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1.
Tag‐recapture studies revealed differences in the growth rate of Haliotis iris between headlands and bays separated by as little as 200 m. Individuals off headlands had a significantly higher incremental growth and reached a higher maximum size than those in bays. These results were consistent with observations of the size composition of H. iris which showed that few individuals of harvestable size (> 125 mm shell length) were found in bays. Differences in the growth rate of H. iris may account for the apparent differences in the size at onset of maturity: individuals off headlands, and from a fast‐growing population off Wellington, reached reproductive maturity at a smaller size compared with those in bays. Yield‐per‐recruit modelling showed greater yields from headlands than bays, but egg‐per‐recruit modelling revealed more egg production was conserved in bays than off headlands. Yields could be increased from bays by decreasing the minimum size to 110 mm. Egg‐ and yield‐per‐recruit models were sensitive to the natural mortality rate (M = 0.03, 0.05 and 0.10 compared). The results show that parameters important in the management of commercial fisheries for abalone can vary over small distances: such variation should be incorporated in management strategies for abalone fisheries.  相似文献   

2.
The New Zealand abalone fishery produces about 1200 t annually, mostly from southern New Zealand. The fishery, based on Haliotis iris, is managed over broad management areas within which fishing intensity is spatially dispersed. The size composition of the commercial catch depends on location within a management area but is similar for divers fishing individual populations of H. iris and reflects the size composition of natural populations. For most populations, length‐frequency distributions of abalone were normally distributed: the mean shell length of H. iris was found to vary within populations over small spatial scales (100s m). The relative abundance of juvenile H. iris was low in relation to adults suggesting, at least for exposed populations, that rates of recruitment to populations of H. iris may be low. The relative abundance and mean shell length of juveniles and adults of H. australis was much less than that of H. iris. The scarcity of H. australis and the lack of separate catch quotas for the two species are factors which explain the low to zero catch of H. australis. The spatial variation in the size composition of H. iris suggests that management would be more appropriate over spatial scales which reflect local populations rather than the large spatial areas which are used to manage the New Zealand abalone fishery.  相似文献   

3.
Short-lived species are extremely dependent on the seasonal and interannual variability of environmental conditions, and determining their stock status is often difficult. This study investigates the effects of environmental variability and fishing pressure on the stock of octopus Octopus vulgaris in Senegalese waters over a 10-year period from 1996 to 2005. Monthly catches-at-age were estimated based on catch-at-weight data and a polymodal decomposition constrained by a given growth curve. Octopus recruitments and fishing mortalities were then estimated using a catch-at-age analysis performed on a monthly basis. Yield and biomass per recruit were simulated using a Thompson and Bell model and used to generate a diagnostic of the fishery's impacts. Results indicate that the high interannual and seasonal variability of the octopus stock biomass is linked to the spring recruitment event, the annual intensity of which was significantly correlated with the coastal upwelling index and sea surface temperature. Yield per recruit varied seasonally but remained almost unchanged from one year to the next. Even when catches vary strongly according to recruitment, the octopus stock appears to be consistently fully exploited, or slightly overexploited in some years. In this context of environmental variability, usual indicators such as the maximum yield per recruit, and the related fishing mortality and spawning potential ratio, remain useful for fisheries management purposes.  相似文献   

4.
An age-structured population model was used to examine the variability of yield created by randomly fluctuating recruitment success in the South African anchovy Engraulis capensis stock. Three different harvesting strategies were examined: (1) constant age-specific pattern of fishing mortality, (2) constant quota with effort limitation and (3) annual quota adjustments by means of an F msy procedure. Variable recruitment was generated by a stochastic stock-recruit relationship, and mean yields, mean spawner biomasses and probability distributions of yield were calculated at each exploitation level after 150 projections from the population structure of anchovy in 1981. Under conditions of constant fishing mortality, the variability of yield increased continuously as the exploitation rate was increased. The maximum average yield (MAY) is the stochastic equivalent of the maximum sustainable yield (MSY) of deterministic models, but an attempt to sustain this MAY by a constant quota resulted in a very high risk of stock collapse. An Fmsy policy based on pre-season adult biomass resulted in more variable yields than were obtained at the equivalent constant fishing mortality at age. Stability of yield therefore demanded the acceptance of lower average yields than could be attained in the long term by quota adjustment. It was considered that the South African purse-seine industry could not cope with the wide fluctuations of yield necessary to attain MAY. Specific management policies sufficiently robust to withstand both fluctuating recruitment success and the uncertainties in the parameters of working population models would be required.  相似文献   

5.
In the mid 1990s, American lobster (Homarus americanus) from Canadian waters was considered overfished based on estimated levels of egg‐per‐recruit production (EPR). In 1997, the Department of Fisheries Oceans responded by adopting a goal of doubling the level of EPR from its 1996 level. In the Magdalen Islands (Quebec), results from a simulation model indicated that doubling EPR could be reached by increasing the minimum legal size (MLS) from 76 mm to 83 mm carapace length. This was done at a pace of 1 mm per year from 1997 to 2003. Following the increase of MLS, the size structure, mean size, and the abundance of ovigerous females improved significantly in the catches. However, levels of fishing effort are still very high. This could cause undesirable effects such as altering sex ratios and size structure of males, which could lead to sperm limitation, and reducing the contribution of multiple spawners to egg production, which could become an issue if egg quality is important. Consideration of these issues leads to the recommendation of a multidimensional approach to lobster conservation.  相似文献   

6.
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.  相似文献   

7.
Indicators of abundance for American lobster (Homarus americanus) based on 8 years of trap catch rates (catch‐per‐unit‐effort, CPUE) were evaluated. Volunteer harvesters recorded count, sex and size of lobsters captured in standard traps on a daily basis during the fishing season in coastal Nova Scotia, Canada. We examined the extent to which standardised CPUEs of prerecruits predict the future catches of legal sizes and explored spatial patterns in the abundance of lobsters of different size and reproductive status. The standardised CPUE of prerecruits was correlated with legal size catches in only one of five areas examined. This area had a strong signal of incoming recruitment. Improving the capacity of prerecruit CPUE for predicting legal size catches several years later most likely lies with model incorporation of variables associated with catchability. The spatial distribution of catch rates showed that the area with the highest historical landings per unit area also had the highest relative abundance of prerecruits. The spatial distribution data point to further areas of research related to recruitment processes in lobster in coastal Nova Scotia.  相似文献   

8.
The carapace length and total length size at first maturity in females of Jasus verreauxi (H. Milne Edwards, 1851) has been investigated by a detailed study of (a) the detection of the smallest size class with external eggs, and (b) the incidence of egg‐carrying females per size‐class interval. For the study 834 females were measured and examined at Spirits Bay, New Zealand, during mid October and late December 1966. The specimens ranged in carapace length from 12.0 cm to 23.9 cm, and in total length from 29.0 cm to 56.9 cm.

The results of the study show clearly that the females reach first maturity at a carapace length or a total length larger than the present minimum legal size, i.e. a carapace length of 10.2 cm (4.02 in.), or a total length of 25.4 cm (10.0 in.). No mature females were found less than 13.5 cm carapace length, or 32.0 cm total length. First maturity was attained by the majority (50% or more) of the J verreauxi females at a carapace length of 15.3 cm or larger, or at a total length of 38.0 cm or larger.

No undersized (smaller than the minimum legal size) females were present in the collections—yet 10.3% of the specimens were found to be immature. Immature females ranged in carapace length from 12.0 cm to 16.9 cm, or in total length from 29.0 cm to 41.9 cm. Being larger than the minimum legal size, all of the immature specimens examined were “harvested"—without having reached first maturity. It appears therefore that the stocks of small, immature female J. verreauxi are not protected by the present New Zealand minimum size regulation.  相似文献   

9.
Growth rates of three species of abalone in New Zealand were estimated by fitting data to the von Bertalanffy growth equation. Length‐frequency analysis and tagging were used for Haliotis iris Martyn, and shell growth‐checks were used for H. australis Gmelin and H. virginea Gmelin. The relative growth coefficient (K) and the asymptotic length (L8) were found to be 0.3104 and 146.2 mm respectively for H. iris, 0.3205 and 86.75 mm for H. australis, 0.4460 and 61.5 mm, and 0.3231 and 64.4 mm for H. virginea from two separate localities. An absolute growth curve could be calculated only for Haliotis iris.  相似文献   

10.
Direct surveys of Cape anchovy Engraulis capensis spawner and recruit biomass in South African coastal waters were made with a 38-kHz calibrated echo-sounder and custom-built digital echo-integrator. The spawning surveys were conducted each November between 1983 and 1985, predominantly on the Agulhas Bank. Recruit surveys were conducted in autumn 1985 and 1986, mainly on the West Coast. The spawning fish were widely distributed across the continental shelf, whereas recruit concentrations were highest close inshore. Employing a target strength per kilogramme expression for herring Clupea harengus, the spawning biomass in 1984 was estimated at 1,06 million tons and that in 1985 at 0,98 million tons. Similarly, recruit biomass in May/June 1985 and June 1986 was estimated at 310 and 466 thousand tons respectively. From an analysis of error in the latter two estimates, it was considered approximately 80 per cent certain that the recruitment in 1986 was greater than in 1985. The estimates of spawning biomass are 3–4 times higher than earlier estimates made by Virtual Population Analysis (VPA) of commercial catch data, and suggest that the stock is less seriously threatened by present catch levels than was indicated by VPA. It is concluded from the distribution and age structure of the spawning population that the VPA estimates are invalid because of the failure of commercial vessels to sample the major part of the adult population.  相似文献   

11.
Polydorid polychaetes can infest cultured abalone thereby reducing productivity. In order to effectively control these pests, their reproductive biology must be understood. The population dynamics and reproduction of polydorids infesting abalone Haliotis midae from two farms in South Africa is described using a length-based, age-structured model. Shells were infested mainly by introduced Boccardia proboscidea. Polydora hoplura and Dipolydora capensis were also present but in numbers too few to identify factors influencing infestation. At both farms, B. proboscidea lived for a minimum of 12 months. Growth rate, size at maturity, maximum size, infestation intensity, recruitment, percentage of the population brooding and mortality appear to be affected by abalone feeding regime and water temperature, and these factors need to be considered in controlling infestation. Brooders and recruits were present throughout the year, but increased significantly during mid- to late winter/early spring when water temperature and day length increased. Treatment measures should therefore be implemented throughout the year but with increased effort when water temperature increases.  相似文献   

12.
The effect of temporal variation in growth rate on per-recruit model outputs was investigated by comparing biological reference points obtained using growth curves derived for Umbrina robinsoni populations from False Bay, on the south coast of South Africa, sampled 10 years apart (1991–1993 and 2001–2003), and applying the two different age–length keys to length frequency data collected over the period 1991–1993 (n = 1 389) for the estimation of total mortality (Z), fishing mortality (F) and spawner biomass per recruit (SB/R). The SB/R and yield per-recruit (Y/R) curves constructed using biological data collected during the two different time periods were very similar, with target and threshold reference points differing only moderately (11%). This suggests that for long-lived species, the frequency of age and growth studies can be at least the lifespan of a cohort if they are to be assessed with a per-recruit approach. The use of the more recent length-at-age data, however, resulted in lower estimates of F (0.61 y?1 vs 1.05 y?1) and a slightly higher spawner biomass per-recruit ratio (19.5% vs 14.5%) for the earlier period. The reduced estimate of F when using the more recent dataset was likely due to a bias towards older fish caused by small sample size, the philopatric nature of U. robinsoni, and possible recruitment collapse. This highlights the importance of having a large, representative sample of length-at-age data for construction of age–length keys.  相似文献   

13.
The effect of climate change on natural oyster recruitment has the potential to disrupt many of the ecosystem services oysters provide. Due to the temperature‐sensitivity of reproduction, oyster recruitment may shift as water temperatures rise. A biological imprint of climate change was revealed in a multi‐decadal time series of recruitment of non‐native Pacific oysters (Crassostrea gigas) in the main stem of Hood Canal, Washington, USA, extracted from historic fishery documents. Water in July and August warmed significantly from 1945 to 1995 (0.028 ± 0.004°C per year [±SE]) and accounted for an increase in Pacific oyster recruitment (7% per year, 0.028 ± 0.006 spat per year on log scale [±SE]); recruitment also strongly tracked inter‐annual variability in summer water temperature. Methods used to collect historical data were repeated in 2013–2015 when recruitment of both Pacific oysters and native Olympia oysters (Ostrea lurida) were recorded in main stem and lower Hood Canal. Both historic and modern data show large variation within and between years for temperature as well as recruitment. The modern data add information regarding spatial variation, in that recruitment patterns in the two regions of Hood Canal were decoupled. As temperatures continue to increase, non‐native Pacific oysters are likely to be favored over Olympia oysters, which recruit earlier at lower temperatures and presently contribute less than half of total oyster recruits. Future recruitment, however, may be limited by environmental factors other than temperature, a point indicated particularly in Hood Canal where many subtidal species already respond strongly to gradients in dissolved oxygen.  相似文献   

14.
Seagrass is an ephemeral habitat for epifaunal sessile invertebrates attaching on seagrass leaves, and spatial and temporal dynamics of seagrasses strongly affect the distribution of epifauna. Zostera caulescens Miki, a seagrass species endemic to Japan, provides a complex habitat for epifauna with two types of shoots: vegetative, less than 1 m tall, and flowering, 5–7 m tall. We conducted monthly field observations and a manipulative field experiment to investigate the effects of the seagrass vertical structure and its temporal variation on the distribution and recruitment of the encrusting bryozoan Microporella trigonellata. The density of M. trigonellata on the leaves of flowering shoots, located at the seagrass canopy, varied temporally, reaching maximum in summer and minimum in winter. In contrast, M. trigonellata density on the leaves of vegetative shoots near the sea floor was consistently low throughout the study period. Early recruit bryozoans also showed this temporal and vertical variation in density; thus spatial and temporal variation in recruitment determined the distribution of the whole colonies. The field experiment revealed that the recruitment rate of M. trigonellata was significantly higher at the higher position of the water column (3 m above the sea floor) than at the lower position (0.5 m) in June. However, the recruitment rate was higher at the lower position in October when most of the flowering shoots started falling down. The temporal change in bryozoan habitat selection is considered to be adaptive to maintain their population on the seagrass leaves that show complex temporal dynamics.  相似文献   

15.
The spawning season of Haliotis iris Martyn, as indicated by a gonad index and examination of ovaries, was late summer to autumn at Kaikoura, New Zealand, in 1967–68. In contrast, H. australis Gmelin spawned twice, once in the spring and again in the late summer to autumn. Both patterns are typical of haliotids. But in the 1968–69 year neither species spawned; full gonads with apparently ripe eggs were maintained through the winter of 1969. No reason for this inconsistency can be offered. Haliotis iris at Taylors Mistake, 154 km south, followed essentially the same pattern as Kaikoura H. iris through 1968, but spawned slightly in autumn 1969.

Haliotis iris first produces mature eggs when it is about 60 mm long and probably spawns substantially for the first time when it is 4 years old. Although fecundities of larger animals reached about 11 million eggs, not all of these were necessarily spawned. Haliotis australis also first produces mature eggs when it is about 60 mm long, but the age at which first spawning takes place could not be determined; maximum fecundity is about 3 million eggs. The sex ratios did not differ significantly from 1: 1 for either species.  相似文献   

16.
The status of a fishery is often defined as the probability of fishing mortality rate exceeding a perilous level for long‐term sustainability. Lobster stock assessments are often subject to large uncertainty in input data and high levels of natural variability in lobster life history processes, which calls for incorporating uncertainty associated with both indicator and management reference points in an evaluation of biological risk of overfishing. Using a Monte Carlo simulation approach, we evaluated the impacts of uncertainty in modelling on the determination of the status of the Taitung spiny lobster (Panulirus penicillatus) fishery (Taiwan), which has not been quantitatively determined despite its commercial importance. The commonly used biological reference points derived from the per recruit model (F 0.1 the fishing mortality rate where the slope of the curve of yield‐per‐recruit model is 10% of the maximum slope and F 4Q%, the fishing mortality rate that reduces the expected egg production for a cohort of female lobsters to 40% of that produced in the absence of a fishery of the egg‐per‐recruit model) were influenced by uncertainties associated with lobster life history and fishery parameters. A large uncertainty in the current fishing mortality rate (F cnr) and estimates of biological reference points (F BRPs) increased the uncertainty in determining the risk of overexploitation throughout the confidence levels of the stochastic decision‐making framework. This simulation study suggests that the target reference point of F 40% is less sensitive to the input parameters’ uncertainty than F 0.1 We suggest a further evaluation of other F‐based references points and development of biomass‐based reference points before final selection and implementation for the management of the Taitung lobster fishery.  相似文献   

17.
Abstract. The population densities, spatial distributions, size frequencies, growth rates, longevity and reproductive activities of sub‐populations of the sea urchin Lytechinus variegatus were investigated over a two‐year period. Sea urchins were examined in three habitats in Saint Joseph Bay, Florida, which is within the northern limits of their distribution. Densities of sea urchins, which ranged as high as 35 individuals ·?2, fluctuated seasonally at all sites and were higher in seagrass beds comprised of Thalassia testudinum than Syringodium filiforme or on a sand flat. A cold front caused large‐scale, catastrophic mortality among adult, and especially juvenile, sea urchins in nearshore habitats of the Bay in the spring of 1993, leading to a dramatic decline in sea urchin densities at the Thalassia seagrass site. The population recovered over 6 months at this site and was attributable to immigration of new adults. Juvenile recruitment displayed both interannual and site‐specific variability, with recruitment being highest in seagrass habitats in fall and spring. The most pronounced recruitment event occurred in fall 1993 at the Thalassia site. Spatial distributions of adult individuals ascertained monthly never varied from random in the seagrass beds (T. testudinum and S. filiforme) or during spring, summer or fall months on the sand flat. Nonetheless, aggregations of adult sea urchins were observed on the sand flat in the winter months and were associated with patchy distributions of plant food resources. Juvenile sea urchins (< 25 mm test diameter) exhibited aggregations at all sites and 67 % of all juveniles under 10 mm test diameter (91 of 165 individuals observed) were found under the spine canopies of adults. Measurements of the inducibility of spawning indicated peak gametic maturity in all three sub‐populations in spring and summer. Gonad indices varied between habitats and years, but distinct maxima were detected, particularly in spring 1993 and late summer 1994. The mean gonad index of individuals at the Syringodium seagrass site was 2‐ to 4‐fold higher than the other sites in spring 1993 and gonad indices were much higher at all sites in spring of 1993 than 1994. Estimates of growth based on changes in size frequency cohorts coupled with measurements of growth bands on lantern demipyramids indicated that L. variegatus in three habitats of Saint Joseph Bay have similar growth rates and attain a mean test diameter of approximately 35 mm in one year. In contrast to populations within the central biogeographical range of the species, which may attain test diameters up to 90 mm, the largest individuals recorded in Saint Joseph Bay were 60 mm in test diameter, and almost all individuals were no more than 45 mm in test diameter or two years of age. The demographics of L. variegatus in the northern limits of their distribution appear to be strongly influenced by latitudinally driven, low‐temperature events and secondarily by local abiotic factors, especially springtime low salinities, which may negatively impact larval development and recruitment.  相似文献   

18.
Optimal and sustainable management of fish resources cannot be ensured without a thorough understanding of the migration patterns and population (demographic stock) structure. Recent studies suggest that these aspects of the economically and ecologically important deepwater hake Merluccius paradoxus are not reflected in the current assessment and management practices for the Benguela Current Large Marine Ecosystem. In this study, we compiled data from multiple demersal trawl surveys from the entire distribution area and applied state-of-the-art geostatistical population modelling (GeoPop) to estimate growth rate, mortality, and spatial and temporal distribution patterns of M. paradoxus. The data and the model enabled us to follow temporal and spatial changes in the distribution and infer movements from the recruitment/nursery areas, through the juvenile phase and the adults’ migration to the spawning areas outside/upstream of the nursery areas. The results indicated one primary recruitment/nursery area on the west coast of South Africa and a secondary less-productive recruitment/nursery area on the south coast near Port Elizabeth. Juveniles initially migrated away from the main recruitment area, followed by natal homing by larger individuals. This pattern was highly consistent through the time-series of the study. This perception of a, primarily, panmictic population that performs transboundary migrations between Namibia and South Africa corresponds largely to the hypothesis and data plots given in recent studies. We recommend that fisheries assessment, advice and management take into consideration these aspects of the distribution and population (stock) structure of M. paradoxus.  相似文献   

19.
Tagging of Haliotis iris Martyn at colonies near low water, and subsequent regular observations over 18 months, showed that significant movement tended to occur only in autumn and winter when rough seas disturbed the habitat. These movements may vary from year to year depending on sea conditions. Tn contrast, subtidal colonies dispersed more gradually, seemingly disturbed by the tagging operation and subsequent rehandling. Other colonies which were not tagged remained apparently unchanged for up to 2 years. Individuals in subtidal H. australis Gmelin colonies also did not move to any great extent over this period: this species inhabits narrow crevices and appears to be a much more active animal than H. iris, both in the field and in the laboratory. Drift and attached algae were virtually absent from the crevices inhabited by H. australis and it seems likely that the species must forage to feed. A homing mechanism may be present because many older H. australis are found in deep scars on the rocks or have characteristically worn shell edges fitting the rock of their particular site.

A 24‐hour watch on a tagged, subtidal colony of Haliotis iris at Kaikoura showed that there was very little movement diurnally, and it seems likely that feeding is passive; drift algae are available at most times. No evidence of homing behaviour was detected.  相似文献   

20.
Recruitment of pelagic larval fishes to the nearshore environment is dependent on a suite of biological and physical processes operating at many spatial and temporal scales. Nearshore circulation processes associated with coastal upwelling are widely upheld as major determinants of year class strength for many rockfishes (Sebastes spp.), but the mechanism by which these processes drive recruitment is largely unknown. We used Standard Monitoring Units for the Recruitment of Fishes (SMURFs) to monitor recruitment of two rockfish complexes (Sebastes spp.) and cabezon (Scorpaenichthys marmoratus) from March to September of 2004 and 2005 at 3 sites along the central California coast. We examined the relationship between recruitment of these fishes and measurements of oceanographic variability associated with upwelling dynamics, including in situ water temperature, AVHRR sea surface temperature, the Bakun upwelling index, and an index of alongshore surface water transport. We found that rockfish comprising the KCGB complex (Sebastes atrovirens, Sebastes caurinus, Sebastes carnatus, Sebastes chrysomelas) recruit during early summer, while fishes of the BYO complex (Sebastes melanops, Sebastes flavidus, Sebastes serranoides), as well as cabezon recruit during late summer. Our results provide limited support for an association between the arrival of juvenile pelagic rockfish and cabezon to the nearshore environment and physical processes related to upwelling and relaxation. Beyond the limitations of our bimonthly sampling scheme, the lack of a clear pattern may be related to the near absence of upwelling–relaxation cycles along this stretch of coast during these two study periods. Moreover, the settlement and recruitment of nearshore fishes may be closely tied to processes occurring earlier in the larval stage.  相似文献   

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