Climate fluctuations and the spring invasion of the North Sea by Calanus finmarchicus

The population of Calanus finmarchicus in the North Sea is replenished each spring by invasion from an overwintering stock located beyond the shelf edge. A combination of field observations, statistical analysis of Continuous Plankton Recorder (CPR) data, and particle tracking model simulations, was used to investigate the processes involved in the cross-shelf invasion. The results showed that the main source of overwintering animals entering the North Sea in the spring is at depths of greater than 600 m in the Faroe–Shetland Channel, where concentrations of up to 620 m⁻³ are found in association with the overflow of Norwegian Sea Deep Water (NSDW) across the Iceland–Scotland Ridge. The input of this water mass to the Faroe–Shetland Channel, and hence the supply of overwintering C. finmarchicus , has declined since the late 1960s due to changes in convective processes in the Greenland Sea. Beginning in February, animals start to emerge from the overwintering state and migrate to the surface waters, where their transport into the North Sea is mainly determined by the incidence of north-westerly winds that have declined since the 1960s. Together, these two factors explain a high proportion of the 30-year trends in spring abundance in the North Sea as measured by the CPR survey. Both the regional winds and the NSDW overflow are connected to the North Atlantic Oscillation Index (NAO), which is an atmospheric climate index, but with different time scales of response. Thus, interannual fluctuations in the NAO can cause immediate changes in the incidence of north-westerly winds without leading to corresponding changes in C. finmarchicus abundance in the North Sea, because the NSDW overflow responds over longer (decadal) time scales.     

The ascent migration of Calanus finmarchicus from overwintering depths in the Faroe–Shetland Channel

The Faroe–Shetland Channel is an important overwintering area for the copepod Calanus finmarchicus in the north-east Atlantic. Stage V copepodites descend to depths of greater than 600 m, where they remain in an arrested development state (diapause) until the following spring. The ascent of the copepods back to the surface waters in the spring was investigated by six surveys in the Faroe–Shetland Channel between October 1993 and May 1995. In February, the first animals emerged from diapause and moulted to become stage VI males; these ascended to depths of 300–500 m. Females constituted a higher proportion of animals that emerged later in the season and these ascended through the layer of males, presumably mating on the way, and proceeded to the upper 100 m to begin spawning in March. Emergence was not synchronous in the population, and was estimated to occur over a 60–70 day period beginning around 10 February. The mean ascent rate of individuals was estimated to be 15–20 m day^–1, so that for any individual the ascent from a mean overwintering depth of 800 m to a depth of 100 m took between 35 and 47 days. The ascent migration resulted in the mass transfer of the C. finmarchicus stock from its overwintering habitat in the cold southerly flowing Norwegian Sea Deep Water of Arctic origin to warm northerly flowing Atlantic water masses. The extent of asynchrony suggests that emergence is probably not triggered by environmental cues. An alternative hypothesis for regulation of the life cycle is proposed that involves reduced development rates during the winter.     

Seasonal changes in respiration rates of copepodite stage V Calanus finmarchicus (Gunnerus)

Copepodite stage V Calanus finmarchicus were collected at locations on the continental shelf north of Scotland, in the Faroe–Shetland Channel and west of Ireland on six occasions covering winter, spring and summer from October 1993 to June 1995. Oxygen consumption by the overwintering and active spring/summer population of animals was determined at temperatures close to in situ temperatures. Laboratory measurements of oxygen consumption were also made at standardized temperatures (0°C, 5°C, 7°C and 12.5°C) to determine the sensitivity of animals to temperature change in the different seasons. Rates of oxygen consumption were very low (7–30 μmol O₂ gC⁻¹ h⁻¹) at in situ temperatures during the winter and early spring and significantly higher (105–219 μmol O₂ gC⁻¹ h⁻¹) for the active surface population in May and June. Animals collected from the overwintering population showed no significant response to changes in temperature. Due to the low respiration rates, the calculated rate of decrease in carbon content in diapausing copepodite stage CV was very low (approximately 0.250 μgC day⁻¹). The respiration rates were used to construct a model to estimate survival of the animals with an initial carbon content equivalent to that expected of animals in October. The results showed that in order to survive during winter and have enough energy for moulting and migration to the surface in the spring, these animals have to live at temperatures close to 0°C and be in a diapause state.     

Basin-scale advection and population persistence of Calanus finmarchicus

Advection of Calanus finmarchicus in the eastern North Atlantic was analysed using a particle-tracking model based on the Hamburg Shelf Ocean Model (HAMSOM). Quasi-static seasonal mean flowfields were simulated, archived and interpolated to represent a climatological-mean annual cycle. Particles had a simple prescribed depth profile comprising deep overwintering, spring ascent, a shallow-water phase followed by descent to overwintering depth. Export routes for C. finmarchicus from the model area were identified to the south of Greenland and to the north of the Lofoten Basin. Self-sustaining overwintering areas were identified by observing how closely particles returned to their origins after one calendar year. Several such areas were found, notably in the Norway and Lofoten Basins, and in the Færoe–Shetland Channel. The particle tracking was run for up to 10 years to demonstrate persistence of these cycles. Known features of the winter and summer distributions of C. finmarchicus were reproduced by the model. The success of the HAMSOM in simulating both the shallow and deep circulation of the eastern North Atlantic and Norwegian Sea was critical to the identification of these spatio-temporal cycles of C. finmarchicus.     

A model of the spring migration into the North Sea by Calanus finmarchicus overwintering off the Scottish continental shelf

A particle tracking model was used to investigate the annual spring invasion of the North Sea by Calanus finmarchicus copepodites which overwinter in deep water off the Scottish continental shelf. Flow fields generated by a hydrodynamic model (HDM) were used to simulate the advection of zero drag particles representing the copepods. Particles were released simultaneously from a regular lattice of start positions at a given depth ( D ₁), and ascended at a fixed rate ( dD/dt ) until they reached a final depth ( D ₂) in the surface layers. The proportion of particles reaching target areas in the northern North Sea was relatively insensitive to a moderate degree of variation (±20%) around chosen default values of the vertical migration parameters ( D ₁, D ₂ and dD/dt ), derived from field data. The inclusion of horizontal diffusion velocities and diel vertical migration in surface layers did not significantly affect the results. Sensitivity to wind direction was investigated by applying flow fields from HDM runs with different wind forcing scenarios. For the default vertical migration parameters, only north-westerly winds resulted in particles entering the North Sea from release locations north of the Iceland–Scotland Ridge, where dense aggregations of overwintering copepods were encountered during field surveys. The particle tracking model predicted that the major routes for the spring Calanus invasion into the North Sea were the East of Shetland Atlantic Inflow and the Norwegian Trench Atlantic Inflow, which agreed with seasonal trends observed in Continuous Plankton Recorder data. Overall, despite its relative simplicity, particle tracking was confirmed as a robust tool to explore the causal mechanisms behind the annual invasion of the North Sea by C. finmarchicus emerging from diapause in the deep waters off the Scottish continental shelf.     

Circulation of the eastern North Atlantic and north-west European continental shelf – a hydrodynamic modelling study

As part of the interdisciplinary investigation of Calanus finmarchicus migrations between oceanic and shelf areas off north-west Europe (the ICOS project), a three-dimensional, prognostic baroclinic circulation model system, consisting of a coarse and a nested fine scale model, was constructed. The aim was to simulate the flow fields around Iceland, the Faroe Islands and the north-west European continental shelf and slope, including the northern North Sea. The coarse scale model covered the northern North Atlantic and provided the far field flow to the finer scale model. The latter was initialized and forced with climatological seasonal (winter and spring) means of temperature, salinity, river run-off, and other parameters necessary for the calculation of heat fluxes. Seasonal average simulations of flow and hydrographic conditions were performed, together with simulations of specific scenarios of uniform wind forcing from different directions. A feature common to all the results was a topographically guided northwards flow along the continental shelf edge, with a southwards counter-flow in the vicinity of the Faroe–Shetland Channel. The extent and strength of the shelf edge current, of its counter flow, and of the re-circulation cell in the northern North Sea, were dependent on the prevailing wind direction. The modelling results were consistent with the accepted circulation patterns in the area, and a comparison with field measurements in the Faroe–Shetland Channel showed that the main characteristics of the flow were well replicated, although the estimated strength of the slope current was somewhat less than in the field observations.     

Variations in spawning ground area and egg density of the Japanese sardine in Pacific coastal and oceanic waters

The spawning ground of the Japanese sardine, Sardinops melanostictus (Schlegel), was distributed over the oceanic waters as well as the coastal waters along the Pacific coasts of western and eastern Japan during 1978–1992. The area of the spawning ground in the coastal waters on the continental shelf has ranged from 95 000 km² in 1992 to 143 000 km² in 1988, constituting 44–77% of the total area of the spawning ground. The area of the coastal spawning ground was relatively constant in spite of the large fluctuations in egg abundance, i.e. size of the spawning population, from 88 trillion (1987) to 668 trillion (1989) in the waters. Spawning adults seemed to extend over the coastal waters irrespective of the size of the spawning population. In contrast to the coastal waters, the spawning area in the oceanic waters offshore of the continental shelf increased from 31 000 km² in 1978 to 183 000 km² in 1988 and then shrank to 83 000 km² in 1992, as a function of the spawning population size. The egg distribution density in the coastal waters stayed less than 6000 m⁻² mo⁻¹, but it reached as high as 27 400 m⁻² mo⁻¹ in the expanded spawning ground in the oceanic waters. The oceanic waters seemed to function as a reserve spawning ground for the sardine in years of extremely high spawning population.     

Creation of artificial upwelling areas for brown trout, Salmo trutta, spawning in still water bodies

Abstract  Brown trout, Salmo trutta L., spawning sites were constructed by creating areas of artificial upwelling water, 252 ± 37 mL m⁻² min⁻¹ (95% CL), through appropriately sized spawning gravel substrate in 3 m² vessels buried in the bottom of a 150-m² pond. Natural spawning occurred in the vessels during autumn 2001–2004, with hatching and alevin swim up the following spring. In areas of upwelling, egg survival was 85–95%, while no live eggs were observed in areas without upwelling. In areas with upwelling, the maximum density of live eggs at the eyed stage was 570–1510 eggs m⁻². In spring 2004 and 2005, the density of alevins was estimated at 322 (±187) m⁻² and 567 (±217) m⁻², respectively, in areas with upwelling water, compared with 35.2 ± 25.4 m⁻² in areas without upwelling water in 2004.     

Ecological variations and El Niño effects off the southern coast of the Korean Peninsula during the last three decades

To explore ecosystem response to environmental changes, we investigated interannual and decadal changes in the marine ecosystem using data collected from the southern coast of the Korean Peninsula during the 1960s–1990s. Water properties such as sea surface temperature (SST) and salinity in April did not show large variation during the 1970s, but a relatively cool water mass appeared from the early to mid 1980s and a warm water mass replaced it after 1989. Chl a concentration in April, which was converted from Secchi disc information, was low (mean=1.18 mg m⁻³) during the period 1968–1980 excluding 1974, but there was a period of greater biomass for 7 years from 1981 (mean=2.78 mg m⁻³). A negative correlation ( r =−0.387, P  < 0.05) between SST and chl a in April matched high chl a during the low-SST period in the early and mid 1980s. Annual zooplankton biomass ranged from 37 to 132 mg m⁻³, but abundance was frequently high after 1984 and low during 1972–1982. The most representative pelagic fishes in the South Sea seemed to respond to the increase in planktonic organisms. Anchovy, mackerel and sardine increased in abundance since the mid 1970s. High catches of anchovy and mackerel lasted until the 1990s, whereas sardine decreased after the early 1990s. From the matrix of simple correlation coefficient, the Southern Oscillation Index (SOI) exhibited a high correlation with SST in December in the study area ( P  < 0.05). Catches of anchovy and mackerel were positively correlated with chl a and zooplankton during their early life periods, as well as with SST in December.     

Cage culture of the Pacific white shrimp Litopenaeus vannamei (Boone, 1931) at different stocking densities in a shallow eutrophic lake

Postlarvae of Litopenaeus vannamei were acclimated and stocked in lake-based cages at the following stocking densities: 10, 20, 30 and 40 shrimp m⁻². Another set of shrimp was stocked in concrete tanks as reference samples at 30 shrimp m⁻². Significant differences were observed among stocking densities throughout the 95-day culture. The final weight at harvest decreased with increasing stocking density: mean weights of 23.3, 15.8, 13.0, 10.9 and 14.6 g for the 10, 20, 30, 40 shrimp m⁻² and reference tanks were observed respectively. There were no significant differences in survival throughout the culture period, ranging between 69% and 77%. Daily growth rates (range: 0.11–0.24 g day⁻¹) and specific growth rates (range: 3.54–4.34%) also differed significantly among stocking densities, both increasing with decreasing stocking density. The feed conversion ratio in the cages did not differ among the stocking densities, ranging from 1.53 to 1.65. The relationship between stocking density and mean individual weight at harvest followed the equation y =81.06 x ^−0.54 ( R ²=0.938) and that of stocking density and production (in g m⁻²) is y =58.01 x ^−0.46 ( R ²=0.834).     

Lipids, buoyancy and the seasonal vertical migration of Calanus finmarchicus

The copepod Calanus finmarchicus remains in diapause for up to 5 months in the cold (<0.5°C) deep (>700 m) waters of the Faroe–Shetland Channel of the north-western approaches to the North Sea. While in diapause, C. finmarchicus has a high lipid content, up to 76% of dry weight, mostly in the form of wax esters. The question we address here is how copepods with such a high content of buoyant lipids can remain in diapause at depth for an extended period of time? The corollary to this is how this lipid content hinders and/or assists the copepods in their seasonal vertical migration? Part of the answer is due to the physical properties of wax esters. These have a thermal expansion and compressibility higher than that of sea water. Thus, depending on their relative composition (i.e. wax esters/water/protein/chitin), a copepod that is positively buoyant in warm surface waters can become neutrally buoyant in cold deep water. We develop a simple three component physical model of a copepod to explore how and where they attain neutral buoyancy, how the lipid content can aid in their ascent, and what fraction of the lipids can be utilized in ascent in gonad/egg formation while maintaining observed ascent rates. As well as being an energy reserve, the results show that rather than being a barrier to vertical migration, wax esters serve as an important regulator of buoyancy.     

Production and predation rates in a cannibalistic Arctic char (Salvelinus alpinus L.) population

Abstract – Production of cannibalistic Arctic char ( Salvelinus alpinus (L.)) and their prey conspecifics were estimated in an allopatric population, situated in an alpine ultraoligotrophic lake in central Norway. Estimates of population abundance were obtained by mark recapture and successive removal experiments. Assuming stable age distribution, we estimated mean yearly biomass (±95% CL) of char in sampled size groups vulnerable to predation (60 to 150 mm) to 7.93 (5.11–14.30) kg · ha⁻¹. Similarly, mean yearly biomass (±95% CL) of cannibalistic char (> 250 mm) was estimated to 0.62 (0.50–1.06) kg · ha⁻¹. Annual production (±95% CL) of char in length groups 60 to 150 mm was estimated to be 4.31 (2.74–8.03) kg · ha⁻¹ · year⁻¹, and production (±95% CL) of cannibalistic char to 0.19 (0.15–0.33) kg · ha⁻¹ · year⁻¹. Depending on the food conversion factor (set to vary from 0.1 to 0.4), the cannibalistic char removed from 10% to 40% of the production of char in sampled size-classes vulnerable to predation yearly. The overall ecological efficiency in energy transfer between the prey and predator population was 4.4%.     

The zooplankton biomass in the Sea of Japan

We examined seasonal, annual variation and horizontal distribution of zooplankton in the Sea of Japan from 1966 to 1990. Zooplankton was most abundant in the spring. The spring maximum appeared in February–March and in April–May in the southern and eastern parts of the study areas, respectively. In the summer and autumn, a secondary peak was most conspicuous in the eastern part. The difference between the estimated biomass at night and day was large in the spring and small in summer and autumn. The biomass in the offshore southern area peaked about every 3 years between 1966 and 1983, and increased abruptly in 1990. The density in the area north of 39°N or 40°N was high. Total biomass estimated in the upper 150 m layer in the Sea of Japan (10⁶ km²) was 9.5 × 10⁶ t in the daytime and 16.6 × 10⁶ t at night.     

The spring/early summer distribution of Calanus spp. in the northern North Sea and adjacent areas

The spring/early summer distribution of Calanus spp. in the north-eastern Atlantic and the northern North Sea was studied from data collected on three cruises between 1992 and 1995, and compared against estimates from Continuous Plankton Recorder (CPR) data obtained between 1958 and 1993. The patterns observed in both data sets were examined in relation to the hypothesis proposed by Backhaus et al . (1994 ) to explain the annual spring invasion of the northern North Sea by Calanus finmarchicus . The results confirmed that arrival of Calanus spp. in the northern North Sea started in March. The main routes of transport into the area were the Norwegian Trench Atlantic Inflow and East Shetland Atlantic Inflow. The highest abundances of Calanus spp. juveniles (stages CI–IV) were found in May in both CPR and cruise data. Although significant differences in absolute abundance of Calanus spp. were found between CPR and cruise data, similar geographical and temporal patterns of spring advection into the North Sea were derived from both data sources, these patterns being consistent with the hypothesized transport mechanisms.     

Physiological responses of juvenile wedge sole Dicologoglossa cuneata (Moreau) to high stocking density

Physiological responses to a high stocking density were tested in juvenile wedge sole Dicologoglossa cuneata (Moreau). Fish were kept at low (1 kg m⁻²), medium (3 kg m⁻²) and high (9 kg m⁻²) stocking densities for 22 days. No differences in the weight, length, survival and hepatosomatic index were observed among treatments. Basal plasma cortisol and osmolality were found to be directly and positively related to stocking density. A mild increase in plasma glucose was seen at medium density, and plasma protein was elevated at medium and high densities. The liver glucose and glycogen content was inversely related to stocking density. The liver triglyceride level was significantly elevated at the highest density, and the α-amino acid content decreased at the highest density. In muscle, glucose levels were significantly higher in fish kept at the lowest density; the α-amino acid content was elevated in fish kept at high density. In conclusion, plasma cortisol levels indicated an increasing stress level depending on the culture density, but significant changes in energy reserves did not occur in tissue (mainly liver and muscle glycogen and glucose reserves were significantly affected).     

Effect of dietary protein levels on growth of Penaeus stylirostris juveniles

The shrimp Penaeus stylirostris is currently produced on a commercial scale in Tahiti and New Caledonia. Both super-intensive (80 animals m⁻²) and semi intensive (25 m⁻²) systems are promoted. Locally produced commercial feed contains 380–400 g kg⁻¹ crude protein (CP) without special consideration for environmental impact. The need for a 'low pollution' diet implies reconsideration of the optimum dietary protein level for this species. Under experimental conditions, six isoenergetic practical diets ranging from 270 g kg⁻¹ to 440 g kg⁻¹ CP were formulated and fed to satiation for 30 days to juvenile P. stylirostris ; average growth rates were between 5.5 and 7.5 g per month with survival rates > 90%.
The lowest protein levels 270–310 g kg⁻¹, gave significantly ( P < 0.05) poorer growth (5.5 g per 30 days) than was observed with 330–430 g kg⁻¹ CP; 330 g kg⁻¹ CP may be recommended, and as it is lower than levels in diets used currently, there is a possibility of reducing nitrogenous waste. In addition to growth response, protein efficiency ratio, protein productive value and food conversion (feed/gain) all supported a recommended dietary protein level of 330 g kg⁻¹. Future prospects for practical feeds with even lower CP levels are considered.     

Temporal changes in European eel, Anguilla anguilla, stocks in a small catchment after installation of fish passes

Abstract  Changes in the abundance of European eel, Anguilla anguilla L., in the River Frémur, France, were examined over an 8-year period. Natural connectivity of the river was disturbed by three high dams that inhibited eel upstream migration and reduced recruitment by elvers and yellow eels. After eel passes were installed, fish became more abundant upstream (mean density 0.5 eel m⁻²). Moreover, except in the more upstream areas, no decline in eel numbers and biomass was found, in contrast to the general decline of eel throughout its distribution range. It was concluded that eel passes are important to conserve and/or to recover eel stocks.     

Growth and mortality of common octopus Octopus vulgaris reared at different stocking densities in Mediterranean offshore cages

Four ongrowing cycles of common octopus ( Octopus vulgaris ), each with two stocking densities, were carried out in an offshore area in order to evaluate the effect of the following variables on growth ( G , % body weight day⁻¹) and mortality ( M , % day⁻¹): weight ( W , 0.5–3.0 kg), temperature ( T , 14–26 °C), stocking density (SD, 6–46 kg m⁻³), size dispersion, expressed as coefficient of variation (CV, 13–42%) and significant wave height (SWH: 0.4–1.2 m). The assays were performed in an 8 m³ stainless-steel floating cage divided into two compartments of 4 m³ each. For the range of temperatures considered (14–26 °C), G depended significantly on T and SWH, with maximum G values being obtained at 18.5 °C and with an important negative effect of SWH. M depended significantly on T, W and CV, mortality being minimal at 18 °C. The two other variables had an antagonistic effect, mortality increasing with greater size dispersion – suggesting that animals should be graded throughout the process – although the effect diminished as the sizes increased. The results point towards two alternatives for the commercial ongrowing of octopus under the particular conditions of the present study: (A) two ongrowing cycles of 3.5 months to reach a final weight of 2.5 kg or (B) one 5-month cycle to reach a final weight of 3.5 kg.     

Dietary methionine requirement of juvenile Arctic charr Salvelinus alpinus (L.)

The dietary methionine requirement of juvenile Arctic charr Salvelinus alpinus (L.) was assessed by feeding diets supplemented with graded levels of DL-methionine (9, 12, 15, 18, 21, and 24 g kg⁻¹dietary protein) for 16 weeks at 12°C. All diets contained 400 g kg⁻¹ protein, 170 g kg⁻¹ lipid, 66 g kg⁻¹ ash and an estimated 17.5 MJ digestible energy (DE) kg⁻¹. When live-weight gain was examined using quadratic regression, the estimate of methionine requirement for optimal growth was 17.6 g kg⁻¹ of dietary protein (DP) or 7 g kg⁻¹ of the diet. Requirements estimated on the basis of carcass protein and energy gains were 18.8 and 17.9 g kg⁻¹ DP, respectively. Plasma methionine concentrations and ocular focal length variability measurements did not provide a sensitive measure of requirement, because each responded in a linear fashion to increasing dietary methionine levels. Based on the prevalence of cataracts, the methionine level required to prevent lens pathology (26.7 g kg⁻¹ DP) appears to be higher than that required for maximum growth.     

Feasibility of pacific white shrimp Litopenaeus vannamei culture in southern Brazil: effects of stocking density and a single or a double CROP management strategy in earthen ponds

Marine shrimp culture at southern Brazil is restricted to the warmer season (November to April). Therefore, farmers must consider culture strategies and competition with shrimp landings from artisanal fishery. The fishing season starts every 1 February; in order to obtain higher prices, farmers may consider shrimp culture in two crops, with a first harvest before the start of the fishing season, and a second harvest after the end of the landings. The present study evaluated the performance and feasibility of Litopenaeus vannamei reared at 10, 25 and 40 shrimp m⁻² either in two short consecutive culture cycles or one longer cycle (LC). The experimental design consisted of two sets of nine pens installed in a 3.8 ha earthen pond. In one set of pens, shrimp were harvested after 75 days and pens were restocked for another 75-day-long rearing period. In the second set of pens, shrimp were cultured for 150 days. Shrimp survival and final weight ranged from 79% to 91% and 6.67 to 14.53 g respectively. Feasibility analysis pointed towards culture at higher densities (25 and 40 shrimp m⁻²) in a LC as productivity (2274–4227 kg ha⁻¹) and shrimp final weight (13.05–13.21 g) resulted in higher profitability (US$7761–12 379).