Adrián Madirolas

Acoustic characterization of gelatinous plankton aggregations: four case studies from the Argentine continental shelf

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.

Pelagic swarms and beach strandings of the squat lobster Munida gregaria (Anomura: Munididae) in the Beagle Channel, Tierra del Fuego

The present article is the first formal record of pelagic swarms and beach strandings of the squat lobster Munida gregaria in the Beagle Channel, southern South America. To describe size composition and natural diet of beach strandings and swarms, samples from strandings were taken in April 2007, March 2008 and March 2010. Samples from swarms were taken in May 2007 and April 2010. Also, during November 2008 and January 2009, two acoustic surveys were carried out to describe both, horizontal distribution and school metrics of swarms. Pelagic swarms and beach strandings were composed of 100% individuals of the morph gregaria of M. gregaria. Mean size of males and females in both, beach strandings and swarms, were similar. The diet of pelagic M. gregaria was composed mainly of crustaceans, unicellular algae and small macroalgae, sediment and particulate organic matter. Swarms of pelagic Munida took the shape of extended layers of varying density and height. Their positions in the water column were also variable: swarms were found at different depths, from the subsurface layer to near the sea bottom. Occasionally, some large swarms occupied most of the water column. In the Beagle Channel, SW winds during spring tides could be a forcing factor for M. gregaria strandings. The absence of shoaling animals in the Beagle Channel between 1997 and 2002 and the recent occurrence of swarms are coincident with their appearance in other locations in Patagonia. In consequence, we hypothesize that the factor influencing the formation of swarms must be acting at a regional scale.

Assessment of fish yield in patagonian lakes (Argentina) : Development and application of empirical models

Abstract The relationship between fish yield and basic limnological information is important to developing sustainable management policies for lake fisheries, particularly when fish data are lacking. Most Patagonian lake fisheries lack the basic statistical information provided by bycatch and effort data. We present a simple first-order model of fish yield based on the relationship between fish biomass derived from hydroacoustic and gill-net surveys developed for 18 small Patagonian lakes. Such a model can be used to develop sound management guidelines that can be applied to the many unsampled small lakes of Patagonia for which basic limnological data are available. We also evaluated the value of well-known models developed from North American data sets. These models consistently underestimated fish yields in the Patagonian lakes, particularly as trophic level increased, illustrating the importance of using region-specific data to develop fish management guidelines. The poor predictive performance of the ...

Acoustic target strength (TS) of argentine anchovy (Engraulis anchoita): the nighttime scattering layer

Engraulis anchoita is a physostomous fish with a dual chambered swim bladder (sb). In situ target strength (TS) measurements on this species are only possible at night, when anchovies disperse and form a scattering layer near the sea surface. A survey data series comprising more than50000 single target detections, recorded from 1995 to 2008, was analyzed in order to study the species specific TS at 38 kHz. The results of this study were recently published at the ICES JMR and a synthesis is presented at RIO Acoustics Symposium given the importance of the several anchovies stocks distributed off South Americas coasts. A TS vs. fish total length (L) equation was obtained from the in situ measurements (TS= 31.9 log L − 82.4 dB; R2= 0.78). When the slope of the regression line was forced to 20 into the TS equation, the resulting value for the constant term (b20) was −68.6 dB. In any case, these results indicate an average difference of +3 dB (higher TS values) when compared with the general model suggested for clupeoid fish. The TS measurements obtained inside the nighttime sound scattering layer exhibited a negative trend with depth. An empirical depth dependence term for the anchovy TS equation was obtained through a three parameter least square fitting of the data [TS= 31.3 log L − 79.6 dB − 4.74 log (1+z/10); R2=0.74]. Anatomical data obtained through high resolution X-Ray Computed Tomography was employed as input for a Prolate Spheroidal Model(PSM). Theoretical TS vs. tilt angle functions were obtained considering the compression of the sb at different depths and under the assumption of different contraction rates. The TS functions were then averaged over different fish tilt angle distributions and used to derive theoretical depth dependence curves of average fish TS. The implications of the adopted sb contraction rate and tilt angle distribution are discussed by comparing the modeled TS(z) curves against the empirical data. Additionally, the potential implications of fish behavior on TS, i.e., nighttime vs. daytime conditions, are discussed.

Winter is cool: spatio-temporal patterns of the squat lobster Munida gregaria and the Fuegian sprat Sprattus fuegensis in a sub-Antarctic estuarine environment

In the sub-Antarctic waters of southern South America, two species that form conspicuous pelagic aggregations are considered both potential fishery resources and ecologically important: the Fuegian sprat Sprattus fuegensis and the squat lobster Munida gregaria. Here, we estimate spatio-temporal patterns of the relative abundance, distribution and potential areas of overlapping of M. gregaria and S. fuegensis in the Beagle Channel. Acoustic data were collected from 18 surveys conducted between 2009 and 2016 in the Beagle Channel. Our observations on the pelagic aggregations revealed that M. gregaria swarms and S. fuegensis schools occurred along the channel throughout the year. However, the occurrence of S. fuegensis was more evident in the inner part of the channel during the winter. There was a negative relationship between the relative abundance of squat lobsters and Fuegian sprats throughout the year. This interaction may result from spatial competition associated with resources, particularly in summer. The vertical distribution and morphology of aggregations of both species showed seasonal differences. Specifically, schools of Fuegian sprats were typically pelagic in summer, whereas in winter they were bigger and associated with the bottom layer. This change could be associated with a seasonal change in the oceanographic regime of the Beagle Channel. The information presented here is necessary to understand processes in a context of possible opening of fisheries targeting these two species that have key ecological roles in this sub-Antarctic ecosystem.

Reconocimiento automático de especies utilizando procesamiento digital de imágenes acústicas

Accurate identification of marine organisms and their numerical abundance calculation using echo detection techniques remains a challenge for researchers. Different approaches using hydroacoustic techniques have been applied, alone or combined, to study marine and fresh water environments. An alternative approach to this problem is to use echoic statistics with confident geometric descriptors obtained from the echo-records processing. To accomplish this task it is necessary to have a development platform that allows reading echo-records from a particular echo-sounder, to detect aggregations or schools and then to calculate the various descriptors that will be use for species identification, in an automatic way. This article describes the application of digital processing algorithms of echo-records for automatic recognition of the ocean seabed, surface and marine organisms. These algorithms are implemented within the EcoPampa software, which is the first Argentinean open source system for the identification of marine species.