Fishing Season of Large Tuna from Purse Seine Fishery in Tumumpa, Manado, North Sulawesi

Knowledge on seasonal fishing pattern is essential in fisheries research in order to increase the effectiveness of fishing and at the same time protecting the fishing ground from excessive fishing practices. Determining the seasonal fishing pattern of large pelagic fishes, namely large tuna (yellowfin and bigeye), small tuna (mackerel) and skipjack tuna were the objective of this study. Data from the 2013-2015 fishing operation time series were used to analyze the fishing season using the percentage average per unit effort (CPUE) method. The main focus of this study was purse seine fishery. The research showed that high fishing season of large pelagic fishes allegedly occurred during May to July, which reached its peak on May. On the other hand, low fishing season occurred during January to April, with the lowest on March. There were similarities between large tuna, small tuna and skipjack in term of their seasonal fishing pattern, although they did not show any good correlation (r<0.5). This might due to fact that even tough large tuna, small tuna and skipjack tuna shared the same ecological niche but apparently, they did not show any interaction (symbiosis) but competition.


Introduction
Fishing season is a consequence on how fishes respond to the environmental changes. It is marked with the higher abundance of fishes' biomass in certain period of time. Usually, this condition associated with the availability of food source, supported by good environmental parameters such as: temperature, chlorophyll-a and sea current (Safruddin et al., 2014). Estimating the fishing season is essential in fisheries research in order to increase the effectiveness of fishing and at the same time protecting the fishing ground from excessive fishing practices.
Previous researches on fishing season of tuna, especially in eastern region of Indonesia mainly focused on skipjack fisheries (Kekenusa, 2006;Kekenusa and Paendong, 2016). Hariati (2011) also gave a glimpse of the mini purse seine fisheries in Banda Sea but unfortunately the information only limited to certain group species of neritic tunas and small pelagic fishes. The main obstacle on analyzing the fishing season based on specific species is there was no clear separation on the catch composition, especially in purse seine fisheries where the production of juvenile tuna is usually lumped together in the logbook.
Determining the seasonal fishing pattern of large pelagic fishes, namely large tuna (yellowfin tuna/Thunnus albacares and bigeye tuna/Thunnus obesus), small tuna (mackerel/Auxis sp; Euthynnus sp.) and skipjack tuna (Katsuwonus pelamis) from purse seine fishery were the solely objective of this study. The result of the study expected to be considered as a basic information in term of making fisheries management policy in the future, especially in the eastern region of Indonesian seas.

Materials and Methods
Tumumpa Coastal Fishing Port (CFP) is located in the coastal area of Tumumpa village, district Tuminting, Manado, with specific geographical location between 01.523 o N and 124.841 o E (Figure 1.). It is only 50 km from the largest fishing port in Sulawesi Utara, namely Bitung Oceanic Fishing Port (PPS). Most of the fleet based in CFP. Tumumpa are purse seiners, ranged from 5-100 GT.

Source of data
Primary data were obtained from daily recording of landing activities based on CFP. Tumumpa. The dataset then combined with the The dataset consists of total catch by group species (kg), fleet definition (purse seine, longline, pole and line, etc), gear type, area of fishing, gross tonnage, and vessel information. The research focused on purse seiners, since it was the dominant fleet in the area (99.36%). Although there was a clear segregation between large pelagic and small pelagic purse seiners fleets mentioned in the SLO (warrant fishing operation) but, through preliminary analysis there was no significant catch composition between two fleets. It means both purse seine fisheries licenses were mostly used to catch large pelagic species. Therefore, both data were combined in the analysis.

Data analysis
Analysis on fishing season pattern of large pelagic fisheries was performed using average percentage method (Spiegel, 1961) with modification from Dajan (1998). The main output is Seasonal Fishing Indices (SFI) in which, hypothetically, high fishing season is when FS>100 (above average); low fishing season is when FS<100 (below average); and if FS=100 it means the season reach its equilibrium. Catch per unit of effort (CPUE) was defined as total catch by group species (kg) divided by total effort (days at sea). Days at sea is the time required for fishermen from the port to the fishing ground and back at the same day. Steps of analysis are presented below: Calculation of monthly CPUE ( ) and average of monthly CPUE in a year ( ̅ ).

Results and Discussion
A total of 7,843 landing data were managed to be extracted from SLO (warrant fishing operation) form during January 2013 to December 2015. Purse seiners dominated the fleet composition (99.36%), while the rest consist of light boat, hand line, pole and line (huhate) and mini pole and line (funae). Based on gross tonnage information, purse seine fleet dominated by size 20-30 GT (51%), and 30-50 GT (39%) ( Figure 2). More than 80% of purse seiners catch composition were dominated by large pelagic fishes (composition of combined skipjack, mackerel, yellowfin and bigeye tuna>50%) (Figure 3.). This result shown that regardless the purse seine fishing license, they all make large pelagic tunas as target.
The monthly average catch for large pelagic tuna group showed strong seasonal pattern, especially for skipjack and mackerel (Figure 4.). High catch of skipjack occurred between May and June and low catch between November and January. The highest catch recorded was in June, 2014 with slightly over than 1 million kg, on the other hand, the lowest catch recorded was in December, 2015 with total 78,000 kg. High catch of mackerel occurred between May to December, while low catch between January and February. Highest production of mackerel was recorded in December 2015, with 534,920 kg, on the other hand the lowest production was recorded in January, 2013 with 47,575 kg. Meanwhile, the catch of tuna relatively stable, with slight increase during June-July and slight decrease between November and January. Three seasonal fishing indices of large pelagic fishes models were successfully created based on moving average method. All three models showed similar trend, where the high fishing season started from May to June and the low fishing season occurred between January and April (Figure 5.).
Based on the licensed given, the purse seine fleets in CFP. Tumumpa, Manado were divided into 2 categories, the first is large pelagic purse seiners (PSPB) and the latter was small pelagic purse seiners (PSPK). But in practical, both licensed chased the same resource, which is large pelagic fisheries, although small portion of the PSPK fleets determined to abide the rules. Given the fact that PSPK fleets have smaller mesh size, it could badly affect the large pelagic resource, since smaller size of juvenile tunas, skipjack and mackerel are more likely to be caught during operation. It is also to be noticed that both PSPB and PSPK allegedly using the same mesh size configuration (1-4 inch) (Hariati, 2011;Kefi et al., 2013;Talakana et al., 2017). Using small mesh sizes was considered as IUU (Illegal, Unreported and Unregulated) fishing, aside from fishing without or fake license, transshipment, double flags and unreported fishing (non-reporting or misreporting, under-reporting of fish catch) (Palma and Tsamenyi, 2008;Edy et al., 2017).
IUU fishing is a serious trans-national problem worldwide, especially in Indonesia. It cost more than IDR 300 trillion/year (Jaelani and Basuki, 2014), as for in Sulawesi Sea such practice has cost a third of total annual value (Palma and Tsamenyi, 2008). It is apparent that most of the IUU fishing related with Filipino and Chinese fishermen. Palma and Tsamenyi (2008)  Skipjack tuna is the main target for purse seine fleets. It also become a target species for other gears, such as: pole and line and hand line (Widodo and Nugraha, 2009, Kefi et al., 2013, Simbolon et al., 2013 with by-product species such as mackerel and juvenile yellowfin and bigeye tuna (Harley et al., 2010;Hampton and Williams, 2011). Most of skipjack tuna caught during May and July, with peak season occurred on May, on the other hand, the low season occurred on January to April with lowest catch appeared on March. This result was supported by previous study from Safruddin et al. (2014) in Mamuju waters, where the most productive fishing season occurred during May, which the skipjack fishing season started in January August and November. But the result is contradictive compared with the finding from Kekenusa (2006) to   June, while the low season started afterward. Although, he used a longer dataset (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) but he only analyzed the catch from pole and line fisheries. So, the result was incomparable since both fisheries have different fishing ground and operational type.
Fishing season identification is very useful. It can be used as an input control where more fleets are allowed to fish when the season occurred and less fishing license are issued when the season is low. Interestingly, the fishing season for all group species have similar trend. This perhaps because the analysis came from the same gear. However, Pearson's correlation index showed that correlation between skipjack CPUE and large tuna CPUE was moderate (r=0.43, n=7843). It is mean, every skipjack caught there is higher probability of juvenile yellowfin and bigeye also caught. On the other hand, the correlation between skipjack CPUE and mackerel CPUE was low, as well as large tuna CPUE and mackerel CPUE (0.21 and 0.36, respectively) ( Figure  6). It means, other than large tuna, mackerel was caught incidentally during the fishing operation, perhaps they do not share the same ecological niche, although it should be proven by stomach content analysis. A study from Malone et al. (2011) explained that yellowfin tuna did not prey on organisms associated with Fish Aggregating Device (FADs), while bigeye tuna was a deep-sea dweller (Brill, 1994), thus it rarely interacts with prey on the surface, except at night (Allain et al., 2012).  Furthermore, juvenile yellowfin usually performing school around FADs during the day and leave at night (Holland et al., 1990). Both juvenile yellowfin and bigeye tuna are likely to stay at the same FADs for 150 and 10 days, respectively (Morgan, 2011). Juvenile yellowfin tuna is more likely be caught alongside skipjack if the set is conducted at day, while juvenile bigeye is rarely caught because of their diurnal movement and short stay at the FADs.
The seasonal fishing indices models fit with monthly CPUE trend for all group species (Figure 7.). This can be explained simply because the index produced from the model solely rely on monthly CPUE as basis, with the exclusion of minimum and maximum value. As and indices, it is not very reliable because fishing season is not only depended on effort but also other influencing variables such as: environmental conditions (temperature, salinity, wave height, wind speed, precipitation level, swimming-layer, sub-surface oceanographic condition and season) (Simbolon and Limbong, 2012;Lan et al., 2013;Rochman et al., 2016) and socio-economy (celebration of religious festivals, man power availability, etc.) . In order to get more precise analysis, a mathematical approach which involved above-mentioned variables could be included into the model. A combining of gears could also be considered to get more robust result.

Conclusion
Fishing season for large pelagic purse seine fishery in North Sulawesi started from May to July, and reached its peak on May. Low fishing season occurred during January to April with the lowest was in March. On every skipjack caught there is higher probability of juvenile yellowfin and bigeye also caught (r=0.43). On the other hand, other than large tuna, mackerel was caught incidentally during the fishing operation (r=0.21 and r=0.36, respectively).