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Watersnakes Prey on Invasive Fishes in an Urban Canal in Southern Florida
Oliver Ljustina

Southeastern Naturalist, Volume 16, Issue 3 (2017): 473–476

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Southeastern Naturalist 473 O. Ljustina 22001177 SOUTHEASTERN NATURALIST 1V6o(3l.) :1467,3 N–4o7. 63 Watersnakes Prey on Invasive Fishes in an Urban Canal in Southern Florida Oliver Ljustina* Abstract - Canals in southern Florida are populated by a variety of invasive fish species, but interactions between these invasive fishes and native species are poorly understood. Watersnakes are documented predators of fishes and are well studied in many portions of their range; however, few studies have addressed their ecology and life history in the expansive southern Florida canal system. I captured individuals of 2 watersnake species in a canal in a residential southern Florida neighborhood to determine which fish species were preyed upon. I recovered 4 prey items, all of which were invasive fishe s. Introduction Invasive species are considered major threats to global biodiversity (Bellard et al. 2016). However, native–invasive species interactions that are beneficial to native species have been demonstrated. Invasive species may provide structure that enhances survival of a native species when it is under threat by another invasive species (Chiba and Roy 2011), or may be a high quality and abundant prey resource (King et al. 2006). There are more than 50 species of invasive freshwater fishes in southern Florida (Schofield and Loftus 2015), some of which are known to displace native fish species (Brooks and Jordan 2010, Harrison et al. 2013). Many of these invasive fishes show a strong association with anthropogenically altered habitats such as canals (Trexler at al. 2000). Despite their prominence in modified systems, a substantial knowledge gap remains regarding how invasive fishes in southern Florida interact with native predators. Nerodia floridana (Goff) (Florida Green Watersnake) is primarily piscivorous, taking a wide variety of fishes as prey (Gibbons and Dorcas 2004). Nerodia taxispilota (Holbrook) (Brown Watersnake) is almost exclusively piscivorous, with larger individuals demonstrating a preference for ictalurid catfishes (Gibbons and Dorcas 2004). Invasive gobies have become the primary food for Nerodia sipedon (L.) (Northern Watersnakes) in Lake Erie, where the snakes have shifted from consuming a variety of native species to almost exclusively preying on gobies (King et al. 2006). An observation in southern Florida suggests that Florida Green Watersnakes also prey on invasive fish species (Krysko at al. 2012), but beyond this observation the topic has not been further studied. I examined the dietary contents of a population of watersnakes in an urban canal in southern Florida to determine to what extent they exploit non-native fishes *Southeastern Louisiana University, Department of Biological Sciences, SLU 10736, Hammond, LA 70402; Manuscript Editor: Kirsten Work Southeastern Naturalist O. Ljustina 2017 Vol. 16, No. 3 474 as prey. I measured various morphological traits on captured watersnakes to provide insight regarding body size of watersnakes occupying canals in urban southern Florida. Methods I captured snakes by hand along a 1.475-km portion of the C-100A canal in Miami–Dade County (beginning at 25°38'21.10''N, 80°18'54.2''W and ending at 25°38'12.8''N, 80°18'53.3''W) between the hours of 20:00 and 02:00. A total of 6 capture nights were conducted between 15 April and 23 April 2017. I palpated the snakes to induce regurgitation of prey items and recorded the following measurements for all captured snakes: weight to the nearest gram using Pesola® 0–300-g and 0–5000-g spring scales, snout–vent length to the nearest mm using a 1480 mm cloth ruler, sexed, and head length from the angular bone to the tip of the snout and head width at the widest point to the nearest 0.1 mm using a SPI® 30-410 manual caliper. Ventral scales were clipped to identify individuals (Brown and Parker 1976). I identified regurgitated prey items to the family level by counting anal fin spines, and to species by observing body shape/coloration. I photographed and disposed of prey items. Results I captured 24 individual snakes representing 2 species: Florida Green Watersnakes (n = 5) and Brown Watersnakes (n = 19). Morphological characteristics are summarized in Table 1. All snakes were found in the water and appeared to be foraging. I recovered 2 prey items from 2 male Florida Green Watersnakes: 1 unidentifiable cichlid species (family: Cichlidae), and 1 Mayaheros urophthalmus (Günther) (Mayan Cichlid). I recovered 2 prey items from 2 male Brown Watersnakes: 1 Mayan Cichlid, and 1 juvenile individual Pelmatolapia mariae (Boulanger) (Spotted Tilapia). Discussion The prey items regurgitated from captured snakes suggest that watersnakes in urban areas of southern Florida prey on invasive fishes. Determining whether this pattern is consistent across all canals containing invasive fishes, and if watersnakes feed preferentially on invasive fish species requires more intensive, long-term sampling. It is likely that diet is reflective of the proportion of available native and invasive fishes. Further study of the quality (e.g., handling time, nutritional value, etc.) of native fishes and invasive fishes as prey would also enhance our knowledge of the interactions of these species. Differences between native and invasive prey may induce physical or behavioral responses in watersnake species that prey on them (Phillips and Shine 2006), and may facilitate or impede the persistence of watersnakes in urban areas. Southeastern Naturalist 475 O. Ljustina 2017 Vol. 16, No. 3 Table 1. Morphological data presented as averages with standard deviations. Captures were not sufficient to perform statistical analysis, but it appears that female watersnakes are substantially larger than males, consistent with watersnake studies across the US (Gibbons and Dorcas 2004). All prey items were recovered from male snakes. Snout–vent Head length Head width Species Sex n length (mm) Mass (g) (mm) (mm) Prey n Nerodia taxispilota (Holbrook) Female 7 699 ± 59.8 356 ± 67.4 36.5 ± 2.75 23.1 ± 2.26 Unidentified Cichlid species 1 Male 12 518 ± 166.6 189 ± 35.0 26.5 ± 2.32 16.3 ± 1.60 Mayaheros urophthalmus 1 Nerodia floridana (Goff) Female 1 1100 ± 0 1849 ± 0 58.4 ± 0 30.6 ± 0 M. urophthalmus 1 Male 4 570 ± 122.7 180 ± 95.8 26.9 ± 5.03 15.8 ± 2.01 Pelmatolapia mariae 1 Southeastern Naturalist O. Ljustina 2017 Vol. 16, No. 3 476 Literature Cited Bellard, C., P. Cassey, and T.M. Blackburn. 2016. Alien species as a driver of recent extinctions. Biology Letters 12(2):20150623. Brooks, W.R., and R.C. Jordan. 2010. Enhanced interspecific territoriality and the invasion success of the Spotted Tilapia (Tilapia mariae) in south Florida. Biological Invasions 12(4):865–874. Brown, W.S., and W.S. Parker. 1976. A ventral-scale clipping system for permanently marking snakes (Reptilia, Serpentes). Journal of Herpetology 10(3): 247–249. Chiba, S., and K. Roy. 2011. Selectivity of terrestrial gastropod extinctions on an oceanic archipelago and insights into the anthropogenic extinction process. Proceedings of the National Academy of Sciences 108(23):9496–9501. Gibbons, J.W., and M.E. Dorcas. 2004. North American Watersnakes: A Natural History. Vol. 8. University of Oklahoma Press, Norman, OK. 496 pp. Harrison, E., J.J. Lorenz, and J.C. Trexler. 2013. Per capita effects of non-native Mayan Cichlids (Cichlasoma urophthalmus; Gunther) on native fish in the estuarine southern Everglades. Copeia 1:80–96. King, R.B., J.M. Ray, and K.M. Stanford. 2006. Gorging on gobies: Beneficial effects of alien prey on a threatened vertebrate. Canadian Journal of Zool ogy 84(1):108–115. Krysko, K.L., S.J. Walsh, and R.H. Robins. 2012. The native Florida Green Watersnake, Nerodia Floridana (Goff 1936), preying upon the nonindigenous African Jewelfish, Hemichromis Letourneuxi Sauvage 1880, in Florida. IRCF Reptiles and Amphibians 19(3):161–162. Phillips, B.L., and R. Shine. 2006. An Invasive Species Induces Rapid Adaptive Change in a Native Predator: Cane Toads and Black Snakes in Australia. Proceedings of the Royal Society of London B: Biological Sciences 273(1593):1545–1550. Schofield, P., and W. Loftus. 2015. Non-native fishes in Florida freshwaters: A literature review and synthesis. Reviews in Fish Biology and Fisheries 25( 1):117–145. Trexler, J.C., W.F. Loftus, F. Jordan, J.J. Lorenz, and J.H. Chick. 2000. Empirical assessment of fish introductions in a subtropical wetland: An evaluation of contrasting views. 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