Importance and Predictability of Cannibalism in Rainbow Smelt

SANDRA L. PARKER STETTER*1

Cornell University Biological Field Station, Department of Natural Resources, Cornell University,900 Shackelton Point Road, Bridgeport, New York 13030, USA

JENNIFER L. STRITZEL THOMSON2

Vermont Cooperative Fish and Wildlife Research Unit, Rubenstein School of Environment and NaturalResources, University of Vermont, Burlington, Vermont 05405, USA

LARS G. RUDSTAM

Cornell University Biological Field Station, Department of Natural Resources, Cornell University,900 Shackelton Point Road, Bridgeport, New York 13030, USA

DONNA L. PARRISH

U.S. Geological Survey, Vermont Cooperative Fish and Wildlife Research Unit, Rubenstein School ofEnvironment and Natural Resources, University of Vermont, Burlington, Vermont 05405, USA

PATRICK J. SULLIVAN

Department of Natural Resources, Cornell University, Ithaca, New York 14850, USA

Abstract.—Cannibalism is a key interaction between young of year (age-0) and older fish in many

freshwater ecosystems. Density and spatial overlap between age-groups often drive cannibalism. Because

both density and overlap can be quantified, the magnitude of cannibalism may be predictable. Our study

considered cannibalism in rainbow smelt Osmerus mordax in Lake Champlain (New York–Vermont, United

States, and Quebec, Canada). We used acoustic estimates of the density and distribution of age-0 and

yearling-and-older (age-1þ) rainbow smelt to predict cannibalism in the diets of age-1þ fish during 2001 and

2002. Experienced density, a measure combining density and spatial overlap, was the strongest predictor (R2

¼ 0.89) of the proportion of cannibals in the age-1þ population. Neither spatial niche overlap (R2¼ 0.04) nor

age-0 density (R2 ¼ 0.30) alone was a good predictor of cannibalism. Cannibalism among age-1þ rainbow

smelt was highest in June, lowest in July, and high in September owing to differences in thermal stratification

and habitat shifts by age-0 fish. Between July and September, age-1þ rainbow smelt consumed 0.1–11% of

the age-0 population each day. This resulted in a 38–93% mortality of age-0 fish due to cannibalism. These

estimated mortality rates did not differ significantly from observed declines in age-0 rainbow smelt

abundances between sampling dates. Age-1þ rainbow smelt are probably the primary predators on age-0

rainbow smelt during the summer and early fall in Lake Champlain.

Cannibalism results in significant mortality of young-

of-year (age-0) fish in many species (Chevalier 1973;

Smith and Raey 1991; Persson et al. 2003). In rainbow

smelt Osmerus mordax, cannibalism by yearling-and-

older (age-1þ) fish on age-0 fish causes cyclical patterns

in population abundance (Henderson and Nepszy 1989;

He and LaBar 1994; Lantry and Stewart 2000), and

modeling indicates that cycle length depends on the age

and abundance of the primary cannibalistic age-group

(He and LaBar 1994; Lantry and Stewart 2000). Such

patterns have been observed in Lake Erie since the

1960s (Henderson and Nepszy 1989; Lantry and

Stewart 2000). Self-regulation makes rainbow smelt

‘‘one of its own worst enemies’’ (Kendall 1926) and

complicates the management of piscivores in systems

dependent on rainbow smelt production. Managing

predator demand and cannibalistic prey supply requires

an understanding of the level of cannibalism relative to

other sources of mortality. Management models would

improve if we could predict the degree of cannibalism

within and between years.

Although cannibalism is often density dependent

(Frankiewicz et al. 1999; Kellison et al. 2002), spatial

overlap between predator and prey is also an important

* Corresponding author: slps@u.washington.edu1 Present address: School of Aquatic and Fishery Sciences,

University of Washington, Box 355020, Seattle, Washington98195-5020, USA.

2 Present address: Annisquam River Marine FisheriesStation, Massachusetts Division of Marine Fisheries, 30Emerson Avenue, Gloucester, Massachusetts 01930, USA.

Received November 9, 2005; accepted July 28, 2006Published online February 1, 2007

227

Transactions of the American Fisheries Society 136:227–237, 2007� Copyright by the American Fisheries Society 2007DOI: 10.1577/T05-280.1

[Article]

predictor of interaction (Lloyd 1967; Williamson and

Stoeckel 1990). This is true for rainbow smelt because

of strong ontogenetic shifts in thermal preferences and

diel vertical migration. Age-1þ rainbow smelt prefer

cool water, remaining in the hypolimnion during the

day but moving into the metalimnion and lower

epilimnion to forage at night (Ferguson 1965; Evans

and Loftus 1987). In the absence of stratification, they

move throughout the water column during foraging

(Nellbring 1989). Conversely, age-0 rainbow smelt are

initially found in warm epilimnetic waters and do not

vertically migrate. In late summer, they begin to shift

down into the metalimnion and overlap with age-1þrainbow smelt (Ferguson 1965; Tin and Jude 1983;

Burczynski et al. 1987; Urban and Brandt 1993).

Although age-1þ fish can forage in suboptimal

temperatures (Kendall 1926; Ferguson 1965), the

thermal structure and timing of habitat shifts by age-0

fish will influence cannibalistic access to these fish.

Thus, both the density of age-0 fish and the degree of

overlap between them and age-1þ fish should influence

the rates of cannibalism.

Our study goals were to determine whether

seasonal cannibalism among rainbow smelt could be

predicted using acoustics and any of three predictors

(spatial niche overlap, age-0 density, or experienced

density) and to apply this knowledge to estimates of

age-0 mortality attributable to predation by age-1þrainbow smelt in Lake Champlain (United States,

Canada). The experienced density predictor (Lloyd

1967; defined in the Methods section) combines

density and spatial overlap in a single metric. Our

approach was to quantify cannibalism in diet analyses

and to estimate predictors using acoustic data under

three thermal periods: weak (June), strong (July), and

weakening (September). Acoustics can provide sepa-

rate estimates of age-0 and age-1þ rainbow smelt

densities even when the age-groups overlap (Parker

Stetter et al., 2006).

Our study had four main objectives: to test the

abilities of age-0 density, spatial niche overlap, and

experienced density to predict observed proportions of

cannibalistic age-1þ fish during different thermal

periods; to estimate nightly age-0 mortality using the

predicted proportion of cannibals, known consumption,

and measured densities of age-1þ and age-0 fish; to

predict the total mortality of age-0 fish due to

cannibalism between June–July and September; and

to compare predictions of age-0 total mortality with

observed changes in acoustic density estimates.

Methods

Study area.—Lake Champlain (United States, Can-

ada) is oriented north–south along the borders of New

York, Vermont, and Quebec. The lake has a total

surface area of 1,140 km2 (Potash et al. 1969). Natural

and artificial barriers divide Lake Champlain into three

areas that differ in depth and strength of thermal

structure: Main Lake, Inland Sea, and Malletts Bay.

The Main Lake is located west of Burlington, Vermont;

Malletts Bay is north of Burlington and east of the

Main Lake; and the Inland Sea is north of Malletts Bay.

The differences among these areas allowed us to

examine cannibalism under different conditions. With-

in the three areas we studied, the Main lake is deepest

(maximum, 120 m; mean, 30 m), the Inland sea is

moderate (maximum, 50 m; mean, 13 m), and Malletts

Bay is shallowest (maximum depth, 30 m; mean depth,

13 m) (Potash et al. 1969).

Survey timing.—Surveys of age-0 and age-1þrainbow smelt were conducted in 2001 and 2002.

Sampling occurred on June 17–21, July 22–26, and

September 16–20, 2001, and on July 21–25 and

September 15–19, 2002. Data for June 2002 were not

available owing to problems with acoustic equipment.

The time periods were selected to represent weak

thermal structure (June), strong thermal structure

(July), and weakening thermal structure during the

overlap between age-0 and age-1þ fish (September).

All work was performed on the University of Vermont

RV Melosira (length, 13.7 m; engine, 275 horsepower

[1 horsepower ¼ 746 W]; cruising speed, 5.7 m/s).

Coupled acoustics and trawling segments were sur-

veyed at night, commencing at least 1 h after sunset

and ending at least 1 h before sunrise. Peak rainbow

smelt feeding occurs after sunset (Parker et al. 2001).

The underlying cruise track was a zigzag survey with a

random start.

Fish collection and diet analyses.—Rainbow smelt

were collected for diet analyses using two trawl types.

A midwater trawl with a 6-mm-square mesh cod end

and 6-m 3 6-m average fishing dimensions was the

primary gear for collecting age-1þ rainbow smelt. A 2-

m 3 2-m Tucker trawl with a 1-mm cod end was also

used for specimen collection. A netsonde was affixed

to the headrope of both nets to monitor trawl depth.

Both nets were deployed and retrieved with hydraulic

winches. All age-1þ rainbow smelt collected for diet

analyses were flash frozen on dry ice. Thermocline

depth was assessed nightly with a temperature–depth

recorder.

Rainbow smelt stomach contents were identified and

counted in the laboratory with a dissecting microscope.

Age-0 rainbow smelt in the stomachs of age-1þ fish

were identified by the presence of pharyngeal teeth,

vertebrae counts, or other morphological features.

Because 99% of identifiable young of year in the diets

of age-1þ fish in this analysis were rainbow smelt, we

228 PARKER STETTER ET AL.

considered partially digested fish to be rainbow smelt

unless they were positively identified otherwise. The

proportion of age-1þ cannibals and the average number

of age-0 fish consumed per cannibal were calculated

for each trawl.

Acoustic analyses.—A Simrad EY500 split-beam

echo sounder (70 kHz, 11.18 half-power beam width,

0.2-ms pulse length, two pings/s) was used to collect

acoustic data. The transducer was mounted on a towed

body and deployed from the starboard stern of the boat.

The unit was calibrated using a standard copper sphere

within a few weeks of each survey.

All acoustic data were processed and exported using

EchoView 3.25 (SonarData 2004). We made correc-

tions to the algorithm-detected bottom and manually

removed electrical or physical noise. No data were used

from within 0.5 m of the bottom or from within 3.0–5.0

m of the surface.

To maximize the coherence between acoustic data

and diet analyses, we analyzed acoustic data only on

transect segments that were sampled by trawl for age-

1þ rainbow smelt diet analyses. Trawls with five or

more age-1þ diet samples and trawl depths less than

22 m were selected for analyses. We chose 22 m

because it was the maximum depth encountered

during trawling in Malletts Bay; it encompassed the

range of vertical overlap between age-0 and age-1þfish; most trawling occurred between 4 and 22 m;

rainbow smelt biomass was concentrated above 22 m

during the night; and the 22-m depth removed the

potential bias in age-0 density estimates from mysids

and low signal-to-noise ratio in deeper water (Parker

Stetter et al. 2006).

Acoustic single-target and echo integration data were

exported in 5-min bins, commencing at the beginning

of each trawl and ending at haul-back. We used 1-m

vertical bins through the zone of overlap and 2-m

vertical bins below the overlap zone, to a maximum

depth of 22 m. We applied a �76-dB threshold to

single-target detections and a �80-dB threshold to

volume backscattering (Sv) data. Single-target data

were exported in 1-dB bins.

The densities of age-0 and age-1þ rainbow smelt

were calculated for each 1- or 2-m analytic bin. First,

we calculated Sawada’s Nv

index (Sawada et al. 1993)

for each cell and included only the cells with an Nv

less

than or equal to 0.10. In situ target strength in cells with

higher Nv

values may be biased (Rudstam et al. 2003).

Echo integration (volume backscattering coefficient, sv

[m2/m3]) was then converted to total density (/m3) for

each analytic bin using a mean acoustic backscattering

cross-section (rbs

) from in situ targets between�76 and

�20 dB. Next, the proportions of age-0 and age-1þ fish

were generated using in situ target strength (TS) ranges

for June, July, and September:�60 to�35 dB (age 1þ,

all months),�76 to�61 dB (age 0, June),�75 and�50

dB (age 0, July), and�68 to�45 dB (age 0, September)

(Parker Stetter et al. 2006). The average TS of age-0

rainbow smelt increases from�68 to�59 dB from June

to September, whereas the TS of age-1þ rainbow smelt

remains around �48 dB throughout this time period

(Rudstam et al. 2003). When the TS of age-0 and age-

1þ fish overlapped in July and September, the ratios of

large to small age-1þ targets were used to attribute

overlapped in situ TS values to both age-groups (Parker

Stetter et al., in press). Finally, the proportions of in situ

targets attributed to age-0 and age-1þ fish were used to

partition total density into age-0 and age-1þ compo-

nents. We averaged the densities for each depth interval

and summed them to calculate total age-0 and age-1þdensity (fish/m2) between 4 and 22 m.

Estimating proportion of cannibals.—We tested

three approaches to predicting the proportion of age-

1þ cannibals in the midwater trawls. These approaches

individually considered spatial niche overlap, age-0

density, and experienced density.

Czekanowski’s index (Feinsinger et al. 1981) was

used to quantify the spatial niche overlap in the vertical

distributions of age-1þ and age-0 rainbow smelt at

depths between 4 and 22 m. We treated location in the

water column as the shared resource. This index is

calculated as

OAge-0;age-1þ ¼ OAge-1þ;age-0

¼ 1� 0:5 �Xm

j¼1

jPAge-1þj � PAge-0jj ;

OAge-1þ,age-0

¼ the overlap between age-1þ and age-0

fish;

OAge-0,age-1þ¼ the overlap of age-0 fish on age-1þ fish;

PAge-1þj

¼ the proportion of age-1þ total density at

depth j of m total depth layers;

PAge-0j

¼ the proportion of age-0 total density at

depth j.

Czekanowski’s index calculates an estimate of

spatial niche overlap using proportional age-1þ and

age-0 distributions and does not account for density

effects.

The second predictor of the proportion of cannibals

in the age-1þ population was average age-1þ density

(DAge-1þ/m3). This estimate was calculated for the 4–22

m depth range as

DAge-0 ¼

Xm

j¼1

Xn

i¼1

DAge-0ij

n � m ;

PREDICTING CANNIBALISM IN RAINBOW SMELT 229

where DAge-0ij is the density of age-0 fish in analytic

bin ij (/m2), j is the vertical depth segment, and i is the

horizontal transect segment of n total horizontal

transect segments.

Experienced density, which is equivalent to Lloyd’s

mean crowding (Lloyd 1967) and Williamson’s density

risk (Williamson et al. 1989), was the third predictor of

cannibalism by age-1þ rainbow smelt. This index is a

measure of the average density of age-0 prey

experienced by an average age-1þ predator, thereby

taking into account both density and spatial overlap. It

has been applied to predation on zooplankton (e.g.,

Williamson and Stoeckel 1990; Folt and Schulze 1993)

and on larval fish (e.g., Frankiewicz et al. 1999;

Garrison et al. 2000; Yamamura et al. 2001) from the

perspective of estimating the risk to a prey species. It

has not, however, been used in understanding canni-

balism. As we apply density risk from the perspective

of the cannibalistic predator, we call it ‘‘experienced

density.’’ Similar concepts have been applied in

spatially explicit bioenergetics models (e.g., Goyke

and Brandt 1993; Luo and Brandt 1993; Stockwell and

Johnson 1997).

Experienced density (E [age-0 density � m�3 � age-1þindividual�1]) was calculated over a standard 4–22 m

depth range as

E ¼

Xm

j¼1

ðPAge-1þj � DAge-0jÞ

m;

where m is the vertical height of the water column over

which the density estimate was made.

Least-squares regression models were used to

determine the relationship between the proportion of

age-1þ cannibals in diet analyses and spatial niche

overlap, age-0 density, and experienced density. All

analyses included an intercept term and were conduct-

ed in S-Plus 6.1 (Insightful Corporation 2002).

Age-0 predicted and observed mortality.—The

proportions of age-1þ cannibals on our transect

segments were predicted from regression results for

experienced density (see Results) to standardize to the

same relationship. The daily mortality of age-0 fish

(MAge-0

) caused by cannibalistic age-1þ fish was then

calculated for each transect segment as

MAge-0 ¼

Xm

j¼1

Xn

i¼1

DAge-1þij

n � m � CAge-1þ � x;

where CAge-1þ is the estimated proportion of age-1þ

cannibals determined from experienced density calcu-

lations, and x is the mean number of age-0 fish per

cannibal stomach.

When cannibalism was predicted but not observed,

the minimum value of 1 age-0 fish per cannibal was

used in calculations. Daily age-0 mortality was then

converted to proportion of the age-0 population

consumed that night (PMAge-0

) as

PMAge-0 ¼MAge-0

DAge-0

� �:

Using our PMAge-0

daily mortality estimates, we

calculated expected mortality between sampling dates

in each area. When more than one estimate of mortality

was available within an area, we used a mean PMAge-0

for that sampling date. We used linear interpolation

between sampling dates to estimate PMAge-0

for each

day. Daily survival was then calculated as 1 – PMAge-0

.

For each day during the sampling period, we multiplied

the population remaining at the beginning of each day

with the daily survival calculated for that day. Total

predicted mortality was calculated between July and

September for all areas in 2001 and 2002. A single

calculation was made between June and September in

Malletts Bay 2001, as June data were only available for

that lake area.

We estimated total observed age-0 rainbow smelt

mortality between sampling dates using acoustic

abundance estimates for each lake area (Parker Stetter

2005). Total observed mortality was calculated as the

difference between age-0 density at the starting and

ending sampling dates. We used a chi-square test to

determine whether predicted age-0 mortality differed

from observed mortality.

Results

Fish Collection and Diet Analyses

Twenty-five trawls were included in this study.

Trawl catches were dominated by age-0 rainbow smelt

in 2001 and by age-1 fish in 2002 (Table 1). Other

species were not important components of trawl

catches, only two trawls having more than 6% other

species (Table 1). As rainbow smelt typically constitute

up to 99% of pelagic trawl samples in Lake Champlain

(Kirn and LaBar 1991; Pientka and Parrish 2002), this

was an expected result. Fifteen trawls contained

cannibals, with the proportion of cannibalistic age-1þfish ranging from 0.04 to 0.70 (Table 1). Age-0

rainbow smelt were positively identified in 88% of age-

1þ stomachs with fish remains.

Cannibalism varied by area and month and broadly

reflected differences in thermal profiles. June and

September 2001 had weak thermoclines and the

highest proportion of cannibalistic age-1þ fish in diet

analyses (Figure 1; Table 1). In July 2001 and 2002,

strong thermoclines increased the spatial separation of

230 PARKER STETTER ET AL.

age-0 and age-1þ in some areas. The highest proportion

of cannibals in July was in the area with the weakest

thermal structure (the Inland Sea in 2001), and the

lowest proportions of cannibals were in the areas with

the strongest thermal structures (Malletts Bay in 2001

and the Inland Sea in 2002) (Figure 1; Table 1).

Acoustic Analyses

The densities of age-0 and age-1þ rainbow smelt

varied between 2001 and 2002. High densities of age-0

fish were observed on our transect segments in 2001,

ranging from 0.038 to 1.750 fish/m3 (Table 2). During

the same year, the densities of age-1þ fish ranged from

0.002 to 0.062 fish/m3 (Table 2). In 2002, the opposite

pattern prevailed; age-0 densities were between 0.007

and 0.123 fish/m3 and age-1þ densities between 0.003

and 0.095 fish/m3 (Table 2). The differences in density

between 2001 and 2002 agree with the proportions of

age-0 and age-1þ fish in the trawl catches (Table 1).

The vertical distributions of age-0 and age-1þrainbow smelt follow thermal profiles. During the

weak thermal structure in June, age-1þ rainbow smelt

were present throughout the water column and

overlapped with age-0 fish in upper waters (Figure

2). In July, the age-1þ distribution began abruptly

below the thermocline, so there was little overlap with

age-0 fish, which remained in the epilimnion (Figure

2). Although a strong thermocline in September pushed

the age-1þ distribution deeper, age-0 movement into

metalimnetic waters increased the overlap between the

age-groups (Figure 2).

Estimating the Proportion of Cannibals

Our three predictors—spatial niche overlap, age-0

density, and experienced density—differed in their

ability to predict the observed proportions of cannibal-

istic age-1þ fish. Spatial niche overlap was a poor

predictor of the observed proportion of cannibals (N¼25, R2 ¼ 0.04; Figure 3, upper panel) in the least-

squares linear regression calculations. Age-0 density

explained only 30% of the variation in the proportion

of age-1þ cannibals (N ¼ 25, R2 ¼ 0.30; Figure 3,

middle panel). However, experienced density was

strongly related to the proportion of cannibals obtained

from diet analyses (N¼ 25, R2¼ 0.89; Figure 3, lower

panel). The highest value in this relationship (0.037,

0.70) had a Cook’s distance (Neter et al. 1996) of 1.2,

indicating that this point had the potential to influence

TABLE 1.—Summary of trawls used for age-1þ rainbow smelt diet analyses. Maximum depth is the maximum site depth during

the trawl, trawl depths are single or stepped trawling depths, trawl style is the gear used (MW¼midwater trawl, TT¼ Trucker

trawl), Prop. Age-1þ, age-0, and other are the proportions of those categories in trawl hauls, and Prop. cannibals is the proportion

of cannibalistic age-1þ fish in diet analyses.

Month and area Maximum depth (m) Trawl depths (m) Trawl style Prop. age-1þ Prop. age-0 Prop. other Prop. cannibals

Jun 2001Malletts Bay 28 20, 12, 9 MW 1.00 0.00 0.00 0.38Malletts Bay 19 16, 10, 1 MW 1.00 0.00 0.00 0.33

Jul 2001Malletts Bay 25 20, 15, 10 MW 0.95 0.05 0.00 0.04Malletts Bay 17 11, 8 MW 0.11 0.89 0.00 0.00Inland Sea 48 30, 20, 10 MW 0.08 0.91 0.01 0.21Main Lake 40 19 MW 0.13 0.86 0.01 0.24

Sep 2001Main Lake 44 31, 25 MW 0.04 0.95 0.01 0.08Main Lake 68 32, 25 MW 0.17 0.82 0.01 0.70Inland Sea 40 20, 15 MW 0.43 0.14 0.43 0.22Inland Sea 48 15 MW 0.01 0.96 0.03 0.17Malletts Bay 28 12 MW 0.48 0.52 0.00 0.00Malletts Bay 26 16 MW 0.06 0.94 0.00 0.16Malletts Bay 26 15, 10 MW 0.67 0.33 0.00 0.21

Jul 2002Malletts Bay 27 15, 10 TT 1.00 0.00 0.00 0.08Malletts Bay 27 6, 2 TT 1.00 0.00 0.00 0.00Malletts Bay 25 20, 15 MW 1.00 0.00 0.00 0.04Inland Sea 41 20 MW 1.00 0.00 0.00 0.00Inland Sea 33 5 MW 0.90 0.04 0.06 0.00Inland Sea 28 25, 15 MW 1.00 0.00 0.00 0.00Main Lake 66 40, 25, 15 MW 0.54 0.00 0.46 0.00Main Lake 46 10 MW 0.02 0.98 0.00 0.00

Sep 2002Malletts Bay 28 15 TT 0.99 0.01 0.00 0.00Malletts Bay 27 15 MW 0.96 0.04 0.00 0.08Malletts Bay 18 15 MW 0.39 0.61 0.00 0.00Main Lake 32 25, 20 MW 0.10 0.90 0.00 0.07

PREDICTING CANNIBALISM IN RAINBOW SMELT 231

the linear regression as an outlier. Without this point,

the regression was CAge-1þ ¼ 15.53E – 0.002 (R2 ¼

0.79, N ¼ 24). An inspection of residuals, fits, and

variance provided sufficient evidence that this point

was valuable for our analyses.

Age-0 Predicted and Observed Mortality

The proportions of cannibals in the age-1þ popula-

tion was predicted for each transect segment using

experienced density. As expected by the high R2 value,

the predicted proportions of cannibals (Table 2) are

very similar to those observed in diet analyses (Table

1). The within-area variability in the predicted number

of cannibals in any given month was generally between

0.01 and 0.08 (maximum less minimum; Table 2).

We estimate that cannibalistic age-1þ fish consumed

between 0.1% and 11.0% of the age-0 population on

the nights we sampled (Table 2). In both 2001 and

2002 there were high estimates of percent age-0 fish

consumed, the top four values occurring in September

2001 (5.2% and 7.3%) and 2002 (5.8% and 11.0%).

When mortality was interpolated as a daily time-step

between sampling dates (Table 3), total predicted age-0

mortality was 63–81% between July and September

(Figure 4). These values do not represent instantaneous

mortality rates. Mortality in the area with June-to-

September data (Malletts Bay 2001) was 86%.

Total observed mortality, based on measured

declines in age-0 acoustic abundance estimates be-

tween sampling dates, ranged from 60% to 90%

(Figure 4). The differences between total observed

and predicted mortality within areas were between

�12% and þ22% (Figure 4). Predicted mortality did

not differ significantly from observed mortality P(v2 .

0.19, df ¼ 5) . 0.90.

Discussion

Quantifying cannibalism is essential for understand-

ing population dynamics, stock–recruitment curves,

and other aspects of a species’ ecology. Our results

suggest that cannibalism by age-1þ rainbow smelt is a

major source of age-0 mortality throughout the

FIGURE 1.—Temperature profiles for Malletts Bay (dashed black lines), the Main Lake (solid black lines), and the Inland Sea

(gray lines) in June, July, and September of (a) 2001 and (b) 2002.

232 PARKER STETTER ET AL.

TABLE 2.—Age-0 mortality estimates for 2001 and 2002. Age-0 density and age-1þ density are the average densities between

4 and 22 m, proportion cannibals is the proportion of cannibals predicted by the least-squares regression equation CAge-1þ ¼

17.48 � E� 0.01, average age-0 fish/cannibal is the mean number of age-0 fish per cannibal stomach from diet analyses, and %age-0 fish eaten/night is the percent of the age-0 population consumed by age-1þ fish on the trawled portion of the transect on the

survey date.

Monthand area

Age-0density

Age-1þdensity

Proportioncannibals

Average age-0fish/cannibala

% Age-0 fisheaten/night

Jun 2001Malletts Bay 0.311 0.009 0.313 2.8 2.7Malletts Bay 0.174 0.018 0.349 1.3 4.5

Jul 2001Malletts Bay 0.104 0.019 0.118 1.0 2.2Malletts Bay 0.304 0.014 0.041 1.0 0.2Inland Sea 0.182 0.005 0.191 2.0 1.0Main Lake 1.750 0.003 0.135 2.2 0.1

Sep 2001Main Lake 0.119 0.002 0.041 1.0 0.1Main Lake 0.967 0.062 0.636 1.8 7.3Inland Sea 0.063 0.008 0.205 2.0 5.2Inland Sea 0.038 0.005 0.127 1.0 1.7Malletts Bay 0.077 0.005 0.006 1.0 0.1Malletts Bay 0.111 0.010 0.234 1.3 2.8Malletts Bay 0.389 0.017 0.360 2.6 4.0

Jul 2002Malletts Bay 0.042 0.052 0.021 1.0 2.8Malletts Bay 0.045 0.095 0.022 1.0 5.0Malletts Bay 0.123 0.073 0.024 1.0 1.5Inland Sea 0.067 0.020 0.009 1.0 0.3Inland Sea 0.122 0.042 0.005 1.0 0.2Inland Sea 0.091 0.039 0.001 1.0 0.1Main Lake 0.007 0.003 0.007 1.0 0.4Main Lake 0.076 0.015 0.053 1.0 1.1Malletts Bay 0.027 0.038 0.040 1.0 5.8Malletts Bay 0.046 0.080 0.062 1.0 11.0Malletts Bay 0.032 0.050 0.010 1.0 1.7Main Lake 0.010 0.014 0.073 1.7 1.0

a This variable cannot be less than 1.0; if cannibalism was predicted but not observed, it was set equal

to 1.0.

FIGURE 2.—Representative proportions of age-0 (YOY; gray lines) and age-1þ (YAO; solid black lines) densities in Malletts

Bay (site depths, 17–22 m) during June, July, and September 2001, along with temperature profiles (dashed black lines).

PREDICTING CANNIBALISM IN RAINBOW SMELT 233

summer. Further, we show that acoustics can predict

the occurrence and magnitude of cannibalism in this

forage species. Our study found that experienced

density, a measure combining age-0 density with the

spatial overlap between age-0 and age-1þ fish, can

predict cannibalism in rainbow smelt. In using acoustic

data for estimates of density and overlap, our analyses

were at finer scales than is possible with conventional

gear types.

We tested the ability of three measures—spatial

niche overlap, age-0 density, and experienced densi-

ty—to predict the proportion of cannibals in the age-1þrainbow smelt population. Spatial niche overlap

considers only the spatial relationship between age-0

and age-1þ fish and does not consider predator and

prey densities. As a result, spatial niche overlap was a

poor predictor of cannibalism. Alternatively, consider-

ing only age-0 density disregards the differences in the

spatial overlap between age-groups that occur because

of water column stratification. Spatial overlap is an

important consideration in a species such as rainbow

smelt, in which age-groups are thermally separated

(Nellbring 1989). Even so, age-0 density was a fair

predictor of the proportion of cannibals in the age-1þpopulation. However, our experienced density index

was the strongest predictor of cannibalism by age-1þfish. This index takes into account both density and

spatial influences. Combining density and spatial

influences is critical in a system such as Lake

Champlain in which there are both directional trends

in age-0 and age-1þ densities (Parker Stetter 2005) and

an internal seiche influencing thermocline depth

(Hunkins et al. 1998).

Age-1þ rainbow smelt consumed a high percentage

of the age-0 population in 2001 and 2002, despite

differences in the proportion of cannibals. This

counterintuitive finding results from differences in

density. A high proportion of cannibals in the low-

density age-0 population caused high age-0 mortality in

2001. Conversely, higher 2002 age-1þ densities offset

the lower proportions of cannibals and resulted in age-0

mortalities comparable to those of 2001. Our results

suggest that cannibalistic age-1þ rainbow smelt are

responsible for most of the observed July–September

age-0 mortality. Predicted cannibalism accounted for

an average of 89% of the observed decreases in

acoustic age-0 density estimates. Our predicted mor-

tality may be conservative, as it is based on peak age-

1þ feeding after sunset (Parker et al. 2001). We did not

examine possible additional consumption of age-0 fish

FIGURE 3.—Least-squares regression relationships between

the proportion of age-1þ cannibals in diet analyses and three

measures of association in June (solid triangles), July (solid

squares), and September 2001 (solid circles) and July (open

squares) and September 2002 (open circles). The estimated

equation in the upper panel is y¼0.16x þ 0.07 (R2¼0.04, N¼25), that in the middle panel is y¼ 0.24x þ 0.07 (R2¼ 0.30, N¼ 25), and that in the lower panel is y¼ 17.48x – 0.01 (R2¼0.89, N¼ 25); YAO¼ age 1þ and YOY¼ age 0 (see text for

additional details).

TABLE 3.—Values used to predict age-0 rainbow smelt

mortality by cannibalistic age-1þ fish. The beginning and

ending dates are the sampling dates (or mean sampling date if

over multiple days) used in mortality calculations, and the

proportions of age-0 fish cannibalized are the mean propor-

tions of the age-0 population consumed by cannibals on those

dates.

Area YearBeginning

dateEnding

date

Proportion age-0fish cannibalized

Beginningdate

Endingdate

Malletts Bay 2001 18 Jun 18 Sep 0.0359 0.0228Malletts Bay 2001 23 Jul 18 Sep 0.0121 0.0228Malletts Bay 2002 22 Jul 15 Sep 0.0309 0.0617Main Lake 2001 26 Jul 16 Sep 0.0005 0.0370Main Lake 2002 25 Jul 18 Sep 0.0073 0.0097Inland Sea 2001 24 Jul 17 Sep 0.0098 0.0345

234 PARKER STETTER ET AL.

during the dawn crepuscular descent but expect it to be

minimal because age-0 fish migrate out of the overlap

zone during this time.

Although we predict that cannibalism is the main

cause of mortality for age-0 rainbow smelt between

July and September, other sources of mortality will also

contribute to the decline. Physical factors such as

extreme weather could cause loss of age-0 fish between

sampling periods. Similarly, predation by stocked or

native piscivores may contribute to age-0 losses, but

these have not been studied in Lake Champlain.

Finally, the growth rates of both age-0 and age-1þrainbow smelt may explain the differences between

predicted and observed age-0 mortality in September

2002. As a result of high 2001 cohort densities, age-0

fish had low growth rates and recruited to the age-1þgroup in 2002 as smaller individuals (Stritzel Thomson

2006). In 2002, low age-0 densities resulted in larger

age-0 individuals (Stritzel Thomson 2006); the smaller

age-1þ fish may have been less successful cannibaliz-

ing the larger age-0 fish due to gape limitations. This

suggests that size dependency be used in future

experienced density predictions of cannibalism by

including only age-0 fish below a critical size.

Cannibalism by rainbow smelt has been estimated in

Lakes Ontario (Lantry 1991; Lantry and Stewart 2000),

Michigan (Lantry 1991; Lantry and Stewart 2000), and

Erie (Parker Stetter et al. 2005). Our estimates of age-

1þ cannibals compare well with Great Lakes data in

July of both years and September 2002. However, June

2001 estimates of the proportion of cannibals are

considerably higher than the 2–6% in Lantry and

Stewart (2000) and the 2–5% in Parker Stetter et al.

(2005). The differences between our estimates and

those of Lantry and Stewart (2000) probably result

from differences in seasonal timing, as Lantry (1991)

assessed diet and cannibalism in April (before age-0

fish were abundant) and again in August and October.

Estimates by Parker Stetter et al. (2005) are based on

May–August data, and the use of a seasonal average

may obscure higher cannibalism in June. Our Septem-

ber 2001 estimates are also higher than those Lantry

and Stewart (2000) and Parker Stetter et al. (2005)

because of high densities of age-0 fish in the metal-

imnion. The levels of age-0 mortality caused by age-1þcannibals in our study are substantially higher than that

needed to cause abundance cycles in population

models (e.g., He and LaBar 1994; Lantry and Stewart

2000).

We identified two limitations to using acoustics:

increasing noise levels with depth and nontarget

organisms. The signal-to-noise ratio increases with

depth, and small age-0 rainbow smelt targets may not

be detectable within that noise. In addition, mysids are

abundant in the Main Lake and may increase the

number of small targets (Gal et al. 2004). To avoid

biases from noise and mysids, we restricted our

analyses to depths less than 22 m. Although 4–22 m

covered the overlap between age-0 and age-1þ fish in

June and July, this depth limit may have excluded the

lower range of vertical overlap in September. Addi-

tionally, by applying the proportion of cannibals to

age-1þ densities only between 4 and 22 m, our

calculations assumed that there were no cannibals in

deeper water.

Cannibalism in forage species requires further study

because it complicates the management of prey fish

and piscivore stocks. Highly cannibalistic species such

as rainbow smelt can have marked and regular

population cycles (Lantry and Stewart 2000) that are

not tracked by constant annual stocking of piscivores.

Both compensatory (e.g., Frankiewicz et al. 1999;

Juanes 2003; Persson et al. 2003) and destabilizing

(e.g., Polis 1981; Hammar 2000; Kellison et al. 2002)

effects of cannibalism have been observed. Therefore,

a better understanding of the relationship between

cannibalism and forage fish population stability is

needed to ensure effective management applications of

stock–recruitment curves (Ricker 1954).

Acknowledgments

Special thanks to crew of the University of Vermont

RV Melosira for assistance in the field. We are grateful

to editor Dennis DeVries, an associate editor, and three

anonymous reviewers for comments that increased the

quality and presentation of this manuscript. This work

was sponsored in part by a grant from the National Sea

FIGURE 4.—Observed and predicted age-0 (YOY) mortality

in 2001 and 2002. Observed mortality is the measured

decrease in age-0 abundance, predicted mortality the estimated

mortality due to cannibalism. Areas are abbreviated as

follows: MB¼Malletts Bay, IS¼ the Inland Sea, and ML¼the Main Lake; the numerals 01 and 02 refer to 2001 and

2002, respectively. All estimates are for July–September

except MB01*, which is for June–September. The dashed line

indicates a 1:1 relationship.

PREDICTING CANNIBALISM IN RAINBOW SMELT 235

Grant College Program, National Oceanic and Atmo-

spheric Administration, U.S. Department of Com-

merce, to Lake Champlain Sea Grant under grant

number NA16RG2206. The views expressed are those

of the authors and do not necessarily reflect the views

of the sponsors. Mention of brand names does not

constitute product endorsement by the U.S. federal

government. This is publication number LCSG-OO-06,

contribution 227 of the Cornell Biological Field

Station. Additional funding was received from the

National Science and Engineering Research Council

(NSERC) of Canada (NSERC postgraduate scholarship

to S.L.P.S.).

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