Vertical distribution and migration of fish larvae in the NW Iberian upwelling system during the winter mixing period: implications for cross‐shelf distribution

The vertical distribution and vertical migrations of fish larvae and implications for their cross‐shelf distribution were investigated in the northern limit of the NE Atlantic upwelling region during the late winter mixing period of 2012. The average positive values of the upwelling index for February and March of this year were far from normal, although the average hydrographic conditions during the period of study were of downwelling and the water column was completely mixed. Fish larvae, most in the preflexion stage, were concentrated in the upper layers of the water column and their distribution was depth stratified, both day and night. However, the larval fish community was not structured in the vertical plane and fish larvae did not show significant diel vertical migration (DVM), although five species showed ontogenetic vertical migration. In regions of coastal upwelling and in the absence of DVM, the location of fish larvae in the water column is crucial for their cross‐shelf distribution. Thus, the cross‐shelf distribution of the six most abundant species collected in this study can be explained by the surface onshore flow associated with coastal downwelling, retaining larvae of the coastal spawning species with a relatively shallow distribution in the shelf region and transporting larvae of slope spawning species onto the shelf. The wide vertical distribution shown by larvae of the offshore spawning species could be an adaptation of these species to ensure that some larvae reach the inshore nursery areas.     

Modelling transport of chokka squid (Loligo reynaudii) paralarvae off South Africa: reviewing,testing and extending the ‘Westward Transport Hypothesis’

Annual landings of chokka squid (Loligo reynaudii), an important fishing resource for South Africa, fluctuate greatly, and are believed to be related to recruitment success. The ‘Westward Transport Hypothesis’ (WTH) attributes recruitment strength to variability in transport of newly hatched paralarvae from spawning grounds to the ‘cold ridge’ nursery region some 100–200 km to the west, where oceanographic conditions sustain high productivity. We used an individual‐based model (IBM) coupled with a 3‐D hydrodynamic model (ROMS) to test the WTH and assessed four factors that might influence successful transport – Release Area, Month, Specific Gravity (body density) and Diel Vertical Migration (DVM) – in numerical experiments that estimated successful transport of squid paralarvae to the cold ridge. A multifactor ANOVA was used to identify the primary determinants of transport success in the various experimental simulations. Among these, release area was found to be the most important, implying that adult spawning behaviour (i.e., birth site fidelity) may be more important than paralarval behaviour in determining paralarval transport variability. However, specific gravity and DVM were found to play a role by retaining paralarvae on the shelf and optimizing early transport, respectively. Upwelling events seem to facilitate transport by moving paralarvae higher in the water column and thus exposing them to faster surface currents.     

Clupeoid larval growth and plankton production in the Benguela upwelling system

Upwelling is a characteristic feature of the Benguela system. Strong upwelling leads to high primary production, which, in turn, supports large biomass of higher trophic levels. To analyse intra‐, interannual and decadal variability of fish recruitment, growth, and distribution, different methods to quantify this upwelling have been used. In the present work, an upwelling index, modelled diatom primary production, based on outputs from a numerical model for the Benguela upwelling system, is proposed. Upwelled water brings large amounts of nutrients into the euphotic zone and this supports primary productivity, which can be considered a preconditioner for the zooplankton that clupeoid larvae feed on. Therefore, it is assumed that high upwelling gives the potential for good feeding conditions for these larvae. This fact has motivated the use of the modelled index for upwelling intensity as a proxy for food availability and an investigation of the interannual variability of observed mean individual weight of Cape anchovy (Engraulis capensis) and sardine (Sardinops sagax) recruits. The study shows that the mean Cape anchovy recruit weight has a linear relationship with upwelling activity along the transport route (from the spawning to the nursery areas) and inside the nursery areas, while sardine have an Optimal Environmental Window (OEW) relationship to the upwelling on the transport route. A possible explanation for these observed differences is the prey the larvae feed on and the change in the plankton community as a function of upwelling activity.     

Impact of spring upwelling variability off southern‐central Chile on common sardine (Strangomera bentincki) recruitment

Off southern‐central Chile, the impact of spring upwelling variability on common sardine (Strangomera bentincki) recruitment was examined by analyzing satellite and coastal station winds, satellite chlorophyll, and common sardine recruitment from a stock assessment model. In austral spring, the intensity of wind‐driven upwelling is related to sea surface temperature (SST) from the Niño 3.4 region, being weak during warm periods (El Niño) and strong during cold periods (La Niña). Interannual changes in both spring upwelling intensity and SST from the Niño 3.4 region are related to changes in remotely sensed chlorophyll over the continental shelf. In turn, year‐to‐year changes in coastal chlorophyll are tightly coupled to common sardine recruitment. We propose that, in the period 1991–2004, interannual changes in the intensity of spring upwelling affected the abundance and availability of planktonic food for common sardine, and consequently determined pre‐recruit survival and recruitment strength. However, the importance of density‐dependent factors on the reproductive dynamic cannot be neglected, as a negative association exists between spawning biomass and recruitment‐per‐spawning biomass. Coastal chlorophyll, upwelling intensity, and SST anomalies from the Niño 3.4 region could potentially help to predict common sardine recruitment scenarios under strong spring upwelling and El Niño Southern Oscillation (ENSO)‐related anomalies.     

Polar light regime and piscivory govern diel vertical migrations of planktivorous fish and zooplankton in a subarctic lake

Abstract –  Seasonal variation in light intensity has strong impacts on invertebrate and vertebrate habitat selection creating trade-offs between foraging gain and risk of death. Diel vertical migration (DVM) has received a particularly interest, but multitrophic level studies in lakes under polar light regime have not been conducted. Here, we examined habitat selection of pelagic zooplankton, planktivores and piscivores in subarctic Lake Muddusjärvi with polymorphic whitefish ( Coregonus lavaretus (L.)). Seasonal change in light was hypothesized to be the most important abiotic factor inducing DVM, whereas predation was considered as an ultimate biotic factor. During period of mid-night sun in June, no DVM was observed at any trophic level, whereas during normal day and night light in September planktivores and zooplankton migrated. DVM was top-down controlled, where piscivorous brown trout ( Salmo trutta L.) used pelagic habitat continuously inducing DVM of its main prey, pelagic whitefish morph, which cascaded to reverse DVM of zooplankton. Top-down control of lower trophic level DVMs by piscivores might be more general pattern in lakes than previously considered.     

Processes controlling retention of spring‐spawned Atlantic cod (Gadus morhua) in the western Gulf of Maine and their relationship to an index of recruitment success

Atlantic cod, Gadus morhua, harvested in US waters are currently managed as a Gulf of Maine stock and as a stock comprising Georges Bank and southern New England populations. Over the past two and a half decades, success of age‐1 recruitment to the Gulf of Maine stock has varied by more than an order of magnitude. To investigate the hypothesis that this variation is related to variation in the transport of larval cod to nursery areas, we carried out model simulations of the movement of planktonic eggs and larvae spawned within the western Gulf of Maine during spring spawning events of 1995–2005. Results indicate that the retention of spring‐spawned cod, and their transport to areas suitable for early stage juvenile development, is strongly dependent on local wind conditions. Larval cod retention is favored during times of downwelling‐favorable winds and is least likely during times of upwelling‐favorable winds, during which buoyant eggs and early stage larvae tend to be advected offshore to the Western Maine Coastal Current and subsequently carried out of the Gulf of Maine. Model results also indicate that diel vertical migration of later stage larvae enhances the likelihood of retention within the western Gulf of Maine. Consistent with model results is a strong correlation between age‐1 recruitment success to the Gulf of Maine cod stock and the mean northward wind velocity measured in Massachusetts Bay during May. Based on these findings, we propose a wind index for strong recruitment success of age‐1 cod to the Gulf of Maine stock.     

Spatial and temporal patterns of vertical distribution for three planktivorous fishes in Lake Washington

Abstract – We sampled three limnetic fish species: juvenile sockeye salmon (Oncorhynchus nerka), three‐spine stickleback (Gasterosteus aculeatus) and longfin smelt (Spirinchus thaleichthys) in Lake Washington to quantify species‐specific patterns of diel vertical migration (DVM). Catch‐per‐unit‐effort data analysed from 15 years of midwater trawling documented seasonal and diel differences in vertical distributions for each species. These results were consistent with the hypothesis that the patterns of DVM in Lake Washington were affected by life history, size and morphology. Sockeye salmon showed clear DVM in spring but essentially no DVM in fall, remaining in deep water, whereas three‐spine sticklebacks were prevalent at the surface at night in both seasons. In fall, distribution patterns may be explained by differences in thermal performance (e.g., sticklebacks favouring warm water), but the patterns were also consistent with inter‐specific differences in predation risk. Younger sockeye salmon and longfin smelt were present in greater proportions higher in the water column during dusk and night periods than older conspecifics. Compared with sockeye salmon, the greater use by three‐spine sticklebacks of surface waters throughout the diel cycle during weak thermal stratification in spring was consistent with the hypothesis that sticklebacks’ armour reduces predation risk, but use of this warmer, metabolically beneficial stratum may also have promoted growth. This study illustrates variation in the vertical distribution of three sympatric planktivores and offers broader implications for the DVM phenomenon and applied lake ecology.     

Variation in the distribution of ocean shrimp (Pandalus jordani) recruits: links with coastal upwelling and climate change

In this analysis, an atypical northward shift in the distribution of age‐1 ocean shrimp (Pandalus jordani) recruits off Oregon in 2000 and 2002–2004 was linked to anomolously strong coastal upwelling winds off southern Oregon (42°N latitude) in April–July of the year of larval release (t?1). This is the first clear evidence that strong upwelling winds can depress local recruitment of ocean shrimp. Regression analysis confirmed a long‐term negative correlation between log_e of ocean shrimp recruitment and April sea level height (SLH) at Crescent City, California, in the year of larval release, for both northern and southern Oregon waters. The regional pattern of ocean shrimp catches and seasonal upwelling winds showed that, although the timing of the spring transition as reflected in April SLH drives ocean shrimp recruitment success off Oregon generally, the strength and consistency of spring upwelling limits the distribution of large concentrations of ocean shrimp at the southern end of the northern California/Oregon/Washington area. A northward shift in 1999 and 2001–03 in the northern edge of this ‘zone of maximum upwelling’ is the likely cause of the weak southern Oregon recruitment and resulting atypical distribution of ocean shrimp observed off Oregon in 2000 and 2002–04, with a return to a more typical catch distribution as spring upwelling moderated in subsequent years. It is noted that a northward shift in the conditions that produce strong and steady spring upwelling winds is consistent with many predictions of global climate models under conditions of global warming.     

Relationships between anchovy (Engraulis encrasicolus) recruitment and environment in the Bay of Biscay (1967–1996)

Various hypotheses have been put forward to explain the mechanisms leading to recruitment variability in fish populations. These have been based on either physical (wind stress, upwelling) or biological (food and predation) processes. In the Bay of Biscay, the hypothesis of a physical influence on anchovy recruitment has been confirmed. Oceanographic conditions in the Bay of Biscay in the spring and summer, influenced by north-easterly winds of medium and low intensity, seem to induce good levels of recruitment to the anchovy ( Engraulis encrasicolus ) population. An index of upwelling was significantly correlated with annual recruitment of anchovy ( P  < 0.0001) for the period 1967–1996. This physical parameter explains about 59% of the variability in the recruitment of the Bay of Biscay anchovy. Two factors affecting productivity in the Bay of Biscay may be linked to north-easterly winds, namely weak upwelling and an extension of the area influenced by river outflows. Both of these factors, together with low turbulence and stability, may act to enhance survival of the early life-history stages of anchovy by increasing food availability. The potential use of this upwelling index to forecast the recruitment of the Bay of Biscay anchovy offers possibilities for improving the management of this population.     

紫霞湖浮游甲壳动物昼夜垂直迁移

2010年9月19～21日,对江苏南京紫霞湖(水深8.5m)浮游甲壳动物昼夜垂直迁移行为进行了调查,分析了影响昼夜垂直迁移的因素。结果表明,奥氏秀体溞(Diaphanosoma orghidani)、象鼻溞(Bosmina sp.)、颈沟基合溞(Bosminopsis deitersi)、台湾温剑水蚤(Thermocyclops taihokuensis)和中华原镖水蚤(Eodiapotomus sinensis)成体均表现出昼夜迁移现象,中华原镖水蚤幼体和无节幼体无明显昼夜迁移。紫霞湖的温度、溶解氧和叶绿素a浓度的垂直变化不是导致浮游甲壳动物昼夜垂直迁移的主导因子,光照强度和鱼类捕食可能是影响浮游甲壳动物昼夜垂直迁移的主要因素。     

Life strategies of cephalopod paralarvae in a coastal upwelling system (NW Iberian Peninsula): insights from zooplankton community and spatio‐temporal analyses

The early life stages of cephalopods ‐ octopods, squids, sepiolids and ommastrephids ‐, are uncommon in zooplankton samples and little is known about their life strategies. Accordingly, cephalopod paralarvae were examined in the upwelling ecosystem of the Ría de Vigo (NW Spain) at night from 2008 to 2010. Multivariate analyses and generalized linear models (GLMs) were used to explore relationships between cephalopod paralarvae and the zooplankton communities that they inhabited in 2008. In addition, the foraging strategy and prey preferences of Octopus vulgaris paralarvae within these communities were determined. Multivariate and GLM results showed a strong association of cephalopod paralarvae with coastal and frontal zooplankton communities. Octopus paralarvae were shown to be specialist predators with a strong preference for decapod zoeae in each of the communities examined. Using the three years of sampling, GLM analyses of paralarval spatio‐temporal variations in relation with the upwelling strength showed a positive relationship with upwelling intensity for O. vulgaris and sepiolids, as well as contrasting temporal, horizontal and vertical distributions for the different paralarvae analysed. Under strong upwelling events, Octopus paralarvae were more abundant in surface waters, whereas the abundance of loliginids and sepiolids was higher in the water column. This vertical behaviour in conjunction with the physical conditions of the Western Iberian Upwelling ecosystem suggests the coexistence of two different life strategies: a coastal strategy displayed by loliginid and sepiolid paralarvae that are retained over the shelf, and an oceanic strategy displayed by O. vulgaris paralarvae that are dispersed far from the shelf.     

Decadal change in abundance of surface migratory myctophid fishes in the Kuroshio region from 1957 to 1994

Decadal change in abundance of surface migratory myctophid fishes was examined in the Kuroshio region of the western North Pacific for the 35 yr from 1957 to 1994 in relation to the Kuroshio regime shift, indicated by changes in its transport volume and sea surface temperature in the early 1970s. Each year, samples were collected from the 0‐ to 1‐m depth layer at night, from January to March, at 83 stations on average. Ten species belonging to five genera occurred. Juveniles [standard length (SL) ≤ 40 mm] and adults (SL > 40 mm) of each species were analysed. In juveniles, Myctophum asperum, M. nitidulum, Symbolophorus evermanni and Centrobranchus brevirostris accounted for 91.1% of the total catch (7419 individuals), while M. nitidulum contributed 82.4% of the total adult catch (494 individuals). The abundance of juvenile S. evermanni, C. brevirostris and M. nitidulum showed prominent peaks in 1971, together with adult M. nitidulum. These peaks corresponded to extraordinarily high zooplankton abundance in the Kuroshio region in 1971, suggesting high recruitment success and low adult mortality in surface migratory myctophids. Smaller changes in abundance in other years were species‐specific, and the fluctuation patterns were mainly explained by year‐to‐year fluctuations in the strength of the Kuroshio and the geographical distribution of each species, i.e. an increased Kuroshio flow volume resulted in an increase in tropical species and a decrease in temperate species, but no change in the pan‐Kuroshio species.     

Modelling the effect of buoyancy on the transport of anchovy (Engraulis capensis) eggs from spawning to nursery grounds in the southern Benguela: an IBM approach

An individual‐based model (IBM) was used to investigate the effects of physical and biological variables on the transport via a jet current of anchovy (Engraulis capensis) eggs from spawning to the nursery grounds in the southern Benguela ecosystem. As transport of eggs and early larvae is considered to be one of the major factors impacting on anchovy recruitment success, this approach may be useful to understand further the recruitment variability in this economically and ecologically important species. By coupling the IBM to a 3D hydrodynamic model of the region called Plume, and by varying parameters such as the spatial and temporal location of spawning, particle buoyancy, and the depth range over which particles were released, we could assess the influences of these parameters on transport success. A sensitivity analysis using a General Linear Model identified the primary determinants of transport success in the various experimental simulations, and model outputs were examined and compared with patterns observed in field studies. Model outputs compared well with observed patterns of vertical and horizontal egg distribution. Particle buoyancy and area of particle release were the major single determinants of transport success, with an egg density of 1.025 g cm^?3 maximizing average particle transport success and the western Agulhas Bank being the most successful spawning area. This IBM may be useful as a generic prototype for other upwelling ecosystems.     

Climate,oceanography, and recruitment: the case of the Bay of Biscay anchovy (Engraulis encrasicolus)

The Bay of Biscay anchovy has experienced, since 2001, a succession of low recruitments, resulting in the collapse of the stock in 2005; this has led to successive closures of the fishery. This study investigates the possible impact of different controlling factors [North Atlantic Oscillation (NAO), East Atlantic (EA) pattern, turbulence, upwelling, and river flow] upon anchovy recruitment and fishery catches. Fifty‐five percent of the recruitment variability of this fishery can be explained by upwelling over the spawning area; this is related, in turn, to the EA pattern. The conceptual understanding of the system proposed for the Bay of Biscay anchovy suggests that negative EA periods are associated with northeasterly wind circulation, which produces weak upwelling over the continental shelf. This pattern results in hydrodynamic stability over the area, leading, probably, to adequate food availability. A positive EA (which extends onwards, from 1998) is associated with southwesterly winds and downwelling over the continental shelf; this leads, probably, to the dispersion of anchovy food and larvae, together with increasing mortality.     

A model‐based meso‐zooplankton production index and its relation to the ocean survival of juvenile coho (Oncorhynchus kisutch)

The ocean survival of coho salmon (Oncorhynchus kisutch) off the Pacific Northwest coast has been related to oceanographic conditions regulating lower trophic level production during their first year at sea. Coastal upwelling is recognized as the primary driver of seasonal plankton production but as a single index upwelling intensity has been an inconsistent predictor of coho salmon survival. Our goal was to develop a model of upwelling‐driven meso‐zooplankton production for the Oregon shelf ecosystem that was more immediately linked to the feeding conditions experienced by juvenile salmon than a purely physical index. The model consisted of a medium‐complexity plankton model linked to a simple one‐dimensional, cross‐shelf upwelling model. The plankton model described the dynamics of nitrate, ammonium, small and large phytoplankton, meso‐zooplankton (copepods), and detritus. The model was run from 1996 to 2007 and evaluated on an interannual scale against time‐series observations of copepod biomass. The model’s ability to capture observed interannual variability improved substantially when the copepod community size distribution was taken into account each season. The meso‐zooplankton production index was significantly correlated with the ocean survival of hatchery coho salmon from the Oregon production area, although the coastal upwelling index that drove the model was not itself correlated with survival. Meso‐zooplankton production within the summer quarter (July–September) was more strongly correlated with coho survival than was meso‐zooplankton production in the spring quarter (April–June).     

Coastal winds and larval fish abundance indicate a recruitment mechanism for southeast Australian estuarine fisheries

Coastal winds transport water masses and larval fish onshore or offshore which may influence estuarine recruitment, yet our understanding of the mechanism underlying this relationship is limited. Here, we combine datasets from a historical database of larval fish off southeast Australia with a high-resolution atmospheric reanalysis model to show that normalised abundance of coastally spawned larvae increased with weak to moderate upwelling favourable winds 14 days prior to sampling. The increase in abundance may reflect increased nutrient and plankton availability for larval fish. Normalised larval abundance decreased following strong upwelling favourable winds but increased after onshore (downwelling favourable) winds, due to wind-driven transport. By combining a commercial estuarine fisheries catch-rate dataset (4 species, 8 estuaries, 10 years) and the high-resolution atmospheric reanalysis model, we show that negative effects of upwelling favourable winds during the spawning period can be detected in lagged estuarine commercial fisheries catch rates (lagged by 2–8 years depending on species' growth rates), potentially representing the same mechanism proposed for larval fish. Upwelling favourable winds in the southeast Australian region have increased since 1850 while onshore winds have decreased, which may have reduced larval recruitment to estuaries. Coastal winds are likely an important factor for estuarine recruitment in the southeast Australian region and future research on the estuarine recruitment of fish should incorporate coastal winds.     

From egg to juvenile in the Bay of Biscay: spatial patterns of anchovy (Engraulis encrasicolus) recruitment in a non‐upwelling region

In this study the spatial distribution of eggs, larvae and juveniles of European anchovy (Engraulis encrasicolus) was followed in 2004 and 2005 during three consecutive cruises each year in spring–summer to test what the limits of retention are in a non‐upwelling area. Eggs, small larvae and large juveniles were mainly distributed over the shelf, whereas large larvae and small juveniles were found mainly off the shelf. Although overall distributions were similar, the 2 yr differed in that there was more of a coastal distribution of individuals in 2004, whereas in 2005 more individuals were found off the shelf. There were no significant differences in the length–weight relationships for individuals found on and off the shelf or between years. The correspondence in circulation patterns and the lack of difference in the length–weight relationships suggest that a single population is present, larvae drifting off the shelf due to currents and returning as mobile juveniles. Quantile regression analysis of the long‐term recruitment index suggests that transport off the shelf may favour good recruitments. This would suggest that in non‐upwelling regions the retention area resulting in good recruitment may not be restricted to the shelf.     

Cohort splitting in bluefish, Pomatomus saltatrix, in the US mid-Atlantic Bight

Atlantic bluefish exhibit cohort splitting, whereby two modes of juvenile recruits originate from spatially distinct spring‐ and summer‐spawning regions in US Atlantic shelf waters. We evaluate the pattern of cohort splitting in a transition area (US Maryland coastal region and Chesapeake Bay) between the two major spawning regions. Spring and summer cohorts were differentially represented in Maryland estuarine (Chesapeake Bay) and coastal waters. The spring cohort was dominant in Chesapeake Bay, but was not well represented in the ocean environment, and the converse true for the summer cohort. We hypothesized that ocean temperatures control the bimodal spawning behavior and extent of cohort splitting. As evidence, we observed an intervening early summer cohort produced in years when shelf temperatures during early summer were suitably warm for spawning. In most years however, two dominant cohorts were evident. We propose that vernal warming dynamics in the mid‐Atlantic Bight influence spawning behavior and the resultant bimodal pattern of seasonal juvenile cohort production commonly observed along the US east coast.     

紫霞湖浮游甲壳动物昼夜垂直迁移及其影响因子分析

2010年9月19～21日,对江苏南京紫霞湖（水深8.5m）浮游甲壳动物昼夜垂直迁移行为进行了调查,分析了影响昼夜垂直迁移的因素。结果表明,奥氏秀体溞（Diaphanosoma orghidani）、象鼻溞（Bosmina sp.）、颈沟基合溞（Bosminopsis deitersi）、台湾温剑水蚤（Thermocyclops taihokuensis）和中华原镖水蚤（Eodiapotomus sinensis）成体均表现出昼夜迁移现象,中华原镖水蚤幼体和无节幼体无明显昼夜迁移。紫霞湖的温度、溶解氧和叶     

The effect of vertical and horizontal distribution on retention of sardine (Sardinops sagax) larvae in the Northern Benguela – observations and modelling

In the present study, a modelling experiment is conducted to simulate the transport of sardine (Sardinops sagax) eggs and larvae in the Northern Benguela. Based on historical and newly obtained data, different scenarios of vertical and horizontal distribution are applied and the effects on retention are discussed. The simulations showed that vertical and horizontal distribution were important for retention of sardine larvae in the Northern Benguela. By using age‐dependent data on vertical distribution, it was shown that retention of particles in the simulation was substantially enhanced compared with a scenario where particles were distributed in the offshore moving Ekman layer. Retention was lowest during October–December (when upwelling intensity is high) and highest during February–April (when upwelling intensity is somewhat lower). When different spawning areas were considered, highest retention was observed in an area near Walvis Bay. It is concluded that the behaviour of sardine larvae is adapted to the circulation system in the Northern Benguela in a way that promotes retention of the larvae in inshore nursery areas.