Catch, bycatch, and mitigation options for endangered sharks in data poor fisheries: a case study on pelagic thresher sharks (Alopias pelagicus) in the eastern Indian Ocean

Study area

Southwest Aceh is a district in Aceh Province located at 3°34′24″ to 4°05′37″N and 96°34′57″ to 97°09′19″E. The Aceh province has one of the largest shark fisheries in Indonesia (Simeon et al. 2020). Like many shark fisheries hotspots in Indonesia, shark fisheries in Aceh started as early as the 1970s when the global fin market was emerging (Pacoureau et al. 2021). The Southwest Aceh district has one of the largest shark fisheries in Aceh, with more than 25 shark-specialised longline vessels operating throughout the year based at two sites in Southwest Aceh: Ujong Serangga fishing port and Lhok Pawoh community landing site (Booth et al. 2018; Simeon et al. 2020) (Fig. 1).

Fig. 1.

Map of pelagic thresher fishing ground (red boxes) for Southwest Aceh shark fishers. Inset: Indonesia.

Data collection

We monitored fishing activities and landings 6 days a week from June 2019 to December 2020. The operational fishing characteristics associated with the catch were recorded by interviewing each boat owner, captain, and crew member and recording the vessel’s name, gear type, and fishing ground. For shark landing information, we recorded the length, sex, and stage of maturity for all pelagic thresher shark landing sites (Nugraha et al. 2020). Sex was determined by the presence of claspers in males or the absence of claspers in females. The degree of calcification and length of claspers determined maturity levels for males. Males were divided into three reproductive stages, immature: short and noncalcified clasper, subadult: long and semicalcified claspers and mature: fully calcified claspers. For females, due to rapid processing of landed sharks, it was not possible to conduct internal examinations of reproduction organs and we only recorded the presence of embryos opportunistically in some processed specimens. Prior to biological and fisheries monitoring we conducted interviews with key actors in the fishery using snowball sampling with shark fishers (n = 17), traders (n = 13) and local leaders (n = 10). During interviews we collected general information about shark fisheries, particularly for thresher sharks including fishing method (vessel type, fishing ground, fishing gear) and socio-economic aspects (price, product types and market destination).

Analysis

Ethics statement

This work was conducted under a Memorandum of Understanding (MoU) and Technical Cooperation Agreement (TCA) between the Wildlife Conservation Society (WCS) and the Ministry of Environment and Forestry (MoEF), Ministry of Marine Affairs and Fisheries (MMAF) and the Marine and Fisheries Agency (MFA) of Aceh Province and supported by ZSL EDGE of Existence Fellowship Programme and Fondation Segré. Due to this MoU and TCA, no specific research permit was required. We collected data by measuring sharks that were already caught, dead, and landed by fishers in Southwest Aceh, with no incentives, compensation, or specific requests for killing sharks for this study. WCS participates in the Conservation Initiative on Human Rights and the rules and guidelines of our Internal Review Board ensures that any research protects the rights of human subjects. We did not apply for an institutional review board permit for this study because our study design focused on collecting fish and fisheries data as opposed to personal socio-economic data. The focus group discussions and interviews were conducted to obtain early scoping information about fishing practices. We used a protocol developed by Ministry Marine Affairs and Fisheries (MMAF) for data collection.

Fisheries characteristics

Of all vessels that landed sharks and rays, we recorded 31 fishing trips from 17 vessels that landed pelagic threshers. Of these 17 vessels that landed pelagic thresher sharks, 12 are shark vessels using bottom longlines. The fishing gear comprised the mainline, branch lines, and 150– 250 hooks. This fishing gear is operated by lowering baited hooks horizontally along the mainline, which is left submerged at about 30 metres deep for several hours. The main target of fishing are requiem shark species such as silky and spot-tail shark (Carcharhinus falciformis and Carcharhinus sorrah), however, thresher sharks are sometimes caught since these species share the same habitat. Fisher used skipjack (Katsuwonus pelamis) or pilchard (Sardinella sp.) as bait. Fishers typically spend 20–25 days at sea with a crew of three to six people. The fishing season is divided into low (January–May) and high season (June–November). According to fishers, the sea condition is calmer in high season, and more boats depart with shorter fishing days, deploying their gear twice a day but with shorter soaking hours.

During the study, 109 pelagic thresher sharks were recorded as caught and landed. Of all pelagic threshers landed, 94% were caught using bottom longline, with the rest by surface longline, gillnet, handline, and purse seine as bycatch in the reef and pelagic fisheries. The average catch rate for pelagic thresher catches using longline was 0.22 (s.d. ±0.19) individuals per soaking hour or about 3–4 individuals per trip. Monthly CPUE fluctuated widely throughout the year, with no seasonal trend (Fig. 2). Most pelagic thresher sharks (94%) are caught around small islands in Western Sumatra in two different provinces: Simuk Island and Nias Island in North Sumatra Province and Bengkaru and Babi Island in Aceh Province (Fig. 1). In this fishing ground, the bottom longline is the only fishing gear used to catch pelagic thresher sharks and not used anywhere else.

Fig. 2.

Monthly CPUE of pelagic thresher sharks.

According to interviews shark meat is made into shark curry and served as a local delicacy, whereas they sent fins to big cities such as Medan and Banda Aceh. The fins are priced higher per kilogram than other fisheries’ products. However, for thresher sharks, the fin price is relatively low (IDR70,000 or around USD5 per kg) compared to the fins of main target species (IDR400,000 or around USD26 per kg), whereas the value of meat (between IDR10,000 and 28,000 or between USD0.7 and 2 per kg) and other body parts is similar to with other shark species. Other than fins and meat, utilised body parts include cartilage, skin, and liver oil.

Biological information

Of the 109 pelagic thresher sharks we recorded, 31 were males, and 78 were females, with a statistically significant skew in the sex ratio of 1:0.39 females:males (X² = 3.84, P < 0.05). The results show no distinctive trends in the average monthly landing (Fig. 3).

Fig. 3.

Average monthly landing of pelagic thresher sharks (individual per month). Sex ratio of 1:0.39 for females:males (X² = 3.84, P < 0.05).

We present the correlation between total length (TL) and precaudal length (PL), which shows a linear trend (Fig. 4). PL is a length from the tip of the snout to the base of the tail. We are using PL as the primary measure of body length due to its higher accuracy for thresher sharks, as their tails are regularly damaged or removed. Sizes ranged from 63–183 cm PL. Male PL varied from 97–183 cm, with a mode of 132 cm, and female PL ranged from 63–152 cm, with a mode of 132 cm. Most individuals are immature, with very few individuals of mature length of either sexes represented in the landings (Fig. 5).

Fig. 4.

Correlation between TL and PL of pelagic thresher sharks.

Fig. 5.

Size distribution of pelagic thresher shark. Length at first maturity (Lm) is 140–145 cm PL for males and 145–150 cm PL for females (Liu et al. 1999).

Management options

Data on the operational fishing characteristics that lead to higher catches of thresher sharks can be used to inform management options via an MH framework (Table 1). Following the mitigation hierarchy, a number of options are possible. The most effective option to prevent fishing mortality could be a ban on specialised shark bottom longlines, as they contribute 94% of the reported pelagic thresher shark catch. Another approach is establishing one or more no-take zones around small islands in Western Sumatra, as these fishing grounds contribute 94% of the reported total pelagic thresher shark catch, however, the efficacy of spatial closures would need to be carefully examined to understand how movement and migration, and the population structure may affect their contribution to reducing thresher shark catches (Chin et al. 2023).

Temporal closure scenarios may reduce total pelagic thresher catches. In 2020, 46% of pelagic threshers were caught in high season. Depending on needs, fisheries managers could choose to limit fisheries activities within a certain time frame (e.g. in high season, low season, or any particular period of the year) to reduce fishing effort, however, the potential effect of displacing this fishing effort to other areas or species would need to be carefully considered and planned in close consultation with fishers to avoid unintended consequences and adverse outcomes (Jaiteh et al. 2016). Other options include effort limits and/or setting catch limits for certain species.

In addition, live release protocols could be used for the remaining residual catch with an estimated 87.5% survival rate based on previous studies of bigeye thresher shark (Alopias supercilious) postcapture survival in Pacific tuna longline fisheries (Hutchinson and Bigelow 2019). Finally, compensatory mechanisms, such as fishers contributing toward critical habitat conservation, could be explored (Booth et al. 2022, 2023). This mechanism can be in form of Payment for Ecosystem Service (PES) where fisheries managers pay fishers to release specific target species.

Biological implications

Subadult individuals of both sexes dominated the size composition of documented pelagic thresher sharks. This composition could indicate population structuring between sizes or age classes, which is evident in many shark and ray populations (Jacoby et al. 2012; Mourier et al. 2013; Araujo et al. 2017; Meyers et al. 2017) and fishing activity might be causing selective mortality on the juveniles in the population.

In terms of fisheries management, the selective harvest of juveniles may actually present a lower risk scenario as for some sharks, restricting mortality to juveniles while protecting breeding adults can actually be a foundational approach to improve sustainability (Prince et al. 2008). However, this approach requires information about the movement and mortality of the adults as well as population modeling to inform catch limits which are also a suggested minimisation measure (Smart et al. 2020) (Table 1). In the future, we also suggest exploring how gear modifications, such as using different hook types or sizes, influence the length of the thresher shark catches. A study in the northern Gulf of Mexico shows that hook type has significantly influenced the catchability and size of Atlantic sharpnose shark (Rhizoprionodon terraenovae) and blacknose shark (Carcharhinus acronotus) (Hannan et al. 2013), and gear modification could be used to reduce catches of pelagic threshers.

Differences in sizes between the sexes were insignificant, though males recorded had a larger maximum PL than females (190 and 163 cm). In contrast, a previous study in Taiwanese waters found females with a larger maximum PL than males (188 cm and 170 cm PL, respectively) (Liu et al. 1999). Nevertheless, the individuals recorded in this study were mostly juveniles and the maximum sizes of males and females in this population, and comparisons on size differences between the sexes, are presently unknown. For comparison, other studies on the pelagic thresher shark in Indonesia found numerous individuals >300 cm TL (Drew et al. 2015). It is possible that fisheries in those locations in Central Java, Bali and Lombok were taking adults from a different portion of this population or a completely different population, however, it is also possible that maximum sizes of thresher sharks have declined over time, which could potentially indicate overfishing. This difference in historical versus present size structures in the pelagic thresher population in Indonesia warrants further investigation.

Lastly, the significant catch of endangered pelagic thresher sharks in this fishery indicates that further catch monitoring is required. At present, catch rates and fishing locations are only estimated as these are based on fisher interviews, with fishers estimating gear soak times and recounting fishing locations. Further quantification of fishing effort using electronic monitoring, onboard observers, and/or low-cost GPS trackers (Lara-Lopez et al. 2012; Hold et al. 2015; Navarrete Forero et al. 2017; Bartholomew et al. 2018) would provide more detailed information on fishing effort. Over the long term, standardised catch-per-unit-effort data would provide baseline data and contemporary information to understand population trends and thus indicate the performance of any introduced management measures.

The female-skewed sex ratio suggests sexual segregation in this thresher shark population. Sexual segregation commonly occurs in elasmobranch populations, where studies on different shark species, such as spurdog (Squalus acanthias) and shortfin mako (Isurus oxyrinchus), also showed similar results (Jacoby et al. 2012; Hannan et al. 2013). The selective fishing of juvenile females as evident in this study will likely affect population dynamics and, given the K-selected life history of this species (Drew et al. 2015), the survival of females should be prioritised to enhance fisheries sustainability. Population viability analyses that integrates existing biological and fisheries data would help mangers understand how this population may respond to different types of fishing pressure and could provide the bases for choosing potential management measures to minimise impacts (Table 1) as has been performed for other shark fisheries and populations (Smart et al. 2017, 2020).

Management options

Our results indicate that the fishing method targeting sharks in Southwest Aceh is highly specialised, with similarities to other small-scale semicommercial shark fisheries in Indonesia, such as Tanjung Luar in Lombok, West Nusa Tenggara (Yulianto et al. 2018; Glaus et al. 2019). Based on the data available, we could adopt a range of potential management measures to reduce thresher shark catches in Aceh, with limitations on specialised shark bottom longlines and spatial closures being the most technically effective. However, these measures require further investigation. The effects of gear modification on catches and resulting population dynamics need to be explored to understand how different modifications may enhance sustainability. Other than that, some strict measures such as banning the most effective gear will be unrealistic in species-specific fisheries, because such measures would most likely be rejected by the fishing community despite their high potential conservation effectiveness.

Likewise, the contribution of a marine protected area (MPA) to protecting a shark population depends on numerous factors. Given that the pelagic thresher shark is a strongly K-selected species (Drew et al. 2015) and that the individuals in this area are juveniles, an MPA in this area may be less effective than an MPA that protects adult females (Chin et al. 2023), which highlights the need to better understand the wider movements and population structure of the species in Indonesia.

However, these results are based only on technical considerations. Besides technical measures, the socio-economic aspects of shark fisheries and compliance management must also be considered, especially in small-scale shark fisheries (Booth et al. 2019; Ichsan et al. 2021). For example, poverty alleviation, alternative livelihood options and incentives for shark conservation are needed to reduce dependence on threatened sharks whilst maintaining or improving wellbeing (Weeratunge et al. 2014; Bennett et al. 2019). Furthermore, introducing management measures that do not account for livelihoods and social and economic factors can lead to broad noncompliance and even cause other unwanted conservation outcomes (Jaiteh et al. 2017).

There is limited information regarding the economic value of specific species and their relative benefits to fisher households. Yet, to implement these measures in Southwest Aceh, understanding the potential socio-economic costs of different management measures to fishers and their relative feasibility will be essential (Booth et al. 2019). Thus, we need more information to determine the cost-effectiveness of management measures and the needs and preferences of fishers to support effective implementation (Muttaqin et al. 2018; Yulianto et al. 2018; Booth et al. 2020; Seidu et al. 2022). This information could also help to identify where novel interventions such as incentives and payments for ecosystem services could be introduced (Bladon et al. 2016; Leduc and Hussey 2019; Wosnick et al. 2020; Booth et al. 2021). Further data on which operational fishing characteristics create the highest risks to thresher sharks, with robust statistical analysis, and considering the results of population viability analyses (PVA) will also help clarify which measures might be the most effective at reducing mortality (Smart et al. 2017).

A recent study on the potential effectiveness of Payment for Ecosystem Services (PES) in artisanal shark fisheries focuses on two CITES-listed and critically endangered taxa, hammerhead sharks (Family: Sphyrnidae) and wedgefish (Family: Rhinidae) in two locations in Indonesia (Booth et al. 2023). This study suggests that PES could save >20,000 hammerheads and wedgefish annually for an estimated cost of USD71,000–236,000. Based on this study, the factors that can influence the effectiveness of PES include incentive design, individual socio-demographic characteristics, and broader contextual and market-related factors. Further, this study suggests that all stakeholders, including government, funders, scientists, and private sector, strengthen partnerships with local communities and create an environment for testing and developing PES projects (Booth et al. 2023). These studies also demonstrate the importance of gaining in-depth knowledge of the social and economic factors that influence fisher behaviours, and a measured, iterative approach in designing and testing PES schemes to maximise positive behaviours and avoid potential adverse outcomes.

More broadly, there is also a need to appropriately manage other fisheries operating in Aceh and the Indian Ocean. Ensuring the survival of highly migratory and threatened shark species in Indonesia needs robust bycatch mitigation in both industrial and small-scale fisheries that interact with these species. Measures for industrial fishing will require a different approach than managing targeted and small-scale fisheries. For example, as a member of IOTC, Indonesia must comply with Resolution 12/09 on the conservation of thresher sharks (family Alopiidae) caught in association with fisheries in the IOTC area of competence, which prohibits fishing vessels flying the flag of IOTC Members and Cooperating Non-Contracting Parties (CPCs) from retaining, transshipping, landing, storing, selling or offering for sale any part or whole carcass of thresher sharks of all the species of the family Alopiidae (The Indian Ocean Tuna Commission (IOTC) 2012). Further, the contracting parties also need to release thresher shark catches and records by observers. However, implementation of this resolution may be constrained by limited monitoring and compliance. Other than that, IOTC only requires a minimum ‘5% observer coverage of the number of operations/sets for each gear type by the fleet of each CPC’ (The Indian Ocean Tuna Commission (IOTC) 2022). More significant investments in observer programs and landings monitoring could help improve compliance with IOTC resolutions, reduce the mortality of thresher sharks in commercial fisheries, and protect marine resources for the livelihoods and wellbeing of coastal communities.

Overall, the data and management options we have presented can be part of a holistic management strategy for conserving the charismatic, evolutionarily distinct, and globally endangered thresher shark in the Indian Ocean. Therefore, this information on data-poor targeted shark fisheries in the Indian Ocean from two sites in Aceh province, Indonesia, could help to inform local and national management for endangered and CITES-listed sharks, such as pelagic thresher sharks (Ichsan et al. 2021; Seidu et al. 2022).