Detection of zooplankton predator-prey interactions in Alboran Sea by combining acoustic backscatter data and different sampling systems

Multifrequency acoustic technology was used to detect the summer epipelagic scattering layer (SESL) associated with a heterogenic zooplankton community in Alboran Sea. Simultaneously, identification hauls by means of two different plankton net (bongo 40 equipped with 250 and 333 μm mesh and bongo 90 equipped with 500 and 2000 μm mesh) were performed to obtain taxonomical and quantitative characteristics of the zooplankton community. Collected data revealed a strong correlation between the abundance of small crustacean (prey) captured by the 250 μm mesh with the abundance of chaetognaths, siphonophores and fish larvae (predators) captured by 500 μm mesh

Acoustic assessment and distribution of the main pelagic fish species in ICES Subdivision 9a South during the ECOCADIZ-RECLUTAS 2017-10 Spanish survey (October 2017).

The present working document summarises the main results obtained during the ECOCADIZ‐RECLUTAS 2017‐10 Spanish (pelagic ecosystem‐) acoustic survey. The survey’s main objective is the acoustic assessment of anchovy and sardine juveniles (age 0 fish) in the recruitment areas of the Gulf of Cadiz. The survey was planned to be conducted by IEO between 12nd and 31st October 2017 in the Portuguese and Spanish shelf waters (20‐200 m isobaths) off the Gulf of Cadiz onboard the R/V Ramón Margalef. However, a serious breakdown of the vessel’s propulsion system detected in the afternoon of the 22th October led to the early termination of the survey in that day. Only the seven (7) easternmost acoustic transects were sampled, which were accompanied by the conduction of eight (8) fishing hauls. The resulting estimates are therefore referred to this surveyed area, which corresponded to the eastern sector of the Spanish shelf waters, comprising the shelf between Doñana and Cape Trafalgar, and they are not comparable to the previous available estimates. Anchovy abundance and biomass in that surveyed area were 1 492 million fish and 7 641 t. The abundance and biomass of age 0 anchovies in the surveyed area were estimated at 1 433 million fish and 7 290 t. This juvenile fraction accounted for 96% and 95% of the total estimated population abundance and biomass, respectively. The estimates for Gulf of Cadiz sardine in the surveyed area were of 591 million fish and 12 103 t. Estimates of age‐0 sardine were of 483 million fish and 8 778 t, 82% and 72% of the total population, respectively. These estimates cannot be compared with the remaining data points in the series because the abovementioned problems with acoustic sampling coverage

Multifrequency study of the epipelagic food web in Alboran Sea

In order to characterize the epipelagic ecosystem from a trophic point of view, a multidisciplinary acoustic survey was carried out in July 2013 in Alborán Sea. A calibrated EK60 scientific echosounder (Simrad) equipped with 5 frequencies (18, 38, 70, 120 and 200 kHz) was used. Fish echotraces were groundtruthed by mean of a pelagic trawl (20mm cod end; 20m vertical opening). The main daytime epipelagic acoustic plankton layer was sampled using plankton nets with 4 different mesh sizes (250, 333, 500 and 2000 µm); the great progress was monitoring in real time the plankton net track. Finally fluorescence was recorded using a CTD (Seabird 19+). Our results show that primary producer (fluorescence data) can´t be acoustically clearly detected; small primary consumers (Copepoda, Cladocera) are detected at 70 kHz frequency near coast (30-45m deep). Going offshore, frequency response change due to the presence of secondary zooplankton consumers like Siphonophora, Chaetognatha, fish larvae and Apendicularia, which are the main targets detected at 18 and 38 kHz frequencies. Tertiary consumers (fish) are detected properly at 18 and 38 kHz

Acoustic identification of krill (Nyctiphanes couchii & Nematoscelis megalops) in the Spanish Mediterranean Sea

During the 2015 routine acoustic survey for stock assessment carried out in the Spanish Mediterranean continental shelf, krill swarms were localized in two different places using an EK60 scientific echosounder, operating at multiple frequencies. Biological identification was performed by mean of a plankton net (bongo 90) equipped with two different mesh sizes (500 to 2000 microns). Although krill frequency response was similar in both localizations, samples analysis revealed the presence of two different species: Nyctiphanes couchii (Bell, 1853) and Nematoscelis megalops (Sars, 1883)

Relationship between anchovy (Engraulis encrasicolus) recruits and anchovy spawners estimated from acoustic surveys in Northwestern Mediterranean Sea

In this study we present the abundance and biomass of European anchovy (Engraulis encrasicolus) estimated by acoustic methods in the Northwestern Mediterranean Sea (GSA06). during two different seasons, winter, the anchovy recruitment season and summer, when the majority of the anchovy stock belongs to the spawner fraction of one year-old. Four standardized acoustic surveys carried out covering the study area during two consecutive years (every six months), two of them during the peak of anchovy recruitment and the other two during the peak of anchovy spawn, allow to observe changes in the anchovy stock abundance and biomass in this area. Acoustic data from summer and winter echo-surveys were analyzed in an effort to better understand the life cycle of anchovy in relation to the survival during its first year of life

Multidisciplinary Acoustic Survey PELACUS0313: Preliminary Results on Fish Abundance Estimates and Distribution

The PELACUS 0313 survey was undertook this year on board R/V Miguel Oliver, an oceanographic research stern trawler vessel similar to R/V Thalassa. The survey was characterised by a very bad weather conditions during the first two weeks which did not allow working properly. Moreover, the weather conditions during the rest of survey were almost similar. As a consequence, most of the coastal pelagic fish community remained very close to the coast, thus not accessible to the pelagic gear samplers. (33% of the total acoustic energy – NASC- was unable to be properly allocated into fish species). Outside the coastal area (>90 m depth) main abundance fish species was mackerel with 380.000 tonnes, corresponding to 1.725 million fish. On the contrary, sardine distribution was scarce, and occurred in small schools (probably as a consequence of the bad weather condition), with only 3.343 tonnes corresponding to 54.0 million fish. Age group 2 was the most abundant, which confirms the high abundance found last year at age group 1 of the 2011 cohort

Sampling uncertainty associated with western Mediterranean pelagic fish abundance estimates derived from acoustic data.

Acoustic surveys are used worldwide for the assessment of pelagic fish stocks. In the Spanish Mediterranean area acoustic surveys are performed annually in late autumn and cover the entire continental shelf between 30 and 200 m depth. This survey was initiated to obtain estimates of anchovy recruitment (Engraulis encrasicolus), but additionally provide a general overview of the whole pelagic fish community in the study area during the time of the year it is performed. Our study area has a diverse assemblage of small and medium-sized pelagic fish (up to nine species) and thus we rely on the proportion of species obtained in middle-water pelagic fishing trawls to attribute the amount of echo corresponding to every single species and estimate their abundance. Although uncertainties may arise from many different sources (e.g. transducer motion, target strength, migration) we focus our study on the estimation of overall sampling uncertainty, one of the main contributors to random error. We apply geostatistical techniques to deal with spatial correlation and discuss benefits and pitfalls in their application. Although transitive geostatistics have seldom been used, most probably due to their inability to produce spatial maps of abundance and variance estimations, they constitute a powerful tool to routinely estimate sampling variance and its variation in time in a multi-specific context. They may also help to assess the effects of varying sampling intensity and could potentially be useful to detect possible processing errors, for example, in echogram scrutinizing. These techniques may therefore potentially help improve both the precision and the accuracy of acoustic dat