Observations on surface-shoaling Cape hake off South West Africa

Cram, Dominic L.; Schülein, F. H.

doi:10.1093/icesjms/35.3.272

Cited by 12 publications

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“…Some of the demersal fish species recorded live near the sea surface as juveniles, including beaked sandfish and bluebottle fish Nomeus gronovii [ 56 ], and are captured by terns at this life stage. Like beaked sandfish, juvenile Cape hake Merluccius capensis can occur near the surface during the daytime [ 57 , 58 ]. Small hake may be captured early in the morning as they migrate to the surface at night to feed on fish and crustaceans [ 56 ].…”

Section: Discussionmentioning

confidence: 99%

Foraging plasticity in seabirds: A non-invasive study of the diet of greater crested terns breeding in the Benguela region

et al. 2018

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Marine predators, such as seabirds, are useful indicators of marine ecosystem functioning. In particular, seabird diet may reflect variability in food-web composition due to natural or human-induced environmental change. Diet monitoring programmes, which sample diet non-invasively, are valuable aids to conservation and management decision-making. We investigated the diet of an increasing population of greater crested terns Thalasseus bergii in the Western Cape, South Africa, during three successive breeding seasons (2013 to 2015), when populations of other seabirds feeding on small pelagic schooling fish in the region were decreasing. Breeding greater crested terns carry prey in their bills, so we used an intensive photo-sampling method to record their diet with little disturbance. We identified 24,607 prey items from at least 47 different families, with 34 new prey species recorded. Fish dominated the diet, constituting 94% of prey by number, followed by cephalopods (3%), crustaceans (2%) and insects (1%). The terns mainly targeted surface-schooling Clupeiformes, with anchovy Engraulis encrasicolus the most abundant prey in all three breeding seasons (65% overall). Prey composition differed significantly between breeding stages and years, with anchovy most abundant at the start of the breeding season, becoming less frequent as the season progressed. The proportion of anchovy in the diet also was influenced by environmental factors; anchovy occurred more frequently with increasing wind speeds and was scarce on foggy days, presumably because terns rely in part on social facilitation to locate anchovy schools. The application of this intensive and non-invasive photo-sampling method revealed an important degree of foraging plasticity for this seabird within a context of locally reduced food availability, suggesting that, unlike species that specialise on a few high-quality prey, opportunistic seabirds may be better able to cope with reductions in the abundance of their preferred prey.

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Section: Discussionmentioning

confidence: 99%

Foraging plasticity in seabirds: A non-invasive study of the diet of greater crested terns breeding in the Benguela region

et al. 2018

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“…In shallow waters Hobson (1966) noted that fishes could be identified to species by the outlines of stimulated bioluminescence they generated as they swam. Movements of fish schools produce intense stimulated bioluminescence at the surface, which has been used commercially to locate schools of pilchard (Sardinops ocellatus), anchovy (Engraulis capensis), and horse mackerel (Tracburtls trachclvus) (Cram and Schulein 1974). Equivalent observations are lacking from midwater environments; however, certain depths have a much greater potential for stimulated bioluminescence than others (Widder et al 1989).…”

Section: Hypothesis 2 : Sperm Whales Use Stimulated Bioluminescence Tmentioning

confidence: 99%

How Do Sperm Whales Catch Squids?

Fristrup¹,

Harbison

2002

Marine Mammal Science

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Vision may play a central role in sperm whale predation. Two complementary hypotheses regarding the detection and capture of prey items are presented, based on a review of mesopelagic ecology. The first hypothesis postulates that sperm whales locate their prey visually, either silhouetted against the midwater "sky," or by searching for bioluminescence produced by the movements of their prey. The second hypothesis postulates that sperm whales create a zone of stimulated bioluminescence around the mouth, which attracts squids and other visual predators. Studies of midwater fishes and invertebrates document the importance of vision in mesopelagic communities. If sperm whales search for silhouetted prey, they should be oriented upside-down to improve visual coverage and to facilitate the transition from search to prey capture. Prey capture events should be marked by excursions toward the surface. If they lure their prey, they should swim at a steady pace, with little rapid acceleration, and spend most of their time foraging at depths with the greatest potential for stimulated bioluminescence.Key words: Pbyseter macrorepbalm, foraging, vision, bioluminescence.The sperm whale, Pbyseter macrocepbalus, regularly dives to depths between 200 m and 1,000 m (Lockyer 1977, Watkins et al. 1993, where it feeds on a variety of squids and fishes (Clarke and MacLeod 1980, Martin and Clarke 1986, Rice 1989. In most parts of the world, squids constitute the major part of their diet (Clarke 1980, Kawakami 1980, Clarke et al. 1988). Clarke et al. (1993 examined specimens in the Azores, and estimated that only 23% of the diet consists of fast-swimming, larger cephalopods; the bulk of the diet consists of smaller, neutrally buoyant, luminescent species. They estimated that about 1,000 prey items are consumed per day. This implies that sperm whales locate and capture their prey efficiently. 42 FRISTRUP AND HARBISON: FORAGING BY PHYSETER 43In the 19th century it was hypothesized that sperm whales might passively lure their prey. Beale (1835) suggested that,". . . when the whale is inclined to feed, he descends a certain depth below the surface of the ocean, and there remains in as quiet a state as possible, opening his enormous mouth, until the lower jaw hangs down perpendicularly, or at right angles, with the body. The roof of his mouth, the tongue, and especially the teeth, being of a bright glistening white colour, must of course present a remarkable appearance, and which seems to be the incitement by which his prey are attracted; and when a sufficient number, I suppose, are within the mouth, he rapidly closes the jaw and swallows the contents." Gaskin (1967), believing that the white lining of the mouth could not attract squids at the great depths to which they dived, proposed that the luminous mucus from previous prey items might coat the mouth and serve to lure more prey. The problem with Gaskin's hypothesis is that living squids are not covered with luminous slime, and that the generalized bioluminescence he reported was d...

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Zooplankton Communities in the West African Upwelling Area

Thiriot¹

1978

Upwelling Ecosystems

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Observations on surface-shoaling Cape hake off South West Africa

Cited by 12 publications

References 0 publications

Foraging plasticity in seabirds: A non-invasive study of the diet of greater crested terns breeding in the Benguela region

Foraging plasticity in seabirds: A non-invasive study of the diet of greater crested terns breeding in the Benguela region

How Do Sperm Whales Catch Squids?

Zooplankton Communities in the West African Upwelling Area

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