Preliminary estimate of post-release survival of immature porbeagles caught with rod-and-reel in the Northwest Atlantic Ocean

: The Northwest Atlantic (NWA) population of porbeagles Lamna nasus is susceptible to capture in rod-and-reel fisheries and most individuals are discarded alive due to catch and size limits. To estimate post-release survival, pop-off satellite archival tags were attached to por - beagles captured with rod-and-reel. Fourteen tags were deployed, of which 13 transmitted. All sharks for which we had data survived, giving a post-release survival rate of 100%. Following release, 6 individuals remained in surface waters for several hours to days, while 2 individuals immediately resumed normal diving behaviors. For the remaining sharks (n = 5), low tag transmission resolution precluded the detection of fine scale post-release behavior. The duration of initial depth-holding behavior was characterized using a break-point analysis of dive track variance, which suggests porbeagles exhibited a median post-release recovery period of 116 h (10th and 90th percentiles = 68.8 and 280.1 h) following capture and handling. Our preliminary study suggests immature porbeagles are resilient to capture and handling, although more data would provide stronger support for management recommendations.


INTRODUCTION
Capture as bycatch is a pressing threat to fish stocks (Davies et al. 2009), and slow-growing shark species with low reproductive output are particularly susceptible to overexploitation (Stevens et al. 2000). Management measures (e.g. catch limits, retention prohibitions, circle hook requirements, wire leader bans) are often employed for overexploited shark species in an effort to reduce fishing mortality (e.g. ICCAT 2015). Although such management measures may effectively reduce directed landings, incidentally captured sharks may still suffer unintended atvessel or post-release mortality from injuries and physiological damage (Kneebone et al. 2013). Therefore, computing total catch and assessing the efficacy of management measures require quantification of both components of unintended fishing mortality (atvessel and post-release mortality; e.g. Musyl & Gilman 2019).
The Northwest Atlantic (NWA) population of porbeagles Lamna nasus is particularly vulnerable to population declines . Historically, this species was targeted commercially or taken as bycatch throughout much of its range (Francis et al. 2008), and population declines have occurred in multiple locations worldwide , Stevens et al. 2006. After the introduction of longline fishing pressure in the early 1960s, the NWA population declined precipitously in abundance, reaching a minimum in 2001 (Campana et al. 2013). Population dynamics modeling suggests fishing mortality must remain low for successful recovery to occur (Campana et al. 2013). In response to declines, catch and size limits have been utilized in the NWA to reduce rod-and-reel fishing mortality (i.e. Campana et al. 2002, NMFS 2007, ICCAT 2015. Despite these management measures, porbeagles remain susceptible to capture in rod-and-reel fisheries targeting tuna and pelagic sharks in the NWA (Hurley 1998, NOAA 2019 and no study to date has investigated the impact of rod-and-reel capture on porbeagles released alive. Therefore, this study utilized pop-off satellite archival tags (PSATs) to estimate post-release survival and characterize behavior following release for porbeagles caught with rod-andreel in the NWA.

MATERIALS AND METHODS
Porbeagles were caught opportunistically in the NWA using rod-and-reel with 200 lb Jinkai fishing line set at a strike drag strength of 40 lb. Rods and reels were equipped with 16/0 non-offset Mustad circle hooks or 12/0 Mustad J-hooks baited with locally caught species (i.e. Atlantic mackerel Scomber scombrus) alive or dead, and whole or chunked. Fight time (time from when the shark was hooked until it was either secured alongside the vessel or brought onboard), sex, fork length (FL), and handling time (time from when the shark was secured alongside the boat or brought onboard until it was released) were recorded. Sharks were assigned an injury code modified from Marshall et al. (2015) and a release condition adapted from Manire et al. (2001) (Table 1). Before release, the hook was removed or the leader was cut.
Different pop-off schedules (in parentheses) characterized the PSATs used to observe post-release survival: Lotek Wireless (LW) PSATLIFE tags (28 d; n = 7), Wildlife Computers (WC) Survivorship PATs (sPATs; 30 d; n = 4), and Microwave Telemetry (MT) High Rate (HR; 30 d; n = 1) and Standard Rate (SR; 9 mo; n = 2) X-Tags. The short-term deployments were assumed to be long enough to detect mortality caused by trauma or stress during capture and handling, while the 2 SR X-Tags were deployed to observe delayed mortality over longer time spans. For 10 porbeagles, tags were attached using a stainless steel dart anchor (Hallprint) inserted into the dorsal musculature. The tags have a buoyancy rating of 8 g and the tag tethers were 2.3 g in water, ensuring tags would float if shed. To reduce the likelihood of premature tag shedding, for 4 porbeagles PSATs were looped through a hole in the dorsal fin (Moyes et al. 2006). In this case, a 5 mm diameter hole was drilled (Ryobi drill) through the dorsal fin, then flexible tubing with 136 kg test monofilament Jinkai fishing line was threaded through the hole and crimped with a stainless steel nicopress crimp sleeve. The PSAT was attached to this loop and centered behind the dorsal fin. PSATs were programmed to collect pressure (i.e. depth) and ambient temperature at 10−90 s intervals and report data at 5 (PSATLIFE, HR X-Tag) or 15−60 (SR X-Tag) min intervals or report daily minimum and maximum data (sPAT). PSATs were programmed to release prematurely if pressure remained constant for a pre determined number of days (LW: ± 5 m for 3 d, MT: ± 3 m for 2 d, and WC: ± 4 m for 1 d), indicating a mortality or a shed tag floating at the surface or washed ashore. PSATs were programmed to release and transmit prematurely if pressure went below a predetermined threshold (LW: 1500 m, MT: 1250 m, and WC: 1400 m).
Post-release survival was determined using depth profiles from PSAT data downloaded from the ARGOS website (www.argos-system.org). To evaluate changes in diving behavior following release, hourly variance in depth was cal culated from the tags transmitting at 5 min intervals (n = 6). A break-point analysis completed in R (v 3.5.1) was used to identify the hour in  Marshall et al. 2015) and release conditions (adapted from Manire et al. 2001) assigned to porbeagles captured with rod-and-reel which variability at the beginning of the dive track differed the most from variability in the latter part of the dive track (Wichern et al. 1976). Extremely low hourly variance only occurred while an individual remained within surface waters, and variance markedly in creased once an individual began utilizing a range of depths. The end of the recovery period was identified from a time series of absolute differences between mean variance prior to and after a given sampling hour. The maximum reduction in the absolute difference in variance occurred once an animal ceased any depth-holding behavior and began to move cyclically through the water column, indicating the end of the recovery period. For tags with lower transmission resolution (i.e. sPATs), sharks that exhibited minimal diving behavior following release were identified from the amount of time they re mained in the top 50 m of the water column.

RESULTS
Opportunistic capture resulted in a random sample of 14 immature (Jensen et al. 2002) porbeagles tagged with PSATs (mean FL (± SD) 122.4 ± 38.8 cm; Fig. 1; Table 2). Data were successfully transmitted from tags on 13 of these individuals (FL 122.5 ± 40.3 cm). Fight time ranged between 1 and 76 min. All 13 (100%) sharks for which we had data survived until the tags popped off (between 12 and 246 d after release). Two tags attached with dart anchors shed   porbeagles exhibiting no recovery period or had insufficient data to conclude a recovery period. Note the y-axes differ in scale. Numbers below error bars indicate sample size. Changes in sample size represent tag shedding or variation in the number of tags with depth data available. Error bars represent ±1 SE the water column, remained below 50 m for 1.2−56% of the time, and made regular dives to ≥ 100 m. Based on break-point analysis of hourly dive variance for individuals with high transmission resolution (example given in Fig. 3), recovery times ranged from 49 to 350 h, with a median of 116 h (10th and 90th percentiles = 68.8 and 280.1 h). For some individuals recovery behavior (based on the time spent in top 50 m of water column or breakpoint analysis of dive variance) could not be determined because of tag transmission resolution. In particular, SR X-tags (n = 2; no. 25 516 and 25 514) did not transmit data for the first several days at liberty and 3 sPATs (no. 416, 193, and 819) transmitted daily maximum depths of approximately > 50 m for the first day at liberty and therefore shorter duration (< 24 h) recovery periods could not be determined. However, the 2 individuals with high transmission resolution tags (no. 1815 and 161 793) that dove to > 50 m immediately following release exhibited relatively consistent hourly dive

DISCUSSION
Our study suggested high (100%) post-release survival for immature porbeagles captured with rodand-reel regardless of capture, handling, and release factors (i.e. injury code, release condition). Shortterm (28 d) survival was the focus of this study and long-term delayed mortality (i.e. from infection or gut hooking and cessation of feeding) may have been underestimated. However, results from the deployment of two 9 mo tags suggests long-term survival may also be high for porbeagles. The survival rate in our study is consistent with those found for other shark species captured with rod-and-reel, including shortfin mako Isurus oxyrinchus (90%; French et al. 2015), blue shark Prionace glauca (87%; Howey et al. 2017), and blacktip shark Carcharhinus limbatus (90.3%; Whitney et al. 2017). Additionally, Sepulveda et al. (2015) found post-release survival of rodand-reel caught common thresher sharks Alopias vulpinus to be 100% when mouth-hooked, although survival for tail-hooked individuals was 66%.
The observed depth-holding behavior was consistent with previously documented post-release behavior modification in porbeagles (Hoolihan et al. 2011), other pelagic fishes (Pepperell & Davis 1999), and other sharks , Hoolihan et al. 2011. Decreased vertical movement may be a consequence of physiological stress associated with capture and handling (Hoolihan et al. 2011), as porbeagles may need to reallocate energy from normal swimming patterns to restore homeostasis (Marshall et al. 2015). This study suggests porbeagles exhibit a recovery period following capture and handling that typically lasts ~116 h (4.8 d). Given the observed recovery behavior increases time spent in surface waters where rod-and-reel fishing for pelagic species occurs, captured and re leased porbeagles may be more vulnerable to recapture and/or predation from larger predators in habiting surface waters.

CONCLUSIONS
Our findings suggest immature porbeagles are resilient to capture and handling in rod-and-reel fisheries in the NWA, and these are the first data of this kind that can be considered in management of this species. Based on our preliminary results, mitigation measures that increase the proportion of porbeagles released alive (i.e. catch limits, size limits) may be viable strategies for minimizing mortality in rod-and-reel fisheries. However, more data is necessary to obtain sufficient statistical power to better support management recommendations (Musyl & Gilman 2019) and evaluate how mortality rates or recovery periods vary with characteristics of the fishery.