© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Acoustic characterization of gelatinous plankton aggregations: four case studies from the Argentine continental shelf
a National Institute of Fisheries Research and Development (INIDEP) CC 175 Mar del Plata 7600, Argentina
b National Council for Scientific and Technical Research (CONICET-INIDEP) CC 175 (7600) Mar del Plata, Argentina
*Correspondence to G. A. Colombo; tel: +54 223 486 2586; fax: +54 223 486 1830. e-mail: acolombo{at}inidep.edu.ar; hermes{at}inidep.edu.ar; adrian{at}inidep.edu.ar.
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 ofjellyfish 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.
Keywords: acoustic characterization, gelatinous-zooplankton aggregations, Río de la Plata estuary, southwestern Atlantic Ocean, spatial-distribution patterns
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Introduction |
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 Top Introduction Materials and methodsResultsDiscussionReferences |
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Blooms and aggregations seem to be the rule in the life history of many gelatinous plankters (Nival and Gorsky, 2001). Periodic blooms of gelatinous zooplankton, leading to high biomass concentrations occupying enormous areas of the oceans, have been observed. These often have a large impact on human activities such as fisheries and other industries that, in turn, affect the ecosystem structure (see Mills, 1995, 2001; Mianzan and Cornelius, 1999).
Regardless of the ecological importance of such aggregations, accurate estimates of their abundance and distribution are scarce because of the methodological constraints characteristic of net sampling. Several alternative methods have been suggested, including acoustic surveys. Traditionally, these organisms were disregarded as conspicuous sources of sound scattering because of the high water content of their tissues with a very low-density contrast at the water–body interface. More recently, various authors have suggested that different species of gelatinous plankton (the salp Iasis zonaria, the hydromedusae Aequorea victoria, and the scyphomedusae Aurelia aurita and Chrysaora hysoscella) are capable of producing significant levels of sound scattering even at low sound frequencies (38–50 kHz) (Toyokawa et al., 1997; Brierley et al., 2001; Mianzan et al., 2001a). Laboratory and in situ analyses of target strength (TS) have been made for several jellyfish species, some of them showing TS values higher than –50 dB (Mutlu, 1996; Brierley et al., 2001). Modelling of their TS has contributed to discrimination between gas-bearing, siphonophora species, and non-gas-bearing (fluid-like) species (Stanton et al., 1996b). Model formulation for umbrella-shaped jellyfish has recently been developed (Monger et al., 1998). However, data are not completely reliable concerning the contribution of jellyfish to the observed echograms during field studies, because of the presence of other non-gelatinous organisms that may contribute to the sound scattering.
During routine acoustic cruises for the assessment of fish abundance in the Argentine Sea, large-scale, plankton-like scattering layers occupying thousands of square nautical miles are commonly observed with echosounders. Net sampling revealed that many of these concentrations are composed mainly of gelatinous zooplankters of large individual size, which aggregate in dense concentrations. As an example, dense aggregations of the salp I. zonaria are capable of producing scattering layers at 38 kHz, at levels high enough to obscure the echo recordings of anchovy schools (Engraulis anchoita) in the Río de la Plata surface-salinity front (Mianzan et al., 2001a).
The objective of this study was to correlate the aggregations of four gelatinous species identified by means of different plankton-net samples with the presence of sound-scattering layers (SSLs). The criteria employed in the selection of the echograms were: first, a clear dominance of a particular gelatinous species, and second, the coverage with plankton nets of most of the local planktonic-size range. As a result of this selection, echo recordings from aggregations of Lychnorhiza lucerna (Scyphozoa), I. zonaria (Salpidae), Mnemiopsis leidyi (Ctenophora), and Aequorea sp. (Hydrozoa) are presented. Features of the spatial distributions of the aggregations are also described and discussed.
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Materials and methods |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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Acoustic and net-sampling data were obtained onboard three research vessels ("Capitán Cánepa", "Doctor Eduardo Holmberg", and "Capitán Oca Balda") of the National Institute of Fisheries Research and Development (INIDEP), over a large part of the northern and central Argentine continental shelf (35°–47°S) (Figure 1). Data were collected along acoustic transects during four routine fish-abundance surveys, where jellyfish were particularly abundant in most of the surveyed area. In order to obtain an unbiased sampling of the whole range of the planktonic organisms present at 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 opening, mesh size reducing from 50 mm to 1500 µm) and a small bottom trawl (piloto) (2.4-m2 mouth opening, 6-m total length, 6-m headrope and groundrope, 25-mm wing-mesh size, 10 mm in the codend, 10-m bridles, and 80-cm vertical opening) were employed to sample the larger organisms, including most of the jellyfish species. The smaller zooplankton fraction was sampled by means of three different nets: a Hydro-Bios Multi-Plankton Sampler Type B (Multinet) (0.25-m2 mouth opening, 300 µm mesh size), a Nackthai high-speed sampler, a modification of the Gulf V high-speed sampler (Nellen and Hempel, 1969) (0.03-m2 mouth opening, 300-µm mesh net), and a 60-cm diameter bongo net (0.28-m2 mouth opening) fitted with 300-µm mesh net (Smith and Richardson, 1977). Filtered volumes by the larger nets were calculated as the product of the travelled distance and the net-mouth area, considering a filtering efficiency of 100%. Digital flowmeters (Hydro-Bios and General Oceanics) were employed to calculate the filtered volumes in the smaller nets. Trawling depth during all the net operations was monitored in real time by means of Scanmar pressure sensors.
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After the net operations, the gelatinous organisms were immediately sorted from the samples and analysed onboard. Taxonomical identification, number, total weight and individual length, weight and volume, or just one of these last two parameters, were obtained. The non-gelatinous organisms were fixed in a 5% formalin solution in seawater for subsequent laboratory analysis. Identification of the gelatinous taxa was based on the studies of Mayer (1912), Kramp (1961), Esnal and Daponte (1999), Mianzan (1999), and Mianzan and Cornelius (1999).
Acoustic data were collected using a calibrated SIMRAD EK500 with a 38 kHz split-beam transducer, except for the data relating to L. lucerna, where a SIMRAD EY500 echosounder and a 120 kHz split-beam transducer were employed. Echograms were recorded with a HP9000 Series graphical station, or with a laptop computer (EY500), and post-processed with the SIMRAD BI500 and SonarData 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).
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From the cruise data, segments of the acoustic transects (Figure 1) were selected to illustrate the patterns observed. The four selected echogram sections (Figure 2) correspond to locations where net sampling revealed a clear dominance of a particular jellyfish species in the total-species composition. The presence of other species, such as small copepods and other crustaceans, at low densities in the net samples was disregarded as a potential source of sound scattering at the working frequencies.
Particular information on each cruise and the equipment employed are presented within the results for each of the gelatinous species considered here.
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Results |
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Lychnorhiza lucerna (Scyphozoa: Lychnorhizidae)
Acoustic data and net samples were obtained along a transect (35°10'S, 56°10'W) during a survey focused on the study of the Río de la Plata salt-wedge regime during February 2000 (Figure 1a). Net sampling consisted of hauls with the small bottom trawl and the Multinet sampler. Echo recordings were obtained at 120 kHz. The scatterers aggregated close to the bottom in the main, where medusae dominated the net catches (Figure 2A).
The section of the echogram selected (Figure 1a) shows the bottom aggregation of L. lucerna (Figure 2A). A catch of 137 kg of L. lucerna (3–31 cm bell diameter, mode=11 cm) was obtained, representing a density of 14 individuals per 100 m3. Some juveniles of demersal fish species (mainly Paralonchurus brasiliensis and Micropogonias furnieri) ranging from 2 to 6 cm (density=5 per 100 m3) and two Chrysaora lactea were also found in the sample (Table 1).
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Iasis zonaria (Thaliacea: Salpidae)
Sampling was carried out during a cruise directed at the assessment of common hake pre-recruits (Merluccius hubbsi) at its northern spawning ground, located between the 50 and 200 m isobaths off the Río de la Plata estuary (35°–38°S) in June 1999 (Figure 1b). Plankton sampling included the use of the IKMT and Nackthai nets towed obliquely to sample the whole water column.
During this survey, high quantities of the salp I. zonaria were collected with both samplers. Up to 46 kg of salps were collected in one single IKMT tow (Figure 2b). No 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 the echosounder over the whole study area. The vertical distribution of salps, as observed in the echo recordings, varied along the ship's track occupying most of the water column and occasionally forming patches of higher densities at variable depths reaching Sv values, averaged by nautical mile, of –62.5 dB.
The selected section of the acoustic track shows a salp distribution as described earlier (Figure 2B). Net samples obtained with the IKMT and Nackthai nets showed that the salps represented up to 99% of total-catch weights. A total of 10 kg of salps (maximum length=7 cm), equivalent to approximately 3300 individuals with an average length of 4 cm, were captured with the IKMT along with eight fishes (3–8 cm) and three squids (2–3 cm). A density of 300 salps (length 2–7 cm) per 100 m3 was obtained from the Nackthai sample. Sparse organisms of a smaller fraction (
1 cm) consisting mainly of small salps and crustaceans (copepods, amphipods, and mysiids) were also found in the Nackthai sample (Table 1).
Mnemiopsis leidyi (Tentaculata: Bolinopsidae)
Data related to this species were obtained in December 1998 during a survey of the El Rincón area focused on the assessment of coastal-fish abundance.
A dense scattering layer (maximum Sv averaged by nautical mile=–62 dB) was observed at night along a transect parallel to the shoreline off Buenos Aires Province (39°S, 60°W) (Figure 1c). Echograms showed the scatterers to be restricted to the lower half of the water column (Figure 2C). The catch of the Nackthai sampler towed at the depth of the SSL consisted of 98 M. leidyi with a length range of 1–8 cm, representing a density of 673 ctenophores per 100 m3, along with 20 anchovy larvae of 1–2 cm total length (Table 1).
Aequorea sp. (Hydromedusae: Aequoreidae)
Acoustic and net samples were obtained during a cruise directed at the assessment of common hake pre-recruits in their southern spawning area off Isla Escondida and Golfo San Jorge (44°–47°S) in June 2000. Acoustic and net samplings during the cruise were conducted between the 50 and 100-m depth contours (Figure 1d).
Conspicuous SSLs were associated with a high incidence of Aequorea sp. in the IKMT samples. Net sampling was carried out after dusk, when the scatterers ascended dispersing through the water column. Catches of this species obtained with the IKMT reached 76 kg (8–20 cm umbrella diameter), representing a numeric density of 2 Aequorea per 100 m3. Bongo catches showed the presence of crustaceans such as copepods, euphausiids, and amphipods, but no relation was observed between their presence and the observed SSLs. No medusae or only a few individuals were captured in the stations where no SSLs were observed. During daylight hours this species aggregates near the bottom, forming dense SSLs (average Sv values of up to –62.1 dB) usually masking the presence of adult bottom fish. Large catches of this species are frequent in the area during hake bottom-trawl surveys.
Diel vertical-migration cycles were observed for this species, during which the organisms ascended at dusk to cover most of the water column and returned to their deeper distribution after dawn. The selected echograms presented in Figure 2D were condensed to show the vertical-migration cycle of the SSL, as described earlier. Only Aequorea sp. specimens and two juvenile hake (5.5–6 cm) were obtained in the IKMT samples at that location (Table 1).
The average TS for Aequorea sp. was estimated from the in situ echo recordings obtained during the night from the most dispersed organisms in the aggregation described earlier. An average of –64.53 dB was determined for the upper 50-m layer (n=4374). In order to validate that estimate, 50 individual target traces were isolated from the echograms and their individual TS distributions analysed, obtaining the similar TS average of –64.15 dB.
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Discussion |
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TopIntroductionMaterials and methodsResultsDiscussionReferences |
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The term "gelatinous zooplankton" is used for a diverse group of marine animals having a body principally composed of "jelly" tissue with a similar density to that of the surrounding medium. Although little sound reflectivity is therefore expected, according to our results jellyfish in fact contribute significantly to the sound scattering even at fairly low frequencies. Several factors may account for this observation. The large size of individuals reached by several species, the hardness of their gelatinous tissue and the fact that they often occur in enormous population densities might combine to produce high levels of sound scattering at several locations of the Argentine shelf waters as observed in the selected echograms.
According to the classification given by Stanton et al. (1996a), jellyfish are in the category of "fluid-like scatterers". Given the absence of an elastic shell, the acoustic-scattering strength of these "weak scatterers" would be mainly a function of their size, shape, and material properties (Stanton et al., 1996b).
The TS of several gelatinous species has been obtained by various authors employing different techniques (Table 2). The size and shape of jellyfish have been shown by Monger et al. (1998) to affect their acoustic response strongly. The simple act of swimming may produce variations of 10 dB in TS values as a result of the rhythmic contraction of the umbrella (Mutlu, 1996; Monger et al., 1998). High TS values have recently been verified for large medusae species at quite low frequencies (18–120 kHz). Laboratory measurements of TS of A. aurita (9.5–15.5 cm diameter) (Mutlu, 1996) and field estimates of C. hysoscella (26.8-cm diameter) and Aequorea aequorea (7.4-cm diameter) (Brierley et al., 2001) have shown a TS range of –46.6 to –68.5 dB. These results are consistent with the strong sound scattering observed in our study. Brierley et al. (2001), in particular, found an average TS of –66.3 dB for a mean umbrella diameter of 7.4 cm for A. aequorea, which closely agrees with the average of approximately –64 dB found in the TS distribution of our in situ observations of the larger Aequorea sp. (umbrella diameters ranging over 8–20 cm).
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Few data are available regarding the scattering from salps and ctenophores. The TS of the ctenophores Bolinopsis sp. (4.5 cm length) (Wiebe et al., 1990) and Pleurobrachia bachei (maximum diameter=1 cm) (Monger et al., 1998) were –80 dB (420 kHz) and –75.9 dB (200–1000 kHz), respectively. The TS of Salpa aspera (2.6-cm length) was approximately –90 dB at 120–200 kHz (Stanton et al., 1994). David et al. (2001) estimated the TS of salps by applying mathematical models to calculate the contribution of the gelatinous bodies and their nucleus separately. They obtained a TS range of –105.8 to –63.2 dB at 38 kHz for a body-length range of 1.4–10 cm. We believe that the strong sound-scattering levels registered from our salp (I. zonaria) were in part related to its large size (2–7 cm) and also to the very rigid nature of the tunic of this species compared with Salpa species (Esnal and Daponte, 1999; Mianzan et al., 2001a). In the case of the ctenophore M. leidyi (maximum length=8 cm in our samples), net samples probably underestimate the larger specimens because of the small mouth opening of the sampler employed on that occasion. Specimens as large as 15 cm are commonly caught with larger nets in the area.
Other factors have been addressed to explain the origin of sound scattering in gelatinous species, and should be considered as contributing to our results. Demer and Hewitt (1994) assumed that most of the scattering produced by Salpa thompsoni results from their spherical stomach, where diatoms, copepods, and other organisms are actively concentrated. Monger et al. (1998) suggested that copepod carcasses in the guts of the ctenophore P. bachei could have accounted for the TS values measured in their study. David et al. (2001) calculated similar contributions to sound scattering from the salps' nucleus and from their gelatinous body. Also, many jellyfish species are known to be associated with small fishes that swim close to their bodies (Mansueti, 1963; Brodeur, 1998). Direct observations made during the surveys indicated that small fish were often seen in close contact and swimming around the oral arms of L. lucerna.
Associated fish could introduce a source of error, causing the volume-scattering strength of jellyfish aggregations to be overestimated, particularly in near-bottom aggregations. However, in the case of L. lucerna, where several fish specimens were caught, the shape and distribution patterns of the observed echo traces could not be attributed to juveniles of demersal-fish species (Figure 2A). In the case of the anchovy larvae found together with M. leidyi, further histological analysis showed that at the measured size range (1–2 cm) the larvae had not yet developed a gas bladder, and they were therefore negligible sources of sound scattering.
The high numerical density of jellyfish present in most of the aggregations may also explain the rather high levels of sound scattering observed at these locations. Extensive aggregations of some of the studied species have been reported as being associated with hydrographic features of the Argentine Sea (Mianzan and Guerrero, 2000; Mianzan et al., 2001a) and the pattern is common to many southern jellyfish populations.
Particular features of the distributions of the aggregations of the four species studied have been observed. At the Río de la Plata surface-salinity front, L. lucerna was found aggregated close to the bottom. In the same area, the salp I. zonaria was found occupying most parts of the water column, with occasional patches of higher density at different depths. The Río de la Plata estuary is characterized by a marked salt-wedge, with a variable layer of denser, saltier water close to the bottom (Mianzan et al., 2001b) and it is possible that L. lucerna would penetrate the estuary following these layers of high salinity. Other species of gelatinous zooplankton have previously been reported as contributing significantly to the sound scattering associated with the estuary halocline (Madirolas et al., 1997). At the same time, the Río de la Plata surface-salinity frontal area represents a border system, there being some salp aggregations formed at this edge or just outside it (Mianzan et al., 2001a).
Acoustic methods allowed us to establish some behavioural patterns of Aequorea sp. This species performed a marked diel vertical migration during which the organisms ascended at dusk, spreading through the water column and returning to the near-bottom layer at dawn. No evidence has been registered yet for migration cycles of M. leidyi. Their aggregations were found mainly in the deeper half of the water column, even though visual observations of their presence near the surface, out of the detection range of the sounder, were common in the area. Consistent with this observation, Costello and Mianzan (2003) reported that M. leidyi appeared to be concentrated either near the bottom or at the surface, with relatively few individuals intermediate between the two depths.
Assessments of the distribution of these organisms in the vast areas they inhabit are possible through the analysis of data from routine surveys of fish abundance involving the use of 38 and 120 kHz transducers. Our results indicate that certain sizes and densities of jellyfish can make a significant contribution to the sound-scattering field, even at acoustic frequencies as low as indicated earlier. The analysis of the horizontal extent of the sound scattering from jellyfish will provide an index of their abundance, allowing for the study of the regional and temporal variations of these species.
The possible increase of gelatinous populations has been postulated recently (Mills, 1995, 2001), with several authors ascribing different causes to these population enhancements (Brodeur et al., 1999; Arai, 2001; Graham, 2001). To prove this assertion, historical data of gelatinous organism distribution and abundance will be needed. Unfortunately, such data are not widely available because gelatinous-plankton organisms were not included in routine monitoring studies until recently. Jellyfish-abundance estimation was neglected due to a general assumption of their low ecological significance (Arai, 1988). The acoustic characterization of jellyfish aggregations could become a valuable tool in the identification of specific distributions from earlier databases, allowing the reconstruction of past scenarios of their abundance and their significance and possible impact on particular ecosystems.
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Acknowledgements |
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The National Institute of Fisheries Research and Development (INIDEP) supported the research-vessel operations and most laboratory facilities. We thank in particular Dr Martín Ehrlich and his laboratory personnel for their support of this research during the hake pre-recruits cruises. We are indebted to Marcela Tobio for her aid with some of the photographs that illustrate this work. The study was partially supported by UNMdP 15/E139, Fundación Antorchas no. 13 817-5 and Agencia Nacional de Promoción Científica y Tecnológica no. 8424 grants to HWM. Travel and living expenses that allowed participation in the Sixth ICES Symposium on Acoustics in Fisheries and Aquatic Ecology were funded by Fundación Antorchas and the SAFAE Committee. This is INIDEP contribution no. 1240.
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References |
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