Multi‐decadal variations in spawning ground use in Northeast Arctic haddock (Melanogrammus aeglefinus)

Despite the importance both in an ecological and management context, much uncertainty remains about the underlying factors controlling spawning ground use in marine fish. Here, we investigate how spawning ground use of Northeast Arctic (NEA) haddock (Melanogrammus aeglefinus) has changed over time. By combining data from a Soviet‐Russian egg survey conducted over a 35 year period (1959–1993) with a biophysical drift model, we estimate annual median spawning latitudes of NEA haddock. Furthermore, we test for statistical association between the median latitude of spawning and covariates that are candidates for shaping the spawning distribution of haddock. The results indicate no association with temperature, as measured at the Kola transect, or demographic structure, as measured by the mean weight of spawners. However, we find a positive and significant association between spawning latitude and spawning stock biomass, indicating that density‐dependence may be more important in shaping spawning location than other factors such as temperature and demographic structure. A potential mechanism for the reported effect of spawning stock biomass may act through the distribution at the feeding grounds, which is reported to expand north‐ and eastwards at high population density.     

The growth of larval cod and haddock in the Irish Sea: a model with temperature,prey size and turbulence forcing

We applied a physiological individual‐based model for the foraging and growth of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae, using observed temperature and prey fields data from the Irish Sea, collected during the 2006 spawning season. We used the model to estimate larval growth and survival and explore the different productivities of the cod and haddock stocks encountered in the Irish Sea. The larvae of both species showed similar responses to changes in environmental conditions (temperature, wind, prey availability, daylight hours) and better survival was predicted in the western Irish Sea, covering the spawning ground for haddock and about half of that for cod. Larval growth was predicted to be mostly prey‐limited, but exploration of stock recruitment data suggests that other factors are important to ensure successful recruitment. We suggest that the presence of a cyclonic gyre in the western Irish Sea, influencing the retention and/or dispersal of larvae from their spawning grounds, and the increasing abundance of clupeids adding predatory pressure on the eggs and larvae; both may play a key role. These two processes deserve more attention if we want to understand the mechanisms behind the recruitment of cod and haddock in the Irish Sea. For the ecosystem‐based management approach, there is a need to achieve a greater understanding of the interactions between species on the scale a fish stock is managed, and to work toward integrated fisheries management in particular when considering the effects of advection from spawning grounds and prey–predator reversal on the recovery of depleted stocks.     

Natural variability of fisheries and lunar illumination: a hypothesis

Long‐term synchronous trends in small pelagic fisheries catches around the world suggest that fish populations are governed by the same global climate forcings. Recent findings regarding the population dynamics of zooplankton during the lunar cycle in sub‐tropical waters may shed light on the influence of the lunar cycle on fish spawning and mortality. Here I hypothesize that, in the short‐term, observed changes in zooplankton abundance during the lunar cycle promote periods of enhanced feeding by adult fish and lower mortality (and increased growth) in their early planktonic stages. Furthermore, a striking 9‐year periodicity in sardine and anchovy mortality was inferred in four major upwelling areas, coinciding with the long‐term variability in lunar illumination. It is suggested that both short‐ and long‐term changes in lunar illumination should be considered when modelling the effect of climate on the natural variability of fisheries.     

Subpolar gyre and temperature drive boreal fish abundance in Greenland waters

As result of ocean warming, marine boreal species have shifted their distribution poleward, with increases in abundance at higher latitudes, and declines in abundance at lower latitudes. A key to predict future changes in fish communities is to understand how fish stocks respond to climate variability. Scattered field observations in the first half of the 20th century suggested that boreal fish may coherently invade Greenland waters when temperatures rise, but this hypothesis has remained untested. Therefore, we studied how local temperature variability and the dynamics of the subpolar gyre, a large‐scale driver of oceanic conditions in the North Atlantic, affect abundance of boreal fishes in a region that sharply defines their lower thermal boundary. We analysed information from demersal trawl surveys from 1981 to 2017, for species distributed from shallow shelf to depths of 1,500 m, collected at over 10,000 stations along ~3,000 km of Greenland. Our results show that local temperature and variability of Labrador and Irminger Sea water in the subpolar gyre region drive interdecadal variability of boreal fish abundance in Greenland waters. Although temperature fluctuations were higher in shallow than deep regions, fish abundance changed as quickly in great depths as in shallow depths. This link between physics and biology provides an opportunity for prediction of future trends, which is of utility in Greenland, where fisheries constitute more than 90% of the national export value.     

Interannual differences in larval haddock survival: hypothesis testing with a 3D biophysical model of Georges Bank

The ultimate goal of early life studies of fish over the past century has been to better understand recruitment variability. As evident in the Georges Bank haddock (Melanogrammus aeglefinus) population, there is a strong relationship between recruitment success and processes occurring during the planktonic larval stage. This research sought new insights into the mechanisms controlling the recruitment process in fish populations using biological–physical modeling methods together with laboratory and field data sets. We created the first three‐dimensional model of larval haddock on Georges Bank by coupling models of hydrodynamics, lower trophic levels, a single copepod species, and larval haddock. Interactions between feeding, metabolism, growth, vertical behavior, advection, predation, and the physical environment of larval haddock were quantitatively investigated using the coupled models. Particularly, the model was used to compare survival over the larval period and the sources of mortality in 1995 and 1998, 2 years of disparate haddock recruitment. The results of model simulations suggest that the increased egg hatching rates and higher food availability, which reduced starvation and predation, in 1998 contributed to its larger year‐class. Additionally, the inclusion of temperature‐dependent predation rates produced model results that better agreed with observations of the mean hatch date of survivors. The results from this biophysical model imply that food limitation and its related losses to starvation and predation, especially from hatch to 7 mm, may be responsible for interannual variability in recruitment and larval survival outside of the years studied.     

Ontogeny matters: Climate variability and effects on fish distribution in the eastern Bering Sea

Analyses of climate effects often ignore differences in life history for individual species. We analyzed a 34‐year time series of eastern Bering Sea fish surveys to evaluate changes in distribution by length and between cold and warm shelf‐wide average water temperatures for 20 species over inhabited depth, temperature, and location. All species showed evidence of ontogenetic migration. Differences in distribution between years with warm and years with cold shelf‐wide water temperatures varied among species and within species at different lengths. For species where shelf‐wide temperature effects were detected, the mid‐sized fish were most active in changing spatial distribution. For aquatic organisms ontogenetic migration occurs because life history stages have different environmental requirements. This study illustrates the need to consider species responses to climate change over different life history stages, and that studies on ecosystem responses should take ontogenetic differences into consideration when assessing impacts.     

Offshore currents explain the discontinuity of a fish community in the seagrass bed along the Japanese archipelago

Oceanographic conditions can affect spatial variability in fish community structures by influencing the temperature‐dependent latitudinal distribution of adult fishes and transport during their young stages. In order to examine latitudinal variability in the fish community structure within a single coastal ecosystem, quantitative sampling was conducted in the sub‐tidal zone of seagrass Zostera marina beds over a broad latitudinal scale (31.31–43.0°N: from subtropical to sub‐boreal zones, covering 80% of the latitudinal range of seagrass distribution in Japan) in the western North Pacific based on a uniform methodology. Cluster analysis with the similarity of fish communities showed that 13 sampling sites were divided into two clusters. The border between the two clusters corresponded with the area of mixing of two dominant currents, Oyashio and Kuroshio, which form a border between the warm temperate zone and the cool temperate zone off the Pacific coast of Japan. Oceanographic properties, such as major currents off the coast, are suggested to affect the latitudinal variability in the fish communities in the coastal ecosystem in the western North Pacific.     

围网养殖对华阳河湖鱼类群落结构的影响

为了发展渔业生产,人们将大量自然湖泊通过围网的方式投放鱼苗进行养殖。目前,这种养殖模式对鱼类群落的影响没有得到有效的评估。本研究于2016年在华阳河湖围网养殖区和非养殖区分别设置采样点对鱼类进行季度调查,探讨围网养殖对鱼类群落结构的影响。结果显示,在围网养殖区共采集鱼类6目12科35属46种,非养殖区采集鱼类6目11科40属57种,其中以鲤形目种类最多,分别占养殖区和非养殖区鱼类总数的65.22%和63.16%。非养殖区的优势种为?和短颌鲚,而鲢、鳙是围网养殖区的优势种。与非养殖区相比,围网养殖区山溪河流性鱼类百分比从21.05%降低至15.22%。通过双因素方差分析解析养殖与季节对鱼类群落结构的影响,发现围网养殖区的鱼类密度、物种数、优势度指数都显著低于非养殖区,而均匀度指数显著高于非养殖区。通过SIMPER分析得出,造成养殖区和非养殖群落结构差异的重要贡献物种为短颌鲚、?、鲤、太湖短吻银鱼、鲫、鱵、鲢和鳙。     

Spatial abundance patterns and recruitment of a virus‐affected commercial mollusc fishery

Infectious pathogens figure prominently among those factors threatening marine wildlife. Mass mortality events caused by pathogens can fundamentally alter the structure of wild fish stocks and depress recruitment rates and yield. In the most severe instances, this can precipitate stock collapses resulting in dramatic economic losses to once valuable commercial fisheries. An outbreak of a herpes‐like virus among commercially fished abalone populations in the south‐west fishery of Victoria, Australia, during 2006–2007, has been associated with high mortality rates among all cohorts. Long‐term records from fishery‐independent surveys of blacklip abalone Haliotis rubra (Leach) enabled abundance from pre‐ and post‐viral periods to be analysed to estimate stock density and biomass. The spatial distribution of abundance in relation to physical habitat variables derived from high‐resolution bathymetric LiDAR data was investigated. Significant differences were observed in both measures between pre‐ and post‐viral periods. Although there was some limited evidence of gradual stock improvement in recent years, disease‐affected reefs have remained below productivity rates prior to the disease outbreak suggesting a reduction in larval availability or settlement success. This was corroborated by trends in sublegal sized blacklip abalone abundance that has yet to show substantial recovery post‐disease. Abundance data were modelled as a function of habitat variables using a generalised additive model (GAM) and indicated that high abundance was associated with complex reef structures of coastal waters (<15 m). This study highlights the importance of long‐term surveys to understand abalone recovery following mass mortality and the links between stock abundance and seafloor variability.     

Seasonal and diel changes in fish distribution in a tropical hydropower plant tailrace: evidence from hydroacoustic and gillnet sampling

Hydropower plant operation can lead to direct impacts on fish near the dam, causing injuries and death. In Brazil, it is more common to observe these impacts in the tailrace. To understand temporal and spatial variations in fish abundance in the tailrace, to assist plant operation management, surveys were performed in Três Marias Hydropower Plant using hydroacoustics and multimesh gillnets. Data were collected in the tailrace backwater over 1 year, during 18 field trips, each covering a 24‐h cycle with six sampling intervals. A greater abundance of fish was found in the tailrace during the wet season, mainly explained by total discharge. More fish were detected by hydroacoustics and gill nets during night sampling. Siluriformes, which are known to exhibit nocturnal habits, were the predominant species in both dry and wet seasons. Thus, damaging operational procedures for fish should be planned in the dry season, low‐abundance period.     

Effects of stocked trout on native fish communities in boreal foothills lakes

Nasmith LE, Tonn WM, Paszkowski CA, Scrimgeour GJ. Effects of stocked trout on native fish communities in boreal foothills lakes.
Ecology of Freshwater Fish 2010: 19: 279–289. © 2010 John Wiley & Sons A/S Abstract – Ecological effects of stocking nonnative trout into lakes are receiving increased attention, especially in alpine environments. We assessed effects of stocked trout on native forage fishes in the boreal foothills of Alberta (Canada) by comparing fish density, population size structure and spatial and temporal activities in stocked and unstocked lakes over 3 years (2005–2007). The numerically dominant dace (primarily Phoxinus spp.) were larger in stocked lakes, consistent with size‐limited predation. Dace were also more crepuscular and concentrated on the lake‐bottom in stocked lakes, compared to more daytime activity in the water column in unstocked lakes. There were, however, no demonstrable effects of trout on the abundance of forage fish. The lack of major population‐level impacts of stocked trout suggests that current stocking practices, characteristics of boreal foothill lakes (e.g. thermal structure, abundant invertebrates, dense macrophytes) and/or behavioural adjustments of forage fish contribute to healthy native fish populations in our stocked lakes.     

Why are so many boreal freshwater fishes anadromous? Confronting ‘conventional wisdom’

Suggestions that anadromy predominates in northern high latitudes because boreal marine seas have high primary productivity are questioned. Instead, post‐Pleistocene invasion of far northern fresh waters, which became inhabitable as Pleistocene temperatures ameliorated, via coastal seas, is suggested as a primary cause for northern distributions of widely anadromous fish families. Also, cold temperature preferences may be implicated in determining the ranges of these families, as well as the inhospitability of far northern fresh waters and the lack of ecosystem space to support the huge populations of boreal anadromous fishes. Moreover, there are significant numbers of tropical anadromous fish species.     

Fish diversity assessment in the headwaters of the Volga River using environmental DNA metabarcoding

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气候变化对长江口鱼类资源密度分布的重塑作用

本研究以2012-2013年长江口鱼类资源密度分布为基础,通过动态生物气候分室模型(DBEM)预估了不同气候变化情景下(IPCC,RCP2.6、RCP6.0和RCP8.5)长江口鱼类资源密度增量分布的变化.在RCP2.6、RCP6.0和RCP8.5这3种气候变化情景下,鱼类资源密度增量、底层鱼类资源密度增量随着时间推移均呈递增趋势,且递增程度和增量重心分布范围随着温室气体排放的增加而扩大(RCP8.5＞RCP6.0＞RCP2.6).鱼类资源密度增量重心主要分布在长江口崇明岛沿岸水域,长江口外侧水域资源密度增量相对较低,并且资源密度增量重心有向南迁移的趋势.     

Interannual variability of the ecosystem of the Kii Channel, the Inland Sea of Japan, as influenced by bottom intrusion of cold and nutrient-rich water from the Pacific Ocean, and a recent trend of warming and oligotrophication

Interannual variability of the ecosystem of the Kii Channel, productive shelf water on the Pacific side of south‐western Japan, was analysed based on physicochemical environmental variables and abundance of major zooplankton taxa collected monthly for 12 yr from 1987 to 1999. The Kii Channel experienced both short‐term (i.e. 3–4 yr) cyclical changes and a long‐term (i.e. decadal) environmental trend. The short‐term variability was primarily associated with year‐to‐year differences in intrusion of subsurface, cold nutrient‐rich water along the bottom of the Kii Channel from the Pacific Ocean. When this bottom intrusion was intense, the Kii Channel experienced a cold, new production‐dominated ecosystem. The bottom intrusion, however, has become less intense in recent years, because of the closer proximity of the Kuroshio flow axis to the Kii Channel. Hence, there was a consistent trend towards warming and a regenerated production‐dominated ecosystem. In accordance with such environmental changes, the zooplankton community shifted towards more oceanic conditions; major herbivorous calanoids (i.e. Calanus sinicus, Paracalanus parvus (s.l.), Clausocalanus spp. and Acartia omorii) decreased, while carnivores (i.e. hydromedusae and Sagitta spp.) increased. The amount of total fish catch also decreased, while the catch of subtropical species increased. These findings lead us to conclude that the bottom intrusion from the Pacific Ocean plays a key role in determining the biological production in the Kii Channel.     

Ocean climate effects on the relative abundance of short-finned (Illex illecebrosus) and long-finned (Loligo pealeii) squid in the northwest Atlantic Ocean

An unusually high abundance of long‐finned squid (Loligo pealeii) was observed around southern Newfoundland in August–September 2000. The prevalence of maturing females and mature males, along with the collection of a single viable egg mop, provide the first evidence of spawning of this species at the northern limit of its geographic range of distribution. Northward expansion of the long‐finned squid population may be related to a general warming trend in Newfoundland near‐shore waters. However, trends in size and abundance of short‐finned squid (Illex illecebrosus) suggest that this expansion may also be related to reduced competition. We hypothesize that these two squid species share, to a large extent, a common niche on the eastern USA shelf and that opposing responses to ecosystem variation affect their relative abundance. We address this hypothesis by applying time‐series analysis of species‐specific fishery‐ and survey‐based abundance indices with biologic and environmental input variables. Our models indicate that direct competition is not important, but that variation in atmospheric forcing, as well as latitudinal position of the Shelf‐Slope Front (SSF), are closely related to direct oceanographic processes that exert opposing effects on these two species. While the direct oceanographic mechanisms that regulate year‐class strength remain unknown, we present a hypothesis to account for opposing population responses to oceanographic variation that operates early in the life history of both species. For the oceanic and highly migratory short‐finned squid, variation in the latitudinal position of the SSF is related to efficiency of downstream dispersal by the Gulf Stream and survival of young stages. For the neritic long‐finned squid, variation in local inshore temperature affects the time exposed to intense predation through its effect on rates of embryonic development and growth. The expansion of long‐finned squid population abundance in 2000 was associated with both warm local water temperatures and an unusual eastward displacement of the atmospheric features associated with the North Atlantic Oscillation (NAO). These conditions may have resulted in an unfavorable oceanic regime for short‐finned squid but a favorable regime for long‐finned squid on the continental shelf as far north as southern Newfoundland.     

Potential impacts of non‐native channel catfish on commercially important species in a Japanese lake,as inferred from long‐term monitoring data

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Understanding imperfect detection in a San Francisco Estuary long‐term larval and juvenile fish monitoring programme

Imperfect detection can present a significant challenge when monitoring for a rare and imperilled species. Here, a long‐term larval and early‐juvenile fish monitoring programme in the upper San Francisco Estuary was examined to evaluate its overall reliability in detecting various fish species, including the imperilled delta smelt, Hypomesus transpacificus McAllister, for which the programme was designed. Using occupancy modelling, detection probability of species with pelagic larval or juvenile life stages was found to be generally high (≥.95) based on the current sampling effort of three larval net tows per site. However, detection probability can vary considerably from year to year depending on the species’ level of larval production. Water temperature and turbidity were identified as important predictors of occurrence for young‐of‐year delta smelt, longfin smelt Spirinchus thaleichthys (Ayres) and striped bass Morone saxatilis (Walbaum), and there was evidence for fish size selectivity by the sampling gear in all three species. These results highlight the need to consider adaptively managing detection probability by increasing sampling effort in years when young‐of‐year delta smelt abundance is expected to be low, especially when information on the species’ occurrence at a particular region is critical.     

A 20‐year census of a rocky littoral fish community in Lake Tanganyika

Takeuchi Y, Ochi H, Kohda M, Sinyinza D, Hori M. A 20‐year census of a rocky littoral fish community in Lake Tanganyika. Ecology of Freshwater Fish 2010: 19: 239–248. © 2010 John Wiley & Sons A/S Abstract – Tropical fish communities are highly diverse with large numbers of allied species coexisting. Concerns have been raised about changes in such communities caused by anthropogenic disturbances, but little is known about long‐term changes. We investigated changes in the littoral fish community of Lake Tanganyika. A 10 × 40‐m quadrat was placed on a rocky shore in the southern part of the lake, and fish censuses were conducted once a year during most years in 1988–2008. In total, 54 cichlid and 6 non‐cichlid species were recorded. Aufwuch feeders were most abundant, followed by plankton feeders, detritus feeders, piscivores, shrimp eaters, zoo‐benthos eaters, and scale eaters. Species richness and abundance were sustained over the study period, with biodiversity, as evaluated by the Shannon–Wiener index, remaining high. However, Mantel tests between the Bray–Curtis index and the difference in years between compared censuses revealed that the species composition of the community gradually changed with time. The fish community gradually underwent a change that may be due to low anthropogenic impacts on food condition. Our results promote awareness about the conservation of diversity in Lake Tanganyika.     

Influence of pejerrey Odontesthes bonariensis (Valenciennes, 1835) broodstock age on gamete quality,reproductive performance and plasma sex steroid levels during the spawning season

This study evaluated the effect of pejerrey Odontesthes bonariensis (Valenciennes, 1835) broodstock age on gamete and spawning quality and its relationship with sex steroid plasma levels. Sperm was analysed using a computer‐assisted sperm analysis (CASA) developed for pejerrey. Semen samples were taken from all fish from mid age groups (5‐ and 7‐year fish), but it was not possible to obtain stripped semen samples from all fish of younger (3‐year) or older (10‐year) groups. The highest relative sperm weight and sperm concentration were recorded in 5‐ and 7‐year groups respectively, while viability was not different between age groups. It was not possible to identify an effect of age on sperm quality by CASA. Non‐viable spawned eggs were obtained in the 3‐year group and, the relative fecundity and the relative number of fertilized eggs decreased with age in other groups. Fertilization and hatching rates showed the highest values in the 10‐year group. However, the estimated number of hatched larvae was similar in 5‐, 7‐ and 10‐year fish. A clear effect of age on 11 ketotestosterone (11‐KT) and estradiol (E₂) levels was identified, with the highest values in the 5‐ and 7‐year groups. These results might be related to the onset of puberty in the 3‐year group and ageing in the 10‐year group. Considering that the estimated number of larvae obtained was similar among age groups, the results of this study suggest that broodstock maintenance cost could be reduced by using mid‐age fish rather than older fish in pejerrey hatcheries.