Australian bass

Australian bass are catadromous — they live in fresh water and breed in salt — and every habitat statement about them depends on that.

ADULTS LIVE IN COASTAL RIVERS AND IMPOUNDMENTS, in structured freshwater habitat: rock bars, snags, undercut banks, weed edges and deep pools. They are ambush predators associated closely with cover, and a snag-free reach holds few of them.

THEY MOVE DOWNSTREAM TO SPAWN IN THE ESTUARY, so a population needs an unobstructed path between its freshwater habitat and brackish water. Weirs and barrages without passage cut that path, and populations above them persist only by stocking.

JUVENILES USE THE ESTUARY AND THE LOWER RIVER, moving upstream progressively as they grow, which means the whole system from salt to headwater is habitat for one life stage or another.

IMPOUNDMENTS HOLD THEM WELL AS ADULTS but cannot support the life cycle, so stocked dam populations are self-sustaining in appearance only.

TEMPERATURE AND FLOW SET ACTIVITY, with fish becoming lethargic in cold water and moving with freshes.

AND STRUCTURE IS THE PRACTICAL HABITAT SIGNAL. Australian bass hold tight to timber and rock and feed on what passes, which is why the fishery is a structure-casting one rather than an open-water search.

Australian bass spawn in estuaries in winter, which is an unusual combination and the reason the species is so sensitive to river regulation.

THE SPAWNING MIGRATION IS DOWNSTREAM. As water cools through autumn and into winter, adults leave their freshwater habitat and move downstream into the estuary, congregating in brackish water. This is the opposite direction from a salmon and it happens in the cold part of the southern year.

SPAWNING REQUIRES SALINITY. Eggs and larvae need brackish conditions to develop, which is why the fish cannot complete its cycle in fresh water however good the adult habitat is.

TEMPERATURE AND SALINITY TOGETHER FORM THE TRIGGER, with spawning occurring in a defined window of cool water and appropriate salinity in the estuary.

JUVENILES THEN MOVE BACK UPSTREAM over months and years, colonising the freshwater reaches as they grow.

BARRIERS BREAK THE CYCLE COMPLETELY. A weir without a fishway between the freshwater habitat and the estuary prevents the spawning run in one direction and the juvenile recolonisation in the other, and it is the principal reason wild populations have declined in several catchments. Stocking maintains fisheries above such barriers without restoring the population.

AND THE WINTER TIMING IS WHY CLOSED SEASONS EXIST in several jurisdictions — a fact this record notes without stating what any current measure permits.

Australian bass are an almost entirely catch-and-release fishery, which shifts the whole handling question from angler safety to fish survival — though the spines will still find an unwary hand.

THE SPINES. A stiff spiny dorsal and sharp gill cover edges, handled by folding the dorsal flat under the palm with the hand sliding forward from the tail. The fish is small and the injuries are minor punctures, but they are frequent because bass are caught on lures with trebles and unhooked quickly.

TREBLES ARE THE REAL HAZARD in this fishery, not the fish. A small bass thrashing with a hard-bodied lure carrying two trebles at hand height is how anglers get hooked, and it is worth pinning the fish in a net before touching the lure.

THE FISH'S SIDE IS THE IMPORTANT HALF. Bass are lipped routinely and the species tolerates it, but a bass held horizontally by the jaw alone, or hung vertically from a grip, is being damaged at the jaw joint. Support the body.

AND THE TIMING QUESTION IS SPECIFIC TO THIS SPECIES. Australian bass are catadromous and undertake a downstream spawning migration to estuarine water, and the fishery is closed over that period in the relevant jurisdictions for exactly that reason. Fish encountered in warm shallow water in summer, or aggregated in the lower system in winter, are at their most vulnerable to handling stress. The closed season is the primary protection and it is not optional; where fishing is open, keep the fish wet, the handling brief, and the air exposure to seconds.

Australian bass seasonality is dominated by a migration that runs the opposite way to a salmon's and for the opposite reason.

THE FISH IS CATADROMOUS: IT LIVES IN FRESH WATER AND BREEDS IN SALT. Adult Australian bass spend most of the year in coastal rivers and their freshwater reaches, and in winter they move downstream into the estuary to spawn in brackish water. That downstream winter migration is the organising event of the species' year.

WHICH MAKES CONNECTIVITY THE BINDING CONSTRAINT. A dam, weir or barrage between the freshwater reaches and the estuary removes the reproductive cycle entirely while leaving a healthy-looking adult population upstream. Many Australian bass populations in impounded systems are maintained by stocking for exactly this reason, and the fish in them will never breed.

SALINITY, NOT TEMPERATURE, IS THE SPAWNING REQUIREMENT. Bass need brackish water of a particular range for eggs and larvae, which is why the estuary is the destination and why freshwater impoundments cannot substitute however large.

THE WARM MONTHS ARE THE FRESHWATER FEEDING SEASON. Through spring, summer and autumn the fish are upstream in the freshwater reaches, feeding actively on insects, crustaceans and small fish, and this is when the surface-lure fishing the species is known for happens — frequently at dusk and after dark in warm weather.

WINTER IS THEREFORE AN UPSTREAM ABSENCE rather than a period of inactivity, and anglers who fish the freshwater reaches in the cold months find them empty for a reason that has nothing to do with feeding.

IMPOUNDMENT POPULATIONS HAVE A DIFFERENT YEAR AGAIN, with fish relating to standing timber, points and thermal structure and showing a temperature-driven rather than migratory pattern — a real fishery, and not the same animal in the same cycle.

AND CLOSED SEASONS EXIST IN THE RELEVANT JURISDICTIONS, frequently timed around the downstream spawning migration. They are set by state authorities, differ, and are not derivable from the biology above.