Study site
Bahia Anegada and Bahia Unión are shallow (less than 2 m during high tides) areas inside the Natural Reserve of Bahía San Blas (NRBSB), with sandbanks, small inner rivers, natural channels and small islands in the northern zone (Figure 1). The northern edge of the NRBSB is highly influenced by discharges of nutrient-rich waters from the Colorado River and the southern edge receives nutrients from the Negro River [9]. The resulting abundance of food coupled with a scarcity of large predators make these bays important spawning and nursery areas for bony [10] and cartilaginous fishes [5, 11].
Tagging task
Five individuals were tagged inside the NRBSB in November 2011 and 2012 (Figure 1). The sharks were captured with sport fishing gear using barbless hooks baited with mackerel (Scomber scombrus) or Brazilian menhaden (Brevoortia aurea). The length (total length) and sex of each individual was recorded before external application of an MK10 pop-up satellite archival transmitter (PSAT) tag [12]. The first three individuals were tagged on the right side of the base of the dorsal fin with a stainless steel harpoon with the tag secured intramuscularly by a polymer dart. The next two individuals were tagged using a plastic sheep ear ID tag to anchor the PSAT on the front side of the dorsal fin. The PSAT tags were programmed for a deployment period of either 180 days (Sharks 1 to 3) or 270 days (Sharks 4 and 5). Tags were programmed to record water temperature, depth and light level at 5-s intervals. These data were automatically binned into 6-hour histograms (depth bins: the first bin for depths shallower than 10 m, then bins at 10-m increments, 10 to 20 m and so on, and the last bin for depths greater than 200 m; temperature bins: the first bin for temperatures below 3°C, then bins at 2° increments, 3°C to 5°C and so on, then the last bin for temperatures above 27°C). The automatic release feature was to be triggered if tags recorded depth variations of ±3 m for 48 hours at a constant depth.
Data analysis
Igor Pro Version 6.2 (WaveMetrics) and the Data Analysis Program 3.0 (Wildlife Computers) were used to analyze the tracking data. Depth and temperature preference distribution histograms during day and night periods were constructed. The vertical speed of ascents and descents as well as maximum dive depths were calculated from depth profiles (Sharks 1 and 2) during the day, at night, and at sunset and sunrise. Local hour intervals were defined as: 9 to 10, 12 to 13, 15 to 16 (day) and 21 to 22, 0 to 1, 3 to 4 (night). The sunset and sunrise period at the study area were obtained from dawn and dusk data for the tracking period using the Navy Hydrographic Service. Vertical speed and dive depths were defined, assuming constant speed for straight ascent and descent movements (0° and 45°) between two consecutive points.
Ethical approval
As Galeorhinus galeus is fished and commercialized in Argentina, it is regulated by fishery acts. This work was carried out according to the code of good practice for sustainable recreational fisheries suggested by the management plan (not implemented yet) of the Natural Reserve of Bahía San Blas, in compliance with act N° 217/07 for sustainable angling fishing in the Province of Buenos Aires for coastal sharks.