Variability in distribution and aggregation behavior of the Peruvian anchovy (Engraulis ringens) analyzed using a fifteen year long series of acoustic surveys (2000–2014)

The Peruvian "anchovy" (Engraulis ringens) maintains the world's largest single species fishery in the world. Acoustic data from research cruises surveying the distribution area of this species provide meaningful information about its behavior and distribution as well as its abundance within years and decades. Acoustic information gathered during 15 years (15 summer and 12 winter "Pelagic Hydroacoustic Evaluation Cruises") along the Peruvian coast from 2000 to 2014, revealed characteristic patterns within the aggregation behavior and distribution of anchovy. Measuring pelagic fish species distribution and abundance for stock assessment were the main objectives of those cruises conducted by the Instituto del Mar del Peru. Acoustic surveys were performed using scientific echosounders Simrad: EK-500 (2000–2007) and EK60 (2008–2014). Frequencies 38, 120 and 200 kHz were deployed but the operative frequency was 120 kHz for analysis and biomass estimation. Echoview was used as acoustic data post-processing program. 27-cruise database was used to calculate statistical indicators such as gravity center, variogram, Lorenz diagram and Gini Index, all associated with anchovy abundance and distribution. Geostatistical analysis were used to determine the gravity center and variogram. Variograms analysis were performed on the EVA (VAriance Estimation) program with spherical model and the fit semivariogram models by the method of least squares. Our main results show that anchovy distribution, as observed in these analyses, was highly variable, with characteristic seasonal behavior (summer and winter), and associated to distinct environmental conditions. Anchovy distributions from 2000 to 2014 have a gravity center between latitudes 07 and 13°S, along the north-central region of the Peruvian coast, area of greatest abundance. In summer, the gravity center fluctuated off Huarmey (10°S) while in winter-spring it showed some southward trend in recent years. El Nino and La Nina phenomena observed during this period did not show any influence on the anchovy gravity center. In summer larger structures on the variograms (variances, variability and clustering of schools of fish) were found with respect to winter. This is likely due to the high concentration of schools and high integration values. In winter with the increased presence of cold water, the favorable habitat of anchovy is wider favoring major school dispersion, lower integration values and lower variability. However, the data structure varies noticeably within the year, each cruise having a particular pattern. Atypical distribution was recorded in winter of 2013, with high abundances and existence of compact well identified schools. The variogram parameters lead to consider that anchovy distribution is structured and spatially dependent, which indicates that anchovy spatial distribution is a continuous process with auto-correlated characteristics. The degree of concentration of the distribution data were evidenced by the Lorenz diagram.