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Application of three-dimensional acoustic telemetry to assess the effects of rapid recompression on reef fish discard mortality


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Geopositioning underwater acoustic telemetry was used to test whether rapid recompression with weighted return-to-depth (descender) devices reduced discard mortality of red snapper (n = 141) and gray triggerfish (n = 26) captured and released at 30-60 m depths at two 15 km 2 study sites in the northern Gulf of Mexico. Cox proportional hazards modelling indicated red snapper released with descender devices had significantly lower discard mortality within the first 2 d (95% CI ¼ 18.8-41.8% for descender-released vs. 44.0-72.4% for surface-released, unvented fish), while there was no significant effect of descender devices on discard mortality of gray triggerfish. Predation by large pelagic predators was estimated to account 83% of red snapper and 100% of gray triggerfish discard mortality. Discard mortality due to predation has likely been overlooked in previous mark-recapture, laboratory, and enclosure studies, suggesting cryptic population losses due to predation on discards may be underestimated for red snapper and gray triggerfish. Large-area three-dimensional positioning acoustic telemetry arrays combined with collaboration and data sharing among acoustic telemetry researchers have the potential to advance our knowledge of the processes affecting discard mortality in reef fishes and other taxa.
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Application of three-dimensional acoustic telemetry to assess
the effects of rapid recompression on reef fish discard mortality
Erin Collings Bohaboy
*, Tristan L. Guttridge
, Neil Hammerschlag
Maurits P. M. Van Zinnicq Bergmann
, and William F. Patterson III
School of Forest Resources and Conservation, Fisheries and Aquatic Sciences, University of Florida, 7922 Northwest 71st Street, Gainesville, FL
32653, USA
Bimini Biological Field Station Foundation, South Bimini, The Bahamas
Saving the Blue, Miami, FL, 33186, USA
Department of Marine Ecosystems and Society, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL
33149, USA
Department of Biological Sciences, Florida International University, North Miami, FL 33181, USA
*Corresponding author: tel: (352) 273-3610; e-mail:
Present address: School of Forest Resources and Conservation, Fisheries and Aquatic Sciences, University of Florida, 7922 Northwest 71st Street,
Gainesville, FL 32653, USA.
Bohaboy, E. C., Guttridge, T. L., Hammerschlag, N., Van Zinnicq Bergmann, M. P. M., and Patterson, W. F. III Application of three-
dimensional acoustic telemetry to assess the effects of rapid recompression on reef fish discard mortality. – ICES Journal of Marine
Science, doi:10.1093/icesjms/fsz202.
Received 18 June 2019; revised 27 September 2019; accepted 2 October 2019.
Geopositioning underwater acoustic telemetry was used to test whether rapid recompression with weighted return-to-depth (descender)
devices reduced discard mortality of red snapper (n¼141) and gray triggerfish (n¼26) captured and released at 30–60 m depths at two
15 km
study sites in the northern Gulf of Mexico. Cox proportional hazards modelling indicated red snapper released with descender devices
had significantly lower discard mortality within the first 2 d (95% CI ¼18.8–41.8% for descender-released vs. 44.0–72.4% for surface-released,
unvented fish), while there was no significant effect of descender devices on discard mortality of gray triggerfish. Predation by large pelagic
predators was estimated to account 83% of red snapper and 100% of gray triggerfish discard mortality. Discard mortality due to predation
has likely been overlooked in previous mark-recapture, laboratory, and enclosure studies, suggesting cryptic population losses due to preda-
tion on discards may be underestimated for red snapper and gray triggerfish. Large-area three-dimensional positioning acoustic telemetry
arrays combined with collaboration and data sharing among acoustic telemetry researchers have the potential to advance our knowledge of
the processes affecting discard mortality in reef fishes and other taxa.
Keywords: acoustic telemetry, descender device, discard mortality, predation, rapid recompression, recreational fisheries, red snapper
Marine recreational fishing is an important economic activity and
source of non-monetary benefits for people around the world
(Arlinghaus et al., 2007;Lovell et al., 2013). In addition to fish
they harvest, recreational fishers often release (discard) a large
portion of their catch. In the US, Canada, and Europe, marine
recreational discarding rates often exceed 50% of the total catch
and approach 100% for some species (Ferter et al., 2013;National
Marine Fisheries Service, 2017;Fisheries and Oceans Canada,
2019). Catch-and-release fishing has historically been practiced in
recreational fisheries as a conservation and management strategy
(Radonski, 2002). However, discarded fish that die as a result of
being captured and released are still removed from the popula-
tion, reducing the intended conservation benefits of release
CInternational Council for the Exploration of the Sea 2019. All rights reserved.
For permissions, please email:
ICES Journal of Marine Science (2019), doi:10.1093/icesjms/fsz202
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(Bartholomew and Bohnsack, 2005). The proportion of live dis-
cards that die following release (discard mortality) may approach
100% in some recreational fisheries and is influenced by multiple
factors including environmental conditions (e.g. depth of capture
and water temperature), fish condition (e.g. species, size, and
presence of hook-related injuries), and handling (e.g. time out of
water and time required for an angler to land a fish once hooked)
(Muoneke and Childress, 1994;Bartholomew and Bohnsack,
2005;Brownscombe et al., 2017). In fisheries where recreational
catches, discarding rates, or discard mortality are high, dead dis-
cards can represent a significant portion of stock removals. In
these cases, reducing uncertainty of discard mortality estimates,
improving understanding of the factors that affect discard mor-
tality, and developing methods to minimize discard mortality
may be instrumental in successful management.
Estimating discard mortality and quantifying the factors affect-
ing it are particularly challenging for marine fish species, largely
due to the difficulty of monitoring the fate of released fish.
Traditional approaches to studying discard mortality, such as
mark-recapture, enclosure, and inferences from physiological
studies, often require numerous assumptions (including emigra-
tion/immigration rates of fish, angler reporting rates, and natural
and fishing mortality), expose fish to the deleterious effects of
prolonged captivity, or fail to account for the effects of predation.
Ultrasonic underwater acoustic telemetry can provide estimates
of three-dimensional positions of tagged fish, and is a reliable
means to track the movement, behaviour, and survival of tagged
fish for months to years in their natural environment. In particu-
lar, predation may be a greatly underestimated contributor to dis-
card mortality because predation events are rarely observed using
approaches traditionally applied in post-release mortality studies
(Raby et al., 2014). Acoustic transmitter tags often are surgically
implanted in the abdominal cavity of fish, requiring sedation, ex-
tended handling, and may rupture over-inflated swim bladders of
fish suffering from barotrauma, or otherwise add extraneous vari-
ables to the process of estimating release mortality. Alternatively,
external attachment of acoustic transmitter tags (e.g. Curtis et al.,
2015;Dance et al., 2016;Runde and Buckel, 2018) reduces han-
dling trauma and preserves barotrauma symptoms, thus improv-
ing estimates of barotrauma-related mortality.
Red snapper (Lutjanus campechanus) are highly sought-after
by recreational fishers in the US Gulf of Mexico (GOM) who an-
nually discard more than 70% of red snapper they catch (SEDAR,
2018). Discard mortality estimates for recreationally caught red
snapper vary from near zero to greater than 80% depending on
factors such as release method, handling, season, capture depth,
and fish condition (Campbell et al., 2014;Drumhiller et al., 2014;
Curtis et al., 2015). Gray triggerfish (Balistes capriscus) are an-
other popular recreational reef fish species in the northern GOM
that are targeted by recreational fishers with hook and line over
artificial reefs where they are also caught incidentally to red snap-
per. GOM gray triggerfish have experienced long-term stock
declines concurrent with reductions in recreational fishing sea-
sons and increased discards; 70% of recreationally caught gray
triggerfish in the eastern GOM are discarded annually (SEDAR,
2015). Discard mortality of recreationally caught and released
gray triggerfish is rarely studied and most estimates are fairly low
based on recapture rates of tagged fish and observations of fish
behaviour at release (0–15%, Patterson et al., 2002;Rudershausen
et al., 2013). Discard mortality was assumed to be 5% in the most
recent GOM gray triggerfish stock assessment (SEDAR, 2015).
However, Runde et al. (2019) used underwater tagging to control
for the effects of barotrauma and estimated that discard mortality
of gray triggerfish in the southeastern US recreational fishery was
much higher (65–66%) and could account for extensive stock
removals when considered together with the magnitude of annual
Fish experience a rapid drop in pressure as they are brought to
the surface by anglers, leading to barotrauma which can be a sig-
nificant contributor to post-release mortality of discarded fish
(Bartholomew and Bohnsack, 2005). Venting (the practice of re-
leasing gas from a fish’s swim bladder with a large gauge hypoder-
mic needle) has been proposed as a means to improve post-
release survival of marine fish suffering from barotrauma, but
results are equivocal regarding the benefits of venting and some
investigators suggest that improperly venting fish can damage
organs and increase discard mortality (Wilde, 2009;Scyphers
et al., 2013;Eberts and Somers, 2017). Releasing fish with
weighted return-to-depth tools, also known as descender devices,
is an alternative means to alleviate barotrauma symptoms and po-
tentially improve post-release survival. Descender devices include
several different designs of weighted hooks, clamps, or cages.
Forcing fish that would otherwise be too positively buoyant at the
surface to resubmerge may reduce predation by delivering reef
fishes as closely as possible to the relative safety of the reef struc-
ture from which they were captured while avoiding the internal
trauma of venting. Evidence supporting the efficacy of descender
devices in reducing discard mortality is largely based on studies
which show fish recompressed in the laboratory or in cages (ab-
sent predation) have increased survival (Parker et al., 2006;Pribyl
et al., 2012). In very few studies, the effect of cage-less descender
devices on discard mortality of marine fish has been examined
(Sumpton et al., 2010;Hochhalter and Reed, 2011;Curtis et al.,
2015). In contrast to the US recreational West Coast rockfish fish-
ery, where the use of descender devices has been widely advocated
(Chen, 2012;California Sea Grant et al., 2014), descender devices
have yet to gain widespread usage among GOM recreational reef
fish anglers (Crandall et al., 2018), and no management regula-
tions exist to require or encourage their usage.
The goals of this study were to investigate whether descender
devices reduce discard mortality in recreationally caught red
snapper and gray triggerfish, and to evaluate the effects of other
variables that might affect survival, including season (air/water
temperature), capture depth, handling time, and fish condition.
A large-area (>15 km
) three-dimensional geopositioning acous-
tic telemetry array was used to monitor fine-scale movement and
behaviour of tagged fish for up to 1 year, providing insight into
the importance of predation on post-release survival, which has
traditionally been overlooked in studies of discard mortality
(Raby et al., 2014). During this study, techniques for quickly ap-
plying external acoustic transmitter tags to red snapper and gray
triggerfish were developed, thus eliminating the need for sedation
and surgery to more closely approximate the handling of recrea-
tionally caught-and-released fish.
Acoustic telemetry array
An array of 60 Vemco (Bedford, Nova Scotia, Canada) VR2
acoustic receivers was deployed 28 km south of Pensacola Beach,
Florida from February 2016 to March 2017 at 28–35 m depth
(hereafter “30-m array”). In September 2016, the array was
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shifted approximately 0.5 km to the south and expanded in the
southeast corner to include additional artificial reefs (Figure 1).
The array was reduced to 46 receivers and moved to a deeper lo-
cation (48–55 m) approximately 80 km south of Orange Beach,
Alabama from August 2017 to July 2018 (hereafter “55-m” array).
Habitat within the study areas of each array deployment consisted
of open sand bottom interspersed with numerous artificial reef
structures (cement pyramids, reef balls, and chicken coops) and
likely also included some natural low-relief limestone hard-
bottom habitat in the 55-m array. Acoustic receivers were placed
in a grid that provided geopositioning capability of tagged fish in
an area >15 km
at each array. A range test was done prior to the
start of the study to evaluate detection efficiency at distances
from 100 to 1000 m. Results of the range test showed that detec-
tion efficiency decreased to 50% at 700 m, so we conservatively
used a maximum distance between receivers of 600 m, allowing
for >50% probability that acoustic tag transmissions within the
array could be detected by at least three receivers simultaneously.
Receivers within each array were a mix of Vemco model VR2Tx
and VR2W receivers. Model VR2Tx receivers were deployed with
the internal synchronization (sync) transmitters set to very high
output (160 dB), while each model VR2W receiver was deployed
with a Vemco V16-5x sync transmitter (set to output 162 dB) sus-
pended 2 m above the receiver on a line attached to a foam buoy.
Each receiver was attached to the top of a 2-m tall PVC support
pipe that was set in a 36-kg cement base. Grab lines were attached
between the cement base and the support pipe that could be used
as hoist points during deployment or retrieval. Most receiver
bases had a line attached to a floating buoy approximately 2 m
above the receiver to increase visibility or suspend a V16-5x sync
transmitter (for model VR2W receivers). Each receiver-base unit
was lowered to the sea floor on a hook and line from the side of
the boat and then located using the boat’s sonar depth sounder to
measure the GPS coordinates and ensure the acoustic receiver
was properly deployed in an upright position.
Acoustic receivers were retrieved midway through each array
deployment in September 2016 and March 2018, respectively, to
clean any fouling from the acoustic hydrophone and offload the
logged acoustic transmission data from each receiver. During re-
trieval, the vessel’s captain would locate the receiver-base unit us-
ing the boat’s sonar depth sounder. A heavy retrieval line
connected to a large mooring hook was attached to a VideoRay
(Pottstown, PA, USA) Pro4 mini remotely operated vehicle
(ROV). The pilot manoeuvred the ROV to attach the retrieval
hook to one of the base’s grab lines. The mooring hook was
mounted on the ROV so that it would easily detach when the
ROV was flown away from the receiver base, thus leaving the re-
trieval line and hook attached to the base. After ensuring the
ROV was free from the retrieval line, the receiver base was raised
to the surface and brought onboard the boat with the assistance
of a stainless steel davit and an electric winch. Using the ROV en-
abled the retrieval of 20–30 receiver-base units in a single day,
and retrieving the cement base in addition to the receiver avoided
leaving marine debris at study sites.
Fish were tagged with Vemco V13P-1x acoustic transmitter tags
which transmitted a 153 dB unique acoustic ID code and pressure
value at random intervals between 1 and 3 min (expected battery
life ¼468 d). Tags were attached to fish externally to minimize
handling time and avoid rupturing the swim bladder which often
occurs when fish suffering from barotrauma are subjected to tag
implantation in the abdominal cavity. There were two different
Figure 1. (a) Map of the study area in the northern Gulf of Mexico
indicating acoustic array locations. (b) The 60-receiver shallow
acoustic array was deployed at 28–35 m depth from February 2016
to March 2017. Receiver locations and the approximate extent of
the array from February 2016 to September 2016 are shown as solid
triangles () and a solid line (—), while receiver locations and array
extent from September 2016 to March 2017 are shown as open
triangles (~) and a broken line (- - -). (c) The deep array was
deployed at 48–55 m depth from August 2017 to July 2018.
Receivers and array extent are shown by solid triangles () and a
solid line (—). Artificial reef locations are denoted by squares ()in
both panels (b) and (c).
Effects of rapid recompression on reef fish discard mortality 3
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tag attachment methods used in this study. For the majority of
tagged fish, the tag was secured to a 2-mm diameter threaded
stainless steel bar with a 6.35-mm nylon-lined locking stainless
steel hex nut. The stainless steel bar was sharpened on one end
and inserted through the fish’s dorsal pterygiophores and secured
with a second 6.35-mm stainless steel hex nut behind a 3.2-mm
thick polyethylene disk. Silicon disks (2.4–3.2 mm thick) were
placed under the tag and under the polyethylene disk to minimize
abrasion (Figure 2a and b).
This stainless steel bar acoustic tag attachment device was tested
on wild-caught red snapper (n¼3) that were held in captivity at
Dauphin Island Sea Lab (Dauphin Island, AL, USA), with the goal
of achieving greater than 2-week tag retention. These tagging trials
were performed with “dummy” V13P tags, which had the same
dimensions, weight, and buoyancy as regular V13P tags but did not
transmit acoustic signals. One fish shed its tag at 39d which was
the result of becoming tangled in the net cover of the aquaculture
tank, causing the entire tag attachment to tear dorsally through the
fish’s back. Another fish lost its tag at 54d when the stainless steel
hex nut unscrewed from the threaded bar. This potential issue was
controlled for in the field by using only new hex nuts where the ny-
lon liner was not compressed from previous use or by slightly
bending the ends of the stainless steel rod after affixing tags to fish
such that nuts could not spin off the end of the threaded bar. The
tag retention trial was terminated after 220 d and the remaining
fish which had retained its tag for the duration was euthanized.
An alternate tag attachment method was used to attach acous-
tic tags to red snapper during late summer 2017 in the deep array.
Tags were attached with this second method to a medium-sized
(20 mm length 10 mm width) Domeier dart head (Domeier
et al., 2005) with approximately 3 cm of polymer-coated braided
stainless steel fishing line. Marine heat-shrink tubing was applied
over the tag cap to reduce movement and friction against the side
of the fish (Figure 2c). Domeier dart heads are constructed of soft
polymer and polyethylene terephthalate surgical fibres and are
designed to heal into the muscle tissue of tagged fish. These dart
heads have been used to quickly attach acoustic and satellite tags
to tunas (Domeier et al., 2005), sharks (Rogers et al., 2013), and
groupers (A. Collins, pers. comm.), minimizing handling time
and producing high tag retention estimates. However, after exam-
ining the acoustic detection data from fish tagged with the
Domeier tag attachment device, it was apparent that red snapper
tag retention, while adequate to estimate release mortality, did
not always allow us to track fish for longer (>1 month) time
periods. Therefore, we returned to the initial tagging method for
the final tagging event in spring 2018.
Red snapper and other reef fishes were captured from artificial
reefs within the study area using hook and line baited with cut
squid or herring. Red snapper were the primary species tagged in
this study (n¼141); however, several gray triggerfish (n¼26)
were also tagged. Fish were tagged during four events: (i) spring
2016 in the 30-m array, (ii) late summer 2016 in the 30-m array,
(iii) late summer 2017 in the 55-m array, and (iv) spring 2018 in
the 55-m array. Each fish was held in a damp V-shaped silicone-
covered measuring board, measured to the nearest mm fork or
total length (FL or TL) and tagged externally with a V13P acous-
tic transmitter tag using either of the methods described above.
Acoustically tagged fish were also tagged with a Floy (Seattle,
WA, USA) dart tag that advertised a $50 reward and toll-free
phone number to report tagged fish. The presence/absence of
traumatic hooking injury, any signs of barotrauma (exophthal-
mia, pronounced bloating, prolapsed intestine or gonads, pro-
truding scales, or everted stomach), and handling time (total time
out of water for dehooking and tagging) were recorded for each
tagged fish. The fight time (time between when a fish was hooked
and reached the surface) was recorded for most fish based on ver-
bal indication of the hooking event by the angler. During the final
tagging event in spring 2018, a Reefnet (Mississauga, Ontario,
Canada) Sensus Ultra depth logger was attached to the terminal
fishing tackle used to capture fish (Murie and Parkyn, 2013).
Depth profiles were examined to identify when a fish was hooked
and subsequently reached the surface to determine hooking depth
and fight time. Bottom water temperature at each artificial reef
where fish were captured was taken from the average daily logged
temperature by the closest VR2Tx receiver. Air temperature on
each day when fish were tagged was acquired from the National
Data Buoy Center at Station 42012 (approximately 48 km west of
the 30-m array; National Oceanic and Atmospheric
Administration and National Weather Service, 2017) and Station
42040 (approximately 77 km west/southwest of the 55-m array;
National Oceanic and Atmospheric Administration and National
Weather Service, 2019).
Each tagged fish was released either at the surface or with a de-
scender device over the reef where it was captured. A SeaQualizer
(Davie, FL, USA) descender device was primarily used to release
fish at depth. However, a second type of descender device was
also used to return seven red snapper to depth in spring 2016, but
its use was discontinued when some fish prematurely detached
Figure 2. Digital images of (a) a 43-cm TL red snapper held in captivity with an external acoustic tag attached with the stainless steel bar
method, (b) an acoustic tag with the stainless steel bar external attachment device, and (c) an acoustic tag with a Domeier dart attachment
4E. C. Bohaboy et al.
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from the device at the surface. A downward looking GoPro (San
Mateo, CA, USA) Hero3 camera was mounted above each de-
scender device to record fish descent and release to evaluate the
performance of the descender device, behaviour of released fish,
and possible predator interactions. Fish released at the surface
were observed and assessed for release condition following
Patterson et al. (2001): condition-1 ¼fish immediately oriented
to the bottom and swam down rapidly; condition-2 ¼fish ori-
ented to the bottom and swam down slowly or erratically; condi-
tion-3 ¼fish remained on the surface; and condition-4 ¼fish was
apparently dead at the surface, including from predation.
Data analyses
Detection data were offloaded from acoustic receivers in
September 2016, March 2017, April 2018, and July 2018 and sent
to Vemco for Vemco Positioning System (VPS) geolocation esti-
mation. Position estimates with horizontal position error (Smith,
2013) in the upper 5th percentile for each of the four datasets were
excluded from further analyses. This level of data filtering elimi-
nated position estimates that were highly uncertain and those that
resulted from false detections (i.e. an acoustic tag was recorded by
a receiver erroneously due to interference from other tags or back-
ground noise). Successive geoposition estimates separated by less
than 10min (600 s) were used to calculate swim speeds of tagged
fish. The average swim speed (v) in metres per second of a tagged
fish as it moved from position 1 with coordinates (Lat
time t
to position 2 with coordinates (Lat
coordinates are in radians, was calculated as
2rarcsin ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
sin2Lat2Lat 1
þcos Lat1
cos Lat 2
where r¼6.371 10
m is the assumed mean radius of the Earth.
Tagged fish were assigned a fate based on estimated swim
speeds, geographical movements, and depth below the surface.
The days to each fate were calculated and fish fates were binned
for some analyses over each of three time periods: immediate
(within 48 h of release), short-term (48 h to 14 d after release),
and long-term (greater than 14 d after release). The possible
assigned fates were predation, emigration, tag loss, surface mor-
tality, harvest, survival, and unknown. Predation and tag loss
were indicated by an abrupt change in tag movement or depth.
Tags from fish that were preyed upon moved faster than 0.5 m
through the array and displayed no affinity for reef structure
(see below for rationale). Tags that were stationary on the bottom
were assumed to have detached from fish and were classified as
tag loss. Fish that were classified as surface mortalities were either
observed dead after release or were detected only at the surface as
the fish drifted from the array. Harvested fish disappeared from
within the centre of the array and were reported by fishers, while
emigrating fish moved toward and then disappeared from the
edge of the array. Tagged fish that were still alive at the end of
each time period were classified as alive and present within the ar-
ray. In instances when position and depth data of tagged fish
were insufficient to differentiate among potential fates, the fate
was classified as unknown.
Mortality (predation or surface mortality) was attributed to
capture and release (i.e. discard mortality) if it occurred within
48 h of release. Point estimates and SEs of discard mortality were
estimated assuming a binomial error distribution (as by Pollock
and Pine, 2007). The point estimate of mortality is ^
where known mortalities (d¼number of fish assigned to either
predation or surface mortality fates) divided by the total number
of “at-risk” tagged fish (n¼number of tagged fish with
known fates excluding individuals assigned the fate of harvest
or emigration). The standard error of estimated mortality is
SE ^
The influence of release method (surface versus at depth with a
descender device), season, reef depth, fish length, presence/ab-
sence of trauma to the mouth or gills from hooking or handling,
presence/absence of barotrauma symptoms, handling time, fight
time, difference between water and air temperature (DTwhere
DT¼Tbottom Tair ), and inadvertent venting (caused by inser-
tion of the anchor tag or a fish’s everted oesophagus or stomach
being punctured by its teeth) on an individual’s probability of
mortality in the immediate time period was explored with non-
linear regression. The predicted probability of mortality for each
individual ( ^
Mifor i¼1... n) was modelled as a function of po-
tential explanatory variables (X
) and model parame-
ters (b
), where ^
1þeb0þb1X1þb2X2þ...þbkXk, with the
total binomial log-likelihood being Pn
Categorical variables such as release method, season, capture
depth, presence/absence of hooking trauma, and presence/ab-
sence of barotrauma were coded as binary variables {0, 1} and M
was the assigned fate of each tagged fish from the telemetry data
(where mortality ¼1, survival to next time period ¼0).
Continuous variables (length, time out of water, fight time, and
DT) were scaled to have mean ¼0 and variance ¼1. For red
snapper where only FL was measured, FL was converted to TL
following TL ¼1:0812 FL 0:950 (in mm; SEDAR, 2018).
Candidate models were evaluated for parsimony with the small-
sample Akaike Information Criterion (AIC
The risk of mortality over time for fish released with a de-
scender device relative to fish released at the surface was evalu-
ated using Cox proportional hazards models (Cox and Oakes,
1984). Cox proportional hazards models describe the risk of
mortality as a function of elapsed time (i.e. are well suited to
staggered-entry designs such as this study where individuals are
tagged and released during multiple events). Proportional haz-
ards models are also well suited to describe the relative risk of
mortality between two different treatments (e.g. fish released at
the surface or at depth with a descender device) and can accom-
modate datasets where individuals leave the study and must be
censored from the model. Fish that were assigned fates of emi-
gration, harvest, tag loss, or were still alive when the acoustic ar-
ray was retrieved were censored at the date of the event. Fish
reported harvested or recaptured either outside the array or af-
ter having lost the acoustic tag were censored from the model
on the date of harvest or recapture, not on the earlier date of tag
loss or emigration. We included the maximum number of cova-
riables (thus minimizing risk of falsely rejecting variables) found
to be informative in the nonlinear regression models of discard
mortality (were within 2 AIC
from the lowest AICc models;
Burnham and Anderson, 2002). Cox proportional hazards mod-
els were fit and model performance evaluated with the
“survival” package in R (Therneau and Lumley, 2019;RCore
Team, 2016). We described goodness of fit (pseudo-r
Effects of rapid recompression on reef fish discard mortality 5
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Cox proportional hazards models as the proportional reduction
in the sum of squared residuals from the null (intercept only) to
the final (with covariables) model.
In total, 141 red snapper ranging from 30.5 to 89.0 cm TL (mean
6SD: 52.1 614.1 cm) and 26 gray triggerfish ranging from 32.1
to 50.0 cm FL (mean 6SD: 41.5 65.5 cm) were tagged with
acoustic transmitter tags and released into the acoustic array
(Table 1). Fight times were mistakenly not recorded for the ma-
jority (n¼30) of red snapper tagged in late summer 2017. A total
of 13 red snapper showed signs of traumatic hooking (i.e. the fish
hook was removed from the fish’s throat, gills, or gut, or bleeding
from the mouth or gills was observed). Approximately half of red
snapper (n¼74) and gray triggerfish (n¼10) were released at
depth using a descender device. There were no observed instances
of a predator removing a tagged fish from the descender device,
although 3 red snapper of 71 descended fish successfully
recorded on video were observed being consumed by a shark
(n¼2) or dolphin (n¼1) shortly after release from the
descender. Sharks or dolphins were present in 25 observed
descender releases (35%).
Offloaded detection data from acoustic receivers yielded posi-
tion estimates for 154 tagged fish. Fish were tracked within the
acoustic array up to 330 d following release. Fate and time to fate
(days post-release) were assigned to each tagged fish by compar-
ing the individual’s movement, swim speed, and depth to known
or inferred behaviour patterns indicating a unique fate.
Movement, swim speed, and depth data available for large sharks
present within our arrays were provided by serendipitous detec-
tion of two large (2.50 and 2.55 m TL) female bull sharks
(Carcharhinus leucas) tagged elsewhere. The calculated mean
swim speed of these two tagged bull sharks within the array was
0.95 m s
(range 0.10–1.54 m s
;Figure 3a). There are no depth
data for these sharks as neither fish’s tag contained a pressure sen-
sor. In addition, movement and depth of feeding sharks were pro-
vided from tags attached to red snapper (45.2 and 46.0 cm TL)
that were observed being preyed upon by large sharks (species un-
certain, estimated length 1.5–2.5 m TL). Both acoustic tags con-
tinued transmitting for several days following consumption by
sharks and provided position estimates up to 50 h following re-
lease. The estimated mean speed of these two consumed tags,
hence sharks, within the array was 0.57 m s
(range 0.00–1.44 m
;Figure 3b). Two tagged red snapper (65.7 and 54.9 cm TL)
were also observed being consumed by bottlenose dolphins,
Tursiops truncatus, following release. The 65.7 cm TL red snapper
was observed by the tagging crew being taken by a dolphin at the
surface and the transmitter tag was detected on the bottom im-
mediately following consumption, suggesting the dolphin did not
consume the tag together with the fish. The 54.9 cm TL red snap-
per was visible on the GoPro video footage being taken into the
mouth of a dolphin at depth following release from the descender
device. That acoustic tag transmitted from depths above the bot-
tom 4 times within approximately 2 min following release, after
which no acoustic transmissions were detected. Although there
were no instances where tagged fish were observed being preyed
upon by dolphins that yielded sufficient detection data to draw
inferences on dolphin movement or behaviour, depth data for a
34.0 cm TL red snapper released at the surface show the tag mov-
ing frequently (several times per hour) from the bottom to within
several metres of the surface for approximately 4 d following re-
lease, suggesting the tag may have been moving with a dolphin as
it travelled frequently to the surface to breathe.
Normal behaviour indicative of living red snapper and gray
triggerfish was inferred from fish that were harvested or recap-
tured after spending time at liberty within the array. Ten tagged
red snapper were recaptured or harvested after being at large be-
tween 30 and 844 d, providing confirmation that these fish were
alive within the array. Although eight of ten red snapper had lost
their acoustic tags prior to being recaptured, review of the VPS
position data revealed that prior to tag loss, harvest, or emigra-
tion from the array, median calculated swim speed was 0.02 m
(range 0.00–0.75 m s
, based on 95 000 positions;
Figure 3c). The frequency distribution of swim speeds was
roughly negative exponential with a mode between 0.0 and 0.1 m
. Two gray triggerfish were harvested by spearfishers after 16
and 20 d at large, respectively. Prior to being harvested, median
calculated swim speed was 0.02 m s
(range 0.00–0.40 m s
based on 8100 positions; Figure 3d). The frequency distribution
of swim speeds for these two gray triggerfish was similar to
the ten recaptured red snapper with a mode between 0.0 and
0.1 m s
A sudden shift in depth or position over time indicated preda-
tion, tag loss, or emigration from the array. For example, a
33.5 cm TL red snapper was tagged and released in the shallow ar-
ray on 14 September 2016. After approximately 8 h, the fish
moved 600 m to an adjacent reef where it remained at depths
greater than 25 m until 19 September 2016 03:00 UTC. A sudden
shift in depth (ranging from the surface to the bottom) and
movement (exiting and entering the array and moving several
kilometers in a single direction) indicated the occurrence of a
predation event (Figure 4a). Tag loss was apparent by a shift from
variable to constant depth and position (Figure 4b). Emigration
events were often assumed to occur when tagged fish left the
acoustic array without first displaying the abrupt shift in speed,
depth, or movement patterns indicative of a predation event
(Figure 4c).
Point estimates of red snapper discard mortality (0–48 h fol-
lowing release) were lowest for descender-released fish in the 30-
m array in late summer (22.7%, SE ¼8.9%) and highest for
surface-released fish in the 55-m array in late summer (80%, SE
¼12.6%) (Figure 5). Overall, red snapper discard mortality for
all tagging events combined was 56.6% (SE ¼6.8%) for surface
released fish and 36.1% (SE ¼6.1%) for descender released fish.
Predation accounted for 77% of all red snapper mortalities and
83% of discard mortalities. Our estimates of discard mortality are
within the upper range of estimates by depth from previous
acoustic telemetry and discard mortality studies of red snapper
(Figure 6;Campbell et al., 2014;Piraino and Szedlmayer, 2014;
Curtis et al., 2015;Williams et al., 2015;Williams-Grove and
Szedlmayer, 2016).
Release method (surface or descender), presence/absence of
traumatic hooking injury, fish length, time out of water, and DT
were all informative variables in regression models of red snapper
discard mortality (Table 2). The Cox proportional hazards model
including these variables (pseudo-r
¼0.73) indicated using a de-
scender device to release red snapper significantly reduced discard
mortality by a ratio of 0.40 (95% CI ¼0.23–0.71; Figure 7a) and
red snapper suffering from traumatic hooking were five times
6E. C. Bohaboy et al.
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Table 1. Summary table of red snapper (RS) and gray triggerfish (GT) that were tagged and released during four tagging events over the
course of this study.
Conditions Fish tagged
(C) Species
Length (cm,
mean 6SD)
Fight time
(s, mean 6SD)
Time out
of water
(s, mean 6SD)
% with
symptoms n
Spring 2016 26
April3 May
28 31 22.7 24.3 20.0 21.4 RS S 45 617 75 661 78 628 30 10
RS D 55 617 63 629 118 639 0 10
GT S 41 6567642 100 635 8 13
GT D 41 6668644 126 627 0 9
Summer 2016
14 September
28 31 26.6 28.8 29.9 RS S 39 6557632 80 617 11 18
RS D 45 612 81 660 95 632 23 22
GT S 45 51 90 0 2
Summer 2017
2 September
51 57 28.5 20.4 21.7 RS S 57 678 ND 74 620 53 15
RS D 59 614 ND 117 674 61 18
Spring 2018
11 24 April
51 57 19.8 20.3 21.0 21.8 RS S 56 612 146 639 94 626 48 23
RS D 58 613 134 636 114 625 83 24
GT S 50 177 85 100 1
GT D 45 168 131 100 1
Length for red snapper is total length and gray triggerfish is fork length.
Figure 3. Calculated swim speeds of (a) tagged bull sharks (n¼2), (b) red snapper that were observed being preyed upon by large sharks in
September 2017 (n¼2), (c) red snapper prior to recapture (n¼10), and (d) gray triggerfish prior to harvest (n¼2).
Effects of rapid recompression on reef fish discard mortality 7
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more likely to experience discard mortality (95% CI ¼2.2–11.2).
Fish length, time out of water, and DTdid not significantly affect
probability of mortality in the Cox proportional hazards model at
alpha ¼0.05 (p¼0.054, 0.283, and 0.630, respectively). However,
the full parameter set chosen a priori based on regression model
selection was retained. When presence/absence of traumatic
hooking injury, fish length, time out of water, and DTwere con-
trolled, estimated discard mortality of surface-released red snap-
per was 60.7% (95% CI ¼44.0–72.4%) and for descender-
released red snapper was 31.2% (95% CI ¼18.8–41.8%).
Gray triggerfish (n¼26) were tagged primarily in spring 2016
in the 30-m array (n¼22). Point estimates of discard mortality
were higher (60.0%; SE ¼15.5%) for descender-released fish
than for surface-released fish (26.7%; SE ¼11.4%). Predation
accounted for 85% of total gray triggerfish mortalities and 100%
of discard mortality. Our dataset did not include enough tagged
gray triggerfish to evaluate the effects of depth, season, baro-
trauma symptoms, traumatic hooking, or accidental venting on
gray triggerfish discard mortality. Of the remaining variables, we
selected the nonlinear regression model for gray triggerfish dis-
card mortality that included fight time, time out of the water, and
fish length, while additional variables such as DTor release
method did not inform the model sufficiently to warrant inclu-
sion (Table 2). The Cox proportional hazards model did not indi-
cate that fight time, time out of water, fish length, or release
method had a significant effect on gray triggerfish survival at the
alpha ¼0.05 level (p¼0.0516, 0.828, 0.098, and 0.339, respec-
tively). When all three covariates were included (pseudo-r
0.77), 95% CIs based on the Cox proportional hazards modelling
of predicted gray triggerfish survival overlapped for surface (95%
Figure 4. Depth (left) and position (right) of (a) 33.5-cm TL red snapper released 14 September 2016 and consumed by a predator 19
September 2016, (b) 37.6-cm TL red snapper released 26 April 2016 and lost the acoustic transmitter tag 13 June 2016, and (c) 43.8-cm FL
gray triggerfish released 14 September 2016 and emigrated from the array 30 December 2016. Normal behaviour of each tagged fish is noted
with dark symbols () whereas squares () indicate the shifted depths and movements associated with each fate (predation, tag loss, and
emigration, respectively). Depth is in m and dates are UTC.
8E. C. Bohaboy et al.
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CI ¼55–100%) and descender (CI ¼22–100%) released fish
(Figure 7b).
Few mortalities of red snapper (seven predation and six
reported harvests of fish after tag loss or emigration from the ar-
ray) or gray triggerfish (one predation, two harvest) were ob-
served beyond the first 48 h following release (Table 3).
Emigration of tagged fish from an array within 14 d following re-
lease was rare, as only 4 red snapper were inferred to have emi-
grated from an array within 48 h following release, but there were
an additional four red snapper and two gray triggerfish that emi-
grated from an array within 14 d. Ultimately, 14 red snapper and
7 gray triggerfish (25 and 54% of at-risk fish for each species) em-
igrated from an array 14–269 d following release. The mean time
to emigration was 44 d (6SD: 46 d) for red snapper and 118 d
(6SD: 99 d) for gray triggerfish. Acoustic tag losses were highest
for the Domeier dart attachment used in late summer 2017
(13.8% tag loss within 48 h, 20.7% within 14 d). Otherwise, the
highest 14-d cumulative tag loss using the threaded bar attach-
ment was 5.1% for red snapper tagged in spring 2018. The
threaded bar attachment for gray triggerfish also had low tag loss
rates. Of 26 gray triggerfish that were tagged in 2016, only one
was estimated to have lost its acoustic tag (330 d after release).
Predation by highly mobile predators within several hours of re-
lease was the dominant source of discard mortality for acousti-
cally tagged red snapper (83%) and gray triggerfish (100%).
Traditional mark-recapture, laboratory, and enclosure
approaches utilized to estimate discard mortality do not explicitly
account for predation, although authors of several studies have
drawn inferences of fish behaviour or included observations
indicating that predation may be an important contributor to dis-
card mortality, at least in the case of red snapper (Campbell et al.,
2010;Drumhiller et al., 2014). Authors of most discard mortality
studies using acoustic telemetry methods identified mortality
events by lack of movement (freshwater/estuarine: Hightower
et al., 2001;Bacheler et al., 2009, marine: Curtis et al., 2015;
Runde and Buckel, 2018), implying that following release fish
succumbed directly to handling injury, starvation, cold-kill, or
disease. Data from the current study suggest, at least in the ma-
rine environment where large predators may be present or abun-
dant, fish that are compromised following capture and release
may be more likely to be consumed by predators before they die
and settle to the bottom. The large spatial coverage and position-
ing accuracy of our acoustic telemetry arrays, combined with
observations of large tagged sharks moving within the arrays or of
sharks consuming tagged fish, provided the unique ability to ex-
plicitly identify predation events. Researchers using acoustic te-
lemetry to estimate mortality of tagged fish have inferred the
occurrence of predation events from movement, depth, or accel-
eration data (Heupel and Simpfendorfer, 2002;Runde and
Buckel, 2018); however, data in our study indicate predation
accounted for the majority of discard mortality observed in
acoustically tagged red snapper and gray triggerfish. This result
may be location-specific, with an abundance of large coastal
sharks or dolphins occurring in our region, or it may indicate
that the large spatial coverage of our study and the inclusion of
depth sensors on tags enabled us to identify predation events that
otherwise would have gone undetected.
Our large-area acoustic arrays enabled us to differentiate be-
tween tagged fish which emigrated from an array under their own
volition and transmitters which moved out of an array with a
predator that had consumed a tagged fish. We observed that fol-
lowing predation by a large shark, transmitters moved away from
Figure 5. Estimated 48-h % mortality for red snapper (RS) and gray
triggerfish (GT) released at the surface versus at depth with
descender devices. Error bars represent 95% CIs as 1.96*SE following
Pollock and Pine (2007). Sample size (number of fish with known
fates at the end of 48 h post-release) is above each point estimate.
Figure 6. Estimated red snapper discard mortality (%) by release
method. Data from prior studies (open symbols) were compiled in
Campbell et al. (2014), with more recent estimates from Piraino and
Szedlmayer (2014),Curtis et al. (2015),Williams et al. (2015), and
Williams-Grove and Szedlmayer (2016).
Effects of rapid recompression on reef fish discard mortality 9
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the tagging site immediately, and often moved out of the detec-
tion area of the array within several hours. Surviving fish, in con-
trast, rarely moved away from the reef within hours of being
tagged. Results of several acoustic telemetry studies have indi-
cated a large number of tagged red snapper and gray triggerfish
apparently leave the detection area surrounding tagging reefs
within 2–6 d post-tagging (Szedlmayer and Schroepfer, 2005;
Piraino and Szedlmayer, 2014;Herbig and Szedlmayer, 2016).
These individuals, which can be >30% of all tagged animals, typi-
cally have been classified as tagging-induced emigrations and
censored from further analyses when the objective of a study was
to examine fish behaviour or survival separate from discard or
tagging mortality. However, when the primary objective is to esti-
mate discard mortality with acoustic telemetry data, it is vital to
accurately distinguish between emigration from the tagging site
and predation, as censoring these individuals from the post-
release survival data may greatly underestimate discard mortality.
Our overall red snapper discard mortality estimates are within
the range of discard mortality estimates reported by other authors
but also generally higher than most previous estimates at
Figure 7. Cox proportional hazards model-estimated survival (695% CIs) for surface-released () and descender-released () fish. (a) Red
snapper models included presence/absence of traumatic hooking injury, fish length, time out of water, and the change in temperature from
the bottom water to the air as covariables (pseudo-r
¼0.73) and (b) gray triggerfish models included fight time, time out of the water, and
fish length as covariables (pseudo-r
Table 2. Nonlinear regression models testing factors affecting discard mortality for red snapper and gray triggerfish.
Model nk -LL DAIC
Red snapper
Mb0þDTþdescender þtrauma þlength 140 5 65.9 0
Mb0þDTþdescender þtrauma þlength þtimeout 139 6 65.7 1.7
Mb0þDTþdescender þtrauma 140 4 68.3 2.6
Mb0þDTþdescender þtrauma þlength þtimeout þseason 139 7 65.5 3.6
Mb0þDTþdescender 140 3 71.4 6.7
Mb0þDT141 2 74.5 10.8
Mb0141 1 79.2 18.1
Gray triggerfish
Mb0þtimefight þtimeout þlength 21 4 8.5 0
Mb0þtimefight þtimeout 21 3 10.1 0.2
Mb0þtimefight þtimeout þlength þDT21 5 8.3 3.1
Mb0þtimefight 22 2 13.6 4.4
Mb0þtimefight þtimeout þlength þDTþdescender 21 6 7.9 6.4
Mb026 1 16.8 8.3
values are shown relative to the model with the lowest AIC
For gray triggerfish, not all levels of some variables contained samples, thus effects of depth, barotrauma symptoms, accidental venting, traumatic hooking, DT,
and season on discard mortality could not be explored.
10 E. C. Bohaboy et al.
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comparable depths. This may be because our approach to estimat-
ing discard mortality, with large-scale three-dimensional tracking
of reef fish over weeks to years, was better able to detect predation
events than existed in previous studies. If we exclude predation
from discard mortality (by censoring all tagged fish that were iden-
tified as predation mortalities), estimated red snapper overall dis-
card mortality in the 30-m array (regardless of release method or
season) would drop from 36.8% (including predation) to 5.3%
(excluding predation), corresponding to a reduction from approxi-
mately the 90th percentile to the 10th percentile of estimates from
previous studies at depths from 25 to 35 m (Figure 6). For red
snapper released in the 55-m array, excluding predation as a source
of mortality reduced estimated discard mortality from 54.4% (70th
percentile of previous studies at depths from 50 to 60 m) to 21.2%
(lower estimate than previous studies at comparable depths;
Figure 6). Many coastal and offshore shark populations in the
southeast US and GOM have begun to recover in recent years, with
positive trends expected to persist (Peterson et al., 2017;SEDAR,
2017). Increased shark abundance, including bull sharks
(Froeschke et al., 2013), could lead to higher predation rates on
recreationally released fish, explaining the high estimates in this
study and suggesting that predation may be an increasingly impor-
tant driver of discard mortality of reef fish in coming years as shark
populations continue to recover.
Descender devices approximately halved estimated red snapper
discard mortality in this study. There have been very few compa-
rable studies of descender devices where released fish were at risk
of predation. In several of these investigations, the authors con-
cluded that releasing fish with descender devices substantially de-
creased discard mortality (e.g. by 67% for red snapper, Curtis
et al., 2015; by 98% for yelloweye rockfish Sebastes ruberrimus,
Hochhalter and Reed, 2011). However, there was no significant
benefit of releasing fish with descender devices for six additional
species investigated in mark-recapture and acoustic telemetry
studies (Sumpton et al., 2010;Eberts et al., 2018). Laboratory and
enclosure studies that exclude predation are much more numer-
ous and are more likely to conclude little or no effect of descender
devices on discard mortality (e.g. Roach et al., 2011;Butcher
et al., 2012;Ng et al., 2015), with Drumhiller et al. (2014) being a
notable exception (red snapper survival was estimated to be 17%
for surface-released and 83% for experimentally recompressed
fish). We found the effect of descender devices on discard mortal-
ity of gray triggerfish was not statistically significant, largely due
to small sample size. Increased handling (fight time and time out
of water) and decreased fish size appeared to have a positive effect
on discard mortality; however, none of the variables investigated
were deemed to have a significant effect on gray triggerfish dis-
card mortality. All gray triggerfish tagged in this study were cap-
tured from shallow (30 m) depth and none were observed to be
suffering from barotrauma, so it is possible that confounding
effects of additional handling and delayed return to depth ob-
scured any benefits of being returned to depth with the descender
Aside from release method, the presence of traumatic hooking
injury greatly affected discard mortality of red snapper, which is
widely supported by results of previous investigations that suggest
throat-, gut-, or gill-hooked fish have low survival probability
(Muoneke and Childress, 1994;Murphy et al., 1995). Reef depth,
presence/absence of barotrauma symptoms, fish size, and season
did not have an apparent effect on discard mortality of red snap-
per or gray triggerfish. This was somewhat surprising given baro-
trauma is a primary contributor to discard mortality and fish size
is expected to influence physiological stress and the degree of
barotrauma that fish experience. However, many barotrauma
symptoms may be cryptic and not detectable without internal ex-
amination of fish tissues and organ systems (Rummer and
Bennett, 2005). Fishing depth profiles from spring 2018 tagging
also indicated fish were rarely captured near the bottom and in-
stead were hooked and retrieved from a range of mid-water
depths. The absence of a depth effect may be due to this observa-
tion error because we had to rely on reef depth in our discard
mortality models since we did not have capture depth data for
most of our tagged fish. Similarly, the categorical variable season
may not have adequately described the physiological stressors
that fish were exposed to during each of the four tagging events.
It is possible that seasonal variation in predation pressure ob-
scured any apparent effect of temperature-induced physiological
stress that tagged fish experienced. For example, fish tagged and
released in the early spring may have experienced less
temperature-induced physiological stress but were exposed to
more actively feeding sharks. Instead of season, we chose to use
the change in temperature between bottom water and air
Table 3. Number of tagged fish in each fate assignment category by
time period following release.
Time post-release
Fate 0–48 h 2–14 d 14þd
Red snapper: surface released
Predation mortality 21 2 4
Surface mortality 9 0 0
Harvest mortality 0 0 0
Emigration 3 1 3
Tag lost 3 0 11
Unknown 7 0 0
Alive and present 23 20 2
Red snapper: descender released
Predation mortality 22 1 0
Surface mortality 0 0 0
Harvest mortality 0 0 1
Emigration 1 3 12
Tag lost 3 3 13
Unknown 9 0 0
Alive and present 39 32 6
Gray triggerfish: surface released
Predation mortality 4 0 1
Surface mortality 0 0 0
Harvest mortality 0 0 1
Emigration 0 2 6
Tag lost 0 0 0
Unknown 1 0 0
Alive and present 11 9 1
Gray triggerfish: descender
Predation mortality 6 0 0
Surface mortality 0 0 0
Harvest mortality 0 0 1
Emigration 0 0 1
Tag lost 0 0 1
Unknown 0 0 1
Alive and present 4 4 0
“Alive and present” includes tagged fish that were identified as alive with
acoustic tag attached and within a study array at the end of each time period
(the end of the 14
d time period was the retrieval of the acoustic array).
Effects of rapid recompression on reef fish discard mortality 11
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temperature to reflect the amount of temperature stress that fish
experienced, which was significant in the red snapper discard
mortality nonlinear regression model (a larger increase in tem-
perature from the bottom water to the air increased mortality).
The overall lowest red snapper discard mortality occurred in late
summer 2016 when the bottom water temperature was greater
than the air temperature (i.e. fish experienced a 2–3C tempera-
ture drop when they were brought to the surface and removed
from the water for tagging). The highest overall discard mortality
occurred in late summer 2017 when air temperature was compa-
rable to late summer 2016 but the bottom water temperature was
7C colder than the air temperature on the day of tagging.
We believe our estimates of discard mortality reduction due
to descender devices may be conservative since seasoned fishers,
in particular crew on for-hire fishing vessels, could streamline
the rigging of the descender device (reducing the amount of
weight and excluding the video cameras to reduce drag and in-
crease retrieval speed) while also reducing handling time relative
to fishes tagged in this study. We did not examine the effect of
venting fish on discard mortality and chose instead to evaluate
the efficacy of descender devices compared with unvented
surface-released fish. Results are equivocal regarding the benefits
of venting and some investigators suggest that many fishers im-
properly vent fish, which can damage organs and increase dis-
card mortality (Wilde, 2009;Scyphers et al.,2013;Eberts and
Somers, 2017). In contrast, neither we nor previous researchers
to our knowledge have presented evidence that descender devi-
ces cause harm to fish. Recreational fishers will undoubtedly
play the primary role in efforts to reduce discard mortality, and
although venting may continue to be the preferred method of
discard mortality reduction among fishers in many instances
(Crandall et al., 2018), further evidence supporting the efficacy
of descender devices could facilitate acceptance among GOM
reef fish fishers.
Continuing advances in geopositioning acoustic technology,
including reduced costs, enable researchers to deploy more
receivers covering larger areas. Greater spatial coverage within
studies, combined with the proliferation of cooperative net-
works that foster equipment sharing and information exchange
among researchers (Hussey et al., 2015), will be instrumental to
improving studies of discard mortality of marine fishes. Future
investigations in the marine environment should be designed to
measure the effects of predation on the survival of discarded
fish or else risk ignoring this potentially significant driver of
mortality. The quantification of dead discards and efforts to
reduce discard mortality will be increasingly vital considerations
in recreational fisheries around the world where recovering
population abundances, harvest prohibitions, or non-
consumptive attitudes of fishers results in large numbers of dis-
carded fish.
We thank research assistants and students at the Universities of
Florida and South Alabama for field and laboratory support.
David Nelson and Michael Nelson at the University of South
Alabama, Department of Mechanical Engineering provided valu-
able assistance and feedback on initial designs of external acoustic
tag attachments. We are indebted to charter boat captains and
crews for collaborating on this research: Captains Johnny Greene,
Gary Jarvis, and Sean Kelley as well as Crew members of F/Vs
Intimidator, Backdown II, and Total Package. Lastly, we would
like to recognize the Integrated Tracking of Aquatic Animals in
the Gulf of Mexico (iTAG) Network for facilitating communica-
tion, data exchange, and collaboration among acoustic telemetry
researchers in the Gulf of Mexico.
We thank the National Marine Fisheries Service Cooperative
Research Program for funding this research (Grant numbers
NA15NMF4540103 and NA16NMF4540086) and the joint
National Marine Fisheries Service/Sea Grant Population and
Ecosystem Dynamics Graduate Research Fellowship for providing
doctoral funding to E.C.B.
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Handling editor: Erika J. Eliason
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... A closer look from a recent study however, suggests that 50% of published articles that use AT to understand fish movement or ecology, fail to incorporate or consider mortality within their study, while those that did estimate an ~11% loss on average of tagged individuals from the system . This is pertinent as transmitters will continue to be detected even after depredation, leading to movement patterns that reflect the predator rather than the prey species (Bohaboy et al., 2020). Even those that survive but leave the array, and thus exhibit different behaviour to individuals typically included in analyses, remain rarely discussed in studies on movement. ...
... Technical and logistical challenges in deploying deep water arrays have constrained the majority of AT studies to depths under 50 m (Loher et al., 2017), and bringing physoclistous species to the surface to tag poses the risk of damage and mortality due to barotrauma and post-release predation (e.g. Bohaboy et al., 2020;Curtis et al., 2015). The increasing use of in-situ tagging methods at depth and improvements to surface tagging protocols such as employing descender devices and rapid tag attachment methods to minimise time at the surface will further unlock the huge potential of AT to study fish movements and population dynamics in the deep sea Runde & Buckel, 2018). ...
... It was recently estimated, using a 3D acoustic positioning array in the Gulf of Mexico, that 83% of red snapper (Lutjanus campechanus) and 100% of gray triggerfish (Balistes capriscus) mortality was a result of post-release depredation. However, for snapper at least, releasing individuals with descender devices (weighted devices that assist in returning the fish to depth), did significantly reduce mortality (Bohaboy et al., 2020). It is important to remember of course that once collected, AT data might also reveal unintended insight. ...
Full-text available
Acoustic telemetry (AT) has become ubiquitous in aquatic monitoring and fish biology, conservation and management. Since the early use of active ultrasonic tracking that required researchers to follow at a distance their species of interest, the field has diversified considerably with exciting advances in both hydrophone and transmitter technology. Once a highly specialised methodology however, AT is fast becoming a generalist tool for those wishing study or conserve fishes, leading to diversifying application by non-specialists. With this transition in mind, we evaluate exactly what AT has become useful for, discussing how the technological and analytical advances around AT can address important questions within fish biology. In doing so, we highlight the key ecological and applied research areas where AT continues to reveal crucial new insight, and in particular, when combined with complimentary research approaches. We aim to provide a comprehensive breakdown of the state of the art for the field of AT, discussing the ongoing challenges, where its strengths lie, and how future developments may revolutionise fisheries management, behavioural ecology and species protection. Through selected papers we illustrate specific applications across the broad spectrum of fish biology. By bringing together the recent and future developments in this field under categories designed to broadly capture many aspects of fish biology, we hope to offer a useful guide for the non-specialist practitioner as they attempt to navigate the dizzying array of considerations and ongoing developments within this diverse toolkit.
... At this time, downloading range test data conducted in situ can also provide helpful in sight into system performance and detection efficiency. Confirmation of post-release behaviour and fate through the use of observational SCUBA surveys and underwater video (Bohaboy et al. 2020) can also be used to inform later data filtering choices. Additional in-water sampling techniques can also be used to confirm theorised behavioural states, e.g. ...
... Filtering steps can include: (1) the removal of detections recorded during post-tagging recovery periods, which depend on perceived animal sensitivity to capture and handling, e.g. those at high risk of barotrauma ) and nocturnal animals (Itakura et al. 2018); (2) the removal of continuous static detections suggestive of direct mortality (see Bohaboy et al. 2020, Everett et al. 2020, Bacheler et al. 2021 for recommendations) or unrealistic swim speeds indicative of a predation event (Barilotti et al. 2020, Schlaff et al. 2020; (3) identification and isolation of periods of poor system performance due to reduced detection efficiency, and; (4) consideration of horizontal positioning error (HPE). Critical data evaluation and sensible data filtering and error assessment ensure that resultant findings are ecologically meaningful and not an artefact of study design and implementation or environmental-habitat conditions. ...
Fine-scale tracking technology has advanced our understanding of aquatic animal behaviour by deriving near-continuous movements of animals ranging in size from small invertebrates to large predatory fish. Commercial fine-scale positioning systems, such as the VEMCO Positioning System, can pinpoint an animal’s location within metres of its true position. While methodological applications of commonly used presence-absence acoustic telemetry have identified factors that can limit array performance, the factors that influence position yield and accuracy and introduce error in fine-scale positioning systems have yet to be synthesised. Evidenced through a systematic review of the literature, we highlight key considerations and potential pitfalls faced when designing and conducting a fine-scale tracking study. Key factors impacting data acquisition are grouped under 4 key categories linked to the study system, species studied, and logistical and technological constraints. Thereafter, in line with these categories, we provide a framework that can be used prior to, during, and post-study to identify sources of error and data loss to optimize system design and acquired results. We provide details on user assessment tools that include a pre-study trial period using fixed tags to assess array geometry and data yield, an in situ checkpoint data download, and a post-study assessment of fixed transmitter performance. We highlight the utility of this framework and integrated assessment tools by presenting a real-world case study that ultimately was compromised. We anticipate that this framework can be used to standardize reporting of essential steps and checks that will generate comparable data for future synthesis, which will further advance fine-scale tracking approaches.
... predicted to resist depth-related trauma. In a recent study in which discard mortality in 2 species, the red snapper and gray triggerfish (Balistes capriscus), was examined, descender device use was found to be associated with a decrease in PRM for red snapper but not for gray triggerfish (Bohaboy et al., 2020). In addition, barotrauma risk increases with capture depth. ...
Fish telemetry is an important tool for studying fish behavior, allowing to monitor fish movements in real-time. Analyzing telemetry data and translating it into meaningful indicators of fish welfare remains a challenge. This is where entropy approaches can provide valuable insights. Methods based on information theory can quantify the complexity and unpredictability of animal behavior distribution, providing a comprehensive understanding of the animal state. Entropy-based techniques can analyze telemetry data and detect changes in fish behavior, or irregularity. By analyzing the accelerometer data, using entropy approach, it is possible to identify atypical behavior that may be indicative of compromised welfareKeywordsShannon entropyfish telemetrybiotelemetrydata processingatypical behaviortypical behaviorfish welfare
Full-text available
Greater amberjack are active swimmers that occur over a range of depths around reefs and artificial structures. In this study, we describe overall diel patterns of depth use and swimming activity for greater amberjack. We tested for impacts of fishing-related stress on diel patterns, and we describe the potential influence of cyclonic storms on diel behavior and site attachment. Fifty-five fish were tagged with depth and accelerometer tags off coastal Alabama, USA, in three field efforts (FEs): FE1, summer–fall 2018; FE2, spring 2019; and FE3, summer–fall 2020. Fish displayed diel depth patterns that varied among individuals: fish that occupied a narrow depth range midway within the water column, fish that spent time both at mid-water and near bottom, and fish with more generalized patterns. Diel depth use was best predicted by both site depth and field effort, while swimming patterns were best predicted by field effort among tested factors: legal or sublegal sizes, field effort, site, and use of a descender device vs. surface release. Swimming activity was elevated on post-release days 1–5, consistent with predicted impacts of fishing stress. For most fish, however, depth use did not vary appreciably following release. Release with a descender did not prevent altered diel patterns, and thus, barotrauma was not likely a factor. Fish occupied shallower depths when Hurricane Laura (2020) passed south of the study area, and swimming activity was elevated during Hurricane Sally (2020). Our study highlights that depth use and swimming activity of greater amberjack are influenced by multiple factors and recreational fishing discards may impart sublethal stress that results in elevated swimming activity.
Full-text available
Shark depredation is a complex social-ecological issue that affects a range of fisheries worldwide. Increasing concern about the impacts of shark depredation, and how it intersects with the broader context of fisheries management, has driven recent research in this area, especially in Australia and the United States. This review synthesises these recent advances and provides strategic guidance for researchers aiming to characterise the occurrence of depredation, identify the shark species responsible, and test deterrent and management approaches to reduce its impacts. Specifically, the review covers the application of social science approaches, as well as advances in video camera and genetic methods for identifying depredating species. The practicalities and considerations for testing magnetic, electrical, and acoustic deterrent devices are discussed in light of recent research. Key concepts for the management of shark depredation are reviewed, with recommendations made to guide future research and policy development. Specific management responses to address shark depredation are lacking, and this review emphasizes that a “silver bullet” approach for mitigating depredation does not yet exist. Rather, future efforts to manage shark depredation must rely on a diverse range of integrated approaches involving those in the fishery (fishers, scientists and fishery managers), social scientists, educators, and other stakeholders.
Full-text available
Red snapper and gray triggerfish are ecologically, economically, and culturally important reef fishes in the northern Gulf of Mexico (nGOM). Scientists and managers have sought to understand the effects of artificial reefs on reef fish ecology by focusing on fish residency and movement at artificial reefs with less attention paid to broader spatial and temporal patterns in reef fish movements among a seascape of artificial reefs and other natural habitats. We used novel large-scale (> 15 km2) geopositioning acoustic telemetry arrays to track the 3-dimensional movements of tagged red snapper (n = 59) and gray triggerfish (n = 15) among multiple nGOM artificial reefs up to 333 days. Tagged fish moved frequently among artificial reefs and had shorter residence times at the release reef (43 days for red snapper and 3 days for gray triggerfish) than reported in previous studies. Both species displayed high individual variability in movement dynamics, as well as seasonally variable diel patterns of habitat use, height above bottom, and distance to reefs, which may have been driven by dynamic influences of predation risk, physiological constraints, or foraging over time and space. The wider seascape view revealed in this study demonstrates the importance of including multiple artificial reefs over long timescales to capture individual, spatial, and temporal variability in reef fish movement.
Descender devices are increasingly recognized as a leading means of barotrauma mitigation for released reef fishes. Yet, some resource users oppose regulations requiring or encouraging descender device use, arguing that predators frequently eat fish during release (depredation), sometimes causing device loss. We synthesized data for over 1,200 descended releases (both videoed and non‐videoed) of 16 species of reef fish off North Carolina, USA. Of 114 videos, we observed possible predators on seven, none of which showed actual depredation. Of 1,176 non‐videoed releases, we lost zero descender devices, indicating that, although cryptic depredation may have occurred, equipment loss was nonexistent. The lack of any evidence of depredation in ocean waters off North Carolina provides information to managers that they can use to reinforce education and outreach to encourage descender use.
Full-text available
Barotrauma causes stress and impairment in fish and can cause mortality after catch and release. Relief of barotrauma symptoms is necessary to reduce mortality, but we currently know little about sublethal effects associated with relief methods. Here, we assess the condition and behavior of tournament-caught Walleye Sander vitreus with barotrauma by using three popular relief methods: 1) swim bladder venting, 2) deep-water release (descending), and 3) livewell reorientation with fin weights. In a short-term ex situ experiment, 50% of untreated fish with barotrauma did not recover sufficiently to be released after 20 h. Fin weighting immediately improved condition by enabling fish to regain correct orientation; however, only 53% of fin-weighted fish recovered sufficiently to be released. All vented fish were negatively buoyant, but 73% were releasable after the holding period. In a concurrent in situ study, acoustic telemetry showed that Walleye without barotrauma (controls) made variable postrelease movements (total distance: 5.1–27.6 km), descended fish behaved similarly to controls (4.7–28.6 km), and vented fish made the shortest movements (2.6–16.7 km). However, there were no statistically significant differences in distance metrics among groups. Control and descended fish used larger areas and volumes of the lake than vented fish. Descended fish also used significantly deeper depths than vented fish, and control fish were intermediate in the depth used. Telemetry did not indicate mortality of any fish in the in situ study. Our data suggest that without treatment, mortality of Walleye with barotrauma could be as high as 50%. Fin weighting is not an effective catch-and-release aid for Walleye with moderate-to-severe barotrauma, and swim bladder venting may alter short-term, postrelease movements and habitat use. The consequences of these short-term changes to Walleye behavior from a fisheries management perspective are unclear. Eliminating catch-and-release angling in deep water is the best means of managing barotrauma in Walleye. If deep-water angling cannot be avoided, we recommend noninvasive descending over venting.
Full-text available
Discard survival of deepwater (>60 m) groupers (Serranidae; Epinephelinae) is often assumed to be 0% given the severity of barotrauma and the inability of fish to submerge. We used acoustic telemetry to study the activity of 19 deepwater grouper after a recompressed release with a descender device, achieved by rapidly returning fish to a depth where expanded gases can contract. The species tested were the Scamp Mycteroperca phenax (n = 8), Snowy Grouper Hyporthodus niveatus (n = 7), and Speckled Hind Epinephelus drummondhayi (n = 4). Individuals of all three species showed post‐recompression variation in water depth and acceleration indicative of survival, whereas information from other tags indicated discard mortality. Nonparametric Kaplan–Meier survivorship procedures yielded a 14‐d survival estimate of 0.50 (95% confidence interval = 0.10–0.91); although low, this estimate is higher than the currently assumed 0% survival. Additionally, our estimate of discard survival is likely biased low because we assumed that no individuals shed their tag, which is unlikely for our attachment method. A technique to increase discard survival of deepwater groupers may lead to better‐constructed regulations for reef fishes in the southeastern USA and in other areas where these species are caught and released.
Full-text available
Descending fish to depths of neutral buoyancy is a promising, less-invasive alternative to swim bladder venting for relieving barotrauma and reducing mortality in sport fish. However, we lack a broad perspective on the relative benefits of these two approaches. We reviewed the most up-to-date literature to evaluate the effectiveness of venting compared to descending treatments. Mean relative risk (RR) based on 76 published comparisons (51 marine, 25 freshwater; 18 genera, 28 species) showed that venting (2.0 ± 4.7 [mean ± SD]) and descending (1.6 ± 1.4) both had positive effects on survival (RR ≥ 1.1). However, RR was generally small and statistically indistinguishable between treatments, providing no strong support for the use of one method over the other. Modeling of factors affecting RR showed that the study design variable “assessment method” was the only important factor affecting RR, having a larger influence on survival than habitat, capture depth, or treatment type (venting versus descending). Biotelemetry and ex situ methods produced significantly higher estimates of RR than other assessments. Our review suggests that the two major approaches to barotrauma relief do not differentially influence survival outcomes and that study design may be an important source of bias. Consequently, we recommend that managers consider barotrauma relief options carefully on a case-by-case basis, and we encourage additional research on sublethal endpoints in addition to mortality. Received November 29, 2016; accepted March 9, 2017
Full-text available
Relative abundance of many shark species in the Atlantic is assessed by compiling data from several independently conducted, but somewhat spatially limited surveys. Although these localized surveys annually sample the same populations, resulting trends in yearly indices often conflict with one another, thereby hindering interpretation of abundance patterns at broad spatial scales. We used delta-lognormal generalized linear models (GLMs) to generate indices of abundance for seven Atlantic coastal shark species from six fishery-independent surveys along the US east coast and Gulf of Mexico from 1975 to 2014. These indices were further analysed using dynamic factor analysis (DFA) to produce simplified, broad-scale common trends in relative abundance over the entire sampled distribution. Effects of drivers including the North Atlantic Oscillation index, the Atlantic Multidecadal Oscillation index, annually averaged sea surface temperature and species landings were evaluated within the DFA model. The two decadal oscillations and species landings were shown to affect shark distribution along south-east US coast. Estimated common trends of relative abundance for all large coastal shark species showed similar decreasing patterns into the early 1990s, periods of sustained low index values thereafter and recent indications of recovery. Small coastal shark species exhibited more regional variability in their estimated common trends, such that two common trends were required to adequately describe patterns in relative abundance throughout the Gulf of Mexico and Atlantic. Overall, all species’ (except the Gulf of Mexico blacknose shark) time series concluded with an increasing trend, suggestive of initial recovery from past exploitation.
Technical Report
Full-text available
In 2014, NOAA Fisheries conducted a nationwide survey of anglers about their annual purchases of durable goods used for saltwater recreational fishing. Data on angler expenditures allows NOAA Fisheries to better understand the economic contributions of marine recreational fishing to the nations’ economy. The sample frame consisted of licensed anglers in coastal states across the U.S. Anglers were asked about their purchases of durable goods such as rods and reels, boats, vehicles, and second homes used for saltwater fishing in the prior 12 months in the state of licensure. Across all coastal states, a total of 10,809 completed surveys were returned with an overall response rate of 11 percent. Mean expenditures for different durable good categories were estimated for both resident and non-resident fishermen in each state. Total expenditures were calculated and used to estimate total economic contributions by state and for the nation. At the national level marine anglers spent $28 billion on fishing equipment and durable goods (e.g., fishing rods, fishing tackle, and boats). These expenditures generated an estimated $49.6 billion in total output, added $29 billion in contribution to gross domestic product, contributed $18 billion to personal income, and supported more than 358 thousand jobs across the United States. I. INTRODUCTION
We estimated condition-specific survival rates of gray triggerfish (Balistes capriscus) using a tag-recapture approach and extrapolated these values to produce an overall discard survival estimate for the US South Atlantic recreational hook-and-line fishery. Tag return rates of fish tagged at the seafloor using SCUBA served as a reference for return rates of fish tagged at the surface. We examined the validity of gross necropsy as a proxy for survival by identifying likely causes of discard mortality. Best-condition surface-released fish (no external trauma) had an estimated mean proportional survival of 0.39 (95% confidence interval 0.28, 0.55). For gray triggerfish exhibiting visible trauma, estimated survival was 0.24 (0.10, 0.60). Floating fish had a survival rate of zero. The necropsy-based estimate of gray triggerfish lacking organ displacement closely matched the tag-based estimate of survival. Mean estimated discard survival across all depths for North Carolina was 0.35 (0.10, 0.59) and for Florida was 0.34 (0.08, 0.59). These results have implications for gray triggerfish management because our estimate of discard survival is substantially lower than previously assumed and for future discard survival research given our findings with gross necropsies.
This monograph contains many ideas on the analysis of survival data to present a comprehensive account of the field. The value of survival analysis is not confined to medical statistics, where the benefit of the analysis of data on such factors as life expectancy and duration of periods of freedom from symptoms of a disease as related to a treatment applied individual histories and so on, is obvious. The techniques also find important applications in industrial life testing and a range of subjects from physics to econometrics. In the eleven chapters of the book the methods and applications of are discussed and illustrated by examples.
We used the Vemco Positioning System (VPS) to estimate mortalities from the fine-scale movements (~1-m accuracy) of Red Snapper Lutjanus campechanus on four artificial reef sites in the northern Gulf of Mexico in 2012, 2013, and 2014. Additional receivers on surrounding reef sites validated emigrations of tagged Red Snapper from the VPS-monitored sites. We tagged and released 86 Red Snapper and tracked 59 fish for extended periods (17 to 1,096 d). Telemetry tracking patterns identified fish status as active, emigrated, caught (F), or dead (M) at monthly intervals. At the end of the study, 17 fish had emigrated, 24 were caught by fishers, and 18 were active on VPS-monitored reef sites. For all years combined, annual fishing mortality was F = 0.44 (0.27–0.65, 95% confidence limit). In 2012, F = 0.72 (0.35–1.31) and was higher than other years, but the number of fish available for recapture at the start of the sportfishing season was low (n = 15). In 2013, F = 0.18 (0.07–0.42; n = 30), and in 2014, F = 0.42 (0.22–0.76; n = 28). One natural mortality (M) was detected in 2012, and M = 0.12 (0.02–0.69); no subsequent natural mortalities were detected in 2013 and 2014 (M = 0). Total instantaneous mortality (Z) for all years was Z = 0.48 (0.30–0.70). We attributed the low M to high fishing mortality but caution that sample sizes were small, which is typical of telemetry studies. The fates of 58 (98%) transmitter-tagged Red Snapper were successfully identified based on the VPS technology. Increases in F from 0.18 (2013) to 0.42 (2014) occurred when the length of the fishing season was decreased (42 to 9 d) and indicated that fishers increased effort during the shortened fishing season, and the management goal of reducing catch may not have been achieved. Received July 20, 2015; accepted April 20, 2016