TROPHIC INTERRELATIONS BETWEEN INTRODUCED COMMON CARP, CYPRINUS CARPIO (ACTINOPTERYGII: CYPRINIFORMES: CYPRINIDAE), AND FISH COMMUNITY IN A EUTROPHIC SHALLOW LAKE

The common carp, Cyprinus carpio Linnaeus, 1758, is a fish species that has been widely introduced for farming and commercial fishing purposes. It was probably one of the first species to be dispersed by humans (Billard 1999, Vilizzi 2012) because its capability to establish itself readily in freshwater ecosystems in subtropical and temperate regions (Zambrano et al. 2006). In Mexico, common carp ACTA ICHTHYOLOGICA ET PISCATORIA (2014) 44 (1): 45–58 DOI: 10.3750/AIP2014.44.1.06

was introduced and cultivated for human consumption as one of the solutions to increase animal-protein intake in rural human communities by national programs during the 1930s (Zambrano and Macías-García 2000). The common carp currently inhabits most freshwater systems of Mexico (>80%) and is established in the larger lakes of central Mexico such as Lago de Chapala, Lago de Cuitzeo, Lago de Pátzcuaro, Lago de Yuriria, and Lago de Xochimilco (Anonymous 2010). Cyprinus carpio was released accidentally into Lago de Pátzcuaro in 1974 from aquaculture pens (Rosas 1976) and currently it is found throughout the lake with spatial segregation, with increased abundance in the southern zone (Ramírez Herrejón unpublished * ).
The feeding behaviour of the common carp can cause increased sediment re-suspension and can directly affect fish habitat and destroy aquatic vegetation cover, and increasing water turbidity in shallow lakes (Zambrano et al. 2001, Leung et al. 2002, Scheffer et al. 2003, Özbay 2008. This kind of changes in habitat characteristics can indirectly cause a cascade of alterations on trophic webs (Khan 2003).
Cyprinus carpio is an omnivorous fish well adapted for bottom feeding; depending on the availability it can consume a wide range of small items like worms, molluscs, zooplankton, aquatic vegetation, plant debris, detritus, and insects (Summerfelt et al. 1971, Eder and Carlson 1977, Crivelli 1981, Powles et al. 1983, Chapman and Fernando 1994, Elías-Fernández and Navarrete-Salgado 1998, Colautti and Remes Lenicov 2001. The diversity of its diet makes this species resistant to food web change and capable of inhabiting a wide variety of habitats (Koehn 2004, Wolfe et al. 2009, Weber and Brown 2011. However, despite knowing the common carp feeding habits, understanding is limited regarding the trophic interrelations with the local fish biota co-inhabiting the lakes. Lago de Pátzcuaro harbours native and introduced fish with different trophic specialization types such as, hervibores, carnivores (zooplanctivorous, insectivorous, and piscivorous) and omnivores (Rosas 1976, Berlanga-Robles et al. 2002. The physical and chemical water properties of this lake are well described (Alcocer and Bernal-Brooks 2002) and their habitat too (Ramírez-Herrejón et al. 2013). For these reasons, we postulated that Lago de Pátzcuaro probably would be a good model for assessing trophic interrelations between common carp and the lake fish fauna as well as the associations of habitat characteristics with trophic interrelations. This study hypothesizes that C. carpio is a omnivorous generalist fish with a diet that overlaps with all fish taxa at all sites regardless of habitat characteristics. To test this hypothesis, we characterized diet composition, diet breadth, trophic position and diet overlap between fish fauna from sites with different habitat characteristics during the wet and the dry seasons.

MATERIALS AND METHODS Study area.
Lago de Pátzcuaro is located in the north-central part of the state of Michoacán,Mexico (19°27′N,101°26′W and 19°44′N,101°53′W) 2038 m above sea level (Fig. 1). The lake belongs to the hydrologic region of the Lerma-Chapala, as part of the Mexican Volcanic Belt. The lake surface has a maximum area of 116 km 2 and a maximum depth of 12.2 m (Gomez-Tagle Chavez et al. 2002). The average depth is 4.9 m, and watershed covers of 9340 km 2 (Bravo-Espinosa et al. 2006). During September and November 2009 (wet season) and February and June 2010 (dry season), we sampled six sites the water properties (total dissolved solids, transparency, and turbidity) and habitat characteristics (depth, floating vegetation cover, and bottom type) as described by Ramírez-Herrejón et al. (2013) -Sosa et al. 2002, Bloom et al. 2009), thereby making presently specific determination not possible. Accumulated prey diversity was measured, using Simpson's index for a finite population to determine the minimum number of guts to characterize the feeding habits of each fish taxa (Magurran 2004 (Hynes 1950) for gut content analysis was used. Prey items were identified to equivalent taxa level (Edmondson 1959, Pennak 1978. We only found remains of insects and it was not possible the identification of insects at a lower taxonomical level. A modified version of the index of relative importance (IRI) of Pinkas et al. (1971) proposed by Yáñez-Arancibia et al. (1976) was used: IRI = FA · 100 -1 where: F is the frequency of occurrence, and A is the area, following Ramírez-Herrejón et al. (2013). This method is used when the gut content is constituted by small feeding components (diatoms, copepods, ostracods, rotifers, cladocerans) or when its quantification is not possible (detritus, plant debris) (Vega Cendejas 1990, Canto-Maza andVega-Cendejas 2008). The graphical method using the A [%] and F [%] proposed by Costello (1990) and IRI expressed as percentages (Cortés 1997) were used to describe and compare easily the importance of prey. Individuals of small sized (<100 mm SL; Goodea atripinnis, Alloophorus robustus, Chirostoma spp.) were categorized in 5 mm intervals while large fish (>100 mm SL; Cyprinus carpio, Oreochromis spp.) were categorized in 10 mm intervals to determine a probable diet change with fish growth. The standardized Levins' Index (B A was used to calculate diet breadth (values from 0 to 1). Values of B A < 0.60 consider the fish to be specialists and a value of B A > 0.60 denotes a generalist fish (Krebs 1989). Feeding behaviour was described by the Omnivory Index (OI); it was calculated as the variance of the trophic levels of a consumer's preys (Christensen and Pauly 1992). The trophic level of each fish taxa was estimated using the TrophLab Program (Pauly et al. 2000). It includes the number of groups in the diet, the prey fraction of the diet (IRI%), and the trophic level of the prey. The diet overlap between fish taxa was assessed with the index of Horn (Krebs 1989). Horn's index varies from 0 when there is no common use of feeding resources to 1.0 when there is complete resource overlap; it is considered as significant overlap when the value exceeds 0.60 (Wallace 1981). Some studies have argued that gut content analysis is biased because it considers ingested but not assimilated food (Bearhop et al. 2004). It is known that δ 15 N is an accurate indicator of fish trophic position because it reflects assimilated food and shows progressive enrichment (3‰ to 5‰) from feeding components (Jardine et al. 2003). For this reason, the trophic position of each fish taxa was corroborated with nitrogen isotope analyses. Approximately 1 g of dorsal muscle tissue was obtained from fish (>30 mm SL) and frozen for later isotope analysis. Phytoplankton were obtained using a 64 µm mesh net during dry and wet seasons, but these samples had important quantities of young zooplankton. Separation between phytoplankton and zooplankton was not accurate, for that reason 1 g of water hyacinth (Eichornia crassipes) root tissue was used to represent the primary productivity of lake. Stable isotope analysis was performed at the University of California-Davis Stable Isotope Facility. Nitrogen stable isotope ratios (δ 15 N) are expressed in delta (δ) notation as parts per thousand (‰). Mean standard error was < 0.1 ‰ for δ 15 N. We estimated the fish trophic position (TP) using δ 15 N following Vander Zanden and Rasmussen (1999) equation: TP = (δ 15 N S -δ 15 N PP · ∆ TP ) + 1 where; δ 15 N S is the value of the fish tissue, and δ 15 N PP is the value of primary productivity. We used 3.4‰ as trophic level enrichment (∆ TP ), as reported by Vander Zanden et al. (2003). The information of Poeciliopsis infans about trophic guild, niche breadth, trophic position and stable isotope analysis were taken from Ramírez-Herrejón et al. (2013). Data analysis. Non-parametric Kruskal-Wallis rank sumstest was used to detect differences of area of food items (mm 2 ), δ 15 N values, and trophic position among sites. Analyses were done for wet and dry seasons. The sum of the ranks was calculated for each group. Then the test statistic H was calculated to represent the variance of the ranks among groups. But, if the null hypothesis is true, the P value corresponding to χ 2 equal to H (McDonald 2009). For this reason, we use χ 2 in the results. If significant differences were found, multiple post-hoc test comparisons were made using the Tukey-Kramer HSD-honestly significant difference- (Zar 1999). Both analyses were performed with JMP 3.1.6.2 software (SAS Institute). Ethical issues. The presently reported study has been carried out in accordance with the Mexico's government regulations on experiments on animals.

RESULTS
We found 12 food items and about 90% of the guts were at least 50% full, ensuring a good description of the diet.
A total of 384 individuals of Cyprinus carpio from 30 to 450 mm SL were analyzed; 237 during the wet season and 147 during the dry season. We found nine food items. Individuals of C. carpio were grouped into three size classes (Table 1): Cc1(< 120 mm SL), Cc2 (120-230 mm SL), Cc3 (>230 mm SL). The three size groups were mainly distributed in the southern zone of the lake at Embarcadero (EMB) and Ihuatzio (IHU) during the wet and the dry season ( Fig. 2).
Cyprinus carpio (Cc3) fed mainly on detritus and secondary on plant debris, zooplankton; chironomids were incidental items at all sites during wet and dry season ( Table 2). The detritus recorded its maximum value (217 ± 108.9 mm 2 , 81%) at EMB during the wet season. The quantity of feeding resources consumed by the biggest sized carp was similar between sites in both seasons. At this size the common  Cyprinus carpio N = 301; χ 2 = chi-squared test, DF = degrees of freedom; Abundance was estimated as mm 2 in the quadrant method; Total surface = total surface represented by a food item; Abundance values are mean ± standard deviation; Superscripts a, b, c refer to differences in data variation between sites (Tukey-Kramer honestly significant difference [HSD] post hoc test, P < 0.05). carp was classified as omnivorous according to the IRI, Costello graph and the OI (Tables 2 and 3, Fig. 3).
Cyprinus carpio was categorized as a specialist according Levins' niche breadth when combining all sites and both seasons (0.35 ± 0.1) as well for any fish size (Cc1 = 0.46 ± 0.1; Cc2 = 0.33 ± 0.1; Cc3 = 0.26 ± 0.1). Oreochromis spp. and A. robustus were determined to be a specialist. Chirostoma spp. was a specialist during the wet season and a generalist only at SAJ and IHU during the dry season. G. atripinnis behaved as a generalist only at UCA during the wet season and at SAJ and NAP during the dry season (Table 4).  Cyprinus carpio at any of the three sized-groups was determined to be a primary consumer according to the TROPH Program during wet and dry season at all sites (Table 5). However, based on δ 15 N, C. carpio (Cc1) was considered as a secondary consumer at IHU and EMB during the wet and dry seasons; at Cc2 it was found to be a primary consumer only at SAJ in both seasons, and at Cc3 only at UCA during the wet season; in the other sites it behaved as a secondary consumer (Table 6). Chirostoma spp. was determined to be secondary consumer during the wet and dry seasons by TROPH Program and δ 15 N analysis. Goodea atripinnis fall as pri-  (Table 5 and 6). The δ 15 N mean of water hyacinth root tissue was 8.2 ± 0.3‰. The δ 15 N mean of each fish taxa by site and both seasons are shown in Table 7. The δ 15 N of muscle and trophic positions based on nitrogen isotopic signatures of Oreochromis spp. were higher at EMB during the dry season (χ 2 = 11.29, DF = 2, P = 0.03, in both analyses) and was less at NAP during the wet season (χ 2 = 8.26, DF = 3, P = 0.04). For Chirostoma spp., the lowest value of δ 15 N of muscle (χ 2 = 19.95, DF = 5, P < 0.01) and trophic positions (χ 2 = 19.87, DF = 5, P < 0.01) were at EMB (Tables 7 and 8). Goodea atripinnis had the highest value of δ 15 N (χ 2 = 11.88, DF = 5, P = 0.03) and trophic position (P = 0.03, DF = 5, χ 2 = 11.90) at EMB and the lowest at NAP. At SAJ, the lowest value of δ 15 N of muscle and trophic position were found during the wet season for C. carpio (Cc2) (χ 2 = 12.82, DF = 3, P < 0.01, both analyses).

DISCUSSION
Cyprinus carpio behaves as an omnivorous fish in Lago de Pátzcuaro. Its food items can be detritus, primary producers and primary consumers. However, this species was classified not as generalist. This peculiar trophic situation derives from the benthic feeding behaviour of common carp; the shallow lake bottom is mainly covered by detritus and plant debris, which are the principal elements available in this eutrophic ecosystem. The similarity in stomach contents and nitrogen isotopic composition between C. carpio and fish fauna reveal no clear patterns of food resource partitioning that could be achieved through selective feeding. The common carp overlapped its diet in a significant way with all introduced fish taxa, with the native Goodea atripinnis at most sites, and with Chirostoma spp. at sizes below 120 mm SL. However, a trend was clear: diet overlapping was more evident at sites with high water turbidity (>200 NTU), detritus (≈100 000 m 3 each year) and plant debris on the bottom, and water hyacinth on the surface (≈44 plant individuals · m -2 ).
Gut content analysis revealed that smaller sizes of C. carpio fed on zooplankton (ca. 50%) and as it grows,  Table 5 Trophic position for fish taxa-based on index of relative importance-during wet and dry season by site in Lago de Pátzcuaro, Mexico Values are mean ± standard deviation; Cc1 = Cyprinus carpio (<120 mm SL), Cc2 = Cyprinus carpio (120-230 mm SL), Cc3 = Cyprinus carpio (>230 mm SL).
3.8 ± 0.4 a 3.8 ± 0.2 a 3.7 ± 0.2 a 3.8 ± 0.2 a 3.6 ± 0.3 a G. atripinnis 2.5 ± 0.5 ab 2.4 ± 0.2 ab 2.5 ± 0.  Table 6 Trophic position for fish taxa during wet and dry season by site in Lago de Pátzcuaro (based on δ 15 N) preferred to feed on hyacinth roots and plant detritus (>60%). The nitrogen stable isotopes analysis showed that common carp can also ingested invertebrates associated to detritus and water hyacinth roots, a feeding practice recorded in "natural" lakes and rivers of different continents (Summerfelt et al. 1971, Eder and Carlson 1977, Crivelli 1981, Powles et al. 1983, Chapman and Fernando 1994, Elías-Fernández and Navarrete-Salgado 1998, Colautti and Remes Lenicov 2001 or highly modified systems as the Xochimilco canals in Mexico City (Zambrano et. al. 2010).
Evidence of the preference of the common carp for water hyacinth and rooted macrophytes as a preferred feeding habitat is the frequency of sizes found along the six study sites. The three groups of sizes of C. carpio were present during the wet and dry seasons at sites with higher aquatic vegetation coverage (IHU, EMB) located in the southern zone of the lake. And few individuals of common carp were captured at habitat with rocks at bottom and <5% of floating aquatic vegetation cover (SAJ) located in the northern zone. In addition, at SAJ, plant debris was scarce in the gut of C. carpio. These findings agree with Britton et al. (2007) in the Lake Naivasha in Kenya, a shallow, warm and high productive lake. These authors argued that after carp initial establishment, the effects of benthic foraging by carp have not altered macrophytes regeneration, because the abundance of macrophytes is directly proportional to carp abundance.
Cyprinus carpio, Oreochromis spp., and Goodea atripinnis in Lago de Pátzcuaro are primary consumers that feed on the most abundant resources. The same occurs for Poeciliopsis infans (see Ramírez-Herrejón et al. 2013). Common carp fed mainly on the bottom (except for juvenile individuals < 100 mm SL), whereas G. atrip-innis and Oreochromis spp. on water column and bottom, and P. infans on bottom and periphyton. However, according to nitrogen stable isotopes C. carpio (Cc1, Cc2, Cc3) and G. atripinnis can be primary and secondary consumers since isotopic analysis show the assimilated food items, despite if these items were or not founded with gut content analysis (Jardine et al. 2003). This situation may be the common in degraded system. Conversely, Mercado-Silva et al. (2009) found in the Laja River system that in the reservoir itself, as well as the tail water exiting the reservoir, the common carp was located at the lowest trophic position, while in river ecosystems common carp where secondary consumers.
Fishes of the genus Chirostoma are known to be specialist that feed on invertebrates (Rosas 1976, García de León and Pérez-Velázquez 1996, Ross et al. 2006, Moncayo-Estrada et al. 2010. However, Chirostoma fishes in this study behaved as predacious with the capacity to behave as generalist during dry season, showing the aptitude to find its food items in different habitats (bottom, periphyton, surface and nekton) and in different trophic webs (nekton, periphyton and terrestrial). This multi-chain feeder behaviour has been described by Vadeboncoeur et al. (2005) for generalist predators that feed on different functional groups and has been found in other silversides (Contente et al. 2011, Strongin et al. 2011). In addition, such behaviour could be related to the analysis of all species together as Chirostoma. Omnivore fish as Poeciliopsis infans also show this type of behaviour in the Lago de Pátzcuaro (Ramírez-Herrejón et al. 2013). Chirostoma spp. and Alloophorus robustus were secondary consumers, the first fed mainly on zooplankton and insects and the second fed principally on insects. Stable isotope analysis located Chirostoma spp. also within tertiary 18.4 ± 1.1 ab 18 ± 1.5 ab 18 ± 1 ab 16.9 ± 1.5 bc 16.6 ± 1.6 c G. atripinnis 13 ± 0 a -13 ± 0.8 a 14 ± 1.4 a 13.6 ± 0.6 a Oreochromis spp.
--10.5 ± 0.5 a 12.5 ± 0.5 a 14.1 ± 0.6 b Values are mean ± standard deviation; Cc1 = Cyprinus carpio (<120 mm SL), Cc2 = Cyprinus carpio (120-230 mm SL), Cc3 = Cyprinus carpio (>230 mm SL); Superscripts a, b refer to differences in data variation between sites (Tukey-Kramer honestly significant difference [HSD] post hoc test, P < 0.05). consumers, coinciding with Moncayo-Estrada (2007) analysis of Chirostoma lucius Boulenger, 1900 from a natural and an artificial system. Diet overlapped substantially between common carp and local fish fauna, an aspect that suggests competition (Lampert and Sommer 2007). However, the shared fooditems are the most typically available in the lake. This result from the increasing eutrophication of the lake associated with the reduction of the water column and the increase of macrophytes where the water hyacinth is the predominant species (Huerto Delgadillo and Amador García 2011). This promotes the availability of feeding resources related to plant detritus in the southern zone of the lake. Water hyacinth cover has increased steadily in Lago de Pátzcuaro, from 9.8% in 1970, before the carp introduction, 22% in 1990, to 35% in 2000(Calderón-García and Ángeles-López 1971, Chacón Torres 1993, Esteva and Reyes 2002. In other words, it seems the eutrophication process has lowered competition in the fish community by increasing the availability of a few food items associated to water hyacinth. An important aspect to be taken into consideration is that, the native fishes in Lago de Pátzcuaro face not only impacts from introduced species, they are also threatened by human activities that has caused trophic and reproductive habitat destruction, a decrease in water quality, unprecedented plant detritus and sediment accumulation, depth loss and overfishing. All these facts also threaten the introduced species, and the decrease of the fisheries in this lake is a clear indicator. In Lago de Pátzcuaro fish capture volumes have declined from~2500 t in 1988 to <30 t in 2007 (Anonymous 2010). Cyprinus carpio catches in Lago de Pátzcuaro have varied from > 600 t in the 1980s (Gaspar Dillanes et al. 2000) to <10 t in 2009 (Zambrano et al. 2011).
The analyses of trophic ecology of the fishes suggest trophic web reduction in Lago de Pátzcuaro. We suggest that this problem is more serious than feeding competition between the common carp and other species. This food web alteration is evidenced by the low energy food items used by all species, the quantities of detritus found in the gut of most fish taxa, and the decline of top predator population such as native Chirostoma estor (>170 mm SL) as well as the introduced largemouth bass, M. salmoides. In 1981 C. estor catches reached 136 t, but by 2000 catches dropped to only 4 t, and in 2006 catches represented only 1% of the total production of the lake (Rojas and Sasso 2006). The results suggest that all the fish, both native and introduced are trying to adapt and survive to the cumulative changes in the food web. However, presently with the data available it is still too difficult to separate the negative impacts of C. carpio on this ecosystem with affects caused directly or indirectly by anthropogenic activities.