Essay
Conservation of Marine Megafauna through
Minimization of Fisheries Bycatch
RAM
¯
UNAS
ˇ
ZYDELIS,
∗
†† BRYAN P. WALLACE,
∗
† ERIC L. GILMAN,‡ AND TIMOTHY B. WERNER§
∗
Center for Marine Conservation, Nicholas School of the Environment, Duke University Marine Laboratory, 135 Duke Marine Lab
Road, Beaufort, NC 28516, U.S.A.
†Center for Applied Biodiversity Science, Conservation International, Arlington, VA 22202, U.S.A.
‡IUCN Global Marine Programme and University of Exeter Centre for Ecology and Conservation, 2718 Napuaa Place, Honolulu, HI
96822, U.S.A.
§New England Aquarium, Central Wharf, Boston, MA 02110 and Biology Department, Boston University, 5 Cummington Street,
Boston, MA 02215, U.S.A.
Abstract: Many populations of marine megafauna, including seabirds, sea turtles, marine mammals, and
elasmobranchs, have declined in recent decades due largely to anthropogenic mortality. To successfully con-
serve these long-lived animals, efforts must be prioritized according to feasibility and the degree to which they
address threats with the highest relative impacts on population dynamics. Recently, Wilcox and Donlan (2007,
Frontiers in Ecology and the Environment) and Donlan and Wilcox (2008, Biological Invasions) proposed
a conservation strategy of “compensatory mitigation” in which fishing industries offset bycatch of seabirds
and sea turtles by funding eradication of invasive mammalian predators from the terrestrial reproductive
sites of these marine animals. Although this is a creative and conceptually compelling approach, we find it
flawed as a conservation tool because it has narrow applicability among marine megafauna, it does not
address the most pervasive threats to marine megafauna, and it is logistically and financially infeasible.
Invasive predator eradication does not adequately offset the most pressing threat to most marine megafauna
populations—fisheries bycatch. For seabird populations, fisheries bycatch and invasive predators infrequently
are overlapping threats. Invasive predators have limited population-level impacts on sea turtles and marine
mammals and no impacts on elasmobranchs, all of which are threatened by bycatch. Implementing compen-
satory mitigation in marine fisheries is unrealistic due to inadequate monitoring, control, and surveillance
in the majority of fleets. Therefore, offsetting fisheries bycatch with eradication of invasive predators would be
less likely to reverse population declines than reducing bycatch. We recommend that efforts to mitigate bycatch
in marine capture fisheries should address multiple threats to sensitive bycatch species groups, but these ef-
forts should first institute proven bycatch avoidance and reduction methods before considering compensatory
mitigation.
Keywords: compensatory mitigation, elasmobranchs, fisheries bycatch, invasive predators, marine conserva-
tion, marine mammals, sea turtles, seabirds
Conservacio´n de la Megafauna Marina Medianate la Minimizacio´n de la Captura Incidental de Pesquer´ıas
Resumen: Muchas poblaciones de megafauna marina, incluyendo aves, mamı´feros y elasmobranquios,
han declinado en de´cadas recientes principalmente debido a mortalidad antropoge´nica. Para conservar
exitosamente a estos animales longevos, los esfuerzos se deben priorizar de acuerdo con la factibilidad y el
grado en que atienden las amenazas con los impactos relativos ma´s altos sobre la dina´mica de la poblacio´n.
Recientemente, Wilcox y Donlan (2007) y Donlan y Wilcox (2008) propusieron una estrategia de conservacio´n
de “mitigacio´n compensatoria” en la que las industrias pesqueras compensan la captura incidental de aves
y tortugas marinas mediante el financiamiento de la erradicacio´n de mamı´feros depredadores invasores
††email zydelis@duke.edu
Paper submitted February 22, 2008; revised manuscript accepted September 30, 2008.
608
Conservation Biology, Volume 23, No. 3, 608–616
C©
2009 Society for Conservation Biology
DOI: 10.1111/j.1523-1739.2009.01172.x

ˇZydelis et al. 609
de las localidades reproductivas terrestres de estos animales marinos. Aunque este es un me´todocreativoy
conceptualmente imponente, lo consideramos deficiente como una herramienta de conservacio´n porque su
aplicacio´n es limitada entre la megafauna marina, no aborda las amenazas ma´s grandes a la megafauna
marina y no es factible log´ıstica y financieramente. La erradicacio´n de depredadores invasores no compensa
adecuadamente la amenaza ma´s fuerte para la mayor´ıa de las poblaciones de megafauna marina — la
captura incidental de pesquer´ıas. Para las poblaciones de aves marinas, la captura incidental de pesquer´ıas y
los depredadores invasores son amenazas que se traslapan poco frecuentemente. Los depredadores invasores
tienen impactos limitados a nivel poblacio´n sobre tortugas marinas y mamı´feros marinos y no tienen impactos
sobre elasmobranquios, todos ellos amenazados por la captura incidental. La implementacio´n de la mitigacio´n
compensatoria en pesquer´ıas marinas no es realista debido a que el monitoreo, control y vigilancia son
inadecuados en la mayor´ıa de las flotas. Por lo tanto, es ma´s probable que la compensacio´n de la captura
incidental con la erradicacio´n de depredadores invasores revierta la declinacio´n de poblaciones y no reduzca
la captura incidental. Recomendamos que los esfuerzos para mitigar la captura incidental en las pesquer´ıas
marinas deber´ıan atender amenazas mu´ltiples a los grupos de especies sensibles a la captura incidental,
pero estos esfuerzos primeramente deber´ıan instituir me´todos de reduccio´n de captura incidental antes de
considerar la mitigacio´n compensatoria.
Palabras Clave: aves marinas, captura incidental de pesquer´ıas, conservacio´n marina, depredadores invasores,
elasmobranquios, mamı´feros marinos, mitigacio´n compensatoria, tortugas marinas
Introduction
To conserve long-lived marine megafauna such as sea-
birds, sea turtles, marine mammals, and sharks, manage-
ment strategies must simultaneously take into account
multiple threats to their populations that affect different
life stages (Mills et al. 1999). One such threat is fish-
eries bycatch, which has been implicated as a major
driver in declines of several marine vertebrate popula-
tions (Tuck et al. 2001; Lewison et al. 2004a, 2004b;
Read et al. 2006). Nevertheless, given their tendency to
have broad geographic ranges and, for some taxa, ter-
restrial reproductive sites, impacts of other threats to
these organisms must also be addressed to halt declines
and recover depleted populations (Grand & Beissinger
1997; Flint 1999; Baker et al. 2002). For example, sev-
eral seabird populations have declined owing to adult
and subadult mortality related to fisheries bycatch, but
also to high rates of depredation by invasive mammals on
eggs and chicks at island nesting rookeries or degradation
of terrestrial habitats on nesting islands (Cuthbert et al.
2004; Priddel et al. 2006). Thus, management strategies
should be prioritized based on relative population-level
impacts of different threats across a population’s range to
strategically allocate limited conservation resources (Ger-
ber & Heppell 2004; Murdoch et al. 2007). To establish
priorities a broad array of parameters should be consid-
ered for each proposed action, including effects on target
populations and other components of the ecosystem, im-
plementation costs relative to conservation benefit, and
logistical feasibility.
Wilcox and Donlan (2007) and Donlan and Wilcox
(2008)—hereafter both publications referred to as
D&W—described a market-influenced conservation strat-
egy of “compensatory mitigation.” The authors propose
that large-scale fishing industries could offset their by-
catch of seabirds and sea turtles by investing in conser-
vation activities that reduce impacts of other threats to
those populations. Donlan and Wilcox suggest that such
a scheme would allow continued fishing while paying
for bycatch that is difficult to avoid or very expensive to
mitigate. Admitting that there could be a range of options
for compensatory mitigation, D&W advocate strongly for
eradication of invasive predators because they see it as a
widely applicable and effective strategy to offset bycatch
impacts on threatened species. Donlan and Wilcox sup-
port the strength of this proposal by stating that most
seabirds and sea turtles threatened by fisheries bycatch
are concurrently threatened by invasive predators; erad-
ication is a relatively low-cost option compared with
bycatch mitigation (area closure in their example) but
has potentially high conservation impact according to
a population model of one species (Flesh-footed Shear-
water [Puffinus carneipes]); fisheries industries would
welcome an opportunity to support conservation efforts
without modifying conventional fishing operations; and
eradications have been effectively carried out on islands
of variable size, thus demonstrating their wide applicabil-
ity. We recognize that compensatory mitigation can use a
variety of models, such as those used in U.S. wetlands law
(Environmental Law Institute 2006). Nevertheless, here
we address the case of bycatch offsets as proposed by
D&W, where measures for avoidance and minimization
of adverse impacts are not required before considering
measures to offset the anticipated losses.
Despite the conceptual appeal of D&W’s proposed
management prescription, various authors have criticized
it on several fronts. For example, Doak et al. (2007) ar-
gue that D&W’s evaluation of population-level impacts
of invasive predator eradication is flawed and leads to an
Conservation Biology
Volume 23, No. 3, 2009