Comparative analysis of ichthyoplankton sampling gear for early life stages of walleye pollock (Theragra chalcogramma)

Comparative gear tows were conducted during the summers of 1987–1990 in the Gulf of Alaska. Efficiencies of four types of ichthyoplankton sampling gear were compared with respect to total catch and size distribution of larval fish. Fish were sampled using the bongo net (333 and 505 μm mesh), 1 m Tucker net, 3 m Tucker net and the Methot frame net. Results indicate that the bongo and the 1 m Tucker nets caught similar numbers and size distribution of fish. The larger 3 m Tucker and Method frame nets caught a larger size class of fish but a smaller total catch. Although there was a slightly significant diel difference in total catch detected in the smaller nets, no diel differences in size distribution of fish caught were detected for any of the nets.     

Influence of mesoscale eddies on ichthyoplankton assemblages in the Gulf of Alaska

Mesoscale eddies (100–200 km in diameter) propagating along the shelf‐break in the Gulf of Alaska are ubiquitous and have been shown to influence the ecosystem, but their influence on ichthyoplankton species composition and diversity has not been described. Evidence for larval fish entrainment in these eddies was examined using data from a cruise in 2005 that sampled three eastern Gulf of Alaska mesoscale eddies, and sampling that compared shelf to slope ichthyoplankton assemblages in the northern Gulf of Alaska (2002–2004). Hierarchical cluster analysis of oceanographic data showed that stations grouped according to location within an eddy. Species hierarchical cluster analysis revealed a latitudinal turnover in species composition, and an abundant species group. Species richness was correlated with distance from eddy center (P = 0.00025), and assemblages within eddies were significantly different (P < 0.05) from those in surrounding basin and shelf waters. These results suggest that mesoscale eddies propagating along the continental shelf‐break influence larval fish assemblages over the shelf and slope, which has implications for the timing and extent of larval fish distribution in the Gulf of Alaska.     

Simulation of physically mediated variability in prey resources of a larval fish: a three-dimensional NPZ model

A three-dimensional biophysical nutrient–phytoplankton–zooplankton model was used to investigate the spatial and temporal dynamics of food resources for young walleye pollock in the western Gulf of Alaska, to further understanding of recruitment processes for pollock. We modeled nitrogen, phytoplankton, a large herbivorous grazer parameterized as Neocalanus spp. (the biomass dominant copepod in the Gulf), and the 13 stages (egg, naupliar and copepodite) of Pseudocalanus spp. (a major constituent of the diet of pollock) so that the appropriate size class of food for each size of larval pollock was represented. Model results identified an area between the Semidi and Shumagin Islands that may not be suitable as a nursery area early in the year due to low prey abundance. Modeled mesoscale eddies, previously hypothesized to be important for larval pollock retention in Shelikof Strait, contained higher prey concentrations than the surrounding waters when they were cyclonic. This work also help to understand the consistency of pollock spawning in time and space in Shelikof Strait, by examining the timing and location of prey availability which, along with transport, narrows the window for optimal spawning.     

Seasonal and inter‐annual variations in the abundance and biomass of Neocalanus plumchrus in continental slope waters off Oregon

Seasonal and inter‐annual variability in the abundance and biomass of copepodid stages of the sub‐arctic oceanic copepod, Neocalanus plumchrus, was studied during the January–May growth season, using an 11‐yr time series of zooplankton samples collected over the upper 100 m of the water column. Abundance and biomass peaks occur in March/April. Abundance and biomass of N. plumchrus were significantly negatively correlated with sea surface temperature and significantly positively correlated with sea surface chlorophyll a, salinity, and density above the pycnocline. The seasonal integrated abundance and biomass of N. plumchrus declined during the warm years (2003–05), and increased during the cold years (2006–08). The date when 50% of the population had passed through stage C5 was significantly negatively correlated with temperature – earlier in warm years and later in cold years. In 3 yr (2003, 2007 and 2008), a second cohort appeared in mid‐May, as indicated by the presence of stages C1 and C2 in the samples. Unusually high abundances of N. plumchrus in the spring of 2007 and 2008 were associated with cool ocean temperatures and an early spring transition in the NCC ecosystem, suggesting that the NCC ecosystem has returned to a cold phase. We discuss the merits of a hypothesis that the N. plumchrus population observed off Oregon is a local population as opposed to one that is expatriated from the Gulf of Alaska.     

长江口两种仔稚鱼网具的采集效率比较

为获得更为准确的数据,提高对长江口鱼类早期资源评估的准确度,本研究同时采用方形网(网口为1.0 m×2.0 m,网目1.0 mm,网衣长5.5 m)和标准网(网口为直径0.8 m圆形,网目0.5 mm,网衣长3.5 m)于2014年4—7月在长江口邻近海域采集仔稚鱼样本,分析比较这2种网具所采集仔稚鱼种类和数量差异,从而选择更适宜采集长江口水域仔稚鱼的采样网具。结果表明:1方形网采集到51种仔稚鱼,多于标准网的37种,两种网具的共有种类有34种,主要优势种组成没有明显差异;24月,前弯曲期仔鱼较多,它们个体小,容易从网眼滤出,导致网目较小的标准网的样品平均密度高于方形网;35—7月,方形网的样本平均密度高于标准网,但样本中仔稚鱼个体发育阶段组成有明显不同。标准网样本中前弯曲期仔鱼占多数,体形为细长形的日本鳀和鳅虎鱼的前弯曲期仔鱼平均密度高于方形网;方形网样品中后弯曲期仔鱼占优势,小黄鱼、日本鳀和鳅虎鱼等优势种的后弯曲期仔鱼平均密度均高于标准网。总体而言,方形网所采集仔稚鱼种类和数量的效果优于标准网;对于细长体形种类(如日本鳀),需结合不同网具以便采集到全部发育阶段个体,而体形较宽种类(如小黄鱼)可仅以方形网采集。     

Walleye pollock egg distribution and mortality in the western Gulf of Alaska

We examine the distribution and mortality of walleye pollock ( Theragra chakogramma ) eggs in the western Gulf of Alaska. Most pollock eggs were found in mid-water, with low proportions in the neustonic and epibenthic layers during all years of sampling. A silhouette camera towed through a high egg density region provided new information on small-scale spatial distributions and provided density estimates at two depth layers similar to those of depth-discrete net sampling. Annual egg production curves and natural mortalities were estimated for 1987-92 based on the abundance of several cohorts relative to their production rate. Production during 1989 and 1990 was lower than in the other four years but 1988 was the only year to show markedly different (higher) mortality than the rest. Fertilization rate was generally very high (>99%) but several collections early in the season contained a substantial fraction of unfertilized eggs. Invertebrate egg predation was mainly due to euphausiids and was variable among locations and years. Egg cannibalism by adult pollock on the spawning grounds was inconsequential (< 1% for all years) compared to invertebrate predation.     

Seasonal changes in pelagic fish biomass around the Chiswell Island Steller sea lion rookery in 2003

Fisheries acoustics surveys were conducted around the Chiswell Island rookery in the northern Gulf of Alaska at night in April and August 2003 to assess seasonal changes in prey available to Steller sea lions (Eumetopias jubatus) foraging around the rookery. Adult walleye pollock (Theragra chalcogramma) ≥28 cm fork length was the dominant biomass in the upper 50 m of the water column in both months, increasing from 122.8 kg/nmi² in April to 457.9 kg/nmi² in August. A similar pattern was observed for Pacific herring (Clupea pallasii), which averaged 2.8 and 65.6 kg/nmi² in April and August, respectively. Incidental trawl catch suggested the appearance of age-0 pollock and juvenile salmonids (Oncorhynchus spp.) around the rookery in August as well. The increased biomass of these key prey species is linked to increased foraging trip durations by lactating sea lions from Chiswell Island, and supports the general view that sea lions in the northern Gulf of Alaska are not food limited during summer months.     

Evaluation of plankton surface pushnets and oblique tows for comparing the catch of diadromous larval fish

A bow-mounted surface pushnet and an obliquely towed plankton net were compared to evaluate gear efficiency and effectiveness in collecting larval fishes under daytime and nighttime conditions. The diadromous species targeted were striped bass Morone saxatilis, white perch, Morone americana, and river herring Alosa sp. We sampled the lower Roanoke River, North Carolina, from March through June of 2002 and 2003. Striped bass, white perch and river herring represented over 90% of the larvae collected during the study period. Mean larval densities (number/100 m³) were 63.4 for striped bass, 26.4 for river herring, and 17.7 for white perch. Striped bass larval densities were significantly higher in the surface pushnet for both years (P ≤ 0.05). In 2002, white perch mean larval density was significantly higher at night in the surface pushnet samples, but in 2003 there were no differences between day and night samples. River herring mean densities were significantly higher in the surface pushnets for both years, but showed no clear patterns between day and night samples. Larger larvae were consistently collected in the surface pushnets for all species. Overall, the surface pushnet was easier to operate. The pushnet was mounted on the bow of a small jon boat and required less specialized gear and fewer personnel than oblique sampling. The method also allows for sampling in shallow water or vegetated habitats. Because larvae were significantly larger in the surface samples, using surface pushnets may not allow for detection of the smaller-sized larvae therefore underestimating the abundance of smaller fish. Depending on the question being asked, we recommend that sampling programs should use both gear types to reduce any gear biases.     

Detection efficiency and habitat use to inform inventory and monitoring efforts: juvenile coho salmon in the Knik River basin,Alaska

Imperfect detection associated with sampling gear presents challenges for wildlife inventory and monitoring efforts. We examined occupancy dynamics and habitat use of juvenile coho salmon, Oncorhynchus kisutch, in shallow lake environments over a summer and early fall season in the Knik River area of south central Alaska using models which control for and estimate sampling gear detection efficiency. In addition, we present statements for the probability that observed absences at a survey site or from a survey area (a collection of sites) are true absences given some amount of sampling effort and analysts' beliefs about site occupancy and sampling gear detection efficiency which can be used to guide inventory and monitoring efforts for juvenile salmon or other wildlife and plant species. Occupancy modelling results demonstrate that minnow traps were effective at sampling juvenile coho in shallow lake environments, with a mean probability of detection across the study period of 0.68 (i.e., probability of detecting the presence of juvenile coho given that they are present at a trap site; SE = 0.03). Juvenile coho salmon migrated into shallow water lakes in late summer and early fall, presumably to seek out overwinter habitat. N‐mixture modelling examination of habitat use demonstrated that once in shallow lake environments, juvenile coho were widely distributed across a range of microhabitats, with some evidence for preference for shallower depths and warmer water temperatures.     

Gear restrictions create conservation and fisheries trade-offs for management

Gear-based management for coral reef fisheries is often overlooked in the scientific literature. Empirical studies have demonstrated the conservation benefits of gear-restricted areas (i.e. prohibiting fishing gears), which can support greater biomass than unrestricted areas and protect species that play key functional roles. However, population dynamics of functional feeding groups of reef fishes under specific gear-restriction regimes remains uncertain. Here, we constructed a multi-species, length-based fisheries model to observe relative biomass and catch of reef fishes under various gear-restriction management scenarios. We used fishery-dependent and fishery-independent data to determine the catchability of functional groups and selectivity of size classes for hook-and-line, net and spear fishing, which are widely used gear types on coral reefs globally. Our model revealed trade-offs involved with gear-restriction management such that no single management strategy was able to maximize biomass or catch of all functional groups simultaneously. Also, we found that spear fishing (i.e. prohibiting hook-and-line and net fishing) maintained the highest total biomass summed across functional groups, whilst hook-and-line fishing (i.e. prohibiting net and spear fishing) and a ban on spears maintained the lowest biomass. However, hook-and-line fishing generated the highest catch-per-unit-effort. Our model results were primarily driven by differential growth rates, maximum per capita production of recruits, and catchability of functional groups targeted by each fishing gear. We demonstrate that gear restrictions can be a critical management tool for maintaining biomass and catch of certain functional groups but will likely require additional management to protect all key functional feeding groups of coral reef fishes.     

The political ecology of gear bans in two fisheries: Florida's net ban and Alaska's Salmon wars

Parametric management in fisheries, which describes the management of how, where and when fishing occurs, is often essential for achieving sustainability. Changes to these parameters likely have impacts on stakeholders, however, for example through the costs and allocative consequences of spatial restrictions or gear changes. Here, I discuss two cases where gear bans have been implemented or proposed in response to conservation concerns: the commercial net ban enacted in Florida in 1995 and the failed ban on set gill‐nets in parts of Alaska. The two cases are remarkably similar, although the outcomes were quite different because of the social context of each fishery. Lessons from the Florida ban, which resulted in numerous negative social and ecological impacts, are informative regarding the impacts that likely would have accompanied the Alaska ban, had it proceeded. In both cases, the gear bans have had or were poised to have notable impacts on allocation, but scientific evidence for their necessity was limited. These cases show how ethical considerations can be inseparable from the ecological aspects of managing fisheries, and that when communities grapple with the sustainability of fisheries, they are simultaneously seeking to define the socially acceptable uses of those resources. I suggest a set of questions that can be asked when proposing parametric changes to fisheries, including how those changes will impact social well‐being and community resilience. These are questions that I argue must be addressed if both ethical and sustainable fisheries are the goal.     

Latitudinal differences in the duration of development of Neocalanus plumchrus copepodites

Quasi‐synoptic sampling along a transect from Alaska to California carried out in spring and summer 2000 using Continuous Plankton Recorders reveals that the abundant calanoid copepod Neocalanus plumchrus (Marukawa) reaches the surface at approximately the same time across the region. However, monthly sampling also reveals that the timing of peak biomass (when 50% of the population consists of copepodites at stage CV) occurs about 5 weeks earlier at the southernmost part of its range than at the northernmost part, with intermediate areas having intermediate timings. Surface water temperature differed by about 4°C between the warmer south and the cooler north, and we suggest that such a difference would reduce development duration by about 3 weeks. If food supply in the north is more conducive to lipid accumulation, as seems possible from phytoplankton biomass data, then the use of the differences in timing of peak N. plumchrus biomass in each area as a measure of the differences in development time may not be accurate. We find that the estimate of the time of peak biomass varying by about 5 weeks between north and south may be an exaggeration of the actual difference in the development duration. Temperature is probably the most important factor governing the shorter development duration of N. plumchrus in southern latitudes.     

闽东渔场张网禁渔区研究

本文首先收集了张网渔船在闽东渔场生产的海区、渔船数量、渔船功率和产量。然后检测了在不同深度海区捕捞的渔获组成并分析和比较其结果,最后提出了如何管理闽东渔场张网的一些建议。张网是一种严重损害经济鱼类幼鱼的渔具,其生产渔场不应扩大;40米等深线外海域的渔获个体较大,表明张网在40米等深线外海域损害幼鱼的程度比40米等深线内海区小,可以把“40米等深线以外海域为张网作业的禁渔区”的规定改为“在拖网禁渔区线以外海域为张网作业的禁渔区”。     

西藏巴松错（湖）夏季浮游植物群落的水平和垂直分布

为揭示西藏巴松错浮游植物群落结构的水平和垂直分布特征及其与环境间的关系，于2020年7月对巴松错出入河流及其湖体15个采样点和湖中心B15采样点11个分层的浮游植物进行了调查，同时测定了水体相关理化指标。调查共采集水样275个，分析了浮游植物物种组成、细胞丰度、生物量和物种多样性等群落结构特征，并探讨浮游植物对水平和垂直环境梯度变化的响应。结果表明：（1）巴松错夏季共鉴定浮游植物167种，隶属于5门8纲20目29科66属，物种组成为硅藻-绿藻-蓝藻型；（2）在水平方向上，平均细胞丰度为1.8453×104 cells/L，平均生物量为0.0117 mg/L；在垂直方向上，平均细胞丰度为1.0055×104 cells/L，平均生物量为0.0061 mg/L；（3）巴松错夏季浮游植物群落结构水平分布较为均匀，群落复杂程度相对较高，较垂直分布更稳定；（4）RDA分析结果表明，EC是影响巴松错夏季浮游植物水平群落变异的主要环境因子，TUR是影响垂直变异的主要环境因子。本研究结果为进一步研究高寒湖泊生态系统提供了基础资料。     

北部湾二长棘犁齿鲷的时空分布特征

二长棘犁齿鲷(Evynnis cardinalis)作为北部湾主要经济鱼类之一,长久以来受过度捕捞的影响,其资源量发生较大波动,将对渔业生产和种群结构有较大影响。文章根据2014年~2015年近海渔业资源调查资料,应用生物量和个体数单位捕捞努力量渔获量(CPUE)综合分析该鱼种资源的季节变化和空间分布。结果表明该鱼种广泛分布于北部湾中国管辖海域内,四季的出现率均在86%以上。生物量和个体数CPUE有明显的季节变化,均表现为最高值在夏季(平均值分别为7.3 kg·h~(-1)和240.83尾·h~(-1)),而冬季最低(平均值分别为1.76 kg·h~(-1)和21.94尾·h~(-1))。从空间分布看,生物量和个体数CPUE高值区主要分布于北部湾20°N以北海域,而低值区则处于海南岛西部海域。近年来在该海域实施的资源养护措施已取得初步成效,相比于20世纪90年代,二长棘犁齿鲷的资源量有所恢复。但二长棘犁齿鲷生物学特征分析结果表明,体长200 mm或体质量80 g以上的群体比重总体偏低,种群低龄化和小型化问题仍较为严峻,需采取新的管理措施(增大最小网目和开捕体长),以降低捕捞强度,改善种群结构。     

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。     

中国沿海刺网网具遗失率影响因子

为掌握影响我国沿海刺网渔船网具遗失率(R_G)的因子及其程度,对中国沿海渔民开展了问卷调查。利用调查数据,结合GAM模型分析了影响我国刺网渔船网具遗失率的因子及其程度。结果显示:(1)影响我国沿海刺网渔船网具遗失率的因子(解释偏差)分别为单航次天数(86.8%)、年作业天数(4.2%)、单航次渔具携带重量(2.8%)、作业水深(2.4%)、渔船吨位(0.9%)和年均渔具采购数量(0.1%);(2)GAM模型能用于研究渔船网具遗失率和影响因子之间的关系;(3)随着单航次天数和年作业天数的增加,R_G呈上升的趋势;(4)随着作业水深的增加、单航次渔具携带重量超过18 000 kg时,R_G呈下降趋势。建议采取以下措施以降低刺网渔船网具遗失率:(1)单航次天数不得超过30 d,减少年作业天数;(2)根据渔船的吨位,确保网具单航次携带重量在18 000 kg以上;(3)根据刺网渔具的高度规定作业水深和作业渔区;(4)渔船总吨位控制在200 t以内。     

Interannual and spatial variation in the population genetic composition of young‐of‐the‐year Pacific ocean perch (Sebastes alutus) in the Gulf of Alaska

Little is known about the population structure of Alaskan rockfishes, including Pacific ocean perch (POP, Sebastes alutus), and how persistent and variable oceanographic features may influence their structures. Moreover, early life history information is sparse for many species. We used data from 14 microsatellite loci to characterize the genetic structure of young‐of‐the‐year Pacific ocean perch collected during 1998–2003 from the Gulf of Alaska and Bering Sea. Broad‐scale geographic variation in genetic structure of the young‐of‐the‐year (F_ST = 0.005, P < 10^?4) had similarities to that observed in a previous adult study. The overall correlation between genetic and geographic distance (isolation by distance) was nearly identical to that observed in the adults. Fine‐scale geographic divergence was also observed and may be the result of oceanographic circulation features within the Gulf of Alaska. Interannual variation (between cohorts) at locations sampled in more than one year is consistent with variable oceanography and fine‐scale population structure rather than the influence of a sweepstakes effect. The similarities of the young‐of‐the‐year with the adults and the pattern of genetic divergence confirm that dispersal of Pacific ocean perch is limited in all life stages.     

Did volcanic ash from Mt. Kasatoshi in 2008 contribute to a phenomenal increase in Fraser River sockeye salmon (Oncorhynchus nerka) in 2010?

The effect of a widely distributed phytoplankton bloom triggered by volcanic ash from Alaska (Hamme et al., 2010. Geophys. Res. Lett. 37) on juvenile Fraser River sockeye is discussed in terms of the timing of ocean migration and trophic structure of the Gulf of Alaska. Our hypothesis is that the occurrence of a massive diatom bloom in the Gulf greatly enhanced energy ascendancy in the ocean at a time of year when adolescent sockeye migrated from the coast in 2008. We contend this increase in food availability was an important factor for the survival and growth of juvenile sockeye which led to one of the strongest sockeye returns on record in 2010 of 34 million, compared with perhaps the weakest return on record of 1.7 million the previous year.     

Springtime abundance of chaetognaths in the shelf region of the northern Gulf of Alaska, with observations on the vertical distribution and feeding of Sagitta elegans

Chaetognatha were among the most dominant macrozooplankton taxa collected in 6 years of springtime collections, both throughout the water column and near-bottom on the continental shelf in the northern Gulf of Alaska. Three species ( Sagitta elegans, S. scrippsae, and Eukrohnia hamata ) were collected in 248 collections, although S. elegans was the numerically dominant species overall during most cruises. Collections taken in epibenthic sleds generally contained about two orders of magnitude more chaetognaths than those that sampled throughout the water column. Two size modes were apparent in the S. elegans size-frequency distributions which were believed to belong to different cohorts. Growth rate was on the order of 2–3 mm per month. The two cohorts showed asynchronous diel vertical distribution patterns, with the smaller individuals found near the surface during the day whereas the larger individuals were near the surface at night. About 7.5% of the S. elegans examined contained food. Copepods made up the majority (∼89% by number) of the diet of both large and small individuals. Euphausiid juveniles and cirripede larvae were also observed, as well as several incidences of cannibalism by large chaetognaths on smaller individuals. Based on the results of two diel series, this species was observed to feed mainly during the night-time.