Stable isotope analysis reveals pelagic foraging by the Southern sea lion in central Chile

Journal of Experimental Marine Biology and Ecology 347 (2007) 123 – 133 www.elsevier.com/locate/jembe

Stable isotope analysis reveals pelagic foraging by the Southern sea lion in central Chile L.A. Hückstädt a,b,⁎, C.P. Rojas b , T. Antezana b a

Ocean Sciences Department. University of California Santa Cruz. Long Marine Lab, 100 Shaffer Road, Santa Cruz, CA 95060, USA b Departamento de Oceanografía. Universidad de Concepción, Casilla 160-C, Concepción, Chile Received 16 March 2007; accepted 27 March 2007

Abstract The diet of Southern sea lion Otaria flavescens is poorly known along the coast of central Chile (32°–39°S), where a population of about 17,300 individuals occurs, in an ecosystem that sustains one of the world's most important fishing industries. The primary objective of this study was to reconstruct the diet and estimate Trophic Positions (TPs) of sea lions and their prey off central Chile using stable isotopes (δ13C and δ15N). Our results showed that the diet of sea lions is primarily composed of pelagic prey, with the jack mackerel Trachurus murphyi as the principal prey item in the diet of sea lion (1–99th percentile: 20–66%), while demersal prey accounted for only 0–2.8%. We also found regional differences on the relative contribution of prey to the diet of sea lions. Animals that were sampled close to major fishing areas showed an increase in the relative contribution of jack mackerel to the diet as opposed to animals sampled away from these areas that displayed a relatively more heterogeneous diet. Trophic Positions (TPs) of sea lions prey items ranged between 3.39 for jack mackerel and 4.48 for pink cusk-eel (Genypterus blacodes). The TP for sea lions was 4.57. Hence, our results showed a community composed by at least 5 trophic levels, with sea lion as the top predator. In summary, our study demonstrates that the Southern sea lion is displaying pelagic foraging off central Chile. We highlight the necessity of conducting further research on the trophic ecology and diving behavior of Southern sea lion to obtain a better understanding of their role as top predator. © 2007 Elsevier B.V. All rights reserved. Keywords: Diet; Marine mammal; Otaria flavescens; Prey; δ13C; δ15N; Trophic level

1. Introduction Most sea lions species have been traditionally characterized as benthic foragers, with the exception of the California sea lion Zalophus californianus, which displays an epi- or mesopelagic diving behavior (Costa et al., 2004). The diving behavior of the Southern sea

⁎ Corresponding author. Tel.: +1 831 459 3112; fax: +1 831 459 3383. E-mail address: [email protected] (L.A. Hückstädt). 0022-0981/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2007.03.014

lion (Otaria flavescens) has not been studied along its Pacific range, but some information is available from studies conducted in Argentina on females (Campagna et al., 2001; Thompson et al., 1998; Werner and Campagna, 1995). Off the Argentinean coast, Southern sea lions forage on the shelf, although dives of N300 m have been recorded (Thompson et al., 1998). This diving pattern is likely to be related to the depth of the continental shelf off Argentina. For instance, the range of the deepest dives recorded for female Southern sea lion (N60 m) is similar to the depth of the shelf in that area (Campagna et al., 2001).

124

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

One should expect significant differences in the diving behavior of Southern sea lion foraging along the Atlantic and Pacific coasts, particularly if we consider the striking differences between the two systems. For instance, the continental shelf off Argentina is considerably wider (up to 400 km) than the shelf off Chile (b66 km in central Chile), and thus we would not necessarily expect foraging to be associated with the shelf as has been suggested for Argentinean waters (Hückstädt and Krautz, 2004). The Southern sea lion is the only species of pinniped regularly inhabiting the coast of central Chile (32°–39°S), where a population of 17,300 individuals has been estimated (Aguayo-Lobo et al., 1998). The information available on diet of the species in Chile is limited and mostly out of date (Hückstädt and Antezana, 2006). Although the species has been historically considered an opportunistic predator with a broad diet spectrum for its Atlantic range (Koen-Alonso et al., 2000; Naya et al., 2000), it appears that Southern sea lion along the Chilean coast feeds on a lower diversity of prey items (Hückstädt and Antezana, 2006). Studies conducted on the diet of Southern sea lions off central Chile in the early 80s and late 90s indicate that they fed mainly on Patagonian grenadier (Macruronus magellanicus), cusk-eel (Genypterus spp.), jack mack-

erel (Trachurus murphyi), hake (Merluccius gayi) and anchovy (Engraulis ringens) (Aguayo-Lobo et al., 1998; George-Nascimento et al., 1985). All of these prey items are commercially exploited by local fisheries, resulting in conflicts between sea lions and commercial and minorscale fisheries (Aguayo and Maturana, 1973; Hückstädt and Antezana, 2003; Oporto et al., 1991). The most common method used to reconstruct diets of pinnipeds is the identification of prey remains collected from stomachs, intestines, scats and regurgitations (Tollit et al., 2003). This approach presents several advantages, such as its low cost and the high likelihood of finding samples with identifiable prey (Bowen and Siniff, 1999; Hammill et al., 2005). Yet, the accurate reconstruction of diet using this method is complicated by erosion and differential digestion of hard parts, and also a high proportion of samples with no prey remains (Bowen, 2000; Bowen and Siniff, 1999; Naya et al., 2000). Naturally occurring isotopes of Carbon (C) and Nitrogen (N) are increasingly used to study trophic relationships and feeding habits of marine mammals, based on the idea of a relatively consistent and predictable relationship between predator and prey (Hobson et al., 1996; Lesage et al., 2001; Phillips, 2001; Post, 2002; Vander Zanden and Rasmussen, 2001). Stable isotopes are also used in

Fig. 1. Study area at the marine ecosystem off central Chile. The dots correspond to the locations where Southern sea lion samples were obtained for stable isotopes analysis and the number of samples for each location is also indicated. The isobath of 200 m represents the break of the continental shelf.

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

studies of food webs based on the fact that isotopes either fractionate or change in a predictable fashion between trophic levels and so reflect trophic position. Dietary studies using stable isotopes provide a longer record than traditional methods, indicating that the prey has not only been ingested, but also assimilated, and unlike stomach or scat analysis, all sampling efforts yield information (Bowen and Siniff, 1999; Burton and Koch, 1999; Hobson et al., 1996; Kurle and Worthy, 2001; Lesage et al., 2001). On the other hand, this method does not allow for the identification of the species that are being consumed, and therefore previous knowledge of potential prey sources is necessary (Hobson et al., 1996; Post, 2002; Vander Zanden and Rasmussen, 2001). Here we present a novel analysis on the diet of the Southern sea lion in the central coast of Chile based on the analysis of stable isotope ratios (δ13C and δ15N). Additionally, we present a first insight to the trophic structure of the coastal ecosystem off central Chile, based on the abundances of δ13C and δ15N from Southern sea lion, previously reported sea lion prey items, Humboldt krill and Particulate Organic Matter (POM). 2. Materials and methods 2.1. Collection and analysis of samples We sampled hair and vibrissae from 27 Southern sea lions found dead along the central Chile coast (n = 7), or incidentally caught during fishing operations of both minor-scale (n = 19) and commercial fishing fleet (n = 1) between November 2001 and October 2002 (Fig. 1). We only sampled freshly dead animals without exterior signs of diseases or injuries. Hair samples were collected by extracting a section of ca. 2 cm2 of skin and hair with a scalpel, followed by removal of the skin. Vibrissae were extracted entirely from the root. Samples were preserved at − 18 °C until analysis. Location and date of sampling were recorded for each sample. Information on morphometrics and sex was available for only eight individuals (all males). Ten sea lion prey species were selected for analysis based on the last information available on diet composition of sea lion in central Chile (Aguayo-Lobo et al., 1998; George-Nascimento et al., 1985; Hückstädt and Antezana, 2006) (Table 1). Samples were collected directly from fisheries landings conducted within the Central-Southern Upwelling Region (CSUR) of the Humboldt Current System (HCS) during the austral spring, 2002. A section of ∼ 1 cm3 of muscle per individual was collected using a scalpel.

125

Table 1 δ13C and δ15N values for Southern sea lion and its prey off central Chile

POM Crustacea Humboldt krill Mollusca Jumbo flying squid Chondricthyes Elephant-fish Osteichthyes Anchovy Black cusk-eel b Pink cusk-eel b Red cusk-eel b Hake Jack mackerel Grenadier Herring Mammalia Sea lion c

n

δ13C (‰)

δ15N (‰)

N/A

− 21.60

8.88

30

− 16.04 a

12.37

3

− 15.06

18.52

4

− 13.63

19.20

15 5 6 3 6 5 6 2

− 15.56 − 13.90 − 13.68 − 13.87 − 14.42 − 16.07 − 15.02 − 14.71

17.83 20.71 20.71 18.47 17.67 17.02 20.12 17.67

27

− 12.48 ± 0.68

20.97 ± 0.77

Values for the Humboldt krill and Particulate Organic Matter (POM) are also included. a Carbonate enrichment factor (0.4‰) applied to δ13C value. b Previous studies on the diet of sea lion did not make distinctions among the three species of the genus Genypterus spp. c Values ± SD.

Samples were then homogenized and preserved at − 18 °C. Humboldt krill Euphausia mucronata samples of abdominal muscle were obtained from plankton collected in spring 2002 within the CSUR, homogenized and preserved at − 18 °C until analysis. POM was obtained from water collected using Niskin bottles at 8 m depth (depth of Chlorophyll maximum) in spring 2002 at 36°20′ S and 73°44′ W. The sample was then filtered through Whatman GF/C filters (1.2 μm). 2.2. Chemical analyses Hair and vibrissae samples were washed with distilled water and soap at least five times. Samples were then rinsed with ether petroleum to extract remaining lipids (Kurle and Worthy, 2002), followed by a second washing with distilled water again and dried at 60 °C for 48 h. A study conducted in an otariid, the Steller sea lion Eumetopia jubatus, demonstrated that sea lions exhibit a consistent growth and year-to-year retention of their vibrissae, with average growth rates of 0.11–0.12 mm/ day (Hirons et al., 2001), while the hair is molted annually. Vibrissae samples were thus analyzed for the first centimeter from the base, allowing us to examine the diet of the sea lions during the last year or 2 months,

126

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

and making it comparable to the isotopic data collected from the hair. Muscle samples from all prey items and Humboldt krill were treated to extract lipids (Bligh and Dyer, 1959), due to the fact that lipids are ∼ 6‰ lighter in δ13C relative to proteins, adding variability to δ13C values (Lesage et al., 2001). After this process, muscle samples were dried at 60 °C for 48 h prior to isotopic analyses. POM samples were washed with HCl 0.12 N to eliminate carbonates, which may add undesirable variability to δ13C (Lorrain et al., 2003). Carbonates may also inflate the δ13C values of invertebrates by about 0.4‰, but treatment with hydrochloric acid adversely affects δ15N (Bunn et al., 1995; Jardine et al., 2003; Lesage et al., 2001). Therefore, a correction factor of 0.4‰ was used for the δ13C of krill. The abundance of stable isotopes is expressed in delta (δ) notation using Eq. (1): dX ¼

Rsample  Rstandard  1000 Rstandard

ð1Þ

where δX is the difference in the isotopic composition between the sample and the standard in parts per thousand (‰), and R is the ratio between the heavier and lighter isotope (i.e. 13C/12C, 15N/14N). The standard for δ13C corresponds to PeeDee Belemnite (PDB), and the standard for δ15N is atmospheric nitrogen. Samples were analyzed using a mass spectrometer Thermo Finnigan Delta Plus coupled with an elemental analyzer Thermo Finnigan Flash EA 1112 (PROFC, Universidad de Concepción). The analytical error for sea lions samples was ± 0.02‰ and ± 0.36‰ for δ13C and δ15N, respectively. The analytical error for all other samples was ± 0.05‰ and ± 0.13‰ for δ13C and δ15N, respectively. 2.3. Data analyses All data were tested for normality using the Kolmogorov– Smironov test. The isotopic values of sea lion were corrected using tissue-specific Trophic Enrichment Factors (TEFs) determined for pinnipeds (Hobson et al., 1996). Differences for the corrected isotopic values between hair and vibrissae samples were examined using t-tests, as well as the differences between the two sampling locations Cobquecura and Talcahuano-San Vicente (Fig. 1). Significance was tested at the α = 0.05 level. Power was calculated for tests at the specific sample size and α = 0.05. The contribution of the prey items to the sea lions diet was estimated using the software IsoSource 1.3.1 (http://

www.epa.gov/wed/pages/models/stableIsotopes/isosource/isosource.htm). The IsoSource model provides a formalized, general procedure by which ranges of source contributions can be determined when the number of sources is too large to permit unique solutions from stable isotope mixing models (Hall-Aspland et al., 2005; Phillips, 2001). A first analysis was conducted including 10 prey items identified for the species. For this analysis source increments of 2% and a mass balance tolerance of ± 0.04 were set. Based on these results, we determined a cutoff of 10% of the relative contribution of jack mackerel to the diet of sea lion (jack mackerel mean relative contribution to the diet of sea lion was 49.3%), following a criteria similar to that applied by Ben-David et al. (2004) for brown bears. We conducted a second analysis with IsoSource, including only those items whose relative contribution to the sea lion diet was above the cutoff value. For this analysis, source increments of 1% and a mass balance tolerance of ±0.02 were set. The dissimilarity in diet composition between locations (i.e. Cobquecura and Talcahuano-San Vicente) was estimated using mean prey contributions to the diet, as obtained from IsoSource. Two indexes were used as indicators of dissimilarity of diet: 1. Euclidean distance (Δjk) between locations: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n  X 2 Xij  Xik Djk ¼

ð2Þ

i¼1

where Xij is the proportion of species (prey) i in sample (location) j, Xik is the proportion of species i in sample k, and n is the total number of prey species. 2. Levin's index of niche breadth (B) for sea lion at each location: 1 B¼P 2 pj

ð3Þ

where pj is the proportion of the prey j in the diet. To estimate the Trophic Position (TP) for all species except the sea lions, δ15N values were converted as follows: TP ¼ 1 þ ðDm  POMÞ=TEF

ð4Þ

where Dm =δ15N value for a consumer, POM=δ15N value of Particulate Organic Matter (POM), and TEF = trophic

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

127

Table 2 Contribution of prey items to the diet of Southern sea lion in central Chile

Fig. 2. Mixing polygon for δ13C and δ15N signatures of prey items (●) and Southern sea lions (○) in central Chile.

enrichment factor in δ15N. TEF is typically assumed as 3.4‰ (Vander Zanden and Rasmussen, 2001). For the sea lion, TP was estimated using Eq. (5) (Lesage et al., 2001). TP ¼ 2 þ ðDm  POM  TEFmmt Þ=TEF

ð5Þ

where TEFmmt = tissue-specific trophic enrichment factor in δ15N calculated for pinnipeds (Hobson et al., 1996). 3. Results 3.1. Descriptive analysis The values of δ13C and δ15N for sea lion ranged between − 14.92 and − 11.43‰, and between 19.75 and 23.45‰, respectively (Table 1). The δ13C and δ15N values for sea lion were normally distributed (Kolmogorov–Smironov, all p N 0.05). The values of δ13C and δ 15 N for prey items ranged between − 16.07 and − 13.63‰, and between 17.02 and 20.71‰, respectively (Table 1). When analyzing differences in isotopic values between tissues we found no significant differences for δ13C (t = 1.03, p N 0.05, power = 0.17) nor for δ15N (t = 1.75, p N 0.05, power = 0.39). Similarly, we found no differences in isotope signatures between sampling location for δ13C (t = − 1.25, p N 0.05, power = 0.23) or δ15N (t = − 0.40, p N 0.05, power = 0.07). In all cases, the statistical power was low to detect an effect.

Prey item

Contribution to the diet 1–99 percentiles (%)

Jack mackerel Anchovy Herring Grenadier Squid Hake Elephant-fish Black cusk-eel Pink cusk-eel Red cusk-eel

20–66 0–58 0–20 8–26 0–26 0–8 0–8 0–12 0–12 0–8

Values are shown as 1–99 percentiles for the corresponding distributions.

isotopic signature) falling near the end dominated by pelagic prey (Fig. 2). With all 10 prey items included in the analysis, jack mackerel appeared as the primary food source (1–99th percentile: 20–66%), followed by anchovy (0–58%). Demersal prey items (hake, cusk-eels, and elephant-fish Callorhynchus callorhynchus) were minor components of the diet (only 0–2.8% of the diet) (Table 2). When we conducted the analysis with the four main prey items selected after the cutoff, the figures changed. Anchovy appeared now as the most important prey item, although its distribution of feasible diet proportion was not well constrained (0–88%). Jack mackerel was now the second most important prey item (3–49%), followed by jumbo flying squid Disodicus gigas (0–40%) and grenadier (7–15%). However, when we considered only four prey, the distributions of feasible diet proportion were not well constrained, limiting our analyses.

3.2. Sea lion diet The mixing polygon of sea lion prey items was relatively broad (Fig. 2), with the mixture (sea lion

Fig. 3. Difference in the mean contribution of prey items to the diet of southern sea lion between the locations of Cobquecura (n = 19) and Talcahuano-San Vicente (n = 5), central Chile.

128

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

3.3. Differences in the diet of sea lions between locations

Table 3 Trophic Positions (TPs) of preys included in the diet of southern sea lion estimated from δ15N and reported stomach content data a

The diet of sea lions from Cobquecura was dominated by jack mackerel, followed by anchovy (Fig. 3). There is also a slight increase in the importance of nonpelagic items with respect to the overall data. The diet of sea lions from Talcahuano-San Vicente was dominated by jack mackerel, whose importance in the diet of sea lion increased with respect to Cobquecura, followed by the grenadier (Fig. 3). The Euclidean distance of the diet of sea lion between locations was 30.3%. The analysis of Levin's niche breadth (B) for each location revealed a relatively more generalistic strategy for sea lions from Cobquecura (BCobquecura =4.1), while sea lions from Talcahuano-San Vicente displayed a narrower niche (BTalcahuano-San Vicente =1.99).

Prey

TPδ15N

TPstomach

Jack mackerel Hake Herring Anchovy Red cusk-eel Jumbo flying squid Elephant-fish Patagonian grenadier Black cusk-eel Pink cusk-eel

3.39 3.59 3.59 3.63 3.82 3.84 4.03 4.31 4.32 4.48

3.56–3.82 4.26 2.69 2.70–2.86 ND ND 3.23–3.45 3.93 ND 4.18–4.34

3.4. Ecosystem structure and TPs The δ 13 C varied by 9.21‰ between POM and the sea lions, while the δ15 N varied by 11.94‰ (Table 1). The δ 13 C for fishes ranged between − 16.07 and − 13.63‰, and the δ15N ranged between 17.02 and 20.71‰. Two carbon pathways which the sea lion partake in could be identified based on the δ 13 C values: the demersal or benthic pathway was represented by species with higher δ 13 C values, while species representing the pelagic pathway had lower values of δ13 C (Fig. 4). The TPs varied between 2 for Humboldt krill and 4.57 for sea lions. The TPs of fishes ranged between 3.47 and 4.48, and the TP of squid was 3.84. The highest TP among the sea lion's prey items was occupied by the

Fig. 4. Trophic Position (TP) of southern sea lion (▲) and its prey in central Chile. The black dots indicate pelagic prey. Open circles correspond to demersal prey.

content data

ND = no data found. a Obtained from FishBase (http://www.fishbase.org).

pink cusk-eel G. blacodes, followed by the black cuskeel G. maculatus and the grenadier (Table 3). 4. Discussion Understanding of the diet of Southern sea lion along the Chilean coast has until now relied on limited research conducted 20 years ago (George-Nascimento et al., 1985). Ours is the first published assessment of the diet of southern sea lions off central Chile since the 1980s, and the first to assess the trophic position of sea lions and their prey. Unfortunately, we had no information on morphometrics, age, and sex classes from most of the individuals included in this study, limiting the analyses that could be conducted in our study. However, as all unidentified samples came from fisheries by-caught animals, it is highly likely that they corresponded to males individuals, given that: (1) males interact predominantly with fisheries, at an approximate ratio of 3:1 ratio over females (Hückstädt and Krautz, 2004; Koen-Alonso et al., 2000), (2) males feed mainly on pelagic items (Oliva, 1984). Additionally, all animals whose sex was recorded were male. Therefore, the diet description presented in this study is more likely to correspond to males alone rather than the entire population of Southern sea lions off central Chile. We also acknowledge the potential bias towards diet corresponding with fisheries in which some animals were by-caught. The isotopic analysis of metabolically inactive tissue such as hair and vibrissae yields information on the diet integrated over a time scale of months. This is a comparative advantage with respect to studies of metabolically active tissues that provide information

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

limited to time scales of days to weeks, and stomach contents and scat analyses studies, which present only an instantaneous view of diet (Hall-Aspland et al., 2005; Hobson et al., 1996).

129

Although vibrissae and hair of pinnipeds are similar in biochemical composition, there are considerable differences between both tissues: vibrissae are large overall, highly innervated, present large blood sinuses

Fig. 5. Comparisons of prey contributions to the diet of southern sea lion (bars) as obtained from George-Nascimento et al. (1985) and 2002 (this study). The data from 1980s correspond to % mass obtained from stomach analyses. The solid line corresponds to fish landings. (Source: Servicio Nacional de Pesca, Chile).

130

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

and are controlled by voluntary muscles (Hirons et al., 2001), while hairs present a central cavity or medulla and lack erector pili muscles (Berta et al., 2006). These differences in the structure could be indicative of different isotopic enrichment factors for each tissue. Studies conducted with animals in captivity indicated a higher enrichment in δ13C for vibrissae in comparison to hair (+ 3.2‰ versus + 2.8‰) (Hobson et al., 1996), while δ15N enrichment showed low variability among high-protein tissue. Our results revealed no significant differences in the δ13C and δ15N values between tissues. The analysis between sampling locations showed no significant differences for both isotopes, although a relatively higher mean value was observed at Cobquecura, likely associated to a particular individual with a δ15N of 23.45‰. Changes in the nitrogen isotopic composition of animal tissues can be used as indicators of change in body condition (Gannes et al., 1997; Kelly, 2000; Vander Zanden and Rasmussen, 2001), and this high value could be an indicator of starvation. On the other hand, the relatively lower values of δ15N found in animals from Talcahuano-San Vicente could be a reflection of the better condition of animals at this location. At Talcahuano-San Vicente animals are exposed to a continuous and easily accessible food supply from discards from fishing vessels and fish processing plants in the area, largely dominated by jack mackerel. 4.1. Diet of sea lions The contribution of prey items to the diet is possible to model using isotopic analyses only if the diet is heterogeneous, being dominated by a few prey items (Hammill et al., 2005; Phillips, 2001). Analyses of niche breath of Southern sea lion along the Chilean coast indicate that its diet is dominated by few species, likely associated to the environmental dominance of resources such as anchovy and jack mackerel (Hückstädt and Antezana, 2006). A recent study on harp seals (Pagophilus groenlandicus) highlighted similarities in diet reconstruction using stable isotopes and stomach contents (Hammill et al., 2005). Given these results and George-Nascimento et al. (1985) study on diet of Southern sea lions using stomach contents, we can hypothesize about a significant shift in the last 20 years from a diet dominated by demersal prey to one dominated by pelagic prey (up to 91% of the diet), as is evident from the δ13C results for prey (Fig. 4). Since the Southern sea lion is a plastic predator (Aguayo and Maturana, 1973; George-Nascimento et al.,

1985; Hückstädt and Antezana, 2006; Koen-Alonso et al., 2000), it is expected that the change in diet would reflect changes in the environmental availability of prey. Nonetheless, it is important to keep in mind this apparent shift in diet may also simply be a result of differences in time scales covered by the two methodologies, as our samples reflect prey items that were consumed by sea lions on a scale of weeks to months, and previous studies reflect items consumed on a scale of hours to days. Due to the lack of information available on the variations of prey biomass, we used fish landings as a proxy for environmental availability of prey and examined our results in light of these changes. However, it is necessary to assume that changes in fish landings are related to changes in the availability (i.e. biomass) of the resource in the environment (Fig. 5), which might be done to some extent. The main change in the diet is related to the increase in importance of jack mackerel in the diet of sea lions (Fig. 5). Landings of this species rose continuously from ca. 600,000 tons in 1979 to a historical maximum of 4.4 million tons in 1995, and then decreased and stabilized at about 1.5 million tons after 1999, reaching 1.5 million tons in 2002. Anchovy was not reported in the diet 20 years ago, but in this study it was the second most important prey for the sea lions. The landings of this species have been erratic, with important interannual variations, yet an increasing trend is evident from the early 1980s with landings of few thousand tons per year (e.g. 300,000 in 1981) to a historical maximum of 2.7 million tons in 1994. This species also experienced a decrease of its landings to about 1.5 million tons in 2002. Herring (Strangomera bentincki) was not previously reported in the diet (George-Nascimento et al., 1985). Landings of herring were about 20,000 tons during the 1980s, and during the 1990s landings increased to the order of 300,000–500,000 tons, and 350,000 tons were landed in 2002. On the other hand, the sardine (Sardinops sagax) corresponded to 4.8% mass of the diet in the 1980s but it was not included in this analysis since it is not reported to occur in the area today. Grenadier decreased in importance in the diet of sea lions, but this is not reflected in the landings of the species which, oppositely, increased between the 1980s and 2002. The squid increased from 0.8% mass to 8.3%, but no information on landings of the squid was available. The contribution of demersal prey to today's diet of sea lion was low, with an overall contribution of less than 9% to sea lion diet. Although the figures presented in this analysis match in general terms, this is intended only as a broad

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

analysis, and should not be considered as a direct correlation between the fisheries trends and the diet of sea lions. It is unknown if the diet of sea lion reflected the trends in the fisheries during the entire period, as there is no concurrent time series data of sea lion diet composition available. Even though no significant differences were found for the isotopic values between locations, the power of all these analyses were low. Therefore, differences in diet of sea lions between locations might still be expected. This was confirmed with our estimation of Euclidean distance and niche breath for each location. The distance and difference in niche breadth between locations was attributable to the higher proportion of jack mackerel in the diet of animals from TalcahuanoSan Vicente compared with animals from Cobquecura. Although the niche breadth analysis revealed animals from Talcahuano-San Vicente as more specialized, the dominance of jack mackerel in the diet of these animals could be reflective of the “free-food” condition in that particular location, as discards from jack mackerel’ fishing vessels and plants are available to sea lions. 4.2. Community structure The central Chile community consisted of at least five trophic levels, with the sea lion as the top predator (TP = 4.57). This value is similar to TPs reported for other pinniped species in the North Atlantic (Lesage et al., 2001). Alternatively, killer whales Orcinus orca have been proposed to predate on marine mammals in this area or, at least, on sea lions (Hückstädt and Antezana, 2004), which would imply a sixth trophic level for this ecosystem. The zooplankton community is dominated by Humboldt krill, the most abundant species in the Humboldt Current, with aggregations comparable to those of Antarctic krill E. superba (Antezana, 2001, 2002). This species appears to be an important prey for many pelagic and demersal fishes and whales (Antezana, 1970). Our results confirm Humboldt krill as a primary consumer of POM, especially considering the ability of krill to feed both at the surface and on the boundary of Oxygen Minimum Layer below the main Humboldt Current, where POM is retained (Antezana, 2002). Trophic Position (TP) estimates can be obtained using the dietary approach, based on the TP of prey organisms and volumetric/gravimetric stomach content data, or using the δ15N approach, based on the consistent isotopic enrichment of this element between prey and predator (Vander Zanden et al., 1997). The pelagic

131

fishes included in this study (jack mackerel, anchovy, herring and grenadier), and the squid, displayed a mean TP of 3.75, while the demersal or benthic species had a higher mean TP (4.04). Some differences were found when the values obtained from this study were contrasted with dietary TP estimates based on stomach content data available online (http://www.fishbase.org). The black cusk-eel and jack mackerel showed similar values to those reported based on stomach contents data. The elephant-fish and anchovy had higher TPs than those previously informed, while hake showed a lower TP (Table 3). Although δ15N is highly variable among systems, its use as an estimate of TP provides a time integrated measure of an organism's TP, and accounts for temporal and spatial variation in feeding, while the dietary data provides only a snapshot in time and presents high variability associated with the estimates of prey TPs (Vander Zanden et al., 1997). The stable isotope analyses revealed clear distinctions between the pelagic and demersal carbon pathways in central Chile, with sea lions participating principally in the pelagic system. As expected, sea lions occupied the role of top predator in the ecosystem but its influence on the dynamics and structure of the ecosystem remains unknown. The use of stable isotopes proved valuable to approach the diet of sea lions and the structure of the ecosystem off central Chile; however the analyses of the data obtained and comparisons are limited by the lack of other studies on the species. Members of the family Otariidae (sea lions) have been traditionally characterized as benthic foragers, with the exception of the California sea lion, which displays an epi- or mesopelagic diving behavior (Costa et al., 2004). Indeed, Southern sea lions have been identified as benthic and mid-water foragers along their Atlantic range (Argentinean Patagonia), where the species is associated with a wide continental shelf that can reach up to 400 km (Campagna et al., 2001; Thompson et al., 1998; Werner and Campagna, 1995). However, along the Pacific coast of South America the continental shelf is considerably narrower or even absent, and therefore a divergence in the habits of sea lions can be expected between the Atlantic and Pacific ranges. Our results suggest that Southern sea lions are epi- and mesopelagic foragers along the Chilean coast, a strategy that is also observed off Peru where sea lions prey predominantly on anchovy (Soto et al., 2006). It then becomes evident that conducting further research on the trophic ecology and diving behavior of Southern sea lions is necessary to obtain a better understanding of their role as top predators in the ecosystem and the different strategies utilized by the species in

132

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133

different ecosystems, as well as their interactions with fisheries. Acknowledgments We thank N. Llanos, E. Cisternas, S. Gacitúa, R. Mena, L.A. Cuevas, I. Municipalidad de Cobquecura, Pescadores Artesanales Caleta Rinconada and Pesquera Itata S. A. for their support during the sampling campaigns. R. De Pol and O. Ulloa provided assistance with the isotopic analyses. We also acknowledge M.C. Krautz and Laboratorio de Inmunología (Dpto. Bioquímica Clínica e Inmunología, UdeC) for their help in processing the lipids. K. Yoda provided valuable assistance with the statistical analyses. S. Simmons, A-L Harrison and the Costa Lab at UCSC provided useful comments on the manuscript. This study was funded by Grants in Aid or Research by Society of Marine Mammalogy (L.A.H), and Beca Jorge Tomicic Karzulovic by Sociedad Chilenas de Ciencias del Mar — Minera Escondida Ltda. (L.A.H). This research was part of L.A.H's M.Sc. thesis (Departamento de Oceanografía, UdeC), supported by Beca de Docencia (Escuela de Graduados, UdeC). L.A.H's doctoral studies are supported by the Fulbright-CONICYT (Chile) doctoral fellowships program. [MC] References Aguayo-Lobo, A., Díaz, H., Yáñez, J., Palma, F., Sepúlveda, M., 1998. Censo poblacional del lobo marino común en el litoral de la V a IX Regiones. Fondo de Investigación Pesquera, Valparaíso, p. 214. Aguayo, A., Maturana, R., 1973. Presencia del lobo marino común (Otaria flavescens) en el litoral chileno. Biología Pesquera (Chile) 6, 45–75. Antezana, T., 1970. Eufáusidos de la costa de Chile. Su rol en la Economía del mar. Revista de Biología Marina 14, 19–27. Antezana, T., 2001. Biodiversidad en el ambiente pelágico. In: Alveal, K., Antezana, T. (Eds.), Sustentabilidad de la biodiversidad. Universidad de Concepción, Concepción, pp. 61–84. Antezana, T., 2002. Adaptive behaviour of Euphausia mucronata in relation to the oxygen minimum layer of the Humboldt Current. In: Färber-Lorda, J. (Ed.), Oceanography of the Eastern Pacific. CICESE, Ensenada, pp. 29–40. Ben-David, M., Titus, K., Beier, L.R., 2004. Consumption of salmon by Alaskan brown bears: a trade-off between nutritional requirements and the risk of infanticide? Oecologia (Berlin) 138, 465–474. Berta, A., Sumich, J.L., Kovacs, K.M., 2006. Marine Mammals Evolutionary Biology. Academic Press, London. 547 pp. Bligh, E.G., Dyer, W.J., 1959. A rapid method of total lipid extraction and purification. Canadian Journal of Biochemistry and Physiology 37, 911–917. Bowen, W.D., 2000. Reconstruction of pinniped diets: accounting for complete digestion of otoliths and cephalopod beaks. Canadian Journal of Fisheries and Aquatic Sciences 57, 898–905. Bowen, W.D., Siniff, D.B., 1999. Distribution, population biology, and feeding ecology of marine mammals. In: Reynolds, J.E., Rommel,

S.A. (Eds.), Biology of Marine Mammals. Smithsonian Institution Press, Washington, pp. 423–484. Bunn, S.E., Loneragan, N.R., Kempster, M.A., 1995. Effects of acid washing on stable isotopes ratios of C and N in penaeid shrimp and seagrass: implications for food-web studies using multiple stable isotopes. Limnology and Oceanography 40, 622–625. Burton, R.K., Koch, P.L., 1999. Isotopic tracking of foraging and longdistance migration in northeastern Pacific pinnipeds. Oecologia 119, 578–585. Campagna, C., Werner, R., Karesh, W., Marin, M.R., Koontz, F., Cook, R., Koontz, C., 2001. Movements and lactation at sea of South American sea lions (Otaria flavescens). Journal of Zoology (London) 255, 205–220. Costa, D.P., Kuhn, C.E., Weise, M.J., Shaffer, S.A., Arnould, J.P.Y., 2004. When does physiology limit the foraging behaviour of freely diving mammals? International Congress Series 1275, 359–366. Gannes, L.Z., O'Brien, D.M., Martínez del Río, C., 1997. Stable isotopes in animal ecology: assumptions, caveats, and a call for more laboratory experiments. Ecology 78, 1271–1276. George-Nascimento, M., Bustamante, R., Oyarzún, C., 1985. Feeding ecology of the South American sea lion Otaria flavescens: food contents and food selectivity. Marine Ecology Progress Series 21, 135–143. Hall-Aspland, S.A., Hall, A.P., Rogers, T.L., 2005. A new approach to the solution of the linear mixing model for a single isotope: application to the case of an opportunistic predator. Oecologia (Berlin) 143, 143–147. Hammill, M.O., Lesage, V., Carter, P., 2005. What do harp seals eat? Comparing diet composition from different compartments of the digestive tract with diets estimated from stable-isotope ratios. Canadian Journal of Zoology 83, 1365–1372. Hirons, A.C., Schell, D.M., St. Aubin, D.J., 2001. Growth rates of vibrissae of harbor seals (Phoca vitulina) and Steller sea lions (Eumatopias jubatus). Canadian Journal of Zoology 79, 1053–1061. Hobson, K.A., Schell, D.M., Renouf, D., Noseworthy, E., 1996. Stable carbon and nitrogen isotopic fractionation between diet and tissues of captive seals: implications for dietary reconstructions involving marine mammals. Canadian Journal of Fisheries and Aquatic Sciences 53, 528–533. Hückstädt, L.A., Antezana, T., 2003. Behaviour of the southern sea lion (Otaria flavescens) and consumption of the catch during purse-seining for jack mackerel (Trachurus symmetricus) off central Chile. ICES Journal of Marine Science 60, 1003–1011. Hückstädt, L.A., Antezana, T., 2004. Behaviour of Southern sea lions in presence of killer whales during fishing operations in Central Chile. Scientia Marina 68, 295–298. Hückstädt, L.A., Krautz, M.C., 2004. Interaction between southern sea lions Otaria flavescens and jack mackerel Trachurus symmetricus commercial fishery off Central Chile: a geostatistical approach. Marine Ecology Progress Series 282, 285–294. Hückstädt, L.A., Antezana, T., 2006. The diet of Otaria flavescens in Chile: What do we know? In: Trites, A., Atkinson, S., DeMaster, D., Fritz, L., Gelatt, T., Rea, L., Wynne, K. (Eds.), Sea Lions of the World. Alaska Sea Grant College Program. University of Alaska Fairbanks, Fairbanks. Jardine, T.D., McGeachy, S.A., Paton, C.M., Savoie, M., Cunjak, R.A., 2003. Stable isotopes in aquatic systems: sample preparation, analysis, and interpretation. Canadian Manuscript Report of Fisheries and Aquatic Sciences 2656, 39. Kelly, J.F., 2000. Stable isotopes of carbon and nitrogen in the study of avian and mammalian trophic ecology. Canadian Journal of Zoology 78, 1–27.

L.A. Hückstädt et al. / Journal of Experimental Marine Biology and Ecology 347 (2007) 123–133 Koen-Alonso, M., Crespo, E.A., Pedraza, S.N., García, N.A., Coscarella, M.A., 2000. Food habits of the South American sea lion Otaria flavescens, off Patagonia, Argentina. Fisheries Bulletin 98, 250–263. Kurle, C.M., Worthy, G.A.J., 2001. Stable isotope assessment of temporal and geographic differences in feeding ecology of northern fur seals (Callorhinus ursinus) and their prey. Oecologia 126, 254–265. Kurle, C.M., Worthy, G.A.J., 2002. Stable nitrogen and carbon isotope ratios in multiple tissues of the northern fur seal Callorhinus ursinus: implications for dietary and migratory reconstructions. Marine Ecology Progress Series 236, 289–300. Lesage, V., Hammill, M.O., Kovacs, K.M., 2001. Marine mammals and the community structure of the Estuary and Gulf of St Lawrence, Canada: evidence from stable isotope analysis. Marine Ecology Progress Series 210, 203–221. Lorrain, A., Savoye, N., Chauvaud, L., Paulet, Y., Naulet, N., 2003. Decarbonation and preservation method for the analysis of organic C and N contents and stable isotope ratios of low-carbonated suspended particulate material. Analytica Chemica Acta 491, 125–133. Naya, D.E., Vargas, R., Arim, M., 2000. Análisis Preliminar de la Dieta del León Marino del Sur (Otaria flavescens) en Isla de Lobos, Uruguay. Boletín de la Sociedad de Zoología, Uruguay (2° época) 12, 14–21. Oliva, D., 1984. Espectro trófico y circaritmos de actividad alimentaria en loberías permanentes y temporarias de Otaria byronia (Blainville, 1820) (Carnivora: Otariidae), Departamento de Oceanología. Universidad de Valparaíso, Valparaíso, p. 111. Oporto, J.A., Mercado, C.L., Brieva, L.M., 1991. Conflicting Interactions Between Coastal Fisheries and Pinnipeds in Southern

133

Chile, Benguela Ecology Programme Workshop on Seal-Fishery Biological Interacions, Cape Town, South Africa, pp. 14. Phillips, D.L., 2001. Mixing models in analyses of diet using multiple stable isotopes: a critique. Oecologia 127, 166–170. Post, D.M., 2002. Using stable isotopes to estimate trophic position: models, methods, and assumptions. Ecology 83, 703–718. Soto, K.H., Trites, A.W., Arias-Schreiber, M., 2006. Changes in diet and maternal attendance of South American sea lions indicate changes in the marine environment and prey abundance. Marine Ecology Progress Series 312, 277–290. Thompson, D., Duck, C.D., McConnell, B.J., Garrett, J., 1998. Foraging behaviour and diet of lactating female southern sea lions (Otaria flavescens) in the Falkland Islands. Journal of Zoology, London 246, 135–146. Tollit, D.J., Wong, M., Winship, A.J., Rosen, D.A.S., Trites, A.W., 2003. Quantifying errors associated with using prey skeletal structures from fecal samples to determine the diet of Steller's sea lion (Eumetopias jubatus). Marine Mammal Science 19, 724–744. Vander Zanden, M.J., Rasmussen, J.B., 2001. Variation in δ15N and δ13C trophic fractionation: implications for aquatic food web studies. Limnology and Oceanography 46, 2061–2066. Vander Zanden, M.J., Cabana, G., Rasmussen, J.B., 1997. Comparing trophic position of freshwater fish calculated using stable nitrogen isotope ratios (Δ15N) and literature dietary data. Canadian Journal of Fisheries and Aquatic Sciences 54, 1142–1158. Werner, R., Campagna, C., 1995. Diving behaviour of lactating southern sea lions (Otaria flavescens) in Patagonia. Canadian Journal of Zoology 73, 1975–1982.