Global fishery development patterns are driven by profit but not trophic level

217

248

Photosynthesis fuels marine food webs, yet differences in fish catch across globally distributed marine ecosystems far exceed differences in net primary production (NPP). We consider the hypothesis that ecosystem-level variations in pelagic and benthic energy flows from phytoplankton to fish, trophic transfer efficiencies, and fishing effort can quantitatively reconcile this contrast in an energetically consistent manner. To test this hypothesis, we enlist global fish catch data that include previously neglected contributions from small-scale fisheries, a synthesis of global fishing effort, and plankton food web energy flux estimates from a prototype high-resolution global earth system model (ESM). After removing a small number of lightly fished ecosystems, stark interregional differences in fish catch per unit area can be explained (r = 0.79) with an energy-based model that (i) considers dynamic interregional differences in benthic and pelagic energy pathways connecting phytoplankton and fish, (ii) depresses trophic transfer efficiencies in the tropics and, less critically, (iii) associates elevated trophic transfer efficiencies with benthic-predominant systems. Model catch estimates are generally within a factor of 2 of values spanning two orders of magnitude. Climate change projections show that the same macroecological patterns explaining dramatic regional catch differences in the contemporary ocean amplify catch trends, producing changes that may exceed 50% in some regions by the end of the 21st century under high-emissions scenarios. Models failing to resolve these trophodynamic patterns may significantly underestimate regional fisheries catch trends and hinder adaptation to climate change.fisheries catch | primary production | ocean productivity | climate change | food webs N early 50 years ago, John Ryther (1) published his seminal work "Photosynthesis and Fish Production in the Sea" in which he hypothesized that differences in net primary production (NPP) alone could not explain stark differences in fish catch across disparate marine ecosystems. Drawing upon "trophic dynamic" principles governing the transfer of energy through ecosystems (2), he hypothesized that synergistic interactions between NPP differences, the length of food chains connecting phytoplankton and fish, and the efficiency of trophic transfers were required to reconcile catch differences. The trophodynamic principles drawn upon by Ryther now underpin diverse models and indicators used in ecosystem-based marine resource management (3-5), yet stubborn uncertainties in the relationship between ocean productivity and fish catch persist. Official catch reports often omit globally significant discards and small-scale artisanal and subsistence catch (6). Uncertain differences in fishing effort and impacts of fishing history complicate attribution of catch differences to "bottom-up" considerations of ocean productivity or "top-down" fishing effects (7). Difficulties directly observing and estimating trophodynamic properties, including NPP (8),...

Section: Discussionmentioning

confidence: 99%

Reconciling fisheries catch and ocean productivity

Stock

John

Rykaczewski

et al. 2017

217

248

“…Where implemented, these adjustments might reduce the resilience of fastgrowing species and put all harvested species at similar risks of decline. In addition, economic forces or management regime may be more important than life history in determining whether fishing effort is successfully controlled (15,16). Small pelagic species, although often possessing a rapid growth rate, are also highly catchable, and therefore susceptible to overfishing (17).…”

mentioning

confidence: 99%

Unexpected patterns of fisheries collapse in the world's oceans

Pinsky

Jensen

Ricard

et al. 2011

209

156

Understanding which species are most vulnerable to human impacts is a prerequisite for designing effective conservation strategies. Surveys of terrestrial species have suggested that largebodied species and top predators are the most at risk, and it is commonly assumed that such patterns also apply in the ocean. However, there has been no global test of this hypothesis in the sea. We analyzed two fisheries datasets (stock assessments and landings) to determine the life-history traits of species that have suffered dramatic population collapses. Contrary to expectations, our data suggest that up to twice as many fisheries for small, low trophic-level species have collapsed compared with those for large predators. These patterns contrast with those on land, suggesting fundamental differences in the ways that industrial fisheries and land conversion affect natural communities. Even temporary collapses of small, low trophic-level fishes can have ecosystem-wide impacts by reducing food supply to larger fish, seabirds, and marine mammals.body size | ecosystem-based management | food webs | life-history theory | marine conservation O verfishing is one of the most serious conservation concerns in marine ecosystems (1), but understanding which species are most at risk remains a challenge. On land, life-history traits are strong predictors of extinction risk (2), and vulnerable species often have large body size and high trophic level (2, 3). In marine ecosystems, the well-publicized declines of large predatory fishes (4, 5) suggest that similar trends may also be common in the sea. However, research to date has found or proposed a wide range of life-history characteristics that cause high vulnerability, including large body size (6-9), late maturity (6, 9), long lifespan (6, 8-11), low fecundity and high parental investment in offspring (11, 12), or high trophic level (2, 13). Understanding which traits, or combinations of traits, are most useful for predicting vulnerability has been difficult because analyses have been limited to regional comparisons or narrow species groups, and because reliable global data have not been available to more broadly test which types of fishes are most likely to suffer fisheries collapse.In addition, there are reasons to believe that regional or terrestrial life-history trends might not apply globally in the ocean. For example, fishery biologists often recommend higher harvest rates for fast-growing, highly productive species, and lower harvest rates for species with lower productivity (14). Where implemented, these adjustments might reduce the resilience of fastgrowing species and put all harvested species at similar risks of decline. In addition, economic forces or management regime may be more important than life history in determining whether fishing effort is successfully controlled (15,16). Small pelagic species, although often possessing a rapid growth rate, are also highly catchable, and therefore susceptible to overfishing (17). Finally, the conflict with human development that is ...

“…However, it has been shown that within the past 50 y of industrial fisheries, the collapse of small-and fast-growing pelagic species has been more frequent than in larger species (16). Since fisheries developed in the 1950s, they have preferentially targeted largebiomass, shallow-water species, such as small pelagics (20). This historical pattern of fisheries development, combined with the increasing global market demand for small pelagic fish as food, fishing bait, fish meal, and oil (21), has probably contributed to their massive declines.…”

Section: Discussionmentioning

confidence: 99%

Global population trajectories of tunas and their relatives

Juan‐Jordá

Mosqueira

Cooper

et al. 2011

121

Tunas and their relatives dominate the world's largest ecosystems and sustain some of the most valuable fisheries. The impacts of fishing on these species have been debated intensively over the past decade, giving rise to divergent views on the scale and extent of the impacts of fisheries on pelagic ecosystems. We use all available age-structured stock assessments to evaluate the adult biomass trajectories and exploitation status of 26 populations of tunas and their relatives (17 tunas, 5 mackerels, and 4 Spanish mackerels) from 1954 to 2006. Overall, populations have declined, on average, by 60% over the past half century, but the decline in the total adult biomass is lower (52%), driven by a few abundant populations. The trajectories of individual populations depend on the interaction between life histories, ecology, and fishing pressure. The steepest declines are exhibited by two distinct groups: the largest, longest lived, highest value temperate tunas and the smaller, short-lived mackerels, both with most of their populations being overexploited. The remaining populations, mostly tropical tunas, have been fished down to approximately maximum sustainable yield levels, preventing further expansion of catches in these fisheries. Fishing mortality has increased steadily to the point where around 12.5% of the tunas and their relatives are caught each year globally. Overcapacity of these fisheries is jeopardizing their longterm sustainability. To guarantee higher catches, stabilize profits, and reduce collateral impacts on marine ecosystems requires the rebuilding of overexploited populations and stricter management measures to reduce overcapacity and regulate threatening trade.