Exploitable biomass and acceptable biological catch (ABC) for spiny lobster Panulirus polyphagus from the coast of North Kalimantan, Indonesia using data-limited fisheries

The acceptable biological catch (ABC) can be applied as a management tool to maintain the lobster stock sustainability in North Kalimantan waters where the overfishing condition of lobsters has been detected. Using data-limited fisheries, this study determines the exploitable biomass and ABC of mud spiny lobster (Panulirus polyphagus). The biomass was estimated based on biomass-based length cohort analysis. The exploitable biomass is considered from the biomass at a size larger than the minimum landing size (≥80 mm). The results showed that the total exploitable biomass was estimated as 51,062 kg, and the ABC can be considered as 14,306 kg. The reduction of yields is needed as 26% from the current annual yield to reach the ABC. The catch quota can be given to each fisherman to manage their fishing effort to fill the allowable catch quota. The cooperation between the government, fisherman and market agents is critical to support the implementation of the catch quota system.


Introduction
Fisheries management needs information on biomass to know the potential sustainable catch for some commercial fisheries resources.Biomass is an important parameter needed to determine the catch quota as a management tool.There are direct methods for estimating biomass, such as visual census, swept area and acoustic methods [1][2].A Previous study conducted the visual transect to estimate the standing stock of ornate spiny lobster [3].The direct estimation of lobster in North Kalimantan waters is challenging to conduct due to its habitat in coral, the habits hide in the shelter area and the high current waters.
Indirect estimation method can be applied to estimate the biomass of lobsters.The indirect methods have commonly been used to estimate the biomass of some fisheries resources.One of the indirect methods, catch depletion, has been used to estimate some biomass of fish [4][5].Another indirect method that can be used to estimate the biomass with limited data fisheries is the biomass-based length cohort analysis (LCA) [6].By using some information on life history parameters and yields, the biomass of mud spiny lobster in North Kalimantan waters was estimated in this present study.
The biomass-based LCA has been conducted to estimate fisheries resources, such as rock sole in the Bering Sea and anchovy in the Iranian waters [6][7].The goal of estimating the biomass is to know the potential yield that can be harvested as acceptable biological catch (ABC) and can be considered as total allowable catch (TAC).Some studies used the surplus production model to estimate the total allowable catch of lobster [8][9].The exploitable biomass and ABC for mud spiny lobster (Panulirus polyphagus) in North Kalimantan waters have not been reported yet.The aim of this study was to measure the exploitable biomass and the acceptable biological catch for fisheries management recommendation of lobster fisheries in North Kalimantan waters, Indonesia.

Data and Methods
The study's coverage was in the North Kalimantan coast area, and the fishing ground of lobsters was commonly found on the coast of Sebatik and Tarakan (Figure 1).The length frequency data was one of the input data to estimate the biomass of lobsters (Figure 2).The annual yield of mud spiny lobster has been reported as 126,263 kg in 2018 based on the statistical data of Directorate General of Capture Fisheries.Some life-history parameters, including length-weight relationship, growth parameters, mortality parameters and some reference points of mud spiny lobster, have been collected from the previous study [10] as inputs to perform the biomass-based LCA (Table 1).

Table 1. Population ecological parameters and biological reference points of P. polyphagus in North Kalimantan waters.
The biomass-based LCA was used by converting length frequency data of number and weights to annual yield.The biomass and fishing mortality was estimated based on the equation [6]: Where BL is biomass in each length class, CL is catch in weight from length l, GL is growth rate, M is natural mortality, FT is terminal F and ∆tL is the time required to grow from length-class L to L+∆L.The total biomass (B) was estimated based on the biomass from the smallest and largest size of the catch.The exploitable biomass (EB) was considered from the biomass from the minimum landing size to the largest size of lobster.The minimum landing size has been declared based on Ministerial decree of MMAF no.17/2021 as 80 mm carapace length.The acceptable biological catch can be estimated using the information on optimal fishing mortality (F40%), the exploitable biomass (EB) and natural mortality (M).The acceptable biological catch was calculated based on the equation: Where ABC1 and ABC2 were the acceptable biological catch based on some different approaches [11][12].

Results and Discussion
The biomass of mud spiny lobster (P.polyphagus) increased with the increase size until it reached the peak of biomass (Figure 3).After reaching the size which gives the peak of biomass, the biomass decreased by increasing in size.The peak of biomass was reached at the size of 72 mm.The lobster fisheries resulted in the reduction of biomass after reaching the size 72 mm.The high fishing mortality at the smaller size can cause the movement of biomass to peak at the smaller size, resulting in the growth overfishing for the lobster fisheries.Therefore, size regulation can be implemented to obtain the optimal catch for lobster fisheries.
The government of Indonesia has declared the minimum landing size for P. polyphagus as 80 mm carapace length.Some studies suggested 75 mm carapace length as the minimum landing size for P. penicillatus and P. argus [13][14].The minimum landing size of 83 mm has been regulated for lobster fisheries in Mauritania [15].However, P. polyphagus has been exploited before that size, which was affected by the selectivity of lobster gillnets.The modification on the lobster gillnet was needed to avoid the catching of small-size lobsters.Growth and recruitment are the main factors contributing to the increasing biomass [2].By capturing the small size lobsters, it can significantly decrease the biomass and threaten its sustainability.Therefore, the exploitable biomass can be considered from the biomass at the optimal size to the largest size, and it is used to estimate the acceptable biological catch (ABC).The exploitable biomass has been estimated considering the biomass at a size larger than the optimal size.The exploitable biomass of P. polyphagus was 51,062 kg (Table 2).The number of lobsters in each mid-length size c The biomass at a size larger than minimum legal size Acceptable biological catch, considered total allowable catch (TAC), can be estimated by some biological reference points on data-limited fisheries.South Africa and Australia has applied lobster's total allowable catch for their fisheries [16][17].Indonesia applied the TAC system for aggregated lobster in a large area, namely as Fisheries Management Area.However, lobster has a limited distribution and different fishing pressure based on the species.The acceptable biological catch of P. polyphagus as the target species in North Kalimantan Waters can be adopted to maintain the lobster stock sustainability in North Kalimantan Waters.
The acceptable biological catch (ABC) of P. polyphagus in North Kalimantan waters was lower than their current annual yield (Ycur).The ABC of P. polyphagus using Cadima (2003) method (ABC2=18,832 kg) was much higher than the ABC estimated by Zhang (2020) method (ABC1=14,306 kg).We recommend using 0.38 as the optimal coefficient on Cadima's method, as  2 = 0.38[  + ()], to obtain a more precautionary approach to the exploited lobster stocks.By using the coefficient of 0.38 on Cadima (2003) method and by using the Zhang (2020) method, the acceptable biological catch was obtained as 14,036 kg.Therefore, the 26% reduction from the current yield is needed to reach the ABC.
The catch quota can be shared with each fisherman so they can control their catch on each lobster species.It can be formed as an individual transferable catch quota (ITQ), which has been reviewed in some studies to obtain multiple fisheries objectives [18][19].The management will be more effectively controlled due to specific areas, and the legal landing site should be used to control the catch.

Conclusion
The biomass at a size larger than the optimum length was considered as exploitable biomass, estimated as 51,062 kg for mud spiny lobster (P.polyphagus) in North Kalimantan Waters.We recommend using 0.38 as the optimal coefficient on the Cadima's method, as  = 0.38[  + ()].The acceptable biological catch can be considered as 14,306 kg.The cooperative management between the government, fishermen and market agents is highly needed to support the implementation of the catch quota system.

Parameters
Values

Figure 1 . 3 Figure 2 .
Figure 1.Scope of study on biomass of spiny lobster P. polyphagus in North Kalimantan waters

Figure 3 .
Figure 3. Biomass and fishing mortality rate in each mid-length of P. polyphagus in North Kalimantan waters based on the analysis of biomass-based length cohort analysis.

Table 2 .
Biomass (B) and exploitable biomass (EB) for P. polyphagus in the coast of North Kalimantan.