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Fish Bulletin 137. Reproduction, Life History, and Ecology of the Round Stingray, Urolophus halleri Cooper

  • Author(s): Babel, John Stanley
  • et al.
Abstract

This is the first comprehensive study of the reproduction, growth, habits, food, and environment of the round stingray. Gametogenesis is described in both sexes. Previously unreported testicular appendages and a corpus luteum of unique origin are revealed. Sperm storage is demonstrated for the first time in a male batoidean. Gestation is described and illustrated. The relationship between certain of the ray's reproductive adaptations and its high biotic potential is discussed; these adaptations include rapid embryonic development, birth of large well-protected young, and a specialized pattern of oogenesis which permits annual ovulation. Male embryos outnumber female embryos, but the sex ratio approaches parity in newborn rays due to a higher male mortality. The sex ratio remains nearly balanced until maturity, after which males become progressively more numerous at depths of less than 7.5 fathoms. Conversely, mature females are more numerous beyond this depth, apparently segregating themselves from the males. The growth rate of U. halleri was determined by four separate means: (i) the Petersen method of width frequencies, (ii) a captivity study, (iii) tagging and recapture, and (iv) double sampling. The species is unique among elasmobranchs for which there are growth data; both sexes attain sexual maturity relatively early in life and at about the same size and age. The significance of early sexual maturation lies in the short time span between generations and the relatively longer reproductive life of the individual. Both factors contribute to a high biotic potential. Movements of U. halleri were traced by tagging and recapture, and from analysis of trawling and seining records. These rays are normally nonmigratory, tending to remain in or return to the same locale. No recaptured individual moved more than 4.75 miles. The most rapid movement was 1 or 2 miles in 4 days. Distances traveled increase with animal size. Young rays remain close to shore and gradually move seaward with growth. Mature females move farthest offshore and return shoreward in June for mating and again in September to bear young. Adults prefer the warmer coastal waters in winter, only entering inlets to forage. Many rays populate the warm inlets, however, during summer. Over 94 percent of the ray's food volume is supplied by the three invertebrate classes—Pelecypoda, Polychaeta, and Crustacea, which are listed in order of importance. Mature rays eat a relatively larger volume of polecypods than do young rays. Urolophus halleri burrows in the substratum for food and concealment. Captive rays locate food by scent as well as by sight. Water temperature appears to limit the depth-distribution of U. halleri. Most rays reside within a narrow coastal zone, at depths of 10 fathoms or less, where temperatures generally remain above 10°C. Latitudinal distribution also seems controlled mainly by water temperature. Local population densities are greatest where a soft substratum exists just offshore and where suitable inlets are available for mating and for bearing young. Inshore dredging and the erection of coastal breakwaters and jetties are improving the ray's environment and may be contributing to a population increase in some locales. The relative abundance of associated fish species was determined from trawling catches. Urolophus halleri ranks fourth among benthic fishes taken in the study area. Little is known of the feeding relationships which exist between U. halleri and other associated fishes; however, the ray is believed to compete with some of the valuable flatfishes. Trawling catches do not accurately reflect the relative abundance of small invertebrates used as food by U. halleri; some of these invertebrates lie beneath the surface of the substratum, while others pass through the net.

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