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FISH FACTS: The future of mulloway

MUCH has been written about the iconic mulloway (Argyrosomus japonicus, a.k.a jewies) in Australia in recent times. The main thread linking together the wall of words on mulloway is the fact that people know they are a great fish. They look great, grow to trophy sizes and taste good too. It seems everyone wants to catch a big mulloway, but their populations throughout Australia are coming under ever increasing pressure from a wide range of factors, all of which are human induced.

The main strongholds for mulloway are in the estuaries of our largest river systems along mainland Australia’s subtropical and temperate coastlines from Shark Bay in WA southwards around to about the Burnett River in Queensland. This is because, like many other members of the Family Sciaenidae (drums and croakers), mulloway use estuaries as nursery areas during their juvenile phase, as well as foraging areas for adult fish. Spawning in mulloway occurs offshore from the mouths of larger river systems, usually after floods or flushes following major summertime rain events.

Juvenile fish then move back into these same river systems, which are used as nursery areas. So in many ways, the health of mulloway populations in Australia is a direct reflection of the health of our estuaries and rivers.

Perhaps because of the failing health of our rivers, but also because of their biology, mulloway are vulnerable. Being apex predators, they grow relatively rapidly to about 45 cm in 2 to 3 years, as they gorge themselves on baitfish and prawns within the estuarine environment. However, during this time they are highly accessible to both recreational and commercial fishers, and easy to find, especially with todays sounder technology. To make matters worse in the larger waterways, prawn trawling in estuaries is a high risk activity as juvenile mulloway stalk the same prawn schools
targeted by trawlers. Where both occur, bycatch interactions are inevitable.

We must remember these estuarine mulloway are almost exclusively immature fish – maturity for female mulloway takes at least 6 or so years, occurring only after they reach 70-85 cm long, depending on water temperatures and estuary productivity.

These recently mature fish are still relatively skinny, especially for a species that can live for over 30 years and grow to over 70 kg. But to the average 21st century angler operating in todays heavily fished estuaries, an immature mulloway of 65-70 cm is often considered a large fish, possibly even a trophy for some people. In contrast, those of us who targeted mulloway in the 20th century, remember how those well conditioned meter-plus fish were commonplace. However, today in many parts of Australia, meter-plus fish are becoming rarer and rarer.

Which leads me to the original reason why I was planning to write about mulloway this month – as I reflect upon a paper written by scientists who were describing the results of their mulloway tagging study in Western Victoria. The study, conducted in 2008-2012 in the Genelg River, used acoustic tags and a listening array of 20 receivers located up and down the river to try to determine habitat preferences and timing of movements of mulloway between the estuary and the open ocean. They
tagged 24 mulloway and followed them for three years. However, during the study period over half of the tagged mulloway (13, or 54%) were caught and kept by anglers. The scientists noted the high catch rate ended up “hampering analysis of mulloway movements, but providing opportunistic data on angling pressure and sizes of captured fish”. The surviving fish exited the estuary and four of them subsequently moved to the mouth of the Murray River over 450 km away, in a presumed spawning migration.

As a first take home point, these studies prove beyond doubt the strong link between mulloway and estuaries as nursery areas as well as attraction areas for adult fish during river mouth spawning events. This point is reinforced by the fact that studies in NSW have found year class strength of greasyback prawns (which live for around a year and make up a high proportion of the diet of juvenile mulloway) are directly related to rainfall, suggesting that rainfall influences recruitment of mulloway twofold by inducing spawning and also increasing subsequent food availability for juveniles.

So, piecing these things together, it is clear that recent declines in mulloway populations throughout Australian estuaries are driven not only by fishing, but also by the reliance of this species on increasingly degraded estuarine habitats experiencing altered freshwater inputs due to human induced changes to river flows in the catchment, and also potentially changing climate. In an evolutionary sense, mulloway probably evolved to have long reproductive lives as a consequence of the need for their populations to persist through extended dry periods of relatively poor recruitment. But of course on top of these natural variations, today we know that fishing generates the majority of mortality of juvenile and adult mulloway. Which means that recovery of depleted mulloway populations must involve fisheries management.

Nowhere is this more evident that in NSW, where it has been concluded that, given mulloway’s heavy reliance on healthy rivers and river flow events for recruitment, that several years of above average rainfall is required to promote multiple good years of mulloway recruitment in order to rebuild their depleted population. However, things become very tricky when heavy fishing pressure is placed into this mix, and Fisho’s own David Rae touched on these issues a couple of years ago in his article on mulloway in crisis.

A decade on from the 2013 management changes to mulloway in NSW (which included raising the minimum legal length to 70 cm and reductions in recreational and commercial catch limits), stock recovery remains limited. This slow response to management intervention, despite the recent wet La-Nina period (and perhaps catalysed by predictions of the upcoming dry El-Nino period), recently lead to NSW Fisheries implementing additional fisheries management controls over mulloway including tighter boat limits for recreational and charter fishers, and more stringent
controls on commercial mulloway catch. These changes were announced as “interim arrangements to provide greater protection to adult fish, pending the finalisation and implementation of the formal Harvest Strategy for Mulloway.”

Another iron in the fire for mulloway recovery in NSW follows on from overseas examples of successful marine restocking programmes for recruitment limited sciaenids, like the red drum (Sciaenops ocellatus) in the coastal waters of Texas.

NSW Fisheries have included mulloway in their own Marine Stocking Program, and as is required during any responsible undertaking like this, the stocked mulloway are being closely monitored by scientists for survival, growth, movements and their contribution to fishery recruitment. Of course, simply stocking fish without also working on healthy habitat and water quality would be a waste of time. This critical point is also recognised by NSW DPI, and no better way is the relationship
communicated than in their own Mulloway Habitat Factsheet, which was first published back in 2010.

An extract from the factsheet states: “Ensuring the best possible water quality in our estuaries will help to improve mulloway populations. Good water quality means healthy seagrass and mangroves, which means healthy populations of the small fish and crustaceans that make up an
important part of the diet of juvenile mulloway. A good supply of food, and shelter, will mean more juvenile mulloway survive and thrive.”

No truer words have been said, especially when you look at their practical hints on how anglers can help, which included “Be mindful of what you wash down the stormwater drain”, “Help work on fish habitat”, “Put the big fish back” and “Continue to abide by bag and size limits”.

In our modern world today, all the finger pointing in the world will not get mulloway back. History has shown with other late maturing and long lived sciaenids (remember those red drum in the USA?) that only with this three pronged approach of:

1) Strong fisheries management;

2) habitat/water quality restoration, and;

3) judicious restocking only when necessary to plug failures in recruitment can we expect to see mulloway populations recover and thrive into the future in Australia’s most populous states.

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