Biology and fishery of the Bay Lobster, Thenus spp.

Bay lobsters of the genus Thenus are a common and valuable bycatch of the shrimp trawl fisheries of northern Australia. Until recently there was thought to be a single species of this genus, but two species are now recognised from Australia, Thenus orientalis and T. indicus, and additional species are likely to be described from other regions. The economic importance of the Australian species was at odds with the meagre biological information available until detailed studies were made in the 1980’s and 1990’s, which generated important information for fisheries management, and revealed the specialized nature of these lobsters. Thenus spp. inhabit the soft, sedimentary mud and sand of the continental shelf, particularly in inter-reef areas along the tropical coastline of Australia. Their morphology and behavior share much in common with other Scyllarids, but also have unique features which reflect successful adaptation to their environment. Most notable are the ability to swim, often long distances, and the capacity to bury into the sediment. T. indicus is generally the smaller of the two species (maximum size approximately 65mm CL), and inhabits shallow, inshore water, 10 to 30m deep in areas characterized by fine sand and silt. T. orientalis grows to a larger size (maximum size approximately 95mm CL) and inhabits waters of 30 to 60m depth, where sediments are characterized by medium to coarse sands. For both species, sexual dimorphism is very subtle. Mating is believed to involve a brief encounter, and the spermatophoric mass is likely to be very short-lived. Mean fecundity is 12,455 for T. indicus and 32,230 for T. orientalis. Reproductive seasonality is marked by two spawning peaks corresponding to spring and mid-summer. Growth is quite rapid, and for both species a CL of 40mm (approximately 40g total weight) is reached within the first 12 months, and approximately 80% of maximum size by 2 years of age. Maximum age for T. indicus is between 2 and 4 years, and between 4 and 8 years for T. orientalis. Both species are nocturnally active, with clear peaks in activity at dusk and just prior to dawn. During daylight hours Thenus spp. bury themselves in the sediment, with only the eyes and antennules exposed. During periods of activity, Thenus spp. occasionally leap from the sediment into a swimming mode effected by contractions of the abdomen, and controlled by movement of the antennae. Such locomotion swimming may be sustained for periods up to 40 minutes, enabling lobsters to move distances of several hundreds of meters. The fishery for Thenus in Australia is managed, and regulations include a total ban on the taking of berried females and a minimum size of 75mm carapace width. CPUE has been stable over the past 2 decades, although total catch has diminished with reductions in fishing effort. For Queensland, a maximum catch of 755 tonnes was recorded in 1997, and more recently, the annual catch has been around 400 tonnes