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1.
The leatherjacket Meuschenia scaber is widely distributed in Australasian waters, and is a valued bycatch of inshore bottom trawl fisheries although little is known of its life history. Here, we describe the reproductive biology of the species based on 651 leatherjackets sampled in the Hauraki Gulf, New Zealand, between July 2014 and March 2016. The maximum total length (LT) recorded for females and males were 320 and 315?mm, respectively, with both sexes present in all size classes. Monthly analysis of gonad condition revealed a clear spawning season from late austral winter to early summer (August–December), and histological analysis of the ovaries revealed that M. scaber is an indeterminate serial spawning gonochorist. The estimated sizes at sexual maturity (L50) for females (189.9?mm LT) and males (188.4?mm LT) did not differ significantly. Relatively small testes, sexual dimorphism and underwater observation of nesting suggest that M. scaber is a paired spawner.  相似文献   

2.
A total of 639 pink dentex Dentex gibbosus was collected in Canary Islands waters between April 1991 and September 1993. Total lengths ranged from 14,2 to 95,2 cm. Females dominated smaller size-classes and males the larger ones. The species was characterized by protogynous hermaphroditism. The overall ratio of males to females was 1 : 1,45. The reproductive period extended from April to September, spawning peaking in June/July. The total lengths at 50% maturity were 34,7 cm for females and 38,6 cm for males. The length-mass relationship for the whole sample can be described by the parameters a = 0,01014 and b = 3,0812. Fish aged 0–16 years were present in the samples. The parameters of the Von Bertalanffy growth equation were: L ∞ = 101,2 cm, k = 0,149·year?1, and t 0 = ?0,111 years. The rates of total mortality Z and natural mortality M were 0,57 and 0,28·year?1 respectively. Rates of fishing mortality F and exploitation E were 0,29 and 0,51·year?1 respectively. The estimated length at first capture (LC 50) was 17,8 cm total length.  相似文献   

3.
Growth and reproduction of the Japanese mantis shrimp, Oratosquilla oratoria, were investigated in the Tongyeong, Korea from July 2014 to August 2015. A total of 2,621 samples (1,380 females and 1,241 males) were collected during the study period. Females were observed more frequently than males. The mean body length (BL) was 128.5 ± 0.38 mm in females and 126.9 ± 0.42 mm in males. The mean body weight (BW) was 31.2 ± 0.28 g in females and 31.1 ± 0.32 g in males. There was a significant difference in the length-frequency distribution between females and males. The relationship between BL and BW was lnBW = 2.85 × lnBL - 10.43 for females and lnBW = 2.87 × lnBL -10.52 for males. The gonadosomatic index (GSI) varied on a monthly basis. The GSI reached a maximum in May and a minimum in November. The highest values of the GSI coincided with the spawning period of O. oratoria. Larger individuals of O. oratoria have their spawning season earlier than smaller ones. The size at sexual maturity of females was estimated as 96.5 mm. The Von Bertalanffy growth function parameters were BL∞ = 184.5 mm, K = 0.72 year?1, C = 0.36 and WP = 0.45 for females and BL∞ = 183.75 mm, K = 0.82 year?1, C = 0.38 and WP = 0.22 for males. The growth of males was slightly faster than females. The present study will help with the fisheries management of O. oratoria based on ecological parameters.  相似文献   

4.
The biology of the redspotted tonguesole Cynoglossus zanzibarensis, a common African cynoglossid inhabiting the Agulhas Bank, South Africa, is described. Growth studies based on sectioned sagittal otoliths revealed that C. zanzibarensis is relatively fast-growing and long-lived, attaining ages >8 years. Growth in length was rapid in immature fish, fish attaining 56% of their maximum size within their first year. By sexual maturity, fish had attained 28% of their maximum age and 68% of their maximum length. Total length-at-age was best described by the Von Bertalanffy growth model with combined-sex growth described as Lt = 354.78(1?e?0.43 (t+1.17)) mm TL. Sexually dimorphic growth patterns were evident, females attaining larger lengths, but at a slower growth rate than males. Despite the similar mean size of adult fish, the trawl-sampled adult population was dominated by females, with a sex ratio of 1 male:2.4 females. Female C. zanzibarensis mature in their second year of life (275 mm TL), after which they spawn small, pelagic eggs throughout the year. Approximations of the rates of total, natural and fishing mortality were estimated to be 0.62, 0.48 and 0.14 year?1 respectively.  相似文献   

5.
The life history of Plesionika edwardsi (Brandt, 1851) around the Canary Islands in the Eastern Central Atlantic was investigated, based on a total of 11 434 shrimps ranging in length between 8 and 40 mm carapace length (CL). The species carries out seasonal migrations; they concentrate in deep water during winter, move shallower in summer and return to deep water again in autumn. Ovigerous females occur throughout the year, but a spawning peak was determined between April and September. The size at maturity for females was approximately 26 mm CL. Shrimp size generally increased with increasing water depth. The growth parameters for males were L ∞ = 25.75 mm CL and K = 0.55 year?1, and L ∞ = 28.28 mm CL and K = 0.66·year?1 for females. The species displays the typical reproductive pattern of tropical pandalids and is dioecious.  相似文献   

6.
The olive ridley is the most abundant seaturtle species in the world but little is known of the demography of this species. We used skeletochronological data on humerus diameter growth changes to estimate the age of North Pacific olive ridley seaturtles caught incidentally by pelagic longline fisheries operating near Hawaii and from dead turtles washed ashore on the main Hawaiian Islands. Two age estimation methods [ranking, correction factor (CF)] were used and yielded age estimates ranging from 5 to 38 and 7 to 24 years, respectively. Rank age‐estimates are highly correlated (r = 0.93) with straight carapace length (SCL), CF age estimates are not (r = 0.62). We consider the CF age‐estimates as biologically more plausible because of the disassociation of age and size. Using the CF age‐estimates, we then estimate the median age at sexual maturity to be around 13 years old (mean carapace size c. 60 cm SCL) and found that somatic growth was negligible by 15 years of age. The expected age‐specific growth rate function derived using numerical differentiation suggests at least one juvenile growth spurt at about 10–12 years of age when maximum age‐specific growth rates, c. 5 cm SCL year−1, are apparent.  相似文献   

7.
The age and growth of filefish, Thamnaconus modestus (Günther 1877) in the southern waters of Korea were investigated. Samples were collected with commercial trawl catches during the period from May 2009 to December 2011. Of the 2,626 specimens collected, the sex ratio was not significantly different from 1:1 (P > 0.05). The total length ranged from 11.3 to 42.1 cm. The gonadosomatic index for both sexes was the highest in May to June, indicating that May to June is the main spawning period. The length of females at sexual maturity was 25.92 cm. The length-weight relationship of the filefish was TW = 0.0121TL3.0536 (n = 1,692, r2 = 0.9034, P < 0.001). The age of the sampled individuals was estimated by counting growth rings recorded on the 5th vertebrae; ages ranged from 0 to 9 years. The filefish of the same age displayed a high individual variation in total length. Length-at-age data were fitted by using the Von Bertalanffy growth model. The estimated Von Bertalanffy growth parameters were L∞ = 42.04 cm, k = 0.21 year?1 and t0 = ?1.56 for females, L∞ = 41.20 cm, k = 0.18 year?1 and t0 = ?2.36 for males, and L∞ = 43.16 cm, k = 0.17 year?1 and t0 = ?2.18 for the combination of both male and female. These data can be used as useful biological information for the future fishery management of filefish resources in Korean waters.  相似文献   

8.
The growth of hottentot is described by the relationship Lf (mm) = 538,015 (1?e?0,097(t + 0,431)). By determining the otolith growth rate and validating it with counts of daily rings identified on scanning electron micrographs of otolith sections, it was found that previous growth curves underestimated the age of the fish by one year. Differences in growth rate between sexes and over geographical regions were negligible. The ratio of males to females in sampled catches was 1:1,383 and 50-per-cent sexual maturity was attained at 220 mm fork length. There are two main spawning seasons, late autumn and summer, although some breeding activity continues throughout the year. Spatial differences in mean gonadosomatic indices were minimal.  相似文献   

9.
The vesicomyid Calyptogena kilmeri is one of the most abundant bivalves inhabiting chemosynthetic environments shallower than c. 1500 m along central California. We estimated the population size structure, biomass, rates of individual growth, somatic production, and mortality for C. kilmeri, based on sampling of seep habitats and tag–recapture studies at chemosynthetic communities in Monterey Bay, California. The composite growth rate of C. kilmeri over all sites was relatively high (K = 0.25), reaching c. 80% of asymptotic length (104.7 mm) in 6.6 years. The density of C. kilmeri was estimated as 938.5 ind. m−2 and biomass density varied from 704 to 2059 g ash free dry mass (AFDM) m−2. Somatic production was also high (294–297 g·AFDM·m−2 year−1), and production/biomass ratios for C. kilmeri varied from 0.14 to 0.42 among sites, related to variation in size–frequency distributions among sites. Instantaneous mortality rates estimated from size distributions ranged from 0.17 to 0.24 year−1. Growth and somatic production by C. kilmeri are in the range reported for chemosynthetic bivalves from hydrothermal vent and seep habitats, as well as photosynthetic‐based assemblages of inshore or intertidal bivalves, and greatly exceed rates reported for heterotrophic deep‐sea benthos.  相似文献   

10.
《Marine Chemistry》2001,73(3-4):291-303
Oxygen and phosphate measurements from two sections across the Norwegian Atlantic Current, the Gimsøy-NW section from 67.5°N 9°E to 71.5°N 1°E and the Bjørnøya-W section along 74.5°N from 7 to 15°E, are used to estimate oxygen fluxes in the surface layer and between the atmosphere and the ocean. Vertical entrainment velocities of 0.9 m day−1 for the winter season and 0.1 m day−1 for the summer season are found and applied to the upper 300 m. The resulting oxygen fluxes to the surface layer driven by this vertical mixing are 0.58±0.05 and 0.27±0.02 mol O2 m−2 year−1 at the Gimsøy-NW and Bjørnøya-W sections, respectively. Oxygen fluxes to the surface layer due to phytoplankton production are 2.6 and 3.4 mol O2 m−2 year−1, which represent the net community production at the two sections. Estimated uncertainties in these numbers are ±15%. The surface water is a sink for atmospheric oxygen during fall and winter and a source during the productive season for both sections. On an annual basis there is a net uptake of oxygen from the atmosphere, 3.4±0.4 mol O2 m−2 year−1 at the Gimsøy-NW section and 4.9±0.5 mol O2 m−2 year−1 at the Bjørnøya-W. A decrease in temperature of 1°C to 1.5°C seen between the Gimsøy-NW section and the Bjørnøya-W section is the main reason for the increased atmospheric flux of oxygen at the latter section. An oxygen budget made for the area bounded by the two sections gives a net advective flux of oxygen out of the area of approximately 10 mol O2 m−2 year−1. The increased concentration of oxygen corresponding to the decrease in surface layer temperatures going northwards in the Norwegian Atlantic Current is mainly attributed to the air–sea oxygen exchange and phytoplankton production in this area.  相似文献   

11.
The total organic carbon (TOC) and total inorganic carbon (CT) exchange between the Atlantic Ocean and the Mediterranean Sea was studied in the Strait of Gibraltar in September 1997. Samples were taken at eight stations from western and eastern entrances of the Strait and at the middle of the Strait (Tarifa Narrows). TOC was analyzed by a high-temperature catalytic oxidation method, and CT was calculated from alkalinity–pHT pairs and appropriate thermodynamic relationships. The results are used in a two-layer model of water mass exchange through the Strait, which includes the Atlantic inflow, the Mediterranean outflow and the interface layer in between. Our observations show a decrease of TOC and an increase of CT concentrations from the surface to the bottom: 71–132 μM C and 2068–2150 μmol kg−1 in the Surface Atlantic Water, 74–95 μM C and 2119–2148 μmol kg−1 in the North Atlantic Central Water, 63–116 μM C and 2123–2312 μmol kg−1 in the interface layer, and 61–78 μM C and 2307–2325 μmol kg−1 in the Mediterranean waters. However, within the Mediterranean outflow, we found that the concentrations of carbon were higher at the western side of the Strait (75–78 μM C, 2068–2318 μmol kg−1) than at the eastern side (61–69 μM C, 2082–2324 μmol kg−1). This difference is due to the mixing between the Atlantic inflow and the Mediterranean outflow on the west of the Strait, which results in a flux of organic carbon from the inflow to the outflow and an opposite flux of inorganic carbon. We estimate that the TOC input from the Atlantic Ocean to the Mediterranean Sea through the Strait of Gibraltar varies from (0.97±0.8)104 to (1.81±0.90)104 mol C s−1 (0.3×1012 to 0.56×1012 mol C yr−1), while outflow of inorganic carbon ranges from (12.5±0.4)104 to (15.6±0.4)104 mol C s−1 (3.99–4.90×1012 mol C yr−1). The high variability of carbon exchange within the Strait is due to the variability of vertical mixing between inflow and outflow along the Strait. The prevalence of organic carbon inflow and inorganic carbon outflow shows the Mediterranean Sea to be a basin of active remineralization of organic material.  相似文献   

12.
Magellania venosa, the largest recent brachiopod, occurs in clusters and banks in population densities of up to 416 ind m?2 in Comau Fjord, Northern Chilean fjord region. Below 15 m, it co‐occurs with the mytilid Aulacomya atra and it dominates the benthic community below 20 m. To determine the question of why M. venosa is a successful competitor, the in situ growth rate of the brachiopod was studied and its overall growth performance compared with that of other brachiopods and mussels. The growth in length was measured between February 2011 and March 2012 after mechanical tagging and calcein staining. Settlement and juvenile growth were determined from recruitment tiles installed in 2009 and from subsequent photocensus. Growth of M. venosa is best described by the general von Bertalanffy growth function, with a maximum shell length (L∞) of 71.53 mm and a Brody growth constant (K) of 0.336 year?1. The overall growth performance (OGP index = 5.1) is the highest recorded for a rynchonelliform brachiopod and in the range of that for Mytilus chilensis (4.8–5.27), but lower than that of A. atra (5.74). The maximal individual production (PInd) is 0.29 g AFDM ind?1 year?1 at 42 mm shell length and annual production ranges from 1.28 to 89.25 g AFDM year?1 m?2 (1–57% of that of A. atra in the respective fjords). The high shell growth rate of M. venosa, together with its high overall growth performance may explain the locally high population density of this brachiopod in Comau Fjord. However, the production per biomass of the population (‐ratio) is low (0.535) and M. venosa may play only a minor role in the food chain. Settling dynamics indicates that M. venosa is a pioneer species with low juvenile mortality. The coexistence of the brachiopod and bivalve suggests that brachiopod survival is affected by neither the presence of potential brachiopod predators nor that of space competitors (i.e. mytilids).  相似文献   

13.
《Marine Geology》2005,216(3):155-167
A total of 83 cores were collected in the Gulf of Lions continental margins and analysed for 210Pbxs (excess 210Pb) in order to understand sedimentation patterns. Apparent sedimentation rates (ASR) range from 0.65 cm year−1 in the vicinity of the Rhône River mouth to 0.01 cm year−1 in the deep basin. Except for the prodelta area, rates decrease with depth linearly with the water depth. On the slope, ASR do not differ between canyons and open slope, except for the western area where the rates are slightly higher in the Lacaze–Duthiers canyon compared to its adjacent, open slope. In the canyon and open slope areas, mass accumulation rates determined from 210Pbxs profiles (0.10 and 0.08 g cm−2 year−1, respectively) are in good agreement with particulate fluxes calculated from 5 years of near-bottom sediment trap data, even when the trap particle fluxes and the apparent accumulation rates are overestimated in response to resuspension and bioturbation effects.However, differences in sediment trap data, between west and east portion of the slope, are not observed in the sedimentation rates calculated with 210Pbxs. The outer shelf area may have an important role in trapping sediment but it is not sufficiently documented. Sediment surface mixed layer depths decrease with water depth, with a mean value for the whole margin of 8±6 cm.210Pbxs inventories in the sediment are systematically higher than the net 210Pb export flux estimated from the above water column. Over the margin, the ratio between accumulated 210Pb and available 210Pb is about 3, suggesting boundary scavenging effects and advective transport.  相似文献   

14.
《Journal of Sea Research》2011,65(4):451-456
We developed an ageing methodology and examined age composition of three flatfish stocks inhabiting the Seto Inland Sea, Japan. Ages were difficult to determine for three-lined tongue sole (Cynoglossus abbreviates) and ridged-eye flounder (Pleuronichthys cornutus) because the first year annulus ring was often indistinct; therefore, we used directional change in otolith growth to distinguish it. Sectioning and etching methods were powerful tools for identifying annual checks for red tongue sole (Cynoglossus joyneri). Using these ageing methods, we determined age–length relationships and growth curves. The age composition of the populations studied and of the landings showed that a large proportion of the latter consisted of individuals under the mean age of sexual maturity, thereby reducing the percent spawning potential ratio (%SPR) to ≈ 20% for all species. These findings suggest that fishing pressure on immature fish is leading to overfishing of these flatfish stocks.  相似文献   

15.
The dynamics of the seagrass Zostera noltii in established and colonizing meadows were assessed in Ria Formosa lagoon, Southern Portugal. Shoot weight, above:belowground biomass ratio, flowering shoot density, meadow production, and biomass–density relationships were investigated. Results indicate that the species population dynamics differ clearly in different development stages of the meadows. The overall mean of flowering shoot density was five times higher in the colonizing (83 flowering shoots m−2) than in the established meadow (16 flowering shoots m−2), revealing a greater contribution of sexual reproduction during the species colonization process. The temporal variation of the biomass–density relationship in the colonizing meadow showed a cyclic seasonal trajectory, a wider range of data, and a simultaneous peak of biomass and density, suggesting no space limitations constraining the internal packing of shoots during the growing season. In the established meadow, density peaked before biomass in agreement to the dominant role of the clonal architecture of seagrasses in the configuration of closed meadows, suggesting the occurrence of self-thinning and/or regulation of ramet formation. Slope of the biomass–density relationships was similar in the established and colonizing meadows, generally suggesting similar nutritional conditions, regardless of their muddy and sandy sediments. Plants of the colonizing meadow invest mainly on the belowground fraction (above:belowground biomass ratio <1), as meadow expansion is mainly controlled by the elongation of horizontal rhizomes. The annual total production (1163 g C m−2 year−1) and the biomass turnover (34.8 year−1) were also higher in this meadow, corroborating the high investment of the species during the meadow expansion. The faster biomass turnover of the colonizing meadow implies a more limited capacity to accumulate biomass, indicating a greater exportation of organic carbon and nutrients to the coastal area. The different biomass turnover rates suggest different trophic and structural roles of Z. noltii communities in established and colonizing meadows.  相似文献   

16.
Orange roughy Hoplostethus atlanticus are unusual fish. They form dense aggregations that have fuelled lucrative fisheries at great depths (600–1 400 m), especially off Namibia, New Zealand and Australia. They are thought to be very long-lived (>100 years, maturity at 22 – 40 years), and to have exceptionally low natural mortality (M = 0.045–0.064 year?1) and slow growth rates (K = 0.055–0.070 year?1). In addition, they spawn large eggs and have low fecundity. These factors combine to make orange roughy highly susceptible to overfishing; most stocks are below 30% of pristine levels. Assessments are obtained from indices of catch rate and trawl, acoustic and egg surveys. Acoustic estimates are the most direct, but are confounded by the species' low target strength (?50 to ?53 dB)–attributable to the wax-filled swim bladder. Extracellular wax esters are stored in abundance and comprise mostly mono-unsaturated fatty acids, with low concentrations of the ω-3 fatty acid family. This unusual composition (resultant from the species' diet) ensures neutral buoyancy. Stock separation has been inferred mainly from biological studies, but genetic studies have also found differences among stocks within New Zealand and Australia. Deep-water habitat may be damaged by trawling operations and may take many years to recover, so in some quarters there is a call for a portion of suitable habitat to be set aside for preservation. Although Namibian orange roughy are shallower, smaller and younger than those in other stocks, the Namibian fishery sustained high catches for only a few years before quotas were reduced, from 12 000 to 1 875 tons. Three management lessons are suggested for developing orange roughy fisheries based on the Namibian experience: (1) imposition of catch limits during exploratory fishing; (2) starting the acoustic surveys earlier in the fishery, if possible; (3) greater reliance on trends in catch rate until a survey series has been established.  相似文献   

17.
The presences of the common macroalgae Ulva sp. and Jania rubens vary between seasons along the Israeli Mediterranean intertidal zone. To reveal some of the potential acclimation mechanisms of these algae, we examined their photosynthetic traits during the year using rapid light curves (RLC) derived from pulse amplitude modulated (PAM) fluorometry. In addition, the relationships between those photosynthetic traits and the relative abundances of the two algae were investigated. Ulva sp. showed high maximal electron transport rates (ETRmax) (49.7–68.8 μmol electrons m−2 s−1) and onsets of light saturation values (Ek) (75.8–85.6 μmol photons m−2 s−1) in the winter months of December–February, while low values were found in the summer months of June–July (5.2–20.6 μmol electrons m−2 s−1 and 6.0–23.7 μmol photons m−2 s−1, respectively). At noon time (during the highest irradiance of the day), the maximal effective quantum yield (Y0) did not vary significantly during the year. These results indicate that seasonal irradiance influences the number of reaction centres per thallus area. Both algae showed depressions in Y0 at noon (23.8% for Ulva sp. and 20.3% for Jania rubens), indicating an efficient non‐photochemical quenching mechanism. A positive correlation between the relative abundance and the photosynthetic parameter Y0 was found for Jania rubens when Y0 was sampled in the morning or noon, indicating that growth rate and primary production for this alga can be estimated from RLCs. No such correlation was found for Ulva sp., indicating that, in addition to photosynthetic traits, the relative seasonal abundance of this alga is influenced by other factors such as grazing and/or catastrophic events.  相似文献   

18.
Community metabolism (respiration and production) and bacterial activity were assessed in the upper water column of the central Arctic Ocean during the SHEBA/JOIS ice camp experiment, October 1997–September 1998. In the upper 50 m, decrease in integrated dissolved oxygen (DO) stocks over a period of 124 d in mid-winter suggested a respiration rate of ∼3.3 nM O2 h−1 and a carbon demand of ∼4.5 gC m−2. Increase in 0–50 m integrated stocks of DO during summer implied a net community production of ∼20 gC m−2. Community respiration rates were directly measured via rate of decrease in DO in whole seawater during 72-h dark incubation experiments. Incubation-based respiration rates were on average 3-fold lower during winter (11.0±10.6 nM O2 h−1) compared to summer (35.3±24.8 nM O2 h−1). Bacterial heterotrophic activity responded strongly, without noticeable lag, to phytoplankton growth. Rate of leucine incorporation by bacteria (a proxy for protein synthesis and cell growth) increased ∼10-fold, and the cell-specific rate of leucine incorporation ∼5-fold, from winter to summer. Rates of production of bacterial biomass in the upper 50 m were, however, low compared to other oceanic regions, averaging 0.52±0.47 ngC l−1 h−1 during winter and 5.1±3.1 ngC l−1 h−1 during summer. Total carbon demand based on respiration experiments averaged 2.4±2.3 mgC m−3 d−1 in winter and 7.8±5.5 mgC m−3 d−1 in summer. Estimated bacterial carbon demand based on bacterial productivity and an assumed 10% gross growth efficiency was much lower, averaging about 0.12±0.12 mgC m−3 d−1 in winter and 1.3±0.7 mgC m−3 d−1 in summer. Our estimates of bacterial activity during summer were an order of magnitude less than rates reported from a summer 1994 study in the central Arctic Ocean, implying significant inter-annual variability of microbial processes in this region.  相似文献   

19.
Fishery-dependent and fishery-independent distribution analyses together reveal four discrete areas of white stumpnose Rhabdosargus globiceps abundance between Port Nolloth and the Kei River off the Cape Province of South Africa: the Western Cape (Saldanha Bay), the South-Western Cape, the Southern Cape and the South-Eastern Cape. On the basis of migratory patterns determined from tagging and catch data, and on differences in growth rate and size-at-maturity, it is concluded that these areas of abundance represent four separate stocks. Each stock apparently disperses offshore in winter (to c. 130 m depth) and concentrates inshore (<60 m depth) in response to ocean ographic patterns during summer. Growth rate and size-at-50% maturity (L 50) increased clinally from the South-Eastern Cape through to the South-Western Cape, and in all three regions males matured at larger size than females. Sizes at maturity for male and female R. globiceps were respectively 18.6 and 15.3 cm (fork length, FL) in the South-Eastern Cape, 22.1 and 18.1 cm in the Southern Cape and 24.3 and 23.6 cm in the South-Western Cape. The fitted Von Bertalanffy growth equations for the three regions were: Lt = 349 (1?e?0.114(t+3.60)) mm for the South-Eastern Cape; Lt = 337 (1?e?0.207(t+1.05)) mm for the Southern Cape; and Lt = 379 (1?e?0.290(t+0.16)) mm for the South-Western Cape. Maximum ages recorded in each region were 21 years for the South-Western Cape, 20 years for the Southern Cape and 10 years for the South-Eastern Cape. Lack of older fish in the South-Eastern Cape sample, attributed to inadequate sample size, has probably resulted in overestimates of both L ∞ and K in this region. Spawning is from August to February, with a peak in spring (September–November). Early juvenile R. globiceps recruit into estuarine and surf-zone marine nursery areas at around 2–5cm (±3 months), but move progressively farther offshore with growth; those trawled deeper than 50 m east of Cape Agulhas were predominantly adults (20–35 cm FL). Because of cooler water temperatures west of Cape Agulhas, adults there are found from the surf zone to depths of only 20 m in summer.  相似文献   

20.
This paper examines the relationships between maturity, length and age of whiting sampled on a length-stratified basis from groundfish surveys of the Irish Sea during spawning in spring 1992–2001. Maturity, defined by the triggering of vitellogenesis or milt production, was a function of both length and age. Proportions of mature individuals in 1-year-old males increased successively from almost zero in length classes below 15 cm to around 0.9 at 25 cm, whilst almost all 2-year-old males were mature from their smallest length of around 19 cm. Maturity in females was more strongly linked to age than to length. Most 1-year-old females were immature, the proportion of mature individuals not exceeding 0.3 in any length class. Most 2-year-old females were mature and immature fish were found in the smallest length classes only (20–25 cm). Almost all 3-year-olds of both sexes were mature in all length classes. Proportions of mature individuals in 1-year-olds increased substantially after 1997, particularly in males. Significant positive cross-correlation between proportion mature and mean length was found for 1-year-olds of both sexes. Length at 50% maturity (L50) averaged around 19 cm in males and 22 cm in females. Variability in L50 was negatively cross-correlated with average sea surface temperature in the preceding year. There is no evidence for substantial changes in maturity of whiting since the 1950s, despite an order-of-magnitude reduction in biomass caused by high fishing mortality. Concomitant decreases in mean length-at-age and weight-at-age in recent decades indicate that conditions may have been unfavourable for compensatory changes in maturation.  相似文献   

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