72
G. Duhamel
of K. anderssoni. The night depth LFD showed a uniformity in the mixture
of species (mainly myctophids) occurring at all the depths.
The results show a total or a partial size structure of the fish populations
in the investigated area, taking into account the known maximal size of the
species [15,21]. All the myctophid species with a complete LFD belong to
the polar subgroup, dominant in terms of specimens (70.8% of the total) in
the sample. This confirms the prevalence of the pattern and a probable
entire life cycle of the species off the islands. Incomplete LFDs are
observed for the myctophid species belonging to the temperate subgroup
which are probably at the southern limit of their distribution or are living
in greater depths in this region. Another explanation would be a functional
division of the habitat as suggested for E. carlsbergi by Zemsky and
Zozulya [22] who consider the SubAntarctic and Subtropical Frontal Zone
as reproduction areas and the Polar Frontal Zone as a feeding ground for a
part of the populations. Such conclusions could also apply to the nonmyctophid species: N coatsi, P. gracilis would relate to the polar
subgroup, while B. tenuis belongs to the temperate subgroup. S. boa, B.
elongata, A. risso, K. macrura, I australis, M. gelatinosum were not
adequately sampled to comment further. Finally the multimodal structure,
observed in some LFD (K. anderssoni, G. braueri, G. nicholsi, P. bolini,
E. subaspera), is probably related to the age structure of these fast length
growing species. Age determination from otoliths will be necessary to
confirm the hypothesis. The increase in the mean size with depth observed
in the case of L. squamifrons indicates a probable ontogenetic migration
from the surface to the deeper layer in postlarvae, before the near-bottom
settlement.
The fish results need now to be compared with the stomach contents and
the feeding strategy of the top predators. The present real time study will
allow us to advance in the knowledge of the relationships between
predators and prey organisms in this particular area of the Southern Ocean.
Acknowledgments
I wish to thank the IFR TP for the equipment used at sea and the opportunity to
participate in the cruise "IPEKER" of "La Curieuse". I appreciate the help of the
master D. Rochard, the crew and J.M. Le Petitcorps from TAAF. The results
would never have been obtained without them. I am indebted to C.A. Bost for the
collaboration during and after the cruise. I have appreciated the help of C. Ravier
and Y. Cherel during the laboratory work and the assistance of P. Pruvost in
finalizing the presentation. I am grateful to P.A. Hulley and referees for useful
comments to improve the manuscript.
G. Duhamel
of K. anderssoni. The night depth LFD showed a uniformity in the mixture
of species (mainly myctophids) occurring at all the depths.
The results show a total or a partial size structure of the fish populations
in the investigated area, taking into account the known maximal size of the
species [15,21]. All the myctophid species with a complete LFD belong to
the polar subgroup, dominant in terms of specimens (70.8% of the total) in
the sample. This confirms the prevalence of the pattern and a probable
entire life cycle of the species off the islands. Incomplete LFDs are
observed for the myctophid species belonging to the temperate subgroup
which are probably at the southern limit of their distribution or are living
in greater depths in this region. Another explanation would be a functional
division of the habitat as suggested for E. carlsbergi by Zemsky and
Zozulya [22] who consider the SubAntarctic and Subtropical Frontal Zone
as reproduction areas and the Polar Frontal Zone as a feeding ground for a
part of the populations. Such conclusions could also apply to the nonmyctophid species: N coatsi, P. gracilis would relate to the polar
subgroup, while B. tenuis belongs to the temperate subgroup. S. boa, B.
elongata, A. risso, K. macrura, I australis, M. gelatinosum were not
adequately sampled to comment further. Finally the multimodal structure,
observed in some LFD (K. anderssoni, G. braueri, G. nicholsi, P. bolini,
E. subaspera), is probably related to the age structure of these fast length
growing species. Age determination from otoliths will be necessary to
confirm the hypothesis. The increase in the mean size with depth observed
in the case of L. squamifrons indicates a probable ontogenetic migration
from the surface to the deeper layer in postlarvae, before the near-bottom
settlement.
The fish results need now to be compared with the stomach contents and
the feeding strategy of the top predators. The present real time study will
allow us to advance in the knowledge of the relationships between
predators and prey organisms in this particular area of the Southern Ocean.
Acknowledgments
I wish to thank the IFR TP for the equipment used at sea and the opportunity to
participate in the cruise "IPEKER" of "La Curieuse". I appreciate the help of the
master D. Rochard, the crew and J.M. Le Petitcorps from TAAF. The results
would never have been obtained without them. I am indebted to C.A. Bost for the
collaboration during and after the cruise. I have appreciated the help of C. Ravier
and Y. Cherel during the laboratory work and the assistance of P. Pruvost in
finalizing the presentation. I am grateful to P.A. Hulley and referees for useful
comments to improve the manuscript.