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

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(T

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)

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

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ho

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an

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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

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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

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pe

r Y

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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.