Current literature considers that Nasolamia velox has a limited distribution along the coastline of the Eastern Pacific with sporadic sightings in the Galapagos Archipelago. This study provides evidence of the occurrence of this species at the Revillagigedo Archipelago (18°99′186″N 112°08′44″W), Mexico, using acoustic telemetry and videos taken from 2014 to 2016. We report here movements from a coastal location (National Park Cabo Pulmo) to a group of oceanic islands (Revillagigedo Archipelago) by one single individual, supporting the idea of the potential connectivity of sharks between the Gulf of California and the Revillagigedo Archipelago. This report extends the known distribution of N. velox to 400 km off the mainland coast of the Americas, thereby increasing the knowledge of the distribution of a species commonly reported in fishery landings of the Eastern Pacific.

Behavioural rhythms in marine species have been mostly investigated in laboratory organisms and their expression within the animals' natural environments remains largely unknown. Here, we studied diel (i.e. 24-hours-based) and intra-diel (i.e. 12-hours-based) rhythmic variations in the abundance of seven shallow rocky-reef fish species, namely Coris julis, Epinephelus marginatus, Sarpa salpa, Serranus cabrilla, Serranus scriba, Sparisoma cretense and Thalassoma pavo, along the rocky shores of Linosa Island (Mediterranean Sea). Data were visually collected by trained volunteers along fixed transects at 3-hourly intervals throughout six consecutive 24-hours periods. Density estimates can vary greatly between consecutive days and during 24-hours periods according not only to the major day–night changeover but also to minor intra-diel variations at the daylight hours. In the case of T. pavo, C. julis, S. cabrilla and S. salpa waveform analyses showed midday troughs in abundance within the 24-hours period but significant variation within the hours of daylight was highlighted only for T. pavo. Although results were not conclusive at the intra-dial level, the employment of volunteers represented a valuable tool for chronobiology, suitable to improve our understanding of fish behaviour in natural systems.

Fish assemblages associated with shallow rocky bottoms along the coast of Cape Milazzo were investigated within three selected habitats, each represented by a different degree of complexity and namely vermetid reef (VR), rocky-algal reef (RR) and boulder and pebbles (BP). The area, located along the northern coast of Sicily (southern Tyrrhenian Sea) was studied by using a non-destructive diver visual census methodology evaluating species composition and abundance. A total of 39 fish taxa belonging to 15 families were recorded over the three habitats investigated. Multivariate and univariate approaches detected significant differences among the three habitats. These differences were either in fish assemblage structure, in the assemblage parameters or in the abundances of some common species. A multidimensional scaling performed on the entire fish density data set showed distinct groupings for the three habitats. The highest values of mean species richness (S), diversity (H′), evenness (J) and density (N) occurred over the more complex habitat, represented by VR, than over rocky-algal reef habitat or BP. Different habitat preferences were evidenced for several fish species. Diplodus sargus, Serranus scriba, Tripterygion tripteronotus, T. delaisi and T. melanurus were significantly more abundant over VR whereas Thalassoma pavo and Symphodus tinca were more abundant both over VR and BP habitats. Statistical comparison did not detect habitat preferences in Symphodus roissali, Oblada melanura, Diplodus vulgaris and Sarpa salpa. As far as the size is concerned, small, medium and large-sized specimens of D. sargus and only large-sized individuals of D. vulgaris were significantly more abundant over the VR habitat side than over the other two. These results suggest that differences in assemblage parameters are related to habitat structure. Greater habitat complexity means greater surface available, thus providing additional resources for adults and juveniles of many species: food, cavities, caves and then new niches, refuges from predation, from artisan fishery nets, as well as resting or mating sites, boosting the total number of individuals and species. The results of this study highlight the importance of VR in sustaining the biodiversity of fish assemblages and emphasize the need for their careful management in an area already proposed as a Site of Community Interest.

We adapted a visual census method, mainly used in demersal and reef fish studies, to characterize fish communities associated to drifting fish aggregating devices (FADs) in the Western Indian Ocean. Drifting FAD associated fishes from both equatorial (Seychelles) and tropical waters (Reunion Island) were examined by divers. A total of 32 species (belonging to 16 families) were observed associated with drifting FADs in equatorial waters, and 24 species (14 families) were found around FADs in tropical waters. Twenty species were found in both regions. The highest number of species observed at a single FAD was 18 (12 ± 2, mean±SD) in equatorial and 13 (10 ± 3) in tropical waters, not counting circumnatant species loosely associated with the FAD. Some species like Kyphosus vaigiensis, Canthidermis maculata, Elagatis bipinnulata, Acanthocybium solandri and Coryphaena hippurus were observed on all or most of the surveys. In this study, the contribution in biomass of the 18 common species associated with drifting FADs (but excluding circumnatant species), represents more than 98% of the biomass. The overall biomass values of closely associated species remains well below tuna biomass estimates for circumnatant tuna schools at FADs, estimated as high as 200 tons. The species that most significantly contribute to the by-catch in tuna purse-seines logically match those that showing the highest biomass values in our surveys (Carcharhinus spp., Elagatis bipinnulata, Coryphaena hippurus, Canthidermis maculata, and Acanthocybium solandri). One of the most abundant and ubiquitous species in our study was the spotted oceanic triggerfish Canthidermis maculata that sometimes formed massive schools of many thousands individuals around the drifting FADs. Future research is needed to explore the role of such non tuna species in the attraction and aggregation processes of tuna around drifting FADs.

Patterns of spatial and temporal variability of coastal fish assemblages were compared between areas subjected to different levels of protection at two different depths using a visual census technique. A no take reserve was compared with recreational fishing areas around a small Mediterranean island at 5 m and 20 m depth. Univariate and multivariate statistical techniques were used to test the hypothesis that protection could have different effects at different depths. A total of 56 species were identified during the survey. The most represented families were Sparidae and Labridae. Results showed an interaction between depth and levels of protection that was not persistent across the study period. This result was consistent for the whole fish assemblage and for the target species; most species showed higher abundance in protected areas at 20 m depth.

Despite its wide use in terrestrial ecology, distance sampling is as yet rarely used in underwater visual censuses. The present study attempts to compare density estimators based on distance sampling and on strip transects. Three stations with increasing densities of Chaetodontidae and Pomacanthidae were sampled twice by two divers of unequal experience, using two different transect types. A total of 96 transects and 2970 records of Chaetodontidae and Pomacanthidae were analysed. Nine estimators based on distance sampling were calculated and only the best fit (DT estimator) was kept for comparison with other estimators. These were either based on the average distance of the fish to the transect (AD estimator), or a 3-m- or 5-m-wide strip transect estimator (FW3 and FW5, respectively). There were no significant differences between the means found by DT, AD and FW3. Lower density estimates were given by FW5 in all cases. FW3 and FW5 did not detect several significant differences between stations which were otherwise detected by DT or AD. The number of transects needed to detect a significant difference between stations was four to ten times higher with FW3 or FW5 than with DT or AD. Diver experience was found to be a significant factor in density estimates. However, this factor was less important than the choice of the density estimator. Transect type or the day of sampling had no consequence for the estimates. The distance distributions of fish were divided into three different patterns which may be explained by a combination of detectability function and a behavioural component.