Variable post-release mortality in common shark species captured in Texas shore-based recreational fisheries

Abstract

The practice of catch and release fishing is common among anglers but has been shown to cause unintended mortalities in some species. Current post-release mortality estimates used in coastal shark stock assessments are typically derived from boat-based shark fisheries, which differ from shore-based operations that expose sharks to potentially more stressful environmental and handling conditions. Recreational post-release mortality rates in shore-based fisheries must be quantified to improve stock assessment models and to create guidelines that protect species from overexploitation. Here, we partnered with experienced anglers acting as citizen scientists to deploy pop-up satellite archival transmitting tags (PSAT, n = 22) and acceleration data loggers (ADLs, n = 22). on four commonly caught sharks including the blacktip shark (Carcharhinus limbatus, n = 11), bull shark (Carcharhinus leucas, n = 14), tiger shark (Galeocerdo cuvier, n = 6), and great hammerheads (Sphyrna mokarran, n = 2). Mortality occurred within minutes to hours post-release. If evidence of mortality occurred after normal diving behavior had been re-established for 10 days, then the mortality was considered natural and not related to the catch-and-release process. Post-release mortality estimates ranged from 0% for bull and tiger sharks to 45.5% for blacktip sharks. Of the two great hammerheads, one died within 30 minutes post-release while the other exhibited mortality characteristics 14 days after release. Moribund blacktip sharks experienced on average 3.4-4.9°C warmer water compared with survivors. Recovery periods were estimated for survivors of each species and were highly variable, differing based on duration of tag deployment. High variability in responses to capture and release between species demonstrates the need for species-specific assessments of post-release mortality in shore-based recreational fisheries.

Fig 1. Map of deployment and pop-off locations for PSAT and ADL tagged sharks.

Fig 2. Example of recovered PSAT data from a blacktip shark (C_lim03).

Consistent changes in depth, temperature, and light level with ambient ocean conditions indicate survival.

Fig 3. Example of recovered ADL data from a blacktip shark (C_lim07).

After tagging, this shark swam away into increasing deeper water, with a tail beat frequency of ~1 Hz. After 40 minutes this shark landed on the seafloor in an upright (normal) posture and stopped displaying regular tailbeats. The shark remained on the seafloor without any tailbeats for a further 21 hours before the release mechanism triggered the tag to detach from the shark. This constant seafloor depth and lack of tailbeats on the seafloor indicates mortality.

Fig 4. Example of ADL tag data from a blacktip shark (C_lim14).

1 hour after tagging, the shark descended to the seafloor, where it remained for 2 hours at constant depth, indicating mortality. In the first hour post release there are clear tail beats, some temperature stratification and the shark is level (Roll ~ 0). However, this shark then dies as evidence of a lack of tailbeats and laying on the seafloor. After ~10 mins on the seafloor it looks like the shark begins to get scavenged as the shark begins to roll into different positions. 1.5 hours after it initially rested on the seafloor the tag appears to be ingested by a predator. The tag is rolled 120 degrees in the predator’s stomach and because the tag is no longer aligned in the same way, we do not see a clear tailbeat signal in the frequency spectrum. Additionally, as this predator swims up and down in the water column we see no temperature stratification. This tag stayed in the predator’s stomach for ~50 Hours before being regurgitated.

Fig 5. Example of PSAT tag data from a blacktip shark (C_lim05).

This individual survived the tagging process and exhibited normal oscillatory diving behavior for 10 days following release. The tag then registers a constant depth at ~30m for five full days, suggesting a natural mortality event occurred, before fluctuating between 28-31m for two weeks and finally releasing at its programmed interval. This tag was not pre-programmed to release when constant depth was detected. This individual was ultimately considered a natural mortality and was not included in the overall PRM rate estimate.

Fig 6

Temperature distributions for blacktip shark survivors (grey) and mortalities (red) across each groups time at large (Left, Kolmogorov-Smirnov two sample test, p<0.0001). (B) Boxplots comparing average initial temperature from the first 10 minutes immediately following release between blacktip sharks that survived capture and release and those that did not (Right, Kruskal-Wallis test, p = 0.045).