Effect of environmental conditions on the seasonal and inter‐annual variability of small pelagic fish abundance off North‐West Africa: The case of both Senegalese sardinella

Abstract

The objective of this study was to assess the effect of environmental variations on the abundance of Sardinella aurita and Sardinella maderensis in Senegalese waters in the upwelling system. Monthly data indicating the abundance of sardinella were first estimated from commercial statistics, using Generalized Linear Model from 1966 to 2011. Abundance indices (AIs) were then compared with environmental indices, at the local scale, a Coastal Upwelling Index (CUI) and a coastal Sea Surface Temperature (SST) index, and on a large scale, the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the Multivariate El Niño Southern Oscillation Index (MEI), using correlations and times series analyses. The results showed that the abundance of sardinella is determined by a strong seasonal pattern and inter‐annual fluctuations. The abundance of S. aurita peaked in spring and in autumn, whereas that of S. maderensis peaked in the warm season (July–September). The trend of the sardinella abundance was significantly correlated with the CUI, especially in autumn and spring. Interannual fluctuations of S. maderensis and S. aurita abundance are, respectively, driven by the precocity and the duration of the upwelling season that is attributed to distinct migration patterns. Both sardinella species also respond with a delay of around 4 years to the winter NAO index and the autumn CUI, and the AMO index, respectively, both related to migration patterns. The wide variations in sardinella biomass are caused by variations in environmental conditions, which should be considered in the implementation of an ecosystem‐based approach in sardinella stocks management.

Contrasted optimal environmental windows for both sardinella species in Senegalese waters

“…These results seem to suggest that at this stage, sardinella prefer oceanographic conditions in the summer. These results are in accordance with previous studies which suggest that variability in abundance and distribution of sardinella is mainly under the influence of oceanographic changes, such as cooling or warming waters (Bacha et al., ; Sabatés et al., ; Thiaw et al., ; Zeeberg et al., ).…”

“…This study is consistent with the findings of previous work which showed that the dynamics of small pelagic are closely linked to oceanographic conditions (Bacha et al., ; Bakun, ; Cury & Roy, ; Diankha, Thiaw, et al., ; Diankha, Wade, et al., ; Fréon, ; Fréon & Mendoza, ; Thiaw et al., ). The use of GAM, which takes into account the potentially complex non‐linear relationships between the covariates, allowed a better illustration of the links between S. aurita, as well as S. maderensis and the oceanographic conditions in off Senegal.…”

“…Map of the study area situated on the Senegalese coast showing the main sardinella landing ports. The landing ports are grouped by sub‐region: the northern coast (e.g., Saint‐Louis, and Cayar), the Dakar region (e.g., Yoff, Ouakam, Soumbédioune and Hann) and the south of the country (Mbour and Joal; reproduced from Thiaw et al., )…”

“…Monthly data concerning five environmental variables often applied to investigate relationships between fish abundance and environmental conditions (Bacha et al., ; Cury & Roy, ; Klemas, ; Thiaw et al., ) were used in our study: sea surface temperature (SST, in °C), chlorophyll‐ a concentration (Chl‐ a, in mg/m 3 ), wind‐induced turbulence (WTI, in m 3 /s 3 ) and a Coastal Upwelling Index (CUI, in m 3 /s 3 per meter of coast) deduced from wind data and North Atlantic Oscillation index (NAO). Except NAO, all variables were averaged from the coast to the 200 m isobaths, i.e., over almost all the Senegalese sardinella habitat, the continental shelf from 12 to 17°N (Figure ).…”

“…New investigations carried out by Thiaw et al. () showed that the variations in sardinella biomass in Senegalese waters were driven by environmental conditions. Similar studies on the second sardinella, i.e., S. maderensis, are scarce.…”