Fig 1 - uploaded by Brendan Talwar
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Diagram of the modified handline, equipped with two line-borne GoFish cameras and an accelerometer, used to capture oceanic whitetip sharks, monitor, and quantify their behavior.
Source publication
Animal behavior varies in response to capture between/within species and fisheries, and its expression may contribute to incidental mortality when behaviors result in physiological ramifications that cannot be resolved. However, this relationship between capture behavior and animal health is poorly understood, and it remains a logistical challenge...
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... captured sharks over three years (2017)(2018)(2019) W). Fishing was conducted with a modified handline constructed of two large polyball floats (37 mm maximum diameter) attached to 8-9 m of nylon line (6 mm diameter), which was connected to four 1.2 m strands of 181.4 kg monofilament leader crimped to an 18/0 non-offset carbon steel circle hook (Fig. 1). Capture behavior was monitored using an accelerometer affixed to the monofilament leader ~1 m from the hook, and two cameras staggered on the nylon line. All research was conducted under Cape Eleuthera Institute research permits (MAF/FIS/9, MAF/FIS/17, MAF/FIS/34) issued by the Bahamian Department of Marine Resources and followed Cape ...
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... Cam, Inc., Austin, Texas, USA) were used in concert with accelerometers to monitor capture behavior for a subsample of capture events. These cameras were attached to handlines to collect video at 1080 pixels (p) and 60 frames-per-second (FPS), or 720p and 30 FPS when observing events longer than one hour (due to battery consumption restrictions; Fig. 1). Initial exploratory trials revealed that a single camera did not provide a sufficient viewing angle to keep the shark within frame throughout capture, regardless of where the camera was positioned on the line (Supplemental Video S1). Therefore, two individual cameras were placed at ~3 and 6 m from the hook. Both placements were ...
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... et al., 2012;Resheff et al., 2014;Ladds et al., 2016Ladds et al., , 2017Brewster et al., 2018;Benaissa et al., 2019;Hounslow et al., 2019; additional details provided in Supplemental Material). Relevant hyperparameters for each SCM were optimized during training to maximize prediction performance to the data structure (Supplemental Table S2 and Fig. S1). Each SCM was used to independently predict the probability of an observation belonging to the three behaviors (i.e., steady swimming, high-energy, and motionless behaviors). This array of behavioral probabilities was then subjected to an ensembling approach that calculated the weighted average of each behavior's probabilities for a ...
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... MLP, RF, and XGBoost) to major (256.9%; SVM) improvements in the overall predictive performance (0.014-0.617 increase in macro F1) of SCMs ( Supplementary Fig. S1). SCMs predicted the three capture behaviors with varying degrees of success (Table 2). ...
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... we extended monitoring periods and maintained enough resolution to observe behaviors clearly. Other cameras may better address the monitoring challenges listed above (e.g., Graham et al., 2004;O'Shea et al., 2015;Bouyoucos et al., 2018;Talwar et al., 2020), but we opted for GoFish cameras because of their design specific to in-line attachment (Fig. 1) and ability to capture quality footage at an affordable price (USD 150). Regardless of the cameras being used, it is important to optimize their placement on the gear to ensure the animal's movements remain in the frame throughout capture (Supplemental Video ...
Citations
... For instance, commercial captures typically occur over many hours (e.g. longline soak times, Campana et al., 2009) and can be passive captures where sharks can spend periods motionless whilst on the line (Knotek et al., 2022b). Recreational captures, in contrast, represent a dynamic interaction between the animal and the fisher, often termed a "fight", typically occurring over minutes (range between one and 14 min; Gurshin and Szedlmayer, 2004;Kneebone et al., 2013Kneebone et al., ,2013bKneebone et al., ,2013cDanylchuk et al., 2014Danylchuk et al., ,2014bDanylchuk et al., ,2014cWeber et al., 2020;Knotek et al., 2022a) and can extend to hours in some cases (Heberer et al., 2010(Heberer et al., ,2010b(Heberer et al., ,2010cFrench et al., 2015;Anderson et al., 2021). ...
In recent decades, the abundance of sharks in the world's oceans has decreased due to overexploitation by commercial fisheries. Over this same period, sharks have been increasingly targeted for sport by recreational anglers. "Catch-and-release" (C&R) angling, where sharks are released alive after capture, has been proposed, and in some situations, mandated as a conservation measure in recreational fisheries. In C&R fisheries, anglers are encouraged to follow best practices, each with the goal of maximising post-release survivorship (PRS) of angled fish. Here for sharks, we review C&R global best practices and the science underpinning them. Despite shark C&R fishing being practiced globally, peer-reviewed research into post-release survivorship is limited to just twelve studies for eight species (Lamniformes, n = 3; Carcharhiniformes, n = 5). PRS in studies ranged from 56% to 100%. Identifying causes for decreased PRS (i.e. mortality) was challenging for studies due to low sample sizes. Of the factors investigated, candidate best practices included: (1) using non-stainless steel circle hooks, (2) not removing sharks from the water, (3) reviving sharks prior to release, and (4) minimising time spent freeing the shark by removing the hook or cutting the line. With the conservation status of many sharks declining, more research is needed to strengthen the scientific basis for these practices to ensure that PRS in C&R is maximised.
... < 2.5 m; [2]), removing shallower hooks in longline sets [14,15] and reducing soak time of hooks [16] has been shown to substantially increase the likelihood of survival for OCS and other species that are more sensitive to the lethal physiological changes of capture related stress [13]. Observable capture behavior can be used to make inferences on the internal physiology of caught species and provide additional information on methods to reduce mortality for longline caught sharks [16,17]. However, collection of such data for threatened species is difficult, as is directly observing the sequence of events that determines the likelihood of survival after capture. ...
... Observing the sequence of behaviors before and during longline capture can provide key information on best practices to increase post-release survival rates for bycaught sharks [17]. When it was first hooked on the line, the OCS observed here exhibited initial increases in overall body activity coupled with erratic narrow vertical movements. ...
... When it was first hooked on the line, the OCS observed here exhibited initial increases in overall body activity coupled with erratic narrow vertical movements. The capture behavior of this OCS was similar to that documented by [17] that showed hooking and capture initiates an immediate stress response and depending on condition, individuals may respond with high intensity burst swimming to escape and/or force oxygenated water over their gills to maintain ram ventilation [1,17]. This prolonged exhaustive activity, over the course of 5 h, likely caused irreparable physiological damage leading to the death of the shark [1]. ...
Background
Bycatch mortality in longline fisheries is a major contributor to global declines in shark populations. The duration of time that an animal is hooked and the impacts of hooking on behavior affect the likelihood of mortality. However, limited information exists on the behavior of sharks to longline capture because of difficulties observing hooking events. Using a fortuitous recovery of an archival satellite tag, we describe the movement of an oceanic whitetip shark (Carcharhinus longimanus) and examine the behavior prior to its mortality in response to hooking on a longline.
Results
A 1.5 m (fork length) C.longimanus was tagged and released in good condition by a fisheries observer following initial capture on a US longline fishing vessel. After release, the shark resumed normal vertical behavior within 5 h. Over 198 days, the shark undertook wide-ranging movements throughout the Pacific between Samoa, Niue, and Tonga. The shark was hooked by a second longline vessel while conducting routine yo-yo diving between 0 and 120 m depth. For the first hour after being hooked the shark exhibited high swimming activity with rapid vertical movements between 20 and 40 m indicative of an initial struggle against the line. After this, the shark struggled at the surface for approximately 5 h, until it succumbed to exhaustion and died on the line.
Conclusion
Fight time has a strong influence on the mortality rates of sharks captured in commercial longline fishing operations. Data obtained from this shark offers further understanding of capture behavior and time to mortality on a longline for C.longimanus which may assist managers as they work on options to reduce mortality rates for this threatened species.
