Title: Re-analysis of Horse mackerel (Trachurus spp) abundance and geographical distribution in relation to environmental effects, using acoustic data from a scientific survey conducted by RV. Dr. Fridtjof Nansen in 2014.

Horse mackerel (Trachurus spp) is one of the major commercial fish species in Angola. Its abundance and distribution are not only affected by the fishery, other factors also have an effect. In this study geographical distribution, the effect of using a different target strength coefficient (72 dB versus 65.2 dB) on abundance estimates, and environmental effects on T. trecae, both on presence/absence and abundance, distribution were explored. Only one-year survey data was used; therefore, it should be taken into consideration when interpreting the results. T. trecae showed a wide geographical distribution surprisingly with more presences in higher salinity areas (35.7 – 36.0 ppt) and higher abundance over a narrow range of dissolved oxygen (2.0 – 4.0 mgL-1). There was no difference in geographical distribution between immature and mature individuals. Temperature is normally used to describe the distribution of pelagic species, surprisingly for this study, no significant influence on neither presence nor abundance of T. trecae was noticed. The use of a target strength coefficient value of 65.2 dB instead of the currently used value of 72 dB, resulted in a 79% decline in estimated biomass. This emphasizes the need of in-situ measurements of T. trecae target strength in Angolan waters.