish can make a significant the volume-scattering strength of jellyfish aggregations to contribution to the sound-scattering field, even at acoustic be overestimated, particularly in near-bottom aggregations. frequencies as low as indicated earlier. The analysis of the However, in the case of L. lucerna, where several fish horizontal extent of the sound scattering from jellyfish will specimens were caught, the shape and distribution patterns provide an index of their abundance, allowing for the study of the observed echo traces could not be attributed to of the regional and temporal variations of these species. juveniles of demersal-fish species (Figure 2A). In the case The possible increase of gelatinous populations has been of the anchovy larvae found together with M. leidyi, postulated recently (Mills, 1995, 2001), with several au- further histological analysis showed that at the measured thors ascribing different causes to these population enhance- size range (1–2 cm) the larvae had not yet developed a gas ments (Brodeur et al., 1999; Arai, 2001; Graham, 2001). To bladder, and they were therefore negligible sources of prove this assertion, historical data of gelatinous organism sound scattering. distribution and abundance will be needed. Unfortunately, The high numerical density of jellyfish present in most of such data are not widely available because gelatinous- the aggregations may also explain the rather high levels of plankton organisms were not included in routine monitoring sound scattering observed at these locations. Extensive studies until recently. Jellyfish-abundance estimation was aggregations of some of the studied species have been neglected due to a general assumption of their low eco- reported as being associated with hydrographic features of logical significance (Arai, 1988). The acoustic character- the Argentine Sea (Mianzan and Guerrero, 2000; Mianzan ization of jellyfish aggregations could become a valuable et al., 2001a) and the pattern is common to many southern tool in the identification of specific distributions from jellyfish populations. earlier databases, allowing the reconstruction of past Particular features of the distributions of the aggregations scenarios of their abundance and their significance and of the four species studied have been observed. At the Rıó possible impact on particular ecosystems. de la Plata surface-salinity front, L. lucerna was found aggregated close to the bottom. In the same area, the salp Acknowledgements I. zonaria was found occupying most parts of the water column, with occasional patches of higher density at The National Institute of Fisheries Research and Develop- different depths. The Rıó de la Plata estuary is characterized ment (INIDEP) supported the research-vessel operations by a marked salt-wedge, with a variable layer of denser, and most laboratory facilities. We thank in particular Dr saltier water close to the bottom (Mianzan et al., 2001b) Martıń Ehrlich and his laboratory personnel for their support and it is possible that L. lucerna would penetrate the estuary of this research during the hake pre-recruits cruises. We are following these layers of high salinity. Other species of indebted to Marcela Tobio for her aid with some of the gelatinous zooplankton have previously been reported as photographs that illustrate this work. The study was partially contributing significantly to the sound scattering associated supported by UNMdP 15/E139, Fundacioń Antorchas no. 13 with the estuary halocline (Madirolas et al., 1997). At 817-5 and Agencia Nacional de Promocioń Cientı´fica y the same time, the Rıó de la Plata surface-salinity frontal Tecnolo´gica no. 8424 grants to HWM. Travel and living area represents a border system, there being some salp expenses that allowed participation in the Sixth ICES Acoustic characterization of gelatinous-plankton aggregations 657

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