# A Method to Accurately Estimate Fish Abundance in Offshore Cages
## 1 Introduction  
In fish farming, knowing the exact amount of fish in a cage is of paramount importance, as it helps to provide the best conditions for the fish under cultivation, thereby guaranteeing the health and growth of the species [1; 2; 3; 4]. As the fish food and other resources are expensive, the knowledge of the fish individuals is necessary for the optimum utilization of these [3; 4]. However, there is no easy and reliable method to keep track of the exact number of fish in a cage, and hence the farmer has to use an intelligent guess out of experience or do manual counting for fish management. Furthermore, no practical solution is available that can be used by third-party companies, such as insurance companies, to verify the fish count claimed by farmers [5]. Therefore, a portable device or a method that gives an accurate estimation of fish count in a cage is highly desirable.  
Over the years, various techniques and methods have been widely used and proposed for the fish counting problem. For instance, the acoustics echo-sounding method, the machine vision-based method, the environmental DNA-based method, and the resistivity counter-based method are some popular methods that have been used in fish counting [6]. Among these, the acoustic echo-sounding method is one of the most popular methods to estimate fish abundance because of simplicity and noninvasive nature [7]. In this technique, echosounder sends an acoustic ping and the reflected echoes from fish are analyzed to estimate fish abundance. The key idea relies on the detection of reflected energy produced by the target as the amount of energy is proportional to the number of targets. The scientific use of acoustics for counting fish has been widespread over the last three decades [8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Modern echosounder and echo processing software implement the echo-integration technique to study fish and their behavior [12; 13; 14; 15; 16; 17]. Even though echo-integration has been widely used in the open sea,when used in a cage, it offers a unique set of challenges [18; 19; 20; 21]. For instance, the reverberation in the cage due to the echoes of the acoustical signal from the boundaries needs to be taken into account when performing echo integration [19; 22]. Therefore, some techniques should be used to remove the cage-boundary signal from the received signal, as discussed in [5]. Another issue that must be taken into account is the behavior of fish in a cage [23]. The accuracy of echo-integration is dependent on the transducer's ability to capture the fish distribution in a cage. Thus, the knowledge of how fish behave and move in a cage at a particular time is salient for accurate estimation. Another problem in fish-abundance estimation by acoustics is the shadowing effect, which can seriously affect the fish count results when dense aggregations of fish are encountered. Compensation for the attenuation of the echo strength due to the shadowing effect is necessary [24]. However, in the case of low fish density and random movement, the shadowing effect is minimum [24].  
The main objective of this research is to investigate the possibility of using a commercial echosounder for fish counting in real time. The aim is to develop a practical, portable, and reliable system to estimate fish numbers in offshore farming cages. For that, firstly, we studied the behavior of fish in a net cage by monitoring continuously for two months using an echosounder and an underwater camera. Based on the observation and analysis, an algorithm and the manual for experimentation were developed. The experimental results verify that when fish counting is performed under the defined guidelines, the estimation accuracy of the proposed algorithm is high. The finding of this research could be beneficial to the researchers and the professionals working in fish management, especially in offshore fish farming.