Dusky flathead

Dusky flathead spawning has one feature that dominates its management: the large fish are all females.

GROWTH IS SEXUALLY DIMORPHIC AND EXTREME. Males remain comparatively small; females grow much larger and live much longer. Every very large flathead is a female, so a fishery that keeps the biggest fish removes breeding females specifically — which is why slot limits protecting large fish have been adopted in several jurisdictions.

SPAWNING IS IN THE WARMER MONTHS, with fish moving towards estuary mouths and adjacent coastal water to spawn as water temperature rises.

EGGS AND LARVAE ARE PELAGIC, and juveniles settle into shallow estuarine habitat — seagrass, sand and mud flats in the upper and middle estuary — where they remain as they grow.

FECUNDITY SCALES STEEPLY WITH SIZE. A large female produces disproportionately more eggs than several smaller fish of the same combined weight, which is the biological argument behind protecting the upper end of the size range rather than only the lower.

MATURITY COMES AT A FEW YEARS, earlier in males than females.

AND THE SPECIES IS ESTUARY-DEPENDENT THROUGHOUT ITS LIFE, so estuarine habitat condition and the state of the mouth affect reproduction as much as anything happening offshore.

Dusky flathead seasonality is a temperature-driven movement within the estuary rather than a migration in or out of it.

WARM MONTHS PUSH FISH SHALLOW AND UP THE SYSTEM. As water warms, flathead spread onto shallow sand and weed flats, into the upper estuary and onto the margins, and they feed actively. This is the peak of the fishery and it is a shallow-water sight and ambush fishery.

COLD MONTHS CONCENTRATE THEM DEEP. As water cools, fish move into deeper channels, holes and the lower estuary, become less active and feed less. They remain catchable but the fishing changes entirely — deeper, slower and more localised.

THE SPAWNING MOVEMENT ADDS A SEASONAL SHIFT TOWARDS THE MOUTH in the warmer months, concentrating larger fish in the lower estuary and adjacent coast.

TIDE IS THE DAILY CYCLE THROUGHOUT. Flathead are ambush predators lying on sand and mud waiting for prey the tide delivers, so the state of tide governs which flats and drains are productive far more than the time of day does.

WATER CLARITY AND RECENT RAIN MATTER, with freshwater pulses pushing fish downstream and changing salinity distribution across the system.

AND THE SOUTHERN HEMISPHERE TIMING IS THE THING NORTHERN READERS INVERT: the warm shallow-water period straddles the turn of the calendar year.

Flathead cause a specific and avoidable injury, and the reason it keeps happening is that the hazard is positioned precisely where instinct puts a hand.

A flathead is flat, wide and slippery, and the natural way to pick one up is across the head — which is where the gill-cover and head spines are. They are sharp, they angle backwards, and a fish that kicks while somebody is holding it that way opens the palm.

The grip that works is across the TOP of the head behind the eyes, thumb and fingers on the bony plate rather than around the gills, or a lip grip with the body supported. Both keep the hand clear of the spines entirely, and both are less comfortable than the wrong grip until they become habit.

The welfare consideration here is unusual and worth stating: where an upper size limit applies, the large fish are breeding females and the entire purpose of the rule is that they go back promptly and in good condition. A big flathead mishandled on the way to a photograph is the specific outcome the regulation exists to prevent, so the handling and the rule are the same subject.

The ground the fish lives on carries its own hazard. Flathead flats also hold stingrays buried in the same sediment, and a wading angler concentrating on a fish is exactly the person who steps on one.

Dusky flathead habitat is one of the most precisely readable in this catalogue, because the fish is an ambush predator that buries and therefore has to be somewhere prey will pass.

THE GROUND IS SAND, MUD AND SEAGRASS EDGE IN ESTUARIES AND SHELTERED BAYS. Flathead lie partly buried on soft bottom with only the eyes and the top of the head exposed, and take fish and prawns moving above them. So the requirement is sediment they can settle into, and traffic overhead.

WHICH PUTS THEM ON EDGES RATHER THAN IN THE MIDDLE OF ANYTHING. The margin of a seagrass bed, the lip of a drop-off, the edge of a channel, a sand hole in weed, the drain from a flat as the tide falls — anywhere prey is funnelled or concentrated. A featureless expanse of sand holds far fewer than the boundary at its edge.

TIDE MOVES THEM AND IT MOVES THEM PREDICTABLY. On a rising tide flathead work up onto shallow flats and into the margins with the prey; as it falls they drop back to the drains, channel edges and deeper holes and wait for the flat to empty over them. That is a habitat that exists only at certain states of tide, which is why the same ground is productive and empty within a few hours.

SALINITY TOLERANCE IS WIDE and they extend well up estuaries into brackish and nearly fresh water, and out onto ocean beaches and inshore ground.

TEMPERATURE MOVES THEM SEASONALLY, deeper and further down the estuary in the cool months and up onto the shallow flats as water warms.

AND THE BIG FISH ARE FEMALE AND USE DIFFERENT GROUND, which is worth knowing: large females tend to be found in deeper channel and lower-estuary habitat rather than up on the shallow flats where the numerous smaller males are, so the ground that produces plenty of fish is frequently not the ground that produces large ones. Slot limits in the relevant jurisdictions exist because of that size-and-sex structure, and what any current limit is remains a matter for the competent authority.