Age and Growth of Electrona Antarctica (Pisces: Myctophidae), the Dominant Mesopelagic ®Sh of the Southern Ocean

Marine Biology (1999) 133: 145±158 Ó Springer-Verlag 1999 T. M. Greely á J. V. Gartner Jr á J. J. Torres Age and growth of Electrona antarctica (Pisces: Myctophidae), the dominant mesopelagic ®sh of the Southern Ocean Received: 12 September 1997 / Accepted: 25 July 1998 Abstract Numerically and in biomass, the lantern®sh simple energy budget using the best information avail- Electrona antarctica is the dominant ®sh in the vast pe- able suggests that a surplus of energy is available to lagic region of the Southern Ocean bounded on the support the observed growth rates (0.05 to 0.07 mm north by the Antarctic Convergence and in the south by d)1). The results of the present study contrast markedly the Antarctic continental shelf. It is an important krill with previous estimates of an 8 to 11 yr maximum age predator, and in turn is important in the diets of ¯ighted for E. antarctica. These results provide important data and swimming seabirds. Further, it is the southernmost addressing the ecology and population dynamics of the and coldest-dwelling representative of the globally dis- pelagic Antarctic ecosystem. E. antarctica is the end- tributed ®sh family Myctophidae. The present study was member species in the continuum of vertically migrating undertaken to estimate the species' growth rate and myctophids that extend from the equator to the polar average life span, to incorporate the information in a circle. Its growth rate is consonant with that of all other basic energy budget, and to compare the growth of myctophid species examined using primary growth in- E. antarctica with more northerly confamilials. Fishes crements to determine age. The present study, in con- were aged using primary growth increments that were junction with earlier studies, suggests that growth rates resolved on sagittal otoliths using three sequential of mesopelagic species are far higher than previously techniques: thin-section grinding and polishing, etching, thought. and scanning electron microscopy (SEM). Based on increment width (0.8 to 1.2 lm), continuity, and previous studies on confamilials, the microincrements were as- sumed to be deposited on a daily basis. Montages of Introduction SEM photomicrographs were constructed for each sagitta to allow the daily rings to be counted over the entire The ®sh fauna within the Southern Ocean is character- life span of 31 individuals representing the entire size ized by a low diversity of species and a high level of range of the species. Results suggest a larval stage of 30 endemism. The most abundant oceanic ®shes in Ant- to 47 d and a maximum life span of 3.5 yr, with females arctic waters are the lantern®shes (family Myctophidae), growing faster than males in the last 1.5 yr of life and deepsea smelts (family Bathylagidae), barracudinas reaching a larger maximum size. Construction of a (family Paralepididae), and bristlemouths (family Go- nostomatidae; Andriashev 1965; Hempel 1985; Kock 1985). Taxa from these families account for >95% of Communicated by N.H. Marcus, Tallahassee the biomass of mesopelagic ®shes in the upper 1000 m of the Weddell-Scotia sea region (Lancraft et al. 1989). T.M. Greely (&) á J.J. Torres Three species of the genus Electrona are abundant in University of South Florida, Department of Marine Science, the Southern Ocean, E. antarctica (GuÈ nther, 1878), 140 7th Avenue South, E. carlsbergi (TaÊ ning, 1932), and E. rissoi (Cocco, 1829). St. Petersburg, Florida 33701, USA Of these, E. antarctica is the numerical dominant in FAX: +(0)813 553-3966 midwater trawl samples taken throughout the Southern e-mail: [email protected] Ocean (Rowedder 1979b; Hulley 1981; McGinnis 1982; J.V. Gartner Jr Linkowski 1987; Lancraft et al. 1989). It is considered to St. Petersburg Junior College, be the only lantern®sh endemic to Antarctic waters, as it Natural Sciences Department, 6605 5th Avenue North, is only found south of the Antarctic convergence (Hulley St. Petersburg, Florida 33710, USA 1981; McGinnis 1982). 146 Electrona antarctica is a strong diurnal vertical mi- family, the data will allow a better understanding of grator, with peak abundance at 0 to 300 m at night and growth in mesopelagic species in general. 650 to 920 m during the day. The diet of postmetamorphic E. antarctica (30 to 110 mm standard length, SL) consists primarily of copepods, ostracods, and eu- Materials and methods phausiids, including the krill, Euphausia superba; (Hop- kins and Torres 1989; Lancraft et al. 1991). Thus, the Sampling lantern®sh Electrona antarctica is signi®cant to the pe- Sampling was conducted in the Southern Scotia Sea (60°S; 40°W) lagic ecosystem as a major component of the ®sh biomass in the austral spring (1983) and in the northwest Weddell Sea (65°S; and as a dominant krill predator (Williams 1985; Kock 46°W) during the austral fall (1986), as part of the Antarctic Ma- 1987; Lancraft et al. 1989). In turn, as an important prey rine Ecosystem Research at the Ice Edge Zone (AMERIEZ) project (Fig. 1). Pelagic ®shes were sampled with an opening-closing item of seabirds in the open waters of the Antarctic re- Tucker trawl (9 m2 mouth opening) at depths between 0 and gion (Ainley et al. 1986), E. antarctica plays a pivotal role 1000 m in the open water near the marginal ice zone. Details of the in the transfer of energy from the macrozooplankton trawling procedures are given in Lancraft et al. (1989). Aboard (e.g. krill, Euphausia superba) to higher trophic levels ship, the myctophid Electrona antarctica was identi®ed, and measured to the nearest millimeter standard length. The sex of indi- (e.g. Antarctic seabirds and mammals). viduals was recorded when possible. Sagittal otoliths were removed There is a notable lack of information available on from each ®sh with forceps, and were stored dry on micropaleon- the growth rates and life spans of Electrona antarctica tological slides. and for Antarctic mesopelagic ®shes in general (Link- owski 1987), particularly compared to the information on tropical, temperate, and subarctic species. Accurate Preparation of otoliths age determinations provide basic life-history informa- Sagittal otoliths were prepared for analysis using three sequential tion and are imperative for describing a species' popu- microstructural techniques: grinding and polishing, thin-sectioning, lation dynamics. Two age-related studies were and scanning electron microscopy (SEM). These techniques have attempted for E. antarctica (Rowedder 1979a; Linkow- proven useful in discerning microstructural growth increments in ski 1987), but they yielded inconclusive results, further the sagittal otoliths of several species (see review by Campana and Neilson 1985). All three techniques were applied consecutively to emphasizing the clear need for this fundamental infor- the same otolith, from the grind- and-polish technique to the more mation. elaborate SEM application. Most aging studies on high-latitude ®sh species are Due to their small size (<2 mm) and the protocol to be fol- based on ages determined from annual rhythmic depo- lowed (e.g. thin-sectioning), sagittae were embedded in a low-vis- cosity epoxy resin following the hard-formula recipe of Spurr sitions (seasonal ``rings'') in otoliths as time marks (1969). After embedding, the whole sagittae were ground by hand (Blacker 1974; Williams and Bedford 1974). The otoliths to the mid-sagittal plane with water and wet/dry sandpaper (400, of Antarctic ®shes do not appear to contain interpreta- 600, 1500, and 2000 grit), and then polished using a polishing cloth ble annual deposition-patterns, perhaps because of the and a 0.05 l gamma alumina slurry. All samples were sonicated lack of strong periodicity in Antarctic hydrographic between successive grinding and polishing to avoid cross-contam- ination of grits and to reduce surface scratches. Abrading revealed conditions (Radtke 1984), particularly in sea-surface three distinct regions within each sagitta. Using the terminology temperature. Similar complications have been docu- and de®nitions of Gartner (1991a), the regions were the premeta- mented for tropical species, where seasonally induced morphic or larval zone (LZ), perinuclear or postlarval zone (PLZ), marks are irregular, indistinct or absent. Otoliths from and postmetamorphic zone (PMZ) (Fig. 2). These regions are de- scribed in ``Results-Internal otolith morphology''. Antarctic ®sh are usually small and dicult to analyze Initially, the external morphologies of sagittae were examined using conventional methods. for analysis of whole-otolith growth (n 117). Two radius mea- Application of microstructural techniques to otoliths surements were recorded using an ocular micrometer. The radius of of Antarctic ®shes has proven successful for the few the entire otolith (total radius; TR) was measured from the central core (nucleus, NU) to the outer edge along the posterior axis (40´ species analyzed. The focus has been almost exclusively magni®cation) and the larval zone radius (LZR) from the central on the age and growth of the dominant, coastal-dwell- core (NU) to the last clear continuous increment (metamorphic ing, demersal species of the suborder Notothenioidei check) in this region (250´ magni®cation). All micrometer mea- (Wohlschlag 1961; North et al. 1980; Townsend 1980; surements were converted to millimeter units. The radii of the Radtke and Targett 1984; Radtke 1987; Radtke and sagittae were then regressed against standard length. Sixty of the original 117 sagittae were further abraded until a Hourigan 1990). These taxa are abundant shallow-water plane through the core was visible. Primary growth increments dwellers that can be maintained in captivity for valida- within the larval zone could then be enumerated under oil using a tion experiments. compound microscope coupled to a high-resolution video camera The focus of the present study was to resolve primary and monitor system. All counts were initiated from a well-de®ned medial increment (i.e.

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