# A Method to Accurately Estimate Fish Abundance in Offshore Cages
## 4 Experimental Results and Discussion  
The echogram of the representative long-term experiment of 92 h is shown in Figure 3. In the figure, the region above the black horizontal line is from the cage, while the bottom half is from the sea bed. The color map is displayed at the right of the figure. The red pixel represents a strong signal from a target such as fish or the sea bed. For the ease of referencing, the first four cycles of days and nights are labeled in the figure. The red sinusoidal graph in the figure represents the change in water level over time. The maximum and the minimum water level measured were 10.6 and 7.5 m, respectively. One interesting pattern that can be observed in the echogram is that the fish tend to stay at the bottom of the net during the day and start to move upward in the evening. Similarly, after sunrise, the fish again move to the bottom. The echogram suggests minimum fish activity during the day, evidenced by the cluster of red pixels at the bottom and mostly blue and white pixels above 3 m. However, at night, the entire cage is full of red pixels, representing high fish activity. Similar mobility patterns for rockfish and other fish species have been reported [35; 36; 37; 38]. The results of fish count performed on the above long-term experiment are demonstrated in Figures 4 and 5. Since there is a clear difference in the echogram during days and at nights, the fish counts are separately shown. The data from 6 am to 6 pm are considered as the day time and the rest as the night. An echogram and its corresponding fish count result are stacked on top of each other, and their x-axes are aligned to time. The window size of 30 min (30 pings) was used to calculate the moving average of the fish count and is shown in the blue graphs. Similarly, the true fish count is shown by the dashed red lines. Figure 3: Echogram for the duration of 92 h performed in Cage1. Figure 4: Echogram and fish estimation during day time from the 92 h experiment performed in Cage1. Figure 4 demonstrates echograms and fish counts for day times. As can be seen from the figures, the fish count varies sharply over time. The volatility of data is related to the spatiotemporal variability of the distribution of fish in the cage at different times of the day. However, some pattern in fish count was observed related to the level of illumination and tidal phenomena. The fish count results illustrate that during slack tides (at high or low tides), which correspond to peaks and valleys of the sea bed in the echograms, the estimation is significantly high, whereas at other times the count is significantly higher. Figure 5: Echogram and fish estimation at night time from the 92 h experiment performed in Cage1. low. The reason that fish count sharply decreases during high water current is because the fish move away from the transducer beam region toward the side of the cage wall blown out by the current, which is evidenced by the echogram, as the thickness of the red pixels in the cage is notably thin compared to slack tides. Since the center of fish concentration is shifted from the center of the cage to sides, the estimation is low. Similarly, the reason for relatively high estimation of fish count during slack tides is because during these periods the current inside the cage is low and fish tend to move toward the center of the cage, forming a school, as reported in [35; 36; 38]. Since all or part of the school is under the transducer beam, the estimation is high. On the other hand, the fish counts at nights are different from those during the day. As can be seen in Figure 5, at night, the estimated fish count tends to stay near to the true value and the fluctuation was significantly lower compared to that in day time. Even though the fish count gradually decreases as we move from night 1 to night 4, the estimation was consistent for a particular night. We note that a similar pattern was observed in other long term experiments also.