Interpreting evidence of dispersal by Haliotis midae juveniles seeded in the wild

Recovery rates of hatchery-raised juvenile abalone seeded in the wild are a result of both survivorship, and dispersal in the time between seeding and sampling. Seeding experiments with juvenile Haliotis midae were conducted at two localities in South Africa to determine recovery rates, which were high at one of the sites (McDougall's Bay) and low at the other (Gouriqua). This difference may reflect the influence of habitat suitability on survivorship, but cannot be interpreted as such without a knowledge of dispersal rates because high dispersal may hinder detection of surviving juvenile abalone. We hypothesized that significant active dispersal by the abalone took place at Gouriqua, where they were seeded in a shallow habitat in which wave and surf activity had a direct impact. Conversely, at McDougall's Bay, a wave-sheltered site, where the juvenile abalone were seeded at slightly greater depth, there was little or no active dispersal by juvenile abalone. We argue that dispersal at Gouriqua took place in response to the effects of wave and surf action, which could be dampened by seeding the abalone at greater depths. Adverse water conditions limit the time and scale of foraging that juvenile abalone can undertake, effectively limiting the number of juveniles that can be accommodated. Within this context, water depth and exposure to adverse ocean conditions must be among the attributes that need to be considered when identifying seeding sites, and interpreting recovery data, since an increase in dispersal makes it more difficult to detect seeded juveniles and therefore will diminish recovery rates without necessarily altering survivorship. (C) 2003 Elsevier Science B.V. All rights reserved.