Long-term trends in the recreational shore-fishery for elf Pomatomus saltatrix (Pomatomidae) along the KwaZulu-Natal coast,South Africa

Elf Pomatomus saltatrix account for between 28% and 80% of the annually recorded recreational shore-angling catch in KwaZulu-Natal, South Africa, and have been subjected to various management regimes over the past 33 years. Management of the elf fishery was amended in 2005 to reduce fishing mortality and enhance egg production, following an assessment in 1997, which considered the elf stock to be overexploited and in need of rebuilding. A compromise was reached whereby the bag limit was reduced from five to four elf per angler per day, but the closed season was shortened, allowing an extra month of fishing for elf. A total of 33 years of recreational elf catches recorded on the National Marine Linefish System was investigated for any long-term trends and how they related to management of this species. Over 3 million angler-outings from the recreational shore-fishery in KZN were analysed, including more than 1.5 million elf in the catch. Despite the fast growth of elf and the latest management intervention seven years ago, which was based on a rigorous investigation of life-history parameters, the present study showed no evidence of an improvement in catch rate. It could not be determined whether this was due to a reduction in the bag limit, which would naturally reduce catch rates or whether other factors, including the shortened closed season, had been detrimental to elf stocks. Some shortfalls in the management of elf are discussed and possible reasons for the great variability in catches, which complicated the assessment of trends, are explored.     

Age and growth of Cape stumpnose Rhabdosargus holubi (Pisces: Sparidae) in the Eastern Cape,South Africa

Rhabdosargus holubi is a small (maximum weight=2.4?kg) yet important fishery species in the estuaries of the south-east coast of South Africa. Little is known of its biology and specifically its growth rate, which is essential for sustainable management of the fishery. We examined and counted the opaque zones in the sectioned otoliths of 134 R. holubi to determine its age and growth parameters. The otoliths from two recaptured fish marked with oxytetracycline confirmed that one opaque zone was deposited annually. The species reached a maximum age of 18 years and growth was adequately described by a von Bertalanffy growth function of the form: L_t = 358.1 (1 – e^{?0.24(t+0.77)}) mm fork length. There were no significant differences between any of the male and female growth parameters (likelihood ratio test: p = 0.3). The growth was slow (omega index: ω = 86.56); however, despite this, the unique life history of R. holubi may provide a degree of resilience to heavy fishing pressure in estuaries.     

An assessment of participation,catch and effort in the KwaZulu-Natal shore-based marine linefishery,with comments on management effectiveness

The primary purpose of this study was to evaluate the effectiveness of current management of the KwaZulu-Natal (KZN) shore-based marine linefishery. Methods used included a stratified-random creel survey and an associated questionnaire survey. The study was undertaken between February 2009 and January 2010. Total participation ranged between 41 283 and 68 087 shore-anglers. Results show that there has been little change in participation in the KZN shore-fishery since 1994–1996. In contrast, total angler effort (779 382–843 702 angler-days y^?1) has decreased substantially over the past 13 years. Overall catch per unit effort (CPUE) for the KZN shore-linefishery amounted to 0.18 ± 0.3 fish angler^?1 h^?1 or 0.07 ± 0.13 kg angler^?1 h^?1. In all, 84 fish species, belonging to 39 families, were recorded in catches of shore-anglers during the study period. Only five species accounted for 75% of the catch recorded along the coast (Sarpa salpa 34.8%, Pomatomus saltatrix 14.7%, Diplodus capensis 14.5%, Pomadasys olivaceum 6.5% and Rhabdosargus holubi 4.9%). The total annual catch for the KZN shore-linefishery was estimated between 249.2 and 276.7 t y^?1 (or 636 589–706 995 fish y^?1). Analysis of overall CPUE, catch composition and total catch in the shore-based linefishery of KZN suggested that it is currently in a relatively stable condition and that little change has occurred over the past 13 years. However, comparisons of species-specific CPUE values from this study with recent literature suggest that some species (e.g. Argyrosomus japonicus) are severely overexploited. Over the past few decades there seems to have been a gradual transition in landings from long-lived, high trophic-level, piscivorous fish (e.g. A. japonicus) to more short-lived, low trophic-level species (e.g. S. salpa). The results therefore suggest that present exploitation levels may not be sustainable for certain slow-growing, long-lived fish species. Current management regulations appear to have limited effectiveness based on poor angler compliance and knowledge of the regulations. In this regard, it is important to consider alternate management measures that may offer more effective protection to fish and at the same time be more easily understood by all anglers in the linefishery. Recommendations for improved management and the importance of the establishment of an effective MPA network are briefly discussed.     

Patterns in abundance,population structure and biology of knife prawn Haliporoides triarthrus on deep-water trawl grounds off eastern South Africa

The deep-water trawl fishery along the KwaZulu-Natal coast of South Africa targets several crustacean species, with the knife (or pink) prawn Haliporoides triarthrus contributing most of the catch. Logbook data of fishing effort and catch between 1988 and 2010 were used to assess the distribution and abundance of H. triarthrus on fishing grounds. Generalised linear models were used to quantify the effects of year, month and depth on catch rates. Standardised trends indicated a general decline in abundance between 1990 and 1998, followed by an increase between 2001 and 2008. Catch rates peaked in March, and they were highest between 200 and 499 m depth. Biological samples collected during commercial fishing were used to assess size and sex composition, growth rates and reproductive activity of H. triarthrus. Females became larger than males and mean carapace length (CL) varied by month. The youngest female cohort appeared in November (modal CL of 25 mm), and dissipated after two years (39 mm). Sex ratios were equal for all data combined, but fluctuated by month and CL. Few reproductively active females were recorded. Length-based methods and the standard von Bertalanffy growth function were used to estimate growth parameters (L_∞ and K) of females (40.6 mm CL and 1.06 y^–1) and males (35.2 mm and 1.27 y^–1) respectively. Our findings were compared with information on H. triarthrus from Mozambican waters.     

Rapid fish stock depletion in previously unexploited seamounts: the case of Beryx splendens from the Sierra Leone Rise (Gulf of Guinea)

Fish stocks associated with seamounts may be particularly susceptible to overexploitation. From January to July 2001, the Spanish Oceanographic Institute (IEO) conducted an experimental fishing survey entitled ‘Palguinea-2001’ on the seamounts of the Sierra Leone Rise. Beryx splendens (commonly called alfonsino) is the main commercial demersal fish associated with this area. The aim of this study was to investigate the effects of a demersal longline fishery targeting B. splendens on the previously unexploited small Machucambo Seamount over a short time-scale, and also to consider trends in pooled catch rate at another four seamounts. During 110 fishing days at Machucambo, a total catch of 207 tonnes of B. splendens was taken with a fishing effort of 1 309 070 hooks. A spectral analysis and red-noise spectra procedure (REDFIT) algorithm was used to identify the red-noise spectrum from the gaps in the observed time-series of catch per unit effort by weight. Our results show the potential impact of longline fishing pressure on an unexploited ecosystem – after approximately 50 fishing days, the stock appeared to decline substantially, as reflected by a marked drop in catch per unit effort. The apparent rapid decline of the stock might be related to the small size and the virgin state of the Sierra Leone seamounts. The results could be extrapolated to similar small seamounts elsewhere.     

The feeding ecology of skates (Batoidea: Rajidae) off the Cape south coast,South Africa

Six species of skates were trawled commonly on the continental shelf off the Southern Cape during routine surveys of demersal fish between 1986 and 1990. Raja miraletus is a small skate found principally in shallow (<50 m) water that feeds mainly on small crustaceans, such as mysids and macrurans. The large Raja alba occurs over most of the continental shelf, is a piscivore throughout its life and preys heavily on flatfish, especially Cynoglossus zanzibarensis. R. cf. clavata is ubiquitous on the shelf but most abundant between 50 and 150 m deep. Feeding predominantly on crustaceans, this skate switches from carids, penaeids and mysids when small to brachyurans and small benthic fish when larger. R. pullopunctata is found over a similar depth range but attains a much larger size. It eats small mysids and macrurans when small, but brachyurans and fish become more important in the diet with increasing size of the predator. R. wallacei is most commonly taken at depths of 80–>200 m, feeds initially on small crustaceans, but switches to fish, including eels, as it grows. The small Cruriraja parcomaculata is found at the edge of the continental shelf and over the upper slope; it preys on small crustaceans, including mysids, isopods and macrurans. The distribution pattern of the skates and their prey suggest overlap in prey taken, the overlap being greatest in those species which feed on crustaceans. The large biomass of skates in the area suggests that they are an important component of the demersal faunal assemblage of the continental shelf there, and also that they may have a significant impact on the benthic fauna.     

Occurrence and feeding of three shark species,Carcharhinus brachyurus,C. obscurus and Sphyrna zygaena,on the Eastern Cape coast of South Africa

The shark species Carcharhinus brachyurus, C. obscurus and Sphyrna zygaena, which attain more than 2 m when adult, occur in coastal waters of the Southern Cape as juveniles. They use these coastal waters as nursery areas and their relative abundance there varies through the year, possibly because of seasonally varying oceanographic conditions. Adult C. brachyurus occur in the same geographic area but usually deeper, although there is some overlap in juvenile and adult depth ranges. Adult C. obscurus prefer the warmer subtropical and tropical waters north of the study area. Adult S. zygaena are found over deep reefs at the edge of the continental shelf. There was considerable dietary overlap between the three species, the prey of all three being dominated by small schooling fish, such as pilchard, and squid. For the three species, animals of total length >2 m took larger prey, such as elasmobranchs, with greater frequency, although they continued to prey on squid and schooling fish. It is postulated that the nursery areas are in the study area because of the abundance of food there, and that avoidance of cannibalism would be enhanced by the use of such nurseries.     

When plenty is not enough: an assessment of the white stumpnose (Rhabdosargus globiceps) fishery of Saldanha Bay,South Africa

White stumpnose Rhabdosargus globiceps is 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^–1 h^–1) was more than 10 times that of recreational boats (0.3 fish angler^–1 h^–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^–1 day^–1, implementing a commercial-sector bag limit, and increasing the minimum size limit to 30 cm TL.     

基于GAM和GWR模型分析环境因子对南极磷虾资源分布的非线性和非静态性影响

分析南极磷虾分布与环境因子的非线性和空间非静态性关系,对南极磷虾的高效捕捞和管理具有重要意义。本研究基于“龙腾”船2015、2016年在南设得兰群岛捕捞作业的渔捞日志数据,应用广义加模型(Generalized additive model,GAM)和地理权重回归模型(Geographical weighted regression,GWR)探究南极磷虾(Euphausia superba)渔场分布与环境因子的非线性和空间非静态性关系,并比较这2种模型的模拟性能,为南极磷虾的渔场渔情预报、资源评估和渔业管理提供基础数据。GAM模型结果显示,2015、2016年单位捕捞努力量渔获量(Catch per unit effort,CPUE)与作业水深均呈显著负相关关系(P<0.01),表明在作业水深范围内,南极磷虾在较浅水域集群密度较高;2015年CPUE与表层水温呈显著正相关关系(P<0.01),但在2016年呈显著负相关关系(P<0.01),推测是由于2年调查作业位置不同所致;CPUE与离岸距离关系不显著(P≥0.05)。GWR模型结果显示,作业水深对CPUE的影响无显著的空间变化(P>0.05);海水表温和离岸距离对CPUE的影响具显著的空间变化(P<0.01),表明这2个因子对南极磷虾渔场分布的影响在空间上不连续,存在显著空间非静态性。GAM模型可用于研究资源分布与驱动因子的一般规律;GWR模型作为全局回归模型的有效补充,可用于探究一般规律不适合的特殊区域,便于发现资源分布的“热点”区域,未来在海洋生物资源分布研究中将有广阔的应用前景。     

Monitoring the recovery of a previously exploited surf-zone fish community in the St Lucia Marine Reserve,South Africa,using a no-take sanctuary area as a benchmark

Limiting accessibility to a fishing area can reduce fishing effort effectively and may therefore promote local recovery of depleted stocks. In January 2002, beach driving was banned in South Africa, thereby reducing angler access to large areas of the coastline, particularly in less-developed areas. In November 2001 a project had been established in the St Lucia Marine Reserve, on the east coast of South Africa, to compare surf-zone fish populations inside a no-take sanctuary zone with those in an adjacent exploited area. Subsequent to the ban, the aim of the project was adapted such that surf-zone fish populations were monitored for potential recovery in what had been the exploited area, located to the north of Cape Vidal, which anglers could no longer access easily, because of the prohibition on beach driving. Standardised research fishing was conducted at two sites in the previously exploited area and two sites in the no-take sanctuary. Conventional stock-status indicators showed evidence of recovery in the four most common species caught in the previously exploited area, in terms of both abundance and biomass. Generalised additive mixed models were used to account for the influence of targeting specific species; however, subtle differences in habitat between the sampling sites, improved angling skill over time, variability in recruitment, and differential species-specific responses complicated interpretation of results. The implications of the findings are discussed in terms of the future monitoring and management of marine protected areas both within the St Lucia Marine Reserve and farther afield.     

Influence of the annual sardine run on catches of large sharks in the protective gillnets off KwaZulu-Natal,South Africa,and the occurrence of sardine in shark diet

Shark catches in the protective nets set off the beaches of KwaZulu-Natal (KZN), South Africa, are strongly influenced by the sardine run, the winter influx of shoals of Sardinops sagax from the south-west. The effect of the sardine run, which is highly variable from year to year, is greatest in June and July at beaches south of Durban. Total annual shark catch and effort are presented for the period 1952–2005, and total monthly shark catch on the KZN south coast for the period May–August, 1965–2005. Measures to reduce catches of sharks associated with the sardine run have been introduced and have been increasingly successful. Reliable species-specific catch data are available for the period 1978–2005 only. For this period, the spatio-temporal distribution of each of 14 species of shark and the frequency of occurrence of sardine in their diets is documented. Occurrence varies according to species, as does the apparent influence of the sardine run on shark distribution. During June and July on the KZN south coast, sardine were found in the diet of all but two species and frequency of occurrence was 40% or greater in eight species. The presence of copper or bronze whaler sharks Carcharhinus brachyurus in KZN waters appears to be strongly associated with the sardine run, as does that of certain life-history stages of dusky sharks C. obscurus. Spinner sharks C. brevipinna and smooth hammerhead sharks Sphyrna lewini are caught in greater numbers in summer than in winter, but appear to shift their spatial distribution seasonally to feed on sardine.     

Indicators of abundance and spatial distribution of lobsters (Homarus americanus) from standard traps

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.     

Spatio-temporal variability of small copepods (especially Oithona plumifera) in shallow nearshore waters off the south coast of South Africa

Although small copepods are one of the main dietary sources for many commercially important fish, their role in the pelagic trophic dynamics has traditionally been underestimated due to the methodology commonly used in plankton sampling. Temporal variation in abundance of adults and nauplii of small copepods (particularly Oithona plumifera) in nearshore waters on the south coast of South Africa was investigated fortnightly over 14 months at site (km) and location (100 m) scales. Sampling was within <500 m of the shore, where depth was ca. 10 m, using vertical hauls of an 80-μm mesh plankton net from 1 m above the seabed to the surface. Twenty-seven adult copepod taxa were recorded, but Oithona spp. was consistently the most abundant. Taxon richness was 7–19 on each sampling occasion. There was strong temporal variation (Oithona varied between 0 and 2300 m⁻³), but much of this was short-term variability (e.g. between consecutive sampling sessions), with no seasonality or other long-term discernable patterns. There were periods of consistently low numbers, but very high numbers often followed samples with low abundances. Nor was there spatial structure at the location scale, though numbers differed between sites. Despite considerable variability at the location scale within sites, Kenton consistently showed higher densities than High Rocks. Separate analyses, with Bonferroni adjustment, showed that this difference was significant on eight out of 21 occasions for Oithona, six for other pelagic copepods and three for nauplii. This suggests that hydrodynamics favour aggregation of plankton at Kenton. A high degree of short-term variability, with a tendency for aggregation of small zooplankton at certain sites has implications for both pelagic processes and food-web links between the benthic and pelagic environments.     

浙江沿岸张网中主要经济幼鱼发生量与当年渔汛渔获量的关系

通过对1998～2005年浙江中北部沿岸3个张网监测点的渔业监测调查,阐述了浙江沿岸张网渔业中的主要经济幼鱼带鱼、小黄鱼、鲳鱼的渔获量比例月变化;同时结合浙江省的渔业统计资料和主要鱼种的生物学测定资料,以张网渔业中主要经济鱼种的平均网获量作为预报当年渔汛补充群体渔获量的主要影响因子,经回归统计分析,分别建立了带鱼和小黄鱼的渔获量单因子预报方程.结果表明:2000～2005年,张网中的带鱼、小黄鱼、鲳鱼3种主要经济鱼类幼鱼合计占总渔获量的月平均比例达22.79%,6～8月为幼鱼的出现高峰期,月平均比例为53.86%,7月份达到最高值,为62.66%.其中5～9月幼带鱼占渔获量的月平均比例范围为11.42%～46.42%,6～11月小黄鱼占渔获量的月平均比例范围为9.73%～14.30%,4～8月鲳鱼占渔获量的月平均比例范围为3.32%～6.52%.设5～9月张网中幼带鱼的日平均网获量(X,kg/网)与当年9～12月渔汛中带鱼补充群体的渔获量(Y,×108尾),经相关分析,可以得到Y关于X的一元一次、一元二次和一元三次函数的预报方程.设6～10月张网小黄鱼幼鱼的日平均网获量(x,尾/网)与当年浙江省小黄鱼的渔获量(y,万t),经相关分析,可以得到y关于x的一元一次、一元二次函数的预报方程.预报方程的相关系数和预报精度基本符合应用要求,表明沿岸产卵场和索饵场附近的张网渔获物中主要经济幼鱼的发生量与当年渔汛补充群体渔获量密切相关.张网渔业的监测调查可以反映当年主要经济幼鱼发生量情况,应加强对沿岸张网渔业的管理及保护主要经济幼鱼.