Garry B. Stenson

Bottom-Up Regulation of Capelin, a Keystone Forage Species

The Northwest Atlantic marine ecosystem off Newfoundland and Labrador, Canada, has been commercially exploited for centuries. Although periodic declines in various important commercial fish stocks have been observed in this ecosystem, the most drastic changes took place in the early 1990s when the ecosystem structure changed abruptly and has not returned to its previous configuration. In the Northwest Atlantic, food web dynamics are determined largely by capelin (Mallotus villosus), the focal forage species which links primary and secondary producers with the higher trophic levels. Notwithstanding the importance of capelin, the factors that influence its population dynamics have remained elusive. We found that a regime shift and ocean climate, acting via food availability, have discernible impacts on the regulation of this population. Capelin biomass and timing of spawning were well explained by a regime shift and seasonal sea ice dynamics, a key determinant of the pelagic spring bloom. Our findings are important for the development of ecosystem approaches to fisheries management and raise questions on the potential impacts of climate change on the structure and productivity of this marine ecosystem.

Temperature signature of high latitude Atlantic boundary currents revealed by marine mammal-borne sensor and Argo data

Results from the development and analysis of a novel temperature dataset for the high latitude North-West Atlantic are presented. The new 1 degrees gridded dataset (ATLAS) has been produced from about 13,000 Argo and 48,000 marine mammal (hooded seal, harp seal, grey seal and beluga) profiles spanning 2004-8. These data sources are highly complementary as marine mammals greatly enhance shelf region coverage where Argo floats are absent. ATLAS reveals distinctive boundary current related temperature minima in the Labrador Sea (-1.1 degrees C) and at the east Greenland coast (1.8 degrees C), largely absent in the widely-used Levitus09 and EN3v2a datasets. The ATLAS 0-500 m average temperature is lower than Levitus09 and EN3v2a by up to 3 degrees C locally. Differences are strongest from 0-300 m and persist at reduced amplitude from 300-500 m. Our results clearly reveal the value of marine mammal-borne sensors for a reliable description of the North-West Atlantic at a time of rapid change.

Seasonal variability of the warm Atlantic Water layer in the vicinity of the Greenland shelf break

The warmest water reaching the east and west coast of Greenland is found between 200?m and 600?m. Whilst important for melting Greenlands outlet glaciers, limited winter observations of this layer prohibit determination of its seasonality. To address this, temperature data from Argo profiling floats, a range of sources within the World Ocean Database and unprecedented coverage from marine-mammal borne sensors have been analysed for the period 2002-2011. A significant seasonal range in temperature (~1-2?°C) is found in the warm layer, in contrast to most of the surrounding ocean. The phase of the seasonal cycle exhibits considerable spatial variability, with the warmest water found near the eastern and southwestern shelf-break towards the end of the calendar year. High-resolution ocean model trajectory analysis suggest the timing of the arrival of the years warmest water is a function of advection time from the subduction site in the Irminger Basin.

Controlling thoracic pressures in cetaceans during a breath-hold dive: importance of the diaphragm

ABSTRACT Internal pressures change throughout a cetaceans body during swimming or diving, and uneven pressures between the thoracic and abdominal compartments can affect the cardiovascular system. Pressure differentials could arise from ventral compression on each fluke downstroke or by a faster equilibration of the abdominal compartment with changing ambient ocean pressures compared with the thoracic compartment. If significant pressure differentials do develop, we would expect the morphology of the diaphragm to adapt to its in vivo loading. Here, we tested the hypothesis that significant pressure differentials develop between the thoracic and abdominal cavities in diving cetaceans by examining diaphragms from several cetacean and pinniped species. We found that: (1) regions of cetacean diaphragms possess subserosal collagen fibres that would stabilize the diaphragm against craniocaudal stretch; (2) subserosal collagen covers 5–60% of the thoracic diaphragm surface, and area correlates strongly with published values for swimming speed of each cetacean species (P<0.001); and (3) pinnipeds, which do not locomote by vertical fluking, do not possess this subserosal collagen. These results strongly suggest that this collagen is associated with loads experienced during a dive, and they support the hypothesis that diving cetaceans experience periods during which abdominal pressures significantly exceed thoracic pressures. Our results are consistent with the generation of pressure differentials by fluking and by different compartmental equilibration rates. Pressure differentials during diving would affect venous and arterial perfusion and alter transmural pressures in abdominal arteries. Highlighted Article: Stiffening the cetacean diaphragm may stabilize thoracic pressures during a breath-hold dive but it could also cause pressure problems for some of the arteries.

The caval sphincter in cetaceans and its predicted role in controlling venous flow during a dive

ABSTRACT A sphincter on the inferior vena cava can protect the heart of a diving mammal from overload when elevated abdominal pressures increase venous return, yet sphincters are reported incompetent or absent in some cetacean species. We previously hypothesized that abdominal pressures are elevated and pulsatile in fluking cetaceans, and that collagen is deposited on the diaphragm according to pressure levels to resist deformation. Here, we tested the hypothesis that cetaceans generating high abdominal pressures need a more robust sphincter than those generating low pressures. We examined diaphragm morphology in seven cetacean and five pinniped species. All odontocetes had morphologically similar sphincters despite large differences in collagen content, and mysticetes had muscle that could modulate caval flow. These findings do not support the hypothesis that sphincter structure correlates with abdominal pressures. To understand why a sphincter is needed, we simulated the impact of oscillating abdominal pressures on caval flow. Under low abdominal pressures, simulated flow oscillated with each downstroke. Under elevated pressures, a vascular waterfall formed, greatly smoothing flow. We hypothesize that cetaceans maintain high abdominal pressures to moderate venous return and protect the heart while fluking, and use their sphincters only during low-fluking periods when abdominal pressures are low. We suggest that pinnipeds, which do not fluke, maintain low abdominal pressures. Simulations also showed that retrograde oscillations could be transmitted upstream from the cetacean abdomen and into the extradural veins, with potentially adverse repercussions for the cerebral circulation. We propose that locomotion-generated pressures have influenced multiple aspects of the cetacean vascular system. Summary: Locomotion may generate oscillations in a cetaceans venous system. Instead of using their caval sphincter to protect their heart from associated flow spurts, cetaceans could allow partial collapse of abdominal veins to smooth flow from the inferior vena cava.

Attendance and nursing patterns of harp seals in the harsh environment of the northwest Atlantic

Harp seals are capital breeders who give birth to single pups and nurse them with fat-rich milk for 12xa0days on pack ice in the North Atlantic under harsh environmental conditions. Our objective was to examine female attendance and nursing patterns under varying environmental conditions to determine whether these patterns change in response to changing weather conditions. The behaviour of 158 harp seal females and pups off north-eastern Newfoundland was recorded every 3xa0min during daylight hours. Air and water temperature, and wind speed were recorded at the beginning of each observation session. We constructed GAMM models to examine the importance of these variables in predicting attendance and nursing patterns. The best model for predicting attendance included time of day, air temperature, wind speed, and the interaction between wind and air temperature. The best model for predicting nursing included wind speed, air temperature and time of day. Females were more likely to attend their pups during the afternoon when solar radiation appeared to be high, but reduced attendance during high winds and/or low temperatures. The likelihood of attending females nursing during these poor weather conditions was greater than when conditions were better. Thus, females were less likely to be present when weather conditions were poor but when present, they were more likely to be provisioning their pups. This strategy may help these females defray the thermoregulatory demands on their limited resources while ensuring that their young attain weights that are likely to increase post-weaning survival and hence maternal fitness.