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 50 mm SL) and euphausiids in the larger sizes. Diet composition shows little apparent seasonal variation Size selectivity in feeding occurs, with G. elongatum preferentially ingesting medium to large zooplankton, a trend which becomes stronger with age. Taxonomic selectivity occurs as well in that this species preferentially feeds on conchoecid ostracods and copepods Pleuromamma spp. There is an obvious selection against non-crustacean prey. A cyclic feeding pattern was observed with most active feeding occurring in the 25 to 250 m zone at night. While G. elongatum has relatively little predation impact on one of its principal foods (Pleuromamma spp.) over the entire zone of occurrence of this prey genus, predation is substantial on large Pleuromamma 3 mm) in certain depth zones (150 to 200 m) at night.