Not to be cited without prior reference to the author ICES/ASC CM 2002/M12 Theme Session on Oceanography and Ecology of Seamounts – Indications of Unique Ecosystems Composition of ichthyoplankton and horizontal and vertical distribution of fish larvae in the Great Meteor Seamount area in September 1998 by Walter Nellen & Silke Ruseler Inst. of Hydrobiology and Fisheries Science Hamburg University Abstract 24 stations above and near the Great Meteor Seamount, central north Atlantic, have been sampled at eight depth strata in late summer 1998 by a modified MOCNESS. Some 18.800 fish larvae were collected and specimens were identified to species and coarser systematic levels, respectively. Taxa typical of the high sea remote from coastal areas dominated the fish larvae assembly. But species which normally are found on the shelf or at the slope occurred as well in the plankton samples, mainly above and fairly close to the 300m deep seamount, obviously keeping away from the oceanic region several thousand meters deep. Eighteen of a total of more than 150 identified fish larvae taxa belonged to the neritic province. One of these species was the third most abundant larvae found so far during this investigation, namely Chlorophthalmus agassizii. Concentration and horizontal and vertical distribution of selected species are discussed as well as migration behaviour in dependence of day and night situation and sea bottom depth. It is considered whether this seamount may be regarded as an isolated, just 1465km² large shallow water area settled permanently by a coastal fish community. The question is treated why this may be so, i.e. which kind of non biotic and biotic factors could be responsible for such a situation. 1 Introduction At 30°00’N and 28°30’W the deep sea character typical for this area of the Atlantic Ocean is considerably disturbed by the Great Meteor Seamount. It is an isolated elevation rising from more than 4000m depth to less than 300m underneath the surface (Fig. 1). Conspicuous features of this seamount are on the one hand a elliptical plateau of 54 km length and 31 km width forming a shallow water area of 1465 km² within the 400m isobathic line and on the other hand very steep slopes. The mean inclination of the latter is about 13° in parts <20° and at the upper part of the slope the inclination is up to 45° (Ulrich, 1971). Already in the late 60ies the question arose whether seamounts are settled by a peculiar community and whether they may be regarded as extremely isolated, more or less self-regulating marine ecosystems on their own. In 1967 and 1970 the Meteor Seamount and its surrounding were subject of several geological, physical, and biological investigations aiming, among other things, at the composition and abundance of fish larvae taxa in February 1970 (Nellen, 1973). A corresponding research program at the Meteor Seamount was carried out almost thirty years later, in September 1998 (Pfannkuche et al. 2000), allowing improved research methods and comparing inspections of former results as well. The following hypotheses were proposed and tried to be proofed though the results of the present analysis are still to a certain extend preliminary and first of all descriptive. Actually, the data base my finally turn out as too small for far reaching statistical testing. 1. The Meteor Seamount is a marine biotope inhabited by a pronounced unique ichthyofauna 2. For the larvae of certain species the epipelagic zone above the seamount’s plateau is a retention area 3. Such larvae species do not make extended vertical migrations 4. One advantage of this behaviour is the avoidance of predators settling the plateau’s sea bottom 5. Ichthyoplankton species living in the vast oceanic surrounding off the seamount perform extended vertical day/night migrations down to water depths below the sea bottom depth of the plateau 6. Such fish larvae may get drifted above the plateau at night when they stay in the surface layer. At day time the may be the pray of benthos organisms settling at the seamount 7. Only relatively few fish species were able to settle the seamount as a niche area, but those which could are abundant there Material and Methods. Between September 1st and 20th 1998 ichthyoplankton was sampled from R.V. METEOR during SEAMEC (Seamount Ecology) Survey M42-3 at 24 stations in the Meteor Seamount area. 12 stations were at shallow water positions, mean sea bottom depth 370m, 9 were at deep water positions, mean bottom depth 3280m, and the positions of 3 stations were above the slope, mean water depth 1330m (Fig. 2). Plankton was sampled with a modified MOCNESS, width of opening rack 1m², netting 335 µm mesh aperture (Nellen et al.1996 and Fig. 3). Oblique step hauls were done from 290m water depth to the surface to allow discrete sampling of seven 2 water layers of defined depths at each station (Fig. 4). The water volume filtered in the respective depth layers was measured by an electrical flow meter when the gear was towed at a ship’s cruising speed of 3 knots while the hauling speed was 1m/s. The plankton was fixed in 4% formalin/seawater solution immediately when a haul was on deck. In the laboratory total plankton displacement volume of the 148 samples obtained was determined and fish larvae were sorted from the entire sample after the formalin had been washed out. The fish larvae were stored later on in “sorting solution” (0,5% Propylen Phenoxetol, 5% Propandiol, 94,5% H20) and then identified to the lowest taxonomic level possible so far. This work was partly done at the U.S. National Marine Fisheries Service, South West Fisheries Science Center, La Jolla , California with help of Dr. G. Moser and his staff. Some specimens were sent to and identified by Dr. W. Richards, South East Fisheries Science Centre, Miami, Florida. Literature used for the taxonomic classification was mainly Fahay (1983), Moser (1984 and1996), Moser & Watson (2001), and Richards (2001) Results. Larval Fish Identification: Altogether some 18 900 specimens of larval fish were found in the samples. 150 taxa were identified. Classification of specimens was successful down to different taxonomic levels: 56 to species (2 of these unknown but distinct species), 53 to genera, 34 to families, and 7 to higher taxonomic levels (Table 1).In all the material included fish larvae of 53 families. More specimens will probably be identified to species when further effort is put into the classification work. For the central issue of this paper, however, this is not deciding. It is likely that the 150 taxa listed in Table 1 correspond closely to the number of species which were actually present in the area in September ’98. All abundant taxa and those which are of specific ecological interest have been identified so far. Description of Diversity: The ichthyoplankton was strongly dominated by larvae of. Gonostomatidae (4136 specimens = 22% of all larvae), Photichthydae (4093 specimens = 21%), the suborder Stomeoidei (390 specimens = 2%), Paralepididae (606 specimens = 3%), and Myctophidae (6047 specimens = 32%). Which genera and species, respectively, were most abundant in the area is shown by Fig. 5a. These seventeen taxa below the family level were reprented with at least 0,9% of all specimens found the rest of 137 taxa was less abundant. One of the very abundant taxa below the family level was Chlorophthalmus agassizii which larvae were found to be the third most abundant ones in September 1998 (6% of all specimens). Samples from the shallow water and slope stations contained slightly more species on average than samples from oceanic stations (Fig. 5b). Taxa of which more than 50 larvae specimens were found occurred at 19 to 24 (80 to 100%) of all stations Some 15 350 specimens or 80% of all larvae belonged to this category which means that the dominating taxa were quite evenly distributed in the research area. Ecological groups. The fish larvae having been caught in the Meteor Seamount area in September 1998 can be placed into three different ecological groups: oceanic (83% of the taxa), slope bound (5% of the tax), and neritic (9% of the taxa) (Fig.6 a, Tab. 1). This finding corresponds closely to the results gained in the same area in 3 February1970 (Fig.6b) when at least 73 different taxa could be identified (Nellen 1973). Regarding the species of adult fish collected in 1998 (Uiblein et al., 1999, Pusch, this theme session) the respective ratio is different with only 62% oceanic taxa (55 species), 14% slope bound taxa (12 species) and 24% neritic ones (21 species, Fig.6c). When the total number of collected larvae specimens which ecological character could be classified is considered, even 90% (N=16242)were oceanic and only 1% (N=215) and 9% (N=1575) slope bound and neritic, respectively in September 1998; whereas in February 1970 92,2% (N=12 398) of some 14 100 fish larvae specimens were of the oceanic type and 7,2% (N=977) were shelf and just 0,5% (N=70) slope bound species (Figs: 6d and e). Horizontal distribution of plankton biomass and of abundance of specific taxa. When looking at the quantities of plankton biomass and total number of fish larvae specimens as well as at the numbers of the more abundant taxa underneath a unit of water surface the influence of the seamount on the distribution seems to be not very conspicuous (Figs. 7-1 to 7-8). In each case positive catches resulted from any of the station groups, but quantities were heterogeneously distributed. For plankton biomass and fish larvae in general the highest concentrations were found outside of the seamount plateau whereas the two relatively abundant neritic species, Chlorophthalmus agassizii and Aulopus filamentosus (Whitehead et al., 1984), occurred mainly at stations on or near to the seamount shallows as did Trachurus picturatus larvae in February 1970 which then made up the main neritic element (Fig.