ASSESSING THE APPLICABILITY OF ENVIRONMENTAL INDICATORS FOR IMPROVING THE FISHERIES ASSESSMENT OF THE ALBACORE (THUNNUS ALALUNGA) UNDER THE A4A APPROACH

In this study we explore the potential for improving the stock assessment of Mediterranean Albacore by integrating environmental indicators. For this purpose we developed a catch at age model within the A4A stock assessment approach. The input data was similar to that used in the official SCRS stock assessment in 2017 but with an updated larval index. The environmental indicator provides information on the interannual variability of the sea surface temperature in the Balearic Sea during the spawning season, and it is included in the “Environmental pressure” component of the Ecosystem Report Card. The indicator is included in the assessment model in different ways, as index of the class age 0, as vector for the Stock/recruitment model, and as productivity value in other stock recruitment models (Ricker, Beverton-Holt). The results showed that incorporating the environmental variability indicators provide a better stock assessment fits (AIC, BIC), and also show the need for more advanced techniques to test stock assessment performance when testing the inclusion of environmental variabilityEn prens

Bluefin tuna larval indices in the western Mediterranean, ecological and analytycal sources of uncertainity

The main objective of this study is to provide the knowledge to design adequate sensitivity analyses on the assessment models used for the Eastern stock of Bluefin tuna. We analyze how different configuration for the same environmental variable (temperature in the mixed layer depth) and different modeling approaches (nonlinear Delta-log,delta-gamma, tweedy and bayesian) affects to the variability of the larval indices of the Eastern bluefin tuna from data collected in the Balearic Sea (Western Mediterranean). We also investigate the effects on the index caused from having differences in the total sampled area among years. We used these results to interpolate larval index values in years with not standard larval surveys but with some ichthyoplankton surveys available, and to propose a “revised version” of the index providing parameters of uncertainty

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

RETROCALCULATED LARVAL ABUNDANCE INDEX OF ATLANTIC BLUEFIN TUNA IN THE WESTERN MEDITERRANEAN SEA, 2001-2020

This document presents the update of the bluefin tuna retrocalculated larval abundance indices from the Balearic archipelago (western Mediterranean). The index has been calculated following methods presented in 2020 (SCRS/2020/067) and 2021 (SCRS/2021/033). The abundance index shows an increasing trend with a maximum value in 2020. A previous version of the index (SCRS/P/2019/055) is also provided for comparison.En prensa

ASSESSING THE SPAWNING STOCK BIOMASS OF ALBACORE (THUNNUS ALALUNGA) IN THE WESTERN MEDITERRANEAN SEA FROM A NON-LINEAR LARVAL INDEX (2001-2019)

Larval abundance indices express retrocalculated abundances of larval densities at hatching time. They provide a proxy for assessing spawning stock biomass and are applied to assess population status of various species in the Gulf of Mexico and in the Balearic Sea. Recently, the methodological approach to calculate the indices was improved to accommodate for non-linear responses of environmental effects on catchability. This improved methodology is routinely applied in the Balearic Sea to assess the bluefin tuna (Thunnus thynnus) spawning stock biomass. Here we apply the same methodology to update the larval index of albacore (Thunnus alalunga) from surveys conducted from 2001 to 2019 in the Balearic Sea, the most relevant spawning ground of this species in the Western Mediterranean. Albacore larval abundances show a decreasing trend and significant lower abundances from 2013 onwards, despite a slight recovery between 2016 and 2017. This larval index, standardized for gears, sampling coverage, salinity, date and sea surface temperature, provides information on the dynamics of the western Mediterranean stock of albacore, which is considered a data poor stock.En prens

LARVAL HABITATS AND CATCHES OF SWORDFISH (XIPHIAS GLADIUS) IN THE BALEARIC ISLANDS (2001-2020): OCEANOGRAPHIC DRIVERS AND OPPORTUNITIES FOR RESEARCH

Since 2001, ichthyoplankton and hydrographic surveys directed to tuna species have been conducted in the Balearic Islands, a main tuna spawning ground in the Mediterranean. These campaigns provide today key information about the interannual changes on larval abundances for Bluefin tuna and albacore, also allowing the investigation of the early-life ecology of various species. The Balearic Islands have been identified as a prominent oceanographic retention area within the western Mediterranean as well as the main spawning area for tuna species. Hence, the regular ichthyoplankton surveys become an opportunity to increase the knowledge of those species whose pelagic early-life stages are encountered during the summer in this area. This is the case of swordfish (Xiphias gladius). Here we analyse the possibility of applying those surveys to investigate the early life ecology of the Mediterranean swordfish, exploring the interannual changes on larval abundances and the hydrographic preferences of larval habitatsEn prens

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