Author：Chen Chao-Ching, Associate Professor of National Kaohsiung University of Science and Technology
Lin Hsin-Jung, Technician of National Kaohsiung University of Science and Technology

I. Overview of the Sergia Lucens Fishery in Taiwan

The sergia lucens (also known as "sakura shrimp") (Fig. 1) fishery in Taiwan primarily utilizes trawl nets to capture shrimp in marine waters. The fishing grounds are mainly located in the seas around Donggang in Pingtung and the vicinity of Turtle Island in Yilan. Among these areas, the fishery in Donggang is the most developed one, accounting for over 80% of the total production. According to statistics, there are nearly 120 fishing vessels engaged in capturing sergia lucens in the Donggang area. The average annual production in the past decade is approximately 1,200 metric tons, with an annual value exceeding 400 million NT dollars.

The sergia lucens fishery in Taiwan is subject to an annual total catch limit, and fishing vessels have reached agreements not to operate on Saturdays and Sundays (a two-day weekend), and even implemented a complete ban on fishing from June to October each year. This allows the sergia lucens sufficient time for recuperation and reproduction, ensuring the sustainability of the fishery. It is considered a benchmark for fishery management in Taiwan. However, as the sergia lucens fishery utilizes trawl nets, it also faces the issue of significant bycatch of juvenile and trash fish. According to our team's 2019 survey, at least 68 species accompany the catch when targeting sergia lucens with trawl gear (hereinafter referred to as "bycatch") (Fig. 2). The major bycatch species include Janicella Spinicauda, Benthosema Pterotum, Pasiphaea Orientalis, Trichiurus Lepturus, Harpadon Microchir, Bregmaceros Lanceolatus, Sergia Talismani, Lestrolepis Japonica, and Plesionika Taiwanica (see Figure 3), with sergia lucens accounting for only 14% of the total catch weight. Based on the average sergia lucens production in Donggang over the past decade, the annual weight of bycatch in the Donggang sergia lucens fishery exceeds 7,000 metric tons. Considering the gravity of the bycatch problem, we believe that it deserves attention and proper management, including proactive measures to mitigate bycatch. This is crucial for enhancing the industry's image and elevating the value of sergia lucens fishery in Taiwan.

II. The Benefits of Implementing Juvenile and Trash Fish Excluder Devices (JTED) in Donggang’s sergia lucens trawl fishery

To mitigate the issue of significant bycatch of juvenile and trash fish in the sergia lucens fishery, we collaborated with the fishing vessel "Yu Shuen Fa No. 2" in the Donggang area. Without altering the vessel's original net structure (e.g., mesh size, net width, and length), we installed the "juvenile and trash fish excluder device" (JTED) developed by our team at the front end of the Codend of the existing gear (Fig. 4). We then conducted 40 net-haul trials in the traditional fishing grounds for sergia lucens in Donggang, as depicted in Fig. 5 and Fig. 6.

The operation time for each net set in the aforementioned trial is approximately one hour. This includes 10 minutes for net throwing, 35 minutes for trawling, and 15 minutes for net hauling. The trawling speed is about 2.24 knots, with the net dragged at a depth ranging from 40 to 200 meters. The depth of seabed in the operational area ranges from 260 to 420 meters. The trial results indicate that the total catch from 40 net sets amounted to 1,512 kilograms, with an average catch per net set of approximately 37.8 kilograms. The catch that escaped back into the ocean through the JTED accounted for 53.18%. However, the effectiveness of the JTED in facilitating escape varies among different types of catch. The escape rates of five main catch species are summarized below (collectively accounting for over 70% of the total catch):

1. Sergia lucens: Approximately 600,000 individuals (223 kilograms) of sergia lucens were caught in this trial, with around 320,000 individuals (121 kilograms) escaping through JTED, posting a loss ratio of 53%.
2. Janicella spinicauda: Approximately 570,000 individuals (439 kilograms) of janicella spinicauda were caught in this trial, with around 290,000 individuals (229 kilograms) escaping through JTED, posting an escape rate of 51%.
3. Benthosema pterotum: Approximately 480,000 individuals (167 kilograms) of benthosema pterotum were caught in this trial, with around 360,000 individuals (127 kilograms) escaping through JTED, posting an escape rate of 75%.
4. Pasiphaea orientalis: Approximately 240,000 individuals (154 kilograms) of pasiphaea orientalis were caught, with around 90,000 individuals (69 kilograms) escaping through JTED, posting an escape rate of 38%.
5. Trichiurus lepturus: Approximately 11,000 individuals (108 kilograms) of trichiurus lepturus were caught, with around 8,000 individuals (27 kilograms) escaping through JTED, posting an escape rate of 73%.

Our team has observed that the current JTED is less effective in screening the escape body length of small fish and shrimp species, such as sergia lucens, janicella spinicauda, benthosema pterotum, and pasiphaea orientalis. In other words, there is no significant difference between the body lengths of the escaped and captured fishes and shrimp. This is primarily due to the higher consistency in the size of small species. However, the probability of small shrimp and fish escaping through the JTED varies significantly, which may be attributed to differences in body structure or swimming ability between fish and shrimp. Regarding larger catch species, the JTED is more effective in screening the body length of trichiurus lepturus. The average body length of escaped ones was 34.77 centimeters, obviously smaller than the average body length of those caught at 52.82 centimeters (P < 0.01). The selected body length for 50% of the catch was 44.68 centimeters. Simply put, a total of 10,882 individuals of trichiurus lepturus measuring less than 45 centimeters were caught, of which 8,280 escaped, resulting in an escape rate of 76%. For those larger than 45 centimeters, 520 were caught, with 117 escaping, resulting in an escape rate of 23%.

III. Conclusion

The “Juvenile and Trash Fish Excluder Device" developed by our team has been proven effective in mitigating bycatch in sergia lucens trawling fishery. It particularly helps to minimize the loss of economically valuable large trichiurus lepturus, while facilitating the escape of a significant number of immature small trichiurus lepturus. However, there remains a need to address the challenge of screening the escape body length for small species. From the perspective of resource and industry sustainability, the application of the "Juvenile and Trash Fish Excluder Device" in sergia lucens trawling fishery can be considered successful. 

On the contrary, if we consider solely the impact on the current income of fishermen, the economic loss for sergia lucens in this trial reaches as high as 53%. This outcome is unlikely to be accepted or embraced by the sergia lucens industry stakeholders. If there is an opportunity in the future to promote the installation of JTED on sergia lucens trawling gear, efforts must be made to further reduce the loss ratio of sergia lucens. This is where our team needs to improve JTED in the future. In addition, addressing the economic losses for industry stakeholders may require better production and marketing strategies and more comprehensive support measures.

Furthermore, the current “Juvenile and Trash Fish Excluder Device" still faces challenges in screening the body length of small fish and shrimp, but shows better results in screening the body length of trichiurus lepturus (larger fish species). Therefore, we boldly speculate that if this device is slightly improved and applied to other bottom trawling which mainly targets large fish species, its effectiveness should be quite promising.