Population shifts in two competing fish species on a degrading coral reef

Abstract

I monitored the population~ of 2 fishes on a coral reef in St. Croix, U. S. Virgin Islands, from 1980 to 1995 to determine if they could track changes in their habitat. Starting in the mid-1970s. white band disease killed all the elkhorn corals Acropora palmata, providing access to a variety of boring organisms that create the cavities inhabited by the spinyhead blenny Acanthemblemaria spinosa and the roughhead blenny A aspera As the corals became more heavily bored, they were weakened and collapsed, providing more of the low habitat occupied by roughheads and less of the high h a b ~ t a t occupied by spinyheads. This structural change was generally gradual but Hurricane Hugo destroyed almost all the remain~ng standing dead corals in 1989 Both specles increased in density until 1989 when the sudden reduct~on of high habitat caused a decrease in spinyhead population density. In contrast, roughheads continued their population growth until 1991 after which they also declined. Although previous work has shown that spinyheads can displace roughheads from preferred habitat, spinyheads did not increase their numbers close to the reef surface as the high corals collapsed. I hypothesize that their inflexible minimum height is due to their previously demonstrated higher metabolic rate which constrains them to the higher locations where planktonic food is more abundant. Consequently, the species mix shifted from one of spinyheads with extremely rare roughheads in 1980 to one dominated by roughheads w t h few splnyheads in 1995. This precise tracking of a changing environment suggests resource limitation and is counter to the widespread view that coral reef fish assemblages are largely unsaturated systems consisting of open, 1-ecruitment-limited populations. The fishes studied here may be atypical, however, and it is suggested that more studies with a mechanistic approach may prov~de insight into reef fish assemblage structure.