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Ultrasonic transducer (UT) systems are used for non-destructive testing and evaluation across a wide range of industries. In the nuclear industry, radiation damage of the UTs can lead to performance degradation or even complete failure. We have investigated the radiation resilience of a new piezo-polymer (PVDF) UT design. The irradiations were performed in a Cobalt-60 gamma facility with total ionising doses up to 10.5 MGy. Continuous pulse-echo measurements were taken in-situ allowing the performance as a function of time to be monitored. In addition to the UT irradiations and measurements, high fidelity Monte Carlo particle transport simulations were performed to study the radiation environments and the ionising dose profiles within the devices under test. These dose profiles were used to gain insight into the behaviour of PVDF under irradiation. Results show that all sensors were able to withstand the doses achieved without suffering complete failure, with the best performing device only losing 47% of its initial amplitude. Dose profiles for the PVDF show piezoelectric performance to increase by up to 7% at doses around 500 kGy after which performance was found to drop linearly at a rate of between 6.5% and 10% per MGy. [ABSTRACT FROM AUTHOR]

The southern mullet Chelon richardsonii (Mugilidae) is the main target species of the inshore net fishery (gillnet and beach-seine) on the west coast of South Africa. The stock has displayed symptoms of overfishing, with a 28% reduction in the standardised catch per unit effort (CPUE) between 2008 and 2016, and a 36.5-mm reduction in mean total length (TL) of fish in the commercial catch between 1998 and 2017. The socioeconomic importance of the inshore net fishery and concern regarding stock status necessitated an assessment of the C. richardsonii stock. This work presents an update of the life-history parameters and the first stock assessment of C. richardsonii at the centre of its distribution—Saldanha Bay and Langebaan Lagoon—using CPUE standardisation and per-recruit analysis. The sexes exhibited distinctive growth, with females growing larger, and their length-at-first-maturity was estimated at 205 mm TL, corresponding to age 2 years. Estimated natural mortality was 0.718 y–1, total mortality was 1.599 y–1, and fishing mortality was 0.881 y–1. Current spawner- biomass-per-recruit was estimated at 24.5% of the pristine (unfished) levels, indicating a stock at risk of recruitment failure. An increase in the minimum stretched mesh size of the gillnets to 51 mm and a reduction in fishing mortality to 0.60 y–1 (approximately 30%) would facilitate the recovery of the spawner biomass to the target of 40% of pristine (SB40). [ABSTRACT FROM AUTHOR]

White stumpnoseRhabdosargus globicepsis the main target of the linefishery in Saldanha Bay. Increased fishing pressure over the last three decades, particularly by the recreational sector, has led to concerns regarding sustainability of the local white stumpnose stock. The fishery was exceptionally productive between 2006 and 2008, with an estimated annual catch of 141.2 tonnes (t). Only 3% of boat outings surveyed were commercial boats targeting white stumpnose, yet this sector accounted for 39.3 t (31%) of the average annual catch. The recreational boat sector accounted for most of the catch (70.0 t), and the recreational shore sector the least (31.9 t). Commercial boat catch per unit effort (CPUE; 3.7 fish angler–1h–1) was more than 10 times that of recreational boats (0.3 fish angler–1h–1). White stumpnose catch length-frequency differed significantly (p< 0.01) between the fishing sectors, with the commercial sector retaining larger fish (34.7 cm [SD 5.9]) than the recreational boat (33.9 cm [SD 5.9]) and shore (30.4 cm [SD 5.8]) sectors. A decline in commercial CPUE (2000–2015) of approximately 40% and a concomitant severe decline (>95%) in survey data for juvenile white stumpnose CPUE (2007–2016) indicate that the current rate of exploitation is not sustainable. Recovery of the white stumpnose stock will require a decrease in fishing mortality. Possible management regulations include sector-specific effort limitations, extending the ‘no take’ marine protected area, reducing the recreational-sector bag limit to 5 fish person–1day–1, implementing a commercial-sector bag limit, and increasing the minimum size limit to 30 cm TL. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Slender baardman Umbrina robinsoni are an important component of recreational shore-angler and spearfisher catches along the eastern seaboard of South Africa. Stocks of U. robinsoni at three sites-False Bay, Stil Bay and the KwaZulu-Natal (KZN) coast-were modelled using a per-recruit approach. Total (Z) and fishing (F) mortality rates were estimated by catch-curve analyses using measures of individual size (length or weight) recorded by researchers, divers (log books) or during spearfishing competitions. Based on estimates of F during the period 2001-2003, spawner biomass per-recruit ratios were estimated to be either at or below the 25% threshold (False Bay SB/R = 21% SB/RF = 0, Stil Bay SB/R = 25% SB/RF = 0, and KZN SB/R = 21% SB/RF = 0), suggesting that rates of F were too high. Reductions in F necessary to achieve target fishing mortality levels (FF = 0) at the current minimum size limit (l 40 cm total length) were 51% for Stil Bay and the KZN coast and 57% for False Bay. Based on the bag frequencies from 927 diver outings in KZN (1989-2003), a reduction in bag limit from the current five to two fish is predicted to reduce F in this region by approximately 25%. Increasing the l to 50 cm is predicted to increase SB/R ratios to 36% SB/RF = 0 in False Bay, 43% SB/RF = 0 in KZN and 52% SB/RF = 0 in Stil Bay, at current levels of F. Owing to the philopatric nature of U. robinsoni and the consequent existence of temporary refugia, catch curves are likely to underestimate fishing mortality. The reductions in F estimated to attain the target reference points are therefore probably conservative. [ABSTRACT FROM AUTHOR]

Biological data for Umbrina robinsoni were obtained from fish sampled monthly during 2001-2002 by shore-angling in the warm-temperate De Hoop Marine Protected Area (MPA) (n = 312), and by means of spearfishing during three trips (May, September and January 2001-2002) in the Kosi Bay region of the subtropical Maputaland MPA (n = 354). Annuli in otolith sections were validated by means of marginal increment analyses and fluorochrome marking (oxytetracycline). Maximum ages recorded were 12 and 16 years at Kosi Bay and De Hoop respectively. Kosi Bay fish obtained a significantly greater asymptotic length than De Hoop fish and mean length-at-age (for ages 2-10 years) was significantly greater. The fitted von Bertalanffy growth equations for combined sexes were: Lt = 594 (1 - e-0.183 (t + 2.42)) for De Hoop and Lt = 875 (1 - e-0.151 (t + 2.49)) for Kosi Bay. Trends in mean monthly gonadosomatic indices and proportions of histologically validated macroscopic gonad stages indicated a summer spawning season (November-February) at De Hoop and year-round spawning at Kosi Bay. Relative condition peaked in both spring and autumn at De Hoop but showed little seasonal variation in the Kosi Bay region. Kosi Bay females attained 50% sexual maturity at a significantly larger size (48 vs 39 cm) but at younger age (2.8 vs 3.4 years) than those at De Hoop. Analysis of maturity schedules indicates that current sizes at maturity are plastic responses that maximise lifetime fecundity within local regimes of somatic growth and natural mortality. The instantaneous rate of natural mortality was substantially higher at Kosi Bay than at De Hoop (M = 0.35 vs M = 0.26). [ABSTRACT FROM AUTHOR]

Fish populations in the Berg Estuary on the west coast of South Africa (32°46'S, 18°09'E) were sampled intensively over a four-year period, 2003-2006, in conjunction with a range of physico-chemical variables as part of the Berg River Baseline Monitoring Programme. A considerable body of historic data also exists for the system, dating back a further 10 years, making the Berg Estuary one of the most intensively sampled estuaries in South Africa. Using these data, this paper presents a detailed analysis of the variations in abundance, community structure and size composition of the fish fauna of the Berg Estuary in relation to intra- and inter-annual changes in freshwater flow and other physico-chemical parameters. It also serves to highlight the importance of the Berg Estuary as a nursery area and feeding ground for fish on the South African west coast and provides some insights into the likely impacts the Berg River dam (recently installed in the upper Berg catchment) on the fish fauna of the estuary. It should be noted that the Berg Estuary is one of the longest and largest in SouthAfrica (length=65 km, area=61 km²), and is rated as one of the most important systems in the country from a conservation perspective. Data analysed for the study includes results of ninepling excursions undertaken between 1992 and 2006 during which 42-46 sites were sampled with a 30 m long beach seine net and in some cases with a fleet of gill nets with mesh sizes from 44-145 mm stretched mesh. Total number of fish collected in the seine net hauls per sampling excursion ranged from 20 402-243 226 and biomass from 80-490 kg. Number of species present in the catches ranged from 14-23 species. Fewer species and lower biomass was generally recorded in samples taken during the winter and autumn (high-flow period) relative to summer and spring (low-flow) months. Overall fish abundance (measured as number of individuals per square meter) increased over the study period with an initial modest increase from 2004 onwards followed by a more marked increased from winter 2005 onwards. Species richness (measured as number of species recorded) also exhibited a modest increase over the entire study period, from around 14-18 species between 1992 and 2003, rising to between 17 and 23 species from 2004 onwards. Grouping fish species in accordance with their degree of association with estuaries revealed that marine migrant species accounted for most of the observed variation in abundance and biomass, with estuarine resident species following a more stable trajectory over time. The relative contribution by different groups of fish exhibited no clearly discernible pattern from year to year in relation to measured physico-chemical parameters including freshwater runoff to the estuary, or between seasons. In terms of species composition, Liza richardsonii was the most abundant species overall (average contribution = 59%), followed by Gilchristella aestuaria (22%), Atherina breviceps (8%), Psammogobius knysnaensis (3%), Caffrogobius nudiceps (2%) and Oreochromis mossambicus (2%). Together these six species accounted for 93-99% of the catches in each year (overall average: 97%). Increases in the abundance of L. richardsonii accounted for the major portion of the increase in fish abundance over time, and is thought to be related to the closure of a historic gill net fishery on the estuary in 2003, the primary target for this fishery. Some (but not all) fish species in the estuary displayed clear and consistent distribution patterns up the length of the estuary with freshwater species typically being most abundant in the upper reaches of the estuary and estuarine resident and marine migrant species being more abundant lower down.… [ABSTRACT FROM AUTHOR]

A total of 248 roving creel surveys along the length of the Berg River Estuary, in the Western Cape, South Africa, recorded 626 shore-angler and 88 boat-angler outings over the period December 2002-November 2005. Catch-and-effort information was obtained from catch inspections with 360 handline and 246 rod-anglers. Average total annual linefishing effort was estimated at 449±29 (mean±SE) boat-angler days, 1 299±118 recreational shore-angler days and 1 394±57 subsistence (handline) shore-angler days. The estimated total shore-based linefish catch (excluding boat-based catches) from the estuary for the years 2004 and 2005 was 37 231±1 326 fish and 26 938±706 fish (approximately 8 t and 7 t) respectively. Compared with other estuaries along the South African east coast where angler catches have been surveyed, species diversity in catches from the cool-temperate Berg River Estuary was low, with only 15 species caught, of which three, elf Pomatomus saltatrix (56%), harder Liza richardsonii (31%) and carp Cyprinius carpio (11%), dominated the catch. Average linefish catch per unit effort of most species increased significantly in the two years subsequent to the closure of the long-existing commercial gillnet fishery in March 2003. Length frequency distributions revealed significant increases in the average size and an increased contribution of larger size class elf and harder to the linefish catch over the monitoring period, suggesting a degree of recovery of the estuarine icthyofauna after more than a century of intensive gillnet fishing. [ABSTRACT FROM AUTHOR]

This study compares the fish assemblages of the permanently open cool-temperate Olifants and warm-temperate Breede estuaries on the west and south coasts of South Africa respectively, and their responses to altered freshwater flows. Both estuaries have experienced a >35% reduction in mean annual runoff (MAR) from a historical reference condition to the present day with a >60% reduction possible under future flow scenarios. With the exception of species that have both marine and estuarine breeding populations, the Olifants Estuary fish assemblage has experienced an overall 20% decrease in abundance from reference (pristine state) to the present day and will gradually decline to 55% of reference with a predicted future 60% reduction in MAR. Consequently, future reductions in flow are likely to result in the Olifants Estuary progressing towards a low biomass, low diversity, marine-dominated system. In contrast, reduced freshwater flows in the Breede Estuary are likely to experience an overall reduction in the abundance of species that breed only in estuaries, and in freshwater and catadromous species. Collectively, entirely estuarine-dependent fish will increase in abundance, but considered individually some important exploited species such as Argyrosomus japonicus and Pomadasys commersonnii will collapse to 50% of historical numbers once there has been a 64% reduction in MAR. Overall, fish abundance in the estuary has increased by 6% from reference to the present day and is likely to increase to 115% of reference with future reductions in flow. Some species with a preference for fresh and brackish water will be all but lost from the system, but overall diversity is likely to increase with the range expansion of warm-temperate and subtropical marine species westward. In all, the fish assemblage of the Breede Estuary will experience a gradual change from a relatively high-diversity, low-abundance, freshwater-rich system under historical flow conditions to a high-diversity, high-abundance, marine-dominated system with future reductions in flow. [ABSTRACT FROM AUTHOR]