Feeding Habits of the Blue Shark (Prionace glauca) in the Northwestern Pacific Based on Stomach Contents and Stable Isotope Ratios1
The blue shark, Prionace glauca (L.), is one of the most abundant pelagic sharks, and determining their feeding habits is important for understanding the ocean ecosystem. Generally, stomach contents analysis has been used to evaluate its feeding habits; however simply analyzing stomach contents reveals short-term feeding habits. In this study, we examined feeding habits of blue sharks in the Northwestern Pacific based on results of both stomach contents and stable isotope analyses. The most dominant species and taxonomic group in the stomach were identified as anchovy and Oegopsida species, respectively. Mean δ13C and δ15N values of blue sharks were 18.5‰ and 12.1‰, respectively, which were higher than those of the prey items. Estimated proportional distribution of prey species varied according to discrimination factors used for stable carbon and nitrogen isotope ratios. Proportion of neritic or mesopelagic nekton, such as anchovy and lanternfish, was higher than that of other prey items including squids. We conclude that blue sharks capture prey ranging from neritic to mesopelagic animals through diel vertical migration.
The blue shark, Prionace glauca (L.), is widely distributed in oceanic waters, from tropical to temperate areas (Compagno 1984). As one of the most abundant pelagic sharks, large numbers of blue sharks are caught by the world’s fisheries, principally as bycatch on longlines and in gill nets (Nakano and Stevens 2008). This species is also abundant in the Northwestern Pacific, where the catch rate declined in the early 1980s but subsequently increased in the 1990s (ISC 2014; Hiraoka et al. 2016; Ohshimo, Fujinami, et al. 2016). Juveniles and semiadults of sharks such as blue shark and shortfin mako (Isurus oxyrhin chus) are distributed north of the Kuroshio–Oyashio transition zone, which is a highly productive area (Nakano 1994, Kai et al. 2015). Sharks play a major role in the exchange of energy between the upper and lower trophic levels in the marine environment (Markaida and Sosa-Nishizaki 2010).
The feeding habits of blue sharks based on stomach contents analysis have been reported from the Northwestern Pacific (Kubodera et al. 2007), Northeastern Pacific (Markaida and Sosa-Nishizaki 2010, Preti et al. 2012, Hernández-Aguilar et al. 2015), South Pacific (Lopez et al. 2010), and the Atlantic Ocean (Stevens 1973, Clarke et al. 1996, McCord and Campana 2003). Those previous studies suggested that blue sharks feed on a large variety of fishes and cephalopods; however, they were conducted in coastal areas or limited by season. To describe the comprehensive feeding habits of blue sharks, samples should be analyzed from a wide area and from a range of shark sizes, as well as from all seasons. In addition, large fluctuations in the population dynamics of small pelagic species, such as anchovy and sardine, have been observed in the Northwestern Pacific and adjacent waters (e.g., Komatsu et al. 2002, Ohshimo et al. 2009). Therefore, the feeding habits of sharks could change owing to such a regime shift in [End Page 21] the pelagic ecosystem. A regime shift is a persistent change in an ecosystem’s structure and function, which can be abrupt and difficult to reverse.
Some authors have criticized stomach contents analyses for being merely a snapshot sample (e.g., Basker et al. 2014), and some biases have occurred from regurgitation during capture (Vignon and Dierking 2011). Therefore, analyzing stomach contents reveals only the short-term feeding habits of animals. In contrast, stable isotope analyses provide information relating to long-term trophic interaction patterns (e.g., MacNeil et al. 2005). Stable carbon and nitrogen isotope ratios are commonly used to determine food web and trophic positions of sharks (e.g., Logan and Lutcavage 2010, Cardona et al. 2012, Malpica-Cruz et al. 2013). These stable isotope ratios provide considerably more robust results when quantifying feeding preferences compared with the stomach contents approach. Recently, Hernández-Aguilar et al. (2015) reported the diet composition of blue sharks off the west coast of Baja California Sur, Mexico, based on results of both stomach content and stable isotope analyses.
In this study, we investigated the feeding habits of blue sharks caught in the Northwestern Pacific from 1999 to 2015 based on combined stomach content and stable isotope analyses to describe the quantitative importance of various prey items in the blue shark diet.
materials and methods
Sampling and Measurements
Blue shark samples for the stomach contents analysis were collected by longline (n = 399), midwater trawling (n = 58), and drift net (n = 3) fisheries operated in the Northwestern Pacific from 1999 to 2014 (Figure 1). The blue shark samples used for the stable isotope analysis were collected by longline (n = 101) and midwater trawling (n = 19) from 2010 to 2015 (Figure 1). The longline bait was mackerel (Scomber sp.) and Japanese sardine (Sardinops melanostictus). Sex was determined by the presence of the copulatory organ. Precaudal length (PCL: length from the tip of the nose to the precaudal pit) and dorsal length (DL: length between the origin of first and second dorsal fin) of blue sharks were measured to the nearest centimeter. As the head and viscera were removed from sharks caught by the commercial vessels, DL was converted to PCL using a conversion equation (male: PCL = 2.51 DL + 12.33; female: PCL = 2.62 DL + 7.48) (Fujinami et al. 2017). The whole stomachs of all blue sharks were removed on board and frozen immediately for the stomach contents analysis. Similarly, the white muscles of sharks were excised and frozen for the stable isotope analysis.
Although the items in the stomach were identified in this study, in many cases the items were almost completely digested, making them unsuitable for the stable isotope analysis. Therefore, the prey species (n = 64) were collected by midwater trawling from 2014 to 2015 (Figure 1). These samples were collected at night when the prey moved to the surface layer and possibility of catch was higher. The net measured 30 m high by 30 m wide, and the towing speed and duration were about 3 knots (5.6 km / hr) and 30 min, respectively. After the trawl net was retrieved, the captured specimens were sorted by species and frozen in seawater. The specimens were brought to the laboratory and measured for body length (standard length for fish and mantle length for squid) and body weight. The muscle of fish or mantle of squid was removed and frozen for the stable isotope analysis.
Stomach Contents Analysis
A total of 460 specimens (303 males, 139 females, and 18 sharks with no sex recorded) was collected for the stomach contents analysis. The size of males ranged from 62.3 to 224.0 cm PCL, and that of females from 60.9 to 209.0 cm PCL (Figure 2). Each stomach was thawed and weighed before its contents were identified. The stomach contents were placed in a plastic pan, and the inner wall of the stomach was rinsed carefully to collect all prey remnants. In some cases, the stomach was empty, and the number of such sharks was [End Page 22] recorded. Undigested and partially digested fish and cephalopods were sorted and identified to the lowest taxonomic level possible based on external morphology, and digested prey items were identified based on the otoliths in fish and the lower beak of cephalopods (Clarke 1986, Rodhouse and Yeatman 1990, Lu and Ickeringill 2002, Xavier and Cherel 2009).
After identifying the species in the stomachs, we calculated the mean number and weight of prey items monthly and annually, the percentage composition by number (%N), the weight percentage (%W), and the percentage frequency of occurrence (%F). The main food items were identified using the index of relative importance (IRI) proposed by Pinkas et al. (1971) and the percentage IRI (%IRI) (Cortés 1997):
where i represents species i. [End Page 23]
A randomized cumulative prey curve (Ferry and Cailliet 1996) was constructed using the vegan package (Oksanen et al. 2010) in R statistical software to determine whether the number of analyzed stomach samples was sufficient to describe the blue shark diet.
Stable Isotope Analysis
Blue sharks for the stable isotope analysis ranged from 33.4 to 230.7 cm PCL for males and from 37.2 to 256.4 cm PCL for females (Figure 2). The sampled lateral white muscle of fish and mantle of squid were freeze-dried, smashed, and placed in a centrifuge tube for the defatting procedure. Subsequently, the samples were treated with a 2 : 1 chloroform-methanol solution and methanol to remove lipids and minimize the differences in13C caused by the variable fatty tissue content among individuals (Schoeninger and DeNiro 1984). The shark tissues were thoroughly rinsed in deionized water for 15 min to remove urea (Kim and Koch 2012, Carlisle et al. 2017).
After removing the lipids and urea, each sample was air-dried at 60°C overnight, and a 1 mg subsample was analyzed. The stable carbon and nitrogen isotope ratios were analyzed at the Isotope Research Institute (Yokohama, Japan) using a stable isotope ratio mass spectrometer (Delta V Advantage, Thermo Fisher Scientific, Yokohama, Japan). The stable isotope ratios are presented as the per mil (‰) deviation from the respective international standards, calculated using the following equation:
where R is the ratio of15N/14N or13C/12C. Atmospheric nitrogen and Vienna Pee Dee Belemnite limestone (V-PDB) were used as the nitrogen and carbon standards, respectively. Analytical precision of isotope analyses was ±0.20‰ for both δ15N and δ13C.
To calculate the relative contribution of potential prey species to the blue shark diet, we used the Bayesian mixing model Stable Isotope Analysis in R (SIAR) (Parnell et al. 2010). The SIAR model parameters were the isotope ratios and the elemental concentrations of potential food sources, the tissue isotope ratio, and the carbon and nitrogen trophic shift (i.e., isotopic enrichment) from prey to predator. Prey-to-predator isotopic discrimination factors for fish, mammals, and loggerhead sea turtles were taken from Reich et al. (2008) and Caut et al. (2009). Although SIAR incorporates uncertainty about diet-tissue isotopic discrimination factors (∆13C and ∆15N) in the form of a standard deviation, we used three combinations of the discrimination factor from Kim et al. (2012) (∆13C = 1.7‰, ∆15N = 3.7‰), Hussey et al. (2009) (∆13C = 0.9‰, ∆15N = 2.3‰), and DeNiro and Epstein (1981) (∆13C = 1.1‰, ∆15N =3.4‰). [End Page 24]
Stomach Contents Analysis
Of the 460 blue shark stomachs that were assessed, 221 (48.0%) contained prey items (Table 1). Few shark specimens were collected during January and March, and the fraction of individuals with empty stomachs ranged from 16.7% in August to 76.9% in July (Table 1). The cumulative prey curve that was created to describe the increasing trend did not reach full asymptotic stabilization (Figure 3).
Five classes of animals were found in the stomachs of blue sharks: Mammalia, Chondrichthyes, Actinopterygii, Cephalopoda, and Malacostraca. The most abundant prey species was anchovy (Engraulis japonicus), followed by krill (Euphausiacea). In this study, krill (euphausiids), amphipods, and decapods were presumed to be secondary prey consumed by the other prey species. The prey species with the greatest percentage weight was mackerel (Table 2); however, because mackerel and Japanese sardine were used as longline bait in this study, these prey species were removed from the analysis. Anchovy and neon flying squid (Ommastrephes bar tramii) were the highest percentage prey species by weight (Table 2). The monthly mean number and weight of prey items were calculated, and the highest values were recorded in August (Table 1) and September (Table 2), respectively. Few blue shark specimens were captured in 2001, 2002, and 2014 (Supplemental Table S1 and Table S2). Anchovy was the prey consumed in the largest number in 2006. The highest mean number and weight of prey items were observed in 2006. In addition, there was no strong variation in prey composition among sampling areas and growth stages (Supplemental Tables S3–S6).
Figure 4 illustrates the %N, %W, and %F of four categories of prey species consumed by blue sharks: anchovy (Clupeiformes), Myctophiformes (lanternfishes), Oegopsida (squids), Octopoda (octopuses), and others. The %N values for anchovy and Oegopsida were 64.7% and 22.5%, respectively. However, the %W value for anchovy (14.0%) was lower than that for Oegopsida (61.1%). The highest IRI species value was for anchovy (410.7), and the IRI prey category value for Oegopsida was 2618.9 (Table 3). In addition, the %IRI anchovy and Oegopsida values were 12.7% and 80.7%, respectively. However, the IRI and %IRI of Oegopsida species values were relatively low except for unidentified Oegopsida. The important prey items in the diets of males and females were similar: the highest %IRI values of prey species for males were Oegopsida (89.5%), anchovy (3.9%), and Octopoda (2.6%) and those for females were Oegopsida (79.0%), anchovy (9.0%), and Myctophiformes (6.9%).
Stable Isotopes Analysis
The mean δ13C and δ15N values for blue sharks were −18.5‰ and 12.1‰, respectively (Table 4). The stable nitrogen isotope ratios of all prey species were lower than those of blue sharks, whereas the carbon isotope ratio of some cephalopods was larger than that of blue sharks. In addition, the stable isotope ratios of Oegopsida were generally higher [End Page 25] than those of fish (Figure 5). The prey species with the lowest mean δ15N value was anchovy(9.3‰), and that with the lowest mean δ13C was Gonatus pyros (−19.9‰). The prey species with the highest mean δ13C was Gonatus berryi (− 17.8‰), and the highest mean δ15N value was for Taningia danae (11.5‰).
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The feasible contribution of each prey species to the diet of blue sharks was calculated using the SIAR package. The proportions of anchovy, Warming’s lantern fish (Cerato scopelus warmingii), and prickly lanternfish (Myctophum asperum) were higher than those of other species (Figure 6) for all combinations of the carbon and nitrogen discrimination factors. The proportions of these species were almost 0.2; however, the other species were below 0.1 even though Oegopsida was often observed in the blue shark stomachs.
Kubodera et al. (2007) reported on the feeding habits of blue and salmon sharks in the transition region of the North Pacific during 1999 and 2000 based on stomach contents analysis. They reported that the important prey species for blue sharks were meso-to bathypelagic cephalopods (e.g., Chiroteuthis calyx, Haliphron atlanticus, Histioteuthis dofleini, and Belonella borealis) and small myctophid fish (e.g., Maurolicus imperatorius). Our stomach contents analysis indicated that the most [End Page 30] important prey for blue sharks was anchovy and Oegopsida. Kubodera et al. (2007) surveyed sharks during April–May, whereas anchovy were observed mainly in August and September in our study. Murase et al. (2012) reported that anchovy were widely distributed in the Kuroshio–Oyashio transition zone, and anchovy was the most abundant pelagic species in this region in the 2000s. Thus, it is likely that the discrepancy between the findings of Kubodera et al. (2007) and this study resulted from differences in the survey area and sampling season. In addition, the number of fish sampled in our study (n = 460) was much larger than that of Kubodera et al. (2007) (n = 70). Therefore, the stomach contents of blue sharks captured in this study may be more representative.
Reported important prey items of blue sharks based on stomach contents analysis varies among studies. For example, Clarke et al. (1996) and Markaida and Sosa-Nishizaki (2010) reported that the most important prey were cephalopods in the waters off the Azores, and in the waters off Ensenada, Baja California, Mexico, respectively. In contrast, McCord and Campana (2003) reported that blue sharks off Nova Scotia feed primarily on pelagic teleost fishes. They also suggested that the differences of prey importance among studies would be due to a seasonal factor or abundance of prey species. In conclusion, blue sharks seem to feed on the spatial-temporally abundant prey species; thus, blue sharks are considered to be opportunistic feeders.
Stable isotope carbon and nitrogen ratios for blue sharks in the Northwestern Pacific have been reported (Takai et al. 2007; Ohshimo, Tanaka et al. 2016). The δ15N (12.0‰) and δ13C (−18.1‰) values reported by Takai et al. (2007) are similar to those of this study (δ15N: 12.1‰, δ13C: −18.5‰), whereas Ohshimo, Tanaka, et al. (2016) reported a higher δ15N value (13.8‰) for the East China Sea. Tanaka et al. (2008) studied differences in the stable isotope ratios of anchovy among sampling areas and found that values in the offshore area of the Pacific were significantly lower than those in coastal areas. In addition, several authors reported geographical differences in δ15N value between the Western and Eastern Pacific (e.g., Madigan et al. 2014) and showed that the δ15N value in the Western Pacific was lower than that in the Eastern Pacific. The δ15N value in our study was also lower than that reported in the Eastern Pacific [15.2‰: Madigan et al. (2012); 16.5‰: HernándezAguilar et al. (2015)]. Although the baseline zooplankton δ15N value varies annually and seasonally (e.g., McMahon et al. 2013), the effect of baseline fluctuations should be analyzed in the future. In addition, to take into account these baseline fluctuations, fine-scale sampling of prey species both temporally and spatially would be valuable.
The ∆13C and ∆15N values are important for calculating trophic position and estimating predator-prey interactions (Hussey et al. 2014) because the discrimination factor highly affects the proportion of prey item calculated by SIAR (Bond and Diamond 2011). Usually, ∆13C and ∆15N values of about 1.0‰ and 3.0–4.0‰, respectively, are used (e.g., DeNiro and Epstein 1981). However, Caut et al. (2009) reviewed ∆13C and ∆15N in animals and found that those values have negative relationships with stable isotope ratios. Hussey et al. (2009) reported that ∆13C and ∆15N values in lipid-extracted muscle of large sharks were 0.9‰ and 2.3‰, respectively. In this study, we evaluated the proportion of prey items in the blue shark diet using three combinations of ∆13C and ∆15N (DeNiro and Epstein 1981, Hussey et al. 2009; Kim et al. 2012), and all results were similar. In all combinations of discrimination factors, our results indicated that the relative contributions of anchovy and small mesopelagic myctophids were higher than that of Oegopsida species, although the results from stomach contents analyses indicated that the %IRI values of anchovy and Oegopsida were high. The stable isotope analysis can predict the actual feeding habits of blue sharks; however, we measured the stable isotope values of only 15 species. Although sampling the other species, such as epipelagic and mesopelagic fish and squid, in the open ocean is difficult, we plan to measure ∆13C and ∆15N values for these other potential prey in the future. [End Page 31]
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Stomach contents analysis has been criticized as providing only a snapshot of predator diet (e.g., Basker et al. 2014). In our study, blue sharks were caught mainly by longline at night. A hooked blue shark cannot capture additional prey, and the prey could be digested. Actually, the cumulative prey curve did not reach an asymptotic value, so our sample size was insufficient despite being much larger than that used in previous studies. In addition, the calculated mean number or weight of blue shark prey items was underestimated because most prey items were already digested. The digestion rate depends on prey type and size (Macdonald et al. 1982). In general, larger and bony fish take longer to digest, and fish otoliths and the squid beak cannot be digested, so squid were more likely to be recorded in the stomach, indicating that squid (Oegopsida) in the stomach contents tended to be overestimated as an important prey species.
Chavez et al. (2003) reviewed the relationships between climate change and population fluctuations of small pelagic fish in the genera Engraulis and Sardinops in the Pacific Ocean. They reported that environmental conditions in the Pacific changed from a cool “anchovy regime” to a warm “sardine regime” in the 1970s, and a shift back to an anchovy regime occurred in the mid- to late 1990s. Murase et al. (2012) reported that 1.5–3.4 million tons of Japanese anchovy were present in the offshore area of the Northwestern Pacific between 2004 and 2007. Some larval Japanese anchovy that hatch on the coastal side of the Kuroshio are believed to be transported offshore by the Kuroshio Extension, where they migrate northward into the Kuroshio–Oyashio transition region (Takasuka and Aoki 2002). Japanese anchovy were mainly observed in blue shark stomachs in August and September. Therefore, although this species is important, it is a seasonally abundant prey item for blue sharks. No reports are available on the stock sizes of lanternfish or squid in the current survey area, but these are mesopelagic taxa and would likely have large stock size. Diel vertical migration of blue sharks has been reported by an electronic tagging study (e.g., Campana et al. 2011), but there are no [End Page 33]
[End Page 34] reports on the horizontal migration of blue sharks in the Northwestern Pacific. Thus, future research should include surveys on blue shark behavior and the horizontal and vertical distributions of its prey items.
Based on our observations of the stomach contents of blue sharks in the Northwestern Pacific, the main prey items were anchovy and squids. The stable isotope ratio analyses with a mixing model indicated that the main blue shark prey species were anchovy and myctophid fish. Our stomach analysis results demonstrated that a large number of samples is needed to describe the overall trophic diversity of the blue shark diet. On the other hand, the stable isotope analysis showed a result similar to that of the stomach contents analysis despite the small sample size. The combination of stomach contents and stable isotope analyses was a better method to avoid observation error bias. The large IRI value of anchovy reflected the capture of this species in summer, when the distribution area of anchovy expands in the North Pacific, and this species was seasonally abundant for blue sharks. We conclude that blue sharks capture prey ranging from neritic to mesopelagic animals through diel vertical migration. [End Page 35]
Corresponding author (e-mail: firstname.lastname@example.org)
We thank the captains and crews of commercial and research vessels for providing the samples. We also thank Y. Semba andY. Hiraoka, Fisheries Research Agency, for their valuable suggestions on the manuscript. Finally, we sincerely wish to thank three anonymous reviewers for their useful comments on the manuscript.
1. This study was supported by the Fisheries Agency, Government of Japan. Manuscript accepted 18 May 2017