Journal of Experimental Marine Biology and Ecology
ELSEVIER
192 (1995)
Can otolith microchemistry
and habitat utilization
David H. Seco?‘*,
JOURNAL OF
EXPERIMENTAL
MARINE BIOLOGY
AND ECOLOGY
15-33
chart patterns of migration
in anadromous fishes?
A. Henderson-Arzapalob, P.M. Piccoli’
“The University of Maryland System, Center for Es&urine and Environmental
Studies,
Chesapeake Biological Laboratory, P.O. Box 38, Solomons, MD 20688, USA
bLeetown Science Center, National Biological Survey, 1700 Leetown Road, Kearneysville,
WV 25430, USA
‘The University of Maryland System, Department of Geology, College Park, MD 20742, USA
Received 11 July 1994; revision received 16 February 1995; accepted 15 March 1995
Abstract
Seasonal and ontogenetic patterns in estuarine and coastal migrations of anadromous fish
species have important consequences to their survival, growth, recruitment, and reproduction. We tested the hypothesis that otolith (sagitta) microchemistry can document the
environmental
history of individual fish across an estuarine salinity gradient. Juvenile
striped bass, Morone suxntilis (Walbaum), (80 days posthatch) were reared for 3 wk in
aquaria at two temperatures and six salinities. The ratio of strontium/calcium
(Sr/Ca)
deposited in the sagittal otoliths of reared juveniles was positively related to salinity.
Temperature and growth rate had relatively minor, but significant effects on the Sr/Ca
ratio. In a second experiment, juveniles (80 days posthatch) were exposed to increasing
salinity (0 ppt to 25 ppt) and then decreasing salinity (25 ppt to 0 ppt) over a 20-wk period.
Electron microprobe examination of the otoliths from these juveniles showed a gradual rise
and decline in Sr/Ca during the experimental period which corresponded directly tiith
experimental changes in salinity. Field data on subadult and adult striped bass corroborated the laboratory analyses and indicated a logistic relationship between ambient salinity
and otolith Sr/Ca ratio. Verification studies support the use of otolith microchemistry to
measure migratory schedules and habitat utilization patterns in anadromous striped bass
populations.
Keywords:
Anadromy;
* Corresponding
Estuary; Migration;
Otolith microchemistry;
author.
0022-0981/95/$09.50
@ 1995 Elsevier
SSDI
0022.0981(95)00054-2
Science
B.V. All rights
reserved
Striped bass; SrlCa
16
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (199-i) 1.5-33
1. Introduction
Prediction of migration patterns in anadromous and estuarine fishes is difficult
because of variability in seasonal, ontogenetic, and sex-related movements among
individuals within populations. Traditional tagging methods can provide information on the spatial and temporal origin of individuals. Telemetric tagging
methods can provide information on more localized movements of individuals
over short periods, dependent upon the species and system. Hydroacoustic
technology can provide instantaneous “snap shots” of spatial occurrence. However, none of these methods can track seasonal and ontogenetic movements of
individuals over long periods. To infer these movements, data on individuals must
be combined. This curtails detailed analyses and can result in biased emphases on
certain segments of the population dependent upon how fish are tagged or
sampled (Ricker, 1975; Waldman et al., 1990; Hilborn & Walters, 1992). Longitudinal life history data on individual movements could reduce these sampling
biases (Secor & Piccoli, 1995).
Recently, otolith microchemistry has been suggested as a method to reconstruct
seasonal and ontogenetic migration patterns of individuals within anadromous
populations (Kalish, 1990; Secor, 1992). The concentration of strontium (Sr) in
seawater is over one order of magnitude greater than in freshwater and varies in
direct proportion to salinity in estuarine environments (Ingram & Sloan, 1992). If
the chemical composition of otoliths are reflective of chemical composition of
their aquatic habitat, then Sr levels of fish exposed to seawater should be
substantially higher than those exposed to freshwater. Further, as an anadromous
individual migrates through a salinity gradient, the Sr level in their otoliths could
record movements among freshwater, estuarine, and marine habitats.
We used the Sr/Ca ratio to describe changes in Sr in the otolith. The Sr/Ca
ratio was originally used as a measure of the precipitation of Sr or Ca from
solution into aragonitic corals (Kinsman & Holland, 1969). Temperature
was
shown to influence this ratio and precedence was established for using the Sr/Ca
ratio in coral and otolith thermometric studies (Smith et al., 1979; Radtke, 1984;
Townsend et al., 1989). The distribution of Sr and Ca within the otolith matrix is
influenced by Sr and Ca levels in the endolymph, but also by physiological factors
like stress and reproductive activity which can result in higher otolith Sr/Ca ratios
(Kalish, 1991). We used Sr/Ca ratio in analyzing the effects of salinity on otolith
Sr because (1) somatic growth and temperature effects on endolymph Sr and Ca
may affect the kinetics of deposition of Sr and Ca onto otoliths (Kalish, 1989,
1991) and, (2) precedence in the literature for the use of otolith Sr/Ca ratio
facilitated comparison of results to other species (Radtke et al., 1988; Kalish, 1989;
Townsend et al., 1989).
To determine whether otolith microchemistry can provide detailed information
on the migratory history of anadromous fishes, we adapted the method for striped
bass Morone saxatilis (Walbaum), a long-lived iteroparous species which has
anadromous populations occurs along the East Coast of North America.Verification of the hypothesis that otolith Sr/Ca ratio is related to ambient salinity thus
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995)
15-33
17
far has been indirect; Sr/Ca ratio measurements taken in different parts of the
otolith (i.e. at different ages or life-stages) are consistent with expectations of
ambient Sr levels associated with freshwater and marine phases of existence
(Casselman, 1982; Radtke et al., 1988; Kalish, 1990; Secor, 1992). Our goal was to
relate otolith Sr/Ca ratio to known salinity and temperature histories. In rearing
experiments of juveniles we addressed three questions related to the validity of
the otolith microchemistry method: (1) Can otolith Sr levels predict the salinity of
freshwater, estuarine, and marine habitats utilized by the fish? (2) How do
temperature and growth rate affect the relationship between otolith Sr deposition
and ambient salinity? (3) What is the spatial resolution of electron probe
microanalysis to detect changes in otolith Sr/Ca ratio associated with temporal
changes in ambient salinity? Prediction of salinity from otolith strontium was also
tested for otolith samples of “sub adult” (females <5 yr in age; males <2 yr in
age) and adult wild fish collected over a range of salinities.
2. Methods
2.1. Validation experiments
Two experiments were conducted on juvenile striped bass: (1) to evaluate the
influence of salinity and temperature
on the Sr/Ca ratio for fish held under
constant conditions, and (2) to determine whether temporal trends in salinity
could be detected using electron-probe microanalysis to detect Sr and Ca levels.
Rearing experiments were conducted at the Aquatic Ecology Laboratory, National Biological Survey, Leetown, WV on Chesapeake Bay juveniles provided by the
Maryland Department of Natural Resources.
In Experiment 1, 10 juveniles per aquarium (80 days posthatch; total length
(TL) = 42.6 mm? 5.10 SE) were reared for 3 wk in 39-l aquaria (2 aquaria/
treatment) at two temperatures (15 and 25°C) and six salinities (0, 5, 10, 15, 20,
and 30 ppt). Juveniles were the progeny of a single female and several males
(n < 6). Aquaria were equipped with undergravel filters, received supplemental
aeration, and immersed in a water bath equipped with a heating element and
chiller to maintain experimental temperatures. A commercial marine salt mix
added to ambient spring water (330 mg/l Ca, 0.65 mg/l Sr) was used to establish
experimental salinities. Data supplied by the manufacturer indicated that Sr levels
in the formulated salt mix would match those expected for marine environments
(12.4 mg/l Sr, 410 mg/l Ca) (e.g. Sverdrup et al., 1946). Temperature and salinity
was monitored daily with a conductivity meter. For all tanks, standard errors for
daily temperatures and salinities over the experimental period were within 6% of
the designated experimental
temperatures
and salinities. Maximum daily departures from designated levels among experimental treatments were 1.5”C and
2.9 ppt. For salinity levels less than 30 ppt, maximum daily departure was 1.9 ppt.
In Experiment 2, 10 juveniles per aquarium of the same age and size used in
Experiment 1 were reared in 585-l aquaria for 21 wk at either 19 or 25°C (2
18
aquaria/treatment).
schedule:
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995)
Salinity
in aquaria
Week
salinity
l-3
4-6
7-9
10-12
13-15
16-18
19-21
0
5
15
25
15
5
0
was adjusted
15-33
according
to the following
Corrected salinity (ppt)
(see explanation
below)
(ppt)
0
2.5
6.1
9.8
6.1
2.5
0
Experimental
adjustments
to salinity were made at a rate of 5 ppt per day.
Therefore,
adjustment
between
5 ppt (Week 6) and 15 ppt (Week 7) occurred
over a 2-day period. Among aquaria, standard errors for daily temperatures
and
salinities were within 4 and 7% of designated
experimental
levels, respectively.
Maximum
daily departures
from experimental
levels were 1.4”C and 1.0 ppt.
For both experiments.
dissolved oxygen and temperature
were measured
daily,
and pH and ammonia
levels were monitored
biweekly and maintained
at levels
favorable
for juvenile
striped bass growth and survival (Nicholson
et al., 1990).
Aquaria water was replaced as necessary
(due to evaporation
or high ammonia
levels) with water of the same temperature
and salinity. Marine salt mix was
reconstituted
with laboratory
spring water and aerated for at least one week prior
to use. Juveniles
were fed dry salmon feed ad libitum
three times each day.
Juveniles
were sacrificed at the end of the experiment,
measured
for length and
weight, and frozen.
Sr concentrations
in Experiment
1 aquaria, measured
at the beginning
of each
week with atomic absorption
spectrophotometry
did not change significantly
over
the 3-wk period of the experiment
among salinity treatment
levels (ANOVA:
II = 18; p = 0.99).
2.2. Salinity corrections
in experimental
treatments
Artificial seawater used for both experiments
was deficient in Sr concentration.
This required
that experimental
salinities
be adjusted
to “natural”
salinities
expected
based upon experimental
Sr levels. Sr concentration
were measured
from water samples collected at the beginning
of each week during Experiment
1
and regressed on experimental
salinity levels (Fig. 1A). Sr levels were much lower
over the range of experimental
salinities than Sr concentrations
measured
across
an estuarine
salinity gradient
by Ingram
& Sloan (1992) (Fig. 1A). Artificial
seawater
was over 2-fold deficient
in Sr when compared
to expected
natural
concentrations.
Experimental
Sr concentrations
were converted
to expected
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995) 15-33
Natural
Salinity:
Sr = 0.228*
Salinity
+
0.0514
F? =0.99
(Ingram
6
T
and Sloan
%
\
4
-
g
2
~.~+o.~~.
(y - ‘.,.,.‘-=
I.,
0
RZ =0.9k
5
10
Natural
@)
*.,*
0 * a.,
20
15
or Experimental
.e_
_..,,.’
_.=
,..*
. t I I I I I I I I.
,
/
,_a’
*..*
1992)
-
;i
19
Salinity
25
9
30
(ppt)
15 -
Adj.
_
Sal.
=
0.367*Salinity
+
0.639
2
!a12 &
x
.c- g _
.z
ZJ z 6_
t;=I ‘2 3 -
oy,,,,.,.,,,,,,,,,,,.,,,,,,,,,,,,
0
5
10
Experimental
15
20
Salinity
25
30
(ppt)
Fig. 1. Salinity corrections
for Experiments
1 and 2. (A) Ambient
Sr versus experimental
or natural
salinity level. Ingram & Sloan’s (1992) relationship
between natural Sr and salinity in the SacramentoSan Joaquin
estuary is indicated
by dashed line. (B) Adjusted
salinity versus experimental
salinity.
Salinity was adjusted by converting
experimental
Sr to natural salinity according to Ingram and Sloan’s
relation.
natural salinity by inputing experimental Sr levels as independent variable into
Ingram and Sloan’s regression of natural salinity on natural strontium level
(expected salinity = -0.059 + 4.35 Experimental
Sr). This conversion was supported by strong correlations which existed between experimental Sr and salinity
levels (Fig. 1A). Corrected salinities were -40% of the designated experimental
salinities levels (Fig. 1B). The reason for the lower Sr concentration
in the
experimental treatments is unknown; it may have been due to the static nature of
20
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995) 15-33
the experimental aquaria, differential absorption of the Sr by the aquarium system
and fish, or reduced Sr concentrations in the formulated salt mix.
2.3. Field samples
Striped bass of various size classes were collected in the field from habitats of
oligohaline, mesohaline, polyhaline, and marine salinities. Young male striped bass
and immature females (~60 cm TL) were sampled during fall and winter
throughout the Chesapeake Bay (United States) as part of a stock assessment
program conducted by the Chesapeake Biological Laboratory (Rothschild et al.,
1992). Otoliths (sagittae) were analyzed from fish collected at mesohaline
salinities (7, 12.5, and 17 ppt) at temperatures between 10 and 15°C. Similar size
striped bass were also collected from a polyhaline impoundment (30 ppt, 15C) in
Salem, Massachusetts during fall 1992. Marine samples (35 ppt, 15°C) of adult
female striped bass (TL>90
cm) were collected by recreational fisherman off
Cape Cod, Massachusetts in Fall 1992.
2.4. Electron microprobe
analysis of otoliths
Preparation
procedures for otolith microanalysis required that otoliths be
sectioned and polished. Otoliths were cleaned in 10% hypochlorite solution and
rinsed with deionized water. They were then embedded in epoxy (Spurr),
sectioned in a transverse plane with an Isomet saw, and mounted on a glass slide.
Otoiiths were polished (Secor et al., 1991) with 3-pm alumina to eliminate surface
pits, cracks or elevations which can cause artifacts in microprobe analysis (Kalish,
1989). Finally, otolith sections were carbon-coated in a high-vacuum evaporator to
minimize ion beam displacement during transmission to the otolith surface.
X-ray intensities for Sr and Ca elements in the otolith matrix were quantified
using a JEOL JXA-840A wave-length dispersive electron microprobe (Central
Facility for Microanalysis, University of Maryland, College Park, MD 20742) with
Calcite (CaCO,) and Strontianite (SrCO,) as standards. Analytical methods for
measuring molar weights of Sr and Ca followed those described by Secor (1992).
Two types of analyses were performed with the microprobe: point probes and
transect probes. Point probes comprised three discrete point (area = 5 ,um’)
measurements taken near the edge of the otolith. The Sr measured by point
probes was assumed to be reflective of the salinity in which the fish most recently
lived. Point probes were conducted for Experiment 1 fish and for otolith samples
from fish collected in the Chesapeake Bay and Massachusetts. Point probe
estimates were the mean of the three point measures. Transect probes were series
of point measurements (area = 5 pm*) of Sr and Ca taken across microstructures
of otoliths at 13.5 to 14-pm intervals. Transect probes of Sr and Ca were used to
indicate temporal variation in exposures to differing salinities and were conducted
on Experiment 2 samples.
The original protocol for Experiment 2 depended upon the use of daily
increments observed in the otolith microstructure to identify the start of the
D.H.
Secor
et al.
I J. Exp.
Mar.
Biol.
Ecol.
192
(1995)
21
15-33
experiment. Otoliths of juvenile striped bass reared in ponds have been demonstrated to form increments at a daily rate until 60 days posthatch (Secor & Dean,
1989). However, daily increments were not easily discernable in the laboratoryreared juvenile otoliths, and the first experimental day could not be determined
accurately in the otolith microstructure. Otoliths from laboratory-reared
larvae
and juveniles often contain increments with poorly resolved optical zones (Secor
& Dean, 1992). Because increment counts could not be used to identify the start
of the experiment, the time series of Sr/Ca ratios for each fish (transect probe)
was plotted from the end of the experiment to the beginning (Fig. 2). The end of
the experiment corresponded to the edge of the otolith. Because daily increments
could not be resolved in the laboratory-reared
juveniles, distance (in microns)
from the edge of the otolith was used as a proxy for experimental days.
3
12
8
4
0 E
0
5
12
4
200
400
6
‘21
‘2fi
l&&e___
8
a
8
4
4
4
0
0k
0
200
400
600
0
200
400
600
0
0
Microns
600
0b
200
200
from
the
400
600
400
600
Otolith’s
0
200
400
600
0
200
400
600
Edge
Fig. 2. Experiment
2. Predicted
salinity transects for individuals
exposed to a cycle of increasing
and
decreasing
salinities at 19°C and 25°C. Salinity was predicted
from otolith SrlCa ratio (see Fig. 5).
Salinity was increased
from 0 ppt to 9.8 ppt (corrected
salinity) and then decreased
from 9.8 ppt to 0
ppt over a 20-wk period. Sr/Ca ratios for each individual
are plotted as distance from the otolith’s
margin which represents
the end of the experiment.
Peak Sr/Ca ratio is indicated by a dashed vertical
line. For individuals #4 and #9, no single peak was observed and a midpoint was visually estimated. A
dashed horizontal
line indicates the experimental
salinity maximum (9.8 ppt).
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995) 15-33
22
“i_‘~
0
400
800
1:~~2,
,_
1/,~~
1200
0
400
800
1200
0
25
25
a
20
20
-3
15
15
15
ux
.G
0
v)
10
10
10
5
5
5
0
0 ~
z
0
400
800
1200
25
14
0
400
800
0 ~
1200
25
20
20
20
15 I
15
15
10
5
5
0
0~
0
400
800
1200
1200
400
800
1200
18
0;
0
Microns
0
25
17
800
15
20
25
IO
400
from
400
the
800
1200
Otolith’s
0
400
800
1200
Edge
Fig. 2b.
2.5. Analysis
Growth
was estimated
G =(Log,W,
by:
- Log,W,)
* t-’
where G = instantaneous
growth rate (day- ’ ), IV, = weight (g) at the end of the
experimental
period, W, = weight (g) at the beginning
of the experimental
period,
and t = experimental
period (d). W, was estimated
from a subsample
of juveniles
(n = 37) on the first day of the experiments
[(W, = 0.843 g 5 0.299 (SE)].
Growth, temperature,
and salinity effects on otolith Sr/Ca ratios were analyzed
using multiple
analysis of variance with growth as a covariate
and temperature
and salinity as treatments.
Bartlett’s test of heteroscedacity
(Zar, 1974) was used
in analyses
of variance.
Log, transformation
of some response
variables
was
necessary to fulfill the assumption
of homogeneity
of variance. Residual analysis
evaluated
the independence
of growth and temperature
effects on otolith Sr/Ca
ratios for Experiment
1. ANCOVA procedures
were performed
to contrast linear
regression
slopes of salinity versus otolith Sr/Ca ratio relationships
derived from
either experimental
or field data. For this comparison,
experimental
data from
only the 15°C factor level were used because field samples were collected from
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995) 15-33
23
temperatures between 10 and 15°C. Field data in this comparison was restricted to
salinities of 0 ppt to 12 ppt to match the range of corrected experimental salinities.
Statistics for the logistic regression of salinity on SrlCa for experimental and field
data were estimated by a Marquadt iterative procedure. Statistical procedures
utilized PC-SAS and STATGRAPHICS.
3. Results
3.1. Experiment 1
Temperature
had a strong positive influence on growth rates. Temperature
accounted for 63% of variation in growth rate; salinity and its interaction with
temperature had no significant effect on growth. Instantaneous growth rates were
0.010 day-’ and 0.032 day-’ for 15 and 25°C levels, respectively.
Analysis of covariance (Table 1) indicated that salinity (p = O.OOOl), growth
rate (p = 0.04), and the interaction between salinity and temperature (p = 0.0001)
explained significant variation in otolith Sr/Ca ratios. Salinity, growth rate and the
interaction term accounted for 86, 0.5, and 5% of the variation in SrlCa ratios,
respectively. Because growth and temperature were likely to have been nonorthogonal in their influences on Sr/Ca ratio, two models were used to investigate
their independent
influences on Sr/Ca ratio. Model 1 omitted growth as a
covariate and Model 2 omitted temperature as a treatment factor (Table 1).
In Model 1 (Table 1B; Fig. 3A), temperature and its interaction with salinity
had a statistically significant influence on otolith SrlCa ratios, accounting for l%,
and 6% of total variation, respectively. However, temperature effects were not
consistent over the range tested. Increasing temperature had a positive effect on
otolith Sr/Ca ratio at 2.5 ppt and a negative effect on Sr/Ca ratio at 4.3 and 8.0
ppt. No temperature effect was observed at 0, 6.1, and 11.7 ppt. Residuals from
Model 1 were regressed on growth rate (Fig. 4A); no significant relation existed
between growth rate and the residuals (p = 0.25). In Model 2 (Table 1C; Fig. 3B),
growth accounted for 1.9% of the variation in Sr/Ca ratios. Analysis of residuals
from Model 2 showed no significant main effects of temperature but a significant
interaction between temperature and salinity (p = O.OOOl),accounting for 53% of
the variation in residuals. This interaction (Fig. 4B) was similar in effect to the
interaction of salinity and temperature on Sr/Ca ratio observed for Model 1 (Fig.
3A) and indicated that a temperature-salinity
interaction may have occurred
which was independent to growth rate effects.
Growth rate effects were also considered by regressing Sr/Ca ratios on growth
rate for each salinity level. Significant regressions and negative slopes occurred for
4.3 (Sr/Ca ratio = 0.00136 - 0.0149 (growth); r2 = 0.66; n = 7) and 8.0 ppt (Sr/Ca
ratio = 0.00252 - 0.0339 (growth); r2 = 0.80, n = 10); regressions for other salinity
levels were nonsignificant (p > 0.05).
For the full model and Models 1 and 2 (Table 2; Fig. 3) least-square adjusted
24
Table 1
Analysis of variance
D.H. Secor et al. I .I. Exp. Mar. Biol. Ecol. 192 (1995)
for otolith
SrlCa
ratio
of juvenile
striped
15-33
bass in Experiment
1
(A) Complete Model: SrlCa ratio = Temp + Sal + Temp * Sal + (cov.) Growth
Sum of squares
F
Effect
df
P
Covariate
Growth
2.44 10 ’
0.04
Main effects:
Temp (A)
Sal (B)
A*B
Residual
1.46
4.47
2.71
2.39
(B) Model I: SrlCa
Effect
ratio = Temp + Sal + Temp * Sal
Sum of squares
Temp (A)
Sal(B)
A*B
Residual
(C) Model 2: SrlCa
Effect
lo-’
lo-’
10 ’
10mh
4.85
4.54
3.20
2.64
to-’
lomr
lo-”
IO-’
ratio=sal+(cov.)
Growth
Sum of squares
1
4.5
1
5
5
44
0.03
164.5
10.0
0.87
0.00001
0.00001
df
F
P
1
5
5
45
8.3
155.1
10.9
0.006
0.00001
0.00001
df
F
P
Covariate:
Growth
9.88 10 ’
1
9.4
Main effect:
Sal
Residual
4.50 IO_’
5.23 lo-’
5
50
85.9
0.03
0.00001
Adjusted salinity treatment
levels were 0, 2.5, 4.3, 6.1, 8.0, and 11.7 ppt. Temperature
treatment
levels
were 15 and 25°C. (A) Analysis of covariance
with growth as covariate and salinity and temperature
as
main effects; (B) Model 1 (omit growth covariate):
analysis of variance with salinity and temperature
as main effects; (C) Model 2 (omit temperature
treatment):
analysis of covariance
with growth as
covariate
and salinity as main effect. Temp=temperature;
Sal=salinity;
cov.=covariate.
means for salinity were significantly
different
among the six salinities
(LSD
multiple range test; p < 0.05). In a predictive regression
analysis, the relationship
between
Sr/Ca ratios and salinity was positive and highly correlated
(r2 = 0.88;
p < 0.001; IE = 57).
3.2. Experiment 2
Growth rate was only slightly influenced
by temperature
(ANOVA;
II = 18;
p = 0.07). Mean growth rates were 0.025 day-’ and 0.027 day-’ for treatments
19
and 25°C respectively.
Sr/Ca ratio values were converted to salinity based upon a predictive regression
(Fig. 5) of Experiment
1 and field measures of Sr/Ca ratios regressed on salinity
(see section 3.3. Field samples below). Salinity levels for Experiment
1 sample had
been corrected
as described
in methods. This conversion
allowed comparison
of
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995) 15-33
25
Model 1
0
2
Adjusted
4
6
Experimental
8
Salinity
10
12
(ppt)
(B)
Model
2
2.8
F
f
Fig. 3. Experiment
1. (A) Means and 95% confidence
intervals for SrlCa ratio in otoliths of juveniles
reared under different
salinities
and temperatures.
Means and variances
estimated
from Model 1
(Table 25). (5) Means and 95% confidence
intervals for least-squares
mean SrlCa among salinity
treatments.
Means were adjusted for the covariate,
growth rate according
to Model 2 (Table 2C).
salinity changes predicted from analysis of otolith Sr/Ca ratio with the experimental cycle of salinity change. Transect probes showed a gradual rise and decline in
salinity (Sr/Ca ratio) during the experimental period for all analyzed juveniles
(Fig. 2). Predicted salinity values for the 25°C treatments were significantly higher
than for the 19°C treatments.
Significant differences between temperatures
occurred in salinity maxima (p = 0.0006), means (p = O.OOOl),but not for minima
(p = 0.32). The mean salinity maxima were 8.65 ppt at 19°C and 16.37 ppt at
25°C representing nearly a two-fold difference. Individual growth rate did not
significantly influence salinity means (p = 0.18), maxima (p = 0.56), or minima
(p = 0.12).
The position of the peak in the Sr/Ca ratio within the otolith microstructure
26
D.H.
Secor et al. I J. Exp.
Mar.
Biol.
Ecol.
192 (1995)
15-33
(A)
I
.
= :
. . .
9 ..
.
.
.
.____
.____~__.~__.._.,-..._...~.**..~~.
3
l
0
-3
9’
.
P
l
I.
* I
.
.
. ... . ...~....~...
..___..._.. . ... . .._
9 .L
.
/
-6 t
c
-9
I
t,
0.02
0
Instantaneous
(8>
0.04
Growth
0;06
Rate
(g g-‘d-l)
.__.
___
-..-...-Al\
1 115
c-4
401
0.08
1
C
025
C
1
-P
_,..__.,,,_,
,,.._.............
___..m
.O_t,,_._._
p
-40
L-
0.64
2.47
4.31
Salinity
6.15
7.99
11.67
(ppt)
Fig. 4. Experiment
1. (A) Residuals for Model 1 (Table 2B) plotted against instantaneous
growth rate.
(3) Interaction
between salinity and temperature
on mean residual for Model 2 (Table 2C). Mean
residual levels are shown for temperature
treatments
15 and 25°C.
was similar among fish for each temperature
treatment.
Peak position occurred
between 200 to 400 pm from the edge of the otolith for the 19°C treatments,
with
most positions (in 8 out of 9 individual
otoliths) occurring less than 320 Frn from
the edge. Peak position for the 25°C treatments
occurred between 200 and 500
pm from the otolith’s edge with most peaks (in 6 out of 8 individual
otoliths)
occurring more than 320 pm from the edge.
Transect
probes did not accurately
estimate
absolute levels of salinity among
individuals.
Four individuals
for the 25°C treatment
had maxima which exceeded
the experimental
maximum (9.8 ppt) by more than 5 ppt. Two individuals
for the
19°C treatment
had maxima which underestimated
the experimental
maximum by
more than 5 ppt.
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995)
Table 2
Review of otolith
Mean
Marine:
1.5
2.7
2.1
3.9
4.2
5.2
2.9
2.8
3.9
5.5
3.3
3.7
3.1
3.1
2.6
1.8
4.8
Estuarine:
2.3
Freshwater:
0.3
0.9
0.8
1.1
0.6
All ratios
Sr/Ca
ratios
( X10m3) reported
for marine,
estuarine,
15-33
27
and freshwater
habitats
Range
Species
Reference
ND-3.1
ND-3.2
2.8-3.8
1.8-6.2
1.5-6.0
2.0-2.1
3.4-4.5
3.9-4.5
5.0-5.4
2.9-3.0
3.7-4.0
4.8-6.2
3.2-3.4
3.3-4.4
2.9-3.2
1.5-5.4
1.5-3.4
Anguilla anguilla
Anguilla rostrata
Gadus morhua
Stenobius genivittatus
Macruronus novaezelandiae
Arripis trutta
Noesebastes scoraenoides
Helicolenus papillosus
Trachurus declivis
Nemadactylus
macropterus
Platycephalus bassensis
Pseudodotabrus
tetricus
Acanthopagrus
butcheri
Thryistes atun
Hoplostethus
atlanticus
Salmo trutta
Oncorhynchus
mykiss
Thunnus thynnus thynnus
1.2-6.0
Clupea harengus
1.7-4.5
1.2-2.7
1.1-8.0
3.0-10.0
4.0-5.7
Nemadactylus
macropterus
Thunnus maccoyii
Haemulon plumieri
Microstomus
pacificus
Morone saxatilis
Casselman,
1982
Casselman,
1982
Radtke, 1984
Radtke et al., 1988
Kalish, 1989
Kalish, 1989
Kalish, 1989
Kalish, 1989
Kalish, 1989
Kahsh, 1989
Kahsh, 1989
Kalish, 1989
Kahsh, 1989
Kahsh, 1989
Kahsh, 1989
Kalish, 1989
Kalish, 1990, 1991
Radtke & Morales-Nin,
1989
Townsend
et al., 1989,
1992; Radtke et al.,
1990
Gunn et al., 1992
Gunn et al., 1992
Sadovy & Severin, 1992
Toole & Nielson, 1992
This study
ND-4.1
0.4-2.5
Morone
Anchoa
Secor,
Secor,
ND-2.1
ND-l.8
ND-l.0
0.4-1.0
0.3-1.8
ND-2.0
Anguilla anguilla
Anguilla rostrata
Stenobius genivittatus
Salmo trutta
Oncorhynchus
mykiss
Morone saxatilis
are given
as molar
fractions.
saxatilis
mitchilli
1992; this study
unpubl.
Casselman,
1982
Casselman,
1982
Radtke et al., 1988
Kahsh, 1989
Kalish, 1990
Secor, 1992; this study
ND=non-detectable.
3.3. Field samples
The rates of change in otolith Sr/Ca ratio with ambient salinity were similar
between regressions based on either Experiment 1 (15“C) data or data from
field-collected subadults and adults (ANCOVA on Log,-transformed
Sr/Ca ratio;
p = 0.33). A logistic model fit well to data combined from field and Experiment 1
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995) 15-33
28
1523.3
Salinity
= 40.302
40
_ N = 54: r2=
30
-
20
-
(1 + 56.337
EXP-
lO*Ratio
-1
)
0.94
G
sk
x
.c
.!L
0
m
,10 -
0
1
2
Sr/Ca
3
Ratio
(X
4
5
4
5
lE-3)
0
-9
,,,,,,,,,,,,,,.~.~~~I~~.~~
0
2
1
Sr/Ca
3
Ratio
(X
IE-3)
Fig. 5. Logistic relationship
between salinity and otolith Sr/Ca ratio for Experiment
1 juveniles and
field collected sub-adults
(<31 ppt) and adults (>30 ppt). Residuals
for the regression
are plotted
against SrlCa ratio in the bottom plot.
(15°C) samples (Fig. 5; r2 = 0.94). Experimental
data tended to have more
positive residuals than field-collected
data. Residuals indicated a maximum
departure of 8 ppt when predicted salinity was 14 ppt but residuals were typically
within 5 ppt of the predicted regression (Fig. 5).
4. Discussion
4.1. Salinity effect on otolith SrlCa
ratios
Experimental and field samples showed a strong positive effect of salinity on
otolith Sr/Ca ratios, explaining over 85% of the variation. The logistic regression
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (1995)
15-33
29
(Fig. 5) indicated that salinity accounted for most variation when Sr/Ca ratios
were between 0.0020 and 0.0042. This range of ratios corresponded to mesohaline
and polyhaline (5 to 30 ppt) segments of the estuary. At lower and higher
salinities, Sr/Ca ratios changed gradually with salinity. Indications of asymptotes
at 0 ppt and 35 ppt suggested that variation in Sr/Ca in freshwater and marine
habitats might occur which is unrelated to salinity. This result agreed with
previous observations of otolith Sr/Ca ratios from marine and freshwater fishes
which showed substantial variation in relatively constant salinity environments
(Table 2). The few studies on otolith Sr/Ca ratios in estuarine fishes indicated
ratios were intermediate
between marine and freshwater fishes (Table 2).
Substantial variation occurs among species within salinity regimes (Kalish, 1989;
Table 2).
The logistic regression for experimental and field samples indicated a precision
of =5 ppt (Fig. 5) in Sr/Ca-based prediction of salinity. The logistic model also
showed that most change in Sr/Ca ratio occurred over salinity ranging from 4 to
30 ppt; thus precision is 225% of the range covered. This precision indicates that
the otolith microchemistry method can be used to distinguish among conventional
salinity-habitat
designations: oligohaline (O-5 ppt), mesohaline (6-18 ppt),
polyhaline (19-30 ppt) and euhaline (>30 ppt). The form of the logistic model
assumes symmetry about a mid-range inflection point. There is no reason to
assume such symmetry; with additional data from spans of salinity not well
represented in the current regression (e.g. polyhaline and hyper-saline habitats),
other models could be more appropriately fit.
Accuracy and precision in the salinity versus otolith Sr/Ca relation were
probably influenced by experimental biases and assumptions made for fieldcollected juveniles. The artificial seawater solutions used in this study were over
two-fold deficient in Sr concentration. This required that a correction be made
which depended upon the accuracy of two relations: (1) experimental Sr and
salinity, and (2) natural Sr and salinity (Ingram & Sloan, 1992). While coefficients
of determination exceeded 95% for both relations, sources of error associated
with water quality measures for two separate studies could have introduced small
errors. In the analysis of wild subadult otoliths, it was assumed that Sr measured
in the most peripheral part of the otolith would reflect the salinity where a fish
was captured. Striped bass are capable of rapid migrations which could transverse
oligohaline, mesohaline, or polyhaline environments
in the space of weeks
(Setzler-Hamilton
et al., 1980). Such rapid movements might not be reflected in
the portion of the otolith which was probed. We speculated that striped bass
migration was curtailed during winter months (Rothschild et al., 1992) and
anticipated that errors associated with short-term movements might be random
over the months and salinities sampled. Good agreement between the experimental and field data (Fig. 5) supported the view that biases may have been random
and were not substantial.
Experimental results indicated that temporal trends in salinity can be detected
by changes in the Sr/Ca ratios in the otolith microchemistry. In Experiment 2, a
discernable trend in increasing, then decreasing salinity (Sr/Ca ratios) was
observed for all analyzed samples. However, absolute levels of expected salinity
30
D.H. Secor et al. I .I. Exp. Mar. Biol. Ecol. 192 (1995) 15-33
were not always observed
among
individuals
(Fig. 2) and may have been
influenced
by procedural
assumptions.
Analytical
limitations
required
that distance from the otolith edge be used as a proxy for days since the end of the
experiment.
This assumption
probably
was not valid because
(1) substantial
growth
rate differences
occurred
among
individuals
which would affect the
relationship
between
otolith radius and age (Secor & Dean, 1992); (2) transect
probe direction
and position probably
varied among otoliths which would cause
daily increments
(age) to be differentially
sampled along a transect (Campana,
1990; Kalish, 1990); and (3) daily increment
widths become narrower
with age
(Secor & Dean, 1989). These sources of error probably
resulted in much of the
variation
observed
in the relative
amplitude
and position
of the salinity peak
among individuals.
4.2. Growth
versus temperature
effects on otolith
SrlCa
A controversy
occurs on the relative importance
of temperature
and growth on
otolith Sr/Ca ratio. Early research used otolith Sr/Ca ratio as a means to measure
temperature
histories (Radtke, 1984; Townsend
et al., 1989; Radtke et al., 1990) in
a manner analogous
to coral thermometry
(e.g. Smith et al., 1979) but provided
only circumstantial
proof of the method. Critical work by Kalish (1989) statistically indicated that a positive relation between growth and otolith Sr/Ca ratios could
explain the inverse relation
between
temperature
and Sr/Ca ratio observed
in
previous studies. Further research correlated
plasma-Sr titer with growth rate and
otolith Sr/Ca ratios in adult Pseudophycis
barbatus (Kalish,
1991). However,
Townsend
et al. (1992) gave strong proof of a temperature
effect which accounted
for a 4-fold variation
in Sr/Ca ratio in larval and juvenile
Cfupea harengus, and
presented
logical rationale
for a temperature-induced
physiological
(stress) effect
on otolith Sr/Ca ratio.
Clearly, it will be difficult to develop
experimental,
sampling,
or analytical
procedures
which can isolate temperature
and growth (physiological)
effects given
the pervasive
effects of temperature
on the physiology
of poikliotherms
(Fry,
1971). In our experiments
we observed
both effects but could not isolate their
relative influences.
In Experiment
1, temperature
and/or
growth rate explained
6% of the variation
in otolith Sr/Ca (Table 2A). Growth and temperature
had
negative
effects on Sr/Ca at 4.3 and 8.0 ppt (Figs. 3, 4 and 5). Negative
(but
statistically
nonsignificant)
temperature
and growth effects also were observed at
6.0 ppt. A negative temperature
influence
on the Sr/Ca ratio is consistent
with
Townsend
et al.‘s (1992) hypothesis of a temperature-induced
physiological
effect
on otolith Sr/Ca because 15°C treatment
juveniles
grew poorly and were in a
stressed condition.
In Experiment
2, both 19 and 25°C are temperatures
which can
support positive growth in striped bass juveniles
(Hartman,
1993). Thus, temperature-induced
stress was probably
not associated
with the 19°C treatment.
Otolith growth can be strongly influenced
by temperature
(Mosegaard
et al., 1988;
Secor & Dean, 1992). Faster growing otoliths with wider increments
for the 25°C
treatment
may have permitted
better spatial resolution
of the imposed salinity
D.H. Secor et al. I J. Exp. Mar. Biol. Ecol. 192 (199.5) 15-33
31
cycle. Thus, the 19°C treatment would have resulted in lower Sr/Ca ratios because
the higher salinity portion of the otolith was incompletely sampled via transect
probes. For instance, if the otolith increment corresponding to the high salinity
phase of the experimental cycle was ~14 pm, then that phase would not have
been sampled. Because somatic growth was only slightly influenced by temperature, Experiment 2 results suggest that temperature may affect otolith Sr/Ca ratio
independently. However, we cannot determine whether temperature influenced
Sr/Ca ratio through increased spatial resolution (daily increment width) or
through physiological factors.
Future investigations on growth and temperature
effects on otolith Sr/Ca
should involve tank experiments of individually-marked
fish exposed to varying
temperature and feeding regimes. Our measure of growth rate was imprecise
because we used a mean size for the initial weight in growth calculations.
Coefficient of variation for W, was 25%, which introduced unexplained variation
into the growth rate estimate. In addition, lower temperatures will affect the
temporal resolution of the otolith microanalysis method by reducing otolith
growth rates. Future studies should evaluate temperature effects on otolith growth
to evaluate for which seasons, otolith Sr/Ca is reflective of habitat utilization.
4.3. Application of the otolith microchemistry method
Application of the otolith microchemistry method to estimate habitat utilization
could answer important ecological and management-related
questions for anadromous and estuarine fishes. For example, in populations which are facultatively
anadromous, the method could measure the proportion of adults that undertake
coastal migrations. At the current resolution limit (5 ppt), the method supports
prediction of movements across oligohaline, mesohaline, polyhaline, and euhaline
salinity zones. The effect of fishing mortality on migration rates could be
examined retrospectively with archived otolith samples collected during periods of
varying exploitation rates. Application of the otolith microchemistry method are
underway for Chesapeake Bay (Secor, 1992) and Hudson River (Secor & Piccoli,
1995) striped bass populations. We are using the method to determine migration
and reproductive schedules, and habitat utilization patterns of anadromous and
resident segments of these populations.
Acknowledgements
We thank personnel at J.P. Manning Hatchery (Maryland Department
of
Natural Resources) and Edenton National Fish Hatchery (United Stated Fish and
Wildlife Service, Region 5) for providing juveniles for the experiments. Technical
support for rearing experiments was provided by J. Howe and G. Powell. M. Trite
assisted in otolith preparation. Ms. L. Fernandez provided assistance in manuscript preparation. Drs. P. Rago and E. Houde provided advice on project design
32
D.H.
Secor
et al. I J. Exp.
Mar.
Biol.
Ecol.
192 (1995)
15-33
and analysis. This research was funded by the United States Fish and Wildlife
Service (Contract No. 14-48-0009-92-934).
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