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The Vertical Distribution a N D Feeding Habits

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The Vertical Distribution a N D Feeding Habits CAlLLlET AND EBELING: FEEDING OF TWO VERTICALLY MIGRATING MESOPELAGIC FISHES CalCOFl Rep., Vol. 31,1990 THE VERTICAL DISTRIBUTION AND FEEDING HABITS OF TWO COMMON MIDWATER FISHES (LEUROGLOSSUSSTILBIUS AND STENOBRACHIUS LEUCOPSARUS) OFF SANTA BARBARA GREGOR M. CAILLIET ALFRED W. EBELING Moss Landing Marine Laboratories Department of Biological Sciences P. 0. Box 450 and Marine Science Institute Moss Landing, California 95039 University of California Santa Barbara, California 93106 ABSTRACT adaptaciones morfol6gicas que le permiten alimen- Leuvoglossus stilbius (Bathylagidae) was abundant tarse por succidn y predar sobre organismos pe- in the nearshore Santa Barbara Basin (SBB), but less quefios y poco activos. A diferencia, la especie S. so in the more offshore Santa Cruz Basin (SCB). leucopsarus est6 mejor adaptada a una alimentaci6n Stenohvachius leucopsarus (Myctophidae) was abun- activa, predando sobre presas mPs grandes y mis dant in both basins. L. stilbius is adapted morpho- escapadizas. L. stilbius se aliment6 principalmente logically to feed by suction and to eat smaller, less de noche en aguas superficiales de la SBB. Durante active organisms. S. leucopsarus is better adapted to todo el aiio el alimento consisti6 de apendicularias y feed by grasping, and to eat larger, faster, and more salpas, reflejando la persistente disponibilidad de es- elusive prey. In the SBB, L. stilbius fed, mostly at tos organismos a lo largo del ciclo anual. El regimen night in surface waters, on larvaceans and salps all alimenticio fue similar en la SCB, si bien la dieta year, reflecting the seasonally consistent abundance vari6 de acuerdo con la variaci6n estacional en la of these prey items. In the SCB, it fed less intensely, abundancia de las presas. No se observaron diferen- mostly at night in surface waters, and its diet varied cias cronol6gicas marcadas en 10s hibitos alimenti- with the seasonal abundance of its gelatinous prey. cios de S. leucopsavus. La dieta estuvo compuesta S. leucopsarus fed mainly on crustaceans, and it did principalmente por crusticeos, en especial de eu- not exhibit a distinct feeding chronology. It ate sim- faQsidos cuando istos eran mis abundantes. El ilar prey all year in both basins, but euphausiids alimento fue similar en ambas cuencas. Por consi- dominated the diet when they were most abundant. guiente, L. stilbius es una especie adaptada a1 am- Thus L. stilbius is well adapted to inshore, eutrophic biente costero eutr6fico donde puede alimentarse de midwater habitats, where it can easily eat abundant apendicularias y salpas en abundancia, y que debido larvaceans and salps, and may act as a trophic link a sus migraciones verticales difusivas es probable between the shallow gelatinous zooplankton and the que represente el eslabon trdfico entre el zooplanc- deep sea through its diffuse vertical migrations. ton gelatinosos de aguas poco profundas y las aguas Offshore, its primary food is less dense, only sea- oceinicas profundas. Lejos de la costa, su alimento sonally available, and restricted to surface waters. principal est5 menos densamente distribuido, y so- S. leucopsuvus is better adapted to eat the more var- lamente disponible estacionalmente y restringido a ied food resources offshore. Its cohesive pattern of las aguas superficiales. La especie L. leucopsartis esti vertical migrations takes it into the food-rich sur- mejor adaptada a un regimen alimenticio oceinico face waters at night, where it consumes crustacean mPs variado. Su patr6n de migraci6n vertical cohe- prey and trophically transports their calories to the sivo le permite predar en aguas superficiales ricas en deep sea. alimento (donde consume crusticeos) y transportar las calorias hacia las aguas profundas. RESUMEN La abundancia de Letivoglosstis stilbius (Bathylagi- INTRODUCTION dae) fue mPs alta en la cuenca de Santa Barbara (SBB) According to Lavenberg and Ebeling (1967) “the que en la cuenca de Santa Cruz (SCB), ubicada a diversity of the mesopelagic and bathypelagic faunas mayor distancia desde la costa. La abundancia de increases with vertical expansion of their habitats Stenobrachiiis leucopsanis (Myctophidae) fue similar offshore. ” This trend is evident when one compares en ambas cuencas. La especie L. stilbius presenta the fish faunas of two deep-sea basins off Santa Bar- bara, California (Ebeling et al. 1970a; Brown 1974). The Santa Barbara Basin (SBB), located inshore of ~~ ~~~ [Manuscript rece~vedFebruary 2,1990 ] the Channel Islands, is relatively shallow (600 m, 106 CAlLLlET AND EBELING: FEEDING OF TWO VERTICALLY MIGRATING MESOPELAGIC FISHES CalCOFl Rep., Vol. 31,1990 with a 425-tn sill), generally isolated froin other ba- lates the sizes of the inshore and offshore populations sins, and enriched by coastal runofc it has a rela- of these species? Of the four key factors listed by tively simple but abundant epipelagic and upper MacArthur and Connell (1966) that regulate popu- mesopelagic fish fauna. In contrast, the offshore lation sizes (reproduction, migration, mortality, aiid Santa Cruz Basin (SCB) is much deeper (>2000 m) food resources), we decided to evaluate the fourth. and is outside the immediate coastal influence, in We investigated the morphological adaptations, closer contact with the deeper oceanic environment; feeding habits, and vertical migration patterns of the it contains a more diverse but less abundant fish two fishes to determine what kinds of prey they fauna. Here, allochthonous species increase in rela- took, how they migrated vertically relative to prey tive abundance with large-scale seasonal changes in availability, how their use of available food might water-mass types, and bathypelagic species occur influence their relative success in the inshore and off- below 500 m. shore areas, and how they might affect the vertical Off California, evidence suggests that phyto- flux of organic material in the water column. plankton production and standing crop are highest inshore (Malone 1971), and the zooplankton diver- sity increases as zooplankton density decreases MATERIALS AND METHODS offshore (Longhurst 1967). The inshore SBB is re- Fishes were collected from 1964 through 1968 garded as highly productive (Emery 1960; Soutar during 33 regular cruises of the RIV Swan off Santa and Isaacs 1969; Sholkovitz and Gieskes 1971). Ebe- Barbara, California (cf Ebeling et ai. 1970a; Brown ling et al. (1970a) reported that catch volumes of 1974). All collections were made with a 1.8-in fishes were higher there, whereas volumes of inver- IKMT, which had a lining of 1-cm stretch mesh tebrate micronekton did not differ significantly be- netting, followed by a standard zooplankton net and tween basins. The variability in the offshore catch a cod-end sampler, divided into four chambers by was greatly influenced by seasonal invasions of electronically closed gates (Aron et al. 1964; Bour- bulky organisms like salps. If salps are eliminated beau et al. 1966). Samples of animals from particular from the analysis, invertebrate standing crop was depth intervals were thus separated by the sequen- also greatest in the SBB. tially closed gates. The trawl’s spreader bar con- The California smoothtongue, Leuvoglossus stil- tained electronic sensors to monitor depth aiid water bius, (Bathylagidae) and northern lampfish, Steno- temperature. A flowmeter measured sampling ef- brachills leucopsartis, (Myctophidae) are the dominant fort in meters trawled. The signals from all sensors fishes in the midwater community of animals that is and flowmeter were transmitted simultaneously especially abundant off southern California (Ahl- through the towing cable to shipboard recorders. strom 1969; Ebeling et al. 1970a, 1970b; Brown Collections were regularly made in the generally 1974). L. stilbius was ranked first, and made LIP 58% recognized major depth zones: (1) epipelagic, in the of all fishes sampled. Its catch rate averaged 6.1 surface wind-mixed layer about 0-200 in; (2) upper adults per kilometer flow through an 1.8-in Isaacs- mesopelagic, within the permanent thermocline, Kidd midwater trawl (IKMT) in the SBB. It ranked about 200-400 in; and (3) lower mesopelagic, in the only third, 16%, and 0.4 km-’ in the SCB. Like- dysphotic depths, below 400 in. Maximum trawl wise, S. leucopsalus ranked second, and had abun- depth varied between 500 aiid 1000 ni, depending dances and catch rates of33% and 5.4 km-’ inshore, on the area sampled. Only shallow and mid-depth and ranked fifth, and had values of 11% and 0.9 samples could be taken in SBB, which lacks a bathy- km-’ offshore. Farther offshore, the numbers of pelagic zone. The more typically oceanic and deeper both species dwindle, but S. leucopsanis is a bit more SCB has a bathypelagic zone that extends well be- abundant than L. rtilbiiis (Pearcy 1976; Willis and low the depth of the lower mesopelagic. Samples Pearcy 1980). Although the distributional centers of were taken throughout the year at night and during both fishes occur off California, L. stilbius ranges the day. A total of 205 stations yielded 631 collec- from Alaska to the Gulf of California, while S. leu- tions, 363 of which were from discrete-depth hauls copsavtis occurs all the way from the Bering Sea to whose vertical excursions were at least 50% within the tip ofBaja California (Miller and Lea 1972; Hart the 200-in depth intervals. An additional 40 samples 1973; Eschmeyer et al. 3 983). through broader depth intervals were used to esti- Ebeling et al. (1970b) theorized that “among me- mate seasonal abundance of potential prey, such as sopelagic fishes L. stilbitis and to a lesser extent salps and euphausiids. S. leucopsants may best exploit the rich inshore basins All fishes were preserved on board in 10% buff- of the borderland. ” What, then, differentially regu- ered Formalin and seawater, and subsequently 107 CAlLLlET AND EBELING: FEEDING OF TWO VERTICALLY MIGRATING MESOPELAGIC FISHES CalCOFl Rep., Vol. 31,1990 changed to 45% isopropyl alcohol ashore. Speci- significantly alter feeding habit results. However, mens of L. stilbius and S. leucopsavtis were routinely Lancraft and Robison (I 980) found simulated prey identified, counted, and measured, along with other items in 23.3% of the guts of S.
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