Indirect Effects Case Study: The Tuna-Dolphin Issue Lisa T. Ballance Marine Mammal Biology SIO 133 Spring 2018
Background
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 Purse-seine Net Setting Phases The problem with setting on dolphins
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 preexploitation 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 backdown procedure) drastically reduced dolphin mortality by 1990 with the expectation that depleted dolphin stocks would recover. 600 DOLPHIN BYCATCH IN ETP TUNA FISHERY NUMBER OF DOLPHINS KILLED (THOUSANDS) 500 400 300 200 100 Total for all dolphins 0 1955 1965 1975 1985 1995 2005 Wade 1995; Marine Mammal Commission 1998
Abundance in Thousands 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). It is unclear to what degree (or whether) these dolphins are recovering. Gerrodette et al. 2008
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.
H1: The purse-seine fishery is reducing dolphin reproduction and survival Social disruption Decreased fecundity The fishery involves chase and encirclement. It may be associated with: Increased fetal/calf mortality Ecological disruption Breaking the tunadolphin bond Removing tuna from the ecosystem Potential to decrease dolphin reproduction and survival
https://www.youtube.com/watch?v=ib96vsn6xpy
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.* *Remember for later Archer et al. 2010
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 purseseine 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 motherinfant (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 mothercalf separations and subsequent mortality of calves. Noren and Edwards 2007
b. Decreased fecundity
Photographs provide a wealth of life For example, Number and proportion of school as dependent calves Length (proxy for age) history information. 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 % school as calves Proportion of school as calves decreased with increased number of purse seine sets for spotted dolphins (but not spinners) # sets to which dolphins were exposed Length of calf at dissociation from mother decreased with increased number of purse seine sets for spotted dolphins (but not spinners) # sets to which dolphins were exposed Cramer et al. 2008
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. nonpregnant females Kellar et al., 2013
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.. Cramer et al. 2008
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 Pantropical Gray s Whitebelly 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 Eastern Perrin and Mesnick 2003
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
Number per Year 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) 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.