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Net feeding in ichthyoplankton samples
from the Paraná River
CARLOS M. FUENTES* AND FEDERICO QUIROGA
DIRECCIÓN DE PESCA CONTINENTAL, SUBSECRETARÍA DE PESCA DE LA NACIÓN, MINISTERIO DE AGRICULTURA, GANADERÍA Y PESCA, PASEO COLÓN
BUENOS AIRES
982,
1063, ARGENTINA
*CORRESPONDING AUTHOR: [email protected] or [email protected]
Received March 5, 2012; accepted in principle July 20, 2012; accepted for publication July 23, 2012
Corresponding editor: Roger Harris
This study investigated to what extent natural predation or net feeding contribute
to the presence of pre-flexion fish larvae in the stomachs of more developed larvae
in ichthyoplankton samples from the Paraná River. A digestion trial and two sampling experiments were conducted. First, pre-flexion larvae were offered to postflexion larvae and prey digestion stages were analyzed hourly. Second, we evaluated the number of prey retained in the mouth and contained in the stomachs of
predators from 1, 5 and 10 min samples taken with a single-standard plankton net
(300-mm mesh size) and with two standard and 1600-mm mesh size nets simultaneously deployed. At time zero, most of the prey were in a fresh condition. Digested
prey accounted for 40% of the prey after 1 h and were dominant after 6 h digestion. Stomachs of predators captured with the standard net contained significantly
more fresh and bitten prey in longer- than in short-duration samples, whereas no
prey were observed in samples collected with the larger mesh size net. Our results
show intra-net predation in the 300-mm mesh net and imply that most prey were
eaten in the net.
KEYWORDS: net feeding; ichthyoplankton; river; South America; Paraná
I N T RO D U C T I O N
The role of predation in the dynamics of early stages of
fish has been widely recognized (e.g. Hunter, 1981;
Bailey and Houde, 1989; Fortier and Villeneuve, 1996;
Houde, 2008). However, the predation rate is difficult to
quantify due to the underestimation of individual prey
caused by difficulties associated with detecting larvae in
the stomach contents of predators which are usually
rapidly digested or regurgitated (Brewer et al., 1984;
Rottiers and Johnson, 1993; Schooley et al., 2008; Legler
et al., 2010). Conversely, there is an overestimation
resulting from net feeding as observed for euphausiids
(Nicol, 1984; Purcell, 1985), chaetognaths (Sullivan,
1980) and fishes (Lancraft and Robison, 1980; Brewer
et al., 1984; Bailey et al., 1993) when predator and prey
are simultaneously captured with the same gear.
To avoid the bias produced resulting from predation
in the net, different sampling and post-sampling procedures have been conducted in studies on feeding
ecology of macrozooplankton and fishes. Some authors
have used a large enough mesh size (Sullivan, 1980;
Fortier and Villeneuve, 1996) and/or a shorter sample
duration (Lass et al., 2001; Takasuka et al., 2004),
whereas others have considered the position and the
degree of digestion of prey in the digestive tract (Fortier
and Villeneuve, 1996; Takasuka et al., 2003, 2004), gut
fullness diel periodicity (Hopkins and Sutton, 1998) or
excluded individuals retained in the anterior digestive
tract of the potential predator (Purcell, 1985).
doi:10.1093/plankt/fbs055, available online at www.plankt.oxfordjournals.org. Advance Access publication August 27, 2012
# The Author 2012. Published by Oxford University Press. All rights reserved. For permissions, please email: [email protected]
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For the last two decades, the ichthyoplankton from
the Paraná River (Argentina) has been sampled by
means of conical plankton nets stationarily deployed for
5 –20 min, with the standard 300 – 500-mm mesh sizes,
with the main purpose of detecting representative size
ranges of drifting larvae (Oldani, 1990; Fuentes et al.,
1998; Fuentes and Espinach Ros, 1998) or studying the
diet of fish larvae (Rossi, 2001, 2008). In these investigations, the presence of pre-flexion larvae in the stomachs
of post-flexion larvae of some well-known adult ichthyophagous species has been reported. As a consequence
of the high frequency in the occurrence of prey, some
researchers support the hypothesis that predation by
post-flexion larvae of Salminus brasiliensis, Pseudoplatystoma
spp. and Pimelodus spp. accounts for the mortality of preflexion larvae of other migrant pre-flexion larvae of
characins such as Prochilodus lineatus and Leporinus obtusidens (Oldani, 1990; Fuentes, 1998; Rossi, 2001, 2008).
Accordingly, it has been hypothesized that downriver
drift is a period of high vulnerability, particularly for
P. lineatus larvae. However, whether the frequent presence
of pre-flexion larvae in stomachs of post-flexion larvae is
either caused entirely by natural predation in the water
column or explained, at least in part, by net feeding, still
remains unclear. This uncertainty is a consequence of
the methodologies used in ichthyoplankton studies.
Mesh size of nets used in the Paraná River (Oldani,
1990; Fuentes, 1998; Rossi, 2001, 2008) is mainly selective for both predators and prey, and sometimes, small
individuals remain in the mouth or foreguts of more
developed larvae (Fig. 1G), suggesting recent ingestion
of prey.
It is thus necessary to address the issue of the relative
importance of net predation on post-larval feeding to
allow a more exhaustive evaluation of the impact of
natural predation on the early stages of fishes. The
purpose of this work is to answer this question by means
of a sequence of experiments performed in the study area.
METHOD
One laboratory and two sampling experiments were
carried out in the Paraná Guazú River (338520 S,
588540 W), Argentina, in January – February (summer)
2007.
Experiment 1: digestion trial
Time for prey digestion is important when evaluating
the occurrence of net feeding. Larval fish remains, products of rapid digestion, can significantly affect the
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evaluation of the source of predation (Legler et al.,
2010) when considering water column predation. Since
there was no information for potential predator species,
we preliminarily attempted to provide a consistent temporal reference for stomach prey digestion stages. To do
this, we captured post-flexion larvae of Pimelodus frequently containing pre-flexion larvae in their stomachs
(SL: 9.6 – 19.84 mm) and starved them for 24 h until
their stomachs were empty. The following day, previously captured pre-flexion larvae of P. lineatus (SL: 6.5–
7.5 mm) were offered to 60 starved “predator” larvae
for 30 min in a 20-L aquarium ( prey density
1000 ind m23) resulting in a prey density 1 or 2
orders of magnitude greater than those commonly
observed in the water column. Prey and predators were
gently separated using a 1600-mm mesh. Six groups of
eight post-larvae randomly chosen were kept at 278C
(the same temperature as the river) and then sacrificed
at the following time points: 0, 1, 2, 3, 4 – 6 and 12 h.
In the laboratory, guts of eight predators from each time
point were dissected under a binocular microscope
(12). For each individual, the number of prey and the
degree of digestion were determined by four observers
blinded as to the time elapsed after the predation trial.
Observers determined the percentage of digestion stages
for each stomach as follows: fresh (F: eyes intact, body
shape not hydrated and skin conserved), slightly
digested (SD: partial trauma in the eye and skin and
body hydration) and clearly digested (CD: eye trauma,
body limits and shape considerably lost, evident
hydration).
Experiment 2: one net (sampling period sample duration)
We evaluated the occurrence of net feeding by
analyzing the stomach contents of Pimelodus and
Pseudoplatystoma post-flexion larvae from samples of different durations (1, 5 and 10 min), under the null hypothesis that, if net feeding does not occur, then the
number of pre-flexion larvae contained in the stomach
of potential predator larvae captured in samples of different duration should be the same. Sample durations
were tested in a two-way design with the period of the
day. Seven and six blocks of samples were taken both
during daylight (DL) and dusk-night-dawn (DND), respectively (Table I), with a conical ichthyoplankton net,
1 m long and 300-mm mesh size, stationarily deployed
for: 1 (five replicates each sample), 5 and 10 min. The
DND period is a period of increased activity in silurid
larvae (Fuentes, 1998).
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Fig. 1. Photographs of predator and prey: (A) fresh larvae at 0 h, (B) CD larvae at 3 h, (C) pre-bolus at 6 h, after predation in Experiment 1,
(D) prey in 15 mm SL stomach of Pimelodus, (E) detail of fresh (n ¼ 6), retained in the mouth (white arrow) and rests, (F) damaged but undigested
larvae, (G) prey retained in the mouth and contained in the stomach (white arrow) of flexion larvae of Pseudoplatystoma, (H) pre-flexion larvae of
P. lineatus damaged (white arrow) after partial mouth retention, (I) four undigested (stomach contained: circle) and one mouth-retained P. lineatus,
from 15 mm SL post-flexion larvae of Pimelodus, (J) fresh P. lineatus in the stomach of S. brasiliensis, (K) remains of silurid larvae (1 min
sample) and (L) fresh pre-flexion Doradidae and rest of fish (10-min sample) in 35 mm SL post-flexion larvae of S. brasiliensis. Horizontal bars
indicate 1 mm.
Experiment 3: two nets (mesh size sample duration)
Treatment of the capture
In a second sampling experiment carried out 20 days
later, two 1-m long nets (one 300- and another
1600-mm mesh size) were simultaneously deployed. In
this experiment, three DL blocks and one DND block
of 1- (five replicated), 5- and 10-min samples were collected, respectively. Due to the reduced number of
blocks available in this experiment, both periods of day
were pooled. Consequently, sample durations were
tested in a two-way design with the mesh size.
After the net was recovered and to reduce the possibility
of post-capture regurgitation and feeding, captured
larvae were anesthetized by immediately placing the
collector cup in a tank with 1000 ppm benzocaine solution. The number of pre-flexion ( potential prey) and
post-flexion larvae was determined for each sample.
When possible, larvae were identified following
Nakatani et al. (Nakatani et al., 2001). Captures of postflexion larvae were dominated by the following taxonomic groups: Pimelodus spp. and Pseudoplatystoma spp.,
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Table I: Abundance of pre-flexion (PRE) and post-flexion (POST) larvae from experiments of different
sample durations
Experiment 2
DL
DND
Experiment 3
300 mm
1600 mm
Number of
blocks
Stage of
development
7
PRE
POST
6
4
4
PRE
POST
PRE
POST
PRE
POST
Species
PIM
PSE
PIM
PSE
PIM
PIM
Larval densities
(ind m23)
Size range (mm)
35.4 + 24.9
6.5 + 9.4
1.2 + 1.3
37.9 + 24
9.5 + 9.7
2 + 1.7
4–7
6.5 –18.7
6.5 –10.8
4–7
6.5 –18.7
6.7 –10.3
87
10.4
2.1
71
24.5
4.4
11 284
1353
282
6792
2341
420
7.2 + 1.8
5.6 + 1.5
—
3.4 + 1.7
4–7
6.5 –18.7
59.3
40.6
—
100
1821
1248
—
648
6.8 –18.3
Capture (%)
Number
Predator/prey
ratio
0.12
0.02
0.34
0.06
0.68
—
DL, daylight; DND, dusk-night-dawn; PIM, Pimelodus spp.; PSE, Pseudoplatystoma spp.; mesh size, 300 and 1600 mm.
and a few (less than 0.1% of captures) S. brasiliensis and
Sorubim lima, whereas pre-flexion individuals were
mostly represented by P. lineatus and members of the
family Doradidae. Each larva was measured to the
nearest 0.1-mm SL. Stomachs of potential predators
were dissected under a binocular microscope and the
average number of prey per predator for each sample,
both retained in the mouth and contained in the stomachs (gut content excluded), either fresh or digested,
was determined (Buckel et al., 1999). For Experiment 3
(sample duration mesh size), the frequency of postflexion larvae of Pimelodus containing remains of insects
was determined for each sample. In both experiments,
larval densities were calculated by dividing the number
of larvae in a sample by the water volume (WV)
sampled estimated from readings of a flowmeter placed
in the mouth of the net.
Statistical analysis
In Experiment 1, the percentage of each digestion stage
(F, SD and CD) in the stomach of a predator was
obtained by averaging the original values determined
by observers. For descriptive purposes, means (+SD,
n ¼ 8) of each stage of digestion were plotted against
elapsed time of digestion (Fig. 2). Since interest was
focused on the importance of change for each digestion
stage, statistical significance for the differences in percentages with time was determined separately for each
variable by means of one-way ANOVA (n ¼ 8), and post
hoc comparisons among fixed time points were determined by the Tukey test (P , 0.05). Due to the lack of
requirements in parametric testing for Experiments 2
and 3, differences in the number of larvae of potential
predators (Pimelodus and Pseudoplatystoma), prey (total preflexion larvae), prey per stomach and prey retained in
Fig. 2. Number (bars, right axis) and percentage of fresh (rhomboid),
SD (square) and CD (triangle) pre-flexion larvae (+SD; mostly
P. lineatus) contained in the stomachs of Pimelodus larvae (left axis) kept
at 278C water temperature in Experiment 1. Different characters
within each symbol series indicate statistically significant differences in
the percentage of digestion stages between time intervals (the Tukey
test, P , 0.05).
the mouth with sample duration were evaluated by
means of the Friedman test (P , 0.05) in both experiments. This test was conducted separately for the
period of the day (Experiment 2) and the mesh size
(Experiment 3). Similarities in trends between both
periods of day in Experiment 2 facilitated pooling
blocks of DL and DND in Experiment 3. Multiple
comparison was used to evaluate differences between
pairs of sample durations (Daniel, 1957).
R E S U LT S
Experiment 1: digestion trial
During the first 4 h after ingestion, the number of prey
found in stomachs of Pimelodus post-flexion larvae
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ranged between 2 and 4 individuals and decreased up
to one individual between 6 and 12 h of digestion. The
percentage of F, SD and CD prey in stomachs of predators sacrificed at subsequent time points showed statistically significant differences (one-way ANOVA, n ¼ 8; F,
P , 0.0001; SD, P , 0.0001; CD, P , 0.0001). At the
beginning of the digestion trial, almost all prey larvae
remained identifiable and were classified by observers
as fresh (F, 85%, Figs 1 and 2) or SD (12%). After 1 h
digestion, the stomach of predators contained a statistically significant lower percentage of fresh larvae, and a
higher percentage of SD larvae than those observed at
0 h (Tukey’s test, P , 0.05). Between 2 and 4 h, SD
prey were dominant (50 – 60%), CD larvae (Figs 1B and
2) were more frequent (20– 30%) and only 10% of individuals were classified as fresh. At 6 and 12 h, significantly more clearly (50 –70%) and less slightly (20 –
30%) digested prey were found in stomachs of Pimelodus,
whereas no prey were classified as fresh. Between 6 and
12 h, larval remains were intimately related forming a
very compact piece difficult to separate into distinct elements (Fig. 1C).
Experiment 2: one net (sampling period sample duration)
A total of 13 blocks of short (five times replicated
1 min, WV: 3.4 + 1.27 m23), medium (5 min, WV:
18.9 + 2.26 m23) and long duration (10 min, WV:
33.1 + 2.6 m23) samples were taken against a water
current between 0.6 and 0.8 m seg21 (Table I). In both
sampling periods, total pre-flexion larvae were abundant accounting for 87% DL and 70% DND of the
captures (Table I). They were dominated by P. lineatus
(DL, 70.2%; DND, 69.1%), and followed by S. brasiliensis
(DL, 17.8%; DND, 19.7), family Anostomidae (DL,
7.7%; DND, 8.17), S. lima (DL, 2.49%; DND, 2.12%)
and Pseudoplatystoma spp. (DL, 1.35%; DND, 0.7%). For
statistical purposes, all pre-flexion potential prey were
pooled. Potential predator post-flexion larvae were dominated by Pimelodus spp. and Pseudoplatystoma spp. (Table I).
Only 50 individuals (.1% of the capture) of S. brasiliensis
were captured during both sampling periods preventing
further statistical analysis. As expected, both during DL
and DND periods, the number of pre-flexion (total) and
post-flexion larvae (both Pimelodus and Pseudoplatystoma)
captured increased significantly with sample duration
(the Friedman test, P , 0.05, Fig. 3).
A total of 2457 mostly fresh P. lineatus (86%) preflexion larvae were found in stomachs of Pimelodus and
Pseudoplatystoma, and regularly, a number of individuals
were retained in the mouth (MR). The number of prey
retained in the mouth and contained in stomachs of
Pimelodus increased in medium- and long-duration
samples, although only prey per stomach was statistically significant (the Friedman test, P , 0.05, Fig. 3), pairs
of 1 – 5 and 1 – 10 min treatments being statistically significant (Multiple comparisons, a ¼ 0.05, P , 0.05).
There was no statistical significance in the number of
prey retained in the mouth of Pimelodus in samples of
Fig. 3. Number (mean + SD) of prey contained in stomachs (black bar, left axis), retained in the mouth (gray bar, left axis), free prey (black
circles, right axis) and predators (white circles, right axis) in the sample, for samples of different duration, taken with a 300-mm mesh size net
during DL (n ¼ 7) and DND (n ¼ 6) periods (Experiment 2). The number of stomachs processed is given in parenthesis. Upper left corner in
parentheses: statistical significance for each variable (the Friedman test). Similar characters in italics indicate no significant differences for
multiple range comparisons (P . 0.05).
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different duration from DL hours (Fig. 3), whereas this
difference was marginally not significant for those taken
during the DND period (Fig. 3). For Pseudoplatystoma, the
number of prey retained in the mouth (for DL samples,
Fig. 3) and contained in stomachs for DND (Fig. 3)
were significantly different from sample duration (the
Friedman test, P , 0.05). Other stomach content components of Pimelodus were mainly remains of insects
and, on some occasions, golden mussel Limnoperna fortunei veligers and a few copepods, largely occupying the
posterior region of stomachs. Some cases of CD larvae
were found only for stomachs of S. brasiliensis, a low
abundance characiform, top predator fish as an adult
(Figs 1J – L and 4).
Experiment 3: two nets (mesh size sample duration)
Simultaneous deployment of 300- and 1600-mm mesh
size nets (water current: 0.3– 0.4 m seg21) produced
four blocks of short- (five times replicated 1 min,
WV300: 3.1 + 0.26 m23; WV1600: 2.57 + 0.50 m23)
medium- (5 min, WV300: 10.05 + 5.12 m23; WV1600:
11.22 + 5.02 m23) and long-duration (10 min: WV300:
WV1600:
25.17 + 1.73 m23)
29.57 + 12.35 m23;
samples. Pre-flexion larvae abundance was lower than
in Experiment 2 and only post-flexion larvae of
Pimelodus were captured (Table I). In the 300-mm mesh
size net, total free pre-flexion larvae accounted for
59.3% of the capture, whereas, as expected, no preflexion larvae were captured in the 1600-mm mesh size
net (Table I).
Total pre-flexion larvae (mostly P. lineatus) contained
in the stomachs of Pimelodus captured in the 300-mm net
totaled 161 individuals. Prey retained in the mouth and
contained in the stomach of these post-flexion larvae
differed significantly with sample duration (the
Friedman test, P , 0.05, Fig. 5), and almost no prey
found in stomachs exhibited signs of digestion.
Statistical significance was found between pairs of 1 –
10 min for prey both retained in the mouth and contained in stomachs (multiple comparisons, a ¼ 0.05,
P , 0.05). Noticeably, no prey were retained in the
mouth or contained in stomachs of post-flexion larvae
collected with 1600-mm nets.
DISCUSSION
In the marine environment, the impact of intra-net predation performed by chaetognaths (Sullivan, 1980),
medusae (Matsakis and Conover, 1991), euphausiids
(Nicol, 1984) and young fish (Lancraft and Robison,
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Fig. 4. Number (mean + SD) of prey contained in stomachs (dashed
bars) and retained in the mouth (white bars) of S. brasiliensis for
samples of different duration, taken with a 300-mm mesh net
(Experiment 2). Both DL and DND periods pooled. The number of
stomachs processed is given in parentheses.
Fig. 5. Number (mean + SD) of prey contained in stomachs (black
bars, left axis) and retained in the mouth (gray bars, left axis), prey
(black circles, right axis) and predators (white circles, right axis) free in
samples of different duration, collected with 300-mm (above) and
1600-mm (below) mesh size nets simultaneously deployed (Experiment
3). The number of stomachs processed is given in parentheses. Upper
left corner in parentheses: statistical significance for each variable (the
Friedman test). No prey contained in the stomachs, retained in the
mouth or free in the samples were observed for a 1600-mm mesh size
net.
1980) has already been described. However, to our
knowledge, this is the first study assessing the occurrence of net feeding by fish larvae in river ichthyoplankton samples. Overall, experiments carried out in this
work show that intra-net predation occurred in the
300-mm mesh size net.
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As expected, Experiment 1 showed blinded observers
agreeing that most preys were in “fresh” condition immediately after ingestion (0 min digestion; Fig. 2).
However, results of the digestion trial indicated that
there could also be some incorrect interpretation when
the source of predation is assigned. The fact that some
individuals appeared to be SD or CD at the same time
(Fig. 2) may be explained by the fact that damage and
even the complete breakdown of small larvae in predator stomachs can occur rapidly, even sooner than our
experimental time of prey exposure, and at high water
temperature (Rottiers and Johnson, 1993; Schooley et al.,
2008; Legler et al., 2010). However, results indicate that
at such a level of stomach fullness, water temperature
and predator size, larval prey would mostly be observed
in a fresh condition when recently ingested and that the
general condition of a significant percentage would tend
to change considerably after only 1 h of digestion.
Any assignment error in the source of predation
resulting from the apparent digestion stage of prey was
a less crucial issue in Experiments 2 and 3, when
sample duration was manipulated. In these experiments,
the evidence of strong net-feeding occurrence was given
by the increasing number of prey in stomachs of predators from samples of increasing duration, and the fact
that even in 1-min samples, post-flexion larvae contained fresh prey in their stomachs and foregut (Figs 3
and 5). Although limited by the low number of individuals in catches, our results suggest that net feeding also
occurred for the characin S. brasiliensis (Fig. 4); however,
CD individuals in stomachs of this species also indicate
natural predation (Figs 1J –L and 4). Additionally, in
Experiment 2, neither a predominance of digested prey
nor diel periodicity in stomach fullness, signs of natural
predation according to Hopkins and Bair (Hopkins and
Bair, 1975) and Hopkins and Sutton (Hopkins and
Sutton, 1998), was detected; the predominant digestion
stage in stomach contents of silurid larvae at a similar
fullness level was “fresh” for both DL hours and DND
periods of low and high activity, respectively (Fuentes,
1998; Fuentes and Espinach Ros, 1998; Gómez et al.,
2011). In Experiment 3, a strong correlation between
prey in the sample and prey per stomach was found,
clearly demonstrating the occurrence of net feeding.
This result is coherent with feeding studies of young
marine fish (Fortier and Villeneuve, 1996; Takasuka
et al., 2003) and euphausiids (Lass et al., 2001), where
the use of nets of multiple mesh sizes appeared to be effective in attempting to control net feeding. In this experiment, we obtained a realistic idea of the magnitude
of this phenomenon in a 300-mm mesh size net and
showed how effective and necessary a larger mesh size
(1600 mm) is to avoid it (Fig. 5). Our results indicate
that a multiple mesh arrangement is essential when
feeding ontogeny of larvae is studied. They also indicate
that results obtained from samples collected with a “not
coarse enough” mesh size net (300 – 800 mm) and sufficiently long-duration hauls (5 – 30 min; Oldani, 1990;
Fuentes and Espinach Ros, 1998; Rossi, 2001) were seriously biased with respect to reporting a large number
of pre-flexion larvae in stomachs of flexion and postflexion silurid and characin fishes.
Mechanisms involved in net feeding are probably
mediated by different factors. It is assumed that prey
and predator densities could have played a significant
role in facilitating net feeding. After cod-end crowding
with potential prey, fishes may either voluntarily or involuntarily ingest items that might not be a “normal”
component of their diets (Sutton, 2005). In addition,
not all fish species appear to be equally efficient in net
predation (Lancraft and Robison, 1980). Probably,
feeding behavior of silurid larvae under dark conditions,
most likely mediated by chemosensory senses, such as
taste buds, distributed in barbells, buccal cavity and
head (Hecht and Appelbaum, 1988; Appelbaum and
Kamler, 2000; Mukai et al., 2008; Mukai and Lim,
2011), makes them good candidates for net feeding. The
endurance of predator larvae in relation to sampling
conditions has also been suggested to be a crucial factor
that may result in intra-net predation by larvae
(Lancraft and Robison, 1980; Hopkins and Sutton,
1998). Flexion and post-flexion larvae of Pimelodus were
found to be in optimal condition after sampling; this
fact may partly explain their intra-net predation capability. However, it is conceivable that high velocity
sampling could preclude intra-net predation by affecting
the survival of potential predators. Results also suggest
that the intensity of net-predation is dependent on the
size range of predators considered. Both prey per
stomach and the difference between that estimation and
the number of individuals retained in the mouth were
greater for the larger Pimelodus than for the smaller
Pseudoplatystoma (Fig. 3). This is probably due to the fact
that larger predators take less time to swallow their
prey; this questions the effectiveness of criteria based on
excluding prey retained in the mouth of predators, especially for more efficient and larger individuals. This is
consistent with the results of Baier and Purcell (Baier
and Purcell, 1997) for chaetognaths, who stated that excluding prey in the foregut from gut content analysis
was appropriate but only in combination with very
short tows.
In Experiment 3, apparently a substantial (if not
total) proportion of the prey contained in stomachs was
predated within the 300-mm net (Fig. 5). Assuming that
something similar occurred in Experiment 2, we could
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importance of larval fish predation by older larvae.
Although our results were developed in the context of a
river system, they could be extended to other environments. In view of our results, it could be concluded that
two or more mesh sizes are necessary when feeding ontogeny of a fish species is studied. When pre-flexion
larvae are the targeted developmental stage and the use
of coarse mesh size is not feasible, short sampling durations can help to mitigate this issue.
FUNDING
Funding by Dirección de Pesca Continental –
Subsecretarı́a de Pesca y Acuicultura – Ministerio de
Agricultura, Ganaderı́a y Pesca – República Argentina.
AC K N OW L E D G E M E N T S
Fig. 6. Number of samples of 1 min (white), 5 min (gray) and 10 min
(black) (upper panel), and the mean predator/prey ratio (lower panel),
versus percentages of total pre-flexion larvae contained in stomachs
(Experiments 2 and 3 pooled).
derive the impact of net predation on the abundance of
pre-flexion larvae. Figure 6 shows that the percentage of
total pre-flexion larvae contained in stomachs of more
developed larvae, appears to depend on the predator–
prey ratio and sample duration. Noticeably, in a single
10-min sample when the predator– potential prey ratio
was well above 1, intra-net predation by post-flexion
Pimelodus larvae significantly reduced up to 80% the
number of free pre-flexion larvae in the sample.
Therefore, when pre-flexion larvae are the targeted developmental stage and consequently a 300- or 500-mm
mesh size is the choice, the shortest sample durations
would be necessary to mitigate this artifact. Additionally,
the relative abundance of potential predator and prey
larvae will give an idea of how likely it is that intra-net
predation could affect pre-flexion larvae estimates.
We thank the comments from two reviewers which
helped to improve the manuscript. We thank SSPyA,
Gerardo Nieto and Ramiro Sanchez for supporting this
work. Thanks are also due to Santiago Sebastiani for
his help in the field work.
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