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Prey and predator distributions influence one another. Understanding the scale and the orientation of predator–prey spatial correlations is crucial in foraging ecology. Growing evidence suggests that predator–prey interactions are more constrained by functional characteristics of both the predator and the prey. Unfortunately, in marine pelagic systems, the scale and orientation of spatial correlations between predators and prey have been only little explored from a functional point of view. We tested the existence of fine-scale association between predators and fish functional groups. Visual predator sightings and acoustic fish records were collected synchronously during oceanographic surveys from 2004 to 2014. Prey biomass was integrated by nautical miles and split into four size classes (<10 cm; 10–20 cm; 20–30 cm; >30 cm) and two depth layers (surface, deep). We computed the relative biomass by prey size and depth category from 0 to 12 nm around predator sightings to determine the predators' proximity to local prey biomass. Two cetaceans (common, bottlenose dolphins) and three seabirds (northern gannets, auks, northern fulmars) were studied. No association was found in fulmars, indicating they probably do not feed on considered fishes in the area. Gannets and auks were positively correlated with local prey biomass for sizes <20 cm at both depth layers. Significant negative relationships were found between common dolphins and prey size classes <20 cm at both depth layers, and between bottlenose dolphins and all size ranges at the deeper layer. Our results suggest that the fine-scale spatial overlap of predator and prey is influenced by their functional traits, and that prey exhibit predator avoidance behaviour in presence of swimming predators but not of flying ones. [ABSTRACT FROM AUTHOR]

Measuring fish target strength (TS) in the wild is challenging because: (i) TS varies versus physical (orientation relative to the incident sound wave, size, and depth) and physiological fish attributes (maturity and condition), (ii) the target species and its aforementioned attributes are difficult to assess in near real time, and (iii) in the case of packed fish schools, accepted echoes may originate from multiple unresolved targets. We propose a method for controlled TS measurements of densely packed small pelagic fish during daytime, based on the joint use of a Remotely Operated Towed Vehicle, “EROC”, with a pelagic trawl fitted with a codend opening system, “ENROL”. EROC, equipped with a 70- kHz split-beam echosounder (Simrad EK60) and a low-light black and white camera, can be moved inside the fishing trawl. Pelagic fish are funnelled into the open trawl and their TS is measured in the middle of the net, where small groups actively swim towards the trawl mouth. The swimming behaviour allows for near-dorsal TS to be measured, minimizing the large effect of incidence angle on TS variability. The EROC camera, located near the open codend, provides optical identification of the species. This method was used to measure the TS of European Anchovy, Engraulis encrasicolus in the Bay of Biscay during 2014. The mean, near dorsal TS was -43.3 dB, for a mean fork length of 12.5 cm. This value is compared to published values of clupeiforms mean TS obtained for a range of natural incidence angles and discussed in the light of TS modelling results obtained for E. encrasicolus. [ABSTRACT FROM AUTHOR]

Petitgas, P., Doray, M., Massé, J., and Grellier, P. 2011. Spatially explicit estimation of fish length histograms, with application to anchovy habitats in the Bay of Biscay. – ICES Journal of Marine Science, 68: 2086–2095.Fish length frequency histograms from research surveys are of prime importance for identifying habitats of different life stages, as well as for stock assessment. However, no method has thus far been available for mapping these histograms as spatially varying curves. Here, a procedure is applied to map spatially connected curves, and detail is given on how it can be applied to map the length frequency histograms. At each sample location, a fish length frequency histogram is given as a vector of non-independent values. The histogram is first modelled as a polynomial expansion on the basis of orthogonal polynomials. Then, the polynomial coefficients are mapped by co-kriging, after fitting a model of co-regionalization. The length frequency map is finally derived by linearly combining maps of polynomial coefficients. An estimation variance associated with the map is also derived. Maps of anchovy length distributions are produced by applying the method to midwater trawl length data from the PELGAS acoustic surveys in the Bay of Biscay. This novel approach extends the application of kriging techniques to curves or functions, opening new perspectives for mapping more complex information than just the values of fish density. [ABSTRACT FROM PUBLISHER]

Mahévas, S., Trenkel, V. M., Doray, M., and Peyronnet, A. 2011. Hake catchability by the French trawler fleet in the Bay of Biscay: estimating technical and biological components. – ICES Journal of Marine Science, 68: 107–118. Several factors affect trawl catchability: (i) gear and vessel technical characteristics, (ii) anthropogenic factors, and (iii) biological factors. The objectives of this study were to assess the relative contribution of each factor to variations in hake (Merluccius merluccius) landings by the French trawler fleet operating on the shelf of the northern Bay of Biscay (ICES Subdivision VIIIa). Using generalized linear models, the impact of technical and anthropogenic factors was evaluated using landings per unit effort (lpue) obtained from logbooks. Variations in hake lpue were explained primarily by anthropogenic factors. For studying the biological components of catchability, the results of a scientific trawl survey in July 2006 involving three similar trawlers of the French trawler fleet were used. Daytime accessibility to large hake was lower than at night, and about zero for small hake (<19 cm). Estimates of spatial variation made using generalized linear mixed models showed a patchy fine-scale spatial distribution, but a random larger-scale distribution of hake over the area surveyed. [ABSTRACT FROM PUBLISHER]

Doray, M., Mahévas, S., and Trenkel, V. M. 2010. Estimating gear efficiency in a combined acoustic and trawl survey, with reference to the spatial distribution of demersal fish. – ICES Journal of Marine Science, 67: 668–676.Few analyses have been performed to estimate the efficiency of trawls targeting demersal fish using the ratio of catches and acoustic densities. In summer 2006, acoustic and fishing data were collected simultaneously over 3 d by three fishing vessels equipped with identical pelagic trawls in the Bay of Biscay. Variography identified moderate spatial autocorrelation in the acoustic backscatter at a mean scale of 3 km, a scale slightly smaller than the mean haul length (3.5 km), indicating that fish horizontal availability did not influence trawl efficiency. Acoustic backscattering densities expressed as nautical area scattering coefficients (NASCs) recorded in the trawled layer were compared with equivalent NASC (ENASC) values calculated from the species composition in the trawl, fish-length structure, and available relationships between target strength and fish length. Estimates of trawl efficiency for hake-dominated trawls were computed as the slopes of the relationships ENASC = 0.008 NASC and ENASC = 0.18 NASC0.31 for trawls made by day and night, respectively. For the whole demersal community, the relationships were ENASC = 0.022 NASC and ENASC = 0.17 NASC0.33 for trawls made by day and night, respectively. [ABSTRACT FROM PUBLISHER]

Doray, M., Petitgas, P., Nelson, L., Mahévas, S., Josse, E., and Reynal, L. 2009. The influence of the environment on the variability of monthly tuna biomass around a moored, fish-aggregating device. – ICES Journal of Marine Science, 66: 1410–1416.Fish-aggregating devices (FADs) are increasingly used worldwide to enhance tuna catches. Meanwhile, ecosystem-based management of this fishery is constrained by a paucity of information regarding the interaction of FAD-associated tuna aggregations with their local environment. This paper reports the results of a nine-month study around a FAD moored near Martinique Island, aimed at assessing the effects of the local environment on the variability of monthly estimates of proximate tuna biomass. Dual-frequency, active acoustics provided high-resolution quantitative data on the pelagic community around the FAD, from fish to micronekton forage. Geostatistics were used to compute biomass estimates of the tuna aggregation comprising most of the FAD-associated fish biomass, with a sampling error of 27%. Environmental variability was summarized by a small set of principal components (PCs) derived from profiles of temperature, salinity, and dissolved oxygen vs. depth; and maps of chlorophyll a derived from remotely sensed, sea surface colour. A generalized linear model was used to relate tuna biomass to environmental PCs and revealed a positive correlation between tuna abundance and: (i) a micronekton layer sensed at 38 kHz and potentially consisting of preferred prey at ∼180-m depth; and (ii) low subsurface salinity (60–80 m). These favourable environmental conditions may be related to the presence of North Brazilian Current eddies that migrating tuna follow when not temporally associated with the FADs. [ABSTRACT FROM PUBLISHER]