Discarded queen conch (<Emphasis Type="Italic">Strombus gigas</Emphasis>) shells as shelter sites for fish

Within the Caribbean millions of queen conch (Strombus gigas Linnaeus) are harvested each year and shells discarded randomly or as middens. Fish use of discarded conch shells was investigated in four different habitat types: sand, seagrass beds, mangrove forests, and coral reefs. The study was carried out in the waters off South Caicos, Turks and Caicos Islands (TCI), between October 2003 and January 2004. The density of discarded shells was greatest near coral reefs; however, the percentage of shells occupied by adult fish was higher in isolated shells on sand and in mangrove habitats. Juvenile fish also showed a preference for sheltering in conch shells relative to other microhabitat types on sandy plains and in mangrove and seagrass habitats. Differences in use of single shells by fish in different habitats were attributed to differences in piscivore abundance and habitat complexity. Although not all isolated shells were occupied by fish, all conch middens deposited by fishermen had fish inhabitants. Examination of fish use of conch middens in three habitat types and conch piles of one, three, and five shells constructed on sand found both fish diversity and abundance increased on conch middens of increasing size. This study suggests that disposal of conch shells as large middens in habitats of low complexity will increase the amount of shelter present and may enhance fish populations in these habitats.Communicated by J.P. Grassle, New Brunswick     

Large-Eddy Simulation of Coastal Upwelling Flow

Large-eddy simulations were carried out to study laboratory-scale realizations of coastal upwelling in an annular rotating tank with a sloping bottom. A two-layer stratified fluid was set into rigid body motion with the tank and then driven by the relative rotation of a solid top lid. The simulation code developed in this work was a three-dimensional incompressible Navier-Stokes solver using the message passing interface. The simulation runs were performed on a distributed memory massively parallel computer, namely, the IBM SP2. The simulation results were able to reveal the evolution of the complex upwelling structures in detail. The results were used to compare with and to complement two relevant series of coastal upwelling experiments. A Rayleigh-Taylor type of instability took place in the top inversion layer due to the unstable stratification after establishment of the upwelling front. The primary upwelling front was unstable to azimuthal perturbations and developed large amplitude baroclinic waves. The frontal wave structure consists of cyclone/anticyclone pairs. Whether cyclonic eddies containing the lower-layer fluid pinch off from the front depends on the _* value. The non-dimensional parameter _*=gh ₀/u _* f_s, which was first introduced by Narimousa and Maxworthy, combines the effects of stratification, rotation and surface stress and can be used to characterize the upwelling flow field. Our studies show that the frontal instabilities are much more intense and the upwelling front itself displays strong unsteadiness and cyclonic eddies containing the lower-layer fluid pinch off from the front when _* is significantly less than 5.8. For _*=5.8, the frontal instabilities are less intense and no pinched-off process is observed. To separate these regimes, a critical value of _* of about 5.4 is consistent with Narimousa and Maxworthy's results.