Australasian snapper

Australasian snapper occupy inshore and shelf waters over a wide range of bottom types, and the shift between them across the life cycle is the defining habitat fact.

JUVENILES ARE ESTUARINE AND SEAGRASS FISH. Young snapper use sheltered estuaries, harbours and seagrass beds as nursery habitat, where structure gives cover and productivity gives food. The condition of those systems drives year-class strength far more than anything happening on the offshore reefs.

ADULTS MOVE OUT TO REEF, FOUL AND ADJACENT SAND. Larger fish occupy broken rocky ground, reef edges and the sand and shell between them, from shallow inshore water down to well over a hundred metres. They use structure for shelter and forage over the softer ground beside it, which is why the productive fishing is generally the boundary rather than the reef top.

TEMPERATURE SETS THE RANGE AND THE DEPTH. Snapper are a warm-temperate species; at the cooler end of their range they occupy shallower, warmer inshore water and are more seasonal, while in the warmer parts they hold deeper and are present year-round.

THEY ARE BENTHIC FORAGERS with a crushing dentition, taking shellfish, crabs, urchins and small fish, so a habitat with a productive invertebrate community holds them.

AND LARGE OLD FISH BECOME SITE-ATTACHED, holding on particular pieces of ground for long periods — which makes local depletion of big fish an easy outcome and a slow one to reverse.

Snapper spawning is an aggregation event, and everything about its management follows from that.

FISH GATHER IN LARGE NUMBERS in shallow inshore waters, harbours and gulfs to spawn, moving in from deeper ground and forming dense aggregations in predictable places. The aggregations are large enough to be significant events in the areas where they occur.

SPAWNING IS BATCH AND EXTENDED. Once aggregated, females spawn repeatedly over a period of weeks, releasing eggs in the evening in most documented cases, with very high total fecundity. Eggs and larvae are pelagic and drift into the nursery estuaries and seagrass grounds.

TEMPERATURE IS THE TRIGGER. Spawning begins when inshore water reaches a threshold and continues while it stays there, so the timing shifts between years and between areas with the warming of shallow water rather than with a date.

MATURITY COMES AT SEVERAL YEARS and the species is long-lived, so a spawning aggregation contains many year classes and represents a large accumulated reproductive investment.

AND THE PREDICTABILITY IS THE VULNERABILITY. A fishery that finds an aggregation can remove a substantial share of a local population's reproductive output in a short period, which is why aggregation areas attract specific management attention in several jurisdictions. This record describes the biology; it does not indicate what any current measure permits, and it is not a suggestion about when or where to fish.

Snapper seasonality is a temperature-driven movement between deep and shallow ground, overlaid by the spawning aggregation.

WARMING WATER BRINGS THEM INSHORE. As shallow inshore water warms through spring, fish move from deeper ground into harbours, gulfs and shallow reef, and the inshore fishery becomes productive. This is the same movement that leads into spawning.

COOLING WATER SENDS THEM BACK OUT. As inshore water cools, fish disperse to deeper reef and shelf ground, and the fishery moves offshore with them. At the cool end of the species' range this produces a sharply seasonal inshore fishery; in warmer areas the movement is less pronounced and fish are available year-round.

SO THE SEASON IS A DEPTH SEASON RATHER THAN A PRESENCE ONE in most of the range. The fish have not left; they have moved down and out.

FEEDING INTENSITY TRACKS TEMPERATURE, with activity and metabolic demand falling in cold water, which changes how fish respond to bait as much as where they are.

THE SOUTHERN HEMISPHERE TIMING IS THE POINT MOST OFTEN GOT WRONG by northern readers: the warm inshore period straddles the turn of the calendar year, and the cool offshore period is the middle of it.

AND YEAR-CLASS STRENGTH VARIES ENORMOUSLY between seasons, driven by conditions in the nursery estuaries during the larval and juvenile phase, so a strong season a decade ago can still be visible in the size structure of the fishery today.

Snapper are among the species where the most consequential handling decision has nothing to do with how the fish is held. It is made at depth, before the fish is on the surface.

THE PHYSICS. Gas volume varies inversely with pressure. A fish at thirty metres is at four atmospheres; at the surface it is at one. The gas in its swim bladder therefore tries to expand to roughly four times its volume on the way up, and it does so faster than the fish can absorb or vent it. The bladder distends, and it distends into everything else — the stomach is forced out through the mouth, the eyes bulge, the intestine protrudes, and internal organs are compressed and displaced.

THE PART THAT MISLEADS EVERYBODY. None of this necessarily kills the fish. What it does is make the fish POSITIVELY BUOYANT, so it physically cannot swim down. A snapper released on the surface with an expanded bladder floats, drifts, and is taken by a gull, a shark or a seal, or dies of exposure and exhaustion on the surface over hours. The angler saw it swim off strongly for two metres and recorded a successful release.

WHY A DESCENDING DEVICE IS THE ANSWER AND VENTING IS THE CONTESTED ONE. A descending device — a weighted clip or inverted hook that carries the fish back down and releases it at depth — solves the actual problem, which is buoyancy. The gas recompresses on the way down and the fish swims away at pressure. Venting, by contrast, means putting a hollow needle through the body wall into the swim bladder to release gas. Done correctly it works. Done in the wrong place, at the wrong angle, or with the wrong implement, it punctures the stomach or the intestine and the fish dies of peritonitis some days later, having swum off convincingly. That is why the guidance has shifted decisively toward descending devices over the past two decades.

AND THE DECISION THAT PRECEDES BOTH. If the intention is to release fish, the most effective intervention is to fish shallower ground. No device is as good as not causing the injury.