Transmissions were received from 37 of the 40 PATs that were applied to blue sharks off the eastern coast of Canada between 20. The selection of the 35 m surface depth value and the 14 u C isotherm as a lower depth were based on the PAT data. Note that the sharks continue to swim horizontally towards their final locations throughout this cycle. The sharks therefore return to the 35 m level sometime during the next 5 hours with the exact timing depending on the local depth of the 14 u C isotherm iii) after reaching the 35 m depth, the shark remains there until 0600 hr at which time the vertical diving cycle is initiated again. In particular, the second shark was given the following diving behaviour, identical for each of the diving sharks: i) at 0600 the sharks start their decent at a rate of 3 cm/s thus descending at a rate of about 500 m in 4 hours ii) when the shark reaches the 14 u C isotherm, it remains at that level until 1800 hr at which time it will start its ascent at 3 cm/s. One of the sharks remained continuously at a depth of 35 m while the diving behaviour of the other was specified similar to the behaviour indicated by the PAT data. The specified diving behaviour was the only difference between the two sharks in each of the paired releases. A constant horizontal swimming speed was specified for each shark so that in the absence of any advection by ocean currents they would arrive at the observed pop-up location at the times indicated by the observations. The numerical sharks were seeded in the model in pairs at the locations where the real sharks were tagged and released. The influence of diving behaviour on 23 ‘numerical sharks’ was evaluated after insertion into the circulation model. The spectral nudging approach is used to avoid model drift and ensure a realistic mean state and eddy variability. There are a maximum of 46 levels in the vertical with thicknesses increasing from 6 m at the surface to 200 m at a depth of 1750 m and reaching the maximum value of 250 m at the bottom. The model has nominal horizontal resolution of approximately J u in longitude with latitude increments chosen to provide roughly square grid cells everywhere. To investigate this possibility, an eddy-admitting model based on the ocean component OPA (Oc ́an Parallelis ́ ) of the NEMO (Nucleus for European Modelling of the Ocean) was used. This introduces the possibility that there may be some advantage to the shark’s diving behaviour by reducing or benefiting from advection by ocean currents. Ocean observations and numerical models show clearly that the speed of ocean currents associated with the Gulf Stream decrease significantly with depth below the surface. Although the contour plots were based on discrete temperature- at-depth measurements made by the PAT, the time-weighted trajectories of individual blue sharks were based on binned time at depth measurements from the PAT, and thus do not represent the exact trajectory between time intervals. This approach allowed us to reflect in the plots all collected data and avoid artefacts or excessive smoothing caused by more sophisticated techniques of data gridding. No extrapolations were made outside of the bounding data points, and the interpolation was only performed over gaps not exceeding 100 m vertically and three days temporally. Then spatial gaps between vertical profiles were interpolated (linearly) with a mixture of observed and interpolated data. A linear interpolation method was used first to fill the missing data for each vertical profile or corresponding grid’s column where at least partial measurements were already present. Each depth-temperature observation was assigned to its closest grid point and if more than one observation was found within a 3-hr (time) by 4-m (depth) range of a certain grid point, those values were averaged before entering the grid. The grids underlying these plots were of uniform time and depth steps of 6 hours and 8 metres respectively, encompassing the entire duration and vertical range of the analysed records. Depth-temperature measurements recorded by the PAT were used to construct time-depth-temperature contour plots, over which the time-weighted diving trajectories of individual sharks were overlaid. on sea surface temperature and ambient light at depth measurements recorded by the PAT, analyzed with the state-space model ukfsst described by.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |