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22001177 SOUTHEASTERN NATURALIST 1V6o(3l.) :1467,3 N–4o7. 63
Watersnakes Prey on Invasive Fishes in an Urban Canal in
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.
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; firstname.lastname@example.org.
Manuscript Editor: Kirsten Work
2017 Vol. 16, No. 3
as prey. I measured various morphological traits on captured watersnakes to
provide insight regarding body size of watersnakes occupying canals in urban
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.
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).
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.
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
2017 Vol. 16, No. 3
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
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
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.
Biological Invasions 2(4):265–277.