GILL RAKER APPARATUS AND FOOD SELECTIVITY AMONG 

 MACKERELS, TUNAS, AND DOLPHINS 



John J. Magnuson and Jean G. Heitz' 



ABSTRACT 



Gill raker morphology and fork length wei-e measured from 411 fish, representing eight species of scom- 

 brids and two species of coryphaenids (dolphin). For each species linear regressions passing through 

 the origin were determined relating mean gill raker gap in millimeters (first gill arch) with fork length 

 in centimeters (/), and log filtering area (first gill arch) with log fork length. Mean gill raker gaps 

 equaled: Avxis rochei — 0.0144/, Katsuwonus pelamis — 0.0211/, Aiixis thazard — 0.0213/, Thunnus alba- 

 cares — 0.0344/, Thunnus alahmga — 0.0365/, Eidhynnus affiuis — 0.0386/, Thunnus obesus — 0.0391/, Sarda 

 chiliensis — 0.0509/, Coryphaena hippurus — 0.0650/, Coryphaena equisetis — 0.0655/, and Acanthocybium 

 solanderi — no gill rakers. Among the species gill raker gap was directly proportional to the number 

 of gill rakers, but no relation occurred between mean gap and filtering areas. Gill raker gap differed 

 markedly among species and lengths of fish. A 50-cm A', pelamis, a 30-cm T. albacares, and a 10-cm Sarda 

 orientalis all had an estimated mean gap of 1 mm. Conversely the gaps of a 50-cm fish of each species 

 were estimated to be ca. 1.0, 1.7, and 4.5 mm respectively. 



Mean gill raker gaps from this study were compared with the percentage of crustaceans in stomachs 

 of Central Pacific fishes based on literature records. Body sizes of fishes and squids in the stomachs were 

 larger than crustaceans. Percent volumes that crustaceans contributed to the stomach content were 

 inversely related to mean gaps (Kendall rank correlation coefl5cient, t = —0.59, n = 16, P<0.001). 

 Partial correlation indicated that gap was more important than fork length in predicting the quantity 

 of crustaceans. Thus, the gill raker gap was related functionally with the quantity of smaller orga- 

 nisms in the stomachs. Presence of euphausids in stomachs of K. pelamis and their absence in T. alba- 

 cares from the eastern tropical Pacific may result from the small size of euphausids and the smaller 

 gill raker gaps of K. pelamis relative to T. albacares. Gill raker gap and the maximum distensi- 

 bility of the esophagus would set physical limits on the size of food eaten. The diverse fauna assem- 

 blage of crustaceans, fishes, and squids within this size range has masked to a great extent the selective 

 feeding that does occur among scombrids and coryphaenids on the basis of food size. 



Most scombrid fishes have a varied diet that 

 includes numerous crustaceans, cephalopod mol- 

 luscs, and fishes. The Indian mackerel, Rastrel- 

 Uger kanagurta (Cuvier), even eat phytoplank- 

 ton (Bhimachar and George, 1952). The high 

 diversity of organisms in their stomach contents 

 has generated the opinion that scombrids are 

 nonselective feeders, preying upon anything 

 they encounter. CoryiDhaenid fishes, dolphins, 

 eat fish predominantly. 



Yet selectivity does exist in food habits of 

 scombrids. Within a species, larger fish contain 

 relatively fewer cru.staceans and more fishes. 

 Crustaceans constituted 44 5r of the stomach vol- 

 ume of skipjack tuna, Katsuwonus pelamis (Lin- 



' Laboratory of Limnology, Department of Zoology, 

 University of Wisconsin, Madison, Wis. 53706. 



Manuscript received January 1971. 



FISHERY BULLETIN: VOL. 69, NO. 2, 1971. 



naeus), shorter than 50-cm fork length but only 

 1.5% of the volume for fish longer than 60 cm 

 (Yuen, 1959). Similarly, crustaceans consti- 

 tuted 35% of the stomach volume of yellowfin 

 tuna, Thunnus albacares (Bonnaterre), shorter 

 than 130 cm but only 1% for those longer than 

 130 cm (Reintjes and King, 1953). Reintjes 

 and King suggested that these differences might 

 result, as the fish grew, from a change in food 

 preference or a change in the ability to search 

 out and capture larger, more mobile prey (fish- 

 es) . Another considei'ation, in our view, is that 

 larger predators have a reduced ability to catch 

 small prey (crustaceans). 



Prevention of food loss through the opercular 

 gap is generally recognized as the primary func- 

 tion of gill rakers. Species with more closely 



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