# Automated Monitoring of Bluefin Tuna Growth in Cages Using a Cohort-Based Approach
## 3 Results
### Straight Fork Length (SFL) Evolution  
Exploring the evolution of tuna lengths over an extended period is an important step in understanding their growth patterns and even assessing their health status. In our initial approach to analyse this length evolution, we estimated the temporal evolution of the mean length within the caged population. Figure 4 illustrates this evolution using a boxplot of SFL measurements. In each box, the lower side of the central rectangle represents the 25 th percentile, whereas the upper side represents the 75 th percentile. The red segment inside the rectangle indicates the median. Notably, the evolution of the median SFL appears inconsistent, showing no discernible progression over time. Subsequently, a second approach was applied with the aim of achieving more accurate results regarding the growth rates of different cohorts within the caged population. This approach relied on the use of [PERSON]'s method described in Section 2.3, applied to the SFL measurements. The outcomes of the modal progression analysis, derived from the application of [PERSON]'s method, are presented in Table 1 in the Appendix A. It is noteworthy that the percentage of measurements and the number of individuals for certain cohorts, specifically the first and last ones (comprising smaller and larger specimens), are relatively low. This occurrence is likely due to the scarcity of specimens with those particular sizes in the monitored cage. Although these fish belong to cohorts distinct from \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline Year 2020 & 28–29 July & 8–11 August & 23–26 & 10–14 & 31 October– & 15–19 & 6–8 & 15–18 \\  & & September & October & 3 November & November & December & December \\ \hline Year 2021 & 21–22 January & 15 February & 24–26 March & 5-7 May & 21–23 May & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Recordings with the automatic stereoscopic system grouped into consecutive-day periods. the well-represented ones, the limited number of individuals precludes fitting the data into a modal group. In other instances, defining a modal group with a small number of specimens is feasible, but the mean lengths of such cohorts may not be accurately represented. Therefore, despite their inclusion in the dataset, these cohorts were not considered for modal progression analyses. The average SFL for each period and its temporal evolution can be observed in Figure 5 and Table A2 in Appendix A (excluding cohorts with few specimens). The results indicate that the length growth from July 2020 to May 2021 ranged approximately between 11 and 26 cm, depending on the initial fish length. The decreasing separation between cohorts aligns with expectations, as annual growth rates typically decrease with age. It is also expected that after one year, the growth of each cohort will either be similar to the distance between consecutive cohorts of the same size range in the initial length distribution or even higher, because of the special conditions in a cage compared to the wild. Our results show that by May 2021, the modal lengths reached or slightly exceeded the length of the next cohorts in July 2020. This suggests that, after one year, each modal SFL surpassed the SFL of the next cohort, indicating accelerated growth in cages, as hypothesised. Variations in the growth tendency in January, February, and early May 2021 can be attributed to the impact of water turbidity on the distance at which fish could be measured. The mean distance (Z) decreases in those samples from 8 to 6 m. Additionally, note that Z is higher in the initial months due to the use of 12 mm focal length lenses, as described in Section 2.1. Figure 4: Boxplot of the evolution of straight fork length (SFL) measurements. NM: Number of measurements. Figure 5: Identification of cohorts and evolution of average straight fork length (SFL) resulting from [PERSON]’s method. Tendencies are represented by dashed lines. Each line represents a cohort from cohorts 2 to 10. Cohorts 1 and 11 with few specimens have been omitted.