Latitudinally distinct stocks of Atlantic cod face fundamentally different biophysical challenges under on-going climate change

The reproductive success of marine ectotherms is especially vulnerable in warming oceans due to alterations in adult physiology, as well as embryonic and larval survival prospects. These vital responses may, however, differ considerably across the species' geographical distribution. Here we investigated the life history, focusing on reproductive ecology, of three spatially distant populations (stocks) of Atlantic cod (Gadus morhua, Gadidae) (50–80° N), in the Irish/Celtic Seas-English Channel Complex, North and Barents Seas, under past and projected climate. First, experimental tracking of spawning behaviour evidenced that the ovulation cycle is highly distressed at ≥9.6 (±0.25)°C (Tup). This knife-edge threshold resulted in erratic spawning frequencies, whereas vitellogenin sequestration remained unaffected, indicating endocrine rather than aerobic scope constraints. Cod in the Celtic Sea-English Channel are, therefore, expected to show critical stock depensation over the next decades as spawning grounds warm above Tup, with Irish Sea cod subsequently at risk. Second, in the relatively cooler North Sea, the northward retraction of Calanus finmarchicus (Calanidae) and Para-Pseudocalanus spp. (Clausocalanidae) (1958–2017) limit cod larvae feeding opportunities, particularly in the southernmost subarea. However, the contrasting increase in Calanus helgolandicus (Calanidae) does not counteract this negative effect, likely because cod larvae hatch ahead of its abundance peaks. Overfishing again comes as a twin effect. Third, in the still relatively cold Barents Sea, the sustainably harvested cod benefit from improved food conditions in the recent ice-free polar region but at the energetic cost of lengthier and faster spawning migrations. Consequently, under climate change local stocks are stressed by different mechanistic factors of varying management severity

Oceanography: Freshened from the south

International audienc

Toward Improved Estimation of the Dynamic Topography and Ocean Circulation in the High Latitude and Arctic Ocean: The Importance of GOCE

The Norwegian Sea circulation plays a key role in maintaining the mild climate of the northwestern Europe via the transport of warm Atlantic Water pole-ward. The first paper addresses the advective currents connecting the two branches of the Norwegian Atlantic Current and shows the general spin up of the Norwegian Sea circulation during winter with the exception of the flow over the Mohn Ridge. The variability in the surface velocities in the Norwegian Sea is found to be deep reaching, which supports the use of altimetry to monitor the variability of the poleward transport of Atlantic Water. A strengthening and weakening of the Atlantic inflow east of the Faroe Islands has a consistent response along the entire slope current. However, a stronger western inflow, observed north of the Faroe Islands, is associated with more flow of Atlantic Water into the slope current. This finding suggest that a substantial fraction of Atlantic Water that eventually enters the Barents Sea or the Arctic through the Fram Strait, may originate from the western inflowing branch of Atlantic Water to the Nordic Seas, and that the two branches of northward flowing Atlantic Water cannot be considered as separate flows. Paper 2 examines the influence of the surface circulation, eddy activity and local heat loss on the spatial distribution and temporal evolution of dense water formation in the Lofoten Basin. Evidence of intrusion of Atlantic Water into the central Lofoten Basin due to buoyant waters in the eastern part of the basin is found. With the support of hydrographic and satellite datasets, the concept of separate western and eastern regions of the Lofoten Basin is introduced and a link between the western Lofoten Basin and Faroe Shetland overflow waters is identified. Paper 3 addresses an anomalous anticyclonic vortex in the Nordic Seas, which is situated in the western Lofoten Basin. The vortex’ surface and vertical characteristics on seasonal, inter-annual, and climatological time-scales are quantified, relevant forcing mechanisms are addressed, and its uniqueness in the Nordic Seas is documented. In the final paper, a new mean dynamic topography (MDT) is estimated for the North Atlantic and the Arctic from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite gravity anomaly data. The new GOCE-based MDT is assessed and compared to independent steric height observations, other state-of-the-art MDTs and three coupled sea-ice-ocean models, showing its usefulness in studies of high latitude ocean circulation