132
for organisms which depend upon them, e.g., marine fishes
and invertebrates.
Marine organisms including fishes and invertebrates have
long been consumed by humans, with the earliest evidence
extending back some 140,000 years to South Africa where
shellfish and shallow-water fishes were consumed (Marean
et al. 2007). Yet the development of fishing equipment is
believed to have arisen 40,000 years later, based upon the
oldest known fishing hooks found in East Timor (O’Connor
et al. 2011). Since then, the evolution of fishing gears and
vessels have supported a transition from small-scale subsistence fishing to modern-day commercial fishing, making
seafood one of the most traded food commodities worldwide
(FAO 2016b). Fishing is now considered to be the most
widespread, unsustainable human impact on the oceans
(Pauly et al. 2002; Halpern et al. 2008; Ricard et al. 2012),
with 31.4% of fish stocks estimated to be fished at biologically unsustainable levels and therefore overfished in
2016 (FAO 2016b). More recently, the Food and Agricultural
Organization of the United Nations (FAO) estimated that
89% of global fish stocks are exploited or overexploited
(Zhou 2017).
On tropical coasts, fishing occurs at both subsistence and
commercial levels, and targets an array of vertebrate (i.e.,
fishes such as snapper, parrotfish and grouper), and invertebrate (e.g., penaeid shrimp and mud crab) species. Many of
these target species are considered ‘mobile links’ (Moberg
and Folke 1999) due to the roles they play in connecting ecosystems across the tropical seascape through diel, seasonal
and/or ontogenetic migrations (Parrish 1989; Cocheret de la
Morinière et al. 2002; Mumby 2006). The larvae of the grey
snapper (Lutjanus griseus) migrate towards their nursery
area among the mangroves where they develop into juveniles, which later migrate to seagrass beds, and finally to
coral reefs as adults, where they reproduce and the cycle
repeats (Fig. 4) (Luo et al. 2009). Penaeid prawns also
undergo a number of habitat shifts during their development,
with the eggs released by adults on offshore waters undergoing two post-larval stages before they migrate to mangrove
areas as juveniles. Late stage juveniles then move towards
alternative habitats such as seagrasses before they transfer to
their offshore adult habitat (Fig. 5) (Robertson and Duke
1987). Several studies have indicated that the abundance and
diversity of fish communities in particular, are higher in
regions where three tropical ecosystems were in close proximity, compared to those where they were a significant distance apart (Unsworth et al. 2008). The transition of
organisms is not only a biological link between tropical ecosystems, it also results in a substantial transfer of organic
matter, nutrients, and energy across ecosystems (Deegan
1993). It can therefore be postulated that the exploitation of
certain species, would have knock-on effects for connectivity pathways among tropical marine ecosystems.
Gulf menhaden (Brevoortia patronus), small euryhaline
clupeid fish found in the waters of the Gulf of Mexico, play
an important role in exporting nutrients and energy between
estuaries and offshore waters (Deegan 1993). They feed on
phytoplankton and detritus and are in turn an important prey
item for larger predatory fishes. They also support the second
largest commercial fishery (by weight) in North America
(Vaughan et al. 2007). When combined, their ecological and
economic values mean this species, along with other
Brevoortia species have been described as “the most important fish in the sea” (Franklin 2007). Although not currently
considered overfished, exploitation of this species correlates
to reduced production of larger pelagic fishes, and may lead
to considerable effects on the trophic structure of ecosystems
in the Gulf of Mexico (Robinson et al. 2015). Further
research is essential to understand the impact of harvesting
B. patronus on nutrient and energy export to adjacent systems. However, it can be seen that the exploitation of organisms with key ecological roles could have adverse effects on
resource transfer among ecosystems and trophic levels. This
theory could be applied to multiple exploited organisms transitioning between tropical ecosystems, however, research
into the ecological roles of many of these organisms remains,
at present, uninvestigated, thus the impact of their exploitation unknown.
One organism, whose role is known, and of vital importance to the health of coral reefs is the Caribbean rainbow
parrotfish (Scarus guacamaia). Adults of this species play a
pivotal role in regulating algal cover on reefs, and consequently preventing phase-shifts (Heenan and Williams
2013). There is evidence that the success of this species is
dependent on the success of nearby mangroves. The juveniles of S. guacamaia are dependent on mangroves as nursery areas, and in Belizean coral reefs it was found that the
density of adult parrotfish was significantly higher in
mangrove- rich regions compared to mangrove-scarce regions
(Mumby et al. 2004). Similar findings were made in Aruba,
where recruitment of juvenile parrotfish from mangroves to
coral reefs was dependent on the maximum distance (10 km)
between these two habitats. S. guacamaia were therefore not
be able to be recruited to coral reefs situated at a greater distance from mangroves (Dorenbosch et al. 2006). Coral reefs
with adjacent mangrove nurseries exhibit increased parrotfish grazing (Mumby et al. 2007), and are consequently considered more resilient to perturbations. However, parrotfish
are highly sensitive to exploitation, and several species
including S. guacamaia are currently classified by the
International Union for the Conservation of Nature as ‘near
threatened’. Exploited populations can only maintain 5% of
a reef in a permanently grazed state compared to 40% in
unexploited populations (Mumby 2006), which has implications concerning increased algal proliferation and its effect
on adjacent ecosystems (see section “A Nutritious Ocean”
H. S. Earp et al.
for organisms which depend upon them, e.g., marine fishes
and invertebrates.
Marine organisms including fishes and invertebrates have
long been consumed by humans, with the earliest evidence
extending back some 140,000 years to South Africa where
shellfish and shallow-water fishes were consumed (Marean
et al. 2007). Yet the development of fishing equipment is
believed to have arisen 40,000 years later, based upon the
oldest known fishing hooks found in East Timor (O’Connor
et al. 2011). Since then, the evolution of fishing gears and
vessels have supported a transition from small-scale subsistence fishing to modern-day commercial fishing, making
seafood one of the most traded food commodities worldwide
(FAO 2016b). Fishing is now considered to be the most
widespread, unsustainable human impact on the oceans
(Pauly et al. 2002; Halpern et al. 2008; Ricard et al. 2012),
with 31.4% of fish stocks estimated to be fished at biologically unsustainable levels and therefore overfished in
2016 (FAO 2016b). More recently, the Food and Agricultural
Organization of the United Nations (FAO) estimated that
89% of global fish stocks are exploited or overexploited
(Zhou 2017).
On tropical coasts, fishing occurs at both subsistence and
commercial levels, and targets an array of vertebrate (i.e.,
fishes such as snapper, parrotfish and grouper), and invertebrate (e.g., penaeid shrimp and mud crab) species. Many of
these target species are considered ‘mobile links’ (Moberg
and Folke 1999) due to the roles they play in connecting ecosystems across the tropical seascape through diel, seasonal
and/or ontogenetic migrations (Parrish 1989; Cocheret de la
Morinière et al. 2002; Mumby 2006). The larvae of the grey
snapper (Lutjanus griseus) migrate towards their nursery
area among the mangroves where they develop into juveniles, which later migrate to seagrass beds, and finally to
coral reefs as adults, where they reproduce and the cycle
repeats (Fig. 4) (Luo et al. 2009). Penaeid prawns also
undergo a number of habitat shifts during their development,
with the eggs released by adults on offshore waters undergoing two post-larval stages before they migrate to mangrove
areas as juveniles. Late stage juveniles then move towards
alternative habitats such as seagrasses before they transfer to
their offshore adult habitat (Fig. 5) (Robertson and Duke
1987). Several studies have indicated that the abundance and
diversity of fish communities in particular, are higher in
regions where three tropical ecosystems were in close proximity, compared to those where they were a significant distance apart (Unsworth et al. 2008). The transition of
organisms is not only a biological link between tropical ecosystems, it also results in a substantial transfer of organic
matter, nutrients, and energy across ecosystems (Deegan
1993). It can therefore be postulated that the exploitation of
certain species, would have knock-on effects for connectivity pathways among tropical marine ecosystems.
Gulf menhaden (Brevoortia patronus), small euryhaline
clupeid fish found in the waters of the Gulf of Mexico, play
an important role in exporting nutrients and energy between
estuaries and offshore waters (Deegan 1993). They feed on
phytoplankton and detritus and are in turn an important prey
item for larger predatory fishes. They also support the second
largest commercial fishery (by weight) in North America
(Vaughan et al. 2007). When combined, their ecological and
economic values mean this species, along with other
Brevoortia species have been described as “the most important fish in the sea” (Franklin 2007). Although not currently
considered overfished, exploitation of this species correlates
to reduced production of larger pelagic fishes, and may lead
to considerable effects on the trophic structure of ecosystems
in the Gulf of Mexico (Robinson et al. 2015). Further
research is essential to understand the impact of harvesting
B. patronus on nutrient and energy export to adjacent systems. However, it can be seen that the exploitation of organisms with key ecological roles could have adverse effects on
resource transfer among ecosystems and trophic levels. This
theory could be applied to multiple exploited organisms transitioning between tropical ecosystems, however, research
into the ecological roles of many of these organisms remains,
at present, uninvestigated, thus the impact of their exploitation unknown.
One organism, whose role is known, and of vital importance to the health of coral reefs is the Caribbean rainbow
parrotfish (Scarus guacamaia). Adults of this species play a
pivotal role in regulating algal cover on reefs, and consequently preventing phase-shifts (Heenan and Williams
2013). There is evidence that the success of this species is
dependent on the success of nearby mangroves. The juveniles of S. guacamaia are dependent on mangroves as nursery areas, and in Belizean coral reefs it was found that the
density of adult parrotfish was significantly higher in
mangrove- rich regions compared to mangrove-scarce regions
(Mumby et al. 2004). Similar findings were made in Aruba,
where recruitment of juvenile parrotfish from mangroves to
coral reefs was dependent on the maximum distance (10 km)
between these two habitats. S. guacamaia were therefore not
be able to be recruited to coral reefs situated at a greater distance from mangroves (Dorenbosch et al. 2006). Coral reefs
with adjacent mangrove nurseries exhibit increased parrotfish grazing (Mumby et al. 2007), and are consequently considered more resilient to perturbations. However, parrotfish
are highly sensitive to exploitation, and several species
including S. guacamaia are currently classified by the
International Union for the Conservation of Nature as ‘near
threatened’. Exploited populations can only maintain 5% of
a reef in a permanently grazed state compared to 40% in
unexploited populations (Mumby 2006), which has implications concerning increased algal proliferation and its effect
on adjacent ecosystems (see section “A Nutritious Ocean”
H. S. Earp et al.