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.
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