Résumé: The avoidance reaction by fish in front of an approaching vessel is a major source of bias in direct biomass assessment and ecological studies based on fisheries acoustics data. An experiment was carried out to compare echosounder data obtained using a small speedboat and a research fisheries vessel generating significant higher noise above conventional reduced-noise standard. The results show that there was no significant difference between the individual fish target strength distributions, and the numbers of schools recorded by both boats, these schools having similar areas and perimeters. However, the schools detected by the noisier vessel were significantly deeper, and unexpectedly had a significantly higher energy level. These findings suggest that noise-reduced vessels trigger a different vessel avoidance reaction. The noise-reduction standard is not sufficient to reduce avoidance behavior. It is also to take into consideration the ambient noise, which could impair perception of the platform by the fish, and the probability that the acoustic stimuli could be less important than visual perception under some local conditions. The paper introduces the concept of partial avoidance and presents a conceptual diagram of the strength of the avoidance reaction. Last, it is not recommended, because of noise reasons, that vessels routinely used for pelagic stock assessment surveys be changed. Indeed standardized time series, which could be disrupted when switching to a new vessel, are more important than the hypothetical gain from change to quieter vessels. Obviously, all long-term surveys must change vessels; best practice will be to estimate the vessel effect before any change to avoid disrupting the time series and/or perform vessel intercalibration surveys.

Résumé: Ensuring the sustainability of fish resources necessitates understanding their interaction with coastal habitats, which is becoming ever more challenging in the context of ever increasing anthropogenic pressures. The ability of coastal lagoons, exposed to major sources of disturbance, to provide resources and suitable habitats for growth and survival of juvenile fish is especially important. We analysed three lagoons with different ecological statuses and habitat quality on the basis of their eutrophication and ecotoxicity (Trix test) levels. Fish abundances were sampled using fishing and horizontal beaming acoustic surveys with the same protocols in the same year. The relative abundance of Anguilla anguilla, Dicentrarchus labrax or the Mugilidae group was not an indicator of habitat quality, whereas Atherina boyeri and Sparus aurata appeared to be more sensitive to habitat quality. Fish abundance was higher in the two lagoons with high eutrophication and ecotoxicity levels than in the less impacted lagoon, while fish sizes were significantly higher in the two most severely impacted lagoons. This leads us to suggest low habitat quality may increase fish growth rate (by the mean of a cascading effect), but may reduce lagoon juvenile abundance by increasing larval mortality. Such a hypothesis needs to be further validated using greater investigations which take into account more influences on fish growth and recruitment in such variable environments under complex multi-stressor conditions. (C) 2013 Elsevier Ltd. All rights reserved.

Résumé: Converging results in different scientific fields (behavioural ecology, fisheries biology, acoustic tagging, fisheries acoustics, behavioural modelling) suggest the existence of “micro-groups” inside fish schools. These would comprise a few (5-10) fish maintaining contact during a period long enough to allow individuals to recognise each other. It is hypothesised that they would prefer to share the space with familiar rather than anonymous conspecifics. To evaluate whether acoustic methods could be used to recognise “micro-structures” inside fish schools and help test the “micro-group” hypothesis we analysed acoustic data from anchovy schools off Peru, and gadoids in the North Sea. Data collection used a multibeam sonar (Reson SeaBat 6012). In the Peruvian case study, the sonar was mounted set horizontally on a drifting research vessel and the internal structure of the schools of anchovies was analysed, although individual fish could not be discriminated. In the North Sea case study, the sonar was orientated vertically above a demersal trawl to allow observation of individual fish entering the trawl. Geostatistical analyses were used to evaluate the existence of small spatial structures in anchovy schools. In these schools, “micro-structures” with a scale as small as 0.5 m were observed acoustically. For the gadoids nearest neighbour distance (NDD) measurements were carried out, suggesting that the fish aggregated in small groups (2 to 25 individuals, with an average of 3.7 fish per group) in the trawl catches. The perspectives and limitations of these results are discussed.