1. Introduction
The family Ophichthidae (Anguilliformes) consists of approximately 323 species grouped into 62 genera; 67 species belonging to 24 genera have been recorded at estuaries in Taiwan [1] [2] [3] [4] [5].
The distribution of rice-paddy eel ((Pisodonophis boro (Hamilton, 1822)) and longfin snake-eel (P. cancrivorus (Richardson, 1848)) extends from the Indo-West Pacific region including the Red Sea, East Africa, India, Indonesia, Taiwan Area, Japan, Korea, Philippines, and Polynesia to Australia [6] [7] [8] [9]. Although the biology, morphology, and fecundity of snake-eels [10] [11] [12] and the diet, migration, habitat, and swimming behavior of P. boro and P. cancrivorus have been reported [13] [14] [15] [16], few studies have investigated their population structure in the southwest coast of Taiwan [17].
Length-weight relationship (LWR) has been used to convert lengths into biomass for management and evaluation of fish in various areas [9] [18] [19]. Furthermore, the estimated LWRs of four species of snake eel juveniles from India were reported to be 3.6 - 58.5 cm [9]. Lampang et al. (2021) [20] studied the body size distribution and ovarian histology of P. boro from Thailand. P. boro eels use mangrove mudflats as homing habitats for feeding, breeding, and migration during the breeding season [21].
Fishermen fish for P. boro on tidal mudflats by setting 5 to 20 lift and trap nets overnight, which trap snake eels hunting for food at night. Another method for catching snake eels is looking in and around the holes that they dwell in. At the southern bank of Jhuoshuei estuary, fishermen catch eels individually; however, in other estuaries and lagoons, a group of two or more fishermen fish for snake eels. Fishermen who are fishing alone are willing to release small snake eels that weigh less than 3 tw-tael (Taiwan weight unit, about 112.5 g) unless they are dead. However, when fishing in groups, fishermen are unwilling to release small snake eels because no law mandates them to do so.
Therefore, the objective of this study is to provide information for fishery assessment using the LWR, fishing methods and attitudes, and management as well as to compare populations of the same species from different regions.
2. Materials and Methods
We collaborated with seven local fishermen who helped us fish samples were used lift nets and trap net fishing at intertidal zone of lagoon and estuarine regions of southern Taiwan (120˚05'04.9''E - 20˚15'50.4''E, 23˚16'18.7''N - 23˚50'46.2''N) [22]. The sampling sites including the Puzih estuary (November 2014-July 2018), Jhuoshuei (Zhuoshui) estuary (July 2013-July 2015), and Beimen lagoon (October 2017-July 2018). collected overnight.
The Jhuoshuei estuary is a bare sandy beach approximately 470 ha, the Puzih estuary and lagoon of are approximately 500 ha and 650 ha, Beimen lagoons is approximately 100 ha, with mangroves covering 14 ha of its riverbank and mudflats [22] [23]. The gravel in the sample areas of the Jhuoshuei estuary was coarse (0.15 - 0.25 mm [38.4%], followed by 0.25 - 0.60 mm [24.5%]). The grain size of the sand in the Puzixikou sample area was fine (0.063 - 0.15 mm [43.0%], followed by 0.15 - 0.25 mm [36.2%]) [22]. The area and the grain size of the intertidal zone not only affect the habitat of P. boro and, P. cancrivorus, but also affect the abundance of their food.
We collected P. boro and P. cancrivorus between November 2014 and July 2018 from the first sampling site Puzih estuary of Taiwan’s southwest coast. Then, we collected P. boro eels only between November 2013 and July 2015 from the second sampling site Jhuoshuei estuary. The main difference of P. boro and, P. cancrivorus in external form is the head and tail [3]. The main difference of P. boro and, P. cancrivorus in external form is the triangle head and rouge tail of the former. All 94 specimens of P. cancrivorus and 13 specimens of P. boro were stored in a freezer at 0˚C and assessed at the laboratory of the Department of Environmental Engineering, Kun-Shan University. The TL values of these specimens were recorded to the nearest 0.1 cm by using digital calipers. The W values of these specimens were recorded to the nearest 0.1 g by using a digital scale. Live P. boro eels are typically used for preparing medicinal liquor in East Asia and they are priced at 600 $/kg in Taiwan. Therefore, measurements were performed by reducing their activity at low temperatures (<15˚C) so there is a large error, its TL with 5 cm, and W with 5 g at the scene.
The natural logarithmic linear relationship between TL and W was calculated according to the linear equation ln(W) = a ln(TL) + b, where Ln is the natural logarithm, a is the slope, and b is the intercept [18] [20] [24] [25]. Compare the slope value of the LWR linear equation of the P. boro and, P. cancrivorus.
The stomach and gut contents of 119 specimens of P. cancrivorus and 11 specimens of P. boro were examined after thawing. All stomach and gut contents (fish otoliths and bones, crab claws, shrimp heads and tails, and shell hinge teeth elements) were preserved in 75% ethanol and subsequently washed, compared and identification [4] [26] [27] [28]. The data are presented as mean ± standard deviation.
3. Results
From July 2013 to July 2018, a total of 420 specimens were analyzed, comprising 288 specimens of P. boro (Hamilton, 1822) and 132 specimens of P. cancrivorus (Richardson, 1848) from the Southwest coast of Taiwan.
3.1. The LWR of Pisodonophis Spp. from Puzih Estuary
From November 2014 to July 2018, at the first sampling site, Puzih estuary of Taiwan’s southwest coast, the average TL of P. cancrivorus was 52.8 ± 9.2 cm (range: 34.0 - 82.0 cm) and the average W was 107.9 ± 73.7 g (range: 20.5 - 555.0 g, N = 119); the average TL of P. boro was 68.4 ± 11.9 cm (range: 47.3 - 143.0 cm) and the average W was 140.4 ± 86.1 g (range: 41.3 - 1012.5 g, N = 194) (Figure 1). P. cancrivorus was smaller than P. boro. The natural logarithmic equations of P. cancrivorus ln(W)c = 3.2168 ln(TL) − 8.2027 (R2 = 0.9341, N = 119) and that of P. boro ln(W)b = 2.4405 ln(TL) − 5.4448 (R2 = 0.796, N = 194) (Figure 2)
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Figure 1. Distribution of the total body length of Pisodonophis boro (N = 194) and P. cancrivorus (N = 119) from the Puzih estuary of Taiwan’s southwest coast between November 2014 and July 2018.
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Figure 2. Natural logarithmic linear relationships between total body length (cm) and weight (g) of Pisodonophis boro (N = 194) and P. cancrivorus (N = 119) from the Puzih estuary of Taiwan’s southwest coast between November 2014 and July 2018.
indicated positive linear relationships between the natural logarithms of TL and W for both P. boro and P. cancrivorus. Furthermore, P. boro is thinner than P. cancrivorus.
3.2. Monthly Population Changes of Pisodonophis Spp. from the Southwest Coast of Taiwan
From the total sampling sites of Taiwan’s southwest coast, for P. cancrivorus, 59 (44.7%), 25 (18.9%), 23 (17.0%), and 17 (12.9%) specimens were collected in November, January, December, and October, respectively (N = 132) (Figure 3). These months constitute the winter season in Taiwan. For P. boro, 81 (28.1%), 75 (26.0%), 49 (17.0%), and 20 (6.9%) specimens were collected in July, September, June, and August, respectively (N = 132) (Figure 3). These months constitute the rainy season in Taiwan.
3.3. Diet of Pisodonophis Spp.
In total, 119 guts of P. cancrivorus and 11 guts of P. boro were examined; 100 specimens of P. cancrivorus and 3 specimens of P. boro had food in their guts, whereas the guts of the 27 specimens were empty. In total, 123 food items were retrieved. P. boro prey on shrimp (N = 3). P. cancrivorus prey on shrimp, crab, fish, and mollusks with frequencies of occurrence of 49%, 21%, 15%, and 2%, respectively (N = 100).
The family levels of these prey items were identified. Shrimp: Palaemonidae and Penaeidae; crab: Grapsidae, Ocypodidae, and Portunidae; fish: Gerreidae and Leiognathidae; and shell: Laternulidae.
3.4. Overfishing of Pisodonophis boro
In the Jhuoshuei estuary during November 2013 to July 2015, the average W of P. boro was 343.2 ± 218.8 g (range: 75.0 - 1, 350 g) (N = 91). The average W of P. boro from Puzih estuary was 40.9% of that of the Jhuoshuei estuary. The Jhuoshuei estuary accounted for 39.6% of specimens weighing more than 300 g, whereas the Puzih and other estuaries accounted for only 2.1% (Figure 4). Obviously, Puzih and other estuaries were overfishing of P. boro.
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Figure 3. Monthly changes in numbers of Pisodonophis boro (N = 288) and P. cancrivorus (N = 132) from the southwest coast of Taiwan between July 2013 and July 2018.
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Figure 4. Distribution of the weight (g) of Pisodonophis boro at the Jhuoshuei estuary from November 2013 to July 2015 and at the other estuaries of Taiwan’s southwest coast from November 2014 to July 2018.
4. Discussion
Although studies have reported that eels use mangrove mudflat habitats for breeding and feeding [21], the result of this study revealed eels thrive on the bare sandy beaches of the Jhuoshuei estuary and other habitats in Taiwan. Therefore, in addition to mangrove habitats, the size of the beach sand and whether food is abundant, are crucial habitat selection criteria.
The LWR of P. boro from the southwest coast of Taiwan is described by ln(W)b = 2.4405 ln(TL) − 5.4448, which differs from that provided in FishBase [1]. Because P. boro is a crucial ingredient in medicinal wine in East Asia, they are overfished. The presented values, which its weight was larger than 3 tw-tael (Taiwan weight unit, about 112.5 g) can provide guidelines for the conservation of continental aquatic ecosystems in estuarine regions.
Acknowledgements
We are grateful to Mr. Kim-Fu Shu, Dong-Hsin Huang, Ming-Choi Juang, Jen-chieh Tsai, Wei-Hsun Tsai, and Shin-Ding Wang for their help. This study was supported by Chiayi County Government, Tainan City Government, and the Construction and Planning Agency of the Ministry of the Interior.