Assigning Fates in Telemetry Studies Using Hidden Markov Models: an Application to Deepwater Groupers Released with Descender Devices

Abstract

Fate assignment is crucial to the results of survival studies, particularly those that utilize acoustic tagging. Most current methodologies are at least partially subjective; thus, having a means of objectively assigning fates would improve the precision, accuracy, and utility of such studies. We released 57 acoustically tagged fish belonging to six deepwater grouper species off North Carolina, USA, via surface release and recompressed release with descender devices. We applied a three-state hidden Markov model (HMM) in a novel way: to distinguish movement patterns between fish that were alive and fish that were dead (and might have been eaten by predators). We assigned fates using two approaches that differed in their reliance on HMMs. When HMMs were the predominant source of fate assignment, we estimated survival of 40 deepwater groupers released with descender devices at the continental shelf break (66-120-m depth) to be 0.46 (95% CI = 0.33-0.65). When a combination of HMMs and prior information was utilized, we estimated survival of the same 40 groupers to be 0.61 (95% CI = 0.47-0.80). Both estimates represented a substantial improvement over the survival of surface-released groupers (n = 9; survival~0.0). Furthermore, HMMs estimated zero survival for an additional five descended groupers at a wreck site in 240-m depth, although one analysis using prior information suggested that survival was possible in that depth. These estimates were aided by the objectivity of HMMs, and we recommend that future survival studies involving acoustic tagging employ similar methodologies. The improved survival of groupers after descending is an important finding for management, as this taxon contains several species of impaired stock status or fishery status.

Discard Mortality of Red Snapper Released with Descender Devices in the U.S. South Atlantic

“…Other than movements that were attributable to typical tides and currents, any movements detected from these "negative control" individuals were considered to be from another animal. This methodology has been utilized in previous telemetry-based survival studies to compare movement data from known-dead individuals with the unobserved fates of live releases (Yergey et al 2012;Muhametsafina et al 2014;Runde et al 2020b).…”

“…For the positive control group, we treated movement data as a positive control only from the day of tagging until the day of re-sighting or recapture (and not after). We attempted to apply hidden Markov models to these data to assign fates (sensu Runde et al 2020b), but these attempts were unsuccessful given similarities in movements among some negative and positive control fish. Instead, from the movements of the two control groups and information provided by the reference tag, we generated a fate assignment decision tree that was used to determine whether each Red Snapper survived, lost its tag, or died (Figure 3).…”

Four decades of reef observations illuminate deep‐water grouper hotspots

“…Large‐bodied groupers have very few predators in the open ocean, but sharks are likely the biggest threat and are relatively common in deep areas of the SEUS. For instance, eight acoustically tagged white sharks ( Carcharodon carcharias , Lamnidae) and one acoustically tagged tiger shark ( Galeocerdo cuvier , Carcharhinidae) were detected near the Snowy Wreck over an 18‐month period (Runde et al., 2020). Moreover, most of the predation upon recently released reef fishes from hook‐and‐line fishing on artificial and natural reefs in the Gulf of Mexico and Atlantic was due to sharks (Bahaboy et al., 2020; Runde et al., 2020).…”

“…For instance, eight acoustically tagged white sharks ( Carcharodon carcharias , Lamnidae) and one acoustically tagged tiger shark ( Galeocerdo cuvier , Carcharhinidae) were detected near the Snowy Wreck over an 18‐month period (Runde et al., 2020). Moreover, most of the predation upon recently released reef fishes from hook‐and‐line fishing on artificial and natural reefs in the Gulf of Mexico and Atlantic was due to sharks (Bahaboy et al., 2020; Runde et al., 2020). Such distributional behavior is supported by experimental evidence that gag exhibit density‐dependent habitat selection, primarily for shelter, on shallow (13 m) artificial reefs in the northeast Gulf of Mexico (Lindberg et al., 2006).…”

Red Drum and Spotted Seatrout Live‐Release Tournament Mortality and Dispersal

“…Therefore, we retained these observed mortalities to provide an encompassing estimate of post-weigh-in mortality. External attachment of acoustic transmitters can overcome surgery effects in postrelease mortality studies (Curtis et al 2015;Dance et al 2016;Runde and Buckel 2018;Bohaboy et al 2019;Runde et al 2020) and should be considered for future studies of live-release tournaments.…”