To sea or not to sea? Flexible life history strategies in diadromous fishes.

A project undertaken at the University of Otago, supervised by Gerry Closs, and research conducted by Andy Hicks.

Background:

Movement patterns of pelagic larvae are a mystery, both in terms of the spatial extent of migration and why they do it. Coral, reef-fish, lobsters, abalone, seastars, fanworms and amphidromous fishes all exhibit a pelagic larval phase that has the potential for widespread dispersal. Until recently, dispersal was assumed to be the primary reason for this larval life-history stage, allowing pelagic larvae to drift and connect what would otherwise be isolated populations of adults. Dispersal was seen as a form of bet hedging against unpredictable local conditions, and an ability to colonise isolated habitats was seen as the primary benefit offered by a dispersive larval phase. Increasingly, this perspective is being questioned in multiple taxa with suggested alternative benefits of a pelagic larval period including gaining access to feeding opportunities, breaking cycles of parasitism, and avoiding predation. Under this alternative hypothesis, dispersal is an 'accidental' benefit rather than the primary reason for a pelagic larval period.

Project aims and methods:

This project will concentrate on amphidromous fishes, taking advantage of the distinct microchemical marker left in otoliths as fish migrate between freshwater, estuarine and marine habitats. Distinct differences in the trace element composition of these habitats are permenantely deposited in fish otoliths, thus allowing us to determine whether fish moved between these habitats. This information can be used to infer whether fish do or do not rear entirely in freshwater, the extent of exchange among populations in different river and lake systems, and whether large scale larval dispersal occurs.

The most convenient (easiest to identify) situation is when the presence of coastal freshwater lakes means fish could complete their entire life within freshwater and lack a marine signature (non-diadromous recruitment). But estuaries, harbours or enclosed bays could all impart a distinctive signature that we could identify by analysing otolith microchemistry.

We will collect a variety of species from different systems around south east Australia and New Zealand. Fish will be collected from areas where we believe larval retention is likely (rivers containing freshwater lakes, large estuaries, or flowing into harbours) and those where we expect larval development will occur in an oceanic environment due to a lack of a suitable coastal pelagic larval rearing environment. We will compare otolith microchemistry to determine whether the fish collected from potential retention areas experienced different water chemistry during their larval period compared to those that were collected from the open coast. By comparing several species in both Australia and New Zealand, we can explore whether patterns in larval retention/larval dispersal are species specific, or if generic rules govern the movement of pelagic larvae.

Outcomes

Our work will benefit Australia and New Zealand by determining where, how and in what numbers, key species of diadromous fish recruit within regions, catchments and sampled sites. Several species (G. argenteus, G. fasciatus, G. truttaceus & G. huttoni) are listed as vulnerable and of conservation significance in Australia & New Zealand. The research will be applied directly to restoration projects being undertaken by national, state and local government agencies, indigenous and local community groups, enabling identification of key requirements for the sustainable management of fish populations, and improving criteria for monitoring the success or otherwise of restoration projects. Government agencies will benefit significantly through improvements to biomonitoring programs that use the composition of fish communities to assess aquatic ecosystem health due to an improved ability to predict where diadromous fishes should occur. Such fish typically form a significant component of coastal catchment freshwater fish communities throughout New Zealand and south east Australia.

Project Team:

Andy Hicks (Phd student – University of Otago)
Dr Gerry Closs (Supervisor – University of Otago)
Dr Jon Waters (Co-Supervisor – University of Otago)
Dr Jonathan Kim (Co-Supervisor – University of Otago)
Dr Bruno David (Collaborator – NZ Department of Conservation)
Dr Rick Stoffels (Collaborator – Murray-Darling Freshwater Research Centre)

For more information please contact Andy Hicks (ashicks@gmail.com) or Gerry Closs (gerry.closs@stonebow.otago.ac.nz)