![Diagrams of the subsurface holding pens used to monitor Walleye survival: (A) sub-surface holding pen suspended 30 cm off the bottom by a rope connected from the holding pen to the top of the ice-fishing hole; and (B) a magnified view of the holding pen, featuring the drawstring used to quickly open and close the holding pen during fish transfers. [Color figure can be viewed at afsjournals.org.]](https://www.researchgate.net/profile/Robert-Lennox/publication/324252135/figure/fig1/AS:614267527127059@1523464199572/Diagrams-of-the-subsurface-holding-pens-used-to-monitor-Walleye-survival-A-sub-surface_Q320.jpg)
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ARTICLE
The Postrelease Survival of Walleyes Following Ice-Angling on Lake
Nipissing, Ontario
W. M. Twardek,* R. J. Lennox, M. J. Lawrence, J. M. Logan, P. Szekeres, and S. J. Cooke
Fish Ecology and Conservation Physiology Laboratory, Department of Biology and Institute of Environmental Science,
Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada
K. Tremblay and G. E. Morgan
Ontario Ministry of Natural Resources and Forestry, 3301 Trout Lake Road, North Bay, Ontario P1B 8G4, Canada
A. J. Danylchuk
Department of Environmental Conservation, University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA
Abstract
Natural resource agencies have developed catch-and-release regulations for Walleyes Sander vitreus of prohibited
size and number to reduce mortality in many recreational fisheries. The efficacy of such regulations is contingent upon
the released fish surviving, but survival data on Walleyes captured by ice-angling are lacking. We estimated the sur-
vival of Lake Nipissing (Ontario, Canada) Walleyes that were captured by both active and passive ice-angling meth-
ods using a variety of hook types and lures baited with Emerald Shiners Notropis atherinoides. We also assessed the
role of de-hooking methods on the survival of deeply hooked Walleyes. After the angling event, Walleyes (n=260)
were held for 24 h in a submerged holding pen to estimate postrelease survival. Average mortality after the 24-h hold-
ing period was 6.9%. Fewer Walleyes captured by active angling were deeply hooked (9.3%) than passively caught
fish (50.4%), and deeply hooked Walleyes were observed to have more frequent postrelease mortality (14.8%) than
shallow-hooked Walleyes (3.0%). There was no significant difference in mortality rates of Walleyes caught by passive
angling (9.8%) or active angling (2.8%); mortality rates of fish caught on circle hooks (6.1%), J-hooks (8.2%), and
treble hooks (5.6%) also did not differ. Neither air temperature nor the presence of barotrauma had a significant
effect on mortality of captured Walleyes. Survival did not significantly differ between deeply hooked fish that had the
line cut (11.1%) and those that had the hook removed (22.6%). Results from this study suggest a relatively high inci-
dence of Walleye survival after catch-and-release angling through the ice.
Angling is a popular recreational activity worldwide
that provides economic, social, and cultural benefits (Tufts
et al. 2015; Barnett et al. 2016). Following capture by
angling, a fish may be harvested or released by the angler.
Catch-and-release (C&R) angling may be more prevalent
where regulations include size-based harvest rules but may
also occur as a voluntary conservation action on the part
of the angler (Arlinghaus et al. 2007). Regardless of the
motive, a substantial number of fish are released from
recreational fisheries based on the premise that released
fish will survive (Wydoski 1977; Cooke and Schramm
2007). Angling capture and handling can impose signifi-
cant fitness costs to individuals, which may scale to the
population level when a substantial number of fish are
captured and released (Post et al. 2002).
Stressors related to the angling event, such as exercise
and air exposure, can induce physiological alterations in
captured fish (e.g., Wood 1991; Ferguson and Tufts 1992;
*Corresponding author: william.twardek@gmail.com
Received June 23, 2017; accepted October 25, 2017
North American Journal of Fisheries Management 38:159–169, 2018
©2017 American Fisheries Society
ISSN: 0275-5947 print / 1548-8675 online
DOI: 10.1002/nafm.10009
159
Cook et al. 2015), while severe physical damage may
occur in instances where fish are forced to rapidly ascend
from deep water to the surface, causing gases to expand
and rupture the swim bladder (i.e., barotrauma; Rummer
and Bennett 2005; Parker et al. 2006). Furthermore, physi-
cal damage may arise when hooks penetrate buccal tissue
and sensitive organs. Deep-hooking and the resulting
bleeding and organ damage are often considered the pri-
mary source of mortality in recreationally caught fish
(Muoneke and Childress 1994; Bartholomew and Bohn-
sack 2005). As a result, numerous studies have evaluated
the importance of various hook specifications (e.g., type,
size, presence of a barb, and presence of bait) on the
hooking locations and mortality of captured fish (Arling-
haus et al. 2008; Rapp et al. 2008; Serafy et al. 2012;
Stein et al. 2012). Other factors, such as the angling
method, have also been shown to affect hooking location:
passively angled Rainbow Trout Oncorhynchus mykiss
were deeply hooked more often than those actively angled
(Sell et al. 2016). Although numerous factors may con-
tribute to angling-induced mortality, the extent to which
each of these components influences the outcome of an
angling event can be highly dependent on the species as
well as the spatial and seasonal context of the fishery
(Cooke and Suski 2005). Despite a growing number of
interspecific evaluations, there is a further need for con-
text-specific evaluations to establish whether there are sen-
sitive periods for fish, such as during spawning (Lowerre-
Barbieri et al. 2003) or during winter, when fish metabo-
lism is considerably slower (Egginton 1997).
Fish are ectotherms and have thermal optimums and pes-
simums that vary across species and contexts (P€
ortner and
Farrell 2008). Although cooler temperatures are suggested
to decrease mortality following C&R (reviewed by Gale
et al. 2013), most studies have been conducted almost exclu-
sively in spring, summer, and fall, with less attention paid to
winter, when temperatures are extremely cold (Lavery
2016). Angling of fish through ice is a unique stressor given
that ambient environmental temperatures can be very cold.
For warmwater fish species, water temperatures near 0°C
may impair the physiological capacity to cope with stress
and may prolong recovery (reviewed by Egginton 1997;
Wendelaar Bonga 1997). The stress responses (blood con-
centrations of glucose, lactate, and cortisol) of ice-angled
Bluegills Lepomis macrochirus, Yellow Perch Perca flaves-
cens, Eurasian Perch Perca fluviatilis, and Northern Pike
Esox lucius are prolonged but diminished relative to those
of summer-caught fish (Acerete et al. 2004; Cook et al.
2012; Cousineau et al. 2014; Louison et al. 2017a, 2017b;
Pullen et al. 2017). A prolonged elevation of glucocorti-
coids such as cortisol can have adverse health consequences
and can decrease overwinter survival (Pickering and Pot-
tinger 1988; O’Connor et al. 2010; Midwood et al. 2017).
Unlike summer fisheries for the Walleye Sander vitreus, ice-
angled Walleyes are exposed to additional stressors, includ-
ing freezing air temperatures upon removal from the water
and more severe barotrauma resulting from short fight
times and rapid ascension. In the former case, the cold air
temperatures can result in extensive damage to epithelial tis-
sue in the eyes, skin, and gills and can greatly impair physio-
logical functioning (Tilney and Hocutt 1987). Ice-angling
may also pose a greater threat to captured fish due to the
extent of passive fishing (passively suspended hooks) that
occurs, increasing the likelihood of hooking injury to criti-
cal organs, as is often the case with passive angling methods
(Lennox et al. 2015; Sell et al. 2016). Together, stressors
associated with a C&R winter fishery may pose a significant
risk to the survival of targeted fish species, particularly in
fisheries where angling pressure is high (Post et al. 2002).
In temperate regions, Walleye fisheries receive consider-
able angling pressure during both the open-water and win-
ter angling seasons (Deroba et al. 2007). Numerous
studies have addressed the C&R mortality rates of Wal-
leyes during the open-water season (Fletcher 1987; Payer
et al. 1989; Schaefer 1989; Bruesewitz et al. 1993; Reeves
and Bruesewitz 2007; Reeves and Staples 2011; Talmage
and Staples 2011), though little research has been done on
the C&R survival of Walleyes after an ice-angling event.
Considering the high angling pressure faced by Walleyes
in many winter fisheries and given the lack of assessments
for postrelease survival of Walleyes after ice-fishing, we
attempted to assess fishing-induced mortality over a 24-h
period after an ice-angling event. The objectives of this
work were to (1) assess the postrelease mortality and
hooking locations of Walleyes captured during the Lake
Nipissing winter fishery and (2) determine what factors
contribute to postrelease mortality of released Walleyes.
The goal of this research was to provide fisheries man-
agers with recommendations for minimizing postrelease
mortality of Walleyes during winter recreational fisheries.
METHODS
Study site.—Lake Nipissing (Ontario, Canada) is an
87,325-ha mesotrophic lake that maintains a diverse fish
community comprising 42 fish species (Morgan 2013). The
lake is surrounded by a human population of approximately
75,000 distributed across North Bay, Callander, West
Nipissing, and nearby areas, as well as a substantial number
of tourists that visit the lake on an annual basis. Lake
Nipissing is the seventh most fished lake in Ontario, sup-
porting indigenous, commercial, and recreational fisheries
(OMNRF 2013). The Walleye is currently the most popular
species in all of these fisheries and is the most targeted fish
species in the lake. Recreational fishing effort for Walleyes
averages 500,000 angler-hours/year (OMNRF 2013), repre-
senting a potential source of anthropogenic-induced stress
with possible population-scale impacts. Additionally, the
160 TWARDEK ET AL.
Walleye is also the main species targeted by Nipissing First
Nation for their commercial fishery (OMNRF 2013).
Together, these influences have resulted in an exploited Wal-
leye population that has undergone a population decline in
recent decades. Consequently, the Ontario Ministry of Natu-
ral Resources and Forestry (OMNRF) has changed the har-
vest regulations for the species in recent years from a
protected 40–60-cm TL slot size limit to a 46-cm TL mini-
mum size limit (Morgan 2013; OMNRF 2013).
Collection method.—Ice-fishing for Walleyes was con-
ducted in South Bay of Lake Nipissing from January 10 to
February 26, 2017. Fishing sites were selected with guidance
from local outfitters and operators and under the advise-
ment of OMNRF personnel. Multiple gear types were used
for targeting Walleyes, encompassing both passive and
active fishing methods that are typical of the local fishery.
Active fishing was conducted by an angler actively jigging
off the bottom with medium- to light-action ice-fishing rods
spooled with 2.72-kg test (6-lb test) monofilament line.
Lures used included 7.09–10.63-g (0.25–0.375-oz), treble-
hooked jigging spoons (Buckshot and Macho Minnow;
Northland Fishing Tackle, Bemidji, Minnesota) and jig-
heads (Cabela’s solid-color barbed jigheads; Cabela’s, Inc.,
Sidney, Nebraska) baited with dead Emerald Shiners Notro-
pis atherinoides. Passive angling included angling techniques
that were not actively presented to the fish by an angler
(e.g., flag tip-ups, doorstop tip-ups, and setlines). All passive
lines were set between 15 and 30 cm off the bottom, with
suspended hooks (Gamakatsu number 4 octopus J-hooks;
Gamakatsu number 4 octopus circle hooks; and 7.09–10.63-
g, treble-hooked jigging spoons) baited with a live Emerald
Shiner and weighted with a 7.09-g lead sinker. Terminal
tackle used for both active and passive fishing methods was
barbed. All passive lines were set out each day, with most
lines (~75%) suspending the octopus J-hooks that were most
frequently used in the fishery. Passive lines were checked
immediately when an indicator was triggered and were
checked routinely throughout the soak time, typically rang-
ing between 0.25 and 0.75 h. Passive and active lines were
fished both inside heated ice huts and outside. Both angling
methods were used in approximately the same ratio at vari-
ous water depths (~1 active line per 10 passive lines). Water
temperature remained at 4°C in the hypolimnion layer,
whereas ambient air temperature varied from −19.4°C out-
side to 15.0°C inside. All air temperature data were
retrieved from the IONTARIO1123 weather station located
in Callander, Ontario (46.217°N, −79.370°E), approxi-
mately 16 km from the study area. The total number of rod
hours was recorded each day for both passive and active
angling across each hook type to the nearest full hour.
Mortality assessment.—Information on fishery-specific
handling practices was used to design postcapture han-
dling treatments based on personal observations and com-
munications with local anglers, outfitters, and operators in
South Bay. Walleyes were angled from depths ranging
from 6.0 to 12.5 m to assess the proximal influences of
barotrauma on postrelease mortality. This depth range
corresponded to a 0.60–1.24-atm change in pressure.
Upon hook set, each Walleye was retrieved to the surface,
where the hook was removed, hook location was noted,
and fish TL was measured to the nearest 5 mm. During
hook removal, physical signs of barotrauma, such as a dis-
tended swim bladder in the buccal cavity, were recorded if
present. A unique identifying combination of fin clips and/
or dorsal spine clips was applied to each fish caught. For
Walleyes that were hooked deeply in the esophagus, gills,
or tongue, the line was either cut immediately or the hook
was removed using pliers. Following this processing per-
iod, the surface temperature of the operculum was mea-
sured using an infrared digital thermometer with a laser
pointer measuring tool (Bearings Canada, Concord,
Ontario). Operculum temperature was measured to evalu-
ate the influence of air temperature on body temperature.
Air exposure did not exceed 45 s during fish processing.
After exposures, fish were transferred into a water-filled
holding tank and were then assessed for the presence of
the equilibrium reflex using the reflex action mortality pre-
dictor (RAMP) as described by Raby et al. (2011). Fish
were then transferred to a custom-made conical, 0.5-m
3
,
subsurface holding pen (0.5-m-diameter ×1.9-m-high con-
ical holding pen constructed of 25-mm-diameter nylon
mesh; H. Christiansen Company, Duluth, Minnesota)
within 90 s of landing. Nets were suspended in the water
approximately 30 cm off the bottom (Figure 1). Each suc-
cessive Walleye captured was added to the same holding
pen, with a maximum of 15 fish held in the pen at a given
time. Walleyes were held for variable periods and at
changing densities throughout the holding period. The
unique order in which fish were put into the net was
recorded to evaluate the potential influence of holding per-
iod and fish density on mortality.
Statistical analysis.—A logistic regression model (R
function “glm,”specifying “family =binomial”; R Core
Team 2015) was used to predict the factors contributing
to mortality and anatomical hook location. Both models
included gear type, hook type, and Walleye TL as
explanatory variables. Anatomical hooking location, pres-
ence of barotrauma, and air temperature (°C) were used
as explanatory variables in the mortality model only.
Anatomical hooking locations were classified as either
superficial (lips or inner mouth) or deep (esophagus, gills,
or tongue) for statistical analysis to maintain a sufficient
sample size for each group. A separate logistic regression
model was fitted with fish order (order of placement into
the holding pen on a given fishing day) as a predictor vari-
able for mortality. After modeling survival data by logistic
regression, we evaluated the applicability of the equilib-
rium reflex test (a RAMP indicator) to predict mortality
WALLEYE POSTRELEASE SURVIVAL AFTER ICE-ANGLING 161
by using a chi-square test of proportions with the R func-
tion “chisq.test”(R Core Team 2015). We also subsam-
pled data to only include deeply hooked fish and used a
chi-square test to compare mortality results from cutting
the line to those obtained from removal of the hook. Dif-
ferences in CPUE across hook types (passive gear only)
and across gear were evaluated using ANOVA with the
“aov”function in R (R Core Team 2015). The CPUE
across hook types was only evaluated for the January 18–
27 period, when all three hook types were used on passive
gear. The influence of air temperature on operculum tem-
perature was modeled using linear regression implemented
with the R function “lm”(R Core Team 2015).
All models were tested for instances of collinearity
prior to further analyses, and plots of the residuals were
examined for any deviance from heteroscedasticity. Logis-
tic regression models were evaluated using Hosmer–
Lemeshow goodness-of-fit tests via the “hoslem_gof”
function in the R package sjstats (L€
udecke 2017). Neither
the mortality model (P=0.43) nor the hooking location
model (P=0.69) had observed values that were signifi-
cantly different from expected values, meeting the
assumptions of logistic regression. A post hoc Tukey test
for general linear hypotheses was used when statistically
significant differences existed for factors with greater than
two levels. Ecologically relevant interactions were
included in statistical models and compared with models
that excluded the interaction term by using Akaike’s
information criterion (AIC; R Core Team 2015). When
there was little to no difference in AIC values, only the
model with fewer predictor variables is presented. Odds
ratios are presented where appropriate. As sample sizes
for mortalities were low and therefore lacked statistical
power, we report mean values. Statistical significance was
assessed at α=0.05.
RESULTS
Size-Classes
In total, 260 Walleyes were angled by active (n=113)
and passive (n=147) fishing methods using octopus J-
hooks (n=133), treble hooks (n=73), and circle hooks
(n=34). The average TL (±SE) of captured fish was
355 ±5 mm, and less than 1% of the fish (n=2) cap-
tured as part of this study were of legal size to harvest.
The average holding period was 22 h, and the order in
which fish were placed into the holding pen had no signifi-
cant influence on mortality (z=−1.03, df =239,
P=0.31). Data were incomplete for 20 of the Walleyes
captured; therefore, those individuals were only included
for the mortality estimate and were excluded from statisti-
cal models.
Catch per Unit Effort
From January 10 to January 27, a total of 167 Wal-
leyes were caught during 3,655 h of fishing. The CPUE
observed when actively fishing (0.21 fish/h) was signifi-
cantly greater than that obtained when passively fishing
(0.04 fish/h; F=7.61, df =1,463, P<0.01; Figure 2;
FIGURE 1. Diagrams of the subsurface holding pens used to monitor Walleye survival: (A) sub-surface holding pen suspended 30 cm off the bottom
by a rope connected from the holding pen to the top of the ice-fishing hole; and (B) a magnified view of the holding pen, featuring the drawstring
used to quickly open and close the holding pen during fish transfers. [Color figure can be viewed at afsjournals.org.]
162 TWARDEK ET AL.
Table 1). The CPUE when passively fishing was signifi-
cantly greater for circle hooks (0.13 fish/h) than for both
octopus J-hooks (0.03 fish/h; F=134.70, df =3,653,
P<0.01; Figure 2) and treble hooks (0.05 fish/h;
P<0.01).
Mortality
Eighteen Walleyes died after capture by ice-angling; the
observed rate of postrelease mortality was therefore 6.9%
(n=260). Mortality of Walleyes caught by passive (9.8%)
and active (2.8%) angling was not significantly different
(z=1.36, df =232, P=0.18; Figure 3; Table 2). Mortal-
ity was not significantly different for Walleyes caught by
treble hooks (5.6%; z=1.56, df =232, P=0.12) or octo-
pus J-hooks (8.2%; z=0.69, df =232, P=0.49) relative
to circle hooks (6.1%; Figure 3; Table 2). Deep-hooking
increased the odds of mortality by 5.19 (z=2.34,
df =232, P=0.02; Figure 3; Table 2). Barotrauma was
observed in 22.2% of captured Walleyes but did not signif-
icantly increase mortality relative to Walleyes without
barotrauma (z=−0.20, df =232, P=0.84; Figure 3;
Table 2). There was no significant effect of fish TL
(z=−1.52, df =232, P=0.13) or air temperature
(z=−0.49, df =232, P=0.62) on mortality rate,
although air temperature was a significant predictor of
Walleye opercular temperature (t=4.57, df =65,
P<0.01). The same model for mortality was analyzed
again with the inclusion of the interaction between hook-
ing location and gear type. This model indicated that pas-
sively caught, deeply hooked Walleyes were not
significantly more likely to succumb to mortality than
actively caught, shallowly hooked fish (z=−0.54,
df =232, P=0.59; Table 2). The model was not signifi-
cantly different from the model that excluded the interac-
tion term (χ
2
=0.60, df =232, P=0.60). Impairment of
the equilibrium reflex was not a significant predictor of
Walleye mortality (χ
2
<0.01, df =1, n=246, P=1.00;
Table 2).
Deep-Hooking
Deep-hooking occurred in 32.5% of fish (n=76).
Deep-hooking was significantly more frequent for passive
(50.4%) than active (9.3%) fishing methods (z=−0.99,
df =235, P<0.01; Figure 4; Table 3). Deep-hooking
was significantly more common for fish captured by octo-
pus J-hooks (42.9%) than for those captured by treble
hooks (9.9%; z=2.48, df =235, P=0.01), but there was
no difference between circle hooks (47.1%) and treble
hooks (z=−1.63, df =235, P=0.43) or between circle
hooks and octopus J-hooks (z=0.76, df =235, P=0.45;
Figure 4; Table 3). The TL of Walleyes was also not a
significant predictor of deep-hooking (z=−0.99,
df =235, P=0.30; Table 3).
Cutting the Line Versus Hook Removal
Deeply hooked Walleyes that had the hook removed
(n=31; 22.6% mortality) exhibited no significant differ-
ence in mortality compared to deeply hooked fish for
which the line was cut (n=45; 11.1% mortality;
χ
2
=1.06, df =1, P=0.30).
DISCUSSION
Walleye mortality after ice-angling was 6.9%. Previous
C&R research on Walleyes caught in the summer (non-
tournament only) have reported hooking mortality rates
FIGURE 2. Walleye CPUE obtained when using (A) active (0.21 fish/h; n=201) and passive (0.04 fish/h; n=3,454) ice-fishing gear; and (B) circle
hooks (0.13 fish/h; n=264), octopus J-hooks (0.04 fish/h; n=987), and treble hooks (0.05 fish/h; n=215) on passive fishing gear. Different
lowercase letters denote a significant difference (Tukey’s honestly significant difference test: P<0.05).
TABLE 1. Logistic regression model outputs predicting Walleye CPUE
during ice-fishing. The models evaluate the difference in CPUE across
gear types and hook types. Significant effects are highlighted by bold ita-
lic font.
Model Ndf FP
Gear type 1,466 1,463 7.61 <0.01
Hook type 3,655 3,653 134.70 <0.01
WALLEYE POSTRELEASE SURVIVAL AFTER ICE-ANGLING 163
from 0.8% to 31% (Fletcher 1987; Payer et al. 1989;
Schaefer 1989; Bruesewitz et al. 1993; Reeves and Bruese-
witz 2007; Talmage and Staples 2011). Most studies used
holding periods longer than 5 d but found mortality rates
typically less than 5% (Fletcher 1987; Payer et al. 1989;
Reeves and Bruesewitz 2007), though this was not the case
for the study by Talmage and Staples (2011), in which
31% mortality was observed. These extended holding peri-
ods account for a longer time-course after release, with
the tradeoff of additional confinement stress and mortality
(Portz et al. 2006). Perhaps the most comparable estimate
is that of Meerbeek and Hoxmeier (2011), who estimated
winter C&R mortality at 12% for congeneric Saugers San-
der canadensis caught at the same depth range (6–12 m)
and similar water temperatures as the Walleyes in our
study. These mortality estimates are particularly wide
ranging and may be partly explained by differences in
study design (e.g., holding pen style, holding duration,
and fish handling) as well as by differences across Walleye
fisheries and the water bodies where the studies occurred.
In two of the studies, water temperatures were shown to
be positively correlated with 5-d mortality (Reeves and
Bruesewitz 2007; Reeves and Staples 2011), while another
two studies identified capture depth as significant sources
of mortality (Bruesewitz et al. 1993; Talmage and Staples
2011). In most cases, mortality was driven by deep-hook-
ing (Fletcher 1987; Payer et al. 1989; Schaefer 1989; Brue-
sewitz et al. 1993; Reeves and Bruesewitz 2007; Reeves
and Staples 2011). Similarly, ice-fishing mortality estimates
for Lake Trout Salvelinus namaycush (10% mortality) and
Northern Pike (1–33% mortality depending on the hook
type) were also driven by hooking location (Dextrase and
Ball 1991; Dubois et al. 1994). Furthermore, percids
FIGURE 3. Mean 24-h mortality of Walleyes after catch-and-release ice-fishing using (A) active (2.8%; n=99) and passive (9.8%; n=141) gear;
and (B) circle hooks (6.1%; n=34), octopus J-hooks (8.2%; n=133), and treble hooks (5.6%; n=73). The 24-h mortality is also shown for (C)
Walleyes that were shallow-hooked (3.0%; n=162) or deeply hooked (14.8%; n=78) and (D) Walleyes that had signs of barotrauma absent (7.3%;
n=184) or present (5.3%; n=56). Different lowercase letters denote a significant difference (P<0.05).
TABLE 2. Logistic regression model output predicting mortality of Wal-
leyes captured by ice-fishing (n=240). The model incorporated two con-
tinuous variables (Walleye TL and air temperature) and four factors.
Inferences for factors are presented relative to reference levels (“active”
for gear type, “circle hook”for hook type, “shallow-hooked”for hooking
location, and “absent”for barotrauma). Significant effects are highlighted
by bold italic font.
Model variable Estimate ±SE zdf P
Intercept −2.73 ±1.84 −1.48 232 0.14
TL (cm) −0.01 ±0.01 −1.52 232 0.13
Gear type: passive 1.23 ±0.91 1.36 232 0.18
Hook type: octopus J 0.59 ±0.84 0.69 232 0.49
Hook type: treble 1.79 ±1.14 1.56 232 0.12
Hooking location:
deep
1.65 ±0.70 −2.34 232 0.02
Barotrauma: yes −0.14 ±0.70 −0.20 232 0.84
Air temperature (°C) −0.02 ±0.04 −0.49 232 0.62
164 TWARDEK ET AL.
appear to suffer higher hooking mortality rates
(mean =19.9%) than any other family of fish (H€
uhn and
Arlinghaus 2011). Abiotic and intrinsic biological factors
have also been shown to explain context-specific differ-
ences in C&R outcomes across a range of recreational
fisheries (Cooke and Suski 2005). In our study, these fac-
tors included differences in weather conditions (cold air
temperatures reaching −19.4°C), fish physiology (J. M.
Logan, M. J. Lawrence, W. M. Twardek, R. J. Lennox,
and S. J. Cooke, unpublished data), and angler behavior
(increased use of passive angling) during the winter
months. As only a small number of mortalities existed, we
often lacked the statistical power to detect significant rela-
tionships amongst the variables.
Hook Selection
Hook selection can have a considerable role in the sever-
ity of anatomical hooking damage following capture
(Cooke et al. 2003). In the present study, we compared the
hooking locations of treble hooks and octopus J-hooks as
well as circle hooks, which have been suggested as a better
alternative to conventional hooks (Serafy et al. 2012).
Hooking location (shallow versus deep) in ice-angled Wal-
leyes was partially related to the type of hook used. Treble
hooks deeply hooked a significantly smaller proportion of
fish (9.9%) compared to octopus J-hooks (42.9%); treble
hooks also deeply hooked a smaller proportion of fish than
circle hooks (47.1%), although not significantly so. This
lower percentage of deep-hooking by treble hooks could be
partly explained by the greater use of treble hooks when
actively fishing, which tended to have lower deep-hooking
rates. Treble hooks may also have reduced deep-hooking
because they are larger in size (being three-dimensional)
than a single hook, making them more difficult for fish to
ingest deeply. A comprehensive review on hooking mortal-
ity across 32 taxa suggested that single hooks generally
result in higher mortality rates than treble hooks (Muoneke
and Childress 1994). However, mortality rates were similar
for treble hooks in our study, suggesting that the additional
stress associated with multiple hooking locations and the
additional handling due to increased difficulty in hook
removal may obscure the potential benefit of reduced deep-
hooking. In some instances, circle hooks have shown pro-
mise as a better alternative to conventional hooks (Cooke
and Suski 2005), but similar to the results reported for Rain-
bow Trout by Sell et al. (2016), we found no evidence of
this (6.1% mortality). Circle hooks did, however, have a
substantially higher CPUE, a finding that is contrary to
most studies comparing catch rates of circle hooks and con-
ventional hook types (Sell et al. 2016; but see Willey et al.
2016). A previous C&R study on Walleyes conducted in the
summer found that circle hooks had significantly lower
hooking efficiencies and lower injuries per strike than octo-
pus J-hooks (Jones 2005). However, the Walleyes captured
during the Jones (2005) study were actively angled, whereas
the fish in our study were caught primarily by passive fish-
ing. The higher CPUE for circle hooks observed in the cur-
rent study may be related to their greater retention of live
minnows compared to both treble hooks and octopus J-
hooks, which were often observed to lose their minnows. A
FIGURE 4. Mean deep-hooking rates for Walleyes captured by (A) active (9.3%; n=99) and passive (50.4%; n=141) ice-fishing gear; and (B)
circle hooks (47.1%; n=34), octopus J-hooks (42.9%; n=133), and treble hooks (9.9%; n=73) during ice-fishing. Different lowercase letters denote
a significant difference (P<0.05).
TABLE 3. Logistic regression model output predicting deep-hooking of
Walleyes captured by ice-fishing (n=240). The model incorporated Wal-
leye TL as a continuous variable and included gear type and hook type
as factors. Inferences for factors are presented relative to reference levels
(“active”for gear type and “circle hook”for hook type). Significant
effects are highlighted by bold italic font.
Model variable Estimate ±SE zdf P
Intercept −1.14 ±1.14 −0.99 235 0.32
TL (cm) −0.01 ±0.01 −1.04 235 0.30
Gear type: passive 2.12 ±0.47 4.56 235 <0.01
Hook type:
octopus J
0.31 ±0.40 0.76 235 0.45
Hook type: treble −1.04 ±0.64 −1.63 235 0.10
WALLEYE POSTRELEASE SURVIVAL AFTER ICE-ANGLING 165
previous C&R evaluation of Walleyes used the exact same
hook type (Gamakatsu size-4 octopus J-hooks) but had con-
siderably different mortality estimates than those generated
here (6.9% [present study] versus 25% [Reeves and Staples
2011]). However, the Reeves and Staples (2011) study
occurred during the open-water season, when temperatures
are considerably warmer. This large discrepancy between
mortality estimates despite use of the same hook type sug-
gests that other factors related to the fishery, such as capture
depth, water temperature, and fishing method, may have
greater roles in influencing mortality.
Gear Selection
Fisheries-related differences in angler behavior (gear,
tackle, handling, etc.) can play an important role in the
outcome of a capture event (Cooke et al. 2017). In our
study, we found that ice-angled Walleyes had a particu-
larly high rate of deep-hooking (32.5% overall), which
may in part be explained by the considerable use of pas-
sive gear (57% of all fish captured). Passively caught fish
had a significantly higher incidence of deep-hooking
(50.4%) than actively caught fish (9.3%). However, active
fishing caught 0.21 fish/h, while passive fishing caught just
0.04 fish/h, suggesting that total deep-hooking levels could
be relatively even (1.95% versus 2.01% fish/h) for the same
fishing effort. Nonetheless, greater deep-hooking rates for
passively angled fish are potentially important, as Walleye
angling during the winter months often employs a greater
use of passive angling gear relative to the open-water sea-
son, when anglers may often only have one line in the
water (OMNRF 2017). This increased deep-hooking rate
of passively angled fish could be a consequence of dimin-
ished line monitoring by anglers, thus allowing the fish to
swallow hooks and bait as well as greater opportunity for
fish to ambush a stationary prey item (Lennox et al.
2015). Deep-hooking is also typically more common in
fish that are captured using live bait (Arlinghaus et al.
2008), which is almost exclusively the case in ice-angling
for Walleyes on Lake Nipissing. The 32.5% deep-hooking
rate for Walleyes captured in the present study is consider-
ably higher than that reported in many other C&R fish-
eries (Sell et al. 2016), probably due in part to our use of
live bait on J-hooks (Bartholomew and Bohnsack 2005)
while passive angling. Although previous research suggests
that deeply hooked fish are at risk of death caused by crit-
ical damage to vital organs and blood loss (Al
os 2008;
Hall et al. 2015), the deeply hooked Walleyes in our study
had a remarkably high survival rate (85%) and a low inci-
dence of bleeding (10%). Minimal blood loss may be
explained in part by the lowered metabolism of fish during
cold temperatures and the correspondingly slower blood
flow (Egginton 1997), and this could ultimately explain
the observed high survival rates. Nonetheless, deeply
hooked Walleyes were indeed significantly more likely to
die than shallow-hooked Walleyes, highlighting the impor-
tance of hooking location in fish survival. As such, any
efforts to reduce damage associated with deep-hooking
would be of benefit to released fish.
De-Hooking Method
Anglers that deeply hook a fish are confronted with the
choice to either remove the deeply lodged hook or cut the
line and release the fish with the hook still embedded.
Hook removal is often considered more damaging to cap-
tured fish due to extra handling and the tearing of vital
organs. Cutting the line has therefore been proposed as an
alternative to removing the hook, with the caveat that the
fish will now have the burden of a hook in its oral cavity.
Previous research on Bluegills indicated a relatively high
ability to shed the hook over the short term (Fobert et al.
2009). However, cutting the line was not an effective
means to reduce mortality in deeply hooked Golden Perch
Macquaria ambigua, which had just 24% survival (Hall
et al. 2015). Consistent with the findings of Reeves and
Bruesewitz (2007), there was no statistically significant dif-
ference in postrelease survival over 24 h for Lake Nipiss-
ing Walleyes, although cutting the line (11.1% mortality)
resulted in a lower average mortality than hook removal
(22.6%). Across species, cutting the line appears to
decrease mortality in most C&R scenarios (Bartholomew
and Bohnsack 2005) and is likely beneficial for deeply
hooked, ice-angled Walleyes.
Abiotic Factors
Abiotic factors such as capture depth can have an
important influence on fish physiology and mortality. Fish
captured at depth experience a rapid decrease in ambient
pressure when brought rapidly to the surface. The result-
ing pressure change leads to the expansion of air in the
swim bladder, rendering the fish unable to swim away
from the surface or unable to maintain normal orientation
(equilibrium; Raby et al. 2011). Barotrauma may also
rupture the swim bladder and tunica externa (the outer
layer of the swim bladder), which are slow healing (Pribyl
et al. 2012), and may cause prolapse of the cloaca, hemor-
rhages, and gastric herniation (Butcher et al. 2013). In our
study, approximately 23% of Walleyes had physical symp-
toms of barotrauma, although fish with barotrauma were
not more likely to die. This result is contrary to an earlier
ice-fishing C&R study completed on Lake Nipissing Wal-
leyes, which highlighted the importance of capture depth
for influencing the presence of barotrauma and mortality
(Rowe and Esseltine 2001). Winter-caught Saugers also
appeared to have mortality increase proportionally with
capture depth: from 2% (at <9 m) to 67% (at 21–24 m).
Our results are consistent with those of Bettoli et al.
(2000) and Reeves and Bruesewitz (2007), who observed
little influence of capture depth and barotrauma on
166 TWARDEK ET AL.
mortality. The high survival of fish with barotrauma in
our study could be related to the submerged holding pens
that forced Walleyes back to their capture depth, allowing
swim bladder gases to recompress (Drumhiller et al.
2014). However, the pens may also have increased stress
and barotrauma, as Walleyes could be brought to and
from the surface several times during fish transfers.
Regardless of the mechanism, several Walleyes in our
study remained at the top of the ice-fishing hole upon
release and were clearly unable to swim as a result of
barotrauma.
Water temperature has been previously identified as an
important factor contributing to C&R mortality, with
warmer temperatures being positively correlated with mor-
tality (Gale et al. 2013) and increased hooking stress
(Wydoski et al. 1976). The peak stress response of ice-
angled fish is typically lower and the corresponding recov-
ery period is longer than those of fish angled in the sum-
mer (Louison et al. 2017a, 2017b). Water temperatures in
Lake Nipissing were constant at 4°C in the hypolimnion
layer where Walleyes were captured, although air tempera-
tures varied greatly from −19.4°C outside to 15.0°C inside
the heated ice huts. Indeed, air temperatures were posi-
tively correlated with operculum temperatures of captured
Walleyes, indicating that fish surface temperatures can be
influenced by air exposure in less than 45 s. Similar to
observations by Rowe and Esseltine (2001), the Walleyes
in our study showed signs of freezing damage to the eyes
and gills, which could result in long-term structural dam-
age due to the formation of both intracellular and extra-
cellular ice crystals (Pegg 1987; Fletcher et al. 1988).
These observations suggest that air exposure should be
minimized during cold winter conditions to reduce physi-
cal damage to released Walleyes.
Management Implications
In many winter Walleye fisheries, a substantial number
of fish are released after capture to comply with provincial
or state fishing regulations. Ice-angling for Walleyes in
Lake Nipissing resulted in relatively low mortality rates
(6.9%) that were in line with other estimates of summer
C&R mortality and lower than the previous estimate of
ice-angling mortality on Lake Nipissing (19%) as reported
by Rowe and Esseltine (2001). In our study, a high pro-
portion of Walleyes were caught using small baited hooks
set on passive lines, resulting in frequent deep-hooking.
Although circle hooks were ineffective for reducing deep-
hooking, larger hooks should be tested to determine
whether they reduce the frequency of deep-hooking, as
this is generally believed to be the most common cause of
mortality among fish captured in recreational fisheries.
However, mortality from deep-hooking in this study was
also modest. Overall, Walleyes were resilient to capture
and handling in the winter recreational fishery, including
handling in air and on ice prior to release. Even fish that
exhibited symptoms of barotrauma had high survival,
despite a previous suggestion that barotrauma is an
important factor causing mortality of Walleyes in the fish-
ery (Rowe and Esseltine 2001). Fisheries managers should
account for differences in catch rates and mortality rates
across gear types, and it may be prudent to suggest that
anglers cut the line for deeply hooked fish.
ACKNOWLEDGMENTS
We extend our gratitude to Mike Young of Lake
Nipissing Ice Fishing Charters for providing valuable
information on fishing locations and techniques specificto
Walleyes on Lake Nipissing. Aaron Zolderdo, Dirk
Algera, Tanya Prystay, Sindre H
avarstein Eldøy, Garrett
Normoyle, Colin Raymond Davis, and the many research
technicians from OMNRF helped make this project possi-
ble. The document was kindly reviewed by Lee Gutowsky
and Dan Taillon. Lee Gutowsky also provided input on
the study design. We thank the staff of H. Christiansen
and Company for their creativity with developing holding
nets that could be deployed in an ice-fishing hole. Primary
funding was provided by OMNRF, with additional sup-
port from the Natural Sciences and Engineering Research
Council of Canada and the Canada Research Chairs Pro-
gram. All research was conducted in accordance with the
guidelines of the Canadian Council on Animal Care as
administered by Carleton University (Protocol 106247).
There is no conflict of interest declared in this article.
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