Summary
Bycatch, the incidental capture of non‐target organisms, occurs in most commercial fisheries. Although immediate bycatch mortality is frequently documented in fisheries, detrimental sub‐lethal effects and potential post‐release mortality remain largely unknown despite the potential population‐level consequences. Turtles are captured as bycatch and their populations are vulnerable to slight increases in adult mortality. In eastern Ontario, turtles are frequently captured as bycatch in a small‐scale freshwater commercial fyke‐net fishery and, currently, the fate of discarded turtles is unknown. We wished to determine the effect of fyke‐net capture on post‐release survival and behaviour in eastern musk turtles Sternotherus odoratus and painted turtles Chrysemys picta. We used biologgers equipped with tri‐axial acceleration, depth and temperature sensors to document locomotor activity, vertical distribution, and temperature use of entrapped (exposed to forced submergence for 4 h) and control turtles upon release. Overall dynamic body acceleration was used as a measure of post‐release activity for the first hour, first 6 h, and first 48 h. Post‐release mortality was not detected. Turtles subjected to entrapment exhibited lower activity during the first 6 h following release, and their vertical distribution and temperature use differed in the first 2 h following release, but these effects disappeared after 48 h, suggesting turtles have the ability to recover. Quantifying the post‐release mortality and sub‐lethal effects of entrapment is important for estimating the population effects associated with bycatch.
Methodology
Study area and turtle collection
All work was conducted on Lake Opinicon (44° 34′ N, 76° 19′W) c. 100 km southwest of Ottawa, Ontario, Canada between 14 May and 19 June 2012 when water temperatures ranged from 18 to 26°C (dissolved O2: 6–8 mg L−1). Turtles were captured with fyke nets (a type of passive trapping gear; details in Larocque et al ., 2012a) set in shallow bays (2–3 m depth) for c. 24 h with floats to provide air pockets. Upon capture, turtles were returned to the field laboratory where they were measured (mass and carapace length), and sexed (based on external characteristics). Individuals were held outdoors in ~700 L fiberglass tanks at ambient temperature for c. 48 h to let capture stress wane. Tanks were supplied with lake water via a flow though system. Turtles were not fed, but were provided with basking platforms exposed to ambient sunlight.
Experimental procedure
We used 32 males of each species evenly split between treatment and control groups. The treatment group consisted of turtles entrapped for 4 h in a closed fyke net set without air access in 1.5 m of water (23 to 29°C), and the control group consisted of turtles placed into a tank with access to oxygen and a basking platform exposed to ambient sunlight for 4 h. Four hours of entrapment was chosen to provide sufficient time for impairment to occur while avoiding immediate mortality, although actual entrapment duration typical of the fishery can be up to 7 days (Larocque et al ., 2012b). We used a matched pair design in which, for each individual undergoing the entrapment treatment, we selected a control individual that was matched for species, size, capture day and capture location. Both individuals from a given treatment/control pair were released simultaneously at the same location (their capture location).
After initial capture, attachment points were created for the accelerometers by drilling two small holes between the 10th and 12th marginal scutes on the left side (Fig. 1). Pairs of control and treatment individuals were then allowed to recover for a minimum of 48 h, after which they underwent their respective treatments. After their 4 h experimental treatment, individuals were removed and tested for impairment using a series of basic behavioural responses (see below). Following behavioural assessment, tri‐axial accelerometer loggers (model CEFAS G6a, 18 g in air, 10 Hz recording frequency, 1 Hz for temperature and depth; CEFAS Technology Limited, Suffolk, UK) were attached to the turtles, using a 13.6 kg stainless steel line. Tri‐axial accelerometers measure both dynamic and static acceleration in units of gravity (g ). Loggers also contained depth (herein referred to as vertical distribution, accuracy: ±1%; resolution: 4 cm) and temperature sensors (accuracy: 0.1°C; resolution: 0.03°C). Accelerometers were set to record for 48 h. To assist with tag retrieval, unique‐frequency radio‐telemetry transmitters (Model BD‐2, 3.2 g in air, 20 cm trailing whip antenna, Holohill Systems Inc., Carp, ON, Canada) were tied to the accelerometers. The entire package weighed ~24 g in air and ~12 g in water. After logger and transmitter attachment (≤3 min procedure), turtles were immediately transported in a 95 L covered plastic container to their capture location for release. Forty‐eight hours post‐release, turtles were located using a hand‐held radio‐tracking receiver (Biotracker, Lotek Engineering, Inc.; Newmarket, ON, Canada) and 3‐element yagi antenna (AF Antronics, Urbana, IL, USA). Recaptured turtles were released following retrieval of both tags.