Late Quaternary sea-surface temperature variations in the southeast Atlantic: a planktic foraminifer faunal record of the past 600 000 yr (IMAGES II MD962085)

A high-resolution (4–5cm/kyr) giant piston core record (MD962085) retrieved during an IMAGES II-NAUSICAA cruise from the continental slope of the southeast Atlantic Ocean reveals striking variations in planktonic foraminifer faunal abundances and sea-surface temperatures (SST) during the past 600 000 yr. The location and high-quality sedimentary record of the core provide a good opportunity to assess the variability of the Benguela Current system and associated important features of the ocean-climate system in the southeast Atlantic. The planktonic foraminifer faunal abundances of the core are dominated by three assemblages: (1) Neogloboquadrina pachyderma (right coiling)+Neogloboquadrina dutertrei, (2) Globigerina bulloides, and (3) Globorotalia inflata. The assemblage of N. pachyderma (right coiling)+N. dutertrei shows distinctive abundance changes which are nearly in-phase with glacial–interglacial variations. The high abundances of this assemblage are associated with major glacial conditions, possibly representing low SST/high nutrient level conditions in the southwestern Africa margin. In contrast, the G. bulloides and G. inflata assemblages show greater high-frequency abundance change patterns, which are not parallel to the glacial–interglacial changes. These patterns may indicate rapid oceanic frontal movements from the south, and a rapid change in the intensity of the Benguela upwelling system from the east. A single episode of maximum abundances of a polar water species N. pachyderma (left coiling) occurred in the beginning of stage 9 (340–330 kyr). The event of the maximum occurrence of this species shown in this record may indicate instability in the Benguela coastal upwelling, or the Antarctic polar front zone position. A winter season SST estimate using transfer function techniques for this record shows primarily glacial–interglacial variations. The SST is maximal during the transitions from the major glacial to interglacial stages (Terminations I, II, IV, V), and is associated with the abundance maxima of a warm water species indicator Globigerinoides ruber. Cross-spectral analyses of the SST record and the SPECMAP stack reveal statistically significant concentrations of variance and coherencies in three major orbital frequency bands. The SST precedes changes in the global ice volume in all orbital frequency bands, indicating a dominant southern Hemispheric climate effect over the Benguela Current region in the southeast Atlantic.     

闽南-台湾浅滩渔场海表温度对鲐鲹鱼类群聚资源年际变动的影响初探

利用闽南-台湾浅滩渔场1986-1997年遥感海表温度资料与同期灯光围网渔业信息船的渔捞日志,对该渔场海表温度与鲐鲹鱼类群聚资源年际变动之间的关系进行了初步的探讨.结果表明,沿着25.5℃等温线形成一个中心渔场,空间位置的年际摆动范围在纬向上小于0.5°;海表温度距平(SSTA)与鲐鲹鱼类资源指数的变动在年际尺度上没有关联,但在年代际尺度上存在一定关联,1988年起SSTA摆动的频度和幅度在正值态大于负值态,而资源指数则呈现逐步上升的趋势.     

Influence of environmental variability on distribution and relative abundance of baleen whales (suborder Mysticeti) in the Gulf of California

The Gulf of California (GC) is one of the most productive seas in the Pacific Ocean because of several oceanographic phenomena that support many marine mammal species, particularly of the cetacean order. The environmental variability of the GC was analysed during the cold and warm periods of 2005 and 2006 and its effects on the distribution and relative abundance of baleen whales using sea surface temperature (SST) and chlorophyll a (Chl a) concentration. Satellite image analysis allowed us to detect important differences in both Chl a concentration and SST including cold to warm periods, even between the two cold periods and between the two warm periods. The cold periods had the highest number of whales: 99 individuals in 2005 and 183 in 2006, which were distributed along the entire gulf. Fewer animals were recorded in the warm period: 46 individuals in 2005 and 30 in 2006, which were mainly distributed in the northern part of the gulf. We concluded that SST influenced the relative abundance of baleen whales while Chl a concentration influenced their distribution.     

Effects of the 1997–1999 El Niño and La Niña events on zooplankton abundance and euphausiid community composition within the Monterey Bay coastal upwelling system

Zooplankton abundance and euphausiid community composition were sampled seasonally (spring, summer, fall) within Monterey Bay, California, between 1997 and 1999. Measurements of sea surface temperature (SST), mixed layer depth, and upwelling indices provided concurrent data on physical oceanographic parameters. Both total zooplankton and krill abundance dramatically declined in the summer of 1997 coincident with a rapid increase in SST and mixed layer depth. Changes in euphausiid community composition occurred in concert with the decline in overall abundance. The relative abundance of the southern neritic Nyctiphanes simplex increased from August to November in 1997, the abundance of cold temperate Euphausia pacifica decreased significantly, and that of the northern neritic Thysanoessa spinifera declined dramatically. The sudden appearance of an adult cohort of N. simplex in July 1997 suggests that rapid poleward flow characteristic of coastally trapped Kelvin waves occurred between June and July of 1997. The persistent presence of warm temperate and subtropical taxa in samples collected between August 1997 and October 1998 indicates that this poleward flow continued in 1998. Zooplankton abundance, euphausiid community composition, and physical oceanographic parameters gradually returned to a more typical upwelling-dominated state in the spring and summer of 1998. E. pacifica and T. spinifera abundances gradually increased during the summer and fall of 1998, while N. simplex abundance abruptly declined in the spring of 1998. However, this recovery was confined to a narrow coastal band as a result of the onshore movement of the oceanic waters of the California Current. This was reflected by higher than normal numbers of the oceanic Nematoscelis difficilis within samples collected during the spring and summer of 1998. By the spring and summer of 1999, both zooplankton and euphausiid abundance had increased to the highest levels recorded during the 3-year study. Both E. pacifica and T. spinifera abundance increased relative to 1998 while N. simplex was completely absent in all samples. These changes reflected the cooler, highly productive environmental conditions associated with the 1998/1999 La Niña.     

不同气候模态下西北太平洋秋刀鱼资源丰度预测模型建立

秋刀鱼（Cololabis saira）资源对海洋环境因素极为敏感,不同气候模态可能对秋刀鱼资源丰度产生不同的影响。根据1990-2014年西北太平洋日本的秋刀鱼渔业中单位捕捞努力量渔获量（CPUE,以此作为资源丰度）,以及相应产卵场、索饵场的海表温（SST）遥感数据,探讨太平洋年际震荡（PDO）指数冷、暖年下,秋刀鱼资源丰度CPUE变化与产卵场、索饵场SST的关系,并分别建立资源丰度的预测模型。研究表明,PDO冷年索饵场4月SST与年CPUE显著相关（P<0.05）,PDO暖年索饵场11月的SST与年标准化CPUE显著相关（P<0.05）。PDO冷、暖年的秋刀鱼资源丰度的预测模型中,CPUE均与索饵场11月的SST、索饵场4月SST呈现正相关的关系,统计学上为显著相关（P<0.05）。PDO冷年（2012年）和PDO暖年（2014年）的CPUE预测值与实际值相对误差分别为14.03%、-16.26%,具有较好的拟合效果。研究认为,不同气候模态下,可用于秋刀鱼资源丰度预测的环境因子不同,上述建立资源丰度模型可用于业务化运行。     

Oceanographic influences on the distribution and relative abundance of market squid paralarvae (Doryteuthis opalescens) off the Southern and Central California coast

Market squid (Doryteuthis opalescens) are ecologically and economically important to the California Current Ecosystem, but populations undergo dramatic fluctuations that greatly affect food web dynamics and fishing communities. These population fluctuations are broadly attributed to 5–7‐years trends that can affect the oceanography across 1,000 km areas; however, monthly patterns over kilometer scales remain elusive. To investigate the population dynamics of market squid, we analysed the density and distribution of paralarvae in coastal waters from San Diego to Half Moon Bay, California, from 2011 to 2016. Warming local ocean conditions and a strong El Niño event drove a dramatic decline in relative paralarval abundance during the study period. Paralarval abundance was high during cool and productive La Niña conditions from 2011 to 2013, and extraordinarily low during warm and eutrophic El Niño conditions from 2015 to 2016 over the traditional spawning grounds in Southern and Central California. Market squid spawned earlier in the season and shifted northward during the transition from cool to warm ocean conditions. We used a general additive model to assess the variability in paralarval density and found that sea surface temperature (SST), zooplankton displacement volume, the log of surface chlorophyll‐a, and spatial and temporal predictor variables explained >40% of the deviance (adjusted r² of .29). Greatest paralarval densities were associated with cool SST, moderate zooplankton concentrations and low chlorophyll‐a concentrations. In this paper we explore yearly and monthly trends in nearshore spawning for an economically important squid species and identify the major environmental influences that control their population variability.     

2009 年春季辽东湾网采浮游植物群落结构

分析了2009 年春季(5 月)渤海辽东湾网采浮游植物物种组成, 丰度分布, 生物多样性指数及群落相似性。春季共鉴定出浮游植物3 门23 属32 种, 主要隶属硅藻门和甲藻门, 生态类型多为温带近岸型和广布型种类, 少部分为暖海性物种或大洋性物种。优势种有夜光藻(Noctiluca scientillans)、翼鼻状藻(Proboscia alata)、旋链角毛藻(Chaetoceros curvisetus)、巨圆筛藻(Coscinodiscus gigas)和尖刺伪菱形藻(Pseudo-nitzschia pungens)等。浮游植物在各站位的细胞丰度分布不均匀, 其平面分布呈现由沿岸海域向海湾中部海域递减的趋势。Shannon-Wiener 指数、Pielou 指数较低, 群落结构简单, 稳定性较差。聚类分析结果表明群落之间的相似性较高。     

西北冰洋表层沉积物中的底栖有孔虫组合及其古环境意义

通过对中国第1~4次北极考察在西北冰洋采集的表层沉积物中底栖有孔虫丰度及其优势种分布特征与环境因素关系的综合研究发现,楚科奇海区低的底栖有孔虫丰度主要受较高的陆源物质输入的稀释作用影响;楚科奇海台和阿尔法脊较高的底栖有孔虫丰度主要受到暖而咸的大西洋中层水的影响;受碳酸钙溶解作用影响的门捷列夫深海平原和加拿大海盆底栖有孔虫丰度较低,并且水深3 597 m的站位出现了似瓷质壳的Pyrgo williamsoni和Quinqueloculina orientalis,说明该区的CCD深度大于3 600 m。根据底栖有孔虫7个优势属种的百分含量分布特征可以划分出5个区域组合:南楚科奇海陆架-白令海峡组合以优势种Elphidium excavatum和Buccella frigida为特征,可能反映受白令海陆架水影响的浅水环境;阿拉斯加沿岸-波弗特海组合以优势种Florilus scaphus和Elphidium albiumbilicatum为特征,可能反映受季节性海冰融化,低盐的阿拉斯加沿岸流以及河流淡水输入的低盐环境;大西洋中层水组合以优势种Cassidulina laevigata为特征,可能反映高温高盐的大西洋中层水影响的环境;北极深层水组合以优势种Cibicides wuellerstorfi为特征,可能反映水深大于1 500 m低温高盐的北极深层水环境;门捷列夫深海平原组合以优势种Oridorsalis umbonatus为特征,可能反映低营养的底层水环境。     

The biogeography and community structure of kelp forest macroinvertebrates

Understanding species distributions and their community structure is increasingly important when taking an ecosystem‐based approach to conservation and management. However, knowledge of the distribution and community structure of species in mid‐range trophic levels (e.g. macroinvertebrates) is lacking in most marine ecosystems. Our study aimed to examine the spatial distribution and community‐level biogeographic patterns of common kelp forest–rocky reef macroinvertebrates in Southern California and to evaluate the effects of environmental gradients on these communities. Quantitative SCUBA surveys were used to estimate macroinvertebrate densities at 92 sites from 2008–2012. Non‐metric multidimensional scaling was used to evaluate spatial patterns of macroinvertebrate communities among Regions. We found that kelp forest–rocky reef macroinvertebrate communities are distinct among different island and mainland regions, and their community patterns exhibited a strong relationship with an environmental gradient (i.e. sea surface temperature) even after controlling for geographic distance between sites. High abundances of urchin species (Strongylocentrotus purpuratus and Strongylocentrotus franciscanus) were strong drivers of regional differences. Macroinvertebrate community patterns were driven by characteristic species that were typically more prevalent at warmer or colder sites. Our results provide the first quantitative analysis of macroinvertebrate community structure within the California kelp forest ecosystem. We also describe the distribution and abundance of 92 conspicuous kelp forest‐rocky reef macroinvertebrates among nine pre‐defined Regions. This study provides important preliminary information on these macroinvertebrate species that will be directly useful to inform management of invertebrate fisheries and spatial protection of marine resources.     

胶州湾浮游植物群落结构及其变化的初步研究

利用2003年胶州湾调查资料，分析了浮游植物群落结构及其与温度和营养盐的相关性。结果表明，胶州湾浮游植物群落主要由硅藻和甲藻组成，生态类型为近岸广布种、温带种和暖温带种以及少数暖水种。浮游植物数量一年中出现2次高峰期，主高峰出现在9月，次高峰出现在2月，低值期在7月和10月至翌年1月期间。浮游植物数量较近几年继续降低，尤其是峰值。浮游植物在春季以角毛藻、丹麦细柱藻、中肋骨条藻和冰河拟星杆藻为优势种群，夏季至秋初暖水性近岸种中华齿状藻、泰晤式旋鞘藻和广温性种星脐圆筛藻占优势地位，而秋季至冬初羽纹硅藻和派格棍形藻逐渐占优势地位，并一直持续整个冬季。溶解无机氮、硅酸盐和磷酸盐与硅藻丰度无明显相关性（R^2〈0．1）。中心硅藻丰度与溶解态无机氮呈线性负相关（R^2=0．4399），而与硅酸盐和磷酸盐无明显相关性（R^2〈0．1）。群落多样性指数与中心硅藻丰度和硅酸盐呈线性相关（R^2=0．3198和0．4443）。     

冬季南黄海浮游动物群落结构及其对黄海暖流的指示

2009年12月和2010年1月对南黄海进行浮游动物采集, 以了解冬季浮游动物群落结构及其对黄海暖流的指示作用。结果表明, 南黄海冬季仍然以温带和暖温带种为主, 中华哲水蚤Calanus sinicus、强壮滨箭虫Aidanosagitta crassa、细足法Themisto gracilipes等温带和暖温带种类在浮游动物数量组成中具有较大优势。与此同时, 一些暖水种在调查海域局部出现。2009年12月暖水种仅分布在南黄海东南部黄海暖流源地附近。位于黄海中部的调查区东侧温盐层化现象明显, 近底层低温、高盐、高营养盐的水文特征体现了黄海冷水团的残留; 2010年1月在35°—36°N区域暖水种种类明显增加, 截平头水蚤Paracandacia truncata、长尾基齿哲水蚤Clausocalanus furcatus出现的位置与暖流路径相吻合, 海洋真刺水蚤Euchaeta rimana数量相比12月有明显向北推进的趋势。主成分分析显示暖水种的分布与温度有良好的相关性。Shannon-Weaver指数、丰富度指数、均匀度指数等没有呈现明显的分布规律, 对黄海暖流的指示作用不如种类明显。     

西北太平洋环境变化对柔鱼冬春生群体资源丰度的影响

During 1995–2011, annual production of winter-spring cohort of Ommastrephes bartramii for Chinese squidjigging fishery has greatly fluctuated, which is closely related to the environmental conditions on the spawning and fishing grounds. To better understand how squid recruitment and abundance were influenced by ocean environmental conditions, biological and physical environmental variables including sea surface temperature(SST), SST anomaly(SSTA), chlorophyll a(Chl a) concentration and the Kuroshio Current were examined during years with the highest(1999), intermediate(2005), and lowest(2009) catches. Catch per unit effort(CPUE) of the squid-jigging vessels was used as an indicator of squid abundance. The results indicated that high SST and Chl a concentration on the spawning ground in 1999 resulted in favorable incubation and feeding conditions for squid recruitment. Whereas the suitable spawning zone(SSZ) in 2009 shifted southward and coincided with low SST and Chl a concentration, resulting in a reduction in the squid recruitment. The small difference of SSZ area in the three years suggested the SSZ provided limited influences on the variability in squid recruitment. Furthermore,high squid abundance in 1999 and 2005 was associated with warm SSTA on the fishing ground. While the cool SSTA on the fishing ground in 2009 contributed to adverse habitat for the squid, leading to extremely low abundance. It was inferred that strengthened intensity of the Kuroshio force generally yielded favorable environmental conditions for O. bartramii. Future research are suggested to focus on the fundamental research on the early life stage of O. bartramii and mechanism of how the ocean-climate variability affects the squid abundance and spatial distribution by coupling physical model with squid biological process to explore transport path and abundance distribution.     

秘鲁外海茎柔鱼栖息地适宜性年代际变动

茎柔鱼广泛分布于东南太平洋海域,是我国重要的远洋捕捞对象之一,其种群易受气候和栖息地环境的影响。利用海表面温度(SST)和海表面高度距平(SSHA)两个关键环境因子,计算1950−2015年1−12月秘鲁外海茎柔鱼栖息地适宜性指数(HSI),对比分析太平洋年代际涛动(PDO)位于冷暖位相下茎柔鱼渔场环境以及栖息地质量的变动。结果显示,1950−2015年PDO呈现冷、暖、冷3个位相变化,其中PDO冷位相内的SST距平(SSTA)和SSHA明显低于PDO暖位相。交相关分析结果表明,PDO指数与SSTA和SSHA均呈显著正相关,而HSI与PDO指数、SSTA和SSHA均呈显著负相关。PDO位于冷位相时,茎柔鱼渔场内水温变冷,海面高度下降,适宜的SST和SSHA范围增加,因此茎柔鱼有利的栖息地面积增大;而PDO位于暖位相时,水温增暖,海面高度上升,适宜的SST和SSHA范围缩减,导致茎柔鱼适宜的栖息地面积缩小。研究认为,太平洋年代际涛动调控了茎柔鱼渔场内的环境变化,进而对茎柔鱼栖息地质量及适宜栖息地范围产生显著影响。     

Growth pattern and changes in abundance of the endangered bat star Asterina stellifera

The sea star Asterina stellifera has declined during the last decade and is currently abundant only in the southern limit of its former range. We surveyed this population over 5 years to model individual growth and explore the relationship of changes in local abundance with variation in environmental factors and the reproductive status of individuals. Our results show that A. stellifera is a species with slow growth and a relatively long lifespan. Contrary to expectations for temperate species, growth rates were fairly constant through the year and therefore models including seasonal oscillations were inappropriate. The abundance of this species increased significantly from early spring to early summer, likely due to augmented activity and small‐scale aggregation during the reproductive season that affected our estimates of abundance. No significant recruitment occurred during the 5 years studied. The lack of recruitment during long periods and the slow individual growth rates make A. stellifera particularly vulnerable to local extinction. This study was performed prior to the arrival in the study area of the invasive kelp Undaria pinnatifida and side‐gilled sea slug Pleurobranchaea maculata, species that threaten the community structure where A. stellifera lives. Therefore, the information reported here will be essential to assessing the impacts of these exotic species on this sea star population.     

北部湾盆地涠西南凹陷古、新近系古生物记录及古环境意义

对北部湾盆地涠西南凹陷WZ5-X-Y井85份沉积样品开展了孢粉和藻类分析,探讨了钻孔所在区域古近纪—新近纪沉积环境的变化和植被类型的演替,并推断了气候条件的变化。研究结果显示,钻孔所在区域始新世—渐新世古湖泊发育,水域范围存在波动,渐新世整体上湖泊水域范围有所收缩,中新世湖泊进一步收缩,并持续受到海水入侵的影响。始新世钻井周边丘陵和低山地区发育以常绿栎和胡桃为主的常绿落叶阔叶混交林,湖泊周边生长桤木林,气候温暖湿润,渐新世的植被类型仍是以常绿落叶阔叶混交林为主,但是常绿林的缩小和针叶林的扩张则指示气温和湿度下降,中新世常绿林的扩张则指示了温度和湿度回升。     

Short-term temporal dynamics of algal species in a subtidal kelp bed in relation to changes in environmental conditions and canopy biomass

Understanding temporal variation at the scale of weeks to months is critical to understanding broad temporal patterns in diversity in the same way as understanding diversity across landscapes relies on understanding variation at the scale of meters. However, whereas small-scale spatial variation in temperate reef algal assemblages has been extensively studied, fine-scale temporal changes have not been well addressed. By sampling the macroalgae of a subtidal reef near Perth (Australia), dominated by the small kelp Ecklonia radiata, every ∼40 days over a 2-year period, we were able to test whether temporal changes in species richness, assemblage structure and species turn-over were related to seasonal changes in surface temperature, solar radiation and wave height. A total of 93 macroalgal taxa were identified, and species richness per sampling time ranged from 25 to 64 taxa 1.25 m⁻². Biomass of E. radiata was positively correlated with changes in sea surface temperature and light, and negatively correlated with wave height. Species richness, assemblage structure and turn-over of other macroalgae were more associated with seasonal changes in kelp biomass than environmental variables per se. We conclude that seasonal changes in environmental conditions drive changes in the kelp canopy, which in turn drive changes in species richness and assemblage structure. This suggests that habitat-formers such as kelps can exert a strong temporal influence on associated communities, analogous to well-described spatial influences. Thus, as kelp canopy biomass expands and retracts over time-scales of weeks to months, so does available space for colonization and growth, resulting in a high species turn-over. Species richness is therefore increased and maintained through time, in the same way as canopy-gap mosaics increase and maintain species richness across spatial landscapes.     

The other 93%: trophic cascades,stressors and managing coastlines in non-marine protected areas

Abstract

This paper reviews interactions involving stands of macroalgae on rocky reefs, and presents new data on changing sea surface temperatures (SSTs), as a contribution to the celebration of the fiftieth anniversary of the Leigh Marine Laboratory (LML) of the University of Auckland. The focus is on trophic interactions involving predators, sea urchins and large brown algae, particularly trophic cascades. Of the 369 publications arising from work at LML, 40 have been on key aspects of these trophic interactions. Quantitative investigations of the structure of kelp bed communities and mechanistic studies involving manipulative field-based experiments, essentially a bottom-up perspective based on habitats and key species, dominated the research through the 1980s. From the mid-1990s onwards, the focus was more on marine reserves and a hierarchical, top-down perspective of community structure, with a particular focus on the role of predatory fish, and marine reserves as a tool of management. I discuss these models of community structure of kelp beds within the wider context of the New Zealand nearshore zone, the varying biogeographic regimes around the coastline, diffuse stressors and the changing nearshore climate. I show there appears to have been a significant warming trend in SST in northeast and northwest New Zealand over the past 30 years. I conclude that a trophic effects model is unlikely to apply to much of the coastline of New Zealand, and that a model involving multiple effects, including bottom-up forces, environmental and climatic influences, species' demographics, and catchment-derived sedimentation is more appropriate for kelp communities over most of the country. New management models are needed to safeguard marine resources and the services they provide.     

2010年胶州湾网采浮游植物种类组成与数量的周年变化

为了研究胶州湾网采浮游植物的种类组成与数量变化,作者根据2010年的胶州湾调查资料,对调查区浮游植物丰度、优势种类和季节变化进行分析研究。结果表明,胶州湾浮游植物主要由硅藻和甲藻两大类组成,硅藻占据绝对优势地位。除了7月以外,甲藻/硅藻比均0.2。生态类型主要以近岸广布种和暖温带种为主,在不同季节也出现了少数暖水种和外洋种。浮游植物丰度表现为明显的"双周期",冬、春两季较高,夏、秋两季较低。2月为全年最高峰,数量为1108×104个/m3,10月为夏、秋两季的高峰,数量为458.7×104个/m3。浮游植物群落的多样性指数平均为1.9。     

烟台月亮湾岩岸潮间带底栖海藻群落结构的季节变化

月亮湾岩岸潮间带底栖海藻群落的种类组成存在显著的季节差异 ,春季群落的种类最丰富 (2 7种 ) ,其次为秋季群落 (2 6种 ) ,夏季和冬季较少 (2 2和 2 1种 ) ,4个季节群落的共有种仅为 7种。各季节群落中均以红藻的种类最丰富 ,褐藻次之 ,绿藻最少。在 4个季节群落中生物量的变化如下 :夏季 >秋季 >冬季 >春季。绿藻和褐藻类群主要为暖温性种类 ,而红藻类群比较复杂 ,即有暖温带性的、温带和暖水性种类 ,也有寒温带的冷水性种类。4个季节群落的物种优势度序列存在明显的差异 ,海黍子在春季和冬季为群落的第 1优势种 ,而孔石莼在夏季和秋季为第 1优势种。不同季节群落多样性指数变化如下 :物种丰富度指数 ,春季 >秋季 >冬季 >夏季 ;物种多样性指数 ,秋季 >夏季 >春季>冬季 ;均匀度指数 ,春季 >夏季 >冬季 >秋季     

Comparison of the response of Atlantic cod (Gadus morhua) in the high-latitude regions of the North Atlantic during the warm periods of the 1920s–1960s and the 1990s–2000s

Concern about future anthropogenic warming has lead to demands for information on what might happen to fish and fisheries under various climate-change scenarios. One suggestion has been to use past events as a proxy for what will happen in the future. In this paper a comparison between the responses of Atlantic cod (Gadus morhua) to two major warm periods in the North Atlantic during the 20th century is carried out to determine how reliable the past might be as a predictor of the future. The first warm period began during the 1920s, remained relatively warm through the 1960s, and was limited primarily to the northern regions (>60°N). The second warm period, which again covered the northern regions but also extended farther south (30°N), began in the 1990s and has continued into the present century. During the earlier warm period, the most northern of the cod stocks (West Greenland, Icelandic, and Northeast Arctic cod in the Barents Sea) increased in abundance, individual growth was high, recruitment was strong, and their distribution spread northward. Available plankton data suggest that these cod responses were driven by bottom-up processes. Fishing pressure increased during this period of high cod abundance and the northern cod stocks began to decline, as early as the 1950s in the Barents Sea but during the 1960s elsewhere. Individual growth declined as temperatures cooled and the cod distributions retracted southward. During the warming in the 1990s, the spawning stock biomass of cod in the Barents Sea again increased, recruitment rose, and the stock spread northward, but the individual growth did not improve significantly. Cod off West Greenland also have shown signs of improving recruitment and increasing biomass, albeit they are still very low in comparison to the earlier warming period. The abundance of Icelandic cod, on the other hand, has remained low through the recent warm period and spawning stock biomass and total biomass are at levels near the lowest on record. The different responses of cod to the two warm events, in particular the reduced cod production during the recent warm period, are attributed to the effects of intense fishing pressure and possibly related ecosystem changes. The implications of the results of the comparisons on the development of cod scenarios under future climate change are addressed.