Aspects of the ecology of the mesopelagic fish Gonostoma elongatum (Gonostomatidae, Stomnformes) in the eastern Gulf of Mexico

Abstract

Gonostoma elongatum is an important mesopelagic fish found throughout the world at subtropical-tropical latitudes. This study examined its distribution, abundance and life history in the eastern Gulf of Mexico. The species was a strong diel migrator, found from 25 to 325 m at night and from 425 to 725 m during the day with small fish occurring shallorver than large fish. Numerical abundance and biomass were high, rivalling the dominant species of myctophids in the Gulf of Mexico. There were no obvious seasonal fluctuations in population abundance. G. elongatum is a protandric hermaphrodite whose population breeds throughout the year and females probably once a lifetime. Growth rate, based on 'daily' ring counts from otoliths, was linear (0.34 mm d-l) and the largest fish (225 mm Standard Length) was estimated to live less than 2 yr G. elongatum feeds primarily on crustaceans, with copepods and ostracods dominating in early juvenile stages ( 3 mm) in certain depth zones (150 to 200 m) at night. INTRODUCTION tivity and feeding chronology. Our data are compared with the findings of others on the nutrition of G. elonGonostoma elongatum, a member of the mesopelagic gatum (e.g. Clarke 1978, Gorelova 1981) and estimates fish family Gonostomatidae, is found throughout most of the predation impact of this species on its zooplankof the world ocean (Mukhacheva 1972) with its maxton food resources in the eastern Gulf are discussed. imum abundance in the circumglobal belt of tropicalsubtropical waters (Badcock 1984). It has been placed among the top 3 numerically dominant stomiiform METHODS species, Cyclothone excluded, in the northern and equatorial Atlantic (Backus et al. 1970, Backus & Gonostoma elongatum was collected during 11 Craddock 1982), the Gulf of Mexico (Hopkins & Lancruises between June 1975 and March 1987 within a 20 craft 1984) and the central Pacific (Clarke 1974). It is a mile radius of 27 N, 86 W in the eastern Gulf of highly fecund protandric hermaphrodite (Fisher 1983). Mexico. The first 3 cruises, on the RV 'Columbus IseWe report on its vertical distribution and abundance lin', were made during the summers of 3 successive (diel and seasonally), and aspects of its growth and years (June 1975, June 1976, September 1977). The 8 reproduction. remaining cruises were on the RV 'Suncoaster' from The present paper also describes the trophic ecology September 1984 to March 1987 (Table 1). of Gonostoma elongatunl including information on diet Three different opening-closing modified Tuckercomposition, changes in diet with ontogeny, prey selectype trawls (Hopkins & Baird 1975) were used: one @ Inter-Research/Printed in F. R. Germany 28 Mar. Ecol Prog. Ser. 49. 27-40, 1988 Table 1. Summary of sampling effort on 11 cruises conducted during the study 1975-1977 discrete depth tows