437
One particularly observable anthropogenic impact is the loss of historic nesting
beaches to coastal development projects. Female sea turtles generally exhibit high
site fidelity to particular nesting beaches, and the availability of natural, undeveloped beaches is dwindling. It is unclear how many nesting beaches may have previously existed in the northern GOM. A 2006 study in the Caribbean estimated that
20% of historic (twentieth century) green and hawksbill nesting beaches in the
Caribbean had been lost and that 50% of the remaining beaches were visited by
fewer than ten nesting females (McClenachan et al. 2006). Beaches in the northern
GOM today have minimal sea turtle nesting relative to the Atlantic regions (except
for Kemp’s ridleys), although previous nesting density is largely unknown
(Hildebrand 1982; Valverde and Holzwart 2017). Currently, most nesting beaches in
the US GOM are in Florida and Texas (see Valverde and Holzwart 2017 for a thorough account of nesting sites and habitats). However, most Kemp’s ridleys found in
US waters hatched on Mexican beaches; thus, conservation measures require an
international perspective.
The majority of the existing data on population-level effects of oil spills on
marine megafauna comes from shallow water spills and their effects on coastal populations. A long-term study of the effects of the Exxon Valdez spill on killer whale
populations indicates that two pods suffered acute losses during the event (33% and
41% of their members) and had not returned to pre-spill numbers nearly 20 years
later (Matkin et al. 2008). One of the two exposed pods appeared to be headed for
extinction at the conclusion of the study (Matkin et al. 2008). Evidence suggests
that lipophilic chemical contaminants are often offloaded from mother to calf in
marine mammals, including bottlenose dolphins (Irwin 2005) and killer whales
(Ylitalo et al. 2001), leading to higher levels of calf mortality.
26.3.1 Study Methods: Pre-DWH
Marine mammal and sea turtle populations in the offshore GOM have historically
been quantified in the offshore GOM using shipboard and aerial visual surveys.
Aerial continental shelf surveys began in 1979, initially conducted by the USFWS
and then by NMFS. Offshore marine mammal surveys were conducted by NMFS in
the spring or summer of 1990–1994, 1996–2001, 2003–2004, and 2009 (Waring
et al. 2015; Mullin and Fulling 2004; Mullin 2007) (Fig. 26.1). Major survey efforts
in the early to mid-1990s (GulfCet study, supported by the Minerals Management
Service) focused on the continental slope region (100–2000 m) and recorded both
marine mammals and turtles over eight surveys in all seasons (Würsig et al. 2000).
Despite these survey efforts, few species had enough sightings to produce robust
population size estimates, and none could be analyzed for long-term population
trends due to low precision and infrequent assessments. As a result, little baseline
information was available on population trends prior to the DWH spill. The only
cetaceans with adequate population data were bottlenose dolphins resident in
26 Impacts of the Deepwater Horizon Oil Spill on Marine Mammals and Sea Turtles
One particularly observable anthropogenic impact is the loss of historic nesting
beaches to coastal development projects. Female sea turtles generally exhibit high
site fidelity to particular nesting beaches, and the availability of natural, undeveloped beaches is dwindling. It is unclear how many nesting beaches may have previously existed in the northern GOM. A 2006 study in the Caribbean estimated that
20% of historic (twentieth century) green and hawksbill nesting beaches in the
Caribbean had been lost and that 50% of the remaining beaches were visited by
fewer than ten nesting females (McClenachan et al. 2006). Beaches in the northern
GOM today have minimal sea turtle nesting relative to the Atlantic regions (except
for Kemp’s ridleys), although previous nesting density is largely unknown
(Hildebrand 1982; Valverde and Holzwart 2017). Currently, most nesting beaches in
the US GOM are in Florida and Texas (see Valverde and Holzwart 2017 for a thorough account of nesting sites and habitats). However, most Kemp’s ridleys found in
US waters hatched on Mexican beaches; thus, conservation measures require an
international perspective.
The majority of the existing data on population-level effects of oil spills on
marine megafauna comes from shallow water spills and their effects on coastal populations. A long-term study of the effects of the Exxon Valdez spill on killer whale
populations indicates that two pods suffered acute losses during the event (33% and
41% of their members) and had not returned to pre-spill numbers nearly 20 years
later (Matkin et al. 2008). One of the two exposed pods appeared to be headed for
extinction at the conclusion of the study (Matkin et al. 2008). Evidence suggests
that lipophilic chemical contaminants are often offloaded from mother to calf in
marine mammals, including bottlenose dolphins (Irwin 2005) and killer whales
(Ylitalo et al. 2001), leading to higher levels of calf mortality.
26.3.1 Study Methods: Pre-DWH
Marine mammal and sea turtle populations in the offshore GOM have historically
been quantified in the offshore GOM using shipboard and aerial visual surveys.
Aerial continental shelf surveys began in 1979, initially conducted by the USFWS
and then by NMFS. Offshore marine mammal surveys were conducted by NMFS in
the spring or summer of 1990–1994, 1996–2001, 2003–2004, and 2009 (Waring
et al. 2015; Mullin and Fulling 2004; Mullin 2007) (Fig. 26.1). Major survey efforts
in the early to mid-1990s (GulfCet study, supported by the Minerals Management
Service) focused on the continental slope region (100–2000 m) and recorded both
marine mammals and turtles over eight surveys in all seasons (Würsig et al. 2000).
Despite these survey efforts, few species had enough sightings to produce robust
population size estimates, and none could be analyzed for long-term population
trends due to low precision and infrequent assessments. As a result, little baseline
information was available on population trends prior to the DWH spill. The only
cetaceans with adequate population data were bottlenose dolphins resident in
26 Impacts of the Deepwater Horizon Oil Spill on Marine Mammals and Sea Turtles