Connectivity of black cod Epinephelus daemelii between Elizabeth and Middleton Reefs (as measured by population genetic structure based on microsatellites)
Drs. L. van Herwerden, O.S. Klanten, Prof. J.H. Choat, D.R. Jerry, and Mr. W.D.
Final report to the Department of Environment, Water, Heritage and the Arts, 2009
- Connectivity of black cod Epinephelus daemelii between Elizabeth and Middleton Reefs (as measured by population genetic structure based on microsatellites) (PDF - 202 KB) | (RTF - 1,024 KB)
This study investigated the population genetic structure of black cod (Epinephelus daemelii) from Elizabeth and Middleton Reefs in the Elizabeth and Middleton Reefs Marine National Nature Reserve. Six independent microsatellite markers were used to screen individuals from these two reefs for population genetic variation in order to determine if there was any evidence of population genetic structure, which would indicate that Elizabeth and Middleton Reef black cod populations are discrete. An additional original aim was to determine if these reefs were genetically distinct from coastal black cod populations on the Australian east coast, but this could not be achieved since tissue samples of only four coastal black cod individuals were available from Museum specimens.
Six of 17 microsatellite markers tested were screened from 78 Elizabeth and Middleton Reefs individuals and four coastal individuals, totalling 82 black cod. These were all variable with observed heterozygosities (Ho) ranging from 0.45 to 0.95. Not unexpectedly, evidence from the genetic structure suggests that the Elizabeth and Middleton Reefs populations are not distinct and are in fact a single stock. Furthermore, there is a suggestion that Elizabeth and Middleton Reefs populations are not distinct from the coastal population either, although the coastal sample was too small to consider this evidence definitive. This leads to four recommendations from the present study:
Recommendation 1: Additional individuals, especially from the Australian coastal population and additional Norfolk Island and New Zealand sites would be required to make a more comprehensive assessment of the linkages between populations of black cod in the southwest Pacific, so that potential sources of stock replenishment can be identified.
Recommendation 2: Additional microsatellite markers are needed to have greater confidence in our ability to determine if connectivity among these two sites is restricted. The present analysis does not have the statistical power to achieve significant results, due to small sample sizes (fewer than 50 individuals per site), restricted sampling (only two sites) and the use of only six microsatellites in this study. The data from an additional three microsatellites used to screen for the same populations (Appleyard and Ward 2007), when combined with the present data and reanalysed, will achieve greater statistical power in the face of the small sample sizes. This will provide greater confidence in the combined results. Importantly, accepting the null hypothesis (populations are not partitioned among study sites) does not prove that the alternative hypothesis (populations are partitioned among the study sites) is not true, particularly given the small sample sizes and number of markers used in this and the previous study.
Recommendation 3: In the absence of relevant data on the impact of recreational fishing on apex predators such as black cod at Elizabeth and Middleton Reefs, it would be wise to manage these reefs to ensure the maintenance of high standing stocks of these apex predators at Elizabeth and Middleton Reefs. In other words, these reefs should be fully protected no take areas, on the basis of research on apex predator standing stocks in Hawaii (Friedlander and DeMartini 2002), since Elizabeth and Middleton Reefs contains the largest reported Australian standing stock of black cod, an apex predator of the same genus studied in Hawaii.
Recommendation 4: Initiate a study to evaluate the impact of recreational and charter fishing activities in partially fished and no-take reserves at Elizabeth and Middleton Reefs and Lord Howe Island Marine Park. Partially protected areas open to recreational fishing in both Hawaii and the Great Barrier Reef have suffered substantial declines in shark and cod numbers compared to areas with complete protection (Friedlander and DeMartini 2002, Robbins et al 2006). Fishing at Elizabeth and Middleton Reefs could deplete the abundances and sizes of apex predators such as black cod (and Galapagos sharks) even if they are not target species (as per van Herwerden et al 2008).