Fishy tale

Unsustainable fishing and habitat destruction is increasing pressure on tropical marine eco-systems. Conservation of coral reef habitats has been prioritized at the expense of other habitats, but the importance of maintaining migration links between habitats has been explicitly recognized during the recent re-zoning of the Great Barrier Reef Marine Park. It is speculated that reef fish species use mangroves and sea grass beds as juvenile nursery areas before migrating to coral reef habitats as adults. The maintenance of seascape connectivity is therefore likely to be necessary to maintain coral reef eco‑system function.

Scientists at The Woods Hole Oceanographic Institution (WHOI) (Massachusetts, USA) have developed a unique method of quantifying fish movements within tropical marine eco‑systems.¹ The investigators quantified seascape connectivity for a commercially important snapper species, Ehrenberg’s snapper (Lutjanus ehrenbergii) in a coral reef ecosystem from the Red Sea.

At the bottom of food webs within mangroves, sea grass beds and reefs there are natural geochemical variation in the levels of δ¹³C that are permanently recorded within the otoliths (ear bones) of reef fish that can be extracted and analysed by gas chromatography (GC). The investigators tracked the migration patterns of Ehrenberg’s snapper fish by analysing the δ¹³C values of five amino acids, threonine, isoleucine, valine, phenylalanine and leucine, contained within otoliths extracted from adult fish on coral reefs.

The five amino acids were selected on the basis of sufficient peak size and baseline GC separation. The multivariate approach identified residence patterns that would not have been identified using conventional bulk stable isotope analysis. This unique and direct estimation of seascape connectivity is applicable to any reef fish species and is critical to the modelling of habitat value.

WHOI biologist and study lead author McMahon commented “Traditional methods of assessing nursery habitats – visual surveys of abundance and size of fish in different locations – provide important but indirect evidence of connectivity among essential habitats. We developed a quantitative method that identifies essential nursery habitats, and allows reconstruction of migration within the seascape. As human activity continues to degrade and fragment tropical seascape habitats, quantitative understanding of the connectivity among them becomes increasingly important”.

1. Kelton W. McMahon, Michael L. Berumen and Simon R. Thorrold, PNAS (2012) doi: 10.1073/pnas.1206378109.

This story originally appeared in The Column. Click here to view that issue.