The Sharkdive, from lucky man Marty's fantastic new crop - and yes, those are links!
Back to the Sharks and to real science.
This new paper summarizes the current status of Sharks. It’s fascinating reading, sometimes counter intuitive and represents the facts as they are known today.
The good news is that the situation is not uniformly catastrophic but that there are great variations among species, habitats and geographic locations. The bad news is that it is fair to say that most large Sharks are acutely endangered.
Required reading for anybody wanting to speak about the topic.
Bold, italics and links are mine.
From
Francesco Ferretti, Boris Worm, Gregory L. Britten, Michael R. Heithaus and Heike K. Lotze
Patterns and ecosystem consequences of shark declines in the ocean.
Abstract
Whereas many land predators disappeared before their ecological roles were studied, the decline of marine apex predators is still unfolding.
Large sharks in particular have experienced rapid declines over the last decades. In this study, we review the documented changes in exploited elasmobranch communities in coastal, demersal, and pelagic habitats, and synthesize the effects of sharks on their prey and wider communities.
We show that the high natural diversity and abundance of sharks is vulnerable to even light fishing pressure.
The decline of large predatory sharks reduces natural mortality in a range of prey, contributing to changes in abundance, distribution, and behaviour of small elasmobranchs, marine mammals, and sea turtles that have few other predators. Through direct predation and behavioural modifications, top-down effects of sharks have led to cascading changes in some coastal ecosystems. In demersal and pelagic communities, there is increasing evidence of mesopredator release, but cascading effects are more hypothetical. Here, fishing pressure on mesopredators may mask or even reverse some ecosystem effects.
In conclusion, large sharks can exert strong top-down forces with the potential to shape marine communities over large spatial and temporal scales. Yet more empirical evidence is needed to test the generality of these effects throughout the ocean.
From the paper
Here, we attempt to synthesize what is known about the ecological role of sharks, which are among the largest and most wide-ranging predators in the ocean.
This topic has received urgent attention over the past decade, as studies have indicated rapid and widespread declines, particularly of large sharks, because of the direct and indirect effects of fishing.
This has prompted questions about the nature and scale of the ecological consequences.
In this study, we begin by briefly reviewing the ecological features of sharks, highlighting differences from other marine predators. We then analyse the current state and history of shark exploitation, searching for general patterns of community change in coastal, demersal, and pelagic ecosystems. Finally, we synthesize the expected and observed effects of sharks on marine ecosystems from experimental, empirical, and modeling studies. In the conclusion, we attempt to explain under which conditions sharks are expected to play a unique role, and how that role may depend on the ecosystem context.
This study is largely based on all major peer-reviewed papers published on this topic over the past decade, but also includes important earlier work.
Vulnerability to fishing
Most chondrichthyans are characterized by low growth rates, late sexual maturity, and low fecundity compared to bony fish, which makes them vulnerable to fishing mortality. A comparison of 26 shark and 151 bony fish populations found that sharks show twice the fishing extinction risk of bony fishes.
Also their ability to recover after depletion is low on average: rebound potential of 26 shark populations ranged between 14% (Mustelus californicus) and 1.7% (Squalus acanthias) per year with variability explained by a combined effect of size and preferred habitat.
In fact, it was highest for small coastal sharks, intermediate for pelagic and minimal for large coastal species. Deep-water sharks may be among the most vulnerable to fishing, with population growth rates 40–60% lower compared with pelagic, and 55–63% lower compared with coastal species. As a life history trade-off, most elasmobranchs invest more into juvenile survival and growth rather than fecundity.
Elasticity analyses show that changes in juvenile and adult survival and age at maturity have the highest contributions to population growth rate.
This explains why elasmobranch populations generally respond strongly to changes in both predation and fishing. While exploitation often leads to decreased ages at maturity and increased fecundities in teleosts , there is little evidence for such compensating responses in elasmobranchs.
Finally, while life history determines the level of mortality sharks can sustain, their vulnerability depends on the combination of life history, sensitivity to habitat loss and exposure to perturbations such as catchability and availability to fisheries.
The latter relates to many factors including geographic range, habitat use, behaviour, and body size.
Only over the past 5–10 years has the conservation status of many elasmobranchs been systematically evaluated by the International Union for the Conservation of Nature (IUCN).
Its shark specialist groups concluded that elasmobranchs are primarily threatened by fishing (96.1%) including directed commercial (31.7%), by-catch (57.9%), recreational (0.7%) and artisanal ⁄ subsistence fishing (5.8%), followed by habitat destruction (2.9%) and pollution (0.4%, http://www.redlist.org).
Of the 1159 chondrichthyans known, 881 species have been evaluated globally with 42.8% listed as data deficient (DD), 25.7% least concern (LC), 13.9% near threatened (NT), 11.2% vulnerable (VU), 4.1% endangered (EN) and 2.4% critically endangered (CR, http://www.redlist.org).
Status varies by region, with the highest proportion of threatened (VU, EN, CR) species in the Mediterranean and NE Atlantic, while in the NW Pacific the situation appears less critical.
However, there is considerable uncertainty as many species are listed as data deficient or not yet assessed. Three regional IUCN assessments further highlight the critical situation in the Mediterranean and NE Atlantic, while providing a more optimistic assessment for Australia.
Coastal ecosystems
These studies suggest that even light fishing pressure by artisanal and subsistence fishing on remote islands or sharknetting programs along continental shores can be sufficient to cause dramatic declines in populations of large coastal sharks.
This would explain why such populations are now rare or absent in more impacted systems such as the Gulf of Mexico, Caribbean, and Mediterranean Sea. Moreover, shark-netting data suggest some patterns of ecological reorganisation. As large coastal sharks declined, catch rates of more fecund and wide-ranging species such as tiger (Galeocerdo cuvier) or hammerhead sharks (Sphyrna spp.) increased, at least temporarily, in shark nets in New South Wales, South Africa, and Queensland.
However, it is unclear at this point to which extent these reflect changes in abundance, distribution, or behaviour.
Demersal ecosystems
More than 90% of elasmobranch species worldwide inhabit demersal ecosystems on continental shelves and slopes which makes them vulnerable to trawl fishing. When trawling begins, catches of elasmobranchs are usually abundant and diverse including both small and large species despite the lower catchability of the latter. As fishing proceeds, this initial diversity and abundance can be eroded very quickly. Large sharks often disappear from catches, and the community becomes dominated by smaller elasmobranch mesopredators.
Moreover, major restructuring of elasmobranch communities can occur through differential vulnerabilities to fishing gears, variation in spatial occurrence relative to fishing areas, and release from predation and competition. Over time, trawl fisheries often expand towards further offshore and deeper grounds where elasmobranch communities are composed of less resilient species. At this stage, domains of developed trawl fisheries often exceed the habitat and dispersal range of many elasmobranch species, leaving no spatial refuges. This is the case of the Mediterranean, where a century of trawl fishing led to the loss of 16 of 31 recorded elasmobranch species in the Tyrrhenian Sea, six of 33 species in the Adriatic Sea and half of the elasmobranch species recorded in trawl fisheries in the Gulf of Lion since the 1950s
Pelagic ecosystems
Industrial fisheries in the open ocean started in the 1950s primarily to catch tuna and swordfish on the high seas. Fisheries statistics and scientific surveys were available from the beginning, and early catch rates essentially reflect unexploited fish communities.
Sharks constituted a substantial by-catch, and often a nuisance in causing damage to hooked target fish. In the Gulf of Mexico and Pacific Ocean, longliners caught about one shark for every two yellowfin tuna and in the Atlantic, 2–3 sharks for every swordfish.
This led to rapid declines in shark catches over the last 50 years.
In the Pacific, standardized catch rates of Carcharhinus falciformis decreased by 91.7%, while in the Gulf of Mexico those of C. longimanus were reduced by > 99% . In the NW Atlantic, 18 coastal and pelagic sharks showed declines in catch rates of 49–89% in <>
At the community level, declines are not uniform across species.
Less resilient carcharhinids usually declined first potentially benefiting more prolific species such as blue and mako sharks (Isurus oxyrinchus). From 1977 to 1994, pelagic fisheries landings in Brazil revealed the disappearance of 14 species of carcharhinids (dominated by C. signatus), and a concomitant increase of mako and blue sharks. Likewise, in the North Pacific, blue shark biomass is estimated to have increased by 20% relative to the 1970s (Sibert et al. 2006), and this species is now considered the most abundant shark in pelagic ecosystems. Mako sharks appear to have declined less than other large species in the Gulf of Mexico, Central Pacific and NW Atlantic.
However, when intense exploitation continues, all large sharks can be virtually eliminated such as in the Mediterranean Sea.
Approximately 21 oceanic elasmobranch species are commonly caught in high seas fisheries; these are broadranging species with circumglobal distribution.
Although there are no documented cases of local species extinctions, 58% of pelagic species are considered threatened by IUCN, more than any other listed group of chondrichthyans.
The high demand for shark fins in Asian markets is an important factor in the decline of pelagic species, which are often highly priced for their fins.
This has motivated new shark fisheries and prompted others to switch from bony fish to sharks. Pelagic sharks range across extensive, poorly monitored areas, thus the amount of sharks taken globally for their fins is estimated to be four times higher than that reported to FAO.
In summary, sharks have been increasingly threatened by the direct and indirect effects of fishing worldwide.
This has caused marked declines in shark populations, particularly larger and less resilient species such as carcharhinids. These declines have coincided with substantial reorganisation of elasmobranch communities, including the rise of smaller sharks and rays in some regions.
ECOSYSTEM CONSEQUENCES
We are just beginning to understand the potential ecological consequences of shark declines, largely because of the difficulties in studying sharks and their prey in their natural environments. Ecosystem models predict that in some situations sharks will exert considerable top-down impacts on their prey, while not in others.
And here follows a cornucopia of examples from across a wide range of habitats.
CONCLUSIONS
Our overview shows that in natural, unexploited systems, sharks often exhibit high abundance and diversity.
Yet even light fishing pressure is sufficient to cause strong population declines in vulnerable species, particularly large sharks. Such trends have been shown for artisanal and subsistence fishing on remote islands, shark netting programs, and in trawl and long-line fisheries in many regions, resulting in community shifts from large- to small-bodied species.
Population declines of large species often exceeded one, sometimes two orders of magnitude with some local extinctions.
Yet some more resilient species have not declined as drastically or have even increased, possibly via reduced competition or predation. Larger shark populations are still seen in some remote or protected areas, particularly in the Pacific, and may provide valuable opportunities to further understand the ecological role of sharks.
Yet, reported catches of sharks and other elasmobranchs are still increasing in most regions, possibly indicating that more fisheries target sharks where they have not been historically exploited, a trend partially driven by the rising demand for highly prized shark fins on Asian markets.
Many large sharks are the sole predators of smaller elasmobranchs and other marine megafauna, and the depletion of large sharks has likely contributed to considerable increases in these species in some regions. With their wide-ranging distribution and predatory role, large sharks in particular can spread their impacts across different ecosystems. Such spatial connectivity has also been shown to be important in freshwater and terrestrial ecosystems and may increase the connectivity and stability of ocean food webs.
Ecosystem models predict that the loss of sharks should result in complex community changes, including trophic cascades, mesopredator release, and consequent declines in some commercial fish.
The strength and persistence of these effects, however, appear to decrease from coastal and reef to demersal and pelagic environments. Observational studies suggest the presence of strong species interactions in some regions, mediated by direct consumption and risk effects, sometimes leading to trophic cascades. As fishing and netting effort has increased, declines in large apex-predatory sharks have coincided with widely documented increases in smaller sharks and rays, as well as mammals and turtles. These mesopredator increases may be partly explained by decreased predation mortality and risk effects, and have in some cases led to increased pressure on prey species, such as invertebrates and teleost fishes or even seagrasses. We must caution that many of the interactions are supported by limited empirical evidence. We are only beginning to study the complex ecological roles that large-bodied, wideranging predators such as sharks play.
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