Assessing Efficacy of Non-Lethal Harassment of
Double-Crested Cormorants to Improve Atlantic Salmon
James P. Hawkes, Rory Saunders, Adam D. Vashon, and Michael S. Cooperman
Northeastern Naturalist, Volume 20, Issue 1 (2013): 1–18
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2013 NORTHEASTERN NATURALIST 20(1):1–18
Assessing Efficacy of Non-Lethal Harassment of
Double-Crested Cormorants to Improve Atlantic Salmon
James P. Hawkes1,*, Rory Saunders1, Adam D. Vashon2,
and Michael S. Cooperman3
Abstract - Salmo salar (Atlantic Salmon) smolts are exposed to predation pressure as
they migrate from freshwater into the estuary and near-shore marine environment. In
particular, Phalacrocorax auritus (Double-crested Cormorants) are a predator of Atlantic
salmon smolts during their estuary and near-shore migration. National Oceanic and
Atmospheric Administration’s (NOAA) National Marine Fisheries Services’ (NMFS)
telemetry data collected prior to this study (1997–2003), suggest that smolts are being
removed from the Narraguagus River on their downstream out-migration. This removal
may be the result of Cormorant predation. We investigated whether smolt survival could
be improved by disrupting normal Cormorant foraging activity by integrating passive
smolt tracking and active harassment techniques. Smolt movement and usage of various
portions of the estuary according to light condition and tidal stage were explored along
with concurrent avian harassment. Although harassment only occurred in approximately
33% of available daylight hours during this study, the impacts were easily recognized.
Non-lethal harassment effectively displaced Cormorants from feeding locations and
reduced loss of emigrating smolts. In 2004, 83.3% (15 of 18) of all smolt mortalities
occurred on days of non-harassment, compared to only 16.7% (3 of 18) on days when
harassment occurred. Similarly in 2005, 87.5% (7 of 8) of all smolt mortalities occurred
on days of non-harassment, compared to only 12.5% (1 of 8) on days when harassment
occurred. Non-lethal harassment appeared to be an effective means to reduce loss of
emigrating smolts in the Narraguagus River estuary.
Salmo salar L. (Atlantic Salmon) populations in the United States are near
historic low abundance (USASAC 2011). The Gulf of Maine Distinct Population
Segment (GoM-DPS) represents the last remnant native stock in the US and
is listed as endangered under the US Endangered Species Act (Fay et al. 2006;
74 Federal Register 29344, 19 June 2009). The freshwater habitats of Maine’s
salmon rivers have been degraded by a variety of land-use practices, chemical
pollution, and introduction of exotic species; and these changes have contributed
to the decline of Atlantic Salmon (NRC 2004). Despite years of intensive efforts
and freshwater habitat restoration coupled with population enhancement via conservation
hatcheries, populations within the GoM-DPS remain low (USASAC
1NOAA, National Marine Fisheries Service, Maine Field Station, 17 Godfrey Drive,
Orono, ME 04473. 2United States Department of Agriculture, Animal and Plant Health
Inspection Service Wildlife Services, 79 Leighton Road, Suite 12, Augusta, ME 04330.
3Conservation International, 2011 Crystal Dr. Suite 500, Arlington, VA 22202. *Corresponding
author - James.Hawkes@noaa.gov.
2 Northeastern Naturalist Vol. 20, No. 1
2011). This lack of recovery of the GoM-DPS is not surprising given the continued
reduction in marine survival of Northwest Atlantic Salmon stocks described
by Chaput et al. (2005).
The seaward migration of Atlantic Salmon smolts can be a time of high mortality
owing to interactions amongst legacy effects as a result of exposure to acid
rain and aluminum during rearing (McCormick et al. 2009), unique physiological
challenges associated with the transition from freshwater to saltwater, and exposure
to abundant and novel predators (Beland et al. 2001, Blackwell and Juanes
1998, Dieperink et al. 2002, Järvi 1989, Martin et al. 2009). Identifying and potentially
managing sources of smolt mortality within estuaries are high priority
research and conservation needs that could improve the effectiveness of recovery
measures through adaptive management practices.
Knowledge of the ecology and migratory behavior of Atlantic Salmon smolts
within estuaries is limited but increasing. Typically, estuarine migration has been
described as ebb-tide transport, whereby smolts move seaward on receding tides
and hold their location on flood tides (Lacroix 2008, McCleave 1978). Recent
studies have demonstrated that appreciable numbers of smolts make pronounced
upstream “reversal” movements during flood tides (Kocik et al. 2009, Martin
et al. 2009). Unlike in freshwater, where smolts typically move downstream at
night presumably to avoid visual predators (Martin et al. 2009, Moore et al. 1995,
Ruggles 1980), smolts within estuaries have been observed moving at all hours
of the day (Kocik et al. 2009; LaCroix et al. 2004; Martin et al. 2009; Moore et
al. 1995, 1998). It is unclear if different estuarine behaviors, such as migration
during day or night, on flood or ebb tides, or rapid transit of the estuary versus
prolonged estuary residency, result in differential survival.
The Narraguagus River, ME supports a naturally reproducing Atlantic Salmon
population, which is supplemented by a river-specific, multi-life stage juvenile
stocking program, designed to recover the natural population. This population is
severely diminished compared to historic levels (USASAC 2011). Estimates of
the number of smolts emigrating from this river over the past decade have ranged
from fewer than 1000 to 2500 annually (USASAC 2011). Prior migration studies
have demonstrated high smolt mortality during the transition from the river
to the estuaries. Specifically, Kocik et al. (2009) reported almost 60% of 581
telemetry-tracked smolts died as they transited though the lower river, estuary,
and inner bay of the Narraguagus system from 1997 to 2004. They observed only
10% mortality in the outer portions of Narraguagus Bay, despite smolts having
comparable residency times in the two areas. Specific sources of mortality were
not identified. Our post-study review of smolt telemetry-tracking data in Kocik
et al. (2009) revealed strong spatial overlap between locations of presumed smolt
losses and our observation of areas occupied by Phalacrocorax auritus Lesson
(Double-crested Cormorant; hereinafter “Cormorants”). We hypothesized the
primary cause of smolt mortality was predation removal by avian sources, particularly
Cormorants are considered generalists, foraging on a variety of seasonally
abundant fresh and saltwater fish and invertebrates from benthic and pelagic
2013 J.P. Hawkes, R. Saunders, A.D. Vashon, and M.S. Cooperman 3
habitats (Blackwell and Krohn 1997, Blackwell et al. 1995). Atlantic Salmon
smolts are a significant prey item; in some cases, smolt losses to Cormorants
can be exceptionally high. For example, Meister and Gramlich (1967) recovered
from Cormorants rookeries 3724 tags applied to hatchery stocked smolts on the
Machias River in 1966 and 1967, representing 4.9 and 8.1%, respectively, of fish
released during these two years. These authors estimated that Cormorants consumed
between 16 to 50% of stocked smolts during this period. Similarly, Krohn
and Blackwell (1996) reported an estimated 7.5 to 9.2% (43,318 to 78,675) of
hatchery-stocked smolts in the Penobscot River were preyed upon by Cormorants
between the years of 1992 to 1994.
Our goal was to test the efficacy of non-lethal harassment and d escribe smolt
survival during harassment and non-harassment periods. Specific objectives were
to describe 1) the distribution of Cormorants within the lowest portion of the Narraguagus
River, 2) the relationship between Cormorant habitat use and Atlantic
Salmon smolt behavior, and 3) the effectiveness of a pilot Cormorant harassment
study. We discuss our findings in the context of management recommendations
and future research needed for limiting smolt mortality in the estuary.
The Narraguagus River is 70 km long with a drainage area of approximately
600 km2. The river flows southeast from its headwaters at 124 m above sea level
to the town of Cherryfield, ME, where the estuary begins (Baum and Jordan
1982). Between Site A and the start of the estuary (above Site B; Fig. 1), the river
is dynamic in composition. The upper third is a slow-moving pool separated by a
dam; traveling further downstream, extreme shifts in gradient and habitat result
in riffle and chutes with coarser substrate consisting of cobble, boulders, and
ledge transitioning to the tidal portion of the watershed. The estuary is 6 km long,
ranges from 50 to 120 m wide, has a mean depth at low water of 3 m and a mean
tidal fluctuation of 3.4 m (Baum and Jordan 1982, Kocik et al. 2009). The tidal
freshwater zone is relatively small, 260 ha, compared to the estuary-mixing zone
totaling 1300 ha (Strategic Assessments Branch 1985). Much of the estuary exhibits
brackish water characteristics, with salinity ranges observed between <1‰
in the upper reaches to as much as 26‰ before entry into the inner bay area (Baird
Software 2004). Our visual surveys of the estuary found that the makeup is primarily
fine sediments and sand in the upper reaches transitioning to constricting
bedrock ledges, boulders, and moving sand bars in the lower portion. There are
four islands in Narraguagus Bay, within 15 km of the mouth of the Narraguagus
River, which supported ≈200 nesting pairs of Cormorants at the time of the study
(Rookery survey–NOAA, Orono, ME, unpubl. data).
The telemetry methods used in our study were identical to those described in
Kocik et al. (2009). Acoustic receivers (VR2 Telemetry Receivers; Vemco Ltd.)
4 Northeastern Naturalist Vol. 20, No. 1
were moored at sites throughout freshwater and the estuary in the Narraguagus
system (Fig. 1). Owing to distinct differences in the habitat structure, salinities,
Figure 1. Map of the study area and telemetry network on the lower Narraguagus River
and Narraguagus estuary, ME. Receivers (gray circles) were deployed at the start of the
study area at Site A. Traveling downstream, the first tidally influenced estuary unit which
is above the salinity gradient is found at Site B. Continuing seaward, the salinity influence
begins at Site C and increases in salinity along the remaining of the study array (Site
J/K) before entering Narraguagus Bay. The upper estuary consists of sites B–D, and the
lower estuary includes sites E–J/K.
2013 J.P. Hawkes, R. Saunders, A.D. Vashon, and M.S. Cooperman 5
and tidal currents in the estuary, we partitioned the estuary into upper (Sites
B–D) and lower (Sites E–J/K) portions (Fig. 1). Additional telemetry arrays were
moored outside our study area in Narraguagus Bay to determine the efficiency
of receiver sites and the fate of tagged smolts. We used two 1.52-m rotary screw
traps to collect emigrating smolts 7.65 km above head of tide on the Narraguagus
River (Fig. 1). We trapped smolts from early April through early June in 2004 and
2005, effectively covering the entire date range of the spring smolt out-migration.
We used smolt run timing and abundance records to deploy telemetry transmitters
(Vemco Ltd. 69 kHz serial coded acoustic transmitters; 2004 = V9-6L, 20 x 9 mm,
3.3 g in air, 20–60 s random delay, 68 d lifespan, 2005 = V7-2L, 18.5 x 1.6 mm, 1.6
g in air, 20–50 s random delay, 74 d lifespan) in proportion to the anticipated daily
abundance of the emigrating population (Kocik et al. 2009). Smolts were anesthetized
using buffered tricaine methanesulfonate (MS-222), surgically implanted
with a transmitter and released within a half-hour of surgery. We tagged smolts
of 145 mm fork length and larger to minimize tagging affects. All smolts were
released at the trapping site at river km 7.65. To minimize surgery effects, only
smolts making it to Site A (River km 2.11) were included in this study.
Detection of tagged smolts at Site A marked entry into the study area. The fate
of each smolt was determined by detection at successive receivers. Successful
smolt movement was defined as whenever the last detection at one VR2 location
was followed by a subsequent detection, either upstream or downstream.
Time of the movement was assigned as the time of arrival to the latter site. An
unsuccessful movement (mortality), was defined as whenever a last detection at
one site, excluding site J and K (the most downstream site in our network), was
not followed by a subsequent detection at another site. When assigning mortalities,
our assumptions were 1) transmitters do not malfunction, 2) transmitters no
longer detected (removed from the system) or which were stationary in the river
had been depredated, and 3) last detection of a smolt deemed “unsuccessful” was
the time which mortality occurred. Each individual smolt could have numerous
successful movements, but only one unsuccessful movement. We classified each
smolt movement as either day or night based on the timing of civil twilight (i.e.,
limit of terrestrial objects to be clearly distinguished) as reported by the US
Naval Observatory (aa.usno.navy.mil) for Milbridge, ME and as flood or ebb
based on tide data obtained from mobile graphics (mobilegeographics.com) for
Cormorant distribution and harassment
During the smolt migration of 2004 and 2005, we documented the presence
of Cormorants in the Narraguagus estuary and conducted non-lethal harassment
activities. From 21 April through 25 May 2004, enumeration and
harassment occurred on weekdays, and involved one person. From 9 May
through 27 May 2005, enumeration and harassment occurred during alternating
four-day periods and involved two people, one each in the upper and
lower estuary. In the freshwater and upper estuary portion of the study area,
Cormorant enumeration and harassment occurred eight hours per day, between
6 Northeastern Naturalist Vol. 20, No. 1
the dam and just below the head of tide (at approximately Site C; Fig. 1). In
the lower estuary, harassment effort was variable. In 2004, hazardous water
levels and difficult access limited Cormorant observation and harassment in
the lower estuary to a three-hour period around the daytime high tides. We
conducted Cormorant harassment for nearly eight hours each day in 2005.
Observation and harassment involved researchers patrolling the study area on
foot or by motorboat. A consistent daily patrol pattern was employed. Activities
began at the top of the study area, with movement seaward, while the remainder
of the day consisted of up and downstream movements for the duration of harassment.
During each event in which Cormorants were observed, time, location,
and number of Cormorants present were recorded. We were unable to identify
individuals; therefore, these data represent Cormorant movement patterns,
with individuals likely sighted several (unknown) times. The specific method
used for each harassment event was selected on a case-by-case basis: approaching
by foot or boat, throwing rocks, or using slingshot, lasers (Avian Dissuader®,
SeaTech, Albuquerque, NM) or pyrotechnics (Bird Bombs® and Bird Whistlers®,
Sutton Agricultural Enterprises, Inc., Salinas, CA; and Shellcrackers®, Stoneco
Inc., Trinidad, CO). We did not use pyrotechnics earlier than 0900 hours or in areas
where Haliaeetus leucocephalus L. (Bald Eagle) were present. Harassment
activities for each event took less than 30 seconds, and Cormorants were observed
leaving the site at the completion of harassment. We considered the different methods
to be equally effective and, for analysis purposes, we grouped them together as
“harassment”. The location and frequency of harassment events were determined
by Cormorant presence at the time of harassment patrol. Hence, the number of harassment
events conducted each day and at each location varied.
We complemented Cormorant data collected via point counts with time-lapse
photography to document diel behavior and daily use patterns during treatment
and control periods. In 2004, we deployed one camera at an isolated location with
a broad vantage point and frequent Cormorant use (Fig. 1). The camera was operated
13 May to 24 May (12 days) and took one image every hour during daylight
hours, yielding 14 pictures daily. In 2005, we attempted to increase spatial coverage
by adding an estuary site and temporal resolution of the images, but were not
successful due to weather-related equipment failures.
Smolt behavior and survival
We determined the number of successful and unsuccessful smolt movements
which occurred during each of the four possible light level and tide direction
combinations (day–ebb, day–flood, night–ebb, night–flood) and plotted each
movement as a function of these variables. For successful and unsuccessful movements,
we summarized proportions of movement during light condition (day or
night) and tide direction (ebb or flood) on the frequency of movements.
Overlap of Cormorants and smolts
We explored the overlap in the timing of smolt movements and Cormorant
presence by plotting the mean (±1 SD) number of first smolt detections (i.e.,
2013 J.P. Hawkes, R. Saunders, A.D. Vashon, and M.S. Cooperman 7
indicative of arrival to a site) that occurred at each hour of the 24-hour daily
cycle. For example, for the 0900 values during the 2004 effort, if smolts moved
through the upper estuary over a 30-day period and there were five sites within
the upper estuary, we determined how many first detections occurred at any of the
five receivers between 0830 and 0930 hours on each of the 30 days and calculated
summary values for the 0900-hour range. We then overlaid a plot of the mean
(±1 SD) number of Cormorants present in photographs on days of no harassment
efforts at each hour of the daily cycle.
Effectiveness of Cormorant harassment
We used one-way ANOVA to test for differences in daily count of Cormorants
present in 2004 photographs on days with and without harassment. We also used
the hourly Cormorant count data from the 2004 photographs to plot the mean
(±1 SD) number of Cormorants present at each hour of the day on both harassment
and non-harassment days. To evaluate the potential for either additive or
declining effectiveness of repeated harassment events within a day with a count
of Cormorants, we fit an inverse first order polynomial regression of the form
y = yø + (a / x) + (b / x2), where y is the number of Cormorants present within
a 1-km section of the river located between Sites A and B, yø is the number of
Cormorants present at the time of the first harassment event of the day, and x is
the number of the harassment events within a day. For example, on the first day
of Cormorant harassment in 2005, we conducted 13 unique harassment events
between Sites A and B, and for each of these 13 events we counted the number of
Cormorants present before each harassment event started. On the second day, we
conducted 27 unique harassment events in this area, and so on for each day of the
year’s field effort. We then calculated the mean (±1 SD) number of Cormorants
present prior to the first harassment event, prior to the second harassment event,
etc., up to the largest number of harassments (i.e., 46 events within a single day),
and we then fit a curve to the resulting plot. In an effort to determine if harassment
of Cormorants affected smolt mortality, we plotted the date and time of each
unsuccessful movement event and overlaid a plot of the timing of harassment
activities and calculated the percentage of unsuccessful movements occurring
during harassment and non-harassment periods.
In 2004, the mean (±1 SD) fork length (mm) and weight (g) of the 66 smolts
in the study was 174.9 mm (14.1) and 53.0 g (13.6), respectively. In 2005, the
mean fork length and weight of the 61 smolts in the study was 169.0 mm (11.8)
and 47.7 g (11.0), respectively. As a comparison, smolts collected by rotary screw
traps for abundance estimates during this period in the Narraguagus had a mean
fork length (n = 569) of 170.7 mm (14.4) and a mean weight (n = 569) of 49.1 g
(13.5). Therefore, the mean length and weight of the smolts in the telemetry study
was representative of the smolt population in 2004 and 2005. For the two years
combined, transmitter weight (wet weight; g) averaged 6.2% (V9-6L tags) or
8 Northeastern Naturalist Vol. 20, No. 1
3.4% (V7-2L tags) of mean smolt weight, and smolt survival was not related to
the ratio of smolt to transmitter weight.
Movement within the upper (Sites B–D) and lower (Sites E–J/K) segments of
the estuary were different. In the upper estuary, approximately 80% of all successful
smolt movements occurred between 2100 and 0400 hours (Fig. 2A, C). In
2004, smolts successfully moved at night regardless of tidal stage. Fifty percent
(n = 144) of the successful movements occurred during ebb-night conditions,
31% (n = 89) during flood-night, 12% (n = 35) during ebb-day, and 7% (n = 20)
during flood-day conditions.
We observed a similar night-use pattern in 2005, but smolt movements were
more common during ebb tides than flood tides. Forty-seven percent (n = 109)
of the successful movements occurred during ebb-night conditions, 32% (n = 74)
during flood-night, 14% (n = 33) during flood-day, and 7% (n = 16) during ebbday
In contrast to the upper estuary, most movements in the lower estuary were
associated with tidal stage rather than the light-dark cycle. In both years of
the study, >72% of successful smolt movements occurred during ebb tides
(Fig. 2B, D). In 2004, there was no indication of an effect of the light-dark cycle
on smolt movements. Forty-two percent (n = 198) of the successful movements
occurred during the ebb-night conditions, 30% (n = 140) during ebb-day, 16%
(n = 76) during flood-night, and 12% (n = 56) during flood-day conditions. In
2005, smolt movements were influenced by ebb tide, but were also associated
with darkness. Fifty-seven percent (n = 199) of successful movements occurred
during ebb-night conditions, 21% (n = 72) during ebb-day, 14% (n = 50) during
flood-day, and 9% (n = 30) during flood-night conditions.
There were 20 unsuccessful smolt movements (mortalities) in 2004. Two of
these mortalities occurred in freshwater at night. The remaining 18 smolt mortalities
occurred in the estuary during daylight hours, with five mortalities in the
upper estuary (ebb-day, n = 3; flood-day, n = 2) and 13 mortalities in the lower
estuary (ebb-day, n = 10; flood–day, n = 3). Twelve of the 18 estuarine smolt mortalities
(66.7%) occurred at or before 1000 hours. The mortality rate for smolts in
2004 was 30.3%.
In 2005, there were 10 unsuccessful smolt movements. Two of these mortalities
occurred in freshwater at night. All eight mortalities in the estuary occurred
during daylight hours, with five in the upper estuary (ebb-day, n = 5) and three in
the lower estuary (ebb-day, n = 2; flood-day, n = 1). Six of the eight smolt mortalities
(75.0%) occurred at or before 1000 hours. The mortality rate for tagged
smolts in 2005 was 16.4% within the study area.
Overlap of Cormorants and smolts
There was notable separation between the timing of peak smolt movements
and peak Cormorant presence within the upper estuary. Most smolt movements
occurred from 2100 to 0400 hours, while Cormorant presence peaked
between 0600 and 1000 hours (Fig. 3).
2013 J.P. Hawkes, R. Saunders, A.D. Vashon, and M.S. Cooperman 9
Figure 2. The timing of both
successful (circles) and unsuccessful
(stars) smolt movements
in (A) upper estuary
2004, (B) lower estuary 2004,
(C) upper estuary 2005, and
(D) lower estuary 2005, as a
function of both the light-dark
cycle and tidal conditions.
Diagonal lines represent high
(solid line) and low (dashed
line) tide conditions and shaded
areas are hours of low light
levels or darkness.
10 Northeastern Naturalist Vol. 20, No. 1
Effectiveness of Cormorant harassment
Harassment affected Cormorant behavior and use in the estuary. In 2004, there
were a total of 344 daylight hours during this study period, in which 108 hours of
harassment (approximately 160 person hours) were conducted, which represents
approximately 32% of daylight hours. 679 harassment events displaced 8132
Cormorants (likely including individuals displaced multiple times) from the study
area. In 2005, there were a total of 274 daylight hours during this study period, in
which 104 hours of harassment (approximately 319 person hours) were conducted,
which represents approximately 38% of daylight hours. 1054 harassment events
displaced 9303 Cormorants. Activity patterns of Cormorants derived from the
2004 time-lapse photography indicate that fewer Cormorants were present on days
of harassment compared to non-harassment days (one-way ANOVA of Cormorant
counts: F[1, 9] = 9.37, P = 0.014) (Fig. 4 A).
On non-harassment days at the fixed camera location, Cormorant activity
peaked between 0600 and 1000 hours, with the hourly mean number of Cormorants
(± 1 SD) observed always greater than 3.7 Cormorants, and with a peak
of 9.5 (±7.3) observed at 0700 hours. On days with harassment efforts, subtle
peaks in the number of Cormorants occurred at 0600 (mean: 2.6 ± 4.5) and
Figure 3. Mean smolt arrival detections to receiver sites used to compare day-night patterns
of successful smolt movement during a 24-hour period in the upper estuary, as
well as the overlap of Cormorants during this period. Circles are mean number of smolt
detections (± 1 SD). Squares are mean hourly Cormorant count (± 1 SD). Data are offset
for easy viewing.
2013 J.P. Hawkes, R. Saunders, A.D. Vashon, and M.S. Cooperman 11
1100 hours (mean: 2.6 ± 4.8). Regardless of harassment intensity, the number of
Cormorants observed after 1100 hours was consistently lower than earlier in the
day, with a mean of 1.25 Cormorants observed per hour (Fig. 4 B). The number
of Cormorants in the study area is correlated with cumulative harassment events
(Fig. 5A, B) in 2004 (inverse first order polynomial regression: r2
adj. = 50.2%, P less than
0.0001) and 2005 (r2
adj. = 75.0%, P less than 0.0001).
Smolts benefited from harassment during both years of this study. In 2004,
there were a total of 251 successful and 18 unsuccessful within-stream movements
of smolts during daylight hours. During periods of non-harassment, 9.0%
(15 of 167) of all smolt movements were unsuccessful. During periods of harassment,
only 2.9% (3 of 102) of all smolt movements were unsuccessful. Further,
of all unsuccessful movements in 2004, 83.3% (15 of 18) of all smolt mortalities
occurred on days of non-harassment, compared to only 16.7% (3 of 18) on days
when harassment occurred.
In 2005, there were a total of 171 successful and 8 unsuccessful withinstream
movements of smolts during daylight hours. During the periods of
non-harassment, 8.2% (7 of 85) of all smolt movements were unsuccessful. During
the periods of harassment, only 1.1% (1 of 94) of all smolt movements were
Figure 4. Data from photographs (n = 168) collected using a fixed-position automated
time-lapse digital camera. A) Daily number of Cormorants observed during harassment
(solid bar) and non-harassment (open bar) days by hour from 13 May–24 May 2004. B)
Mean (± 1 SD) count of Cormorants on days of harassment (solid square) and non harassment
(open square). Data are offset for easy viewing.
12 Northeastern Naturalist Vol. 20, No. 1
unsuccessful. Further, of all unsuccessful movements in 2005, 87.5% (7 of 8)
of all smolt mortalities occurred on days of non-harassment, compared to only
12.5% (1 of 8) on days when harassment occurred (Fig. 6 A, B).
Atlantic Salmon smolts in the upper and lower Narraguagus River estuary
responded differently to light levels and tidal conditions. These behaviors are
consistent with smolt movements in other systems (Martin et al. 2009, McCleave
1978, Moore et al. 1995). In the upper estuary, where tidal influence begins, smolts
moved mostly at night and were not strongly influenced by the tidal cycle, effectively
a continuation of freshwater behavior. In contrast, in the lower portions of
the estuary, where tidal influence is significant, movements were primarily associated
with ebb tides, but were still more common during darkness than daylight.
During our study, there were 30 unsuccessful smolt movements that we classified
as mortalities. The vast majority of mortalities occurred during daylight
(particularly during early morning hours) and ebb tide conditions. We believe
that the four mortalities classified as occurring during nighttime might have occurred
during daylight hours before the next detection occurred because of the
area where they occurred (heavy use by Cormorants) and when they occurred.
This pattern of mortality is consistent with predation by visual predators, which
Figure 5. Mean (± 1 SD) Cormorant number by successive harassment event for A) 2004
adj. = 50.2%) and B) 2005 (r2
adj. = 75.0%).
2013 J.P. Hawkes, R. Saunders, A.D. Vashon, and M.S. Cooperman 13
is a leading hypothesis as to why smolts avoid moving during daylight (Mc-
Cormick et al. 1998, Ruggles 1980). We observed a strong temporal separation
between smolt movements and Cormorant “on-water” counts, which suggests
that innate smolt behaviors (e.g., a reduction in movement) likely improve survival
when visual predators would be actively foraging or present.
Cormorants were at least partially responsible for losses during this study. We
searched for transmitters at the four largest nearby Cormorant rookeries in Narraguagus
Bay and found one of the 30 missing telemetry transmitters. The extent
for which Cormorants are responsible for the remainder of the missing transmitters
is unclear, but mortality of smolts in early hours of daylight on ebb tide is
consistent with the foraging behavior of Cormorants (Anderson et al. 2004, Dunn
1975). Predation by other piciverous birds species is also possible. We regularly
observed several species, including Pandion haliaetus L. (Osprey), Bald Eagle,
Larus argentatus Pontoppidan (Herring Gull), Larus delawarensis Ord (Ringbilled
Gull), Ardea Herodias L. (Great Blue Heron), Lophodytes cucullatus L.
(Hooded Merganser), Mergus merganser L. (Common Merganser), and Mergus
serrator L. (Red-Breasted Merganser). However, the number of Cormorants encountered
during field activities indicates that Cormorants are the most abundant
picivorous bird in the study area.
We observed Cormorants throughout the entire study area, but they most
commonly congregated at constriction points within the estuary (ledge, mud
Figure 6. Harassment period and last detection of unsuccessful smolts (circle) by day in
A) 2004 and B) 2005. Open boxes represent periods of harassment (start and stop times)
and shaded area (top and bottom of figure) represents low-light level or dark conditions.
14 Northeastern Naturalist Vol. 20, No. 1
or sand banks, etc.) at low tide conditions. Presumably, Cormorants used these
constriction points to efficiently forage for smolts and other prey. Harassment
successfully displaced Cormorants away from the areas targeted in the Narraguagus
estuary, as evidenced by fewer Cormorants present on the water on days
with harassment and a reduction in Cormorant count with cumulative harassment
events. Most smolt mortality occurred during daylight hours outside of harassment
time and location. The proportion of mortality observed during the day was
significantly less when harassment activities were taking place. Mortality during
harassment periods was as low as 12.5% during harassment, compared to 87.5%
during non-harassment periods. In 2004, harassment effort only consisted of
one field staff, who had limited access to the lower estuary where two (of three)
mortalities occurred during harassment. In 2005, there were fewer smolts lost
overall. Increased harassment effort (two people vs. one person; 319 vs 160 man
hours) likely resulted in fewer birds in feeding areas (Fig. 5) and lower smolt
losses. Survivorship in 2005 may also have been enhanced as a result of greater
river discharge in that year. Several rain events between 15 April and 1 June 2005
resulted in mean discharge of 43.4 m3/s, twice the discharge for the same time
period in 2004 (16.5 m3/s; USGS 2004, 2005). Hvidsten and Hansen (1988) and
Milton et al. (1995) suggest high discharge events, coinciding with smolt outmigration
and stocking, likely result in an accelerated migration time, potentially
aiding in survival of out-migrating smolts. It is possible that these pulses of high
discharge resulted in an improvement in survival in 2005. We believe that harassment
improved survival further because the shallow nature of the estuary would
still make smolts vulnerable under ebb and low tide conditions.
Once harassment efforts began, photographs at our fixed-point location showed
fewer Cormorants observed on the first day of harassment during each harassment
period with a subsequent increase and variability in Cormorant counts during the
remainder of each harassment period. We considered the possibility of Cormorants
becoming conditioned to our treatment, as observed by Stickley et al. (1995), who
used fixed-position human-effigy units (i.e., inflatable scarecrows) to harass Cormorants
at catfish aquaculture ponds. Unlike Stickley et al. (1995), we did not have
spatial or temporal consistency between events, which they found to be necessary
for conditioning to occur; therefore, conditioning in our study is less likely.
During the egg-laying and chick-rearing periods, Cormorants often forage
early in the morning and then later in the day (Coleman and Richmond 2007).
In Cormorant colonies near Milbridge, ME, breeding commences in late May
or early June (B. Allen, Maine Inland Fisheries and Wildlife, Bangor, ME, pers.
comm.). Therefore, activity on the Narraguagus system at the time of the year
of this study appears to be either pre-breeding or a different foraging pattern
entirely. Photographs, harassment data, and data on unsuccessful smolt movements,
suggest Cormorant foraging in this area occurs primarily early in the day
(0400–1000 hours) followed by sporadic events during the remainder of daylight
hours. Since Cormorant breeding occurs just after the peak of the smolt run, timing
of this study may explain why we did not document a pulse of Cormorant
foraging activity on the ebb tide in the evening.
2013 J.P. Hawkes, R. Saunders, A.D. Vashon, and M.S. Cooperman 15
Acoustic telemetry allowed us to passively track smolt success on an individual
basis through the estuary. Combining passive smolt tracking and active
Cormorant harassment activities revealed that smolts actively moved and may
have benefited from Cormorant disturbance during daylight hours. Cormorants
were successfully displaced, which effectively altered their normal behavior.
Cormorant harassment in the estuary successfully provided protection to
smolts during daylight movements and improved smolt survival. The efforts
during the two years of this study only carried out harassment activities approximately
33% of total daylight hours. We can realistically expect to observe
a population effect if we expand this study to a larger scale. While population
mortality reductions were not the objective of this study, we could expect to
see improvements of survival similar to those observed by Kocik et al. (2009)
from the river to the inner bay. It would be reasonable to expect an increase of
smolts entering the near-shore environment and GoM, with mortality trending
near the 12.5% observed during the second year of this study. This result may
require additional harassment effort, both temporally and spatially. If used as a
management tool, harassment should occur throughout the emigration window
(mid-April through early June) and focus the bulk of activities in the early morning
(0400 to 1000 hours) with a reduced effort later in the day. Maintaining
elevated activities beyond this period may exhibit diminishing returns and overextend
resources. Integrating a Cormorant-count program with smolt monitoring
could further increase effectiveness by allowing low-cost monitoring until bird
counts or abundance and smolt emigration were at target levels, which would
trigger harassment. Management activities such as this could present new options
to improve, or at least maintain, remnant Atlantic Salmon populations.
Harassment is a moderate intervention targeted on minimizing predation and
managing ecosystem structure and function. Such manipulations are an especially
attractive management approach when it is suspected that a predator is at abnormally
high abundance and prey at low abundance. During the mid- to late 1900s,
we believe the ecosystem may have become out of balance because Cormorant
abundance was at record high levels (Krohn et al. 1995, Wires et al. 2001) while
Atlantic Salmon and other diadromous species were at historic low abundance
(Saunders et al. 2006, USASAC 2011). With recent Cormorant declines being
attributed to populations exceeding their carrying capacity and the resurgence of
the Bald Eagle, a known predator of juveniles (Wires et al. 2001), there is a possibility
that the ecosystem is trending towards a lower Cormorant predation rate
on smolts. Ongoing restoration activities (dam removal, habitat alterations, and
restoration hatchery practices) should result in increased abundance of multiple
diadromous species. With decreased predator abundance and an increased prey
field for piscivores, the combined effects should result in a higher percentage of
smolts successfully entering the GoM. It is, however, unclear whether this would
ultimately increase adult returns because it is not known whether early marine
mortality is compensatory or additive. Given the relatively large spatial scale of
the Northwest Atlantic and relatively small number of Atlantic Salmon in that
area, the assumption of additive mortality (i.e., mortality that is not constrained
16 Northeastern Naturalist Vol. 20, No. 1
by density dependence) seems reasonable. Thus, our results suggest that predator
manipulation and management can be a reasonable and important management
tool to temporarily protect endangered populations until co-occurring prey species
(and the concomitant ecological functions they perform) can recover.
We gratefully acknowledge the many individuals who worked on the Narraguagus
Cormorant harassment project. Particular thanks are extended to T. Trinko Lake for GIS
assistance, and D. Schick of the US Fish and Wildlife service for rookery site visits on
coastal islands. We also thank M. Renkawitz, P. Music, J. Kocik, D. Bean, and two anonymous
reviewers for helpful comments on earlier versions of this manuscript. We further
acknowledge the understanding people in the towns of Cherryfield and Milbridge, and
the Crane family for their tolerance of a sometimes noisy management activity.
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