Temperature and Shading Effects on Surf Smelt, Hypomesus pretiosus, Egg Survival

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1 Temperature and Shading Effects on Surf Smelt, Hypomesus pretiosus, Egg Survival Linda Rossell 1 and Paul Dinnel 2 1 Cedar Crest College, Allentown, PA 2 Shannon Point Marine Center, Western Washington University, Anacortes, WA Keywords: Surf smelt, spawning, temperature, shade, egg survival Abstract Much of the vegetative shading on Puget Sound surf smelt (Hypomesus pretiosus) spawning beaches has been lost to shoreline development. As a result, sand-gravel beaches experience higher substrate temperatures and light intensity. Because surf smelt are ecologically important, field and laboratory experiments were used to determine if shade and temperature have an effect on surf smelt egg survival. Eggs were incubated at various temperatures in the laboratory for eight hours to determine their temperature tolerance. Between 3 and 35 C, survivorship of eggs dropped from 11% to %. A field experiment was set up from July 14-22, 26, using screening mesh and opaque Plexiglas to simulate different degrees of shading to determine how shade affects egg survivorship. Eggs under full shaded conditions had significantly higher survivorship than eggs under partial shaded and no shade conditions. Our results showed that there is a strong relationship between shade, temperature and the survival of summer-spawned surf smelt eggs. Introduction The surf smelt (Hypomesus pretiosus) is a schooling marine forage fish that is an important trophic link in nearshore food webs (Penttila 21). Surf smelt provide a food source for marine mammals, sea birds, and larger fish, such as salmon. It is a member of the family Osmeridae, which also includes whitebait smelt, capelin, longfin smelt and eulachon. Surf smelt range from Monterey Bay, California to Prince William Sound, Alaska, and are also found throughout the Puget Sound region. Surf smelt are indiscriminate plankton feeders, eating copepods, amphipods, crab larvae, marine worms and larval fish (DFO 22). Surf smelt have evolved distinctive reproductive habits that allow them to utilize the upper intertidal zone for spawning (Misitano 1977). Spawn is deposited on coarse sand and gravel beaches during slack high water, concentrated within the 7-foot and 11-foot high tide levels in Puget Sound (Garrison and Miller 1982). Wave action buries eggs -6 inches (-15 cm) in the gravel. The eggs are adhesive, sticking to the substrate and remaining attached when the tide recedes. Surf smelt eggs hatch in 2-5 weeks after spawning, and hatch in less time during warmer conditions (DFO 22). Spawning occurs throughout the year, except during late March and April. There are two distinct surf smelt spawning stocks in the Puget Sound region: a fall-winter spawning stock, which spawns September through March; and a summer spawning stock, which spawns from May through October (WDFW 1997). Summer spawn, however, is subjected to much higher temperatures than winter spawn. The fall-winter spawning stock suffers lower egg mortality because average substrate temperatures are usually moderate. Eggs are very resistant to freezing temperatures and spawning can continue during cold snaps (Middaugh et al. 1987; Penttila 1996). In contrast, on calm, sunny days during the summer months in the Puget Sound region, surface temperatures on sand-gravel beaches can reach at least 4 C (14 F). Although they 1

2 have considerable tolerance for such conditions, when exposed to direct sunlight, surf smelt eggs are vulnerable to heat shock and desiccation (Penttila 21). Brennan and Culverwell (24) observed that there is a sharp contrast in the substrate moisture levels and temperatures between shaded and unshaded beaches in the Puget Sound region. Vegetative shading, which helps block solar radiation, is considered to be an important limiting factor in the distribution, abundance and species composition of organisms in the upper intertidal zone (Brennan and Culverwell 24). Penttila (21) found in an a posteriori study that the shading provided by terrestrial vegetation has a positive effect on the survival of surf smelt eggs incubating on sand-gravel beaches of the upper intertidal zone during the summer in the Puget Sound region (Penttila 21). Rice (26) also found that modified, unvegetated beaches had substantially lower surf smelt egg survival than did naturally shaded beaches. However, both of these studies relied on correlative data instead of experimental work. The first objective of our study was to determine the lethal temperature threshold for surf smelt eggs using laboratory tests. The second objective was to determine if lack of shade and high substrate temperatures have an impact on the survival of surf smelt eggs using field experiments. Methods Laboratory Temperature Tolerance Test A temperature tolerance test was performed to determine what temperature is lethal to surf smelt eggs. Fresh spawn was collected from Weaverling Spit (South Fidalgo Bay, Washington, Fig. 1) and stored at 1 C. Four replicate egg/gravel samples were placed in deep Petri dishes and held at 1 C as the control. Egg/gravel samples were also incubated in deep Petri dishes at 45, 4, 35, 3, 25 and 2 C for 8 hours under lowlight conditions (~1 foot candles). There were four replicate samples for each temperature and one temperature set was incubated per day. After the incubations were complete, eggs were left for 7 days at 1ºC to develop further. Then, approximately 1 eggs per replicate were counted to determine the number of live eggs vs. dead eggs. Live eggs were those that had eyespots present; all other eggs were assumed to have died at an earlier Figure 1. Map of northern Fidalgo Island, Washington. Weaverling Spit is highlighted in red. 2 stage of development. The percent mortality was then calculated for each sample. Field Experiment -- Nine plots were set up near the high water line (~ +8 MLLW) on the Samish tribal beach on Weaverling Spit (Fig. 1) from July 14-22, 26. Three plots each were randomly assigned to be fully shaded, partially shaded or unshaded. Fresh surf smelt eggs were collected from Egg Cove located approximately 2 yd east of the experimental site. Egg Cove was a naturally shaded area that is considered partially to fully shaded. Collected eggs were poured into 3 deep holes in the center of each plot. Data loggers that recorded temperature every 15 minutes were buried just below the surface of one plot for each condition.

3 The shaded and partially shaded plots were covered with 3 by 3 PVC pipe square frames on 6 PVC pipe legs (Fig. 2). Shaded plot covers had opaque black Plexiglas cable-tied to the frames. Partially shaded plot covers had 6 mm Vexar mesh cable-tied to the frames, which reduced light intensity by approximately 4%. Unshaded plots had no coverings. All frames were secured to the substrate with rebar stakes to prevent movement during high tides. Plots were monitored and debris removed daily. Surface temperature, air temperature and light intensity measurements were also made daily. After eight days, two egg samples were taken from the center of each plot: one surface sample and one 2-3 deep sample. Similar samples were taken from three spots at the location where the eggs were collected ( Egg Cove ). A subset of each sample was counted to determine the number of live eggs and number of dead eggs. When possible, 1 eggs per sample were counted, but if eggs in the samples were too sparse, 2-4 eggs were counted. Percent mortality was then calculated for each sample. Temperature/Depth Profile -- A temperature profile was created on one occasion by measuring gravel temperatures at the surface and down to 12 in depth. A digital temperature probe was inserted horizontally into the gravel at the surface and then at every subsequent inch down to 12. A profile was created for unshaded conditions and for naturally shaded conditions ( Egg Cove ). Results Temperature Tolerance Test -- The 1 C control had the highest mean percent survival, at 65.1%. After eight hours of incubation, the 2 C sample had the next highest mean percent survival, at 22.6%, followed by 25 C and 3 C, at 19.9% and 11.5%, respectively. The 35, 4 and 45 C temperature treatments had % survival (Fig. 3). Field Experiment -- The mean percent egg survival in the shaded plots was 35.7% at the surface and 88.1% at 2-3 deep. The mean percent survival for the partially shaded and unshaded plots were % at both depths. The mean percent survival for Egg Cove was 5.8% at the surface and 5.2% at 2-3 deep (Fig. 4). The results of an ANOVA test found a significant difference among the percent survivals of the different conditions (p<.1). The results of an ANOVA test showed that there was a significant difference between the daily maximum temperatures of the shaded and partially shaded plots (p=.3) and the shaded and unshaded plots (p<.1). There was no significant difference between the daily maximum temperatures of the partially shaded and unshaded plots (p=.65). Figure 6 summarizes the daily temperature readings of the shaded, partially shaded and unshaded plots. Maximum daily temperatures in the unshaded plot exceeded 3 C on all but one day. The mean maximum temperature of the unshaded plot was 34.6 C. Maximum daily temperatures in the partially shaded plot exceeded 3 C on four occasions. The mean maximum temperature of the partially shaded plot was 31 C. Maximum daily temperatures in the shaded plot remained below 25 C on Figure 2. Frame of plot cover (not to scale).

4 four occasions, and reached 3 C once. The mean maximum temperature of the shaded plot was 25.1 C. 1 9 Control 8 7 Percent Survival Temperature ( C) Figure 3. Percent survival of surf smelt eggs following 8 hours of incubation at varying temperatures. Bars represent one standard deviation. Survival was zero for the three highest temperatures. Results of an ANOVA test showed that there was a significant difference between the light intensity of the shaded and partially shaded plots (p<.1), the shaded and unshaded plots (p<.1) and the shaded and Egg Cove plots (p<.1). There was also a significant difference between the light intensity of the unshaded and partially shaded plots (p<.1) and the unshaded and Egg Cove plots (p<.1). There was no significant difference between the partially shaded and Egg Cove plots (p =.13). The daily light intensity readings are summarized in Figure 6. Temperature/Depth Profile -- The air temperature at Weaverling Spit was 26 C on the day that temperature profile data were collected. The surface temperature of the unshaded site was 4. C, and the 12 deep temperature was 25.4 C. The surface temperature of the shaded ( Egg Cove ) site was 23. C, and the 12 deep temperature was 19.4 C. The results of the temperature profile measurements are summarized in Figure 7. 4

5 Mean Percent Survival Percent Survival Shaded Surface Shaded 2" Deep Partially Shaded Surface Partially Shaded 2" Deep Unshaded Surface Unshaded 2" Deep Egg Cove Surface Egg Cove 2" Deep Condition Figure 4. Mean percent survival for different egg exposure conditions in the field experiment. Bars represent one standard deviation. Survival was zero for four of the treatments. 45 Daily Field Temperature Readings Shaded Partially Shaded Unshaded 4 35 Temperature( C) /14/6 1:24:46. 7/14/6 22:54:46. 7/15/6 11:24:46. 7/15/6 23:54:46. 7/16/6 12:24:46. 7/17/6 :54:46. 7/17/6 13:24:46. 7/18/6 1:54:46. 7/18/6 14:24:46. 7/19/6 2:54:46. Date/Time 7/19/6 15:24:46. 7/2/6 3:54:46. 7/2/6 16:24:46. 7/21/6 4:54:46. 7/21/6 17:24:46. 7/22/6 5:54:46. Figure 5. Summary of daily field temperature readings taking at 15 minute intervals. 5

6 1 9 Daily Light Intensity Measurements Shaded Partially Shaded Unshaded Egg Cove 8 Light Intensity (Foot Candles) /17/26 7/18/26 7/19/26 7/2/26 7/21/26 7/22/26 Date Figure 6. Daily light intensity measurements for each field experiment condition. Temperature Profile 45 Unshaded 4 "Egg Cove" 35 3 Temperature (C) Depth (inches) Figure 7. Temperature/depth profile of unshaded and shaded ( Egg Cove ) sites by depth. 6

7 Discussion The results of this study indicate that lack of shading can have a strong negative effect on the survival of surf smelt eggs. The temperature tolerance test showed that survivorship of eggs drops from 11% to % between 3 and 35 C. It can be concluded that if temperatures on sandgravel beaches reach above 3 C for a period of about 8 hours, the survival of surf smelt eggs on those beaches will likely be very low. In the field experiment, the surface temperatures of the unshaded and partially shaded plots reached above 3 C approximately four times. The unshaded plot reached above 35 C on four occasions. The shaded plot, however, only reached 3 C on one occasion and survivorship was substantially higher than the unshaded and partially shaded plots. This experiment showed that shading can significantly reduce the surface temperature of sand-gravel beaches, sometimes by more than 1 C. This reduction in temperature can provide tolerable conditions for surf smelt eggs to survive in. The shaded site temperature/depth profile showed that temperatures from the surface to 12 deep were well below temperatures that are lethal to eggs. The temperature/depth profile of an unshaded site, however, showed that on a sunny day, the temperatures in the range of depth surf smelt eggs are typically found (-6 ) exceeded the temperatures that are tolerable for egg survival. These results help show the importance of vegetative shading along shorelines. Natural shading is provided primarily by overhanging trees, such as the bigleaf maple (Acer macrophyllum), the red alder (Alnus rubra) and willows (Salix spp.). These species can tolerate exposure to seawater and root-mass exposure to the supralittoral zone. They are also fast growing, so they have a quick response to natural changes in the shoreline (Penttila 21). With increases in shoreline development in the Puget Sound region, much of the vegetation has been removed for bulkheading, enhancing views of the waterways, building homes, etc. The loss of shaded beaches could reduce the size of surf smelt populations. High egg mortality can limit the number of juveniles entering the population that would replace adult surf smelt when adults reach the post-reproductive life stage. Small surf smelt population sizes can cause problems for species that depend on this forage fish for food. Forage fish are the dominant part of the diets of fish such as Chinook salmon and coho salmon. In British Columbia, there is a positive correlation between the amount of forage fish (including surf smelt) present and the abundance of the Chinook and coho salmon. Around Vancouver Island, Chinook salmon on the west side of the island are larger than Chinook salmon of the same age on the east side of the island; the size difference is related to the lack of food such as forage fish on the east side of the island (Bargmann 1998). A decline in the surf smelt populations in Puget Sound could also impact seabird and marine mammal populations. Springer et al. (1984) found that there have been large-scale reproductive failures among fish-eating seabirds which have been linked to collapses of prey resources. Harbor porpoises are known to prey heavily on surf smelt and herring (Bargmann 1998), so a decline in surf smelt populations could prove detrimental to the harbor porpoise populations. The results of this study, as well as those of Penttila (21) and Rice (26), show that vegetative shading is a critical aspect of the survival of summer surf smelt spawn in the Puget Sound region. Stricter regulations regarding shoreline development would be beneficial in 7

8 preserving not only surf smelt populations, but the organisms that rely on surf smelt and other nearshore spawning fish as well. Acknowledgments We thank the National Science Foundation (NSF) for funding Shannon Point Marine Center s Research Experience for Undergraduates (REU) program; the Samish Tribe for allowing us to conduct our field experiments on their beach; and Shannon Point Marine Center staff and students, notably Gene McKeen, Rich Hoover, Brandon Jenson, Nate Schwarck, and Sarah Bayer, for their assistance in setting up our experiments. Literature Cited WDFW. (Washington Department of Fish and Wildlife) Surf smelt fact sheet. WDFW, Olympia, WA. 2 pp. Bargmann, G Forage fish management plan. Washington Department of Fish and Wildlife, Olympia, WA Brennan, J.S., and H. Culverwell. 24. Marine Riparian: An assessment of riparian functions in marine ecosystems. Washington Sea Grant Program, Seattle, WA. DFO (Department of Fisheries and Oceans). 22. Surf Smelt. DFO Can. Sci. Advis. Sec. Stock Status Rep. B6-9 (22). Garrison, K.J. and B.S. Miller Review of the early life history of Puget Sound fishes. Report FRI-UW-8216, Fisheries Research Institute, Seattle, WA. Middaugh, D.P., M.J. Hemmer and D.E. Penttila Embryo ecology of the Pacific surf smelt, Hypomesus pretiosus (Pisces:Osmeridae). Pacific Science 41 (1-4): Misitano, D.A Technique for incubating and hatching eggs of surf smelt for bioassay. The Progressive Fish Culturist 39(4):187. Penttila, D.E Surf smelt spawning ecology information pertinent to bioassays. Washington Department of Fish and Wildlife, Olympia, WA Penttila, D.E. 21. Effects of shading upland vegetation on egg survival for summer-spawning surf smelt, Hypomesus, on upper intertidal beaches in Northern Puget Sound. In: Proceedings of Puget Sound Research, 21 Conference. Puget Sound Action Team, Olympia, WA. Rice, C.A. 26. Effects of shoreline modification on a northern Puget Sound beach: Microclimate and embryo mortality in surf smelt (Hypomesus pretiosus). Estuaries and Coasts 29 (1): Springer, A., D. Roseneau, E. Murphy and M. Springer Environmental controls of marine foodwebs: food habitats of seabirds in the eastern Chukchi Sea. Can J. Fish Aquat. Sci. 41: