In situ acoustic target strength of anchovy kilka (Clupeonella engrauliformis) in the Caspian Sea (Iran)

• The new TS model is suggested for anchovy kilka.
• About 83% of the observed variance in the target strength of anchovy kilka can be explained by changes in the percentage of maturing females.
• Fish depth was introduced into the relationships as the second regressor and the contraction rate of the backscattering cross section of the fish with depth was estimated to be – 0.604.

Understanding the factors that influence the target strength (TS) of fish is important for calculating accurate estimates of fish biomass and abundance using acoustic-based stock assessment techniques. The purpose of this study was to derive a robust TS model for anchovy kilka (Clupeonella engrauliformis), an important commercial-targeted fish species, based on its biological and morphological characteristics (length, weight, female maturity status, condition factor, depth of fish occupancy), and season. In situ measurements of TS were collected in the southern Caspian Sea during winter (February-March) and spring (May) 1997 using a Simrad EK500 echosounder operating a 38-kHz split-beam transducer. The results showed high variability in the mean TS estimates of anchovy kilka, despite a relatively small range in the sizes of fish encountered across the study site (range in mean fish lengths = 3.2 cm). A multiple linear regression analysis showed that the mean TS of anchovy kilka was significantly (P < 0.05) correlated with indices of female maturity status and depth of fish occupancy, but not size (P >   0.05), suggesting that biological characteristics other than size were important drivers of this variation. Our estimated TS model for anchovy kilka conformed to the equation: TS=20logL−0.096(%MF)−6.04log(Z10+1)−65.9 when the percentage of maturing females (MF, %) and the depth of fish occupancy (Z, m) were included in the analysis. These results help to improve the accuracy of acoustically-derived estimates of anchovy kilka abundance and biomass during routine stock assessments, which is important for robust resource management strategies in the Caspian Sea region.