The Pelagic Upper Layers Ichthyocenoses of the East Atlantic Tropical Zone
Alexander
R. BOLTACHEV
synopsis of the Ph.D. dissertation
Actuality of research. The World Ocean High Waters ichthyofauna investigations began comparatively not long ago. Suffice it to say that overwhelming majority of ocean fishes have been described first for the last 100 - 150 years. Those investigations were considerably accelerated in the early seventies in connection with increased interest for the problem of search and exploitation of new fishing resources. As a result a significant in its volume material was stored, on the grounds of analyses of which, with attracting hydrobiologic and oceanologic knowledge, the ocean ichthyology was formed as an independent part of science treating of fishes (Parin, 1987). For a comparatively short period a number of revisions was carried out which embraced majority of families; the zoogeography, some peculiarities of ecology and biology of separate species were studied as well. All that allowed to proceed to summarizing ichthyologic reports on some World Ocean areas, in particular, on the North and South-East Atlantic, however, we haven't available such ones concerning the tropical zone of this ocean. Simultaneously with faunistic investigations ecosystem ones develop very actively at present. The Open Ocean is considered a single biotope, that is simultaneosly a hierarchical system of biotopes of different spatial scale (Beklemishev, 1969; Beklemishev et al., 1977).
Within the Ocean cenoses the most important element are the fishes which form ichthyocenoses in their composition (Parin, 1968, 1987). Definition of fishes role in the structure and functioning of cenoses is not only of self-independent interest for ichthyologists and ecologists, but it is also of practical importance since this knowledge is of a great significance when drawing-up recommendation for reasonable exploitation of bio-resources of Open Waters. On the whole, the knowledge on ocean fishes in particular and ichthyocenoses in the whole is evidently insufficient, that's why investigations in these directions undoubtedly are of a certain theoretical and practical interest - all this has determined the expediency and actuality of this work.
The aim of research. Studying the taxonomic composition, some elements and spatial position of ichthyocenoses in pelagic upper layers of the East Atiantic Tropical zone. The purposes of research. Studying the species composition,peculiarities of horizontal spreading and vertical distribution of fishes within the upper 450m.Layer of the East Atlantic Tropical zone.2.On the base of original and literary data each of species found here should be given zoogeographical and biotopic characteristics and some peculiarities of separate species ecology is to be considered as well. 3. Choosing geographical ichthyocenoses, examination of their composition, some elements of structure and spatical position. 4. Studuing the composition and bathymetric position of ichthyocenoses in upper layers of water thickness in dependance upon some abiotic factors.
Scientific novelty. For the first time the taxonomic and biotopic composition of fishes in Open Waters of pelagic upper layers of the East Atlantic Tropical zone have been studied and a summarizing ichthyofaunic report has been given on the base of that. New finding spots were shown for 59 species, for 25 species changes of belonging their areas to a certain type were offered, 10 species were attributed to other ecologic groups but not to those as it had been considered before. The Sast-Equatorial, Angolese and Bengela geographic ichthyocenoses were isolated, their composition and partical position of their bounds were studied. The composition and vertical position of ichthyocenoces in water thickness were examined and the day-time ecotone episopelagic ichthyocenosis was fixed. Some additions on ichthyologic material the scheme of bathymetric division; according to them there is a transitional zone between epi- and mesopelagic zones timed to subsurface water masses. First an active schooling form of epimesopelagic fishes was found out and the conclusion was drawn, that interzonal mesopelagic fishes do not make their vertical migrations every day.
Practical significance. The results of investigations can be used for drawing up summarizing reports on the ichthyofauna of examined region. Schooling species of small fishes perspective for fishing have been described. Obtained data on fishes spreading can be used for working out recommendations on protection and reasonable exploitation of ocean pelagic zone resources.
Approbation of the thesis. The principle results of the dissertation were reported at the Scientific-Practical Conference on Fishing Forecasting Methods (Murmansk, 1983); at All-Union Conferences The Nature Environment and Biological Sea and Ocean Resources (Leningrad, 1984); Study and Rational Utilization of the Open Ocean Resources (Moscow, 1984); the 3-th Regional Conference The Complex Study of the Atlantic Ocean Nature (Kaliningrad, 1985) and at other sittings and discussions in ATLANTNIRO and INBYUM as well. Publications. 13 works were published and 2 are being in the press on the Dissertation subject.
The volume of the work. The Thesis consists of the Preface, 5 chapters, conclusions, the list of cited literature and an appendix. The principle text is stated on 99 pages of type-written text with four tables and 45 pictures. The list of literature includes 167 names, 88 of them are in foreign Languages. The appendix volume is 14 papes.
Content of the Work
Chapter 1. Material and Methods.
Material for the dissertation was collected during expeditional trips on the SRV "Professor Vodyanitsky" (Jan.- Apr. 1981; Dec.1981 -Jan. 1982; Jul.-Sept. 1987); SRV "Vozrozhdeniye" (May - Aug. 1980); SRV "Novoukrainka" (May - Sept. 1981); SRV "Ichthyandr" (Apr.- June 1982); SRV "Titanit" (Jan.- March 1984). The main research area is Located in the East Atlantic Open Waters beyond 200-mile economic zones of coastal countries between 10 North - 40 South Eastward from 24 West. For fishing midwater unlocked nets were used, their vertical opening was from 6 up to 20-50 metres. In all 332 ichthyologic samples of 332 catches were treated: 292 of them were processed within the main research area and 40 - beyond the pointed bounds, the results of the latters served for more precise definition of zoogeoraphic spreading of species found out in the area. The specific belonging of more than 304 thousand specimen was defined exactly. With the purpose of studying peculiarities of dayly vertical distribution of fishes collecting the material was carried out during day-time (77 catches), twilight (11) and night-time (204). At nights the towing were carried out at levels about 150 m from the surface, and up to 450 m at the day-time. 27 towing were carried out in by-bottom variation at the depth range 200-720 m near underwater mountains, one of those towings was carried out at the depth of 1195 m. Hydroacoustic observations were made by means of devices "Sargan-K", "Priboy-101", "Elac Super Lodar" at working frequence of radiated signal of 20 kc and "Simrad-EK"-38 kc. Befor trawlings the water temperature was taken in the lager 0-20 m or hydrologic observation were carried out down to the depth of 500 m.
Each catch was weighed, taxonomic composition was examined as well as the ratio of species, mass measurement of fishes was carried out when the standard length of body (SL) was measured with the accuracy to 1 mm. For counting the number of fin rays and vertebrae the fry was painted with alizarin by M.Yakubovsky's method (1970). The identification correctness of some complicated for systematization fishes was checked up by leading specialists of 10 RAN and InBYUM.
Chapter 2. Oceanology Conditions Characteristics
Since the object of investigation are the fishes which inhabit the upper and middle layers of pelagic zone, the hydrologic structure of surfase, sub-surface and intermediate waters was examinet. On the base of analysing the structure and dynamics of waters (using expedition and literature data) three main sections could be determined: East-equatorial, Angolese and Bengela ones, first of them by the water mass structure, according to Beklemishev K.V.(1968), being oceanic, and two others far-neritic.
Chapter 3. The Ichtyofauna
The list of species found out within the bounds of explored area includes 65 families, 141 genera and 223 reliably identified species of 132 genera.
Prevailing position in variety of collected ichthyologic material belongs to the family of Myctophidae which makes up 33.6 per cent of the whole ichthyofauna, 24 per cent from which belong to the genus of Diaphus, 11 per cent-to Lampanyctus; the family of Melanostomiidae from 5.2 per cent, Sternoptychdae-4.7 per cent, Paralepididae-4.3 per cent, Gonostomatidae-3.9 per cent and Astronesthidae-3.9 per cent. Such specific ratio is typical for the World Ocean Open waters ichthyofauna.
Chapter 4. Geographic Spreading of Fishes.
For 59 species forming a fourth part of the faunistic composition new spots of their finds were pointed out.
Spreading of macroplanktonic fishes is lealized by passive transferring with the streams and the areas bases of this group, as a rule, are timed to exclusive rotations; nektonic species realize active horizontal migrations within the bounds of area keeping to spots with optimal inhabitance conditions (Parin, 1968; 1979; 1987; 1988; Beklemishev at al., 1977). Within the collected ichthyologic material 182 species relate to macroplanktonic and 41 species - to nectonic fishes what makes 86.6 per cent and 18.4 per cent accordingly.
Macroplanktonic fishes. The system of waters cirkulations of explored area have a very complicated structure and this fact defines the variety of its zoogeographic elements. On the whole, the prevailing species are those with oceanic type of areas the centres of which are situated within the main rotations. They make more than 62 per cent of all macroplanktonic fishes, the zoogeagraphy of which is studied rather thoroughly.
On the base of obtained original and literary data relating the following species there appeared suppositions about changes or specification of their areas belonging to a certain type. The following species were attributed to the Wide-Tropical area group: Bathophilus pawneei, Echiostoma barbatum, Flagestomias boureei, Thysanactis dentex, only in the Atlantic Ocean Lepidophanes guentheri; spreads as widely; to the Equatorial-Central group: Photostomias guernei, Lestidiops jayakari, Bentosema suborbitale, Hygophum reinhardtii, Symbolophorus rufinus, Diaphus bertelseni, Stylepforus chordatus. Apparently Bonaparta pedaliota should be atributed to the Equatorial-Nort-Central group Bathophihus digitatus, Aristostomias grimaldii, A. polydactilus, A. tittmani, Opistoproctus grimaldii, Notoscopelus caudispinosus, Evermannella balbo; to the Equatorial-Peripheral group; Diaphus meadi - to the Equatorial-South-Peripheral group. Obviously Myctophum affine, Lestidium atlanticum as well as Lestrolepis intermedia and Pontosudis advena spreaded in the Western part of the Ocean, should be considered Equatorial species.
The remote-neritic species areas centres (the species from 17 per cent) are timed to neutral regions rotations. All 3 categories of remote-neritic species defined in the Attantic Ocean by K.V. Beklemishev (1969) are represented within the collected material. Margretia obtusirostra is a typical representative of widely-remote-neritic category. In the North and Tropical-neutral regions of the East Atlantic Gonostoma denudatum and Diaphus holti are spreaded. The tropic remote-neritic species are notable for their specific variety. The most extensive transequatorial area among them is peculiar to two Atlantic endemics: Astronesthes richardsoni and Diaphus dumerillii, the first being more numerous in the West tropical neutral region, the second - in the Eastern one. Lampadena urophaos atlantica inhabits mainly in the Eastern remote-neritic zone and Diaphus garmany - in the Western one. The group of East-Atlantic endemics uniting 15 species is of a high interest. Some of those species, e. g. Diaphus vanhoeffeni, penetrate up to 30 W, others spread less widely, mainly in the remote-neritic zone, the third remote-neritic category is presented by 7 transitional species. The Indo-Pacific group includes 7 species, 6 of which are oceanic, and 1 is remote-neritic.
Nektonic fishes. There prevail oceanic species, forming 55 per cent of all nectonic species. These are the widely-tropical (4 species) and tropical (16); also there were found bisubtropic Scomberesox saurus and North-Atlantic endemic Ptericombus brama. The adults of other nektonic fishes populate continental and talass zone (45 per cent).
The Geographic position of ichthyocenoses. On the basis of analysing the areas, particuliarities of spatial distribution of macroplanktonic fishes within their connection with the structure of water masses of the explored area three ichthyocenoses were defined: East-Equatorial, Angola and Bengela ones. Within the composition of the East-Equatorial ichthyocenosis oceanic species prevail, and their areas are directly connected with equatorial waters, their quantity is 81 per cent of all macroplanktonic fishes, inhabiting here (table 1.)
Table 1. Zoogeographic Composition of Macroplanktonic Fishes of the East-Equatorial (1), Angoles (2) and Bengela (3) Ichthyocenoses
| Type of Area | 1 | 2 | 3 | In all within |
| Wide- Tropical | 17/49 | 12/20 | 11/16 | 17/51 |
| Equatorial- Central | 13/27 | 11/13 | 7/9 | 13/27 |
| Equtorial- Peripheral | 13/21 | 14/15 | 9/10 | 17/25 |
| Equatorial | 8/14 | 8/8 | 5/7 | 8/14 |
| Central | 4/5 | 7/8 | 7/7 | 9/10 |
| Peripheral | 1/2 | 4/4 | 7/9 | 9/12 |
| Remote- Neritic | 11/18 | 13/18 | 9/13 | 16/23 |
| Ttransitional | 7/7 | 7/7 | ||
| Indo- Pacific | 1/1 | 6/6 | 7/7 | |
| Total: | 68/137 | 69/86 | 68/84 | 103/176 |
Note: 1) in numerator - number of the "main"
families, dominator - the whole number of defined species
(explanations in the text);
2) widely-remote-neritic species are given together with
remote-neritic.
The west bounds of the spot, evidently is located somewhat more westward of 24 West. 11 endemics of 15 East-tropical ones were defined on 24 West, and the most mass ones - Diaphus vanhoeffeni, Hygophum macrochir were found in 75 per cent of catches. Nevertheless the total decrease in number and biomass of this two species in comparison with the East part of the spot was observed. Simultaneously on 24 W. first or more often than in other areas, we met west- oceanic species (about 10), e.g. Diaphus garmani.
The Angola area is characterized by total decreasing of the quantity of fishes areas of which are connected with equatorial waters (65.1 per cent) and the simultaneous increasing of central (9.3 per cent), peripheral (4.6 per cent) and remote-neritic (20.9 per cent) species. Equatorial species with the waters of South-Subtrade-wind countercurrent are spreading to the South and they are spreaded relatively evenly up to 15 South. In the system of transversal circulation of the South trade-wind current the penetration of central species is directed to the North-East. Thus the most plentiful Diplospinus multistriatus was available almost in every catch, when fishing approximately south wards of 9 South and West ward of 9 East.
In the North-West direction up to 13 South 4 peripheral species spread widely and Scopelopsis multipunctatus was registered in the position 8 23 S. 4 54 W. Representatives of remote-neritic group inhabit everywhere and in the Angolese "cupola" region a sharp domination of one species - Diaphus vanhoeffeni was noted. It's very difficult to find out the position of border between East-Equatorial and Angolese ichthyocenoses, but approximately it coincides with the North periphery of the Angolese "cupola" between 7 - 9 South. In the Bengela current divergention zone approximately between 15 - 18 S. there is a sharp decrease in the specific varieties of equatorial species and East-Tropical endemics. The transitional zone between the Angolese and Bengela ichthyocenoses is timed to this area, but some specific individuals relating to these zoogeoraphical elements can be fonnd up to 20 S: Lampanyctus nobilis, Gonichthys cocco, Diaphus vanhoeffeni. Simultaneously there increases the number of peripheral species and southern transitional species appear e. g. Lampichthys procerus was found on 16 38 S. and southward of 18 S. all 7 species of this group are invariably available in all catches.
The Bengela ichthyocenosis is characterized by decreasing the portion of oceanic species connected with tropical rotations up to 50 per cent. The number of central ones is changed insignificantly but the peripheral species number increases up to 9 per cent (10.8 per cent). The percentage of remote-neritic and transitional species is about 31. The ratio of zoogeographic elements is typical for remote-neritic communities. In the western periphery of the Bengela current there is an increase of finding frequency and number of central and equatorial- central species, what was used when determining the ichthyocenosis west bounds position. Southward of 34 S. Indo-Pacific species can be found and as a whole this area up to 40 S. is the transition zone between the South subtropic and the Ahullas ichthyocenoses.
The comparison of zoogeographic structure of determined ichthyocenoses was carried out in the area of 103 species of "main" families: Gonostomatidae (9 species), Sternoptychidae (11), Photichthyidae (4), Myctophidae (77), Gempylidae (2), spreading of which is better studied and all they within their majority are not very rare (table 1.) The coefficient of faunistic alikeness between the East-equatorial and Angolese ichthyocenoses is 0.79 and it is somewhat lower between the Angolese and Bengela ichthyocenoses - 0.69, and 0.59 between the first and the third ones. Thus inspite of insignificant number of common for ichthyocenoses species, as well as interchange and mixing the species, there are certain differences among ichthyocenoses, what serves as the confirmation of their existance. The East- equatorical ichthyocenosis is a typical oceanic one, but the Angolese and Bengela ones are remote-neritic.
We have noted that at some underwater mountains we caught (sometimes in large numbers) rather rare or usually not available in adjoining oceanic waters species. E.g. near one of underwater mountains of the Vavilov Ridge the West-oceanic Myctophum obtusirostre makes 86 per cent of all fishes quantity in the catches. This allows to suppose the existence of depended populations of macroplankton species timed to local closed circulations near underwater heights. It is not excepted that these circulations may be considered to be ecotonic biotops of specific ichthyocenoses.
Chapter 5. Vertical Disribution of Fishes.
At present a significant number of ocean fauna vertical zone schemes is offered, the critical analysis of the most important of them as well as the substantiation of their personal hypotheses are reflected in the works by M.E.Vinogradov (1968, 1977), N.B. Parin (1968, 1979), K.V. Beklemishev (1969). It is universally recognized that there exist transitional zones (ecotones) among biotopes contacting on the vertical. As a rule a significant faunistic variety and increasing of the population density is typical for contacting layers, and this fact allows to record them with hydroacoustic devices, e.g. Sound Scattering Layers (SSL). There is epipelagic ichthyocenosis in the upper and middle pelagic layers and it is timed to the upper isothermal layer, the epimesopelagic one - to the layer of the "leap", the mesopelagic one to the main thermocline.
Biotopic composition. The main quantity of the researched species included representatives of various oceanic ichthyocenoses - about 94 per cent of the ichthyofauna and the rest 6 per cent were the fishes inhabiting costal biotopes. The group of holoepipelagic fishes was represented within our catches incompletely mainly by alevins (jevenals), what was connected with the fishing tactics. This group made 5.8 per cent of the whole species composition.
The fishes inhabiting the upper and middle pelagic layers more or less constantly were picked up comparatively not long ago into the epimesopelagic group (Parin, 1988). Vinciguerria nimbaria and Diplospinus multistriatus can be included into its composition, their juvenile, alevin and adult individuals can be found within the upper 500-metre layer independingly upon the day time. Their part in the catches is not large: 4.5 per cent. The leading role in the biotopic composition of Open Waters belongs to the representatives of mesopelagic ichthyocenosis - 164 species (73.6 per cent), 156 of which are macroplanktonic fishes and the rest are nektonic ones. In our opinion the following species which formerly were considered epi- and epimesopelagic should be attributed to the mesopelagic ones: Scombrolabrax heterolepis, Brama dussumieri, Gempylus serpens, Nealotus tripes, Cubiceps capensis, C. pauciradiatus, Psenes arafurensis, Tetrgonurus atlanticus. The bathypelagic ichthyocenosis is represented by 5 species of alevins. The bottom-pelagic grouping of fishes includes 15 species (6.7 per cent), but the Gonostoma denudatum, Polyipnus polli, Diaphus dumerillii are usual over continental and insular slopes as well as in the oceanic waters. They may be considered the elements of two ichthyocenoses: mesobenthopelagic and mesopelagic ones. About 7 per cent of all found out species are "chance" elements of the fauna, mainly they are representatives of the neritic zone.
Vertical migrations and peculiarities of bathymetric distribution of fishes. Perhaps of all found holoepipelagic fishes only the representatives of Exocoetidae and Hemirhamidae families inhabit directly the surface layer, but the rest can be found in subsurface and some of them in transitional watermasses. It's evident that the upper bounds of the "leap" layer are not strict bounds for inhabatance of the most of epipelagic fishes. The group of epimesopelagic fishes is not homogeneous ecologically, e.g. the representatives of Regalecidae, Trachipteridae and Lophotidae families constantly make populations in the thickness of waters from the surface down to 500 metres, some down to 1000 m (Regalecus glesne) and do not make dayly vertical migrations. As well, regular circade migrations are not typical for Nomeus gronovii, the rest nektonic species actively shift within the limits of these depths. For Vinciguerria nimbaria and Diplospinus multistriatus two forms of bathymetric distribution and conduct are typical:
1) the interzonal one with everynight ascent of the most part of population up to surface and subsurface waters and every day descent down to 300-400 meters depth, where they join the fauna composition of the Day Time Sound Scattering Layer (DTSSL), in this case an individual reaction to irritants is characteristic for fishes;
2) the active schooling one with the day-time forming schools within the depth range from 10 to 420 m-s, in the case of danger and food searching all the individuals simultaneously shift in vertical and horisontal directions i.e. they demonstrate a sort of collective (school) behaviour. To mesopelagic animals vertical migrations are the characteristic ecologis peculiarity. To overwhelming majority of the species the onthogenetic migrations are peculiar i.e. division of different-age groups ihabitance depths, in this case, as a rule, the alevins develop in the surface layers (Vinogradov, 1968; Parin et al., 1977).
Depending upon the 24-hours bathymetric distribution of the adult part of the population the mesopelagic hydrobionts are subdivided into two main ecologic groups: interzonal migrants and stationary (non-migrating or weakly-migrating) species. Within the whole number of mesopelagic and mesobenthopelagic species (they were 179) 113 (63.1 per cent) may be attributed to interzonal migrants. Some of them cross the upper limit of the thermocline (picnocline) during the migration period, these are so-called nictoepipelagic or by-surface species, the others concentrate themselves mainly within the "leap" layer - these are "layer" species (Parin, 1968; 1988; Becker, 1983). The latter take part in the night by-surface SSL. In the upper isothermal layer there were found adult individuals of about 30 species, usually they are known as nictoepipelagic species, as well as typical "layer" ones: Diplophos taenia, Benthosema suborbitale, Diaphus vanhoeffeni, Diogenichthys atlanticus, Chauliodus schmidti, Ceratoscopelus warmingii, Lampanyctus alatus, Lepidophanes guenheri, Notolychnus valdiviae, Notoscopelus resplendens, Bregmaceros atlanticus.
The fauna of "layer" species is the most various. The are more than 90 species, which are representatives of the families:
Gonostomatidae(3), Sternoptychidae(1), Photichthyidae(1), Astronesthidae(9), Chauliodontidae(2), Stomiidae(3), Melanostomiidae(10), Malacosteidae(4), Microstomidae(2), Myctophidae(51), Bregmacerotidae(1), Caproidae(1), Bramidae(1), Scombrolabracidae(1), Gempylidae(1), Nomeidae(2).
In the day time the most of interzonal fishes move down to 50-1000(1500)m depth and only some of them concentrate themselves at less depths, where they join the composition of multy-specific assotiations, which are considered the main DTSSL. They will be spoken about below. But it is possible that night assents up to the surface layers have place not every 24 hours. According to the data of the catches by means of locking fish gears a part of this species population remains at large depths at nights (Karnella, 1987 and many other). According to the data of visual observation from a submersible apparatus of Pakhorukov N.P. and Telegin O.N. the Myctophidae can be found at day-time as well as at night down to 1500 m depth. Some of them don't demonstrate any motive activity. The analysis of vertical migration velocity, which is about 1.7- 2.8 m/min by our observation, may be an indirect confirmation of that fact. To move down to 500-1000 m depth at minimum velocity they will spend about 5-10 hour, at maximum speed - 3-6 hours correspondingly and it is for the case if their migration velocity would be invariable with the change of the depth. In this case the interzonal species simply wouldn't have enough time for completing the migration cycle during 24 hours. The supposition about non-everyday periodicity of migrations strengthens the position of McLaren's hypothesis on energetic gain of migrants.
The stationary mesopelagic fishes vital cycles (66 species) are more closely associated with deep waters. These are representatives of the following families:
Gonostomatidae(5), Sternoptychidae(10), Photichthyidae(2), Stomiidae(1), Idiacanthidae(1), Microstomidae(1), Opistoproctidae(4), Notosudidae(2), Myctophidae(9), Scopelarchidae(1), Evermannellidae(1), Paralepididae(9), Giganturidae(1), Nemichthyidae(3), Macrouridae(2), Melanonidae(1), Stylephoridae(1), Diretmidae(2), Melamphaidae(3), Anoplogasteridae(1), Epigonidae(1), Chiasmodontidae(2), Gempylidae(1), Nomeidae(1), Tetragonuridae(1). The stationary species in their turn are subdivided into upper and lowermesopelagic ones, the bounds between their inhabitance are at 400-500 (450) m depth (Parin at al., 1977).
The inhabitants of the upper mesopelagic layer are available directly within our collections, as they are included into the composition of DTSSL (22 species). The peculiarities of bathymetric distribution and vertical migrations of the main part of lower-mesopelagic fishes can be learned from literature data as well as indirectly from the results of our catches for lack of adult individuals within them.
Ichtyocenoses of pelagic layers. We have undertaken an attempt to analyse the collected material from the standpoint of the views on the vertical zonalty of pelagic layer which were stated in the beginning of the chapter.
Epipelagic ichthyocenosis. In the upper quasihomogeneous layer, we have found 82 species (table 2) 13.4 per cent of which are epi- and epimesopelagic fishes. The group of meroepipelagic fishes is the most numerous (82.9 per cent). As a rule these are larvae and alevins of deep water or rarer - of coastal species the earlier phases of development of which are directly connected with epipelagic layer. It should be accentuated that we didn't determine the specific belonging of the most of larvae. As it was stated above adult individuals of about 30 interzonal species populate in surface waters at night-time.
The night epimesopelagic ichthyocenosis. At night (table 2) the fauna of epimesopelagic ichthyocenosis is the most various. The bathymetric position of epimesopelagic ichthyocenosis corresponds to the depth of the main night SSL. As usual the greatest density of SSL is timed to the "leap" layer but in some cases it can be recorded in the upper quasihomogeneous layer as well, e. g. in waters lowering zones. The Moon light influences on the laying depth significantly what was noted by some authors as well (Vinogradov, 1968; Parin, 1988). Simultaneously with the Moon rising deepening of echo- scattering layers at several dozens meters was observed as well as its exfoliation and reflected signal intensity diminution. This factor is determining in many cases where the gradient layer is not experessed, e. g. in the Bengela area at serene moonlit nights the densest SSL is recorded at depths of 80- 130m, but at moonless nights at 30-90 m. It should be noted that the specific variety of transition ichthyocenosis decreases at the full Moon (Ovacharov and others, 1990).
Table 2.The Biotopic Composition of Epipelagic (EP), Night (NEMP) And Day-time (DEMP) Epimesopelagic Ichthyocenoces
|
EP | NEMP | DEMP | Specios in all |
|---|---|---|---|---|
| Epipelagic | 7 | 8/4 | 4 | 13 |
| Epimesopelagic | 4 | 9/4 | 6 | 10 |
| Mesopelagic | 68 | 143/22 | 68 | 169 |
| Bathypelagic | 2/4 | 1 | 5 | |
| Coastal-Continental | 3 | 11/6 | 6 | 16 |
| Total | 82 | 173/40 | 85 | 213 |
- Note: For epipelagic ichthyocenosis the species number isadduced for night period only, for day epimesopelagic one - for day period and for night epimesopelagic one - for the night in numerator, for day-time in denomenator.
At the day-time a very weak SSL is recorded within the "leap" layer; cinophores and tunicates, rarer - crustacea and fish larvae are parts of its composition. In total catches within the upper 120 m layer and in layer- by- layer trawlings above the horizon of upper bounds of the main DTSSL (250 - 450m), we found adult individuals of epi- and epimosopelagic fishes only, other ichthyocenoces were represented exclusively by their larvae and by alevins sometimes. Thus, obvious is the fact of absence of this transitional ichthyocenosis in the "leap" Layer as well as within the depths range from the surface down to the upper bound of the main DTSSL. Day-time epimesopelagic ichthyocenosis. As day-time a comparatively dense echo- scattering layer is recorded within the depth range of 250- 450 and more often - 300- 400m. Approximately within the limits of these depths of the explored area there is a transition layer between surface and intermediate waters. The usual thickness of the main DTSSL is from 30 to 100m. The catches results proved that it is formed by hydrobionts of several taxonomic groups the leading position among which belongs to fishes. The vertical position of the main DTSSL directly depends upon the Sun height, cloudness and water transparency. 85 species were fixed within the composition of the main DTSSL ichthyofauna (table 2). About 85 per cent of them are the mesopelagic fishes, mainly they are representatives of the families of Gonostomatidae, Sternoptychidae, Photichthyidae, Myctophidae. The constant component of the fauna consists of upper- mesopelagic (22 species), some interzonal (3) and epimesopelagic (2) fishes, the rest can be found incidentally. The findings of the alevins of holoepipelagia and csenoepipelagic fishes within the main DTSSL are of a great interest. All this is an illustration of mixing the epi- and mesopelagic faunas within the main DTSSL.
A distinctive peculiarity of the most constant species within the main DTSSL is the combination of morpho-ecologic features of by-surface and deep-water fishes. Many of them have a dark spinal part and light (silvery colorations on each side) what is typical for the upper pelagic layer inhabitants and simultaneously, they have developed luminescent organs as the holomesopelagic fishes have. And what is more some of them are notable for mimicry: the dark coloration of sides at night, which Argyropelecus hemigimnus and Valenciennellus tripunctulatus have is changed for a light coloration at day-time and in this case the darkening of photophores is observed (Baird, 1971; Howell, Krueger, 1988). As it is known when there is no natural illumination, the dark (black, dark-red) coloration of body, which has a very low reflectivity, serves a good camouflage against the most usual bioluminescent light of predators - the blue one, and when there is the ulumination the coloration is silvery, which have a high reflectivity (Denton, 1990). The change of dark-brown night coloration for a lighter on at day-time is observed on D. vanhoeffeni and Lobianchia dofleini. A very characteristic feature of stationary fishes of the main DTSSL is lateraly- thickened from of their bodies, what gives the greatest reflextion effect within the horisontal plane at day-time, and the dorsal plate and ventral keel (e. g. Argyropecus) makes them imperceptible within the vertical plane. Some species have the eyes of a telescopic from, what is a consequence of their adaptation to the conditions of twilight zone vision. By their conduct the constant species differ from interzonal ones as well. At day-time they are very active and they feed intensively.
In our opinion the agregate of all fishes of the main DTSSL composition is the border ichthyocenosis - a part of epimesopelagic ecotone community between sub-surface and intermediate water mass. This ichthiocenosis is formed of specific stationary and inhabiting adjacent biotopes species. If to classify the ichthyocenoses of the water thicknes according to their biotopic composition we shall find out that at day-time the epipelagic one is located between the surface and 250- 450m depths what corresponds to the upper bounds of the ecotonic ichthyosenosis, the lower border of the latter, in its turn, corresponds to the beginning of the mesopelagic ichthyocenosis. Thus the bathymetric position of the bounds between epi- , mesopelagic ichthyocenoses changes in accordance with the photoperiod from several dozens (at night) to hundreds (at day-time) meters, and this fact brings in an element of uncertainty when finding out the depths of their laying. On the base of stated above it follows that there are the night and day-time epimesopelagic ichthyocenoses differing in their bathymetric positions, compositions and structures. The position of the first one more or less coincides with the upper transition zone between the surface and sub-surface waters, and the position of the latter one - between the sub-surface and intermediate waters. Thus, the whole sub-surface layer is an ecotonic epimesopelagic biotope. The depth of location of day and night ichthyocenoses, composition and structure of the first of them significantly depend upon the Moon and the Sun illumination correspondingly. The following scheme of the upper pelagic layers division would be proposed: the epipelagic layer, timed to surface waters, epimesopelagic one which is timed to sub-surface ones and mesopelagic layer, timed to the intermediate ones specific ecotonic epimesopelagic species which combine morpho-ecologic features and peculiarities of the vertical distribution and conduct of the epi- and mesopelagic fishes are V. nimbaria and D. multistriatus.
Conclusion
1. For the first time the ichthyofauna of the oceanic upper pelagic layers of the Tropical Atlantic Easter part has been revised. The species list includes 223 reliably identified species, 141 genera of 65 families.
2. New spots of findings for 59 species were determined, for 25 species changes or more precise definitions of their areas belonging to a certain type were proposed. 3. It is settled that among macroplanktonic fishes the species of oceanic area type predominate, they make 62 per cent of all fishes, the part of remote-neritic is 17 per cent and more than half of the latter ones are the East-tropical endemics.
4. On the base of the areas analyses as well as analyses of peculiarities of macroplanktonic fishes spatial distribution in connection with the structure of water masses within the explored area three ichthyocenoses were determined: the East Equatorial, Angolese and Bengela ones, the transition zones between the first and the second spread from 7 S up to 9 S, and between the second and the third one - from 15 up to 18 S.L. In the South- East part of the Exloration area between 34 and 40 S the zone of the Atlantic and Pacific Oceans ichthyofaunas inter-mixing has been defined. The East-Equatorial ichthyocenoses is oceanic, the rest two ichthyocenoses are remote-neritic. It is possible that there exist some specific ichthyocenoses in local rotations near under-water heights.
5. According to their biotic belonging mesopelagic fishes take the leading position - about 74 per cent.
6. The most of epipelagic species populate within surface and sub-surface water-masses. In the tropical waters within the upper quasihomogeneous layer adult indviduals of about 30 interzonal mesopelagic species were found out. The interzonal species come to the surface layers not every day.
7. There have been determined the night and day epimesopelagic ichthyocenoses which differ in their bathymetric position, composition and structure. The position of the first one is timed to the upper transitional zone between the surface and syb-surface waters, the second one's between the sub-surface and intermediate waters. Thus, the whole subsurface layer is the ecotonic epimesopelagic biotope. The depth of location of the night and day ichthyocenoses, the composition and structure of the first of them, significantly depend upon the Moon and Sun illumination correspondingly.
8. Vinciguerria nimbaria and Diplospinus multistriatus are the specific ecotonic epimesopelagic species which combine morpho-ecologic features and peculiarities of vertical distribution and conduct of the epi- and mesopelagic fishes.
9. The following scheme of the pelagic upper layers division is proposed: the epipelagic layer, which embraces the surfase water masses, epimesopelagic one, which embraces the sub-surface waters, the mesopelagic, which embraces the intermediate waters.
Copyright © A.Boltachev