Indicator variables for a robust estimation of an acoustic index of abundance

Abstract

In North West Africa, pelagic fisheries are an essential economic sector. However, the scientific community fails to perform satisfactory assessments of key pelagic species like sardinella, owing to a lack of relevant indices of abundance to tune the model. This paper provides an alternative acoustic index based on a semiquantitative modelisation of acoustic densities. Acoustic energy is split into binomial variables coding for null, low, medium, large, and very large densities. A multivariate geostatistical approach allows (i) mapping the spatial distribution of classes of densities and (ii) computing a new acoustic index of abundance for Sardinella aurita and Sardinella maderensis. We used the surveys of RV Fridtjof Nansen (1995–2006) and RV Al-Awam (2007–2010). Our results indicated that empirical spatial structures were highly stable over time for both between areas and surveys. Co-kriging maps also showed that sardinella had stable hot spots of distribution. The indices of abundance developed in the present study were tested in an assessment procedure and outperformed all the indices used routinely by the FAO–CECAF (Fishery Committee for the Eastern Central Atlantic) assessment working group.

Environmental effects on the spatio‐temporal patterns of abundance and distribution of Sardina pilchardus and sardinella off the Mauritanian coast (North‐West Africa)

“…This is because the two species are seasonally migrating and characterized by large‐scale shifts ihn their distributional boundaries along the NW African coast (Binet et al ., ; Kifani, ). Studies conducted off NW Africa highlighted the seasonal hydrological variability as a key factor that drives and structures small pelagic fish distribution and seasonal migrations (Boëly, ; Zeeberg et al ., ; Arístegui et al ., ; Bez et al ., ). Both sardine and sardinella showed clear and opposite seasonal variation in abundances.…”

“…The annual migration pattern of sardinella up and down the northwestern African coast between 12°N (Senegal) and 22°N (Western Sahara) was shown to be associated with spawning activity (Ter Hofstede et al ., ). Acoustic surveys conducted during the last decade have confirmed the high abundance of sardinella in Mauritanian waters until December (Bez et al ., ). During winter, catches of sardinella in Mauritania waters decline, presumably because the fish migrate southward towards Senegal where the water temperature remains above 21°C and hence attempt to remain within a range of preferred temperature (Ter Hofstede et al ., ; Zeeberg et al ., ).…”

El Niño as a predictor of round sardinella distribution along the northwest African coast

“…There are also intrinsic limitations in round sardinella catch data associated with the difficulty of distinguishing environmental influences (i.e., not directly related to changes in human activities) and human-related influences. The second includes variability of the market demand, changes in fishing gear and fleet composition, and improvements in fishermen technical skill over time (see e.g., discussion in Bez and Braham, 2014). The recent and interesting observational analyses of the CCUS (see e.g., Braham et al, 2014) do not clarify the multiple environmental factors impacting round sardinella spatio-temporal variability.…”

Comprehensive Spatial Distribution of Tropical Fish Assemblages from Multifrequency Acoustics and Video Fulfils the Island Mass Effect Framework

“…The availability of target strength measurement for this species 86 opened the eld for estimating its biomass. To account for the strong dissymmetry of s A histograms, we used a non-linear geostatistical approach [97][98][99] . Observations were reduced to 5 classes of acoustic biomass, i.e.…”