Ocean surgeonfish Acanthurus bahianus. I. The geography of demography

Abstract

The ocean surgeonfish Acanthurus bahianus is a common inhabitant of reefs throughout the west and central Atlantic. We examined geographic variation in its demography among 10 locations spanning 56°of latitude. These populations exhibit a great diversity of growth trajectories, as well as 3-fold variation in terminal size and maximum longevity. There are strongly contrasting patterns of habitat variation in demography at 2 sites: at Bermuda, fish settle inshore, grow to about asymptotic size and then, when 2 to 6 yr old, relocate permanently to outer reefs, where they can reach 32 yr. At Belize, fish settle and attain 10 yr on both inner and outer reefs, but grow faster and reach a ~50% greater asymptotic weight on inner reefs. Habitat differences in growth at Belize largely span the range of variation among NW Atlantic locations. A. bahianus exhibits the fastest growth known for an acanthurid, and the strongest spatial variation in demography known for a tropical reef-fish. Maximum age, adult survivorship, terminal size and absolute growth rate are inversely related to temperature. However, relative growth rate is not: in all populations a similar percentage of mean asymptotic standard length (L ∞ ) is achieved at 1, 2 and 3 yr, and then growth effectively ceases. Variation in longevity is related to temperature, and is largely independent of size. Variation in growth and size is related strongly to both habitat and temperature effects, and the rate of growth appears to set terminal size. High longevity at 3 isolated oceanic islands seems to reflect a temperature effect, rather than an island effect.

Ocean surgeonfish Acanthurus bahianus. II. Fishing effects on longevity, size and abundance?

“…Effects on longevity and survivorship: Robertson et al (2005) found inverse linear relationships between mean sea temperature and both maximum longevity and annual survivorship across 10 locations. They suggested that this pattern is related to a decline in mortality costs of reproduction with increasing latitude.…”

“…Maximum size, which is a good proxy for asymptotic size, is directly related to mean sea temperature at the 10 locations where we made population samples (Robertson et al 2005). We expected that, if fishing affects size in Acanthurus bahianus, then maximum size at locations with heavy trap-fishing would be less than the size predicted from the size-temperature relationship at locations where there is little or no such fishing.…”

“…The methods we used to take population samples of Acanthurus bahianus to obtain data on size, growth, survivorship and longevity at 10 localities spanning 56°of latitude are described in Robertson et al (2005). To ensure that we obtained a representative sample at each location, we made collections at at least 5 sites separated by distances of kilometres, and across a range of habitats used by the ocean surgeonfish at the different sites.…”

“…Robertson et al (2005) analysed otolith growth increments to age individual Acanthurus bahianus taken in population samples at 10 locations, and obtain data on maximum and asymptotic size, age-size relationships, growth rates and lifetime growth trajectories, maximum age and average annual survivorship of adults.…”

“…Here we present the results of the second part of a study that had 2 main aims. Our first aim (Robertson et al 2005, this volume) was to pro-vide a basic description of range-wide variation in the demography of a common and widely distributed tropical reef-fish, and assess the extent and natural causes of variation in its longevity, survivorship, growth and size, both across a large latitudinal gradient and among habitats within locations. The second aim, which is discussed here, was to assess effects of fishing on a common, small, reef-fish that experiences great variation in fishing pressure throughout its range.…”