Download Joel E. Cohen, Laboratory of Populations, Rockefeller University

Joel E. Cohen, Laboratory of Populations, Rockefeller University
Body size, population variability, and food webs: observations and speculations
The bigger the average sizes of trees in an area, the smaller the number or population density per
unit area of those trees, in many studies. A quantitative form of this relationship is known as the
self-thinning law. The allometric negative scaling of population density with increasing body mass
has been widely claimed, and sometimes disputed, for many animals as well. Using forestry data, we
examined empirically and theoretically some relations between body mass (estimated by aboveground biomass) and population mean density and variability. We speculate on some possible
consequences of such relations for food webs. A principal goal of this talk is to learn of data or
analytical ideas that could be used to test these ideas and speculations.
Richard Law, Department of Biology, University of York
On balanced exploitation of marine ecosystems
A balanced approach to exploitation of marine ecosystems has recently been suggested by some
fisheries biologists. This entails setting exploitation rates in proportion to the natural productivity of
the components of the system; these components may be species, body sizes, or both. However,
balanced harvesting does require a different perspective in fisheries. Our current paradigm is that
harvesting small fish is bad and harvesting large fish is good. Yet it is typically the small fish that
have high productivity and large fish that have low productivity. This talk will give some results
showing that, unconventional though it would be to harvest small fish and leave large fish, there are
potential benefits in doing so. Such harvesting generates substantial biomass yields, while retaining
resilience to perturbations and having relatively little impact on the size structure of the system as a
whole.
Jeppe Kolding, Department of Biology, University of Bergen
Size selective mortality in fisheries and nature
Fishing mortality is a combination between the quantity of fishing effort and the selectivity of the
fishing gear, and size selective regulations, aiming at protecting the young, has a century long history
in fisheries. Life history theory predicts that the balance between growth and reproduction is
determined from the ratio between adult and juvenile survival, and this is reflected in generalised
evolutionary life strategies such as r- and K-selection, where the latter is considered the more
vulnerable to human exploitation. Productivity, the creation of tissue per time unit, in Nature is
largely a function of body size, which explains the ubiquitous pyramidal shape of aquatic trophic
foodwebs, but the slope of the pyramid is again determined by the ratio between adult and juvenile
survival. Comparing life history theory with fisheries theory reveals a deep conceptual mismatch that
has adverse unintended consequences for human food production in the short term and for the
resilience, dynamics and conservation of natural ecosystems in the long term.
Shijie Zhou, CSIRO Marine and Atmospheric Research, Queensland
Quantitative ecosystem approach to fisheries
Overfishing, stock collapse, and concerns over ecological effects of fishing have called for an
ecosystem approach to fisheries (EAF) worldwide. EAF is considered to be a holistic approach for
healthy ecosystems and sustainable food production. However, these two fundamental inclusive EAF
goals have been viewed as conflicting, and data deficiency has often become an excuse for
inappropriate management. Research related to EAF has been a core part in CSIRO. In this talk, I will
present some of our recent and current research on theory of biodiversity management, stock
assessment, population estimation, and ecological risk assessment, particularly in data poor
situations. The examples may provide a sketch of opportunities for applied mathematics and
statistics and inspire a discussion on potential collaboration under the EAF paradigm
Julia Blanchard, Department of Biology, University of Sheffield
Projecting climate change impacts on large marine ecosystems and global fisheries
In the absence of detailed knowledge on how fish species will respond to climate change, simple
macroecological approaches can be usefully employed to investigate the potential impacts on
production of marine ecosystems across a body size range typically encompassing mesozooplankton
and fish larvae up to the largest predatory fishes. Simple theory based on the transfer of energy
from smaller to larger organisms and temperature dependence on metabolic rates can be used to
predict the abundance-body size distribution from primary productivity and temperature. Dynamic
size spectrum models allow for time-dependent processes to be taken into account and testing
effects of different fishing scenarios on the size spectrum.
Using a coupled physical-biogeochemical model, climate-mediated changes in primary productivity
and temperature were modelled across 12 regional domains, encompassing 28 large-marine
ecosystems around the world, under future climate change scenarios. This information was used to
force a dynamic size-structured community model to estimate the potential abundance-size
distributions of marine predators, across size ranges that are typically dominated by fish and squid.
The dynamic size spectrum model is then used to investigate the effects of different fishing
strategies on potential yield of the overall system. We compare potential production estimates from
size-based models in the absence of exploitation to present day observed catches of small pelagic,
demersal and large pelagic fishes and discuss the potential implications for fisheries in the future.