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Feeding by a larval fish community: Impact on zooplankton

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Feeding by a larval fish community: Impact on zooplankton

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We studied patterns in the diurnal fluctuations in gut fullness in 11 species of larval fish in Conception Bay, Newfoundland, Canada. From these data and empirical information on prey selection, we estimated the daily consumption rate of microzooplankton by the entire larval fish community for the period May to September in 1985 and 1986. In general, <0.1% of the available prey were consumed by larval fish per day, which is considerably less than the P/B ratio typical for temperate copepods found in this region. We conclude that larval fish are unlikely to exert significant grazing pressure on their prey and that density-dependent growth is unlikely to occur during this phase of the life cycle in fish species that use coastal Newfoundland waters as spawning or nursery areas.
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MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 204: 199–212, 2000 Published October 5
INTRODUCTION
For food competition to influence early life survivor-
ship, larval fish must exert a substantial impact on their
prey. Cushing (1983) and Jones (1983) argued that this
is only possible during later stages of development (i.e.
juvenile and beyond). However, these and other stud-
ies viewed density-dependent processes from the per-
spective of single species. Does competition and food
limitation become more likely if we consider consump-
tion of prey by the entire ichthyoplankton community?
Eggs and larvae of a single species of fish are gener-
ally scarce (<1 m–3
) relative to other zooplankton
(Smith & Lasker 1978), and the total biomass of a
cohort decreases from spawning until larvae reach
approx. 10 mm in length (Houde 1997). Throughout
much of the larval stage, most species feed on similar
prey (nauplii and early copepodite stages of calanoid
copepods) (Arthur 1976, Last 1978a,b, Economou 1991,
Pepin & Penney 1997). Because the majority of fish
species in the north Atlantic spawn during the spring
and summer (Sherman et al. 1984), the common feed-
ing patterns and high levels of larval production of
many species of fish may increase the potential for
interspecific competition among larvae.
This study investigated the feeding patterns of larval
fish in Conception Bay, a physically dynamic coastal
ecosystem which serves as a spawning site (Laprise &
Pepin 1995) and potential nursery (Anderson & Dalley
1997) for several species of fish. We describe the diurnal
periodicity in stomach contents, estimate consumption
rates, and assess the impact of the entire ichthyoplank-
ton community on the zooplankton prey on which larval
fish feed. We apply our estimates of consumption over
the primary ichthyoplankton production season, May to
September. Data from our previous study (Pepin & Pen-
ney 1997) are used to take into account species- and size-
dependent patterns of prey selection.
MATERIALS AND METHODS
Sample collection and processing. Plankton was
sampled fortnightly from late May until late September
in 1985 and 1986 at 3 locations on the eastern side of
Conception Bay (47° 37’ N, 52°55’ W) (see Pepin & Pen-
ney 1997 for details). Ichthyoplankton samples were
obtained using a 0.75 m ring net fitted with 165 µm
© Inter-Research 2000
*E-mail: pepin@athena.nwafc.nf.ca
Feeding by a larval fish community: impact on
zooplankton
Pierre Pepin*, Randy Penney
Fisheries and Oceans, PO Box 5667, St. John’s, Newfoundland A1C 5X1, Canada
ABSTRACT: We studied patterns in the diurnal fluctuations in gut fullness in 11 species of larval fish
in Conception Bay, Newfoundland, Canada. From these data and empirical information on prey
selection, we estimated the daily consumption rate of microzooplankton by the entire larval fish com-
munity for the period May to September in 1985 and 1986. In general, <0.1% of the available prey
were consumed by larval fish per day, which is considerably less than the P/B ratio typical for tem-
perate copepods found in this region. We conclude that larval fish are unlikely to exert significant
grazing pressure on their prey and that density-dependent growth is unlikely to occur during this
phase of the life cycle in fish species that use coastal Newfoundland waters as spawning or nursery
areas.
KEY WORDS: Fish larvae · Feeding · Ingestion · Microzooplankton · Density-dependence
Resale or republication not permitted without written consent of the publisher
Mar Ecol Prog Ser 204: 199–212, 2000
Nitex and a General Oceanics flow
meter, and towed from a 7.5 m boat at
a depth of 8 m and a speed of 0.3 to
0.5 m s–1 for 10 min. This provided a
representative sample of the popula-
tion, as all the stages of species of
larval fish are found predominantly in
the upper water column and show little
evidence of diurnal vertical migra-
tion (Pepin unpubl. data). Zooplankton
samples were obtained using a similar
ring net fitted with 64 µm Nitex which
was towed over the same path immedi-
ately following ichthyoplankton sam-
pling. Samples were preserved in 4 %
buffered formaldehyde. Some addi-
tional ichthyoplankton and microzooplankton samples
were collected using 23 m vessels towing 0.6 m bongo
nets fitted with 165 and 64 µm Nitex and General
Oceanics flow meters. Sampling times on both vessels
were selected to provide as thorough a coverage of the
diurnal feeding cycle as possible. Logistic constraints
prevented continuous sampling over a complete cycle
on any one occasion. We have therefore combined all
samples from different sampling dates to estimate the
diurnal feeding pattern.
All fish larvae were sorted from the samples, identi-
fied to the lowest taxonomic level possible, and mea-
sured for standard length to the nearest millimeter
using an ocular micrometer. A subsample of the 11
dominant species of larval fish (Table 1) was taken for
stomach analysis. Specimens were selected to ensure
that observations of feeding from each species spanned
the entire period of their seasonal occurence. For each
specimen, the digestive tract was removed and teased
apart using fine dissecting needles. Presence or ab-
sence of a yolk sac and the state of digestion were
noted. The contents were identified to the lowest taxo-
nomic and life stage possible. If the item was unidenti-
fiable, or if a species had continuous growth, its width
was measured to the nearest 10 µm. Otherwise, the
size was estimated from the means of the microzoo-
plankton samples described below, to avoid the distor-
tion characteristic of partially digested items.
Zooplankton samples were processed by first remov-
ing all the large organisms not contributing to the diet
of larval fish (fish, medusae, pteropods and amphi-
pods). Each sample was poured into a 64 µm mesh
sieve, rinsed with tap water, and suspended in a cali-
brated beaker filled to the 1 l mark. A 1 ml sample was
transferred with a Hensen-Stempel pipette and placed
in a 100 ml calibrated beaker. A second 10 ml subsam-
ple was taken from this volume. In combination, the
1/10000 and 1/1000 subsamples usually contained suf-
ficient organisms for accurate estimation of total abun-
dance. At least 500 organisms were counted. Animals
were identified to the lowest taxonomic level possible,
although it was generally difficult to identify to species
level some naupliar stages of copepods (with the
exception of Temora longicornis). For animals with
definite stages (e.g. copepods), widths of 5 individuals
of each species and life stage were measured to the
nearest 5 µm. For animals with continuous growth, or
those that could not be identified to species, the width
of each individual was measured to the nearest
100 µm, or to 50 µm for copepod nauplii. Ten percent of
the samples were re-analyzed for quality control.
Total weight of prey ingested was the summed
weight of all prey in the stomachs of each larva. Prey
widths were converted to dry weight using Pearre’s
(1980) empirical relationship for copepods and unpub-
lished data for other species (Pepin unpubl. data).
Analysis. To correct for differences in body size
among individual larvae, the total weight of prey in the
gut of each larva was expressed as gut fullness (GFI ):
(1)
where Np (i) is the number of prey of category i, Wp(i) is
their dry weight, and WLis the dry weight of the larva
reported by Pepin (1995). GFI is a useful basis for com-
parison among larval fish as long as it is isometric (i.e.
W1), because any allometry in GFI would prevent an
assessment of diurnal periodicity using larvae of differ-
ent body size. To determine if this was an accurate
assumption, an analysis of covariance was used to
assess whether there was any significant difference in
the allometric relationship in GFI among species.
Although we used 129 to 245 individual larvae per
species (except for Stichaeus punctatus) (Table 1), all
parametric analyses represent the mean within a unit
of body length (1 mm length intervals converted to dry
weight) for each species to provide conservative esti-
GFI
NW
W
pi pi
i
L
() ()
=
200
Species No. of Length Period
specimens range (mm) (day of year)
Clupea harengus 131 6– 14 232 –244
Gadus morhua 173 3– 14 213 –296
Glyptocephalus cynoglossus 245 4– 14 239 –276
Hippoglossoides platessoides 160 3– 14 206 –239
Liparis sp. 168 3– 14 173 –244
Mallotus villosus 129 5– 14 273 –276
Pleuronectes ferrugineus 194 2– 14 213 –276
Pleuronectes americanus 202 2– 71198 244
Stichaeus punctatus 91 6– 14 172 –181
Tautogolabrus adspersus 183 2– 91273–276
Ulvaria subbifurcata 151 6– 14 198 –251
Table 1. Larval fish-sample characteristics
Pepin & Penney: Impact of larval fish on zooplankton
mates. Each observation was weighted by the inverse
of the standard deviation for each unit of measure-
ment. We use F-values calculated when each variable
is the last added to the model (Type III).
Consumption rates were estimated by calculating
the diurnal change in GFI per hour for each species of
larval fish, after taking into account evacuation rates.
We chose to use the cumulative probability distribution
(CDF) of GFI instead of some measure of central
tendency (e.g. mean or median) to provide a fuller
description of the changes in stomach contents taking
place over a diurnal cycle. This approach is based on
Hall et al.’s (1995) model, with 2 important differences.
Our estimates of the CDF are based on the use of local
non-parametric density estimators (Davison & Hinkley
1997) which make no assumptions about the underly-
ing form of the density distribution or the parameters
which describe it, and which are generally less sensi-
tive to outliers than methods which estimate central
tendency. (The approach is described in greater detail
by Evans & Rice 1988 and Pepin et al. 1999.) Conse-
quently, we could not use Hall et al.’s (1995) iterative
minimization scheme to estimate the parameters that
best describe the distribution of meal sizes required to
achieve the changes in the distribution of GFI from one
period to the next. We did not wish to make any
assumptions about the underlying distribution of ob-
servations or of the processes that lead to them. Conse-
quently, use of minimization schemes would have been
unlikely to have provided a unique solution. As a re-
sult, the difference between distributions of GFI over
time represents an integrated measure of the feeding
rates required to achieve such changes. This approach
effectively underestimates the extremes of the distrib-
utions of consumption rates.
In order to describe diurnal patterns in stomach con-
tents using non-parametric local density estimators, we
face the statistical question: what is the probability dis-
tribution of a random variable y (GFI), and how does it
depend on some other variable x(time of day)? The cu-
mulative probability distribution F(y) is the probability
that a value chosen at random will be less than y. To
compute the local influence of xon y, we applied the
generalized concept of locally-weighted estimates of the
CDF using kernel smoothing. We made a local assump-
tion: observations near to the target xare more relevant
for estimating the distribution at x. More formally, fol-
lowing Davison & Hinkley (1997), the CDF is:
(2)
where H(z) is the Heaviside function (0 for z < 0 and 1
for z > 0). The ogive is a step function whose steps are
of different heights, decreasing as the corresponding xi
is further from x. The step sizes depend only on the dis-
tances between xand the different xi, where the steps
occur depends only on yi. We used the weighting func-
tion w(d) = e–d
, where d= |xix|/b, and bis a band-
width parameter which describes how far local ex-
tends. In this study, the weighting reflected the cyclic
nature of diurnal feeding patterns (e.g. estimates at
23:00 h were influenced equally by observations at
20:00 and 02:00 h). The value of bcan be chosen by
cross-validation but in this study we set b = 3 (h) to
reflect the influence which food previously ingested
might have had on stomach fullness because catches of
larvae were uneven during the day; 3 h represent the
maximum clearance rate measured in previous studies
as well as the e-folding scale (time required for stom-
ach contents to decrease by ~63%) for the longest
observation (8 h) (Sumida & Moser 1980, Boehlert &
Yoklavich 1983, Tilseth & Ellertsen 1984, Young &
Davis 1990, Canino & Bailey 1995, Johnston & Mathias
1996, Lough & Mountain 1996).
To determine if the diurnal change in median gut
fullness for each species occurred by chance alone, we
use a randomization test. (We computed the median at
a representative set of times for the original data, and
also for 500 synthetic data sets in which the assignment
of pairs of variables [time, GFI] is randomized. The
median at a given time is significantly greater/less
than average if it is greater/less than 97.5% of the
medians for randomized data sets at that size.) Signifi-
cance was evaluated only at times where there were
observations for a species. Randomization was also
used to determine if variability in gut fullness fluctu-
ated diurnally. As a measure of variability we chose
the difference in GFI between the 10th and 90th per-
centiles which, for brevity, we shall refer to as the
scatter throughout this paper. Large numbers of obser-
vations are required to accurately describe the obser-
vations in the extreme tails of the distribution beyond
the 10th and 90th percentiles.
Weight-specific ingestion rates were calculated
based on the difference between the distribution of
log-transformed GFI for each hour of a diurnal cycle
after accounting for gut evacuation. Gut evacuation for
all species of fish larvae was assumed to follow an
exponential decay that is 95% complete in 6 h, which
is at the mid-point of values estimated from laboratory
studies (Sumida & Moser 1980, Boehlert & Yoklavich
1983, Tilseth & Ellertsen 1984, Young & Davis 1990,
Canino & Bailey 1995, Johnston & Mathias 1996,
Lough & Mountain 1996). For each 1 h time period, tto
t+ 1, we subtracted the amount of material which
passed from the gut for each point of the cumulative
probability distribution (F(y)) as
(3)
F C F GFI F GFI kF GFI
ttt
() ( ) ( ) ( )=−
[]
+1
Fy Hy y w x x b
wx x b
ii
i
ˆ() ()()/
()/
=−−
{}
{}
201
Mar Ecol Prog Ser 204: 199–212, 2000
where kis the gut evacuation rate set at 0.5 h–1, and C
is the instantaneous weight-specific ingestion rate (h–1)
required to change the distribution (Fig. 1).
Because we had to make assumptions about evacua-
tion rates, we performed a sensitivity analysis of our
calculations using the extremes obser-
ved in other studies (3 and 8 h). The im-
pact of each species of larval fish on the
zooplankton community was estimated
by separating the total consumption ac-
cording to the empirical distribution of
prey widths observed in the stomachs
of each length category of each species
of larvae (reported in Pepin & Penney
1997). The total amount consumed by
the ichthyoplankton community was
calculated by summing over all species
and length intervals.
RESULTS
Stomach contents
The total weight of prey ingested in
relation to larval weight followed an
allometric relationship for all species
(Fig. 2, Table 2). There is considerable
variation in the height (i.e. intercept) of
the relationship among the various
species studied, with the clupeoids (Clupea
harengus, Mallotus villosus) showing the
smallest average GFIs. Other species (e.g.
flatfishes) showed little similarity among
closely related taxa. Although there was an
indication of significant differences in the
allometric slopes among the various species,
the overall contribution in terms of explained
variance was very small (< 0.2 % of the vari-
ance), suggesting that all species follow a
similar isometric relationship between mean
weight of prey in the stomachs and body
weight.
Feeding periodicity
The distribution of stomach contents showed
a skewed distribution, approaching a log-
normal, at all times and for all species (Fig. 3).
GFI varied diurnally, with gut fullness show-
ing a marked increase late in the afternoon or
in the early evening in 10 of 11 species and
reaching a peak around midnight. Only cun-
ners (Tautogolabrus adspersus) exhibited a
different pattern, whereby stomach fullness increased
after dawn and began to decrease in the late after-
noon. Randomization tests showed that the observed
pattern in gut fullness exhibited significant diurnal
variation in all species. Species with the highest GFI
202
(A)
Species Intercept (SE) Slope (SE) R2p
Clupea harengus 1.29 (0.18) 0.66 (0.17) 0.71 0.009
Gadus morhua 2.07 (0.09) 0.86 (0.09) 0.9 0.0001
Glyptocephalus cynoglossus 2.07 (0.12) 1.08 (0.11) 0.92 0.0001
Hippoglossoides platessoides 1.98 (0.064) 1.18 (0.049) 0.98 0.0001
Liparis sp. 2.18 (0.065) 0.87 (0.073) 0.95 0.0001
Mallotus villosus 1.37 (0.081) 0.84 (0.063) 0.96 0.0001
Pleuronectes ferrugineus 1.99 (0.13) 0.93 (0.087) 0.93 0.0001
Pleuronectes americanus 2.66 (0.37) 1.26 (0.18) 0.93 0.002
Stichaeus punctatus 2.06 (0.085) 0.91 (0.11) 0.9 0.0002
Tautogolabrus adspersus 1.79 (0.035) 0.77 (0.025) 0.99 0.0001
Ulvaria subbifurcata 1.95 (0.08) 1.00 (0.15) 0.9 0.001
(B)
Source df Type I SS Type III SS FIII p
Weight 1 3.35 0.9 574 0.0001
Species 10 0.81 0.19 12.1 0.0001
Species × Weight 10 0.1 0.1 4.22 0.0001
Error 79 0.12
Table 2. Allometric models and analysis of total stomach contents for individual
species (log10 [wt of stomach contents] = a+ blog10 [larval wt], where aand bare
intercept and slope, respectively, and all weights are in mg dry wt) (A), and
results of analysis of covariance contrasting those relationships (B). F-values
reported are based on Type III sums of squares
Fig. 1. Schematic diagram showing approach taken to estimate con-
sumption rates of each species of larval fish based on distribution of
gut fullness index (GFI). Evacuated portion of gut contents (k) is sub-
tracted from cumulative probability distribution (CDF) of GFI at time t.
Consumption (C ) required at each point of CDF is then estimated as
difference between observations at times t + 1 and t(after correcting
for evacuation)
Pepin & Penney: Impact of larval fish on zooplankton 203
Fig. 2. Allometric relationships for
total dry weight of gut contents in
relation to larval dry weight. Species
full names and details are listed in
Table 2
Mar Ecol Prog Ser 204: 199–212, 2000204
Fig. 3. Diurnal pattern in cumulative
probability distribution of GFI (log10-
transformed) (left axis) for each species
of larval fish. Continuous lines: 10th,
30th, 50th, 70th and 90th percentiles of
CDF; dotted lines: probability (right
axis) of measuring estimated median
GFI relative to a randomization of data
for each species (if median is signifi-
cantly greater/less than would be
expected from randomization of data,
then dotted line appears in upper/
lower 2.5% of randomized data sets
generated in simulations); dashed
lines: upper and lower 2.5% marks
Pepin & Penney: Impact of larval fish on zooplankton
showed the greatest degree of diurnal periodicity in
stomach fullness.
Scatter in gut fullness (10th to 90th percentiles)
spanned about 1 order of magnitude in species with
straight guts whereas it spanned approx. 1.3 orders of
magnitude in the majority of species with complex
convoluted guts (Fig. 3). In 9 of 11 species, there was
no significant variation in the scatter in gut fullness
throughout the day. Only in Clupea harengus and in
Tautogolabrus adspersus did randomization tests indi-
cate a significant decrease in the scatter in gut fullness
between the hours of 09:00 to 14:00 and 08:00 to 15:00,
respectively, relative to the remainder of the day.
Estimates of weight-specific ingestion rates indicated
that despite decreases in GFI during the day (the oppo-
site was true for Tautogolabrus adspersus) (Fig. 3), feed-
ing by the larval fish population continued throughout
the day to some degree (median ~1 to 2 % of body weight
per hour) (Fig. 4), even with the low evacuation rates
used in our calculations. Species with straight guts
(Clupea harengus, Mallotus villosus) had the lowest
ingestion rates, whereas species with well-developed
digestive systems had higher ingestion rates. Winter
flounder (Pleuronectes americanus) had the highest
relative ingestion rate (~2 to 3 times that of most other
species). Overall changes in the amount of food in the
stomachs were greatest, in absolute terms, in the upper
percentiles of the distribution but in relative terms, in-
gestion rates were most variable in the lower percentiles
of the distribution of stomach contents. There was gen-
erally a 4- to 6-fold difference between the lowest and
highest hourly individual ingestion rates within a given
species at times of peak feeding rates.
The median hourly integrated weight-specific inges-
tion rate (Ct· F(Ct)) of most species varied between 1
and 4% of body wt h–1 (Fig. 5). Exceptions to this were
Clupea harengus and Mallotus villosus, which had
feeding rates below 1% body wt h–1, and Pleuronectes
americanus, which ingested between 4 and 12% of its
body wt h–1. When integrated over a complete diurnal
cycle, the average larva ingested between 15 and
150% of its body weight, depending on the species
(Fig. 5). Most species ingested 30 to 70% of their body
weight daily, whereas C. harengus and M. villosus
ingested an average of 15% of their body weight. P.
americanus ingested approx. 10 times that amount.
Because our observations were nearly log-normal
within any species and time interval, the overall esti-
mates of total daily consumption are similar to those
obtained using the cycle in median gut fullness.
Sensitivity analysis revealed that estimated con-
sumption rates of each species increased by about 50%
if the evacuation time was reduced to 3 h (k= 0.99 h–1),
and decreased by 27% if evacuation times were in-
creased to 8 h (k= 0.37 h–1) (Fig. 6).
Ichthyoplankton community
Larval fish abundance and overall community struc-
ture show a marked seasonal variation (Fig. 7). The
number of species, and their overall abundance and
biomass increased from May (Day 141) through
July/August (~Day 210), after which community struc-
ture began to decrease in complexity, but the total
number of larvae remained high (Fig. 8). Stichaeus
punctatus, Hippoglossoides platessoides and Liparis
sp. dominated the larval fish community in May and
June (Days 140 to 180), with abundance of ~1 larva
50 m–3
. Atlantic cod (Gadus morhua) was present
throughout the study period but generally comprised
a minor element of the larval fish community, except
in early July when it was the second most abundant
species. American plaice (Pleuronectes platessoides)
showed a similar pattern, but it disappeared from the
plankton in August (~Day 240), as the larvae under-
went metamorphosis and settled to the bottom. The
community was dominated by the appearance of
capelin (Mallotus villosus) in July through September
(Day 200 and beyond): at its peak their abundance was
at least 10 to 30 times greater than the next most
abundant species (Fig. 7). In late summer, cunners
(Tautogolabrus adspersus) and witch flounder (Glypto-
cephalus cynoglossus) appeared as important ele-
ments of the ichthyoplankton community. In most
species, there was no major increase in cohort bio-
mass after the peak in larval numbers (Fig. 7), in
contrast to observations by Houde (1997) in Chesa-
peake Bay. Often, the overall biomass of a species
remained relatively level or decreased slightly after
numerical abundance had peaked. Growth and mor-
tality (which includes both biological loss and trans-
port) appear to have been balanced, or with the latter
slightly higher.
Total biomass of the ichthyoplankton community fol-
lowed a pattern similar to that of total abundance
(Fig. 8). Lowest biomass levels occurred in late May
and early June (Days 150 to 170: ~0.005 mg m–3
),
increased gradually from June until August, and then
remained relatively high into September (~0.3 to
0.5 mg m–3
: Fig. 8). From May through July, the peak
biomass moved from large (~10 mm) to smaller
(~5 mm) larvae. This reflected a change in community
from one dominated by Stichaeus punctatus to one
with large numbers of capelin (Mallotus villosus) and
cunner (Tautogolabrus adspersus) larvae. The subse-
quent development of the latter species moved bio-
mass into larger size categories by September. In May,
most of the biomass was found in S. punctatus,
whereas in June the biomass distribution was shared
between Atlantic cod, American plaice, S. punctatus
and Liparis sp.
205
Mar Ecol Prog Ser 204: 199–212, 2000206
Fig. 4. Contour diagram of the diurnal
cycle of cumulative probability distrib-
ution in estimated hourly weight-spe-
cific ingestion rates for each species.
See ‘Materials and methods’, Eq. (3)
and Fig. 1 for details of calculations
Pepin & Penney: Impact of larval fish on zooplankton
Impact on zooplankton
There was relatively little change
in the size distribution of avail-
able prey for fish larvae from May
to September, although larger size
categories were somewhat less
abundant in May (Fig. 9). The over-
all impact of the ichthyoplankton
community on zooplankton abun-
dance was negligible: generally
<0.1% of the zooplankton, of any
size category, was consumed by the
entire larval fish community on any
given day (Fig. 9). This conclusion
is largely unchanged by the value
of the evacuation rate used in our
calculations (Fig. 6). The seasonal
pattern in total prey consumption
mimicked the seasonal changes in
overall biomass of larval fish. There
was a seasonal progression in the
size category of microzooplankton
most heavily preyed upon by the
larval fish community; this moved
from 100 µm in May to 200 µm in
September. There was almost no
impact on size categories of zooplankton >350 µm in
width until July, when larger fish larvae begin to
appear in the community.
DISCUSSION
Despite relatively large numbers of larval fish, and a
community composed of a diversity of species, and
relatively high, daily, weight-specific ingestion rates
(mean 53% d–1, range 15 to 150), ichthyoplankton
appear to have a very small impact on the abundance
of their prey in Conception Bay. Less than 0.1% of the
available prey is consumed on a given day; this is far
below the production-to-biomass ratio of 3 to 8% d–1
typical for the dominant zooplankton species in this
area (Tremblay & Roff 1983). The potential for density-
dependent food limitation of pelagic larval fish ap-
pears to be minimal in this coastal ecosystem. This is
consistent with suggestions by Cushing (1983) and
Jones (1983).
Evacuation rates for larval fish are poorly known;
there have been relatively few observations, and their
temperature-dependence has not been described.
Slight variations in this critical parameter affects esti-
mates of the consumption rate of an individual larva
(Fig. 6). Surface mixed-layer temperature in Concep-
tion Bay varies over a range of ~10°C (5 to 14°C;
207
Fig. 5. Median relative hourly weight-specific ingestion rate (Ct·F(Ct)) for 11 spe-
cies of fish larvae; scale is shown on left-hand axis. Points at extreme right of
graph show integrated daily median weight-specific ingestion for the 11 species;
scale for this is shown on right-hand axis
Fig. 6. Results of sensitivity analysis of evacuation rates,
showing revised estimated daily consumption rates for 11
species considered in this analysis in relation to those esti-
mated as part of this study. Original weight-specific ingestion
rate represents value estimated using evacuation rate of
0.5 h–1. Results are shown for sensitivity analysis using faster
(k= 0.99 h–1; s) and slower (k= 0.37 h–1; d) evacuation rates.
Each point represents the estimate for an individual species.
Values on right hand side: slope of the relationships
Mar Ecol Prog Ser 204: 199–212, 2000208
Fig. 7. Seasonal absolute abundance (left column), total biomass (centre column), and relative abundance (Ni/iNi, where Ni= the
abundance of species iin a sample) (right column) patterns of larval fish in Conception Bay based on individual sample catches
Pepin & Penney: Impact of larval fish on zooplankton
Laprise & Pepin 1995, Pepin unpubl. data) during the
period when larval fish are present in the system. Since
most metabolic processes have a characteristic Q10 2,
this could have altered both larval evacuation and zoo-
plankton production rates during the course of the
study. This might result in a seasonal variation in con-
sumption rates relative to that of zooplankton produc-
tion, but this would likely have a negligible effect on
the predicted impact of larvae on zooplankton since
we would expect both prey and predator to respond
similarly to seasonal fluctuations in physical condi-
tions. Finally, the impact of larval fish on zooplankton
calculated in this study is very small, no matter what
value we assign to the evacuation rate. As a result, our
conclusions are unaffected by our use of an inter-
mediate value for the evacuation rate.
Microzooplankton in Conception Bay is substantially
more abundant than has been observed in most of the
studies summarized by MacKenzie et al. (1991) in their
review. In general, concentrations are 5 to 10 times
209
Fig. 8. Seasonal patterns (May to September) of total abun-
dance and biomass of entire larval fish community (top graph)
with contoured breakdown of length-dependent biomass dis-
tribution (bottom graph)
Fig. 9. Monthly pattern of abundance of dominant groups of
microzooplankton prey available to fish larvae in relation to
prey width (top graph) and cumulated daily impact on prey of
larval fish community (bottom graph)
Mar Ecol Prog Ser 204: 199–212, 2000
those observed in other shelf and coastal ecosystems.
In part, this difference may be an artefact of the choice
of sampling gears used to estimate microplankton
abundance. Frank (1988) highlighted limitations in
previous studies which had not used mesh sizes appro-
priate for the capture of small prey typically eaten by
most fish larvae. This may have led some investigators
to suggest that larvae or juveniles could reduce their
prey population, which would eventually lead to den-
sity-dependent competition because of underestimates
of prey concentrations. However, the impact of the lar-
vae on their prey in Conception Bay is so small that
even a 10-fold reduction in prey availability would be
unlikely to lead to competition among larval fish.
Potential microplankton production would still exceed
the impact which fish larvae have on their food re-
sources.
Houde (1997) found that after an initial decrease in
the biomass of a cohort caused by mortality exceeding
growth, total population biomass began to increase
when larvae reached about 10 mm in body length. In
contrast, we found no evidence that the biomass of any
species of larval fish in Conception Bay was increasing
with increasing length, even though the total biomass
of larval fish in the system increased from May to
September. There were clear peaks in the time- and
length-dependent distribution of larval fish biomass
(Fig. 7). However, these reflected shifts in the relative
proportion of different species with increasing length.
The hatching and release of small larvae tended to
dominate certain size categories, but as different spe-
cies grow at different rates, the relative composition of
any larval fish community changes with increasing
size. Such shifts in community structure may indicate
differential survival rates among cohorts of different
species. Alternatively, the lack of an overall biomass
increase within individual species as larvae aged may
have been due to the dynamic nature of the circulation
in Conception Bay (deYoung et al. 1994, Laprise &
Pepin 1995, Pepin et al. 1995). Continuous advection of
larvae, particularly capelin, out of Conception Bay may
contribute to the lack of increased biomass as well
as changes in ichthyoplankton community structure.
Transport of larvae in and out of Conception Bay may
also have limited the impact that ichthyoplankton
might have had on their prey by advecting larvae out
of the area and thus limiting growth in biomass of the
ichthyoplankton community.
The abundances reported in this study represent
local area averages based on large numbers of sam-
ples. However, both Frank & Leggett (1982) and
Laprise & Pepin (1995) found evidence of spatial seg-
regation of ichthyoplankton according to water-mass
characteristics. Frank & Leggett (1982) and Taggart &
Leggett (1987) argued that the pattern of association
not only reflected emergence mechanisms for the
release of larvae for several fish species with demersal
eggs but also adaptations to place larvae in environ-
ments with low numbers of predators and high con-
centrations of available prey. Although the strength of
this pattern weakens once larvae begin to disperse
into the wider circulation system of Conception Bay
(Laprise & Pepin 1995), there is still considerable
patchiness in the distribution of larvae. Larval fish
densities in excess of 30 larvae m–3 are not infrequent.
Such high densities may lead to local depletions in the
prey community. How this may impact the dynamics
of fish larvae is unknown. Given the high degree
of mixing which takes place throughout the bay
(deYoung et al. 1994, Laprise & Pepin 1995), it
seems unlikely that individual larvae would be sub-
ject to depleted food conditions over an extensive
time period.
Although larval fish feed predominantly on copepod
nauplii, they gradually shift their feeding to the larger
copepodite stages which provide a greater energy
return per unit ingested (Arthur 1976, Last 1978a,b,
Economou 1991, Pepin & Penney 1997). This gradual
shift to larger prey, which varies considerably in nature
among species (Pepin & Penney 1997), not only allows
more efficient feeding by the larvae but may also serve
to reduce the possible impact which larvae have on
their food resource. All the species considered in this
study exhibit protracted spawning and larval release
periods. Shifts in prey preference within a species
might serve to limit the degree of competition among
cohorts. However, differences in the shift to larger prey
among the diverse species in Conception Bay do not
show strong evidence of a partitioning of the prey field
(Pepin & Penney 1997). Most size categories of larvae
feed on a broad range of prey sizes and species, and
there is considerable overlap in the prey field between
the smallest and largest larvae (Pepin & Penney 1997).
There is also substantial overlap in the types and sizes
of prey eaten by different species that co-occur. Since
predation pressure by the larvae on the microzoo-
plankton does not appear substantial, it should not be
unexpected that there is little reason to partition
resources, either in terms of prey species or sizes eaten
by the larvae.
Our use of the complete cumulative distribution of
gut fullness provides an indication of the extremes
likely to be encountered within a population of fish
larvae. Although we were not able to estimate the
extremes in the distribution of meal sizes and their
frequency, we can infer a conservative estimate of the
fraction of the population of a species which does not
ingest sufficient food to satisfy their basic metabolic
requirements. Based on laboratory studies, Checkley
(1984) found that energy intake exceeded metabolic
210
Pepin & Penney: Impact of larval fish on zooplankton
demands when the daily weight-specific ingestion
rates reached 0.04% d–1, based on a comparative
analysis of several species. This implies that an aver-
age ingestion rate below 0.0016% body wt h–1 over a
full day could lead to an energy deficit which could
in turn lead to lower condition (Buckley 1984,
Clemmesen 1996). Whether this could lead to even-
tual starvation or greater vulnerability to predators is
unclear. However, when we consider the 11 species
of this study, we find that such low feeding rates
were seldom observed within the 10th to 90th per-
centiles considered in this study. At the other ex-
treme, maximum hourly weight specific ingestion
rates could reach ~5% h–1 in most species with com-
plex guts, and ~1.2% h–1 in species with straight guts.
Govoni et al. (1986) give no a priori evidence to sug-
gests that such differences are related to gut mor-
phology, as development of the alimentary tract dif-
fers little among larval fish until transformation to the
juvenile stage. The difference in estimated ingestion
rates may reflect differences in the inherent feeding
ability among the species considered here. Because
there is evidence that patterns in survivorship are
related to the persistence in growth rates or condition
within individual fish larvae (Meekan & Fortier 1996,
Pepin et al. 1999), the extremes in the distribution of
gut fullness may provide an indication of the survival
potential among individuals in the population. To val-
idate this hypothesis, there is a need to understand
the factors that would allow individual larvae to ob-
tain high rations relative to others in the population,
and how this translates into growth of each indi-
vidual.
Although the impact of the ichthyoplankton com-
munity may be negligible, the cumulated impact of
all carnivorous plankton on the microzooplankton
may not. Medusae and crab zoea are important
members of the plankton, reaching average densities
between 100 and 1000 m–3 (Frank & Leggett 1982,
Paradis & Pepin in press). Without further knowledge
of the diet of these organisms, it is impossible to
assess the potential for competition between fish
larvae and the remainder of the zooplankton com-
munity. Future studies to assess the impact of graz-
ing on microzooplankton communities need to be
broadened to consider all possible predators in the
ecosystem.
Acknowledgements. We wish to thank J. Anderson and T.
Shears for their assistance in various phases of this project.
Nell Stallard, of LGL Limited Environmental Research Associ-
ates, performed the analysis of microzooplankton samples
and larval fish gut contents. We thank P. Snelgrove and 3
anonymous referees for comments and suggestions concern-
ing this manuscript.
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212
Editorial responsibility: Kenneth Sherman (Contributing
Editor), Narragansett, Rhode Island, USA
Submitted: September 29, 1999; Accepted: May 1, 2000
Proofs received from author(s): September 7, 2000
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Larvae of mesopelagic fishes usually dominate in oceanic larval fish assemblages, but detailed investigations of their ecology are limited and thus preclude full assessment of the ecosystem structure and dynamics in oceanic waters. Here, we examined the growth and mortality of six taxa of numerically dominant mesopelagic fish larvae and their predatory impact on zooplankton in the Kuroshio region off southern Japan during late winter. The weight-specific growth coefficient (G w) ranged from 0.077 (Sigmops gracilis) to 0.156d⁻¹ (Vinciguerria nimbaria), and the instantaneous daily mortality coefficient (M) from 0.067 (S. gracilis) to 0.143d⁻¹ (Myctophum asperum). The ratio G w /M, an index of stage-specific survival of the larvae, was from 0.90 (Notoscopelus japonicus) to 1.24 (V. nimbaria), without a significant difference from a value of 1 in all species. Based on the reported relationship between G w and ingestion rate of the larval fishes, the daily ration of each species was calculated to be 32-57% of body dry weight d⁻¹. Mean and 95% confidence interval of food requirements of the six taxa of larvae was 1.41 ± 0.55mgCm⁻² d⁻¹. Predatory impact of the mesopelagic fish larvae on the production rate of the available prey was estimated to be approximately 3.5-5.2%, implying that the larvae have a low level but consistent effect on zooplankton production in the oligotrophic Kuroshio region.
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Analysis of post-emergent larval capelin Mallotus yrlJosus density in a small embayment in eastern Newfoundland during 1981 to 1983 demonstrated that as sampling scales approached those relevant to the larvae (ca 2 h, 200 to 400 m) the degree of positive association between microzooplank- ton and latvae decreased. Forty sequential estimates of post-emergent iarval mortality were not significantly correlated with either the density of edibie microzooplankton (90 to 130 prm size-classes), the density of potential predators (chaetognaths and jellyfish), or the density of other macrozooplankton. Correlatrons with potential predator densities, while insignificant, were positive. We suggest that positive correlations between predator density and mortality are more likely to occur when analysing fleld data that is collected near-instantaneously because of the near-instantaneous effect predators have on population losses relative to the longer-term cumulative effects that result from food limitation (starvation). There was no slgnificant relation between larval density and larval mortality and the hypothesis of density-dependent short-term mortality in post-emergent larvae was rejected.
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Larval dry weight, temperature and food density explained 85% of the variance in the laboratory derived ingestion rates of 11 species. After removing the effects of larval size and water temperature on ingestion rates, larval functional response was steepest at food densities <185 μg l-1; beyond this level, ingestion rates were independent of food density. Comparison of the laboratory functional response with natural microzooplankton densities shows that 1) larvae are unlikely to feed at maximal rates in the sea; 2) larval feeding rates are most sensitive to changes in food abundance across the range of food densities that are most likely to occur in nature. However, in situ ingestion rate estimates for 8 species of marine fish larvae indicate that these larvae fed at rates independent of food density and near-maximally, despite relatively low food densities. The difference between in situ and laboratory estimates of ingestion rates as a function of prey density results primarily from the failure of most integrated census estimates of prey density to adequately represent the real contact rate of larvae with their prey and the failure of most laboratory experimental designs to incorporate relevant variables known to influence prey encounter rates and selection. -from Authors
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Onshore winds induced rapid water mass exchange in coastal Newfoundland resulting in the replacement of cold, high-saline, predator-laden waters with warmer, less-saline waters in which the abundance of an important fraction of the predator community was reduced 3-20 fold and zooplankton densities in the edible size fraction were increased 2-3 fold. The synchronous emergence of larval capelin during onshore winds, coupled with the reduced predator density at this time, results in predator satiation. Wind-induced elevations of the biomass of the edible zooplankton size fraction can produce 5-fold increases in the daily growth rate of larval capelin. Capelin larvae thus initiate their drift and first feeding in a wind-induced 'safe site'. The abundance of eggs and larvae of 11 other marine fish species were also associated with this 'safe site' water mass.-from Authors
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1. In this paper we show that analysis of changes in the distribution of stomach contents weights over time offers opportunities for improved understanding of fish feeding using data gathered from four of the larger fish species exploiting a sea-loch on the west coast of Scotland. 2. A simple dynamic model of the feeding and digestion process was developed to help understand complex feeding patterns, for which explanatory mechanisms are not immediately obvious. This model characterizes the 24-h cycle in food uptake rate by a small group of parameters whose values are estimated from the data set taken as a whole. 3. As a result of its higher immunity to noisy data, our model has advantages over previous methods for calculating food uptake rates.
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Recent studies which have concluded that larval fish distributions are independent of the abundance and distribution of their prey, used coarse-mesh nets incapable of retaining the edible zooplankton for fish larvae to provide quantitative estimates of the larval food resource. In waters off SW Nova Scotia the biomass of edible zooplankton for young larval fish was highly concentrated in the nearshore region, progressively lower levels were evident offshore on the shelf, and the mesoscale distributional pattern did not accurately reflect the total zooplankton biomass retained by a 333 μm mesh net. A more logical explanation for the reports of "paradoxical' distributions of fish larvae and their prey is to be found in the inefficiency and bias in the sampling methods used to evaluate the larval food resource. -from Author
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Larvae of Atlantic herring Clupea harengus were reared on wild plankton and Artemia salina nauplii in the laboratory at 7 to 9OC for 95 d. Between ages of 20 and 38 d, larvae were fed only Artemia nauplii and the specific rates of ingestion and growth were measured and compared. Relations of rate and efficiency of growth to ingestion were similar in terms of carbon and nitrogen. Growth was linearly related to ingestion (r2 = 0.89, n = 9). Starved larvae lost mass at a specific rate of 0.03 d-I (3 % dl) until death at 14 d. A specific ingestion rate of 0.04 d1 was required to balance defecation and metabolism. Gross growth efficiency (growth ratehngestion rate) rose from - 1.2 at a low ingestion rate (0.015 dl) to 0.4 at the greatest observed ingestion rate (0.11 d'). Condition factor (dry weight length3) was significantly related to both ingestion rate and length (r2 = 0.69, n = 20). These results, combined with those for other fish larvae, indicate an asymptotic relation between rates of growth and ingestion such that gross growth efficiency is maximal (0.4) at intermediate ingestion rate. Fish larvae surviving in the sea appear to maximize their ingestion rate and thus grow rapidly but with a reduced efficiency.
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The larval diets of five gadoids in the northern North Sea in May in both 1978 and 1979 are described and compared. Cod, Norway pout, saithe, and whiting larvae consumed similar types of prey and showed a general tendency to select increasingly larger prey with increasing body size. The haddock larvae exploited a wider range of prey types, and at comparable lengths they consumed smaller and slower moving organisms than did the other gadoid larvae. The diets of sandeels and long rough dab which were abundant in the area were also examined. The species-specific selectivity patterns with respect to size and mobility of prey fell into two categories: those dictated by the basic body morphology, and those determined by behavioural factors, which were intimately linked to adult behavioural patterns. Competition for food was potentially possible between late larvae, but it could not be identified as a factor causing shifts in dietary characteristics.