In many parts of Victoria, trout stocking is a vital part of trout fishery management. For example, the majority of our trout lakes rely on stocking to sustain a fishery – natural recruitment is either non-existent or insufficient to maintain a viable trout population. There are also a few rivers in south-west Victoria where stocking is essential to maintain the trout fishery because conditions suitable for natural spawning are inadequate.
With any trout stockings, one of the challenges fisheries managers face, is simply understanding what becomes of those stocked trout. For example, does stocking fish of a certain size produce a better return to anglers? We know from past Fisheries research that in some waters, stocked fry/fingerling sized trout hardly ever make it to a catchable size, while yearling sized trout certainly do. But given larger trout are more expensive to grow, what’s the ‘sweet spot’? In other words, for a particular water, what’s the size that – dollar for dollar – provides the best angler return? Or in waters where there’s some natural recruitment, how does the contribution of stocked trout compare to wild trout?
Until now, these questions and more have been challenging to answer. The best option has been to fin-clip hatchery trout before stocking. This method leaves a distinct, easily observable mark that researchers and informed anglers can subsequently use to determine the origin of a recaptured fish – and significantly, the fish doesn’t have to be killed to gather this information.
However fin-clipping also has its limitations:
- Fin-clipping is labour intensive, as each individual fish has to be clipped by hand. With over half a million trout stocked in Victoria each year, there is no way more than a fraction of those trout can be fin-clipped.
- For various reasons there are really only three different ‘clips’ that are practical to use on trout. So this limits the number of stocking variations that can be tried in a given water or interconnected waters.
- Only having three clips can lead to confusion with longer-lived trout like browns, whose lifespan in a stocked water may exceed 3 years and thus cause ‘overlapping’ between younger and older trout of the same fin clip.
- Very small trout can’t be fin-clipped.
But as explained at the Conference, thanks to a collaborative effort by Fisheries and Melbourne Uni. scientists (based on some Norwegian salmon research) there’s now a way to mark and track large numbers of hatchery trout of any size relatively cheaply, easily and safely. It turns out that dipping eggs or larvae in a solution of barium leaves trout with a lifelong mark on their otolith (inner ear bone). Further, a range of barium isotopes means lots of different ‘batches’ of hatchery trout can be distinctively marked.
The only downside of barium marking is that trout have to be killed to ‘read’ the otolith, but given the future capability for safe, reliable mass marking of almost any trout Snobs Creek Hatchery produces, this seems a small price to pay.
So, where does all this fit with the Wild Trout Program? Unlike many trout fisheries in the west of Victoria, the wild trout waters of the north-east usually have first-rate spawning facilities and therefore the value of trout stocking isn’t clear. So, there are a couple of questions the Wild Trout Program wants to answer:
- In the case of Lake Eildon, which is periodically stocked with trout, how do those fish contribute to the fishery in the inflowing streams?
- Can stocking make a positive contribution to our wild trout streams?
To help answer the first question, Snobs Creek brown trout with a unique barium mark will be stocked into Lake Eildon in 2016 and researchers will look for these fish in the inflowing streams in 2017.
To help answer the second question, researchers have already been looking to recapture distinctively fin-clipped stocked brown trout among wild trout surveyed in the Howqua and upper Goulburn rivers. Additionally, in 2016, brown trout stocked into these rivers will also carry a barium mark (different to those stocked into Lake Eildon) and a new fin-clip. So this will be an added way for researchers to differentiate stocked fish in the Howqua and upper Goulburn rivers with wild bred fish.
As for results, the researchers want to wait until they have a three year sample before they start making any conclusions about the contribution of stocked trout to the Howqua and upper Goulburn wild trout fishery. And as for the contribution of brown trout stocked in Eildon to the inflowing river fisheries, early findings should come in later 2017.
Ultimately, thanks to barium marking, some fin-clipping and the Wild Trout Program, in a couple of years we should have answers to these key trout management questions.