Results
Relationships between size distribution and
depth
We observed a significant increase (p-value
< 0. 001) in fish size (total length) with depth at the
community level (Table I). The median individual size increased
consecutively between the epipelagic, upper mesopelagic, and lower
mesopelagic depth layers (median individual size equal to 7.0, 9.0, and
10.6 cm respectively; Figure 2). Median individual size then decreased
slightly between the lower mesopelagic and bathypelagic layers, with a
median individual size of 10 cm in the bathypelagic layer. Furthermore,
while the first three depth layers had an unimodal distribution, the
bathypelagic layer presented a bimodal distribution with a peak of
around 8 cm and another around 13 cm.
The relationship between size distribution and depth was also tested at
the species level (Table I). Only four of the 12 species showed a
significant relationship. Melanostigma atlanticum, Lampanyctus
crocodilus , and Xenodermichthys copei showed a significant
increase in individual size with depth. Myctophum punctatum was
the only species showing a significant decrease in the individual size
with depth.
Lampanyctus crocodilus showed an increase in individual size
between the upper and lower mesopelagic layers, from a median size
(total length) of 10.0 cm to 12.0 cm (Figure 3). This was followed by a
stabilisation between the lower mesopelagic and bathypelagic layers with
the same median size of 12.0 cm. Melanostigma atlanticum showed a
continuous increase in the size of individuals with depth, with median
individual sizes of 6.0, 7.45, and 9 cm respectively.Xenodermichthys copei showed a maximum median size in the lower
mesopelagic layer (= 10.2cm total length). Myctophum punctatumshowed small differences in individual size between depth layers, with a
median size of between 6.6 and 7.0 cm. Relationships for other species
are available in Appendix 3.
Relationships betweenδ 15N values and
size
δ15N values were determined for seven migratory and
five non-migratory or short migratory species, for a total of 682
individuals (= isotopic dataset). Mean δ 15N
values ranged from 9. 49 ± 0. 57‰ for Serrivomer
beanii to 12. 36 ± 0. 33‰ for Aphanopus carbo (Table
II). For this dataset, mean values of standard length ranged from
6. 3 ± 1. 7 cm for Argyropelecus olfersii to
77. 3 ± 10. 8 cm for Aphanopus carbo , withSerrivomer beanii having the widest size range (45.0 cm between
the minimum and the maximum length) and Lampanyctus macdonaldithe narrowest (3.3 cm).
The relationship between δ 15N values and
individual size was first investigated at the community level (Figure
4). The linear model results showed a significant increase inδ 15N values with individual size. However, theR 2 was very low (= 0.01), indicating high
variability in the values.
The relationship between δ 15N values and size
was then investigated at the species level (Figure 5A). The results of
the linear models showed that six species had a significant increase ofδ 15N values with increasing individual size:Myctophum punctatum , Melanostigma atlanticum ,Lampanyctus crocodilus , Stomias boa , Serrivomer
beanii, and Aphanopus carbo . Only Arctozenus risso had a
significant decrease of δ 15N values with
increasing size.
The other five species had no significant relationship between the two
variables (Figure 5B). The δ 15N values were
stable regardless of the size increase. However, the coefficient of
variations allowed to distinguish species with low interindividual
variability (e.g., N. kroyeri ) from species with high
interindividual variability (X. copei ).
Variance partitioning
Results of the variation partitioning analyses showed that five species
(the same as above) had their δ 15N values
significantly influenced by individual size (Figure 6, NB: S. boaand A. carbo not considered in these analyses due to small depth
range). L. crocodilus had the highest proportion of variation inδ 15N values explained by size (25.7%),
followed by M. punctatum (24.7%), M. atlanticum (10.1%),S. beanii (13.2%) and A. risso (5.8%). Alternatively, inA. olfersii and X. copei , variations inδ 15N were significantly explained by depth, at
a proportion of 5.7% and 5.5% respectively.
Summary of relationships at specific and community
levels
According to the different relationships, diverse species patterns can
be described (Table III). First, two species (L. crocodilus andM. atlanticum ) showed an ontogenetic change both in their
vertical distribution and theirδ 15N values. The largest individuals of these
species were caught at greater depth and theirδ 15N values increased with individual size.
Four other species showed an ontogenetic change in theirδ 15N values but no change in their vertical
distribution (from trawling data): A. risso , S. beanii ,S. boa, and A. carbo . Among the species that did not show
any significant change in their trophic ecology (according toδ 15N values) with increasing size, differences
appeared in the dispersion of δ 15N values. Some
species such as N. kroyeri and L. macdonaldi had a
restricted range of δ 15N values whatever the
size of individuals (i.e. variation coefficients = 2.15 and 2.68,
respectively) whereas some species such as X. copei showed a high
dispersion of δ 15N values (CV = 6.57). Finally,
three species showed no relationships among the variables tested:N. kroyeri , L. macdonaldi , and S. koefoedi .
However, for L. macdonaldi , a particularly low size range was
sampled for isotopic analysis (= 3 cm between the minimum and the
maximum individual size). At the community level, an increase in
individual size with depth was observed, as well as an increase in
δ15N values with increasing size (Table III).