The offshore marine ecosystem of the Canadian Beaufort Sea faces the double pressure of climate change and industrialization. Polar cod (
Boreogadus saida
) is a pivotal forage species in this ...ecosystem, accounting for 95 % of the pelagic fish assemblage. Its vertical distribution over the annual cycle remains poorly documented. Hydroacoustic records from 2006 to 2012 were analysed to test the hypothesis that age-0 polar cod segregate vertically from larger congeners. Trawls and ichthyoplankton nets validated the acoustic signal. Fish length, weight, and biomass were estimated from new regressions of target strength and weight on standard length. Polar cod were vertically segregated by size in all months, with small age-0 juveniles in the epipelagic (<100 m) layer and larger age-1+ deeper in the water column. From December to March, the biomass of age-1+ peaked in a mesopelagic layer between 200 and 400 m. With increasing irradiance from April to July, the mesopelagic layer deepened and extended to 600 m. Starting in July, age-0 polar cod formed an epipelagic scattering layer that persisted until November. From September onward, age-0 left the epipelagic layer to join small age-1+ in the upper mesopelagic layer. Low biomass in the mesopelagic layer from February to September likely resulted from large polar cod settling on the seafloor to avoid diving marine mammals. Longer ice-free seasons, warmer sea-surface temperatures, or an oil spill at the surface would likely impact epipelagic age-0, while mesopelagic age-1+ would be vulnerable to an eventual oil plume spreading over and above the seafloor.
Euphausiids are a keystone species in coastal food webs due to their high lipid content and seasonally high biomass. Understanding the habitat and environmental drivers that lead to areas of high ...biomass, or 'hotspots', and their seasonal persistence, will support the identification of important foraging regions for mid- and upper- trophic level predators. We quantify the distribution of hotspots of the two dominant species of euphausiid in the north-east Pacific Ocean: Euphausia pacifica and Thysanoessa spinifera, as well as euphausiid larvae (mixed species). The Canadian coast encompasses the northern California Current Ecosystem and the transition zone to the Alaska current, and is a highly productive region for fisheries, marine mammals, and seabirds. We used spatiotemporal modelling to predict the distribution of these three euphausiid groups in relation to geomorphic and environmental variables during the important spring-summer months (April through September) when euphausiid biomass is highest. We quantified the area, intensity, and persistence of biomass hotspots across months according to specific oceanographic ecosections developed for marine spatial planning purposes. Persistent hotspots of both adult species were predicted to occur along the 200 m depth contour of the continental slope; however, differences were predicted on the shallower Dixon shelf, which was a key area for T. spinifera, and within the Juan de Fuca Eddy system where E. pacifica hotspots occurred. The continental slope along the west coast of Vancouver Island was the only persistent hotspot region common between both adult species and euphausiid larvae. Larval distribution was more correlated with T. spinifera than E. pacifica biomass. Hotspots of adults were more persistent across months than hotspots of euphausiid larvae, which were seasonally patchy. The persistence of biomass hotspots of forage species through periods of low overall biomass could maintain trophic connectivity through perturbation events and increase ecosystem resilience to climate change.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Subsurface foraging is an important proportion of the activity budget of rorqual whales, yet information on their behaviour underwater remains challenging to obtain. Rorquals are assumed to feed ...throughout the water column and to select prey as a function of depth, availability and density, but there remain limitations in the precise identification of targeted prey. Current data on rorqual foraging in western Canadian waters have thus been limited to observations of prey species amenable to surface feeding, such as euphausiids and Pacific herring (Clupea pallasii), with no information on deeper alternative prey sources. We measured the foraging behaviour of a humpback whale (Megaptera novaeangliae) in Juan de Fuca Strait, British Columbia, using three complimentary methods: whale-borne tag data, acoustic prey mapping, and fecal sub-sampling. Acoustically detected prey layers were near the seafloor and consistent with dense schools of walleye pollock (Gadus chalcogrammus) distributed above more diffuse aggregations of pollock. Analysis of a fecal sample from the tagged whale confirmed that it had been feeding on pollock. Integrating the dive profile with the prey data revealed that the whale's foraging effort followed the general pattern of areal prey density, wherein the whale had a higher lunge-feeding rate at the highest prey abundance and stopped feeding when prey became limited. Our findings of a humpback whale feeding on seasonally energy-dense fish like walleye pollock, which are potentially abundant in British Columbia, suggests that pollock may be an important prey source for this rapidly growing whale population. This result is informative when assessing regional fishing activities for semi-pelagic species as well as the whales' vulnerability to fishing gear entanglements and feeding disturbances during a narrow window of prey acquisition.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Previous work found that an earlier ice breakup favors the recruitment of juvenile polar cod (
Boreogadus saida
) by enabling early hatchers to survive and reach a large size by late summer thanks to ...a long growth season. We tested the hypothesis that, in addition to a long growth season, an earlier ice breakup provides superior feeding conditions for young polar cod by enhancing microalgal and zooplankton production over the summer months. Ice cover and surface chlorophyll
a
were derived from satellite observations, and zooplankton and juvenile cod biomass were estimated by hydroacoustics in ten regions of the Canadian Arctic over a period of 11 years. Earlier breakups resulted in earlier phytoplankton blooms. Zooplankton backscatter in August increased with earlier breakup and bloom, and plateaued at chlorophyll
a
> 1 mg m
−3
. Juvenile cod biomass in August increased with an earlier breakup, and plateaued at a zooplankton backscatter > 5 m
2
nmi
−2
, supporting the hypothesis that higher food availability promotes the growth and survival of age-0 fish in years of early ice melt. However, there was little evidence that late summer biomass of either zooplankton or age-0 polar cod benefitted from ice breakup occurring prior to June. On average, zooplankton standing stock was similar in the Southern Beaufort Sea and the North Water-Lancaster Sound polynya complex, but juvenile cod biomass was higher in the Beaufort Sea. Intense avian predation could explain the lower biomass of juvenile cod in the polynya complex, confirming its reputation as a biological hotspot for energy transfer to higher trophic levels.
Arctic cod (Boreogadus saida) is the dominant pelagic fish in Arctic seas and a staple food of many arctic predators including several seabird species. Marginal ice zones are known as important ...feeding locations for seabirds. The hypothesis that thick-billed murre (Uria lomvia), northern fulmar (Fulmarus glacialis) and black-legged kittiwake (Rissa tridactyla) congregate in areas of high Arctic cod food resource and low ice concentration was tested at different spatial scales. Arctic cod biomass was estimated by hydroacoustics as a resource proxy, and seabirds were counted and sampled for stomach analysis along eight longitudinal transects across the marginal ice zone in southern Baffin Bay in June–July 2016. With increasing length, the epipelagic age-0 Arctic cod migrated from open waters to ice-covered areas. Subsequently, age-1 and age-2 Arctic cod tended to concentrate in a subsurface layer (40–100 m) within the epipelagic layer. Arctic cod 5.7–16.1 cm long (late age-0 to age-5) were the main fish prey of the three seabird species, which preferentially captured age-1 cod (6–11.5 cm). At large spatial scale (western versus eastern Baffin Bay), thick-billed murre, northern fulmar and their Arctic cod resource proxy were generally more abundant on the western ice-covered side of Baffin Bay. No clear spatial match was found, however, when comparing seabird abundances and their food-resource proxy in different ice concentrations across the marginal ice zone or at small scale (5 km). At medium scale (12.5 km), only murre density was influenced positively by its Arctic cod resource. A lack of schooling behavior and a successful strategy to avoid predation by hiding under the ice could explain the absence of any strong spatial match between Arctic cod and its seabird predators at these different scales.
Antarctic silverfish (
Pleuragramma antarcticum) is a key link between plankton and the community of top predators in the shelf waters of the Ross Sea. In spite of their abundance and important role ...in Antarctic food chains, very little is known of many ecological and biological aspects of this species. A combined trawl and acoustic survey of silverfish was carried out on the western Ross Sea shelf during the New Zealand International Polar Year Census of Antarctic Marine Life research voyage on R.V.
Tangaroa in February–March 2008. Multi-frequency acoustic data (12, 38, 70, and 120
kHz) allowed discrimination of silverfish marks from those of krill and other associated species. Mark identification was achieved using targeted midwater trawls. Additional midwater and demersal trawls were carried out at randomly selected locations over the shelf as part of the core biodiversity survey.
Silverfish were widely distributed over the Ross Sea shelf. Adult silverfish tended to form layers at 100–400
m depth and were sometimes present close to the bottom, where they were frequently caught in demersal trawls shallower than 500
m. A weak layer at about 80
m depth was associated with juvenile silverfish of 50–80
mm standard length. Acoustic backscatter strength from both silverfish and krill marks increased with increasing frequency (i.e., was highest at 120
kHz), which is characteristic of species without an air-filled swimbladder. Acoustic target strengths (TS) for silverfish at 12, 18, 38, 70, and 120
kHz were estimated from anatomically detailed scattering models based on computed tomography (CT) scans of frozen specimens. The relationship between TS and fish length at 38
kHz was sensitive to estimates of density and sound speed contrast within the fish, especially for small specimens (less than 110
mm SL). Our best estimate of the acoustic biomass of silverfish in the study area was 592
000
t (95% confidence interval 326
000–866
000
t). However, the biomass of juvenile silverfish was highly uncertain due to large differences between TS model results.
We assess optimal deterministic nonlinear income taxation in a Mirrlees economy with a continuum of risk‐averse agents whose utilities are quasilinear in labor. A weak redistribution motive makes ...random taxes more likely socially dominated by the deterministic policy where after‐tax income lotteries are replaced with their certainty equivalents.
Changing ecosystem conditions present a challenge for the monitoring and management of living marine resources, where decisions often require lead-times of weeks to months. Consistent improvement in ...the skill of regional ocean models to predict physical ocean states at seasonal time scales provides opportunities to forecast biological responses to changing ecosystem conditions that impact fishery management practices. In this study, we used 8-month lead-time predictions of temperature at 250 m depth from the J-SCOPE regional ocean model, along with stationary habitat conditions (e.g., distance to shelf break), to forecast Pacific hake (Merluccius productus) distribution in the northern California Current Ecosystem. Using retrospective skill assessments, we found strong agreement between hake distribution forecasts and historical observations. The top performing models (based on out-of-sample skill assessments using the area-under-the-curve (AUC) skill metric) were a generalized additive model (GAM) that included shelf-break distance (i.e., distance to the 200 m isobath) (AUC = 0.813) and a boosted regression tree (BRT) that included temperature at 250 m depth and shelf-break distance (AUC = 0.830). An ensemble forecast of the top performing GAM and BRT models only improved out-of-sample forecast skill slightly (AUC = 0.838) due to strongly correlated forecast errors between models (r = 0.88). Collectively, our results demonstrate that seasonal lead-time ocean predictions have predictive skill for important ecological processes in the northern California Current Ecosystem and can be used to provide early detection of impending distribution shifts of ecologically and economically important marine species.
Commodity taxes and taste heterogeneity Gauthier, Stéphane; Henriet, Fanny
European economic review,
January 2018, 2018-01-00, 2018-01, Letnik:
101
Journal Article
Recenzirano
Odprti dostop
We study optimal linear commodity taxes in the presence of non-linear income taxes when agents differ in skills and tastes for consumption. We show that optimal commodity taxes are partly determined ...by a many-person Ramsey rule when there is taste heterogeneity within income classes. The usual role of commodity taxes in relaxing incentive constraints explains the remaining part of these taxes when there is taste heterogeneity between income classes. We quantify these two parts using French consumption microdata and find that commodities taxes are only shaped by many-person Ramsey considerations.
This paper studies fiscal competition under the origin principle. It identifies a pattern of consumers' taste heterogeneity under which the first‐best world social optimum arises as a noncooperative ...Nash equilibrium. Consumers' tastes are characterized by the strength of their preference for home and foreign goods. Nash implementation of the first‐best obtains when in every tax jurisdiction the number of consumers who display a home bias (those consumers who prefer purchasing the home good to shopping abroad at equal prices) equals, for every magnitude of the home bias, the number of consumers who display an “import bias” (those who instead prefer shopping abroad) equal in magnitude.
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