JELLYFISH BLOOMS
Acoustic survey of a jellyfish-dominated ecosystem
(Mljet Island, Croatia)
G. Alvarez Colombo Æ A. Benovic
´
Æ A. Malej Æ
D. Luc
ˇ
ic
´
Æ T. Makovec Æ V. Onofri Æ M. Acha Æ
A. Madirolas Æ H. Mianzan
Published online: 15 October 2008
Ó Springer Science+Business Media B.V. 2008
Abstract Acoustic techniques have been proposed
as a new tool to assess jellyfish populations. However,
the presence of mixed echoes from jellyfish and other
organisms that share their distribution often prevent
accurate estimates of their abundance and distribution
being obtained. The isolated population of Aurelia
inhabiting the Veliko Jezero (Big Lake-BL) of Mljet
Island, in the South Adriatic Sea, offered a good
opportunity to employ acoustic techniques to assess an
entire jellyfish population. During October 2–5, 2006,
combined video and acoustic methods were used in BL
to determine the vertical distribution of medusae. Two
synoptic acoustic surveys were performed during the
day and night. In the daylight echograms, medusae
were clearly discernible from the acoustic data, and
their presence verified by video camera images, as
forming a layer of varying density at and below the
thermocline (15–30 m). The depth of the jellyfish layer
also coincided with the depth of maximum dissolved
oxygen concentration. The echointegration of these
daylight data enabled quantification the Aurelia pop-
ulation, at a frequency of 120 kHz. In the night
echograms, the acoustic signals of Aurelia were at least
partially masked by pelagic and demersal fish, which
disaggregated from schools and formed a layer asso-
ciated with a strong thermocline at 15 m. An average
target strength (TS) of -76.4 dB was obtained in situ
corresponding to a mean length of 10.8 cm and a mean
wet weight of 134 g measured from sampled medusae.
These results were combined with echo-integration
values to provide an estimate of 4,238,602 individuals
and a biomass of 568 tons of Aurelia in BL. This study
provided a synoptic view of Mljet Lake and illustrated
the potential of acoustic surveys of jellyfish popula-
tions to contribute to ecosystems studies.
Keywords Hydroacoustics Á Medusae Á
Scyphozoa Á Aurelia Á In situ target strength Á
Aggregation structure Á Distribution Á
Biomass assessment
Guest editors: K. A. Pitt & J. E. Purcell
Jellyfish Blooms: Causes, Consequences, and Recent Advances
G. Alvarez Colombo (&) Á M. Acha Á
A. Madirolas Á H. Mianzan
Instituto Nacional de Investigacio
´
n y Desarrollo Pesquero,
Mar del Plata, Argentina
e-mail: acolombo@inidep.edu.ar
A. Benovic
´
Á D. Luc
ˇ
ic
´
Á V. Onofri
Institute for Marine and Coastal Research, University
of Dubrovnik, Dubrovnik, Croatia
A. Malej Á T. Makovec
Marine Biology Station, National Institute of Biology,
Piran, Slovenia
M. Acha Á H. Mianzan
CONICET Consejo Nacional de Investigaciones
Cientı
´
ficas y Te
´
cnicas, Buenos Aires, Argentina
M. Acha
Facultad de Ciencias Exactas y Naturales, UNMDP,
Mar del Plata, Argentina
123
Hydrobiologia (2009) 616:99–111
DOI 10.1007/s10750-008-9587-6