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Mortality of Lingcod Towed in a Net as Related to
FishLength, Seawater Temperature, and Air Exposure:
ALaboratory Bycatch Study
a
Michael W. Davis & Bori L. Olla
b
a
Alaska Fisheries Science Center, National Marine Fisheries Service, Hatfield Marine Science
Center, Newport, Oregon, 97365, USA
b
Oregon State University, Hatfield Marine Science Center, Newport, Oregon, 97365, USA
Available online: 08 Jan 2011
To cite this article: Michael W. Davis & Bori L. Olla (2002): Mortality of Lingcod Towed in a Net as Related to FishLength,
Seawater Temperature, and Air Exposure: ALaboratory Bycatch Study, North American Journal of Fisheries Management, 22:4,
1095-1104
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North American Journal of Fisheries Management 22:1095–1104, 2002
q Copyright by the American Fisheries Society 2002
Mortality of Lingcod Towed in a Net as Related to Fish
Length, Seawater Temperature, and Air Exposure: A
Laboratory Bycatch Study
MICHAEL W. DAVIS*
Alaska Fisheries Science Center, National Marine Fisheries Service,
Hatfield Marine Science Center, Newport, Oregon 97365, USA
BORI L. OLLA
Downloaded by [Oregon State University] at 16:42 22 August 2011
Oregon State University, Hatfield Marine Science Center,
Newport, Oregon 97365, USA
Abstract.—The mortality of discarded bycatch is a critical problem in the management of fisheries
worldwide. Little is known about the key principles involved in the mortality of discarded bycatch.
These principles are best elaborated under controlled conditions in the laboratory where the actions
and interactions of stressors found in fishing practices can be investigated independently. The goal
of this study was to investigate the principles involved in the mortality of lingcod Ophiodon
elongatus by testing hypotheses concerning the factors that may control trawl bycatch mortality.
Lingcod were towed in a net and exposed to increased seawater temperature and to air, two stressors
that occur during the processes of trawl capture, retrieval through a thermocline, and landing on
deck. Mortality occurred after exposure to more than 45 min in air, after exposure to 4 h towing
in a net followed by more than 30 min in air, or after 4 h towing followed by exposure to seawater
above 16.08C for 30 min and air for 15 min. In treatments of equal stressor intensity, smaller fish
(41–51 cm total length) had higher rates of mortality than larger fish (52–67 cm). The effects of
net towing and air—as well as of towing, increased seawater temperature, and air—were additive.
Lingcod bycatch mortality may be reduced by decreasing trawling times and exposure to increased
seawater and air temperatures during warmer seasons or by restricting fisheries that produce bycatch
to seasons of cooler temperatures. The sorting, handling, and release of bycatch on deck after
capture may be conducted in a manner that would probably enhance survival if fish are released
within 30 min of capture. Because smaller lingcod had higher rates of mortality, further information
about the mortality rates of relevant size-classes of fish is needed to validate the assumptions of
management rules for released, undersized bycatch that are designed to enhance recruitment.
The mortality of fish released after capture (discarded bycatch) because of harvest restrictions
(i.e., number or size limits, or incidental catch as
a nontarget species) is a critical problem in the
management of fisheries worldwide. Discarded bycatch is estimated to be 25% of the total world
catch (Pascoe 1997). Solutions to the bycatch
problem have focused on avoiding areas of potential bycatch and modifying fishing gears to reduce
the capture of bycatch or to allow for bycatch to
escape through grids, panels, or increased mesh
sizes (Kennelly and Broadhurst 1995; Broadhurst
2000). While a great deal of progress has been
made in reducing the capture of bycatch through
improvements in fishing gear selectivity, there remains the problem of undetected bycatch mortality
for fish that leave fishing gear or are discarded
after landing on deck.
* Corresponding author: michael.w.davis@noaa.gov
Received October 17, 2001; accepted January 17, 2002
Little is known about the key principles that
underlie bycatch stress and mortality and how the
interactions of fish with fishing gear may induce
mortality. It is not possible to generalize from field
experiments the processes that cause stress to and
the mortality of bycatch, as fishing processes and
environmental conditions are highly variable and
the prolonged holding of fish to evaluate mortality
is difficult (Chopin and Arimoto 1995; Chopin et
al. 1995). The principles of bycatch mortality may
best be understood in the laboratory where the
interactions of stressors and the dynamics of mortality can be studied under controlled conditions.
Examples of these principles include (1) the interaction of stressors which magnify mortality, (2)
the importance of temperature and air exposure,
and (3) the differences in sensitivity to stress related to size and species. These principles can be
used in combination with economic and sociological constraints to develop scientifically based
management strategies and fishing practices.
This laboratory study used lingcod Ophiodon
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DAVIS AND OLLA
elongatus as an example to develop an understanding of bycatch mortality. Lingcod is important in
marine food webs and comprises valuable recreational hook and line and commercial trawl and
hook and line fisheries, as well as incidental bycatch in other directed fisheries along the west
coast of the USA and Canada. The total catch of
lingcod in the USA and Canada was 7,412 metric
tons in 1993, falling to 2,628 metric tons by 1999;
the total estimated exvessel value was US$6.2 million in 1993 and $4.6 million in 1999 (FAO 2001;
PacFIN 2001). Because of declining stocks and
catches, conservation is a concern for lingcod in
the USA and Canada. In 1999 the Pacific Fishery
Management Council designated lingcod as an
overfished species in California, Oregon, and
Washington, and imposed regulations for minimum size and trip limits on the directed fishery
and on other fisheries that produce lingcod bycatch. These restrictions have led to an increased
discarding of smaller lingcod and the early closure
of fisheries with excess lingcod bycatch. In Canada, fishing for lingcod has been restricted because
of depressed stocks, and some areas of the Strait
of Georgia have been closed since 1990. Lingcod
is a potential candidate for the reduction of bycatch
mortality rates because it does not have a gas bladder and is relatively resistant to the effects of capture and handling, with little scale loss or damage
to skin. Mortality is very high in discarded fish
with a gas bladder or organs which inflate after
capture, while mortality may be variable and
sometimes quite low in fish that do not have a gas
bladder, or have a gas bladder that does not inflate
after capture (Alverson et al. 1994). While the
mortality of discarded lingcod from trawl fisheries
has been hypothesized, estimates of this mortality
are not available (Jagielo 1999).
The intent of this work was not to precisely
duplicate widely varying fishing conditions and
estimate bycatch mortality rates. That task is best
performed in future studies in the field using physiological measures of fish condition and mortality
that have been correlated with key principles of
bycatch mortality. The goal of this study was to
investigate the principles involved in lingcod mortality by using tested hypotheses concerning the
classes of stressors that may control bycatch mortality. These stressors, including towing in a net
and exposure to increased temperature and to air,
have been found in the processes of capture, retrieval through a thermocline, and landing on deck.
Fish from two size-classes (41–51 cm and 52–67
cm total length [TL]) were exposed to stressors
and then held for 60 d after stress induction to
assess delayed mortality. The results of this
study—along with previous laboratory studies of
stress and mortality in walleye pollock Theragra
chalcogramma, sablefish Anoplopoma fimbria, and
Pacific halibut Hippoglossus stenolepis—can be
used to elaborate the general principles of bycatch
mortality and to suggest improvements in the design of field bycatch experiments (Olla et al. 1997,
1998, 2000; Davis and Olla 2001; Davis et al.
2001).
Methods
Preliminary laboratory experiments with stress
in lingcod indicated that there may be a size effect
in the induction of stress. Therefore, efforts were
made to capture fish in a range of sizes for this
study. Eighty nonreproductive lingcod (41–67 cm
TL) were captured in May 2000 by trawling with
four 30-min tows at 1.2 m/s, using a commercial
bottom trawl in the Pacific Ocean (448459N,
1248279W) offshore from Depoe Bay, Oregon. The
tow depth was 150 m, the tow depth temperature
was 7.58C, and the surface water temperature was
13.08C. Fish were released from the cod end onto
the deck after each tow, and all fish appeared to
be in excellent condition. Forty fish less than 52
cm TL (small fish) and 40 fish equal to or greater
than 52 cm TL (large fish) were sorted into eight,
450-L totes (106 3 92 3 46 cm) within 2 min of
landing on the deck. Each tote contained 10 fish
of a size-class and was supplied with seawater
pumped from depth (7.58C, 32.5 g/L salinity). The
fish were transported within 4 h to the laboratory
at the Hatfield Marine Science Center, Newport,
Oregon. Small and large lingcod were held separately at a density of 20 fish per tank in 15,904-L
circular tanks (4.5 m diameter, 1.0 m depth) and
supplied with seawater (20 L/min, 7.0–8.08C, 30–
32 g/L salinity, O2 . 90% saturation). The tanks
were in low light conditions (daylight fluorescent,
5,000 K) at 0.5 mmol photons·m22·s21 with a photoperiod of 12 h light: 12 h darkness. Fish were
initially fed every other day with one live surf
smelt Hypomesus pretiosus per fish until all fish
had begun feeding and later were fed ad libitum
twice per week with frozen Pacific herring Clupea
pallasi. Large lingcod began feeding after capture
and transport to the laboratory within 2 d, while
small lingcod began feeding within 8 d. The resumption of feeding was the criterion for recovery
from stress imposed by capture, transport to the
laboratory, and laboratory stressors. Experiments
were begun 45 d after the fish had been brought
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NET-TOWED LINGCOD MORTALITY
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FIGURE 1.—Length frequency of lingcod in two size-classes (small [,52 cm], N 5 27; and large [$52 cm], N
5 27) that died as a result of experimental stressor treatments.
into the laboratory. Fish from the two size-classes
that died as a result of experimental treatments
were measured (N 5 27 small, 27 large; Figure 1).
The two size-classes probably represented different age-classes, although this was not confirmed
by aging techniques.
Air exposure.—Lingcod were exposed to air for
various periods of time, similar to the exposure
that would occur on the deck of a boat (Figure 2).
The fish were transferred by dip net from a holding
tank into an empty rectangular tank (90 3 60 3
30 cm). Thirty fish were used in the air experiments (12 small and 18 large fish). Small fish (N
5 6 replicate fish for each time period) were held
in air (20.08C) for 45 and 60 min, while large fish
(N 5 6 replicate fish for each time period) were
held in air for 45, 60, and 75 min. After stressor
treatment, fish were transferred to 3,140-L circular
tanks (2.0-m diameter, 0.8-m depth) supplied with
seawater (10 L/min, 7.0–8.08C, 30–32 g/L salinity,
O2 . 90% saturation) and mortality was noted for
up to 60 d after exposure to the stressors.
Net towing and air exposure.—Lingcod were
towed in a net for 4 h and then exposed to air,
which is similar to the conditions found in trawling
(Figure 2). Thirty-six fish were used in the net
towing and air experiments (12 small and 24 large
fish; N 5 6 replicate fish per period of air exposure
and fish size). The fish were transferred by dip net
from a holding tank into towing nets, located in a
tank, as previously described (Olla et al. 1997,
1998). In brief, the apparatus had two nets suspended at the ends of two rotating arms in a tank
(4.5-m diameter, 1-m depth). The nets were cylindrical (1.2 m length, 0.7 m diameter) and constructed with nylon, single-knot 2.5-cm diamond
mesh (single strand twine, 0.11-cm diameter). Nets
were towed for 4 h at 8.0 6 0.28C (mean 6 1 SE)
in lighted conditions (1.0-mmol photons·m22·s21)
at 1.1 m/s. At this speed, lingcod did not swim
and remained pressed against the mesh of the net.
Comparisons between the laboratory current speed
and current speeds inside commercial trawls was
not possible, since the current speeds in trawls
have not been measured and may be quite low
when the trawls are filled with fish that block water
flow. After being towed for 4 h, fish were transferred to the air tank (20.08C) as previously described and small fish were held for 30 and 45
min, while large fish were held for 30, 45, 60, and
75 min. After stressor treatment, the fish were
transferred to 3,140-L circular tanks (2.0-m diameter, 0.8-m depth) supplied with seawater (10
L/min,7.0–8.08C, 30–32 g/L salinity, O2 . 90%
saturation), and mortality was noted for up to 60
d after exposure to stressors.
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DAVIS AND OLLA
FIGURE 2.—Flow chart of experimental stressor treatments for lingcod. Fish were exposed to three different
treatments: (1) air exposure at 20.08C for 30, 45, 60, or 75 min, followed by return to a recovery tank (upper
panel); (2) net towing for 4 h in 8.08C seawater and air exposure at 20.08C for 30, 45, 60, or 75 min, followed by
return to a recovery tank (middle panel); and (3) net towing for 4 h in 8.08C seawater, exposure to seawater at
8.0, 14.0, 16.0, 18.0, or 20.08C for 30 min, and exposure to air at 16.88C for 15 min, followed by return to a
recovery tank (lower panel).
Net towing, increased temperature, and air exposure.—Lingcod were towed in a net for a 4-h
period, followed by an abrupt exposure to increased seawater temperature and then exposure
to air (Figure 2). During retrieval onboard a fishing
vessel while trawling in the warmer seasons, fish
may be exposed to 45–60 min of increased temperature (either from seawater or air) which will
warm them. These conditions can be found if fish
are exposed to (1) a slow passage through thermoclines, (2) time in the surface water before hauling aboard, (3) time in the warm air on deck, and
(4) time in the surface water after discarding. The
fish were towed as previously described, and 72
fish were used (36 small fish and 36 fish large)
with N equal to 6 replicate fish per temperature
tested (except at 18.08C when N 5 12 replicate
fish used). After towing, the fish were exposed to
control (8.08C) or increased seawater temperatures
(14.0, 16.0, 18.0, or 20.08C) for 30 min in a circular tank (3.0-m diameter, 1.0-m depth), followed
by 15 min in air (16.8 6 0.28C) in an empty rectangular tank (90 3 60 3 30 cm). After stressor
treatments, the fish were transferred to 3,140-L
circular tanks (2.0-m diameter, 0.8-m depth) supplied with seawater (10 L/min, 7.0–8.08C, 30–32
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NET-TOWED LINGCOD MORTALITY
1099
g/L salinity, O2 . 90% saturation), and mortality
was noted for up to 60 d after exposure to stressors.
Statistical analysis.—In these studies, individual
fish used for the three stressor treatments were
considered independent replicates and analyzed
with the sign test. The assumption of fish replicate
independence was reasonable because towing conditions were uniform among all trials, temperatures were controlled, and conditions did not vary
among holding tanks for fish after stress induction.
Statistical analysis for significant mortality associated with stress treatment was made using the
one-tailed sign test which required a minimum of
N equal to 5 to demonstrate a 100% mortality effect (at P 5 0.05). A one-tailed test was used because the untreated fish were not expected to experience mortality (based on previous experience).
The results were reported in percent mortality for
clarity of presentation, although statistical analysis
was not performed on values for percent mortality.
The survivors from initial trials in which fish were
towed and exposed to increased temperature and
to air were reused 90 d later for trials with exposure
to air and exposure to towing and air. As animal
care and ethical considerations in studies with
mortality as an endpoint required that the study
use minimum numbers of fish, surviving fish were
reused in additional experiments.
Results
When lingcod were exposed to air (20.08C) for
45 min, no mortality resulted in either small or
large fish (N 5 6, 6; Figure 3). In contrast, lingcod
exposed to air for 60 min resulted in significantly
higher mortality in small fish (100%) than in large
fish (33%)(N 5 6, 6; one-tailed sign test, P ,
0.016; Figure 3). The exposure of large lingcod to
air for 75 min resulted in 100% mortality (N 5 6;
Figure 3). All mortality occurred within 1 d of
exposure to air, with sustained flaring of opercula
as an indication of mortality.
When lingcod were towed in a net for 4 h at
8.08C and then abruptly exposed to air (20.08C),
no mortality was observed in small or large fish
after 30 min of air exposure, while after 45 min
of air exposure, mortality was significantly higher
in small fish (100%) than in large fish (33%; N 5
6, 6; one-tailed sign test, P , 0.016; Figure 3).
The mortality in large fish continued to increase
as time in air increased after towing, with 66%
mortality after 60 min and 100% after 75 min (N
5 6, 6; Figure 3). All mortality (with sustained
flaring of opercula) occurred within 1 d of exposure to towing and air.
FIGURE 3.—Mortality of lingcod subjected to air exposure only or to both net towing and air exposure. Small
fish (,52 cm) were exposed to air at 20.08C for 45 or
60 min (air only treatment) or subjected to 4 h of towing
in 8.08C seawater followed by exposure to air at 20.08C
for 30 or 45 min (towing plus air treatment). Large fish
($52 cm) were exposed to air at 20.08C for 45, 60, or
75 min (air only) or subjected to 4 h of towing in 8.08C
seawater followed by exposure to air at 20.08C for 30,
45, 60, or 75 min (towing plus air). No mortality is
indicated by 0; no data is indicated by nd (i.e., the experimental treatment was not performed because 100%
mortality was already reached at lower stress).
When lingcod were towed in a net for 4 h at
8.08C and abruptly transferred to 8.08C for 30 min
and then to air (16.88C) for 15 min, no mortality
resulted in either small or large fish (N 5 6, 6;
Figure 4). Towed lingcod exposed to 14.08C and
then to air showed 17% mortality in small fish and
0% in large fish (N 5 6, 6; Figure 4), while exposure to 16.08C and then to air resulted in a mortality of 17% in small and large fish (N 5 6, 6;
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DAVIS AND OLLA
F IGURE 4.—Mortality of lingcod exposed to net towing, increased temperature, and air. Small and large fish
were exposed to 4 h of towing in 8.08 C seawater, 30
min in seawater at 8.0, 14.0, 16.0, 18.0, or 20.08 C, and
then air at 16.88 C for 15 min. No mortality is indicated
by 0.
Figure 4). Exposing towed lingcod to 18.08C and
then to air, resulted in a significantly higher mortality in small fish (42%) than in large fish (0%;
N 5 12, 12; one-tailed sign test, P , 0.031; Figure
4). Exposure of towed lingcod to 20.08C and then
to air resulted in a mortality of 100% in small and
large fish (N 5 6, 6; Figure 4). All mortality (with
sustained flaring of opercula) occurred within 1 d
of exposure to towing, temperature, and air.
Discussion
The mortality in lingcod exposed to bycatch processes was magnified by combinations of increased temperature and exposure to air and towing. The exposure of lingcod to increased temperature after 4 h of towing in a net caused mortality to increase as temperature increased between
16.08C and 20.08C. Similarly, while exposure to
air without previous towing caused mortality to
increase as time in air increased from 45 to 75
min, after 4 h of towing the effects of exposure to
air were magnified, causing a reduction in the time
of air exposure that led to 100% mortality.
The stress and mortality induced in fish as a
result of bycatch processes often result from a
combination of several stressors, including capture, environmental factors, and handling. The
magnification of stress caused by the interactions
of stressors is probably a common occurrence
(Wedemeyer et al. 1990; Barton and Iwama 1991).
Previous studies with walleye pollock, sablefish,
and Pacific halibut in the laboratory demonstrated
that the interactions of capture stressors, temperature, and handling caused significant increases in
stress as measured by changes in behavior, blood
physiology, and mortality (Olla et al. 1997, 1998;
Davis and Olla 2001; Davis et al. 2001). Field
studies of bycatch mortality related to trawling
have shown that significant factors which control
mortality rates often include net entrainment, mesh
passage, crushing and wounding, sustained swimming until exhaustion, changes in pressure, towing
time and depth, time on deck, surface water and
air temperatures, and the mean size of fish (Neilson
et al. 1989; Chopin and Arimoto 1995; Richards
et al. 1995; Ross and Hokenson 1997). Interacting
stressors in the field may also degrade the potential
for liberated fish to evade predators in the short
term and disrupt reproductive activity in the long
term (Schreck et al. 1997, 2001). Laboratory studies have shown that predator evasion was diminished in walleye pollock, sablefish, and Pacific halibut after towing in a net (Olla et al. 1997; C. Ryer,
Alaska Fisheries Science Center, personal communication). Because conditions are highly variable and uncontrolled in field studies, it has not
been possible to quantify the relative strength for
the induction of stress and mortality by stressors
and their interactions.
Lingcod was relatively resistant to air exposure,
with or without towing for 4 h prior to air exposure. A similar resistance to the effects of air after
capture by trawl was noted in winter flounder Pleuronectes americanus, which did not show measurable mortality until after 45 min on deck (Ross
and Hokenson 1997). In contrast, other studies of
bycatch in the field have shown that mortality induced by exposure to air may occur rapidly, with
exposure to air for 15 min being sufficient to induce mortality in Pacific halibut, witch flounder
Glyptocephalus cynoglossus, American plaice Hippoglossoides platessoides, and pollock Pollachius
virens (Hoag 1975; Richards et al. 1995; Trumble
et al. 1995; Ross and Hokenson 1997). In a field
experiment with lingcod captured by trawl, deck
time was more stressful than trawling. As deck
time was increased from 10 to 60 min, the mortality in fish (55–75 cm TL) rose from 0% to 94%
(50% mortality at 30 min), while increased towing
time from 1 and 5 h prior to time on deck did not
increase mortality (S. Parker, Oregon Department
of Fish and Wildlife, personal communication).
With lingcod, the laboratory and field results suggested that the sorting, handling, and release of
bycatch on deck after capture may be conducted
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NET-TOWED LINGCOD MORTALITY
in a manner that would probably enhance survival
if fish are released within 30 min of capture. Of
course, the possibility of reducing bycatch mortality by the rapid liberation of fish depends on the
type of handling they receive. If fish are discarded
using a gaff or other tools which cause external or
internal injury, then mortality rates will be greatly
increased.
Sensitivity to towing in a net in the laboratory
differed among species. Lingcod, sablefish and Pacific halibut did not experience mortality when
towed in a net for 4 h at a speed at which they did
not swim (1.1 m/s), followed by exposure to 8.08C
seawater for 30 min and air for 15 min (Olla et al.
1998; Davis and Olla 2001; Davis et al. 2001).
However, walleye pollock appeared to be more
sensitive as 100% mortality occurred 14 d after
towing in a net for 15 min at 0.85 m/s, a speed at
which the fish did not swim (Olla et al. 1997). The
speeds at which these species were towed and did
not swim appeared to be similar to the observed
maximum sustained swimming speeds (1.1–1.2
m/s) for a variety of marine species (He 1993).
The towing speeds and current speeds were the
same in our laboratory nets. The current speeds
inside our laboratory nets cannot be compared with
the speeds inside commercial trawls since the latter
have not been measured and are probably lower
than the actual trawl towing speeds because of
trapped water and fish.
Lingcod mortality increased with increasing
temperature after net towing, and the exposure to
increased temperature after capture has generally
been found to be a significant source of stress and
mortality for fish in both laboratory studies (Olla
et al. 1998; Davis and Olla 2001; Davis et al. 2001)
and field studies (Plumb et al. 1988; Barton and
Iwama 1991; Muoneke and Childress 1994; Schisler and Bergersen 1996; Ross and Hokenson 1997).
Seasonal increases in temperature associated with
thermoclines and deck conditions would probably
increase the mortality of bycatch that might otherwise survive capture and release in cooler seasons. In fisheries observer programs, it would be
useful to record water and deck temperatures and
body core temperatures, in addition to recording
the characteristics of capture and handling processes as well as fish condition and length. Realistic estimates of bycatch mortality related to temperature could be made for various fisheries by
combining such observer information with field
measurements of the relationship between fish
body core temperature, stress, and mortality.
Comparing the results of experiments performed
1101
in the laboratory with results obtained in the field
must be done with caution. The experiments in
this study were generally conservative in their effects relative to field capture conditions and did
not attempt to duplicate highly variable field conditions. There were some minor stressors that were
unavoidably present in these laboratory experiments, and these were considered part of the overall stressor treatments (including dipnetting and
transfer to tanks between treatments). The capture
of lingcod by trawl would also include exposure
to the effects of depth and pressure changes, crushing and wounding, changes in temperature that are
generally less rapid than those simulated in the
laboratory, handling on deck during discard of bycatch, the presence of predators, and exposure to
thermocline conditions after discarding from the
deck. The conclusions from the present study emphasize the principles of stressor action in bycatch
mortality. Ongoing work in our laboratory seeks
to combine the principles of bycatch mortality with
the physiological measures of stress in an effort
to develop portable measures of fish condition
(Davis et al. 2001). Once the principles of bycatch
mortality are correlated with the physiological
measures of fish condition, then appropriate field
experiments can be designed to estimate bycatch
mortality by measuring the condition of fish that
are captured in various fisheries.
Delayed mortality is an important factor to consider in the design of bycatch experiments. When
lingcod were exposed to capture stressors, there
was no delayed mortality after 1 d. Delayed mortality in laboratory studies has been observed after
capture for up to 3 d in sablefish, up to 14 d in
walleye pollock, and up to 30 d in Pacific halibut
(Olla et al. 1997, 1998; Davis and Olla 2001). In
the field, delayed mortality has been observed for
Atlantic mackerel Scomber scombrus, plaice Pleuronectes platessa, sole Solea solea (also known as
Solea vulgaris), Atlantic herring Clupea harengus,
and lingcod (Lockwood et al. 1983; Van Beek et
al. 1990; Suuronen et al. 1996a, 1996b; Albin and
Karpov 1998). Although our results suggest that
holding lingcod for 1 d after capture is sufficient
to evaluate potential mortality, delayed mortality
has been observed for lingcod up to 12 d after
capture by hook and line; therefore experiments
with lingcod bycatch mortality should include a
12-d period of observation for delayed mortality
(Albin and Karpov 1998).
Lingcod mortality resulting from towing and air
exposure was greater in small fish (41–51 cm TL)
than in large fish (52–67 cm TL). In a previous
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DAVIS AND OLLA
field study, a similar effect of size was hypothesized for lingcod captured by hook and line, although this could not be confirmed because of
small sample sizes (Albin and Karpov 1998). The
relationship between fish size and rates of mortality induced by capture processes has been observed previously in field studies of fish that were
captured and passed through trawl gear into a net
pen for observation of potential delayed mortality.
Under these conditions, species in which smaller
fish showed greater mortality included haddock
Melanogrammus aeglefinus, European whiting
Merlangius merlangus, vendace Coregonus albula,
and Atlantic herring (Suuronen et al. 1995, 1996a,
1996b; Sangster et al. 1996). This size effect was
generally attributed to fatigue in smaller fish from
swimming as they passed down the net and to
greater injury from abrasion as they passed along
and through the net mesh. Studies of trawl-caught
Atlantic halibut and Pacific halibut that were held
in tanks after capture also found that injury and
mortality were greater in the smaller fish (Neilson
et al. 1989; Richards et al. 1995). The smaller
lingcod were more sensitive to increased temperature after net towing, and similar size effects were
also suggested in laboratory studies on sablefish
and Pacific halibut as the body core temperature
warmed faster in smaller fish than in larger fish
(Olla et al. 1998; Davis and Olla 2001; Davis et
al. 2001). Size differences in fish sensitivity to
increased temperature would be expected in general, based on the observed inverse relationship
between the rate of body core temperature increase
and fish size (Spigarelli et al. 1977).
Fisheries management strategies often rely on
minimum size rules to enhance recruitment and
yield, with the assumption that the release of undersized fish may contribute to future recruitment.
Unless there is a significant avoidance of undersized bycatch in the fishery, the consequences of
differential fishing mortality among size-classes
are dependent on the mortality rates of released
undersized bycatch. The mortality rates of relevant
size-classes of fish are needed to validate the assumptions of management rules for released, undersized bycatch (Wilson and Burns 1996; Bettoli
and Osborne 1998).
Minimum size regulations have been applied
widely to lingcod fisheries. A limit of 61 cm TL
was imposed on all recreational and commercial
fisheries for lingcod in California, Oregon, and
Washington in 1998. Canadian commercial and
sport fisheries are subject to a minimum size regulation of 65 cm TL for lingcod. Our results
showed that while the mortality of released lingcod
larger than 52 cm may be low enough to support
increased yields, the mortality of fish smaller than
52 cm exposed to temperatures greater than 16.08C
or to air for periods longer than 30 min may be
too high for the size rule to result in effective
fishery management. High-grading (the practice of
discarding smaller fish and landing larger ones for
economic reasons) would not be recommended
when smaller fish have higher mortality rates, as
this would disproportionally increase bycatch mortality. To confirm potential size effects, research
must be performed in the field to determine how
capture processes and seasonal conditions may affect the mortality of undersized lingcod bycatch
that are released.
The management of lingcod stocks off Washington, Oregon, and California in 2001 allowed a
total catch of 611 metric tons, with retention of
lingcod only during May–October for trawl gear,
fixed gear and open access, trip limits of 182 kg/
month for each gear type, and 1–2 fish bag limit
in sport fisheries (depending on the season and
state of jurisdiction). In Canada, the management
of lingcod in 2001 relied on a combination of
closed areas for protection, trip limits (which were
higher than in U.S. fisheries), and a total catch of
950 metric tons. Trip limits were 6,818 kg/month
for trawl during January–December (with various
closed areas and measures to reduce towing time
and to avoid the capture of bycatch), 6,818 kg/
month per month for hook and line during April–
November, and 1–3 fish bag(s) in sportfishing during June–September or April–November, depending on the jurisdictional zone.
Unfortunately under present management scenarios, discarded bycatch and mortality continue
to increase as trip limits are reduced and bycatch
limits are reached sooner, and as efforts shift to
those species with remaining quotas to be fished.
For example, under the present management for
trawling, the increased fishing mortality associated
with lingcod bycatch would eventually result in a
severe depression of stocks and the closure of other
fisheries that impact lingcod. One radical alternative would be to retain all fish caught, close
seasons during periods of warmer weather and water temperatures, and close seasons once bycatch
limits have been reached. This alternative is not
acceptable to fish processors and fishermen because it would reduce the duration of fishing seasons, make fish unavailable during warmer parts
of the year, require the use of expensive observer
programs, and produce a larger quantity of smaller,
NET-TOWED LINGCOD MORTALITY
lower value fish. Sport fishermen would be excluded from the summer months when recreation
has its highest value. The most politically and economically feasible management alternatives appear to be the avoidance of areas with lingcod, the
reduction of trawling times to 1–3 h, and the implementation of shorter handling times (,30 min)
of discarded bycatch on deck as a result of smaller
catches per tow.
Downloaded by [Oregon State University] at 16:42 22 August 2011
Acknowledgments
We thank Steve Parker and Polly Rankin for the
collection and transport of lingcod to the laboratory. Michele Ottmar, Mara Spencer, Erick Sturm,
and Rich Titgen provided excellent technical assistance during rearing and experimentation. The
protocols used in this research conform to the
guidelines for the ethical treatment of experimental animals prescribed by Oregon State University.
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