By Dr Alan Williams CSIRO Oceans and Atmosphere, Hobart
Grilled with garlic, oven baked, or lightly pan fried with a hint of lemon, blue-eye trevalla is one of Australia’s premium fish, and an iconic fish species for commercial fishers and seafood lovers alike! Given its popularity, it’s then surprising to realise the extent of the knowledge gaps for blue-eye in Australian waters, and how these create uncertainty for its stock assessment. Some of this uncertainty has been addressed in a just-completed project funded by FRDC, CSIRO and VicDPIE which has shed new light on the ecology and spatial distribution of Blue-eye stocks in Australian waters.
Surprisingly, the blue-eye’s early life history remains something of a mystery. Whilst it turns up on fishing grounds aged about two years old and measuring about 40 cm in length, only four juveniles have ever been seen in Australia (these were dip-netted by a commercial fisher off eastern Tasmania). A good outcome from the project is a more complete picture of the biology and ecology (including early life history) for Australian blue-eye – a process helped considerably by the input of knowledgeable commercial fishers and SETFIA, and input by colleagues in New Zealand.
Surface dives in open ocean off NZ discovered juvenile blue-eye (~ 5 cm in length) under kelp floating at the surface.
Other key knowledge gaps for the blue-eye assessment were uncertainties about the species distribution and the possibility of there being different stocks. It was known that blue-eye ranges widely across southern Australia, lives on offshore seamounts, and extends to sub-tropical latitudes on both east and west coasts, but without detailed spatial mapping it could only be managed as a single stock. This meant it was difficult to identify the most appropriate total allowable catch (TAC) and determine a long term level of sustainable catch.
Building on life-history information, the project then mapped patterns in three different characteristics of blue-eye to look for evidence of stock structure. Each of the three analyses showed there was some separation of stocks within Australian waters: spatial differences in age and growth and in fish ear bone (otolith) chemistry of the adult life stage implied there was local and regional residency by adults. Dispersal in ocean currents indicated that a broader scale connectivity amongst regional populations was likely during early life.
LEFT: Stock areas are defined by boundaries (green lines) within the geographical limits of the blue-eye’s Australian distribution (red lines), and the narrow depth range occupied by adult Blue-eye Trevalla (depth contours)
By overlaying these spatial patterns, we identified four broad Blue-eye ‘stock areas’ (see the map below): West, South, East and Seamounts-Lord Howe. Each of these stock areas represents an interconnected ‘meta-population’, i.e. a group of discrete adult sub-populations resident on the continental slope and seamounts without extensive migration between them. Stock areas do not reflect truly separated biological stocks because there is some exchange between them during pelagic early life history, and some of the adult sub-populations act as larger ‘sinks’ than others, i.e. benefiting more from recruitment derived from ‘upstream’ spawning areas.
These patterns, together with spatial patterns in catch and effort which have also been mapped up in detail by the project, will help AFMA and fishers to develop additional management options for the blue-eye fishery. In turn, new options may provide a greater confidence in the stock assessment leading, potentially, to a less risk-averse setting of the blue-eye trevalla TAC.