Fact Sheet: Western Rock Lobster

Western rock lobster

Panulirus cygnus

©R.Swainston/www.animafish.com

Unlocking lobster secrets

Colourful and protected by a strong carapace, the western rock lobster is one of the family of ‘spiny’ lobsters – and the target species of WA’s largest and most valuable fishery.

Spiny by nature

The western rock lobster is often commonly referred to as ‘crayfish’ here in Western Australia (WA), but they’re actually a lobster. Here’s how to spot the difference:

• You caught it in saltwater and it has no claws… It’s a rock lobster
• You caught it in saltwater and it has claws… It’s a lobster
• You caught it in freshwater and it has claws… It’s a crayfish.

Western rock lobster can live for over 20 years and reach sizes over 5 kg, although lobster over 3kg are rare.

This lobster species belongs to the spiny lobster family, which get their name from the hundreds of tiny forward pointing spines that cover their body and carapace, as well as their most prominent feature – the 2 huge antennae that protrude from their head. Whilst these antennae are vital for spiny lobsters to find their way around, they also form a crucial defensive weapon and communications tool.

A temperate species

Around 8 species of rock lobster are found in WA waters, but the most abundant by far is the western rock lobster.

A temperate species, western rock lobsters are only found on the continental shelf off the coast of WA, with most living in the area between Perth and Geraldton. In the southern areas of its distribution, western rock lobster mature at 6 to 7 years old, at a carapace length of about 90 mm. In the northern waters near Kalbarri and at the Abrolhos Islands, they mature at smaller sizes, usually at about 70 mm carapace length, owing to the relatively warmer water.

Distribution of Western rock lobster

Life cycle

1. Western rock lobster (Panulirus cygnus) juveniles live on shallow inshore reefs and seagrass meadows, where they grow through a series of moults over 3-4 years.
2. In late spring, sub-adult lobsters undergo a synchronised moult, changing their normal red shell colour to a creamy white/pale pink. Known as ‘whites’, these lobsters then migrate in large groups at night to spawning grounds in deep water. Their red colour returns a few months later.
3. Western rock lobster mature at 6-7 years of age. Mating occurs in late winter to spring, where the male attaches a packet of sperm, known as a ‘tarspot’, to the underside of the female between the hind most legs. When females mature, they develop setae on their swimmerettes, known as setose.

underside of female with tarspot

4. Female lobsters develop eggs around September to February. The eggs are fertilised using the tarspot as they are swept backwards and become attached to the sticky setae (fine hairs) on the swimmerettes beneath the tail. Females carrying eggs are known as ‘berried’.

‘berried’ female with eggs

5. The eggs hatch in four to eight weeks, depending on water temperature, releasing 1.8 mm long planktonic larvae called ‘phyllosoma’ into the water.
6. The phyllosoma drift offshore and spend 9 to 11 months in the open ocean, generally between 400 and 1,000 km off the Western Australian coast. The larvae grow to 35 mm in a series of moults, changing in size and shape in stages. The majority die, but the survivors are eventually carried by winds and ocean currents back towards the continental shelf.
7. The late stage larvae undergo a moult that resembles minature transparent lobsters known as ‘pueruli’. At this stage they swim up to 60 km across the continental shelf with help from prevailing currents, to settle on inshore habitats. Here they quickly develop red colouration and grow to become juvenile rock lobsters.

transparent pueruli

Predators and prey

Western rock lobster are opportunists and feed on a wide range of items – from coralline algae and detritus (dead and dying marine matter) to molluscs and crustaceans.

When they are juveniles, lobster are eaten by a number of fish species, and as adults are prey for octopus and a variety of large fish.

As part of the moulting process, rock lobsters can regrow any legs and antennae that have been lost – usually from skirmishes with hungry predators such as octopus.

Armour-plated

Rock lobsters are an aquatic animal, a crustacean like crabs and prawns. They have jointed legs, a hard shell and no backbone.

Once the larvae moult to become pueruli, they get a suit of armour that they continue to regrow for the rest of their lives. The simple fact is that spiny lobsters like western rock lobster need some sort of protection, as they feed by conducting sudden raids on reefs and ocean floors at night.

This involves a lot of looking around with their large bulbous eyes, feeling with their long antennae and small sensory hairs on their body called ‘setae’, tasting the water with their clawlike feet (strange but true) and smelling with their antennules – small antenna-like protrusions just above the mouth.

Another good reason for them being armour-plated – and having a couple of large spiny antennae that they can swing at a predator – is finding a safe home on a crowded piece of reef. Each reef has a limited number of small holes that suit a single rock lobster – and these are sometimes occupied by octopus, which like the taste of lobster.

reefs have a limited number of holes that suit rock lobster. Photo: Sue Morrison

As a result, the lobster will often seek safety in numbers and take over larger caves and rock ledges that are large enough to provide shelter for a whole group of them. A common defensive strategy employed by lobsters is for them to back into caves, so their carapace spines and spiny antennae point aggressively forward.

Western Australia’s major fishery

Western Australia’s western rock lobster fishery has received international acknowledgement as one of the best managed and most sustainable in the world, and was the first in the world to be accredited by the Marine Stewardship Council (MSC) as an ecologically sustainable fishery.

Fishing for western rock lobster occurs year-round. The commercial fishery, which involves around 230 boats, is managed under a quota framework. The recreational fishery, managed though bag, boat, possession and gear limits, involves around 50,000 fishers.

The key goal of the quota management system is to ensure the integrity of the Total Allowable Commercial Catch (TACC) that has been set for the western rock lobster fishery. The TACC can be varied, depending on factors like the success of puerulus settlement.

Several additional protective measures are used to ensure the sustainability of WA’s rock lobster populations including size limits, protection for any females in breeding condition and controls on the type of gear used (including a requirement to use escape gaps on pots to allow small undersized lobsters to escape).

a rock lobster pot

SLEDS

To reduce the interaction of Australian sea lion pups with rock lobster pots, sea lion exclusion devices (SLEDs) have been fitted to pots in areas where sea lion pups are common.

Fishy science

The stock status of the western rock lobster fishery is monitored through a long-term scientific program that provides detailed information to be able to predict catches up to 4 years in advance. This scientific program also provides estimates of breeding stock levels, along with a range of information on natural variations in the fishery, as it responds to both fishing activity and environmental variations.

To protect populations into the future, the Department of Primary Industries and Regional Development (DPIRD) uses a number of methods to monitor the status of lobster populations and forecast future catch levels. This involves fishery-independent monitoring of puerulus settlement and breeding stock levels.

Artificial seaweed collectors are placed at various sites along the coast. These can look rather like an oversized bottlebrush and are used to monitor the levels of puerulus that settle on inshore reefs.

puerulus collector

DPIRD also monitors the activities of commercial rock lobster fishers and processors through compulsory daily ‘catch and effort’ logbooks as well as through departmental staff directly measuring commercial fishers’ catch onboard their vessels. A sub-set of fishers also deploy remote camera systems within their pots to monitor the marine diversity throughout their fishing grounds. These programs provide comprehensive data that are used for stock modelling and assessment.

Good and bad years

‘Recruitment’ is a term used by researchers to describe the addition of lobsters or fish (juvenile or legal size) to an adult ‘fishable’ population, either by breeding or migration.

variation in western rock lobster recruitment at Dongara WA over time

Levels of recruitment of puerulus to WA’s western rock lobster population fluctuates considerably, depending on the strength of the Leeuwin Current, and the frequency and intensity of low pressure weather systems that generate westerly winds.

In years when the Leeuwin Current is flowing strongly, a higher proportion of larval lobster are able to return to the coast. In years when the Leeuwin Current is weak, recruitment is usually poorer. Westerly winds at the time of year when the pueruli are ready to ‘settle’ may also help more of these larval lobster to reach the shallow reefs along the coast.

Glossary

Berried: When a female rock lobster carries eggs attached to the swimmerettes on the underside of her tail

Breeding stock: Mature rock lobsters in a population that are able to breed

Carapace: A protective shell covering the rock lobster’s back, head and upper body

Distribution: The places where a species occurs

Phyllosoma: Early larval stage in the development of a rock lobster

Pueruli: Late larval stage in the development of a rock lobster, when the larvae become fully-fledged but miniature rock lobsters

Recruitment: Addition of rock lobsters (or fish) to a population as a result of reproduction, migration or growth to legal size

Settlement : A type of behaviour in fish where they reside in/inhabit/ occupy/ populate an area for a period of time

Sustainability: A condition where catch is removed from a fish stock without causing the stock to decline to unsustainable levels

Swimmerettes: Small paddle-like structures located under the abdomen that assist in movement and reproduction

Temperate: In the southern hemisphere, seas that lie between the Antarctic and the tropic of Capricorn

Fish illustrations ©R.Swainston/www.animafish.com