Biomass and Abundance Biases in European Standard Gillnet Sampling

The European Standard EN 14757 recommends gillnet mesh sizes that range from 5 to 55mm (knot-to-knot) for the standard monitoring of fish assemblages and suggests adding gillnets with larger mesh sizes if necessary. Our research showed that the recommended range of mesh sizes did not provide a representative picture of fish sizes for larger species that commonly occur in continental Europe. We developed a novel, large mesh gillnet which consists of mesh sizes 70, 90, 110 and 135mm (knot to knot, 10m panels) and assessed its added value for monitoring purposes. From selectivity curves obtained by sampling with single mesh size gillnets (11 mesh sizes 6 – 55mm) and large mesh gillnets, we identified the threshold length of bream (Abramis brama) above which this widespread large species was underestimated by European standard gillnet catches. We tested the European Standard gillnet by comparing its size composition with that obtained during concurrent pelagic trawling and purse seining in a cyprinid-dominated reservoir and found that the European Standard underestimated fish larger than 292mm by 26 times. The inclusion of large mesh gillnets in the sampling design removed this underestimation. We analysed the length-age relationship of bream in the Římov Reservoir, and concluded that catches of bream larger than 292mm and older than five years were seriously underrepresented in European Standard gillnet catches. The Římov Reservoir is a typical cyprinid-dominated water body where the biomass of bream > 292mm formed 70% of the pelagic trawl and purse seine catch. The species-specific relationships between the large mesh gillnet catch and European Standard catch suggested that the presence of carp (Cyprinus carpio), European catfish (Silurus glanis), tench (Tinca tinca) or bream warrants the use of both gillnet types. We suggest extending the gillnet series in the European Standard to avoid misinterpretation of fish community biomass estimates.     

Age, growth and reproduction of the barrelfish Hyperoglyphe perciformis (Mitchill) in the western North Atlantic

Otoliths ( n = 847) and gonads ( n = 817) were collected from barrelfish Hyperoglyphe perciformis that were captured by commercial fishermen in the waters off South Carolina and Georgia in 1995, 1997 and 2001–2006. Of the otoliths collected, 97% were aged successfully, and specimens sampled ranged from 5 to 85 years, with a median age of 12 years. The von Bertalanffy growth parameters yielded the equation: L_t = 857·8{1 − e^{−0·0985[ t −(−8·95)]}}, where L_t is fork length ( L _F) at time t . Through histological examination, 94% of the gonads assessed were assigned to a sex and reproductive class. Females spawned from September to May with a peak from November to January. Males spawned year round, but had a peak from September to April. The sex ratio (M:F) for this population was 1:1·34. The smallest mature female was 605 mm L _F and the youngest immature female was 697 mm L _F. Estimates of L _F and age at 50% maturity ( L ₅₀ and A ₅₀) for females were 660 mm L _F (95% CI = 633–667 mm L _F) and 6·08 years (95% CI = 3·50–7·27 years), respectively. The youngest mature male was 575 mm L _F and the oldest immature male was 762 mm L _F, and no estimates of L ₅₀ or A ₅₀ were made for males. It was determined that barrelfish exhibit the typical characteristics of long life span, slow growth and high age at maturity seen in other deepwater fishes, and that care should be taken to manage this species accordingly.     

A comparison of different structures and methods for estimating age of northern-form Dolly Varden <Emphasis Type="Italic">Salvelinus malma malma</Emphasis> from the Canadian Arctic

Assessment of anadromous northern-form Dolly Varden Salvelinus malma malma in the Western Canadian Arctic requires reliable methods for estimating ages. Additionally, conservation efforts warrant determining whether fin rays provide a non-lethal alternative to otoliths. Precision and bias of whole and sectioned otoliths, and sectioned pectoral and pelvic fin rays were examined. Two age readers with different levels of experience ageing this species read each structure three times. Coefficient of variation (CV) was calculated to measure precision, and age bias plots were created for each method of preparation/structure within and between readers. The experienced reader demonstrated the highest precision with sectioned otoliths (CV = 1.6 %) followed by whole otoliths (CV = 4.2 %) while pectoral and pelvic fins were the lowest, CV = 7.7 % and 7.5 %, respectively. The age bias plot showed little difference between whole and sectioned otoliths, although greater imprecision/bias was evident for whole otoliths at age ≥9. Compared to otoliths, fin rays produced younger age estimates starting at 5 years; however, pelvic fins were more biased towards younger estimates than pectoral fins. The less experienced reader had greater inconsistencies, tending to overage younger and underage older samples for all methods compared to the more experienced reader, underscoring the importance of experience when estimating age for this species. We conclude that both types of fin rays are a poor non-lethal alternative to otoliths for fish ≥5 years and recommend an experienced ager could use whole otoliths up to age 8 and sectioned otoliths for fish ≥9 years (>500 mm fork length).     

The fish species composition and variation of catch from the small-scale gillnet fishery before, during and after the 1997-1998 ENSO event, central Mexican Pacific

In the tropical and subtropical coastal zone, were highly diverse fish communities occur, it is important to study the small scale fisheries exploiting these communities. For this study, 219 fishing days were carried out in Bahia de Navidad, Jalisco, Mexico. Four gillnets with different mesh sizes (76.2, 88.9, 101.6 and 114.3 mm) were used for the fishing operations, from April 1994 to March 1995 and from January 1998 to December 2000. A total of 26126 organisms weighting 11680 kg were caught. One hundred and eighty three species belonging to 57 families and 19 orders were identified. Ten species accumulated more than 60% of the total abundance and biomass, the remaining species (173) individually contributed with less than 2% of the total abundance and biomass. The most important species in the catch were Microlepidotus brevipinnis, Caranx caballus, Haemulon flaviguttatum, Scomberomorus sierra, C. caninus, C. sexfjasciatus, Lutjanus guttatus and L. argentiventris. The catch per unit of effort showed a large variation during the study, both in number and biomass. On average, 110 fishes and 48.5 kg fishing day(-1) were caught. The maximum values were recorded during January and October 1998 (250 fishes and 100 kg fishing day(-1)), and the minimum values were recorded during September and November 1994 (25 org. and 10 kg fishing day(-1)). The total abundance and biomass was lower during the 1994-95 period (F=6.16, 8.32, P<0.05). There were statistically significant differences among the annual catch from each gillnet. All the environmental changes caused by the ENSO event had ecological and economic effects that can be rated from moderate to severe.     

Assessment of errors associated with harbour seal (Phoca vitulina) faecal sampling

Six harbour seals, ages 4–8 years, were held as pairs in a 10 times 20 times 2 m tank filled with sea water, and on 60 occasions were fed a meal of a specific species of fish or cephalopod of known size. The tank was drained periodically, and harbour seal faeces were collected on a 0.5 mm sieve. Number and size of otoliths and beaks found in faeces were determined. Fifty-eight percent of 670 fish and 37% of 36 cephalopods fed to harbour seals were represented by their otoliths or beaks in faeces. Estimated number of prey consumed was determined from the greatest number of left or right otoliths or upper or lower beaks collected in faeces. Estimated length ofprey was determined from measurements of otoliths and beaks recovered in the tank and relationships of otolith and beak measurements to prey length. Estimated number of fish eaten was not significantly different among pairs of harbour seals, but was different among species of fishes. Only 24–35% of fish species with small otoliths were represented in faeces, whereas more robust otoliths from other species were less apt to be completely dissolved. Estimated length of fishes was significantly less than lengths of fishes fed to harbour seals in 39 (76.5%) of 51 trials. Cephalopod beaks were not affected by passage through the harbour seal digestive tract. Amount of otolith dissolution was not related to species of fish; estimated fish length was underestimated by an average 27.5%. Although some (7.4%) of the otoliths were collected within 100 h after the fish were ingested, more than 90% were recovered within 24 h after the fish was eaten. Correction factors were developed which will allow researchers to estimate more reliably number and size of fish and cephalopod prey eaten by harbour seals.     

丹江口水库鱼类群落特征及其历史变化

为了解丹江口水库鱼类群落现状及历史变化, 作者于2018-2019年采用多网目复合刺网采样和渔民渔获物调查的方式对丹江口水库进行了鱼类群落调查。本次调查共采集鱼类33种, 隶属5目11科29属, 以鲤科鱼类为主, 占总数的63.6%。基于多网目复合刺网采样的鱼类群落多样性指数和单位努力捕捞量(catch per unit effort, CPUE, g·day ^-1·m ^-2)在各采样点间具有一定的差异。目前丹江口水库以定居性、喜静缓流、杂食性和肉食性鱼类为主。与20世纪50年代和80年代的历史资料相比, 丹江口水库鱼类物种丰富度先升高后下降; 而生态类型则呈现杂食性鱼类、喜流水性鱼类和洄游性鱼类比例持续下降的趋势。2018-2019年丹江口水库鱼类组成与20世纪80年代的相似性指数为0.35 (中等不相似), 与20世纪50年代的相似性指数为0.29 (中等不相似)。调查结果表明半个世纪以来丹江口水库鱼类群落结构发生了较大变化, 目前趋于小型化和简单化。这些变化可能是由大坝建设、渔业放养、不合理捕捞、外来鱼类引入等人类活动所致。     

Movement response of Orange-Vaal largemouth yellowfish (<Emphasis Type="Italic">Labeobarbus kimberleyensis</Emphasis>) to water quality and habitat features in the Vaal River,South Africa

Many threatened fish species that utilize riverine habitats are faced with habitat degradation and subsequent deterioration in their ecological surroundings. Habitat degradation is a consequence of water quality parameters associated with anthropogenic activities including mining, industrial, agricultural and urban activities. We examined how the movement behaviour of radio-tracked Orange-Vaal largemouth yellowfish (Labeobarbus kimberleyensis) responded to a suite of water quality chemical parameters and habitat features in the Vaal River, South Africa. We found that the probability of their movement increased with a decrease in water clarity, presence of emergent and overhanging marginal vegetation and fast flowing rapids. High mobility in conditions of low water clarity was probably related to low prey capture success of this piscivorous fish. High movement of largemouth yellowfish in emergent and overhanging marginal vegetation areas and rapid habitat biotopes were attributed to accessibility of prey within these important cover features. When water quality parameters were considered, the probability of largemouth yellowfish movement increased with increasing levels of dissolved chloride (Cl) and silicon (Si), whereas movement decreased with high phosphate (PO₄) concentrations and increased with alkalinity levels in the river. High nutrient levels associated with eutrophication caused reductions in the movement of largemouth yellowfish. The association of increased movement of largemouth yellowfish with increasing Cl and Si is indicative of degraded habitat condition in the Vaal River system. Our study showed the importance of monitoring fish behavioural movement patterns to multiple environmental parameters, as these fish are important ecological indicators when appropriate conservation and management plans of freshwater ecosystems are required.     

Monthly variability of hydroacoustic fish stock estimates in a deep lake and its correlation to gillnet catches

Monthly (April to November) hydroacoustic surveys and parallel gillnet catches were used to determine vendace Coregonus albula abundance, biomass and population dynamics in a deep oligotrophic lake. By hydroacoustic surveys, recruitment of 0+ year vendace could clearly be detected. In contrast, gillnet catches resulted in low numbers of 0+ year fish, and similarly under-represented proportions of the oldest vendace. Consequently, the correlation between hydroacoustics and gillnets with respect to fish numbers and geometric mean fish total length was high only for the age groups 1 + to 4+ years. Annual variability in hydroacoustic estimates offish abundance and biomass was high (CV=26–29%) which reflects the seasonal population dynamics of vendace.     

Quality and status of fish stocks in lakes: gillnetting,seining, trawling and hydroacoustics as sampling methods

All sampling methods give selective or biased estimates of fish species abundance, distribution and size structure. This creates problems, e.g. in regard to the Water Framework Directive of the European Union, which demands evaluation of the quality and status of fish stocks in lakes. We compared fish sampling by means of Nordic multimesh gillnets, seining, trawling and hydroacoustics in two Finnish lakes in summer 2007 and 2008. Sampling methods were used ‘as such’, i.e. no special design was implemented for method comparison. In the shallow eutrophicated lake the species’ composition of gillnet sampling and seining were very different. The biomass-% of percids dropped from gillnet (61%) to seining (9%) and that of cyprinids grew from 39 to 90%, respectively. In the deep pelagic area of the oligotrophic lake, vendace and smelt predominated in trawl catches. The number of fish caught by gillnetting in that area was too small to make any conclusions about the species composition. In the eutrophicated lake, the combined length distribution for all fish species differed significantly between gillnetting and seining. In the oligotrophic lake, the gillnet catches were too small for any comparison of fish size. The difference in the length distribution of fish between trawl and echosounding was significant in most analysed depth layers. In upper depth layers acoustics sampled larger fish than trawling, and in deeper layers smaller fish. Using a combined acoustic-trawl method, the pelagic fish biomass was estimated to be approx. 17 kg ha⁻¹ in the deep and oligotrophic lake. We conclude that in large and deep-water areas, the use of active gear is enough in fish sampling to evaluate the quality and status of fish stocks. Gillnetting together with seining is an appropriate method to work out the quality and status of fish stocks in shallow and littoral areas of large lakes. Variation in the catch selectivity of fish sampling gear requires a discrete ecological classification for each type of gear.     

The effects of food-web management on fish assemblage dynamics in a north temperate lake

There was an intensive trawl fishery in the Enonselkä basin in Lake Vesijärvi (southern Finland) in 1989–1993 and thereafter the fishing intensity declined. The fish assemblage dynamics were studied both during the years of intensive trawling and for 3 years afterwards. Roach Rutilus rutilus dominated the fish assemblage before the mass removal. The intensive trawling effectively diminished the roach stock and the present fishing intensity has been sufficient to prevent its recovery. There were significant decreasing trends in the gillnet cpue (catch per unit effort) of bream Abramis brama (L.) and bleak Alburnus alburnus (L.) while increasing trend was observed in cpue of ruffe Gymnocephalus cernuus (L.) and vendace Coregonus albula (L.). Perch Perca fluviatilis L. and pikeperch Stizostedion lucioperca (L.) showed variations in cpue but no trends existed. The proportion of cyprinids decreased while that of percids and coregonids increased in the catches. The changes in the fish assemblage were induced by the intensive fishing and by the concomitant decline in the ecosystem productivity.     

长江中游典型草型湖泊与藻型湖泊鲫种群结构和生长比较(英文)

浅水湖泊牛山湖和月湖均位于长江中游武汉市境内,前者水质清澈、沉水植物茂盛,属典型的草型湖泊;后者长期受生活污水和工业废水排放的影响,水质恶劣、沉水植物稀少,属典型的藻型湖泊。文章选择这两个湖泊的共同优势鱼类——鲫(Carassius auratus)作为研究对象,通过比较两类不同湖泊环境条件下鲫种群结构和生长上的差异,评估人类活动引起的水环境改变对鱼类种群生存状况的影响。2006年春季(3—5月),于牛山湖和月湖各采集鲫种群样本713尾和641尾,研究结果表明:(1)牛山湖和月湖鲫种群的全长频数分布经kolmogorov-Smirnov检验均符合正态分布,但前者的分布范围(63—271 mm)大于后者(72—191 mm),前者的全长峰值(190 mm)亦大于后者(110 mm);(2)牛山湖鲫种群的年龄组成(6个年龄组)较月湖(仅3个年龄组)完整,但月湖各龄鲫的全长均显著大于牛山湖(P<0.05);(3)两个湖泊性成熟鲫的雌、雄鱼比均随全长的增长而增加,即小个体性成熟鲫以雄鱼为主,大个体性成熟鲫以雌鱼为主,该现象在月湖更为明显;(4)牛山湖和月湖鲫全长–体重回归关系(W=a Lb)分别为W=0.00001 L3.060(R2=0.989)和W=0.000006 L3.246(R2=0.971),协方差分析表明两个湖泊鲫种群的b值差异极显著(P<0.01);另一方面,月湖鲫种群各全长组雌、雄鱼的肥满度均显著大于牛山湖(P<0.05)。作者认为影响牛山湖和月湖鲫生长和种群结构的主要因素是两个湖泊饵料组成和捕捞压力等方面的差异。     

An indirect estimate of bimodal gillnet selectivity of smelt

This study showed that it is possible to model a bimodal gillnet selectivity curve without knowing how individual fish have been caught. A bimodal model based on a modified normal density distribution was fitted over the length ranger of smelt Osmerus eperlanus from 8·9 to 17·0 cm. The model had eight parameters, and the height of the second peak was allowed to change with fish length. The coefficient of determination ( r ²) was 0·915 and the residuals were distributed symmetrically against variable axis. Corrected relative length distribution did not differ statistically from the relative length distribution of the trawl catch. In earlier studies of other species, pooled relative efficiency of multimesh gillnets composed by a geometric series mesh-size combination has been constant on the average. Because the model increased along fish length, the pooled relative efficiency was increasing also with fish length. Therefore, it is suggested that the gillnet catches of smelt always have to be corrected for gillnet selectivity.     

The variability in catches from multi-mesh gillnets fished in three Canadian lakes

Three types of multi-mesh gillnet were fished in three lakes in Alberta during spring and summer of 1983. Eight species offish were caught in a total of 89 lifts. The variability of the catches was analysed after a transformation into y = log (x+1). For the major species (apart from yellow perch) there were remarkable similarities between species for error variances associated with catches from nets and from meshes. The total error variance associated with a catch from any net was 0-0575 (C.V. = 59-7%). Total error variance associated with a catch from a single mesh was 0-0852 (C.V. = 75-6%). These values are compared to those from other studies and an error variance estimate of 0-0868 (C.V. = 76-5%) was considered to be an appropriate value for gillnet catches in fresh waters. In sampling programmes more than six nets would need to be used for confidence limits to be below half or more than twice the geometric mean.     

Catch composition of drift gillnets in a multi-species fishery with an emphasis on kingfish (Scomberomorus commerson) in Northern Persian Gulf

Gillnets are the primary fishing gear used in tropical multi-species fisheries along the Iranian southern coastal waters. Therefore, it is necessary to investigate the catch composition and performance of fishing gear for the possible negative effects on the stocks. Catch data and length frequency distribution (just for Kingfish, S. commerson) were examined for three kinds of drift gillnets: 1) small-meshed nets, 2) medium-meshed nets, and 3) nets with a single mesh size of 165 mm. This study was conducted in the northern part of the Persian Gulf from 2014 to 2015. Overall, 60 species of 32 families from four fish groups (i.e., Pelagic, Demersal, Benthopelagic, and Reef-associated) were recorded. Each net-type was used to target more than one species, with most species being considered as bycatch. Commercial fish species were caught the most, accounting for around 83% of the total catches. S. commerson is targeted by all kinds of nets, with medium-meshed nets being most efficient. The majority of S. commerson caught by the nets were immature. Overall, the total ban of gillnets with small mesh sizes is recommended to protect young S. commerson and prevent the possible risk of growth overfishing. Studies on gillnet selectivity are needed to set minimum mesh regulations for S. commerson and other commercial species if the potential impacts of gillnet are known.     

Schindleria parva,a new species of Schindler's fish (Teleostei: Schindleriidae: Schindleria) from Jeddah,Saudi Arabia,Red Sea

Schindleria parva, a new species of the family Schindleriidae, is described from two specimens collected from the central Red Sea of Saudi Arabia. The new species is characterized by lack of pigmentation on the body, possession of an inconspicuous gas bladder and the presence of small teeth on the premaxillae. The holotype is a female of 11 mm standard length (SL) (11.9 mm total length) and the paratype is a male of 9 mm SL. Dorsal fin rays 10 (9) anal fin rays 9 (7). The body depth at pectoral-fin origin 5% (4%) of SL, depth at anal-fin origin 8% (7%) SL, predorsal length 63% (65%) SL, preanal length 72% (72%) SL, the first anal-fin ray situated below the fourth dorsal-fin ray), a total of 23 + 16 myomeres. The female contained a series of 30 rectangular eggs in a single row, whereas the male is characterized by a short rod-like urogenital papilla. Species of the genus Schindleria are likely the smallest marine vertebrates on the planet and S. parva is likely the smallest Schindleria species in the Red Sea. The global diversity of Schindleria is likely underestimated due to the paedomorphic features of this genus. Its fast generation times make it a species-rich genus of high turnover rates, thus potentially highly important for the trophic food webs of coral reefs. Thus, this finding advances knowledge on the biodiversity of the Red Sea, highlights its conservation significance, and contributes towards the understanding of the complexity of the coral-reef fish community.     

Developing ultraviolet illumination of gillnets as a method to reduce sea turtle bycatch

Fisheries bycatch of marine animals has been linked to population declines of multiple species, including many sea turtles. Altering the visual cues associated with fishing gear may reduce sea turtle bycatch. We examined the effectiveness of illuminating gillnets with ultraviolet (UV) light-emitting diodes for reducing green sea turtle (Chelonia mydas) interactions. We found that the mean sea turtle capture rate was reduced by 39.7% in UV-illuminated nets compared with nets without illumination. In collaboration with commercial fishermen, we tested UV net illumination in a bottom-set gillnet fishery in Baja California, Mexico. We did not find any difference in overall target fish catch rate or market value between net types. These findings suggest that UV net illumination may have applications in coastal and pelagic gillnet fisheries to reduce sea turtle bycatch.     

A successful population of largemouth bass, Micropterus salmoides, in a subtropical lake in Mozambique

Sectioned otoliths were used for age and growth determination of the alien Micropterus salmoides from Lake Chicamba, a hydroelectric dam in central Mozambique. Marginal zone analysis showed that annulus formation occurred during winter in August and September. The maximum estimated age was 5 years. Growth was described by the 3 parameter von Bertalanffy model as L_t=465.51 (1 – e^{-1.175(t+0.009)}) mm FL. Juvenile growth rate was 23.1 mm per month during their first 7 months of life. The age at 100% maturity was 0.9 years, and spawning occurred during August and September. The juvenile fish fed sequentially on the most abundant prey items. Spawning occured before any of the indigenous fish species and so, there is little inter-specific competition. Conspecific juveniles formed the most important constituent in the adult M. salmoides diet from October to December when alternative prey was limited. The total annual mortality rate (Z) for the species in Lake Chicamba was 1.27 yr^-1, the mean empirical estimate of natural mortality (M) was 0.73 yr^-1, and fishing mortality (F) was calculated at 0.54 yr^-1. The success of M. salmoides in Lake Chicamba was attributed to limited inter-specific and intra-specific competition for prey as a consequence of the timing of the spawning, cannibalism and high mortality rates.     

Fish prey of the Wandering Albatross Diomedea exulans at South Georgia

Summary The fish diet (45% of total diet by weight) of Wandering Albatrosses rearing chicks at South Georgia during the austral winters of 1983 and 1984 was investigated using otoliths retrieved from regurgitations. These provide the first quantitative data for this species and for any albatross. By number of identified otoliths (32% could be identified only as ?Macrouridae and ?Moridae), Pseudochaenichthys georgianus (35%), Muraenolepis microps (33%) and Chaenocephalus aceratus (20%) predominated, with Notothenia gibberifrons, Pagothenia hansoni and Champsocephalus gunnari (together 12%) also present. Composition by weight (estimated from otolith length) of the main species was Pseudochaenichthys 51%, Muraenolepis 14%, Chaenocephalus 27%; if digestion and wear had reduced otoliths by 10% the values would be Pseudochaenichthys 54%, Chaenocephalus 25%, Muraenolepis 13%. Composition by weight (actual or corrected values) was almost identical between years but epipelagic fish were significantly more abundant in 1983 than 1984. All identified fish eaten by Wandering Albatrosses are common on the South Georgia continental shelf and most of them are caught in the commercial fishery there. However, two of the three main target species of this fishery in 1983–1984, Notothenia rossii and Champsocephalus were not, or rarely, caught by Wandering Albatrosses. It seems unlikely, therefore, that the albatrosses depend greatly on the fishery for acquisition of fish prey but how they catch several species, including Muraenolepis, which are mainly benthic in habit is unknown.     

Mn<Superscript>2+</Superscript> concentrations in coastal fish otoliths: understanding environmental and biological influences from EPR

The Mn²⁺ concentrations in the sagittae otoliths of 12 fish families (and 19 species) that co-occur in a coastal area of southeastern Brazil (~21°S) were quantified using electron paramagnetic resonance (EPR). Inferences were made about the relationship between fish habitat and trace element incorporation. Inferences were made on the relationship between trace element concentration and otolith shape. The differences in Mn²⁺ concentrations among the species suggest that habitat (and feeding habits) might drive the incorporation of this trace element into fish otoliths, with higher values in bottom-associated fish species than in surface-associated species. In surface-associated fish species, the correlation between trace element concentrations and otolith shape was stronger than in bottom-associated species. Thus, while the Mn bioavailability in a fish’s habitat, especially from feeding resources, is a local driving influence of trace element incorporation in sagittae otoliths, species-specific requirements also have an influence. Quantitative EPR is a non-destructive technique that is very useful when the available samples cannot be damaged, like with otolith collections.