A field assay was used to identify whether the presence of larger individuals of each of three candidate species (Gomphosus varius, Pseudocheilinus hexataenia and Thalassoma quinquevittatum; the three species most similar to T. hardwicke in terms of habitat use; see Results) negatively effected the survival of transplanted T. hardwicke. 32 reefs within 80 m of the reef crest were selected. Reefs consisted of a live Porites base, surmounted by 1-2 Pocillopora verrucosa colonies and small patches (< 10% surface area) of macroalgae. Reefs had an average area of 5.43 m2 (SE = 0.34), a mean height of 0.77 m (SE = 0.04), and were isolated from their nearest neighbour by 4.71 m (SE = 0.25). Of these 32 reefs, eight were inhabited by two to five (mean = 2.8; SE = 0.4) T. quinquevittatum; eight were inhabited by two to five (mean = 3.4; SE = 0.4) G. varius; and eight were inhabited by two to four (mean = 3; SE = 0.3) P. hexataenia. The SL of individuals of all candidate species ranged between 12 and 30 mm. All reefs lacked fish of the other species (e.g., reefs with G. varius did not have P. hexataenia or T. quinquevittatum). In addition, eight reefs lacked G. varius, P. hexataenia and T. quinquevittatum. All reefs were interspersed with each other within the lagoon. Before the field assay, all resident T. hardwicke were removed from all reefs. Thalassoma hardwicke settlers were collected from reefs ~ 600 m from the study site using the fish anaesthetic eugenol (clove oil) and hand nets. All fish were held in tanks with running seawater for 12 h, then individually tagged with different colors of Visible Implant Elastomer (VIE) (Northwest Marine Technology, Shaw Island, Washington) forward of the caudal peduncle. VIE tags are clearly visible through the skin of the fish by observers in the field so it was not necessary to recapture individuals to determine their identity. VIE tags do not have adverse effects on other fishes (Beukers et al. 1995, Imbert et al. 2007, Simon 2007), have been used to tag fish as small as 8 mm (Frederick 1997), and a preliminary study confirmed no tag-induced mortality of 20 tagged T. hardwicke held in aquaria for 72 h. Tagging and handling effects were therefore assumed negligible. After tagging, fish were returned to aerated aquaria for 12 h before being measuring to the nearest 0.1 mm SL. Three tagged T. hardwicke individuals (14.8 mm SL; SE = 0.2; approximately two weeks post-settlement age) were then transplanted onto each reef (which is representative of natural densities). To estimate the survival rates of transplanted individuals, reefs were surveyed daily (~ 9 a.m.) for five days (beginning June 1 2005). At the same time, the abundance of competitors was recorded and any untagged T. hardwicke (two untagged T. hardwicke were removed, both < 10 mm) were removed. Neighboring non-experimental reefs were also searched for tagged immigrant T. hardwicke, none were found. Previous research has suggested that recent T. hardwicke settlers rarely move between reefs separated by as little as three meters (Shima 2001b). The disappearance of a tagged fish was therefore assumed to be due to mortality rather than migration. The mean proportion of T. hardwicke remaining in each treatment on the last day of the assay was used as the response variable because by the end of the assay survival trajectories had stabilized.
BCO-DMO Blog
©2024 Biological and Chemical Oceanography Data Management Office.
