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When analyzed together, scales of A. gigas with different sizes, showed a trend that
was opposite that found for O. bicirrhosum.
Because scales from different locations showed distinct isotopic composition, it may be
possible to identify the origin of captured fish.
Assessing feeding history as recorded in tropical fish scalesTomas F. Domingues 1, M. C. Martinelli 2, L. A. Martinelli 3 and J. R. Ehleringer 1
1 - University of Utah, 2 - IPAM - Brazil, 3 - Universidade de São Paulo - CENA - Brazil
Introduction
Stable isotope analysis can be a useful technique providing
information about the life history of organisms, if the
organisms have a recording device to register events
throughout their life time. We evaluated the linear distance
along fish scales as a recorder of a fish’s feeding habits.
Scales are mainly composed of collagen with a long turnover
time, allowing fish scales to potentially be accurate indicators
of dietary inputs. Mineralization creates an ossified layer on
scales. The presence of calcium carbonates in this layer may
pose a difficulty for evaluating feeding history.
To test the hypothesis that scales register fish feeding history,
we evaluated scales from two large fish species occurring in
the Amazon Basin, a region that shows strong seasonality in
food resource availability.
The link between forest and fish
The interaction between fish and forest is modulated by the
annual flooding of the Amazon River. For most fish species
food availability changes with water level, since seasonally
flooded forests are a large portion of the aquatic system and
represent a very different ecosystem than the main river.
During high waters, fish disperse and colonize lateral lagoons,
which may became isolated from the main channel during low
waters.
Arapaima gigas and Osteoglossum bicirrhosum are both
large predators, with the former feeding on fish and the latter
on allochthonous insects.
Conclusions
Fish scales do record feeding history, with a suggestion of dietary change with
the size of the fish.
A. gigas showed signs of being influenced by the seasonality of the Amazon
Basin, perhaps reflecting fish that were once restricted to lakes now having access
to open river water.
The13C values of the calcified layer were not statistically different from that of the collagenous layer
indicating that carbonates, if present, do not alter the carbon isotopic signature of the scale.
However, 15N, C% and N% values were statistically different between ossified colagenous layers.
O. bicirrhosum showed a progressive depletion for both 13C and 15N as fish aged. This trend may be
explained by a progressive importance of allochthonous food to this species diet. The annual water
level fluctuation does not seem to influence the feeding behavior of this species.
No temporal trend was common to most A. gigas evaluated, and a high variation in
13C and 15N was observed. It was also found that shifts in isotopic composition were
frequent.
Individual preferences and exploration of different habitats may explain the high
variability among individuals while shifts in isotopic composition may be caused by
migration between lateral lagoons and the main channel.
Arapaima gigas
Methods
Samples were taken from the posterior portion of the scales,
following a straight line from its focus to its edge, representing
the chronological nature of scale growth.
A dual inlet Finnigan Mat IRMS coupled with a Carlo-Erba EA,
operating at continuous flow was used to determine carbon and
nitrogen isotope ratios and concentrations. Analytical precision
was ± 0.15 ‰.
Osteoglossum bicirrhosum