Estimating relative abundance of kawakawa ( Euthynnus affinis Cantor, 1849) landed in eastern indian ocean using catch and effort data

. Kawakawa ( Euthynnus affinis ) is a neritic tuna commodity with high economic value, especially for coastal states such as Indonesia. However, there is a lack of information about kawakawa abundance in the Eastern Indian Ocean. This research aims to estimate the relative abundance of kawakawa using catch and effort data, which is often used as a method in fisheries stock assessments. The size distribution of kawakawa was also analyzed in this study. The data was collected through daily monitoring from 16 landing sites along the Fisheries Management Area (FMA) 572 to 573 from 2015 to 2021. The data recorded included catch, day of fishing, fishing gear, and biological data (length and weight). The annual catch per unit of effort (CPUE) analysis of kawakawa reached its maximum in 2018, while the minimum value occurred in 2015. Furthermore, the monthly CPUE demonstrated that the highest catch occurred during the observation years in February and March. The highest catch proportion of kawakawa reached 87.21% in 2017. Finally, the gross tonnage (GT) of the gear showed the most significant factor in the catch.

Fish abundance in the water can be estimated using nominal catch per unit effort (CPUE), and utilization is estimated using the distribution of total catch and fishing effort.The fill index, frequently used to assess the absolute and relative health of fish populations, is one of the fundamental indices used in fisheries biology to describe the pattern of fish growth status factors [7].
Kawakawa is one of the neritic tuna commodities with high economic value, especially for coastal states such as Indonesia [8].The implementation of stock assessment in all Regional Fisheries Management Organizations (RFMOs) has made significant progress not only for the high economic value targeted species but also for non-targeted species (e.g., neritic tuna) [9].The recommendation has been made to improve data-limited stock by increasing the data collection, including catch, effort, and life history data [9].However, time series data on the catch and effort, as well as the biology of kawakawa, were lacking, especially in Indonesia.This research objective is to estimate the relative abundance of kawakawa using catch and effort data, which is often used as a method in fisheries stock assessments.

Sample collection
Enumerators collected 23,844 individuals from the 16 landing sites in FMA 572 and 573 (Figure .1).Kawakawa (Figure .2) was measured for its fork length to the nearest 0.5 cm FL using the ruler or mini caliper.The data was gathered from all fishing gears, including gill net, purse seine, troll line, and hand line.The parameters recorded include catch, day of fishing, fishing gear, and biological data (length and weight) from the daily monitoring activities.The enumerators also interview the fishers, vessel owners, fishers' cooperation staff, and fishing port officers.

Data analysis
The data were analyzed using both Microsoft Excel and R software.The daily length data were aggregated monthly and yearly.Catch per unit effort (CPUE) calculation was based on the formula [10] as follows: Where, C = catch (kg) E = effort (days at sea) Further analysis was done using the General Linear Model (GLM) with four parameters, i.e., year, quarter, month, and Gross Tonnage (GT).In this study, the GLM was applied based on the previous study for CPUE standardization using observer data in the Indian Ocean [11].

Results and discussion
The results show that the annual nominal CPUE for kawakawa from all gears during the observation years 2015-2021 reached the highest in 2018 and 2019, while the lowest nominal CPUE occurred in 2015 (Fig. 3).On the other hand, fishing activities happened every month starting from below 500 Kg in January then continue to reach the peak in February and March (Fig. 4).The catch decreased gradually from April to June and continued increasing slightly in July.It declined by half in August and September, then rose again in October.It then continued falling a bit in November and December.Catch per unit effort (CPUE) generated from catch rate values can be used to explain the abundance of fish [12].The annual nominal CPUE for kawakawa in the present study describes the fluctuation of the catch and effort itself.Temporal trends of CPUE are influenced by environmental conditions, fishing practices, government policies, and social [11].The unchanging in the CPUE describes sustainable harvesting.Meanwhile, overexploitation happens when there is a decreasing CPUE [13].Catch proportion for kawakawa from the two species, i.e., frigate and bullet tuna, is essential in fisheries science.These two species are at the same school with kawakawa in the water [14].Thus, these three species were mainly caught together when the fishers deployed the fishing gears.
Table 1 provides the number of samples analyzed during the observation years 2015-2021 with the mean length of kawakawa per year.The highest sample number was measured in 2016, with 7,913 fish.On the contrary, the smallest sample number occurred in 2018, with only 394 fish.The mean length of kawakawa was between 30.47 cm FL and 43.75 cm FL.The size frequency distribution analysis of kawakawa shows that the catch was generally above the length at first maturity (Fig 6).In this study, we used the IOTC benchmark for Lm with 38 cm FL as the smallest length at the first maturity of kawakawa found in the Indian Ocean [8].The time series biological data of certain species is critical to obtaining the goal of fisheries science, specifically to manage the fish stock [15].One of the key parameters is fish length, which is applied to determine the maturity and fish age [1,16,17].In this study, the size distribution of kawakawa ranged from 16 to 77 cm FL.The result aligned with the previous studies in the Western Sumatera, Java Sea, Persian Gulf, Sea of Oman, and eastern Mediterranean coast [6,14,16,18].Most of the lengths of kawakawa landed in the Eastern Indian Ocean in the present study were above the length at first maturity (>38 cm FL).The previous study off the coast of India found that kawakawa maturity was at size 49 cm FL for females and 49.7 cm FL for males using ova diameter measurement through microscopic examinations [19].This finding was still in the range of the IOTC research on length at first maturity of kawakawa at 38 to 55 cm FL [8].Fish size distribution in the waters is influenced by the overexploitation of fish, fishing gear, and environmental conditions [20].
Initially, the simple GLM analysis was applied in this study to test which parameters highly influenced the nominal catch of kawakawa.The result shows the Gross Tonnage (GT) of the gear and month showed the most significant factor to the catch with P = 0.000112 and P = 0.003250, respectively (Table 2).In the past ecological science, environmental variables were widely used to predict the model based on the linear relationship [13].Unfortunately, ecological parameters often create data errors, resulting in a normal distribution [12,13].In the development of ecological science, the normal assumption was substituted by the generalized linear models (GLM) and generalized additive models (GAM) [21][22][23].In the present study, the model applied was the simple GLM with the parameters, i.e., year, quarter, month, and gross tonnage (GT) as the characteristic of the confounding factors.The vessel power and month are likely the most substantial factor in the nominal catch of kawakawa in the eastern Indian Ocean.

Conclusion
In general, the population of kawakawa in the eastern Indian Ocean is considered sustainable, with most of the size distribution being above the length at first maturity.Furthermore, the nominal CPUE also showed an increasing value.The parameters influenced mainly by the nominal CPUE were gross tonnage (GT) and month.In contrast, the year and quarter were less significant.Further research and development are highly recommended to maintain the sustainability of kawakawa fisheries, especially in the eastern Indian Ocean.

Acknowledgment
The data courtesy of the Research Institute for Tuna Fisheries year 2015-2021.The authors would like to thank the researchers and enumerators of the RITF who were involved in the data collection and analysis in this paper.We are also grateful to the group of R-software experts who assisted in the nominal CPUE analysis.

Fig. 4 .Fig. 5 .
Fig. 4. Monthly catch of kawakawa during the observation years 2015-2021.The proportion of kawakawa catch among bullet tuna and frigate tuna during the observation years 2015-2021 is presented in Fig. 5.The highest catch proportion of kawakawa reached 87.21% in 2017 and the lowest in 2018 with 0.69%.

Fig. 6 .
Fig. 6.Size distribution of kawakawa during the observation years 2015-2021.The red line describes the length at first maturity.

Table 2 .
The parameters for calculating kawakawa nominal CPUE using GLM.