1 ICES CM 2002/M:28 Theme Session on Oceanography and Ecology of Seamounts Indications of Unique Ecosystems The role of Zenopsis spp. as a predator in seamount and shelf habitats Heike Zidowitz, Heino O. Fock, Hein v. Westernhagen Alfred Wegener Institute for Polar and Marine Research, PO Box 12 01 61, D- 27515 Bremerhaven, Germany [tel + 49 471 4831 1382, fax +49 471 4831 1425, email: initial first [email protected]] Abstract The genus Zenopsis Gill 1862 consists of three species. Zenopsis conchifer occurs in the Atlantic and Indian Oceans whereas Zenopsis nebulosus has a wide distribution in the (Indo-) Pacific Ocean. Zenopsis oblongus, described in 1989, is closely related to Z. nebulosus and only known from the Nazca Ridge in the SE Pacific. Feeding studies are reviewed for all three species. At the Great Meteor Seamount (GMR) (NE Atlantic), Z. conchifer shows a shift in prey selectivity with ontogenetic development. Smaller specimens (<46 cm TL) preyed on constituents of the sound scattering layer (SSL) (myctophids, stomiids). Diet of larger specimens (>52 cm) predominantly consisted of bentho-pelagic Macroramphosus spp., which was the most abundant fish species on that seamount. A similar ontogenetic shift was observed for Z. conchifer on the Namibian shelf. However, prey for the larger specimens consisted mainly of the pelagic species Trachurus trachurus and Synagrops microlepis. In the Pacific, Z. nebulosus (oblongus) obtains a similar trophic position with regard to migrating components of the SSL. However, though being also very abundant at the Nazca-Ridge seamounts, no Macroramphosus spp. were eaten. Large specimens of Z. nebulosus were found to prey on bentho-pelagic rockfishes (Sebastidae). Zenopsis spp. appears to be an off- bottom pelagic predator with preference for mesopelagic food components in all areas considered. It is suggested that larger specimens abandon the off-bottom pelagic feeding mode and that body size thus determines the capabilities of Zenopsis spp. to prey on bentho- pelagic species. 2 Keywords: Zenopsis conchifer, Zenopsis nebulosus, Zenopsis oblongus, feeding ecology, sound scattering layer, Abbreviations: GMR – Great Meteor Seamount (follow Wilson and Kaufmann 1987), SSL – sound scattering layer Introduction: The genus Zenopsis spp. Gill 1862 consists of three species, Zenopsis conchifer, Zenopsis nebulosus and Zenopsis oblongus. These species are very similar in body plan with laterally flattened bodies with an upwardly pointing face and a silvery coloration (Swaby & Potts 1999). Z. conchifer occurs in the Atlantic and Indian Oceans (fig.1). In the eastern Atlantic it inhabits shelf areas from Ireland to South Africa, most abundant at 20° N (north-west Africa) (Maurin & Quéro 1982). The distribution in north-western European areas expanded since the 1960ies reaching Ireland in the 1980ies (Swaby & Potts 1999). Besides the shelf habitats it is also found around Islands (Canary islands) and on seamounts far off the continental rises e.g. at the Great Meteor Seamount (GMR). In the western part of the Atlantic its distribution ranges from Nova Scotia to North Carolina (Scott & Scott 1988) where it gets more abundant. It is also known from Brazil (Haimovici et al. 1994) to Argentina (Quigley & Flannery 1995). In the Indian Ocean Z. conchifer is found off the SW coast of India and off southern Africa from Walvis Bay to Kenya (Smith & Heemstra 1986) up to Somalia and in Indonesia (Froese & Pauly. Eds. 2002. FishBase). Z. conchifer reaches a total length of 80 cm (Smith & Heemstra 1986) and a body weight up to 3.2 kg (Robins & Ray 1986). Specimens of ca. 60 cm are estimated to be 12-14 years old (this paper), larger specimens probably get older than 20 years. Z. nebulosus is known in the Indo-Pacific region, from Japan, Northwest shelf of Australia to Broken Bay in New South Wales, New Zealand (Kailola et al. 1993), and elsewhere in the region (Philippines (Anon 2001)) (fig.2). In the Eastern Pacific it is known off central and southern California, USA (Eschmeyer et al. 1983) and from several seamounts in the western Pacific (e.g. Hancock Seamount, Kammu Seamount (Hawaiian Ridge); Jumeau Seamount (Richer de Forges 2000), Stylaster Seamount (Norfolk Ridge) (Froese & Pauly. Eds. 2002. FishBase) Specimens caught at seamounts of the Nazca Ridge (SE Pacific) have been 3 investigated by Parin et al. 1988. An analysis of otolith increments revealed an age of 13 years at a standard length of 46 cm. Different from Z. chonchifer, Z. nebulosus reaches only a size up to 70 cm in total length and a maximum weight of 3.0 kg (Williams 1990), so that it can be estimated that the species reaches an age of more than 20 years. These investigations on the ecology of Z. nebulosus of the Nazca Ridge (Parin et al. 1988) are more likely to refer to Z. oblongus. Z. oblongus was first described in 1989 and is very closely related to Z. nebulosus, but differs by a lower body and larger number of osseous scutella above the anal fin (Parin 1989). It is only known from the Nazca Ridge in the Eastern Pacific Ocean as a probable endemic species of the area (Parin et al. 1997) (fig.3). Z. conchifer was caught in depths ranging from 50 – 730 m (Saldanha 1968) but mainly occurs on the upper slope at 200 – 400 m (Quigley & Flannery 1995, Quéro 1998). It is frequently encountered in coastal waters (Quéro et al. 1990) and found in midwater or near the bottom (Swaby & Potts 1999) and it is probable that it occurs in schools (Berry 1978, Whitehead et al. 1986). Z. conchifer is regarded as a bentho-pelagic species of the deep-water (Froese & Pauly. Eds. 2002. FishBase), a mesopelagic species (Scott & Scott 1988) or bathypelagic species (Quéro & Pariente 1977). Z. nebulosus is considered as a bathydemersal deep-water species with occurrences in depths ranging from 30 - 800 m (Froese & Pauly. Eds. 2002. FishBase). Parin et al. (1997) consider Z. oblongus as a bentho-pelagic species in a wide sense inhabiting depths between 180 – 330 m. They also indicate the species as an off-bottom pelagic species, which can swim far away from the bottom, rising in midwater during diel vertical migrations. Compared to fisheries of shelf areas, deepwater fisheries show a shift in families of fishes which are commercially exploited. On the continental shelves primary families of exploited fishes are Gadidae, clupeoids, salmonids, scombrids and Pleuronectidae whereas deepwater fisheries are based on entirely different orders, such as Beryciformes, Zeiformes and Scorpaeniformes (Koslow et al. 2000). Differences at this taxonomic level indicate fundamental shifts in body plan and ecological strategy as well as in evolutionary lineage (Koslow et al. 2000). Many deepwater species aggregate on seamounts and form a distinct guild based on common features of their body plan, proximate composition, physiology and metabolism, ecology and life history (Koslow 1996, 1997). They tend to be robust and deep- 4 bodied in order to manoeuvre in the strong currents characteristic of this environment (Koslow et al. 2000). These fishes generally do not migrate vertically, but depend on the influx of meso- and bathypelagic organisms past the seamount and on intercepting mesopelagic migrators on their downward migration (Isaac and Schwartzlose, 1965; Genin et al. 1988; Koslow 1997). Some predators developed an expectant hunting strategy along plateau margins and an increased habitat dependent resource utilisation rate at locations of sound scattering layer interception (Fock et al. 2002). To explain living conditions for seamount populations in often impoverished nutritional conditions in the ambient oceanic regions, the sound scattering layer-interception hypothesis (Isaacs and Schwartzlose 1965) has been developed. It implies a primarily pelagic food utilisation for bentho-pelagic fishes, increased habitat dependent utilisation rates at locations of interception with the sound scattering layer, diel changes in utilisation rates due to availability of prey and sufficient resource partitioning among species in order to avoid competitive exclusion (Fock et al. 2002b). It was suggested that it represents a large enough prey source to maintain populations at seamounts (Hesthagen 1970, Rogers 1994, Parin et al. 1997). A very important reason for the success of the deepwater families is the far K-selected end of the life-history spectrum (Koslow et al. 2000). It is characterised by longevity, slow growth and delayed maturity. With these features the species fill a gap in the distribution of teleost life-history patterns (Roff 1984). Because in deepwater habitats the recruitment appears to be episodic and e.g. orange roughy and Sebastes spp. undergo extended periods (decade or more) of very low recruitment to the adult population (Leaman and Beamish 1984), Murphy 1968 and Stearns 1976 hypothesised an evolutionary link between longevity and recruitment. The adaptations on the life-history level could constitute the population’s ability to withstand extended periods of poor recruitment (Koslow et al. 2000). Episodic recruitment can also be assumed for commercially not exploited genera like Zenopsis, Antigonia, Capros. Morids (Moridae), cusk-eels (Brotulidae) and hakes (Merlucciidae) are robust-bodied Gadiformes and active predators (Koslow et al. 2000). Hakes form a small but widely distributed family (Koslow et al. 2000). Species of Merluccius are voracious predators inhabiting the continental shelf and upper slope (Froese & Pauly. Eds. 2002. FishBase) and are often dominant piscivores over upper portions of the continental slope and typically migrate vertically into the upper waters at night to feed (Bulman and Blaber 1986). Their 5 productivity is thereby linked directly to the near-surface food web (Koslow et al. 2000) which makes them less adjusted to seamount habitats. Fock et al. (2002) showed that the Gadiformes contributed an extremely low share to overall catch at the Great Meteor seamount although they dominate on the shallower shelf of the NE Atlantic. Materials and methods Sampling Sampling was carried out at GMR during R/V Meteor cruise M 42/3 in September 1998.