Vital population statistics of the exploited eel stock on the Swedish west coast

The population dynamics and exploitation of the yellow eel (Anguilla anguilla (L.)) stock on the Swedish west coast were studied. In contrast to a generally observed reduction in the recruitment of glass eels in Europe, including in Swedish waters, there was no indication of a decline in the total eel fishery yields along the Swedish west coast. Long-term records of daily catches as well as by test fishing results also shown that this stability in eel fishery yields has not been maintained by an increase in fishery effort, as the catch-per-unit-efforts in the past 20 years have been more or less unchanged. These findings implied that the number of recruits to the fishery has been rather stable, possibly indicating that density-dependent factors at the elver and yellow eel stages may moderate variations in glass eel recruitment. Total instantaneous rate of mortality was estimated from records on eel length distribution in the professional fyke-net fishery. The estimated total mortality rate in an isolated archipelago population on the west coast was chosen as an approximation of the instantaneous rate of natural mortality and net emigration in the west coast eel stock. The differences between these two estimates could, thus be regarded as the mortality that occurred due to fishing. It was found that the eel fishery was very intense and most fish were caught in small sizes, resulting in a low escapement rate of maturing fish.     

Can fishermen allocate their fishing effort in space and time on the basis of their catch rates? An example from Spermonde Archipelago,SW Sulawesi,Indonesia

Spatial and temporal patterns in catch rates and in allocation of fishing effort were analysed for the coastal fishery in Spermonde Archipelago, Indonesia, to assess whether fishermen can optimise their strategy from catch information, or whether they fish under great uncertainty and merely minimise risks. On average 517 fishing units operated in the 2800 km² area, catching 21 t fish day⁻¹. Major gear categories were hook and line (59% of total effort and 5% of total catch), and lift nets (16% of total effort and 70% of total catch). The size of individual resource spaces varied with gear type and was smaller in unfavourable weather conditions. Although spatial patterns in catch rates at the scale of the whole archipelago were evident, fishermen could not differentiate between locations, as catch variance within their individual resource spaces was high relative to the contrasts in spatial patterns. The aggregated distribution of fishing effort in Spermonde must be explained by factors such as the small scale of operations, rather than fish abundance.     

Stock assessment of Japanese eels using Japanese abundance indices

The paper compiles a catch history of Japanese eels Anguilla japonica in East Asia and some Japanese relative abundance series. Maximum likelihood estimates of stock abundance of eels have been obtained using the abundance series and various biological parameters, such as growth, maturity and natural mortality. Age- and sex-structured models have been used to express the dynamics of stock abundance, and the Beverton and Holt model has been used to express the relationship between stock and recruitment. Data for estimations are standardized catch per unit effort of commercial fishery for exploitable stock (1954–2006 and 1968–2008) and for glass eel (1954–2010, 1972–2004, and 1973–1997). From the results of the base case scenario of estimations, the estimated stock size of individuals aged ≥1 year was 18.7 thousand tons in 2010, which was 24 % of the carrying capacity. The estimated stock size has recovered since 1990. Maximum sustainable yield was 4,180 tons if only the exploitable stock were utilized, and 266 tons if only the glass eel were utilized. These results and issues relating to estimation and management for reducing the fishery impact on stock are discussed.     

Inherent bias in using aggregate CPUE to characterize abundance of fish species assemblages

We have analyzed the practice of assessing an assemblage of fish species in a multispecies fishery on the basis of aggregate catch per unit effort (CPUE), which is the summed catch of all species per unit of effort. We show that at the onset of fishing or of a large positive or negative change in fishing effort, aggregate CPUE will be hyper-responsive, that is, relative change of aggregate CPUE will be greater than that of aggregate abundance. We also show that as the fishery reaches equilibrium, the aggregate CPUE in most circumstances will continue to be hyper-responsive, with a greater relative change from its value at the start than the aggregate abundance. However, there are less likely circumstances in which the aggregate CPUE will be hyper-stable compared to aggregate abundance. The circumstances leading to hyper-responsiveness or hyper-stability depend on the distribution of productivity and fishery vulnerability parameters among the species in the aggregation.     

Co‐management in Latin American small‐scale shellfisheries: assessment from long‐term case studies

Co‐management (Co‐M), defined as the sharing of management tasks and responsibilities between governments and local users, is emerging as a powerful institutional arrangement to redress fisheries paradigm failures, yet long‐term assessments of its performance are lacking. A comparative analysis of five small‐scale Latin American shellfisheries was conducted to identify factors suggesting success and failure. In Chile, Uruguay and Mexico Co‐M produced positive effects, including stabilization of landings at low levels, increase in abundance, CPUE, unit prices and revenues per unit of effort, and reduced interannual variability in several fishery indicators, particularly in landings. Co‐M was successful because it was mainly bottom‐up implemented and accompanied by‐catch shares (spatial property rights and community quotas). By contrast, Co‐M implementation was unable to prevent the collapse of the Galapagos sea cucumber fishery, as reflected by a decrease in abundance and CPUE. Negative effects were also observed in the Galapagos spiny lobster fishery during Co‐M implementation. However, recovery was observed in recent years, reflected in a stabilization of fishing effort and the highest CPUE and economic revenues observed since the beginning of the Co‐M implementation phase. The combined effects of market forces, climate variability and a moratorium on fishing effort were critical in fishery recovery. We conclude that Co‐M is not a blueprint that can be applied to all shellfisheries to enhance their governability. These social–ecological systems need to be managed by jointly addressing problems related to the resources, their marine environment and the people targeting them, accounting for their socioeconomic and cultural contexts.     

福建海区单拖渔业现状浅析

本文根据1998-2003年福建海区拖网渔业资源监测以及1995-1997年间闽南台湾浅渔场单拖渔业资源调查、1998年和2000-2001年福建海区拖网定点调查的资料,论述和分析福建海区单拖渔业的现状。结果表明:目前整个单拖渔业虽然船数削减,实际却加大渔船功率,作业时间延长,促使年产量、单位产量不断提高,导致渔获组成发生很大变化,由低质小型鱼类、生命周期短的头足类、虾蟹类支撑,渔获鱼类质量较差,经济效益下降;另一方面头足类、虾蟹类资源量年间波动性较大,易受捕捞过渡影响,很难长时间承担目前捕捞压力。为此,文中提出加强严格实行"双控制度"及控制网目规格等管理建议和意见,为有关渔业管理和生产部门提供参考依据。     

The eel fishery in Lough Neagh,Northern Ireland – an example of sustainable management?

Abstract  Lough Neagh produces over 500 t of grown eel annually and employs 300 people fishing yellow and silver eel. Glass eel are transported upstream and stocked into the Lough. Since glass eel returns crashed in the 1980s, additional glass eel seed has been purchased from other fisheries. The fishery now faces ecological, social and economic pressures. Prices for the product have fallen; recruitment to the fishery has declined, and seed has decreased in availability and increased in price. Fishers are less inclined to take up the hard work required to make a living fishing eel and the fisher population is ageing. The European Commission has recognised the decline of eel and proposed emergency measures, which may further affect the viability of the fishery. The sustainability of the fishery is examined, based on the relationship between glass eel input and grown eel outputs over a period of 45 years, set against increasing environmental, socio-economic, and natural resource pressures. Spawning escapement of silver eel is estimated by mark–recapture experiments.     

Comparison of glass eel stages of American eel and speckled worm eel in a northeast Florida estuary

Since 2001, a glass American eel, Anguilla rostrata (Lesueur), survey has been conducted at Guana River Dam, Florida, USA. Although present in earlier samples, glass speckled worm eel, Myrophis punctatus Lütken, were not identified until a large proportion appeared in samples in 2006. To determine whether the species could be separated in historical samples, weight–length relationships were compared between the two species. Then, with data from the full survey period, relationships between catch rate and annual percent contribution of glass speckled worm eel were explored in relation to temperature, water flow and salinity. Glass speckled worm eel were found to weigh significantly less and had distinguishing physical characteristics that could be used for identification. Glass speckled worm eel constituted between 0 and 34% of the catch, with no detectable pattern in timing or abundance among years. In 2006 and 2012, which had the two largest catches of glass speckled worm eel, relationship between nightly catch rate and measured environmental variables was inconsistent, even when measured variables had a similar range in values. It is recommended that south‐eastern, including Gulf of Mexico, USA, states restrict harvest of glass American eel to prevent the incidental capture and exportation of glass speckled worm eel. Efforts should be made to ensure proper species identification in glass eel surveys.     

Influence of stock levels, fishing effort and environmental factors on anglers' catches of Atlantic salmon, Salmo salar L., and sea trout, Salmo trutta L.

Abstract. Monthly catches of Atlantic salmon, Salmosalar L., and sea trout, Salmo trutta L., by anglers in a west of Ireland fishery were analysed. Data were available for 50 months from the years 1971 to 1981. The most important single determinant of catch was found to be fishing effort measured in boat-days. An additional 10 boat days were found to correspond to a catch of almost 6 salmon and 23 sea trout. The relationship between catch and stock was weaker, though a relatively high catchability of sea trout at low stock levels was recorded. Slock levels, fishing effort and environmental factors accounted for much, but not all, of the variation in catch from month to month and from year to year. Most of the effect of rainfall, sunshine and water level was attributable to variations in fishing effort associated with these factors.     

A new spatial framework incorporating uncertain stock and fleet dynamics for estimating fish abundance

Relative abundance indices based on catch and effort data can become biased unless consideration is given to the spatial dynamics of the fishery such as changes in either the spatial distribution of fishing effort or the range of the stock over time. The construction of such indices therefore needs to take into account features of the fishery itself. In this paper, a general framework is presented for developing more appropriate abundance indices based on fishery catch and effort data. In developing this framework, it adopts the approach of (i) developing a range of hypotheses which encompass the uncertainties in the spatial–temporal dynamics of the stock and the fishing effort, (ii) identifying the hypotheses underlying the different CPUE series, and (iii) evaluating the available information relative to these hypotheses as the basis for evaluating CPUE indices. Observations from the fishery for southern bluefin tuna (Thunnus maccoyii) are used to illustrate various hypotheses about the nature of the fishery which can be used to construct indices of stock abundance while a simple simulation framework is used to explore the implications of some of these hypotheses on the accuracy of such indices.     

Assessment of salmon stocks and the use of management targets; a case study of the River Tamar, England

Abstract  Over recent years the rod and net catch of Atlantic salmon, Salmo salar L., on the River Tamar in south-west England has decreased markedly, resulting in a consistent failure to meet the minimum egg deposition target (conservation limit). Compliance with the target is by annual assessment using rod catch as the major input variable. Further analysis suggested a disproportionate deterioration in the rod fishery performance of the Tamar compared with rivers locally, regionally and nationally. A concomitant decrease in rod licence sales and fishing effort, above both national and regional trends was also evident. However, examination of juvenile electric fishing and adult fish counter data revealed a different trend for the past 10 years, indicating a stable fish population, albeit at a lower level of abundance than previously. The analyses suggested that without consideration of changes in effort and rod exploitation rate, rod catch alone is not a reliable indicator of stock abundance and hence should not be used as such in stock assessment.     

Recreational charter fishery attributes and variation in key species catches and discards: Resource management considerations

Strategic long‐term sampling programmes that deliver recreational catch, effort and species demographic data are required for the effective assessment and management of recreational fisheries and harvested organisms. This study used a spatially and temporally stratified observer programme to examine variation in the rates, quantities and lengths of retained and discarded catches of key species in a recreational charter fishery. Geographic region, but not season, significantly influenced catch rates of key demersal species, being driven by temporally persistent latitudinal clines in environmental conditions influencing species distributions. There was considerable trip‐to‐trip variation in catch rates that were attributed to localised differences in fishing operations, locations, environmental conditions and client preferences. Broad trends in retained and discarded catch rates were nevertheless, similar across different fishing effort standardisations (per‐trip, per‐hour, per‐client, per‐client/fished hour), demonstrating that the coarsest unit of effort could be used in fishery assessments. Discard rates of organisms were variable and driven by a combination of mandated legal lengths, individual client and operator preferences for particular species and sizes of organisms, and not due to attainment of catch quotas or high‐grading. This study has identified important fishery attributes that require consideration in assessing charter fisheries and stocks of recreational fish species.     

Impacts of fishing,river flow and connectivity loss on the conservation of a migratory fish population

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Variation in numbers of eel Anguilla anguilla caught by constant effort in an Irish lake, 1981–1994

Abstract– A total of 8,903 eels were collected in 503 fyke-net samples during the months March to December from 1981 to 1994 in a small bay in Lough Derg, a mesotrophic lake. Variations in catch between months and between years are described. In addition to expected decreases in catch in the colder months, a regular reduction was observed in June. Low catches were observed from 1985 to 1987, coinciding with low water temperatures. A very low catch in August 1994 may have been a chance variation but could have been a first indication of the effects of poor recruitment or of increased commercial fishing effort. The results support the initial hypothesis that a long-term study is required to determine the extent of variation in an eel population.     

Catch rate standardization for yellowfin tuna (Thunnus albacares) in Taiwan's distant-water longline fishery in the Western and Central Pacific Ocean,with consideration of target change

Catch per unit of effort (CPUE) needs to be standardized to remove the effects of factors such as fishing time and location, before it can be used as an index of abundance in fish stock assessments. One of the most substantial effects arises from a change of target species. This is particularly important for the Taiwanese distant-water longline fishery, which has a long history of fishery data from two fleets that target various tuna species across three oceans. We review the development of the Taiwanese distant-water longline fishery and compare five designs for standardizing the catch rate of yellowfin tuna (Thunnus albacares) in the western and central Pacific Ocean, using generalized linear models with lognormal and delta-lognormal error assumptions. Two approaches to address targeting effects were tested: separating fishing fleet data based on observer records, and including four target indicators calculated from catch data. Four statistical regions (relating to major fishing grounds) were treated as a single factor in the first three cases and were treated separately for the last two (one independent run for each region). The last case, which involved independent analyses for each fishing fleet for each region, and using the delta-lognormal approach, was considered to provide the most informative standardized CPUE trends for yellowfin tuna.     

Decadal trends in abundance,size and condition of Antarctic toothfish in McMurdo Sound,Antarctica, 1972–2011

We report the analyses of a dataset spanning 39 years of near‐annual fishing for Dissostichus mawsoni in McMurdo Sound, Antarctica, 1972–2011. Data on total length, condition and catch per unit effort (CPUE) were derived from the > 5500 fish caught, the large majority of which were measured, tagged and released. Contrary to expectation, the length frequency of the McMurdo Sound catch was dominated by fish in the upper two‐thirds of the overall distribution exhibited in the industrial catch for the Ross Sea shelf. Fish length and condition increased from the early 1970s to the early 1990s and then decreased. Fish length positively correlated with Ross Sea ice extent in early spring, a relationship possibly caused by more ice encouraging larger fish to move farther south over the shelf and into the study area. Fish condition positively correlated with the amount of open water in the Ross Sea during the previous summer (Feb), perhaps reflecting greater availability of prey with the higher productivity that more open water brings. Decreasing fish size corresponds to the onset of the fishery, which targets the large individuals. CPUE was constant through 2001 and then decreased dramatically. We hypothesize that this decrease is related to the industrial fishery, which began in the 1996–97 austral summer, and concentrates effort over the ice‐free Ross Sea continental slope. As a result of limited prey choices and close coupling among mesopredators of the region, Antarctic toothfish included, the fishery appears to be dramatically altering the trophic structure of the Ross Sea.     

涉海建设项目中深水海域渔业资源调查问题的研究

为解决涉海工程深水渔业资源评估的问题,于2014~2015年在南海北部局部海域开展底拖网与灯光罩网联合调查渔业资源活动。结果表明:在深水区底拖网渔获组成与灯光罩网有明显的不同,并受到季节与海域的影响(P0.05)。拖网渔获物中鱼类平均占渔获总尾数的93.36%,头足类占3.53%,甲壳类占3.11%;灯光罩网渔获物中鱼类平均占渔获总尾数的63.33%,头足类占36.65%,甲壳类占0.02%。两种调查的相同渔获物1~6种,主要为鱼类和头足类,在拖网中所占比值较低,平均占渔获总尾数的2.07%,而在灯光罩网中比值极高,平均占渔获总尾数的67.03%。灯光罩网与底拖网调查的渔业资源密度直接比值平均为0.06,灯光罩网与底拖网扣除相同渔获种类后的资源密度比值平均为0.06,相应的质量密度直接比值平均为0.62,扣除相同渔获种类后的比值平均为0.67。研究认为,深水海域的渔业资源量应该是两种渔具调查结果之和,对两种渔具调查中出现的相同渔获物,宜取其评估数据的平均值。由于各站点调查结果差异较大,采用所有站点的平均值来计算评估海域的损害赔偿较为合适。建议对SC/T9110-2007技术规程修订,规范中上层渔业资源调查方法。     

利用栖息地指数模型预测秘鲁外海茎柔鱼热点区

利用栖息地指数模型准确地预测了秘鲁外海茎柔鱼的热点区。根据2008-2010年1-12月期间我国鱿钓渔船在秘鲁外海的生产数据,结合实时的海表温及海表面高度数据,分别建立以作业次数、单位捕捞努力量渔获量为基础的适应性指数。利用算术平均数模型建立基于海表温和海表面高度的栖息地指数模型,并利用2011年生产及环境数据对栖息地指数模型进行验证。结果显示,以作业次数为基础的适应性指数符合正态分布,而以单位捕捞努力量渔获量为基础的适应性指数显著性检验不显著,因此,只建立以作业次数为基础的模型。结果表明,以作业次数为基础的栖息地指数模型都高估了茎柔鱼热点区的范围,但大体范围基本一致,这说明其能较好地预测茎柔鱼的热点区。     

Standardising the CPUE for the <Emphasis Type="Italic">Illex argentinus</Emphasis> fishery in the Southwest Atlantic

The catch per unit effort (CPUE) is a widely used index for assessing the abundance of exploited populations in fishery management. To obtain appropriate CPUE values, it is essential to standardise catch-effort data from fisheries. This task is particularly important for squid fisheries because squid generally have a short life-span and are vulnerable to environmental variability, and thus effective fishery management should take such factors into account. In this study, we analysed unit catches of paired vessels operating under similar fishing conditions to calculate their relative fishing power (RFP) in order to standardise the CPUE of the Taiwanese fleet jigging for Illex argentinus in the Southwest Atlantic. To evaluate the appropriateness of the method, we used a logbook dataset covering eleven years (1993–2003), in which 93.5% of the total catch during the period was included. The results indicate that 98.7% of the fishing effort can be standardised according to the estimated RFP. Compared to nominal CPUE, the standardised CPUE values projected an explainable temporal pattern, indicating an increasing trend in abundance from 1995 to 1999 and a subsequent sharp plunge from 1999 to 2003. However, the RFP was not related to apparent physical factors of the vessel, such as gross tonnage or vessel length. Our evaluations suggest that the RFP method is appropriate for standardising the CPUE, so that it can serve as an abundance index that reflects the annual recruitment size of the squid fishery, because the quality of the method can potentially take possible affecting factors into account in order to satisfy the general assumptions of standardisation criteria. However, the effects of varying the settings of parameters should be carefully examined prior to applying this standardisation method to other squid fisheries.