1. Introduction
The Kuroshio Current flows along the eastern waters of Taiwan and brings an upwelling with high primary production and rich nutrients when it hits seamounts, making this area a conventional fishing ground for migratory fish species such as dolphinfish (Coryphaena hippurus Linnaeus 1758), tunas, billfishes and sharks. This area is also one of the best cetacean-watching areas in Taiwan due to the high occurrence of cetaceans.
According to the Fisheries Statistics of Taiwan [
1], annual fishery production of the Hsinkang fishing port, southeastern Taiwan, is 6164 tons and is valued at USD 24.7 million. The major fisheries in this port include the small-scale tuna longline (STLL, <100 gross tonnages (GRT)), large-mesh drift net and harpoon fishery in the winter. The annual catch of the STLL is 3361 tons, which is 54.52% of the total annual catch and is valued at USD 10.7 million. Cetacean and shark depredations often occur when fishes were hooked by longline vessels. This issue affects, in different modalities, both cetaceans and fishers. The catch in terms of quality and quantity decreased when encountering cetacean interactions during fishing operations. On the other hand, depredations may result in the damage or mortality of cetaceans due to their being hooked or entangled by fishing gears [
2].
There have been 31 cetacean species recorded in Taiwanese waters, and large cetaceans are commonly found in the eastern Taiwan waters [
3]. The cetacean-watching activities started in the late 1990s after the cetacean-harvesting policy ban was implemented [
3]. Several fishing ports in eastern Taiwan were transformed to recreational fishing ports for cetacean-watching ecotourism. The major areas for cetacean-watching were within 20 miles of the coast, and the sighting rate was 64–100% [
4]. Risso’s dolphin (
Grampus griseus Cuvier G., 1812), spinner dolphin (
Stenella longirostris) and Pantropical spotted dolphin (
S. attenuate) were the most sighted species, followed by the common bottlenose dolphin (
Tursiops truncates), Fraser’s dolphin (
Lagenodelphis hosei) and false killer whale (
Pseudorca crassidens) [
4]. Based on sales records, the shark landings at the Hsinkang fishing port decreased from 953 tons in 2001 to 436 tons in 2010, and the major shark species were the blue shark (
Prionace glauca), shortfin mako (
Isurus oxyrinchus), bigeye thresher (
Alopias superciliosus), pelagic thresher (
A. pelagicus), scalloped hammerhead (
Sphyrna lewini), spinner shark (
Carcharhinus bervippina) and tiger shark (
Galeocerdo cuvier).
Various studies have documented cetacean and shark depredations in different areas around the world, particularly in the Indian Ocean [
5]. Ramos-Cartelle and Mejuto [
6] documented that the false killer whale depredations in Spanish billfish longline fishery were severe in the Indian Ocean. Rabearisoa et al. [
7] reported toothed whale and shark depredations from the Reunion Island and Seychelles pelagic longline fisheries and concluded that depredation levels in Seychelles were among the highest observed in the world. Mitchell et al. [
8] reviewed 61 studies and found shark depredation rates between 0.9% and 26% in commercial and recreational fisheries, respectively. Hamer et al. [
9] and Gilman et al. [
10] reviewed the interaction between cetaceans and longline fisheries. The issue of the depredation conflict between large marine predators and fisheries has been addressed [
11]. The cetacean/dolphin depredations and mitigation for various fisheries have been well documented [
12,
13,
14,
15,
16,
17,
18]. Carmody et al. [
19] demonstrated that greater fishing efforts in a concentrated area may lead to high rates of shark depredations in a commercial trolling fishery in sub-tropical Australia.
The economic losses due to depredations have also been well documented in various areas. Liu [
20] estimated the economic loss due to cetacean and shark depredations in a longline fishery in northeastern Taiwan. Luciano and Secchi [
21] reported that the economic loss from killer whale depredations was higher than that from shark depredations in tuna longline fisheries in the southern waters of Brazil. Hernandez-Milian et al. [
22] suggested that cetacean depredations were related to the moon phase, sea surface temperature and operation depth in the Atlantic Ocean. Muñoz-Lechuga et al. [
23] described the depredation of Portugal billfish longline fleets in the Indian Ocean using general added models and identified the hotspot of depredations in the southwest Indian Ocean. Maccarrone et al. [
24] and Fader et al. [
25] reported the economic losses due to cetacean depredations in Sicily and Hawaii waters, respectively.
There are conflicts between commercial fisheries and recreational fisheries, particularly in terms of cetacean-watching activities. Since the ban on cetacean harvesting in Taiwan waters was implemented in 1993, fishermen argued that the reduction in fish catch was partially because the of increase in cetaceans that consumed large amounts of fish and interfered with fishing operations. In addition, the overlapping between cetacean-watching areas and conventional fishing grounds, as well as cetacean depredations, also resulted in the conflict between the two parties. Despite Yeh’s [
26] description of the interaction of cetaceans in longline and troll fisheries and Liu’s [
20] work on cetacean and shark depredations and the economic loss in the longline fishery in the northeastern Taiwan waters, depredation information is still lacking in the southeastern Taiwan waters. Hence, this study aims to estimate cetacean and shark depredations and to identify major factors affecting depredations in the STLL fisheries in the southeastern Taiwan waters based on fishermen interviews and logbook data of sampling vessels. The total economic loss due to depredations in the STLL in the southeastern Taiwan waters was also estimated.
4. Discussion
This study provided the first information on cetacean and shark depredations in the STLL fishery in the southeastern waters of Taiwan. The results derived from this study can be used as a reference for the future stock assessment and management of tuna, billfishes and sharks in this region.
4.1. Uncertainty in Estimation
Several uncertainties may have affected the results of our estimations. Chiang et al. [
31] documented that the drift net and harpoon were the major fishing gears for the coastal fishery, but the STLL was the major fishing gear for the offshore fishery in eastern Taiwan. Fishermen usually switched fishing gears depending on the target species and season. Some of the STLL vessels switched to drift nets to catch billfishes in certain seasons and even used harpoons to catch white marlin in the winter. As the fishing gear information was incomplete in the sales records of the Hsinkang fishing port, the estimation of the total longline catch and economic losses due to depredations at this port may have been overestimated.
The fishing efforts of the 12 sampling vessels varied by month. Therefore, the estimated monthly catch and depredation may have been biased when lower fishing effort was reported in certain months. Uncertainty also existed in the identification of cetacean or shark depredation when only a small part of the fish carcass was left, which also biased the subsequent estimation of the DRW.
4.2. Cetacean and Shark Depredation
Liu [
20] reported that the damage rates of cetaceans and sharks for a longline fishery in the northeastern Taiwan waters were 33.24% and 22.44%, with a DRN of 10.20% and 2.10%, DRW of 9.52% and 2.22%, and DI of 4.62 and 0.88, suggesting that the cetacean depredations were higher than shark depredations. Luciano and Secchi [
21] also concluded that the economic loss due to cetacean depredations (12.38%) was higher than that from shark depredations (9.10%) in the southern Brazilian waters. Similar results were found in this study.
The higher cetacean depredation can likely be attributed to the fact that cetaceans need more energy than sharks to fulfill their energy requirements [
31]. In addition, cetaceans, with their higher intelligence, often swim along the branch lines of longline fishing vessels and depredate the hooked fish [
32]. Adult cetaceans even teach juveniles to depredate the fish hooked by longline vessels [
33]. Sivasubramaniam [
34] mentioned that when cetacean depredations occurred, if the fishing operation continued, the situation would grow worse in the Indian Ocean. To avoid fishing losses, fishermen tended to stop operations or change fishing areas when they encountered cetaceans.
4.3. Depredation Rates in Different Areas
IOTC [
4] reported that the DRN of cetaceans was 2.6% in the southwest Indian Ocean from December 2004 to December 2005. Ramos-Cartelle and Mejuto [
6] documented that the cetacean DR was 2% for the Spanish swordfish longline fleets, and the DRN were 1.1–1.8%, 0.5–2.6% and 0.1–0.3% in the Atlantic, Indian and Pacific Ocean, respectively.
As for the cetacean depredations in the longline fisheries in eastern Taiwan waters, Yeh [
26] reported a DR of 11% based on interviews of 58 fishermen and observations of 73 trips on two sampling vessels. Liu [
20] estimated a DR of 46.84%, DRN of 9.89%, DRW of 12.17% and DI of 3.36/1000 hooks for an offshore longline fishery in the northeastern Taiwan waters based on nine sampling vessels. However, the depredation estimates from the aforementioned studies were larger than our estimates of the STLL vessels in the southeastern Taiwan waters with DR = 19.02%, DRN = 2.13%, DRW = 3.11% and DI = 0.93/1000 hooks. One of the possible reasons for the difference is the different sizes of fishing vessels in the two areas. The fishing vessels in northeastern Taiwan were mainly 20–50 GRT and 50–100 GRT; however, those in southeastern Taiwan had smaller sizes (10–20 GRT and 20–50 GRT). Larger fishing vessels can travel farther from the port and deploy more hooks, thus increasing the catch [
35]. As the soaking time increased with the vessel size, more depredations may have occurred, which resulted in higher economic loss [
27]. Hernandez-Milian et al. [
11] mentioned that the cetacean sightings increased with the catch. Garrison [
36] suggested that cetacean depredation was positively correlated to the length of the branch line. The mean number of hooks per set was 1442 and 1005 for the longline fishing vessels in northeastern and southeastern Taiwan waters, respectively. More hooks lead to longer soaking times, which may increase the opportunity for depredation. In addition, different marine environments in the two areas may be another reason for the difference in depredations. The continental shelf in eastern Taiwan is very narrow, and the water depth drops sharply to thousands of meters within a couple of miles from the coast. Thus, different cetacean species are found in the two areas [
37,
38]. Liu [
20] documented that the major cetacean species were Risson’s dolphin, short-beak common dolphin (
Delphinus capensis) and pantropical spotted dolphin in the northeastern Taiwan waters. However, the spinner dolphin, pantropical spotted dolphin and short-finned pilot whale were the major species sighted in southeastern Taiwan. These sighting records agree with the distribution of cetaceans in eastern Taiwan that short-finned pilot whale occur in deep waters [
4]. The GLM indicated that the operation depth was a significant factor that affected the DRN, DRW and DI. Therefore, different depredation rates may have resulted from various cetacean species inhabiting different areas and water depths.
The sighting rates of cetaceans were 3.63/100 km and 1.63/100 km, with 263.45 individuals/100 km and 122.34 individuals/100 km in northeastern and southeastern Taiwan waters, respectively [
37,
39]. The higher number of cetaceans in northeastern Taiwan may result in the higher cetacean depredation.
4.4. The Preference of Cetacean and Shark Depredations
Liu [
20] pointed out that the major fish depredated by cetaceans were dolphinfish and yellowfin tuna in the longline fishery in northeastern Taiwan. Similar results were found in this study. The major species depredated by sharks were dolphinfish, oilfish and billfishes. However, the major species depredated by sharks in this study were dolphinfish, yellowfin tuna and sailfish. The dolphinfish was the most common species in cetacean and shark depredations in the two areas. This was likely because the dolphinfish is the most abundant species caught by longline fisheries in eastern Taiwanese waters [
40]. The dolphinfish catch peaked from April to June and from October to February [
30]. When abundant dolphinfish were caught by longline fisheries, the hooked dolphinfish provided good sources for cetaceans [
14]. Thus, the two peak seasons of dolphinfish catch corresponded to high cetacean depredations in southeastern Taiwan. Harwood [
41] mentioned that, when many prey items were found, marine mammals could pick their preferred species. It is likely that the dolphinfish is the major fish species depredated by cetaceans and sharks in southeastern Taiwanese waters.
4.5. Cetacean Depredation and Dolphinfish Catch
The present study found that the catch-in number of dolphinfish and its interaction with season had a significant effect on the cetacean damage rate. Although dolphinfish were caught year-round, the number of catches from March to September was higher than that from October to February, which corresponded to the higher cetacean depredations of dolphinfish. Chiang et al. [
30] documented that the catch of dolphinfish peaked from April to June, which is the period of the highest fishing effort for our sampling vessels. During this period, more hooks were deployed and longer operation and soaking times increased the cetacean depredation opportunities. Therefore, the number of hooks, catch of dolphinfish and cetacean depredations varied by season.
4.6. Economic Loss of Longline Fishery Due to Depredation
The economic losses of the longline fishery in Hsinkang were estimated based on the logbook data of the sampling vessels. However, some of the longline fishing vessels switched to drift nets or harpoons in certain seasons. As information regarding the seasonal switches of fishing method was not available in the sales records, the estimation of economic loss due to depredations of the longline fishery in Hsinkang fishing port may be biased. Future research should focus on collecting fishery switch information to improve the estimation.
Some Regional Fishery Management Organizations (RFMOs) have implemented the quota system on certain tuna or tuna-like species, but the cetacean and shark depredations have not been taken into account, which may lead to underestimations of the total removal of the species in stock assessment and mislead the subsequent management measures. The total depredation loss of dolphinfish was 44 and 10 tons, while the total loss of yellowfin tuna was 37 and 4 tons from cetacean and sharks, respectively. Consistent monitoring of the depredation is recommended to ensure the accuracy of the catch estimates in future stock assessments.
5. Conclusions
Cetacean and shark depredations in the STLL fishery in the southeastern Taiwan waters, estimated based on the logbooks from 12 sampling vessels, indicated damage rates of 19.26% and 11.56%, respectively. The depredation rates, in terms of number and weight, from cetaceans were estimated to be 2.21% and 3.23%, respectively, and were significantly higher than those from sharks, which were estimated to be 0.51% and 0.47% in terms of number and weight, respectively. The annual economic losses due to cetacean and shark depredations for the STLL fishery in Hsinkang port, southeastern Taiwan, were estimated to be USD 478.2 thousand and USD 63.6 thousand, respectively, which corresponded to 4.5% and 0.6% of the total sales of the longline fishery. The yellowfin tuna had the highest depredation loss, followed by the dolphinfish. The catch in number of dolphinfish and the operation depth were significant factors that affected cetacean depredations. Future research should collect more environmental data, increase the sample size and apply alternate models, such as general addictive models, to improve the results of the estimation.