A comparative study of the movement habits and their relationship to buoyancy compensation in two species of shallow reef rockfish (Pisces, Scorpaenidae)
Date
1979
Authors
McElderry, Howard Ingram
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Abstract
An assessment was made of the potential for restricted vertical movement in t wo species of shallow reef rockfish caused by concurrent changes in buoyancy. Movement habits of a schooling form, Sebastes melanops, were compared with those of a benthic crevice dweller, Sebastes nebulosus, on a moderate exposure rock-reef in Barkley Sound, British Columbia. Through multiple observations of tagged fish against a geographically fixed landmark system, distribution and movement of individuals was determined. Tagged s. melanops remained on the study reef for periods of about three weeks; but, while present, were distributed in the water column at select locations. Movements in this species occurred during 46 percent of successive observations with distances frequently greater than 15 meters, extending as much as 10 meters vertically. The solitary s. nebulosus were resident for most of the field study period (2 months) having a more limited distribution, apparently territorial. Their movements are less frequent (26 percent of successive observations involved movement) and usually less than 10 meters horizontally with little of no vertical aspect.
In the laboratory, s. melanops had a much greater capacity to use their swimbladder as a buoyancy device under changing pressure situations than s. nebulosus. Although nearly identical in general swimbladder morphology, measurements of the secretory epithelium showed that s. melanops have over twice the secretory area than s. nebulosus of similar body weight. In a simulated descent from the surface to 20 meters depths. melanops required 90 hours to restore neutral buoyancy while s. nebulosus required 450 hours. Ascending from this depth using a stepwise decompression procedure to avoid damage through overexpansion, s. melanops could achieve neutral buoyancy in about 5 hours while it took s. nebulosus at least 20 hours.
Despite the comparatively faster rates of buoyancy adjustment in s. melanops it was clear that short term vertical movement in this species was not accommodated by buoyancy adjustment through secretion or resorption of swimbladder gas. Instead, vertical movement results in periods of non-neutral buoyancy. A survey was made of the various buoyancy strategies that facilitate vertical movement in other fish species; the relationship of those species to the two study species is discussed.