Spatial variation in size at onset of maturity of female southern rock lobster Jasus edwardsii around Tasmania, Australia
Abstract
The size at onset of maturity (SOM) of female Jasus edwardsii (Hutton, 1875) was estimated at 50 sites around Tasmania, Australia, based on the presence of ovigerous setae. There was a distinct spatial cline with the largest SOM being found at northwestern sites and the smallest at southwestern sites. Variation in SOM between sites was substantial and ranged from 59 min to 112 mm carapace length. The observed decline in SOM from north to south was the reverse of that described for the same species at similar latitudes in New Zealand, which suggests that SOM in J. edwardsii is regulated by factors in addition to temperature. The effect of density on female SOM was investigated by comparing SOM estimates from two marine reserves with adjacent fished sites; however, there was no evidence of a decline in SOM with increasing density as predicted. A model of SOM predicted by latitude and longitude is described to facilitate spatial modelling of lobster stocks. The substantial and predictable spatial variation in SOM implies that management of this fishery would be improved by incorporating spatial elements, such as regional legal minimum size limits.
... Although monitoring methods for some crustaceans are similar to methods used for fish species, many crustaceans have different life histories and population dynamics and hence a range of more tailored approaches are used. Most lobster fisheries utilise lobster pots (or traps) as the primary capture or monitoring method (Ziegler et al., 2002, Gardner et al., 2006, Courchene and Stokesbury, 2011, Tuffley et al., 2018. Several lobster fisheries utilise puerulus (final larval stage) monitoring to inform on stock abundance, based on pioneering trials conducted for the West Australian lobster Panulirus cygnus (WRL) fishery (Phillips, 1972, Caputi et al., 1995. ...
... Sponge (Pauly et al., 2022b); jellyfishes (Palomares & Pauly, 2009); planarian (Whitney, 1942); chaetognaths ; cephalochordates ; horseshoe crabs (Suzuki et al., 2008); fishes (Pauly, 2019(Pauly, , 2021a Sponge (Pauly et al., 2022b); planarians (Whitney, 1942); polychaetes (Forbes & Lopez, 1990); trilobites (Gutiérrez-Marco et al., 2009); chaetognaths (John, 1933;; fishes (Pauly, 2019) See Figure 6 for rock lobster in West Australia; Gardner et al. (2006) 5 Large individuals of a given taxon are more sensitive than small ones to hypoxia and/or high temperatures ...
Gill surface area (S) and respiration (R) in juvenile and adult crustaceans scale with their body weight (W) such that S ∝ R ∝ Wd, with d ranging mostly between 0.6 and 0.9, but always <1, as in other water-breathing ectotherms (WBE). The growth of adult crustaceans therefore approaches an asymptote, whether or not seasonal growth oscillations are explicitly considered in the model used to describe that growth (e.g., von Bertalanffy growth function). On the other hand, the variation in asymptotic size (L∞ or W∞) among crustaceans is primarily determined by water temperature, which impacts on the oxygen requirements of WBE. Through multiple examples, this and related aspects of the Gill-Oxygen Limitation Theory (GOLT), first developed for fishes and later extended to other WBE, are shown to also apply to the growth of a wide range of crustacean taxa. The GOLT also explains certain aspects of crustacean reproduction, such as the relationship between size at first maturity and maximum size, and, possibly, the feature that female crustaceans hold their eggs outside of their bodies instead of internally.
... The longer larval duration of Southern Rock Lobster (Villacorta-Rath et al., 2016) likely results in non-local recruitment conditions driving their distributions. Changes in upwelling patterns and resulting reductions in temperatures can have an impact on the growth rate and size of maturity of Southern Rock Lobster (Gardner et al., 2006;Linnane et al., 2008). It is expected that settlement of larvae will become even more variable with time as climate change alters the oceanographic environment at higher rates in the future (Linnane and Crosthwaite, 2009). ...
Parks Victoria Technical Series Number 118
... Although there was no apparent impact in this study, egg-bearing females may behave and compete differently, requiring further study (Campbell 1990, Figler et al. 1997, Mello et al. 1999). Second, due to different growth rates and size at maturity between J. edwardsii and S. verreauxi (Montgomery 1992, Gardner et al. 2006, matched size pairs of lobsters were of different ages and not at the same stage of sexual maturity in this study. However, since size is an important predictor of competitive dominance in crustaceans (Thomas et al. 2003), future studies will need to assess this factor by simulating natural size variations. ...
Species redistributions are one of the most prevalent changes observed in oceans worldwide due to climate change. One of the major challenges is being able to predict temperature-driven changes to species interactions and the outcome of these changes for marine communities due to the complex nature of indirect effects. In the ocean-warming hotspot of south-east Australia, the ranges of many species have shifted poleward. The range of the eastern rock lobster Sagmariasus verreauxi has extended into warming Tasmanian waters inhabited by the resident southern rock lobster Jasus edwardsii , which may lead to increased competitive interactions between the species. Using video monitoring, we investigated how the 2 species compete for food at current (18°C), future (21°C) and future heatwave (24°C) summer temperatures. Behavioural competition occurred in 80% of experiments, during which J. edwardsii won 84% of competitive interactions and showed more aggressive behaviour at all temperatures. This indicates that resident J. edwardsii is not only more dominant in direct food competition than the range-shifting S. verreauxi but, surprisingly, also sustains competitive dominance beyond its physiological thermal optimum under predicted future ocean warming and heatwave scenarios.
... Reproductive characteristics of crustaceans, however, are influenced by a suite of factors, including environmental variables (Fisher, 1999;Dawe et al., 2012;Gallardo et al., 2017), nutrition (Pollock, 1995;Berkenbusch & Rowden, 2000), predation and disease (Shields, 1991;Haig et al., 2016;Olson et al., 2018), population density (Queirós et al., 2013;Murphy, 2021) and fishing pressure (Lester et al., 2014;Tu et al., 2018). Water temperature, in particular, is well-recognised to influence the onset of sexual maturity (Annala et al., 1980;Gardner et al., 2006;Kuhn & Darnell, 2019;Santana et al., 2021), timing and duration of mating and spawning (Aragón-Noriega & Alcántara-Razo 2005; Castilho et al., 2008;van de Kerk et al., 2016) and the number and viability of eggs produced (Pinchuk & Hopcroft, 2006;Fischer & Thatje, 2008;Perez-Miguel et al., 2020). Robust management therefore requires knowledge of how these reproductive parameters vary over time as well as throughout the geographic distribution of a species. ...
Sound fisheries management requires knowledge of how the reproductive biology of targeted species varies over space and time and responds to external drivers. We determined the size at maturity and fecundity of the blue swimmer crab Portunus armatus (A. Milne-Edwards, 1861) (Brachyura: Portunidae) along the west coast of Australia at monthly, annual and interdecadal (1990s versus 2010s) time scales. The size at maturity of females determined by abdominal morphology (carapace width (CW) at 50% maturity (CW50)) was highest in sub-tropical Shark Bay at 26°S (104 mm), and declined latitudinally to temperate Geographe Bay at 34°S (93 mm). A notable exception was in a marine embayment at 32°S (Cockburn Sound), where CW50 was significantly lower than all other locations (83 mm). In contrast, batch fecundity was lower in sub-tropical (830,000 eggs at 127 mm CW) than temperate waters (950,000–990,000 eggs). Temporally, CW50 was significantly higher during January-June (typical growth period, encompassing the warmest months of the year) than during July-December (breeding period), and also differed between months, years, and decades. Positive correlations between water temperature, monthly and annual estimates of CW50, and functional maturity (size at first ovigery), were detected in most locations. Annual CW50 also increased with population density, particularly in locations where abundances and biomass were lowest. Results highlight plasticity in the reproductive biology of this portunid in response to environmental drivers such as temperature. Regular review of such parameters is important for ensuring robust fisheries management and stock assessments.
... However, it is likely that the influence of sex would be negligible as lobsters were not mature. The size at maturity for this species and region is 75-80 mm CL (see Gardner et al., 2006). Spiny lobsters undergo changes in social behaviours (e.g. ...
Predation risk can strongly shape prey ecological traits, with specific anti-predator responses displayed to reduce encounters with predators. Key environmental drivers, such as temperature, can profoundly modulate prey energetic costs in ectotherms, although we currently lack knowledge of how both temperature and predation risk can challenge prey physiology and ecology. Such uncertainties in predator–prey interactions are particularly relevant for marine regions experiencing rapid environmental changes due to climate change. Using the octopus (Octopus maorum)–spiny lobster (Jasus edwardsii) interaction as a predator–prey model, we examined different metabolic traits of sub adult spiny lobsters under predation risk in combination with two thermal scenarios: ‘current’ (20°C) and ‘warming’ (23°C), based on projections of sea-surface temperature under climate change. We examined lobster standard metabolic rates to define the energetic requirements at specific temperatures. Routine metabolic rates (RMRs) within a respirometer were used as a proxy of lobster activity during night and day time, and active metabolic rates, aerobic scope and excess post-exercise oxygen consumption were used to assess the energetic costs associated with escape responses (i.e. tail-flipping) in both thermal scenarios. Lobster standard metabolic rate increased at 23°C, suggesting an elevated energetic requirement (39%) compared to 20°C. Unthreatened lobsters displayed a strong circadian pattern in RMR with higher rates during the night compared with the day, which were strongly magnified at 23°C. Once exposed to predation risk, lobsters at 20°C quickly reduced their RMR by ~29%, suggesting an immobility or ‘freezing’ response to avoid predators. Conversely, lobsters acclimated to 23°C did not display such an anti-predator response. These findings suggest that warmer temperatures may induce a change to the typical immobility predation risk response of lobsters. It is hypothesized that heightened energetic maintenance requirements at higher temperatures may act to override the normal predator-risk responses under climate-change scenarios.
The Norway lobster ( Nephrops norvegicus ) is one of the most important decapod crustacean seafood species in the Adriatic Sea. Previous research has identified significant differences in growth rates and maturation timing of Nephrops in the Pomo/Jabuka Pits area compared to other subpopulations in Adriatic fishing grounds. Here, we use 1,623 genome-wide single nucleotide polymorphisms (SNPs) to investigate whether the Pomo Pits subpopulation is genetically different from other sites in the Adriatic and neighbouring seas. We found no genetic differentiation among all sampled Adriatic sites, suggesting high gene flow between Pomo Pits Nephrops and those of surrounding areas. We also found genetic homogeneity between the Adriatic sites and single-site samples from the Aegean and Tyrrhenian Seas. However, we detected distinct genetic differentiation between all Mediterranean sites and an Atlantic site in western Scotland, which provides evidence for a phylogenetic break between the Atlantic and the Mediterranean. Our results indicate that Pomo Pits Nephrops are not genetically different from others sampled in the Adriatic and that key biological parameters in Pomo Pits Nephrops could be driven by spatial variation in fishing pressure and/or environmental factors rather than geographic isolation.
Crustacean fisheries, which are influenced by environmental drivers, can benefit from spatial management solutions. More size limit zones or other controls afford finer tuning of management solutions, but can also increase complexity in fishing operations. Defining management boundaries, and the feasibility of that compliance can be a balancing act between numerous stakeholders from commercial and recreational fishers, environmental groups and the science which informs such decisions. In Tasmania, the Southern Rock Lobster, SRL (Jasus edwardsii), exhibits dramatic growth variation across its range from the north to south of the fishery. A known proxy for growth is the Onset of Size of Maturity (SoM), which we have calculated for SRL at specific locations around Tasmania using a General Additive Model. Five assessment methods (objective functions: absolute difference and squared difference, both with and without equal zone weighting, and directionally penalised absolute difference) were used to identify possible management boundaries. Objective function inputs were the same for each method, and comprised of 100000 permutations of sampled SoM and spatial boundary locations for assessed regimes of 1–5 spatial boundaries. Statistical methods were applied to make zonation optimal for biology based on SoM. The results show that number of spatial zones required to yield “good” results did not require overly complex management solutions, nor unwieldy enforcement. Specifically, an investigation into two distinct spatial boundaries yielded solutions in which a critical, narrow boundary was complemented by a more flexibly located, secondary boundary for results of similar quality. The recommendations provided are in ranges of feasible locations, and likely to satisfy all stakeholders.
Waters along Australia’s most densely populated south-east coast are warming at 3.8 times the global average rate, the most rapid change in the Southern Hemisphere. Ecosystems in this region are therefore likely to be severely impacted by climate change and significant biodiversity change is expected. The rapid nature of these ecosystem changes requires science-based decisions about where, how and when to apply adaptive management interventions. Well informed predictive models are needed to estimate likely ecological changes and inform management actions such as spatial closures to protect vulnerable habitats, translocation of key predators, or direct manipulation of abundances of threatening and or threatened species. Our study addressed these challenges using a mix of long-term (up to 20-yr) monitoring records of fishes, invertebrates and macro-algae in, and adjacent to marine reserves in the region undertaken as part of University and/or State agency research programs. This was coupled with spatially extensive species abundance data derived from the Reef life Survey citizen science program (http://reeflifesurvey.com/) to examine past, and predict future ecological responses to warming, including assemblage changes, kelp decline and predator-prey relationships.
The phyllosoma larva of the southern rock lobster, Jasus edwardsii, is thought to be among the longest larval phases of any planktonic larva, with estimates in the literature ranging from 12 to 24 months. In the present study, we have used an extensive archive of samples (over 2800 samples with 680 phyllosoma) to refine the estimate of the duration of the pelagic phase. The distribution through the year of larval stages suggested that larvae from two separate spawning events were present in any 12-month period. Using regression analysis, we have estimated the duration of the phyllosoma phase to be 547±47.5 days (~18.2±1.6 months). A new model of J. edwardsii phyllosoma development is presented and compared with data on known hatching and settlement patterns. The new model will improve the paramiterisation of stage-specific biophysical models of larval dispersal and regional connectivity, to better inform management of the southern rock lobster fisheries.
This report summarises information from simulations of the effects of large Marine Protected
Areas (MPAs) on Tasmanian rock lobster Jasus edwardsii stocks. Simulations are based on
the declaration of large MPAs in conjunction with a total allowable catch (TAC, or quota)
management system using the Tasmanian rock lobster assessment model. This model
incorporates extensive fisheries and biological data1 and fits are good by resource assessment
standards. Spatial information is limited to the division of Tasmanian waters into eight regions
and no migration between regions was considered. Migration was not included in the model
as it was considered important to initially establish if MPAs affect biomass and egg
production: migration of adults across reserve boundaries simply tends to dampen any effect.
Modelling provides a technique for assessing the complex set of interacting factors that will
influence any harm or benefit from MPAs. On its own, experimental evidence of a change in
abundance or diversity inside MPAs is of almost no value for assessing the impacts of MPAs
on fisheries under TAC management. Rather, we need to know the effect of MPAs on
animals both inside and outside the reserve.
While several published studies have examined the effect of marine reserves on commercial
species, none have directly considered the effect of increased fishing effort on open areas due
to effort displacement from the reserves. No study has assessed objectively if there is a yield
or propagation reduction/benefit from MPAs when management is by TAC. This project
aimed to examine two assumptions that are usually made with MPAs - these were:
1) does an increase in egg production/biomass inside an MPA actually lead to an overall
increase in egg production/biomass across a TAC managed fishery ?
2) does the displacement of fishing effort from a MPA lead to increased fishing pressure that
outweighs any benefit from the MPA ?
On the scale of the total fishery, both of these factors may cause MPAs to harm, rather than
benefit, fish stocks and habitats. The possibility that marine reserves may harm commercial
fish stocks and associated ecosystems complicates their implementation. Marine reserves are
sometimes perceived as a foolproof management system for ensuring sustainability of fisheries.
But sustainability is easy to achieve in fisheries management when catch is simply reduced to
very low levels. The difficulty in fisheries management arises through attempting to maximise
harvest without jeopardising sustainability, and the use of MPAs must be considered in this
perspective.
Two factors relating to the modelling process are important in understanding the results:
1) the statewide annual catch of rock lobsters was constrained by total allowable catch
(TAC). Under this system, MPAs do not reduce total catch (by weight) - effort is just
shifted elsewhere.
2) the shift of effort is incorporated into the model based on the historical patterns in
behaviour of fishers in relation to season, region, and catch rates.
The analysis of MPAs was intended to assess the potential for reducing or increasing egg
production and yield through the use of MPAs, rather than prescribing location and size of
MPAs to minimise their negative impact, or to maximise any fisheries benefit. It was
considered that if a fisheries benefit could be demonstrated, spatially explicit work would be
warranted.
In general, results from projections were consistent with observations from inside existing
Tasmanian MPAs - the abundance, mean size and egg production of rock lobsters increased
inside MPAs at the expense of egg production and yield in areas outside the MPA.
Important new conclusions from this work were that:
1) Outcomes of MPAs were not always positive. Under a TAC of 1500 tonnes, sized
biomass in open areas with MPA scenarios was generally reduced by around 50% after
10 years, relative to sized biomass without MPAs.
2) the location of large MPAs influenced their effect because aspects of the biology of
lobsters vary dramatically around Tasmania.
3) Large MPAs in areas important to the fishery caused severest depletion of biomass and
egg production (and ecological impacts ?) in areas open to fishing.
4) Total statewide biomass became reduced when MPAs were placed in southern areas.
This drop in biomass reduces the viability of the fishery and may have secondary
ecological impacts. MPAs in southern areas cannot increase yield, regardless of the extent
of migration across reserve boundaries.
5) MPAs in areas south of 42°S on the east coast and 41°S on the west coast either had no
impact or a negative impact on total egg production. A large MPA in the southwest
severely depleted total statewide egg production due to the large proportion of males in
commercial harvests from this region.
6) MPAs in northern areas raised statewide egg production and biomass, albeit at the
expense of other areas. Egg production in the north is low so it may be appropriate to
sacrifice a portion of egg production in southern areas to improve egg production in
depleted northern regions.
7) MPAs provided benefits to egg production and biomass in areas where size limits are
currently sub-optimal. Similar results could also be achieved by other spatial management
techniques such as split size limits around the State, or regional quotas.
The economic value of the Tasmanian rock lobster fishery contributes significantly to the State’s regional economy. The fishery has recreational and commercial components and this annual analysis of the state of the resource is important for management advice in both sectors and to identify potential for growth.
Opportunities for economic growth exist through both increasing catches (through improved management of spatial components of the fishery) and increasing the value of harvests. However, in the commercial fishery trading conditions are currently difficult due to continued pressure on margins through a combination of increased product globally, exchange rate, and most recently the closure of the Chinese border to Australian lobsters.
The state of the resource was formally evaluated against a series of performance indicators (Table 1). These were assessed for the most recent quota year: March 2003 to February 2004. The recreational catch was estimated through surveys and the latest estimate had almost reached the trigger point of 10% of the commercial catch, which indicated that a review was required. Other performance indicators were not close to trigger points.
The Australian Government Department of Environment and Heritage reviewed the monitoring and management of the Tasmanian rock lobster fishery as part of wildlife export accreditation. This process provided recommendations to focus research onto ecosystem based management issues (Table 2). While all recommendations were met at least in part there is scope for improvement, which provides direction for future research.
Several other measures of the resource were analysed aside from the formal indicators. Many of these showed some problems with the resource, especially in sheltered and shallow areas off the north and east.
• Catch rates were standardised to remove effects of depth, block, month and skipper. Assessment Areas 1 (SE), 8 (SW), and 7(W) all exhibited continued increases in standardised catch rates relative to 2002/2003, and this was despite the catches taken from Areas 1 (SE) and 8 (SW) increasing over those taken in 2002/2003.
• In assessment Areas 2 (E), 3 (E), 4 (NE), and 5 (NW) the standardised catch rates all exhibited a strong reduction during 2003/2004. This reversal of the previous trend of growth was assumed to be the result of fished down of a strong year class that arose in 1999/2000.
• There was a trend of increasing commercial catch in shallow waters (<20 m) from Areas 2 (E) and 3 (E). The recreational sector removed a large proportion of the total catch from shallow waters (<20m) in these Areas (around 1/3 of the total). Thus the recent trend of steadily increasing recreational catch compounds the concern in the fishery.
• Non-standardised commercial catch rates in Areas 3 (E) and 4 (NE) fell by around 20% over the last year.
• Non-standardised commercial catch rates in all east coast Areas remains below 0.9 kg/pot lift, which is lower than most southern rock lobster management areas in Australia. Recent declines in Area 3 have resulted in very low mean annual catch rates (0.64 kg/potlift).
• Absolute commercial catch taken from shallow waters (<20 m) has not tended to increase in most Areas, however, the proportion of catch from shallow waters increased due to reduced effort in deeper waters. This led to a trend of stock rebuilding in deeper waters.
• Model projections for biomass indicated stability in northern Areas 4 (NE) and 5 (NW), although this did not consider possible shifts between depths.
• Model projections for egg production indicated that is unlikely that the management target of 25% virgin egg production in Areas 4 (NE) and 5 (NW) will be achieved in the foreseeable future under the current management system.
• Puerulus catches indicate future recruitment to the fishery in Areas 2 (E), 3 (E), and 4 (NE) will be low. Puerulus catches for Areas 2 and 3 have been low for a prolonged period with the last 8 years being lower than 5 of the 6 years previous. Likewise, puerulus catches for the last 4 years in Area 4 (NE) have been lower than the 5 years previous. These trends are of concern because puerulus-monitoring data appears to correlate with model estimated recruitment (which is derived from commercial catch rates).
It is recommended that management be cognisant of apparent declines in eastern and northern regions when considering options for the fishery. The fishery increasingly appears to be one of spatial extremes with concerns for the north and east coasts while western deep-water stocks appear to be undergoing strong stock rebuilding. These spatial disparities are driven by accessibility and also regional differences in the biology of lobsters.
Satellite-ua(ked drifters, satellite imagery, a research vessel and moorings in the waters of Tasmania, excluding Bass Strait, revealed a southward Cllrrent, sometimes called the Zeehan Current, on the outer continental shelfofwesternTasmania. The current has a peak in winter, wilen it reaches around southern Tasmania as far north as Schouten Island before veering southeastward under the influence of the East Australian Current. In summer, the Zeehan Current is weaker, while the East Australian Current is stronger and overshoots southward past Tasmania by about 200 km, entraining the Zeehan Current in the process. Off western Tasmania, well seaward of the Zeehan Current, the residence time for satellite-tracked drifters was as much as one year. More than 200 km south of Tasmania in summer there was weak flow to the west. Eleven of 13 satellite drifters released in Tasmanian waters eventually meandered eastward towards New Zealand, with a travel time of over 18 months from the time of release.
The spiny lobster Jasus tristani is exploited at the isolated volcanic islands of Tristan da Cunha, Nightingale, Inaccessible and Gough in the south central Atlantic Ocean. Tagging studies indicated consistent inter-island differences in lobster growth rates and size composition which were unrelated to fishing intensity. A combined diving/trapfishing survey was conducted during March/April 1989 to investigate possible reasons for observed differences in lobster growth, size composition and sizes at maturity at Nightingale and Inaccessible, two small islands just south of Tristan da Cunha. Differences in water temperature were not considered to be relevant owing to the two islands' proximity to each other. Although it was thought that food availability to the lobster stocks might be the underlying cause for the inter-island differences, an intensive diving survey failed to reveal any obvious differences in the benthic biota of the two islands. Predation by fish on small lobsters is intense at all islands, but at Inaccessible the topography of the shallow subtidal zone (where most puerulus larvae settle) affords more shelter to young lobsters than at Nightingale. Survival of juvenile lobsters is thus relatively enhanced, resulting in higher densities of recruiting lobsters at Inaccessible, a feature confirmed by diving as well as analysis of trap catch per unit effort and size composition. The observed inter-island differences in lobster population structure therefore appear to be caused by differences in density-dependent growth and survival of young lobsters. The implications of these findings are discussed in relation to yield per recruit, egg production per recruit and the effects of minimum size regulations.
Studies some years ago suggested that yields of Jasus lalandii in the northern Cape and Namibia had declined as a result of severe overfishing and progressive reductions of minimum size limits, especially between 1960 and 1970. Although catches were temporarily boosted to artificially high levels by reductions in minimum size, evidence is presented which suggests that sustainable yields may have declined largely as a result of environmental changes during and after the 1960s. It is postulated that a progressive expansion of oxygen-deficient shelf water may have forced lobsters to occupy a much-reduced habitat in shallow waters, where overcrowding has resulted in reduced growth and survival, and production and yields have declined accordingly. It is suggested that increased competition for food and space in the better-oxygenated shallows has led to a diminution in the size at sexual maturity of female lobsters and reduced adult growth rates. In addition, low levels of dissolved oxygen may have direct physiological effects on rates of feeding, growth and mortality. Possible reasons for the likely change in oxygen concentrations after the mid 1960s are discussed. Reduced grazing of phytoplankton by planktonic herbivores (zooplankton and clupeoid fish) as well as increased phytoplankton production per se are considered to be possible causative factors leading to the carbon-overloading prevalent in the central and northern Benguela system during the past two decades.
The fecundity and size at onset of sexual maturity of Nephrops norvegicus was estimated at different locations within the Firth of Clyde, S.W. Scotland. The “potential” fecundity was estimated from oocyte counts in mature ovaries, while estimates of “actual” fecundity were made from counts of the eggs attached to the pleopods of ovigerous females. Size at onset of maturity of Nephrops norvegicus was estimated from the study of primary (ovary maturity and eggs on pleopods) and secondary (allometric) sexual characteristics.
Relationships between potential fecundity and female carapace length were found to conform to a power function (F=aLb). Comparison of the logarithmic formulations of the relationships for seven locations revealed no differences in power terms, but there were significant differences in the constants. Results suggested that differences in fecundity mainly reflected a geographical variation in oocyte volume (expressed in terms of mean dry weight) which appeared to be related to growth. Where growth rate was characterized by a low value for the asymptotic length of the carapace (L∞), females appeared to have smaller oocytes and larger size-specific fecundity.
Estimates of size at onset of sexual maturity varied over small geographic scales (tens of km) and ranged between 21–34 mm carapace length for females and 29–46 mm for males, the estimate obtained from the different approaches being similar. Size at onset of maturity was positively related to L∞(p<0.05) and negatively related to adult density (p<0.05). Age at onset of maturity appeared relatively constant geographically but varied between the sexes (males, 4–4.5 years; females, 3–3.5 years).