Species identification of small pelagic fish schools by means of hydroacoustics in the Eastern Mediterranean Sea

  Reliable biomass estimates by means of hydroacoustics largely depend on the correct identification of acoustic targets. Data collected during five summer acoustic surveys (2004-2008) in the North Aegean Sea (Greece) were analyzed to explore effective discrimination of small pelagic fish schools according to the species they belong. Discriminant Function Analyses (DFA) using bathymetric, energetic and morphometric school descriptors as explanatory variables were applied per research cruise as well as to pooled data from all surveys. Results revealed that the schools can be successfully classified into the five species considered (anchovy Engraulis encrasicolus, sardine Sardina pilchardus, round sardinella Sardinella aurita, blue whiting Micromessistius poutassou, and Atlantic mackerel Scomber scombrus). The percentage of correct classifications in terms of number of schools was higher in the analyses of the annual cruises (75.6%-95.36%) than in the analysis of pooled data (≈72%). This is because of (i) the lower number of species, as well as (ii) the reduced intraspecific variability, occurring in each separate cruise. Significant differences were detected among school descriptors for the different species, revealing discrete aspects of schooling behaviour for each species. The benefit of the specific approach is that the classification functions of the DFAs can be used to classify a larger set of schools, which has not been possible to assign to specific species. Overall the approach constitutes an objective, more automated and less time consuming procedure for the analysis of acoustic data and can contribute to the improvement of biomass estimates in the area

Applying a two-stage Bayesian dynamic model to a short lived species, the anchovy in the Aegean Sea (Eastern Mediterranean). Comparison with an Integrated Catch at Age stock assessment model.

Two different stock assessment models were applied to the North Aegean Sea anchovy stock (Eastern Mediterranean Sea): an Integrated Catch at age Analysis and a Bayesian two-stage biomass based model. Commercial catch data over the period 2000-2008 as well as acoustics and Daily Egg Production Method estimates over the period 2003-2008 were used. Both models results were consistent, indicating that anchovy stock is exploited sustainably in relation to an exploitation rate reference point. Further, the stock biomass appears stable or increasing. However, the limitations in age-composition data, potential problems related to misinterpretation of age readings along with the existence of missing values in the survey data seem to favour the two-stage biomass method, which is based on a simplified age structure.

Effects of sampling intensity and biomass levels on the precision of acoustic surveys in the Mediterranean Sea

Acoustic surveys represent the standard fishery-independent method worldwide for evaluating the biomass and spatial distribution of small pelagic fish populations. Considering the peculiarities of the spatial behaviour of pelagic fishes, the efficiency of the survey design in determining their biomass and spatial distribution is related to its ability to capture the portion of the patches accounting for larger part of the total biomass. Yet, the spatial structure of the patches could be strongly influenced by ecosystem characteristics as well as by changes in total biomass related to a density-dependent mechanism. This is of particular interest for anchovy and sardine which are known for their wide fluctuations and high sensitivity to the environment. In this study, we analysed the efficiency of acoustic surveys, targeting European anchovy (Engraulis encrasicolus) and European sardine (Sardina pilchardus), in 10 different areas of the Mediterranean Sea across three years of different biomass levels. Using the geostatistical Coefficient of Variation (CVgeo) of the average occurrence probability of high/medium density values, we showed different patterns in terms of survey design efficiency among areas and species. Anchovy usually showed lower CVgeo than sardine, but in the Alboran Sea. In 4 out of 20 cases, CVgeo values showed a consistent decrease with increasing biomass while in the remaining cases the CVgeo did not follow any clear pattern suggesting the presence of important environmental effects. Higher survey design efficiency was found in high productive sectors influenced by river run-off, letting us to hypothesize that higher productivity along with the presence of well-localized enrichment mechanisms could favour a spatially consistent distribution and coherent organization of fish population leading to higher precision estimates with a given transect design. While most surveys displayed CVgeo close to 10% or less even at low biomass, evidencing generally good performances of the survey design, a few areas exhibited higher CVgeo, yielding discussions about a potential need to decrease the inter-transect distance, always keeping in mind that survey should be as synoptic as possible.Versión del edito

New Fisheries-related data from the Mediterranean Sea (October 2015)

In this third Collective Article, with fisheries-related data from the Mediterranean Sea, we present the historical length distribution of Lophius budegassa in the catch of commercial trawlers in the Greek seas; length-weight and length-length relationships of five flatfish species (Lepidorhombus boscii, L. whiffiagonis, Platichthys flesus, Pegusa lascaris and Solea solea) from different coastal areas of Turkey (Black Sea and Eastern Mediterranean Sea); growth of settled Polyprion americanus and length-weight relationships of this species and of Deltentosteus quadrimaculatus, Capros aper and three commercially important groupers in the Eastern Mediterranean Sea; the age, growth and mortality of Zosterisessor ophiocephalus in the Eastern Adriatic Sea; the length-weight relationship and condition factor of Atherina boyeri in a Central Mediterranean semi-isolated lagoon, and also the length-weight and length-length relationships of three Alburnus species from different inland waters in Turkey

Habitat Suitability Modeling to Identify the Potential Nursery Grounds of the Atlantic Mackerel and Its Relation to Oceanographic Conditions in the Mediterranean Sea

Our knowledge for the distribution of Atlantic mackerel (Scomber scombrus) in the Mediterranean Sea is limited and fragmented. In the current work habitat suitability modeling was applied to summer acoustic surveys data of Atlantic mackerel juveniles derived from the north part of the Mediterranean (i.e., acoustic data from the Gulf of Lions, pelagic trawls held during acoustic surveys in Spanish Mediterranean waters, south Adriatic Sea, Strait of Sicily, and North Aegean Sea) using generalized additive models (GAMs) along with satellite environmental and bathymetry data. Bathymetry along with sea surface temperature and circulation patterns, expressed through sea level anomaly and the zonal component of the absolute geostrophic velocity, were the environmental variables best to describe nursery grounds. The selected model was used to produce maps presenting the potential nursery grounds of Atlantic mackerel throughout the Mediterranean Sea as a measure of habitat adequacy. However, the assessed potential nursery grounds were generally marked as “occasional,” implying that although there are areas presenting high probability to encounter Atlantic mackerel, this picture can largely vary from year to year stressing the high susceptibility of the species to environmental conditions. In a further step and toward a spatial management perspective, we have estimated and visualized the overlap between Atlantic mackerel and anchovy/ sardine juvenile grounds throughout the basin. Results showed that although the degree of overlapping was generally low, not exceeding 15% in general, this varied at a regional level going up to 30%. The potential of the output of this work for management purposes like the implementation of spatially-explicit management tools is discussedVersión del edito

Trammel net catch species composition, catch rates and metiers in southern European waters: A multivariate approach

We identified and quantified the effect of season, depth, and inner and outer panel mesh size on the trammel net catch species composition and catch rates in four southern European areas (Northeast Atlantic: Basque Country, Spain; Algarve, Portugal; Gulf of Cadiz, Spain; Mediterranean: Cyclades, Greece), all of which are characterised by important trammel net fisheries. In each area, we conducted, in 1999-2000, seasonal, experimental fishing trials at various depths with trammel nets of six different inner/outer panel mesh combinations (i.e., two large outer panel meshes and three small inner panel meshes). Overall, our study covered some of the most commonly used inner panel mesh sizes, ranging from 40 to 140 mm (stretched). We analysed the species composition and catch rates of the different inner/outer panel combinations with regression, multivariate analysis (cluster analysis and multidimensional scaling) and other 'community' techniques (number of species, dominance curves). All our analyses indicated that the outer panel mesh sizes used in the present study did not significantly affect the catch characteristics in terms of number of species, catch rates and species composition. Multivariate analyses and seasonal dominance plots indicated that in Basque, Algarve and Cyclades waters, where sampling covered wide depth ranges, both season and depth strongly affected catch species compositions. For the Gulf of Cadiz, where sampling was restricted to depths 10-30 m, season was the only factor affecting catch species composition and thus group formation. In contrast, the inner panel mesh size did not generally affect multidimensional group formation in all areas but affected the dominance of the species caught in the Algarve and the Gulf of Cadiz. Multivariate analyses also revealed 11 different metiers (i.e., season-depth-species-inner panel mesh size combinations) in the four areas. This clearly indicated the existence of trammel net 'hot spots', which represent essential habitats (e.g., spawning, nursery or wintering grounds) of the life history of the targeted and associated species. The number of specimens caught declined significantly with inner panel mesh size in all areas. We attributed this to the exponential decline in abundance with size, both within- and between-species. In contrast, the number of species caught in each area was not related to the inner mesh size. This was unexpected and might be a consequence of the wide size-selective range of trammel nets. (c) 2006 Elsevier B.V All rights reserved

Density dependence in the spatial behaviour of anchovy and sardine across Mediterranean systems

A spatial indicator—the spreading area index—is used to describe anchovy and sardine spatial distribution in relation to biomass variation and to look for ecosystem differences within the Mediterranean basin. Specifically, the variation in the spreading area index in relation to biomass was examined for different areas of the Mediterranean Sea (i.e. Aegean Sea, western Adriatic Sea, Strait of Sicily, Gulf of Lion, and Spanish Mediterranean waters). In order to capture the spatial variability of the population at different levels of fish density, acoustic survey data for the years of highest, lowest, and intermediate abundance were used. In a subsequent step standardized values of spreading area and biomass were estimated to allow comparisons. Results showed pronounced area differences. A significant relationship was revealed in the case of anchovy for areas with extended continental shelf (i.e. Aegean Sea, Adriatic Sea, and Gulf of Lion), indicating an increase in biomass with an increase in the spreading area. No relationship was found for areas dominated by narrow continental shelf and strong currents (i.e. Spanish Mediterranean waters and the Strait of Sicily). With regard to sardine, an increase in biomass was followed by an increase in the spreading area when estimates from the Aegean Sea, the Adriatic Sea, and the Strait of Sicily were considered together. The relationship was even more Abstracts–Theme Session B 9 pronounced when analysis was limited to the Aegean Sea and the Strait of Sicily. No relationship was found for the Spanish Mediterranean waters and the Gulf of Lion. This clearly implies that spatial indicators should be integrated into ecosystem management, taking into account that they can be area‐ or ecosystem‐dependent