Illumination of trawl gear by mechanically stimulated bioluminescence

The role of bioluminescence stimulated by moving fishing gear was investigated using an ISIT video camera mounted inside a pelagic and demersal trawl. Data presented here show that trawling is responsible for generating bioluminescent light. In two Norwegian fjords, the mean number of bioluminescent events per metre of head rope per second were 5.14 ± 2.17 and 2.39 ± 1.0 for a pelagic trawl at ∼270 m depth and a demersal trawl at ∼500 m depth, respectively, travelling at 2–4 knots. By extrapolation of these data, thousands of point-source flashes occur over the entire mouth of a trawl every second during a tow, considerably increasing its visual presence to fish. The occurrence of stimulated bioluminescent flashes on a trawl in deep-water may provide sufficient illumination to permit herding in the absence of ambient space light. The aim of this study is to bring to the attention these preliminary observations of mechanically stimulated bioluminescence by mobile trawl gear as possible visual detection stimulus for fish and its potential to affect catch rates by altering herding, escapement and avoidance behaviour.     

Further evidences of deep-sea recruitment of Aristeus antennatus (Crustacea: Decapoda) and its role in the population renewal on the exploited bottoms of the Mediterranean

Data were collected during four trawl surveys (April, May, June and September 2006) in the north-western Ionian Sea (Eastern Mediterranean) with the aim of investigating the recruitment and population structure of Aristeus antennatus and evaluating whether population processes on bottoms deeper than 800 m could have a role to the renewal of the exploited population on fishing grounds, above 800 m. Samples were taken randomly between 400 and 1200 m using a professional fishing vessel equipped with an experimental otter trawl net, with 20 mm stretched mesh size in the cod-end. No variation in density index was shown across the whole depth range while the negative trend of biomass index with depth was due to the increasing proportion of small individuals and males. The recruitment was observed on a wide depth range with an increasing frequency of occurrence at greatest depths. It seems to occur as a discrete phenomenon. A significant bigger-shallower phenomenon was detected in the female population indicating upwards displacement from virgin grounds to fishing ones. The exploited population mostly consists of large females. Mating and reproduction also occur in deep waters. The contribution to the renewal of the fishing population by the virgin one seems to be mostly due to the recruitment on unexploited deep areas and displacement of individuals from these areas to fishing ones. The deep waters act as a refuge area mostly for the recruitment of A. antennatus and, to lesser extent, for spawners. On the basis of all these results the authors discuss the optimum exploitation conditions often detected for this shrimp throughout the Mediterranean highlighting the fact that, since the exploited population mostly consists of reproductive females, conditions of “recruitment overfishing” might occur in A. antennatus contrary to other Mediterranean demersal resources.     

Global fishing effort (1950–2010): Trends,gaps, and implications

According to a recent World Bank report, the intensification of global fishing effort and the ensuing depletion of marine fish stocks causes economic losses of 50 billion US dollars annually. Data deficiencies, however, currently hamper analysis of global fishing effort. We analyzed data from the Food and Agricultural Organization of the United Nations (FAO), the EUROPA fishing fleet registry, and peer-reviewed and other publications, to determine the global trends in fishing effort from 1950 to 2006. Our results show that global fishing effort, expressed as total engine power and the number of fishing days in a year (kilowatt days), was roughly constant from 1950 to 1970, and then steadily increased up to the present. Europe dominated global fishing effort, followed by Asia. Projecting current trends suggests that Asia will soon surpass Europe. Trawlers contribute a major fraction of global fishing effort, as do vessels greater than 100 gross registered tons. Current estimates of global fishing effort, the number of vessels, and total vessel tonnage are, however, underestimates given the data gaps that we have identified. Our results are useful in the following ways: (1) they may encourage researchers in academia and government to improve global fishing effort databases; (2) they allow deeper global analyses of the impact of fishing on marine ecosystems; (3) they induce caution in accepting current underestimates of economic losses of global fisheries; and (4) they reinforce calls for a reduction in global fishing effort.     

Evaluation of a trawl to estimate the inventory of catfish in commercial ponds

Most commercial catfish farms use multiple-batch production systems that contain fish sizes ranging from recently stocked fingerlings to fish weighing more than 1.5 kg. Currently, there are no accurate and cost-effective techniques to estimate pond inventories of catfish. Three trials were conducted in earthen catfish ponds with a 12 m × 8 m trawl (bar mesh at mouth of 3.8-cm; 2.54-cm at cod end). The first two trials included trawling with and without feed as bait in 4-ha commercial catfish ponds (Trial 1) and in 1.6-ha experimental ponds (Trial 2). In Trial 3, the trawl was pulled three times without feeding in 1.6-ha experimental ponds. Mean individual weights and size distributions of catfish caught by the trawl were statistically similar to those of catfish caught by the seine (Trial 1) and to stocking data (Trial 2). Single pulls with the trawl resulted in 73% error in estimated fish inventories in Trial 3. However, if pulled two times, the estimated catfish inventory error dropped to 7%, more accurate than other inventory methods, but was variable (coefficient of variation = 186%). The trawl appears to have potential as a means to sample commercial catfish ponds to estimate means and frequency distributions of fish size, but additional refinement and correction would be needed for it to be used reliably to estimate total pond inventory.     

Distribution and intensity of bottom trawl fisheries in the Patagonian Shelf Large Marine Ecosystem and its relationship with marine fronts

Trawling is a major concern worldwide and there is considerable debate about its impact on marine ecosystems. The Patagonian Shelf Large Marine Ecosystem (PSLME) is an important fishing area in the Southwest Atlantic where bottom trawling is the dominant fishing method. We investigated the distribution of bottom trawl fishing within this region, defining the areas of highest trawling intensity (hotspots) and evaluating their relationship with marine fronts. We focused on the three main oceanographic fronts, the shelf‐break front, the southern Patagonia front and the mid‐shelf front. To estimate fishing effort and trawled areas, we used VMS data from 2006 to 2012. Despite being almost a fully trawlable shelf, we found that the spatial distribution of trawling activity is patchy and trawling hotspots were small, comprising annually <5% of the shelf extension or <7% of the total trawlable area. Contrary to what is believed worldwide, our findings suggest that over the PSLME the magnitude of habitat effects as a result of bottom trawling is relatively small. Regarding the three frontal systems studied, only the shelf‐break front showed a positive relationship with trawl fishing activity. Although trawling hotspots did not overlap with marine fronts, the shelf‐break front receives more trawling effort than expected. We hypothesize that this pattern is due to aggregation of species near or at the front taking advantage of the opportunities provided by this area.     

Vulnerability to a small-scale commercial fishery of Lake Tana's (Ethiopia) endemic Labeobarbus compared with African catfish and Nile tilapia: An example of recruitment-overfishing?

In 1986 a motorised, commercial gillnet fishery was introduced in Lake Tana, Ethiopia's largest lake (3050 km²) in addition to the artisanal, predominantly subsistence fishery conducted from reedboats. The three main species groups targeted by this fishery are a species flock of endemic, large Labeobarbus spp., African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus). The commercial gillnet fisheries was monitored during 1991–1993 and in 2001 (CPUE, effort allocation, catch composition, yield) and the development of the stocks of the three species groups was determined using an experimental trawl program during 1991–1993 and 1999–2001.In 1991–1993 the vast majority of fishing activities took place in the southern Bahar Dar Gulf (71%). Each species group contributed roughly one third to the total catch. In 2001, 41% of the effort was allocated to the north-eastern shores of Lake Tana. Both the CPUE and the contribution of O. niloticus to the total catch had doubled. However, the proportion of large specimen (>50 cm TL C. gariepinus; >20 cm FL O. niloticus) in the trawl surveys decreased significantly. A three-fold decline in abundance of the anadromous Labeobarbus species occurred. The 15 large labeobarb species are long-lived, ecologically specialised endemics. The seven riverine spawning Labeobarbus species form aggregations in the river mouths in August–September, during which period they are targeted by the commercial gillnet fishery.A sharp decrease in abundance by ca. 75% of the migratory riverine spawning Labeobarbus species in the sublittoral and pelagic zones of the lake, areas where no fishing takes place and the collapse of juvenile Labeobarbus (between 5 and 18 cm FL: by 90%) during the 1990s suggest recruitment-overfishing. To prevent the possible extinction of the unique Labeobarbus species flock all fishing effort should be severely restricted near the river mouths and on the upstream spawning areas during August–September (peak breeding period) to protect the vulnerable spawning aggregations.     

Proximity of Pacific cod to the sea floor: Using archival tags to estimate fish availability to research bottom trawls

The percentage of Pacific cod available to Alaskan bottom trawl surveys was estimated from the proximity of tagged cod to the seafloor. Archival tags recorded time and depth data at 15- or 30-min intervals. The distance of a cod above the bottom was computed by subtracting tag depth from bottom depth, estimated as the maximum depth recorded during each 24-h day. These estimates of bottom depth are based on the assumption that cod approach the seafloor at least once a day, and do not undergo migrations up or down bottom gradients. To ensure that migrations over variable bottom gradients did not confound our estimates of bottom depth, we narrowed our analysis to 11 tagged cod that were recaptured in areas of flat bathymetry, and only analyzed data recorded within one month prior to recapture (N = 29,462 depth recordings). Pacific cod maintained short distances above the seafloor that often differed between day and night. Over 95% of the daytime tag recordings occurred within 10 m of the seafloor. Average effective headrope heights for survey bottom trawls currently used in the eastern Bering Sea and Alaska Gulf/Aleutian Islands groundfish surveys are approximately 2.5 and 7 m, respectively. In the absence of any behavior responses to an approaching trawl, we would expect 47.3% of the cod within the water column are available to the trawl used on the eastern Bering Sea survey and 91.6% are available to the trawl used on the Gulf/Aleutian Islands surveys. This study demonstrated that Pacific cod are highly demersal, and current values of trawl survey catchability (Q) used in current stock assessments are consistent with estimates of cod availability to the trawl gear.     

Hydroacoustic separation of rainbow smelt (Osmerus mordax) age groups in Lake Champlain

Separate assessment of young-of-year (YOY) and yearling-and-older (YAO) fish is desirable from both ecological and management perspectives. Acoustic assessments provide information on fish population size structure in the target strength (TS) distribution, but interpretation of TS distributions must be done carefully, as single age groups can produce multiple TS modes. We assessed the ability of in situ TS distributions to identify Lake Champlain rainbow smelt (Osmerus mordax) age groups in June, July, and September of 2001 using mobile and stationary surveys, knowledge of vertical distribution preferences, and predicted TS from trawl catches. YAO rainbow smelt (93–179 mm total length) had wide TS distributions between −60 and −35 dB in all 3 months with two modes at approximately −50 and −40 dB. Most stationary survey single-fish tracks attributed to YAO had targets in both TS modes and a wide TS range often over 15 dB. Between June and September, YOY rainbow smelt TS increased, but single-fish tracks were unimodal, and the TS range was smaller (6 dB). Overlap in TS attributed to YOY and YAO increased from no overlap in June (YOY TS −76 to −61 dB, 15–25 mm) to moderate overlap in July (−76 to −50 dB, 25–63 mm) to considerable overlap in September (−68 to −45 dB, 33–80 mm). In June and July, the TS distribution changed abruptly at the thermocline, indicating almost complete separation of the two groups. A more gradual TS transition was evident in September, indicating substantial overlap between YOY and YAO. Separate estimates can be obtained in September by decomposing TS overlap into components attributed to YOY and YAO rainbow smelt. However, this decomposition introduces additional uncertainty and an assessment in July or possibly August is preferable to obtain separate abundance estimates of YOY and YAO.     

浙江省帆张网捕捞强度分布的提取方法

捕捞强度是渔业资源管理和评估领域的重要参数之一。传统的计算方法无法满足实时、大范围、快速统计时空分布的需要。本文提出根据VMS数据对渔船生产活动划分作业阶段;结合帆张网渔船的作业特点,采用阈值划分和密度聚类算法提取网位,计算各航次的捕捞时长;划分地理格网并累加其范围内的捕捞时长,以各格网平均每平方公里累计捕捞时长(h/km2)作为帆张网捕捞强度的量化依据,并可视化捕捞强度空间分布。本研究以2017年浙江省所属帆张网渔船的北斗VMS船位数据为研究对象,共提取有效作业航次733个,网位6021个,累计捕捞时长736 761.78h。在121.6E~126.5E,27.6N~33.9N范围内划分0.1°×0.1°地理格网,2017年上半年帆张网捕捞相对分散,各格网平均捕捞强度11.26 h/km2;下半年相对集中,平均捕捞强度12.83 h/km2;全年平均14.76 h/km2。其中,在125.4E~126.1E,31.2N~32.1N范围内捕捞强度最大,平均28.51 h/km2。本文设计的网位提取、捕捞时长、捕捞强度空间分布,为分析帆张网渔船作业状态和捕捞强度量化提供新的研究思路。     

The efficiency of three fry trawls for sampling the freshwater pelagic fry community

Avoidance reactions of young-of-the-year fish assemblages were investigated in the offshore zone of three reservoirs in the Czech Republic during late summer. Pikeperch (Stizostedion lucioperca) strongly dominated in the Lipno Reservoir in 2003, bream (Abramis brama) and bleak (Alburnus alburnus) were the most abundant species in the Slapy Reservoir in 2004 and roach (Rutilus rutilus) and bleak dominated in the Želivka Reservoir in 2004 and 2005. Densities of fry ranged between 1.09 ind./100 m³ in the Lipno Reservoir and 2.95 ind./100 m³ in the Slapy Reservoir. In each reservoir, three trawl nets of different widths (dimensions 1.5 m × 3 m, 3 m × 3 m, 6 m × 3 m) were used in the pelagic area at night. For each trawl the estimated fish density was determined as the catch per 100 m³ of water and was compared between all three trawls. The fish length–frequency distribution was also compared between all the trawls. Day catches were much lower compared with those taken at night. Adult fish showed very clear avoidance of fry trawls. At night, no important differences in estimated fry density, obtained by these trawls, were observed in any locality for any fish species, nor in the length distribution of important species between trawls. According to these results, the avoidance reactions of fry in the late summer (August, September), at night, are unimportant. At this time of year, nighttime pelagic trawling appears to be an effective method for quantitative fry sampling. We recommend the 3 m × 3 m trawl for quantitative night sampling of the fry community as a good compromise between assured efficiency and relatively easy handling.     

Evaluating habitat suitability indices derived from CPUE and fishing effort data for Ommatrephes bratramii in the northwestern Pacific Ocean

Neon flying squid (Ommastrephes bartramii) plays an important role in the pelagic ecosystem and is an international fishery resource with high commercial value in the North Pacific Ocean. The west stock of winter–spring cohort of this species is an important target for the squid-jigging vessels of Japan, Korea and China (including Taiwan). The squid has a life span of less than 12 months, and its population dynamics is heavily influenced by its environment. Thus, a good understanding of its interactions with the habitats, often quantified with a habitat suitability index (HSI) model, is critical in developing a sustainable fishery. In this study, using the Chinese commercial squid-jigger fishery data and corresponding environmental variables we conducted HSI modeling to evaluate the habitat of the west stock of winter–spring cohort of neon flying squid in the northwestern Pacific Ocean. We compared catch per unit effort (CPUE) and fishing effort data in HSI modeling. This study suggests that the CPUE-based HSI model tends to overestimate the ranges of optimal habitats and under-estimate monthly variations in the spatial distribution of optimal habitats. We conclude that a fishing effort-based HSI model performs better in defining optimal habitats for neon flying squid. According to the fishing-effort-based HSI model, the optimal ranges of the following key habitat variables are defined: from 16.6 to 19.6 °C for SST, from 5.8 to 12 °C for temperature at depths of 35 m, from 3.4 to 4.8 °C for temperature at depth of 317 m, from 33.10 to 33.55 psu for SSS and from ?20 cm to ?4 cm for SLH.     

The fishery for Nephrops norvegicus (Linnaeus, 1758) in the central Adriatic Sea (Italy): Preliminary observations comparing bottom trawl and baited creels

The Norway lobster, Nephrops norvegicus, is mainly targeted by bottom trawlers whose selectivity (mesh size = 40 mm stretch) with respect to this species is poor producing large quantities of discards both in terms of demersal fish species and undersized Nephrops. The Pomo pit (central Adriatic Sea) is a very important nursery area for European hake, Merluccius merluccius, but is also the main trawling ground for the Adriatic fleet exploiting Nephrops. The use of baited creels may constitute an alternative to bottom trawling. This paper summarises experimental data collected from the western Pomo pit in order to compare bottom trawl and baited creel catches with the aim of evaluating the ecological consequences of creeling. Because of high scavenger activity in the area, the effect of this on the creel fishery was also investigated.A large fraction of the trawl catch was composed of juvenile undersized individuals of commercial species, the same was not true for creels. Composition of Nephrops catches differed markedly between creels and the bottom trawl. Scavenger activity was very high. The results indicate that ecologically the use of baited creels appears to be a valid alternative to bottom trawling but, unfortunately, may not comprise an economically viable solution in the investigated area, which is 30 nautical miles offshore.     

Estimation of basket trap selectivity for changeable nassa (Nassarius mutabilis) in the Adriatic Sea

Size selectivity of basket traps for changeable nassa (Nassarius mutabilis) was estimated through comparative fishing trials with traps varying in mesh size and net colour. Selectivity curves of two commercial (19 mm white twine and 19 mm black twine) and three experimental (23, 26 and 28 mm white twine) traps were estimated using SELECT method. In addition, theoretical size-selectivity curves were calculated from morphometric data using a simplified version of Sechin method.Differing from the opinion of fishermen, N. mutabilis did not show any preference for dark coloured twine. Retention curves estimated with SELECT method had a very sharp selectivity and well fitted catch data in all cases. Theoretical size-selectivity curves were very close to size-selectivity curves obtained from experimental data. The two types of curve were very similar in shape, although the theoretical ones were shifted towards larger size classes and showed a slightly steeper slope than SELECT curves. The mesh size commonly used by fishermen is not adequate for rationally exploiting the resource, as catches mainly included undersized specimens, whilst the 23 and 26 mm mesh sizes would represent a good compromise between reduced occurrence of undersized individuals and satisfactory yields of commercial product.     

Size selective capture of Atlantic cod (Gadus morhua) in floating pots

Little is known about the size selectivity of cod in static fishing gears such as pots and traps. In this field study, floating fishing pots were equipped with 40, 45 and 50 mm square mesh escape windows in order to estimate the size selectivity of cod at different mesh sizes. Relationships between selectivity parameters and mesh size, as well as an optimal mesh size for the escape window with respect to current minimum landing size of cod in the Baltic Sea were sought. The results show that the floating pot is not only species selective when used in the Baltic Sea cod fishery, the implementation of an escape window reduced the proportion of undersized bycatch in the pots by more than 90%. The estimated length at 50% retention was found to be a direct function of fish body length (girth) and mesh size of the escape window, while the estimated selection range remained unchanged regardless of mesh size. Optimal mesh size, with respect to the length at 50% retention and current minimal landing size of 38 cm in the Baltic, was determined to be 45 mm. Strong indications (significant on the 0.01 probability level in the case of 50 and 40 mm escape windows) suggested that the relative fishing power of the pots increased with the implementation of an escape window. This result could be explained as a “saturation effect”, i.e. the probability of cod entering the pot is likely to be negatively dependent on cod density in the pot. The high species selectivity and the low catch rate of undersized fish have consolidated the floating pots position as a highly benign fishing method and the foremost alternative gear for the future coastal cod fishery in the Baltic Sea.     

闽南-台湾浅滩渔场单船拖网作业调查与分析

根据2009—2010年闽南-台湾浅滩渔场单船拖网作业监测调查资料和生产统计数据,阐明单船拖网作业的渔业地位、渔场分布、网具类型,着重分析该作业渔获种类组成及主要捕捞对象的资源动态。结果表明:单船拖网作业产量位居闽南-台湾浅滩渔场各种海洋捕捞作业的首位,大吨位、大功率渔船数量连续多年不断增长;渔具型式以有翼单囊网具为主;部分渔船使用的拖网网囊尺寸远远低于国家标准;带鱼、二长棘鲷幼鱼损害现象较为严重;秋季8—10月蓝圆鲹和鲐鱼群体绝大多数个体可达到最小可捕规格。调查研究结果还表明,2002年以来单船拖网渔业资源结构已发生较大变化,生命周期短、营养级较低的蟹类、枪乌贼类、乌贼类和虾类资源渔获比重显著增长;原优势种类绿布氏筋鱼资源已出现严重衰竭;二长棘鲷资源量逐年减少,渔获群体明显趋向小型化、低龄化。     

Bycatch and discarding in the South African demersal trawl fishery

Observers aboard commercial trawlers collected data on the total catch composition of 614 and 479 hauls made by vessels operating off the south and west coasts of South Africa, respectively. On the south coast, four fishing areas were identified on the basis of target species and fishing depth. On the west coast, hauls were separated into those targeting hake Merluccius spp. in four depth ranges (0–300, 301–400, 401–500, and >500 m) and those targeting monkfish Lophius vomerinus. For each area, the catch composition was calculated and the species assemblages were investigated using cluster analysis and multi-dimensional scaling. Finally, for each coast, the weight of fish discarded annually was estimated. On the south coast, although hake dominated, between 21% and 47% of the catch was not hake, depending on the fishing area. In comparison, hake dominated west coast catches, the proportion of hake increasing with depth. For each fishery investigated, approximately 90% of the catch was processed and landed. However, estimates of annual discards indicate that the south and west coast fisheries may annually discard 9000 or 10,000 t and 17,000 or 25,000 t, of undersized and unutilizable fish and offal, respectively, depending on the estimation method used. When developing strategies to limit or enhance utilization of bycatch, cognisance should be taken of the differences in catch composition between the south and west coasts and of the importance of bycatch revenue to south coast fishing companies.     

Contribution of bait to lobster production in an oligotrophic marine ecosystem as determined using a mass balance model

The fishery for the western rock lobster (Panulirus cygnus) in Western Australia is Australia's largest trap-based fishery, deploying 8.8 million pot lifts, landing on average 11,000 tonnes of lobster and using approximately 14,000 tonnes of bait annually. A mass balance model was constructed to determine the potential contribution of this bait to the diet of western rock lobsters. Bait is potentially a significant subsidy given the oligotrophic nature of Western Australia's marine environment. The mass balance model was constructed on the principle that the biomass of the lobster population reflects the difference between inputs (growth, immigration) and outputs (natural and fishing mortality and emigration). Biomass calculated using this approach was within 7% of biomass calculated from independent estimates based on depletion analysis, indicating the model is robust. The food required to explain observed growth was then calculated, allowing the potential contribution of bait to lobster diet to be assessed. The abundance of natural diet items on the benthos was sufficient to explain the observed growth of lobsters, with bait contributing a maximum of 13% of lobster food requirements over the whole ecosystem. This contribution of bait will differ spatially and temporally reflecting uneven distribution of fishing effort and may be as high as 35% during some months of the fishing season, a result consistent with dietary studies based on stable isotopes. Given observed effects of organic matter addition on ecosystem processes as observed in trawl fisheries and aquaculture operations, it is likely that the effects of bait addition on ecosystem function are more widespread than lobster production.     

Pelagic longline gear depth and shoaling

Temperature-depth recorders (TDRs) were attached to pelagic longline gear in the Hawaii-based commercial fishery to obtain actual fishing depths and to test the accuracy of catenary algorithms for predicting fishing depths. Swordfish gear was set shallow by typically deploying four hooks between successive floats. The observed depth of the settled deepest hook had a median value of 60 m for 333 swordfish sets. Tuna longline gear deployed more hooks between floats (mean = 26.8), and the observed median depth of the deepest hook was 248 m (n = 266 sets). Maximum gear depth was predicted from estimates of the longline sag ratio and catenary algorithms; however, depth was not predicted for all TDR-monitored sets because estimating sag ratios proved problematic. Swordfish sets had less slack in the main line and correspondingly smaller catenary angles (median = 54.2°) than tuna sets (median = 63.7°). Median values of the predicted catenary depth were 123 m for swordfish sets (n = 203) and 307 m for tuna sets (n = 198). Shallow swordfish sets reached only ∼50% of their predicted depth, while deeper tuna sets reached about 70%. These values indicated that capture depths using traditional catenary equations may be biased without the benefit of TDRs affixed to longlines. Generalized linear models (GLMs) and generalized additive models (GAMs) were developed to explain the percentage of longline shoaling as a function of predicted catenary depth and environmental effects of wind stress, surface current velocity, and current shear. The GAM explained 67.2% of the deviance in shoaling for tuna sets and 41.3% for swordfish sets. Predicted catenary depth was always the initial variable included in the stepwise process, and the inclusion of environmental information in the GAM approach explained an additional 10–17% of the deviance compared to the GLMs. The explanatory ability of the environmental data may have been limited by the scale of the observations (1° in space; weekly or monthly in time) or the geometric (transverse versus in-line) forcing between the environment and longline set. Longline gear models with environmental forcing affecting shoaling may be improved in future studies by incorporating contemporaneous environmental information, although this may restrict analyses to fine-scale experimental longlines.     

浙江省单拖渔业现状分析与研究

利用2001~2006年浙江省单拖渔业典型性调查数据和2005~2008年的单拖作业监测资料,从浙江省单拖捕捞力量和产量分布、渔获对象和资源密度变化及经济效益、劳动力组成等几个方面进行了初步分析。结果表明,近几年浙江省单拖作业船数保持在2200艘左右,产量在40×104t左右,单船净利润存在年间波动,以2005年最高;单拖作业渔场分布较广,渔获物以头足类和中下层鱼类为主;近年来经济鱼类比例下降,而其它低值鱼类比例上升。文章最后提出了当前浙江单拖渔业存在的问题,并提出了相应管理建议以供参考。     

闽中地区单拖作业捕捞能力的因子分析

单拖作业是福建省闽中地区重要的作业渔具之一。在捕捞生产过程中渔具受到诸多因素的影响,各种因素对渔具捕捞能力的影响程度不同,给捕捞力量的量化计算和管理带来困难。为了了解各因子对单拖作业捕捞能力发挥的影响程度,本文选择影响单拖捕捞能力发挥的12个因子指标,采用因子分析法对其进行分析。结果表明,可将影响单拖捕捞能力发挥的诸多因素归并为网具结构、渔船作业能力、船龄、渔民捕捞技能4个公共因子,它们在单拖捕捞过程中所做的贡献率依次为网具结构52.87%,渔船作业能力24.46%,船龄7.05%,捕捞技能4.73%。本文还计算和讨论了各样品船的因子得分,通过各因子的得分和综合得分,可对各样品船的捕捞能力发挥情况进行综合评价,为渔业生产和管理提供参考。