The IUCN/SSC Shark Specialist Group
Shark News 7: June 1996
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Release mortality studies in
Massachusetts
Gregory Skomal and Bradford Chase, Massachusetts Division of
Marine Fisheries, Vineyard Haven, Massachusetts
Extensive recreational fisheries for tunas, billfish and sharks occur off
the coast of New England from June through October each year. Those
species commonly targeted by offshore anglers include: bluefin tuna
Thunnus thynnus, yellowfin tuna T. albacares, albacore tuna
T. alalunga, bigeye tuna T. obesus, skipjack tuna Katsuwonus pelamis,
Atlantic bonito Sarda sarda, false albacore Euthynnus alletteratus,
white marlin Tetrapterus albidus, blue marlin Makaira nigricans, blue
shark Prionace glauca, and mako shark Isurus oxyrinchus.
In recent years, there has been an increasing trend in the release
of angled gamefish by the offshore recreational fishing sector. Catch
and effort data compiled by the Massachusetts Division of Marine
Fisheries from 60 big game fishing tournaments held in Massachusetts
from 1987 to 1995 show that 5,821 large pelagics were caught by
tournament anglers during 29,345.5 boat hours of fishing effort.
Overall, 75.7% of these fish were released and 21.4% were tagged
before release. Notably, 78.9% (8.4%) of the bluefin tuna, 96.7%
(74.5%) of the white marlin, and 92.0% (20.8%) of the blue sharks
were released (tagged).
While some of this is due to the imposition of management
measures such as minimum sizes and bag limits designed to reduce
mortality on immature fish, there have also been changing attitudes
among recreational anglers and tournament organisers. Cooperative
tagging programmes have contributed greatly to the education of
fishermen relative to fish conservation and the importance of biological
study. The angler that tags and releases fish now feels a sense of
contributing to causes that will enhance the fishery. Discussions
about numbers of fish tagged have slowly replaced those about
numbers of fish killed among the sport fishing community.
Little is known of the mortality associated with the release of
pelagic gamefish. Evidence from National Marine Fisheries Service
(NMFS) Cooperative Tagging Programs shows a higher recapture rate
for sharks (4%) (N.E. Kohler, NMFS, NEFC, pers. comm.) than billfish
(1.1%) (Bayley and Prince 1994) and non-bluefin tunas (2.6%)
(D. Rosenthal, NMFS, SEFC, pers. comm.). Although low recapture
rates can be attributed to tag shedding, emigration, stock size, natural
mortality, and reporting failure, mortality associated with angling
stress cannot be discounted.
In general, fish react to the acute stress of capture, severe exercise,
and handling with more exaggerated disruptions to their physiology
than those seen in higher vertebrates (see reviews by Wood 1991 and
Milligan 1996). Nearly all species of fish have a substantial proportion
of their myotomal muscle mass (80%-95%) as anaerobic white
swimming muscle which reflects an ability for high work output in
short bursts (Driedzic and Hochachka 1978). Angling practices cause
increased anaerobic activity, muscular fatigue, and time out of water,
resulting in marked respiratory and metabolic changes (Wood 1991;
Ferguson and Tufts 1992).
Since fish blood comprises only 3%-6% of the body weight and
white muscle over 30%, changes in muscle biochemistry will be
reflected strongly by the composition of the blood (Wells et al. 1986).
Therefore, measuring the changes in various haematological
parameters relative to the degree of physical exhaustion can provide
useful indices of stress.
The objective of our ongoing study is to elucidate the
physiological effects of angling-induced stress on the survivor-ship
of pelagic species commonly caught offshore of New England. In
contrast to previous studies, fish are captured, tagged, and released
utilising standard angling practices and equipment. Field-collected
blood samples provide a 'snapshot' of the physiological status of each
animal taken. The response of each blood constituent to varying levels
of stress is quantified. While it is understood that this study, like its
predecessors, cannot overcome the difficulty of obtaining blood from
stress-free fish, we do attempt to measure sub-lethal and lethal
disturbances due to the effects of the various angling practices.
Hypotheses on release mortality are tested using acoustic telemetry.
To date, we have sampled 289 gamefish comprised of 12 species
of sharks, tunas and billfish. Due to sample size limitations, the bulk
of our analyses have been confined to bluefin tuna, yellowfin tuna and
blue sharks. Preliminary findings show that these fish exhibit
fluctuations in blood pH and blood levels of hormones, electrolytes
and metabolites due to the fight associated with rod and reel angling.
Each species was found to have a different physiological response to
angling. For example, the metabolic by-product of anaerobic glycolysis
is lactic acid. We found that blood lactate levels in angling-stressed
tunas were significantly higher than those in sharks and marlin.
Moreover, bluefin tuna possessed extremely high levels of blood
lactate relative to other species sampled. Since blood lactic acid
readily dissociates into the lactate anion and hydrogen protons, the
amount of this metabolite in the blood contributes to the acidity of the
blood. By measuring the pH of the blood, we can determine the extent
of the acidosis. Extreme acidosis can cause more complex physiological
disturbances which may severely impede normal behaviour and
ultimately compromise survivorship (Wood et al. 1983).
For each species, changes in blood chemistry can be compared to
several variables which are associated with the fight such as tackle
type, fight time, water temperature and fish size. Most of the
correlations we have conducted to date are associated with fight
time. The following gives a brief preliminary synopsis of what
happens physiologically to
bluefin tuna, yellowfin tuna and
blue sharks during the angling
event.
Anglers may apply tags without bringing the shark into the boat.
This should help to reduce stress and improve survivorship. Photo: H. Wes Pratt.
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Bluefin tuna
This species exhibits immediate
drops in blood pH due to the
build-up of carbon dioxide and
metabolic by-products in the
blood. This acidosis seems to
drive the pH to its lowest level
in fish that have been fought
for 20 to 25 minutes.
Yellowfin tuna
The blood pH measurements
made on yellowfin tuna fought
on rod and reel are much lower
than those reported as 'normal'
by other researchers for this
species. Although the degree
of acidosis fluctuates greatly
with fight time, lowest pH levels
are reached after as little as 10
minutes of fighting.
Blue shark
The magnitude and nature of
blood disturbances appear to
be less dramatic in the sharks
when compared to the tunas.
Blood gas measurements indicate that the blue shark is not hampered
by respiratory problems when fought on rod and reel; blood oxygen
levels remain relatively high. Blood pH does decrease slowly to a low
at a fight time of about 40 minutes. This can probably be attributed to
the slow increase in metabolic by-products like lactate. Nonetheless,
pH levels remained appreciably higher in this species relative to the
tunas fought for similar durations.
Survivorship
Can these species recover from this physiological disturbance? Short
and long term recovery from the acute stress associated with exhaustive
exercise was evaluated from tag-recapture and ultrasonic tracking
studies. Both methods allow for inferences on the effects of tagging.
We have tracked two blue sharks, three bluefin tuna, and one
yellowfin tuna after exposure to prolonged fights on rod and reel,
blood sampling, and tagging. Minimum tracking periods for these fish
were eight hours, with the exception of one blue shark which was
followed for four hours. All fish survived this tracking and appeared
to recover from the physiological effects of exhaustive exercise.
Tag recaptures of two blue sharks and one yellowfin tuna that
were previously blood-sampled, by the study provided long term
evidence that these fish were not physiologically compromised by the
angling experience or the tagging.
It is very important to emphasise the scope of this study. We are
specifically attempting to quantify and assess the physiological effects
of rod and reel angling. In doing so, we encounter varying degrees of
physical trauma as well. The rough handling of fish, the use of gaffs,
internal hook damage, poor tagging, and excessive time out of
water can cause irreparable damage to a fish which is released.
Recovery may take days or months if the fish survives. While some
degree of physical trauma can be assessed in this study, short term
ultrasonic tracking may not be sufficient to measure the long term
effects of such trauma. Tag recaptures of our sampled fish do help to
rectify this. Physiological stress can be minimised by reducing fight
and handling time. However, physical trauma can only be reduced
through the conscious efforts of anglers when choosing to tag and
release a fish. Hook design, handling methods, tagging tools, and
experience all play a major role in the proper tag and release of
gamefish.
Photo: H. Wes Pratt.
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The importance of tagging large pelagic species of sharks, tunas
and marlin cannot be over-emphasised. These are species of fish
which cannot easily be maintained in captivity for biological studies.
What we know of their complex biology, we must derive from dead
specimens or from tagging studies. A single recapture can provide
important information on migration, distribution, age, growth, longevity
and reproductive biology without killing the fish. The recreational
angler has been an integral component of our tagging programmes for
decades and has thus contributed to the pool of knowledge that
scientists now have to work with. Only through these efforts can
scientists provide a valid foundation on which wise measures of
conservation and utilisation of these species can be built.
References
Bayley, R.E., and Prince, E.D. 1994. A review of tag release and
recapture files for Istiophoridae from the Southeast Fisheries
Center's Cooperative Gamefish Tagging Program, 1954 to present.
ICCAT, Coll. Vol. Sci. Pap. 41: 527-548.
Driedzic, W.R., and Hochachka, P.W. 1978. Metabolism in fish
during exercise. In: Hoar, W.S., and Randall, D.J. (eds.). Fish
Physiology, Vol. VII, Locomotion. Pp. 503-543, Academic Press,
NY.
Ferguson, R.A., and Tufts, B.L. 1992. Physiological effects of brief air
exposure in exhaustively exercised rainbow trout ( Oncorhynchus
mykiss): implications for 'catch and release' fisheries. Can. J. Fish.
Aquat. Sci. 49: 1157-1162.
Milligan, C.L. 1996. Metabolic recovery from exhaustive exercise in
rainbow trout. Comp. Biochem. Physiol. 113A(1): 51-60.
Wells, R.M.G., McIntyre, R.H., Morgan, A.K., and Davie, P.S. 1986.
Physiological stress responses in big gamefish after capture:
observations on plasma chemistry and blood factors. Comp.
Biochem. Physiol. 84A(3): 565-571.
Wood, C.M. 1991. Acid-base and ion balance, metabolism, and their
interactions after exhaustive exercise in fish. J. Exp. Biol. 160:
285-308.
Wood, C.M., Turner, J.D., and Graham, M.S. 1983. Why do fish die
after severe exercise? J. Fish Biol. 22(2): 189-201.
Gregory Skomal and Bradford Chase, Massachusetts Division of
Marine Fisheries, P.O. Box 68, Vineyard Haven, MA 02568-
0068, USA. Fax: + 1 508 693 4157. Email:
gskomal@whsun1.wh.whoi.edu
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