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|Egg Mass Surveys|
Egg mass surveys may work for other species, but these are the only species for which we currently have literature citations using this technique.
Rana sylvatica, Wood Frog; Rana luteiventris, Columbia Spotted Frog; Ambystoma gracile, Northwestern Salamander; Ambystoma jeffersonianum, Jefferson Salamander; Ambystoma macrodactylum, Long-toed Salamander; Ambystoma maculatum, Spotted Salamander
Egg mass surveys can be used in inventory or monitoring programs to assess presence of wood frogs and spotted salamanders and/or to provide estimates of abundance, reproductive output and population trends of these species at scales ranging from single wetlands to state or regional levels. Some states, such as Massachusetts, New Hampshire and Maine use the presence of egg masses of wood frogs or spotted salamanders to certify a wetland as a vernal pool, which can afford the wetland greater protection (Kenney 1995, Tappan 1997, Calhoun 1999, Kenney and Burne 2000, Burne 2001). Wood frogs and spotted salamanders that breed in ephemeral or vernal pools deposit large globular egg masses, often in communal aggregations, that are relatively persistent (typically remaining evident for 3-4 weeks from initial deposition to hatching) and easy to find (Crouch and Paton 2000, Egan 2001). Crouch and Paton (2000) found that egg mass counts are an effective, relatively accurate and precise survey method to monitor wood frog populations. They determined that egg mass counts were highly correlated with numbers of females and males entering the pond. Egg mass counts can reliably estimate numbers of breeding females for some species (e.g., wood frog, northwestern salamander) in which each female oviposits one egg mass per year (Thoms et al., 1997). However, egg mass counts can underestimate female populations if not all females reproduced in a year, or if only part of a larger area of oviposition sites for the population is sampled. Alternatively, egg mass counts can overestimate populations if females produce more than one egg mass in a year (e.g., Ambystoma maculatum, Ambystoma macrodactylum) (Thoms et al., 1997, Petranka 1998).
By correlating egg mass abundance, trend and distribution patterns with covariates such as within-pond or landscape variables, it is also possible to gain insight into factors potentially affecting population changes over time. For example, Egan (2001) found that wood frog egg mass counts were higher in landscapes with more forested uplands and forested wetlands and were lower at ponds with low-density development and road density greater than 12 m per ha in a 1 km area surrounding the pond. Windmiller (1996) found that the area, integrity and habitat characteristics of the upland forest within 300 m of breeding ponds were the most significant correlates of spotted salamander abundance. Blem and Blem (1991) conducted egg mass counts of spotted salamanders over an 8 year period at many ponds, and found declines in populations at sites with high aluminum, copper, silicon and zinc concentrations in the water. Rowe and Dunson (1993) found that Jefferson salamander egg mass counts were positively correlated with pH and alkalinity, but negatively correlated with aluminum. Other studies have examined spotted salamander egg masses at field sites to determine impacts of acidic water or road salt on hatching success and development (Pough 1976, Cook 1983, Turtle 2000).
Wetland sites should be monitored closely to determine the best time to conduct egg mass surveys. The goal is to obtain counts when the maximum number of eggs are present at a pool, allowing the best picture of the overall cumulative reproductive output for each vernal pool, but also surveying egg masses as early in development as possible. This requires conducting the egg mass surveys twice (or more) as soon as possible when maximal breeding activity is observed. An Egg Mass Count data sheet (Fig. 1) with a grid can be used to map the site and egg mass locations. Before conducting egg mass surveys, observers can draw a site map with landmark features (e.g., fallen trees, islands) onto the grid to make drawing the egg mass locations on the map easier. Egg mass counts are best conducted during the day when the sun is out (9 am - 3 pm). When it is darker, it is harder to see into vernal pools, particularly those that are stained by tannic acid from decaying leaves. Two observers are required to conduct the survey. Both observers can start at one end of the pond (north end) and circumnavigate and thoroughly search the pond together. It is important that the entire pond is searched to ensure that all egg masses are counted (Crouch and Paton 2000). Alternatively, if ponds are extremely large, subsampling methods can be used (e.g., quadrats). Observers can use polarized glasses to minimize sun glare and to aid in detection of egg masses. Often observers use a visual/tactile method to count egg masses, using eyes and hands (cupping under the egg mass) to count and feel the egg masses in the pond. It is extremely important, however, to avoid disturbing the egg masses as much as possible during the survey. Observers must walk very slowly through the entire pond. Most egg masses are near the edges of the ponds, but egg masses can also be in the middle of ponds, particularly shallow ponds. Crouch and Paton (2000) report that 16% of wood frog egg masses may be deposited away from communal aggregations. Egg mass locations can be flagged during the first survey to avoid double counting masses on subsequent surveys, although full counts on each survey are recommended as sometimes additional egg masses are added to areas where other egg masses were laid previously (Windmiller 1996, Egan 2001).
A double-observer dependent technique can be used to better estimate egg mass abundance and detection rates (Cook and Jacobson 1979, Nichols et al. 2000). Observer 1 counts and points out egg masses to Observer 2. Observer 2 records what Observer 1 reports, but also writes down in a separate column any egg masses that Observer 1 missed or counted twice. Observer 2 does not make any comments to Observer 1 about the counts. The double-observer dependent method allows calculation of detection probabilities for each observer, providing adjustments to the number of egg masses based on each observer's skills. Double-observer techniques do not correct for other forms of bias such as those caused by weather events and sampling at the wrong time of year. The double observer technique requires that the participants switch roles halfway around the pool. Observer 1 for the first half of the pool becomes Observer 2 for the second half of the pool. It is essential to indicate when observers switch roles by drawing a line after counts on the data sheet.
Things that could bias your counts
It takes some time to develop the search image for egg masses and to be able to distinguish among egg masses of various species. Training and experience are necessary for egg mass surveys. There is a lot of variation in egg mass shape and appearance within species. For example, spotted salamander egg masses can appear cloudy, opaque, clear, green or orange-colored with sizes varying from grape- to orange-sized. Egg masses can be twisted, round or long. If egg masses are covered in silt, they can also be more difficult to identify. In the field, egg masses of different Ambystoma species can be confused with one another as can pickerel with wood frog egg masses.
Weather greatly influences when egg masses are laid. Wood frogs and spotted salamanders migrate to vernal pools in the winter and early spring on rainy or foggy nights when night temperatures are above 10-12°C .Because egg mass surveys involve counting egg masses that are relatively immobile as opposed to animals that move around, weather is less of a factor biasing the count of actual numbers of eggs present. At Patuxent, we (REJ, PN, Edward Schwartzman) have observed that weather conditions can influence detection of egg masses. Sky conditions can affect the visibility of egg masses, particularly those of Ambystomatid salamanders laid further under water. Less favorable conditions for egg mass surveys include: 1) overcast days, 2) rainfall, and 3) excessive sunshine. For 1), gray skies can be reflected on the surface of the water and can obscure the grayish tone of some Ambystoma maculatum egg masses (see also Egg Mass Appearance below), thus decreasing the likelihood of detecting these masses. If it is raining, raindrops on the surface of the water can mask or mimic the appearance of egg masses found closer to the surface of the water, such as those of Rana sylvatica, again affecting the detection of these masses. Finally, sunshine can cause a glare on the water's surface, making it difficult to see what lies beneath. This is the least problematic, and the glare can be minimized by wearing polarized sunglasses, which allow surveyors to see through the surface of the water. However, the sunglasses may not completely eliminate glare, thus some egg masses may still go undetected.
Another source of variation and potential bias in egg mass surveys is the onset of breeding behavior. Advent of the breeding season for wood frogs may begin earlier in the late winter or early spring if the weather is wet and warm early on. Considerable annual variation in the onset of breeding can occur and even within a given Refuge or Park, breeding phenology can vary quite a bit among vernal pools only kilometers apart. Below are recommended sampling windows for various states in the Northeast for wood frog and spotted salamander egg mass surveys:
Because breeding can be particularly explosive for wood frogs (i.e., all breeding occurs within 2-6 day or 4-8 day periods; Herreid and Kinney 1967, Waldman 1982), it is important to conduct egg mass surveys during this short time frame as close to the end of the peak period of egg laying to best assess the population breeding at a site. This often requires multiple surveys (Egan 2001). If you wait too long and the weather is very warm, eggs develop quickly and egg masses become more difficult to count.
Conducting counts during the day (between 9 am-3 pm) is best (see weather section).
Every researcher, technician, or volunteer differs in their visual acuity, search speed, carefulness, ability to discern species, ability to estimate large sets of egg masses, experience, and their eye for finding egg masses under difficult circumstances. Training and testing of observers equalizes many of these differences within a project. Double-observer techniques permit the calculation of individual observer correction factors for the number of egg masses in pools (Cook and Jacobson 1979, Nichols et al. 2000). Egan (2001) reported no significant differences between observers in egg mass counts (Egan 2001). Crouch and Paton (2000) reported a 12% difference in wood frog egg mass counts between two observers.
Egg mass abundance can be analyzed in relation to covariates such as within-pond habitat (e.g., water quality, vegetation characteristics) and/or landscape (e.g., road density and forest cover within 1 km radius of pond) variables using multivariate regression analyses. Presence/absence data based on egg mass surveys can be analyzed in relation to covariates using logistic regression analyses. Egg mass counts can be analyzed over time to determine temporal trends in population sizes.
Berven (1990) found the numbers of wood frog egg masses in breeding ponds vary annually with a coefficient of variation (CV) of approximately 95%. Based on data from Clay (1997), Petranka and Smith (1995) and Petranka et al. (1995) (see below), the average CV for spotted salamanders egg mass counts is 37%.
Also, see our Amphibian CV database
Berven, K.A. 1990. Factors affecting population fluctuations in larval and adult stages of the wood frog (Rana sylvatica). Ecology 71:1599-1608.
Blem, C.R., and L.B. Blem. 1991. Cation concentrations and acidity in breeding ponds of the spotted salamander, Ambystoma maculatum Shaw (Amphibia: Ambystomatidae), in Virginia. Brimleyana 15:67-76.
Burne, M. R. 2001. Massachusetts Aerial Photo Survey of Potential Vernal Pools. Natural Heritage & Endangered Species Program, Massachusetts Division of Fisheries & Wildlife, Westborough, MA.
Calhoun, A. 1999. Maine Citizen's Guide to Locating and Documenting Vernal Pools. Maine Audubon Society. 98 pp.
Clay, D. 1997. The effects of temperature and acidity on spawning of the spotted salamander, Ambystoma maculatum, in Fundy National Park. Pp. 226-232 in D.M. Green (ed.), Amphibians in Decline: Canadian Studies of a Global Problem. SSAR, St. Louis, MO.
Cook, R.P. 1983. Effects of acid precipitation on embryonic mortality of Ambystoma salamanders in the Connecticut Valley of Massachusetts. Biological Conservation 27:77-88.
Cook, R.D., and J.O. Jacobson. 1979. A design for estimating visibility bias in aerial surveys. Biometrics 35:735-742.
Crouch, W.B., and P.W.C. Paton. 2000. Using egg-mass counts to monitor wood frog populations. Wildlife Society Bulletin 28:895-901.
Egan, R.S. 2001. Within-pond and landscape-level factors influencing the breeding effort of Rana sylvatica and Ambystoma maculatum. M.S. Thesis in Environmental Sciences, University of Rhode Island.
Herreid, C. F., II, and S. Kinney. 1967. Temperature and development of the wood frog, Rana sylvatica, in Alaska. Ecology 48:579-589.
Hunter, M. L., Jr., A. J. K. Calhoun, and M. McCollough. 1999. Maine Amphibians and Reptiles. The University of Maine Press, Orono, Maine.
Kenney, L.P. 1995. Wicked big puddles: A guide to the study and certification of vernal pools. U.S. Government Printing Office, Washington, D.C.
Kenney, L.P., and M.R. Burne. 2000. A field guide to the animals of vernal pools. Massachusetts Division of Fisheries and Wildlife, Natural Heritage and Endangered Species Program and Vernal Pool Association. Westborough, MA.
Nichols, J.D., J.E. Hines, J.R. Sauer, F. Fallon, J. Fallon, and P.J. Heglund. 2000. A double-observer approach for estimating detection probability and abundance from avian point counts. The Auk 117:393-408.
Petranka, J.W., C.K. Smith, and A.F. Scott. 1995. Final Report: Implementation of a plan for long-term monitoring of amphibians in selected parks of the southeast region. National Park Service Report. Cooperative Agreement No. CA-5140-1-9001.
Petranka, J.W. and C.K. Smith. 1995. Protocol manual for long-term monitoring of amphibians in GRSM-BLRI-CUGA-MACA. Subagreement No. CA-5140-3-9002, Cooperative Agreement No. CA-5140-1-9001.
Petranka, J.W. 1998. Salamanders of the United States and Canada. Smithsonian Institution Press, Washington, D.C.
Pough, F. H. 1976. Acid precipitation and embryonic mortality of spotted salamanders, Ambystoma maculatum. Science 192:68-70.
Rowe, C. L., and W.A. Dunson. 1993. Relationships among abiotic parameters and breeding effort by three amphibians in temporary wetlands of central Pennsylvania. Wetlands 13:237-246.
Ruth, B. C., W. A. Dunson, C. L. Rowe, and S. B. Hedges. 1993. A molecular and functional evaluation of the egg mass color polymorphism of the spotted salamander, Ambystoma maculatum. Journal of Herpetology 27:306-314.
Tappan, A. 1997. Identification and documentation of vernal pools in New Hampshire. New Hampshire Fish and Game Department, Nongame and Endangered Wildlife Program.
Thoms, C., C. C. Corkran, and D. H. Olson. 1997. Basic amphibian survey for inventory and monitoring in lentic habitats. Northwest Fauna 4:35-46.
Turtle, S.L. 2000. Embryonic survivorship of the spotted salamander (Ambystoma maculatum) in roadside and woodland vernal pools in southeastern New Hampshire. Journal of Herpetology 34:60-67.
Waldman, B. 1982. Adaptive significance of communal oviposition in wood frogs (Rana sylvatica). Behav. Ecol. Sociobiol. 10:169-174.
Windmiller, B. S. 1996. The pond, the forest, and the city: spotted salamander ecology and conservation in a human-dominated landscape. Ph.D. Thesis, Tufts University, Boston, MA.
Wright, A.H., and A.A. Wright. 1949. Handbook of Frogs and Toads of the United States and Canada. Comstock Publ. Co., Ithaca, NY.
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