Rock Lobster Jasus Lalandii Research Articles

Simulation models of rock-lobster populations from areas of widely divergent yields on the Cape west coast

Since the 1960s, yields of the rock lobster Jasus lalandii have declined greatly in the Namaqualand (northern Cape) region from c. 3l°50′S (Doring Bay) north to the Orange River. The population of rock lobsters in this region is crowded into a subtidal fringe within the kelp beds whereas, farther south, animals approaching sexual maturity tend to move offshore to deeper waters, so that adults are found in depths up to 70 m or more. Females attain sexual maturity at a smaller size in the Namaqualand region (and in Namibia) than farther south and growth rates of adults are retarded. Simulation models are used to show how stunted juvenile growth and increased rates of natural mortality can result in the typical size distributions observed in the Namaqualand region. This is contrasted with simulations for more typical grounds to the south, where growth rates of both adults and juveniles are faster. The models also illustrate the effects of reduced growth and survival on population fecundity, fecundity per recruit and yield per recruit, assuming constant average annual recruitment in all areas. The simulations demonstrate that overcrowding and stress, brought about by a diminution in the habitat area available to lobsters in the Namaqualand region, could have led directly to reduced production and yields. This region is characterized by the presence of very low levels of dissolved oxygen in waters near the sea bed just outside the kelp beds. Oxygen deficiency is thought to have increased in the central and northern Benguela system since the 1960s as a result of changes in the rate of phyto-detrital deposition and decay in this highly eutrophic system.

Variations in fecundity and size at sexual maturity of female rock lobster Jasus lalandii in the Benguela ecosystem

Fecundity of female rock lobster Jasus lalandii was studied in order to examine whether regional differences existed along the west coast of South Africa, where growth rates (at least of males) were known to vary considerably. The study, in August/September 1985, included all major rock-lobster Fishing grounds from Port Nolloth in the north to Olifantsbos on the Cape Peninsula in the south. Earlier studies showed that growth rates of adult rock lobsters are fast in areas where per capita food availability is high (the south-central region of the West Coast), whereas growth rates were slower where food supplies are limited. The results presented indicate that fecundity of females is also higher in areas where growth rates of males are fast. Year-to-year variations in fecundity of females may possibly be an indicator of growth rates of adult rock lobsters.

Nitrogen excretion by the Cape rock lobster Jasus lalandii and its possible contribution to the inshore Benguela System

The Cape rock lobster Jasus lalandii is a major predator in the inshore Benguela system. The mean density and biomass at Oudekraal is 0,48 individuals ·m−2 or 49,75 g dry mass ·m−2. The main component of its diet is Aulacomya ater, the ribbed mussel, which has a mean biomass of 1,15 kg dry mass ·m−2. Daily consumption of carbon and nitrogen from this source reaches a maximum in summer and, when J. lalandii feed on mussels, 14,1 per cent of the flesh is lost to the environment as a result of "messy feeding". The absorption efficiency of ingested nitrogen is 86,2 per cent. Ammonia and urea excreted in the first 12 h after feeding represent 6,7 and 1,6 per cent respectively of the nitrogen ingested. Endogenous nitrogen excretion has a mean rate of 1,9 μg N·g (dry mass)−1·h−1 The range of estimates for combined figures of kelp and phytoplankton nitrogen requirements are 76,4 – 86,7 g N·m−2·year−1 J. lalandii returns 6,3 g N·m−2·year−1 to the system, accounting for 7,2 – 8,2 per cent of annual kelp and phytoplankton requirements. This could be of particular importance during downwelling when the supply of new nitrogen is limited.

Review of the Fishery for and Biology of the Cape Rock Lobster Jasus lalandii with Notes on Larval Recruitment

The South African fishery for the rock lobster Jasus lalandii currently yields about 3800 metric tons (8.4 million animals) of which more than 90% are males. Males grow much faster than females, and a size limit which is fairly large in relation to the size and age at maturity protects females from exploitation for most of their lifespans, thereby ensuring a high level of larval production from the population. Exploitation rates in the fishery are controlled by means of quotas on fishing companies, as well as catch limits on individual fishing grounds. Quotas are reassessed annually to maintain instantaneous fishing mortality rates of approximately 0.3 on all grounds. Catch rates on most grounds have been increasing since area catch limits were introduced in 1980, and seasonal averages now vary between 4 and 12 kg∙trap-day−1. Although annual indices of puerulus larval recruitment are not available, and the stock–recruitment relationship is unknown, it appears that relatively stable yields can be maintained despite fluctuations in puerulus recruitment. Density-dependent regulatory mechanisms operative in juvenile and young adult stages tend to dampen large variations in the numbers of adults entering the fishable size range each year. Variations in puerulus recruitment are likely to result from changes in the paths and velocities of extensive offshore currents which eventually return larvae toward the coast of South Africa after many months of pelagic existence.

Postural Reflexes Coordinating Walking Legs in a Rock Lobster

Postural Reflexes Coordinating Walking Legs in a Rock Lobster

Predator-prey relationships between the rock lobster Jasus lalandii and the mussel Aulacomya ater at Robben Island on the Cape West Coast of Africa

The natural diet and mode of feeding of the rock lobster Jasus lalandii (H. Milne Edwards) was determined in a rock-lobster sanctuary near Cape Town, South Africa. Field observations were tested and confirmed by means of aquarium studies. Rock lobsters feed mainly upon ribbed mussels Aulacomya ater (Molina), which comprise the largest component of the sessile benthic fauna. Mussel remains were found as the major constituent in 97% of the rock-lobster stomachs examined. The density of rock lobsters averaged 8,100 per hectare (0.81 m-2), while mussel biomass averaged more than 5 kg (wet whole weight) m-2 within the same depth range (12 to 30 m). More than 80% of the mussel biomass comprised large individuals between 60 and 90 mm in length. Large rock lobsters (mainly males) were capable of feeding on all sizes of mussels, although many of these were inaccessible to predation. Smaller rock lobsters became progressively more limited in the size range of mussels which they could crack open and consume. Competition between rock lobsters for small mussels appeared to be intense, as mussels of suitable size for feeding were generally in short supply to most of the rock-lobster population. Hence, feeding and growth rates of rock lobsters are likely to be affected by the relative population densities of predator and prey. Growth rates of rock lobsters could be limited by food supplies even in areas where mussel biomass is comparatively large.

Growth of the rock lobster Jasus lalandii and its relationship to benthos

Growth increments of the male rock lobster Jasus lalandii vary systematically with location within a small (38 km2) fishing ground off the South African west coast. High growth rates were recorded from an area where the availability of benthos suitable as rock-lobster food was also high. Increments could also be shown to vary significantly in different years, but over the size range examined, growth did not appear to be a function of size. Benthic biomass was found to be inversely related to depth. However, comparisons between biomass values from the same depths in two different areas of the fishing ground suggested that biomass was higher in areas which were in proximity to shallow water, where extensive kelp beds were found. Mussels (Aulacomya magellanica) formed an important constituent of the rock lobster's diet; these were prevalent at depths between 20 and 40 m, and especially in areas where rock-lobster growth rates were high.

A Mass Stranding of Rock Lobsters Jasus Lalandii (H. Milne Edwards, 1837) At Elands Bay, South Africa (Decapoda, Palinuridea)

Die sudafrikanische Languste (Jasus lalandii) fuhrt in der Gegend von Elands Bay (etwa 160 km nordlich von Kapstadt) jahreszeitliche Wanderungen zur Kuste durch. Dieses Verhalten wird verursacht durch die Anwesenheit einer sauerstoffarmen, bodennahen Wasserschicht, die sich, infolge winderzeugter Auftriebsbewegung, zur Kuste hin verlagert. Im Marz 1968 waren die Langusten in Strandnahe zusammengedrangt und konnten wahrend der Springflut bei ruhiger See in die sanft ansteigende Gezeitenzone vordringen. Bei einsetzender Ebbe sind die Langusten dann in grosser Zahl am Strand zuruckgeblieben und umgekommen. Das Massensterben war von solchem Ausmass, dass die Fange der nachsten Saison dadurch beeintrachtigt waren.

A symbiotic shell-encrusting bryozoan provides subtidal whelks with chemical defence against rock lobsters

The subtidal whelk Burnupena papyracea co-occurs with a voracious predator, the rock lobster Jasus lalandii, in situations where other potential prey are largely eliminated. The survival of B. papyracea has been ascribed to a symbiotic bryozoan, Alcyonidium nodosum, which characteristically encrusts the shells of this mollusc and deters feeding by J. lalandii. Although B. papyracea shells encrusted with A. nodosum were found to be stronger than non-encrusted shells, this bryozoan-induced physical defence was insufficient to deter predation by J. lalandii. However, laboratory bioassays using individual J. lalandii suggested a chemical basis for feeding deterrence originating from A. nodosum and highlighted the advantages of using ecologically relevant bioassays to monitor chemically mediated interactions between species in the marine environment. The nature of the chemical compound(s) responsible for the acquired chemical defence passively imparted by A. nodosum to B. papyracea is unknown.

Temporal changes in kelp forest benthic communities following an invasion by the rock lobster Jasus lalandii

The rock lobster Jasus lalandii expanded its centre of distribution south-eastwards into an area known as ‘East of Cape Hangklip’ on the south-west coast of South Africa in the early 1990s. Using historical and present data, we analysed differences in the abundance of key species and functional groups between the pre- and post-rock lobster invasion periods at two sites along that coast: Cape Hangklip and Betty's Bay. Pre-1989, lobsters were absent, but after 1995 they reached densities approximating 0.4–0.8 m−2. Benthic community composition also changed significantly with herbivores being abundant whereas macroalgae and sessile invertebrates were scarce pre-invasion. We attribute the decline of herbivores to the direct effects of lobster predation, in turn indirectly promoting macroalgae. Post-invasion sessile invertebrates and macroalgae increased by 2 600% and 453% respectively, whereas herbivores declined by 99.3%. The virtual elimination of the sea urchin Parechinus angulo-sus by rock lobsters has substantial implications for the commercial harvesting of the abalone Haliotis midae, because its juveniles are intimately associated with this urchin. The lobster invasion has thus not only led to a regime shift of the ecosystem but has also substantial economic consequences, which calls for an ecosystem approach to the management of the pool of commercially exploited resources in this region.

The Cape Rock Lobster Jasus lalandii from the Bonteberg Excavation

The Cape Rock Lobster Jasus lalandii from the Bonteberg Excavation