Boyra, G., Martínez, U., Cotano, U., Santos, M., Irigoien, X., and Uriarte, A. 2013. Acoustic surveys for juvenile anchovy in the Bay of Biscay: abundance estimate as an indicator of the next year's recruitment and spatial distribution patterns. – ICES Journal of Marine Science, 70: . A series of acoustic surveys (JUVENA) began in 2003 targeting juvenile anchovy (Engraulis encrasicolus) in the Bay of Biscay. A specific methodology was designed for mapping and estimating juvenile abundance annually, four months after the spawning season. After eight years of the survey, a consistent picture of the spatial pattern of the juvenile anchovy has emerged. Juveniles show a vertical and horizontal distribution pattern that depends on size. The younger individuals are found isolated from other species in waters closer to the surface, mainly off the shelf within the mid-southern region of the bay. The largest juveniles are usually found deeper and closer to the shore in the company of adult anchovy and other pelagic species. In these eight years, the survey has covered a wide range of juvenile abundances, and the estimates show a significant positive relationship between the juvenile biomasses and the one-year-old recruits of the following year. This demonstrates that the JUVENA index provides an early indication of the strength of next year's recruitment to the fishery and can therefore be used to improve the management advice for the fishery of this short-lived species.

Identifying the role that environmental factors and biotic interactions play in species distribution can be essential to better understand and predict how ecosystems will respond to changing environmental conditions. This study aimed at disentangling the assemblage of the pelagic predator–prey community by identifying interspecific associations and their main drivers. For this purpose, we applied the joint species distribution modelling approach, JSDM, to the co-occurrence patterns of both prey and top predator communities obtained from JUVENA surveys during 2013–2016 in the Bay of Biscay. Results showed that the co-occurrence patterns of top predators and prey were driven by a combination of environmental and biotic factors, which highlighted the importance of considering both components to fully understand the community structure. In addition, results also revealed that many biotic interactions, such as schooling in prey (e.g. anchovy–sardine), local enhancement/facilitation in predators (e.g. Cory’s shearwater–fin whale), and predation between predator–prey species (e.g. northern gannet–horse mackerel), were led by positive associations, although predator avoidance behaviour was also suggested between negatively associated species (e.g. striped dolphin–blue whiting). The identification of interspecific associations can therefore provide insights on the functioning of predators–prey network and help advance towards an ecosystem-based management.

Decreased body size is often cited as a major response to ocean warming. Available evidence, however, questions the actual emergence of shrinking trends and the prevalence of temperature‐driven changes in size over alternative drivers. In marine fish, changes in food availability or fluctuations in abundance, including those due to size‐selective fishing, provide compelling mechanisms to explain changes in body size. Here, based on three decades of scientific survey data (1990–2021), we report a decline in the average body size—length and weight—of anchovy, Engraulis encrasicolus L., in the Bay of Biscay. Shrinking was evident in all age classes, from juveniles to adults. Allometric adjustment indicated slightly more pronounced declines in weight than in total length, which is consistent with a change toward a slender body shape. Trends in adult weight were nonlinear, with rates accelerating to an average decline of up to 25% decade⁻¹ during the last two decades. We found a strong association between higher anchovy abundance and reduced juvenile size. The effect of density dependence was less clear later in life, and temperature became the best predictor of declines in adult size. Theoretical analyses based on a strategic model further suggested that observed patterns are consistent with a simultaneous, opposing effect of rising temperatures on accelerating early growth and decreasing adult size as predicted by the temperature‐size rule. Macroecological assessment of ecogeographical—Bergmann's and James'—rules in anchovy size suggested that the observed decline largely exceeds intraspecific variation and might be the result of selection. Limitations inherent in the observational nature of the study recommend caution and a continued assessment and exploration of alternative drivers. Additional evidence of a climate‐driven regime shift in the region suggests, however, that shrinking anchovy sizes may signal a long‐lasting change in the structure and functioning of the Bay of Biscay ecosystem.

This paper presents measures of target strength (⁠$TS; dB re 1 m2$⁠) and models of $TS$ vs. fork length (L; cm), i.e. $TS=20logL+b20$⁠, for skipjack tuna associated with fish aggregating devices (FADs) in the Central Pacific Ocean. Measurements were made using 38-, 120-, and 200-kHz split-beam echosounders on a purse-seine workboat during fishing operations. To mitigate potential bias due to unresolved targets, $TS$ measurements were rejected if they were not simultaneously detected with multiple echosounder frequencies in approximately the same location. The filtered TS and concomitantly sampled L data were used to estimate b₂₀ = −76, −71, and −70.5 dB for 38, 120, and 200 kHz, respectively, using the method of least squares. For comparison, quasi-independent estimates of TS and b₂₀ were calculated from acoustic echo-integration and catch data representing entire aggregations around the FADs. The results differed by ≤1 dB for all three frequencies. The sensitivities of these results to variations in fish morphology and behaviour were explored using a simulation of $TS$ for fish without swimbladders. The utility of the results on acoustic properties of skipjack tuna and next research steps to achieve selective fishing at FADs are discussed.

Aim: Understanding the mechanisms that allow the coexistence of species is key to preserve full ecosystem functioning. In dynamic environments, the study of ecological niches faces the complexity associated to the three dimensionality of the habitat and requires information that reflects such heterogeneity. Within this context, this study intends to identify the segregation mechanisms behind the co‐occurrence of five phylogenetically related pelagic birds by applying a functional perspective based on seabirds' vertical ranges and prey availability features such as depth and body size.Location: Bay of Biscay.Methods: Based on the hypothesis that niche differentiation may occur in any of the three dimensions of the marine environment, we (a) identified the biologically meaningful vertical range affecting seabird species, (b) modelled their environmental and trophic niches, (c) estimated an environmental and trophic overlap index for each pairwise species, and (d) developed a conceptual framework with the most plausible segregation hypotheses.Results: The application of the conceptual framework revealed that in this particular area, pelagic birds coexist through environmental and trophic niche partitioning and potentially through vertical segregation, based on the different biologically meaningful vertical ranges we identified for each species. Indeed, some species responded to prey and oceanographic conditions on the surface (10 m), while others responded to the conditions on deeper waters (above the depth of maximum temperature gradient). These different responses could be interpreted as an additional mechanism to reduce competition, although seabirds diving records would be needed to contrast this hypothesis.Main conclusions: Niche differentiation was found to be primarily driven by trophic and environmental niche partitioning, although species were also influenced by conditions on the vertical dimension. Considering all the dimensions of the niche is essential to fully understand how diving seabirds coexist in dynamic systems and provides insights on species' 3D niches that may help advance into their management.