Environmental influences on the spatio-temporal distribution of Coilia nasus in the Yangtze River estuary

The Yangtze River estuary (YRE) is an important migration channel and foraging habitat for Coilia nasus. Due to its ecological significance and a prioritization of this species’ protection, the need to investigate and analyze environmental relationships of the abundance of Coilia nasus in the YRE as well as develop an understanding of their temporal and spatial distributions is becoming exceedingly important. Using fishery data and environmental survey data from 2009 to 2016, three models including generalized additive mixed models (GAMM), generalized additive models with zero-inflated Poisson distribution (ZIP-GAM) and two-step GAM were used to analyze relationships between environmental factors and the distribution of Coilia nasus in the YRE. The results showed that model fitting of GAMM was more consistent with observations and revealed influences of water temperature, salinity, chlorophyll, and pH on distribution. GAMM demonstrated that higher Coilia nasus abundances were located in waters with water temperature values at 15°C and 30°C, and lower Coilia nasus abundances were located in areas with water temperature values at 10°C and 20°C. All models indicated that the effect of salinity on abundance of Coilia nasus present a multimodal pattern including three peaks at 5, 15, and 25 ppt respectively. Additionally, abundance of Coilia nasus increased with the increase of chlorophyll A in its range of 0–4 mg/L. In a range of 8.0–9.5, higher PH value was more suitable for the aggregation of Coilia nasus. Cross validation was used to evaluate the predictive performance of models and GAMM was found to be the best. The predicted abundance distribution of Coilia nasus in the summer and autumn of 2016 was relatively higher overall than that in winter and spring. The predicted zero abundance distribution pattern was consistent with the sampling presence distribution which was obtained using fishery independent survey data of the year 2009–2015. Facing the urgency protection of Coilia nasus in YRE, results of this study could be used for Coilia nasus conservation and reserve planning.     

Characterization of the pelagic shark‐fin trade in north‐central Chile by genetic identification and trader surveys

Shark fins have become a highly valued commodity with the major Asian fin‐trade centres supplied from global sources, including Chile. With growing concerns about the resilience of shark populations to heavy fishing pressure, there is a need for better information on shark landings to aid management efforts. In the widespread absence of shark landing records especially by species, monitoring the fin trade has been proposed as a way to assess species exploitation levels. Here, the first species assessment of the Chilean shark‐fin trade was provided. The goals of this study were to (1) determine the species composition and relative species proportion of sharks utilized in the fin trade, (2) determine the relationship between fin trader market names and species and (3) assess trader accuracy in identifying shark fin species based on fin photographs. Fins were analysed from two different fin drying facilities (n = 654) (secaderos) and two fin‐storage warehouses (n = 251). In contrast to official government landing records that only document four species in the landings, molecular species identification of the fins demonstrated that at least 10 pelagic shark species are present in the north‐central Chilean shark fin trade: Alopias superciliosus, Alopias vulpinus, Carcharhinus obscurus, Galeorhinus galeus, Isurus oxyrinchus, Isurus paucus, Lamna nasus, Prionace glauca, Sphyrna lewini, Sphyrna zygaena. The species composition of the fins from the secaderos was P. glauca (83·9%), I. oxyrinchus (13·6%), L. nasus (1·7%) and A. superciliosus (0·2%). There was generally good agreement between market names and single shark species for the trade categories ‘Azulejo’, ‘Tiburon’, ‘Tintorera’, ‘Cola de zorro’ and ‘Martillo’. In contrast, the market category ‘Carcharhinus’ consisted of a mixture of at least five species. The molecular results also identified two species (S. lewini and I. paucus) not previously recorded in Chilean waters. The fin identification survey given to nine regional traders demonstrated that they were highly accurate in recognizing pictures of fins from P. glauca and I. oxyrinchus. The overall strong concordance between market categories and fins from single species and the trader accuracy in survey fin identification suggests that monitoring the Chilean fin trade by market names will provide a reasonably accurate picture of the volume of sharks landed by species.     

应用脂肪酸组成研究热带东太平洋同域中上层鲨鱼营养生态位分化

物种对食物资源利用方式的差异,即营养生态位分化是物种共存的先决条件之一,对种间营养生态位的比较研究有助于了解同域分布物种的共存机制。脂肪酸组成可反映生物较长时间尺度的摄食信息,对探讨物种间营养生态位分化具有重要指示作用。热带东太平洋主要栖息有8种大型中上层鲨鱼,大青鲨(Prionace glauca)、大眼长尾鲨(Alopias superciliosus)、镰状真鲨(Carcharhinus falciformis)、长鳍真鲨(Carcharhinus longimanus)、浅海长尾鲨(Alopias pelagicus)、尖吻鲭鲨(Isurus oxyrinchus)、路氏双髻鲨(Sphyrna lewini)和锤头双髻鲨(Sphyrna zygaena),通过比较其肌肉脂肪酸组成,分析种间食性差异,营养关系及营养生态位分化。结果表明,尖吻鲭鲨营养级相对较高,大青鲨相对较低。3种鼠鲨与5种真鲨存在食性差异或栖息地隔离。浅海长尾鲨与大眼长尾鲨营养生态位重叠程度较高,存在激烈的资源竞争。大青鲨与镰状真鲨生态位宽度较大,表征其对环境的可塑性较强;尖吻鲭鲨和路氏双髻鲨生态位宽度较小,表现为其食性的特化。本研究解释了脂肪酸组成分析在鲨鱼摄食研究中的潜在应用,对分析大洋性鲨鱼的营养生态位分化,资源分配方式及同域共存机制有一定的应用价值。     

A case of genetic polyandry in the shortfin mako Isurus oxyrinchus

This article documents a case of genetic polyandry in the oceanic and pelagic shortfin mako Isurus oxyrinchus and briefly comments on the implications of this finding.     

Multiple freshwater invasions of the tapertail anchovy (Clupeiformes: Engraulidae) of the Yangtze River

Freshwater fish evolved from anadromous ancestors can be found in almost all continents. The roles of paleogeographic events and nature selection in speciation process often are under focus of research. We studied genetic diversity of anadromous and resident tapertail anchovies (Coilia nasus species complex) in the Yangtze River Basin using 4,434 nuclear loci, and tested the history of freshwater invasion of C. nasus. We found that both C. brachygnathus and C. nasus were valid species, but the resident C. nasus taihuensis and the anadromous C. nasus were not different genetically based on Bayes factor species delimitation (BFD*). Maximum likelihood tree, Network, PCA and STRUCTURE analyses all corroborated the results of BFD*. Two independent freshwater invasion events of C. nasus were supported, with the first event occurring around 4.07 Ma and the second happened around 3.2 Ka. The time of the two freshwater invasions is consistent with different paleogeographic events. Estimation showed that gene flow was higher within ecotypes than between different ecotypes. F‐DIST analyses identified 120 disruptive outliers by comparing C. brachygnathus to anadromous C. nasus, and 21 disruptive outliers by comparing resident C. nasus to anadromous C. nasus. Nine outliers were found to be common between the two comparisons, indicating that independent freshwater invasion of C. nasus might involve similar molecular pathways. The results of this study suggest that adaptation to landlocked freshwater environment of migratory fish can evolve multiple times independently, and morphology of landlocked ecotypes may cause confusion in their taxonomy.     

Spatial and temporal changes in the fish communities from a mangrove‐dominated creek system near Karachi,Pakistan

The objectives of the study were to inventory the fish community, examine how the fish communities changed in these creeks temporally (season), and compare the fish communities between creeks of varying structure (width). During February 1999 to December 2001, a survey was undertaken to monitor the distribution and abundance of fishes in the Korangi‐Phitti Creek (KPC) system (24°45′N, 67°20′E) near Karachi, Pakistan. Fish were collected from 123 stations using a combination of monofilament gillnets of 8.9 cm, 5.7 cm, and 3.8 cm stretched mesh sizes. A total of 17 023 fish representing 86 species were collected during the study; Sardinella gibbosa and Nematalosa nasus were the most abundant taxa (in terms of numbers of individuals) collected, accounting for over 60% of the total catch. Physical conditions in the study area were found to have significant seasonal (monsoon) patterns related more to temperature and salinity than to rainfall. Seasonal comparisons indicated that the communities were significantly different between the northeast monsoon (NEM) and southwest monsoon (SWM) seasons. Taxa primarily responsible for distinguishing the NEM were the mullets (Moolgarda cunnesius, Liza carinata, Mugil cephalus). The SWM was characterized by Anodontostoma chacunda, Scomberoides commersonnianus, and the engraulid (Thryssa hamiltonii). The community sampled in wide creeks (shipping channels) was very different in composition than the community in small creeks within this system. Wide creeks were characterized by schooling pelagic species (S. gibbosa, N. nasus), while the smaller creeks were characterized by the presence of mullets (M. cunnesius, M. cephalus, L. carinata) and scat (Scatophagus argus). The KPC system was shown to contain a highly diverse fishery, which in turn contributes to regional offshore fisheries (shad, mackerel, tuna and perches). The various negative natural and anthropogenic events in this region highlight the need for information regarding the fish community, water quality, and the mangrove forest to evaluate ecosystem changes, the effects of disturbances, and for the development and making of sound management decisions.     

Genetic variability in Chondrostoma from Austrian, French and Greek rivers (Teleostei, Cyprinidae)

Allozyme variation in Chondrostoma from Austrian, French and northern Greek rivers was investigated. Populations of C. nasus from the upper Danube catchment are genetically very similar and apparently connected by high level of gene flow. Chondrostoma nasus has conserved all major allozyme polymorphisms during its recent range expansion into the Rhone basin. Chondrostoma vardarense from the Axios and Aliakmonas rivers share most of their allozymes with C. nasus from the upper Danube catchment, but have less genetic variation, a possible consequence of a genetic bottleneck following river capture.     

Eating catch of the day: the diet of porbeagle shark Lamna nasus (Bonnaterre 1788) based on stomach content analysis,and the interaction with trawl fisheries in the south-western Atlantic (52° S–56° S)

This work examined the diet of the porbeagle shark Lamna nasus in the south-west Atlantic Ocean (SWAO, Argentina, 52° S–56° S) by analysing the stomach content information obtained by scientific observers who sampled specimens captured as by-catch on-board commercial fishing vessels from 2010 to 2020. A total of 148 fishing sites were analysed, in which the estimated catch was composed mainly of hoki Macruronus magellanicus (56.00%) and southern blue whiting Micromesistius australis (33.13%). From 413 porbeagle sharks sampled (292 females and 121 males) ranging from 71 to 241 cm total length (L_T) (mean: 179.76 ± 26.74 cm), 310 (75.06%) contained food in the stomachs. The forage fish were mainly hoki M. magellanicus (23.53%) and southern blue whiting M. australis (19.05%), followed by the Patagonian sprat Sprattus fuegensis (4.48%) and nototheniids (1.4%). Cephalopods and crustaceans accounted for 10% of the diet. The estimated trophic level was 4.35. Generalized linear models revealed that the consumption of hoki M. magellanicus and southern blue whiting M. australis increased with the L_T of the porbeagle shark. Moreover, smaller porbeagle sharks preyed upon both small and large teleost fish, whereas larger porbeagle sharks predated exclusively upon large fish. The diet of porbeagle shark involved interactions with fisheries as it fed upon the fish species that constituted the main catch in the analysed fishing sites, as well as the main catches of the austral trawl fisheries. The ecological role of porbeagle shark observed in the SWAO exposed implications for fisheries management from a multispecies perspective.     

Record of a clupeoid fish nematalosa nasus without an anal fin

This paper records the catch of Nematalosa nasus without an anal fin, from Karachi. The specimen is a mature male of 165.5 mm in total length. Comparison of the abnormal specimen with a normal Nematalosa nasus, shows considerable changes in the muscles of the anal fin region, posterior part of the axial skeleton, the course of the intestine and especially the extent and shape of the body cavity. Comments are also made to account for such an abnormality.     

Evidence for dynamic resource partitioning between two sympatric reef shark species within the British Indian Ocean Territory

Stable-isotope analyses (δ¹³C, δ¹⁵N and δ³⁴S) of multiple tissues (fin, muscle, red blood cells and plasma), revealed ontogenetic shifts in resource use by grey reef sharks Carcharhinus amblyrhynchos and resource partitioning with silvertip sharks Carcharhinus albimarginatus within the British Indian Ocean Territory marine protected area (MPA). Resource partitioning varied temporally, with C. albimarginatus feeding on more pelagic prey during October to January, potentially attributable to an influx of pelagic prey from outside the MPA at that time. Reef sharks may therefore be affected by processes outside an MPA, even if the sharks do not leave the MPA.     

Plasticity of ontogenetic trajectories in cyprinids: a source of evolutionary novelties

Research in evolutionary developmental (evo‐devo) biology is making an increasingly important contribution to our understanding of the molecular mechanisms underlying the establishment of complex morphological traits. Deciphering the ontogenetic trajectories leading to the differentiation of sister species (and the existence of hybrids) is a new challenge in our understanding of speciation processes. In the present study, we characterized the ontogenetic trajectory of lower lip morphology in two cyprinid species and their hybrids. Chondrostoma toxostoma has an arched lower lip and a generalist diet. Chondrostoma nasus has a straight lower lip and a specialist diet. An analysis of 99 C. toxostoma, 99 C. nasus and 25 first‐generation (F₁) hybrid individuals demonstrated that the difference between arched and straight lip morphology was found to depend strongly on the height/width ratio of the lower lip. A comparison of the ontogenetic trajectories of these morphometric traits showed that the height of the lower lip was isometric to body length in both species, whereas developmental changes involving an acceleration and a hypermorphosis of the widening of the lower lip led to a straight lip morphology in C. nasus. F₁ hybrids were characterized by an extreme phenotype resulting from a rate of lower lip widening slower than that in the two parent species. Therefore, we rejected a codominance hypothesis and concluded that the first stage of hybridization provides the foundations of evolutionary novelty. These results have important evolutionary implications because lower lip shape is linked to dietary behaviour in many fish species. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 342–355.     

Diet overlap between the newly introduced <Emphasis Type="Italic">Lamprichthys tanganicanus</Emphasis> and the Tanganyika sardine in Lake Kivu,Eastern Africa

This study evaluates the possible competition for food between Lamprichthys tanganicanus, recently introduced in Lake Kivu, and Limnothrissa miodon, which has been the basis of the pelagic fishery in this lake for several decades. Since 2006, L. tanganicanus has expanded in the lake and its numbers have increased in the captures, raising concern for the sardine fishery. We carried out a 2-year monthly survey, based on experimental captures in littoral and pelagic stations, which demonstrated the invasive dispersal of L. tanganicanus in littoral and pelagic waters. The diet of both species was determined on the basis of gut content analyses, taking into account the influence of site and season, and a diet overlap index was calculated. In the pelagic zone, where almost all size classes of both species were present and essentially fed upon mesozooplankton, the diet overlap was high. This situation stems from the fact that L. tanganicanus has colonized the pelagic zone in Lake Kivu, likely in search for more abundant mesozooplankton. Inshore, the diet overlap between the two species was lower, as L. tanganicanus consumed a broad range of food, whereas L. miodon strongly selected insects and, chiefly for the largest specimens, fishes. These results suggest a likelihood of interspecific competition, particularly offshore, where mesozooplankton is the main available food type, and call for further monitoring of the sardine fishery, to assess a possible impact of the invader.     

Global population genetic dynamics of a highly migratory,apex predator shark

Knowledge of genetic connectivity dynamics in the world's large‐bodied, highly migratory, apex predator sharks across their global ranges is limited. One such species, the tiger shark (Galeocerdo cuvier), occurs worldwide in warm temperate and tropical waters, uses remarkably diverse habitats (nearshore to pelagic) and possesses a generalist diet that can structure marine ecosystems through top‐down processes. We investigated the phylogeography and the global population structure of this exploited, phylogenetically enigmatic shark by using 10 nuclear microsatellites (n = 380) and sequences from the mitochondrial control region (CR, n = 340) and cytochrome oxidase I gene (n = 100). All three marker classes showed the genetic differentiation between tiger sharks from the western Atlantic and Indo‐Pacific ocean basins (microsatellite F_ST > 0.129; CR Φ_ST > 0.497), the presence of North vs. southwestern Atlantic differentiation and the isolation of tiger sharks sampled from Hawaii from other surveyed locations. Furthermore, mitochondrial DNA revealed high levels of intraocean basin matrilineal population structure, suggesting female philopatry and sex‐biased gene flow. Coalescent‐ and genetic distance‐based estimates of divergence from CR sequences were largely congruent (d_corr = 0.0015–0.0050), indicating a separation of Indo‐Pacific and western Atlantic tiger sharks <1 million years ago. Mitochondrial haplotype relationships suggested that the western South Atlantic Ocean was likely a historical connection for interocean basin linkages via the dispersal around South Africa. Together, the results reveal unexpectedly high levels of population structure in a highly migratory, behaviourally generalist, cosmopolitan ocean predator, calling for management and conservation on smaller‐than‐anticipated spatial scales.     

SEM images reveal intraspecific differences in egg surface properties of common nase (Chondrostoma nasus L.)

Scanning electron microscopy (SEM) has been widely used to describe interspecific differences in egg quality of teleost freshwater fish, but potential intraspecific differences are poorly studied. Eggs of many rheophilic cyprinids are covered with adhesive structures such as attaching villi facilitating egg attachment at substrates of spawning grounds with high currents. Recent findings indicate that the egg quality of the rheophilic cyprinid common nase (Chondrostoma nasus L.), a target species of conservation, differs in the adhesiveness between spawning populations, potentially explaining differences in recruitment success. In this study, a SEM image-based standardized protocol was established to assess egg surface quality of Chondrostoma nasus eggs. Multivariate statistics detected significant differences of egg surface properties among individual females and among three different populations. These differences were mainly attributed to length variability and merging of adhesive villi as well as to coating and filament-like connections of these structures. The findings of this study highlight the need for further investigations to better understand the relationship of egg surface properties, egg stickiness and hatching success to understand the recruitment ecology and performance of early life stages in freshwater fish.     

Spiral valve parasites of blue and common thresher sharks as indicators of shark feeding behaviour and ecology

This study documented the parasite faunas of the spiral valves of blue sharks Prionace glauca (L. 1758) and common thresher sharks Alopias vulpinus (Bonnaterre, 1788) caught in the California Current Large Marine Ecosystem (CCLME) north of the Mexican border. The spiral valves of 18 blue and 19 thresher sharks caught in the CCLME from 2009 to 2013 were examined for parasites. Seven parasite taxa were found in blue sharks and nine in threshers. The tetraphyllidean cestode Anthobothrium sp. (78% prevalence) was the most common parasite in blue sharks, and the phyllobothriid cestode Paraorygmatobothrium sp. (90% prevalence) was the most common in threshers. An adult nematode of the genus Piscicapillaria was found in threshers for the first time and may be a new species. Adult individuals of Hysterothylacium sp. were found in both shark species. The adult acanthocephalan Rhadinorhynchus cololabis and remains of the parasitic copepod Pennella sp. – both parasites of Pacific saury, Cololabis saira – were found in the intestines of threshers, indicating recent feeding on saury. This study paves the way for a more comprehensive examination, including more samples and a wider variety of shark species, to provide a greater understanding of shark feeding behaviour and possibly provide information on shark population biology.     

Movement patterns and spawning activity of individual nase Chondrostoma nasus (L.) in flow-regulated and weir-fragmented rivers

Eighteen adult nase Chondrostoma nasus (L.) (37.6–48.2 cm FL) from three highly fragmented Belgian rivers were tagged with surgically implanted radio transmitters and manually tracked from the banks of the rivers for 3–34 weeks. Their movements were mainly recorded during the circum reproduction period in 2001, 2002 and 2006 in river stretches delimited by physical barriers and/or minimum flow conditions. To reach the spawning grounds, nase displayed different patterns of movements that were mainly influenced by the configuration of the study site, the date of observation and the associated environmental conditions. Nase never cleared any physical obstacle but were able to spawn in minimum flow conditions or just downstream from physical obstacles. Nase were present on spawning grounds in late March in the warmest spring (2001) and from mid‐April to early May in the coldest spring (2006). After spawning, a major proportion of C. nasus migrated a substantial distance downstream, whereas others remained near their capture site and showed frequent movements, sometimes corresponding to group displacements from one river to another.     

The vertical distribution and abundance of <Emphasis Type="Italic">Gammarus lacustris</Emphasis> in the pelagic zone of the meromictic lakes Shira and Shunet (Khakassia,Russia)

The vertical distribution and abundance of Gammarus lacustris in the pelagic zone of two fishless meromictic lakes, L. Shira and L. Shunet, in Southern Siberia (Russia), was studied with the underwater video recording system and using vertical hauls. In both lakes, during summer stratification, Gammarus was distributed non-homogenously, with a stable peak in the metalimnion. The average depth of Gammarus population in the pelagic zone was significantly correlated with the depth of the thermocline. Gammarus abundances obtained using vertical plankton hauls with net were quite comparable with those obtained from video records. The peak abundance of Gammarus in the pelagic zone of the lakes observed with underwater video amounted up to 400 individuals m⁻², while the peak animal densities in the metalimnion reached 50 ind. m⁻³. The data are compared with previously published abundances of Gammarus in the littoral of Lake Shira. Both littoral and pelagic can be equally important habitats for amphipods in meromictic lakes. The absence of fish in the pelagic zone, high oxygen concentration, low water temperature, increased seston concentration, elevated water density in the metalimnion and the anoxic hypolimnion can be the most probable combination of factors that are responsible for the peak of Gammarus in the metalimnion of these lakes.     

Comparative retinal anatomy in four species of elasmobranch

Using both light and transmission electron microscopy, we examined the retinal anatomy of four elasmobranch species with differing ecologies: the bull shark Carcharhinus leucas, Port Jackson shark Heterodontus portusjacksoni, epaulette shark Hemiscyllium ocellatum and pink whipray Himantura fai. Their retinas are typical of other vertebrates, having three nuclear and two synaptic layers, but are characterised by very large horizontal cells, low densities of ganglion cells (many of which are displaced to the inner nuclear and inner plexiform layers) and the presence of numerous myelinated axons within the nerve fibre layer. Carcharhinus leucas, H. fai and H. ocellatum have duplex retinas containing both rods and single cones. The peak ratio of rods to cones is much lower in C. leucas (4:1) and H. fai (3:1) compared to H. ocellatum (19:1), reflecting differences in diel activity patterns. No cones were observed in the retina of H. portusjacksoni, which is strongly nocturnal. The cones of H. fai lack a distinct myoid and their nuclei are located in a discrete layer sclerad to the external limiting membrane (ELM), whereas those of C. leucas and H. ocellatum have an obvious myoid, and their nuclei are located vitread to the ELM. No double/twin cones were observed in any species. Incorporating data from other studies, there is a clear correlation between rod outer segment volume and visual ecology in elasmobranchs, with smaller volumes found in partly diurnal pelagic species and larger volumes in benthic nocturnal species. This trend may reflect fundamental differences in visual temporal resolution between active and more sedentary species. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

The biology of Lates species (Nile perch) in Lake Tanganyika, and the status of the pelagic fishery for Lates species and Luciolates stappersii (Blgr.)

Three Lates species, L. mariae (Stdr.), L. microlepis (Blgr.), L. angustifrons (Blgr.) and Luciolates stappersii (Blgr.) which belongs to a monotypic genus occur only in Lake Tanganyika. They are the principal predators in the pelagic and benthic fish communities. Data are presented on distribution, growth, length-weight relationships, condition, reproduction and food in the Lates spp. After a post-larval pelagic phase, each Lates sp, spends 1 year in littoral weed. Thereafter, L. mariae adopts a benthic habitat moving into deep water, L. microlepis becomes exclusively pelagic and L. angustifrons occurs from the littoral to near the limit of the oxygenated zone. Cycles in breeding correspond with seasonal maxima in clupeid prey, and changes in growth rate within a year are probably related to physiological cycles rather than to variations in the physical environment. Salient morphological and ecological adaptations are discussed. Local abundance and vagility of Lates stocks seem related to basin morphometry and depth of the oxygenated layer. L. microlepis has most vagility. The Lates appear highly susceptible to intensive fishing by purse-seines, and have been fished-up in the southeast arm in Zambia and in the Burundi sector, where only small populations remain. With reduced predation, the clupeid biomass has increased. The change to dominance of small species may be accomplished rapidly in L. Tanganyika under such conditions of exploitation. However, the type of species successions well-known in large temperate lakes are unlikely to occur because of the high degree of specialization in these fishes. A simplified pelagic community is foreseen, consisting almost entirely of clupeids and Luciolates, in which the total fish biomass will be determined above some intensive level of fishing and from year to year by the environmental factors controlling survival of young and plank-tonic food supply. Quantitative equilibrium between the few species will be determined mainly by gear selectivity and total fishing stress, and the community may be sufficiently robust by reason of its rapid turn-over rates to permit a largely empirical approach to management.