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Indirect EffectsCase Study: The Tuna-Dolphin Issue
Lisa T. Ballance
Marine Mammal Biology – SIO 133
Spring 2016
Background: The Tuna-Dolphin Issue
The association between yellowfin tuna, spotted and spinner
dolphins, and tuna-dependent seabirds forms the basis for
one of the world’s largest tuna fisheries.
• A hallmark of the eastern tropical Pacific is the prevalence of multi-species aggregations of spotted and spinner dolphins, yellowfin tuna, and large, speciose flocks of seabirds.
• The dolphins and seabirds provide a clear visual signal of the location of surface tuna schools.
• By the 1950s, tuna fishers were using “dolphin sets” to catch large schools of large tuna. Dolphin schools were visually located, and chased by speed boats into a purse seine net. The co-schooling tuna followed the dolphins and were also captured.
Fishing Gear 101: Purse Seines
• The problem with setting on dolphins
Purse-seine Net Setting Phases
High incidental mortality of dolphins in the early
years of the fishery resulted in declaration of three
stocks as “depleted” under the U.S. Marine
Mammal Protection Act.
• It is estimated that more than 6 million dolphins have been killed since the fishery began, 4 million by 1972, when the MMPA was enacted.
– Northeastern Offshore Spotted Dolphins have been estimated to be between 19 and 28% of pre-exploitation levels.
– Eastern Spinner Dolphins have been estimated to be at 44% of pre-exploitation levels.
Wade 1995, Wade et al. 2007
Revised fishing practices (including, notably, the “back-
down” procedure) drastically reduced dolphin mortality by
1990 with the expectation that depleted dolphin stocks would
recover.
DOLPHIN BYCATCH
IN ETP TUNA FISHERY
0
100
200
300
400
500
600
1955 1965 1975 1985 1995 2005
NU
MB
ER
OF
DO
LP
HIN
S K
ILL
ED
(T
HO
US
AN
DS
)
Total for all dolphins
Wade 1995; Marine Mammal Commission 1998
The 2006 point estimates of abundance for
the three depleted stocks are higher than
estimates from 1998-2000.
• From 1998-2006, Northeastern Offshore Spotted, and Eastern Spinner dolphins have increased at rates of 0.035, and 0.092, respectively.
• However,– The uncertainty surrounding point
estimates of abundance is substantial (due to the large size of the study area).
– The 95% confidence intervals of growth rate over the 20-year time series, and since 1998, for both stocks include zero (= no recovery).
Gerrodette et al. 2008
Ab
un
da
nc
e i
n T
ho
us
an
ds
It is unclear to what degree (or whether)
these dolphins are recovering.
There are three main hypotheses to explain these abundance
trends.
H1: The purse-seine fishery is reducing dolphin reproduction and survival
H2: An oceanic regime shift has changed the ecosystem so that recovery to pre-exploitation numbers cannot occur
H3: Our expectation of immediate and rapid recovery is too simple
These are not comprehensive, and not mutually exclusive.
Ecological
disruption
• Breaking the tuna-
dolphin bond
• Removing tuna from
the ecosystem
Social
disruptionIncreased
fetal/calf
mortality
Decreased
fecundity
Potential to decrease dolphin reproduction and survival
The fishery involves chase and
encirclement.
It may be associated with:
H1: The purse-seine fishery is reducing dolphin reproduction
and survival
Indirect Effects*
a. Separation of females and dependent calves
b. Decreased fecundity
c. Social disruption of mating systems
*Perturbations that reduce reproduction or survival, but do not directly kill
But first - Quantifying the Perturbation
• Using recorded number of purse-seine sets, estimates of dolphin abundance and school size, and …
• assuming (falsely) that sets and dolphins are randomly spread in space and time,
– A given individual dolphin may be chased 10 times and set upon 3 times/yr
– “Large” schools (carrying more tuna) are set on 2-8 times/yr
Perkins and Edwards 1999, Reilly et al. 2005
• Using recorded number and location of purse-seine sets, recorded location of dolphin schools, and dolphin movement patterns based on tracking studies, an “Exposure Index” can be created.– Spotted dolphins are exposed to purse-seine sets more than
eastern spinner dolphins– Exposure to purse-seine sets is greater in the “Eastern Pacific
Warm Pool” for dolphins of both species
• This index (or similar) can be correlated with other dolphin traits to investigate the impact of purse-seine sets.*
Archer et al. 2010*Remember for later
a. Separation of females and dependent calves
Dependent calves are missing from the school encircled in the purse-seine net.
• Data from 9,397 sets w/ kill, 77,361 dolphins killed, two spotted dolphin stocks, 1973-2000
• Observers aboard vessels collected life history data (e.g., sex, teeth for age, reproductive status, length)
• 75-95% of lactating females killed in purse-seine sets did not have their nursing calves with them.
• This calf deficit represents a ~14% increase in number of calves killed as reported by fisheries observers.
• The estimated total “calf deficit” ranges from 10s to 8300/yr. Archer et al. 2004
Echelon swimming is energetically beneficial to a calf, and costly to a mother.
• Kinematic analysis of bottlenose dolphin mother-infant (0-1 mo) pairs in echelon position versus lone infants and lone mothers
• Infants in echelon position have:– 28% increase in mean swim speed– 22% reduction in fluke stroke amplitude– 19% increase in distance/stroke– >1/3 time gliding
• Mothers in echelon position have:– 76% of mean maximum swim speed– 13% reduction in distance/stroke
Noren 2007, Noren et al. 2008
Evasive behavior of mothers, coupled with the developmental state of calves, provides a plausible mechanism for set-related mother-calf separations and subsequent mortality of
calves.
Noren and Edwards 2007
b. Decreased fecundity
Photographs provide a wealth of life history information.
• For example, – Number and
proportion of school as dependent calves
– Length (proxy for age)
• 1987 – 2003– Eastern spinner: 11466
dolphins counted (88 schools); 1280 measured (33 schools)
– NE spotted: 9700 dolphins counted (124 schools); 298 measured (31 schools)
Cramer et al. 2008
• Length of calf at dissociation from mother decreased with increased number of purse seine sets for spotted dolphins (but not spinners)
• Proportion of school as calves decreased with increased number of purse seine sets for spotted dolphins (but not spinners)
Cramer et al. 2008
% s
cho
ol a
s ca
lves
# sets to which dolphins were exposed
# sets to which dolphins were exposedLen
gth
of
calf
at
dis
soci
atio
n
Additionally, proportion of pregnant females in a school reflects exposure to purse-seine sets.
• Blubber progesterone concentrations in dolphin biopsy samples indicate pregnancy state
• 212 biopsy samples from female spotted dolphins collected 1998-2003
• 11.8% of sampled females were pregnant
Fishery Exposure was significantly lower for pregnant vs. non-pregnant females
Kellar et al., 2013
Cramer et al. 2008
“ …the effect of dolphin sets on both measures of reproduction for NEPS [spotted] dolphins demonstrates that the practice of setting on dolphins has population-level effects beyond the direct kill recorded by observers on fishing vessels. ….”
c. Social disruption of mating systems
Spinner dolphins show a geographical range in mating systems.
• Samples collected from 1,678 male spinner dolphins, 1969-1992
• Sexual dimorphism in dorsal fin shape higher in Eastern (79.2%) than Whitebelly (3.0%) males
• Testis size lower in Eastern (max 843 g) than Whitebelly (max 1354 g) males
• High sexual dimorphism and low testis size indicate polygyny in a wide variety of mammal species
Eastern spinner dolphins are more polygynous than other spinner dolphin ecotypes
Perrin and Mesnick 2003
“Eastern”
“Whitebelly”
“Pantropical Gray’s”
• Recall that the fishery exposure index is greatest in highest density area of eastern spinner dolphins
• Relatively few males may be involved in successful matings, as is the case with polygynous species
If chase and capture in the purse-seine fishery breaks up school structure, it may negatively impact reproduction in eastern spinner dolphins
Perrin and Mesnick 2003
Meanwhile, purse-seine sets on dolphin schools continue.
• The number of sets on dolphins has been relatively stable for the past 20 years, at ~10,000 sets per year.
• There have been drastic reductions in observed dolphin kills since 1990 (~99% of captured dolphins are released alive), but that number remains in the 1000s.
-- 3,885 dolphins
reported killed
-- (0.33 dolphins
per set)
Nu
mb
er
pe
r Y
ea
r
IATTC 2008; IATTC unpublished
Reading
• Competition with Fisheries. in: Perrin,
W.F., B. Würsig, and J.G. M. Thewissen.
2009. Encyclopedia of marine mammals.
Second edition. Academic Press.