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Feeding Ecology of Four Hatchetfishes (Sternoptychidae) in the Eastern Gulf of Mexico

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Feeding Ecology of Four Hatchetfishes (Sternoptychidae) in the Eastern Gulf of Mexico BULLETIN OF MARINE SCIENCE, 36(2): 260-277, 1985 FEEDING ECOLOGY OF FOUR HATCHETFISHES (STERNOPTYCHIDAE) IN THE EASTERN GULF OF MEXICO Thomas L. Hopkins and Ronald C. Baird ABSTRACT Vertical distribution and trophic ecology of hatchet fishes were investigated in the eastern Gulf of Mexico. The four principal species, Argyropelecus aculeatus. A. hemigymnus, Ster- noptyx diaphana and S. pseudobscura, ranged in abundance from 21-53 x 103 km-2 in the upper 1,000 m. There is strong evidence for time-space and food resource partitioning among these species. Depth of habitat and diet characteristics are reflected in cryptic adaptations and functional morphology of the two genera. A. aculeatus appeared to feed in the epipelagic zone « 200 m) early at night. Ostracods and copepods were the most important {biomass} food for smaller size classes, and pteropods, euphausiids and fish for larger individuals. A. hemigymnus apparently foraged in late after- noon in the 300-500-m zone. Ostracods and copepods were the principal food of all size classes. Cyclic feeding was not evident in either species of Sternoptyx. S. diaphana. which occurred primarily at 500-800 m, fed largely on copepods, ostracods and amphipods as juveniles and on amphipods and euphausiids at maturer sizes. S. pseudobscura which occurred mostly below 800 m, ingested primarily copepods, polychaetes and euphausiids as juveniles and took proportionately more amphipods and fish as adults. Non-random food choice was apparent, the Argyropelecus species selectively feeding on ostracods and S. diaphana on ostracods and amphipods. Much of the food of S. pseudobscura. inexplicably, was epipelagic in origin. Diet, depth and morphological characteristics of these hatchetfish species support the case for reduction of intraspecific competition through evo- lution. Knowledge of the feeding strategies of organisms important to the trophic hierarchy is central to our understanding of oceanic ecosystems. Through accu- mulation of information of the trophodynamics of key species we can ultimately construct a picture of how pelagic ecosystems function. Among the dominant micronektonic zooplanktivores in the ocean are midwater fishes and their impact on zooplankton populations must be considered in any model of oceanic ecosys- tem dynamics. At tropical-subtropical latitudes the mesopelagic fish community is characterized by high diversity with a hundred or more species often found in the upper 1,500 m (Gibbs and Roper, 1970; Badcock, 1970; Rass, 1971). Most of these species are zooplanktivorous (Legand and Rivaton, 1969; Merrett and Roe, 1974; Hopkins and Baird, 1977; Clarke, 1978; 1980; Gorelova, 1980, and many others) and it is of considerable theoretical interest to elucidate the under- lying factors regulating the coexistence of species in these complex assemblages. In the eastern Gulf of Mexico there are four relatively common species of hatchetfishes (Sternoptychidae) which together constitute an important compo- nent ofthe mesopelagic fish fauna (Maynard et aI., 1975, and the authors' unpubl. data). The four species have pan-oceanic distributions between 40"N-40oS and all are zooplanktivores (Merrett and Roe, 1974; Repelin, 1972; Hopkins and Baird, 1975). It would be predicted that these related taxa would tend to evolve life strategies directed towards minimization of competitive interactions and opti- mization of partitioning of available food resources in time and space. The vertical distributions and diets of the four species, Argyropelecus aculeatus, A. hemigym- 260 HOPKlNSANDBAIRD:HATCHETFISHFEEDINGECOLOGY 261 nus, Sternoptyx diaphana and S. pseudobscura, are examined in light of this concept. Hydrographic Setting. - The sampling that constituted the basis of this research was conducted around 27°N, 86°W in the eastern Gulf of Mexico between 1970 and 1977 during the months of May through October. During these months the eastern Gulf is dominated by the flow of the Subtropical Undercurrent which is an anticyclonic gyre of Caribbean water (Leipper, 1970; Nowlin, 1971; Maul, 1977; Molinari and Mayer, 1980). Known as the Loop Current, it flows into the eastern Gulf through the Yucatan Channel and exits through the Florida Straits. In some years the edge of the loop nearly reaches the Mississippi delta. The principal sampling locale lies east of the Loop Current axis and the water typically found at 27°N, 86°W is boundary or Transition water. This water can be identified by the depth of the 22°C isotherm which lies between 50 and 150 m; in the Loop Current it occurs between 150 and 200 m (Nowlin, 1971; Jones, 1973). The two bodies of water are biologically similar in that they are oligotrophic and have similar species composition (EI-Sayed, 1972; Michel and Foyo, 1976; Hopkins, 1982). Transition water, however, appears to be somewhat more pro- ductive (Jones, 1973; Hopkins, 1982). Temperature profiles at 27°N, 86°W during the summer show surface water temperatures to be 28-30°C, with the mixed layer extending to 30-50 m. The portion of the thermocline with the greatest slope extends from the bottom of the mixed layer to approximately 150 m, at which depth the temperatures are 15- 18°C. At 500 m, and the deepest horizon sampled, 1,000 m, temperatures are 8- 9°C and 4-5°C, respectively. Our data (unpubl.) show Transition water to be well oxygenated at all depths, with the minimum, 2.7-2.9 ml O2 per liter, occurring between 400 and 500 m. METHODS Information on the vertical distribution of hatchetfishes presented here is restricted to collections from around 27°N, 86°W (Table 1). Individuals taken at other locations in the eastern Gulf, however, were included in diet analysis. Closing Tucker trawls having either 1.8 x 1.8-m or 1.8 x 3.6-m mouth dimensions, a 4-mm mesh (1.1-cm stretched) trawl body and either a 0.3-, 0.5- or l.O-mm mesh codend plankton net were used in sampling. The trawls were opened and closed with a messenger operated release system. The volume of water filtered was estimated using dial-type meters which were modified so that they recorded only when the trawls were open and fishing. Filtration efficiency was assumed to be 100%. Trawl depth was monitored through wire angle measurements or with a conducting cable-depth transducer system. Towing accuracy using the depth transducers was ± 10 m at depths shallower than 200 m and ± 20 m below this depth. Depth distributions of the four hatchetfish species were determined from horizontal discrete depth tows made with the transducer system on three RV COLUMBUSISEUNcruises between 1975 and 1977. The abundances of the four hatchetfish species were estimated from 19 oblique sweeps of the upper 1,000 m made on RV BELLOWScruise VIII in 1981. These collections were also made using a depth transducer. Trawling speed was approximately two knots and the angle of the trawl mouth, estimated from tows just beneath the surface, varied from 30-45° depending on weight loading of the trawl. Mouth angle data were used in adjusting calculations for determining volumes filtered. Zooplankton was sampled concurrently using collapsible I62-ltm mesh plankton nets suspended in the mouth of the trawl. These were either 44 x 44 cm or 66 x 66 cm in mouth dimensions. Row through the plankton nets was also estimated with mechanical dial-type flowmeters. Further details on the construction and operation of the trawl-plankton net system are reported in Hopkins et al. (1973) and Hopkins and Baird (1975; 1981). Zooplankton and micronekton collections were preserved in 5-10% v/v borax buffered formalin. Fishes were subsequently transferred to 50-70% isopropyl alcohol. All stomach analyses and mor- phometric measurements were made using alcohol preserved specimens. In the laboratory, hatchetfishes were sorted from collections, identified, then measured to the nearest I mm (standard length, SL). For diet analysis the entire digestive tract was removed. The stomach was defined as the muscular, darkly pigmented, anterior portion of the digestive tract extending from 262 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.2, 1985 Table 1. Hatchetfish collection data No. tows with hatch- A. A. hem;· S. S. pseudob· Cruise Date Location etfishes aculealUS gymnus diaphana SCUTa D-I 7/1971 27"00'N, 86"00'W 1 M-II 3/1972 25°50'N,85°30'W 2 2 8 B-1 8/1972 27"00'N,86"00'W 9 2 2 26 M-III 8/1973 27"00'N, 86000'W 4 2 2 I 27°36'N, 88°40'W 5 7 12 20 28°28'N, 88°56'W 5 6 20 48 29°19'N, 87"01 'W 3 1 1 3 B-II 8/1973 24°30'N, 84°IO'W 2 I 2 24°38'N,85°IO'W 1 I B-III 8/1974 27"00'N, 86"00'W 10 23 31 28 5 C-I 6/1975 27"00'N, 86000'W 10 70 2 I C-II 6/1976 27"00'N,86"00'W 37 20 50 15 2 27006'N,85°16'W 2 4 B-IV 5/1977 27"09'N, 85002'W 1 I 27"09'N, 85006'W 6 2 2 1 5 27"06'N,85°16'W 2 4 27"04'N, 85°40'W 2 8 10 1 C-III 9/1977 27"00'N, 86000'W 31 7 8 21 93 B-V 6/1978 27"00'N, 86"00'W 1 1 B-VI 6/1980 27"00'N,86"00'W 10 7 13 68 8 B-VIII 9/1981 27000'N, 86"00'W 19 20 50 51 43 Totals 163 174 209 299 164 the esophageal opening to the origin of the thin-wall, slightly pigmented intestine. Separate records were kept on stomachs and intestines.
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