Elements in otoliths may elucidate the contribution of estuarine recruitment to sustaining coastal reef populations of a temperate reef fish

Abstract

Estuaries and associated seagrass habitats are thought to be important nursery areas for many fishes. There is, however, no direct evidence for movement of fish from estuaries to reefs. The aim of this study was to determine if populations of Achoerodus viridis (Labridae) on rocky reefs were sustained by (1) recruitment to estuarine seagrass habitat followed by movement to rocky reels, (2) direct recruitment to rocky reefs, or (3) a combination of the two. Recruits were collected from estuarine seagrass and rocky reef habitats and elements in their otoliths analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to determine if different 'elemental fingerprints' could be found. Higher concentrations of Zn, Al, Pb, Mn, Ba and Co were found in otoliths of recruits from estuarine seagrass habitat than in otoliths of recruits from coastal reefs, the latter 3 elements showing significant differences. Strontium occurred in significantly higher concentrations in otoliths of recruits from coastal reefs. Differences in concentrations of some elements in the otoliths of recruits allowed fish from the 2 environments to be distinguished with a high degree of accuracy, enabling the contribution of estuarine recruitment to sustaining reef populations to be determined. Elemental composition of the juvenile core of otoliths from adults on reefs was related to the composition of otoliths of recruits from each environment to identify historical recruitment environments. Discriminant function analysis showed that 41% of adults had recruited to estuaries and 59% had recruited to reefs, but these figures may be overestimated because adults must be assigned to 1 of the 2 groups. There was evidence to suggest that some adults may form a third intermediate group. Further validation (e.g. comparison with laser or probe based methods and lagging techniques) of our approach is warranted. Elemental techniques may have great potential for resolving fisheries problems and identifying broader scale effects of environmental degradation.