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Observations on the Distribution of Ichthyoplankton within the Saco River Estuary System
Andrew M. Wargo, Charles E. Tilburg, William B. Driggers, and James A. Sulikowski

Northeastern Naturalist, Volume 16, Issue 4 (2009): 647–654

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2009 NORTHEASTERN NATURALIST 16(4):647–654 Observations on the Distribution of Ichthyoplankton within the Saco River Estuary System Andrew M. Wargo1, Charles E. Tilburg1, William B. Driggers2, and James A. Sulikowski1,* Abstract - Determining planktonic larval species composition and abundance data can serve to elucidate local patterns of distribution, determine an area’s importance as a nursery ground, and help clarify broad-scale trends of adult distribution and spawning ranges. Although the Saco River and estuary is the fourth largest waterway system in Maine, this ecosystem has remained relatively unstudied over the last thirty years, and research describing the temporal ichthyoplankton composition and distribution is virtually absent. The present study examined the structure of the ichthyoplankton community and determined the temporal and spatial variation in species diversity and abundance within the Saco Bay estuary system. Weekly sampling trips during the months of June, July, and August in 2007 revealed ten species of ichthyoplankton present in the study area. Ulvaria subbifurcata (Radiated Shanny) and Tautogolabrus adsperus (Cunner) dominated the abundance data followed by Hippoglossoides platessoides (American Plaice) and Myoxocephalus octodecimspinosus (Longhorn Sculpin). Introduction Estuaries are typically considered to be unstable systems due to their highly variable abiotic conditions, such as temperature and salinity (Faria et al. 2006). Despite the variability in abiotic characteristics, these systems support high abundances of organisms due to high localized productivity. In addition, in many cases these areas function as important nursery grounds where ichthyoplankton encounter suitable conditions for enhanced development into juveniles (Hettler and Hare 1998). Recent studies suggest that Maine’s estuaries and coastal river systems play an important role in the early life history of many marine species within the Gulf of Maine (GOM; Lazzari 2001). For example, the importance of these coastal areas to ichthyoplankton survival has been documented for the Damariscotta (Laroche 1980), Sheepscot (Graham 1972), and Sullivan Harbor (Townsend 1983) estuarine systems. Although the Saco River watershed is the fourth longest (201 km) and sixth largest in the GOM drainage area (4395 sq ha; NRC 2004), limited information exists for the ichthyoplankton community in the coastal waters in and around this coastal ecosystem. Information regarding ichthyoplankton distribution in this system is important, since previous research by Reynolds and Casterlin (1985) suggests that this system serves 1Marine Science Center, University of New England, 11 Hills Beach Road, Biddeford, ME 04005. 2National Marine Fisheries Service, Southeast Fisheries Science Center, Mississippi Laboratories PO Drawer 1207, Pascagoula, MS 39568. *Corresponding author - jsulikowski@une.edu. 648 Northeastern Naturalist Vol. 16, No. 4 a nursery function for several juvenile species within the Gulf of Maine. Thus, the objective of the current study was to gain a basic understanding of the ichthyoplankton diversity in and around the Saco River and the adjacent coastal ocean. Field-site Description The current study examined the coastal region directly outside and in the mouth of the Saco River. The southernmost sample was obtained at 43º26.26'N, 70º19.98'W and the northernmost sample was obtained at 43º28.76'N, 70º20.86'W (Fig. 1). Over 67,000 people reside within the Saco River watershed. The Saco River is a narrow bedrock-cut estuary that is characterized by a salt-wedge profile during low discharge conditions, but the river plume can extend to the coastal ocean during spring snow melt (Kelley et al. 2005), when discharge can exceed 600 m3 s-1. Salinity in the region is affected by the Western Maine Coastal Current, and discharge from the Saco River itself and can vary between 0 practical salinity units (PSU) during strong discharge events and 32 PSU during summer months when there is relatively little river discharge. Surface temperature varies between 4 °C during winter and greater than 15 °C during summer. Tides are semi-diurnal, and mean tidal range is 2.7 m. The river empties into a glaciated, passive continental margin. Bottom depth varies between 5 and 35 m over the study site. Materials and Methods Twelve weekly sampling trips were performed between June 1 and August 31, 2007. Since the net used in the study was not equipped with a doubletrip mechanism (for subsurface sampling), each sampling trip consisted of two surface tows at each of four designated stations: Station 1 (43º26.26'N, 70º19.98'W), Station 2 (43º28.76'N, 70º20.86'W), Station 3 (43º27.579'N, 70º20.389'W), and Station 4 (43º27.713'N, 70º21.624'W) (Fig. 1). All sampling was conducted during daylight hours using a 0.5-m diameter, 350-micron mesh plankton net at approximately high tide. All tows were 15 minutes in length at an average speed of 2.5 knots. Except for larval samples collected on June 30th and August 31st, a vertical profile of salinity and temperature was collected with a hand-deployed conductivity-temperature-depth recorder (SBE 25 SEALOGGER CTD, Sea-Bird Electronics) after the tows were completed at each station. All biological samples were fixed in 5% percent buffered formalin for 24 hours and then preserved in 70% ethanol for later identification. In the laboratory, all specimens were identified to species level using a dissecting microscope fitted with a Nikon CoolPix S-4 digital camera and D’VIMS version 4.0 professional imaging software. Since no flow meter was used in the study, the volume of water, V, that passed through the net was estimated using the following equation: V = v x Δt x A, Where v = average speed through water during tow (m/s), Δt = time of tow (s), and A = cross-sectional area of net (m2). 2009 A.M. Wargo, C.E. Tilburg, W.B. Driggers, and J.A. Sulikowski 649 Figure 1. Map of study area. Black lines at mouth of Saco River indicate location of jetties. Stations sampled weekly with a 0.5-m plankton net in the Saco River Estuary System are indicated by circled numbers: Station 1 (43º26.26'N, 70º19.98'W), Station 2 (43º28.76'N, 70º20.86'W), Station 3 (43º27.579'N, 70º20.389'W), and Station 4 (43º27.713'N, 70º21.624'W). 650 Northeastern Naturalist Vol. 16, No. 4 Speed through the water was estimated using a Garmin GPS receiver (GPSmap 276c). While average speed derived from the GPS unit is obviously speed over ground (not through the water), tows were conducted at slack high tide when water velocities were significantly less than the tow speed (≈2.5 knots), minimizing errors in the calculation of water volume. Although the lack of a flow meter does limit the accuracy of the results, every effort was made to ensure that the calculated densities were representative of the sample tows (i.e., examining for clogging of net mesh). The two tows at each station were pooled for statistical analysis. Statistical analysis A power analysis was performed to determine the minimum number of samples necessary to detect differences in larval densities, temperature, and salinity, when using weekly and monthly groupings as replicates. A post hoc power analysis indicated that a minimum of 16 samples per site would need to be collected to detect differences among variables. As our sample size at each site (n = 10 for salinity, n = 12 for ichthyoplankton) was below the threshold needed to detect differences at α = 0.05, data collected inside and outside of the plume were compared in aggregate. When data were normally distributed, as indicated by kurtosis and skewness, a t-test was used to determine if statistical differences between the variables existed. For comparisons between non-normally distributed data, the Mann-Whitney test was employed to test for differences between sample medians. All tests were considered significant at α = 0.05. Results Fish larvae diversity A total of 1055 larvae were collected, encompassing ten species over the three month period. Of the ten observed species, Ulvaria subbifurcata Storer (Radiated Shanny) and Tautogolabrus adsperus Walbaum (Cunner) comprised 56% and 40% of total ichthyoplankton catch, respectively. The remaining 4% Table 1. Monthly abundance data for ichthyoplankton collected in the Saco River Estuary system in the summer of 2007. The numbers to the right of each species represent the total number of larval fish collected during the respective months. Species June July August Total Radiated Shanny (Ulvaria subbifurcata) 392 191 15 598 Longhorn Sculpin (Myoxocephalus octodecimspinosus) 2 0 0 2 Winter Flounder (Pseudopleuronectes americanus) 1 1 0 2 Four-bearded Rockling (Enchelyopus cimbrius) 0 11 0 11 American Plaice (Hippoglossoides platessoides) 0 12 8 20 Lumpfish (Cyclopterus lumpus) 0 2 0 2 Cunner (Tautogolabrus adsperus) 0 127 290 417 Sandlance (Ammodytes americans) 0 1 0 1 Cusk (Brosme brosme) 0 1 0 1 Northern Pipefish (Syngnathus fuscus) 0 1 0 1 Total 395 347 313 1055 2009 A.M. Wargo, C.E. Tilburg, W.B. Driggers, and J.A. Sulikowski 651 of ichthyoplankton included Myoxocephalus octodecemspinosus Mitchill (Longhorn Sculpin), Pseudopleuronectes americanus Walbaum (Winter Flounder), Enchelyopus cimbrius L. (Four-bearded Rockling), Cyclopterus lumpus L. (Lumpfish), Ammodytes americans DeKay (Sandlance), Brosme brosme Ascanius (Cusk), Syngnathus fuscus Storer (Northern Pipefish), and Hippoglossoides platessoides (Fabricius) (American Plaice) (Table 1). Temporal ichthyoplankton distribution Although a visual difference appeared to exist (Fig. 2), power analyses indicated that the sample size was not large enough to examine for statistical differences among the ichthyoplankton densities. Although ichthyoplankton densities did not statistically change over time, a marked decline in ichthyoplankton density was observed on our last sampling date, August 31, 2007 (Fig.3). Effects of temperature and salinity Of the two abiotic parameters measured, only salinity was found to be associated with differences in larval abundance. Based on sea surface CTD measurements, the fresh water entering the coastal region from the Saco River creates a plume of low salinity water that surrounded stations 1 and 2 (average surface salinity for stations 1 and 2 [n = 20] was 24.8 PSU), but did not reach stations 3 or 4 (average surface salinity for stations 3 and 4 [n = 20] was 29.8 PSU). When the stations were grouped based on surface salinity, (normal distribution: skewness inside the plume = -0.07 and outside the plume = -0.20; kurtosis inside the plume = -1.28 and outside the plume = -0.58) a significant difference (t-test: t = -3.26, P = 0.004) in salinity of the plume was found between the stations inside (1 and 2) and outside (stations 3 and 4) (Fig. 4). In addition, ichthyoplankton abundance at stations outside Figure 2. Monthly densities of larval fish (± SD) at each sampling station (4 samples per station, per month). Station 4 had the largest larval fish densities, which were dominated by Ulvaria subbifurcata (Radiated Shanny) and Tautogolabrus adsperus (Cunner). 652 Northeastern Naturalist Vol. 16, No. 4 the plume was found to be significantly greater than stations inside the plume (Mann-Whitney W = 137.0 , P = 0.0076). Discussion Larval fish The Radiated Shanny and Cunner were the most abundant species captured, comprising 96% of the total catch. Total densities of Radiated Shanny found in the current study were similar to those observed in other ichthyoplankton surveys along the Maine coast, including Penobscot Bay (0.5/100 m3; Lazzari 2001). However, unlike the results of Lazzari (2001), Cusk, Northern Pipefish, and Lumpfish were found within our sampling area. The Lumpfish observation is surprising. Although not normally found in coastal areas, this species can drift in with debris from a storm or other oceanographic event (Collette and Klein-MacPhee 2002). Northern Pipefish are found in estuaries during summer months, and larval forms have been documented in Gulf of Maine coastal areas (Collette and Klein- MacPhee 2002). Although only one Cusk larva was recorded in our study, its presence is not unexpected since this species spawns in late spring and early summer within this region (Collette and Klein-MacPhee 2002). Density variance The ichthyoplankton densities varied depending on sampling location. Moreover, the sharp increases and declines in total ichthyoplankton density Figure 3. Weekly ichthyoplankton density (number/100 m3 ± SD; n = 4 stations per sampling date) collected over the sampling period. The large increase in late June corresponds with the dramatic increase in Ulvaria subbifurcata (Radiated Shanny), while the large change in total density in late July was due to the arrival of Tautogolabrus adsperus (Cunner) to the sampling sites. 2009 A.M. Wargo, C.E. Tilburg, W.B. Driggers, and J.A. Sulikowski 653 were due to the observed changes in Radiated Shanny and Cunner abundance over the course of the sampling period. Although Radiated Shanny was observed in every month of the study, this species was most abundant in late June. This species produced large increases in ichthyoplankton densities from 4.0/100 m3 on June 22nd to 80.8/100 m3 five days later. However, a dramatic decline in the density of this species was observed twenty days later, reducing the total observed ichthyoplankton density to 10.0/100 m3. Interestingly, immediately following the marked decline in Radiated Shanny abundance, a similar event occurred in Cunner abundance. For example, this species markedly increased the total ichthyoplankton density from 20.4/100 m3 to 118.7/100 m3 over a twenty-two day period between July19th and August 10th. Twenty-one days (on August 31st) after the highest recorded ichthyoplankton density, ichthyoplankton abundance substantially decreased to the lowest recorded levels of 1.0/100 m3. Neither the Radiated Shanny nor Cunner was observed in the lowest density sample. The large changes in ichthyoplankton densities produced by the Radiated Shanny and Cunner are consistent with Collette and Klein-MacPhee (2002), which suggests that these species may have recently spawned just before their respective peaks in abundance observed in the current study. Figure 4. Comparison of total larval density outside of Saco River freshwater plume to those inside the plume over the course of the 3-month study. The larval densities outside the plume were statistically greater (denoted by asterisk) than larval densities found inside the plume (Mann-Whitney W: W = 137.0 , P = 0.0076 ± SD). In addition, the salinity outside the plume (n = 20) was significantly different than the salinity inside the plume (n = 20; t-test: t = -3.26, P = 0.004 ± SD). See text for more details. 654 Northeastern Naturalist Vol. 16, No. 4 An interesting observation from this study is the potential impact of a large fresh-water plume on ichthyoplankton density. Larval density in the stations found outside the freshwater plume were significantly larger than the larval density found within the plume and accounted for over 80% of the observed ichthyoplankton captured during the study. The significantly lower densities of ichthyoplankton observed inside the plume when compared to the densities outside the plume suggests that the reduced salinity environment within the plume may have affected the distribution of ichthyoplankton. However, further research needs to be conducted before any conclusions can be drawn. Acknowledgments We thank Tim Arienti, Nathan Furey, and Angela Cicia for their help in sample collections. Thanks are further extended to Dr. John Olney for his help in identifi- cation of the ichthyoplankton. This project was supported by a University of New England Marine Science Center (MSC) Summer Fellowship to J.A. Sulikowski and C.E. Tilburg. This manuscript represents MSC contribution number 20. Literature Cited Collette, B.B., and G. Klein-MacPhee. 2002. Bigelow and Schroeder’s Fishes of the Gulf of Maine. Smithsonian Institution Press. Washington, DC. 748 pp. Faria, A., P.M. Morais, and A. Chicharo. 2006. Ichthyoplankton dynamics in the Guadiana estuary and adjacent coastal area, South-East Portugal. Estuarine, Coastal, and Shelf Science 70:85–97. Graham, J.J. 1972. Retention of larval herring within the Sheepscot estuary of Maine. Fishery Bulletin 70(2):299–305. Hettler, F.W., and J.A. Hare. 1998. Abundance and size of larval fishes outside the entrance to Beaufort Inlet, North Carolina. Estuaries 21(3):476–499. Kelley, J.T., D.C. Barber, D.F. Belknap, D.M. Fitzgerald, S. van Heteren, and S.M. Dickson. 2005. Sand budgets at geological, historical, and contemporary time scales for developed beach system, Saco Bay, Maine, USA. Marine Geology 214:117–142. Laroche, J.L. 1980. Larval and juvenile abundance, distribution, and larval food habits of the larvae of five species of sculpins (Family: Cottidae) in the Damariscotta River. PhD. Dissertation. University of Maine, Orono, ME. 169 pp. Lazzari A.M. 2001. Dynamics of larval fish abundance in Penobscot Bay, Maine. Fishery Bulletin 99:81–93. National Research Council (NRC). 2004. Atlantic Salmon in Maine. A report of the National Research Council of The National Academies, Washington, DC. 240 pp. Reynolds, W.W., and M.E. Casterlin. 1985. Vagile macrofauna and the hydrographic environment of the Saco River Estuary and adjacent waters of the Gulf of Maine. Hydrobiologia, 128:207–215. Townsend, D.W. 1983. The relations between larval fishes and zooplankton in two inshore areas of the Gulf of Maine. Journal of Plankton Research 5(2):145–173.